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Title: An investigation into Nd and Sr isotopes in marine sediments and their application to paleoceanography
Author: Bayon, Germain
ISNI:       0000 0001 3452 2040
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2002
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Studies of past ocean circulation and, in particular, the behaviour of North Atlantic Deep Water (NADW), are crucial to improving understanding of the potential links between the oceans and climate-change. Previously, conventional paleoceanographic proxies have provided a wealth of information concerning deep ocean circulation throughout the Late Quaternary. However, development of additional, novel tracers continues to help improve our understanding of this complex natural environment. In this thesis, the use of radiogenic isotopes (Sr and Nd) as paleoceanographic proxies has been investigated. The study has focussed on five sediment cores from the southeast Atlantic (Angola and Cape Basins), a key area for monitoring the behaviour of NADW over glacial-interglacial time-scales. In these sediments, the Sr and Nd isotopic compositions of both detrital and Fe-Mn oxyhydroxide fractions have been analysed to investigate evidence for changes in deep ocean circulation in the SE Atlantic throughout the Late Quaternary. A sequential leaching procedure for marine sediments has been developed which can be used to separate both Fe-Mn oxides and the detrital fraction for isotopic analysis. For any given sample, this procedure allows one to make use of two independent paleoceanographic proxies; a) the isotopic composition of the detrital fraction - as a tracer of the provenance and flow trajectory of local bottom-water at the time of deposition; b) the isotopic composition of the Fe-Mn component - taken to record the isotopic composition of contemporaneous deep-water. Isotopic analysis of the detrital fractions from the studied cores has revealed three main sources of terrigenous material to the deep SE Atlantic: a) clays delivered to the Atlantic by the Congo River and transported south by NADW; b) material from the SW Atlantic province, advected north by Circumpolar Deep Water (CDW); c) aeolian dust blown from the Namib Desert. Nd isotopic compositions in the detrital fractions are consistent with the modern-day hydrography of the deep South Atlantic, in surficial samples, and exhibit glacial-interglacial cyclicity down-core. Sr isotopes do not exhibit such regular cyclicity because Sr-rich authigenie barite strongly influences detrital compositions. A mixing model shows that the influence of Congo River material decreased in the glacial SE Atlantic, whilst the contribution from southwestern Atlantic clays increased. This is in agreement with previous studies which have indicated that the relative influence of NADW in the SE Atlantic decreased during the last glacial maximum. Here, modelling of mass accumulation rates of each individual detrital source provides new evidence concerning absolute changes in CDW and NADW flow. While efficient southward transport of Congo River clays (by NADW) apparently persisted throughout the Late Quaternary, northward advection of SW Atlantic clays (by CDW) appears to have increased significantly during glacial maxima. This would be consistent with an enhanced flow of CDW being responsible for any apparent weakening of NADW in the glacial South Atlantic Ocean. The major, trace, rare earth element (REE) and Nd isotopic compositions of Fe-Mn oxyhydroxide fractions have also been analysed to evaluate their potential for paleoceanographic studies. Our data show that early diagenetic processes, specifically the adsorption of P onto Fe-oxyhydroxides, can both control and fractionate REE distributions in sediment FeMn fractions. This suggests that diagenetic re-distribution of the REE may lead to an homogenisation, at least in part, of the Nd isotopic composition of Fe-Mn leachates. In addition, this study confirms that contributions from aeolian and riverborne Fe-Mn oxides can influence the Nd isotopic composition of Fe-Mn oxyhydroxide fractions significantly. Nevertheless, a glacial-interglacial cyclicity is observed for the Nd isotopic composition of Fe-Mn leachates from the northern Cape and Angola Basin cores; this is attributed to variations in the flux of aeolian Fe-oxides from the Namib desert. Such wind-blown Fe-oxides appear to be resilient to early diagenetic processes, even under strongly reducing conditions. This suggests that similar aeolian material may represent a significant fraction of the Fe-Mn oxyhydroxide component in other marine sediments. This study has demonstrated an exciting potential for the future use of radiogenic isotopes (Nd, Hf?, Os?) in marine sediments for paleoceanographic investigations. While the case for detrital/provenance determinations may be relatively straightforward, isotopic investigations of Fe-Mn fractions may be more problematic - specifically one should pay particular attention to the diagenetic/redox history of any samples under consideration.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Geochemistry