Phytoplankton community, seasonality, and water chemistry modulate the lacustrine diurnal carbon engine and carbonate δ¹³C values

Abstract. Stable isotopic measurements of carbon in carbonate rocks (δ¹³C_carb) have long been used to investigate paleoclimate and global carbon cycle dynamics. Recent work documented the diurnal carbon engine effect, which describes the impact of daily photosynthetic cyclicity on dissolved inorganic carbon (DIC) and, in turn, δ¹³C_carb and δ¹³C_DIC values, in low-latitude shallow marine environments. Because carbonate precipitation is temporally structured over the diurnal cycle, the resulting δ¹³C_carb reflects a precipitation-weighted bias rather than a simple daily average. However, this system has not yet been explored in lakes. Here we present an adapted diurnal carbon engine model and constrain this effect for three end-member lake systems: Great Salt Lake (UT, USA), Green Lake (NY, USA), and Morrison Lake (MT, USA). We document how differing lake water chemistry and phytoplankton communities modulate local diurnal carbon engines. Because of geochemical and ecological differences, the impact of the diurnal carbon engine on δ¹³C_carb values varies greatly by lake system. We estimated Δ¹³C offsets, which represent the daily variability expected resulting from the diurnal engine and precipitation weighting, of ~0.36 ‰ in Great Salt Lake and ~0.24 ‰ in Green Lake. We also modeled how seasonality impacts the diurnal carbon engine in Great Salt Lake, which resulted in ~0.10 ‰ of δ¹³C_carb variability. Modeled diurnal variability is consistent with observed dynamics in Green Lake. Predicted δ¹³C_carb values are consistent with those measured from most carbonate sedimentary facies in each lake. Lacustrine δ¹³C_carb shifts may reflect local environmental and ecological conditions, rather than changes in long-term lake δ¹³C_DIC values or the global carbon cycle, complicating lacustrine δ¹³C_carb record interpretations.