Rapid environmental changes in the Ross Sea Embayment using a geochemical approach

Gerhard.Kuhn [ at ] awi.de


Enhanced glacier and polar ice sheet melting during the last decades is one of the major focuses of geosciences. The understanding of the effects of future global warming is important due to raising sea level. Antarctic ice masses play a key role in global climate. Melting of Antarctic ice would result in a sea level rise of about ~3-6 m due to the West Antarctic Ice Sheet and 60 m due to the East Antarctic Ice Sheet. Because of its marine-based nature, especially the West Antarctic Ice Sheet is sensitive to rising temperatures. Additionally, the Ross Sea, as a part of the west Antarctic system, and its shelf are a key region stabilizing the west Antarctic ice masses. It is thus essential to understand the processes and changes in this area in order to interpret the past and predict the future climate developments. Sedimentary archives are a unique opportunity to get insights into past climate variability and the ice response due to increased temperature (~3°C), as the earth had experienced in the last 14 Ma since the mid Miocene cooling. The multi-national drilling program ANDRILL (ANtarctic geological DRILLing, McMurdo Ice Shelf project, MIS) focuses on the changes of climatic influences on the West Antarctic Ice Sheet during the past ~14 Ma. During austral summer 2006/07, an approximately 1300 m long sedimentary succession beneath the northwestern Ross Ice Shelf was cored. In this study geochemical investigations were carried out and interpreted using a multiproxy approach. The results of major element measurements from X-ray fluorescence on discrete samples and high-resolution non-destructive XRF analyses on split-cores, using an XRF core scanner, total organic carbon, total inorganic carbon, opal, and mineral data, as well as optical microscope and visual colour reflectance investigations were used to reveal different processes controlling the depositional environment in the southern Ross Sea. Provenance analyses reveal three main sources for fine-grained terrigenous sediments at the MIS site. First, close-by McMurdo Group volcanoes (MVG) are a source for sedimentary deposits of AND-1B (mainly diamictites) during times of extended glaciations. Second, in interglacial periods, sediment composition (mainly mudstones) is controlled by southern Transantarctic Mountains (S TAM) and finally a geochemical mixture of both sources is visible in the record, which also can be indicative for a western Transantarctic Mountain source (W TAM). According to sediment architecture (McKay, 2008) and source, different transport mechanisms are existing. MVG sediments are mainly transported by subglacial erosion, whereas S TAM and W TAM sediments represent ice proximal to distal conditions with transport processes such as fluvial meltwater and gravity flows. The entire AND-1B core can be divided into 5 major sections of provenance. These geochemical facies (GCF) represent colder phases during Late Miocene and Late Pleistocene (GCF1, dominated by MVG); warm intervals during early Pliocene and late Pliocene are dominated by input from southern provenances (GCF 2), and oscillations between material from MVG and W TAM dominate the mid Pliocene (GCF 3). In sedimentary successions indicating glacial terminations and extended sea ice conditions dolomite, Fe-dolomite siderite and other traces of carbonate minerals were found. Micritic crystal size and distribution within the sediment matrix reveal an authigenic, early diagenetic precipitation. These precipitates form in concert with freezing processes that result in a hyper-saline brine formation, saturating pore waters with respect to dolomite phases. Within the layers of hyper-saline brines, dolomite or unstable carbonate phases occur that are later on recrystallized to dolomite. An additional factor for dolomite formation is sulphate reduction, indicated by pyrite and biomarkers of sulphate reducing bacteria. The features of the AND-1B dolomite are likely to be transferable to global cap-carbonates of Precambrian times and can be likely an explanation for dolomite formation due to rapid deglaciations at glacial terminations. During open water conditions in the southern Ross Sea, when no West Antarctic Ice Sheet had existed (Naish et al., 2009), mass accumulation and paleoproductivity estimations from the AND-1B record reveal the Ross Sea as a high production area during Pliocene and Pleistocene. Opal accumulation is in the same ranges as for modern analogues in front of the present Ross Ice Shelf edge and even higher during specific times in the Pliocene. In contrast, paleoproduction and organic carbon accumulation rates are low and reveal a decoupling between organic carbon and opal preservation, where opal evidences good preservation efficiency while organic carbon shows low preservation rates. These results suggest that the Ross Shelf was an oxic environment with strong organic carbon degradation during the Pliocene. Nevertheless, accumulation of organic carbon was high enough to make the Ross Shelf a major sink for organic carbon. In a global comparison, the Ross Sea area removed more carbon from the atmospheric CO2 cycle as some highly productive up-welling systems do.

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Monien, D. (2010): Rapid environmental changes in the Ross Sea Embayment using a geochemical approach , PhD thesis, FB5, Geosciences.

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