Changes in biogenic opal productivity with Milankovitch cycles during the last 1.3 Ma at IODP Expedition 323 Sites U1341, U1343, and U1345 in the Bering Sea

Abstract

Sediments from three Sites (U1341, U1343 and U1345) of IODP Expedition 323, located in the Bering Sea, were analyzed for variation of biogenic opal representing surface water paleoproductivity. Initial age models of three Sites (bottom age, U1341: 4.3 Ma, U1343: 2.4 Ma, U1345: 0.5 Ma) were established onboard by biostratigraphic and paleomagnetic investigations. The initial age models were tuned by δ18O stratigraphy, which was established on shore. Biogenic opal contents of sediments were analyzed for the interval of 1.3 Ma to the present. Generally, the %biogenic opal values at Site U1341 (Bowers Ridge) were significantly higher than those at the Beringian slope sites (U1343 and U1345). Higher natural gamma radiation (NGR) counts and magnetic susceptibility (MS) at the Beringian slope sites suggested a greater contribution of lithogenic matter than at Site U1341 on Bowers Ridge. The percentages of sea-ice diatoms at the Beringian slope sites are also higher than those at Bowers Ridge Sites. This is primarily attributed to the fact that the sediments from the Beringian slope area were significantly diluted by lithogenic matter derived from the Alaskan continent, and were largely affected by sea-ice cover. The cyclicity of biogenic opal accumulation rates at Site U1341 is determined to be about 160, 40, and 30 ky, whereas that at Site U1343 is determined to be about 330, 100, 40 and 20 ky. If these had been reflecting a global climate change, the cyclicity of biogenic opal contents at all sites would be consistently the same. However, the results do not show such a consistency. Obliquity is observed at both sites of the Bowers Ridge and Beringian slope. A strong eccentricity was observed only at U1343, but no signature for eccentricity was detected at U1341. Therefore, the Milankovitch cycle with 100 ky periodicity was only observed at the Beringian slope area. The results suggest that the %biogenic opal at Site U1341 indicated a local characteristic for biogenic productivity changes. Thus, the absence of the 100 ky cycle of biogenic opal at Site U1341 seems to be due to local phenomena such as water circulation, although the changes in biogenic opal productivity at Site U1341 are attributed to summer insolation.