Diagenetic aluminum uptake into diatom frustules and the preservation of diatom-bound organic nitrogen

The chemistry and isotopic composition of siliceous diatom frustules and the organic matter bound within them have been used to reconstruct past changes in the environment. However, the alteration of biogenic silica in the sediments and its influence on the chemical properties of the diatom opal and diatom-bound organic matter has not been broadly studied. We use inductively coupled plasma atomic emission spectrometry and energy dispersive X-ray analysis to investigate the concentrations of various cations, especially aluminum, in cleaned diatom frustules extracted from sediments at three sites from the Bering Sea, where the opal content in the sediments and preservation conditions vary both spatially and over the last glacial cycle. Relative to cultured diatoms, cleaned diatom frustules from the sediments are found to have a high Al/Si ratio, most likely by homogenous Al incorporation but possibly also by formation of an aluminosilicate coating of significant thickness relative to the diatom walls. Variations in the Al/Si ratio of the cleaned diatoms among samples are strongly correlated with the aluminosilicate to opal ratio of the bulk sediment, suggesting that sedimentary clays are the primary source of the aluminum taken up by the diatoms. The aluminum and nitrogen contents of the diatom frustules are negatively correlated, which is best explained by the loss of intrinsic N during the chemical changes associated with Al incorporation. The lack of a consistent relationship between nitrogen isotopic composition and the aluminum content of the cleaned opal suggests that the diagenetic organic N loss does not fractionate N isotopes, which is as expected given that this loss process relates to the geochemistry of the opal, not the organic matter. Our findings help to explain the previous observations that opal from clay-rich sediments (1) requires harsher chemical cleaning to remove diagenetic N and (2) has diatom-native N that is better protected from alteration by excessively harsh chemical cleaning. The diagenetic alteration of the opal may have led to partial protection of the external N from the surrounding sediments. At the same time, the diatom frustules have been chemically 'hardened' by the diagenesis associated with Al uptake, reducing the chemical accessibility of the diatom-native N that remains.