USING REACTIVE IRON DISTRIBUTION TO CHARACTERIZE REDOX CONDITIONS IN THE LATE-HOLOCENE RECORD OF MARTIN LAKE, USA

Iron geochemistry is used as a key approach to assess local oxygen conditions in ancient marine environments. The Fe-based redox proxies are based on the observation that iron- mineral phases (oxides, carbonates, and magnetite) that are highly reactive (Fe_HR) towards hydrogen sulfide are enriched relative to the total iron (Fe_T) pool in sediments that are deposited within anoxic waters (Raiswell et al., 2018). To our knowledge these Fe-based proxies for determining dissolved oxygen conditions have not been applied to lacustrine water columns. Like marine basins, lakes experience fluctuations in oxygen availability, which is why we propose the application of Fe-based proxies in lacustrine environments, such as our study site, Martin Lake.

Martin Lake is a kettle lake in LaGrange County, Indiana. Despite its hydraulically open conditions, Martin Lake remains seasonally stratified through the warm-season from April through November, which causes bottom water anoxia from March through December. A regional 2100 year-long precipitation reconstruction following the Pacific North American pattern has already been created by Bird et al (2017), from a multi-proxy study of Martin Lake. New research focusing on geochemical proxies, will help bridge the gap between the climate and its effect on the geochemical processes within the lake.

I hypothesize Martin Lake became oxic during time periods of low water levels and anoxic when water levels were higher. Times of increased precipitation will show up in the record as an enrichment in the ¹³C and an increase in anoxia, due to stable bottom water and prolonged seasonal thermal stratification. During extreme drought periods, the shallow water will allow for complete mixing and should remain oxic. This hypothesis presents the idea that under the two extremes in lake level there will be notable differences in important geochemical markers that can reveal details of past redox conditions and oxygen levels. Between extremes there were likely prolonged intermediate periods, but it is known from a past study (Bird et al., 2017), that Martin Lake did experience these two extremes during the transition between the Medieval Climate Anomaly and the Little Ice-Age during the Holocene.