Surficial Pore Water Research Articles

Atypical Diagenesis and Geochemistry of Redox-Sensitive Elements in Hydrothermal Sediments of the Southern Okinawa Trough

Early diagenesis processes and the geochemistry of redox-sensitive elements (RSEs) in four sediment cores in an active hydrothermal field in the southern Okinawa Trough (OT) was investigated. Dissolved Fe, Mn, ΣHS–, and several other RSEs (Mo, U, and V) in pore water were measured. We also studied metal distribution in sediments using sequential extraction methods. Very high dissolved Fe concentrations (∼140 μmol L–1) but insignificant dissolved Mn were observed in surficial pore waters in the station adjacent to the hydrothermal vent, where highly reactive Fe, Mo, U in the sediments were also measured. Such an atypical diagenetic sequence found in those cores could be driven by the overwhelmingly high reactive Fe fraction (mostly Fe oxides) delivered from the vents. Consequently, significant upward benthic fluxes of Fe and Mo were estimated for the studied stations. In addition, we performed a principal component analysis (PCA), together with relative ratios of carbonate-related elements (Sr, Ca, Mg), to identify particles’ origins in the hydrothermal field; two endmembers being the hydrothermal source and hydrogenous processes. This comprehensive study on a unique set of samples collected by advanced technology provided valuable data to demonstrate distinctive geochemical features that occur in hydrothermal sediments.

Nutrient addition increases rotifer abundance and diversity in a temperate bog

Rotifers are among the smallest metazoan animals living in lakes, ponds, rivers and in interstitial water within Sphagnum mats. When in sufficient numbers they are capable of regenerating large amounts of nitrogen (N) as NO3 and NH4 and phosphorus (P) as PO4; all of these nutrients are immediately available to bog plants. The ecological role of rotifers in bog ecosystems has been recognized at both local and global scales, but the effect of N and P addition on rotifers in bogs remains unknown. Here we present data on the influence of N and P addition on rotifers at a temperate Canadian bog dominated by Sphagnum mosses. We found that rotifer species abundance and diversity were significantly higher in surficial pore water after long-term exposure to increased N and P addition than in the control. A change in plant species distribution and increased rotifer abundance may partly explain the faster decomposition rates, jeopardizing the carbon sink capacity of bogs.

Activated carbon thin-layer placement as an in situ mercury remediation tool in a Penobscot River salt marsh

The efficacy of thin layer in situ soil amendments was tested as a potential tool for methylmercury (MeHg) risk mitigation in Penobscot River, ME, salt marsh. Salt marshes are sites of high MeHg accumulation within the Penobscot, and key targets for remediation. The study was a fully-crossed small plot study, with four treatments (activated carbon (AC), biochar, FeCl2, and lime) and unamended controls at two sites. Plots were monitored for two years. Porewater MeHg concentrations were the main endpoint, with impacts on sediment biogeochemistry as a secondary study goal. AC-based SediMite™ was effective in reducing MeHg, and to a less extent total Hg, in surficial pore waters. AC reduced MeHg concentrations by >90% at the one month time point, and continued to significantly reduce pore water MeHg through about a year. AC was less effective in reducing total Hg in pore water, yielding about 70% reduction at one month, and 50–65% reduction at 8months. Biochar provided lower, and more variable reduction in porewater MeHg, but was not effective in reducing total Hg. Biochar amendment also increased soil MeHg. Neither FeCl2 nor lime amendments reduced pore water Hg or MeHg levels. About 50% of AC treatment applied as SediMite™ pellets was retained in marsh soils after one year. This study is one of the first field trials of in situ amendment for MeHg remediation. Our results show that thin-layer AC placement is a potential remediation tool for Hg risk to biota, especially in marshes where net MeHg accumulation is often strong.

Coupled effects of hydrodynamics and biogeochemistry on Zn mobility and speciation in highly contaminated sediments.

Porewater transport and diagenetic reactions strongly regulate the mobility of metals in sediments. We executed a series of laboratory experiments in Gust chamber mesocosms to study the effects of hydrodynamics and biogeochemical transformations on the mobility and speciation of Zn in contaminated sediments from Lake DePue, IL. X-ray absorption spectroscopy (XAS) indicated that the oxidation of surficial sediments promoted the formation of more mobile Zn species. Bulk chemical measurements of porewater, overlying water, and sediment also suggested that this process liberated aqueous metals to porewater and facilitated Zn efflux to the overlying water. In addition, sediment resuspension events increased the release of aqueous metals to both surficial porewater and the overlying water column. XAS analysis indicated that resuspension increased dissolution of Zn-sequestering mineral phases. These results show that both steady slow porewater transport and rapid episodic resuspension are important to the release of metal from fine-grained, low-permeability contaminated sediments. Thus, information on metals speciation and mobility under time-varying overlying flow conditions is essential to understanding the long-term behavior of metals in contaminated sediments.

Mercury and methylmercury cycling in sediments of the mid‐Atlantic continental shelf and slope

We present a detailed study of the biogeochemical factors controlling mercury (Hg) distribution, methylmercury (MeHg) production, and MeHg efflux in sediments of the mid‐Atlantic continental shelf and slope. The mildly reduced surface sediments of the shelf and slope provide ideal conditions for MeHg production. They are sufficiently reduced to support microbial sulfate reduction, but contain very low dissolved sulfide concentrations. The redox zonation of sediments determined the depth distribution of MeHg production, whereas the bioavailability of inorganic Hg for methylation appeared to be the dominant driver of spatial patterns across the shelf and slope. Sediment total Hg concentrations were well predicted by sediment organic matter (SOM) content, with the highest concentrations of Hg and MeHg in the fine‐grained organic clays of the slope. However, SOM‐normalized Hg concentrations decreased with distance from shore. The changing character of organic matter with distance from shore appeared to affect Hg partitioning and bioavailability for methylation. The percentage of Hg in sediments as MeHg was well predicted by measured methylation rates, but not by demethylation rates. On the basis of measured concentrations in bottom waters and surficial pore waters, the average diffusive efflux of Hg(II) and MeHg from sediments to coastal waters was estimated to be 26 and 0.8 pmol m−2 d−1, respectively. Extrapolated globally, the diffusive input of MeHg from shelf and slope sediments is estimated to be 0.01 Mmol per year. As the actual fluxes can be substantially higher than diffusive fluxes, we suggest that shelf and upper slope sediments are a major source of MeHg to the coastal ocean.

Sediment-water exchange of Mn, Fe, Ni and Zn in Galveston Bay, Texas

In-situ benthic flux studies were conducted at three stations in Upper Galveston Bay twice during March 1996 to directly measure release rates of dissolved Mn, Fe, Ni and Zn from the sediments. Results showed reproducible increases with time in both replicate light and light–dark benthic chambers, resulting in average fluxes of −1200±780, −17±12, −1.6±0.6 and −2.4±0.79 μmol m−2 day−1 for Mn, Fe, Ni and Zn, respectively. Sediment cores collected during 1994–1996 showed that surficial pore water concentrations were elevated compared to overlying water column concentrations, suggesting diffusive release from the sediments. Diffusive flux estimates of Mn and Zn agreed in direction with chamber fluxes measured on the same date, but only accounted for 5–38% of the measured flux. Diffusive fluxes of Fe agreed with measured fluxes at the near Trinity River station but overestimated actual release in the mid and outer Trinity Bay regions, possibly due to inaccurate determination of the Fe pore water gradients or rapid oxidation processes in the overlying water at these stations. In general, measured fluxes of Mn and Ni were higher in the mid Trinity Bay region and suggested a mechanism for the elevated trace metal concentrations previously reported for this region of Galveston Bay. However, the fluxes of Fe were highest in close proximity to the Trinity River, supporting the elevated Fe concentrations measured in this region during this and other studies, and decreased towards middle and outer Trinity Bay. Trace metal turnover times were between 0.1 and 1.2 days for Mn, between 1.3 and 4.6 days for Fe, and between 27 and 100 days for Ni and 12–20 days Zn, and were considerably shorter than the average Trinity Bay water residence time (1.5 years) for this period. Comparing area averaged benthic inputs to Trinity River inputs shows the sediments to be a significant source of trace metals to Galveston Bay. However, while benthic inputs of trace metals were measured, water column concentrations remained low despite rapid turnover times for Mn and Fe, suggesting removal of these metals from the water column after release from the sediments.

Methylmercury concentrations and production rates across a trophic gradient in the northern Everglades

Methylmercury (MeHg) concentrations and production rates were examined along with sulfur biogeochemistry in Everglades sediments in March, July and December, 1995, as part of a large, multi-investigator study, the Aquatic Cycling of Mercury in the Everglades (ACME) project. The sites examined constitute a trophic gradient, generated from agricultural runoff, across the Everglades Nutrient Removal (ENR) Area, which is a re-constructed wetland, and Water Conservation Areas (WCA) 2A, 2B and 3 in the northern Everglades. MeHg concentrations and %MeHg (MeHg as a percent of total Hg) were lowest in the more eutrophic areas and highest in the more pristine areas in the south. MeHg concentrations ranged from 0.1 ng gdw 1 sediment in the ENR to 5 ng gdw 1 in WCA3 sediments; and MeHg constituted 0.2% of total Hg (Hg T) in ENR, but up to about 2% in two sites in WCA2B and WCA3. Methylation rates in surficial sediments, estimated using tracer-level injections of 203 Hg(II) into intact sediment cores, ranged from 0 to 0.12 d 1 , or about 1 to 10 ng g 1 d 1 when the per day values are multiplied by the ambient total Hg concentration. Methylation was generally maximal at or within centimeters of the sediment surface, and was never observed in water overlying cores. The spatial pattern of MeHg production generally matched that of MeHg concentration. The coincident distributions of MeHg and its production suggest that in situ production controls concentration, and that MeHg concentration can be used as an analog for MeHg production. In addition, the spatial pattern of MeHg in Everglades sediments matches that in biota, suggesting that MeHg bioaccumulation may be predominantly a function of the de novo methylation rate in surficial sediments. Sulfate concentrations in surficial pore waters (up to 400 m), microbial sulfate-reduction rates (up to 800 nm cc 1 d 1 ) and resultant pore water sulfide concentrations (up to 300 m) at the eutrophic northern sites were all high relative to most freshwater systems. All declined to the south, and sulfate concentrations in WCA2B and in central WCA3 resembled those in oligotrophic lakes (50-100 m). MeHg concentration and production were inversely related to sulfate reduction rate and pore water sulfide. Control of MeHg production in the northern Everglades appears to mimic that in an estuary, where sulfate concentrations are high and where sulfide produced by microbial sulfate reduction inhibits MeHg production.

Sediment metabolism in a transitional continental - marine area: The Albufera of Majorca (Balearic Islands, Spain)

The concentrations of nutrients in sediment pore water and the fluxes of nutrients at the water-sediment interface were measured in a channel that joins continental and marine areas in the Albufera of Majorca in order to evaluate the role of sediments in the nutrient dynamics in this system. Upstream, surficial pore water presented lower values of Eh, which became negative in summer, whereas downstream Eh remained positive. Nutrient concentrations were especially high upstream, reaching 1000 mol L-1 of NH4 and 75 μmol L-1 of PO4 during summer. In summer, measured fluxes showed intense respiration upstream, with an oxygen consumption of 130 mg m-2 h-1 and a respiratory quotient near 4, which indicates dominance of anaerobic respiration. Total CO2 efflux and nutrient fluxes were also high, reaching 30.50 mmol m-2 h-1 for CO2, >2000 μmol m-2 h-1 for NH4 and 58 μmol m-2 h-1 for PO4. A substantial amount of the total CO2 efflux (14 mmol m-2 h-1) was due to calcium carbonate redissolution. Downstream, oxygen consumption, respiratory quotient and ammonium fluxes were lower (around 70 mg m-2 h-1, between 2 and 3, and <20 μmol m-2 h-1, respectively), which indicates a moderate rate of decomposition activity and suggests denitrification as the main respiratoy process. Differences between fluxes measured 'in situ' and those calculated from pore-water concentrations indicated non-diffusive fluxes upstream and suggest substantial denitrifying activity downstream. Extra keywords: benthic chambers, sediment fluxes, pore water, ammonium, phosphate.

Trace metals in metalliferous sediments, MANOP Site M: interfacial pore water profiles

We present data showing enrichments of metals Mn, Cu, V, Cr, Cd and Ni and dissolved organic matter (DOM) in surficial pore waters, depths < 1 cm, at a metalliferous sediment site in the Pacific. We also present a model of metal release during particulate organic carbon (POC) oxidation and opal dissolution at the sediment-seawater interface. A comparison of measured interfacial metal concentrations with those predicted from the model suggest that Cd, Ni and Cr are released during oxidation of POC at the sediment-seawater interface. However, for V, Cu and Mn additional ‘source(s)’ and/or processes are required to account for the measured interfacial concentrations. For Cu, recycling via interfacial remineralization and release to bottom waters, with subsequent scavenging out of bottom waters by particulates, probably provides the major additional ‘source’. For V, we suggest that, in addition to some contribution from a benthic flux-bottom water scavenging scenario, complexation with DOM at the interface serves as a trap to accumulate V released from particulates in the zone of oxygen reduction. For manganese, we suggest that most of the interfacial pore water content is produced from reduction of sedimentary Mn(IV) by reactions with DOM at the sediment-seawater interface. The calculated benthic flux of manganese as a result of this reaction may account for a large fraction of the particulate manganese rain that is apparently not preserved in the sediment at this site.