『Abstract
The depth to which seawater and fresh water circulate through
modern carbonate platforms may be estimated with 87Sr/86Sr
isotope ratios of dissolved Sr2+ that is enriched through
carbonate mineral dissolution and recrystallization. In 23 water
samples from onshore San Salvador Island and Long Island, Bahamas,
carbonate mineral dissolution and aragonite-to-calcite transformations
elevate Sr2+ concentrations to twice seawater values
in water with near seawater salinity, and to about 130 times the
expected value for seawater that has been mixed with fresh water.
carbonate mineral dissolution enriches Ca2+ concentrations
to around 30 times seawater concentrations only in the mixed waters;
water with seawater salinity has approximately seawater Ca2+
concentrations. Assuming two end-member mixing between seawater
Sr2+ and mineral-derived Sr2+, model estimates
indicate that mineral-derived Sr2+ of 19 samples have
87Sr/86Sr ratios equivalent to modern seawater
within error of the measurement, indicating alteration of shallow
buried Late Pleistocene to Holocene carbonate minerals. Four samples
have mineral-derived Sr2+ with 87Sr/86Sr
ratios lower than modern seawater value. These low ratios reflect
alteration of carbonate minerals that were deposited around 1
mybp, although the measured 87Sr/86Sr values
could reflect ages as great as 4.6 Ma considering the analytical
uncertainty of the measurements. These estimated ages are likely
to be minimum values because alteration of modern carbonate minerals
at the surface would provide an unknown, but probably large amount
of Sr2+ with modern seawater isotope signatures, thereby
overprinting any low 87Sr/86Sr ratios of
non-modern mineral-derived Sr2+. Three of the four
samples with low 87Sr/86Sr ratios have high
salinities and were collected from the interior of the islands.
They reflect seawater flow paths at least tens of meters deep
may link the ocean to water several kilometers inland. The fourth
sample is from the fresh-water lens below a Pleistocene beach
ridge (〜125 ka) only 100 m from the shore line. This sample suggests
the lens may be thicker than expected based on estimates of recharge,
hydraulic conductivity and size of the ridge.
Keywords: Carbonate platforms; Sr isotope ratios; Hydrogeology;
Bahamas; Aragonite; Calcite』
1. Introduction
2. study area
3. Sampling and analytical methods
4. Results
4.1. Cl, Ca and Sr concentrations
4.2. Sr isotope ratios
5. Discussion
5.1. Sources of Ca2+ and Sr2+
5.2. Sr isotope ratios and possible ages of carbonate fraction
5.3. Assessment of depth of flow paths
6. Conclusions
Acknowledgements
References