FLUCTUATION OF BIOGENIC AND ABIOGENIC SEDIMENTATION ON THE SHATSKY RISE IN THE WESTERN NORTH PACIFIC DURING THE LATE QUATERNARY

Abstract

Sedimentation of biogenic and abiogenic components was studied in cores NGC108 (36°36.85'N, 158°20.90'E; water depth 3390 m) and S2612 (32°19.84'N, 157°51.00'E; water depth of 2612 m) from the Shatsky Rise to understand fluctuations in primary productivity and abiogenic sedimentation in the mid-latitude of the western North Pacific during the late Quaternary. The mean COrganic/N atomic ratio of 6.0-7.8 in both cores indicates that organic matter is mainly marine in origin. Organic carbon is positively correlated with biogenic opal in core NGC108 in contrast to a weak correlation in core S2612. Although the maxima of paleoproductivity estimates in both cores generally occur during glacial times, the paleoproductivity estimates, biogenic opal/carbonate ratios and the COrganic/CCarbonate ratios have always been higher in core NGC108 than in core S2612 during the last 180 kyr, suggesting that the surface water at site NGC108 could have been influenced more by Subarctic water mass than at site S2612. However, the opal/carbonate ratio in core S2612 remains fairly constant relative to that in core NGC108, which might mean that the transition zone between Subarctic and Central water was narrower in latitude in at the oxygen isotope stage (OIS) 2/3 boundary, OIS 4 and OIS 6. Sedimentation of 13 inorganic elements has been measured in both cores. These elements are classified into four groups based on correlation between each element in content: (1) terrigenous components (Al, Ti, Fe), (2) biogenic calcareous material (Ca, Sr), (3) biogenic-scavenged elements (Mg, Zn, Cr, Be), and (4) the other elements (Mn, Ba, Cu, Ni). The terrigenous mass accumulation rates were elevated in OIS 2, 3 and 4 and late OIS 6 in core NGC108 while they were higher in early OIS 1, OIS 2, 4 and 6 in core S2612. MnO2 and Ba might be redistributed during the sub-surface reduced condition. Especially precipitation of particle-reactive Be, which could be accelerated by both enhanced terrigenous input and biogenic vertical transport, has fluctuated largely in response to climatic change because of its short residence time (on the order of the oceanic mixing time).