Clumped isotope paleothermometry of marine calcite cements from the Late Ordovician Kullsberg and Boda Limestones, Sweden

18O/16O ratios (δ18O) in well-preserved marine carbonates provide a critical archive of information about ancient climates and seawater chemistry, but this proxy suffers from a fundamental limitation. Because δ18O reflects a temperature-dependent equilibrium fractionation between the measured mineral and the water with which it equilibrated, neither the temperature nor the isotopic composition of this water (δ18Owater) can be determined directly from δ18O of carbonate without independent knowledge of one of the controlling variables. Lack of such knowledge has been the major factor preventing resolution of a long-running debate over the interpretation of secular trends in δ18O. Clumped isotope paleothermometry is a recently developed thermodynamic proxy that permits independent determination of carbonate crystallization temperature and can place unique constraints on δ18Owater. Most clumped isotope analyses have focused on calcitic fossils, but marine calcite cements are common in carbonate-rich successions and provide a complementary archive that may shed light on both primary temperature trends and their diagenetic alteration. We examined the clumped isotope systematics of samples from the Late Ordovician Kullsberg and Boda Limestones of central Sweden. These units have never been substantially buried but may have affected by the nearby Siljan impact of Late Devonian age. Both were deposited at tropical to low subtropical latitudes and contain large micritic buildups rich in "stromatactis" voids containing multiple cement generations. Previous work (Tobin and Walker, 1996) characterized the Kullsberg cements and posited that translucent fibrous calcites record unaltered to slightly altered marine signatures and turbid fibrous calcites record variable alteration by restricted marine fluids, with subsequent generations of equant calcite recording meteoric and/or burial fluids. Translucent and turbid fibrous calcite in the Kullsberg exhibit narrow δ18O ranges (-2.2 to -4.5‰ VPDB) but significant variations in temperature (31° to 51°C), implying partially closed system diagenetic behavior. Many samples capture physical mixtures of relatively primary phases with 18O-depleted, high temperature meteoric or burial phases (mean δ18O = -8.1 ± 0.7‰ VPDB, mean temp. = 58 ± 3° C). Boda cements record generally higher temperatures, consistent with other evidence suggesting that the Boda has experienced more diagenetic alteration. Consequently, we interpret the lowest-temperature Kullsberg samples as most likely to record a primary marine signature. Samples yielding temperatures <35°C exhibit δ18Owater values from 0.94 to -0.14‰ VSMOW. Minimum δ18Owater values inferred from these sampled are indistinguishable from those inferred from well preserved calcitic fossils from the contemporaneous Decorah Formation of the U.S. midcontinent that record lighter δ18O values and higher temperatures. This suggests that both least-altered Kullsberg cements and Decorah Formation fossils were sampling isotopically similar seawater in the 0 to -1‰ VSMOW range but precipitated at different temperatures, an observation consistent with their relative latitudinal positions.