Sedimentology and age models of cores from the Antarctic continental margin in the eastern Weddell Sea

To reveal the late Quaternary paleoenvironmental changes at the Antarctic continental margin, we test a lithostratigraphy, adjusted to a stable isotope record from the eastern Weddell Sea. The stratigraphy is used to produce a stacked sedimentological data set of eleven sediment cores. We derive a general model of glacio marine sedimentation and paleoenvironmental changes at the East Antarctic continental margin during the last two climatic cycles (300 kyr). The sedimentary processes considered include biological productivity, ice-rafting, current transport, and gravitational downslope transport. These processes are controlled by a complex interaction of sea-level changes and paleoceanographic and paleoglacial conditions in response to changes of global climate and local insolation. Sedimentation rates are mainly controlled by ice-rafting which reflects mass balance and behaviour of the Antarctic ice sheet. The sedimentation rates decrease with distance from the continent and from interglacial to glacial. Highest rates occur at the very beginning of interglacials, i.e. of oxygen isotope events 7.5, 5.5, and 1.1, these being up to five times higher than during glacials. The sediments can be classified into five distinct facies and correlated to different paleoenvironments: at glacial terminations (isotope events 8.0, 6.0, and 2.0), the Antarctic cryosphere adjusts to new climatic conditions. The sedimentary processes are controlled by the rise of sea level, the destruction of ice shelves, the retreat of sea-ice and the recommenced feeding of warm North Atlantic Deep Water (NADW) to the Circumpolar Deep Water (CDW). During peak warm interglacial periods (at isotope events 7.5, 7.3, 5.5., and 1.1), the CDW promotes warmer surface waters and thus the retreat of sea-ice which in turn controls the availability of light in surface waters. At distinct climatic thresholds local insolation might also influence sea-ice distribution. Primary productivity and bioturbation increase, the CCD rises and carbonate dissolution occurs in slope sediments also in shallow depth. Ice shelves and coastal polynyas favour the formation of very cold and saline Ice Shelf Water (ISW) which contributes to bottom water formation. During the transition from a peak warm time to a glacial (isotope stages 7.2-7.0, and 5.4-5.0) the superimposition of both intense ice-rafting and reduced bottom currents produces a typical facies which occurs with a distinct lag in the time of response of specific sedimentary processes to climatic change. With the onset of a glacial (at isotope events 7.0 and 5.0) the Antarctic ice sheet expands due to the lowering of sea-level with the extensive glaciations in the northern Hemisphere. Gravitational sediment transport becomes the most active process, and sediment transfer to the deep sea is provided by turbidity currents through canyon systems. During Antarctic glacial maxima (isotope stages between 7.0-6.0, and 5.0-2.0) the strongly reduced input of NADW into the Southern Ocean favours further advances of the ice shelves far beyond the shelf break and the continous formation of sea ice. Below ice shelves and/or closed sea ice coverage contourites are deposited on the slope.为揭示南极大陆边缘晚第四纪（late Quaternary）古环境变化，我们以威德尔海东部的稳定同位素记录为参照，对岩石地层学（lithostratigraphy）方案进行了校正并开展测试。该地层方案被用于构建11根沉积岩芯的堆叠沉积学数据集，据此我们推导了过去两个气候周期（30万年）内，东南极大陆边缘的冰海沉积作用与古环境变化的通用模型。 本研究涉及的沉积作用过程包括生物生产力（biological productivity）、浮冰搬运（ice-rafting）、洋流搬运以及斜坡重力搬运。这些过程受海平面变化、古海洋与古冰川条件的复杂交互作用调控，而这些条件响应全球气候变化与局地太阳辐射变化。沉积速率主要受控于浮冰搬运作用，该作用可反映南极冰盖的物质平衡与动态特征。沉积速率随与大陆的距离增加而降低，且从间冰期向冰期逐渐减小。最高沉积速率出现在间冰期伊始，即氧同位素事件7.5、5.5与1.1时期，其速率最高可达冰期的5倍。 研究区沉积物可划分为5种截然不同的沉积相，并可与不同古环境对应：在冰消期（同位素事件8.0、6.0与2.0时期），南极冰冻圈会调整以适应新的气候条件。此时沉积作用过程受海平面上升、冰架破坏、海冰退缩以及暖态北大西洋深层水（North Atlantic Deep Water, NADW）重新补给环极深层水（Circumpolar Deep Water, CDW）的共同调控。 在暖盛间冰期（同位素事件7.5、7.3、5.5与1.1时期），环极深层水（CDW）会使表层海水升温，进而引发海冰退缩，而海冰退缩又会调控表层水体的光照可获得性。在特定气候阈值下，局地太阳辐射也会影响海冰分布。此时初级生产力（Primary productivity）与生物扰动（bioturbation）均有所增强，碳酸钙补偿深度（Calcium Carbonate Compensation Depth, CCD）上升，且斜坡沉积物甚至在浅水区也会发生碳酸盐溶解作用。冰架与沿岸冰间湖有利于形成极寒高盐的冰架水（Ice Shelf Water, ISW），该水团可为底层水形成提供贡献。 在从暖盛期向冰期的转型阶段（同位素阶段7.2-7.0与5.4-5.0），强浮冰搬运与底流减弱的叠加作用会形成一种典型沉积相，该相的特定沉积过程对气候变化的响应存在显著滞后。随着冰期启动（同位素事件7.0与5.0时期），受北半球广泛冰川作用引发的海平面下降影响，南极冰盖会发生扩张。此时重力沉积物搬运成为最活跃的过程，沉积物通过峡谷系统的浊流（turbidity currents）被输送至深海。 在南极冰期盛期（同位素阶段7.0-6.0与5.0-2.0），北大西洋深层水（NADW）向南大洋的输入量大幅减少，这有利于冰架进一步向陆架坡折（shelf break）以外区域推进，并持续形成海冰。在冰架下方及/或被封闭海冰覆盖的区域，等深流沉积（contourites）会在斜坡上沉积下来。