Major and minor element geochemistry of black shales from the Cenomanian Turonian Boundary Event (CTBE): Paleoenvironmental constraints
ICBM, University of Oldenburg, P.O.Box 2503, D-26111 Oldenburg, F.R.Germany
Drastic global changes in floral and faunal assemblages, climate, sea level, oceanic circulation patterns, oxygen levels of deep and intermediate waters, redox state at the sediment/seawater interface, etc. should be reflected in variations of major and minor element abundances of sedimentary sequences. To distinguish changes in terrigenous background sedimentation from dilution effects caused by biogenic carbonate or silica, element ratios should be used instead of absolute data. This requires the analysis of all major elements by analytical techniques of high precision and accuracy at high stratigraphic resolution. Furthermore great care has to be taken during sample collection. Materials from outcrops often are influenced by weathering effects, i.e. organic matter and suphides are easily oxidised and may alter element abundances.
Major element variations in sedimentary sequences often are associated with changes in clay mineralogy reflecting different weathering conditions in the source areas of the terrigenous load or variations in transport conditions. These signals often are altered by diagenesis. In contrast to most major element ratios, minor element ratios are more affected by paleoenvironmental changes. This seems to be directly related to the higher relative abundance of certain minor elements in seawater compared to solid phases.
Cretaceous black shales from the Cenomanian/Turonian Boundary Event (CTBE or OAE 2) are characterized by the enrichment of specific trace metals, in particular Ag, Cd, Mo, Zn, V, Cu, Ni, and others. Under present day oceanographic conditions many of these metals are involved in nutrient-type regeneration processes. For this reason elements like Cd, which shows a depth distribution similar to P in seawater, are only accumulating in areas of high productivity and rapid burial and not in deep-sea environments.
The occurrences of many CTBE black shales in deep-water settings and the combination of low sedimentation rates and high organic carbon (TOC) and trace metal contents favour deposition of these strata under severe oxygen depletion in the ocean. Water column regeneration must have been suppressed and early diagenetic (Mo, V, U) as well as authigenic (metal-sulfide precipitation) element enrichment processes are important factors. An extreme example for this "sapropel-type" black shale has been recovered at ODP Site 641A. At this deep-water site metal concentrations, in particular Mo, Cd, Zn, etc., are among the highest ever reported for any CTBE occurrence.
In contrast to this paleoenvironmental model, large parts of the open Tethys have probably been characterized by an expanded oxygen-minimum-zone (OMZ). This zone may have served as a conveyor belt for the transport of anoxic waters, enriched in elements like Mn, from the quite enclosed Proto-Atlantic towards the Proto-Pacific. An example is given from ODP Legs 122 and 123 off Australia, where TOC-rich sediments have been recovered from relatively shallow paleodepths within the OMZ and Mn-rich sediments from below this zone. The sediments are extremely enriched in Mn, Co, V, and Cu, but much less pronounced in Cd, Zn, and Pb. These sediments represent exceptional deep-sea clays and are indicative of oxic conditions in the deep waters in the open Tethys during this time interval.
Simple mass balance considerations reveal that seawater alone does not explain element abundances in CTBE black shales. Fluvial and hydrothermal (e.g. Zn) inputs are important metal sources. By making assumptions about the metal input rates, an estimate for the duration of the CTBE (0.3 to 1 Ma) can be given.
A possible scenario for the development of widespread black shales during the Mid-Cretaceous seems compatible with an increase in volcanic activity (mantle plume hypothesis?). Volcanism provides CO2 for global warming (greenhouse-effect) and is essential for the excess TOC burial observed. Due to the warmer climate circulation in the Proto-Atlantic became more restricted and anoxic conditions could develop. At the same time the increase in seafloor spreading rate induced transgressive pulses, thus enlarging schelf area and diminishing pelagic sedimentation.