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Geochemistry of meteoric diagenesis in carbonate islands of the northern Bahamas: 1. Evidence from field studies
Author(s) -
Whitaker Fiona F.,
Smart Peter L.
Publication year - 2007
Publication title -
hydrological processes
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.222
H-Index - 161
eISSN - 1099-1085
pISSN - 0885-6087
DOI - 10.1002/hyp.6532
Subject(s) - aragonite , geology , carbonate , diagenesis , meteoric water , calcite , dissolved organic carbon , mineralogy , groundwater , oceanography , chemistry , geotechnical engineering , organic chemistry
Processes driving carbonate diagenesis in islands of the northern Bahamas are investigated using major ion, dissolved oxygen and dissolved organic carbon analyses of water samples from surface and ground waters, and measurements of soil gas P CO 2. Meteoric waters equilibrate with aragonite, but reactions are water controlled rather than mineral‐controlled and drive dissolution rather than concurrent precipitation of calcite. Surface runoff waters equilibrate with atmospheric P CO 2and rapidly recharge the vadose zone, limiting subaerial bedrock dissolution to only 6·6–15 mg l −1 Ca. P CO 2of soil gas measured in the summer wet season ((7·4 ± 3·7) × 10 −3 atm) is elevated compared with that of the atmosphere, despite the thin skeletal organic nature of the soil and the discontinuous soil cover. Soil waters retained in surface pockets are equilibrated with respect to aragonite and have dissolved 51 ± 19 mg l −1 Ca. This is substantially less than the 93 ± 18 mg l −1 Ca in samples from pumping boreholes that sample meteoric waters from the freshwater lens. The high P CO 2of the freshwater lens ((16 ± 8·3) × 10 −3 atm for pumping boreholes) suggests that significant additional CO 2 may be derived by oxidation of soil‐ and surface‐derived organic carbon within the lens. The suboxic nature of the majority of the freshwater lens and the observed depletion in sulphate support this suggestion, and indicate that both aerobic and anaerobic oxidation may take place. Shallow lens samples from observation boreholes are calcite supersaturated and have a lower P CO 2than deeper lens waters, indicating that CO 2 degasses from the water table, driving precipitation of calcite cements. We suggest that the geochemical evolution of waters in the vadose zone and upper part of the freshwater lens may be determined by the presence of a body of ground air with P CO 2controlled by production in the freshwater lens and soil and by degassing to the atmosphere. Copyright © 2007 John Wiley & Sons, Ltd.