δ18O AND δ13C VARIABILITY IN MODERN AND HOLOCENE BRACHIOPODS, SOUTHERN AUSTRALIAN SHELF

The δ¹⁸O and δ¹³C values of brachiopod shell calcite are commonly used as proxies for ancient environmental marine conditions and secular changes in ancient ocean chemistry. The variability of δ¹⁸O and δ¹³C across modern shelf settings is, however, not well documented. This study presents δ¹⁸O and δ¹³C data from 407 brachiopods collected from 220 sites across ~2,600 km along Australia’s southern shelf, the largest cool-water carbonate shelf in the modern world. Significant isotopic variability is present within separate specimens, at individual sites, and across the entire region. Individual specimens analyzed at multiple shell positions have an isotopic range of values up to 1.3‰ and 1.9‰ for δ¹⁸O and δ¹³C respectively. Multiple brachiopods at a single site have values that vary as much as 2.1‰ for δ¹⁸O and 1.8‰ for δ¹³C. Regional variability ranges on the order of 3.6‰ and 3.4‰, for δ¹⁸O and δ¹³C respectively. The distribution of intrasite variability can be divided into two distinct modes. Mode 1 areas are characterized by low variability and occur in zones of relatively consistent shelf water conditions. Mode 2 zones exhibit high variability and usually correspond to areas of seasonal upwelling onto the shelf and this aspect is interpreted to be the main cause of isotopic variability. Cold upwelled water affects δ¹⁸O of shells being precipitated whereas upwelled nutrient elements promote active phytoplankton growth that alters seawater δ¹³C and that of calcareous filter feeders growing during these periods. The extensive spatial coverage across this vast latitude-parallel region provides a valuable baseline illustrating the isotopic range that could occur in a single layer in the rock record. Furthermore, changes in isotopic signatures measured across stratigraphy does not necessarily reflect secular changes in ocean chemistry but, instead, could be recording local changes in shelf circulation and upwelling intensity.