High-resolution organic carbon isotope chemostratigraphy of the lower Aptian and the expression of Oceanic Anoxic Event 1a in the Tibetan Himalaya

Abstract

Variations in carbon isotope (δ¹³C) values of Aptian marine strata are globally comparable and regarded as an important tool for the stratigraphic correlation of Aptian successions. The most remarkable feature of the δ¹³C curve of the Aptian is an abrupt negative excursion followed by a prominent positive excursion, which defines the early Aptian Oceanic Anoxic Event (OAE1a, ∼120 Ma). However, a complete and precise OAE1a record from eastern Tethys has yet to be established. Based on previously investigated integrated biostratigraphy, we present a high-resolution lower Aptian δ¹³C curve from bulk organic carbon through an expanded succession at the Chaqiela section in the Tibetan Himalaya. The depositional environment, couple with TOC/TN data, indicate that the organic matter was predominately sourced from marine plankton. As such, our results provide a record of secular changes in the δ¹³C of the dissolved carbon pool of the shallow sea on the southern margin of eastern Tethys.

The co-occurrence of glauconites and enrichments of the redox-sensitive trace elements (RSTEs) indicate largely suboxic bottom water conditions in the shallow marine eastern Tethys Ocean during OAE1a. Other paleoclimatic proxies suggest a relatively warm and humid paleo-environment during this time interval, with moderate to intense chemical weathering conditions revealed by chemical index of alteration (CIA) values.