T238 Marine Redox Evolution: Controls and Consequences I

Wednesday, 3 November 2010
8:00 AM - 12:00 PM
Room 203

DIGESTION METHODS FOR TRACE ELEMENT MEASUREMENTS IN SHALES: PALEOREDOX PROXIES EXAMINED

Paper number: T238-11
Program: Technical Sessions
Time: 10:40 AM – 10:55 AM

Authors

  • Guangping Xu Presenting Author
    Colorado State University and Geological Survey of Norway, Fort Collins, CO
  • Judith L. Hannah
    Department of Geosciences, Fort Collins, CO
  • Bernard Bingen
    Geological Survey of Norway, Trondheim, N/A
  • H. Stein
    Colorado State University and Geological Survey of Norway, Fort Collins, CO
  • S. Georgiev
    dual affiliation with AIRIE Program, Department of Geosciences, Colorado State University, Trondheim, N/A

Abstract

Trace elements like U, Mo, Re, V and Ni in marine shales are useful to probe paleoenvironmental conditions. These elements have either a detrital or a seawater source. The seawater contribution is often modeled from bulk-sediment analyses. Using bulk trace element ratios as paleoredox proxies, however, presents limitations. In this study, we compare three different methods of digestion using Middle Triassic black shales from Svalbard and the Barents Shelf (Svalis Dome), and Late Permian black shales from the Mid-Norwegian Shelf and East Greenland. Multi-acid digestion provides whole rock composition. Aqua regia and inverse aqua regia digestions dissolve organic matter, sulfides, carbonates and apatite, but also release trace metals from clay minerals, leaving behind detrital silicates. Thus, trace element concentrations by aqua regia digestions provide a better approximation of the seawater component.

For most trace elements, the concentrations and their ratios derived from aqua regia and inverse aqua regia dissolutions agree within 10%, roughly the uncertainty of the analytical procedures. However, inverse aqua regia may not dissolve apatite completely, which leads to slightly lower Ca, P, Sc, U and Pb than aqua regia dissolution. Whole rock and aqua regia analyses yield comparable abundances of Fe, P, S, Co, Ni, Mo, Cu, Zn and Mn, which are dominantly of seawater origin. However, U, Pb, V, Sc and Cr derived from aqua regia dissolution are lower than those from whole rock dissolution, suggesting significant contributions from both detrital and seawater sources.

We conclude that: (1) trace metals released by aqua regia or inverse aqua regia better represent the seawater components, and thus have value as paleoredox proxies, (2) when using the absolute trace metal concentrations to infer the paleoredox conditions, the total amount of organic carbon and sulphur needs to be taken into account, and (3) values for trace metal ratios (such as Th/U, V/Sc, V/V+Ni) based on whole rock composition are not globally applicable to infer redox conditions, as these ratios reflect a combination of both detrital and seawater components.

Funding: NFR Petromaks 180015/S30, Statoil, Eni Norge.