5Corg chemostratigraphy of the Permian-Triassic boundary in the Maitai Group, New Zealand: evidence for high-latitudinal methane release

Carbon isotopic studies from marine organic matter of the Permian-Triassic Maitai Group, New Zealand, reveal a significant 5Corg shift toward more negative values within the Little Ben Sandstone Formation. These isotopic data chemostratigraphically define the previously debated position of the Permian-Triassic boundary in the Maitai Group. The Permian-Triassic record of the Maitai Group is also important because of its high paleolatitudinal setting and the deposition at intermediate depths in the ocean (c. 400 m) within a volcanic arc-related basin. Marine Permian-Triassic strata deposited at water depths deeper than shelf areas are rare. High latitude Permian-Triassic boundary sections document a significantly larger isotopic offset across the boundary compared with lower latitude settings. Carbon isotopic values decrease rapidly by an average of 7%o from homogeneous values (x -25%o) in the Tramway and lower Little Ben Sandstone Formation to highly lluctuating and very depleted values (x -32%o) within the Little Ben Sandstone Formation. The lowermost Big Ben and Tramway Formations are considered to be Permian in age, based on their homogeneous and comparably heavier carbon isotopic values and supported by fossil atomodesmatinid bivalves. Based on the distinct 5Corg excursion toward negative values and the concurrent onset of strong isotopic fluctuations, the Permian-Triassic boundary is placed in the lower half of the Little Ben Sandstone Formation. G98047 Received 18 December 1998; accepted 14 September 1999 Very depleted 8C values in the Little Ben Sandstone Formation of-38%o indicate a contribution from isotopically light methane. A possible methane source is clathrates, released by large submarine slides or warming-induced melting of permafrost. The Little Ben Sandstone Formation has been interpreted as a massive event deposit from a submarine slide (Landis 1980). This hypothesised methane release could have been in part responsible for the larger Permian-Triassic isotopic shift in high latitudes compared with low latitudes because large volumes of clathrates are trapped in continental shelves and high-latitude permafrost.