Factors controlling the burial of organic carbon in laminated and bioturbated sediments off NW Mexico: implications for hydrocarbon preservation

Factors controlling the burial of organic carbon (OC) in Late Quaternary sediments on the NW Mexican continental margin are assessed using a suite of box and piston cores strategically located on the shelf-slope rise with respect to the intense oxygen minimum in this region. An OC concentration maximum occurs on the mid-slope, below the core of an intense water-column O₂ minimum, due to current winnowing on the outer shelf, the preferential accumulation of organic matter in fine-grained deposits, and the offshore decrease in the settling flux of organic detritus. The organic matter at all water depths is overwhelmingly marine. Hydrogen indices (HI) are higher on the slope (>300 mg HC/g TOC) than on the shelf (<300 mg HC/g TOC), where current winnowing has promoted organic matter degradation, but there is no difference in HI in slope sediments accumulating under well oxygenated and O₂-deficient conditions. The degree of winnowing appears to be the primary factor affecting the preservational quality of organic matter deposited on this margin.Rates of accumulation of OC and opal are all higher in the interglacial intervals when compared with the glacial deposits over the last 140,000 yr. However, matrix-corrected HI values in the mid- and lower-slope cores are invariant and are similar to values in the laminated intervals from the oxygen-minimum site. Thus, cyclic changes in organic carbon accumulation on this margin have been controlled by production variations rather than differential preservation. HI values in Late Quaternary sediments from several continental margins, including NW Mexico, and euxinic basins correspond to type II kerogen, irrespective of bottom water O₂ concentrations. Therefore, the preservation of oil-prone kerogen in productive margin settings does not appear to be restricted to sediments deposited under conditions of low bottom water O₂ concentrations as envisioned in models of petroleum source-rock deposition.     

Fate of organic carbon reaching the deep sea floor: a status report

The rates of organic carbon oxidation by O₂, NO₃^?, MnO₂, Fe₂O₃ and SO₄^? have been calculated for five pelagic Pacific and Atlantic sites using simple diffusion-reaction models. O₂ everywhere oxidizes > 90% of the raining C_org; the fraction oxidized by the secondary oxidants decreases as the rain rate of organic C to the seafloor decreases. A large fraction of the C_org escaping oxidation by O₂ is oxidized by the secondary oxidants. Hence while these oxidants play a small role in remineralization, they are important in regulating the burial of organic matter and the consequent removal from the oceans of reduced carbon and nutrients.     

Carbon Cycling and the Coupling Between Proton and Electron Transfer Reactions in Aquatic Sediments in Lake Champlain

We used fine-scale porewater profiles and rate measurements together with a multiple component transport–reaction model to investigate carbon degradation pathways and the coupling between electron and proton transfer reactions in Lake Champlain sediments. We measured porewater profiles of O₂, Mn²⁺, Fe²⁺, HS⁻, pH and pCO₂ at mm resolution by microelectrodes, and profiles of NO₃ ⁻, SO₄ ²⁻, NH₄ ⁺, total inorganic carbon (DIC) and total alkalinity (TA) at cm resolution using standard wet chemical techniques. In addition, sediment–water fluxes of oxygen, DIC, nitrate, ammonium and N₂ were measured. Rates of gross and net sulfate reduction were also measured in the sediments. It is shown that organic matter (OM) decomposes via six pathways: oxic respiration (35.2%), denitrification (10.4%), MnO₂ reduction (3.6%), FeOOH reduction (9.6%), sulfate reduction (14.9%), and methanogenesis (26.4%). In the lake sediments, about half of the benthic O₂ flux is used for aerobic respiration, and the rest is used for the regeneration of other electron acceptors produced during the above diagenetic reactions. There is a strong coupling between O₂ usage and Mn²⁺ oxidation. MnO₂ is also an important player in Fe and S cycles and in pH and TA balance. Although nitrate concentrations in the overlying water were low, denitrification becomes a quantitatively important pathway for OM decomposition due to the oxidation of NH₄ ⁺ to NO₃ ⁻. Finally, despite its low concentration in freshwater, sulfate is an important electron acceptor due to its high efficiency of internal cycling. This paper also discusses quantitatively the relationship between redox reactions and the porewater pH values. It is demonstrated here that pH and pCO₂ are sensitive variables that reflect various oxidation and precipitation reactions in porewater, while DIC and TA profiles provide effective constraints on the rates of various diagenetic reactions.     

Modeling of subsurface calcite dissolution, including the respiration and reoxidation processes of marine sediments in the region of equatorial upwelling off Gabon

Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O₂, pH, pCO₂ and Ca²⁺ (in situ), pore-water concentration profiles of NO₃⁻, NH₄⁺, Fe²⁺, and Mn²⁺ and SO₄²⁻ (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O₂, NO₃⁻, Fe(OH)₃ and SO₄²⁻, reoxidation reactions involving Fe²⁺ and Mn²⁺, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 μmol C cm⁻² yr⁻¹, of which 77% were due to O₂, 17% to NO₃⁻ and 3% to Fe(OH)₃ and 3% to SO₄²⁻. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d⁻¹ and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO₃ flux to the sediment was estimated to occur at a rate of 42 g m⁻² yr⁻¹ in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m⁻² yr⁻¹, thus indicating that ∼90% of the calcite flux to the sediment is redissolved.     

Uranium diagenesis in sediments underlying bottom waters with high oxygen content

We measured U in sediments (both pore waters and solid phase) from three locations on the middle Atlantic Bight (MAB) from the eastern margin of the United States: a northern location on the continental shelf off Massachusetts (OC426, 75 m water depth), and two southern locations off North Carolina (EN433-1, 647 m water depth and EN433-2, 2648 m water depth). These sediments underlie high oxygen bottom waters (250-270 μM), but become reducing below the sediment-water interface due to the relatively high organic carbon oxidation rates in sediments (EN433-1: 212 μmol C/cm²/y; OC426: 120 ± 10 μmol C/cm²/y; EN433-2: 33 μmol C/cm²/y). Pore water oxygen goes to zero by 1.4-1.5 cm at EN433-1 and OC426 and slightly deeper oxygen penetration depths were measured at EN433-2 (∼4 cm).All of the pore water profiles show removal of U from pore waters. Calculated pore water fluxes are greatest at EN433-1 (0.66 ± 0.08 nmol/cm²/y) and less at EN433-2 and OC426 (0.24 ± 0.05 and 0.13 ± 0.05 nmol/cm²/y, respectively). Solid phase profiles show authigenic U enrichment in sediments from all three locations. The average authigenic U concentrations are greater at EN433-1 and OC426 (5.8 ± 0.7 nmol/g and 5.4 ± 0.2 nmol/g, respectively) relative to EN433-2 (4.1 ± 0.8 nmol/g). This progression is consistent with their relative ordering of ‘reduction intensity’, with greatest reducing conditions in sediments from EN433-1, less at OC426 and least at EN433-2. The authigenic U accumulation rate is largest at EN433-1 (0.47 ± 0.05 nmol/cm²/y), but the average among the three sites on the MAB is ∼0.2 nmol/cm²/y. Pore water profiles suggest diffusive fluxes across the sediment-water interface that are 1.4-1.7 times greater than authigenic accumulation rates at EN433-1 and EN433-2. These differences are consistent with oxidation and loss of U from the solid phase via irrigation and/or bioturbation, which may compromise the sequestration of U in continental margin sediments that underlie bottom waters with high oxygen concentrations.Previous literature compilations that include data exclusively from locations where [O₂]_bw < 150 μM suggest compelling correlations between authigenic U accumulation and organic carbon flux to sediments or organic carbon burial rate. Sediments that underlie waters with high [O₂]_bw have lower authigenic U accumulation rates than would be predicted from relationships developed from results that include locations where [O₂]_bw < 150 μM.     

Changes in organic matter-mineral interactions for marine sediments with varying oxygen exposure times

Density fractionation, X-ray photoelectron spectroscopy (XPS) and amino acid analyses were used to evaluate the physical form of preserved organic carbon (OC) in sediments from transects in the north east Pacific Ocean off the Mexican (Mazatlan) and Washington coasts. Low density (i.e. mineral-free) organic material dominated the OC in sediments with very short oxygen exposure times (OET <1 yr; total wt% OC 7.9-9.3, ∼70% in ρ < 1.9 fractions). Quantities of low density OC decreased with increasing OET, suggesting that organic-rich debris was oxygen-sensitive. As OET increased from years to decades, OC was found predominately in the form of organic-mineral aggregates (58-77% of total OC was in the 1.9-2.2 ρ mesodensity fraction). Aggregated OC then decreased in abundance as a function of OET, suggesting that long-term oxygen exposure leads to destruction of organic-mineral aggregates. At OET values of centuries to millennia, the dominant forms of organic matter were OC sorbed to mineral surfaces (∼50% of total) and OC locked within biogenic diatom frustules (∼22% of total). These two physical forms of OC in marine sediments may provide the best long-term protection for OC.     

Microelectrode Study of Oxygen Uptake and Organic Matter Decomposition in The Sediments of Xiamen Western Bay

Sediment cores were sampled from Xiamen Western Bay at five sites during the summer and winter of 2006 and Hg–Au microelectrodes were used to make on board measurements of the concentration gradients of dissolved oxygen, Mn²⁺, and Fe²⁺ within the sediments. The O₂ concentrations decreased sharply from about 200 μmol L⁻¹ in the bottom seawater to zero within a depth of a few millimeters into the sediment. Dissolved Mn²⁺ was detected below the oxic zones with peak concentrations up to 600 μmol L⁻¹, whereas dissolved Fe²⁺ had peak concentrations up to 1,000 μmol L⁻¹ in deeper layers. The elemental contents of organic carbon and nitrogen within the sediments were analyzed and their C/N ratios were in the range of 9.0 to 10.1, indicative of heavy terrestrial origin. Sediments from two sites near municipal wastewater discharge outlets had higher organic contents than those from the other sites. These high organic contents corresponded to shallow O₂ penetration depths, high dissolved Mn²⁺ and Fe²⁺ concentrations, and negative redox potentials within the sediments. This indicated that the high organic matter content had promoted microbial respiration within the sediments. Overall, the organic content did not show any appreciable decrease with increasing sediment depths, so a quadratic polynomial function was used to fit the curve of O₂ profiles within the sediments. Based on the O₂ profiles, O₂ fluxes across the seawater and sediment interface were estimated to be in the range 6.07 to 14.9 mmol m⁻² day⁻¹, and organic carbon consumption rates within the surface sediments were estimated to be in the range 3.3 to 20.8 mgC cm⁻³ a⁻¹. The case demonstrated that biogeochemistry within the sediments of the bay was very sensitive to human activities such as sewage discharge.     

Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis

Pore water profiles of total-CO₂, pH, PO^3?₄, NO^?₃ plus NO^?₂, SO^2?₄, S^2?, Fe²⁺ and Mn²⁺ have been obtained in cores from pelagic sediments of the eastern equatorial Atlantic under waters of moderate to high productivity. These profiles reveal that oxidants are consumed in order of decreasing energy production per mole of organic carbon oxidized (O₂ > manganese oxides ~ nitrate > iron oxides > sulfate). Total CO₂ concentrations reflect organic regeneration and calcite dissolution. Phosphate profiles are consistent with organic regeneration and with the effects of release and uptake during inorganic reactions. Nitrate profiles reflect organic regeneration and nitrate reduction, while dissolved iron and manganese profiles suggest reduction of the solid oxide phases, upward fluxes of dissolved metals and subsequent entrapment in the sediment column. Sulfate values are constant and sulfide is absent, reflecting the absence of strongly anoxic conditions.     

Solid-state C NMR analysis of size and density fractions of marine sediments: Insight into organic carbon sources and preservation mechanisms

Burial of organic carbon (OC) in ocean sediments acts as the ultimate long-term sink for both terrestrial and marine carbon, however, the mechanisms controlling the preservation of this carbon are poorly understood. To better understand these mechanisms, we applied solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy, along with elemental, stable carbon isotopic (δ¹³C) and lignin phenol analyses, to size and density fractions of sediments influenced by either mixed terrestrial and marine OC inputs (Washington Coast slope) or dominantly marine inputs (Mexican Margin). Elemental, isotopic and lignin analyses all reveal that within the Washington Coast sediment, the OC mixes linearly between nitrogen-poor and ¹³C-depleted, lignin-rich OC in the large and light fractions and nitrogen-rich and ¹³C-enriched, lignin-poor OC in the small and dense fractions, suggesting that this sediment contains a two-component mixture of terrestrial vascular plant- and marine-derived OC. The integral areas of each of seven NMR spectral regions in the different samples trend linearly when plotted versus δ¹³C signature, with most R² values of 0.78 or greater, demonstrating that the NMR spectra of the two sources of carbon also mix linearly between the two endmembers. The terrestrial endmember in this sediment appears to be dominated by lignin and black carbon whereas the source of the marine endmember is less clear from the NMR spectra. In contrast, all of the analyses indicate that OC in the Mexican Margin sediment fractions is homogenous and derives almost exclusively from marine sources. It appears that selective preservation of (bio)chemically recalcitrant lignin and black carbon is the primary mechanism of preservation of terrestrial OC, whereas mineral-protection is the dominant mechanism preserving marine OC in the Washington coast sediment. There is little evidence showing that either preservation mechanism functions in the Mexican Margin sediments.     

Microbial nitrogen removal in a developing suburban estuary along the South Carolina coast

The conversion of undisturbed coastal regions to commercial and suburban developments may pose a threat to surface and groundwater quality by introducing nitrate-nitrogen (NO₃ ^?-N) from runoff of land-applied wastewater and fertilizers. Microbial denitrification is an important NO₃ ^?-N removal mechanism in coastal sediments. The objective of this study was to compare denitrification and nitrate conversion rates in coastal sediments from a golf course, suburban site, undeveloped marsh, and nonmarsh area near rapidly developing Hilton Head Island, South Carolina. Nitrous oxide was measured using gas chromatography and nitrate and ammonium concentrations were measured using a flow injection autoanalyzer in microcosms spiked, with 50 μg NO₃ ^?-N gdw^?1. The two marsh sites had the greatest ammonium production, which was correlated with fine sediment particle size and higher background sediment nitrate and surface water sulfate concentrations. The golf course swale had greatest denitrification rates, which were correlated with higher total carbon and organic nitrogen in sediments. Nitrate was consumed in golf course sediments to a greater extent than in the undeveloped marsh and upland freshwater sites, suggesting that the undeveloped sites and receiving estuaries may be more susceptible to nitrate contamination than the golf course swale and marsh under nonstorm conditions. Construction of swales and vegetated buffers using sediments with high organic carbon content as best management practices may aid in removing nitrate and other contaminants from runoff prior to its transport to the receiving marsh and estuary.     

Organic carbon 13C12C variations in estuarine sediments

The δC¹³ value for sedimentary organic carbon in four estuaries of the Gulf of Mexico increases with radial distance from the river mouth. Mass balance calculations indicate that terrestrial organic carbon is limited to sediments within a relatively short distance from the river mouth. This distance is a function of the discharge rate of the river. For the Mississippi River, terrestrial organic carbon is limited to sediments within 69 km of the mouth of Pass à Loutre and 61 km of South Pass. These data indicate that the low δC¹³ (< ?22%.) values reported for Pleistocene sediments in the Gulf of Mexico may be the result of factors in addition to the postulated large influx of terrestrial organic carbon.     

Transient States in Diagenesis Following the Deposition of a Gravity Layer: Dynamics of O<Subscript>2</Subscript>, Mn,Fe and N-Species in Experimental Units

Biogeochemical processes induced by the deposition of gravity layer in marine sediment were studied in a 295-day experiment. Combining voltammetric microelectrode measurements and conventional analytical techniques, the concentrations of C, O₂, N-species, Mn and Fe have been determined in porewaters and sediments of experimental units. Dynamics of the major diagenetic species following the sudden sediment deposition of few cm-thick layer was explained by alternative diagenetic pathways whose relative importance in marine sediments is still a matter of debate. Time-series results indicated that the diffusion of O₂ from overlying waters to sediments was efficient after the deposition event: anoxic conditions prevailed during the sedimentation. After a few days, a permanent oxic horizon was formed in the top few millimetres. At the same time, the oxidation of Mn²⁺ and then Fe²⁺, which diffused from anoxic sediments, contributed to the surficial enrichment of fresh Mn(III/IV)- and Fe(III)-oxides. Vertical diffusive fluxes and mass balance calculations indicated that a steady-state model described the dynamic of Mn despite the transitory nature of the system. This model was not adequate to describe Fe dynamics because of the multiple sources and phases of Fe²⁺. No significant transfer of Mn and Fe was observed between the underlying sediment and the new deposit: Mn- and Fe-oxides buried at the original interface acted as an oxidative barrier to reduced species that diffused from below. Nitrification processes led to the formation of a NO₃⁻/NO₂⁻ rich horizon at the new oxic horizon. Over the experiment period, NO₃⁻ concentrations were also measured in the anoxic sediment suggesting anaerobic nitrate production.     

Organic carbon isotope ratios of recent sediments from coastal lagoons of the Gulf of Mexico,Mexico

The stable carbon isotope composition sedimentary organic carbon was determined in the sediments of seven coastal lagoons of the Gulf of Mexico, Mexico. For most of the lagoons the δ¹³C values for sediments ranged from ?20.1 to ?23.9%. Anomalously low values, ?26.8 to 29.3%. were determined in sediments of two of the studied lagoons, probably due to the presence of organic carbon from anthropogenic sources, naturally absent in these environments. The δ¹³C values determined in the tissues of oysters collected at the same time in the different lagoons were very similar to those recorded in the sediments.     

Biogeochemistry of pyrite and iron sulfide oxidation in marine sediments

Pyrite (FeS₂) and iron monosulfide (FeS) play a central role in the sulfur and iron cycles of marine sediments. They may be buried in the sediment or oxidized by O₂ after transport by bioturbation to the sediment surface. FeS₂ and FeS may also be oxidized within the anoxic sediment in which NO₃⁻, Fe(III) oxides, or MnO₂ are available as potential electron acceptors. In chemical experiments, FeS₂ and FeS were oxidized by MnO₂ but not with NO₃⁻ or amorphous Fe(III) oxide (Schippers and Jørgensen, 2001). Here we also show that in experiments with anoxic sediment slurries, a dissolution of tracer-marked ⁵⁵FeS₂ occurred with MnO₂ but not with NO₃⁻ or amorphous Fe(III) oxide as electron acceptor. To study a thermodynamically possible anaerobic microbial FeS₂ and FeS oxidation with NO₃⁻ or amorphous Fe(III) oxide as electron acceptor, more than 300 assays were inoculated with material from several marine sediments and incubated at different temperatures for > 1 yr. Bacteria could not be enriched with FeS₂ as substrate or with FeS and amorphous Fe(III) oxide. With FeS and NO₃⁻, 14 enrichments were obtained. One of these enrichments was further cultivated anaerobically with Fe²⁺ and S⁰ as substrates and NO₃⁻ as electron acceptor, in the presence of ⁵⁵FeS₂, to test for co-oxidation of FeS₂, but an anaerobic microbial dissolution of ⁵⁵FeS₂ could not been detected. FeS₂ and FeS were not oxidized by amorphous Fe(III) oxide in the presence of Fe-complexing organic compounds in a carbonate-buffered solution at pH 8. Despite many different experiments, an anaerobic microbial dissolution of FeS₂ could not be detected; thus, we conclude that this process does not have a significant role in marine sediments. FeS can be oxidized microbially with NO₃⁻ as electron acceptor. O₂ and MnO₂, but not NO₃⁻ or amorphous Fe(III) oxide, are chemical oxidants for both FeS₂ and FeS.     

Tepexpan Palaeoindian site, Basin of Mexico: multi-proxy evidence for environmental change during the late Pleistocene–late Holocene

The Tepexpan Palaeoindian skeleton was discovered in 1947 close to the former Lake Texcoco margin, in the Basin of Mexico. The find has been the object of considerable interest and discussion over the last 60 years regarding its real age and archaeological interpretation. Here we report new AMS radiocarbon dates associated with the sedimentary succession at Tepexpan with ages between 19,110 ± 90 and 612 ± 22 ¹⁴C years BP and a new uranium-series date for the skeleton with an age of 4700 ± 200 years BP that indicates a mid Holocene age. The sedimentary succession was studied in detail using: stable isotopes, diatoms, organic geochemistry and tephrochronology. The multi-proxy evidence suggests large changes around the margins of Lake Texcoco in terms of the balance between aquatic and terrestrial plants, C₃ and C₄ plants, saline, alkaline and freshwater conditions, volcanic activity, marginal reworking of lake sediments and input from the drainage basin through the late Pleistocene–late Holocene. These changes had large impacts on the prehistoric human populations living by the lake shores since the late Pleistocene in the Basin of Mexico.     

Land-derived organic matter in surface sediments from the Gulf of Mexico

Lignin oxidation products and ¹³C/¹²C ratios were compared as indicators of land-derived organic matter in surface sediments from the western Gulf of Mexico. Whole sediments were reacted with cupric oxide to yield phenolic oxidation products that indicated the types and relative amounts of the lignins that were present.Measurements of lignin concentration and carbon isotope abundances both indicated a sharp offshore decrease of land-derived organic matter in most areas of the western Gulf. This decrease results primarily from mixing of terrestrial and marine organic matter. The terrestrially derived material in these sediments has a lignin content similar to that of grasses and tree leaves. Flowering plants contribute most of the sedimented lignin compounds. These lignins apparently occur in the form of well-mixed plant fragments that are transported to sea by rivers and deposited primarily on the inner continental shelf.     

Early diagenesis in sediments from Danish coastal waters: Microbial activity and Mn-Fe-S geochemistry

Depth distributions of bacterial respiration of O₂, NO₃⁻ and SO₄²⁻ were compared with geochemical data for Mn, Fe and S in coastal sediments from water depths between 26 and 520 m. As water depth increased, the zone of SO₄²⁻ respiration was found deeper in the sediment and was eventually separated from the surface-located activity of O₂ and NO₂⁻ respiration. At the deepest station additional SO₄²⁻ reduction activity was observed in small, detrital aggregates on the sediment surface. Dissolved Mn²⁺ and Fe²⁺ appeared between the O₂- plus NO₃⁻-containing surface layer and the H₂S-plus FeS-containing sediment below. This was a result of Mn and Fe reductions coupled to either the oxidation of sulfide or the mineralization of organic matter. Tracer experiments showed that both FeS, FeS₂ and S⁰ were important radiolabelled products of sulfate respiration in this intermediate zone. In the same zone, the overall degradation of organic matter seemed to be underestimated by the assay of SO^2-₄ respiration and additional mineralization by Mn and Fe reductions was likely.     

Authigenic carbonates in methane seeps from the Norwegian sea: Mineralogy, geochemistry, and genesis

Authigenic carbonates in the caldera of an Arctic (72°N) submarine mud volcano with active CH₄bearing fluid discharge are formed at the bottom surface during anaerobic microbial methane oxidation. The microbial community consists of specific methane-producing bacteria, which act as methanetrophic ones in conditions of excess methane, and sulfate reducers developing on hydrogen, which is an intermediate product of microbial CH₄ oxidation. Isotopically light carbon (δ¹³C_av =−28.9%0) of carbon dioxide produced during CH₄ oxidation is the main carbonate carbon source. Heavy oxygen isotope ratio (δ¹⁸O_av = 5%0) in carbonates is inherited from seawater sulfate. A rapid sulfate reduction (up to 12 mg S dm⁻³ day⁻¹) results in total exhausting of sulfate ion in the upper sediment layer (10 cm). Because of this, carbonates can only be formed in surface sediments near the water-bottom interface. Authigenic carbonates occurring within sediments occur do notin situ. Salinity, as well as CO ₃ ²⁻ /Ca and Mg/Ca ratios, correspond to the field of nonmagnesian calcium carbonate precipitation. Calcite is the dominant carbonate mineral in the methane seep caldera, where it occurs in the paragenetic association with barite. The radiocarbon age of carbonates is about 10000 yr.     

Characterization of the reactivity of riverine heterogeneous sediments using a facies-based approach; the Rhine–Meuse delta (The Netherlands)

Large groundwater resources are found in densely populated lowland areas, which consist often of young unconsolidated and reduced sediments. When anthropogenic activities lead to oxygenation of the aquifer, breakdown of the main reduced fractions, i.e. sedimentary organic matter (SOM) and pyrite, could lead to severe groundwater deterioration such as acidification, heavy metal mobilization, and increased hardness. The characterization of the reactive properties of these sediments is important in predicting groundwater deterioration, but is often complicated by the high degree of heterogeneity of these sediments. In this study, the potential reduction capacity (PRC, based on SOM and pyrite content), the potential buffer capacity (PBC, based on carbonate content), potential acidification capacity (PAC, based on the potential acid production by sulfide oxidation), and the measured reduction capacity (MRC) of five facies, which are typical of the riverine sediments in the Rhine–Meuse delta (The Netherlands) were determined. A universal facies-classification model was used to classify the deposits into more homogeneous sub-units based on lithologic and geogenic properties, with a further sub-division into oxic or anoxic redox environment based upon groundwater data and field observations. The bulk chemical data show strong variation across facies for the median values of PRC (186–9093 mmol O₂ kg⁻¹), PBC (17–132 mmol O₂ kg⁻¹), and PAC (36–1530 mmol H⁺ kg⁻¹). The MRC was measured as reactivity to molecular O₂ exposure and was 0.5–567.3 mmol O₂ kg⁻¹. Steady-state oxidation rates were in the wide range of 0.001–10.355 mmol O₂ kg⁻¹ day⁻¹ but were typically about 3–8 times faster in fine facies than in coarse facies. Both the PRC and MRC depend strongly on grain size, but also on the syn/post-depositional environment and redox conditions. The main part of the PRC consists of SOM, but pyrite reactivity is higher than SOM reactivity as shown by the relative depletion of pyrite in oxic subfacies and the preferential oxidation during the oxidation experiments. Some facies are very prone to acidification because the PAC is higher than the PBC, but the oxidation experiments also show that acidification could already start before the PRC is fully exhausted. This study, is one of the few that combines bulk chemical data, groundwater data, and reactivity measurements and shows that a facies-based approach is a practical tool in characterizing the reactivity of heterogeneous deposits.