SEDIMENTOLOGY AND MINERALOGY OF ARCTIC OCEAN SEDIMENTS - WHAT CAN WE LEARN FOR PALEOCEANOGRAPHY?
VOGT, CHRISTOPH . Central Laboratory for Crystallography and Applied Material Science (CECAM), Geowissenschaften, Universitæt Br.
Knies, Jochen . Geological Survey of Norway, Leiv Eiriksson vei 39, N-7491 Trondheim, Norway.
Matthiessen, Jens . Alfred-Wegener-Institut f…r Polar- und Meeresforschung, Bremerhaven, Columbusstr. 27515 Bremerhaven, Germany.
M…ller, Claudia . Alfred-Wegener-Institut f…r Polar- und Meeresforschung, Bremerhaven, Columbusstr. 27515 Bremerhaven, Germany.
Wahsner, Monika . Alfred-Wegener-Institut f…r Polar- und Meeresforschung, Bremerhaven, Columbusstr. 27515 Bremerhaven, Germany.
Stein, Ruediger . Alfred-Wegener-Institut f…r Polar- und Meeresforschung, Bremerhaven, Columbusstr. 27515 Bremerhaven, Germany.
Fischer, Reinhard X.. Central Laboratory for Crystallography and Applied Material Science (CECAM), Geowissenschaften, Universitæt Br.
The highly sensitive sea-ice cover of the Arctic Ocean, the exchange of surface and deep water masses with the global ocean and the coupling with the atmosphere interact directly with global climatic changes. The terrigenous content of Arctic Ocean sediments is an outstanding archive to investigate changes in the paleoenvironment. As conditions are harsh for life, and remnants and fossils are sparse, it is only the lithology and minerology of Arctic Ocean sediments which can always be used for paleoceanographic reconstructions. Despite many sedimentological-lithological investigations, detailed mineralogical investigations are comparatively sparse, in particular in the central Arctic Ocean. But the potential for the investigation of sediment sources, transport pathways, and sedimentation environments is high (e.g. Darby et al., 1989, Nürnberg et al., 1994, Vogt et al., 2001).
The impact of sea-ice cover on the sedimentation is of particular interest as its large extent compared to its small thickness results in an extreme sensitivity to climatic change. Predominantly fine grained material is entrained in the shallow shelf regions (< 30 m water depth) and in particular in the polynyas where most sea ice is formed. Newly produced sea-ice transports smectite-rich sediments from the Laptev and Kara Seas to the Fram Strait through the Southern Eurasian Basin of the Arctic Ocean (Pfirman et al., 1997). Northwest of Svalbard relatively warm Atlantic Water of the northward flowing Westspitsbergen Current submerge at the Polar Front beneath the cold sea-ice covered Polar Water. The position of the Polar Front and the sea-ice edge depends on the strength of the WSC and the outflow of the Polar Water. Fine grained sediments, just released from melting sea-ice may be deposited on the sea-floor very close to the actual position of the marginal ice zone due biologically accelerated sedimentation. As sea-ice contains high amounts of smectite, bottom sediments derived from sea-ice should contain increased smectite contents. This is one example how the mineralogical composition of sediments may help to reconstruct paleoenvironmental conditions such as drift paths of sea-ice and source areas of sediments.
Additionally, the mineral content of different grain size classes can give even more detailed information on the origin of terrigenous sediments. As an example we show here the sediment record of Polarstern Core PS2757 from the southernmost tip of the Lomonosov Ridge, close to the Laptev and East Siberian shelf seas. The stratigraphic framework is based on the correlation to adjacent sediments cores which have a well established chronostratigraphy (Stein et al., 2001). Here we concentrate on mineralogical indicators which reflect sediment input from the Putorana Flood Basalts (PFB). The erosional products are transported by the Khatanga River to the western Laptev Sea. These sediments may be then redistributed northeastward by currents and sea-ice to the core position on the southernmost tip of the Lomonosov Ridge. The flood basalts can be traced in the sediments by different marker minerals such as the smectite group in clay fraction investigation and the (clino) pyroxenes in the heavy mineral fraction. In the bulk fraction analysis the PFBs are represented by the Expendable Minerals group (Exp. Min.), which is the sum of the montmorillonite, smectite and expandable mixed-layer clay contents in the bulk sediment. The comparison of the Exp.Min. to the the clay fraction smectite group curve shows some similarities but many dissimilarities. While the clay fraction data illucidates the dominance of the PFB derived material in the clay fraction the Exp.Min. accumulation rate is several times strongly related to the clay fraction accumulation rate. Most strikingly this can be seen between 26,000 and 12,000 calendar years. While the clay mineral content data does not show an increase in PFB material input the Exp.Min. accumulation rate together with the pyroxene in the bulk material and in particular the clinopyroxene contents in the heavy mineral analysis are increased.
At this point one has to realize that all different mineral fractions have to be regarded for the paleoceanographic reconstruction. This particular enigma in the mineral signal can be explained by the paleosituation of the Laptev Sea during the last glaciation. The shallow shelf was subaerially exposed and the rivers cut their beds to the shelf edge. Current transport from the western to the eastern Laptev Sea shelf was not possible, and boundary currents along the continental slope might not have been as active as today. Therefore, the clay mineralogy of the sediment core was probably strongly influenced by sediment input from the adjacent paleo-Lena. On the other hand the clay fraction became dominant and the clay accumulation rate doubled around 26,000 years before today. The heavy minerals have been assumed to be transported mainly by sea-ice (Behrends, 1999). Today, sea-ice contains mainly (very) fine fraction sediment due to the uptake as frazil ice in late autumn during freeze up. If the same process had been active in the vicinity of the former Khathanga mouth at the western Laptev Sea shelf, a lot of sediment with PFB signature could have been transported eastward to the sediment core position. This is only possible if the Arctic Ocean circulation is the same as today. Several sediment cores on the northern Barents Sea continental slope do show that Atlantic water influx to the Arctic Ocean was absent in particular during the last glaciation.
In our presentation we will discuss more examples, how the mineral content of sediments could be used for Arctic Ocean paleoceanographic reconstructions.
Behrends, M., 1999, Reconstruction of sea-ice drift and terrigenous sediment supply in the Late Quaternary: Heavy-mineral associations in sediments of the Laptev-Sea continental margin and the central Arctic Ocean: Bremerhaven, Alfred Wegener Institute for Polar and Marine Research, Reports on Polar Research, v. 310, 167 p.
Biscaye, P.E., 1965. Mineralogy and sedimentation of recent deep-sea clays in the Atlantic Ocean and adjacent seas and oceans. Geological Society of America Bulletin, 76: p. 803-832.
Darby, D.A., Naidu, A.S., Mowatt, T.C. and Jones, G., 1989. Sediment composition and sedimentary processes in the Arctic Ocean. In: Y. Herman (Editor), The Arctic Seas - Climatology, Oceanography, Geology, and Biology. van Nostrand Reinhold, New York, pp. 657-720.
M…ller, C., 1999, Reconstruction of the paleoenvironmental conditions at the Laptev Sea continental margin during the last two glacial/interglacial cycles based on sedimentological and mineralogical investigations: Bremerhaven, Alfred Wegener Institute for Polar and Marine Research, Reports on Polar Research, v. 328, 146 p.
N…rnberg, D. et al., 1994. Sediments in Arctic sea ice: Implications for entrainment, transport and release. Marine Geology. v.119, p. 185-214.
Pfirman, S.L., Colony, R., N…rnberg, D., Eicken, H. and Rigor, I., 1997. Reconstructing the origin and trajectory of drifting Arctic sea ice. Journal of Geophysical Research, v. 102(C6): p. 12,575-12,586.
Stein, R., Boucsein, B., Fahl, K., Garcia de Oteyza, T., Knies, J., and Niessen, F., 2001, Accumulation of particulate organic carbon at the Eurasian continental margin during the late Quaternary times: Controlling mechanism and paleoenvironmental signifcance: Global and Planetary Change, v. 31 (1-4), p. 87-102.
Vogt, C., Knies, J., Spielhagen, R.F. and Stein, R., 2001. Detailed mineralogical evidence for two nearly identical glacial/deglacial cycles and Atlantic water advection to the Arctic Ocean during the last 90,000 years. Global and Planetary Change, 31(1-4): p. 23-44
Figure 1. Fig. 1 Mineralogical Data and Sediment Accumulation rates of grain sizes and minerals of PS2757-8 from the southernmost tip of Lomonosov Ridge, Arctic Ocean (81î09¸6N, 140î12.0¸E; 1230 m water depth). From left to right: Bulk sediment accumulation rate (ACC), ACC of the Expandable Minerals Group in the bulk sediment, ACC of the clay fraction (<2 µm), content of the clay mineral group smectite (calculated according to Biscaye, 1965) in the clay fraction, integrated area of the most pyroxene peaks in the X-ray diffractogram of the bulk material as direct measure of pyroxene content in the bulk sample, ACC of the sand fraction (63-2000µm), content of the pyroxenes from a heavy mineral fraction investigation (based on counting of grains in a slide through light microscopy of the seperated heavy mineral fraction; Behrends, 1999).
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