USING PERMIAN BRACHIOPOD SHELL STABLE ISOTOPES TO DETERMINE MID-TO-HIGH LATITUDE SEASONALITY

Stable isotope paleothermometry capitalizes on the relationship between δ¹⁸O values found in carbonate or phosphate materials, δ¹⁸O values of seawater (δ¹⁸O_w), and seawater temperature to produce accurate paleotemperature records. However, this useful paleothermometer is limited by the mineral’s δ¹⁸O co-dependence on both the temperature and δ¹⁸O of the environmental waters from which the mineral precipitated. When studying temperatures of climates long past, the data are further complicated by the increased potential for burial and diagenesis.

This study presents high-resolution snapshots of the seasonal ranges in δ¹⁸O values found in Late Carboniferous to Permian, mid to high latitude, brachiopods. To do so, δ¹⁸O and δ¹³C data were generated by serially sampling pristine, nonluminescent brachiopod shells from South America, Arctic Canada, West Spitsbergen, and Western Australia. Samples were examined in thin section and cathodoluminescence to determine preservation and were milled along visible growth-associated banding. The resulting δ¹⁸O records exhibited systematic, cyclic variation across the specimen which is interpreted as seasonal changes in the environmental conditions over the lifetime of the shell. δ¹⁸O values average -0.32‰, and are more positive compared with data produced from low-latitude Sakmarian brachiopods, and in line with data from high-latitude Australian brachiopods (Korte et al., 2008, PPP). Intraspecimen δ¹⁸O variation is significant, with ranging from 1.1‰ to 2.1‰. These regular, cyclical trends are not found in the concurrent δ¹³C data, which are not significantly variant and average ~4.2‰.

These seasonal records characterize variations in local seawater geochemistry and temperatures. If a constant δ¹⁸O_w of -1‰ is assumed, the seasonal paleotemperature range averages ~ 6 °C, with an average minimum paleotemperature of ~8.4°C and an average maximum paleotemperature of 14.6 °C. The δ¹⁸O ranges in these brachiopods are less than that of the Permian bivalve Eurydesma from Australia (Ivany and Runnegar, 2010, Geology), but seasonal changes in salinity due to fresh water mixing and melt water may have amplified that δ¹⁸O range.