Resolving the 'opal paradox' in the Southern Ocean

In the Southern Ocean, high accumulation rates of opal-which forms by precipitation from silica-bearing solutions-have been found in the sediment in spite of low production rates of biogenic silica and carbon in the overlying surface waters. This so-called 'opal paradox' is generally attributed to a higher efficiency of opal preservation in the Southern Ocean than elsewhere(1,2). Here we report biogenic silica production rates, opal rain rates in the water column and opal sediment burial rates for the Indian Ocean sector of the Southern Ocean, which show that the assumed opal paradox is a result of underestimated opal production rates and overestimated opal accumulation rates. Our data thus demonstrate that the overall preservation efficiency of biogenic opal in this region is substantially lower than previously thought(2), and that it lies within a factor of two of the global mean(3). The comparison of our revised opal preservation efficiencies for the Southern Ocean with existing values from the equatorial Pacific Ocean and the North Atlantic Ocean shows that spatial differences in preservation efficiencies are not the primary reason for the differences in sedimentary opal accumulation. The reconciliation of surface production rates and sedimentary accumulation rates may enable the use of biogenic opal in the reconstruction of palaeo-productivity when the factors that affect the Si/C ratio are better understood.