The composition of the continental river weathering flux deduced from seawater Mg isotopes

This study establishes the magnesium isotopic composition of seawater and evaluates its constancy as a function of depth and geographic location. It also provides results on the magnesium isotopic composition of river water samples draining specific lithologics. In combination, the results are used to obtain an average magnesium isotopic composition of +1.54 parts per thousand for the global continental weathering flux, compared to the +2 parts per thousand value of the global ocean (both relative to NIST SRNI 980). Analyses of modern and fossil carbonate-secreting echinoderms suggest that these can serve as archives of seawater delta M-26 values. It is demonstrated that these records of the magnesium isotopic composition of ancient oceans can be used to make important inferences about the relative contribution of different lithologies to the global continental weathering flux, particularly carbonate versus silicate weathering. Preliminary results suggest that the composition of the continental weathering flux has not undergone dramatic changes during the Phanerozoic. (c) 2004 Elsevier B.V. All rights reserved.