Paleoenvironmental indications and cyclostratigraphic studies of sediments from tropical Lake Towuti obtained from downhole logging

点击翻译按钮获取中文摘要

An extensive scientific drilling project was started at Lake Towuti, a tropical pacific lake (Sulawesi, Indonesia), in spring 2015. The objectives of the international science party were to investigate its paleoenvironmental, tectonic, and evolutionary biological history over the past five to six glacial-interglacial cycles (approx. 650,000 years). Pre-site studies on shallow sediment cores, spanning the past 60,000 years, have shown that the dynamic of past environmental changes in between wet (warm) and dry (cold) periods are indicated by physical and chemical properties. Therefore, boreholes were drilled down to a depth of 174 m below sea floor at three sites to extend this study to the early lake sediments. In addition to coring, downhole logging was carried out at two drill sites to determine crucial parameters under in situ conditions. In this project, the acquired logging data will be analyzed to derive paleoenvironmental indicators. Suitable parameters are most likely magnetic susceptibility and K contents but the use of U concentrations and high resolution resistivity variations will be tested. Within a final multi-proxy study, the achieved results from all working groups will be compared and merged to achieve the most comprehensive picture. Potential climate-sensitive parameters will additionally be studied by cyclostratigraphic methods. This enables to derive an age-depth model for Lake Towutis past which will be interpreted in conjunction with results from dating of core materials (tephra layers). Because of event deposits (e.g. tephra layers) which occasionally disturb the quiet pelagic sedimentation, cyclostratigraphic studies might be difficult to perform on Lake Towutis sediments. However, experiences from Lake Van in Turkey have shown that cyclostratigraphic methods can be successfully applied after removing these events from the records. Additional objectives, useful for the entire project group, are: (a) construction of a continuous lithological profile, e.g. to close data gaps in the core record, (b) identification of tephra layers that are crucial age control points, (c) correction of coring depths, which might be affected by pressure release, and (d) calibration of seismic profiles (transformation of travel time into depth) using seismic borehole measurements.