Shannon M Owings

Laurie Bréthous

Eryn M Eitel

Benjamin P Fields

Anthony Boever

Jordon S Beckler

Bruno Bombled

Bruno Lansard

Édouard Metzger

Christophe Rabouille

Edouard Metzger

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

Keywords:

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Riverine-dominated continental margins (RiOMars) represent carbon remineralization hot spots as a result of the significant input of natural organic matter and inorganic particulate material from terrestrial origin [START_REF] Bianchi | Centers of organic carbon burial and oxidation at the landocean interface[END_REF]. In addition, the nutrient supply from the underlying sediment [START_REF] Aller | Remineralization rates, recycling, and storage of carbon in Amazon shelf sediments[END_REF][START_REF] Grégoire | Nitrogen budget of the northwestern Black Sea shelf inferred from modeling studies and in situ benthic measurements[END_REF][START_REF] Morse | Benthic biogeochemistry beneath the Mississippi River plume[END_REF] or riverine inputs [START_REF] Turner | Characterization of nutrient, organic carbon, and sediment loads and concentrations from the Mississippi River into the northern Gulf of Mexico[END_REF] promotes primary production on continental shelves that may enhance the overall input of natural organic matter to the seafloor [START_REF] Aller | Remineralization rates, recycling, and storage of carbon in Amazon shelf sediments[END_REF][START_REF] Waterson | Sources of sedimentary organic matter in the Mississippi River and adjacent Gulf of Mexico as revealed by lipid biomarker and δ13CTOC analyses[END_REF] and increase diagenetic reactivity by a priming effect [START_REF] Bianchi | The role of terrestrially derived organic carbon in the coastal ocean: a changing paradigm and the priming effect[END_REF]. The supply of organic matter and lithogenic material to continental shelves depends on the intensity of riverine discharge, which typically varies seasonally (Bianchi et al.,2 2007; [START_REF] Mossa | Sediment dynamics in the lowermost Mississippi River[END_REF]. At low river discharge, the density difference between freshwater and seawater impedes the seaward transport of freshwater and its particulate load such that freshwater and sediment are both trapped in the estuary or backwater reaches of rivers [START_REF] Allison | A water and sediment budget for the lower Mississippi-Atchafalaya River in flood years 2008-2010: implications for sediment discharge to the oceans and coastal restoration in Louisiana[END_REF][START_REF] Geyer | The transport, transformation and dispersal of sediment by buoyant coastal flows[END_REF]. In contrast, the momentum of the river outflow at high discharge is strong enough to overcome this effect and transport water and sediment offshore. As a result, sediment trapped in estuaries or backwater reaches during low discharge seasons is biogeochemically processed until the next high discharge, such that the inorganic [START_REF] Allison | Discharge controls on the sediment and dissolved nutrient transport flux of the lowermost Mississippi River: implications for export to the ocean and for delta restoration[END_REF][START_REF] Meiggs | The effect of riverine discharge on biogeochemical processes in estuarine sediments[END_REF][START_REF] Sutula | Effect of seasonal sediment storage in the lower Mississippi River on the flux of reactive particulate phosphorus to the Gulf of Mexico[END_REF] and organic [START_REF] Cowie | Carbohydrate sources in a coastal marine environment[END_REF][START_REF] Cowie | Sources and reactivities of amino acids in a coastal marine environment[END_REF][START_REF] Sampere | Burial and degradation of organic carbon in Louisiana shelf/slope sediments[END_REF] content of surrounding shelf sediments is affected seasonally. In turn, the lithologic input to continental shelves is greater during high discharge as a result of increased erosion upstream [START_REF] Meiggs | The effect of riverine discharge on biogeochemical processes in estuarine sediments[END_REF]. Riverine dissolved and particulate material eventually delivered to the ocean flocculates and aggregates into fine-grained material that settles into the benthic boundary layer (BBL) 0.1-2 m above the seabed and forms mobile muds, an ephemeral layer that is often remobilized by physical mixing generated by tidal currents or storms [START_REF] Aller | Carbonate dissolution in Nearshore Terrigenous muds: the role of physical and biological reworking[END_REF][START_REF] Aller | Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors[END_REF][START_REF] Mckee | Transport and transformation of dissolved and particulate materials on continental margins influenced by major rivers: benthic boundary layer and seabed processes[END_REF]. These mobile muds are eventually deposited on continental shelves and slopes, with selective dispersal that depends on particle size, density, morphology, or composition [START_REF] Mckee | Transport and transformation of dissolved and particulate materials on continental margins influenced by major rivers: benthic boundary layer and seabed processes[END_REF], and enhance carbon remineralization by exposing sediment to oxygenated bottom waters [START_REF] Aller | Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors[END_REF][START_REF] Blair | The fate of terrestrial organic carbon in the marine environment[END_REF][START_REF] Hartnett | Influence of oxygen exposure time on organic carbon preservation in continental margin sediments[END_REF] and co-metabolizing fresh, planktonic organic carbon [START_REF] Canfield | Factors influencing organic matter preservation in marine sediments[END_REF]. As a result, sediment distribution across continental margins and their associated diagenetic processes may vary widely depending on the organic carbon and lithologic composition of the top sediment layers [START_REF] Canfield | Factors influencing organic matter preservation in marine sediments[END_REF][START_REF] Grégoire | Nitrogen budget of the northwestern Black Sea shelf inferred from modeling studies and in situ benthic measurements[END_REF][START_REF] Taillefert | Early diagenesis in the sediments of the Congo deep-sea fan dominated by massive terrigenous deposits: part II -Iron-sulfur coupling[END_REF].

The complex interplay between abiotic and biotic processes influences the availability of electron acceptors and efficiency of carbon remineralization processes. After aerobic respiration, sulfate reduction often plays a large role in marine sediments exposed to significant inputs of natural organic matter [START_REF] Bowles | Global rates of marine sulfate reduction and implications for sub-sea-floor metabolic activities[END_REF][START_REF] Ferdelman | Sulfate reduction in surface sediments of the Southeast Atlantic continental margin between 15 degrees 38'S and 27 degrees 57'S (Angola and Namibia)[END_REF][START_REF] Jørgensen | Mineralization of organic matter in the sea bed-the role of sulphate reduction[END_REF][START_REF] Lee | Organic carbon accumulation and sulfate reduction rates in slope and basin sediments of the Ulleung Basin, east/Japan Sea[END_REF][START_REF] Lin | Sulfate reduction and iron sulfide mineral formation in the southern East China Sea continental slope sediment[END_REF] mainly due to the high concentration of sulfate in seawater and the efficient carbon remineralization yield of this process (i.e., 2 mol of carbon per mole of sulfate reduced). In RiOMars and shelf to slope transition zones of passive continental margins, however, manganese (Mn) and iron (Fe) reduction may contribute to a large fraction of carbon remineralization processes [START_REF] Aller | Bioturbation and manganese cycling in hemipelagic sediments[END_REF][START_REF] Beckler | Importance of microbial iron reduction in deep sediments of river-dominated continental-margins[END_REF]Canfield et al., 1993a;Canfield et al., 1993b;[START_REF] Devereux | Changes in northern Gulf of Mexico sediment bacterial and archaeal communities exposed to hypoxia[END_REF][START_REF] Hyun | Manganese and iron reduction dominate organic carbon oxidation in surface sediments of the deep Ulleung Basin, East Sea[END_REF][START_REF] Law | Manganese, iron, and Sulphur cycling on the Pakistan margin[END_REF][START_REF] Oldham | The speciation and mobility of Mn and Fe in estuarine sediments[END_REF][START_REF] Sørensen | Early diagenesis in sediments from Danish coastal waters: microbial activity and Mn-Fe-S geochemistry[END_REF][START_REF] Thamdrup | Microbial manganese and sulfate reduction in Black Sea shelf sediments[END_REF]. The differential kinetics of Mn and Fe transformations also separates Mn 2+ and Fe 2+ vertically in the sediment column. Indeed, microbial Mn(III/IV) oxide reduction is thermodynamically more favorable and much faster than microbial Fe(III) reduction [START_REF] Arnold | Reductive dissolution of fe(III) oxides by Pseudomonas sp. 200[END_REF][START_REF] Dollhopf | Kinetics of Fe(III) and Mn(IV) reduction by the Black Sea strain of Shewanella putrefaciens using in situ solid state voltammetric Au/Hg electrodes[END_REF][START_REF] Larsen | Manganite reduction by Shewanella putrefaciens MR-4[END_REF][START_REF] Lovley | Novel mode of microbial energy metabolism: organic carbon oxidation coupled to Dissimilatory reduction of Iron or manganese[END_REF][START_REF] Myers | Bacterial manganese reduction and growth with manganese oxide as the sole Electron acceptor[END_REF] whereas abiotic Fe 2+ oxidation is much faster than abiotic (Stumm and Morgan, 1996) and biologically mediated Mn 2+ oxidation even if abiotic and biotic Mn 2+ oxidation rates in natural environments are difficult to disentangle [START_REF] Richard | Kinetics of manganese adsorption, desorption, and oxidation in coastal marine[END_REF]. As a result, Mn 2+ diffuses out of the sediment and is slowly reoxidized either close to the sediment surface [START_REF] Mouret | Benthic geochemistry of manganese in the Bay of Biscay, and sediment mass accumulation rate[END_REF] or in the overlying waters [START_REF] Trefry | Manganese fluxes from Mississippi Delta sediments[END_REF], whereas Fe 2+ is oxidized immediately at the oxic-anoxic transition [START_REF] Millero | The oxidation kinetics of Fe(II) in seawater[END_REF] and may either precipitate as Fe(III) oxides or remain soluble under the form of organic-Fe(III) complexes in the presence of natural organic ligands [START_REF] Taillefert | Reactivity of freshly formed Fe(III) in synthetic solutions and (pore)waters: voltammetric evidence of an aging process[END_REF][START_REF] Beckler | The origin, composition, and reactivity of dissolved iron(III) complexes in coastal organic-and iron-rich sediments[END_REF]. These processes and the oxidation of reduced sulfur minerals are promoted by bioturbation and bioirrigation which enhance the oxygen supply to the first tens of centimeters of sediment [START_REF] Aller | The sedimentary Mn cycle in Long Island sound: its role as intermediate oxidant and the influence of bioturbation, O 2 , and Corg flux on diagenetic reaction balances[END_REF]Canfield et al., 1993b). Simultaneously, Fe 2+ is less mobile in comparison to Mn 2+ due to rapid precipitation with H 2 S [START_REF] Pyzik | Sedimentary iron monosulfides: kinetics and mechanism of formation[END_REF]. Mn(III, IV) oxides are therefore dispersed and redeposited in surface sediments where they become important electron acceptors for organic carbon oxidation [START_REF] Hyun | Manganese and iron reduction dominate organic carbon oxidation in surface sediments of the deep Ulleung Basin, East Sea[END_REF][START_REF] Law | Manganese, iron, and Sulphur cycling on the Pakistan margin[END_REF], whereas Fe(III) oxides are typically immobilized below the oxygen penetration depth in the sediment where they can eventually be utilized for organic matter degradation, with possible production of organic-Fe(III) complexes as soluble intermediates [START_REF] Taillefert | Shewanella putrefaciens produces an Fe(III)-solubilizing ligand during anaerobic respiration on insoluble Fe(III) oxides[END_REF][START_REF] Jones | Shewanella oneidensis MR-1 mutants selected for their inability to produce soluble organic-Fe(III) complexes are unable to respire Fe(III) as anaerobic electron acceptor[END_REF]. Differentiating these processes in RiOMar sediments may help quantify anaerobic carbon remineralization processes more accurately.

This study investigated the main redox processes involved in the transformation of natural organic matter in continental shelf and slope sediments of the northern Gulf of Mexico (nGoM). Two cross slope transects were studied to examine the effect of lithogenic and organic carbon inputs on diagenetic processes. The first transect is influenced by inputs from the Mississippi River and the other, to the east of the Mississippi River plume, is not exposed to significant inputs. At each station, depth profiles with potentiometric, amperometric, and voltammetric microelectrodes in intact sediment cores were combined with conventional pore water analyses and solid phase chemical extractions of the same sediment cores. Finally, a one-dimensional transient reactive transport model run to steady-state was also used to estimate carbon remineralization rates and identify the role of lithogenic inputs on microbial metal reduction.

Methods

Field site and sediment sampling

The continental margin of the nGoM is well known for its seasonal hypoxic zone (defined as <63 μM O 2(aq) ) resulting from the high input of freshwater and nutrients from fertilizers from the Mississippi-Atchafalaya River System (MARS) [START_REF] Rabalais | Hypoxia in the northern Gulf of Mexico: does the science support the plan to reduce, mitigate, and control hypoxia? Estuar[END_REF]. The limited circulation and strong water column stratification based on the differences in density of fresh and marine waters, together with nutrientstimulated local production promote the depletion of dissolved oxygen in the deep waters [START_REF] Fennel | Biogeochemical controls on coastal hypoxia[END_REF]. In addition, the continental margin of the nGoM is a unique region to study the effect of riverine inputs on diagenetic processes in RiOMar sediments as the continental margin receives approximately 2.1 × 10 8 tons of sediment per year [START_REF] Milliman | World-wide delivery of river sediment to the Oceans[END_REF]) and an annual mass load of 7.38 × 10 7 tons of metals [START_REF] Reiman | Metals geochemistry and mass export from the Mississippi-Atchafalaya River system to the Northern Gulf of Mexico[END_REF] from the MARS under the form of a particulate plume that covers as much as 2400 ± 400 km 2 in the summer [START_REF] Green | An organic carbon budget for the Mississippi River turbidity plume and plume contributions to air-sea CO 2 fluxes and bottom water hypoxia[END_REF][START_REF] Xu | Seabed erodibility variations on the Louisiana continental shelf before and after the 2011 Mississippi River flood[END_REF]. The transport of material extends beyond the hypoxic region and can be transferred to the nearby Mississippi canyon [START_REF] Bianchi | Rapid export of organic matter to the Mississippi canyon[END_REF] via lateral down-slope accretion of material from the margins and/or mass wasting [START_REF] Rowe | Sediment community oxygen consumption in the deep Gulf of Mexico[END_REF].

Sediment cores (9.6 cm inner diameter x 70 cm) were collected using a multi-corer (MC 800, Ocean Instruments) onboard the R/V Savannah along the Louisiana shelf and slope during an oceanographic cruise from July 25 to August 6, 2017 (Fig. 1 and Table 1). Station locations were selected along a transect near the outlet of the Mississippi River across the slope and toward the Mississippi canyon stations referred to as the 'western transect' with respect to positioning of the Mississippi River: St. 2b (65 m), St. 13 (510 m), and St. 11 (925 m) (Table 1 and Fig. 1). Two additional stations were selected to the east of the Mississippi River along a transect across the continental slope which is referred to as the 'eastern transect': St. 15 (274 m) and St. 12 (989 m) (Table 1 and Fig. 1). Stations were named based on the labelling of stations from previous expeditions [START_REF] Beckler | Importance of microbial iron reduction in deep sediments of river-dominated continental-margins[END_REF].

O 2 and pH microprofiles and porosity profiles

Bottom water samples were collected as close to the seafloor as possible (i.e. 2 m) using the rosette of 12 L Niskin bottles (OceanTest Equipment, Inc). Bottom water pH was measured using the m-cresol purple spectrophotometric method [START_REF] Clayton | Spectrophotometric seawater pH measurements: total hydrogen ion concentration scale calibration of m-cresol purple and at-sea results[END_REF][START_REF] Dickson | Guide to best practices for ocean CO2 measurements[END_REF][START_REF] Rassmann | Carbonate chemistry in sediment porewaters of the Rhône River delta driven by early diagenesis (northwestern Mediterranean)[END_REF]) and recalculated at in situ temperature using salinity and total alkalinity (TA). Bottom water O 2 concentrations were determined by Winkler titration [START_REF] Grasshoff | Methods of Seawater Analysis[END_REF] and oxygen saturation levels related to maximum oxygen concentrations (100% saturation) determined at the measured temperature and salinity with USGS DO tables (https://water.usgs.gov/software/ DOTABLES/).

In situ oxygen and pH profiles were collected at St. 2b in 100 μm increments using an autonomous benthic lander with a high-resolution micromanipulator (Unisense®) equipped with a resistivity probe [START_REF] Revsbech | An oxygen microsensor with a guard cathode[END_REF] and O 2 [START_REF] Andrews | Measurements of diffusivity near the sediment-water interface with a fine-scale resistivity probe[END_REF] and pH microsensors. For all other stations, ex situ O 2 and pH microprofiles were obtained with a 200 μm resolution in the same core using the same microsensors deployed on a laboratory micromanipulator (Unisense®). During ex situ profiling, overlying water temperature, oxygen concentrations, and pH were monitored and maintained at bottom water conditions by bubbling a mixture of air, N 2 , and CO 2 using a 3-way valve and circulating the overlying water through a thermostatic bath. The pH probes were practically calibrated with NBS buffers at pH 4, 7, and 10, but the pH profiles were adjusted using the spectrophotometrically measured bottom water pH at the in situ temperature [START_REF] Rassmann | Carbonate chemistry in sediment porewaters of the Rhône River delta driven by early diagenesis (northwestern Mediterranean)[END_REF]. All pH values are reported on the total proton scale (pH T ).

Oxygen sensors were calibrated using oxygen concentrations determined in bottom waters and their responses in the anoxic zone [START_REF] Cai | Oxygen penetration depths and fluxes in marine sediments[END_REF]. Two pH and two oxygen profiles were obtained successively in most of the sediment cores within 2-3 h. After completion of the pH and O 2 microprofiles, the same core was sectioned in 3-60 mm sections with higher resolution at the surface of the core. Sediment samples were frozen (-20 • C) and porosity was determined back at the LSCE by measuring the difference between wet and dry sediment weight after heating at 60 • C for 10 days and using a sediment density of 2.65 g cm -3 .

Voltammetric ex situ profiling

A replicate core collected from the same MC800 deployment was profiled in millimeter increments with 100 μm gold/mercury (Au/Hg) voltammetric microelectrodes deployed on a computer-controlled micromanipulator with DLK 70 or 100 potentiostat (Analytical Instrument Systems, Inc.) in a three-electrode system (Ag/AgCl reference, Pt counter, and Au/Hg working electrodes). Au/Hg microelectrodes were fabricated as previously reported [START_REF] Luther | Use of voltammetric solid-state (micro)electrodes for studying biogeochemical processes: laboratory measurements to real time measurements with an in situ electrochemical analyzer (ISEA)[END_REF] and prepared daily by electrodeposition of mercury [START_REF] Brendel | Development of a gold amalgam Voltammetric microelectrode for the determination of dissolved Fe, Mn, O2, and S(-II) in Porewaters of marine and freshwater sediments[END_REF].

Table 1

Station location, water column depth, and corresponding bottom water parameters (temperature, salinity, oxygen concentration, pH) along the western (St.2b,13,11) and eastern (St. 15 and 12) transects. O 2 % sat calculated relative to saturated O 2 estimated from in situ temperature and salinity Dissolved oxygen was measured using linear sweep scans from -0.1 to -1.8 V and a scan rate of 200 mV s -1 after a conditioning step at -0.1 V for 10 s (Minimum Detection Limit: MDL ~ 5 μM) [START_REF] Luther | Use of voltammetric solid-state (micro)electrodes for studying biogeochemical processes: laboratory measurements to real time measurements with an in situ electrochemical analyzer (ISEA)[END_REF].

At each depth, cathodic square wave scans with a preconditioning step at -0.1 V for 10 s were run in at least triplicate from -0.1 to -1.8 V with a scan rate of 200 mV s -1 . These scans are used to quantify dissolved manganese(II) (Mn 2+ , MDL ~ 15 μM), dissolved iron(II) (Fe 2+ , MDL ~ 25 μM), thiosulfate (S 2 O 3 2-, MDL ~ 10 μM), and total dissolved sulfide (ΣH 2 S=H 2 S + HS -+ S 0 + S x 2-, MDL ~ 0.2 μM) in addition to qualitatively detect organic complexes of Fe(III) (org-Fe(III)) [START_REF] Taillefert | Reactivity of freshly formed Fe(III) in synthetic solutions and (pore)waters: voltammetric evidence of an aging process[END_REF] and aqueous iron sulfide clusters (FeS (aq) ) [START_REF] Theberge | Determination of the electrochemical properties of a soluble aqueous FeS species present in Sulfidic solutions[END_REF]. When ΣH 2 S or org-Fe(III) species were present, an additional preconditioning step was applied at -0.9 V for 10 s before each scan to avoid memory effects. Voltammetric signals were integrated using a semi-automated VOLTINT software package [START_REF] Bristow | VOLTINT: a Matlab®-based program for semiautomated processing of geochemical data acquired by voltammetry[END_REF]. Before profiling, working electrodes were calibrated in a cell stand with MnCl 2 (0-400 μM Mn 2+ ), and Fe 2+ , ΣH 2 S, and S 2 O 3 2-were quantified from the pilot ion method with Mn 2+ as pilot ion [START_REF] Brendel | Development of a gold amalgam Voltammetric microelectrode for the determination of dissolved Fe, Mn, O2, and S(-II) in Porewaters of marine and freshwater sediments[END_REF]. Oxygen concentrations in the overlying waters were calibrated using the bottom water oxygen concentrations determined by Winkler titration. As the chemical composition of org-Fe(III) complexes and FeS (aq) is unknown, these species cannot be quantified [START_REF] Taillefert | Reactivity of freshly formed Fe(III) in synthetic solutions and (pore)waters: voltammetric evidence of an aging process[END_REF][START_REF] Theberge | Determination of the electrochemical properties of a soluble aqueous FeS species present in Sulfidic solutions[END_REF] and are reported in normalized current intensities [START_REF] Meiggs | The effect of riverine discharge on biogeochemical processes in estuarine sediments[END_REF].

Pore water extraction and analyses

After voltammetric profiles were completed, the same core was immediately transferred to a glove bag (Sigma Aldrich) with a N 2 atmosphere for sectioning in 7-10, 20, 50, and 100 mm increment depending on core depth. Sediment sections were partitioned into 50 mL falcon tubes, centrifuged for 10 min at 3000 rpm under N 2 atmosphere to extract pore waters, and the leftover sediment sections were frozen at -20 • C. Extracted pore waters were then filtered through 0.2-μm Whatman® nitrocellulose Puradisc syringe filters into 15 mL falcon tubes under N 2 atmosphere and immediately split for onboard analyses (dissolved iron and manganese speciation, dissolved orthophosphate, dissolved inorganic carbon, and total alkalinity). The leftover pore water was preserved at -20 • C until analysis at Georgia Tech (dissolved ammonium, chloride, bromide, sulfate, nitrate, nitrite).

Total alkalinity (TA) measurements were conducted according to [START_REF] Dickson | Guide to best practices for ocean CO2 measurements[END_REF], but with a 3 mL open-cell potentiometric titration [START_REF] Rassmann | Carbonate chemistry in sediment porewaters of the Rhône River delta driven by early diagenesis (northwestern Mediterranean)[END_REF] using 0.01 M HCl, which was calibrated every day using Dickson standard batch #150 [START_REF] Rassmann | Carbonate chemistry in sediment porewaters of the Rhône River delta driven by early diagenesis (northwestern Mediterranean)[END_REF]. Error in this analysis represents the standard deviation from the mean of duplicate samples if available or propagation of analytical error for single samples (1%). Dissolved inorganic carbon (DIC) was determined by flow injection analysis, if necessary after 5 mM ZnCl 2 amendment to avoid dissolved sulfide interference [START_REF] Hall | Rapid, small-volume, flow injection analysis for SCO2, and NH4+ in marine and freshwaters[END_REF]. Dissolved Mn(III) (Mn 3+ ) and total dissolved Mn (Mn d ) were determined in duplicate using the porphyrin kinetic spectrophotometric method [START_REF] Madison | Simultaneous determination of soluble manganese(III), manganese(II) and total manganese in natural (pore)waters[END_REF] modified to account for dissolved Fe 2+ interferences using external calibrations with Fe 2+ -spiked Mn 2+ standards and independent pore water Fe 2+ measurements. Mn d was quantified by converting the final absorbance to concentration using the molar absorptivity (ε = 91.2 ± 0.4 × 10 -3 μM cm -1 ) determined from external calibration curves with Mn 2+ (MDL: 0.11 μM, 3σ of blanks, n = 24). Mn 3+ was quantified by fitting the spectrophotometric kinetic curve as an exponential rise to a maximum curve but was below detection limit (MDL < 1.5 μM) in the majority of pore water samples. Dissolved orthophosphates (ΣPO 4 3-)

were quantified with the methylene blue method after correction for silica interference [START_REF] Murphy | A modified single solution method for the determination of phosphate in natural waters[END_REF]. Finally, dissolved Fe 2+ was quantified using the ferrozine method without hydroxylamine while total dissolved Fe (Fe d ) was determined by reacting a separate aliquot with hydroxylamine hydrochloride (0.2 M) in the dark for 24 h. Dissolved Fe(III) (Fe(III) d ) was obtained by subtracting the dissolved Fe 2+ concentration from total dissolved Fe [START_REF] Stookey | Ferrozine-a new spectrophotometric reagent for Iron[END_REF][START_REF] Viollier | The ferrozine method revisited: Fe(II)/Fe(III) determination in natural waters[END_REF]. At Georgia Tech, NH 4 + was quantified by the indophenol blue method [START_REF] Strickland | A Practical Handbook of Sea-Water Analysis[END_REF] and anions via HPLC with UV detection using a 3.2 mM NaCO 3 / 1.0 mM NaHCO 3 buffer (SO 4 2-, Cl -, Br -; MDL: 218, 422, 114 μM respectively) and 54 mM NaCl (NO 2 -NO 3 -, MDL = 0.12 and 0.11 μM) eluents [START_REF] Beckler | Development of single-step liquid chromatography methods with ultraviolet detection for the measurement of inorganic anions in marine waters[END_REF]. All standard deviations reported for pore water measurements represent analytical error propagated from calibration curves except for SO 4 2-at St. 13, St.

11, and St. 12 which represent the standard deviations from duplicate measurements.

Solid phase analysis

Solid phase manganese and iron were extracted in duplicate from sediments following classical sequential and parallel extraction protocols [START_REF] Anschutz | Interactions between metal oxides and species of nitrogen and iodine in bioturbated marine sediments[END_REF][START_REF] Hyacinthe | Early diagenetic processes in the muddy sediments of the Bay of Biscay[END_REF][START_REF] Hyacinthe | An authigenic iron phosphate phase in estuarine sediments: composition, formation and chemical reactivity[END_REF][START_REF] Kostka | Partitioning and speciation of solid phase iron in saltmarsh sediments[END_REF][START_REF] Tessier | Sequential extraction procedure for the speciation of particulate trace metals[END_REF]. A 0.2-0.5 g pellet of wet sediment was added to a 15 mL falcon tube, and the adsorbed fraction (absorbed Fe 2+ , absorbed Mn 2+ ) was first extracted for 2 h on a rotary wheel using 8 mL MgCl 2 (0.5 M, pH 7) [START_REF] Tessier | Sequential extraction procedure for the speciation of particulate trace metals[END_REF]. The supernatant was removed for analysis and replaced by a 8 mL sodium acetate solution (0.5 M adjusted to pH 5 with acetic acid) to extract Fe 2+ and Mn 2+ bound in the carbonate phases (carbonate-bound Fe 2+ , carbonate-bound Mn 2+ ) on a rotary wheel for 5 h [START_REF] Tessier | Sequential extraction procedure for the speciation of particulate trace metals[END_REF]. Finally, the supernatant was replaced by 8 mL of an ascorbate solution (pH 7.4) consisting of sodium citrate (0.2 M), sodium bicarbonate (0.6 M), and 0.1 M L-ascorbic acid for the extraction of poorly crystalline Fe(III) and Mn(III/IV) phases on the rotary wheel for 24 h [START_REF] Hyacinthe | Early diagenetic processes in the muddy sediments of the Bay of Biscay[END_REF][START_REF] Kostka | Partitioning and speciation of solid phase iron in saltmarsh sediments[END_REF]. The ascorbate fraction reductively dissolves poorly crystalline Fe(III) oxides (i.e., ferrihydrite, [START_REF] Kostka | Partitioning and speciation of solid phase iron in saltmarsh sediments[END_REF][START_REF] Raiswell | The determination of labile Fe in ferrihydrite by ascorbic acid extraction: methodology, dissolution kinetics and loss of solubility with age and de-watering[END_REF] and reactive Mn(III/IV) oxides [START_REF] Anschutz | Speciation, oxidation state, and reactivity of particulate manganese in marine sediments[END_REF] but not crystalline iron oxides (e.g., hematite, goethite, magnetite) or FeS (s) [START_REF] Kostka | Partitioning and speciation of solid phase iron in saltmarsh sediments[END_REF]. In parallel, total reactive Fe (i.e., siderite, poorly crystalline Fe(III), crystalline Fe(III) oxides, and FeS (s) ) was extracted in duplicate sediment pellets with a dithionite reagent (0.35 M ammonium acetate, 0.2 M sodium citrate, 0.03 M sodium dithionite, pH 5) at 60 • C in a water bath for 4 h [START_REF] Kostka | Partitioning and speciation of solid phase iron in saltmarsh sediments[END_REF]. The carbonate and ascorbate concentrations were subtracted from the dithionite fraction to quantify crystalline Fe(III) + FeS (s) . Between each extraction step, the sediment slurry was centrifuged for 10 min at 3000 rpm and the supernatant was stored in a separate falcon tube at 4 • C until analysis. The supernatant was measured for total extracted Fe 2+ using the ferrozine method [START_REF] Stookey | Ferrozine-a new spectrophotometric reagent for Iron[END_REF], whereas samples for total dissolved manganese (Mn d ) from the extractions were stored at 4 • C until analysis by Inductively Coupled Plasma -Mass Spectrophotometer (ICP-MS) using appropriate analytical procedures [START_REF] Owings | Development of a rate law for arsenite oxidation by manganese oxides[END_REF]. In addition, two sediment samples were dried at 50 • C for 3-5 days, allowed to cool in the oven, and weighed to normalize concentrations to the weight of dry sediment in grams (gdw). All errors reported represent the standard deviation of the average of duplicate extractions. Finally, the acid volatile sulfides (AVS) fraction was extracted in triplicate by cold distillation of wet sediment (0.5-0.7 g) with 5 mL of 3 M HCl and entrainment of the H 2 S gas product by a pre-purified (using a Cu trap heated at 400 • C to remove O 2 traces) ultra-high purity (UHP) N 2 stream into a 1 M NaOH solution trap at low flow rate [START_REF] Henneke | Determination of inorganic Sulphur speciation with polarographic techniques: some preliminary results for recent hypersaline anoxic sediments[END_REF]. Total dissolved sulfide trapped after 4 h was quantified by absorbance at 230 nm using NaS 2 standards dissolved daily in degassed 1 M NaOH [START_REF] Ellis | Spectrophotometric determination of the acid dissociation constants of hydrogen sulphide[END_REF].

Calculations

Flux calculations

Dissolved oxygen uptake (DOU) rates and the diffusive fluxes of DIC, NH 4 + , and ΣPO 4 3-, were calculated from their concentration gradients across the sediment-water interface (SWI) using Fick's first law (Eq. ( 1)), assuming diffusion mainly controlled transport.

F i = -φD s dC i dz (1)
In these calculations, φ represents the average porosity of the sediment layers where the diffusive flux was determined, D s is the apparent diffusion coefficient after correcting for tortuosity, and dCi dz is the concentration gradient of species i as a function of depth z. For all species D s = D0 θ 2 , where D 0 is the molecular diffusion coefficient and tortuosity θ 2 = 1 -ln(φ 2 ). For dissolved oxygen, D 0 = 4.79 10 -9 T μV 0.6 b , where T is the temperature, μ the dynamic viscosity, and V b the molar volume, whereas for ions D 0 = (m 0 + m 1 T) 10 -6 cm 2 s -1 , where m 0 and m 1 are empirically derived constants [START_REF] Boudreau | Diagenetic models and their implementation[END_REF]. The diffusion coefficients for HCO 3

-and HPO 4 2-were used for the calculations of the diffusive fluxes of DIC and ΣPO 4 3-as these species constituted the majority of the carbonate and phosphate speciation in representative solutions for each station modeled using Visual MINTEQ version 3.1 [START_REF] Gustafsson | Visual MINTEQ, version 3.1[END_REF]. Positive fluxes in these calculations represent downward fluxes into the sediment, whereas negative fluxes represent upward fluxes across the SWI. Data from O 2 microsensors (Unisense®) were used for DOU rate calculations given their high spatial resolution in the top 20 mm of sediment.

Diagenetic modeling

The Matlab™-based one-dimensional transient reactive transport model MATSEDLAB [START_REF] Couture | Non-steady state modeling of arsenic diagenesis in Lake sediments[END_REF] was used to quantify the depth-integrated rates of aerobic and anaerobic respiration at each of the station investigated. In this model, the transport parameters reflect conventional bioturbation, irrigation, sedimentation, compaction, and diffusion in sediments [START_REF] Boudreau | Diagenetic models and their implementation[END_REF][START_REF] Wang | A multicomponent reactive transport model of early diagenesis: application to redox cycling in coastal marine sediments[END_REF]. Porosity was assumed constant at the mean porosity determined in each core (Fig. S1), sedimentation rates were fixed at each station with values from the literature [START_REF] Corbett | Nature of decadal-scale sediment accumulation on the western shelf of the Mississippi River delta[END_REF][START_REF] Yeager | Sediment accumulation and radionuclide inventories (239,240Pu, 210Pb and 234Th) in the northern Gulf of Mexico, as influenced by organic matter and macrofaunal density[END_REF]. Diffusion was assumed molecular and modified to account for salinity, temperature, and tortuosity [START_REF] Boudreau | Diagenetic models and their implementation[END_REF]. Finally, bioirrigation and bioturbation were considered in the model and assumed to decrease exponentially with depth in the sediment as in previous modeling efforts [START_REF] Soetaert | A model of early diagenetic processes from the shelf to abyssal depths[END_REF][START_REF] Wang | A multicomponent reactive transport model of early diagenesis: application to redox cycling in coastal marine sediments[END_REF][START_REF] Boudreau | Diagenetic models and their implementation[END_REF].

The bioirrigation coefficient at the sediment-water interface was the same for all species and the decrease in bioirrigation with depth was fitted using the ammonium profiles [START_REF] Wang | A multicomponent reactive transport model of early diagenesis: application to redox cycling in coastal marine sediments[END_REF]. The bioturbation coefficient at the sediment-water interface was predicted from an empirical relationship as a function of water depth [START_REF] Boudreau | Diagenetic models and their implementation[END_REF], and the maximum bioturbation depth was set empirically at each station according to the Fe(OH) 3 profile. Organic carbon degradation that produces NH 4 + and DIC as by-product is described by Michaelis-Menten rate laws with respiratory inhibition [START_REF] Soetaert | A model of early diagenetic processes from the shelf to abyssal depths[END_REF][START_REF] Wang | A multicomponent reactive transport model of early diagenesis: application to redox cycling in coastal marine sediments[END_REF] using C:N ratios from the literature [START_REF] Sampere | Burial and degradation of organic carbon in Louisiana shelf/slope sediments[END_REF][START_REF] Waterson | Sources of sedimentary organic matter in the Mississippi River and adjacent Gulf of Mexico as revealed by lipid biomarker and δ13CTOC analyses[END_REF]. Reduced compounds produced during remineralization (i.e., NH 4 + , Mn 2+ , Fe 2+ , ΣH 2 S) are partly removed from the system by precipitation of sulfide (FeS) or carbonate minerals (MnCO 3 , FeCO 3 ) and oxidation in the presence of O 2(aq) or other oxidants (MnO 2 , Fe(OH) 3 ) using explicit kinetic formulations [START_REF] Wang | A multicomponent reactive transport model of early diagenesis: application to redox cycling in coastal marine sediments[END_REF]. The whole suite of reactions and rate laws included in the model are provided in the appendix (Tables S1-S4). Boundary conditions included Dirichlet conditions at the sediment surface, using either bottom water concentrations of each dissolved constituent or optimized concentrations of particulate material (i.e. organic carbon, Mn(III/IV) oxides, or Fe(III) oxides), and zero flux (Neumann) conditions at the bottom of the domain. As kinetic parameters and rate laws of most abiotic processes are relatively well characterized, they were fixed in the model (Table S5). In turn, microbial respiration rate constants (Table S6) and other important parameters (Table S7) were optimized to fit depth profiles of the main redox species measured in a self-consistent manner at each station. Data from O 2 microsensors (Unisense®), voltammetric microelectrodes (ΣH 2 S), pore water extractions (DIC, NH 4 + , NO 3 -, Mn 2+ , Fe 2+ , and SO 4 2-), and solid phase extractions (poorly crystalline Fe(III) oxides and Mn(III/IV) oxides, carbonate-bound Fe 2+ , carbonate-bound Mn 2+ , and AVS) were used in these calculations.

Results

The stations investigated in this study (Table 1) include St. 2b located in the zone of highest sediment accumulation rate (>2.5 cm yr -1 , [START_REF] Corbett | Nature of decadal-scale sediment accumulation on the western shelf of the Mississippi River delta[END_REF])), while the other stations display relatively low sedimentation rates (<0.1 cm yr -1 , [START_REF] Corbett | Nature of decadal-scale sediment accumulation on the western shelf of the Mississippi River delta[END_REF][START_REF] Yeager | Sediment accumulation and radionuclide inventories (239,240Pu, 210Pb and 234Th) in the northern Gulf of Mexico, as influenced by organic matter and macrofaunal density[END_REF]. Percentage of organic carbon content varies between 1.0 and 1.5%, with the highest percentages found at the shallowest stations of each transect (St. 2b, St. 15) (Table 2). The sediment texture and color reflected the different geochemical conditions at each station. St. 2b consisted mainly of dark gray/black mud (Fig. 1) with a band of black sediment below 45 cm (not pictured). St. 13 and St. 11 cores contained light gray muds with flocculant rust and brown colored sediments present in the top 5 and 10 cm, respectively (Fig. 1). The sediments at St. 12 displayed a light orange/brown color in the top 10 cm that transitioned rapidly to gray sediments with black patches after 10 cm (Fig. 1). St. 15 was made of gray mud with rust colored patches.

Bottom water parameters and oxygen, pH, and porosity microprofiles

Bottom waters, though oxic (Table 1) and outside the hypoxic zone for that season [START_REF] Rabalais | Summary of 2017 Research Cruise Press Release (Gulf of Mexico)[END_REF], were significantly undersaturated (44-72%) along the continental slope. Dissolved oxygen concentrations decreased exponentially from the SWI to below detection limit at depth. The oxygen penetration depth (OPD) increased across the shelf to slope transects from 1.6 mm at St. 2b to 8.3 and 14.0 mm at St. 13 and 11 along the western transect, and to 7.4 and 9.0 mm at St. 15 and 12 along the eastern transect (Table 2). At St. 15, a pocket of oxygen was detected below the OPD (Fig. 2) likely due to bioirrigation. Bottom water pH T collected from overlying waters of the core varied between 7.76 and 7.86 (Table 1), reflecting the widespread acidification of sediment overlying waters in this environment [START_REF] Cai | Acidification of subsurface coastal waters enhanced by eutrophication[END_REF]. The pH microprofiles collected over the first 5-8 cm at St. 13,11,15,and 12 Table 2 Sedimentation rate and Organic Carbon (OC) content obtained from the literature and average porosity, oxygen penetration depth (OPD), and diffusive oxygen uptake rate (DOU) determined along the western (St. 2b,13,11) displayed the same trend, with abrupt decrease in pH within the first 1-1.5 cm below the SWI to minimum pH values ranging from 7.0-7.3 followed by a rebound to relatively constant values between 7.0 and 7.8 at depth (Fig. 2). At St. 2b, the pH profile followed a similar trend, however, the pH decrease occurred within a few millimeters (versus centimeters at the other stations) from the SWI. Furthermore, the pH decreased again after the initial rebound in pH around 10 mm and remained at its lowest value (i.e., pH 7.2) below 30 mm in the profile (Fig. 2). Little to no differences were observed between ex situ and in situ oxygen and pH microprofiles (Fig. 2), confirming the integrity of the sediment cores collected during this study. Sediment porosity ranged from 0.86 to 0.95 in surface sediments, decreased by 0.1-0.15 units within the top 50-100 mm, and remained relatively constant at depth (Fig. S1) with average values ranging from 0.75 to 0.87 (Table 2).

Overall, stations along the eastern transect displayed lower porosities and steeper decreases in porosity with depth compared to the western transect (Fig. S1), indicative of more significant inputs of fine-grained sediments along the western transect. At each station, the water content of the different sections decreased with depth, suggestive of increased compaction.

Voltammetric depth profiles of redox species

Dissolved oxygen measured by Au/Hg microelectrodes also decreased rapidly with depth (Fig. 3) but OPDs, although increasing with water depth, were 3-6 mm shallower than detected with the amperometric microelectrodes. At St. 2b, pore water Mn 2+ concentrations uniformly increased immediately below the sediment water interface (SWI) to a maximum concentration of 1039 ± 96 μM at 56 mm. Mn 2+ concentrations then rapidly decreased to below detection limits at 62 mm, and reappeared below 165 mm albeit in lower concentrations (ranging from 141 to 351 μM). Although Mn 2+ concentrations also increased with depth to concentrations ranging between 295 and 347 μM at St. 13, 11, 15, and 12, the onset of Mn 2+ production was detected much deeper at 103, 65, 82, and 150 mm below the SWI, respectively (Fig. 3). In comparison, Fe 2+ concentrations remained below detection limits throughout each core, even though org-Fe(III) was detected in all cores except at St. 11. At St. 2b, a strong peak in org-Fe(III) was observed (372 ± 14 nA maximum) that coincided with the Mn 2+ peak and progressively decreased to below detection limit at 150 mm. At St. 13, org-Fe(III) was produced between 110 and 146 mm to approximately 20 nA and abruptly increased to 96 ± 5 nA at 156 mm followed by a decrease to below detection limit within 30 mm. Across the eastern transect, org-Fe(III) was detected in much lower current intensities. At Station 15, org-Fe(III) was produced at 82 mm in low current intensities (ca. 5 nA) and increased to a maximum of 37 ± 6 nA at depth, whereas org-Fe(III) appeared at 151 mm at St. 12 and remained low deeper (9-12 nA). Dissolved ΣH 2 S was only detected at St. 2b, where it was produced below 49 mm and reached a maximum of 219 ± 14 μM at 67 mm before decreasing regularly to below detection limit at 214 mm. Molecular clusters of FeS (FeS (aq) ) were also detected just above the onset of ΣH 2 S production (50 mm) at St. 2b, although their current intensities remained relatively low (3-38 nA) across the profiles and decreased to below detection limit at 178 mm. Finally, FeS (aq) was also detected in low current intensities (max 10 ± 2 nA) below 150 mm at St. 13, but remained below detection limit at all other stations (Fig. 3).

Pore water profiles of carbon respiratory products, sulfate, and dissolved metals

Depth profiles of the main redox species reflected the intensity of respiratory processes as revealed by DIC, TA, ΣPO , reflecting what was found with Au/Hg voltammetric microelectrodes (Fig. 3). Station 2b displayed a large Fe(III) d peak (253 ± 1.3 μM) over the first 100 mm of sediment that coincided with the org-Fe(III) peak measured by voltammetry (Fig. 3a) and was accompanied by small increases in Fe 2+ measured voltammetrically (Fig. 3a) and pore water Fe 2+ (Fig. 4 a 3 ). In the deeper stations along the western transect, the onset of Fe(III) d production was located at greater depths of 34 and 144 mm at St. 13 and St. 11, and concentrations increased gradually with depth to concentrations of 128 ± 1.0 and 92 ± 1.5 μM respectively (Fig. 4 b 3 ,c 3 ). Although org-Fe(III) voltammetric signals did not match the Fe(III) d profiles at St. 13 and 11, likely due to the highly localized voltammetric measurements compared to the longitudinally-averaged pore water Fe(III) d concentrations in heterogeneous sediments, org-Fe(III) current intensities were similarly higher at St. 13 compared to St. 11. Along the eastern transect, two subsurface peaks in Fe 3+ with maximum concentration of 129 ± 2.1 and 398 ± 6.3 μM were found over the first 50 mm and between 200 and 300 mm at St. 15 (Fig. 4 d 3 ) that generally matched the org-Fe(III) voltammetric profile (Fig. 3d). A gradual increase in Fe 3+ concentration with depth to 90 ± 0.6 μM was observed at St. 12 (Fig. 4 e 3 ) that was reflected by the general increase in org-Fe(III) below 150 mm (Fig. 3e).

Solid manganese, iron, and sulfur profiles

At St. 2b, the highest reactive Mn concentration in the solid phase (20.7 ± 0.4 μmol gdw -1 ) was present within the first 7 mm of the sediment and included 2.7 ± 0.1 μmol gdw -1 of Mn(III/IV) oxides (Fig. 4 St. 2b in comparison with the other stations as well as the change in scale of Mn and Fe solid phase speciation and AVS between each station to observe general trends. '0's in the sixth panels represent AVS analyses of surface sediments that were below detection limit.

a 4 ). The of the core consisted of reactive Mn concentrations ranging between 10 and 16 μmol gdw -1 , the majority of which (8-14 μmol gdw -1 ) was in carbonate-bound Mn 2+ (80-91%), a smaller fraction (0.9-1.6 μmol gdw -1 , 6-12%) as Mn(III/IV) oxides, and a much smaller adsorbed Mn 2+ fraction (0-1.6 μmol gdw -1 , 0-11%). The distribution of solid phase Mn was typically similar at stations St.13 and St. 11, with an enrichment of Mn(III/IV) oxides at the surface (207 ± 8.0 and 151 ± 1.9 μmol gdw -1

, respectively) that rapidly decreased to 1.2 ± 0.2 μmol gdw -1 at 53 mm at St. 13 but only progressively to 1.3 ± 0.2 μmol gdw -1 at 215 mm at St. 11 (Fig. 4 b 4 , c 4 ). At both stations, carbonate-bound Mn 2+ represented the dominant fraction (56-86% at St. 13 and 0.5-71.3% at St. 11) across the sediment column (Fig. S2) and increased in concentrations below the Mn(III/IV) oxide maximum. This increase, however, was much more rapid at St. 13 (approx. 1.3 μmol gdw -1 per mm on average between 7 and 20 mm) than at St. 11 (approx.

0.3 μmol gdw -1 per mm on average between 3 and 127 mm). Adsorbed Mn 2+ , which was rapidly produced below 20 mm at St. 13 and much more progressively below 70 mm at St. 11 (Fig. 4 b 4 and c 4 ), remained relatively low in concentration at both stations (0.9-3.7 μmol gdw -1 at St. 13 and 1.2-5.2 μmol gdw -1 at St. 11), not exceeding 30% of the sum of Mn (Fig. S2). Along the eastern transect, Mn(III/IV) oxides and carbonate Mn 2+ displayed similar trends with the highest Mn(III/IV) oxide concentrations found at the sediment surface (1.1 ± 0.1 μmol gdw -1 at St. 15, 1.9 ± 0.5 μmol gdw -1 at St. 12) but <1 μmol gdw -1 Mn(III/IV) oxides across the rest of the sediment column. Similarly, the carbonate Mn 2+ fraction, though dominating the solid phase speciation at both St. 15 and St. 12 (Fig. S2), was highest at the surface (7.63 ± 0.09 at St. 15, 14.12 ± 2.03 at St. 12) and below 5 and 8 μmol gdw -1 , respectively, across the remainder of the sediment column (Fig. 4 d 4 , e 4 ). Interestingly, the difference in Mn solid phase speciation between St. 15 and St.

12 manifested in the adsorbed fraction which increased only to 0.3 μmol gdw -1 at 245 mm at St. 15, in contrast to St. 12 where adsorbed Mn 2+ was detected as high as 1.5-3.0 μmol gdw -1 between 10 and 183 mm (Fig. 4 d 4 ,e 4 , Fig. S2).

The distribution of total reactive Fe in St. 2b sediment was relatively homogeneous with depth and included relatively high concentrations of poorly crystalline (30-55 μmol gdw -1 or 40-100% of total reactive Fe) and crystalline (0-47.7 μmol gdw -1 or 0-51% of total reactive Fe) Fe (III) (Fig. 4 a 6 , Fig. S2), and generally low carbonate-bound Fe 2+ (< 10%) (Fig. 4 a 5 , Fig. S2). The distribution of total reactive Fe was more variable at St. 13 (Fig. 4 b 5 , Fig. S2), where the surface sediment (0-53 mm) was depleted in poorly crystalline Fe(III) oxides (36-83 μmol gdw -1 or 29 ± 10% of total reactive Fe) and enriched in crystalline Fe (III) oxides (112-269 μmol gdw -1 or 57-77%), and the deep sediment was enriched in poorly crystalline Fe(III) oxides (μmol gdw -1 or %) and depleted in crystalline Fe(III) oxides (34-112 μmol gdw -1 or 22-79%). Although low at the surface (0-4.4 μmol gdw -1 or 0-1.6%), carbonatebound Fe 2+ concentrations increased at 73 mm to reach 5-13 μmol gdw -1 or 3.8-8.7% of total reactive Fe between 73 and 525 mm (Fig. 4 b 5 , Fig. S2). The inverse trend was apparent at St. 11 (Fig. 4 c 5 ), where poorly crystalline Fe(III) oxides rapidly increased below the SWI to 90 μmol gdw -1 at 20 mm and constituted the majority of total reactive Fe (80-93%, Fig. S2), then decreased progressively to 22.8 ± 2.5 μmol gdw -1 at depth. Although generally low at St. 11 (34-43 μmol gdw -1 , Fig. 4 c 5 ), crystalline Fe(III) oxides were proportionally lower than the poorly crystalline Fe(III) oxides fraction in surficial sediment (< 20%) but represented the dominant fraction of total reactive Fe in the deep sediment (50-56%) with a small contribution from the carbonate fraction (9.6-10.4 μmol gdw -1 or ~12%) between 215 and 395 mm (Fig. 4 c 5 , Fig. S2). On the eastern transect, poorly crystalline and crystalline Fe (III) oxide distributions were similar at both stations: The first 4 mm consisted mainly of poorly crystalline Fe(III) oxides (63.1 ± 2.0 and 58.5 ± 4.8 μmol gdw -1 at St. 15 and St. 12) (Fig. 4 d 5 ,e 5 ). Below the first section, poorly crystalline Fe(III) oxides constituted slightly more than half (58 ± 37% at St. 15 and 55 ± 15% at St. 12) of total reactive Fe (Fig. 4 d 5 ,e 5 , Fig. S2). On the other hand, differences in the carbonate fraction were evident along the eastern transect, where carbonatebound Fe 2+ concentrations increased from 2.4 ± 1.2 at the surface to 9.03 ± 2.6 μmol gdw -1 at depth at St. 15, whereas the carbonate fraction averaged 7.4 ± 1.5 μmol gdw -1 below the first 7 mm at St. 12 (Fig. 4 d 5 ,e 5 ).

At all stations, AVS concentrations were low in the top 3-10 mm of sediment, ranging from below detection limit to the highest value of 0.56 ± 0.05 μmol S gdw -1 at St. 11 (Fig. 4 a 6 , b 6 , c 6 , d 6 and e 6 ). The highest AVS concentrations throughout the core were found in St. 2b, with a peak in concentration of 40.3 ± 1.7 μmol S gdw -1 at 42 mm, followed by the lowest concentration measured in the core of 5.3 ± 0.4 μmol S gdw -1 at 580 mm (Fig. 4 a 6 ). AVS concentrations at St. 13 were generally low but increased from 0.35 ± 0.12 μmol S gdw -1 at the surface to 5.6 ± 1.6 μmol S gdw -1 at the bottom of the core (525 mm) (Fig. 4 b 6 ). AVS concentrations were even much lower at Station 11, remaining below 1 μmol S gdw -1 throughout the core (Fig. 4 c 6 ). Along the eastern transect, AVS concentrations were also low at St. 15, with a maximum concentration of 2.7 ± 1.13 μmol S gdw -1 at 145 mm, whereas AVS concentrations were generally higher and ranged between 1.8 and 4.6 μmol S gdw -1 at St. 12, with a maximum concentration of 16.4 ± 7.0 μmol S gdw -1 at 105 mm (Fig. 4 d 6 and e 6 ).

Discussion

Intensity of carbon remineralization and main diagenetic pathways from the continental shelf to the slope

Although bottom waters were oxygenated, St. 2b, 13, 11, and 15 displayed lower percent saturations (44-48%) in comparison to those previously found (55-100%) in the nGoM shelf and slope region during August-September 2006 [START_REF] Hu | Geochemical environments of continental shelf-upper slope sediments in the northern Gulf of Mexico[END_REF]. Low oxygen saturation in the bottom waters could be caused by upwelling of O 2 -depleted waters, respiration in the water column, or O 2 consumption by the sediments along with low vertical mixing rate due to limited winds or absence of storm events. As the surface area of the hypoxic zone during the summer of 2017 was the largest ever recorded in the northern Gulf of Mexico (22,720 km 2 ) [START_REF] Rabalais | Summary of 2017 Research Cruise Press Release (Gulf of Mexico)[END_REF], it is likely that strong respiration in the water column and low vertical mixing conditions were responsible for the depleted O 2 levels in the overlying waters. The highest DOU rate was measured on the western transect and decreased from 13.8 mmol m -2 d -1 on the shelf (St. 2b) to 3.6 on the slope (St. 13) and 3.9 mmol m -2 d -1 at the bottom of the slope (St. 11) (Table 2). DOU rates along the eastern transect also decreased with distance from the shelf but were about 17 and 42% lower on the slope and bottom of the slope than along the western transect (Table 2). These DOU rates are comparable to sediment community oxygen consumption [START_REF] Rowe | Sediment community oxygen consumption in the deep Gulf of Mexico[END_REF] and DOU rates measured at the deep stations along the continental shelf and slope [START_REF] Hu | Geochemical environments of continental shelf-upper slope sediments in the northern Gulf of Mexico[END_REF]. In turn, DOU appears to vary seasonally on the shelf near St. 2b [START_REF] Lehrter | Sedimentwater fluxes of dissolved inorganic carbon, O2, nutrients, and N 2 from the hypoxic region of the Louisiana continental shelf[END_REF][START_REF] Rowe | Sediment community metabolism associated with continental shelf hypoxia, Northern Gulf of Mexico[END_REF]. The higher DOU rates and compressed pH profile at St.2b are indicative of intense carbon remineralization and re-oxidation in a thin sediment section near the SWI due to large labile organic carbon deposition from MARS (Table 2). The reactive transport model that optimized respiration rates to reproduce the experimental depth profiles (Fig. 5 and Figs. S3-S6) confirms that aerobic respiration represents the main O 2 consumption process along these continental shelf and slope sediments except at St. 13, where the OPD was much larger than any other station and the contribution of reduced metabolites reoxidation to O 2 removal reaches around 60% (Table 3). Except for St. 2b, where high Mn 2+ fluxes were calculated (-1325 ± 70 μmol m -2 d -1 ), Fe 2+ and Mn 2+ diffusive fluxes across the SWI were either absent or low (Table S9), suggesting most of the reduced metabolites produced during anaerobic respiration processes were buried under the form of sulfide or carbonate minerals in the deep sediment (Fig. 4). In contrast, DIC, NH 4 + , and even ΣPO 4 3-diffused across the SWI (Figs. 4,6), demonstrating input of nutrients into the overlying waters that may help fuel new primary (Fig. 6). The trends in flux values indicate a decrease in overall anaerobic carbon remineralization rates from the shelf to the continental slope in agreement with respiration measurements in incubations of Louisiana shelf and slope sediments [START_REF] Lin | Sulfate reduction and iron sulfide mineral formation in Gulf of Mexico anoxic sediments[END_REF] and integrated carbon remineralization rates calculated with the reactive transport model (Table 3). Interestingly, the stations in the middle of the slope (i.e., St. 13 for the eastern transect and St. 15 for the western transect) also showed much lower DIC and NH 4 + diffusive fluxes (Fig. 6)

and integrated carbon remineralization rates (Table 3) than the deepest stations (i.e., St. 11 and St. 12). As riverine inputs of particulate matter extend widely on the slope [START_REF] Green | An organic carbon budget for the Mississippi River turbidity plume and plume contributions to air-sea CO 2 fluxes and bottom water hypoxia[END_REF], these findings suggest the accumulation of particulate material may be transient along the steepest part of the slope (Fig. 1). Integrated carbon remineralization rates calculated with the model are between 20% and 3-4 folds higher than DIC fluxes at the SWI calculated from pore water measurements (Fig. 6), except at St. 11 where carbon remineralization rates are 38% lower. The generally higher carbon remineralization rates than DIC fluxes is attributed to the precipitation of calcium-magnesium carbonate minerals (not determined and not considered in the model), as precipitation of Fe (Fig. 4 a 4 -e 4 ) and Mn (Fig. 4 a 5 -e 5 ) carbonate minerals was observed but their burial fluxes do not significantly remove carbonates from the pore waters (Table S9). In turn, calcium-magnesium carbonate dissolution may be responsible for the missing DIC flux at St. 11. Using mass balance, the missing carbonate precipitation or dissolution flux is estimated to range between 0.2 and 4.5 mmol m -2 d -1 at all stations, except at St. 12 where the missing flux may reach 7 mmol m -2 d -1 . The rapid decrease in NO 3 -concentrations below the SWI is attributed to intense denitrification in Gulf of Mexico sediments [START_REF] Mccarthy | Denitrification dominates sediment nitrogen removal and is enhanced by bottom-water hypoxia in the northern Gulf of Mexico[END_REF]. Denitrification plays a significant role in the top 2 cm of sediment, suggesting its effect on carbon remineralization processes may be important. Indeed, the reactive transport model ran at steady-state reveals that denitrification constitutes between 15 and 25% of carbon remineralization rates and is more prominent in mid-slope sediments (St. 13 and St. 15) where bottom water NO 3 -concentrations are larger (Table 3). However, these calculations do not account for anaerobic nitrification processes that may explain the low, yet above detection limit, NO 3 -concentrations below the apparent denitrification zone (< 2

Table 3

Percent O 2 consumption by reoxidation of reduced metabolites and relative importance of aerobic respiration, denitrification, Mn(III/IV) oxide reduction, Fe(III) oxide reduction, and sulfate reduction on carbon remineralization processes calculated by the MATSEDLAB reactive transport model at steady-state. [START_REF] Beckler | Importance of microbial iron reduction in deep sediments of river-dominated continental-margins[END_REF][START_REF] Rickard | Kinetics of pyrite formation by the H2S oxidation of iron (II) monosulfide in aqueous solutions between 25 and 125 degrees C: the mechanism[END_REF][START_REF] Taillefert | Reactivity of freshly formed Fe(III) in synthetic solutions and (pore)waters: voltammetric evidence of an aging process[END_REF][START_REF] Theberge | Determination of the electrochemical properties of a soluble aqueous FeS species present in Sulfidic solutions[END_REF]. In contrast, AVS was only observed in small concentrations and FeS (aq) was not detected at St. 11 (Fig. 3). Finally, ΣH 2 S either reduce Mn (III/IV) oxides [START_REF] Herszage | Mechanism of hydrogen sulfide oxidation by manganese(IV) oxide in aqueous solutions[END_REF] and Fe(III) d or precipitate Fe 2+ rapidly [START_REF] Taillefert | Reactivity of freshly formed Fe(III) in synthetic solutions and (pore)waters: voltammetric evidence of an aging process[END_REF]. Thus, the low Mn(III/ IV) oxide concentrations below 5 cm (St. 2b and 13), generally low Fe (III) d concentrations and org-Fe(III) current intensities below 10 cm (St. 2b) and 20 cm (St. 13), and decreasing Fe 2+ concentrations with depth (St. 2b) (Figs. 3 and4) present additional evidence that sulfate reduction is most active in shelf and upper slope sediments of the western transect.

Comparatively, Fe(III) d concentrations and org-Fe(III) current intensities were much lower or below detection limit at St. 11, whereas Mn (III/IV) oxides and Fe 2+ concentrations declined at deeper depths in the sediment, suggesting that sulfate reduction is much less significant deeper along the slope. In contrast to the western transect, sulfate reduction appears insignificant along the eastern transect mid-slope sediment (St. 15) as sulfate concentrations did not decrease significantly (Fig. 4 d 2 ), in agreement with previous studies on the Louisiana shelf [START_REF] Zhuang | Heterotrophic metabolism of C1 and C2 low molecular weight compounds in northern Gulf of Mexico sediments: controlling factors and implications for organic carbon degradation[END_REF] and slope [START_REF] Lin | Sulfate reduction and iron sulfide mineral formation in Gulf of Mexico anoxic sediments[END_REF]. Neither FeS (aq) nor ΣH 2 S were detected (Fig. 3d), org-Fe(III) (Fig. 3d) and/or Fe 3+ (Fig. 4 d 3 ) were observed at depth, and AVS concentrations were much lower (Fig. 4 d 6 ), in agreement with visual inspection of the cores which did not reveal any black FeS (s) in sediments at St. 15. In turn, sulfate reduction appeared much more important in the deep sediments of the eastern transect (St. 12). Although FeS (aq) and ΣH 2 S (Fig. 3e), indicative of active bacterial activity, were not detected, sulfate decreased noticeably in the first 12 cm (Fig. 4 e 2 ), Fe(III) d concentrations remained low, Fe 2+ decreased deep in the sediment (Fig. 4 e 3 ), and AVS concentrations were much higher than in the mid-slope sediments (Fig. 4 e 6 ) as corroborated by visual FeS patches at St. 12 (Fig. 1). The reactive transport model confirms that the contribution of sulfate reduction to carbon remineralization rates (Table 3) is highest on the shelf of the western transect (64% at St. 2b) and decreases as water depth increases (27% at St. 13 and 6% at St. 11). These calculations also demonstrate that the contribution of sulfate reduction to carbon remineralization processes is low along the slope of the eastern transect (0.2% at St. 15) but much higher in the deep sediments (30% at St. 12). Simultaneously, geochemical evidence suggests microbial Mn(III/ IV) and Fe(III) oxide reduction are active in the surface sediments of St. 2b and 13 and relatively important across the sediment column at St. 11 and 15, but much less at St. 12 (Fig. 3,4). The presence of org-Fe(III) and Fe(III) d indicates ΣH 2 S was not produced significantly in the top 5 cm at St. 2b (Figs. 3a and4c), top 15 cm at St. 13 (Figs. 3b and4h), and across the profiles at St. 11 and 15 (Fig. 3c and d,Fig. 4 c 3 -d 3 ). The formation of org-Fe(III) at depths much deeper than oxygen penetration and the build-up of Fe 2+ in the pore waters (Fig. 4 a 3 -c 3 and e 3 ) before the decrease in sulfate concentration also suggests microbial Fe(OH) 3 reduction was active at depth in these sediments [START_REF] Beckler | Importance of microbial iron reduction in deep sediments of river-dominated continental-margins[END_REF][START_REF] Taillefert | Early diagenesis in the sediments of the Congo deep-sea fan dominated by massive terrigenous deposits: part II -Iron-sulfur coupling[END_REF]. In turn, the absence of org-Fe(III), much lower Fe (III) d concentrations, and decrease in Fe 2+ with depth below 80 mm at St. 12 suggest that sulfate reduction was more prominent at the deepest station of the eastern transect. Although the reactive transport model indicates that Mn(III/IV) oxide reduction contributes between 5 and 24% of carbon remineralization rates at St. 2b and 13 compared to 9% at St. 11 (Table 3), the contribution of microbial Fe(III) oxide reduction is much smaller at these stations (< 6% of carbon remineralization rates). These findings are explained by the shallow location of the sulfate reduction zone on the shelf at St. 2b (Fig. 5), mid-slope at St. 13 (Fig. S3), and bottom of the slope at St. 11 (Fig. S4) that compresses the microbial metal reduction zone against the SWI and therefore favors microbial Mn (IV/III) reduction and abiotic Fe(III) oxides reduction by ΣH 2 S with eventual precipitation of MnCO 3 and FeS minerals. Along the eastern transect, microbial metal reduction rates are much lower, and the anaerobic respiration of carbon is mainly driven by denitrification (19% at St. 15 and 18% at St. 12) with minor contribution by microbial Mn (III/IV) oxide reduction (< 5%). These findings corroborate with the generally low sediment deposition rates predicted across the slope at this location [START_REF] Xu | Dispersal of Mississippi and Atchafalaya sediment on the Texas-Louisianashelf: Model estimates for the year 1993[END_REF].

Dissolved metal transport mechanism and enrichment of Mn(III/IV) oxides on the Louisiana slope

The Mississippi River supplies a large load of particulate material during its high discharge period from the winter to early spring that is eventually deposited on the Louisiana shelf [START_REF] Corbett | Nature of decadal-scale sediment accumulation on the western shelf of the Mississippi River delta[END_REF][START_REF] Corbett | An evaluation of mobile mud dynamics in the Mississippi River deltaic region[END_REF][START_REF] Devereux | Spatially variable bioturbation and physical mixing drive the sedimentary biogeochemical seascape in the Louisiana continental shelf hypoxic zone[END_REF]. These sediments form mobile muds that are often remobilized by physical mixing generated by tidal currents or storms [START_REF] Sampere | Burial and degradation of organic carbon in Louisiana shelf/slope sediments[END_REF]. This process eventually sorts material by size, morphology, or composition [START_REF] Mckee | Transport and transformation of dissolved and particulate materials on continental margins influenced by major rivers: benthic boundary layer and seabed processes[END_REF], transports terrestrial organic carbon [START_REF] Waterson | Sources of sedimentary organic matter in the Mississippi River and adjacent Gulf of Mexico as revealed by lipid biomarker and δ13CTOC analyses[END_REF] and inorganic material [START_REF] Trefry | Manganese fluxes from Mississippi Delta sediments[END_REF] to the deep sediment, and may increase diagenetic reactivity by a priming effect [START_REF] Bianchi | The role of terrestrially derived organic carbon in the coastal ocean: a changing paradigm and the priming effect[END_REF]). An enrichment of Mn(III/IV) oxides is observed at the sediment surface of St. 13 and more particularly St. 11, but not St. 2b, nor along the eastern transect (St. 15 and 12) where Mn(III/IV) concentrations remain low across the sediment (Fig. 4 a 4 -e 4 ). Indeed, the total reactive Mn:Fe ratios (log of the sum of reactive Mn(II/IV) oxides, adsorbed Mn 2+ , and carbonate-bound Mn 2+ relative to dithionite-extracted Fe) in the top 5 cm of St. 13 and in the top 25 cm of St. 11 are far greater (Fig. 7) than the total reactive Mn:Fe ratios at St. 2b, 15, and 12 (Fig. 7) which remain relatively constant with depth. Surface enrichment in Mn(III/IV) oxides such as that observed at St. 13 and 11 is commonly reported in marine sediments along water depth gradients [START_REF] Aller | The sedimentary Mn cycle in Long Island sound: its role as intermediate oxidant and the influence of bioturbation, O 2 , and Corg flux on diagenetic reaction balances[END_REF][START_REF] Law | Manganese, iron, and Sulphur cycling on the Pakistan margin[END_REF][START_REF] Mouret | Benthic geochemistry of manganese in the Bay of Biscay, and sediment mass accumulation rate[END_REF][START_REF] Sundby | Pathways of manganese in an open estuarine system[END_REF][START_REF] Trefry | Manganese fluxes from Mississippi Delta sediments[END_REF][START_REF] Wade | Trace elements and polycyclic aromatic hydrocarbons (PAHs) concentrations in deep Gulf of Mexico sediments[END_REF]. Such feature is attributed to the diffusive flux of Mn 2+ formed at sediment depth in reduced conditions to the oxic zone and eventual heterogeneous reoxidation by oxygen [START_REF] Sundby | Manganese fluxes in the benthic boundary layer1[END_REF][START_REF] Trefry | Manganese fluxes from Mississippi Delta sediments[END_REF]. As the oxidation of Mn 2+ is generally slow in sediments, Mn(III/IV) oxides are not recycled efficiently [START_REF] Richard | Kinetics of manganese adsorption, desorption, and oxidation in coastal marine[END_REF]. Rates of Mn 2+ oxidation, however, are enhanced by bioirrigation which increases both the penetration of dissolved oxygen into the sediment [START_REF] Aller | The effect of biogenic irrigation intensity and solute exchange on diagenetic reaction rates in marine sediments[END_REF] and the exchange of Mn 2+ with the waters [START_REF] Boudreau | Diagenetic models and their implementation[END_REF]. In turn, Mn 2+ oxidation rates are decreased by the precipitation of MnCO 3 minerals in pore waters [START_REF] Duckworth | Role of molecular oxygen in the dissolution of siderite and rhodochrosite[END_REF]. Evidence of bioirrigation exists in the oxygen profile at St. 15, where a pocket of oxygen was detected below the OPD (Fig. 2) and has been documented in the vicinity of the slope stations [START_REF] Xu | Seabed erodibility variations on the Louisiana continental shelf before and after the 2011 Mississippi River flood[END_REF]. Low adsorbed Mn 2+ concentrations were observed at depth in cores along both transects (Fig. 4 a 4 -e 4 , Fig. S2). Additionally, adsorbed Mn 2+ was absent in the surface sediments, likely due to enhanced Mn 2+ oxidation in the oxic zone by surface catalyzed reactions on Fe(III), Al, and Si oxide surfaces [START_REF] Davies | Manganese(II) oxidation kinetics on metal oxide surfaces[END_REF][START_REF] Lan | Mechanisms of Mn(II) catalytic oxidation on ferrihydrite surfaces and the formation of manganese (oxyhydr)oxides[END_REF]. In the shelf and mid-slope sediments of the western transect (St. 2b and St. 13), microbial Mn(IV/III) reduction and reduction of Mn(IV) oxides by ΣH 2 S generate a high upward flux of Mn 2+ to the sediment surface (Fig. 4 a 3 -b 3 ) that leads to a significant diffusive flux across the SWI at St. 2b and small, but detectable at St. 13 (Table S9), even though a significant fraction of Mn 2+ is immobilized under the form of carbonate minerals (Fig. 4 a 4 -b 4 and Fig. S2). In fact, the diffusive flux of Mn 2+ at the SWI is much larger than the average burial fluxes of MnCO 3 at St 2b, suggesting that the shelf station represents an important source of Mn 2+ to the overlying waters. Compounded by the low dissolved O 2 concentrations of the overlying waters at St. 2b (Table 1), model calculations demonstrate that O 2 consumption by reoxidation of reduced metabolites in the sediment is relatively small (< 20%) except at St. 13 (Table 3). As a result, Mn d is able to reach the bottom waters at the shelf station (Table S9) where it is likely removed by particle adsorption [START_REF] Trefry | Manganese fluxes from Mississippi Delta sediments[END_REF], oxidized heterogeneously [START_REF] Davies | Manganese(II) oxidation kinetics on metal oxide surfaces[END_REF], then eventually transported by mobile muds and deposited along continental slope sediments (St. 13 and St. 11). Ultimately, these Mn-enriched materials generate a Mn 'conveyor belt' from the shelf to the bottom of the slope, as observed with Mn and Fe in a variety of continental margins (e.g., [START_REF] Lyons | A critical look at iron paleoredox proxies: New insights from modern euxinic marine basins[END_REF][START_REF] Scholz | Early diagenesis of redox-sensitive trace metals in the Peru upwelling arearesponse to ENSO-related oxygen fluctuations in the water column[END_REF]2013;[START_REF] Jilbert | Iron and manganese shuttles control the formation of authigenic phosphorus minerals in the euxinic basins of the Baltic Sea[END_REF][START_REF] Scholz | Shelf-to-basin iron shuttle in the Guaymas Basin, Gulf of California[END_REF][START_REF] Lenstra | Controls on the shuttling of manganese over the northwestern Black Sea shelf and its fate in the euxinic deep basin[END_REF]. Although the upward diffusive flux of Mn 2+ was not significantly lower at St. 11 (136 ± 10 μmol m -2 d -1 ) than the mid-slope sediment (172 ± 17 μmole m -2 d -1 at St. 13), Mn 2+ did not reach the SWI (Fig. 4 c 3 and Table S9) due to the enhanced thickness of the Mn 2+ oxidation zone, as evidenced by the deeper OPD, pH minimum (Fig. 2), and position of the dissolved Mn peak (Fig. 4 b 3c 3 ).

Differential Iron and Mn cycling

The pore water and solid phase speciation of Mn and Fe as a function of depth in these sediments is consistent with the predictions of the thermodynamic redox ladder regarding the dynamics of Fe and Mn redox cycling [START_REF] Froelich | Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis[END_REF]. Whereas Mn 2+ concentrations were greater than Fe 2+ in most sediment pore waters studied, the inverse is true in the solid phase (Fig. 4), except for a few depths near the sediment surface at St. 11 where Mn(III/IV) oxides were enriched relative to Fe (III) oxides (Fig. 4 c 4 -c 5 ). Abiotic and biotic Mn(III/IV) oxides reduction are typically faster than Fe(III) oxide reduction [START_REF] Arnold | Reductive dissolution of fe(III) oxides by Pseudomonas sp. 200[END_REF][START_REF] Dollhopf | Kinetics of Fe(III) and Mn(IV) reduction by the Black Sea strain of Shewanella putrefaciens using in situ solid state voltammetric Au/Hg electrodes[END_REF][START_REF] Herszage | Mechanism of hydrogen sulfide oxidation by manganese(IV) oxide in aqueous solutions[END_REF][START_REF] Larsen | Manganite reduction by Shewanella putrefaciens MR-4[END_REF][START_REF] Lovley | Novel mode of microbial energy metabolism: organic carbon oxidation coupled to Dissimilatory reduction of Iron or manganese[END_REF][START_REF] Myers | Bacterial manganese reduction and growth with manganese oxide as the sole Electron acceptor[END_REF], whereas Mn 2+ oxidation is much slower than Fe 2+ oxidation (Stumm and Morgan, 1996;[START_REF] Richard | Kinetics of manganese adsorption, desorption, and oxidation in coastal marine[END_REF]. The large abundance of poorly crystalline (51 ± 21 μmol gdw -1 ) and crystalline Fe(III) oxides (46 ± 45 μmol gdw -1 ) throughout each profile, representing about 94 ± 5% of total reactive Fe in these sediments (Fig. S2) and the low AVS concentrations in all sediments except St. 2b and the deep layers of St. 13 (Fig. 4 a 6 -e 6 ) suggest that these sediments have been exposed to dissolved oxygen. Although bioturbation was not directly assessed, depth profiles of dissolved constituents (e.g., NH 4 + , DIC) appeared diffusive all the way to the SWI (Fig. 4 a 1 -d 1 ) whereas depth profiles of Fe(III) oxides were not significantly enriched near the SWI (Fig. 4 a 5 -d 5 ), suggesting that bioturbation was not intense at these stations. As evidence for intense bioturbation is lacking, it is likely that exposure to dissolved oxygen occurred during resuspension as mobile muds prior to eventual deposition and accumulation. In contrast to Mn 2+ , however, Fe 2+ is mostly retained in the sediment (Table S9) either by oxidation near the SWI or precipitation under the form of carbonate or sulfide minerals. These findings are different than the recent Mn [START_REF] Lenstra | Controls on the shuttling of manganese over the northwestern Black Sea shelf and its fate in the euxinic deep basin[END_REF] and Fe [START_REF] Lenstra | The shelf-to-basin iron shuttle in the Black Sea revisited[END_REF] conveyer belt observed in the Black Sea, although the benthic flux of these metals from the coastal stations was attributed to intense bioturbation which enhanced the supply of organic matter and dissolved oxygen and stimulated both Fe and Mn reduction close to the SWI.

The data collected in this study immediately at the end of the high discharge period confirm that sedimentary material is transported all the way to the bottom of the slope (St. 11) during the high discharge period and preferentially enriched in Mn(III/IV) oxides relative to reactive Fe. Whereas Mn(III/IV) oxide reduction was intense in these sediments, the reduction of Fe(III) oxides was apparently not significant (i.e., low Fe 2+ , FeCO 3 minerals, and FeS (s) ), providing evidence for the differential kinetics of microbial Mn and Fe reduction in a natural setting. The reactive transport model confirms these findings by demonstrating that the Mn:Fe input flux ratio at the SWI is increased with depth across each transect (Table S8) and metal respiration processes on the slope during that time period were mainly controlled by microbial Mn reduction and not microbial Fe reduction (Table 3). As suggested by thermodynamic calculations (Fig. S7), microbial Fe(III) oxide reduction likely dominates after depletion of the Mn(IV) oxides, which would explain the high concentrations of reduced Fe 2+ previously observed near St. 12 [START_REF] Beckler | Importance of microbial iron reduction in deep sediments of river-dominated continental-margins[END_REF]. However, it is not clear whether the stock of Fe(III) oxides is large enough to prevent sulfate reduction from dominating anaerobic respiration processes during the rest of the summer and fall. The enhanced production of carbonates under sulfate reducing conditions (i.e., one mole SO 4 2-reduced produces two moles of carbonate) should at least bury Mn 2+ , thus preventing its diffusion across the SWI and reducing the efficiency the Mn conveyor belt at the end of the high discharge period. Overall, these findings indicate continental slope sediments exposed to large riverine inputs are highly dynamic systems that may remineralize natural organic matter using different terminal electron accepting processes depending on riverine discharge intensity.

Conclusions

The main redox processes involved in the transformation of natural organic matter in RiOMars sediments were investigated along two transects in the northern Gulf of Mexico to examine the effect of particulate material inputs from the Mississippi River plume on diagenetic processes after a high riverine discharge period. Results suggest organic carbon deposition control the intensity of anaerobic carbon remineralization processes and select the dominant anaerobic carbon remineralization pathways. Although sulfate reduction dominates at the shelf station (65 m water depth), microbial Mn reduction appears to represent a significant anaerobic respiration process along the continental slope closest to the Mississippi River plume whereas iron mainly remains under the form of Fe(III) oxides and does not play a major role in carbon remineralization. These findings suggest the differential kinetics of Mn and Fe redox transformations coupled with the high discharge of the Mississippi River during the winter and spring influence carbon remineralization processes on the continental slope. The fast Fe 2+ oxygenation kinetics near the sediment-water interface and high sedimentation rates maintain iron under the form of Fe(III) oxides and prevent sulfate reduction from being significant, whereas the much slower kinetics of oxygenic oxidation allows diffusion of Mn 2+ across the sediment-water interface. Exposure to bottom water oxygen and mobile muds typical of deltaic sediments during high riverine discharge likely promote the formation and downslope transport of Mn(III/IV) oxides, forming a Mn 'conveyor belt' that selectively enriches Mn(III/IV) oxides relative to Fe (III) oxides in the deep slope sediment. It is not clear whether the stock of Fe(III) oxides is large enough to prevent sulfate reduction from dominating anaerobic respiration processes over time. The enhanced production of carbonates under sulfate reducing conditions should at least reduce the efficiency of the Mn conveyor belt and efficiently bury this important terminal electron acceptor during the low riverine discharge period. In contrast, the intensity of 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 Mn cycling in continental slope sediments exposed to large riverine inputs may be more important than previously considered, and carbon remineralization via Mn reduction could contribute significantly to benthic DIC and alkalinity production.

Declaration of Competing Interest

None.

Fig. 1 .

 1 Fig. 1. a) Map of the stations across the Louisiana shelf and slope in the northern Gulf of Mexico where benthic landers were deployed and sediment cores collected during a research cruise in July and August of 2017. The two transects, east and west, are labeled with arrows. Depth contours (m) indicate ocean bathymetry; b) pictures of the cores showing contrasting sediment colors at the different sampling locations and color variations with depth in each core. Thin, medium, and thick white and black lines represent depth intervals of 1, 2 and 5 cm respectively.

Fig. 2 .Fig. 3 .

 23 Fig. 2. O 2 (red) and pH (green and blue) microprofiles in sediment cores along the western (St. 2, St. 13 and St. 11) and eastern transects (St. 15, St. 12). Replicate profiles are indicated by solid and dotted lines. Oxygen profiles at St. 2b were collected in situ, while all others were collected ex situ after core collection. The pH profiles at St. 2b were collected both in-situ with a benthic lander (pH-green) and ex situ in sediment cores (blue). The sediment water interface (depth = 0 mm) is indicated by the dashed horizontal line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4 3 - 3 -

 33 , and NH 4 + profiles. DIC, TA, ΣPO 4 3-, and NH 4 + concentrations increased from the SWI to maxima at depth at all stations, but maximum concentrations decreased with increasing water depth in the order St. 2b (Fig. 4 a 1 ) > > St. 13 (Fig. 4 b 1 ) > St. 11 (Fig. 4 c 1 ). In the eastern transect, DIC, ΣPO 4 3-, and TA did not vary throughout the core at St. 15 (Fig. 4 d 1 ), indicating little respiratory activity relative to the other stations. In turn, slight increases in DIC, TA, ΣPO 4 3-, and NH 4 + concentrations with depth were observed at St. 12 (Fig. 4 e 1 ). Sulfate concentrations decreased regularly from 29.9 ± 0.5 to 21.3 mM at 500 mm at St. 2b (Fig. 4 a 2 ), only slightly to 26.7 ± 1.13 mM between 365 and 525 mm at St. 13 (Fig. 4 b 2 ), and remained relatively constant throughout the profiles at the other stations (Fig. 4 c 2 , d 2 , and e 2 ). With respect to the dissolved nitrogen species, NO concentrations were highest in the bottom waters (ranging between 12 and 31 μM), decreased rapidly within the first 10-20 mm to approximately 1-5 μM, and remained relatively constant around these values at depth (Fig. 4 a 2 , c 2 , d 2 , and e 2 ) as observed previously (Zhuang et al., 2019). In turn, NO 2 -concentrations remained below 5 μM throughout each core (Fig. 4). St. 2b displayed the highest Mn d concentration in the overlying water (12.2 ± 0.2 μM) of all stations (1.7 ± 0.4 μM at St. 13 and < MDL at St. 11, St. 15, and St. 12) (Fig. 4 a 3 , b 2 , c 3 , d 3 , e 3 ), indicative of a benthic flux as observed by the voltammetric measurements (Fig. 3a). At each western transect station, Mn d was produced immediately below the SWI and generally displayed a subsurface peak with a maximum concentration ranging between 239 and 400 μM, the position of which increased with depth across the continental slope (10 mm at St. 2b, 43 mm at St. 13, and 127 mm at St. 11) (Fig. 4 a 3 , b 3 , c 3 ). Below this peak, Mn d concentrations decreased to a minimum of approximately 100 μM at 600 mm at St. 2b, whereas it rebounded below 108 mm to a maximum of 277 ± 6 μM at depth at St. 13 and slightly decreased to a minimum of 191 ± 2 μM at depth at St. 11. The Mn d profile at the deepest station of the eastern transect (St. 12), displayed a similar profile as the deepest station of the western transect (St. 11), with increase in Mn d to a maximum of 317 ± 1 μM at 75 mm but only a slight decrease in concentration with depth (Fig. 4 d 3 ). Atypical to all other stations, Mn d concentrations at St. 15 did not display a peak, rather concentrations remained relatively low and gradually increased to 73 ± 2 μM at 215 mm followed by a decrease in concentration deeper (Fig. 4 e 3 ). Overall, pore water Fe 2+ concentrations were lower than pore water Mn d and Fe(III) d concentrations in all cores, suggesting sediments across the continental slopes were suboxic. Although a shallow Fe 2+ peak (max. 62 ± 0.6 μM at 90 mm) was present at St. 2b (Fig. 4 a 3 ), pore water Fe 2+ was present in low concentrations (0-15 μM) throughout the cores of all other stations (Fig. 4 b 3 , c 3 , d 3 , e 3 )

Fig. 4 .

 4 Fig. 4. Depth profiles of DIC, total alkalinity (TA), ΣPO 4 3-, and NH 4 + (first panel), NO 3 -, NO 2 -, and SO 4 2-(second panel), dissolved Mn(II), Mn(III), Fe(II), and Fe(III) d (third panel), total solid Mn and Fe partitioned as adsorbed, carbonate, noncrystalline, and crystalline phases (fourth and fifth panels), and 'acid volatile sulfides' (sixth panel) at: St. 2b (a 1 -a 6 ); St. 13 (b 1b 6 ); and St. 11 (c 1 -c 6 ) along the western transect and at: St. 15 (d 1 -d 6 ); and St. 12 (e 1 -e 6 ) along the eastern transect. The sediment water interface is indicated by the dashed horizontal line. Note the change in scale of alkalinity, DIC, ΣPO 4 3-, and NH 4 + at

Fig. 5 .+,

 5 Fig. 5. Example of modeled depth profiles of the main species involved in diagenetic processes at St. 2b (plain lines) compared to experimental depth profiles (scatter points). Only respiration rates (aerobic, denitrification, Mn reduction, Fe reduction, and SO 4 2-reduction) were optimized to fit the experimental data at steady-state. Rate constants of abiotic processes and biological oxidation of NH 4 + , ΣH 2 S, and Mn 2+ by O 2(aq) were fixed at each station.

Fig. 6 .+

 6 Fig. 6. Diffusive fluxes of DIC, NH 4 + , ΣPO 4 3-, and SO 4 2-across the sedimentwater interface. Error bars represent the standard deviation extrapolated from porosity.

Fig. 7 .

 7 Fig. 7. Depth profiles of total reactive Mn to Fe ratios (logarithmic scale) at St. 2 (red), St. 13 (blue), and St. 11 (green) along the western transect and St. 15 (brown) and St. 12 (black) along the eastern transect. Total reactive Mn includes the sum of reactive Mn(III/IV) oxides, adsorbed Mn 2+ , and carbonatebound Mn 2+ , whereas total reactive Fe was quantified by the dithionite extraction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

  and eastern (St. 15 and 12) transects i .

	Station	Sed rate (cm/ year)	OC (%)	Average porosity g	OPD (mm) h	DOU (mmol/ m 2 /d)
	St. 2b St. 13 St. 11	>2.5 a <0.7 b 0.07 ± 0.06 c	1.5 d 1.1 d 1.2 e	0.85 ± 0.04 0.87 ± 0.04 0.85 ± 0.05	1.6 8.3 14.0 ±	13.8 3.6 3.9 ± 0.3
	St. 15 St. 12	0.09 ± 0.05 c	1.5 f 1.0 e	0.80 ± 0.05 0.75 ± 0.05	1.1 7.4 ± 0.6 9.0 ± 0.3	2.6 ± 0.1 2.3 ± 0.2
	a Proximal (Type 1) sediment accumulation rates using 210 Pb from Corbett
	et al., 2006. b Distal (Type 3) sediment accumulation rates using 210 Pb from Corbett et al.,
	2006. c From 210 Pb measurements from Yeager et al., 2004 (stations MT3 and S36). d TOC% St. 2 and St. 13 values from Waterson and Canuel, 2008 (stations 80 m
	and H9).

e % OC from

Morse and Beazley, 2008 (stations MT3 and HiPro)

. f %POC from

Zhuang et al., 2019 (Taylor Energy Site)

. g Average porosity and standard deviation over entire core, porosity profiles in supplemental material. h OPD and DOU represent mean values from two ex situ profiles, except for St.

2 and 13 which were profiled once and St 2b which consisted of two in situ profiles.

i See methods section for calculations.

  ) which would enhance denitrification even more. In turn, multiple proxies suggest sulfate reduction is most active on the shelf of the western transect (St. 2b) and decreases in intensity with water depth. First, a significant decrease in sulfate concentrations on the shelf (St. 2b) and mid-slope (St. 13) that was not observed at the bottom of the slope (St. 11), and the detection of ΣH 2 S at St. 2b only, provide evidence for sulfate reduction on the upper slope but not in deep sediments. Second, AVS was present throughout the sediment at St. 2b and deep in the sediment at St. 13, and the simultaneous detection of FeS (aq) at these stations indicates active precipitation of FeS

	Station	O 2 Consumption by reduced metabolites (%)	Aerobic respiration (%)	Denitrification (%)	Mn reduction (%)	Fe reduction (%)	SO 4 2-reduction (%)	Rate cox (mmol m -2 d -1 )
	St. 2b	17.7	11.7	15.1	5.3	4.1	63.8	16.5
	St. 13	60.5	20.3	24.0	23.7	5.4	26.6	3.6
	St. 11	0.6	68.5	16.5	8.9	0.2	5.9	4.5
	St. 15	3.6	73.1	19.1	2.8	4.8	0.2	1.8
	St. 12	17.4	47.1	17.7	4.4	0.6	30.2	10.3

cm

Acknowledgements

We would like to thank the captain and crew of the RV Savannah as well as Emily Buckley, Julien Richirt, and Andrew Stancil for help with sample collection and/or analysis. This research was supported by the National Science Foundation (OCE-1438648) to M. Taillefert, EC2CO-DRIL MissRhoDia project to C. Rabouille, National Academies of Science, Engineering, and Medicine Gulf Research Program (Early Career Grant 2000007281) to J. Beckler, and the Chateaubriand Fellowship of the Office for Science & Technology of the Embassy of France in the United States awarded to S. Owings. Finally, we thank the editor and two anonymous reviewers for their insightful comments and suggestions which helped improve this paper significantly.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.marchem.2020.103908.