Climatic and tectonic controls on source-to-sink processes in the tropical, ultramafic catchment of Lake Towuti, Indonesia

Humid tropical landscapes are subject to intense weathering and erosion, which strongly influence sediment mobilisation and deposition. In this setting, we aimed to understand how geomorphology and hydroclimate altered the style and intensity of erosion and sediment composition in a tropical lake and its tectonically active catchment. Lake Towuti (2.75 degrees S, 121.5 degrees E) is one of the oldest and deepest lakes in Indonesia, with uninterrupted lacustrine sedimentation over several glacial-interglacial cycles. Here we present results from a novel set of Lake Towuti surface sediment, bedrock and soil samples from the catchment, and two existing sediment cores that extend to 30,000 and 60,000years before present. We studied the catchment morphology, soil properties, geochemistry, and clay and bulk mineralogy. Results from several river long profiles show clear signs of tectonic activity, which enhances river incision, favours mass movement processes, and together with remobilisation of fluvial deposits, strongly influences modern sedimentation in the lake. Material from the Mahalona River, the lake's largest inflow, dominates modern sediment composition in Towuti's northern basin. The river transports Al-poor and Mg-rich sediments (mainly serpentines) to the lake, indicating river incision into the Mg-rich serpentinised peridotite bedrock. Relatively small, but important additional contributions of material, come from direct laterite-derived input and the Loeha River, which both provide Al-rich and Mg-poor sediment to the lake. Over time, the Al/Mg and kaolinite-to-serpentine ratios varied strongly, primarily in response to lake-level fluctuations driven by hydroclimatic changes. In the past 60,000years, both the Al/Mg and kaolinite-to-serpentine ratios showed variations sensitive to changes in climate boundary conditions across glacial-interglacial cycles, while tectonic activity had less influence on changes in sediment composition on these short time-scales

Iron Mineralogy and Sediment Color in a 100 m Drill Core from Lake Towuti, Indonesia Reflect Catchment and Diagenetic Conditions

Iron is the most abundant redox-sensitive element on the Earth’s surface, and the oxidation state, mineral host, and crystallinity of Fe-rich phases in sedimentary systems can record details of water-rock interactions and environmental conditions. However, we lack a complete understanding of how these Fe-rich materials are created, maintained, and oxidized or reduced in sedimentary environments, particularly those with mafic sources. The catchment of Lake Towuti, Indonesia, is known to contain a wide range of abundant crystalline Fe oxide, and the lake has a long sedimentary history. Here we study a ∼100 m long drill core from the lake to understand patterns of sedimentation and how young iron-rich sediments are affected by diagenesis through geologic time. We use visible/near infrared and Mössbauer spectroscopy, X-ray diffraction, bulk chemistry measurements, and statistical cluster analysis to characterize the core sediment. We find that the core sediment can be divided into three statistically different zones dominated by Mg serpentine, Al clay minerals, and Fe2+ carbonate, respectively. The entire core is rich in nanophase Fe, and elemental correlations and Fe mineralogy vary between these zones. The nanophase Fe is highly complex with both ferrous and ferric components, and contributes to, but does not dictate, variations in sediment color. We propose that the distinctive zones are the result of structural basin changes (notably river capture and shifting drainage patterns), and diagenetic overprinting caused by deep burial of reactive Fe. This complex record has implications for disentangling depositional and diagenetic trends in other mafic lacustrine systems

Modern sedimentation processes in Lake Towuti, Indonesia, revealed by the composition of surface sediments

Lake Towuti on Sulawesi Island, Indonesia, is located within the heart of the Indo-Pacific Warm Pool. This tropical lake is surrounded by ultramafic (ophiolitic) rocks and lateritic soils that create a unique ferruginous depositional setting. In order to understand modern sediment deposition in Lake Towuti, a set of 84 lake surface sediment samples was collected from across the entirety of the lake and samples were analyzed for their physical, chemical, mineralogical and biological constituents. End-member analyses were carried out to elucidate modern sediment origin, transport and depositional processes. This study found that allochthonous sediment, characterized by the concentrations of the elements Mg, Fe, Si and Al, as well as the clay and serpentine minerals, is dominated by fluvial supply from five distinct source areas. Granulometric data and the occurrence of organic matter of a terrestrial origin suggest that, in the southern and north-eastern parts of the lake the near-shore sediments may additionally be influenced by mass wasting. This is due at least partly to the particularly steep slopes in these areas. Furthermore, sediment composition suggests that sediment transport into deeper parts of the lake, particularly in the northern basin, is partly controlled by gravitational and density-driven processes such as turbidity currents. Directional sediment transport by persistent lake currents, in contrast, appears to be less important. Organic matter deposition in the ultra-oligotrophic lake, albeit limited, is dominated by autochthonous production, but with some contribution of fluvial and gravitational supply. Biogenic silica deposition, primarily from diatom frustules and sponge spicules, is very limited and is concentrated in only a few areas close to the shoreline that are characterized by shallow waters, but away from the areas of high suspension loads at the mouths of the major inlets. The results of this study build upon current and published work on short piston cores from Lake Towuti. Conversely, the results will support the interpretation of the depositional history and past climatic and environmental conditions derived from the composition of much longer records, which were obtained by the Towuti Drilling Project in May 2015 and are currently under investigation

Characterization of Iron in Lake Towuti sediment

Sediments collected from Lake Towuti, an ultramafic-hosted lake in Indonesia, preserve a visible alternating pattern of red and green sediments due to variations in clay mineral and Fe-oxide composition and abundance consistent with changes in iron oxidation state through time. Spectral, mineralogical, and chemical analyses on soils, river, and sediment samples from across the lake and its catchment were carried out to better understand the starting composition of these sediments and the processes that affected them before and after deposition. Despite high Fe abundances in all samples and abundant Fe oxides in lateritic source regions, mineralogical analyses (X-ray diffraction (XRD) and Mossbauer spectroscopy) of the modern lake sediment show almost no well-crystalline iron oxides. In addition, sequential Fe extractions suggest an increasing proportion of easily extractable, poorly crystalline (X-ray amorphous) material with burial depth. XRD, bulk chemistry, and visible-near infrared (VNIR) spectral reflectance measurements demonstrate that clay mineralogy and bulk chemistry can be inferred from VNIR data. These results provide evidence for variations in Fe mineralogy and crystallinity based on location in this source to sink system. Understanding how the mineralogy and chemistry of sediments within a ferruginous lake basin are affected by transport, chemical alteration, physical alteration, and deposition from source to sink on Earth, and the degree to which these trends and underlying processes can be inferred from chemical and spectral properties, may provide useful direction in assessing paleoenvironmental conditions in other terrestrial lakes as well as ancient lacustrine environments preserved in the stratigraphic record of Mars

The late quaternary tectonic, biogeochemical, and environmental evolution of ferruginous Lake Towuti, Indonesia

There is a paucity of long and continuous continental records from South East Asia suitable to inform on past changes and underlying causes of the region's climate and associated diverse ecosystem evolution during the late Quaternary. In 2015, the Towuti Drilling Project (TDP) collected a series of sedimentary drill cores from the tectonic, ferruginous, and highly biodiverse Lake Towuti, Sulawesi, one of Indonesia's oldest lakes. The drill cores contain similar to 1 Myr of uninterrupted lacustrine sedimentation to document long-term environmental and climatic change in the tropical western Pacific, the impacts of geological and environmental changes on the biological evolution of aquatic taxa, and the geomicrobiology and biogeochemistry of metal-rich, ultramafichosted lake sediment. Here we use lithostratigraphic, mineralogical, geochemical, and geochronological datasets to elucidate Lake Towuti's tectonic emergence and its biogeochemical responses to climatic and volcanic forcings since lake formation. Our data document that Lake Towuti emerged during a phase of accelerated tectonic subsidence from a landscape characterized by active river channels, shallow lakes and swamps into a permanent lake at -1 Ma. The lacustrine sediments feature quasi-rhythmic alternations of green organic rich and red sideritic clay beds reflecting changes in lake mixing and biogeochemistry as a response to temperature and hydrological changes driven by orbital-scale changes in insolation and continental ice volume through the midto late Pleistocene. Clay deposition is interrupted by two beds of diatomaceous oozes composed primarily of planktonic diatoms that reflect phases of substantially increased primary productivity. The occurrence of these diatomaceous oozes in close association with multiple tephra beds suggests atrophic state change driven by the addition of volcanically sourced P, possibly in combination with a lake mixing state that supports recycling of P. Data on lake age and ontogeny are also in agreement with molecular-clock estimates of similar to 0.7 Ma (0.18-1.37 Ma) for the divergence of Lake Towuti's Telmatherinid fishes from a riverine ancestor. Our data therefore are compatible with an evolutionary model in which Lake Towuti's endemic fauna is a result of geographic speciation in the Malili Lakes, a set of large lakes in Southeast Sulawesi, driven by physical or chemical dispersal limits imposed by the regional rivers and lakes. More detailed chronological constraints and refined climate and environmental proxy datasets are currently in preparation and will help to paint a more detailed history of the region's climate and environmental history in future studies