Sedimentology of cores from the Bellingshausen Sea, Antarctic

On the continental rise west of the Antarctic Peninsula there are nine large mounds interpreted as sediment drifts, separated by turbidity current channels. Drift 7 is 150 km long, 70 km wide and up to 700 m high and is asymmetric, with steep sides on the south-east (towards the continent) and south-west, and gentle slopes to north-west and north-east. Cores on the gentle sides of the drift show a cyclicity between brown, bioturbated, diatom-bearing mud with foraminifera and radiolarians, and grey, laminated, barren mud. Biostratigraphic evidence is consistent with a Late Quaternary age. Detailed lithostratigraphy and magnetic susceptibility data allow precise correlation over distances of tens of kilometres. On the basis of chemostratigraphy, the brown sediment is interpreted as interglacial (isotope stages 1 and 5) and the grey as glacial (stages 2-4 and 6). Sedimentation rates are 3.0-5.5 cm/ka. Cores on the steep sides of the drift recovered a condensed section with thinner cycles and hiatuses. Fine grain size, very poor sorting and the absence of a mode in the silt size range indicate deposition from suspension with only weak current activity, There is little evidence for cyclic changes in bottom current strength. Supply of sediment to the benthic nepheloid layer was by entrainment ofmud from turbidity currents, and by settling ofpelagic material (biogenic grains, IRD, sediment suspended in meltwater plumes). Cyclic changes in sediment supply include more biogenic supply in interglacials with less sea ice cover, more terrigenous supply from turbidites in glacials with ice sheets grounded to the shelf edge, and changes in IRD content.在南极半岛西侧的大陆隆（continental rise）上，发育9处被解释为沉积漂移体（sediment drifts）的大型丘状体，彼此被浊流水道（turbidity current channels）分隔。7号沉积漂移体长150千米、宽70千米，最高可达700米，形态不对称，其东南侧（朝向大陆一侧）与西南侧地势陡峭，西北侧与东北侧则坡度平缓。该漂移体平缓侧的岩心显示，沉积物呈现周期性交替：一类为含孔虫与放射虫的生物扰动含硅藻棕色泥质沉积物，另一类为灰色、纹层状、无生物的泥质沉积物。生物地层学（biostratigraphy）证据表明其形成年代属于晚第四纪（Late Quaternary）。详实的岩石地层学（lithostratigraphy）与磁化率（magnetic susceptibility）数据支持在数十千米尺度内开展精确地层对比。基于化学地层学（chemostratigraphy）分析，棕色沉积物被认定为间冰期（同位素阶段1与5）沉积，灰色沉积物则对应冰期（同位素阶段2至4及6）沉积。沉积速率为3.0~5.5厘米每千年。该漂移体陡峭侧的岩心则获取了一套沉积旋回更薄且发育沉积间断的凝缩段（condensed section）。该沉积物具有细粒级、极差分选性以及粉砂级粒径区间无众数的特征，表明其形成于仅伴随微弱水动力活动的悬浮沉积环境；几乎没有证据显示底层流（bottom current）强度存在周期性变化。底栖雾状层（benthic nepheloid layer）的沉积物补给主要来源于两部分：一是浊流裹挟的泥质，二是远洋物质（生物成因颗粒、冰筏碎屑（IRD，Ice Rafted Debris）、融水羽流中悬浮的沉积物）的沉降。沉积物补给的周期性变化包括：海冰覆盖度更低的间冰期内生物成因补给占比更高，冰盖固结于陆架边缘的冰期内浊积岩提供更多陆源补给，以及冰筏碎屑含量的变化。