Mid to late Holocene oceanographic changes offshore Adélie Land, Antarctica: Ultra-high resolution foraminiferal assemblage and isotopic records from IODP Expedition 318 Site U1357
Abstract:Foraminiferal biofacies as well as δ18O and δ13C records from IODP Site U1357 reveal significant changes in deep and surface water properties that include temperature, ventilation, and productivity during the mid to late Holocene. Site U1357 is located in the Adélie Trough, a glacially scoured valley on the continental shelf ~50km off the coast of East Antarctica. Sediment samples were taken at 10cm intervals resulting in an approximate time step for each sample of 7yr resolution based on extensive C14 dating and visible band counting exercises. As part of a collaborative effort between Queens College and Stanford University, samples from the upper part of the core were used in this study, which spanned from near Recent to 6kyr BP. Neogloboquadrina pachyderma make up nearly 50% of all foraminifers counted and was used to construct pelagic stable isotope records. From nearly 10,000 foraminifers counted, ~34 foraminiferal species were identified.
The highest δ18O values occur from ~3.0 to ~6.1kyr and ~1kyr to Recent, with the lowest occurring from ~1.4 to ~3.8kyr. The highest δ13C values occur when δ18O are low. Total benthic foraminiferal abundances are highest during two intervals: Recent to ~1.4kyr and ~3.6 to ~6.1kyr. For ~2 to ~3.6kyr, the agglutinated species are the most dominant with calcareous benthic foraminifers being mainly absent.
The higher δ18O values observed are consistent with lower surface water temperatures and decreased melt water from icebergs, with lower δ18O values ascribed to increased melt waters and possibly higher surface water temperatures. Previous studies indicate that cooler waters occurred when we observe lower foraminiferal δ18O values. This suggests that the N. pachyderma δ18O record was influenced primarily by the δ18O of seawater (e.g., iceberg melt waters), with temperature being a minor control. Higher δ13C values are associated with lower δ18O, which implies increased water column stratification coupled with high productivity possibly owing to less sea ice. This increase in stratification is associated with the absence of calcareous benthic forms, which is interpreted to be due to decreased bottom-water ventilation.