A molecular evaluation of bulk organic carbon-isotope chemostratigraphy for terrestrial correlations: An example from two Paleocene–Eocene tropical sequences

Abstract

The dynamics associated with the carbon cycle and the linkage between the oceans, the atmosphere, and land plants provide an opportunity to correlate marine and terrestrial sedimentary sequences using stable isotopes of carbon ( δ 13C), but few studies have tested this approach. For instance, it has been proposed that changes in plant community (e.g., gymnosperm-dominated vs. angiosperm-dominated) could have significantly altered/amplified the carbon-isotope ratios of bulk sedimentary organic matter derived from land plants ( δ 13C TOM), compared to that of the marine carbonates ( δ 13C carbonate). Here, δ 13C TOM values in a terrestrial sequence of the Colombian tropics are compared to the composite Paleocene–Eocene δ 13C carbonate curve from Zachos et al. (2001) to evaluate the use of δ 13C TOM values as a reliable chronostratigraphic tool. Sediments of the studied terrestrial sequences were deposited in a transitional setting dominated by mudstones, coals, and small lenses of sandstones (Late Cretaceous–Middle Paleocene sediments) and in a mixture of deltaic and fluvial conditions (Late Paleocene–Early Eocene sediments). The biostratigraphic control was based on palynological zones for the region. The δ 13C TOM values for the studied terrestrial sequence show three carbon-isotope excursions, which correlate closely with those present in the marine record. The δ 13C TOM values decreased from − 24.2‰ to − 26.5‰ in sediments accumulated during Early to Middle Paleocene, increased from − 26.5‰ to − 23.8‰ during the Late Paleocene, and decreased from − 23.8‰ to − 26.5‰ near the Paleocene–Eocene boundary (52–50 Ma). Selected biomarkers indicate that most of the organic matter derived from both gymnosperms and angiosperms. Moreover, the analyses of selected biomarker ratios (CPI, Pr/Ph, P aq, and ββ/ββ + αβ hopanes) show some diagenetic transformation in the preserved organic matter. However, no correlation between diagenesis and δ 13C TOM values was detected, thus suggesting that δ 13C TOM could be correlated with δ 13C carbonate values. The close correspondence that was found between δ 13C TOM and δ 13C carbonate values (Δ 13C TOM–carbonate ~ − 27‰) provides support to the hypothesis that a tight land-plant–oceans linkage exists through geologic timescales via atmospheric carbon dioxide.