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Abstract
Carbon-isotopic compositions of total carbonate, inoceramid carbonate, micritic carbonate,
secondary cements, total organic carbon, and geoporphyrins have been measured in 76
different beds within a 17-m interval of a core through the Greenhorn Formation, an
interbedded limestone and calcareous shale unit of Cretaceous age (Cenomanian-Turonian)
from the Western Interior Seaway of North America. Results are considered in terms
of variations in the processes of primary production (which led to the biosynthesis
of the molecular precursors of the geoporphyrins) and in secondary processes (those
mediating the transformation of primary organic material into sedimentary total organic
carbon). It is shown that the porphyrin isotopic record reflects primary isotopic
variations more closely than the TOC isotopic record, and that, in these sediments,
TOC is enriched in 13C relative to its primary precursor by 0.6 to 2.8%. This enrichment
is attributed to isotope effects within the consumer foodweb and is associated with
respiratory heterotrophy. Variations in this secondary enrichment are correlated with
variations in the isotopic composition of marine carbonate. This correlation is attributed
to effects of environmental changes on the marine foodweb. These may have included
increased atmospheric oxygen associated with the Cenomanian-Turonian oceanic anoxic
event. The isotopic fractionation associated with fixation of carbon by primary producers
is observed to have varied by 1.5% during the interval of deposition. It is suggested
that this change is due to a variation in the makeup of the community of primary producers
and/or to a decrease in the atmospheric abundance of CO2 during the oceanic anoxic
event.