Paleolake Deposits on Mars: Perspectives on Source-to-sink Mineralogy from Lake Towuti, Indonesia

Introduction: Ancient paleolake basins are amongst the best evidence for surface fluvial activity early in martian history [e.g., 1-3]. Many of these paleolakes contain sedimentary deposits formed when there was standing water within the basin [e.g., 1,2,4]. Study of lacustrine sedimentary deposits on Earth can provide fundamental constraints on martian paleolake deposits, which are largely restricted to remote sensing analyses. To provide perspective on how remote sensing data can be used to study lacustrine sediment, we have studied the source-to-sink mineralogy of Lake Towuti, a modern terrestrial lake, using visible to near-infrared (VNIR) reflectance spectroscopy as the primary dataset. Our results may also aid in the interpretation of combined in situ and remote sensing data for Gale crater, a closed-basin paleolake [2,5] and the site of exploration for the Mars Science Laboratory (MSL). Lake Towuti is a large (area = ~560 km 2 ), hydrologically open lake on the island of Sulawesi, Indonesia [6,7]. The ~1280 km 2 catchment area is largely composed of lateritic soils derived from, and overlain on, the ultramafic East Sulawesi Ophiolite [8,9], compositionally comparable to the basaltic martian crust [10,11]. We analyzed the bedload and suspended load (source sediment) from the Mahalona River, the primary input to Lake Towuti, and a piston core (sink sediment) from the distal margins of a delta deposit at the mouth of this river. Methods: Input Sediment. Bedload samples were freeze-dried and dry sieved to >1 mm, 0.25-1 mm, 63-250 μm, 32-63 μm and <32 μm size fractions. Suspended load samples were freeze-dried, wet sieved to the same size fractions, and freeze-dried again. The reflectance spectra of all 5 size fractions were measured with an Analytical Spectral Devices (ASD) FieldSpec 3 over the wavelength range 350-2500 nm. Major element chemistry of the samples was analyzed using an inductively coupled plasma atomic emission spectrometer (ICP-AES) with flux fusion sample preparation [12]. X-ray diffraction (XRD) data were collected using a Bruker D2 PHASER instrument to validate the mineralogy inferred from VNIR spectroscopy Core. The analyzed core is ~10.5 m in length, and has a basal age of ~20 ka [7]. A small, full-length subsection (Uchannel) of the core was collected and freeze-dried. Reflectance spectra of this core were measured at 1 cm depth intervals using an ASD FieldSpec 3 attached to a Geotek multisensor core logger. Eleven sub-samples were collected from the core, freeze-dried, and analyzed with the ASD FieldSpec 3, ICP-AES and XRD instruments. Results: Input Sediment. The input sediment samples have a series of distinct spectral absorptions, primarily indicative of phyllosilicate mineralogy (Fig. 1). The spectral characteristics are dominated by absorption features of Mg-rich serpentine, including vibrational absorptions centered near ~1390, 2100 and 2320 nm (Fig. 1) [13,14]. These absorptions are due to the first overtone of the OH stretch (~1390 nm absorption) and a combination tone of the Mg-OH bend and OH stretch (~2320 nm absorption) [13,14]. The absorption centered near ~2100 nm is characteristic of serpentine; however, its cause is not fully understood, although it is likely to be related to Mg-OH vibrational modes [14]. The input sediment spectra also have a strong absorption feature centered near ~1900 nm, caused by a combination tone of OH stretch and H-O-H bend from structural H2O [14]. The dominance of serpentine in the input sediment is also confirmed with XRD data.