IRON ISOTOPE EVIDENCE FOR RAPID CHANGE OF THE REDOX STATE OF THE LATE CRETACEOUS OCEANS

The Upper Cretaceous oceanic sediments record a major change from organic-rich black shales formed during Oceanic anoxic event 2 (OAE2) to Upper Cretaceous oceanic red beds (CORBs). Iron (Fe) isotope compositions of 43 samples from the Chuangde section (CD) in southern Tibet and ODP Leg 103 Site 641 A in North Atlantic (NA641) have been measured to constrain their deposition environment and to trace Fe geochemical cycling of the Late Cretaceous ocean. The OAE2 grey shales from the CD section have elevated mean δ⁵⁶Fe_IRMM-14 value of 0.191±0.073‰ (1SD, N = 10) with depleted Fe/Al ratios compared to lithogenic background, recording the loss of isotopically light Fe from the sediments by reduction in an anoxic environment. By contrast, the overlying red shales have uniformly constant δ⁵⁶Fe values of 0.101±0.017‰ (1SD, N= 7) with relatively elevated mean Fe/Al ratios of 0.79, showing a δ⁵⁶Fe versus Fe/Al correlation observed in modern marine sediments deposited in an oxic deep shelf below oxygen minimum zones (OMZ) in open oceans. Hence, δ⁵⁶Fe versus Fe/Al correlations from the CD section reveal a deposition environment change from an anoxic to oxic shelf below OMZ, and thus might record an OMZ shoaling process during the transition from OAE2 to CORBs. By contrast, the NA641 section has a different δ⁵⁶Fe versus Fe/Al correlation. Organic-rich black shales of OAE2 have negative mean δ⁵⁶Fe of -0.190±0.092‰ (1SD, N= 2) with elevated Fe/Al of 0.6. These shales may be deposited in an euxinic environment with net input of isotopically light Fe from anoxic shelf or hydrothermal fluids. The lower interval of the red shales have variably negative δ⁵⁶Fe from -0.400±0.042‰ (2se) to -0.008±0.048‰ (2se) with the Fe/Al_mean=0.62, probably reflect an oxic deposition environment below OMZ in a relatively restricted basin. The upper interval of the red shales has constant δ⁵⁶Fe values (0.039±0.010‰, 1SD, N= 7) with slight Fe enrichment, indicating an oxic deposition environment below OMZ in an open ocean. Thus, δ⁵⁶Fe versus Fe/Al correlations of NA641 suggest a deposition environment gradually changing from euxinia during OAE2 to oxic state after OAE2, and may record the evolution of North Atlantic from a restricted basin to an open marine in Late Cretaceous.