Nitrogen isotopic composition of sediments from the eastern Tethys during Oceanic Anoxic Event 2

Abstract Low nitrogen isotopic compositions (δ 15 N) have been reported in deposits from the proto-North Atlantic and western Tethys during Oceanic Anoxic Event 2 (OAE2) at the Cenomanian-Turonian (C-T) transition. It has been proposed that the low δ 15 N values may have resulted from enhancement of N 2 fixation and/or incomplete assimilation of the upwelled ammonium due to expansion of anoxic/euxinic conditions. However, few δ 15 N measurements are available for the sediments from the eastern Tethys and the Pacific and therefore the global significance of the N cycle and its environmental implications remains to be tested. Here, we present δ 15 N data from OAE2 deposits in the Gongzha section in the Tingri area of southern Tibet, which was located on the southern margin of the eastern Tethys during the mid-Cretaceous. The δ 15 N data reveal a large perturbation of the N cycle during OAE2. The pronounced negative shift in δ 15 N prior to OAE2, as well as the persistent low δ 15 N values during OAE2, imply the significant contribution of N 2 fixation. The changes in the marine N cycle suggest that the expansion of an anoxic environment may also have occurred in the southern continental margin areas of eastern Tethys. The coincidence of the marine faunal turnover and the perturbation of the N cycle suggests that the expansion of anoxic water masses may have played an important role in the extinction of marine organisms across the C-T boundary in eastern Tethys.