Records of Late Permian Surface Temperatures in Continental Gondwana in Isotope Geochemistry of Upper Permian Early Diagenetic Calcite Concretions
Keddy Yemane and Kerry Kelts
We present geochemical and isotopic evidence of paleotemperatures from freshwater continental deposits from 55°S in interior southern Gondwana. Lacustrine shales host spheroidal concretions with abundant septarian cracks. Cement carbonate varies from 65% at the centre to 15% at the edges. Ostracode remains are preserved. Septarian calcite occurs in dull and bright bands, with three distinct generations of vein-fills. 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) are: host shales, ^dgrv13C = -4.36 to 0.77 ^pmil, ^dgrv18O= -12.73 to -17.12 ^pmil; concretion cements, ^dgrv13C=+0.26 to -5.38 ^pmil, ^dgrv18O= -9.89 to -9.34 ^pmil; and vein-fills, ^dgr 13C= -7.05 to +1.09 ^pmil, ^dgrv18O= -8.28 to -18.24 ^pmil. 13C and 18O ratios are depleted from the center of concretions to the periphery, as well as from the centre of veins to the tip.
Near-surface cementation is suggested by textural evidence. ^dgrv18O in the range of -12.636^pmil to -8.989 SMOW is calculated for the meteoric palaeowaters, from which average annual surface temperatures of 5-8°C are inferred. ^dgrv18O of early-formed cements also yields a mean annual surface temperature of 2-6°C. Based on continentality and palaeolatitudes of northern Malawi during the Late Permian, we propose that mean annual surface palaeotemperatures may have been as high as 10°C, similar to those found in modern continental temperate climates.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California