Dolomitization by Near-Normal Seawater? Field Evidence from the Bahamas

  1. Bruce Purser,
  2. Maurice Tucker and
  3. Donald Zenger
  1. F. F. Whitaker1,
  2. P. L. Smart1,
  3. V. C. Vahrenkamp2,
  4. H. Nicholson3 and
  5. R. A. Wogelius4,†

Published Online: 14 APR 2009

DOI: 10.1002/9781444304077.ch8

Dolomites: A Volume in Honour of Dolomieu

Dolomites: A Volume in Honour of Dolomieu

How to Cite

Whitaker, F. F., Smart, P. L., Vahrenkamp, V. C., Nicholson, H. and Wogelius, R. A. (1994) Dolomitization by Near-Normal Seawater? Field Evidence from the Bahamas, in Dolomites: A Volume in Honour of Dolomieu (eds B. Purser, M. Tucker and D. Zenger), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304077.ch8

Author Information

  1. 1

    Department of Geography, University of Bristol, Bristol BS8 1SS, UK

  2. 2

    Koninklijke/Shell Exploratie en Productie Laboratorium, Volmerlaan 6, 2080AB Rijswijk (Z-H), The Netherlands

  3. 3

    B. P. Research, Sunbury Research Centre, Chertsey Road, Sunbury on Thames, TW16 7LN, UK

  4. 4

    Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UK

  1. Argonne National Laboratory, University of Chicago, Illinois 60439, USA

Publication History

  1. Published Online: 14 APR 2009
  2. Published Print: 25 MAY 1994

ISBN Information

Print ISBN: 9780632037872

Online ISBN: 9781444304077

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Keywords:

  • mixing-zone and seawater dolomitization models;
  • dolomitization by near-normal seawater - field evidence from Bahamas;
  • dolomitization by seawater — the Bahamas;
  • saline groundwater circulation;
  • geochemical evidence for dolomitization by saline groundwaters;
  • dolomitization - cannot occur via primary precipitation;
  • aqueous geochemistry of saline groundwaters;
  • estimation of rate of dolomitization by saline groundwaters;
  • shallow dolomites from South Andros

Summary

Measurements of salinity, temperature and groundwater discharge indicate active circulation of groundwaters of near-normal seawater composition through the Great Bahama Bank beneath Andros Island. Waters of slightly elevated salinity derived from the bank surface flow eastwards beneath the island, and then mix with cold normal-salinity seawaters from the adjacent oceans. Saline groundwaters are significantly depleted in magnesium compared to Great Bahama Bank and open ocean seawaters. In the absence of major calcium depletion, this provides direct evidence for present-day replacement dolomitization. The saline groundwaters are characterized by elevated pco2 and, as a consequence, lowered carbonate mineral saturation indices, due to subsurface oxidation of organic matter principally by sulphate reduction under anoxic conditions. In Stargate Blue Hole, South Andros, sparse replacement dolomites and dolomite cements are observed only in rock samples from the depth of the present saline zone. Stable isotope and trace element analyses suggest precipitation from a fluid of near-seawater composition under slightly reducing conditions, and at temperatures in the lower range of those observed for modern saline groundwaters. Strontium isotopes yield a maximum age of 0.4–0.8 Ma for the dolomites, substantially younger than that estimated for initial deposition (1.5–2.5 Ma). Order-of-magnitude calculations based on estimates of groundwater flow combined with the observed magnesium depletion in saline groundwaters indicate dolomitization at a rate of 0.18–3.0 × 10−5% a−1. Over the long term, given the likely effect of Pleistocene sea-level fluctuations on the saline circulation system, this would be sufficient to generate 0.3–4.5% dolomite, a figure comparable to the observed abundance in the Stargate rock samples.