THE SOURCES AND DEPOSITION OF ORGANIC-MATTER IN THE LATE QUATERNARY PYGMY BASIN, GULF OF MEXICO

The concentration and carbon isotopic composition (δ13C) of sedimentary organic carbon (Corg), N/C ratios, and terrigenous and marine δ13Corg endmembers form a basis from which to address problems of Late Quaternary glacial-interglacial climatic variability in a 208.7 m hydraulic piston core (DSDP 619) from the Pigmy Basin in the northern Gulf of Mexico. While interpretations of sedimentary δ13Corg time series records are often not unique, paired analyses of δ13Corg and N/C are consistent with the hypothesis that the Corg in the Pigmy Basin is a climatically determined mixture of C3-photosynthetic terrigenous and marine organic matter, confirming the earlier δ13Corg model of Sackett (1964). A high resolution (∼ 1.4-2.9 Ka/sample) δ13Corg record shows that sedimentary organic carbon in interglacial oxygen isotope (sub)stages 1 and 5a-b are enriched in 13C (average ±1 σ values are -24.2 ± 1.2%o and -22.9 ± 0.7%o relative to PDB, respectively) while glacial isotope stage values 2 are relatively depleted (-25.6 ± 0.5%). Concentrations of terrigenous and marine sedimentary organic carbon are calculated for the first time using δ13Corg and Corg measurements, and empirically determined terrigenous and marine δ13Corg endmembers. The net accumulation rate of terrigenous organic carbon is 4.3 ± 2.6 times higher in isotope stages 2-4 than in (sub)stages 1 and 5a-b, recording higher erosion rates of terrigenous organic material in glacial times. The concentration and net accumulation rates of marine and terrigenous Corg suggest that the nutrient-bearing plume of the Mississippi River may have advanced and retreated across the Pigmy Basin as sea level fell and rose in response to glacial-interglacial sea level change. © 1990.