SOURCE TO SINK MINERALOGY IN LAKE TOWUTI, INDONESIA FROM REFLECTANCE SPECTROSCOPY: INSIGHTS INTO PALEOLAKE DEPOSITS ON MARS

We studied the mineralogy and chemistry of sediment from the hydrologically open Lake Towuti, Indonesia to gain insights into source to sink mineralogy and potential in situ aqueous alteration, with application to martian paleolakes. The Lake Towuti watershed largely consists of lateritic soils derived from and overlain on the ultramafic East Sulawesi Ophiolite, and so offers a unique study area for comparison with the mineralogically and chemically similar basaltic martian crust.

We analyzed bedload and suspended load sediment from the Mahalona river, the primary input to Lake Towuti. The samples were separated into five grain size fractions and analyzed with visible to near-infrared (VNIR) reflectance spectroscopy. We also analyzed the VNIR spectral signatures of two piston cores from the lake located at the distal margins of a delta deposit of the Mahalona river, at 1 cm spacing intervals.

Our results indicate the input sediment is spectrally dominated by Mg-rich serpentine, with minor spectral contribution from an Al-rich phyllosilicate (e.g., kaolinite). The lake core spectral data show that modern sediment has a strong Al-rich phyllosilicate spectral signature, which transitions to Mg-rich serpentine-bearing sediment at approximately the Last Glacial Maximum (LGM; ~20 ka). In modern inputs, Al-rich phyllosilicates are most abundant in fine grained sediment, and Mg-rich serpentine sediment is coarser-grained. Therefore we interpret the observed trend to represent changes in mineralogy of the input sediment controlled by changes in sediment grain size over time, consistent with previous work that suggests a lower lake level and basinward migration of the Mahalona river mouth at the LGM.

Our results provide little evidence for authigenic alteration phases associated with transport and deposition of sediment in this ultramafic, lacustrine environment. This result is similar to observations of martian paleolake deposits from remote analyses of mineralogy, where the period of lacustrine activity is not genetically related to the production of alteration minerals. Furthermore, our results emphasize the importance of physical sorting from fluvial transport on the mineralogy of lacustrine sediment, which must also be considered in interpreting compositional variations in paleolake deposits on Mars.