Application of the Sm-Nd Isotope System to the Late Quaternary Paleoceanography of the Yermak Plateau (Arctic Ocean)

摘要

By storing and transporting vast amounts of energy derived from solar insolation, the oceans play an important role in shaping Earth’s climate. On the largest scale, ocean currents smooth the temperature gradients between the equator and the poles by redistributing excess energy from the tropics to higher latitudes. Much of this excess heat is transported by the so-called Ocean Conveyor Belt (Broecker, 1991), a global network of ocean currents driven by thermohaline convection. Changes in the pattern and strength of thermohaline circulation affect the redistribution of heat, and thereby significantly influence climate on local to global scales.The reconstruction of paleocurrents has long been a subject of paleoceanographic research. Among the various methods employed in tracing paleocurrents (and modern currents), the Sm-Nd isotope system is experiencing ever increasing attention. First applied in an oceanographic context by O’Nions et al. (1978), it is by now established as a standard tool, as shown by numerous recent publications (e.g. Rutberg et al., 2000; Tütken et al., 2002; Weldeab et al., 2002; Benson et al., 2003; Farmer and Barber, 2003; Piotrowski et al., 2004; Bayon et al., 2002, 2003, 2004; Lacan and Jeandel, 2001, 2004, 2005, and many more). Two lines of application of the Sm-Nd isotope system to oceanography/paleoceanography can be distinguished, both of which were followed for this thesis.The first approach uses the isotopic composition of Sm and Nd hosted in detrital minerals to infer the provenance of terrigenous sediments. This information can be used to draw conclusions about the direction and distance of sediment delivery. The second approach uses the isotopic signature of Nd as a tracer of different water masses. Due to the oceanic residence time of Nd being shorter than the global turnover rate of seawater (500-1000 years vs ~1000 years; Tachikawa et al., 2003), different bodies of water acquire distinct Nd isotopic signatures as a function of the age of adjacent continents. Apart from directly analyzing the Nd isotopic compositions of water samples to trace the modern distribution of different watermasses (e.g. Lacan and Jeandel, 2001, 2004), suitable archives of seawater-derived Nd can be employed to study paleocurrents. Possible archives are fossil remains of marine organisms (e.g. foraminifers; Burton and Vance, 2000), or, most widely used for the recent geological past, Fe-Mn nodules and crusts (e.g. Frank et al., 2002). With slow growth rates on the order of mm/Ma, however, Fe-Mn nodules do not offer the high temporal resolution necessary to study Late Quaternary climate change. Attention has therefore recently turned to authigenic Fe-Mn oxyhydroxides finely dispersed throughout the sediment column (e.g. Rutberg et al., 2000; Bayon et al., 2002, 2003, 2004; Piotrowski et al., 2004).For this thesis, both lines of application of the Sm-Nd isotope system to paleoceanography were followed. The samples were taken from a sediment core collected from the Yermak Plateau in the north-eastern Fram Strait. Situated between Greenland and the Svalbard Archipelago, the Fram Strait is the only deep connection between the Arctic Ocean and, via the Greenland-Iceland-Norwegian (Nordic) Seas, the North Atlantic. The Nordic Seas are an area of deep-water formation important for the global thermohaline circulation. There, the processes of deep-water formation are in a state of equilibrium that is most sensitive to changes in surface water salinity, which, in turn, is strongly influenced by the outflow of water of low salinity from the Arctic Ocean. This makes the history of water exchange between the Atlantic and the Arctic Ocean through the Fram Strait a subject of key interest for climate research.In particular, it was attempted to reconstruct the provenance of sediments deposited on the western Yermak Plateau over the last 129 000 years. This was done by analyzing samples from the sediment core and from potential source areas for their Sm-Nd isotopic compositions. The current understanding is that under present interglacial conditions sediment is delivered to the Yermak Plateau by ice drift from the Siberian shelf areas (Kara- and Laptev Sea) and as suspended load of Atlantic water advected from the south. To resolve these assumed differences in provenance and transport mechanism, the majority of the samples was split into the grain-size fractions clay, fine silt, coarse silt, and sand for Sm-Nd analyses. The position of the investigated core on the upper slope of the western Yermak Plateau limits delivery of sand-size (or coarser) material to ice rafting. The sand fractions of the core samples were therefore interpreted to be exclusively of ice rafted origin, and thus used as an indicator of changes in the pattern of surface currents. Clay- to silt-size material, on the other hand, yields a mixed signal of ice rafting and suspended-load delivery. Based on a comparison of the isotopic compositions of the core samples with those of the samples from potential source areas, a number of conclusions can be drawn: Most core sample show only little isotopic variation between their constituent size fractions (mostly less than analytical uncertainty). Only sand fractions show considerable differences. This can probably be explained by the sand samples’ small sample size relative to their coarse grain size; as a result, most sand fractions probably are not representative.The generally good agreement between the isotopic compositions suggests a common origin of ice rafted detritus (IRD) and suspended load. The possibility of suspended particulate matter transport from the Siberian shelf areas of the Kara- and Laptev Seas to the Yermak Plateau in significant amounts can be excluded. An origin of IRD in the Kara and Laptev Sea is therefore equally unlikely. Instead, a common provenance of IRD and suspended particulate matter from the Svalbard/Barents Sea area is a plausible scenario, supported by isotope-independent data from the literature (e.g. grain-size distribution, mineralogical composition, faunal abundance, etc.).The moderate downcore Nd isotopic variation suggests that, despite repeated large-scale glaciations in the Svalbard/Barents Sea area, the general modern-type circulation in the Fram Strait area has been active for most of the last 129 000 years. The largest deviation from modern conditions is indicated for the peak of the last glacial phase, approximately 20 000 years ago. Then, large amounts of IRD were delivered to the Yermak Plateau by icebergs calving from the Scandinavian ice sheet. Moreover, the occurrence of chalk fragments confirms iceberg drift from as far south as the North Sea. A similar finding has previously been reported for samples from the southern Fram Strait by Spielhagen (1991).Regarding the second analytical approach, i.e. the Nd isotopic analysis of finely dispersed authigenic Fe-Mn oxyhydroxides, implementation of the experimental technique was targeted first. The method of Fe-Mn oxyhydroxide extraction by means of leaching with a mixed reagent (acetic acid and hydroxylamine-hydrochloride) largely is based on the work of Chester and Hughes (1967). Modifications of their method have been reported in Tessier et al. (1979), Chao and Zhou (1983), and Hall et al. (1996), and have recently been compared by Bayon et al. (2002). Based on the experimental protocol described by Bayon et al. (2002), five core samples were processed and analyzed for their rare earth element (REE) concentrations by ICP-MS at the European Union Large Scale Geochemical Facility at the University of Bristol, England, financed by the EU. In addition, nine core samples were processed and the leachates analyzed for their Nd isotopic composition in Munich.The REE patterns of the leachates show an enrichment of the middle REE that is atypical for authigenic Fe-Mn phases. The isotopic analysis also yielded controversial results: downcore, the Nd isotope curves for the leachates and the detrital phases run approximately parallel, suggesting a systematic genetic relationship between the analyzed Nd fractions. A similar relationship appears to exist between data reported in Rutberg (2000), Rutberg et al. (2000), and Piotrowski et al. (2004) for a sediment core from the south-eastern Atlantic.To answer the questions raised by these controversial results, a sequential leaching experiment was designed. Several aliquots of one core sample were treated for different durations with different concentrations of the leaching reagents, and at intermediate steps were analyzed for their Sm-Nd isotopic composition. The results of this leaching experiment point towards a conceptual weakness of the method. In order to avoid contamination by non-authigenic sediment components, all experimental methods described in the literature focus on adjusting the concentration of the hydroxylamine-hydrochloride used to reduce Fe and Mn to their soluble states. This approach, however, does not take into account the dissolution of acid-soluble phases by acetic acid, which in all cases is used at a strength of 4.4 mol·l-1. Consequently, the leaching reagent is sufficiently corrosive to attack easily-soluble detrital minerals and release non-seawater-derived Nd (Hannigan and Sholkovitz, 2001; Dubinin and Strekopytov, 2001). Phosphatic phases are therefore a likely source of nonseawater-derived Nd. Apatite, for instance, is a common component of clastic sedimentary rocks, is easily dissolved by weak acids, and can account for the middle REE enrichment in the leachates. Its high Nd concentrations would mask any seawater signal. To conclude, it appears as though the available extraction techniques are not yet sufficiently refined to reliably determine the Nd isotopic composition of finely dispersed Fe-Mn oxyhydroxides as a proxy for paleoseawater composition.