Clay mineralogy of ODP Leg 178 sites

The clay mineral compositions of upper Miocene to Quaternary sediments recovered at Ocean Drilling Program (ODP) Leg 178, Sites 1095 and 1096, from the continental rise west of the Antarctic Peninsula were analyzed in order to reconstruct the Neogene and Quaternary Antarctic paleoclimate and ice dynamics. The clay mineral assemblages are dominated by smectite, illite, and chlorite. Kaolinite occurs only in trace amounts. Analysis of a surface-sample data set facilitates the assignment of these clay minerals to particular source areas on the Antarctic Peninsula and, thus, the reconstruction of transport pathways. In the ODP cores, clay mineral composition cyclically alternates between two end-member assemblages. One assemblage is characterized by <20% smectite and >40% chlorite. The other assemblage has >20% smectite and <40% chlorite. Illite fluctuates between 30% and 50% without a significant affinity to one end-member assemblage. By comparison with a Quaternary sediment sequence from gravity core PS1565, the clay mineral fluctuations can be ascribed to glacial and interglacial periods, respectively. The cyclic changes in the clay mineral composition suggest that glacial-interglacial cycles, repeated ice advances and retreats, and changes in the Antarctic ice volume were already a main control of the global climate in late Miocene time. Throughout the late Neogene and Quaternary, the clay mineral records in the drift sediments exhibit only slight long-term changes predominantly attributed to local changes in glacial erosion and supply of source rocks. The absence of clear long-term trends associated with major climatic or glaciological changes points to an onset of vast glaciation in the Antarctic Peninsula region before ~9 Ma and to relative stability of the Antarctic ice sheet since then.

针对南极半岛以西大陆隆区大洋钻探计划（Ocean Drilling Program, ODP）第178航次1095与1096站位钻获的上中新统至第四纪沉积物，本研究对其黏土矿物组成开展系统分析，旨在重建新近纪及第四纪南极古气候与冰盖动力学演化历史。该套沉积物的黏土矿物组合以蒙脱石（smectite）、伊利石（illite）和绿泥石（chlorite）为主，高岭石（kaolinite）仅以痕量产出。通过对表层样品数据集的解析，可将上述黏土矿物归属于南极半岛的特定物源区，进而厘清沉积物的搬运路径。在ODP岩芯中，黏土矿物组成呈现两种端元矿物组合的周期性交替：其一以蒙脱石占比小于20%、绿泥石占比大于40%为特征；其二则以蒙脱石占比大于20%、绿泥石占比小于40%为特征。伊利石含量在30%~50%区间内波动，与任一端元组合均无显著相关性。通过与重力岩芯（gravity core）PS1565的第四纪沉积序列对比，可将上述黏土矿物的周期性波动分别对应至冰期与间冰期。黏土矿物组成的周期性变化表明，中新世晚期以来，冰期-间冰期旋回、冰盖反复进退以及南极冰盖体积变化，已然成为全球气候的核心控制因素。在整个新近纪晚期至第四纪期间，漂移沉积物中的黏土矿物记录仅呈现微弱的长期演化趋势，该趋势主要由区域冰川侵蚀作用变化与物源岩供给变动所致。本研究未观测到与重大气候或冰川学变革相关的显著长期趋势，这一结果表明南极半岛地区大规模冰川作用始于约9 Ma之前，且自彼时起南极冰盖始终保持相对稳定。