The Towuti Drilling Project:paleoenvironments, biological evolution, and geomicrobiology of a tropical Pacific lake

The Towuti Drilling Project (TDP) is an international research program, whose goal is to understand 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, ultramafic-hosted lake sediments through the scientific drilling of Lake Towuti, southern Sulawesi, Indonesia. Lake Towuti is a large tectonic lake at the downstream end of the Malili lake system, a chain of five highly biodiverse lakes that are among the oldest lakes in Southeast Asia. In 2015 we carried out a scientific drilling program on Lake Towuti using the International Continental Scientific Drilling Program (ICDP) Deep Lakes Drilling System (DLDS). We recovered a total of  ∼ 1018 m of core from 11 drilling sites with water depths ranging from 156 to 200 m. Recovery averaged 91.7 %, and the maximum drilling depth was 175 m below the lake floor, penetrating the entire sedimentary infill of the basin. Initial data from core and borehole logging indicate that these cores record the evolution of a highly dynamic tectonic and limnological system, with clear indications of orbital-scale climate variability during the mid- to late Pleistocene

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

Non-destructive particle analyses from different surface sediment samples of Lake Towuti, Indonesia

At the University of Cologne, Germany, a subsample was taken from one aliquot and used to produce smear slides for identification of sedimentary components using transmitted light microscopy. On selected samples, sponge spicules and diatom frustules were additionally investigated using a Zeiss Gemini Sigma 300VP scanning electron microscope (SEM; Carl Zeiss AG, Oberkochen, Germany). Furthermore, some magnetic mineral grains were identified with energy dispersive X-ray spectroscopy (EDX) of the Sigma SEM system. Based on smear slide analyses, a set of 40 samples that contain sponge spicules, diatoms and/or tephra particles were selected for automated, non-destructive particle image analyses using a dynamic imaging system (Benchtop B3 Series VS FlowCAM®; Fluid Imaging Technologies, Inc., Scarborough, ME, USA) to quantify the abundance of these particles. Aliquots of wet bulk samples were treated with hydrogen peroxide (H2O2; 30%) for seven days at room temperature to remove organic matter (OM) and disaggregate the siliceous biogenic particles, and were subsequently sieved with 25 and 80 µm meshes. The pre-treated sample fractions were diluted with deionized water (samples 80 µm). Particle recording in the 80 µm fraction was recorded using a 300 µm flowcell, a 4x objective lens without collimator, and a 5 ml syringe-pump (flow rate 0.6 ml/min). Data were acquired using the software VisualSpreadsheet (Fluid Imaging Technologies, Inc., Scarborough, ME, USA) until 10.000 images were recorded or 30 ml of the sample was investigated. An automated catalogue based on training sets developed for sponge spicules, diatoms and tephra particles was compiled to differentiate and group components with comparable characteristics in the measured sample fractions

Geochemical analyses from different surface sediment samples of Lake Towuti, Indonesia

For granulometric, geochemical and mineralogical analyses, approximately 25 ml of each surface sample was frozen for 24 hours and subsequently lyophilized using a Christ BETA 1-8 LDplus (Martin Christ Gefriertrocknungsanlagen GmbH, Osterode am Harz, Germany). The freeze-dried samples were homogenized and split into two aliquots. The other aliquot of the freeze-dried surface samples was ground to <63 µm with a Planetary Mill Pulverisette 5 (FRITSCH GmbH, Idar-Oberstein, Germany) and used for mineralogical and geochemical analyses. For quantitative analyses of the inorganic element composition of the surface samples, including concentrations of selected major, minor and trace elements (Ti, K, Al, Mg, Ca, Fe, Cr and Mn), 0.5 g of dry and ground bulk sample material was digested using a near-total digestion protocol with HCl, nitric (HNO3), perchloric (HClO4) and hydrofluoric (HF) acids in heated and closed teflon vessels. Measurements were performed by means of inductively coupled plasma-mass spectroscopy (ICP-MS) at Activation Laboratories Ltd., Ancaster, ON, Canada. Separate Si measurements were conducted by energy-dispersive X-ray fluorescence (ED-XRF) using a portable analyzer (NITON XL3t; Thermo Fisher Scientific, Waltham, MA, USA) at the University of Cologne, Germany. Triplicate measurements were performed on pellets of freeze-dried and ground sample aliquots, which were pressed into teflon rings under 12 bars, and subsequently covered with a 4 µm polypropylene film (X-ray film, TF-240-255, Premier Lab Supply, Port St. Lucie, FL, USA). Measurements were performed using a gold anode X-ray source (70 kV) and the 'mining-minerals-mode'. The secondary X-rays of element-specific photon energies were detected with a silicon drift detector and processed by a digital signal processor. Si concentrations (in ppm) were calculated from the element-specific fluorescence energies and compared with external and internal reference materials (STDS-4, BCR142R and BCR-CRM 277)

Grain-size distribution from different surface sediment samples of Lake Towuti, Indonesia

A digital elevation model (DEM) of Lake Towuti and its surrounding was calculated using ArcGIS (Esri, Inc., Redlands, CA, USA). The model is based on open source satellite data for Sulawesi provided by the United States Geological Survey (Aster Global DEM based on the Shuttle Radar Topography Mission carried out by the National Aeronautics and Space Administration at 1 arc-second 30 m spatial resolution). Spatial interpolation of the analytical surface sediment data was carried out with the software Surfer 9 (Golden Software Inc., Golden, CO, USA) using the kriging method. Statistical analyses employed on the surface sediment data sets comprise end-member (EM) unmixing, principal component analysis (PCA) and a redundancy analysis (RDA). EM analyses were carried out on normalized and standardized grain-size (EMGS), chemical (EMChem) and mineralogical (EMMin) data sets. Assuming a sedimentary mixture from different sources the mixing model in all cases can be written as: X = AS + E (1) where X represents the n-by-m matrix of n samples (one per row) and m variables (relative abundance of individual data). Matrix A (n-by-l) denotes the mixing proportion of l end-members for the n samples, S represents the m properties of the l EMs and E is the error matrix of residuals. The uncertainties of the EM analyses are controlled by the errors of the data sets used. The EM algorithm developed by Heslop and Dillon (2007) adopting the approach of Weltje (1997) was applied. The decision criterion of how many EMs are included in the three models is based partly on the coefficients of determination derived from the PCA. Nevertheless, the number of the respective EMs should also be reasonable in the geological context of the data set (Weltje, 1997; Weltje and Prins, 2007). Residuals of the EM models include analytical errors and non-identified additional sources of variability. All other multivariate statistical analyses were carried out with the Excel-based software Addinsoft XLSTAT (STATCON GmbH, Witzenhausen, Germany) The PCA was conducted with the sand content and the concentrations of selected elements determined by ICP-MS and XRF analyses (Fe, Mg, Al, Si, K, Ca, Cr and Ni). In the RDA, the results derived from the PCA are expanded by the concentrations of major minerals, the MS and TOC values and the C/N ratio. The correlation matrix includes all data except 13COM and the concentrations of diatom frustules, sponge spicules and tephra particles, which all were determined on a subset of the surface samples only

Geochemical analyses (CNS and TOC) from different surface sediment samples of Lake Towuti, Indonesia

For granulometric, geochemical and mineralogical analyses, approximately 25 ml of each surface sample was frozen for 24 hours and subsequently lyophilized using a Christ BETA 1-8 LDplus (Martin Christ Gefriertrocknungsanlagen GmbH, Osterode am Harz, Germany). The freeze-dried samples were homogenized and split into two aliquots. The other aliquot of the freeze-dried surface samples was ground to <63 µm with a Planetary Mill Pulverisette 5 (FRITSCH GmbH, Idar-Oberstein, Germany) and used for mineralogical and geochemical analyses. Total organic carbon (TOC) as well as total carbon (TC), total nitrogen (TN) and total sulfur (TS) were measured with a vario MICRO cube and vario EL cube combustion elemental analyzers (Elementar Analysesysteme Corp., Langensebold, Germany), respectively. For the TOC measurements, 15 mg of sediment powder was placed into metallic silver containers, heated to 100 to 120°C, and treated three times with a few drops of HCl (32 %) to dissolve carbonates. The metallic silver containers were then wrapped and pressed into silver paper, and the resulting pellets were analyzed for their TOC concentration using the vario EL cube. All concentrations are given as mean values of duplicate measurements. For TC, TN and TS measurements with the vario MICRO cube, 10 mg of sediment powder was placed in zinc containers, with 20 mg of tungsten (VI) oxide (WO2) added to catalyze oxidation. The total inorganic carbon (TIC) was calculated as the difference between TC and TOC. Analytical errors were determined on internal and external reference material. The C/N ratio is calculated as the weight ratio of TOC and TN. The carbon isotopic composition of bulk OM (δ13COM) in the sediment was measured on a set of 42 subsamples at Brown University, Providence, RI, USA. For that purpose, ca 50 mg of sediment was acidified in HCl (2 N) for one hour at 80ºC to remove carbonate minerals. The acid-treated samples were subsequently rinsed in deionized water and centrifuged four times to remove any excess HCl. The samples were then freeze-dried and homogenized prior to isotopic analysis. The δ13COM values were measured using a Carlo Erba Elemental Analyzer coupled to a Thermo DeltaV Plus isotope ratio mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The analytical precision determined through replicate measurements of internal sediment standards was 0.16 ‰. All results are reported relative to the Vienna PeeDee Belemnite (VPDB) standard

Mineralogical composition from different surface sediment samples of Lake Towuti, Indonesia

For granulometric, geochemical and mineralogical analyses, approximately 25 ml of each surface sample was frozen for 24 hours and subsequently lyophilized using a Christ BETA 1-8 LDplus (Martin Christ Gefriertrocknungsanlagen GmbH, Osterode am Harz, Germany). The freeze-dried samples were homogenized and split into two aliquots. The other aliquot of the freeze-dried surface samples was ground to <63 µm with a Planetary Mill Pulverisette 5 (FRITSCH GmbH, Idar-Oberstein, Germany) and used for mineralogical and geochemical analyses. The bulk mineralogy was determined on powder samples using an X-ray diffractometer (D8 Discover; Bruker, Billerica, MA, USA) with a Cu X-ray tube (λ = 1.5418 Å, 40 kV, 30 mA) and a LYNXE XE detector (opening angle = 2.9464°). The spectrum from 3° to 100° 2-theta was measured in 0.02° steps at 1 second exposure time. Mineral identification was carried out using the software packages SEARCH (Stoe and Cie GmbH, Darmstadt, Germany) and Match! (Crystal Impact 2014, Bonn, Germany), supported by the data base pdf2 (ICDD 2003, Newton Square, PA, USA). The concentration of the minerals was evaluated using the program TOPAS Rietveld (Coelho Software, Brisbane, Australia), which yields a standard deviation of analyzed minerals varying from +/- 2 % (for quartz) to +/- 5 to 10 % (for feldspars and clay minerals; Środoń et al., 2001; Vogt et al., 2002). For the clay mineral group illite the error range can be even higher (Scott 1983). Given these uncertainties, a detection limit of 5 % is considered in the discussion of the mineralogical composition of the surface sediments

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