Surface and deep water response to rapid climate changes at the Western Iberian Margin

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Abstract

Rapid climate changes at the onset of the last deglaciation and during Heinrich Event H4 were studied in detail at IMAGES cores MD95-2039 and MD95-2040 from the Western Iberian margin. A major reorganisation of surface water hydrography, benthic foraminiferal community structure, and deepwater isotopic composition commenced already 540 years before the Last Isotopic Maximum (LIM) at 17.43 cal. ka and within 670 years affected all environments. Changes were initiated by meltwater spill in the Nordic Seas and northern North Atlantic that commenced 100 years before concomitant changes were felt off western Iberia. Benthic foraminiferal associations record the drawdown of deepwater oxygenation during meltwater and subsequent Heinrich Events H1 and H4 with a bloom of dysoxic species. At a water depth of 3380 m, benthic oxygen isotopes depict the influence of brines from sea ice formation during ice-rafting pulses and meltwater spill. The brines conceivably were a source of ventilation and provided oxygen to the deeper water masses. Some if not most of the lower deep water came from the South Atlantic. Benthic foraminiferal assemblages display a multi-centennial, approximately 300-year periodicity of oxygen supply at 2470-m water depth. This pattern suggests a probable influence of atmospheric oscillations on the thermohaline convection with frequencies similar to Holocene climate variations. For Heinrich Events H1 and H4, response times of surface water properties off western Iberia to meltwater injection to the Nordic Seas were extremely short, in the range of a few decades only. The ensuing reduction of deepwater ventilation commenced within 500–600 years after the first onset of meltwater spill. These fast temporal responses lend credence to numerical simulations that indicate ocean–climate responses on similar and even faster time scales.

Introduction

Our view of Earth's climate system is largely influenced by the traditional perspective of long-term stable and favourable conditions. From the historical record of the Little Ice Age (approximately 1400–1800 AD) we learned, however, that even a subtle reduction in annual average temperatures of 1–2 °C may seriously affect our living conditions at mid-northern latitudes. Under the scope of ongoing Greenhouse warming and the ever-increasing frequency of natural hazards, the probability, velocity, and severeness of rapid climate changes increasingly attract public attention.

Rapid climate changes are defined as shifts in the operational mode of the combined climate–ocean system that take place abruptly, in a manner of decades or few centuries (Clark et al., 1999 and references therein). During the late Pleistocene, they comprise the onset and cessation of event-type, reversible, and non-analog conditions such as the sporadic Heinrich meltwater events and the Dansgaard-Oeschger oscillations in the North Atlantic region Heinrich, 1988, Bond et al., 1992, Bond et al., 1993, Alley et al., 1993, Dansgaard et al., 1993, Taylor et al., 1993, Broecker, 1994. Variations in concentrations of greenhouse gases are believed to exert some control on rapid climatic changes (Brook et al., 1994). As atmospheric mixing times are short, the associated climate changes should be felt instantaneously worldwide. Yet, temporal leads in the onset of Dansgaard-Oeschger and post-Heinrich Event warmings during Oxygen Isotope Stage 3 as well as an early onset of the last Glacial–Interglacial transition are observed in southern hemisphere records Blunier et al., 1998, Jouzel et al., 1995, Vidal et al., 1999. This pattern has been used to invoke a higher sensitivity of high southern latitudes to insolation changes (Kim et al., 1998). On the other hand, the temporal lead may also be explained by a retainment of marine heat in the southern hemisphere due to reduced thermohaline convection in the North Atlantic (bipolar seesaw, Broecker, 1998, Stocker, 1998, Stocker, 2000). In this case, North Atlantic Deep Water (NADW) formation is exerting primary control on the interhemispheric climatic pattern. Changes in NADW production have been regarded as part of a continuously oscillating and interrelated ocean–climate system (Blunier and Brook, 2001). Pacemaker and possible triggering processes are seen in ice-volume changes, rapid tropical climate changes and their teleconnections to high northern latitudes, or interference with a globally recognised but as yet undefined 1.47 ka periodicity Bond et al., 1997, Rühlemann et al., 1999, Cane and Clement, 1999, Schulz et al., 1999, Peterson et al., 2000, Schmittner et al., 2000, Shackleton et al., 2000, Bond et al., 2001.

One of the most profound changes within the climate–ocean system is the last Glacial–Interglacial Transition (Severinghaus and Brook, 1999). The onset is associated with melting of Northern Hemisphere and Antarctic ice sheets. Two pulses of meltwater spill induced rapid raises of the sea level at the onset of the Bölling/Alleröd Interstadial and after the Younger Dryas cold interval (Fairbanks, 1989). Atmospheric warming and increase in snowfall over Greenland was achieved over 20 and 50 years only (Alley et al., 1993). The trigger for this change that has been nonreversible on a millennial scale is not well understood. Threshold conditions that bear on ice sheet stability may have been involved, as well as hysteresis response patterns of North Atlantic thermohaline circulation MacAyeal, 1993, Manabe and Stouffer, 1997, Ganopolski and Rahmstorf, 2001 and atmospheric forcing through raising greenhouse gas concentrations (Monnin et al., 2001).

The large majority of past rapid change studies have been conducted at high northern latitudes where amplitudes of these changes are largest. In view of recent human-induced environmental changes it is of immediate importance to assess the response time to these changes of climates at mid-latitudes. This appears notably important as recent oceanographic surveys have revealed a fast spreading and transfer of changing conditions throughout the North Atlantic basin, within a few years only (Sy et al., 1997). Studies on the late Pleistocene thermohaline circulation history in the northern North Atlantic and Nordic Seas have likewise demonstrated that rapid climatic changes impacted instantaneously the deepwater production Zahn et al., 1997, Dokken and Jansen, 1999, Shackleton et al., 2000, Austin and and Kroon, 2001.

Past patterns of sea surface and deepwater circulation have been monitored with geochemical tracers such as stable oxygen and carbon isotopes, alkenones, trace metal concentrations, and biotic indicators. In particular planktonic foraminiferal assemblages have been used for reconstructions of past sea surface temperatures (e.g. Imbrie and Kipp, 1971, Ortiz and Mix, 1997). Benthic foraminiferal faunal indices have been applied only occasionally to infer deepwater conditions Lutze et al., 1986, Thomas et al., 1995, Rasmussen et al., 1996a, Guichard et al., 1999. Yet, benthic Foraminifera may serve as reliable indicators for the origin, amount, and quality of particulate organic matter reaching the sea floor Fariduddin and Loubere, 1997, Altenbach et al., 1999, Morigi et al., 2001. Oxygen as the other limiting variable for life in the deep sea is considered to be of minor importance for benthic Foraminifera in recent mesotrophic environments Jorissen et al., 1995, Van der Zwaan et al., 1999, Fontanier et al., 2002. Nonetheless, once a drawdown in bottom-near water oxygenation has occurred, a faunal response follows that can be used to later reconstruct the changes of deep-sea environmental conditions Kaiho, 1994, Bernhard et al., 1997, Cannariato et al., 1999. Specific adaptations of dysoxic species provide a competition advantage over other species Goldstein and Corliss, 1994, Schönfeld, 2001. In particular Globobulimina affinis occurs with increasing abundance in the fossil record under oxygen limited conditions Caralp, 1987, Baas et al., 1998.

In this study we present high-resolution paleoceanographic records from two IMAGES cores from the western Iberian margin to assess (1) how sea surface parameters, benthic communities, and ambient deepwater masses responded to past rapid climatic fluctuations at mid-latitudes, (2) the temporal relations between variations of sea surface hydrography and deepwater masses during periods of rapid change, (3) the link of these changes to marine environmental changes in high-latitude areas, and (4) the speed of the signal transmission from high to mid-latitudes.

Section snippets

Material and methods

Paleoceanographic records from two CALYPSO cores from the northern Portuguese margin (MD95-2039, 40°34.71′N/10°20.91′W, 3381-m water depth; MD95-2040, 40°34.91′/9°51.67′W, 2465-m water depth) are used in this study (Fig. 1). Core MD95-2039 was routinely sampled at 10 cm intervals on board “Marion Dufresne” for initial information on stratigraphic range and basic sedimentological parameters Bassinot and Labeyrie, 1996, Thomson et al., 1999, Thomson et al., 2000. Core MD95-2040 was continuously

Stratigraphy and core correlation

In order to better understand the surface and deep-ocean response patterns to rapid changes we concentrated on two intervals, the onset of Termination I and Heinrich Event H4 (Fig. 2). The surface waters in the Atlantic off western Iberia were subjected to strong meridional gradients during the last Glacial Abrantes, 1991, Duprat, 1983, de Abreu et al., 2001. Atlantic-wide implications and a synoptic description of paleoceanographic signals from both cores were achieved by first deriving a

Discussion

A variety of considerable rapid changes and fluctuations of benthic and planktonic proxies are seen in our records during the early Termination I and across Heinrich Event H4. The significance and possible explanations of the observed pattern has been noted and briefly discussed above. In this section, we place these observations into the broader paleo-environmental context of climatic and oceanographic change. Timing and temporal lead-lag patterns that we discuss below depend on the accuracy

Conclusions

Fine-scale isotope and faunal records from the Western Iberian Margin reveal a consistent pattern in the succession of events that were common to Heinrich Events 1 and 4. Both events were preceded by meltwater spill in the Nordic Seas. This affected the surface circulation in the northern Atlantic and promoted the southward protrusion of subpolar surface waters along the trans-Atlantic and Eastern Boundary Current systems. Changes in surface water circulation systems continued to change even

Acknowledgements

M. Higginson, Dartmouth, and N. Thouveny, Aix-en-Provence, made samples from the MD95 IMAGES cores available for this study, and A. Sturm, Kiel, helped the senior during sampling at Marseilles. D. Reese, Kiel, and M. Hall, Cambridge, provided technical support for mass spectrometer operation. N. Shackleton, Cambridge, contributed ideas and made valuable suggestions in an early stage of the study. L. Labeyrie, Paris, and L. Vidal, Aix-en-Provence, provided sound and helpful comments on an

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