Glacimarine sedimentation of muds in Hornsund Fjord, Spitsbergen

Krzysztof Görlich

Abstract


Glacjalno-morska sedymentacja mułów we fjordzie Hornsund na Spitsbergenie

The Hornsund fjord is fed with sediment-laden meltwaters by eight major tidewater glaciers. Two glacimarine mud facies are described in bottom sediments of Hornsund: (1) laminated mud and (2) homogeneous-bioturbated mud. The laminated mud facies is ice-proximal and is deposited in a ponded mode at high accumulation rates (> 5 cm/a). It reveals organic-debris-induced lamination and little or no fauna. The clay mineral assemblage in this facies preserves compositional features typical of suspended load discharged from a given source glacier. The homogeneous to bioturbated mud facies is ice-distal and is deposited at relatively low accumulation rates (< 5 cm/a). It has no organic-induced lamination and is rich in fauna. The composition of clay-mineral assemblage in this facies is not directly related to any particular source of suspended load, due to the conspicuous effects of blending and differential settling of suspension along transportation path. In the homogeneous to bioturbated mud facies, slight seaward coarsening is observed, accompanied by enhanced grain-size bimodality. The latter is due to relatively increasing contribution of ice-rafted debris. The clay minerals are inherited almost unchanged by bottom sediments from the reeks which crop out in the glaciers’ catchments. Lithological indices are introduced to express this property of the glacial environment. Settling of clay suspension is electrochemically ccerced by sea water. Flocculation is not preferential toward any specific clay-mineral species or grain size. The suspended load undergoes gravitational sorting which affects also clay fraction. 1 Md illite and diagenetic chlorite are preferentially carried in suspension out of the fjord. Seaward decay of sedimentation rate is exponential but, for individual tidewater-glacier sources, it occurs with different steepness of the rate-distribution curve. This is attributed to compositional and textural differences between the initial suspended loads, and to different values of vertical salinity gradients within near-source parts of the basins. Variable but always large vertical salinity gradients in front of tidewater glaciers enforce mass settling of suspension. This phenomenon is much less pronounced in fjord-type estuaries of glacier-fed rivers due to diffuse halocline and intensive vertical mixing. Hence, efficiency of entrapment of suspension in the near-source zone is the greatest in tidewater-glacier systems.

Zbadano mineralogicznie i sedymentologiczniemuły morsko-lodowcowezalegające dno fiordu Hornsund na Spitsbergenie. W osadzie wyróżniono prowincje mineralne odzwierciedlające pochodzenie materiału osadowego. Określono przestrzenny rozkład stopy akumulacji mułów. Wykładniczy spadek stopy akumulacji w kierunku od czoła lodowca zachodzi w poszczególnych basenach przyczołowych z różną prędkością. Prędkości te zależą od pionowego gradientu zasolenia wody morskiej i wyjściowego składu zawiesiny. Materiał osadowy jest segregowany grawitacyjnie także w obrębie frakcji ilastej. Szczególnie silna flokulacja zawiesiny w przyczołowych basenach lodowców uchodzących do morza powoduje, że zasadnicza część materiału osadowego zostaje zatrzymana w strefie przyźródłowej i nie dostaje się do oceanu.

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