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

Abstract

<p><strong>Abstract.</strong> The eastern tropical South Pacific (ETSP) represents one of the most productive areas in the ocean that is characterized 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 utilized by microbial communities. The degradation of POM is associated with dissolved organic matter (DOM) production and reworking. The release of recalcitrant DOM to the overlying waters may represent an important organic matter escape mechanism from remineralization within sediments but 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 <i>in situ</i> fluxes of DOC and DON in order to investigate processes affecting DOM cycling at the sediment–water interface along a transect 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 and DON accumulated with increasing sediment depth, suggesting an imbalance between DOM production and remineralization within sediments. High DON accumulation resulted in very low pore water DOC / DON ratios (> 1) which could be caused by either an <q>imbalance</q> in DOC and DON remineralization, or to the presence of an additional nitrogen source. Diffusion driven fluxes of DOC and DON exhibited high spatial variability. They varied from 0.2–0.1 mmol m<sup>−2</sup> d<sup>−1</sup> to 2.52–1.3 mmol m<sup>−2</sup> d<sup>−1</sup> and from −0.042–0.02 mmol m<sup>−2</sup> d<sup>−1</sup> to 3.32–1.7 mmol m<sup>−2</sup> d<sup>−1</sup>, respectively. Generally low net <i>in situ</i> DOC and DON fluxes as well as steepening of spectral slope (<i>S</i>) of CDOM and accumulation of humic-like FDOM at the near-bottom waters over time indicated active microbial DOM utilization at the sediment–water interface, potentially stimulated by nitrate (NO<sub>3</sub><sup>−</sup>) and nitrite (NO<sub>2</sub><sup>−</sup>). The microbial DOC utilization rates, estimated in our study, may be sufficient to support denitrification rates of 0.2–1.4 mmol m<sup>−2</sup> d<sup>−1</sup>, suggesting that sediment release of DOM contributes substantially to nitrogen loss processes in the ETSP off Peru.</p>

Molecular Composition of Dissolved Organic Matter in Sediment Porewater of the Arctic Deep-Sea Observatory HAUSGARTEN (Fram Strait)

“…The DOM composition in the upper 10 cm of sediments suggests that the signal of recent phytodetritus was restricted to surface sediment porewaters, indicating that the signal is rapidly lost after deposition, due to porewater DOM alteration by the bacterial communities in the sediment. Our observations are in agreement with the increase of more refractory DOM with increasing sediment depth detected by nuclear magnetic resonance (Fox et al, 2018) and optical properties (Loginova et al, 2020) of porewater DOM in other marine sediments.…”

“…Nevertheless, a comparison of bottom waters from eastern and western Fram Strait indicated that the latter had higher abundances of unique CRAM-like compounds (absent in their surface porewaters) compared to the East, consistent with less input of fresh material at these permanently ice-covered stations. These data suggest that Arctic sediments are a source of CRAM and fresh DOM to the overlying bottom water, whose contributions vary depending on the quality and quantity of the OM deposited, as previously suggested for other environments (Fox et al, 2018;Loginova et al, 2020).…”

“…4500 m water depth) and Antarctic Ocean (<2500 m water depth), DOC fluxes in the western Fram Strait were in the lower range, while DOC fluxes in the eastern Fram Strait (HG, S1-3 and N4 stations) were up to 9 fold higher (37-259 and 66 mmol/m 2 /yr for Weddell Sea and for Atlantic, respectively; Hulth et al, 1997;Hall et al, 2007). DOC fluxes from Arctic shelves located far from our sampling area, and from productive systems were similar or higher than those from the eastern shallower stations in our study (Hulthe et al, 1997;Loginova et al, 2020), supporting the hypothesis of strong links to productivity regimes and the availability of OM at the seafloor.…”

“…± SD of 134 ± 34 and 212 ± 78 µmol/L, respectively; Rossel et al, 2016). Compared to other marine systems, DOC concentrations in Arctic porewaters of Fram Strait were in the lower range, and those of the Central Arctic substantially lower than that of other oceans (Burdige and Komada, 2015;Schmidt et al, 2017;Fox et al, 2018;Loginova et al, 2020). Nonetheless, the gradients between bottom water and porewater are in line with previous reports indicating at least one magnitude higher DOC concentration in porewater compared to bottom water, supporting the role of POM diagenesis in surface sediments for the production of DOC and flux to overlying bottom water (Schmidt et al, 2011;Burdige and Komada, 2015;Loginova et al, 2020).…”

“…Porewater DOM is further reworked during a degradation continuum that begins in surface sediments, with the uptake of monomeric molecules (e.g., amino acids, sugars and fatty acids) by benthic organisms dominated in biomass by bacteria, and continues in deeper sediment layers, with the utilization of more complex substrates by subsurface microbial communities (Burdige and Komada, 2015). Compared to the amount remineralized in the seabed, a significant fraction of this pool escapes diagenesis via diffusion to the water column (Burdige et al, 1992(Burdige et al, , 1999Burdige and Komada, 2015;Rossel et al, 2016;Loginova et al, 2020), sorption onto minerals (Arnarson and Keil, 2001;Aufdenkampe et al, 2001), formation of complexes with metals (Seidel et al, 2014(Seidel et al, , 2015aLinkhorst et al, 2017) or due to sulfurization, which favors preservation of OM under anoxic conditions (Sinninghe Damste and de Leeuw, 1990;Schmidt et al, 2014;Jessen et al, 2017;Pohlabeln et al, 2017). Although previous studies have evaluated the composition and reactivity of porewater DOM (Burdige and Komada, 2015 and references therein), the complexity of this pool limits our understanding of its role in the preservation and degradation of OM.…”