Diagenetic behavior of structural materials formed by benthic macrofauna

Abstract

The role of biogenic tube material produced by polychaete worms in the cycling of organic matter in marine sediments is relatively unknown. Here I characterize the fate of the constructional material of one polychaete, Chaetopterus variopedatus. Preliminary experiments showed little to no degradation under oxygenated conditions or in anoxic seawater, so degradation was followed in anoxic sediment. SEM studies revealed distinct morphological changes during anoxic decomposition. The tube walls are made from a well-formed cross-woven mesh of fibers, which are individually typically 100 nm in diameter. In incubated tubes, the integrity of this mesh structure is lost during decomposition. SEM and X-ray studies of tube material also revealed the presence of swathes of manganese micro-nodular structures in fresh tubes that are also lost during anoxic decomposition. The time-scale of organic matter release from these tubes is on the order of months, with a higher pulse release of organic matter in the first 60 days, followed by a more gradual release. In addition to the slow release of organic matter, the tube material also acts as a sink for redox-sensitive metals including manganese and iron. Manganese is typically 40X enriched compared to surrounding sediments, whereas the iron is typically 2.5X enriched. Manganese is lost more rapidly and continuously from the tubes over time compared to iron, which after an initial decrease stays fairly constant. The time-scale of release of manganese is on the order of weeks, with >50% being lost within the first month. This has significant effects on the concentration of manganese in surrounding sediments and porewater. In localized areas where they are found, C. variopedatus tube production can account for ~12% of the average annual POC flux to the benthos from water column primary production, ~11% of the PON flux and >100% of the POP flux to the benthos.