Opal‐based isotopic proxies of paleoenvironmental conditions

Abstract

The δ30Si and δ15Ndiatom of diatom opal provide a view of nutrient utilization in past oceans and are used to formulate and test hypotheses concerning Southern Ocean productivity and fluctuations in atmospheric CO2 over glacial cycles. Water column profiles of the Si and N isotopic composition of nutrients and the δ15Ndiatom of sediment core tops support the use of δ30Si and δ15Ndiatom as tracers of silicic acid and nitrate utilization, but some issues remain concerning the use of these proxies for paleoceanographic reconstructions. If average marine δ30Si changes over time, it could contribute to the observed down core variations in δ30Si. Reconstruction of deepwater δ30Si using opal from sponges or deep‐dwelling radiolarians would address this concern. Cleaning and measurement methods for δ15Ndiatom need to be standardized between laboratories and in general suffer from our lack of knowledge of how much organic matter a clean diatom frustule should contain and what its C/N ratio should be. Corresponding shifts in the δ15N and C/N of diatom opal with species composition suggests that changes in species composition contributes to the measured down core variations in N and possibly Si as well. This could be due to changes in the ecological niche represented in the sediments or, in the case of N, to species specific fractionation factors. Separation of opal sediments into something more closely resembling monospecific samples is a key development that needs to be made and may be possible using laminar flow systems like “split‐flow lateral‐transport thin fractionation” (SPLITT). In the meantime, information on the species composition of each sieved and cleaned sample analyzed needs to be collected alongside the isotopic data.