Generation and fate of glacial sediments in the central Transantarctic Mountains based on radiogenic isotopes and implications for reconstructing past ice dynamics Quaternary Science Reviews

The Nd, Sr and Pb isotopic compositions of glacial tills from the Byrd and Nimrod Glaciers in the central Transantarctic Mountains (TAM) in East Antarctica were obtained to assess the sources of detritus transported by these ice masses. Tills from lateral moraines along the entire extent of both glaciers have isotopic compositions consistent with their derivation predominately from erosion of adjacent bedrock. Fine- (<63μ) and coarser-grained (0.5 mm−2 mm) sediment from these tills have identical isotopic characteristics, indicating that fine-grained detritus is the product of further comminution of coarser sediments. Comparison of present-day till isotopic data to existing data from fine-grained LGM tills in the central Ross Sea confirm that these were deposited from East Antarctic ice that expanded through the TAM and indicates that the LGM sediments are mixtures of detritus eroded along the entire path of ice transiting the TAM. If specific lithologies were preferentially eroded as ice passed through the TAM, it is not clearly evident in the Ross Sea till isotopic compositions. Our data do demonstrate, however, that glacial tills generated from erosion of inboard regions of the mountain belt yield sediment with a larger component of 560 Ma to 600 Ma detrital zircons and lower average εNd(0) values (<−5) than that produced further downstream. As a result, past retreat of ice grounding-lines up the narrow valleys of the TAM resulting in active erosion of inboard region should recognizable in glacial sediments deposited in the Ross Sea and so provide a means to identify times when the East Antarctic ice sheet was smaller than today. This study highlights both the value and necessity of utilizing multiple provenance methods in evaluating glacial erosion and transport when reconstructing past ice sheet dynamics.

为评估东南极洲横贯南极山脉（Transantarctic Mountains, TAM）中部伯德冰川与尼姆罗德冰川所携带碎屑物质的来源，本研究获取了两处冰川冰碛物的钕（Nd）、锶（Sr）及铅（Pb）同位素组成。沿两处冰川全延伸范围采集的侧碛冰碛物，其同位素组成与主要源自相邻基岩侵蚀的产物一致。从这些冰碛物中分离出的细粒（<63μm）与粗粒（0.5 mm~2 mm）沉积物具有一致的同位素特征，表明细粒碎屑物质是粗粒沉积物进一步磨蚀的产物。将现代冰碛物同位素数据与罗斯海中部采集的细粒末次盛冰期（Last Glacial Maximum, LGM）冰碛物已有数据进行对比，结果证实这些沉积物由穿越横贯南极山脉的东南极冰体沉积形成，同时表明末次盛冰期沉积物是沿横贯南极山脉冰流路径全程侵蚀形成的碎屑混合物。若冰体穿越横贯南极山脉时存在特定岩性的优先侵蚀现象，该现象在罗斯海冰碛物的同位素组成中并未得到清晰体现。但本研究数据确实表明，相较于山地带下游区域侵蚀形成的冰碛物，源自山地带内侧区域侵蚀的冰碛物所产出的沉积物中，560 Ma至600 Ma年龄的碎屑锆石占比更高，且平均εNd(0)值更低（<−5）。因此，东南极冰盖的冰架接地带沿横贯南极山脉狭窄谷地上溯后退、进而引发内侧区域活跃侵蚀的古事件，应当能在罗斯海沉积的冰碛物中得到识别，这也为确定东南极冰盖规模小于现代的时期提供了可行手段。本研究强调了在重建古冰盖动力学过程中，采用多种物源分析方法评估冰川侵蚀与搬运过程的价值与必要性。