Resolving the 'opal paradox' in the Southern Ocean

In the Southern Ocean, high accumulation rates of opal-which forms by preci pitation from silica-bearing solutions-have been found in the sediment in s pite 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 elsewher e(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 secto r of the Southern Ocean, which show that the assumed opal paradox is a resu lt of underestimated opal production rates and overestimated opal accumulat ion rates. Our data thus demonstrate that the overall preservation efficien cy of biogenic opal in this region is substantially lower than previously t hought(2), and that it lies within a factor of two of the global mean(3). T he comparison of our revised opal preservation efficiencies for the Souther n Ocean with existing values from the equatorial Pacific Ocean and the Nort h Atlantic Ocean shows that spatial differences in preservation efficiencie s are not the primary reason for the differences in sedimentary opal accumu lation. The reconciliation of surface production rates and sedimentary accu mulation 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.