Abstract
AbstractAragonite 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.
Highlights
The 87Sr/86Sr ratios of unaltered marine biogenic carbonates are routinely compared to the well-established secular variation of Sr isotopic composition of sea water (Hess et al, 1986; Howarth & McArthur, 1997; McArthur et al, 2001) to provide chronostratigraphic control
Previous work had shown that aragonite bivalve fragments recovered from ANDRILL AND-2A core in Southern McMurdo Sound, Antarctica, passed generally accepted criteria for unaltered aragonite and showed Sr concentration contrast along growth lines of the middle layer
Within the uncertainties associated with the incorporation of Sr into aragonite, the observed high-Sr concentration of the shells was modelled here using AND-2A porewater
Summary
The 87Sr/86Sr ratios of unaltered marine biogenic carbonates are routinely compared to the well-established secular variation of Sr isotopic composition of sea water (Hess et al, 1986; Howarth & McArthur, 1997; McArthur et al, 2001) to provide chronostratigraphic control. Given that carbonate secreting organisms do not fractionate Sr when they incorporate it in their skeletons, the 87Sr/86Sr ratio of the open oceanic sea water can be reconstructed through geological time using unaltered biogenic carbonate material. In this context, well-preserved samples can be used to estimate Sr sources and to model past mixing and weathering rates, these relationships are not straightforward and numerous components of the climate system appear to play different roles of variable importance at different points in time. Modelling suggests that most of the increase in sea water 87Sr/86Sr since the mid Pliocene can be explained solely by the relative rise in phyllosilicates weathering prompted by global cooling (Li et al, 2007), while Kashiwagi et al (2008) conclude that sea water 87Sr/86Sr cannot be used as a direct proxy for silicate weathering and atmospheric CO2 decrease during the Cenozoic
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