The benthic cycle of biogenic opal at the Bermuda Atlantic Time Series site

Measurements of the particulate rain of biogenic opal, the flux of dissolved SiO2 across the sea water-sediment interface, pore-water profiles of dissolved SiO2, and opal burial at the Bermuda Atlantic Time Series site are reported for a 5-year period, 1988–1993. Opal rain was determined on samples collected by a sediment trap deployed at 3200 m, 1200-m above the sea floor. Samples were collected almost continuously over the 5-year period (94% of the time). Benthic dissolution fluxes were measured in situ with benthic chambers several times each year. SiO2 diagenesis was defined with pore-water profiles of SiO2 concentration determined on samples collected in situ. The data collected are used to describe the benthic biogeochemical cycle of biogenic opal for this oligotrophic ocean environment. Despite strong seasonal variation in the rain of opal at 3200 m, driven by the cycle of primary production in the surface waters, dissolution fluxes from the sediments were constant throughout the period of our study. Pore-water profiles indicate that ∼90% of SiO2 dissolution within the sediments takes place in the upper 5 cm, and that the residence time of opal in this interval is 100–350 years. The long residence time is consistent with the lack of response to seasonal variation in opal input. Dissolution fluxes, determined with benthic chambers and estimated from pore-water profiles, provide no evidence of SiO2 dissolution at the water-sediment interface. The average particulate opal rain and the benthic dissolution flux, 0.0052 and 0.0047 μmol cm−2 day−1, respectively, are not significantly different, indicating little opal burial. The burial efficiency of opal based on mass balance considerations is estimated to be ∼6%.