科研之友
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摘要
The use of molybdenum isotope data (delta Mo-98) from organic-rich shales to draw inferences concerning marine paleoredox conditions at a global scale is predicated upon the assumptions of (1) a residence time of Mo in seawater much greater than the ocean mixing time, and (2) quantitative removal of Mo from a strongly euxinic ([H2S](aq) %26gt; 11 mu M) water column to the sediment, thus preserving the seawater delta Mo-98 signature. In this study we analyze Mo isotopic variation in the Hushpuckney Shale, a 73-cm-thick unit representing the late transgressive to early regressive stages of a glacio-eustatic cyclothem (Swope Formation) deposited in the Late Pennsylvanian Midcontinent Sea (LPMS) of North America. The Hushpuckney can be subdivided into four stratigraphic zones of distinctive geochemical character. Zones I and III, which accumulated under weakly euxinic conditions, acquired relatively high delta Mo-98 values (+0.9 to +1.1 parts per thousand), whereas Zone II, which accumulated under intensely euxinic conditions, acquired lower delta Mo-98 values (similar to+0.6 parts per thousand). Zone IV, which accumulated under suboxic conditions in the water column, acquired the heaviest delta Mo-98 values ( +1.1 to + 1.8 parts per thousand). These results contrast with the pattern of redox - delta Mo-98 covariation in modern marine environments, in which the heaviest delta Mo-98 values are found in the most intensely euxinic facies. %26lt;br%26gt;We evaluate three different hypotheses to account for the Mo isotopic patterns of the Hushpuckney Shale. One hypothesis, seawater-freshwater mixing, is rejected owing to isotopic mass balance considerations. A second hypothesis is a local control on delta Mo-98 by water-column redox cycling of Mn, with particulate Mn-oxyhydroxides adsorbing isotopically light Mo and transferring it to the sediment, a process that was most active during deposition of Zone II. The significance of this scenario is that euxinic black shales may not reliably record global seawater delta Mo-98 in areas where a Mn-particulate shuttle is operative. A third hypothesis is based on rapid secular variation of the Mo isotope composition of Late Pennsylvanian global seawater. In order to account for delta Mo-98 trends within the Hushpuckney Shale, seawater delta Mo-98 must have varied by similar to 1.2 parts per thousand at a similar to 100-kyr timescale, which would have been possible only if the residence time of Mo in Late Pennsylvanian seawater was %26lt; 100 kyr. Although both the second and third hypotheses are viable based on the present limited delta Mo-98 dataset, we discuss how each model might be tested through additional Mo isotope data.
- 出版日期2012-9-24
