Early Diagenesis in the Hypoxic and Acidified Zone of the Northern Gulf of Mexico: Is Organic Matter Recycling in Sediments Disconnected From the Water Column?

Abstract

Hypoxia and associated acidification are growing concerns for ecosystems and biogeochemical cycles in the coastal zone. The northern Gulf of Mexico (nGoM) has experienced large seasonal hypoxia for decades linked to the eutrophication of the continental shelf fueled by the Mississippi River nutrient discharge. Sediments play a key role in maintaining hypoxic and acidified bottom waters, but this role is still not completely understood. In the summer 2017, when the surface area of the hypoxic zone in the nGoM was the largest ever recorded, we investigated four stations on the continental shelf differentially influenced by river inputs of the Mississippi-Atchafalaya River System and seasonal hypoxia. We investigated diagenetic processes under normoxic, hypoxic, and nearly anoxic bottom waters by coupling amperometric, potentiometric, and voltammetric microprofiling with high-resolution diffusive equilibrium in thin-films (DET) profiles and porewater analyses. In addition, we used a time-series of bottom-water dissolved oxygen from May to November 2017, which indicated intense O2 consumption in bottom waters related to organic carbon recycling. At the sediment-water interface (SWI), we found that oxygen consumption linked to organic matter recycling was large with diffusive oxygen uptake (DOU) of 8 and 14 mmol m–2 d–1, except when the oxygen concentration was near anoxia (5 mmol m–2 d–1). Except at the station located near the Mississippi river outlet, the downcore pore water sulfate concentration decrease was limited, with little increase in alkalinity, dissolved inorganic carbon (DIC), ammonium, and phosphate suggesting that low oxygen conditions did not promote anoxic diagenesis as anticipated. We attributed the low anoxic diagenesis intensity to a limitation in organic substrate supply, possibly linked to the reduction of bioturbation during the hypoxic spring and summer.

Sources and sinks of bottom water oxygen in a seasonally hypoxic fjord

“…In the organic-rich Bedford Basin sediment (Fader and Miller, 2008), DOU is proportional to bottom water oxygen concentration, implying that the benthic O 2 flux is diffusion limited. Similar observations have been made in other studies (Rasmussen and Jorgensen, 1992;Rabouille et al, 2021). The annual mean DOU in 2019, based on four seasonal measurements in this study, was 11.5 ± 4.8 mmol m -2 d -1 , and model derived mean benthic uptake was 12.6 ± 7.9 mmol m -2 d -1 .…”

Temporal and spatial variations in benthic nitrogen cycling in a temperate macro-tidal coastal ecosystem: Observation and modeling

“…The gradual decrease of O2 and low O2 penetration depth suggest rapid O2 consumption related to OM degradation (Pastor et al, 2011;Rabouille et al, 2021). This is consistent with our model results, which indicate that OM, especially the OM1 pool, was decomposed primarily at the sediment surface using O2 (Table 1).…”

“…In addition, decomposition of the deposited phytoplanktonic material can rapidly consume O2 at the sediment surface (within hours to several days) and can be followed by O2 depletion in the bottom water, especially during summer. This pattern may also be valid for other coastal ecosystems with similar characteristics as the VB, as was reported in oxic sediments from the North Sea (De Borger et al, 2021) and in the seasonally hypoxic sediments of the northern Gulf of Mexico (Rabouille et al, 2021). The overall results lead to conclude that temporal and spatial variations of the benthic DIN and DON fluxes in the VB depend predominantly on the sedimentation of the diatom blooms rather than on the composition of the sediment column itself.…”

“…The mineralization of OM in the sediment leads to a build-up of NH4 + in the sediment porewater resulting in the benthic efflux of NH4 + (Herbert, 1999;. The absence of a significant change in the NH4 + pore-water profiles during the temporal study carried out in 2015 agrees with the lack of temporal variations in diffusive NH4 + fluxes, implying that a large part of deposited OM is decomposed at the sediment surface before integrating the sediment column, as suggested by Rabouille et al (2021) for the Louisiana shelf. Relatively low diffusive NH4 + fluxes compared to the measured fluxes confirms that significant NH4 + production took place through aerobic OM mineralization at the SWI in contact with the bottom water.…”

Exceptionally high respiration rates in the reactive surface layer of sediments underlying oxygen-deficient bottom waters

“…Higher phosphate fluxes—a possible proxy for OC respiration—have been measured in chamber incubations than calculated from sediment profiles in unbioturbated sediments underlying hypoxic or anoxic bottom waters in the Peruvian oxygen-minimum zone (lander-based estimates were 50–210% higher; [55]) and the Pakistan margin oxygen-minimum zone (lander-based estimates were orders of magnitude higher; [56]). Additionally, a recent study in the hypoxic Gulf of Mexico estimated a seven times higher respiration rate based on high-resolution oxygen microsensor profiles compared with core-based DIC profiles [57]. Because of the higher vertical advective velocity at higher MARs, the respiration rate offsets are expected to decrease with increasing MARs.…”