The potential biogenic origin of black layers in glacimarine sediments of Disenchantment Bay, Alaska

Black layers have been noted in many late Holocene sediment cores from Disenchantment Bay, Alaska, but their origin has been enigmatic. This study focused on the investigation of diatom abundance and assemblages as they relate to the black layers documented throughout many of the sediment cores collected from Disenchantment Bay. Studies in similar environmental settings with high sedimentation rates have often correlated these black layers to a spring diatom bloom. In order to substantiate that the same is occurring in Disenchantment Bay, specific sediment cores were chosen as priority cores based on the abundance of black layers, relative distance from the glacial front and variance in distance from shore. The chosen cores' sub-samples were processed and mounted on slides to be viewed under the microscope in order to determine relative abundance of diatoms in the black and surrounding layers. After general analysis of diatom assemblage and abundance distribution throughout the cores, some variation in diatom distribution and absolute abundance was observed. This variation doesn't seem to be significant enough to be the direct cause of the observed black layers in the sediment cores. It is most likely that the occurrence of these black layers are due to some other environmental factors in combination with the semi-annual diatom blooms such as blooms of other phytoplankton species, the decay of organic matter, anoxia or sulfide-producing bacteria.