PALEOPRODUCTIVITY AND GEOCHEMICAL PROXIES: INDICATORS OF WATER-COLUMN CONDITIONS, FRASNIAN-FAMENNIAN BOUNDARY, NEW ALBANY SHALE, INDIANA

The Frasnian-Famennian Late Devonian boundary is marked by episodes of organic matter (OM) burial. This initial study of 30 New Albany Shale core samples from the Illinois Basin investigates the various controls on carbon production, accumulation, and preservation (i.e., carbon sequestration) within a low-latitude intracratonic seaway. Abrupt changes in geochemical and biotic proxies for OM type and environmental conditions across the Frasnian-Famennian boundary coincide with a distinct lithological change, characterized by laminated, brownish-black Famennian shales unconformably overlying alternating bioturbated, greenish-gray and non-bioturbated, dark-gray Frasnian mudstones.

In addition to high concentrations of major elements Al, Na, Mn, and Ti, Frasnian green-gray shales are enriched in trace metals: Nb, Ru, Th, Ba, and La. Si and K values are slightly more elevated than those in the overlying Famennian shales whereas C_org, P, Ni, and C/N values are much lower. Oxic bottom-water conditions are indicated by low trace-element redox indicators (Mo, Ni+Co, V/Cr). Acritarchs, considered remains of normal marine phytoplankton, dominate the samples.

Trace-metal and major elemental data confirm major shifts in water–column conditions across the boundary. Famennian shales contain significant amounts of C_org, P, and N, and have elevated C/N ratios. The shales are enriched in trace metals: U, Mo, Cu, Ni, Pb, V, and Zn. High Mo contents and V/Cr and V/(V + Ni) redox proxies imply dysoxic-anoxic conditions during sediment accumulation. Green algae (notably Tasmanites) predominated surface waters. Sporadic Zn and low La concentrations as well as rare spores and wood, and the small size of black phytoclasts (<20 µm) indicate a distal, basinal setting during the Famennian.

Changes in C/N, δ¹³C, and C_org values across the boundary suggest metal enrichment may be dependent upon OM type as well as water-column conditions. This is directly related to a significant shift from acritarch- to prasinophyte-sourced OM. Biogeochemical proxies and palynological data signify a direct influence by phytoplankton on nutrient and elemental distributions, as well as a direct response by phytoplankton to changing paleoceanographic chemistry during times of black shale formation in the Upper Devonian Illinois Basin.