Isotopic and minor element chemistry of Devonian-Carboniferous abiotic marine calcite.

Abstract

Multi-tracer analysis ($\delta\sp{18}$O, $\delta\sp{13}$C, $\sp{87}$Sr/$\sp{86}$Sr, Sr, Mg, Fe and Mn) of late Devonian and early Mississippian marine cements from the Alberta Basin of Western Canada indicates that the $\delta\sp{18}$O value of seawater increased by 3 to 4% at the Devonian-Carboniferous boundary ($\sim$360 Ma). This increase is the largest variation observed during the Phanerozoic, and effectively separates lower Paleozoic marine carbonates (low $\delta\sp{18}$O values) from post late Devonian marine carbonates (high $\delta\sp{18}$O values). Mechanisms responsible for this change may include an increase in heat flow and/or the relative volume of oceanic crust associated with high-temperature exchange reactions. This change may be associated with a large tectonic reorganization of ridge systems rather than an increase in spreading rates. Using a maximum range in age estimates, the duration of this transition is 20 m.y., yielding a rate of $\delta\sp{18}$O value change that is approximately twice as fast as previous estimates. Sr/Mg ratios of modern biotic and abiotic marine calcite are compared with those of ancient analogues to estimate the Mg/Ca and Sr/Ca ratios of ancient seawater. The Mg/Ca ratio of late Devonian seawater was 80 to 90% lower and the Sr/Ca ratio was 50 to 60% higher than that of modern seawater. Moreover, modern abiotic marine calcite has significantly lower Sr contents than modern biotic calcite, indicating that abiotic calcite precipitates several orders of magnitude more slowly than biotic calcite, and is therefore relatively free of kinetic effects. This supports findings that abiotic marine calcite is also precipitated in oxygen and carbon isotopic equilibrium with ambient seawater. These findings have particular significance because, in many studies of secular variation in the $\delta\sp{18}$O values of marine carbonates, brachiopods were chosen for analysis over marine cements because the low Mg content of brachiopod calcite is potentially more resistant to diagenetic alteration. However, modern brachiopods generally do not precipitate calcite in oxygen isotopic equilibrium with ambient seawater, thus limiting their effectiveness as indicators of the temperature or $\delta\sp{18}$O value of ancient seawater.