『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 (Mn4+) bto
epicontinental carbonates (Mn2+) is inferred to have
occurred during expansion of oceanic oxygen-minimum zones with
associated changes in redox gradients and boundaries. Simultaneous
excursion 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.』
Background
Cenomanian-Turonian carbon isotope excursion
Manganese enrichment in carbonate rocks
Geochemical methods
Isotope values
Trace element values
Cenomanian-Turonian boundary sequence of Haute Normandy
Lithology
Isotopic data
Trace element distribution: The manganese event
Cenomanian-Turonian boundary sequence in U.S. Western Interior
The Pueblo section
Lithology
Isotope excursion
Manganese
The Ponca State Park section
Lithology
Isotopic excursion
Manganese and other trace elements
Discussion
Conclusion
Acknoeledgments
References