Oxygen and carbon isotope composition of cold-water Berriedale Limestone (Lower Permian), Tasmania, Australia

Abstract

The Berriedale Limestone formed at about 80°S paleolatitude and contains many glacial dropstones. It formed during a period of major Gondwana deglaciation. The Berriedale Limestone contains mostly bryozoans, brachiopods and bivalves, with some intraclasts and rare pellets. The faunal diversity is low and the fauna are similar to the modern cold-water foramol faunal assemblage. Micrite, microspar and spar occur as equant to well developed rhombs of calcite. The coarse spar cements are bored and are ruptured by dropstones, indicating submarine origin of low-Mg calcite at water-temperatures of around 3°C. The mixing zone cementation was preceded by erosion of early formed crystals. The eroded crystals occur as inclusions in mixing zone cements. The fauna are characterized by heavy δ 13C and light δ 18O. The whole-rock field of δ 18O- δ 13C falls at the edge of “Normal Marine Limestone” and deviates to lighter δ 18O values (down to −16.7‰ PDB). Lightest δ 18O values ( ∼ −22‰ PDB) of fresh-water sparry calcite cement are similar to those in the Early Permian continental tillites, suggesting that the Permian sea was diluted by isotopically light melt waters. Micrite δ 18O values (−9.2 to −12.6‰ PDB) are within the range of whole-rock values. The δ 18O values of calcite in shales are lighter than limestone values. The δ 18O values of the fauna give an unrealistic range of sea-water temperatures because the fauna have equilibrated with variable amounts of melt waters. However, calculated original δ 18O values of the fauna indicate temperatures < 4°C. The heaviest δ 18O of fauna gives cold temperatures of 9°C (with δ w −2.8‰) and −3°C (with δ w −6‰ ). The lightest values of sparry calcite cements ( −22‰ PDB) indicate that the limestone reacted with cold melt waters. The δ 18O of Permian sea is estimated to be about +1.2‰ and was diluted by melt waters as light as −27‰ SMOW.