In this study, we undertook simple analyses of the bulk chemistry of the biogenic opal fractions that we had extracted from sediments for diatom-bound N isotope analysis. Even from extremely low-opal, high-clay samples, we find that our standard physical separation protocol for sedimentary opal
JNK-IN-8 only trivial amounts of aluminosilicate (i.e. clay) in the opal sample, so far as studies of organic matter are concerned. Nearly all of the aluminum in our opal samples derives from the incorporation of diagenetic aluminum into the opal, which occurs in rough proportion to the aluminosilicate content of the sediments. Aluminum content appears to be inversely correlated with the diatom-bound nitrogen content, but without clear correlations with nitrogen isotopic composition. The loss of opal-bound organic nitrogen appears to result from diagenesis of the mineral material and associated exposure of once-protected diatom-native organic matter, such that
cell plate loss process does not cause significant isotopic change in the remaining diatom-protected N. The observations can also explain the different behaviors of oxidative cleaning protocols in opal extracted from clay-rich and -poor sediments. Finally, the evidence for diagenetic aluminum incorporation into biogenic opal, even in opal-rich samples, raises concerns for any chemical or isotopic analysis of the biogenic opal itself that is intended to reconstruct surface ocean conditions.
