Evidence from the Northeastern Atlantic basin for variability in the rate of the meridional overturning circulation through the last deglaciation

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Abstract

A first study from the subtropical western Atlantic, using 231Pa/230Th ratios as a kinematic proxy for deep water circulation, provided compelling evidence for a strong link between climate and the rate of Meridional Overturning Circulation (MOC) over the last deglaciation. However, these results warrant confirmation from additional locations and water depths because the interpretation of the sedimentary 231Pa/230Th ratio in terms of circulation vigor can be biased by variations in particle flux and composition. We have measured 231Pa/230Th in a core from the Iberian margin, in the Northeastern Atlantic basin, and have compared these new results to the data from the western Atlantic basin. We find that the reduction in the circulation during H1 and YD and the subsequent increases first recognized in the sediment deposited on Bermuda Rise are also evident in the eastern basin, in a totally different sedimentary regime, confirming that sedimentary 231Pa/230Th ratios record basin-wide changes in deep water circulation. However, some differences between the eastern and western records are also recognized, providing preliminary evidence to differentiate between renewal rates in the two North Atlantic basins and between shallower and deeper overturning. Our results suggest the possible existence of two sources of Glacial North Atlantic Intermediate Deep Water (GNAIW), one in the south Labrador Sea and another west of Rockall Plateau. Both sources contributed to the meridional overturning but the two had different sensitivity to meltwater from the Laurentide and the Fennoscandian ice sheets during the deglaciation. These results indicate that additional information on the geometry and strength of the ventilation of the deep Atlantic can be obtained by contrasting the evolution of sediment 231Pa/230Th in different sections of the Atlantic Ocean.

Introduction

Abrupt climate changes during the last glacial period and deglaciation have been related to changes in the Atlantic Meridional Overturning Circulation (MOC) [2], [3]. There is widespread evidence for changes in deep water chemical composition from the sedimentary record of nutrient proxies [4], [5], but these reconstructions do not provide direct constraints on the rate of overturning [6]. The 231Pa/230Th ratio in North Atlantic sediment has been proposed as a dynamic proxy that could more directly constrain changes in the rate of MOC [7], [8], and document their impact on past climate variability. A first high-resolution record of sedimentary 231Pa/230Th from Bermuda Rise, in the deep western basin of the North Atlantic, documents large increases in the 231Pa/230Th ratio during the two periods of abrupt cooling of the last deglaciation [9], which suggest a near cessation of Atlantic MOC during the catastrophic iceberg discharge Heinrich event H1, and a sharp but brief decline in MOC during the Younger Dryas cold event. Rapid increases in MOC were concurrent with the two most dramatic deglacial warmings occurring at the onset of the Bølling-Allerød and the Preboreal. Although these results provide compelling evidence for a strong link between climate and the rate of MOC, they nevertheless warrant confirmation from additional locations and water depths. Indeed, the interpretation of the sedimentary 231Pa/230Th ratio in terms of circulation vigor can be obscured by changes in the mineralogy, flux, and geographic distribution of settling particles [10], [11], [12], [13]. Since the circulation signal is expected to be basin-wide, analyzing sediment deposited under different sedimentary regimes will provide a means of assessing the importance of the more local scavenging effects. In order to start documenting these trends, we have measured 231Pa/230Th in a well dated core from the Iberian margin, in the less ventilated Northeastern Atlantic basin (SU81-18; 37°46′N, 10°11′W and 3135 m depth (Fig. 1 and Table 1)) [14], [15], and we have compared these new results to the recently published data from the Bermuda Rise (OCE326-GGC5; 33°42′N, 57°35′W, 4550 m).

Section snippets

Cores and sites description

SU81-18 was taken from the Iberian continental margin and contains a thick deglacial section of hemi-pelagic sediment. The lithogenic fraction consists mainly of fine-grained silty clay derived directly from the adjacent continent. Ice-rafted debris (IRD) is also found in the section corresponding to Heinrich event H1 [14] but in a very limited amount (fewer than 100 grains > 150 μm/g) compared to the IRD belt (several thousands grains > 150 μm/g) [16], [17]. The carbonate content in the core

231Pa/230Th ratio as a paleocirculation proxy

The use of sedimentary 231Pa/230Th measurements to constrain past changes in the rate of the Atlantic meridional overturning circulation (MOC) is based on the difference in particle reactivity of both isotopes, which are produced in seawater at constant rates from the radioactive decay of dissolved 235U and 234U (βPa = (2.33 ± 0.05) × 10 3 dpm/m3/yr; βTh = (2.67 ± 0.05) × 10 2 dpm/m3/yr) [7]. Both are rapidly removed from seawater by adsorption on settling particles, resulting in relatively short residence

Analytical procedures

Uranium, thorium and protactinium from SU81-18 were measured by isotope dilution on a Finnigan MAT Element 2, single collector, sector field, inductively coupled plasma mass spectrometer [29]. The samples (typically 0.3–0.4 g) were ground with mortar and pestle, then spiked with 233Pa and 229Th before total dissolution in HNO3, HF and HClO4. An aliquot of the resulting solution was removed, spiked with 236U and 229Th, and analyzed for 238U and 232Th. The remaining solution was used for 231Pa

General trends in the 231Pa/230Th profiles

The first order variations in 231Pa/230Th in core SU81-18 are similar to those previously documented in OCE326-GGC5 (Fig. 2). Both profiles show an increase in 231Pa/230Th, reaching values close to the production rate (0.093) during the catastrophic iceberg discharge Heinrich event H1, and a sharp but brief raise of the ratio during the Younger Dryas cold event. Following these cold events, 231Pa/230Th decreases simultaneously in both cores. The close similarity between the two profiles from

Conclusions

This study further demonstrates the potential of sedimentary 231Pa/230Th in the North Atlantic for reconstructing past changes in Atlantic MOC. By comparing the 231Pa/230Th in cores from the Eastern and Western basins and from different depths, we are able to better document changes in the rate of formation of different water masses. This study provides further evidence for a weakened deep water ventilation in the North Atlantic during the LGM, consistent with the record of nutrient tracers,

Acknowledgments

The 231Pa/230Th measurements have been run at the Woods Hole Oceanographic Institution with funds from NSF Grant OCE-0099176. Dave Schneider from the ICP-MS Facility Laboratory, Alan Fleer and Suzan Brown-Leger from the Geology and Geochemistry Department are acknowledged for their analytical and technical support. Thanks to T. Marchitto and an anonymous reviewer for their helpful comments. The LSCE-WHOI cooperation has been supported by an NSF-CNRS cooperative grant NSF INT-0233483. Analytical

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