(Note: reprints /PDF files of all most cited UMCP papers are currently available upon request)

5. STABLE MOLYBDENUM ISOTOPES AS TRACERS OF RECENT AND ANCIENT REDOX CONDITIONS IN OCEANS AND ESTUARIES.

Molybdenum isotopes provide a powerful means to trace paleoenvironmental changes, specifically those changes related to oxygen depletion in water columns. The prevalence of anoxic signatures in geologic and historic sediment records indicates that oxygen depletion has been a significant and long standing problem for life over much of geologic time. The ability to track and interpret episodes of environmental stress using a simple this geochemical tool has already attracted wide scientific interest over its first decade of use. The power of the Mo isotope system lies in the relative simplicity of its behavior in oxygenated, versus anoxic, water. In the former, Mo is present as a soluble form (molybdate), while in the latter, insoluble forms (oxythio- and thiomolybdates) occur.

Using a double spike method, coupled with multi-collector ICP-MS analysis, we have undertaken to answer several questions using the Mo isotope system. The first of these projects is essentially right in our Maryland backyard-- the ongoing problem of coastal eutrophication and seasonal anoxia in Chesapeake Bay. In the Chesapeake Bay, coastal eutrophication is linked to an increase in severity and occurrence of summer anoxic episodes since ~1900. We have obtained Mo isotope data for sediments from multiple locations within and around the bay, as well as samples from two cores in the main channel of the Bay. To support conclusions drawn from the sediment samples, water samples from the Bay and its main tributary, the Susquehanna River, were also collected and analyzed (see figure below). Collectively, the samples support the notion that suboxic conditions have increased in severity through the last century.

An additional study currently underway that exploits the strength of Mo isotopes as a tracer of anoxia involves the formation of organic-rich sediments (sapropels) within the Mediterranean basin. The sapropels appear cyclically, in tune with Northern Hemisphere orbital insolation maxima. Their formation is therefore thought to be driven by the influence of climatic factors on the basin. Climatic shifts stimulated the development of anoxic episodes, considered by some previous studies to be the most important event involved in sapropel formation. We have obtained samples from Ocean Drilling Program Leg 160, Site 969D. These samples cover the time frame surrounding the Pliocene-Pleistocene boundary. Our intent is to use Mo isotopes from the sapropels to assess the changing redox conditions in the Mediterranean basin, and the nature of the water column during 'normal' and sapropel deposition. Sapropels have anomalously high concentrations of trace metals, and part of this work addresses the question of the source of the Mo to the basin, as well as questions of possible changes in the Mo isotopic composition of the water during intense Mo drawdown.

Top: Salinity (psu) versus Mo concentration (μg/L) for water samples from the Chesapeake Bay. Solid line is a 3rd-order polynomial regression between Susquehanna River water (SRW) and salinity normalized seawater (Collier, 1985). The relationship between Mo concentration and salinity is non-linear, as shown by the deviation from a linear trend (dashed line). This implies that removal of Mo is taking place in the Chesapeake Bay, particularly between salinities of 7-20 psu. Water with that range of salinities overlies the seasonally anoxic main channel.

Lower: Salinity (psu) versus Mo isotopic composition for water samples from the Chesapeake Bay. Solid line is a sigmoid curve fit to the data with 95% confidence intervals (dotted lines). The δ98Mo value of Chesapeake Bay water is dominated by seawater.

Ph.D. student Kate Scheiderich collecting core samples for Mo isotope analysis at the Indiana Geologic Survey, 2007.

To learn more about our research concerning the application of molybdenum isotopes to the study of the chemical evolution of the oceans, please refer to:

Pietruszka A.J., Walker R.J. and Candela P.A. (2006) Determination of mass dependent molybdenum isotopic variations by MC-ICP-MS: an evaluation of matrix effects. Chem. Geol. 225, 121-136.

Scheiderich K., Helz G.R. and Walker R.J. (2010) Century-long record of Mo isotopic composition in sediments of a seasonally anoxic estuary (Chesapeake Bay). Earth Planet. Sci. Lett. 289, 189-197.

Scheiderich K., Zerkle A.L., Helz G.R., Farquhar J. and Walker R.J. Molybdenum isotope, multiple sulphur isotope, and redox-sensitive element behaviour in early Pleistocene Mediterranean sapropels. Chemical Geology, in review.

Last Revised September 2010.