Mediterranean outflow strengthening during northern hemisphere coolings: A salt source for the glacial Atlantic?

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Abstract

High-resolution grain size and magnetic susceptibility records from the eastern Gulf of Cadiz (site MD99-2339; 1170 m water depth) reveal contourites formed by the Mediterranean Outflow (MOW) during the last 47 kyr BP. Oscillations in the MOW's intensity occurred in phase with Greenland temperature variations with a stronger outflow during northern hemisphere coolings such as Dansgaard-Oeschger stadials, Heinrich events, and the Younger Dryas. Benthic δ13C data implies the Western Mediterranean Deep Water as one of the main sources feeding the outflow current, while differential changes in the properties of the Mediterranean source and entrained North Atlantic Central Water largely control the MOW's strength. Detailed studies for Heinrich events 1, 4 and 5 show that the flow strength peaked only when subtropical surface waters prevailed in the eastern Gulf of Cadiz, while incursions of icebergs and subpolar surface water were not favorable for the MOW's intensification. As the MOW was strong when the Atlantic's thermohaline circulation (THC) was weakened, the heat and salt injected by the MOW into the intermediate North Atlantic waters might have preconditioned the THC to switch from the stadial to the interstadial mode.

Introduction

Millennial-scale Dansgaard-Oeschger (DO) cycles, consisting of a cold stadial or Heinrich event (H) phase and a warm interstadial phase, were first observed in Greenland ice core records [1], [2] and later in climate records worldwide [3]. Surface and deep water records from the North Atlantic region [4], [5], [6] as well as climate models [7], [8] indicate the thermohaline circulation (THC) as a major force controlling these oscillations. The glacial THC had three modes of overturning: the stadial, basic mode with a reduction in North Atlantic Deep Water (NADW) formation [7]; the Heinrich mode with a collapsed NADW formation; and the interstadial mode, which was similar to the present-day circulation. The stadial and Heinrich modes were both forced by freshwater derived from the calving and melting northern hemisphere ice sheets. The reason for the switch into the interstadial mode, however, is still under debate. Possible trigger mechanisms include salt concentration during sea ice formation in the North Atlantic's convection areas [6], southern hemisphere forcing in a bipolar seesaw [8] or an oceanic feedback involving the Mediterranean Outflow (MOW) [9]. Changes in the properties of mid-depth water masses in the Pacific and Indian Oceans have been linked to millennial-scale climate change, e.g. [10], [11], [12], but outside of the Bahamas bank [4], [13] little is known about the North Atlantic's intermediate water masses that also include the MOW.

Section snippets

Modern hydrography

The MOW's Mediterranean water consists of changeable parts of Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW), which contributes an estimated 0.2 Sv [14], [15] to the 1 Sv outflow volume [14], [16]. Both water masses are formed in winter when outbursts of cold dry air from the European continent induce surface water cooling and deep convection in the Gulf of Lions (for WMDW), in the southern Adriatic Sea, in the southern Aegean Sea, and in the Levantine Basin

Material and methods

The MOW's history for the last 47 kyr Before the Present (kyr BP) is studied in Calypso piston core MD99-2339 (35.88°N, 7.53°W; 1170 m water depth; Fig. 1), retrieved from a mud wave field [23] in the eastern Gulf of Cadiz during leg 5 of the IMAGES V campaign [29]. MOW related data of core MD99-2339 is compared to the Mediterranean Sea records of Calypso cores MD95-2043 and MD99-2343 (Fig. 1). Core MD99-2343 (40°29.84'N; 4°1.69'E; 2391 m) was recovered north of Menorca during IMAGES V. Site

Primary age model of site MD99-2339

The stratigraphy of core MD99-2339 is based on 20 AMS 14C ages and three tie points to the GISP2 ice core chronology [2] via δ18O tuning (Table 1). The tuning points are within H 2, at the transition from H 5 to IS 12 and within IS 13. H 2 is identified by its IRD peak (J. Schönfeld, unpublished data). As the warm SST at the beginning of IS 12 were not favorable for G. bulloides, the correlation point at the onset of the H/IS transition is also based on the δ18O record of G. ruber white (see

Millennial-scale variability

The δ18O record of G. bulloides, a species absent or sparse in parts of the Holocene, has values ranging from − 0.05‰ to 2.87‰ (Fig. 5B). The heaviest planktonic δ18O values coincide with a precursor event to Heinrich event (H) 2 at 25.6 kyr BP and with the beginning of H 1, previously denoted as Last Isotopic Maximum (LIM) [45]. In this paper, the H 1 interval is equal to the Greenland Stadial GS-2 [46] and encompasses the H 1 ice-rafting events sensu strictu and the remaining cold period prior

Conclusions

The records from site MD99-2339 show that the MOW underwent large changes during the last glacial. With the intensified MOW a significant amount of heat and salt was added into the intermediate depth North Atlantic during times when the THC was in stadial or Heinrich mode. This is also true if the contourites formed at site MD99-2339 are caused by a displacement of the lower MOW's flow path rather than a real intensification of the lower core. In the only climate model that includes the MOW in

Acknowledgements

We thank J.C. Faugeres and T. Mulder for the generous access to core MD99-2339, Yvon Balut, IPEV and the shipboard party for their support and expertise during the IMAGES-GINNA cruise, and the laboratory team of the Departamento Geologia Marinha for their efficient work. A.V., S.L. and F.A. were funded by the Fundação para a Ciência e a Tecnologia (FCT) project MOWFADRI. High-resolution magnetic susceptibility measurements were made possible by an EC Access to Research

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