Glacial rapid variability in deep-water temperature and δ18O from the Western Mediterranean Sea
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
References (61)
Cadmium, zinc, copper and barium in foraminifera test
Earth and Planetary Science Letters
(1981)- et al.
Comparison of Atlantic and Pacific palaeochemical records for the last 215.000 years: changes in deep ocean circulation and chemical inventories
Earth and Planetary Science Letters
(1985) - et al.
Evidence of enhanced Mediterranean thermohaline circulation during rapid climatic coolings
Earth and Planetary Science Letters
(2000) - et al.
Response of the western Mediterranean Sea to the rapid climatic variability that occurred during the last 50,000 years: a molecular biomarker approach
Journal of Marine Systems
(2002) Lateglacial to recent deep-sea benthic foraminifera from the Northeastern Atlantic (Cadiz Gulf) and Western Mediterranean (Alboran Sea): Palaeoceanographic results
Marine Micropaleontology
(1988)- et al.
Ocean surface water response to short-term climate changes revealed by coccolithophores from the Gulf of Cadiz (NE Atlantic) and Alboran Sea (W Mediterranean)
Palaeogeography, Palaeoclimatology, Palaeoecology
(2004) - et al.
Calibrations for benthic foraminiferal Mg/Ca palaeothermometry and the carbonate ion hypothesis
Earth and Planetary Science Letters
(2006) - et al.
Late Pleistocene evolution of the ocean's carbonate system
Earth and Planetary Science Letters
(2001) - et al.
Variable bottom water in the Western Mediterranean Basin
Progress in Oceanography
(1985) - et al.
Controls on magnesium and strontium uptake in planktonic foraminifera determined by live culturing
Geochimica et Cosmochimica Acta
(1999)
Benthic foraminiferal Mg/Ca-palaeothermometry: a revised core-top calibration
Geochimica et Cosmochimica Acta
Quaternary deep sea temperature histories derived from benthic foraminiferal Mg/Ca
Earth and Planetary Science Letters
The Gulf of Lions hydrodynamics
Continental Shelf Research
Environmental processes of the ice age: land, oceans, glaciers (EPILOG)
Quaternary Science Reviews
Saharan dust transport and high-latitude glacial climatic variability: the Alboran Sea record
Quaternary Research
Millennial-scale variability in the productivity signal from the Alboran Sea record, Western Mediterranean Sea
Palaeogeography, Palaeoclimatology, Palaeoecology
Links between marine and atmospheric processes oscillating at millennial time-scale. A multi-proxy study of the last 50,000 yr from the Alboran Sea (Western Mediterranean Sea)
Quaternary Science Reviews
Assessing the reliability of magnesium in foraminiferal calcite as a proxy for water mass temperatures
Geochimica et Cosmochimica Acta
Western Mediterranean planktonic foraminifera events and millennial climatic variability during the last 70 kyr
Marine Micropalaeontology
The oxygen and carbon isotope distribution in the Mediterranean water masses
Marine Geology
Variability of the large scale general circulation of the Mediterranean Sea from observations and modelling: a review
Palaeogeography, Palaeoclimatology, Palaeoecology
On the contribution of the Mediterranean Sea outflow to the Norwegian-Greenland Sea
Deep-Sea Research
Review and new aspects concerning the formation of eastern Mediterranean sapropels
Marine Geology
Temperature control on the incorporation of magnesium, strontium, fluorite and cadmium into benthic foraminiferal shells from Little Bahama Bank: prospects for thermocline palaeoceanography
Geochimica et Cosmochimica Acta
Rock-magnetic detection of distal ice rafted debris: clue for the identification of Heinrich layers on the Portuguese margin
Earth and Planetary Science Letters
Mediterranean outflow strengthening during northern hemisphere coolings: a salt source for the glacial Atlantic?
Earth and Planetary Science Letters
Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series
Palaeoceanography
A study of cleaning procedures used for foraminiferal Mg/Ca palaeothermometry
Geochemistry, Geophysics, Geosystems
Planktonic foraminiferal Mg/Ca as a proxy for past oceanic temperatures: a methodological overview and data compilation for the Last Glacial Maximum
Quaternary Science Reviews
Budgets of the Mediterranean Sea. Their dependence on the local climate and on the characteristics of the Atlantic waters
Oceanologica Acta
Cited by (108)
Surface water dynamics of the last 20 kyr documented by coccolithophores in the Gulf of Cadiz
2023, Palaeogeography, Palaeoclimatology, PalaeoecologyPlant use and vegetation trends in Algeria from Late Glacial to Middle Holocene: Charcoal and seeds from Gueldaman GLD 1 cave (Babors d'Akbou)
2022, Review of Palaeobotany and Palynology