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

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 87 Sr/ 86 Sr 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 4Á5 AE 0Á6 mole% SrCO 3 ; ca 25 mmol mol À1 Sr/Ca) and was depleted in 18 O and 13 C compared to the middle layer, both features inconsistent with pristine material. The d18 O 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.

On the reconciliation of biostratigraphy and strontium isotope stratigraphy of three southern Californian Plio-Pleistocene formations

“…McArthur et al (1994) reported aberrant 87 Sr/ 86 Sr ratios when attempting to use mollusks with seemingly well-preserved aragonite (assessed using multiple methods) to construct the seawater strontium curve for the U.S. Western Interior Seaway. Marcano et al (2015) cited multiple studies in which aragonitic specimens with no indication of diagenetic alteration by cathodoluminescence or Fe/Mn concentration yielded ambiguous 87 Sr/ 86 Sr ratios and proposed that the recrystallization of the original aragonite into inorganically precipitated aragonite was the cause of the anomalous values. We propose a similar explanation for the San Diego Formation, since coastal San Diego has been hydrologically dynamic over the last 800 k.y.…”

The fossil bivalve Angulus benedeni benedeni: a potential seasonally resolved stable isotope-based climate archive to investigate Pliocene temperatures in the southern North Sea basin

“…No blocky calcite was observed in the EBSD images of any of the three specimens (Cusack 2016;Casella et al 2017). This alone does not preclude diagenetic alteration, as calcite is not formed in the earliest stages of diagenesis (Cochran et al 2010;Marcano et al 2015;Ritter et al 2017) and there is not necessarily a difference in grain size (due to the presence of larger, secondary grains) between pristine and altered aragonite (Casella et al 2017). Further analysis of the EBSD maps did not, however, produce any evidence for diagenetic alteration.…”

Investigating the environmental interpretation of oxygen and carbon isotope data from whole and fragmented bivalve shells

“…The isotope composition of carbonate from the sequential analysis of intact fossil bivalves is being increasingly utilised in palaeoenvironmental reconstructions (Schöll-Barna et al, 2012;Lewis et al, 2017). However, little is known about the relevance of the isotope composition of shell fragments, which are commonly preserved in the sedimentary record (Wagner et al, 2014;Marcano et al, 2015). To better understand the efficacy of shell fragments as proxy in palaeoenvironmental studies, we utilise sediment cores recovered from Lake Dojran (Balkan Peninsula) that have been previously investigated using multi-proxy techniques and contain both whole shells and fragments of the bivalve Dreissena carinata.…”