Evaluating the Silicic Acid Leakage Hypothesis in the Pacific Sector of the Southern Ocean

The Silicic Acid Leakage Hypothesis (SALH; Matsumoto et al., 2002; Glob. Biogeochem. Cycles 16, 10.1029/2001GB001442) posits that lower atmospheric carbon dioxide concentrations during glacial times resulted from a net transfer of dissolved silicic acid from the Southern Ocean to the tropics, coupled with a corresponding shift in phytoplankton taxa in tropical waters toward a greater dominance by diatoms (relative to today). If valid, then the SALH requires that opal burial rates in the Southern Ocean were lower than today during the Last Glacial Maximum (LGM), while contemporary opal burial rates in tropical regions were greater than today. Previous studies in the Indian and Atlantic sectors of the Southern Ocean have shown little to no net change between the LGM and the Holocene when opal burial rates are integrated across Subantarctic and Antarctic zones. This study aims to test a sub-hypothesis suggested by Chase et al. (Deep-Sea Research-II, v.50, 2003, 799-832) that silicic acid was exported only out of the Pacific sector. Piston cores from the SW Pacific sector of the Southern Ocean were analyzed to determine opal burial fluxes. Magnetic susceptibility and Eucampia antarctica abundances were used to determine glacial cycles and identify cores containing both LGM and Holocene sediment. Fluxes of opal, calcium carbonate, and lithogenic material were evaluated by normalizing to ²³⁰Th to correct for sediment focusing. Our results show opal flux at, and to the south of, the Antarctic Polar Front (APF) to have been lower during the LGM compared to the Holocene, consistent with previous results from throughout the Southern Ocean. North of the APF, two cores show higher opal fluxes during the LGM. However, due to a possible hiatus in one of the cores, the degree to which opal flux during the LGM was greater than during the Holocene has a large uncertainty. Additional analyses are planned to help reduce this uncertainty.