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Title: The geochemistry and geomicrobiology of a salinity-stratified coastal carbonate aquifer, Yucatan Peninsula, Mexico
Author: Smith, Samantha Lisa
ISNI:       0000 0001 2449 9158
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2004
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Geochemical and geomicrobiological data combined with in situ experiments and numerical modeling were used to assess the rates and distribution of modern carbonate diagenesis along the east coast of the Yucatan Peninsula platform (Mexico). The platform hosts a salinity-stratified aquifer where a thin meteoric lens is separated from underlying saline water by a well-defined mixmg zone. An extensive network of flooded caves discharges brackish lens water to the Caribbean Sea, while seawater flows into the caves at depth. Lens waters are enriched in calcium (Caxs = 2.19 ± 0.55 mM, n = 77) relative to the dilution of seawater. However, within the 'freshwater system', the majority (61 %) of CaC03 dissolution occurs in the 10 m-thick vadose zone due to high ground air pC02 (-1.18%). Caxs combined with recharge estimates indicate a vadose dissolution rate of 22.9 metric tons/km2/a (porosity generation = 8.64 xl 04%/a). Some (",,9.5% of the total) dissolution may also occur in lens waters due to organic matter oxidation, however precipitation in the upper lens via degassing and mixing with the lens base is also likely. Comparatively, dissolution potential is enhanced in the fresh-salt water mixing zone, however, geochemical modeling alone cannot explain the locus or extent of undersaturation. Model results from the mixing of brackish and saline water estimate maximum calcite undersaturation at -75% seawater (SW), while field data shows two zones of undersaturation. These zones occur near the top (-35% SW) and bottom (-95% SW) of the mixing zone and in situ dissolution experiments reveal associated enhanced calcite dissolution. The maintenance of undersaturation is driven by increases in bacterial activity (evidenced by increases in inferred reproduction rates, decreases in pH and dissolved oxygen and increases in pC02). Most bacteria are heterotrophs, although acidproducing sulfur-oxidising bacteria (SOX) may also be important in driving undersaturation. Where bacterial activity approaches zero, calcite saturation (SI-C) approaches equilibrium. Despite the thermodynamic potential for Dorag dolomitisation within some (-50-80% SW) mixing zone waters, no significant amount of dolomite is found in the associated wall-rock and Mg2+and Ca2+ concentrations are not significantly different from those expected from fresh-salt water mixing. However, replacement dolomitisation may occur in the underlying saline zone where waters are depleted of magnesium (Mgxs = -1.16 ± 3.31 mM) and enriched in calcium (Caxs = + 1.24 ± 3.01 mM) (n = 48). This study offers a unique insight into the diagenetic processes within the zone of saline groundwaters and geochemical data reveal that there are two distinct 'sub-zones': shallow «40 m water depth) and deep (>40 m). In the shallow saline zone, bacteria continue to maintain calcite undersaturation via organic matter breakdown and pC02 production, although the shallow saline ground waters remain supersaturated with respect to disordered dolomite. In this zone, dolomitisation may also be associated with the (bacterial?) re-oxidation of reduced sulfur species (S04XS= +0.82 ± 0.40 mM, n = 34). In the deep saline zone, however, sulfate enrichment is due to the dissolution of anhydrite (AN) clasts within breccia deposits located ~ 110 m below the surface (evidenced by CaXS:S04XSratios, l)34S-S0t values and SI-AN increases with depth). Concurrent Mg2+ depletions and positive Caxs (beyond that due to CaS04 dissolution) indicate replacement dolomitisation is also occurring. Petrographic results suggest wall-rock dolomite does not occur outside the saline zone, and may only occur at coastal sites, the dolomites are of seawater origin and that the amount of dolomite increases where there is interconnected porosity, reinforcing the importance of Mg-rich fluid circulation. Five shallow saline zone sites have chemistries resembling the deep saline zone. These 'outliers' may offer insight into deep saline zone circulation patterns, where upwelling of geothermally-heated water occurs through regions of increased cavernous porosity rock in response to outflowing lens waters at low tide. This brackish water outflow also results in a compensatory east to west inflow of seawater at depth. This circulation 'pattern' explains the previously unexplained co-variance of maximum saline groundwater inflow rates with low tide and minimum inflow rates with high tide.
Supervisor: Whitaker, Fiona Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available