Diagenetic incorporation of Sr into aragonitic bivalve shells: implications for chronostratigraphic and palaeoenvironmental interpretations

Authors

Maria C. Marcano, University of Michigan
Tracy D. Frank, University of Nebraska at LincolnFollow
Samuel B. Mukasa, University of New Hampshire
Kyger C. Lohmann, University of Michigan
Marco Taviani, ISMAR

Date of this Version

2015

Citation

The Depositional Record 2015, 1(1):38–52

Comments

© 2015 The Authors.

Open access

doi: 10.1002/dep2.3

Abstract

Aragonite is easily altered during diagenesis, therefore presumed pristine when present. In effect, beyond polymorphic transformation to calcite, alteration paths of aragonite remain poorly understood despite heavy reliance on such material to produce palaeoenvironmental and chronostratigraphic interpretations. Previous work on core material from Southern McMurdo Sound, Antarctica, showed that unlike their calcitic counterparts, seemingly unaltered aragonite shell fragments invariably produced older than expected ⁸⁷Sr/⁸⁶Sr ages. In this study, we pursued additional analyses of these aragonite shells and of the porewater of the core to understand this discrepancy. Aragonite mineralogy was reconfirmed and elemental mapping of shell fragments revealed growth lines within the middle layer suggestive of good preservation. The outer layer, however, showed anomalously high Sr concentrations (average 4^.5 ± 0^.6 mole% SrCO₃; ca 25 mmol mol^-1 Sr/Ca) and was depleted in ¹⁸O and ¹³C compared to the middle layer, both features inconsistent with pristine material. The δ¹⁸O values and Sr concentrations of the porewater were used to model outer layer compositions reasonably well. Coincidentally, porewater Sr isotope composition was in general agreement with the age model of the core only at the aragonite-bearing interval suggesting that Sr-isotopic disequilibrium between porewater and the carbonates was the rule rather than the exception in the core. The Sr isotope compositions of the aragonite shells are most likely the result of early diagenesis as suggested by the inconsistent O and C isotope compositions between shell layers and the anomalously high Sr concentrations. We conclude that knowledge of Sr concentration and distribution in shells is critical to determine the viability of Sr stratigraphy and the scale at which it may be applied. Reliance on traditional indicators of lack of alteration, such as cathodoluminescence, Mn-Fe concentration, and the presence of labile mineralogies to assert chronostratigraphic and palaeoenvironmental questions may produce erroneous conclusions due to obscurely altered material.