APPLICATIONS OF XRF TO INVESTIGATE PLEISTOCENE CLIMATE VARIABILITY AT HOLE 1208A, SHATSKY RISE, NORTH PACIFIC OCEAN

Ocean Drilling Program (ODP) Hole 1208A is located on the Shatsky Rise, a submarine plateau off the coast of Japan, where changes in terrigenous and biogenic sediment deposited during the Pleistocene reflect cyclic climate change. An alternation of light and dark intervals, visible within the sediment and shown in color reflectance patterns, suggested influence of orbital forcing on biological productivity. Core 8 was scanned using X-ray fluorescence (XRF) to obtain elemental data as a proxy allowing us to additionally investigate and evaluate changes in terrigenous and biologically-controlled inputs. Measurements were obtained for 14 elements at 2 cm intervals between 62 and 70 mbsf. Light-colored sediments contain higher abundances of Ca, generally higher abundances of Mn, and higher Ba/Al and Si/Al ratios. The high Ca abundance is consistent with the carbonate ooze lithology, and the Ba and Mn are probably biogenically derived. In contrast, dark-colored sediments contain higher abundances of Al, Ti, Si, Fe, generally higher abundances of P and S, and higher Ti/Al ratios. The increases in Al, Ti, Si, and Fe indicate greater terrigenous inputs, consistent with the more clay-rich (nannofossil clay) lithology. The higher Ti/Al ratio may indicate an increase in grain size as well. Higher P in the dark intervals may be caused by increased nutrient supply via upwelling, or from shifting water masses (Kuroshio to Oyashio Currents) over the site. Either of these circulation changes would support an increase in siliceous productivity, and is consistent with the shipboard observation of more diatoms in the dark intervals. Elemental abundance data have a periodicity of ~42kyr, indicating obliquity orbital forcing. Consistent with earlier studies, we interpret the light intervals to represent interglacials, and the dark intervals to represent glacials. However, while previous studies suggest that the cycles represent climatically-driven changes in carbonate dissolution and/or changes in bioproduction, we propose that the dilution of the biogenic fraction by terrigeous sediments would have been an additional influence. During glacials the East Asian winter monsoon gains strength. This would result in greater aeolian transport of terrigenous sediments to Site 1208, diluting the biogenic fraction.