CONCLUSIONS

Comparison between the distribution of carbonate minerals and pore-fluid chemistry in sediments associated with marine gas hydrates on the Blake Ridge, offshore of southeastern North America, reveals three distinct, depth-stratified, diagenetic zones.

  1. The carbonates from the upper 20 mbsf (corresponding to the sulfate reduction zone) are primarily composed of calcite from biogenic sources and exhibit no evidence of diagenetic overprint.
  2. Between 20 and 80 mbsf, authigenic dolomite with 13C-depleted isotopic values becomes a dominant carbonate component. Both dolomite and calcite precipitation is active within these sediments, especially near the sulfate/methane interface where anaerobic methane oxidation takes place.
  3. Below 140 mbsf, siderite is ubiquitous and displays enriched 13C and 18O values that are consistent with the present-day isotopic values of the CO2 and DIC pool, and with predicted equilibrium 18O values of siderite between 120 and 450 mbsf. These properties indicate that siderite forms above and within the gas hydrate-bearing section of the sediment column, where high alkalinity is conducive to carbonate precipitation.
  4. Siderite found beneath the present-day gas hydrate zone is texturally and isotopically indistinguishable from siderite within the gas hydrate zone, and thus appears to be the buried remnants of early diagenesis. Siderite with enriched 13C values may, therefore, serve as a proxy for paleo gas hydrate-bearing sedimentary sections.

The distribution of carbonate minerals that we see today is not only reflective of currently active diagenesis, but also reflects how changes in sedimentation rates have affected diagenetic processes acting upon these sediments in the past. The presence of dolomite and calcite between 20 and 80 mbsf with isotopic signatures reflective of the uppermost methanogenic zone (where anaerobic methane oxidation and methane production by CO2 reduction occur) may reflect decreases in sedimentation rates since the Pliocene. During the Miocene and Pliocene, high sedimentation rates prevented this "concentrated" zone of diagenesis from staying in place long enough to leave a "lasting" record of diagenesis.

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