30. EARLY DIAGENESIS IN AMAZON FAN SEDIMENTS1Stephen J. Burns2 |
ABSTRACTEarly diagenesis in sediments of the Amazon Fan is strongly influenced by the high iron content of the pre-Holocene fan sediments. The diagenetic mineral assemblage is composed almost exclusively of iron-bearing phases: iron sulfides (hydrotroilite and greigite), vivianite (FePO4· 8H2O), and siderite (FeCO3). Iron sulfides form in the zone of sulfate reduction. Millimeter-sized vivianite nodules are common and begin to form as shallow as 0.5 meters below seafloor (mbsf), and continue to form to ~20 mbsf. Siderite is also common, forming as dispersed 1- to 5-µm crystals from the upper few mbsf to ~20 mbsf. Numerical modeling of sulfate reduction and ammonium production demonstrates that the modern pore-water profiles are not in a steady state, but have been migrating through the sediments in response to ~100“ decrease in sedimentation rates at 9.5 ka. Calculated initial rates of sulfate reduction and ammonium formation range from 1.7 to 6.0 “ 10-13 moles/liter/second (M/s) and 2.5 to 5.0 “ 10–14, respectively. The pre-Holocene depths to sulfate depletion were on the order of 20-50 mbsf, rather than the 5–10 mbsf presently observed. The zones of diagenetic mineral formation were thus considerably deeper than indicated by present pore-water chemistry. |
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