John P Jasper

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

Abstract 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.

The relationship between sedimentary organic carbon isotopic composition and organic biomarker compound concentration

A study of organic biomarker compounds which could serve as tracers of terrigenous and marine sedimentary organic matter sources was performed on samples from a 208.7 m hydraulic piston core hole (DSDP Hole 619) from the hemipelagic Pigmy Basin in the northern Gulf of Mexico. Organic carbon-normalized concentrations of total long chain (C37–C39) alkenones and some individual C27–C29 desmethyl sterols were determined to be useful proportional indicators (tracers) of preserved marine and terrigenous organic carbon, respectively. The alkenones, whose only known source is marine phytoplankton of the class Prymnesiophyceae, generally occurred in higher concentrations in interglacial isotope stages 1 and 5a-b than in the intervening glacial stages. Sterols (C27-C29), apparently of a dominantly terrigenous origin, occurred in lower concentrations during interglacial stages than in glacial stages. Tracers of both terrigenous and marine organic matter appear to be affected by the differential diagenetic alteration of the biomarker/Corg ratios, as indicated by a simple, first-order kinetic model. The lack of any desmethylor 4a-methylsterol which is linearly related to the proportion of marine sedimentary organic matter (as scaled by δ13Corg) indicates that either n1. n(1) sedimentary diagenesis has obscured the biomarker/Corg vs. δ13Corg record n n2. n(2) phytoplanktonic assemblage changes caused variations in the biomarker/Corg ratio of the primary input. n n nPreferential preservation of terrigenous sterols may result in a biased sedimentary record of sterol input which could be misinterpreted as indicating solely terrigenous sterol sources. A simple model which characterizes the effects of sedimentary diagenesis on the relationship between Corg-normalized biomarker ratios and δ13Corg demonstrates the potential problems of long-term, differential-diagenetic skewing on those tracer records.

Migration of C1-C8 hydrocarbons in marine sediments

Abstract Light hydrocarbon (C 1 -C 8 ) profiles are compared for three wells of varying maturities: two immature DSDP wells (Site 397 near the Canary Islands and Site 530A near the Walvis Ridge in the south-east Atlantic) and a mature well, the East Cameron well in the Texas Gulf Coast. Primary migration of C 1 and C 2 appears to be occurring in all of the sedimentary rocks examined. Primary migration of C 3+ components becomes important only as fine-grained sedimentary rocks enter the catagenetic hydrocarbon generation zone or over short distances in more permeable sections. Lateral migration along bedding planes was more important than vertical migration in sedimentary rocks of all maturities. The lightest (methane, ethane and propane gases) hydrocarbon show greater fractionation than do the C 4 -C 8 alkanes which generally show minimal fractionation during the migrational process. Subsurface diffusion coefficients for these p.p.b. quantities of C 2 -C 5 alkanes from immature sediments from DSDP Site 530 are estimated to be several orders of magnitude less than values reported in the literature for diffusion of much larger amounts of these compounds from mature water wet sediments into air or sandstones. Since our calculations suggest light hydrocarbons are present in amounts less than their reported solubilities in pure water at 25°C, we postulate that the sediment organic matter has a substantial effect on retarding the movement of these light hydrocarbons.