Glacial rapid variability in deep-water temperature and δ18O from the Western Mediterranean Sea

https://doi.org/10.1016/j.quascirev.2006.10.004Get rights and content

Abstract

Deep-water temperatures (DWTs) from the Western Mediterranean Sea are reconstructed for the last 50 ka based on the analysis of Mg/Ca ratios in benthic foraminifera from core MD95-2043 collected in the Alboran Sea. The exceptionally high sedimentation rates of this core and the robust chronology available allow discussion of the results in the context of the Dansgaard–Oeschger (D–O) rapid climatic variability. The applicability of Mg/Ca thermometry in the Western Mediterranean Deep-Water mass (WMDW) is first tested by the analysis of different benthic species in a collection of Mediterranean core tops. The results indicate the need of a readjustment of the existing Cibicidoides spp. calibrations in order to reconstruct present Western Mediterranean DWT values (12.7 °C). Different physiological effects in the Mg uptake between the C. pachydermus living in different regions could account for this offset in the Mediterranean samples. Consequently, the obtained DWT record still has many uncertainties in absolute terms but trends provide valuable information on past changes in WMDW conditions. The DWT record shows significant oscillations in relation to the D–O cycles, colder values occurred during the time of D–O stadials and warmer ones during D–O interstadials. Surprisingly, the coldest DWTs occurred during the time of Heinrich Event 4 (HE4) and not during the Last Glacial Maximum (LGM) when DWTs were mostly warm. These and other particular features of the DWT reconstruction mimic changes in the vegetation from the Eastern Mediterranean indicating the control of the Mediterranean climate on the DWT record. Paired analyses of Mg/Ca and δ18Occ (calcite δ18O) provide the opportunity to reconstruct deep-water δ18O (δ18Odw) and past salinities and hence changes in past WMDW density. Due to the large error associated with these calculations, they can only be discussed in terms of relative changes between different intervals. The results suggest the dominance of a heavier water end member during glacial times and a lighter one during the early Holocene in relation to the δ18Odw conditions present today. Densest WMDW were formed during most of Marine Isotopic Stage (MIS) 2 and during the D–O Stadials not associated with HEs, while lightest WMDW dominated during D–O Interstadials. The δ18Odw record shows a D–O variability pattern likely controlled by changes in the composition and intensity of the local run-off and also to changes in the δ18Ow signal of the Atlantic inflow. Changes in the residence time of the Mediterranean waters, governed by the global sea level, are also considered to exert an important role governing Mediterranean δ18Ow and salinity, particularly during MIS 2. Overall, our results are consistent with the formation of dense WMDW during D–O stadials and even denser during most of MIS 2.

Introduction

The semi-arid climatic regime of the Mediterranean region leads to a negative precipitation–evaporation balance and hence this semi-enclosed basin acts as a concentration basin (Béthoux, 1979; Pinardi and Masetti, 2000). Relatively dense water masses are formed and sink to a great depth in very specific regions such as the Gulf of Lions where Western Mediterranean Deep Water (WMDW) is formed (Lacombe et al., 1985). The Eastern Mediterranean Sea has two main convection cells, one in the Levantine Basin where Levantine Intermediate Water (LIW) forms and the other in the Adriatic Sea (sometimes switching to the Aegean Sea) where the Eastern Mediterranean Deep-Water mass is formed (EMDW). All these convection cells are interconnected as LIW is one of the main contributors to the EMDW and WMDW (Pinardi and Masetti, 2000).

Deep-water overturning is controlled by regional evaporation but also by the local wind systems over these areas. The flow of relatively dry and cold north winds intensifies water evaporation and promotes cooling but also adds kinetic momentum, which allows water to sink (Lacombe et al., 1985; Millot, 1990). As a consequence, changes in the intensity of these overturning Mediterranean cells can provide a good diagnosis of the dominant climatic conditions in the region. These Mediterranean water masses are also relevant to the North Atlantic Ocean as they export the Mediterranean Outflow Water (MOW) that is fed by a mixture of modified LIW and WMDW. The MOW forms a salt injection into the intermediate Atlantic Ocean and can have a potential impact on deep-water production in the Nordic Seas (Reid, 1979). It has been hypothesized that changes in the MOW intensity could have been more relevant in the North Atlantic deep overturning in the past (Bigg et al., 2003). In particular, model results suggest that brief but large increases in MOW strength could have led the North Atlantic to return to strong overturning mode (Bigg and Wadley, 2001). In this respect, changes in the MOW intensity in relation to glacial–interglacial climatic variability and sapropel production have been documented previously (Zahn et al., 1987; Rohling, 1994). More recently, high resolution records from the Western Mediterranean Sea have provided solid evidence for centennial–millennial changes in the ventilation rate of WMDW following D–O climatic variability (Cacho et al., 2000; Sierro et al., 2005). Drift deposits from the Gulf of Cadiz support MOW strengthening during times of enhanced WMDW overturning (Voelker et al., 2006).

The application of Mg/Ca palaeothermometry on benthic foraminifera brings a unique opportunity to reconstruct, in absolute terms, changes in deep-water properties. Use of Mg/Ca palaeothermometry is increasing rapidly in palaeoceanographic reconstructions particularly in planktonic foraminifera (Barker et al., 2005 and references therein). However, benthic reconstructions are still very scarce and the calibrations available are very limited in number and species and need further improvement (Rosenthal et al., 1997; Lear et al., 2002; Martin et al., 2002; Marchitto and deMenocal, 2003; Marchitto et al., in press). This study presents the first reconstruction of deep-water temperature (DWT) and δ18O (δ18Odw) from the Western Mediterranean Sea. This reconstruction is based on paired measurements of Mg/Ca and stable isotopes in C. pachydermus from the IMAGES core MD95-2043 recovered in the Alboran Sea (Western Mediterranean Sea). Previous studies have documented the high quality of this core for palaeoceanographic and palaeoclimatic studies covering the last 50 ka (Cacho et al., 1999; Pérez-Folgado et al., 2003; Moreno et al., 2005 and references therein). The benthic isotopes from this core were reported in a previous study but at lower resolution than presented here and concentrating only on MIS 3 (Cacho et al., 2000). This previous study proposed a model of D–O variability for WMDW formation consisting of stadial stimulation in deep ventilation produced by a reinforcement of the north westerlies flowing over the WMDW source area (Gulf of Lions). A subsequent study performed on a higher resolution record from the North of Menorca (MD99-2343) confirms this model of variability but suggests a more complex pattern for those stadials in which Heinrich Events (S-HE) occurred (Sierro et al., 2005).

The present study provides the first reconstruction of the WMDW temperature changes associated with D–O variability during the MIS 3 and the LGM and, at lower resolution, during deglaciation and Holocene periods. Since C. pachydermus disappears during the deglaciation, the record is supplemented by a few analyses performed on alternative benthic species such as Gyroidina altiformis and Gyroidina neosoldanis. In order to test the suitability of the current benthic calibrations in the context of this Mediterranean region, we have also analysed different benthic species in a set of core top samples taken from a wide area of the Western Mediterranean Sea. These core top results are compared to water measurements to give an insight on the feasibility of our geochemical approach for reconstructing reliable temperature, δ18Odw and salinity conditions of WMDW.

Section snippets

Material and methods

Reconstructions of past Mediterranean conditions are based on the analysis of core MD95-2043 (36°8.6′N; 2°37.3′W; 1841 m water depth) recovered from the Alboran Sea (Western Mediterranean Sea) during the 1995 IMAGES-I Calypso coring campaign onboard R/V Marion Dufresne (Fig. 1). The chronological model for this core was previously presented and discussed (Cacho et al., 1999). Briefly, it is based on 17 calibrated AMS 14C dates for the last 20 ka and on visual correlation between the alkenone sea

Potential interferences in the Mg/Ca record

Mg/Ca ratios in foraminifera tests can be affected by post-depositional processes which induce inaccuracies in the temperature reconstructions. Carbonate dissolution and the presence of silicates or diagenetic minerals are the main potential sources for these Mg/Ca biases. Analyses in planktonic foraminifera have shown that partial dissolution of the carbonate tests results in a lowering of the Mg/Ca ratio since the Mg-enriched carbonate dissolves preferentially (Brown and Ederfield, 1996).

Conclusions

Paired isotopic and trace element measurements on different benthic species from a core top collection from the Western Mediterranean Sea suggest that this is a valid approach for the reconstruction of realistic WMDW conditions. These results also indicate the need to review the benthic Mg/Ca calibration for different regions and suggest that a correction of the global Cibicidoides calibration is needed for its application to the Western Mediterranean Sea, but this does not seem to be the case

Acknowledgements

We thank the specialist assistance of Mike Hall and Mervin Greaves; discussions and support with the ICP-OES measurements from Stephanie de Villiers; helpful reviews and comments on the manuscript by Luke Skinner, Stephen Barker and Chronis Tzedakis; and assistance with S–T plots from L. Pena. This research has been funded by the “POP project” (EC Grant: EVK2-2000-00089) and NERC Grant GR3/12889. IC thanks the support from the Comer foundation for abrupt climatic change research (USA) and from

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