title: Depositional modes and post-depositional mineral formation in a Pleistocene sediment record from Lake Towuti, Indonesia
Creators: name: Morlock, Marina Alexandra
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 550 Earth sciences &amp; geology
subject: 560 Fossils &amp; prehistoric life

description: This thesis is part of an international collaboration of scientists, who drilled Indonesian Lake Towuti in 2015. In a combined scientific effort, the Towuti Drilling Project recovered around 1,000 m of sediment cores, including cores of the entire 165 m thick sediment infill at the main coring location, Site 1. The main objectives of the project were to study climate history in the heart of the Indo-Pacific Warm Pool, one of three major centres of atmospheric convection, to get insights into iron recycling and microbial processes in modern and ancient sediment, analogous to Archean ocean sediments, and to explore biologic evolution in a lake where numerous species are endemic.
The work presented here focuses on geochemical, mineralogical, and geotechnical aspects of the lake sediment record and the modern catchment, contributing to all of the above-mentioned areas of research. Adopting a source-to-sink approach, the modern lake system is initially characterised by identifying the main sedimentation sources and depositional processes in tropical Lake Towuti. Samples of bedrock, soils, and lake surface sediments reveal differential fault activity in the catchment and alternating wet and dry climate phases as the main drivers of modern and recent (last 60,000 years) sedimentation.
Building upon these results, the 100-m long continuous lacustrine sequence as well as the underlying, more heterogeneous ~65 m of sediment are studied in detail. Mineralogy and grain-size patterns record long-term basin evolution and changes in transport processes related to lake-level fluctuations. In the early extensional phase, repeated cycles of silting and peat formation, with standing water bodies, swamps and rivers in close proximity, offered diverse habitats to aquatic organisms. The formation of a permanent lake likely corresponds to the initial lake colonisation by riverine species, before subsidence and rapid lake surface expansion formed the lake we find today.
In greater detail, high-resolution X-ray computed micro-tomography (μCT) imaging visualises authigenic siderite (FeCO3) and millerite (NiS) in unprocessed sediments from Lake Towuti. This technique allows the study of post-sedimentary overprinting of the sediments and helps to identify processes related to the formation of the authigenic minerals. Particularly intriguing is the high within-sample heterogeneity of probable mechanisms of mineral formation, which suggests that geochemical and isotopic proxy interpretation of iron carbonates in ferruginous sediments could be errorneous. The analysis adds a new perspective to the highly controversial approach of reconstructing Precambrian seawater composition from iron carbonate isotopes.
Lastly, a high-resolution geochemical data set is produced from X-ray fluorescence (XRF) core scanning, to establish an objective stratigraphy of the 100-m long lacustrine record. End-member modelling as a statistical tool to unmix large data sets robustly separates sedimentation processes identified in earlier chapters of this thesis. It facilitates the generation of an objective, quantitative, and consistent characterisation of depositional processes and post-depositional alterations in the complex setting of Lake Towuti.
Overall this thesis ties the regional history of Lake Towuti to aspects of environmental, climatic, tectonic, microbial, and biodiversity research, highlighting the multifaceted outcome of international scientific collaboration in tropical Southeast Asia, an area which already played a key role in establishing the theory of evolution more than 150 years ago.