Application of Sr-Nd-Pb isotopes to the late quaternary paleoceanography of the Atlantic Ocean
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Abstract
The Atlantic Meridional Overturning Circulation (AMOC) plays a crucial role in distributing heat around the globe and impacts strongly on Earth’s climate. The long-lived radiogenic systems Sm-Nd, Rb-Sr and Th-U-Pb have been developed and used extensively in oceanography as robust tracers of water mass provenance and water mass mixing, in particular, in relation to variability in the AMOC and its correlation with climate change and glacial-interglacial cycles. These applications of the Sr-Nd-Pb radiogenic isotope systems to paleoceanography are explored here by analyzing hydrogenetic components and terrigenous phases from Atlantic deep-sea sediment cores.
The reconstruction of paleocurrents has long been a subject of paleoceanographic research. Many of these studies have focused on documenting variations in North Atlantic Deep Water (NADW) intensity on millennial and glacial-interglacial timescales. Changes in AMOC strength and formation of NADW during the Last Glacial Maximum (LGM) still remain very uncertain despite being discussed and argued about in many studies. Here, we present Sr and Nd radiogenic isotope records of Fe-Mn oxide leachates of bulk sediment - which reflect the composition of deep water - from cores located in the South Atlantic, to reconstruct past variability in NADW circulation over the last glacial cycle. The Fe-Mn leachate ƐNd records show a coherent decreasing trend from glacial radiogenic values towards less radiogenic values during the Holocene. Our results, along with published multi-proxy records, suggest that (a) a shallower Southern Component Water penetration northwards during the LGM, (b) there was continuous production and export of Northern Component Water over the last 24 kyr, and (c) the AMOC was reinvigorated progressively to its full strength today over the Holocene period.
A second study presented here uses the radiogenic isotopic compositions of Sr, Nd and Pb hosted in detrital minerals extracted from ocean sedimentary cores to infer the provenance of air-borne dust deposited into the equatorial Atlantic, and to discuss these data in relation to glacial-interglacial climate forcing. The down-core Sr-Nd-Pb isotopes in the detrital fraction from a core located on the Sierra Leone Rise (SLR), in the eastern tropical Atlantic, imply that the predominant dust sources are located in Mauritania, Northern Mali and the Bodélé Depression. The eolian inputs from the Mali and Bodélé sources are present in roughly equal proportions throughout the past 180 kyr. However, more Mauritanian dust input, having a more radiogenic Pb isotopic composition, is found during African Humid Periods, when vegetation spread northwards and many now-dry lakes were filled, suppressing dust emission from the Mali and Bodélé Depression regions. Dust delivery to the SLR occurs mainly during the boreal winter by wet deposition, and is blocked during the summer monsoon months as the Intertropical Convergence Zone migrates northwards.
In the western equatorial Atlantic, we have investigated the detrital fraction of a sediment core located on the Ceara Rise (CR), which reflects suspended load output from the Amazon River plume entering the western Atlantic. Systematic variations are seen in the detrital fraction Sr-Nd-Pb isotope records from this core: these variations correlate extremely well with glacial-interglacial cycles. Within the catchment, terrigenous supply is from the highland Andes (in the west) and the lowland Guiana Shield (in the east), and there is a greater contribution from the highland Andes during glacial versus interglacial periods. Good correlations are observed between isotopic compositions, relative sea level and Northern Hemisphere summer insolation, showing that climate within the Amazon Basin as a whole was strongly modulated over glacial-interglacial cycles, exerting control on the provenance of the suspended load delivered by the Amazon River.
In summary, this thesis provides new insights into changes in AMOC strength over the last glacial cycle. Further, it is shown how North African eolian dust blown into the Eastern Atlantic was affected by climate over the past 200 kyr; similarly, we have obtained new insights into how the suspended load off the Amazon River changed in provenance over the past several glacial-interglacial cycles.