Marine Isotope Stage 3 in Southern South America, 60 KA B.P.-30 KA B.P.

This volume was conceived during the Symposium “El Estadio Isotópico 3 en la Argentina y el sur de América del Sur: 60.000 a 25.000 años atrás” (The Marine Isotope Stage 3 (MIS 3) in Argentina and southern South America: 60,000 to 25,000 years ago) held in June 2013, in La Plata, Argentina. The main purpose of this meeting was to promote the interaction of the leading scientists in various disciplines of the Geological and Paleontological Sciences of the Late Cenozoic of South America in order to update the existing knowledge on the core issues cited in the title of the symposium (e.g., geology, geomorphology, vertebrate and invertebrate paleontology, palynology, paleomagnetism, paleoenvironmental and paleoclimatic studies, etc). This was the first time ever that these topics related to MIS 3 were publicly discussed in Argentina. The Quaternary geological history is characterized by cyclical climatic changes (glacial and intervals). These cyclical changes generated periodic reorganizations of the landscape and the environmental system. MIS 3 was an interstadial stage, a relatively warmer climatic period which developed roughly between 60 ka B.P. and 25 cal. ka B.P. Several very cold periods (Heinrich events) developed during MIS 3, and several paleoclimatic moments with relatively warmer conditions (Dansgaard-Oeschger events) took place in between. We wish that this first attempt to compile data about MIS 3 in the southern part of South America will be appreciated by our colleagues around the world who are interested in this fascinating period of our climatic history, that the available evidence will be thoroughly analyzed and discussed, and that our data and interpretations will be compared with the existing information emerging from the rest of the planet, and particularly, from other regions of the Southern Hemisphere.

The Marine Isotope Stage 3 (MIS 3) was an interstadial stage, a relatively warm climatic period which developed roughly between 60 and 50 and 30 cal. ka B.P. Several very cold periods, known as Heinrich (H) events, developed during MIS 3 as a result of partial collapse of the North American ice sheet margins, with formation of huge amounts of icebergs which, after melting in more temperate latitudes, would have inundated the North Atlantic Ocean with low salinity waters which would have impeded the penetration of the Gulf Stream into the North Atlantic Ocean. Several paleoclimatic moments with relatively warmer conditions, known as the Dansgaard–Oeschger (D-O) events, took place in-between the Heinrich (H) events, throughout MIS 3. These H and D-O cycles would have been very short in geological terms (perhaps even only around 1 kiloyears (kyr) each in some cases) and quite intense, with mean annual temperatures, for instance in the area of Beringia (the land bridge between Siberia and North America) ca. 5–8 °C higher than those active at the Last Glacial Maximum (LGM; ca. 24 cal. ka B.P.) and perhaps close to those occurring in past interglacial periods, respectively. Even though climate was warmer than during the LGM, total melting of the continental ice sheets did not take place; thus, global sea level was perhaps lower than today during the entire MIS 3. It was low enough to allow the persistence of Beringia, without any interruptions throughout the whole of MIS 3. The aim of this paper is to present basic paleoclimatic and paleogeographic information about MIS 3, which may be useful to understand the nature and evolution of the South American terrestrial and marine ecosystems later on during the LGM.

The origin of the Dansgaard–Oeschger (D/O) events and its variability between different isotope stages is at present under debate. Evidence is herein presented that during the Holocene the “Bond cycle,” which is, a reduced version of the D/O one, is related to a signal on solar activity that it has been baptized as the “Long Trend.” A mechanism by which solar storms, which effect on the Earth’s atmosphere strongly depends on the geomagnetic field morphology, may explain this relationship is proposed, and how the differences between the D/O events along different isotope stages and between the two hemispheres may help in disentangle, at least partially, the ultimate origin of climate change is discussed.

The present authors propose in this paper that a connection exists between the variations of the Earth’s magnetic field during polarity reversal and climate change. The mechanism by which the variations of the internal magnetic field could trigger climate changes would be produced by the influence of the internal magnetic field on Galactic Cosmic Rays (GCR), since the geomagnetic field (GF) provides shielding to such radiation.

The climate in the North Atlantic Ocean during the Marine Isotope Stage 3 (MIS 3)—roughly between 80,000 years before present (B.P.) and 20,000 years B.P., within the last glacial period—is characterized by great instability, with opposing climate transitions including at least six colder Heinrich (H) events and fourteen warmer Dansgaard–Oeschger (D-O) events. Periodic longer cooling cycles encompassing two D-O events and ending in a colder Heinrich episode occurred lasting about 10 to 15 ky each, known as the Bond cycle. Heinrich events occurred less frequently than D-O events. These were recurrent every 1.5 ky on average, while ~10 ky elapsed between two H events. Neither of the two types of events is strictly periodical, however. After H events abrupt shifted to warmer climate, the D-O events followed immediately. During an H event, abnormally large amounts of rock debris transported by icebergs were deposited as layers at the bottom of the North Atlantic Ocean. The various theories on the causes include factors internal to the dynamics of ice sheets, and external factors such as changes in the solar flux and changes in the Atlantic Meridional Overturning Circulation (AMOC). The latter is the most robust hypothesis. At certain times, these ice sheets released large amounts of freshwater into the North Atlantic Ocean. Heinrich events are an extreme example of this, when the Laurentide ice sheet disgorged excessively large amounts of freshwater into the Labrador Sea in the form of icebergs. These freshwater dumps reduced ocean salinity enough to slow down deep-water formation and AMOC. Since AMOC plays an important role in transporting heat northward, a slowdown would cause the North Atlantic Ocean to cool. Later, as the addition of freshwater decreased, ocean salinity and deep-water formation increased and climate conditions recovered. During the D-O events, the high-latitude warming occurred abruptly (probably in decades to centuries), reaching temperatures close to interglacial conditions. Even though H and D-O events seemed to have been initiated in the North Atlantic Ocean, they had a global footprint. Global climate anomalies were consistent with a slowdown of AMOC and reduced ocean heat transport into the northern high latitudes. The bipolar pattern with warming conditions in the Northern Hemisphere (NH) and cooling in the Southern Hemisphere (SH) is discussed from the information published by various authors who have used the limited data available for the SH, and palaeoclimatic simulations obtained by numerical modelling. Results show that the SH mid-latitude anomalies presented much smaller magnitude than those of the NH.

A broad range of processes act today and have acted simultaneously during the Quaternary, producing relief from the Chilean coast, where the Pacific Ocean floor is sinking underneath the South American margin, to the Brazilian and Argentine Atlantic Ocean platform area. This picture shows to be complex and responds to a variety of processes which are just started to be considered. These processes involve mountains created in a passive margin setting along vast sections of the Brazilian Atlantic Ocean coast and regions located inland, to “current” orogenic processes along the Andean zone. On one hand, mountains in the passive margin seem to be created in the area where the forearc region eastwardly shifts at a similar rate than the westward advancing continent and, therefore, it can be considered as relatively stationary and dynamically sustained by a perpendicular-to-the-margin asthenospheric flow. On the other hand, the orogenic processes associated with the eastern Andes show to be highly active at two particular areas: the Subandean region, where the trench is stationary and the Pampean flat subduction zone to the south, where a shallower geometry of the Nazca plate creates particular conditions for deformation and rapid propagation of the orogenic front generating a high-amplitude orogen. In the Southern Central and Patagonian Andes, mountain (orogenic) building processes are attenuated, and other mechanisms of regional uplift become dominant, such as the (i) crustal weakening and deformation linked to the impact of mantle plumes originated in the 660 km mantle transition, (ii) the retirement of ice masses from the Andes after the Pleistocene producing an isostatic rebound, (iii) the dynamic topography associated with the opening of asthenospheric windows during the subduction of the Chile ridge and slab tearing processes, (iv) the subduction of oceanic plateaux linked to transform zones and (v) the accretion of oceanic materials beneath the forearc region. Additionally and after recent geodetic studies, (vi) forearc coastal uplift due to co-seismic and post-seismic lithospheric stretching associated with large earthquakes along the subduction zone, also shows to be a factor associated with regional uplift that needs to be further considered as an additional mechanism from the Chilean coast to presumably the arc zone.

A depositional unit called DU2 identified for the period MIS 3 (ca—30,000–60,000 year B.P.) formed by only one sedimentary facies (F3) was found in the Luján and Salto-Arrecifes rivers basins. F3 is a fluvio–lacustrine unit that overlies in erosive unconformity over eolian sediments with ages of 56,400 ± 6500 and 50,400 ± 10,200 years B.P. and is unconformably covered by another eolian vitroclastic sandy loess deposit, dated as 32,000 ± 4000 years (Infrared Stimulated Luminescence, IRSL) (Blasi et al. 2009a). It represents the recurrence of ephemeral fluvial streamlets and the development of temporary pools by subsequent damming of channels. It corresponds lithologically to sandy muddy gravel, gravelly muddy sand, gravelly mud olive to pale olive, feldspar and quartz sands, bearing extinct mollusks such as Heleobia ameghini and Diplodon lujanensis. Radiocarbon chronologies obtained on monospecific samples of Cyprideis salebrosa hartmanni and Heleobia ameghini yielded ages of 37,710 ± 840 years ¹⁴C B.P. and >40,000 years ¹⁴C B.P., respectively. Furthermore, the age obtained through the IRSL technique was of 44,000 ± 6500 years. Based upon the analyzed bioproxies (malacological, phytoliths and diatomological content) F3 accumulated under variable climatic conditions, ranging from temperate to colder and from subhumid to drier. According to the exhaustive stratigraphic identification, it is proposed that in N-E Buenos Aires Province, the so-called Undulated Pampa region, the sediments that were accumulated during MIS3 occur only in the central portion of the studied fluvial basins. This prompted two hypotheses related to the existence of a particular drainage pattern for the Late Pleistocene, different from the present one, and subsequent tectonic controls that allowed the identification of DU2 sediments only in some of the analyzed sections.

Marine Isotopic Stage 3 (MIS 3) was a period of rapid climatic changes and sea level fluctuations. Regarding these fluctuations some doubts were based on the limit of the radiocarbon dating method (about 50,000 years B.P.). However, the modelling of the isotopic oxygen ratios is also indicating sea level fluctuations. In this sense, only at certain depths it is possible to accept these ages at stable coasts, and taking note about the taphonomic conditions within the sequence. Shells located at depths higher than 60 m on stable and wide continental shelves as that of the Northern Patagonia have been analyzed here in that sense.

The preparation of a digital model showing the rising and lowering of relative sea level, by means of using the Global Mapper 10 program, allowed an approximate reconstruction of the paleogeographic evolution of the Atlantic coast of South America during the Marine Isotopic Stage 3 (MIS 3). To elaborate this digital model, the curve of global sea level variations as proposed by Lambeckand Chappell (2001), for the last glacial cycle, was taken into consideration. The model shows the development of an extensive coastal plain, which extended almost continuously from Staten Island (Isla de los Estados; southeastern end of Argentina) until the Panama isthmus. The surface of this coastal plain varied from a maximum expansion of around 1,182,000 km², when sea level achieved its minimum level of approximately −80 m below present sea level (b.p.s.l.) within the Marine Isotope Stage 3, in between 40,000 and 30.000 cal. years B.P., and a minimum area of approximately 954,000 km², when sea level was at its highest position of −60 m b.p.s.l. (in between 57,000 and 63,000 cal. years B.P.). These figures represent an overall surface variation in the order of only 20 % between both extreme paleogeographic configurations, proving that the coastal plain was a permanent, stable feature of the landscape of eastern South America, not only during the Late Pleistocene glacial stages MIS 4 and MIS 2, but even also during MIS 3. Its average width varied between 76 ± 73 km and 61 ± 71 km, showing significant latitudinal variations. Thus, South America increased its total surface by 6.6 and 5.3 % during MIS 3. This coastal plain had its maximum extent at the latitude of the eastern Argentina province of Buenos Aires (between approximately 35° and 40° South latitude), when the Río de la Plata estuary did not exist. Its minimum amplitude occurred in central Brazil and in front of the Caribbean coast of Venezuela and Colombia. The information provided by the present model sustains the importance of the Atlantic coastal plain of South America as a distinctive feature of the South American landscape and proves its paleogeographic continuity even during the warmest periods of Marine Isotope Stage 3. These facts suggest that the coastal plain, most of it submerged today, has been a major element of the landscape during most of the Quaternary.

Marine Isotope Stage 3 (MIS 3) is characterized by high climatic variability resulting from numerous centennial to millennial scale events. The environmental and climatic reconstruction of this interval is restricted by the sparsity of high-resolution (centennial-scale) terrestrial records in most of South America. This contribution is an attempt to reconstruct the general environmental and climatic conditions of southern South America during MIS 3 by means of continental records located in central Argentina; this is an extensive and heterogeneous region made up of diverse geomorphological settings under different climatic conditions. Therefore, the main features of several aeolian and fluvial records situated in different geomorphological settings across the region are overviewed. The results indicate the predominance of regional aggradation during MIS 3 with differences in the accumulation rates and dominance of either aeolian or fluvial deposits depending on the geomorphological setting. The aggradation process was interrupted by stability intervals evidenced by paleosols in the San Rafael plain, the San Luis paleo-dunefield, the eastern Sierras Pampeanas piedmont and the eastern Pampean plain. The paleosols might represent lapses of decreasing aeolian input and perhaps more humid conditions. In addition, paleobiological indicators from alluvial sequences suggest higher temperatures and water availability between 35 and 31 ka in the Andean piedmont, while dry subhumid or strongly seasonal conditions with alternating subhumid-humid phases were inferred in the eastern Pampean plain during MIS 3. These intervals tend to cluster during the second part of MIS 3, and might reflect the environmental responses to some of the climatic oscillations that occurred during MIS 3. Detailed analysis and a more adjusted chronology are needed to correlate the aeolian and fluvial episodes along with the stability intervals at regional and continental scales.

The Sopas Formation is a late Pleistocene continental unit that includes trace fossils, woods, fresh-water mollusks, and vertebrates with mammals being the predominant taxa. Likely, relationships with the Last Interglacial Stage or with the Last Interstadial were proposed. The paleontological content of the Sopas Formation is updated, and the climatic and environmental signals provided by the fossil content are evaluated. Radiocarbon AMS dates ranging from 33,560 ± 700 year B.P. (cal 36,089 − 39,426 year) to 39,900 ± 1,100 (cal 42,025 − 45,389 year) and TL/OSL ages from 27,400 ± 3,300 to 71,400 ± 11,000 year (being the 45–28 ka time interval better represented), support a relationship with Marine Isotopic Stage 3 (MIS 3) in most outcrops. In the fossil assemblage are taxa that indicate open habitats, savannahs, and woodlands including gallery forests and perennial rivers; living representatives of taxa related to benign climatic conditions (mostly tropical to temperate climates), some taxa that suggest arid to semiarid environments, migrants, and seasonality indicators. A replacement versus mixed faunal models is discussed in the light of available evidence.

This chapter reviews information about the extinct fauna that lived in the Brazilian Intertropical Region (BIR) between 64 and 10 ka B.P. Data from the available literature regarding dating (¹⁴C, ESR, U-series) and paleodiet reconstruction (δ¹³C) for some of taxa of the BIR are herein presented. Furthermore, paleoenvironmental reconstructions of two climatic moments are presented, one at 64 ka, and another between 27 and 10 ka B.P.

Paleontological sites in Argentina with continental vertebrates corresponding to the Marine Isotope Stage 3 (MIS 3) interval are scarce or poorly known. This situation is mainly due to the lack of absolute ages for Pleistocene fossil remains or their bearing sediments that would allow the verification of the chronology established for this interval. However, a few isolated evidences show that continental vertebrates responded to the abrupt temperature changes that characterized the MIS 3 (Heinrich colder events and Dansgaard–Oeschger warmer events). Up to date, continental vertebrate remains of this age have been found mainly in Buenos Aires province, but also in a few sites of northeastern Argentina (such as Entre Ríos, Corrientes, Formosa and Chaco provinces). In Buenos Aires province: (1) Paso Otero, in the Río Quequén Grande valley, evidence of warmer and more humid conditions were found in sediments dated in 37,800 ± 2300 radiocarbon years before present (RCYBP); (2) Mar del Sur, General Alvarado County, coastal marine sediments with continental mammals were dated in 25,700 ± 800 and 33,780 ± 1200 RCYBP; (3) Balneario Saldungaray, in the Río Sauce Grande valley, Tornquist County, gastropods associated with mammal remains were dated in 32,300 ± 1800 and 27,500 ± 670 RCYBP; (4) Los Pozos, Marcos Paz County, sediments dated between 29,000 and 33,000 RCYBP are associated with remains of mammals, birds, reptiles and amphibians; (5) San Pedro, San Pedro County, sediments bearing vertebrate fauna have two OSL datings of 37,626 ± 4198 and 41,554 ± 3756 years B.P. (YBP). In Entre Ríos province, Río Ensenada valley, Diamante Department, some levels of the Tezanos Pinto Formation with OSL datings between 9000 and 35,000 YBP yielded remains of grazer megamammals and other taxa characteristic of the modern Patagonian Domain. In the province of Corrientes, Arroyo Toropí, Bella Vista, vertebrate remains dated with OSL from 36,000 to 52,000 YBP show a clear taxonomic change in response to climatic fluctuations. In Formosa province, Río Bermejo, Villa Escolar sediments of the Fortín Tres Pozos Formation, bearing vertebrate fauna have an OSL age of 58,160 ± 4390 YBP. In the province of Chaco, Charata locality, gastropods associated with mammal remains were dated between 22,000 and 27,000 RCYBP. A larger amount of absolute datings of the bearing sediments and especially taxon dates are needed to determine more accurately the response of the fauna to the climate changes characteristic of MIS 3.

Uruguay has several marine deposits of undoubtedly Late Pleistocene age, but there is conflicting evidence when comparing ages obtained by different methods. While ¹⁴C datings suggest younger ages (related to Marine Isotope Stage 3—MIS 3), OSL, where available, indicate older times (related to Marine Isotope Stage 5—MIS 5). The analysis of the abundant molluscan fauna and the presence of extralimital warm water taxa points to a higher than present water temperature for the Uruguayan coast. The referred discrepancies are discussed and a MIS 5 age is preferred according to all available evidence.

Data from terrestrial pollen records in the Chilean sector show that Marine Oxygen Isotope Stage 3 (MIS 3) was characterized by the Heinrich (stadials)/Dansgaard–Oeschger (interstadials) oscillations. At central Chile (32–38° S) an open beech/podocarp woodland was apparently established during the last ice age under cold and humid climates, where nowadays grows a semiarid, broad sclerophyllous vegetation, while episodes of aridity with rise of temperature were indicated by expansion of chenopods–amaranths. At the Southern Lake District and northern Isla Grande de Chiloé (40°–42° 30′ S), the stadial events were characterized by higher amounts of grasses indicative of the Subantarctic Parkland vegetation. This vegetation implied summer temperatures of ~6 °C (~8 °C below present). The interstadials were represented by expansion of the Valdivian-North Patagonian Evergreen Forest-Subantarctic Evergreen Forest implying summer temperatures of ~12 °C. In the Argentine sector, the steppe environmental conditions prevailed. Interstadial conditions are pointed out at 39° S, in NW Patagonia. Meanwhile, in southern Patagonia at 51°–52° S, and Tierra del Fuego at 54° S, the climatic conditions during MIS 3 are interpreted as colder and drier than today.

Diatoms are very useful proxy indicators to reconstruct past climate changes. Studies are based on qualitative and quantitative analyses that allow to infer variables related directly to climate as temperature, or indirectly as salinity, depth, productivity and pH. Reconstructions based on these methods rely on the general assumption that past environmental requirements of the fossil diatom taxa have remained similar to those of their closest living representatives. In this way, the environmental information obtained from living organisms can be used as analogs and extrapolated to the fossil record, particularly in Late Quaternary studies. Diatom records from lacustrine deposits from Argentina, ancient lakes from South America, and marine cores from Southeastern Atlantic Ocean and Eastern Equatorial Pacific were reinterpreted with the aim to correlate them to climatic changes during Marine Isotope Stage 3 (MIS 3) in the Southern Hemisphere. Marine records allowed paleo-reconstructions of productivity and upwelling conditions; at the same time continental records were used to interpret the lake-level histories. The high temporal resolution of diatom assemblages in both environments makes it possible to identify abrupt climate changes between ca. 60 and 30 cal. ka B.P. The future integration of diatom datasets constructed from different environments will solve the analogy problems between fossil and modern assemblages and increase the potential for reliable quantitative reconstructions of Late Quaternary climate in southern South America.

Silicophytoliths are amorphous silica biomineralizations deposited in intracellular or extracellular spaces of plant tissues. Due to their taxonomic value and their high preservation in a variety of soils and sediments, they are widely used as indicators of past plant communities. Numerous phytolith studies show the presence of past grass-dominated ecosystems in the Late Cenozoic, including changes between glacial and interglacial periods. Studies in South America are scarce, particularly those associated to the temporal interval corresponding to the Marine Isotopic Stage 3 (MIS3). A synthesis of silicophytolith studies on pedosedimentary sequences of MIS3 age in South America is herein presented and, particularly, our own work carried out in Argentina. Integrated profiles’ representatives of typical pedostratigraphic sequences from two regional geomorphological units (Mesopotamia and the Pampean Plain) were analyzed. Samples from pedostratigraphic sequences were subjected to routine analysis. Silicophytoliths were extracted after the elimination of carbonates, organic matter, and clay; and their morphologies were described under optical and scanning electron microscopes (SEMs). Profiles from both regions show the presence of conspicuous paleopedological levels, developed in the MIS3 interval. C₃ grasses (Pooideae and/or Panicoideae subfamilies) and, in a lesser proportion, C₄ grasses (Chloridoideae and/or Panicoideae subfamilies) were present in both areas. This indicates the development of mesothermal grass-dominated ecosystems, which nowadays grow mainly in warm-temperate regions. Within the MIS3, frequent climatic environmental variations during the Late Pleistocene may have led to a fluctuation in biogeographic connections between the Mesopotamian region and other parts of South America, closely linked to the Chaco-Pampean plain and, at other times, to inter-tropical regions.