Porewater pH and authigenic phases formed in the uppermost sediments of the Santa Barbara Basin

Porewater pH and authigenic phases formed in the uppermost sediments of the Santa Barbara Basin In this paper porewater and solid phase analyses are used in combination with in situ O 2 and pH microelectrode measurements to characterize early diagenetic processes in the uppermost sediments of the Santa Barbara Basin, California. Rapid reduction of dissolved oxygen, nitrate, solid phase manganese and iron, and dissolved sulfate is observed. Between sediment depths of 0 and 2 cm, reductive solubilization of ferric iron phases releases Fe 2+ , adsorbed phosphate, and fluoride to the porewaters and contributes to a sharp increase in porewater pH. Between 2 and 4 cm, sulfate reduction rates peak, pH levels off, and acid volatile sulfides and pyrite become the dominant forms of solid phase iron. Saturation state calculations, which depend largely on pH, indicate that the porewaters of the Santa Barbara Basin become saturated with respect to carbonate fluorapatite and calcite within the first 0.25 mm of the sediment and are highly supersaturated by and below 2 cm. In spite of this result, porewater evidence of phosphate and fluoride removal into a solid phase is observed only in the first ∼5 cm of some cores, whereas dissolved Ca profiles suggest dispersed calcite precipitation throughout the sediment column. This finding is interpreted as an indication of the nonsteady state nature of the surface reactions that may, given sufficient nucleation sites and time, lead to carbonate fluorapatite genesis in anoxic sediments. Finally, microelectrode pH profiles from two other basins in the California Borderlands are presented. These demonstrate that the porewaters of the Santa Barbara Basin are more alkaline than those of other basins. This outcome is attributed to the lack of particle mixing and a unique interplay between Fe liberation and FeS precipitation reactions in the Santa Barbara Basin. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geochimica et Cosmochimica Acta Elsevier

Porewater pH and authigenic phases formed in the uppermost sediments of the Santa Barbara Basin

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Publisher
Elsevier
Copyright
Copyright © 1996 Elsevier Ltd
ISSN
0016-7037
eISSN
1872-9533
D.O.I.
10.1016/S0016-7037(96)00231-1
Publisher site
See Article on Publisher Site

Abstract

In this paper porewater and solid phase analyses are used in combination with in situ O 2 and pH microelectrode measurements to characterize early diagenetic processes in the uppermost sediments of the Santa Barbara Basin, California. Rapid reduction of dissolved oxygen, nitrate, solid phase manganese and iron, and dissolved sulfate is observed. Between sediment depths of 0 and 2 cm, reductive solubilization of ferric iron phases releases Fe 2+ , adsorbed phosphate, and fluoride to the porewaters and contributes to a sharp increase in porewater pH. Between 2 and 4 cm, sulfate reduction rates peak, pH levels off, and acid volatile sulfides and pyrite become the dominant forms of solid phase iron. Saturation state calculations, which depend largely on pH, indicate that the porewaters of the Santa Barbara Basin become saturated with respect to carbonate fluorapatite and calcite within the first 0.25 mm of the sediment and are highly supersaturated by and below 2 cm. In spite of this result, porewater evidence of phosphate and fluoride removal into a solid phase is observed only in the first ∼5 cm of some cores, whereas dissolved Ca profiles suggest dispersed calcite precipitation throughout the sediment column. This finding is interpreted as an indication of the nonsteady state nature of the surface reactions that may, given sufficient nucleation sites and time, lead to carbonate fluorapatite genesis in anoxic sediments. Finally, microelectrode pH profiles from two other basins in the California Borderlands are presented. These demonstrate that the porewaters of the Santa Barbara Basin are more alkaline than those of other basins. This outcome is attributed to the lack of particle mixing and a unique interplay between Fe liberation and FeS precipitation reactions in the Santa Barbara Basin.

Journal

Geochimica et Cosmochimica ActaElsevier

Published: Nov 1, 1996

References

  • Anomalous accumulation of acid-volatile sulphides (AVS) in a coastal marine sediment, Saguenay Fjord, Canada
    Gagnon, C.; Mucci, A.; Pelletier, E.
  • Carbon fluxes and burial rates over the continental slope and rise off central California with implications for the global carbon cycle
    Reimers, C.E.; Jahnke, R.A.; McCorkle, D.C.

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