Orbitally driven redox fluctuations during Cretaceous Oceanic Anoxic Event 2 (OAE2) revealed by a new magnetic proxy

Abstract

Recent studies of the Cretaceous sedimentary records have documented that rapid redox changes between euxinic (sulfidic) and ferruginous conditions occurred during the perturbations to the global carbon cycle associated with oceanic anoxic events. This observation has major implications for the dynamics of and feedbacks on marine primary productivity and organic carbon burial during these events. The high-resolution rock magnetic study of a significantly expanded OAE2 interval in southern Tibet, presented here, provides further insight into the redox dynamics during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) (~94 Ma). The ratio of hard isothermal remanent magnetization to magnetic susceptibility, HIRM/χ, is used as a proxy for bottom-water redox conditions and shows repeated fluctuations between relatively oxic and dysoxic/anoxic conditions in the eastern Tethys of the Southern Hemisphere during OAE2. Spectral analysis of a log (HIRM/χ) time series reveals cyclical variations at orbital frequencies and the subtle cyclic redox changes are found to occur within the obliquity frequency band. Moreover, variability in redox conditions at the onset of OAE2 correlates well with that recently documented for the OAE2 interval at Tarfaya, Morocco, implying that the cyclic redox variations spanned both hemispheres. Notably, the presence of obliquity signals, which dominantly influence high-latitude climate processes, in the tropics and subtropics of both the Northern and Southern Hemispheres further confirms previously hypothesized more vigorous meridional overturning circulation (MOC) during OAE2 and implies the interhemispheric/global nature of this major shift in oceanic circulation.