TRACE METAL MOBILITY/SEDIMENT MATURITY IN BLACK SHALES DERIVED FROM VARIATIONS OF CARBON PROXY VALUES: V, Mo, Zn
Pangloss Foundation, 1735 Highland Place
Berkeley, CA 94709
Timothy W. LYONS
Department of Geological Sciences, University of Missouri
Columbia, MO 65211 firstname.lastname@example.org
Mary S. QUINBY-HUNT
Lawrence Berkeley Laboratory
Berkeley, CA 94720 email@example.com
Various authors have reported on the high correlation of carbon with such trace metals as V, Mo, and Zn, in many black shales. First order regressions of these correlations vary in slope and intercept between various sample sets even for the same metal. Considering that each metal presumably shows differential mobility as a function of mineralogy, anion availability, redox conditions etc. even under the same environmental conditions, this is not surprising. Also, estimates for the original organic carbon differ when regression equations for different metals from the same data set are applied to other sample suites. Such differences may reflect relative mobility of the metals as a function of sediment maturity.
We have calculated apparent TOC % for the Paleozoic Marine Sciences Group data base using Mo/Al, V/Al, and Zn/Al regressions to test the validity of this approach in various black shale settings in time. The sample base is limited to Group IV black shales (Mn < 260 ppm, Fe < 36000 ppm, V > 330 ppm) or high values of V, Mo, Zn, and relatively low values of Fe and Mn. Based on Eh-pH diagrams such samples should show the most reduced initial conditions. This approach should be used to span the greatest possible environmental variation over geologic time in order to assess both the relative mobility of the metal and the degree of thermal maturation of sediment during carbon transport.
In general, V/Al and Zn/Al proxies have the highest statistical confidence and give the highest TOC values compared to those based on Mo/Al. This suggests that V and Zn are relatively more stable once deposited than Mo. The difference in calculated TOC values can be used to back calculate the 'original' metal value which may be used to estimate its mobility and by comparison with other parameters (DOP, C/S etc.) the maturity of the particular sample or suite of samples. Unquestionably, the relationship with metal-sulfur proxies must be considered.
The long term goal is to understand the patterns and mechanisms reflected in the various metal-TOC regressions spanning a range of stratigraphic units across geologic time in a consistent geochemical manner.