Abstract

The boundary interval between the Cenomanian and Turonian stages of the Upper Cretaceous shows a positive excursion in the carbon isotopic composition of organic matter and carbonate in many epicontinental and oceanic sections of the Atlantic-Tethyan realm. Some of the studied epicontinental sections contain an excursion in the manganese content of carbonates coincident with the shift in carbon isotopic values. In a key reference section at Pueblo, Colorado, the stratigraphic position and magnitude of the manganese excursion are described with respect to the carbon isotopic excursion. Coincident excursions in manganese contents and carbon isotopic values are verified for a section in Normandy, France. The depositional setting for these two sections is persistently to intermittently well oxygenated with the accumulation of interbedded chalky and marly sediments. No shift in manganese values was detected through the isotopic excursion in a section from northern Nebraska, where fossils are preserved only as bedding-plane impressions due to extensive early diagenetic dissolution of carbonate. The carbon-isotopic and manganese excursions begin at the same stratigraphic position, but manganese contents return to background values within the lower half of the carbon-isotopic excursion. The relative timing and duration of these signals suggest the existence of a large manganese reservoir that was rapidly mobilized. Transfer of manganese from deep-sea oxides (Mn (super 4+) ) to epicontinental carbonates (Mn (super 2+) ) is inferred to have occurred during expansion of oceanic oxygen-minimum zones with associated changes in redox gradients and boundaries. Simultaneous excursions in carbon isotopic values and manganese contents can occur at sites of carbonate accumulation when the carbon and manganese cycles are linked through the reducing potential of widespread oxygen-depleted water masses.

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