The sources and deposition of organic matter in the Late Quaternary Pigmy Basin, Gulf of Mexico

Abstract

The concentration and carbon isotopic composition (δ 13C) of sedimentary organic carbon (C org), N/C ratios, and terrigenous and marine δ 13C org 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 δ 13C org time series records are often not unique, paired analyses of δ 13C org and N/C are consistent with the hypothesis that the C org in the Pigmy Basin is a climatically determined mixture of C 3-photosynthetic terrigenous and marine organic matter, confirming the earlier δ 13C org model of Sackett (1964). A high resolution (∼ 1.4–2.9 Ka/sample) δ 13C org 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 δ 13C org and C org measurements, and empirically determined terrigenous and marine δ 13C org 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 C org 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.