Stratigraphic evidence of changes in Amazon shelf sedimentation during the late Holocene

Late Holocene sedimentation on the Amazon shelf was investigated using a combination of seismic, sedimentological, and geochronological methods. Piston cores collected in the subaqueous delta region during A Multidisciplinary Amazon Shelf Sediment Study ( AmasSeds) penetrated a regionally traceable (1.25 × 10 4km 2) seismic discontinuity, which corresponds to a dense relict-mud reflector overlain by acoustically transparent sediment. Distinct offsets in profiles of wet-bulk density, P-wave velocity, and 210Pb activity are noted in cores at the reflector horizon (unconformity), signifying a past change in sedimentation conditions on the shelf. Bulk-sediment 14C dating indicates net depositional conditions for the subaqueous delta from > 1800 yr B.P. to 700 yr B.P. and a net erosional phase from about 700 yr B.P. to 100 yr ago. Sedimentological evidence of the erosional phase includes lag deposits and burrows present at the unconformity surface in piston cores. The deposits above and below the unconformity are similar with respect to grain-size parameters and sedimentary structures, suggesting that environmental conditions during the two depositional periods were similar. According to 14C dates of coastal deposits, the shoreline of Amapa State, Brazil served as a major depocenter of Amazon mud during the erosional phase on the shelf. 210Pb geochronology indicates that the most recent phase of sediment accumulation in the study area commenced about 100 yr ago, resulting in rapid progradation of topset deposits and burial of relict strata. In the present environment, oceanographic and meteorologic processes cause along-shelf variations in sediment dispersal and depositional patterns, and about 1400 km 2 of the erosional relict deposits are still exposed at the seabed. The short time scales over which Amazon shelf strata respond to changes in the physical environment preclude the influence of relative sea-level cyclicity on recent sedimentation patterns. During the late Holocene, sedimentation and strata formation on the Amazon shelf are linked to fluvial, oceanographic and meteorologic processes that affect sediment supply, deposition and erosion.