Neogene environmental history deduced from glacigenic sediments on James Ross Island, northern Antarctic Peninsula

摘要

The stability, configuration and volume of the Cenozoic Antarctic Ice Sheet (AIS) are poorly known and contentious. Our investigation of late Neogene glacial and interglacial sediments from James Ross Island, northern Antarctic Peninsula, will inform this debate by providing critical new data from a part of the AIS that is particularly sensitive to climatic variability. James Ross Island contains the greatest number of Neogene sedimentary outcrops in Antarctica. Understanding the genetic history of these sediments will reveal critical information on the past behaviour and parameters of the Antarctic Peninsula Ice Sheet (APIS). The sedimentary lithofacies on James Ross Island include massive and bedded diamict and conglomerate, and laminated mudstone with dropstones. Our sedimentological analyses, including individual clast characteristics and fabrics, reveal a combination of basal tills, remobilised debris flow deposits, and glaciomarine sequences. Till micromorphology shows a high percentage of subglacial sediment deformation structures suggesting significant ice-bed coupling by the APIS. Whereas some glacigenic deposits contain only locally-derived clasts, others contain a high percentage of Antarctic Peninsula-derived detritus, indicating the influence of two scales of ice masses: a regional-scale APIS and a local ice cap similar to the glacial cover on James Ross Island today. The contact relationships between the glacial sediment and overlying volcanic rocks indicate that glaciation and volcanism were essentially contemporaneous, and the volcanic units have provided an excellent chronology for the glaciations, which is absent from Neogene sedimentary sequences elsewhere in Antarctica. A polythermal glacial regime is suggested for the Neogene glacial cover on James Ross Island, with conditions similar to the high Arctic today.