CLUMPED AND OXYGEN ISOTOPE SYSTEMATICS OF MARINE CALCITE CEMENTS FROM THE LATE ORDOVICIAN KULLSBERG AND BODA LIMESTONES, SWEDEN

Deep-time paleoclimate reconstructions are inhibited by inability to discriminate between temperature and reservoir effects on ¹⁸O/¹⁶O ratios (δ¹⁸O) of marine carbonate minerals. Clumped isotope paleothermometry is a recently developed thermodynamic proxy that allows direct determination of carbonate crystallization temperature and constrains the oxygen isotopic composition of the water with which the mineral equilibrated (δ¹⁸O_water). It thus has the potential to help resolve long-running arguments about the Phanerozoic history of δ¹⁸O_water. Most clumped isotope analyses of ancient sedimentary rocks have focused on calcitic fossils, but marine calcite cements are common in carbonate successions and may provide a complementary archive. To investigate the potential of this archive we examined the clumped isotope and δ¹⁸O systematics of the Late Ordovician Kullsberg and Boda Limestones of central Sweden. These units were deposited at tropical to low subtropical latitudes and contain large micritic buildups with “stromatactis” voids containing multiple cement generations. Previous work (Tobin and Walker, 1996) has posited a precipitation sequence and alteration trajectory for these cements. Putatively primary to moderately altered fibrous calcites in the Kullsberg exhibit a narrow δ¹⁸O range (-2.2 to -4.5‰ VPDB) relative to their temperature range (31° to 51°C), implying partially closed system diagenetic behavior. Many samples capture 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 for greater diagenetic alteration. We therefore interpret the lowest-temperature Kullsberg samples as most likely to record a primary marine signature. These exhibit δ¹⁸Owater values similar to those inferred from 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 sampled isotopically similar seawater in the 0 to -1‰ VSMOW range but precipitated at different temperatures, an observation consistent with their relative latitudinal positions.