Characteristics of ice-rafted pebbles from ODP Leg 178 holes

Pebbles (>10 mm) sampled from three drill sites on the continental rise west of the Antarctic Peninsula during Ocean Drilling Program Leg 178 were classified by shape and roundness. In addition, pebble lithology and surface texture were visually identified. To increase the pebble sample number to 331, three sites that were drilled 94 to 213 km from the continental shelf edge were integrated into the data set using magnetostratigraphy for core correlation. Pebbles were compared in three groups defined by the same stratigraphic intervals at each site: 3.1-2.2 Ma (late Pliocene), 2.2-0.76 Ma (late Pliocene-late Pleistocene), and 0.76 Ma to the Holocene. Pebble lithologies originate from sources on the Antarctic Peninsula margin. Most pebbles are metamorphic and sedimentary pebbles are rare (<6%), whereas mafic volcanic and intrusive igneous lithologies increase in abundance upsection.
Pebbles from 3.1 to 0.76 Ma, plotted on sphericity-roundness diagrams, indicate original transport as basal and supraglacial/englacial debris. Pebbles are abundant and of diverse lithology. From 0.76 Ma to the present, the number of pebbles is low and their shape characteristics indicate they originated as basal debris. Observed changes in ice-rafted pebbles can be explained by growth of an ice sheet and inundation of the Antarctic Peninsula topography by ice ~0.76 Ma. Prior to this, outlet and valley glaciers transported debris at high levels within and at the base of the ice. The mass accumulation rate of sand fluctuates and includes rounded quartz grains. Ice-sheet growth may have been accompanied by overall cooling from subpolar to polar glacial regimes, which halted meltwater production and enhanced the growth of ice shelves, which consequently reduced sediment supply to icebergs.

本数据集的研究样本为南极半岛西侧大陆隆上3个钻探站位的粒径大于10 mm的砾石，采自大洋钻探计划（Ocean Drilling Program）第178航次。研究人员对这批砾石的形态与圆度开展了分类工作，同时通过目视识别的方式确定了砾石的岩性与表面纹理特征。
为将砾石样本总量扩充至331件，研究人员纳入了距大陆架边缘94~213 km的另外3个钻探站位的数据，通过磁地层学（magnetostratigraphy）完成岩心对比（core correlation）后将其并入本数据集。依据各站位一致的地层间隔，将砾石划分为3组：3.1~2.2 Ma（上新世晚期，late Pliocene）、2.2~0.76 Ma（上新世晚期-更新世晚期，late Pleistocene）以及0.76 Ma至全新世（Holocene）。砾石的岩性均源自南极半岛陆缘物源区，其中绝大多数为变质岩砾石，沉积岩砾石占比极低（<6%）；而基性火山岩与侵入火成岩类的丰度随地层向上逐渐升高。

针对3.1~0.76 Ma时段的砾石，通过绘制球度-圆度图（sphericity-roundness diagrams）进行分析后可知，其初始搬运类型为底部碎屑与冰面/冰内碎屑。该时段砾石数量丰富且岩性多样。0.76 Ma至今的时段内，砾石数量稀少，其形态特征表明其起源为底部碎屑。观测到的冰筏砾石（ice-rafted pebbles）变化特征，可通过冰盖扩张以及约0.76 Ma时冰体覆盖南极半岛地形的事件得到合理解释。在此之前，冰流与山谷冰川（outlet and valley glaciers）将碎屑搬运至冰体内部及底部的较高位置。
砂的质量累积速率（mass accumulation rate）存在波动，其中包含磨圆的石英颗粒。冰盖扩张可能伴随了从亚极地到极地冰期环境的整体降温，这一过程终止了融水产生并促进了冰架（ice shelves）的发育，进而减少了输送至冰山（icebergs）的沉积物供给。