Influence of water column anoxia and sediment supply on the burial and preservation of organic carbon in marine shales

Abstract

Previous work has suggested that the laminated, organic-rich and bioturbated, organic-poor shales of the Camp Run Member of the Late Devonian-Early Mississippian New Albany Shale formed under anoxic and oxygenated bottomwater conditions, respectively, and that the interbedding of the two faciès was due to the vertical oscillation of a water-column anoxic/oxic boundary where it impinged on the basin margin. We have extended this analysis by examining the chemical and mineralogical differences between the two shale facies in a single borehole core, by seeking evidence for deposition of the laminated shales under bottom-water oxia or anoxia, and by determining whether the laminated shales formed when the carbon supply to the sea floor was higher. The results of this study show that the laminated and bioturbated shales are mineralogically and chemically distinct; relative to Al, an index of the aluminosilicate content, Si, Ti, Fe, P, Na, Ba, Co, Cr, Cu, Mo, Ni, V, Zn, and Zr are all higher, whereas Mn, Ca, Mg, and Sr are lower in the laminated compared with the bioturbated shales. The differences are due to a higher quartz, feldspar, titanite/ilmenite, and zircon content in the laminated shales, probably indicating a coarser grain-size, and the greater abundance of manganoan calcite in the bioturbated shales. Dissolved oxygen was present in bottom waters during the deposition of some of the laminated shale intervals because of the presence of manganoan calcite, a phase that can only form in sediments with an oxic surface. In addition, the organic matter preserved in the two shale types is isotopically different; δ 13C organic values are 1.9z.permil; lighter on average in the laminated compared with bioturbated intervals, possibly indicating a larger fraction of terrestrial organic matter in the latter. δ 15N values are 1.9z.permil; lighter on average in laminated compared with bioturbated intervals, possibly indicating a larger fraction of terrestrial organic matter in the latter. δ 15N values are 1.9z.permil; lighter on average in laminated compared with bioturbated intervals, suggesting that nutrient drawdown was less during the deposition of the organic-rich, laminated shales. The chemical, mineralogical, and isotopic contrasts between the two shale facies of the Camp Run Member indicate that the conditions of sedimentation were different during their deposition. The difference was possibly related to variations in sea level, which would have caused the Camp Run shoreline to move closer to and farther from the core site, causing, in turn, the deposition of coarser and finer grained sediments that contained different mixtures of marine and terrestrial organic matter. Bottomwater conditions were anoxic during deposition of most laminated intervals. Bottom-water anoxia or dysoxia led to decreased burial and preservation of the essential nutrient phosphorus in the laminated, organic-rich shales relative to the rocks deposited beneath better oxygenated bottomwaters. Increased availability of phosphorus in the water column on long timescales leads to increased productivity and a higher settling flux of organic matter, causing bottom-water oxygen levels to fall. This is consistent with the nitrogen isotope evidence suggesting that production was probably higher during the deposition of the organic-rich shales. Thus, production variations coupled with enhanced sedimentary regeneration of phosphorus from sediments related to low oxygen bottom-water concentrations provide a general mechanism for the formation of the alternating facies of this member of the New Albany Shale.