The Red Sea

This introduction gives a brief summary of the main features of the Red Sea and its surroundings, including the topography and bathymetry, the geology and geophysics, as well as its tectonic setting. Some of the concepts regarding its formation are mentioned and some geophysical and earthquake data are also shown. The oceanography as well as some environmental and biological aspects of the Red Sea are also outlined. These subjects are discussed in more detail, including current ideas, in the individual chapters in this volume.

Nowhere on the present-day Earth can the transition from a continental to an oceanic rift be observed and studied better than in the Red Sea region, where three rifts in different stages of evolution meet in a triple point located in the Afar region. A thermal and/or compositional mantle plume may have risen from the upper mantle below Afar already at ~30 Ma, and may have triggered, at least in part, the rifting process. The axial area of the rifts is marked by intense seismicity. While the East African is a fully continental rift, the Gulf of Aden rift experienced oceanic crust accretion between Arabia and Somalia starting already at 17 Ma with a progressive westward propagation that impacted against Africa in the Afar Triangle starting at <1 Ma. The axial zone of oceanic crustal accretion in the Gulf of Aden is segmented by several small (<30 km) offsets, including two major transform-fracture zones, the Socotra (offset ~50 km) and the Alula-Fartak (offset 180 km). Spreading is asymmetric, faster in the northern (Arabia) side (11–13 mm/a) than in the southern (Somalia) side (8 mm/a). The Afar Triangle is a topographically depressed region, located between the continental blocks of Nubia, Somalia, and the Danakil Alps, that separate it from the southern Red Sea. It is an area of thin crust, seismicity related to extension, and intense intrusive and extrusive, mostly basaltic, magmatism. Intrusive basaltic magmatism appears to be important in triggering the rifting process in Afar. Northern Afar displays basaltic ranges oriented parallel to the axis of the Red Sea, such as the Erta Ale, with a crestal permanent lava lake. These ranges represent an incipient oceanic accretionary plate boundary separating Africa from Arabia. At the northern tip of Afar, the plate boundary is displaced to the axial zone of the southern Red Sea, an elongated basin oriented ~N30°W. Its southern part is characterized by an axial rift valley floored by oceanic basalt and accompanied by parallel Vine-Matthews magnetic anomalies, suggesting initial oceanic crust accretion at ~5 Ma, although alternative interpretations suggest that the entire width of the southern Red Sea is underlain by oceanic crust. Moving still farther north, the axial valley becomes discontinuous and the initial accretion of oceanic crust appears to take place in discrete cells that become younger northward. Propagation from these initial nuclei will result in a continuous axial zone of oceanic accretion. Some of these axial “deeps” are the locus of intense hydrothermal activity and metallogenesis. Moving north, the oceanic rift impacts against the Zabargad fracture zone, a major topographic-structural feature that crosses the Red Sea in a NNE direction, offsetting its axis by nearly 100 km. Zabargad island, located at the SSW end of the fracture zone, exposes a sliver of sub-Red Sea lithosphere, including mantle peridotite bodies, Pan-African granitic gneisses criss-crossed by basaltic dykes, gabbro intrusions, and a sedimentary sequence starting with pre-rift Cretaceous deposits. North of the Zabargad Fracture zone, the Red Sea lacks an axial rift valley; it probably consists of extended thinned and faulted continental crust injected by gabbros and basaltic dykes. The activation of the NNE-trending Aqaba-Dead Sea fault at about 14 Ma has deactivated rifting in the Gulf of Suez. Basalt chemistry suggests that the degree of melting of the Red Sea subaxial mantle decreases from south to north, in parallel with a decreasing spreading rate and a lesser influence of the Afar plume.

The Red Sea is part of an extensive rift system that includes from south to north the oceanic Sheba Ridge, the Gulf of Aden, the Afar region, the Red Sea, the Gulf of Aqaba, the Gulf of Suez, and the Cairo basalt province. Historical interest in this area has stemmed from many causes with diverse objectives, but it is best known as a potential model for how continental lithosphere first ruptures and then evolves to oceanic spreading, a key segment of the Wilson cycle and plate tectonics. Abundant and complementary datasets, from outcrop geology, geochronologic studies, refraction and reflection seismic surveys, gravity and magnetic surveys, to geodesy, have facilitated these studies. Magnetically striped oceanic crust is present in the Gulf of Aden and southern Red Sea, active magma systems are observed onshore in the Afar, highly extended continental or mixed crust submerged beneath several kilometers of seawater is present in the northern Red Sea, and a continental rift is undergoing uplift and exposure in the Gulf of Suez. The greater Red Sea rift system therefore provides insights into all phases of rift-to-drift histories. Many questions remain about the subsurface structure of the Red Sea and the forces that led to its creation. However, the timing of events—both in an absolute sense and relative to each other—is becoming increasingly well constrained. Six main steps may be recognized: (1) plume-related basaltic trap volcanism began in Ethiopia, NE Sudan (Derudeb), and SW Yemen at ~31 Ma, followed by rhyolitic volcanism at ~30 Ma. Volcanism thereafter spread northward to Harrats Sirat, Hadan, Ishara-Khirsat, and Ar Rahat in western Saudi Arabia. This early magmatism occurred without significant extension or at least none that has yet been demonstrated. It is often suggested that this “Afar” plume triggered the onset of Aden–Red Sea rifting, or in some models, it was the main driving force. (2) Starting between ~29.9 and 28.7 Ma, marine syn-tectonic sediments were deposited on continental crust in the central Gulf of Aden. Therefore, Early Oligocene rifting is established to the east of Afar. Whether rifting propagated from the vicinity of the Sheba Ridge toward Afar, or the opposite, or essentially appeared synchronously throughout the Gulf of Aden is not yet known. (3) By ~27.5–23.8 Ma, a small rift basin was forming in the Eritrean Red Sea. At approximately the same time (~25 Ma), extension and rifting commenced within Afar itself. The birth of the Red Sea as a rift basin is therefore a Late Oligocene event. (4) At ~24–23 Ma, a new phase of volcanism, principally basaltic dikes but also layered gabbro and granophyre bodies, appeared nearly synchronously throughout the entire Red Sea, from Afar and Yemen to northern Egypt. The result was that the Red Sea rift briefly linked two very active volcanic centers covering 15,000–25,000 km

2

in the north and >600,000 km

2

in the south. The presence of the “mini-plume” in northern Egypt may have played a role somewhat analogous to Afar vis-à-vis the triggering of the dike event. The 24–23 Ma magmatism was accompanied by strong rift-normal extension and deposition of syn-tectonic sediments, mostly of marine and marginal marine affinity. The area of extension in the north was very broad, on the order of 1,000 km, and much narrower in the south, about 200 km or less. Throughout the Red Sea, the principal phase of rift shoulder uplift and rapid syn-rift subsidence followed shortly thereafter. Synchronous with the appearance of extension throughout the entire Red Sea, relative convergence between Africa and Eurasia slowed by about 50 %. (5) At ~14–12 Ma, a transform boundary cut through Sinai and the Levant continental margin, linking the northern Red Sea with the Bitlis–Zagros convergence zone. This corresponded with collision of Arabia and Eurasia, which resulted in a new plate geometry with different boundary forces. Red Sea extension changed from rift normal (N60°E) to highly oblique and parallel to the Aqaba–Levant transform (N15°E). Extension across the Gulf of Suez decreased by about a factor of 10, and convergence between Africa and Eurasia again dropped by about 50 %. In the Afar region, Red Sea extension shifted from offshore Eritrea to west of the Danakil horst, and activity began in the northern Ethiopian rift. (6) These early events or phases all took place within continental lithosphere and formed a continental rift system 4,000 km in length. When the lithosphere was sufficiently thinned, an organized oceanic spreading center was established and the rift-to-drift transition started. Oceanic spreading initiated first on the Sheba Ridge east of the Alula-Fartaq fracture zone at ~19–18 Ma. After stalling at this fracture zone, the ridge probably propagated west into the central Gulf of Aden by ~16 Ma. This matches the observed termination of syn-tectonic deposition along the onshore Aden margins at approximately the same time. At ~10 Ma, the Sheba Ridge rapidly propagated west over 400 km from the central Gulf of Aden to the Shukra al Sheik discontinuity. Oceanic spreading followed in the south-central Red Sea at ~5 Ma. This spreading center was initially not connected to the spreading center of the Gulf of Aden. By ~3 to 2 Ma, oceanic spreading moved west of the Shukra al Sheik discontinuity, and the entire Gulf of Aden was an oceanic rift. During the last ~1 My, the southern Red Sea plate boundary linked to the Aden spreading center through the Gulf of Zula, Danakil Depression, and Gulf of Tadjoura. Presently, the Red Sea spreading center may be propagating toward the northern Red Sea to link with the Aqaba–Levant transform. However, important differences appear to exist between the southern and northern Red Sea basins, both in terms of the nature of the pre- to syn-rift lithospheric properties and the response to plate separation. If as favored here no oceanic spreading is present in the northern Red Sea, then it is a magma-poor hyperextended basin with β factor >4 that is evolving in many ways like the west Iberia margin. It is probable that the ultimate geometries of the northern and southern Red Sea passive margins will be very different. The Red Sea provides an outstanding area in which to study the rift-to-drift transition of continental disruption, but it is unlikely to be a precise analogue for all passive continental margin histories.

The rifting apart of continents involves interaction of tectonic and magmatic events that reflect the strain-rate and temperature-dependent processes of solid state deformation and decompression melting within the Earth. The spatial and temporal scales over which these mechanisms localize extensional strain, allowing continental rifts to evolve towards seafloor spreading, remain controversial. Here we show the role played by magmatism during the transition from a continental to an oceanic rift based on geophysical and geochemical data from the Thetis and Nereus Deeps, the two northernmost oceanic cells in the central Red Sea. The Thetis segment is made by coalescence of three sub-cells that become shallower, narrower and younger from south to north. Magnetic data reveal that the initial emplacement of oceanic crust is occurring today in the Thetis northern basin and in the southern tip of Nereus. The intertrough zones that separate the Thetis “oceanic” cell from the Nereus cell to the north, and the Hadarba cell to the south, contain thick sedimentary sequences and relicts of continental crust. A seismic reflection profile running across the central part of the southern Thetis basin shows a ~5 km wide reflector about 3.2 km below the axial neovolcanic zone, interpreted as marking the roof of a magma chamber or melt lens and as a last step in a sequence of basaltic melt intrusion from pre-oceanic continental rifting to oceanic spreading. The spatial evolution of mantle melting processes across Thetis and Nereus is evaluated from the chemical composition of 22 basaltic glasses sampled along 100 km of the rift axis. Trace and major element compositions corrected for crystallization show relationships with age of initial emplacement of the oceanic crust and preserve a clear signal of mantle melting depth variations. While Zr/Y and (Sm/Yb)

n

decrease, Na*/Ti* increases slightly from south to north. Na

8

correlates positively with Fe

8

, and Zr/Y and (Sm/Yb)

n

with both Fe

8

and Na

8

. This indicates that an increase in the degree of melting corresponds to a decrease in the mean pressure of melting, suggesting active mantle upwelling beneath Thetis and Nereus. The inferred sharp rift-to-drift transition marked by magmatic activity with typical MORB signature and a relatively high degree of mantle melting, with no contamination by continental lithosphere, suggests that lower crust and mantle lithosphere had already been replaced by active upwelling asthenosphere before separation of the Nubian and Arabian plates.

The transition from continental rifting to seafloor spreading can be observed along the 2,000 km length of the Red Sea Rift system. Whereas the southern Red Sea shows seafloor spreading since 5 Ma and the central part gives evidence of a transitional stage, the northern Red Sea is thought to represent the latest stage of continental rifting. Ocean deeps along the rift axis are considered to be first seafloor spreading cells that will accrete sometime in the future to a continuous spreading axis. The northern Red Sea deeps are isolated structures often associated with single volcanic edifices in comparison with the further developed larger central Red Sea deeps where small spreading ridges are active. Our analysis of the northern Red Sea deeps showed that not all deeps can be related to initial seafloor spreading cells. Two types of ocean deeps were identified: (a) volcanic and tectonically impacted deeps that opened by a lateral tear of the Miocene evaporites (salt) and Plio-Quaternary overburden; (b) non-volcanic deeps built by subsidence of Plio-Quaternary sediments due to evaporite subrosion processes. These deeps develop as collapse structures. The volcanic deeps can be correlated with their positions in NW–SE-oriented segments of the Red Sea which are consequently termed volcanic segments. The N–S segment, linking the volcanically active NW–SE segments, is termed as “non-volcanic segment” as no volcanic activity is known, in agreement with the magnetic data that show no major anomalies. Accordingly, the deep that was analyzed in this segment is interpreted as a collapse-related structure. However, collapse-type ocean deeps are not limited to the non-volcanic segments as subrosion processes due to hydrothermal circulation are possible at any part of the axial depression. The combined interpretation of bathymetry and seismic reflection profiles gives further insight into lateral salt gliding. Salt rises are present where the salt flows above basement faults. The internal reflection characteristic of the salt changes laterally from reflection-free to stratified, which suggests significant salt deformation during the salt deposition. Acoustically transparent halite accumulated locally and evolving rim synclines were filled by stratified evaporite facies.

The structure of the crust beneath the Red Sea is obscured by thick evaporites, impeding progress in understanding the processes associated with continental rifting and leaving the nature of the crust (whether oceanic or continental) still controversial. Here, version 18.1 of the marine gravity field derived from satellite altimetry measurements by Sandwell and Smith (1997) is examined because the gravity field has the potential to reveal structure associated with the topography of basement beneath the evaporites and with crustal density variations. Comparing the satellite-derived data with gravity data from expeditions of RRS

Shackleton

in 1979 and RV

Conrad

in 1984, discrepancies are found to have standard deviations of 6.1 and 4.9 mGal, respectively, somewhat higher than parts of the gravity data from the open oceans. Coherent features in maps of these discrepancies suggest that some systematic errors in version 18.1 of the gravity field still remain. Nevertheless, they appear not to affect the short-wavelength structure of the data because simple image processing reveals some striking structural features in plan-view. The satellite-derived gravity data are enhanced by showing them with artificial shading and as directional second derivatives. The maps reveal lineaments that cross the central Red Sea. Many of them die out towards the coastlines and have convex-north-west shapes. These features are interpreted as evidence for migrating volcanic segments of oceanic crust, here suggesting a 1.5 mm year

−1

along-axis movement of the sub-axial asthenosphere away from the Afar plume. This migration velocity is modest compared with the plate-opening rate (~12 mm year

−1

here) and compared with velocities deduced from V-shaped ridges near Iceland and the Azores.

We use geodetic, plate tectonic, and geologic observations to quantitatively reconstruct the geologic evolution of the Red Sea and Gulf of Aden since separation of Arabia from Africa in the Late Oligocene. Rifting initiated at 22 ± 3 Ma roughly simultaneously along the full strike of the proto-Red Sea and Gulf of Aden. Rifting began along pre-existing zones of weakness associated with a Pan-African Precambrian collisional suture shortly after the Afro-Arabia Plate was weakened by impingement of the African hot spot (~30 Ma). The initial phase of continental rifting followed a roughly linear trend from the Gulf of Suez in the north, to the Bab-al-Mandab in the south where the Afar Triple Junction (junction of Red Sea, Gulf of Aden, and East African rifts) was located at that time. The initial rate of extension across the rift was roughly half the present-day rate. At 11 ± 2 Ma, the rate of rifting doubled to the present-day rate (24 ± 1 mm/year in the south [~12°N] and 7 ± 1 mm/year in the north [~27°N]) and the configuration of rifting changed in both the northern and southern Red Sea. This time corresponds to the initiation of ocean spreading (i.e., complete severing of the continental lithosphere and intrusion of rift basalts) along the full extent of the Gulf of Aden. The changes in the S Red Sea involved the propagation of the Afar Triple Junction westward to its present location (~11.5°N, 42°E), the transfer of rifting from the S Red Sea (Bab-al-Mandab) to the more N–S-oriented Danakil Depression, and accompanying CCW rotation of the Danakil Block with respect to Africa. In the northern Red Sea, rifting transferred from the Gulf of Suez to the more N–S-oriented Gulf of Aqaba/Dead Sea fault system. The rate of rifting has not changed significantly since that time (i.e., 11 ± 2 Ma). The initiation of rifting at 22 ± 3 Ma corresponds temporally with slowing of Africa–Eurasia convergence by a factor of ~2 and the changes at 11 ± 2 Ma with a second phase of slowing of Africa–Eurasia convergence, while Arabia–Eurasia convergence has remained roughly unchanged since >30 Ma. These observations are consistent with simple models where changes in Africa–Arabia–Eurasia relative plate motions are the fundamental cause of post-Oligocene Middle East and Mediterranean tectonics. Based on the simultaneity between full ocean spreading along the Gulf of Aden and a doubling of the extension rate across the Red Sea, and the change to more N–S-oriented rifting in both the northern and southern Red Sea, we hypothesize that slowing of Africa–Eurasia convergence resulted from a decrease in slab pull on the African Plate across the evolving AR-AF plate boundary.

It was thought that the Kingdom of Saudi Arabia was characterized by low seismic activity and that the Arabian shield was relatively stable. After establishing the Saudi National Seismic Network and occurrence of some felt earthquakes in the Arabian Shield, this assumption changed, especially for the western part of the shield. A study of the most recent felt earthquakes in the Red Sea coastal plain area that were recorded by the Saudi National Seismic Network can help in better understanding the Red Sea rifting. Moment tensor inversion of the waveform data offers a unique insight into the active tectonics of the region as well as providing information for seismic hazard analyses of the western part of Saudi Arabia and the Gulf of Aqaba. Moment tensor analyses of recent earthquakes in the Red Sea coastal plain show that some Cenozoic faults are seismically active, with normal faulting under NE–SW tension and NW–SE compression. This is seen in the northern Red Sea, Gulf of Aqaba, and the Badr and Harrat Lunayyir earthquakes, whereas published moment tensor solutions of the largest earthquakes in and around the Arabian plate indicate that most of the earthquakes in eastern Saudi Arabia reflect reverse faulting with NE forces due to collision with the Eurasian plate. There is also an E–W tension axis acting on the Makkah–Madinah–Nafud volcanic line, in agreement with the fault plane solution of the Qunfudah earthquake. It is recommended that two seismically active regions on either side of the Red Sea (Abu Dabbab and Harrat Lunayyir) should be studied further, including their relation to the rifting of the Red Sea. The Saudi National Seismic Network plays an important role in a better understanding of the seismicity and seismotectonic setting of Saudi Arabia in particular and the Arabian plate in general, including providing data for hazard evaluations.

The seismicity of the southern Red Sea is driven by the geological and geophysical processes taking place at and around the Afar Triple Junction, where the Red Sea, East African, and Gulf of Aden Rifts meet. From this plate tectonic node, the Nubian, Arabian, and Somali Blocks are moving away from each other. The consequent structures being predominantly extensional, almost all focal mechanisms of earthquakes in the region express normal faulting, although local tectonic complications are revealed in subordinate strike-slip mechanisms. The

b

-value, which indicates regional stress conditions and corresponding modalities of energy release, ranges between 0.5 and about 1.5. Seismic episodes are mostly of the swarm type, comprising small to moderate magnitude earthquakes at shallow focal depths. These episodes are occasionally accompanied by volcanism, either basaltic or silicic. Four recent major seismic events in the region, and their significant economic and social consequences, are briefly considered. As the economic development of the region proceeds, the impacts of these natural hazards will inevitably become more severe. This invites the preparation of appropriate mitigation procedures.

The first volcanic eruption known to occur in the southern Red Sea in over a century started on Jebel at Tair Island in September 2007. The early phase of the eruption was energetic, with lava reaching the shore of the small island within hours, destroying a Yemeni military outpost and causing a few casualties. The eruption lasted several months, producing a new summit cone and lava covering an area of 5.9 km

2

, which is about half the area of the island. The Jebel at Tair activity was followed by two more eruptions within the Zubair archipelago, about 50 km to the southeast, in 2011–2012 and 2013, both of which started on the seafloor and resulted in the formation of new islands. The first of these eruptions started in December 2011 in the northern part of the archipelago and lasted for about one month, generating a small (0.25 km

2

) oval-shaped island. Coastal erosion during the first two years following the end of the eruption has reduced the size of the island to 0.19 km

2

. The second event occurred in the central part of the Zubair Islands and lasted roughly two months (September–November, 2013), forming a larger (0.68 km

2

) island. The recent volcanic eruptions in the southern Red Sea are a part of increased activity seen in the entire southern Red Sea region following the onset of a rifting episode in Afar (Ethiopia) in 2005.

A new conceptual model, called ‘the hydrothermal salt model’, predicts that salt may accumulate in the marine sub-surface from the hydrothermal circulation of sea water. The hypothesis is based on the physicochemical behaviour of supercritical sea water; when sea water is driven into its supercritical high-temperature and high-pressure domain (407 °C, 298 bars), it loses its solubility for the common sea salts (chlorides and sulphates). Consequently, a spontaneous precipitation of salts takes place in the water-filled pore spaces. The same process may occur when porous rocks containing saline pore water are exposed to sufficiently high temperature and pressure, for example in the subduction of oceanic crust. Salts will also precipitate sub-surface or sub-marine during boiling, in contact with high heat flow sources, such as magma intrusions. Large accumulations of salt are found within the central trough and along both sides of the Red Sea rift. Many of the associated accumulation features are difficult to explain in terms of the conventional ‘evaporite’ model for salt deposits. The features are as follows: (1) several kilometre thick salt deposits on both flanks of the Red Sea, (2) thick (~3 km) salt deposits inside the central graben, (3) dense, hot brines inside some of the central graben deeps (Atlantis II Deep, Conrad Deep, etc.), (4) up to 40-km-long walls and ridges of exposed salt on the northern Red Sea seafloor, (5) tall walls of salt adjacent to the Conrad Deep central graben, and (6) large flows of salt in the Thetis Deep. Furthermore, these huge accumulations of salt took place in a relatively short geological period of time. On the basis of these pertinent structures and features, it is concluded that accumulation, deformation, and transportation of salts may have several drivers and origins, including a process closely associated with hydrothermal activity. Whereas hydrothermally produced salts are lost directly to sea water in mid-ocean spreading hydrothermal systems, they are protected by sediments and ponded high-density brines on the seafloor in deep-spreading centres like that of the Red Sea. Because the Red Sea is the closest active analogue to the rifting and rupturing of ‘Atlantic-type’ continental lithosphere, it may also be the key to understanding the accumulations of huge underground salt deposits and salt-related structures in some of the world’s ancient deep-water rifted margins.

The central Red Sea is a nascent oceanic basin. Miocene evaporites, kilometers in thickness, were deposited during its continental rifting phase and early seafloor spreading. With further seafloor spreading, increasing dissolution due to increasing hydrothermal circulation as well as normal fault movements removed lateral constraint of the evaporites at the walls of the axial rift valley. Because halite is a ductile material that forms a large part of the evaporite sequence, the evaporites started to move downslope, passively carrying their hemipelagic sediment cover. Today, flowlike features comprising Miocene evaporites are situated on the top of younger magnetic seafloor spreading anomalies. Six salt flows, most showing rounded fronts in plan view, with heights of several hundred meters and widths between 3 and 10 km, are identified by high-resolution bathymetry and DSDP core material around Thetis Deep and Atlantis II Deep, and between Atlantis II Deep and Port Sudan Deep. The relief of the underlying volcanic basement likely controls the positions of individual salt flow lobes. On the flow surfaces, along-slope and downslope ridge and trough morphologies parallel to the local seafloor gradient have developed, presumably due to extension of the hemiplegic sediment cover or strike-slip movement within the evaporites. A few places of irregular seafloor topography are observed close to the flow fronts, interpreted to be the result of dissolution of Miocene evaporites, which contributes to the formation of brines in several of the deeps. Based on the vertical relief of the flow lobes, deformation is taking place in the upper part of the evaporite sequence. Considering a salt flow at Atlantis II Deep in more detail, strain rates due to dislocation creep and pressure solution creep were estimated to be 10

−14

1/s and 10

−10

1/s, respectively, using given assumptions of grain size and deforming layer thickness. The latter strain rate, comparable to strain rates observed for onshore salt flows in Iran, results in flow speeds of several mm/year for the offshore salt flows in certain locations. Thus, salt flow movements can potentially keep up with Arabia–Nubia tectonic half-spreading rates reported for large parts of the Red Sea.

The major geochemical characteristics of Red Sea brine are summarized for 11 brine-filled deeps located along the central graben axis between 19°N and 27°N. The major element composition of the different brine pools is mainly controlled by variable mixing situations of halite-saturated solution (evaporite dissolution) with Red Sea deep water. The brine chemistry is also influenced by hydrothermal water/rock interaction, whereas magmatic and sedimentary rock reactions can be distinguished by boron, lithium, and magnesium/calcium chemistry. Moreover, hydrocarbon chemistry (concentrations and δ

13

C data) of brine indicates variable injection of light hydrocarbons from organic source rocks and strong secondary (bacterial or thermogenic) degradation processes. A simple statistical cluster analysis approach was selected to look for similarities in brine chemistry and to classify the various brine pools, as the measured chemical brine compositions show remarkably strong concentration variations for some elements. The cluster analysis indicates two main classes of brine. Type I brine chemistry (Oceanographer and Kebrit Deeps) is controlled by evaporite dissolution and contributions from sediment alteration. The Type II brine (Suakin, Port Sudan, Erba, Albatross, Discovery, Atlantis II, Nereus, Shaban, and Conrad Deeps) is influenced by variable contributions from volcanic/magmatic rock alteration. The chemical brine classification can be correlated with the sedimentary and tectonic setting of the related depressions. Type I brine-filled deeps are located slightly off-axis from the central Red Sea graben. A typical “collapse structure formation” which has been defined for the Kebrit Deep by evaluating seismic and geomorphological data probably corresponds to our Type I brine. Type II brine located in depressions in the northern Red Sea (i.e., Conrad and Shaban Deeps) could be correlated to “volcanic intrusion-/extrusion-related” deep formation. The chemical indications for hydrothermal influence on Conrad and Shaban Deep brine can be related to brines from the multi-deeps region in the central Red Sea, where volcanic/magmatic fluid/rock interaction is most obvious. The strongest hydrothermal influence is observed in Atlantis II brine (central multi-deeps region), which is also the hottest Red Sea brine body in 2011 (~68.2 °C).

The Atlantis II Deep is a 65 km

2

topographic depression located in the axial trough of the Red Sea at 2,000 m depth. The depression traps 17 km

3

of hot and dense brines fed by hydrothermal fluids. This chapter reviews numerous data collected during the last 50 years. Chemical and isotopic data suggest that the processes that lead to the formation of the Atlantis II Deep brines are similar to those that produce open ridge black smoker fluids, but the recharging fluid is sea water in the case of sediment-free ridges, whereas it is sea water that has dissolved evaporites in the case of the Atlantis II Deep. The monitoring of temperature indicates that the heat flux was 0.54 × 10

9

W between 1965 and 1995. After 1995, the heat flux became 10 times lower. The substratum of the Atlantis II Deep consists of MORB-type basalts, which are covered with 0- to 30-m-thick metalliferous sediments. The solid fraction contains biogenic calcareous and/or siliceous components and silico-clastic detrital particles diluted by metalliferous sediment, which consists of metal oxides, sulphides, carbonates, sulphates, and silicates that precipitated from the hydrothermal fluids. The redox interface between the deepest brine layer and sea water is a major place of mineral precipitation. During glacial periods before the Holocene, the redox boundary was located above the brine–sea water boundary, so that hydrothermal metals spread over a large area of the Red Sea bottom.

After two companies were awarded a 30-year license for the exploration and exploitation of metalliferous sediments in the Atlantis II Deep (Red Sea) in 2011, we herewith present conclusions and recommendations derived from an environmental risk assessment, the Metalliferous Sediment Atlantis II Deep (MESEDA) study, conducted in the period 1977–1981. For economic reasons, this program was discontinued before final report delivery and fell dormant for 30 years. The effects of environmental disturbances of the benthic and the near-bottom water layer habitats in and around the mining site deserve further and more modern risk assessments. We examine the relevance of our 1981 recommendations and of subsequent publications to the extended period of resource extraction planned for this century and recommend more up-to-date risk assessment investigations and evaluations.

This chapter discusses the saturation states of the Red Sea with respect to both calcite and aragonite and their possible biogeochemical impacts as a result of ocean carbonate chemistry changes. The saturation levels of the Red Sea surface waters are several-fold supersaturated with respect to calcite and aragonite; they range from 634 to 721 % and from 446 to 488 %, respectively. The saturation levels of the deep waters range from 256 to 341 % with respect to calcite and from 177 to 230 % with respect to aragonite. They generally increase from south to the north. The lowest values of seawater supersaturation with respect to both calcite and aragonite were found at water depths >1,400 m. Changes in the seawater acid–base chemistry due to excess CO

2

emission and oceanic acidification affect the saturation states of calcium carbonate. Based on reported results of the excess CO

2

sink in the northern part of the Red Sea (Krumgalz et al.

1990

), the estimated degree of saturation with respect to calcite and aragonite was higher by 1.9 ± 0.4 % at >200 m, 4.9 ± 0.7 % at 200–600 m, and 2.5 ± 0.1 % at >600 m in preindustrial times than in 1982. A projected drop in pH by a 0.1 unit decreases the saturation level by a factor of 1.2, whereas a drop by 0.4 pH unit decreases the saturation level by a factor of 2.1. These changes in saturation levels will have major impacts on the calcifying pelagic and benthic organisms as well as the distribution and depth of coral reefs. Low magnesian calcite and pure calcite are expected to be the dominant carbonate minerals at these low supersaturation levels.

The sedimentary characteristics of the lagoons along the Red Sea coast of Saudi Arabia are governed by the potential of the wadis (seasonal streams) to transport sediment, while the sediment distribution is influenced by the water current patterns that are mostly generated by the wind. Human interference and anthropogenic input are the two main issues that change the dynamics and morphology of the lagoons. The absence of permanent fluvial systems keeps the water transparent and helps coral growth, while the exchange of water between the lagoons and the Red Sea is restricted and leads to pollution in the lagoons. The sediment yield varies from north to south depending on the amounts of rainfall. The sediment veneer in the southern half of the Red Sea is mostly terrigenous because of relatively higher input from the wadis, whereas in the northern half where rainfall is low, the numerous wadis contribute terrigenous material in varying amounts. The central part is mainly devoid of terrigenous input where anthropogenic input prevails. The lagoons are mainly blanketed by carbonate of biogenic origin, and therefore the associated Red Sea environment is regarded as a carbonate-rich environment. Some wadis that drain directly into the Red Sea or into the lagoons are either dumping sites or are used as farm land, and lagoons close to major cities are being reclaimed and used as resorts. Although the coastal belt is still somewhat pristine, the health of the marine environment may deteriorate with excessive human interference and irresponsible actions.

Sea levels are always changing, for many reasons. Some changes are rapid, while others take place very slowly. The changes can be local, or extend globally. Sea levels, particularly extremes, are important for coastal flooding and coral reef development, both of which may be impacted by climate change. In this chapter, we review Red Sea sea-level changes, before looking at the various processes involved in more detail and relating them to basin development and dynamics. There is no systematic review of Red Sea levels: most scientific studies have been local and piecemeal; measurements are few and limited to widely spaced harbour facilities. This chapter is a brief overview of sea-level changes and a source of references for further studies. On increasing timescales, we review tidal, weekly, seasonal and long-term changes. Finally, we link to changes of sea level in the recent geological record.

This chapter discusses the horizontal circulation of the Red Sea and the surface meteorology that drives it, and recent satellite and in situ measurements from the region are used to illustrate properties of the Red Sea circulation and the atmospheric forcing. The surface winds over the Red Sea have rich spatial structure, with variations in speed and direction on both synoptic and seasonal timescales. Wintertime mountain-gap wind jets drive large heat losses and evaporation at some locations, with as much as 9 cm of evaporation in a week. The near-surface currents in the Red Sea exhibit similarly rich variability, with an energetic and complex flow field dominated by persistent, quasi-stationary eddies, and convoluted boundary currents. At least one quasi-stationary eddy pair is driven largely by winds blowing through a gap in the mountains (Tokar Gap), but numerical simulations suggest that much of the eddy field is driven by the interaction of the buoyancy-driven flow with topography. Recent measurements suggest that Gulf of Aden Intermediate Water (GAIW) penetrates further northward into the Red Sea than previously reported.

The Red Sea experiences strong atmospheric forcing through both wind stress and air–sea buoyancy fluxes. Direct observations and modeling experiment show a robust response that consists of a strong and complicated three-dimensional circulation pattern with intense seasonal variability, involving water masses that are locally formed in the Red Sea or enter it from adjacent basins. Two thermohaline cells are identified related to intermediate (Red Sea Outflow Water—RSOW) and deepwater (Red Sea Deep Water—RSDW) formation processes. Results from numerical simulations indicate that the permanent cyclonic gyre in the northern end of the basin is the most probable location for the RSOW formation to take place, but further investigation with observations and numerical modeling techniques is needed to better understand the processes involved as well as the role of the Gulfs of Suez and Aqaba in the regional thermohaline circulation. The Red Sea circulation is closely linked to the flow pattern in the Strait of Bab-al-Mandab where the exchange of the Red Sea with the Indian Ocean takes place. The exchange is of an inverse estuarine type, which compensates for the strong heat and freshwater fluxes in the basin, but with very strong seasonal and synoptic variability related to remote and local forcing. Although important progress has been achieved during the last two decades in describing and understanding basic processes of the Red Sea dynamics, several features are not yet understood and explained. Further observational and modeling activities are required to improve our understanding of these processes and should be combined in an interdisciplinary approach to improve our monitoring and forecasting capabilities for environmental management and protection.

The Red Sea is one of the few regions of the world where the balance of the surface heat fluxes at the air–sea interface can be compared favourably with the advective heat flux at Bab-al-Mandab. Various estimates of the surface heat fluxes and advective oceanic heat flux are discussed and their average values are derived. The annual average values for the absorbed solar radiation at the sea surface, net long-wave radiation, latent heat and sensible heat fluxes are, respectively, 220 ± 6, 70 ± 5, 165 ± 4 and −3 W m

−2

for the Red Sea as a whole. This gives a heat loss of −12 W m

−2

at the air–sea interface which is almost in agreement with the net advective flux of 13 ± 4 W m

−2

to the Red Sea at Bab-al-Mandab. Because of its location in an arid area, the evaporation is high and precipitation is very low. The average precipitation along the eastern side of the Red Sea is about 0.05 m per year. The estimates of evaporation vary in time and space, and the average is about 2 m per year for the whole Red Sea. Based on the Comprehensive Ocean Atmospheric Data Set (COADS) from 2000 to 2010, the monthly averages of latent heat flux are calculated for the southern, central and northern regions of the Red Sea. The variability of the net long-wave radiation and sensible heat flux for these regions is also shown. The surface heat fluxes over the Red Sea show a wide temporal and spatial variation.

This chapter evaluates the impacts of projected climate change (period: 2020–2100) on the hydrologic systems along the Eastern Red Sea coastal plain. Shuttle Radar Topography Mission (SRTM) data and Landsat Thematic Mapper (TM) scenes were used to delineate watersheds and their channel networks (155 watersheds) that collect precipitation over the Red Sea Hills in the Kingdom of Saudi Arabia (KSA) and drain toward the Red Sea coastal plain. Continuous rainfall-runoff models were constructed and calibrated for each of the 19 major watersheds (watershed area: 1,825–107,769 km

2

; total area: 176,683 km

2

). Regionalization techniques were applied to extrapolate catchment-specific parameters from proximal calibrated catchments (i.e., Wadi Girafi, which has an area of 5,057 km

2

). The calibrated Soil Water and Assessment Tool (SWAT), and climatic inputs from the Climate Forecast System Reanalysis (CFSR) database, and from Tropical Rainfall Measuring Mission (TRMM) measurements were used to calculate the partitioning (runoff, evapotranspiration, and potential groundwater recharge) of precipitation for the time period from 1998 to 2010. TRMM data were validated by comparing them to rain gauge data (period: 1998–2012; number of rain gauge stations: >100) across the study area. Downscaled outputs of the Community Climate System Model (CCSM4) were used to simulate (for the period 2020–2100) the effects of climatic changes on the selected 19 Red Sea watersheds. Findings indicate that (1) the average annual precipitation is 130 mm and the total annual amount is 23 × 10

9

m

3

; (2) with time, precipitation will increase over the northern and central watersheds and decrease over the southern watersheds; and (3) global warming is causing a rise in sea surface temperature, enhancing evaporation, intensifying northerly and northwesterly winds and precipitation over the northern and central watersheds, and possibly causing shifts in monsoonal fronts and modifying precipitation patterns over the southern watersheds.

Along the Red Sea, narrow coastal plains ascend directly into fault-bounded blocks within a few kilometers of the shoreline. Littoral areas on both sides of the Red Sea are characterized by mixed sedimentation relating to a complex system of fringing and barrier reefs and alluvial fans. These marine sediments are uplifted to altitudes rarely exceeding 50 m. However, although terraces are well developed on both sides of the basin there is no apparent correlation, the possible exception being the youngest level situated about 2 m above present sea level. Raised coral reefs result from either the corrosive action of waves or from local erosion by occasional torrents creating low cliffs and exposures just above high-tide level. Each reef unit exhibits in a short distance lateral facies changes, which begin at the shore with the beach facies, mainly composed of siliciclastics, and end at the reef crest zone with transition to the fore slope made up of carbonate sediments. A strong similarity can be noticed between sedimentary facies of ancient Pleistocene sediments and those now present in modern fringing reefs. Reefs with their siliciclastic associations occur in the form of repeated cycles reflecting tectonic effects and/or sea level changes. Reef sequences exhibit different degrees of diagenetic alteration, which are reflected by a gradual change of skeletal particles and the early-formed cement, from aragonite and high Mg-calcite to low Mg-calcite. Tectonism controls the areal distribution of the depositional systems and influences the number, thickness, extension, and elevations of the reef sequences. Each sequence in each area can be uniquely correlated to the overall (global) population of dated terraces. Coastal areas of the Red Sea are under stress from a variety of human activities and many have experienced widespread degradation, especially around Hurghada and Jeddah. Hotel, resort and other developments along the coast of Egypt are growing rapidly, destroying raised reefs and threatening valuable coral reef ecosystems. Some areas along the coast suffer from construction problems that are associated with coral reefs. These problems include ground settlement and low bearing capacity which are mainly due to the low shear resistance and high porosity of reef sediments. These problems greatly affect the safe and economic land utilization of the coasts. Prediction of the future changes along the Red Sea coast would give guide lines to what will happen due to the varying nature of the coast. Such predictions would have implications for future social and economic development along the coast. Effective and integrated coastal zone management programs are critical to sustaining the natural resources of the Red Sea.

In this chapter, we examine the different processes that control the orientation and arrangement of shallow water coral reef environments in the Red Sea. Particular focus is paid to the diversity, distribution, and abundance of coral reefs in Saudi Arabia, where shallow water (<30 m) environments have been mapped into one of sixteen types of ‘coral reef system’. Each of these types of coral reef system represents the terminal node in a decision tree that differentiates reef environments based on distinctive planar morphology, that is, viewed from above and audited from high-resolution satellite imagery. The wide variation in morphology of the Red Sea reefs is primarily governed by the width of the coastal shelf. Coral reef systems can be described as either ‘shore-attached’ or ‘detached’ on the basis of the depth of water separating them from a landmass. The morphology of attached systems ranges from the simple arrangement of fringing reefs and sediments to more complex forms that extend into the Red Sea basin and are incised by channels, large lagoons, and repetitive reef lineaments. There is also considerable diversity in morphology in detached coral reef systems, which are as abundant overall as those that are shore-attached. Several morphological types of coral reef system are restricted to narrow regions of latitude. Such distributional trends may be explained in process terms by the rift tectonics of the Red Sea basin, spatial variability in the presence of sub-seafloor evaporites, the input of siliciclastic (non-carbonate) detritus onto the coastal shelf via wadis, and eustatic sea-level control. These processes act in concert, but at different spatial and temporal scales, to deliver diverse coral reef morphologies throughout the Red Sea basin.

The Red Sea hosts a high diversity of modern coral reef morphologies and non-accreting coral communities which exist under a wide range of environmental regimes. The majority of research into the coral reefs in the Red Sea has focused on the steep fringing reefs’ characteristic of the northern Red Sea and other reef types along the Arabian coastline. This chapter explores the less well-known coral reefs and other coral communities in the central region and southern region, along the coasts of Sudan, Eritrea, Djibouti, and Yemen. In the central region, the Red Sea widens and fringing reefs diminish in scale, as nearshore water depths shallow on account of the presence of broad shallow shelf platforms. Other more unusual reef types and coral communities occur, growing on carbonate, granitic, and volcanic lava flows. Environmental conditions become progressively more extreme towards the south and exhibit cross-shelf and pronounced seasonal gradients. Corals demonstrate a high degree of local acclimation and adaptation to these shifts in conditions. The central region and southern region are transitional zones between the optimal Red Sea reef conditions and the more marginal conditions characteristic of the Gulf of Aden and Arabian Sea.

Several features have favored the development of coral reefs in the Red Sea including the semi-enclosed nature, situation in an arid area with no permanent rivers or significant upwelling, its warm sea water and a reduced tidal range with moderate winds. The fact that the Red Sea coral reefs are the best developed in the western Indian Ocean is not surprising; more than 60 different genera of reef forming corals are found in the Red Sea alone with an exceedingly large recorded number of species. However, reef development varies from north to south in the Red Sea. North of 20

o

N reefs are well developed, occurring as narrow fringing reefs with steep slopes that drop into very deep water, particularly in the Gulf of Aqaba. South of 20

o

N the continental shelf widens and therefore reefs are less well developed vertically and often occur in more turbid water. Nevertheless, reef health is generally good throughout the Red Sea, with 30–50 % live coral cover at most locations and more than 50 % total cover on average. General threats to coral reefs and coral communities of the Red Sea include land filling and dredging for coastal expansion, destructive fishing methods, shipping and maritime activities, sewage and other pollution discharges, damage from the recreational scuba industry, global climate change, and insufficient implementation of legal instruments that affect reef conservation such as Marine Protected Areas (MPAs). The Sudanese reefs consist of three primary coral habitats along the Sudanese coastline: barrier reefs, fringing reefs and the Sanganeb atoll. They are considered to be in moderate to good health, with good fish fauna health. Raised fossil reefs that form coastal cliffs are characteristics of some sites such as Suakin and Dungonab Bay, while Sanganeb Marine National Park and Dungonab Bay–Mukawwar Island are the only MPAs in Sudan. Many of the present problems with coral reef conservation in Sudan are attributed to a lack of law enforcement, a lack of awareness, a weak legal framework, and the absence of surveillance. The crown-of-thorns starfish (COTS)

Acanthaster planci

was not recorded in plague numbers at any of the Sudan reefs. However, in 1999, bleached corals were estimated to cover 14 % of the substrate. In addition to the Jeddah Consolidated Convention, the Red Sea countries have become signatories to a number of international, regional, bilateral or multilateral agreements, and other legal instruments. Each country also possesses a relatively complete set of national laws and regulations. However, the implementation of these remains generally poor, and in some cases, there is no implementation or enforcement. The Regional Organization for the Conservation of the Environment of the Red Sea and Gulf of Aden (PERSGA) has made significant efforts to assist its member states, including the Sudan, to conserve their coastal and marine ecosystems and key species. Nevertheless, there is a need for further continued research on coral reefs and an information dissemination programme to enhance community participation and awareness.

The Saudi Arabian Red Sea (SARS) contains diverse shallow water coral reef systems that include attached (fringing and dendritic reefs) and detached (platform, patch, tower, ribbon, and barrier reefs) reef systems extending up to 90 km offshore. To better understand the current status of coral reefs in SARS, the Living Oceans Foundation conducted assessments of representative reef environments in the Farasan Islands (2006), Ras Al-Qasabah (2007), Al Wajh and Yanbu (2008), and the Farasan Banks (2009). A combination of belt transects and quadrats was used to assess the diversity, size structure, partial mortality, condition, and recruitment of the dominant reef-building corals. Most sites had high structural complexity, with up to 52 genera of scleractinian corals recorded from a single region. Living corals varied in abundance and cover by region, habitat, and depth, with the highest species richness documented in the south (Farasan Banks), followed by Al Wajh and Yanbu and lowest at Ras Al-Qasabah. On most reefs, a single species was dominant. The reef architecture was constructed by massive and columnar

Porites

, with unusually large (1–4 m diameter) colonies in shallow water (up to 80 % live cover in 2–10 m depth) and a deeper reef

Porites

framework that was mostly dead.

Porites lutea

was the single most abundant coral throughout SARS, and the dominant species on leeward reef crests and slopes, while reef slopes and deeper coral carpets were predominantly

Porites columnaris

and

P. rus.

Faviids (

Goniastrea

and

Echinopora

) were the next most abundant corals, especially in areas that had experienced a disturbance, although these were small (most <15 cm diameter) and made up a small fraction of the total live coral cover. Multi-specific

Acropora

assemblages often formed large thickets, but these were restricted in distribution.

Pocillopora

was the dominant taxon in Yanbu, widespread in Al Wajh, and much less common in northern and southern sites. Coral cover throughout the region averaged about 20 %, with higher cover (often >50 %) in shallow water and rapid decline with increasing depth. In each region, many reefs (15–36 %) showed signs of damage and had less than 5 % live coral cover. These degraded sites were characterized by extensive dead skeletons in growth position, substrates colonized by thick mats of turf algae and soft corals (

Xenia

), and surviving massive and plating corals that were subdivided by partial mortality into numerous small (<10 cm) ramets. Mortality was attributed to bleaching events, disease, and outbreaks of corallivores occurring over the last 10–15 years. Several sites also exhibited signs of recent mortality from crown of thorns sea stars (

Acanthaster

), coral-eating snails, and coral disease. In many cases, the

Porites

framework had been recolonized by faviids, acroporids, and other corals and these had subsequently died. Most degraded areas appeared to be rebounding, as substrates had high cover of crustose coralline algae (CCA), little macroalgae, and high numbers of coral recruits and juvenile corals.

Coral reefs are the most abundant shallow water ecosystems in the Red Sea, harboring a high species diversity and habitat complexity over large environmental gradients. At the same time the semi-enclosed ocean basin and its partly extreme environmental conditions may promote species evolution being distinct from Indo-Pacific coral reefs. Extreme conditions are found in the southern Red Sea, where temperatures reach up to 33 °C in summer and where nutrient input is high. Mechanisms of organism adjustment to these conditions are of particular interest in the light of climate change research. Towards the north, conditions become more ‘coral-promoting’ finally reaching temperatures between 21–27 °C (winter-summer) and clear waters at the northern end of the Red Sea (Gulf of Aqaba). In this chapter, we summarize the current knowledge about the biology of shallow water, symbiotic, reef-building corals of the Red Sea. We start with an overview on the environmental conditions of the Red Sea, the history of coral reef research in this region and a general introduction into coral biology, before we describe the ecophysiology of Red Sea corals. Coral ecophysiology is presented in the context of varying environmental conditions over depth (e.g., light), between seasons, and over latitudes (e.g., light, temperature, nutrients). Mechanisms and patterns of coral reproduction are discussed in the context of seasonal and latitudinal environmental changes. Finally, we briefly describe anthropogenic influences on Red Sea coral reefs. Acclimatization mechanisms of corals to changing conditions over a depth gradient (mainly light reduction) have been well studied in the Gulf of Aqaba and include the following metabolic adjustments with depth: (i) an upregulation of light-harvesting pigments (chlorophyll a) and a downregulation of photo-protective pigments (xanthophyll), (ii) an increase of heterotrophy, and (iii) a decrease of metabolic activity (e.g., calcification and growth). In addition, a change in the symbiont composition (

Symbiodinium

clade and/or type) over depth was observed in some coral species. Seasonal environmental changes (mainly light availability, temperature, nutrients) lead to various metabolic responses of the corals, including (i) changes in zooxanthellae pigmentation and density and (ii) changes in the metabolic activity. In particular, changes in calcification and growth rates can be observed with lowest rates during low temperatures in winter. Interestingly, however, this reverses in the southern Red Sea, where calcification rates are higher in winter than in summer. This kind of latitudinal shift is also evident in the timing of reproduction, which occurs earlier in the year (January–March) in the south compared to the north (March–August). This indicates that growth and reproduction are strongly linked to temperature, following a single temperature optimum, which occurs at different times throughout the year from north to south. Furthermore, this hints towards a high phenotypic plasticity (acclimatization) rather than local genetic adaptation of the investigated coral species. A clear shift in the genetic population structure from north to south in another coral species, however, indicates local adaption. Adjusting mechanisms need to be further understood in order to provide indication for predicted climate change effects.

The semi-enclosed and narrow Red Sea basin is characterized by bathyal zones in its axial sectors. It is determined by extreme hydrological parameters regarding its deep-water salinity and temperature which are a serious challenge to be coped with by deep-water benthos. Besides, it is separated from the adjacent Indian Ocean (Gulf of Aden) by a remarkably shallow sill that not only opposes easy transit for deep-water benthos but also exerts a strong control on the basin’s hydrology budget during sea-level fluctuations, likely causing pulsing basin-wide extinctions at times of low stands. Among the relevant macrobenthic groups inhabiting the deep Red Sea, Mollusca stand out as the more diverse phylum. Although the full taxonomic appreciation of the Red Sea deep-water molluscs is still unresolved, as many as 262 species are recorded to date from depths below 400 m (163 Gastropoda, 94 Bivalvia, 4 Scaphopoda and 1 Polyplacophora). Part of this fauna is represented by eurybathic species with a wide bathymetric range. A substantial aliquot is equipped with a larval strategy (planktotrophy) in principle enabling the crossing of the shallow sill from the Gulf of Aden. Various taxa occur also in the Indo-West Pacific, and only a few are putatively considered as Red Sea endemics.

Sea slugs have been making a comeback in recent years, with international research producing papers on their taxonomy, biochemistry, biology, and biogeography. Several large expeditions have been made to the western Pacific, resulting in numerous papers and books. Sadly, the Red Sea has not benefitted from all this funding, and a Google Scholar search reveals only the older papers, all of which have already been covered in my book (Yonow

2008a

) and last paper (Yonow

2008b

). The book generated much interest and to date, a further 73 species have been reliably recorded from the Red Sea. The original lists of species with their zoogeographical distributions are updated with these records and corrections, and are provided in this chapter. Some previously unnamed species now have names which, in many cases, reflect known distributions; others have no names, but photographic records have been confirmed from elsewhere, and more research needs to be done to see whether any of the older names apply or whether they are indeed new species, but at least they are well recorded. Finally, many really are species new to science, which are known either to be endemic to the Red Sea or to have wider distributions, while a small group are so complex that the literature and photographic database are insufficient to determine their distributions. This chapter will present some of the species “discovered” since 2007, with comments on their probable identifications and distribution implications. A discussion of changes in distribution patterns and endemism over the last seven years concludes the chapter.

Marine turtles are long-lived reptiles that appeared on Earth in the late Triassic. There are seven extant species worldwide, five of which can be found in the Red Sea: the green turtle, the hawksbill turtle, the loggerhead turtle, the olive ridley turtle and the leatherback turtle. Marine turtles—as all top predators—have a prominent role in maintaining balanced and healthy ecosystems, in particular seagrass beds and coral reefs, but also in transporting nutrients towards naturally nutrient-poor ecosystems (the nesting beaches), and providing food and transportation for other marine species (e.g., barnacles and commensal crabs). Marine turtles also play an important role in the economy of the tourism industry. In fact, because they can usually be observed in coastal areas frequented by people, marine turtles are the primary attraction for numbers of snorkellers and divers, and contrary to other pelagic species, they are easily accessible even from the shore. In the Red Sea, very few data are available on marine turtles: green and hawksbill turtles are known to feed and nest in the area, although quantitative data are not precise. It is estimated that approximately 450–550 and 450–650 females of green and hawksbill turtles, respectively, nest every year along the Red Sea coast. Loggerhead turtles are known to nest outside the Gulf of Aden on the Socotra Islands (Yemen) in great numbers but no nesting activity has been reported within the Red Sea. Only one nest from an olive ridley turtle has been reported in the region, from 2006 in Eritrea. The leatherback turtle is only sporadically seen feeding, usually following jellyfish blooms, but no nesting activity has been recorded. Data for turtles in their feeding grounds and in-water habitats are greatly lacking for most countries. In Egypt it was found that there are at least 453 green turtles using 13 shallow bays as feeding and developmental areas. No regional assessment has been undertaken to quantify the impact of human-induced mortality on marine turtles in the region; however, major threats have been identified as bycatch, habitat destruction, directed harvest and pollution. Marine turtles are legally protected in all countries bordering the Red Sea both through national laws and international agreements; however, enforcement at sea and on nesting beaches is substantially lacking.

Although the phytoplankton of the Red Sea has been studied since 1900, our information is still inadequate and information is scattered; the last review of the plankton was done by Halim (1969). The primary goal of this chapter is to give an overview of the phytoplankton of the entire Red Sea, not only of its composition and distribution, but also of the primary production and biomass. Earlier sources of information relating to both expeditions and individual works on this subject are considered in this review. The Red Sea is an oligotrophic basin characterized by special features due to its enclosed position. The phytoplankton of the entire Red Sea currently comprises 389 species and varieties, an increase of 181 species since Halim’s review. Both the Gulf of Suez and Gulf of Aqaba are less diverse than the main Red Sea. There is a gradual decrease in phytoplankton richness from the southern Red Sea to the Gulf of Suez. Primary production and chlorophyll biomass increases from north to south, consistent with the distribution of nutrients.

Mangroves of the Red Sea have particular biogeographic and ecological significance, as they constitute the boundary for mangrove distribution in the Indo-Pacific region.

Avicennia marina

is the most abundant mangrove species in the Red Sea region.

Rhizhophora mucronata

stands coexist in a few areas. Both species show various growth performances in the region, depending on local environmental conditions. Previous studies reported the earlier existence of

Ceriops tagal

and

Bruguiera gymnorizha

in several areas of the region; however, recent reports indicate that the existence of these species is currently confined to Djibouti and Eritrea.The vast majority of the mangrove stands in the region are mono-specific, consisting of

A. marina,

which typically forms narrow forests along the shoreline and on nearshore and offshore islands, or fringing tidal creeks and channels. Although they are mostly narrow, such forests vary considerably in extent from a few tens of meters to several kilometers along the shore. Many mangrove areas in the region were reported to be degraded at various rates. Unless intervention through restoration and conservation takes place, degradation of mangroves will possibly have adverse impacts on fisheries, coastal stability, and on adjacent habitats connected to mangroves such as coral reefs and sea-grass beds. The main obstacles to conservation efforts in the region include the lack of institutional and human capacities, inadequacy in legislation and weak compliance, gaps in information and data, poor awareness, and rapid coastal development. This chapter presents a brief overview of the available information on mangroves of the Red Sea with emphasis on their distribution, characteristics, significance, and status. The chapter also describes and discusses mangrove conservation efforts and challenges facing them in the region.

This chapter summarises current knowledge about the deep history of human occupation in the Arabian Peninsula and more specifically examines the likely role of the Red Sea escarpment and coastal region both as a major zone of human occupation in early prehistory and as a key pathway for the movements of people and the transmission of cultural ideas between Africa and Eurasia. This is a highly topical issue in the international literature at present both because of new archaeological investigations that are providing new dates for early Stone Age settlements in various parts of the Arabian Peninsula and because of genetic studies that highlight the southern Red Sea and southern Arabian Peninsula as a major ‘corridor’ of early human settlement and connection between Africa and Asia. The time range of these processes covers at least the past 150,000 years and could extend to 1 million years or more and therefore places a high premium on new understandings about the impact of climate change, sea-level change and other geological processes on the suitability of different areas of the Arabian landscape for human settlement and dispersal. This chapter discusses the archaeological and climatic evidence for Quaternary occupation, the effect of sea-level changes on the possibility of sea crossings of the southern Red Sea, the evidence for coastal archaeological settlements demonstrating early human interest in the exploitation of marine resources and seafaring, and new investigations in the Farasan Islands region that are searching for traces of submerged landscapes and archaeological sites formed at periods of lower sea level.