BIOGEOCHEMICAL AND SEDIMENTOLOGICAL RESPONSES TO CLIMATE CHANGE IN THE LARGE, FERRUGINOUS LAKE TOWUTI, INDONESIA

Lake Towuti is the largest and one of the oldest tectonic lakes in Indonesia. The lake’s location provides an important opportunity to reconstruct long-term changes in terrestrial climate in the Western Pacific warm pool. Moreover, Lake Towuti is housed in the ultramafic East Sulawesi Ophiolite which provides extraordinarily high concentrations of iron and other metals to the lake and its sediments, resulting in the deposition of a variety of iron minerals. In 2015 the Towuti Drilling Project recovered over 1,000 meters of new sediment core, including cores through the lake’s entire sediment column to bedrock to investigate paleoenvironmental changes system and their influence on this unique lake system.

The basal units the composite sedimentary sequence from Towuti consist of lacustrine clays and silts, fluvial sands and gravels, and peats, indicating alternating shallow lakes, rivers, and swamps that formed during the initial stages of basin formation. The upper 100 m consists of fine-grained clay, indicating continuous lacustrine environments, with an age for this transition of ~600 kyr BP. Within this unit, we observe alternations between reddish-grey and greenish-grey clays, which reflect orbital-scale changes in climate, with the reddish clays deposited under drier conditions that occur during high-latitude glacial maxima. Despite their oxidized appearance, the reddish clays are characterized by high concentrations of reduced iron minerals (magnetite and siderite). Geochemical, mineralogical, and isotopic analyses suggest that the siderite and magnetite, and the alternation between red and green clays, form in response to changes in lake mixing. Today, Lake Towuti is thermally stratified with an anoxic and ferruginous bottom. Lake mixing during drier and colder periods promotes the deposition of reactive iron oxides that are subsequently transformed into siderite and magnetite in the sediment. Biological indicators suggest reduced primary productivity during intervals of lake mixing, linking changes in iron mineralogy with the lake’s carbon cycle. These data provide insight into sedimentation and the mechanisms behind iron-rich sediments in the Earth’s early ferruginous oceans, and potentially Martian lakes.