HOW DOES DIURNAL CARBON CYCLING IN LAKES INFLUENCE LACUSTRINE CARBONATE CARBON ISOTOPE RECORDS?

Sedimentary rocks formed in lacustrine environments preserve valuable records of continental climate dynamics across Earth history. One of the most important types of these records comprises measurements of stable carbon isotope ratios of calcium carbonate minerals (δ¹³C_carb), which have been interpreted as records of rapid climate change events and other environmental changes. However, such regional- and global-scale environmental changes are not the only processes that can drive variations in δ¹³C_carb records. We are investigating the consequences of one of these other types of processes—the diurnal engine effect. The diurnal engine describes the daily transition from local carbon cycling dominated by aquatic photosynthetic organisms during the day to carbon cycling dominated by heterotrophy at night. The net effect of this process is that more carbonate precipitates when δ¹³C_DIC is elevated by photosynthesis during the daytime, meaning that bulk δ¹³C_carb values of precipitated minerals will tend to more closely match daytime δ¹³C_DIC values rather than daily averages. Although phototrophs and heterotrophs are abundant in most lakes and shallow marine environments, the magnitude of net photosynthetic forcing can vary significantly between and within ecosystems. This forcing affects the magnitude of change in δ¹³C_DIC, so when this forcing is small, δ¹³C_carb values closely match the values predicted for carbonate minerals precipitating in equilibrium with average δ¹³C_DIC, a key observation that forms the basis of using δ¹³C_carb values as proxies for environmental change. In contrast, when the magnitude of photosynthetic forcing is large, the diurnal engine can lead to large differences (up to ~7‰) between actual and predicted δ¹³C_carb values, significantly undermining the interpretative power of this important geochemical proxy. We will present field data and modeling results from Great Salt Lake (UT, USA) that will constrain the magnitude of photosynthetic forcing in this environment, with implications for how we interpret lacustrine δ¹³C_carb records.