Oxic Pore Waters Research Articles

The importance of the soluble and colloidal pools for trace metal cycling in deep-sea pore waters

Physical and chemical trace metal speciation are important for our understanding of metal cycling and potential toxicity to marine life. Trace metals can behave differently in diffusion processes or particle-solution interactions and have different bioavailabilities depending on their physical and chemical forms, which often depend on redox conditions. Here we investigated dissolved (< 0.2 µm) and soluble (< 0.02 µm) concentrations of Mn, Co, Ni, Fe, Cu, V, Mo, U, Cd, and As in oxic and suboxic deep-sea sediments of the central equatorial Pacific Ocean. Vanadium, Mo, U, As, and Cd showed no significant concentration differences between their dissolved and soluble forms, suggesting that they are present as inorganic ionic species or organic complexes in the truly dissolved or small colloidal fraction. In contrast, the colloidal fraction (> 0.02 µm < 0.2 µm) of Mn, Co, Ni, and Cu increased with depth in oxic pore waters and Fe had the largest but variable colloidal pool. Soluble Mn, Co, and Ni were released in the uppermost 2-4 cm in the sediment because of reductive dissolution. The increasing colloidal fraction with depth suggests a decrease in the concentration of small organic ligands with depth, that are abundant in the surface sediment pore waters, and instead an increasing importance of larger (> 0.02 µm) inorganic nanoparticles and colloids such as Mn and Fe (oxyhydr)oxides that control Mn, Fe, and Co cycling at depths > 10 cm. The distribution of Ni and Cu cannot be exclusively explained by inorganic nanoparticles and a shift from low to larger high molecular weight organic ligands might occur. These findings provide new insights into trace metal distributions in the dissolved phase, highlighting the diversity of metal complexes and the need to incorporate these in future calculations of benthic metal fluxes and ecotoxicity assessments, especially in oxic pore waters.

Residence Time Controls on the Fate of Nitrogen in Flow‐Through Lakebed Sediments

AbstractFor many glacial lakes with highly permeable sediments, water exchange rates control hydrologic residence times within the sediment‐water interface (SWI) and the removal of reactive compounds such as nitrate, a common pollutant in lakes and groundwater. Here we conducted a series of focused tracer injection experiments in the upper 20 cm of the naturally downwelling SWI in a flow‐through lake on Cape Cod, MA. We systematically varied residence time and reactant controls on nitrate processing, using isotopically labeled 15N nitrate to monitor the effect of these changes on nitrate removal via denitrification. The addition of acetate, a labile carbon compound, triggered the lake SWI to switch from net production to net removal of nitrate. When acetate was combined with increased residence time created by controlled reductions in water flux, we observed a fivefold increase in nitrate removal, a 26‐fold increase in N2 production, and a 42‐fold increase in N2O production. We demonstrate that water residence time is an important control on the fate of nitrate in these lake SWIs and illustrate that seasonal conditions that alter lake exchange rates and variability in lake carbon may predict dynamic nitrate removal across the SWI. Additionally, observed N2O production during the oxic pore water experiments paired with geophysical characterization of the sediment porosity revealed that the lake SWI has less mobile pores occupying upward of 50% of the total porosity volume, which function as reactive microzones for nitrate processing.

Organotin persistence in contaminated marine sediments and porewaters: In situ degradation study using species-specific stable isotopic tracers

This paper provides a comprehensive study of the persistence of butyltins and phenyltins in contaminated marine sediments and presents the first data on their degradation potentials in porewaters. The study’s aim was to explain the different degradation efficiencies of organotin compounds (OTC) in contaminated sediments. The transformation processes of OTC in sediments and porewaters were investigated in a field experiment using species-specific, isotopically enriched organotin tracers. Sediment characteristics (organic carbon content and grain size) were determined to elucidate their influence on the degradation processes. The results of this study strongly suggest that a limiting step in OTC degradation in marine sediments is their desorption into porewaters because their degradation in porewaters occurs notably fast with half-lives of 9.2 days for tributyltin (TBT) in oxic porewaters and 2.9±0.1 and 9.1±0.9 days for dibutyltin (DBT) in oxic and anoxic porewaters, respectively. By controlling the desorption process, organic matter influences the TBT degradation efficiency and consequently defines its persistence in contaminated sediments, which thus increases in sediments rich in organic matter.

Long-term in situ oxidation of biogenic uraninite in an alluvial aquifer: impact of dissolved oxygen and calcium.

Oxidative dissolution controls uranium release to (sub)oxic pore waters from biogenic uraninite produced by natural or engineered processes, such as bioremediation. Laboratory studies show that uraninite dissolution is profoundly influenced by dissolved oxygen (DO), carbonate, and solutes such as Ca(2+). In complex and heterogeneous subsurface environments, the concentrations of these solutes vary in time and space. Knowledge of dissolution processes and kinetics occurring over the long-term under such conditions is needed to predict subsurface uranium behavior and optimize the selection and performance of uraninite-based remediation technologies over multiyear periods. We have assessed dissolution of biogenic uraninite deployed in wells at the Rifle, CO, DOE research site over a 22 month period. Uraninite loss rates were highly sensitive to DO, with near-complete loss at >0.6 mg/L over this period but no measurable loss at lower DO. We conclude that uraninite can be stable over decadal time scales in aquifers under low DO conditions. U(VI) solid products were absent over a wide range of DO values, suggesting that dissolution proceeded through complexation and removal of oxidized surface uranium atoms by carbonate. Moreover, under the groundwater conditions present, Ca(2+) binds strongly to uraninite surfaces at structural uranium sites, impacting uranium fate.

A physical explanation for the development of redox microzones in hyporheic flow

AbstractRecent observations reveal a paradox of anaerobic respiration occurring in seemingly oxic‐saturated sediments. Here we demonstrate a residence time‐based explanation for this paradox. Specifically, we show how microzones favorable to anaerobic respiration processes (e.g., denitrification, metal reduction, and methanogenesis) can develop in the embedded less mobile porosity of bulk‐oxic hyporheic zones. Anoxic microzones develop when transport time from the streambed to the pore center exceeds a characteristic uptake time of oxygen. A two‐dimensional pore‐network model was used to quantify how anoxic microzones develop across a range of hyporheic flow and oxygen uptake conditions. Two types of microzones develop: flow invariant and flow dependent. The former is stable across variable hydrologic conditions, whereas the formation and extent of the latter are sensitive to flow rate and orientation. Therefore, pore‐scale residence time heterogeneity, which can now be evaluated in situ, offers a simple explanation for anaerobic signals occurring in oxic pore waters.

Respiration and denitrification in permeable continental shelf deposits on the South Atlantic Bight: Rates of carbon and nitrogen cycling from sediment column experiments

Nitrogen (N) cycling and respiration rates were measured in sediment columns packed with southeastern United States continental shelf sands, with high permeability (4.66×10 −11 m 2) and low organic carbon (0.05%) and nitrogen (0.008%). To simulate porewater advection, natural shelf seawater was pumped through columns of different lengths to achieve fluid residence times of approximately 3, 6, and 12 h. Experiments were conducted seasonally at in situ temperature. Fluid flow was uniform in nearly all columns, with minimal dead zones and channeling. Significant respiration (O 2 consumption and ∑CO 2 production) occurred in all columns, with highest respiration rates in summer. Most (78–100%) remineralized N was released as N 2 in the majority of cases, including columns with oxic porewater throughout, with only a small fraction released as NO 3 − from some oxic columns. A rate of 0.84–4.83×10 10 mol N yr −1, equivalent to 1.06–6.09×10 −6 mmol N cm −2 h −1, was calculated for benthic N 2 production in the South Atlantic Bight, which can account for a large fraction of new N inputs to this shelf region. Metal and sulfate reduction occurred in long residence time columns with anoxic outflow in summer and fall, when respiration rates were highest. Because permeable sediments dominate continental shelves, N 2 production in high permeability coastal sediments may play an important role in the global N cycle.

Reconstruction of changes in export productivity during Pliocene sapropel deposition: a biomarker approach

Biomarkers (alkenones, loliolide/isololiolide, dinosterol and C 30 1,15-diol/keto-ol) representative of four major classes of marine primary producers (haptophytes, diatoms, dinoflagellates, eustigmatophytes, respectively) from three sites of an east–west transect spanning the entire eastern Mediterranean basin were studied for changes in phytoplankton export production before, during and after deposition of a Pliocene, total organic carbon-rich (up to 27%) sapropel (2.943 Ma). Biomarker contents showed substantial differences between non-sapropelic and sapropelic sediments. In non-sapropelic sediments, small contents are observed reflecting extensive oxic degradation in oxic pore waters during deposition. In the sapropels, biomarker accumulation rates showed a substantial increase from the base to the centre, likely resulting from increased biological productivity since the bottom waters were continuously anoxic and preservation conditions are similar. The diatom biomarker, loliolide/isololiolide, became distinctly more abundant in the sapropels indicating that the eastern Mediterranean basin was enriched in nutrients. This caused enhanced export production by the major phytoplankton classes leading to periods of sapropel deposition. Alkenone-based sea surface temperature reconstructions indicated that this change in phytoplankton composition was not influenced by changes in sea surface temperature, but was mainly nutrient-controlled. Compound-specific δ 13C measurements did not provide additional evidence for substantially increased primary production rates: the δ 13C values of C 37:2 alkenones and loliolide/isololiolide showed a slightly negative shift instead of the anticipated positive shift. This was probably caused by the enrichment of dissolved inorganic carbon in the upper water column, caused by enhanced recycling of respired CO 2 due to the shallow chemocline.

Production, sedimentation, and isotopic composition of organic matter in Lake Ontario

Organic matter and its carbon and nitrogen isotopic composition were measured in sequential sediment trap and core samples from the Rochester Basin of Lake Ontario to evaluate their usefulness in reconstructing historic changes in lake productivity. The greatest flux of organic matter from the epilimnion occurred during late summer and coincided with whiting events, indicating that calcite precipitation is an effective mechanism for sedimenting organic matter. Carbon isotopes of organic matter were low prior to the onset of stratification, increased to maximum values in late summer, and then decreased following fall overturn. This pattern is controlled mainly by the timing of stratification and primary productivity, which preferentially removes 12CO2 from the epilimnion. The physiological effect of decreased carbon isotopic fractionation with decreasing supplies of [CO2]aq may have also contributed to increased δ13CorgC. Nitrogen isotopes showed a seasonal pattern opposite to that of carbon, whereby δ15N values were low during the summer stratified period and high for the remainder of the year. Seasonal variability in δ15NorgN probably reflects changes in the source of sedimented organic particles, which is dominated by isotopically depleted phytodetritus during the stratified period and isotopically enriched organic matter from heterotrophic or detrital sources during the mixed period. A comparison of organic carbon accumulation rates and δ13CorgC between sediment cores collected in 1987 and 1993–1994 confirms earlier predictions that diagenetic processes reduce the mass accumulation of organic carbon in the zone of oxic pore waters, but will not change the δ13CorgC values. All cores analyzed for δ13CorgC display the reproducible pattern of a progressive increase in the 19OOs, peaking in the early to mid‐1970s, and then decreasing to the present. This pattern matches the historical trends of phosphorus loading to the basin, suggesting that δ13C of organic carbon is a reliable proxy for paleoproductivity and responds to spring phosphorus supplies in the water column. The δ15N of sedimentary organic matter increased linearly from 1840 to 1960 at a rate of 0.3ppt per decade, and remained relatively constant thereafter except for an increase in the upper few centimeters of sediment. The increase in δ15NorgN reflects a combination of factors, including early forest clearance by Europeans, increased sewering by municipalities after 1940, and increased nitrate utilization as productivity increased in the lower Great Lakes. Increased rates of denitrification in the central basin of upstream Lake Erie from the 1930s to the early 1970s may have also contributed to the rise in δ15NorgN values.

Nature and reactions of dissolved organic matter in the interstitial waters of marine sediments

Two organic rich sediments, an oxic muddy sand and a silty mud containing sulphate reducing and methane producing metabolic zones, were sampled from Loch Duich, a fjord type estuary in the N.W. coast of Scotland. Dissolved organic carbon (DOC), as measured by dry combustion and UV absorption, remained constant (8.3–15.8 mg C/l) with depth in the oxic pore waters at a concentration at least twice that of the overlying seawater. DOC in the anoxic pore waters increased linearly with depth from 13.6 at the surface to 55.9–70.5 mg C/l at 80cm. Most of the DOC was present in the high molecular weight (HMW) fraction as separated by ultrafiltration; the low molecular weight (LMW) fraction remained constant (10.0 mg C/l) in both oxic and anoxic pore waters. Spectroscopic data showed the ‘humic’ fraction of the HMW dissolved organic matter was mainly fulvic acid, a small proportion (approx 1%) of humic acid, and a third fraction, possibly melanoidins, which increased relative to fulvic acid with depth. These data confirm the pathway of humification (NissenBaum et al, 1971; nissenbaum and Kaplan, 1972) where HMW organic matter accumulates in pore waters as condensation products of LMW organic substances.