Fluorescent dissolved organic matter in marine sediment pore waters

Abstract

Fluorescent dissolved organic matter (FDOM) in sediment pore waters from contrasting sites in the Chesapeake Bay and along the mid-Atlantic shelf/slope break was studied using three-dimensional fluorescence spectroscopy. Benthic fluxes of FDOM were also examined at the Chesapeake Bay sites. The major fluorescence peaks observed in these pore waters corresponded to those observed in the water column. These included peaks ascribed to the fluorescence of humic-like material (peaks A, C and M), as well as protein-like peaks that appear to result from the fluorescence of the aromatic amino acids tryptophan and tyrosine. In these pore waters we also observed a fourth humic-like fluorescence peak (A′). These four humic-like peaks appeared to occur in pairs (peaks A and M in one pair and peaks A′ and C in another pair) with near identical emission maxima but different excitation maxima. Peaks A′ and C were red shifted relative to peaks A and M. Humic-like fluorescence increased with sediment depth at almost all stations, and was closely correlated with total DOC. This fluorescence appeared to be a tracer for the refractory, relatively low molecular weight pore water DOM that accumulates with depth during sediment diagenesis. Fluorescence–DOC relationships indicated that larger relative amounts of humic-like FDOM were seen in anoxic sediments versus sub-oxic or mixed redox sediments. By extension, these observations suggest that refractory humic-like compounds (in general) are preferentially preserved in sediment pore waters under anoxic conditions. A simple conceptual model is presented here which proposes that different types of organic matter (e.g., marine vs. terrestrial) as well as internal transformations of DOM or FDOM may lead to the occurrence of these humic-like fluorophores. This model is consistent with a wide range of data on FDOM in marine as well as freshwater systems. Protein-like fluorescence showed no coherent depth trends in sediment pore waters, other than the fact that pore water fluorescence intensities were greater than bottom water values. Protein-like fluorescence in pore waters may be associated with refractory DOM, although this observation is somewhat equivocal. In contrast, the results of benthic flux studies suggested that here protein-like fluorescence was associated with reactive DOM intermediates of organic matter diagenesis (e.g., dissolved peptides and proteins) produced near the sediment–water interface. Furthermore, the interplay between transport processes and the depth zonation of DOM cycling in bioirrigated sediments leads to molecular diffusion (rather than bioirrigation) playing a much more important role in transporting protein-like fluorescence out of the sediments. In contrast, bioirrigation dominates sediment–water exchange of humic-like fluorescence (and therefore most DOC in general). Finally, benthic flux studies indicated that sediments represent a source of chromophoric DOM to coastal waters, although further work will be needed to quantify their significance in terms of other known sources of this material (e.g., riverine input, phytoplankton degradation products).