SOURCES OF CARBONATE PRECIPITATION IN MONO LAKE SEDIMENTS: δ18O AND CARBONATE CLUMPED ISOTOPES AS LAKE LEVEL PROXIES

From 1941-1994, LA County diverted water from the Mono Lake basin, drastically decreasing water level. The coupling of high sedimentation rate (0.7 cm/yr) and rapid human-induced environmental change results in a unique opportunity to investigate the isotopic fingerprint of lake level change at a much finer scale than typically preserved in the geologic record. Further, Mono Lake water level change is well-documented during this interval, providing relatively well-constrained time stamps on lake level proxy data. In this work, we reconstruct δ¹⁸O_water of Mono Lake for the past ~116 years using δ¹⁸O_carb and clumped isotope thermometry (T(∆₄₇)) within a 70 cm sediment core. Today, the δ¹⁸O_w value of Mono Lake is ~0‰VSMOW, reflecting the hydrological balance of input (Sierran runoff and hydrothermal springs: ~-15‰VSMOW) and output (evaporation increases δ¹⁸O_w). However, as expected in a high-pH lake, we see isotopic contributions from both lakewater and authigenic carbonate precipitation. Carbonate from the upper 10cm of the sediment core yield a T(∆₄₇) of 26°C, reflecting surface water carbonate precipitation during late summer. Carbonates from deeper in the core yield a consistent T(∆₄₇) of 9.6°C, suggesting dissolution-precipitation of “warm” carbonate deposited from the water column with “cold” water of a different δ¹⁸O_w than Mono Lake water. We employ a clumped isotope mixing model (Defliese & Lohmann, 2015) to calculate the relative contributions of the two reservoirs. Preliminary modeling suggests that ~53% of the carbonate found in the lower 60 cm of the sediment core is authigenic. Mirrored shifts in δ¹³C_org and δ¹³C_carb down-core corroborate this isotopic mass balance. Although the δ¹⁸O_w values calculated using δ¹⁸O_carb and T(∆₄₇) measurements do not directly reflect lake level change, we are able to “unmix” the carbonate reservoirs contributing to the isotopic composition of Mono Lake lacustrine sediments through modeling efforts and additional isotopic constraints (i.e. δ¹⁸O of biotic material or δD of organics), and restore the primary δ¹⁸O_w as a proxy for lake level change. Our ability to determine the origin of carbonate precipitate is critical to our ability to make lake level and other paleo-environmental assessments using lacustrine carbonate in the modern and ancient.