Paleoceanographic Significance of Cerium Anomalies During the OAE 2 On the NW African Margin

Abstract

Abstract Upper Cenomanian–lower Turonian carbonates from the Preafrican Trough (South Morocco) and Agadir Basin contain biological (benthic and planktonic organisms) and chemical (redox-sensitive trace metals) markers indicating the occurrence of dysoxic conditions with a significant increase in productivity in shallow-water areas on the NW African margin during the Oceanic Anoxic Event 2. The geochemistry of carbonate rocks from three sections along a W-E transect, perpendicular to the margin, is studied here to elucidate the origin of the increased productivity and to test the applicability of cerium anomalies as markers of redox-sensitive and/or indicators of paleoenvironmental or paleoceanographic conditions. The rare earth element (REE) data show typical seawater patterns and negative Ce/Ce* anomalies which appear to indicate the persistence of oxic conditions during the late Cenomanian–early Turonian. This interpretation is in contradiction with that inferred from chemical data (ratios of various redox-sensitive elements to aluminum) and paleontological assemblages. To resolve this paradox, our data are compared with current oceanographic data and examined using a geochemical model that simulates the REE patterns of seawater, specifically the cerium anomaly. There is a satisfactory agreement between measured and calculated REE values for the three studied sections which allows us to distinguish Cenomanian and Turonian water masses. Taking all these geochemical data together, we suggest that the Cenomanian–Turonian stratigraphic interval in the Preafrican Trough and the Agadir Basin records a change of water mass during a transgressive event on an upwelling margin. The intense upwelling of nutrient-rich waters explains the high marine productivity throughout the studied area during the Turonian. The dissolved-oxygen concentrations in waters are not straightforwardly recorded in the trace-metal contents of carbonates. Nevertheless, these data can be deconvoluted by taking account of the saturation state of seawater and the role of particles. Even if negative cerium anomalies in carbonates are classically correlated with dissolved-oxygen concentration, this study proposes that they are also dependent, possibly to a large extent, on particle concentration and water-mass circulation. This would explain why several proxies are required to reconstruct paleo-redox conditions in carbonate facies.