『Abstract
Subterranean estuaries are characterized by the mixing of terrestrially
derived groundwater and seawater in a coastal aquifer. Subterranean
estuaries, like their river water-seawater counterparts on the
surface of the earth, represent a major, but less visible, hydrological
and geochemical interface between the continents and the ocean.
This article is the first in a two-part series on the biogeochemistry
of the subterranean estuary at the head of Waquoit Bay (Cape Cod,
MA, USA). The pore-water distributions of salinity, Fe and Mn
establish the salt and redox framework of this subterranean estuary.
The biogeochemistry of Fe, Mn, P, Ba U and Th will be addressed
from the perspective of the sediment composition. A second article
will focus on the groundwater and pore-water chemistries of Fe,
Mn, U and Ba. Three sediment cores were collected from the head
of Waquoit Bay where the coastal aquifer consists of permeable
sandy sediment. A selective dissolution method was used to measure
the concentrations of P, Ba, U and Th that are associated with
“amorphous (hydr)oxides of iron and manganese” and “crystalline
Fe and Mn (hydr)oxides.” The deeper sections of the cores are
characterized by large amounts of iron (hydr)oxides that are precipitated
onto organic C-poor quartz sand from high-salinity pore waters
rich in dissolved ferrous iron. Unlike Fe (hydr)oxides, which
increase with depth, the Mn (hydr)oxides display midcore maxima.
This type of vertical stratification is consist with redox-controlled
diagenesis in which Mn (hydr)oxides are formed at shallower depths
than iron (hydr)oxides. P and Th are enriched in the deep sections
of the cores, consistent with their well-documented affinity for
Fe (hydr)oxides. In contrast, the downcore distribution of Ba,
especially in core 3, more closely tracks the concentration of
Mn (hydr)oxides. Even though Mn (hydr)oxides are 200-300 times
less abundant than Fe (hydr)oxides in the cores, Mn (hydr)oxides
are known to have an affinity for Ba which is many orders of magnitude
greater than iron (hydr)oxides. Hence, the downcore distribution
of Ba in Fe (hydr)oxide rich sediments is most probably controlled
by the presence of Mn (hydr)oxides. U is enriched in the upper
zones of the cores, consistent with the formation of highly reducing
near-surface sediments in the intertidal zone at the head of the
Bay. Hence, the recirculation of seawater through this type of
subterranean estuary, coupled with the abiotic and/or biotic reduction
of soluble U(VI) to insoluble U(IV), leads to the sediments acting
as a oceanic net sink of U. These results highlight the importance
of permeable sediments as hosts to a wide range of biogeochemical
reactions, which may be impacting geochemical budgets on scales
ranging from coastal aquifers to the continental shelf.』
1. Introduction
2. Study area
3. Experimental methods and procedures
3.1. Field methods
3.2. Laboratory methods and analysis
4. Results
4.1. Distribution of pore-water salinity, Fe, Mn and sulfate
4.2. Sedimentology and type of iron (hydr)oxide precipitates
4.3. Total oxide concentrations
4.4. Elemental concentrations in the amorphous and crystalline
oxides
5. Discussion
5.1. Hydrogeochemical model
5.2. Oxides of Fe and Mn
5.3. Organic matter and redox conditions
5.4. Sorption of P onto Fe (hydr)oxides
5.5. Uranium enrichment
5.6. Th enrichment
5.7. Ba sorption to Fe and Mn (hydr)oxides
6. Conclusions
Acknowledgments
References
Appendix