Elsevier

Quaternary Science Reviews

Volume 42, 24 May 2012, Pages 59-73
Quaternary Science Reviews

Field observations and modelling of Holocene sea-level changes in the southern Bay of Biscay: implication for understanding current rates of relative sea-level change and vertical land motion along the Atlantic coast of SW Europe

https://doi.org/10.1016/j.quascirev.2012.03.014Get rights and content

Abstract

The absence of basal peat in the stratigraphic sequences of the southern Bay of Biscay has long precluded the development of Holocene sea-level curves. We have approached this problem by combining the indicative depositional meaning (derived from the micropalaeontological composition and sand content) with radiocarbon ages of 55 borehole samples obtained from three estuarine areas of the southern Bay of Biscay. These new sea-level index points have produced the first complete Holocene sea-level curve from this area. We further reviewed all available sea-level data from SW Europe to provide the regional trend and use these data to calibrate a recently developed isostatic model. Field data and model reconstructions present a good agreement for the region considered. A north-south trend is apparent in the data and this is shown to be dominated by the influence of the deglaciation of Eurasian ice sheets, as suggested by previous studies for this region. However, some data-model discrepancy in the south of the Iberian Peninsula suggests that local factors tend to dominate during the late Holocene. On comparing our results to estimates of recent sea-level rise obtained from tide gauges and high-resolution proxy records, it is clear that this region has experienced a significant acceleration in sea level during the past century or so.

Highlights

► A new Holocene sea-level curve for the southern Bay of Biscay has been developed. ► The sea-level trend provides a value of 0.7–0.3 mm yr−1 since 7000 cal yr BP. ► The results might suggest a small fall and then rise of RSL during the late Holocene. ► New trends have been calculated for the 20th century for SW Europe.

Introduction

Changes of relative sea level have occurred throughout geological time due to tectonic, isostatic and Earth rotation processes (Milne and Shennan, 2007) as well as variations in ocean volume and mass induced (primarily) by climatic changes (Lambeck and Chappell, 2001; Vink et al., 2007). During the Quaternary, the main control on sea-level changes was the exchange of mass between ice sheets and oceans with ice sheet growth inducing sea-level lowstands (Lambeck and Chappell, 2001). However, local and regional changes are superimposed on the global signal. These local/regional changes become more important as the temporal scale resolution increases (Lambeck and Chappell, 2001). This is a result of the combination of eustatic sea-level changes (global mean change due to changes in seawater volume which are a function of time only), changes in the gravity field and vertical land movements (both of which are functions of time and position) (Vink et al., 2007), producing substantial spatial and temporal variability in sea-level changes, even when localities lie close to each other (Lambeck and Chappell, 2001). High quality relative sea-level (RSL) data can resolve these spatial and temporal variations and are used for many applications, ranging from calibrating models of Earth rheology to predicting future sea-level change for hazard and risk assessment in populated coastal lowlands (Mauz and Bungenstock, 2007; Leorri et al., 2008b).

Current concerns regarding potential sea-level rise associated with anthropogenic warming of the atmosphere and oceans and its impacts on coastal resources have resulted in increased interest in former RSL fluctuations (Church et al., 2008). Rates of sea-level rise obtained from the geological record represent an important reference to compare with the historical and present day changes. They provide a benchmark against which the additional sea-level rise that has occurred over the last 100–150 years can be measured (Church and White, 2006; Holgate, 2007). Available sea-level data from the North Atlantic provide a broad picture of relatively fast sea-level rise since 15,000 yr BP – from 100 to 120 m below current level – until 6000 or 5000 yr BP when sea level approached its present elevation. Since then, sea level has been relatively stable (Lambeck, 1997).

In the southeastern Bay of Biscay (Fig. 1) where salt marshes are few, fragmentary and restricted to the inner parts of the estuarine areas (Cearreta et al., 2002), the supply of minerogenic sediments exerts the dominant control on their growth.Becreasing accumulation rates are associated with increasing marsh surface elevation and decreasing tidal inundation (Pethick, 1981; Allen, 2000) reflecting the colmatation of the accommodation space, sometimes in response to changes from aggradational to progradational sedimentation (Dabrio et al., 2000; Lario et al., 2002). Therefore, salt marsh surfaces aggrade relatively slowly and are under-represented in Holocene sedimentary sequences. This problem has led to a very low number of Holocene sea-level curves in the Iberian and French Atlantic coasts (Lambeck, 1997; Granja, 1999; Dias et al., 2000).

Different approaches have been developed to achieve sea-level index points (SLIPs) in the absence of coastal peat (Mauz and Bungenstock, 2007). In temperate regions, intertidal foraminifera have become the most widely used tool to infer past tide levels mainly due to the development of foraminifera-based transfer functions (e.g., Gehrels et al., 2002; Edwards et al., 2004; Hayward et al., 2004; Horton and Edwards, 2006); including sometimes low intertidal and subtidal foraminiferal assemblages (Horton et al., 1999; Hayward et al., 2002; du Chatelet et al., 2005). Additionally, sedimentological approaches can provide further insight of former tidal variations. This approach is based on the elevational zonation of the tidal flats (i.e., “sand flat”, “mixed flat”, and “mud flat”), where the “mixed flat” (50–90% sand content) approximates mean tidal level (see Mauz and Bungenstock, 2007 for description of the model).

Previous studies in the southern Bay of Biscay have determined that foraminifera strongly correlate with elevation above mean sea level, while few Holocene sea-level curves exist for areas on the eastern margin of the North Atlantic (French and Iberian coasts; Fig. 1) and no such information is available for the southern Bay of Biscay. Consequently, we hypothesize that foraminiferal and sedimentological analyses combined with 14-C dating of the Holocene estuarine infilling would provide high-quality data to reconstruct changes in RSL for the Bay of Biscay.

In this paper, we use the inferred relationship of foraminiferal assemblages with elevation derived from different estuarine areas (Leorri et al., 2008a) to provide a Holocene sea-level curve. However, we argue that a single estuarine area could reflect local rather than regional forcing factors. Consequently, we study here multiple cores from three different estuaries (Bilbao, Urdaibai and Deba; Fig. 1) and we compare the reconstructed sea-level curve with other similar data produced in the North Atlantic. We further analyse previously published data from SW Europe (from Brittany, France to Algarve, Portugal) and provide a best fit isostatic model for this region.

The Bay of Biscay coastal area is a typical inundation coastline formed as a consequence of sea-level rise since the Last Glacial Maximum (LGM). Erosive processes are dominant along the southern area (named Cantabrian coast, northern Spain), as constant wave attack causes active cliff destruction. Leorri and Cearreta (2004) interpreted the infill of the southern Bay of Biscay estuaries as a depositional sequence within a fourth-order eustatic cycle in the sense of Vail et al. (1991). Fluviatile gravels and coarse sands were deposited during the LGM low sea-level conditions, followed by a marine transgression that lasted from 8500 to 3000 cal yr BP. The materials accumulated since 3000 cal yr BP (until the 19th century human reclamation) were considered to be deposited under stabilized sea level similar to present day position.

On the other hand, the eastern area of the Bay of Biscay (known as Aquitaine coast; southwestern France) consists of straight and continuous Holocene beaches bordered by eolian sand dunes (Tastet et al., 2008). In this region, Klingebiel and Gayet (1995) suggested that sea level reached the present day position 2000 years ago in the Arcachon bay (Fig. 1), supported by the development of continental marsh sediment during the last 2000 years in the lower estuary of Gironde (Medoc peninsula) together with the isolation of the marsh environment and extensive peat development since 2100 cal yr BP in La Perroche (Fig. 1) (Diot and Tastet, 1995; Pontee et al., 1997; Clave et al., 2001). These events are concomitant with the onset of mud deposition in the inner shelf adjacent to the mouth of the Gironde estuary (around 2000 cal yr BP) (Lesueur et al., 1996, Lesueur et al., 2002). In this region, present day sea-level elevation might have been reached ca 3000 cal yr BP, supported by the presence of a transgressive trend dated between 3600 and 2100 cal yr BP (Diot and Tastet, 1995; Pontee et al., 1997; Clave et al., 2001) and active dune fields since 3000 cal yr BP (Tastet and Pontee, 1998). This is similar to the data derived the southern Bay of Biscay (Leorri and Cearreta, 2004). However, in the Gironde estuary (Fig. 1) present day sea level has been reported occurring even earlier, at about 4000 yr BP (Allen and Posamentier, 1993).

The chronological differences of the main events of coastal evolution identified for the last 8500 years from different localities in the Bay of Biscay might result from a variety of local and regional factors combined with the eustatic sea-level rise (ESLR) which occurred since the LGM. During the LGM, large ice sheets covered the high latitudes of Europe and North America, and the Antarctic ice sheet was larger than today with eustatic sea level around 130 m below present mean level (Lambeck et al., 2002). The post-LGM sea-level rise is marked by changes in rates; the averaged rate was 10 mm yr−1 and peak rates were about 40–45 mm yr−1 (see Locker et al., 1996; Church et al., 2008). Sea level rose much more slowly over the last 7000–6000 years (order mm yr−1 or less) during which it reached its current position (Lambeck et al., 2002; Harvey and Nicholls, 2008).

Lambeck (1997) studied the sea-level change along the French Atlantic coast (between 45° and 50° N) and concluded that regional sea-level reached 20 m below mean sea level (bmsl) about 10,000 yr BP, 15 m bmsl at 8000 yr BP and that there may have been only a small (∼3 m) increase in sea level over the past 6000 years. A north-south trend of decreasing sea-levels in response to the melting of the Fennoscandian ice sheet with a superimposed east-west trend as a consequence of the isostatic response to the addition of meltwater to the Atlantic Ocean was also identified.

In the northwest Iberian Peninsula proposed sea-level curves (e.g., Dias et al., 2000) are poorly constrained due to the lack of precise SLIPs (Bao et al., 2007), although some estimations have been proposed. In the Galician coast (northwest Spain) the sea level rose fast from 24 m bmsl at 8000 yr BP until 6000–5000 yr BP, reaching ca 7 m bmsl around 6000 yr BP and placing the maximum sea-level position at 5000 yr BP (Chao et al., 2002; Margalef, 1986 from Garcia-Garcia et al., 2005; Bao et al., 2007). Further south, in the north of Portugal, Dias et al. (2000) proposed a rapid sea-level rise of about 40 m between 10,000 and 8000 yr BP, reaching present sea level around 3500 years ago. However, this area has been claimed to be affected by neotectonism (Granja, 1999).

In central Portugal (Tagus estuary), a rapid sea-level rise from ca 40 m bmsl at ca 12,000 cal yr BP to its present level reached at ca 7000 cal yr BP has been inferred (Vis et al., 2008; see also; Van der Schriek et al., 2007). In southwest Portugal (the Melides lagoon and Algarve coast), data suggest a rapid sea-level rise between 11,000 and 7000 yr BP (25 m bmsl at 10,000 yr BP), a period of deceleration between 7000 and 5000 yr BP, and a stable sea level for the last 5000 years, or even slightly above current mean sea level (Teixeira et al., 2005; Cearreta et al., 2007; Moura et al., 2007; Zazo et al., 2008).

In the southern Iberian Peninsula (Gulf of Cadiz, Spain), sea level rose similarly fast from 49 m bmsl at 13,000 and 30 m bmsl at 10,000 yr BP until 6500 yr BP when values similar to present day were reached (Dabrio et al., 2000; Zazo et al., 2008).

In the French-Iberian Atlantic coast, ESLR can be considered the main factor responsible for coastal development during the postglacial interval and first part of the Holocene (between 10,000 and 5500 yr BP). However, decelerating rates of eustatic sea-level rise led to other factors becoming more important in governing the coastal evolution during the late Holocene. These are primarily local factors, such as sediment availability, local wave climate or land management practices (Cearreta et al., 2007).

The coastal area of the southeastern Bay of Biscay is characterized by Mesozoic–Cenozoic sedimentary rocks forming high cliffs interrupted by short, narrow estuaries that are separated from the open sea by small sand bars, beaches and dune deposits. The morphology and extent of the different estuarine sedimentary environments are constantly altered by erosion and deposition of sediments, and they are sensitive to even subtle environmental changes (Leorri and Cearreta, 2004).

This study was conducted in three estuaries with similar mesotidal ranges (mean tidal range: 2.5 m; Leorri et al., 2008b). The Bilbao estuary was originally the most extensive estuarine area on the Cantabrian coast of northern Spain. The modern estuary is 15 km long and is formed by the tidal part of the Nervion River, although four other rivers (Kadagua, Asua, Galindo and Gobelas) discharge into the main course (Fig. 1). Today the Bilbao estuary is a largely artificial system which bears little resemblance to the original estuary (Leorri and Cearreta, 2004). The Urdaibai estuary is formed by the tidal part of the Oka River, covers an area of 765 ha, and occupies the flat bottom of the 11.6 km long, 1 km wide alluvial valley (Leorri et al., 2008b). On the other hand, the Deba estuary is significantly smaller, with a length of 5.5 km and less than 300 m width on average (Borja et al., 2003).

This coastal area was selected because 1) it can be considered tectonically stable at a temporal scale of 10,000 years or so; 2) it is relatively far from the former centres of glaciation and therefore the average rate of eustatic change will be the major contribution to sea-level changes (Lambeck, 1997; Chao et al., 2002); and 3) it is strategically placed between two areas relatively well studied regarding sea-level changes, the French and Portuguese Atlantic coasts (Pirazzoli, 1991; Lambeck, 1997), and that provides an independent constrain for isostatic models.

The combination of three different estuarine areas and multiple boreholes seeks to minimize the footprint of local environmental variables controlling the sedimentation (e.g., sediment availability, autocompaction, erosion) which are expected to differ from one estuary to another and within each estuary. Furthermore, the relatively short distance between the three estuaries (less than 30 km between Urdaibai and Bilbao and Deba) allows the combination of all data sets to construct a regional sea-level curve (Lambeck, 1997). Therefore, our sampling strategy will introduce some variability in the data set, but it will enhance reconstruction abilities for regional sea level, since combined sets from various estuaries provide a more realistic reconstruction rather than local data sets alone (Gehrels et al., 2001).

Section snippets

Drilling and sample preparation

In order to establish the general framework of the sea-level rise during the Holocene at the regional level, 55 samples recovered from 21 boreholes and one trench were selected and analysed for sedimentological (relative abundances of sand/mud/gravel) and micropalaeontological content and radiocarbon dated (Fig. 1; Table 1). Samples were chosen as representative of different estuarine sub-environments and elevations. Depths are referred always to local ordnance datum (lowest tide at the Bilbao

Results and discussion

Fig. 2 illustrates observations of sea-level change during the past ca 12,000 years for the study area (data are summarized in Table 1). From the new 55 samples analysed in Zone B (Cantabrian coast), forty eight are high quality SLIPs (i.e. provide a well-defined constraint on past mean sea level), ranging from ca 10,000 cal yr BP to ca 200 cal yr BP and seven are limiting dates which define only an upper or lower bound on past mean sea level. Three of the latter samples correspond to fluvial

Conclusions

We have provided a new Holocene sea-level curve for the southern Bay of Biscay using 55 new sea-level index points (SLIPs) ranging from ca 10,000 cal yr BP to ca 200 cal yr BP and seven are limiting dates.

The overall trend shows two main phases: (1) rapid relative sea-level rise from ∼27 m bmsl at ca 10,000 cal yr BP to ∼5 m bmsl at ca 7000 cal yr BP; and (2) a relatively slow sea-level rise ca 7000 cal yr BP until present.

The estimation of relative sea-level change rates derived from all the

Acknowledgements

Nieves González kindly helped with the graphical design and typing of the manuscript. Dr Mallinson (East Carolina University) kindly provided very helpful comments and suggestions. The research was funded by the following projects: TANYA (MICINN, CGL2009-08840), 80IT365-10 (Harea-Coastal Geology Research Group), K-Egokitzen II (Climate Change: Impact and Adaptation, Etortek 2010), UFI11/09 (UPV/EHU Unidad de Formación e Investigación en Cuaternario), Acção Integrada Luso-Espanhola n°

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