Silica fluxes and opal dissolution rates in the northern Arabian Sea

Abstract

Box cores of sediments were collected at eight stations along a NE–SW transect starting on the Pakistan continental slope in the moderately productive northeastern sector of the Arabian Sea, via Murray Ridge to the highly productive upwelling zone off Oman. The dry sediments were analyzed for carbonates, organic matter and opal, and pore waters were analyzed for silicic acid concentrations. Asymptotic silicic acid concentrations in pore waters reflect the trend in primary production: high near the SW-monsoon-driven upwelling zone off Oman and gradually diminishing away from that zone. The concentrations were not significantly related to the ratio of detritus to opal content. To fit the silicic acid profiles, we used the diagenetic pore-water model of Rabouille et al. (Deep-Sea Res. II 44(1997) 1151), in which we additionally took into account the bioturbation depth. Using a numerical solution of the diagenetic equations, we derived values for the opal dissolution constant ( K B) as a function of depth and for the benthic flux of silicic acid. The K B values at the benthic boundary layer varied between 0.03 and 0.19 yr −1 . Normalized to 0°C, these values correlate positively with calculated opal rain rates. The profiles of K B show a rapid drop in opal reactivity within the top few centimeters of the sediments. The model-derived benthic fluxes of dissolved silica decrease with water depth in a NE to SW direction, but increase again at stations closer to the upwelling zone off Oman. The burial rates of biogenic silica, calculated from the sedimentary opal content in combination with the mass accumulation rate, also decrease in the NE to SW direction. At steady state, the sum of the benthic and burial fluxes should theoretically equal the fluxes of opal arriving at the sea floor. On the Pakistan margin, these calculated opal rain rates decreased from 177 mmol m −2 yr −1 on the upper slope to 99 mmol m −2 yr −1 on the lower slope; on Murray Ridge, the rain rates were quite similar (160±9 mmol m −2 yr −1) ; further to the southwest, the rain rates sharply increased from 234 to 535 mmol m −2 yr −1 . The highest rain rates were derived for stations closest to the main upwelling zone. The burial efficiencies of biogenic silica gradually decrease in the NE to SW direction, from 0.27 for a station on the upper continental slope off Pakistan to about 0.03 for stations near the Oman upwelling zone.