P. N. Shukla

Geochemistry of some K/T sections in India

Abstract A number of marine as well as continental K/T sections in India, particularly around the Deccan volcanic regime, have been analyzed to obtain concentration profiles of some platinum group and other trace elements, specifically to search for anomalous levels of iridium. Except for the Um Sohryngkew River (USR) section in Meghalaya, where iridium is found to occur at an anomalously high level in the K/T boundary layer, all other sections show low levels in the range of 10 to ∼120 pg/g. The iridium profile in the USR section is characterized by a sharp peak of 12.1 ng/g in a 1.5 cm thick limonitic layer, superimposed on a broad band of ∼0.2 ng/g extending over a 50±20 cm thick section, beyond which Ir decreases to the base level of ∼0.02 ng/g, both in the Tertiary as well as in the Cretaceous. High Ir is accompanied by high Os but the Os/Ir ratio in the peak (0.37) is different from the Os/Ir ratio of ∼2 measured in the broad band. The Ir profile in the USR section seems to be consistent with fallout of cometary debris preceding and following the main impact. The measurements at other sites in the Deccan suggest that the iridium inventory of the Deccan is too low to account for the high iridium observed in the KTB layer all over the world, but may be sufficient to give rise to its enhancement adjacent to the KTB peak.