Edward T. Tipper

Ocean acidification and the Permo-Triassic mass extinction

Ocean acidification and mass extinction The largest mass extinction in Earths history occurred at the Permian-Triassic boundary 252 million years ago. Several ideas have been proposed for what devastated marine life, but scant direct evidence exists. Clarkson et al. measured boron isotopes across this period as a highly sensitive proxy for seawater pH. It appears that, although the oceans buffered the acidifiying effects of carbon release from contemporary pulses of volcanism, buffering failed when volcanism increased during the formation of the Siberian Traps. The result was a widespread drop in ocean pH and the elimination of shell-forming organisms. Science, this issue p. 229 A rapid injection of massive amounts of carbon into the atmosphere acidified the oceans, causing mass extinction. Ocean acidification triggered by Siberian Trap volcanism was a possible kill mechanism for the Permo-Triassic Boundary mass extinction, but direct evidence for an acidification event is lacking. We present a high-resolution seawater pH record across this interval, using boron isotope data combined with a quantitative modeling approach. In the latest Permian, increased ocean alkalinity primed the Earth system with a low level of atmospheric CO2 and a high ocean buffering capacity. The first phase of extinction was coincident with a slow injection of carbon into the atmosphere, and ocean pH remained stable. During the second extinction pulse, however, a rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota.

On discrimination between carbonate and silicate inputs to Himalayan rivers

We review new and published analyses of river waters, bedloads and their constituent minerals from the Dhauli Ganga and Alaknanda, headwaters of the Ganges in Garhwal, and the Marsyandi in Nepal and their tributaries. These data are used to discriminate between the inputs of major cations and Sr from silicate and carbonate sources. Methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in Sr-Ca-Mg-Na-K-87Sr/86Sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component. This results in factor-of-two overestimates of the fractions of silicate-derived Ca, Mg and Sr. To correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate-derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of Sr-Ca-Mg-Na-K-87Sr/86Sr space. The compositions of the carbonate and silicate end-members in the bedload are determined by sequential leaching. The results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electron-microprobe analyses of the minerals in the bedload. In the upper Marsyandi catchment, which drains low-grade Tethyan Sedimentary Series formations, a set of mainstem samples collected over a two-year period define tight correlations in Sr-Ca-Mg-Na-K-87Sr/86Sr space. Modeling of the magnitude of secondary carbonate precipitation and fractions of silicate-derived Ca, Mg and Sr in Sr-Ca-Mg-87Sr/86Sr space gives self-consistent results that are compatible with both the calculations of mineral modes and published Mg-isotopic compositions, if the ratio of chlorite to biotite weathering is high or if there is another silicate source of Mg. These calculations imply that between 12 and 31 percent of the Sr and 44 and 72 percent of the Mg is derived from silicate minerals where the range reflects the seasonal change in the ratio of silicate-derived to carbonate-derived cations. Modeling in Sr-Ca-Na and/or K space is inconsistent with the Sr-isotopic and Mg-isotopic constraints and we conclude that in this catchment dissolution of Na and K are incongruent relative to Sr-Ca-Mg. Potassium is preferentially retained in micas whereas the controls on Na are unclear. Modeling of the catchments underlain by High Himalayan Crystalline and Lesser Himalayan Series in Garhwal is complicated by the presence of dolomite as well as calcite in the carbonate and the results imply that dolomite dissolves faster in the acetic acid leaches than in nature. Up to 60 percent of the Sr in the catchment on High Himalayan Crystalline Series and 20 to 30 percent of Sr in the catchments on Lesser Himalayan Series are estimated to be derived from silicates. However it should be noted that the element budgets are not all self-consistent and the use of bedrock-element ratios to model the sources of chemical inputs to river waters remains subject to uncertainties.

Global Ca Cycles: Coupling of Continental and Oceanic Processes

Calcium is one of the most important mobile metals that can migrate easily between major geochemical reservoirs at the Earth’s surface; the hydrosphere and the biosphere and crust. In doing so calcium plays a key role in regulating climate over million year time-scales, transferring carbon from the atmosphere and storing it as calcium carbonate. Calcium isotopes potentially provide a way of tracing the mobility of calcium within the Earth surface environment. This chapter reviews the steps where calcium isotopes are fractionated in the weathering and ocean environments, and how these fractionations can be used to constrain mass transfers on both the continents and in the oceans.