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(26)M 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

Carbonate diagenesis in a high transmissivity coastal aquifer, Biscayne Aquifer, southeastern Florida, USA

Cores collected through the Biscayne Aquifer (Plio-Pleistocene) in Hollywood, southeastern Florida, as part of the Hollywood Coastal Salinity Barrier Project provided an opportunity to examine the diagenesis of limestones and sandstones at the meteoric to marine water transition of one of the most transmissive aquifers in the world. The saline water front, and thus coastal mixing zone, has migrated landward approximately 1 km in the Hollywood area as the result of wellfield withdrawals. No changes in mineralogy (such as dolomitization), cement types and abundances, paragenetic sequence, or porosity are evident that can be correlated with the current or likely pre-development (wellfield withdrawals) location of the mixing zone. An approximately 2parts per thousand downhole increase in calcite delta(18)O values is present in a core (HMW-6D) that penetrates the pre-development mixing zone, which may be related to either a down hole increase in salinity or to the interaction of meteoric waters with marine carbonate sediments during calcite cementation. The Biscayne Aquifer in Hollywood is currently a relatively quiescent diagenetic environment. The limited current diagenesis appears to consist of the dissolution of trace skeletal aragonite remaining in the aquifer, as suggested by a meteoric water Sr/Ca ratio similar to that of molluscan aragonite. It is proposed that a 'punctuated equilibria' model may be applicable to diagenesis in the Biscayne and other aquifers, in which limestones and sandstones entered a long period of diagenetic stasis after a period of relatively rapid textural and mineralogical stabilization