Interpretative Challenges and Opportunities in Oxygen and Strontium Isotope Compositions of Bivalve Shells.

Abstract

Oxygen and strontium from biogenic carbonates (mostly bivalves) were used to evaluate paleoenvironmental interpretations on the basis of modern analogs along the North American east coast and to offer chronostratigraphic, paleoenvironmental, and diagenetic constraints to Neogene sequences from Southern McMurdo Sound, Antarctica. These are relevant pursuits in light of imminent climatic changes, which heighten the need for reliable paleoenvironmental interpretations and for data from climate–sensitive regions. In Vineyard Sound, Massachusetts, U.S.A., d18O from several coexisting shallow marine taxa and well–constrained environmental parameters were used to test the extent to which the known local annual variation of these parameters was represented in the shell carbonate. This relation was explored through the construction of time series from bivalve d18O versus shell distance profiles and the calculation of growth rates and isotopic fractionation factors. It was discovered that the marine temperature range recorded in the biogenic carbonate extended to lower temperatures than expected and was, in its entirety, better represented not by a single taxon but by the concurrent use of multiple taxa. Modeling of bivalve d18O of modern shallow marine environments along the North American east coast showed that large salinity ranges complicate environmental interpretations. However, modeled clams showed that d18O seasonal variability can discriminate between tropical and temperate shallow water marine climates. Higher variability characterizes the winter of tropical zones while the opposite is true for temperate regions. 87Sr/86Sr and Sr concentration of unaltered calcite bivalves from ANDRILL’s core AND–2A produced reliable ages and confirm contrasting marine climate conditions across the Middle Miocene of Southern McMurdo Sound, Antarctica. In contrast, seemingly unaltered aragonite shells invariably produced older than expected ages. Additional analyses of the aragonite shells and 87Sr/86Sr compositions of pore water were carried out to reconcile these results. d18O and Sr concentrations of pore water and aragonites suggested that the anomalous ages are likely the result of early diagenetic alteration, even in the absence of mineralogical change.