Isotope geochemistry of Upper Permian early diagenetic calcite concretions: Implications for Late Permian waters and surface temperatures in continental Gondwana

This paper presents comprehensive geochemical and isotopic studies on calcite concretions from Upper Permian freshwater continental deposits that were laid down at ≈55°S in the interior of southern Gondwana. The shales hosting the concretions are partially bioturbated and calcareous. The concretions are predominantly spheroidal and contain abundant septarian cracks. The maximum content of cement carbonate in concretion matrix varies from 65% at the centre to 15% at the edges. Ostracod remains are exclusively preserved within the concretions. Septarian calcite occurs in dull and bright bands, with three distinct generations of vein-fills: brown, euhedral calcite with saw-tooth habit; coarse fibrous white calcite, and minor fibrous white calcite with sutured termination at the centre of the veins. Early-formed cement both in concretions and septarian veins is magnesium-rich whereas, later-formed carbonates are pure calcite. Carbon- and oxygen-isotope ratios (PDB) of the carbonates are as follows: host shales, ∂13C = −4.363 to 0.777‰, ∂18O = −12.732 to −17.118‰; concretion cements, ∂13C = +0.256 to −5.378‰, ∂18O = −9.882 to −9.336‰; and vein-fills, ∂13C = −7.053 to +1.094‰, ∂18O = −8.280 to −18.241‰. 13C and 18O ratios are depleted from the centre of concretions to the periphery, as well as from the centre of veins to the tip. urface cementation as suggested by textural evidence and maximum burial temperatures of 50–70°C established from organic matter and clay mineral composition allow independent estimates of surface palaeotemperatures from 18O ratios. ∂18O in the range of −12.636‰ to −8.989 SMOW is derived for the meteoric palaeowaters, from which average annual surface temperatures of 5–8°C is inferred. ∂18O of early-formed cements also yields a mean annual surface temperature of 2–6°C. Considering the continentality of the palaeolatitudes of northern Malawi during the Late Permian, we contend that mean annual surface palaeotemperatures may have been as high as 10°C, similar to those found in modern continental temperate climates. This is the first independent palaeotemperature data from the interiors of southern Gondwana.