B.A. Cragg,2 K.M. Law,2 A. Cramp,3 and R.J. Parkes2


Bacterial populations were quantified at two Ocean Drilling Program (ODP) Leg 155 sites in the Amazon Fan system to depths of 108 and 245 meters below seafloor (mbsf) in water depths of ~3400 and 3200 m, respectively. Bacteria were present in all samples, and populations decreased gradually with depth. However, divided/dividing cells were absent in some samples below 70 mbsf. Sediment accumulation rates in the Amazon Fan periodically have been very rapid (to 25 m/k.y.), and the structure of the sediment at both sites was dominated by turbidites, debris flows, and levee slumps. Bacterial profiles at both sites indicated that bacterial populations could be related to changes in the lithostratigraphy. Zones of frequent, small turbidites coupled with bioturbation and zones of massive mud flows or slumps were generally associated with unusually constant bacterial profiles. Conversely, outside these zones where sedimentary input was more dominated by material from the water column, the expected gradual decrease in bacterial populations with depth was observed. A particularly striking change was a strongly positive increase ( 3.7) of bacterial numbers (P << 0.001) coincident with sediment layers rich in plant remains and wood fragments between 17 and 26 mbsf at Site 934. Despite this, the overall profiles conformed to a bacterial depth distribution derived from other ODP sites (exclusively from the Pacific Ocean) indicating that deep bacterial populations are characteristic of all marine sediment. A few tens of centimeters of sediment have accumulated since the fan has been inactive, and near-surface bacterial populations (5.6 - 6.0 108 cells/cm3) are consistent with a trend in near-surface populations with varying water depth observed at Pacific Ocean sites.

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).
2Department of Geology, University of Bristol, Bristol BS8 1RJ, United Kingdom. b.cragg@bristol.ac.uk
3Department of Earth Sciences, University of Wales, Cardiff, P.O. Box 914, Cardiff, CF1 3YE, United Kingdom.