The records of opal contents and opal accumulation rates in drift sediments on the continental rise west of the Antarctic Peninsula provide signals of paleoproductivity; even then the opal records are overprinted by dissolution in the water column and the sediment column and by bottom current-induced lateral sediment supply (focusing). Previously reported positive correlations between opal and biogenic barium records in upper Quaternary sedimentary sequences suggest that the primary signal of opal deposition is not masked crucially by dissolution processes and that opal records represent a more suitable proxy for paleoproductivity rather than TOC and CaCO3 records.
Opal contents at ODP Leg 178 Sites 1095, 1096, and 1101 show similar long-term trends throughout the Neogene and Quaternary but are affected by dilution with lithogenic matter. In contrast, the opal accumulation rates that reflect deposition and preservation of opal exhibit marked discrepancies between the sites. Because the influence of bottom current-delivered biosiliceous particles on the opal accumulation rates is assumed not to be crucial at the drift sites, the main cause for the discrepancies in opal accumulation is seen in locally different opal preservation linked to local variations of bottom current-induced lateral supply of lithogenic detritus. These variations are clearly expressed in local variability of sedimentation rates. A correlation between sedimentation rate and opal preservation found for continental margin deposits in the Ross Sea, which represents an environmental and depositional setting comparable to that in the Bellingshausen Sea, can be applied to decipher the signal of original opal deposition on the seafloor in the Bellingshausen Sea from the opal accumulation in the drift deposits. On long-term timescales, the reconstructed opal depositional rates show patterns similar to those of the opal contents and a much better coherency between the different locations on the Antarctic Peninsula continental rise.
Supposing that sea-ice coverage within the Antarctic Zone was the main factor controlling biological productivity in the Bellingshausen Sea, and thus the estimated opal depositional rates on the continental rise, we reconstructed paleoceanographic long-term changes during the Neogene and Quaternary considering the climatic control on regional and on global scales. During the late Miocene, temperatures in the Bellingshausen Sea as well as in the Atlantic sector of the Southern Ocean were slightly warmer than at present. Because heat injection by northern source deep-water masses was reduced during that time, regional Antarctic climate was more important for paleoproductivity than global deep-water circulation.
Higher productivity was reconstructed for the early Pliocene and early late Pliocene (to ~3.1 Ma) than during the late Miocene. A strong productivity increase in the Bellingshausen Sea before the closure of the Panama Isthmus might indicate that initial strengthening in NCW flow reported at ~5 Ma was at least partly triggered by early warming in Antarctica and the adjacent Southern Ocean. A positive linkage between Antarctic climate and NCW input is assumed for maintaining the Pliocene warm period, when sea-ice coverage in the Bellingshausen Sea was strongly reduced, if not absent. Productivity in the Bellingshausen Sea decreased between 3.1 and 1.8 Ma, indicating expansion of annual sea-ice coverage. This climatic deterioration probably was caused by the reduction of NADW inflow due to increased glaciation in the Northern Hemisphere. We interpret the relatively constant level of productivity during the Quaternary as a consequence of stable climatic conditions in the Antarctic Peninsula area.