『Abstract
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
lithologies. In combination, the results are used to obtain an
average magnesium isotopic composition of +1.54‰ for the global
continental weathering flux, compared to the +2‰ value of the
global ocean (both relative to NIST SRM 980). Analyses of modern
and fossil carbonate-secreting echinoderms suggest that these
can serve as archives of seawater δ26Mg 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 ha not undergone dramatic changes
during the Phanerozoic.
Keywords: Magnesium isotopes; Echinoderms; Phanerozoic; River
water; Continental weathering』
1. Introduction
2. Methods
2.1. Water samples
2.2. Echinoderm samples
2.3. MC-ICP-MS magnesium isotope measurements
3. Results and discussion
3.1. The δ26Mg composition of modern seawater
and river water samples
3.2. The δ26Mg composition of echinoderms, modern
and Phanerozoic
3.3. The δ26Mg composition of the continental weathering
flux and Mg mass balance
3.4. The Phanerozoic seawater δ26Mg record and implications
for weathering
4. Conclusion
Acknowledgements
References