Concentrations of DCHOs ranged from 0 to ~1500 µM C in the interstitial waters at the seven sites studied (Table T1). Depth profiles showed no consistent trends (Figs. F1, F2). At some sites (1226, 1227, 1228, and, perhaps, 1231), DCHO concentrations decrease with depth, whereas at Site 1225 DCHO concentrations increase with depth. At the remaining sites (1229 and 1230) DCHO concentrations show minimal downhole gradients. Of equal importance, both the magnitude and direction of these downhole gradients showed no consistent trends between open-ocean (Sites 1225, 1226, and 1231) and Peru margin sites (1227, 1228, 1229 and 1230) (Figs. F1, F2).
Relative DCHO concentrations (i.e., as a fraction of the total DOC concentration in the interstitial waters) did, however, appear to vary consistently among the different sites (Fig. F3; DOC data from Smith, this volume). Although there is scatter in the data, at the open-ocean sites relative DCHO concentrations were ~0.8, suggesting that the DOC in these sediment interstitial waters is extremely carbohydrate rich. In contrast, relative DCHO concentrations were lower at the Peru margin sites (~0.3 at Sites 1227, 1228, and 1229 and ~0.1 at Site 1230). When compared with maximum interstitial water dissolved inorganic carbon (DIC) concentration at these sites (data from D'Hondt, Jørgensen, Miller, et al., 2003), relative DCHO concentrations decreased with increasing values of DICmax (Fig. F4).
If we assume that values of DICmax can be used as a first-order proxy for rates of sediment organic matter remineralization, the results in Figure F4 suggest that dissolved carbohydrates become an increasingly less important component of the interstitial water DOC pool as remineralization rates decrease. This trend is consistent with results from shallower estuarine and continental margin sediments (Burdige et al., 2000). In this study, these observations were interpreted as being related to changes in the overall controls on sediment organic matter remineralization, based on the assumption that dissolved carbohydrates are preferentially found in the pool of reactive HMW-DOC intermediates produced and consumed during sediment organic matter remineralization.
This explanation assumes that as rates of sediment organic matter remineralization decrease, that oxidative or hydrolytic processes affecting the initial HMW-DOC intermediates of sediment organic matter remineralization exert increasingly more control on remineralization. This, therefore, allows HMW-DOC intermediates to increase in the pore waters to a greater extent (also see related discussions in Burdige and Gardner, 1998). Conversely, as remineralization rates increase, fermentative or perhaps terminal respiratory processes exert increasingly more control on sediment organic matter remineralization (e.g., see Brüchert and Arnosti, 2003, and references therein).
At the same time though, most DOC in sediment interstitial waters appears to be refractory low molecular weight material (Burdige and Gardner, 1998). Therefore, the high relative DCHO concentrations observed at the open-ocean sites could also suggest that some (or all) of these dissolved carbohydrates are refractory to microbial degradation, yet are still chemically recognized by the assay used here. Additional studies will be needed to more critically examine all of these suggestions.