Clumped-isotope" thermometry is an emerging tool to probe the temperature history of surface and subsurface environments based on measurements of the proportion of 13 C and 18 O isotopes bound to each other within carbonate minerals in 13 C 18 O 16 O 2 2À groups (heavy isotope "clumps"). Although most clumped isotope geothermometry implicitly presumes carbonate crystals have attained lattice equilibrium (i.e., thermodynamic equilibrium for a mineral, which is independent of solution chemistry), several factors other than temperature, including dissolved inorganic carbon (DIC) speciation may influence mineral isotopic signatures. Therefore we used a combination of approaches to understand the potential influence of different variables on the clumped isotope (and oxygen isotope) composition of minerals.We conducted witherite precipitation experiments at a single temperature and at varied pH to empirically determine 13 C-18 O bond ordering (D 47 ) and d 18 O of CO 3 2À and HCO 3 À molecules at a 25°C equilibrium. Ab initio cluster models based on density functional theory were used to predict equilibrium 13 C- 18 O bond abundances and d 18 O of different DIC species and minerals as a function of temperature. Experiments and theory indicate D 47 and d 18 O compositions of CO 3 2À and HCO 3 À ions are significantly different from each other. Experiments constrain the D 47 -d 18 O slope for a pH effect (0.011 ± 0.001; 12 P pH P 7). Rapidly-growing temperate corals exhibit disequilibrium mineral isotopic signatures with a D 47 -d 18 O slope of 0.011 ± 0.003, consistent with a pH effect. Our theoretical calculations for carbonate minerals indicate equilibrium lattice calcite values for D 47 and d 18 O are intermediate between HCO 3 À and CO 3 2À. We analyzed synthetic calcites grown at temperatures ranging from 0.5 to 50°C with and without the enzyme carbonic anhydrase present. This enzyme catalyzes oxygen isotopic exchange between DIC species and is present in many natural systems. The two types of experiments yielded statistically indistinguishable results, and these measurements yield a calibration that overlaps with our theoretical predictions for calcite at equilibrium. The slow-growing Devils Hole calcite exhibits D 47 and d 18 O values consistent with lattice equilibrium.Factors influencing DIC speciation (pH, salinity) and the timescale for DIC equilibration, as well as reactions at the mineral-solution interface, have the potential to influence clumped-isotope signatures and the d 18 O of carbonate minerals. In fast-growing carbonate minerals, solution chemistry may be an important factor, particularly over extremes of pH and salinity. If a crystal grows too rapidly to reach an internal equilibrium (i.e., achieve the value for the temperature-dependent mineral lattice equilibrium), it may record the clumped-isotope signature of a DIC species (e.g., the temperature-dependent equilibrium of HCO 3 À ) or a mixture of DIC species, and hence record a disequilibrium mineral composition. For extremely slow-growing cry...

No abstract

[1] A large body of paleoceanographic data for the Paleocene-Eocene Thermal Maximum (PETM) is based on foraminiferal stable carbon and oxygen isotope composition (d 13 C and d 18 O). However, the proxy records could be biased due to a "pH effect" on stable isotopes during times when the ocean became more acidic, as has been demonstrated for modern planktonic foraminifera. In this paper, we calculate the possible ranges of the pH effect on d 13 C and d 18 O during the PETM based on the relative pH decline (DpH) from the preperturbation steady state simulated by a carbon cycle model and the empirical relationships obtained from culture experiments with planktonic foraminifera. The model is configured with Eocene paleogeography and simulates DpH for surface, intermediate, and deep water in the major ocean basins in response to various carbon input scenarios (2000 to 5000 Pg C). For an array of scenarios, the modeled DpH of the surface ocean ranges from 0.1 to 0.28 units. This suggests that d 13 C of planktonic foraminifera may be increased by up to 2.1‰ and d 18 O may be increased by up to 0.7‰ (corresponding to over 3°C error in paleotemperature estimate). Under conditions in which the model best simulates the global CaCO 3 dissolution pattern, we find marked differences in the deep-sea DpH between the Atlantic (−0.4) and Pacific oceans (−0.1). This would imply that the magnitude of the negative d 13 C and d 18 O excursions of benthic foraminifera in the Atlantic Ocean was dampened by up to 2.8‰ and 0.9‰ at maximum, respectively, relative to a constant pH scenario.Citation: Uchikawa, J., and R. E. Zeebe (2010), Examining possible effects of seawater pH decline on foraminiferal stable isotopes during the Paleocene-Eocene Thermal Maximum, Paleoceanography, 25, PA2216,

No abstract

[1] Ocean uptake of anthropogenic CO 2 causes a decline in seawater pH, a process known as ocean acidification, which may adversely affect marine organisms. We investigate whether continental weathering can mitigate future ocean acidification by sequestering atmospheric CO 2 . We conducted simulations under a suite of carbon emission scenarios with different weathering parameterizations. The short-term impact of a strong weathering feedback was only notable for large emissions with slow injection. This mitigation by enhanced weathering, however, is an order of magnitude smaller than the expected maximum pH decline based on the default parameterizations. Thus on short timescales, weathering has little effect on future atmospheric CO 2 and ocean acidification, regardless of the assumed weathering feedback strength. But on longer timescales and for large emissions, different weathering parameterizations introduce large uncertainties regarding the time when pCO 2 will return to climatically relevant levels of, say, 400 matm in the future. Citation: Uchikawa, J., and R. E. Zeebe (2008), Influence of terrestrial weathering on ocean acidification and the next glacial inception, Geophys. Res. Lett., 35, L23608,