δ18O and δ13C Values in Living and Holocene Brachiopods and the Relationship with Oceanographic Variability across Australia’s Vast Southern Shelf

Abstract:

Carbon and oxygen isotopic compositions of brachiopods are commonly used to reconstruct secular changes in ocean chemistry through the Phanerozoic but few studies have focused on the variations that occur laterally and concurrently across a single vast depositional system. Previous studies have identified significant isotopic variability to occur within an individual stratigraphic layer and the scatter in values has been attributed to diagenesis, non-equilibrium fractionation effects, and variability in oceanography. In order to further investigate these hypotheses, this study evaluates the δ¹⁸O and δ¹³C values from 346 living and Holocene brachiopods collected from surficial sediments across the latitude-parallel southern Australian shelf, a lateral distance of ~3000 km. Modern oceanographic measurements were used to calculate the range in δ¹⁸O values of calcite precipitated in apparent equilibrium with ambient seawater. A total of 84% of δ¹⁸O values from brachiopod samples (n = 684) fall within the range of calculated equilibrium calcite and accurately record a combination of normal shelf water conditions, winter downwelling across the shelf, and local summer upwelling. Most δ¹⁸O outliers are attributed to seasonal upwelling (90 of 108 outliers) and imply that either upwelling occurred in an area that has not been well established as an upwelling zone, or it occurred in a known upwelling area but the intensity was greater than previously measured. The δ¹³C values of brachiopods increase with increasing depth, which is the opposite of what is reported elsewhere. This unusual δ¹³C trend is caused by deeper slope currents being sourced from surface water southwest of Tasmania, an area with relatively high δ¹³C of DIC. The δ¹³C values of living specimens are consistently lower by 0.5-1.0‰ than most dead specimens, which is attributed to the decrease in δ¹³C values in the carbon cycle due to combustion of isotopically light fossil fuels over the last 200 years.