Underestimation of the authigenic fraction of Cu and Ni in organic-rich sediments

Abstract

There are two possible techniques to estimate the degree of authigenic (or non-lithogenic) metal enrichment in sediments. First, the authigenic metal fraction can be calculated according to Mauth=Mtotal−(M/Albackground·Altotal) while M is the metal of interest and Al is aluminum. Second, a metal enrichment factor (EF) is calculated according to EF=M/Altotal/M/Albackground. Both techniques rely on the appropriate use of the M/Al ratio of the lithogenic background. For the latter, the M/Al ratio of average shale or crust is widely applied as lithogenic background in marine geochemistry. However, as we show here, the lithogenic background based on average shale or crust is too high for Cu and Ni. As a consequence, the authigenic fraction of Cu and Ni in sediments and suspended particulate matter (SPM) is significantly underestimated as shown for examples from various environments (upwelling areas of Peru, Chile and the Gulf of California, the Norwegian Kyllaren fjord and the German Wadden Sea). By correlating Cu/Al and Ni/Al with TOC in the samples the apparent M/Al ratio of the background can be estimated. A prerequisite for this technique is high linearity and high quality of correlation (r2≥0.8) otherwise the lithogenic background cannot be exactly determined. The resulting higher authigenic level of Cu and Ni may promote their use as productivity indicators while the status of Cu and Ni may shift from moderately to highly enriched when the enrichment is assessed via EFs.