Permian Gondwanaland paleoenvironment inferred from carbon and oxygen isotope records of brachiopod fossils from Sydney Basin, southeast Australia

Abstract

The δ 13C and δ 18O records of Permian fossil brachiopods with biostratigraphical age control from southeastern Australia have been constructed to investigate the high latitude paleoenvironment of Gondwanaland. All samples were thin sectioned and examined using petrographic and cathodoluminescence microscopy for evaluating shell preservation. Elemental compositions of select brachiopod shells were measured using electron probe microanalysis to further evaluate shell preservation. Within these intervals the average δ 13C values of the best preserved (non-luminescent; NL) portion of the brachiopod shells are all greater than + 5.1‰ with the highest value, + 7.0‰, at Lower Wandrawandian Siltstone (late Kungurian). These carbon isotope values are similar to those of contemporaneous tropical eastern Pangea (Russian Platform) but heavier than those of western Pangea (North America). The oxygen isotope difference between brachiopods of high southern latitudes and low latitudes fluctuated during Early to Middle Permian. The mean δ 18O values of the high-latitude NL brachiopod shells were more positive than those of the low latitudes in early Sakmarian (− 0.3‰), early Late Artinskian (− 1.1‰), late Kungurian (− 1.0‰), Roadian (− 0.2‰), and Wordian (− 0.7‰); comparable to those of tropical western Pangea in early Artinskian (− 2.1‰) and early Kungurian (− 2.5‰); and more negative than those of tropical western Pangea in late Late Artinskian (− 3.9‰). Assuming the oxygen isotope value for Permian seawater in southeast Australia was − 1‰, the calculated oxygen isotope temperatures were mostly between 12 °C and 16 °C reflecting southeast Australia's cool high latitude temperature conditions. Higher apparent oxygen isotope temperatures for the Upper Pebbly Beach Formation (29 °C, late Late Artinskian), Berriedale Limestone (21 °C, early Artinskian), and Snapper Point Formation (22 °C, early Kungurian) may be attributed to a combination of warmer climate and lower seawater δ 18O in this region.