Stephen J. Burns2


Early 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.

1Flood, R.D., Piper, D.J.W., Klaus, A., and Peterson, L.C. (Eds.), 1997. Proc. ODP, Sci. Results, 155: College Station, TX (Ocean Drilling Program).
2Geologisches Institut, Universität Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland. burns@geo.unibe.ch