Dataset Information

Carbon and Oxygen Isotope Records from Tridacna derasa Shells: Toward Establishing a Reliable Proxy for Sea Surface Environments.

ABSTRACT: We report the carbon (?13C) and oxygen (?18O) isotope records of three modern Tridacna derasa shells from Ishigaki-jima, southwestern Japan. The high-resolution ?13C profiles of samples from the inner shell layer on cross-sections fall within similar narrow ranges and display no regular variations or trends, such as an ontogenetic trend or abrupt short-term drops likely to be related to reproductive activity. This suggests that the calcification site of this species is not likely affected by photosynthetic CO2 uptake or CO2 incorporation during respiration. The ?18O profiles show distinct seasonal cycles. The intraspecific variability in the ?18O values is small in parts of the shell precipitated in the adult stage, but is not negligible in the juvenile and senescent stages. The differences in the monthly and seasonally resolved ?18O values among shells are less than 0.51‰ and 0.76‰, respectively. The shell ?18O values are nearly identical or close to the ?18O values for aragonite precipitated in oxygen isotope equilibrium with ambient seawater (?18OEA). The largest differences between the shell ?18O and ?18OEA values calculated from the monthly and seasonally resolved data correspond to an overestimate of the seawater temperature by as much as 1.7°C and 2.3°C, respectively. However, these differences are smaller in the adult stage (<0.25‰) than in the other stages. This small difference allows an accurate reconstruction of the seawater temperature with an error of <1.1°C. Consequently, we recommend that multiple shell records be obtained because of the non-negligible intraspecific variations in their ?18O values. Growth banding, composed of alternating narrow white bands and wide light-grey bands, is discernible on cross-sections of the inner shell layer. The ?18Oshell data indicate that they were formed in winter and the other seasons, respectively.

SUBMITTER: Yamanashi J

PROVIDER: S-EPMC4915723 | biostudies-literature | 2016

REPOSITORIES: biostudies-literature

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Carbon and Oxygen Isotope Records from Tridacna derasa Shells: Toward Establishing a Reliable Proxy for Sea Surface Environments.

Yamanashi Junpei J Takayanagi Hideko H Isaji Ayaka A Asami Ryuji R Iryu Yasufumi Y

PloS one 20160621 6

We report the carbon (δ13C) and oxygen (δ18O) isotope records of three modern Tridacna derasa shells from Ishigaki-jima, southwestern Japan. The high-resolution δ13C profiles of samples from the inner shell layer on cross-sections fall within similar narrow ranges and display no regular variations or trends, such as an ontogenetic trend or abrupt short-term drops likely to be related to reproductive activity. This suggests that the calcification site of this species is not likely affected by pho ...[more]

PMID: 27327430

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Project description:The application of U isotopes in carbonates as a paleo-ocean oxygenation proxy is based on the critical assumption that the calcareous shell-building organisms incorporate U into their shells without fractionation relative to the U isotopic composition of ambient seawater. Recent studies claim a small, but resolvable, isotopic offset during abiotic and biogenic aragonite precipitation, whereas no isotope fractionation has been recorded during calcite precipitation. Although aragonite is meta-stable and not preserved over geological timescales (>1 Myr) and U precipitates during diagenesis, the U isotope composition of biogenic aragonite is important because aragonite precipitation is an important U sink to carbonate sediments. In contrast, low-magnesium calcite (LMC) is preserved over geological timescales and may provide a reliable fingerprint of ancient ocean chemistry. Therefore, a more general study is needed that compares U isotope compositions of primary marine biogenic carbonate precipitates. We report the U isotope compositions of 32 modern samples from geographically distinct localities in the Atlantic Ocean including corals (Scleractinia, Octocorallia), brachiopods (Articulata), molluscs (Tellina Listeri, Codahia Obicularis) and barnacles as well as one fossil mollusc. These samples reflect variable primary minerals, water temperatures, water depths, pH-values of ambient water, and U concentrations. Several seawater samples have also been measured to compare our methods with those of previously published studies. The analyzed modern corals and brachiopods display U isotopic compositions that are indistinguishable from modern seawater. This suggests that these carbonates have the potential to faithfully record the U isotopic composition of the surrounding seawater in which they form. The analyzed brachiopods are of particular interest as they are composed of the calcium carbonate polymorph LMC that is stable over geological timescales. While this study shows for the first time that LMC phases are robust targets in ancient samples, their low U abundance presents analytical challenges for precise U isotope analyses. We also show that two barnacle shells collected with ambient seawater have U isotopic compositions that are both lighter and heavier than the ambient seawater. The mechanism to explain this offset is not determined, but it demonstrates that at least barnacle shells are not representative of the seawater in which they last lived. Two of three partially fossilized mollusc shells also show resolvable offsets from seawater, likely indicating secondary processes that are known to shift or fractionate U isotopes. Collectively, our new data indicate that: 1) aragonite delivers U with a seawater composition to carbonate sediments, and 2) LMC shells of brachiopods that are stable over geological timescales may be more suitable for reconstructing the U isotope composition of ancient oceans.

Dataset Information

Carbon and Oxygen Isotope Records from Tridacna derasa Shells: Toward Establishing a Reliable Proxy for Sea Surface Environments.

Publications

Carbon and Oxygen Isotope Records from Tridacna derasa Shells: Toward Establishing a Reliable Proxy for Sea Surface Environments.

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