17 Multi-Proxy Investigations of Black Shales: What Can They Tell Us?

Thursday, 20 April 2006
1:40 PM - 3:20 PM

DEVELOPING A MORE ROBUST GEOCHEMICAL FRAMEWORK FOR INFERRING PALEOREDOX CONDITIONS DURING BLACK SHALE ACCUMULATION: FOCUS ON THE OHIO AND NEW ALBANY SHALES

Paper number: 17-1
Program: THEME SESSIONS
Time: 1:40 PM – 2:00 PM

Authors

Abstract

The widespread deposition of black shale facies during key episodes of Earth history documents distinct changes in paleoceanographic conditions that necessarily represent depositional responses to tectonic and/or climatic evolution. Multiple driving mechanisms have been invoked in order to explain the occurrence of black shale formation, including 1) increases in the rate and/or spatial extent of primary productivity associated with increases in nutrient input, 2) increases in organic preservation prompted by ocean anoxia and/or increases in clastic burial rate, and 3) a mixture of mechanisms 1 and 2. Defining the fundamental significance of marine black shales in the geologic record and evaluating the potential biogeochemical impact on ocean chemistry and productivity require the integration of various datasets, a working knowledge of modern ocean processes (productivity, particle transport, depositional environments, burial rates), and the assumption that the ancient ocean was not entirely unlike the modern ocean.

One key requirement for assigning driving mechanisms of black shale formation is to develop the paleoredox framework, i.e., to define the redox state of the system during black shale accumulation. The (untenable) goal is to construct a quantitative index of redox state in the formation of interest. Developing a robust, qualitative index is a more tenable aspiration. Recent studies of marine black shale accumulation generally take an integrative methodology, coupling geochemical and sedimentological approaches in order to unravel changes in paleoredox. The purpose of the present research is to define down-core changes in concentration of redox-sensitive trace elements (RSTE) for sections of the Ohio and New Albany shales in the context of their significance as tracers of redox conditions. Specifically, an attempt is made to partition the terrestrial, planktonic, and seawater fractions of several RSTE in order to infer their enrichment as a result of 1) organic deposition, and 2) a shift in redox state. The partitioning of RSTE fractions may yield important insight for differentiating productivity- versus preservation-driven conditions.