Sediment release of dissolved organic matter to the oxygen minimum zone off Peru

Abstract

Abstract. The eastern tropical South Pacific (ETSP) represents one of the most productive areas in the ocean that is characterised by a pronounced oxygen minimum zone (OMZ). Particulate organic matter (POM) that sinks out of the euphotic zone is supplied to the anoxic sediments and utilised by microbial communities, and the degradation of POM is associated with the production and reworking of dissolved organic matter (DOM). The release of DOM to the overlying waters may, therefore, represent an important organic matter escape mechanism from remineralisation within sediments but has received little attention in OMZ regions so far. Here, we combine measurements of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) with DOM optical properties in the form of chromophoric (CDOM) and fluorescent (FDOM) DOM from pore waters and near-bottom waters of the ETSP off Peru. We evaluate diffusion-driven fluxes and net in situ fluxes of DOC and DON to investigate processes affecting DOM cycling at the sediment–water interface along a transect at 12∘ S. To our knowledge, these are the first data for sediment release of DON and pore water CDOM and FDOM for the ETSP off Peru. Pore water DOC accumulated with increasing sediment depth, suggesting an imbalance between DOM production and remineralisation within sediments. High DON accumulation resulted in very low pore water DOC ∕ DON ratios (≤1) which could be caused by an “uncoupling” in DOC and DON remineralisation. Diffusion-driven fluxes of DOC and DON exhibited high spatial variability and ranged from 0.2±0.1 to 2.5±1.3 mmolm-2d-1 and from -0.04±0.02 to 3.3±1.7 mmolm-2d-1, respectively. Generally low net in situ DOC and DON fluxes, as well as a steepening of spectral inclination (S) of CDOM and an increase in humic-like DOM at the sediment–water interface over time, indicated active microbial DOM utilisation. The latter may potentially be stimulated by the presence of nitrate (NO3-) and nitrite (NO2-) in the water column. The microbial DOC utilisation rates, estimated in our study, are potentially sufficient to support denitrification rates of 0.2–1.4 mmolm-2d-1, suggesting that the sediment release of DOM may on occasion contribute to nitrogen loss processes in the ETSP off Peru.

Sedimentary supply of humic-like fluorescent dissolved organic matter and its implication for chemoautotrophic microbial activity in the Izu-Ogasawara Trench

“…Consequently, the sediment fluxes for C1 and C2 were estimated to be ~ 100 (RU L) m -2 yr −1 and be ~ 250 (RU L) m −2 yr −1 , respectively. These results were within the previous results 7 , 24 , 27 . Differently from this study, the sediment fluxes of C1 and C2 were similar to each other or those for C1 were greater than those for C2 in previous studies 7 , 24 , 27 .…”

“…These results were within the previous results 7 , 24 , 27 . Differently from this study, the sediment fluxes of C1 and C2 were similar to each other or those for C1 were greater than those for C2 in previous studies 7 , 24 , 27 . Obviously, the reason why the sediment flux of C2 was higher than that of C1 needs to be further investigated.…”

“…The other caveat is that other possible mechanisms contributing to the increase in FDOM H with depth in the Izu-Ogasawara Trench may exist. In this study, we clarified that the steady sedimentary flux of FDOM H within the previously reported range 7 , 24 , 27 can explain our observations. Previous studies in the Izu-Ogasawara Trench hypothesized that dissolved and particulate substances could be intermittently released and suspended from sediments in the hadal zones by episodic phenomena, such as earthquakes and/or submarine volcano eruptions 4 , 5 .…”

“…Because silicate and 222 Rn are dissolved constituents and likely transferred from the sediments at the slope and bottom of the trench to the overlying waters through pore water 5 , dissolved organic matter (DOM) and electron donors are also likely to be transported from the sediments to the overlying waters. Actually, various studies suggested that DOM and electron donors such as ammonium, nitrite, and hydrogen sulfide are released from sediments to overlying waters 6 , 7 . These substances may affect microbial respiration in the water column 7 , 8 .…”

“…Actually, various studies suggested that DOM and electron donors such as ammonium, nitrite, and hydrogen sulfide are released from sediments to overlying waters 6 , 7 . These substances may affect microbial respiration in the water column 7 , 8 . Hiraoka et al 1 showed that anaerobic environments are formed in the bottom sediments of the Izu-Ogasawara Trench, and that ammonium concentrations in the sediments increase with depth.…”