Early Diagenesis in Sediments of the Venice Lagoon (Italy) and Its Relationship to Hypoxia

This work focuses on sediments of a shallow water lagoon, located in a densely populated area undergoing multiple stressors, with the goal of increasing the understanding of the links between diagenetic processes occurring in sediments, the dynamics of dissolved oxygen (DO) in the water column, and potential consequences of hypoxia. Sediment data were collected over three consecutive years, from 2015 to 2017, during spring-summer, at five stations. Measured variables included: sediment porosity, grain size and organic carbon content, porewater microprofiles of O-2, pH and H2S, porewater profiles of dissolved inorganic carbon (DIC), total alkalinity (TA), NH4+, NO3-, dissolved Fe, and SO42-. In addition, long-term time series of oxygen saturations in the water column (years 2005-2017) were utilized in order to identify the occurrence and duration of hypoxic periods. The results show that the median DO saturation value in summer months was below 50% (around 110 mu mol L-1), and that saturation values below 25% (below the hypoxic threshold) can persist for more than 1 week. Sediment stations can be divided in two groups based on their diagenetic intensity: intense and moderate. At these two groups of stations, the average DIC net production rates, estimated trough a steady-state model (Profile) were, respectively, of 2.8 and 1.0 mmol m(-2) d(-1), SO42- consumption rates were respectively 1.6 and 0.4 mmol m(-2) d(-1), while diffusive oxygen uptake fluxes, calculated from the sediment microprofile data, were of 28.5 and 17.5 mmol m(-2) d(-1). At the stations characterized by intense diagenesis, total dissolved sulfide accumulated in porewaters close to the sediment-water interface, reaching values of 0.7 mM at 10 cm. Considering the typical physico-chemical summer conditions, the theoretical time required to consume oxygen down to the hypoxic level by sediment oxygen demand ranges between 5 and 18 days, in absence of mixing and re-oxygenation. This estimation highlights that sediment diagenesis may play a crucial role in triggering and maintaining hypoxia of lagoon waters during the summer season in specific high intensity diagenesis zones. This role of the sediment could be enhanced by changes in regional climate conditions, such as the increase in frequency of summer heat waves

Modelling biogeochemical processes in sediments from the north-western Adriatic Sea: response to enhanced particulate organic carbon fluxes

This work presents the result of a study carried out in the north-western Adriatic Sea, by combining two different types of biogeochemical models with field sampling efforts. A longline mussel farm was taken as a local source of perturbation to the natural particulate organic carbon (POC) downward flux. This flux was first quantified by means of a pelagic model of POC deposition coupled to sediment trap data, and its effects on sediment bioirrigation capacity and organic matter (OM) degradation pathways were investigated constraining an early diagenesis model by using original data collected in sediment porewater. The measurements were performed at stations located inside and outside the area affected by mussel farm deposition. Model-predicted POC fluxes showed marked spatial and temporal variability, which was mostly associated with the dynamics of the farming cycle. Sediment trap data at the two sampled stations (inside and outside of the mussel farm) showed average POC background flux of 20.0–24.2 mmol Cm2 d1. The difference of organic carbon (OC) fluxes between the two stations was in agreement with model results, ranging between 3.3 and 14.2 mmol Cm2 d1, and was primarily associated with mussel physiological conditions. Although restricted, these changes in POC fluxes induced visible effects on sediment biogeochemistry. Observed oxygen microprofiles presented a 50%decrease in oxygen penetration depth (from 2.3 to 1.4 mm), accompanied by an increase in the O2 influx at the station below the mussel farm (19–31 versus 10–12 mmol O2 m2 d1/ characterised by higher POC flux. Dissolved inorganic carbon (DIC) and NHC 4 concentrations showed similar behaviour, with a more evident effect of bioirrigation underneath the farm. This was confirmed through constraining the early diagenesis model, of which calibration leads to an estimation of enhanced and shallower bioirrigation underneath the farm: bioirrigation rates of 40 yr1 and irrigation depth of 15 cm were estimated inside the shellfish deposition footprint versus 20 yr1 and 20 cm outside. These findings were confirmed by independent data on macrofauna composition collected at the study site. Early diagenesis model results indicated a larger organic matter mineralisation below the mussel farm (11.1 versus 18.7 mmolm2 d1/, characterised by similar proportions between oxic and anoxic degradation rates at the two stations, with an increase in the absolute values of oxygen consumed by OM degradation and reduced substances re-oxidation underneath the mussel farm

Impact of spatial heterogeneities on oxygen consumption in sediments: Experimental observations and 2D numerical modeling

International audienc

Spatial Variability of Organic Matter and Phosphorus Cycling in Rhône River Prodelta Sediments (NW Mediterranean Sea, France): a Model-Data Approach

International audienceThe Mediterranean Sea (MS) is a large oligotrophic sea whose productivity is sensitive to riverine nutrient inputs. More specifically, phosphorus (P) river supply is crucial for the MS, with an important role of the estuarine/deltaic filter especially for the storage and recycling in sediments. A benthic dataset from the Rhône River prodelta was used to derive P budgets, by means of an early diagenetic model including the benthic P cycle. The model was fitted to pore water profiles of oxygen, nitrate, sulfate, dissolved inorganic carbon, ammonium, oxygen demand units, dissolved inorganic phosphorus (DIP) and solid data (organic carbon (OC), Fe-bound P, Ca-bound P and organic P). Results indicated that the intensity of biogeochemical processes occurring below the sediment–water interface decreased from the river mouth to the adjacent continental shelf with decreasing integrated rates of OC mineralization (160–10 mmol m−2 day−1). The organic P mineralization was intense near the river mouth and decreased offshore (1196–80 μmol m−2 day−1). Its contribution to DIP release was large (> 90%). Fe-bound P had a key role in transferring P to deeper layers. These deltaic sediments played an important role as a source of regenerated DIP. A significant part of DIP was recycled to the overlying waters (72–94%), representing 25% of the riverine DIP discharge. Simultaneously, 6–28% of DIP produced in sediments was buried as Ca-bound P. Overall, this study highlighted the importance of deltaic sediments as an additional source of DIP to the coastal sea, and a minor but permanent sink of phosphorus as solid P burial

Characterization of the benthic biogeochemical dynamics after flood events in the Rhône River prodelta: A data-model approach

At the land-sea interface, the benthic carbon cycle is strongly influenced by the export of terrigenous particulate material across the river-ocean continuum. Episodic flood events delivering massive sedimentary materials can occur, but their short-term impact on carbon cycling is poorly understood. In this paper, we use a coupled data-model approach to estimate the temporal variations of sediment-water fluxes, biogeochemical pathways and their reaction rates during these abrupt phenomena. We studied one episodic depositional event in the vicinity of the Rhône River mouth (NW Mediterranean Sea) during the fall-winter of 2021–2022. The distribution of dissolved inorganic carbon (DIC), sulfate (SO42−) and methane (CH4) were measured in sediment porewater collected every 2 weeks before and after the deposition of a 25 cm sediment layer during the main winter flood event. Significant changes in the distribution of DIC, SO42− and CH4, concentrations were observed in the sediment porewaters. The use of an early diagenetic model (FESDIA) to calculate biogeochemical reaction rates and fluxes revealed that this type of flooding event can increase the total organic carbon mineralization rate in the sediment by 75 % a few days after deposition, essentially by increasing the sulfate reduction contribution to total mineralization relative to non-flood depositional period. It predicts a short-term decrease of the DIC flux out of the sediment from 100 to 55 mmol m−2 d−1 after the deposition of the new sediment layer with a longer-term increase by 4 %, therefore implying an initial internal storage of DIC in the newly deposited layer and a slow release over relaxation of the system. Furthermore, examination of the stoichiometric ratios of DIC and SO42− as well as model output over this five-months window shows a decoupling between the two modes of sulfate reduction following the deposition – organoclastic sulfate reduction (OSR) intensified in the newly deposited layer below the sediment surface, whereas anaerobic oxidation of methane (AOM) intensified at depth below the former buried surface. This depth-wise bifurcation of both pathways of sulfate reduction in the sediment column is clearly related to the deepening of the sulfate-methane transition zone (SMTZ) by 25 cm after the flood deposition. Our findings highlight the significance of short-term transient biogeochemical processes at the seafloor and provide new insights on the benthic carbon cycle in the coastal ocean

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?

International audienceHypoxia 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

Differential manganese and iron recycling and transport in continental margin sediments of the Northern Gulf of Mexico

International audiencePore water and solid phase geochemical profiles of sediment cores collected along two transects on the western and eastern sides of the Mississippi River mouth in the northern Gulf of Mexico were incorporated into a reactive transport model to determine the role of manganese and iron in the remineralization of carbon. Reactive transport model calculations indicate that sedimentation rates control the intensity of anaerobic carbon remineralization and select for the dominant anaerobic carbon remineralization pathways. Although sulfate reduction dominates the shelf station (65 m water depth), denitrification and microbial manganese reduction appear equally significant anaerobic respiration processes along the continental slope the closest to the Mississippi River, whereas microbial iron reduction does not represent an important process in these sediments. These findings suggest that the differential kinetics of manganese and iron redox transformations influence carbon remineralization processes on the continental slope. The fast kinetics of Fe2+ oxidation near the sediment-water interface and high sedimentation rates maintain Fe under the form of Fe(III) oxides and thermodynamically prevent sulfate reduction from dominating carbon remineralization processes on the slope, whereas the much slower Mn2+ oxygenation kinetics allows diffusion of Mn2+ across the sediment-water interface of the shelf station closest to the river mouth. Exposure to oxygenated bottom waters and entrainment within mobile muds typical of deltaic sediments during high riverine discharge likely promote the formation and downslope transport of Mn(III/IV) oxides within the nepheloid layer. This phenomenon appears to form a manganese ‘conveyor belt’ that selectively enriches Mn(III/IV) oxides relative to Fe(III) oxides in the deep sediment. In contrast, the intensity of anaerobic carbon remineralization processes along the eastern continental slope the farthest from the Mississippi River plume is much lower due to the low organic and lithogenic inputs, and denitrification dominates anaerobic respiration. Overall, these findings suggest that manganese cycling and its role in carbon remineralization processes in continental slope sediments exposed to large riverine inputs may be more important than previously considered

A new device to follow temporal variations of oxygen demand in deltaic sediments: the LSCE benthic station

International audienceA new benthic station equipped with oxygen microelectrodes and environmental sensors was developed by Laboratoire des Sciences du Climat et de L'Environnement (LSCE) and Division Technique of the Institut National des Sciences de L'Univers (DT‐ INSU) to perform in situ time series monitoring of sediment oxygen demand, linked to the mineralization of organic matter. The time series typically cover periods of 2‐ 3 months, with a base frequency of 1 set of oxygen profiles per day. The profiling head assessed the lateral heterogeneity of the sediment oxygen demand at the beginning of the time series over a 0.8‐ m long rectangle to discriminate spatial and temporal variability. A continuous recalibration is performed using a moored oxygen optode anchored to the benthic station together with a set of environmental sensors. These sensors (turbidity, temperature, salinity, and oxygen) can trigger a high‐ frequency profiling mode to investigate the fate of particulate organic matter delivered during floods, resuspension, and deposition events. Deployments of the benthic station were performed in the Rhone River subaqueous delta (Mediterranean Sea). We show that “stable” periods (when neither floods nor storms occur) were characterized by a stable oxygen demand. In the case of resuspension events, an increase of the sediment oxygen demand by a factor of 2‐ 3 with a relaxation time of 1 day was observed, indicating that the new benthic station can adequately capture the impact of resuspension events on the oxygen demand in deltaic sediments

Summer hydrography and circulation in Storfjorden, Svalbard, following a record low winter sea‐ice extent in the Barents Sea

Storfjorden, Svalbard, hosts a polynya in winter and is an important source region of Brine-enriched Shelf Water (BSW) that, if dense enough, feeds the Arctic Ocean deep water reservoir. Changes in the BSW production may thus have far-reaching impacts. We analyze the water mass distribution and circulation in Storfjorden and the trough south of it, Storfjordrenna, using hydrographic sections occupied in July 2016, following a winter characterized by the lowest ice coverage recorded in the Barents Sea. These observations reveal an unusual hydrographic state, characterized at the surface by the near absence of Melt Water and Storfjorden Surface Water, replaced by a saltier water mass. At depth, BSW (maximum salinity of 34.95) was found from the bottom up to 90 m, above the 120-m deep sill at the mouth to Storfjordrenna. However, no gravity driven overflow was observed downstream of the sill: the dome of BSW remained locked over the depression in a cyclonic circulation pattern consistent with a stratified Taylor column. Observations further reveal a previously unreported intrusion of Atlantic Water far into the fjord, promoting isopycnal mixing with entrapped Arctic Water. This intrusion was possibly favored by positive wind stress curl anomalies over Svalbardbanken and Storfjordrenna. The bottom plume exiting Storfjordrenna was weak, carrying Polar Front Water rather than BSW, too light to sink underneath the Atlantic Water layer at Fram Strait. Whether Storfjorden switched durably to a new hydrographic state, following the observed Atlantification of the Barents Sea after 2005, remains to be established. Key Points Changes in the hydrography and circulation in Storfjorden a decade after a regime shift in the Barents Sea linked to its Atlantification The less saline Brine-enriched Shelf Water remained entrapped in Storfjorden within a cyclonic circulation pattern with no overflow Storfjorden was flooded with Atlantic Water, promoting isopycnal mixing with Arctic Water and the local formation of East Spitsbergen Water Plain Language Summary Storfjorden, east of Spitsbergen, plays an important role in Arctic Ocean climate through formation of dense water as salt is added to the ocean when sea ice forms. This dense water accumulates in winter before spilling toward the deep ocean into autumn, fueling the global ocean’s circulation. We analyze observations from a research cruise in July 2016, following a winter season characterized by the lowest ice coverage in the Barents Sea ever recorded. These observations reveal striking differences from previous reports, which are mostly based on data prior to the 2005 regime shift in the Barents Sea characterized by warmer temperature and reduced ice cover, an expression of its ”Atlantification” reported by many authors. First, the expected overflow of the locally formed dense water was absent. The latter, less saline than usual, was instead trapped in Storfjorden suggesting an intermittent discharge regime usually observed in the fall. Another notable observation is the intrusion of Atlantic Water far inside the fjord. Such a flooding episode, increasingly frequent in the fjords of the west coast of Spitsbergen, is previously unreported in Storfjorden. These observations made in the wake of an exceptionally mild winter could prefigure more permanent changes in this important region