Abstract
Abstract. Stable carbon and oxygen isotopic compositions (δ13C and δ18O) of benthic foraminiferal carbonate shells have been used to reconstruct past bottom-water environments. However, the details of factors controlling the isotopic disequilibrium between the shells and the surrounding bottom seawater (so-called the "vital effect") are still ambiguous. In this study, we analyzed the isotopic composition of individual benthic foraminifera of multiple species by using a customized high-precision analytical system, and found that the magnitude of the isotopic disequilibrium between benthic foraminiferal shell and the surrounding bottom seawater (δ13CDIC and δ18Owater) in different species is correlated with inter-individual isotopic variations. As a result, we can choose suitable species as bottom-water proxies by using the inter-individual isotopic variations. In addition, by using the simplified interpretation of the inter-individual and inter-species isotopic variations established in this study, we could reconstruct the δ13C values of dissolved inorganic carbon in bottom water by correcting foraminiferal isotopic compositions for the isotopic shift resulting from the isotopic effects (vital effect, microhabitat effect, and many other reported isotopic effects). Our findings will allow the use of isotope data for benthic foraminifera as more reliable proxies for reconstructing past bottom-water conditions and evaluating global carbon cycling.
Highlights
Variations in the stable carbon and oxygen isotopic composition (δ13C and δ18O) of foraminiferal carbonate shells have been used for over half a century to estimate paleoenvironmental parameters such as temperature, to quantify global changes in sea level and deep-sea circulation, and to document events such as large seafloor methane releases (e.g. Emiliani, 1955; Shackleton and Opdyke, 1973; Kennett et al, 2000; Zachos et al, 2001)
The magnitude of the inter-species and inter-individual isotopic variations can be simplified to its correlation with the mass of the individual calcite shell
We found that the species with smaller interindividual isotopic deviations are more suitable as environmental proxies, some of those species (U. akitaensis and the Cassidulina group) had carbon isotopic values that were slightly negative relative to ambient δ13C of DIC in bottom water (δ13CDIC) ( δ13C ≈ −2 ‰)
Summary
Variations in the stable carbon and oxygen isotopic composition (δ13C and δ18O) of foraminiferal carbonate shells have been used for over half a century to estimate paleoenvironmental parameters such as temperature, to quantify global changes in sea level and deep-sea circulation, and to document events such as large seafloor methane releases (e.g. Emiliani, 1955; Shackleton and Opdyke, 1973; Kennett et al, 2000; Zachos et al, 2001). Variations in the stable carbon and oxygen isotopic composition (δ13C and δ18O) of foraminiferal carbonate shells have been used for over half a century to estimate paleoenvironmental parameters such as temperature, to quantify global changes in sea level and deep-sea circulation, and to document events such as large seafloor methane releases Δ13C and δ18O of benthic foraminiferal shells have been used as tracers to reconstruct past bottom-water environments. There is still ambiguity concerning the factors controlling the isotopic disequilibrium between the isotopic compositions (δ13C and δ18O) of benthic foraminifera and the environmental factors (δ13C of dissolved inorganic carbon (DIC) and δ18O of seawater) (Grossman, 1987; McCorkle et al, 1990; Spero et al, 1997; Zeebe et al, 1999; Bijma et al, 1999; Erez, 2003; Schmiedl et al, 2004). T. Ishimura et al.: Tracers for quantifying the magnitude of isotopic disequilibrium
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