Antarctic cryogenic sediments: Biotic and inorganic facies of ice shelf and marine-based ice sheet environments
Antarctic cryogenic sediments: Biotic and inorganic facies of ice shelf and marine-based ice sheet environments
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南极低温沉积物：冰架和海洋冰盖环境的生物和无机相
DOI：
10.1016/0031-0182(88)90122-8
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发表时间：
1988
期刊：
影响因子：
--
通讯作者：
D. Kellogg
中科院分区：
文献类型：
--
作者：
T. B. Kellogg;D. Kellogg
低温沉积物被定义为与冰（冰山、海冰、冰架或海洋冰盖）一起形成的海洋沉积物，在高纬度地区广泛存在。现有的低温沉积过程模型的效用有限，因为(1)缺乏直接的观测数据，(2)依赖于可以从中得出推断性结论的较老的陆地暴露部分，(3)使用不充分的冰川学和海洋学概念，以及(4)未能充分纳入这些沉积物中生物含量的信息。作为开发低温沉积综合模型的第一步，我们利用南极大陆架的数据将现代冰架和海洋冰架环境的冰川学、沉积学和生物过程联系起来。海洋冰原下的主要沉积物是沉积物，其中通常含有重新加工和破碎的化石物质。这片土地在所有重要的沉积学方面都与陆地沉积物相同。海洋沉积物通常在地理上形成广泛的薄片，这些薄片仅限于大陆架遗址，并且可以通过其高度压实和重新加工的化石与其他低温沉积物区分开来。在冰川底部融化的碎片穿过冰和冰床之间的薄水层（最多几米）落下的地区，在接地冰下可能形成水坑。水碛物通常缺乏化石物质，除非冰下碎屑的源物质是化石的，并且比沉降物不致密。大型极地冰架（如Ross、Filchner-Ronne冰架）下的沉积主要局限于接地线附近的一个区域（宽度可能小于100公里）。在这一地区，可能会形成一次和二次低温沉积的复合体（包括水耕、冰架漂流碎屑、流耕、滑塌等）。控制这种沉积的过程包括：冰盖和冰架的局部基础条件（冻结或融化）；冰下流出物、海流和潮汐作用的存在或不存在；沉积物供应;长、短期接地线移动；海底地形;和水深。ISRD的沉积在这些大冰架下面的其他地方是有限的，因为高的表面堆积率导致了冰和碎片向下的颗粒路径。因此，大多数碎片在通过接地线后不久就被释放出来。冰架下的生物活动在产犊边缘和接地线之间的多样性和丰度迅速下降。产犊边缘向海的沉积速率比大冰架下的要高得多。在这里，来自出口冰川和直接终止于海洋的冰流的冰山可能携带大量冰下、冰上或冰上碎屑，这些碎屑在冰山融化时通过各种机制释放出来。这些冰筏状的碎屑与浮游生物的生物物质混合在一起，形成化合物IBRD。如果水流存在于水柱或海底，分选可能会形成残留的IBRD。在浅滩上，携带冰下碎屑的搁浅冰山可能在原地融化，形成冰山耕地，而冰山对沉积物的翻耕可能形成冰山湍流。这两种沉积物可能很难与沉积物区分开，但它们都局限于现代水深小于≈450米的地区。
Cryogenic sediments, defined as marine deposits that form in association with ice (icebergs, sea ice, ice shelves or marine-based ice sheets), are widespread in high latitude regions. Existing models for cryogenic depositional processes are of limited utility because of (1) lack of direct observational data,(2) reliance on older land-exposed sections from which inferential conclusions have been drawn,(3) employment of inadequate glaciologic and oceanographic concepts, and (4) failure to incorporate adequately information on the biotic content of these deposits. As a first step toward developing a synthetic model for cryogenic deposition, we relate glaciologic, sedimentologic, and biotic processes of the modern ice shelf and marine-based ice sheet environments using data from Antarctic continental shelves. The principal deposit beneath grounded marine-based ice sheets is lodgement till, which often contains reworked and fragmented fossil material. This till is identical in all important sedimentologic respects with terrestrial lodgement deposits. Marine lodgement tills commonly form geographically extensive sheets which are restricted to continental shelf sites, and which may be differentiated from other cryogenic deposits by their high degree of compaction and reworked fossils. Waterlain till may form beneath grounded ice in areas where debris melting out of the glacier sole falls through a thin water layer (up to a few m) between the ice and its bed. Waterlain till usually lacks fossil material, unless the source material for the subglacial debris is fossiliferous, and is less compact than lodgement till. Deposition beneath large polar ice shelves (eg, Ross, Filchner-Ronne) is largely restricted to a zone (probably< 100 km in width) near the grounding line. In this zone, a complex of primary and secondary cryogenic deposits (including waterlain till, ice shelf rafted detritus (ISRD), flow till, slumps, etc.) is likely to interfinger. Processes controlling this deposition include: local basal conditions of the ice sheet and ice shelf (freezing or melting); presence or absence of subglacial outflow, marine currents, and tidal action; sediment supply; long-and short-term grounding line movement; sea-floor topography; and water depth. Deposition of ISRD is limited elsewhere beneath these large ice shelves because high surface accumulation rates cause downward particle paths for ice and debris. Thus most debris is released soon after passing the grounding line. Biotic activity beneath ice shelves declines rapidly, in diversity and abundance, between the calving margin and grounding line. Deposition rates seaward of the calving margin are much higher than beneath the large ice shelves. Here, icebergs derived from outlet glaciers and ice streams that terminate directly in the sea, may carry significant loads of subglacial, englacial, or superglacial debris, which is released by a variety of mechanisms as the icebergs melt. This ice-rafted detritus mixes with biotic material from planktonic organisms as it falls through the water column to form compound IBRD. If currents are present in the water column or at the seafloor, sorting may occur to form residual IBRD. On shallow banks, grounded icebergs carrying subglacial debris may melt in situ forming iceberg till, and iceberg ploughing of sediment may form iceberg turbate. Both these deposits may be difficult to distinguish from lodgement till but both are restricted to areas with modern water depths less than≈ 450 m.