Physical and Biological Controls on Post Depositional Alteration and Strata Formation In a Temperate, Turbid Outwash Fjord: Simpson Bay, Alaska

Simpson Bay is a turbid outwash fjord located in south central Alaska. The controls on sediment distribution and accumulation were studied to determine the processes which govern strata formation and preservation in non-glaciated, macrotidal fjord. Box and piston cores were taken throughout the fjord to determine modern sedimentation rates and investigate long term changes in sediment deposition. Geochemical and geotechnical proxies were used to investigate different sedimentary environments. Short lived radioisotopes were used to determine accumulation rates at a number of locations throughout the fjord. Evidence of both physical and biological post-depositional modification was found throughout the system. Depositional environments ranged from physically dominated delta front environments to biologically dominated deepwater environments, as well as non-depositional relict glacial deposits. X-rays were used to identify sedimentary structures, validate surface mixed layers, and determine physically versus biologically mixed environments. ²¹⁰Pb_xs derived accumulation rates varied from 0.23 to 1.2 cmyr^-1 throughout the system. Most cores show a bioturbated surface mixed layer followed by steady state accumulation. Delta front cores showed laminated strata indicative of gravity flow erosion and deposition. ²¹⁰Pb_xs profiles in the diamicton deposits on the morainal banks show a log-linear decrease in activity indicative of steady state accumulation, but because these areas are non-depositional relict glacial deposits, the decrease in activity is modeled as biodiffusivity rather than accumulation. Activity profiles were also used to look at the response and recovery of the seafloor to large earthquakes. Profiles throughout Simpson Bay show evidence of a rapidly deposited layer, correlated with the 1964 Good Friday Earthquake, identified by a unit of constant activity bounded above and below by units of logarithmically decreasing activity. The first occurrence of ¹³⁷Cs is found below the layer, further validating this observation. Lignin biomarkers were used to determine the source of the material preserved in these layers.