Date of Award

Spring 5-22-2021

Degree Type


Degree Name

Master of Science (MS)


Earth Sciences


Greg D. Hoke


calcite, carbonate, clumped, fractionation, isotope, lacustrine

Subject Categories

Biogeochemistry | Chemistry | Earth Sciences | Geochemistry | Physical Sciences and Mathematics


Lacustrine carbonates are important paleoclimate archives, but unknowns in the seasonal timing and depth of precipitation obscure the interpretation of temperatures associated with them. The clumped isotope paleothermometer records the formation temperature T(D47) of carbonates. This study examines a set of lacustrine carbonates formed in the water column, in addition to carbonate from a sediment core, to better understand the depth and seasonality of carbonate formation in a lacustrine setting. We test that, regardless of season and formation temperature, lacustrine carbonates form in isotopic equilibrium with respect to D47 and d18O fractionation. Further, we account for the effect of our sampling regime, glass-fiber filters mixed with water column calcite, on D47, d18O and d13C. Carbonate and water was sampled from Fayetteville Green Lake at depths of 0.5, 2.5, 5, 7.5, 10, and 15 meters below the lake surface. Short core segments, from no deeper than 12 cm depth, were analyzed to compare against water column calcites. Water column carbonate was present throughout the entire season with maximum concentrations of 1.4 mg/l sampled on July 11, 2019 at 7.5 meters depth. Average water column carbonate d18O was -9.64 ∓ 0.32‰ VPDB while the average d18O from surface water was -9.10 ∓ 0.20‰ VSMOW. T(D47) from core carbonate was calculated to be 24 ∓ 7℃ and d18O from core carbonate was -9.80 ∓ 0.22‰ VPDB. Our results suggest that even in lakes prone to whiting events and large temperature and calcite saturation fluctuations, carbonates precipitate in isotopic equilibrium with respect to d18O and D47. T(D47) recorded in the sediments of Fayetteville Green Lake is indistinguishable from modern summer mean surface water temperatures recorded in June, July, and August. This largely supports the work of Hren and Sheldon in showing that T(D47) is a warm season proxy and reflects the season over which the bulk of the carbonate preserved in sediments is precipitated. Carbonate clumped isotope results suggest lacustrine carbonate forms in the upper 2.5 meters of the water column.


Open Access



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