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2017
Luo, M, Gieskes J, Chen L, Shi X, Chen D.  2017.  Provenances, distribution, and accumulation of organic matter in the southern Mariana Trench rim and slope: Implication for carbon cycle and burial in hadal trenches. Marine Geology. 386:98-106.   10.1016/j.margeo.2017.02.012   Abstract

It is generally accepted that organic matter content and accumulation rate at the seafloor attenuate with increasing water depth. However, hadal trenches, which represent the deepest portion of the hydrosphere on Earth, do not abide by this universal rule. It has been speculated that hadal trenches would serve as organic matter depocenters, where bacteria-mediated organic matter degradation intensifies. Here we examine the contents of total organic carbon (TOC) and total nitrogen (TN), as well as the δ13C values of TOC in three gravity cores, two box cores, and three grab samples with water depth from 4900 to 7118 m, in order to reveal the provenances of organic matter, spatial distribution and accumulation rates of particulate organic carbon (POC) in the southern Mariana Trench rim and slope. Although the deepest area was not sampled, trench rim and slope is also of importance in terms of transportation and accumulation processes of organic matter. A vast majority of the sediment samples have bulk TOC/TN molar ratios and δ13C values of TOC ranging from 4.2 to 11 and from − 21.8 to − 18.9‰, respectively, implying that the organic matter was primarily sourced from marine algae. Two exceptions have been found at 101 cm and 201 cm depths of core GC05 with a possible input of terrestrial material suggested by an abrupt increase in TOC contents and TOC/TN molar ratios accompanied by marked decreases in δ13C values of TOC. Moreover, TOC contents in surface sediments basically increased with water depth. Based on the published excess 210Pb data and by assuming a simplified one-dimensional sediment loading despite of frequent occurrences of mass wasting transports in hadal trenches, the sedimentation rate in the southern Challenger Deep (6037 m) was estimated to be 0.02 cm yr− 1. This much higher sedimentation rate compared to the globally averaged values in deep ocean sediments coupled with the trend of increasing in TOC contents with water depth may serve as evidence for the lateral transport of sediment particles induced by the funnel-like topography and localized current dynamics within the trench. The average POC accumulation rate within the southern Challenger Deep roughly amounts to 1.5 × 10− 5 g cm− 2 yr− 1, equivalent to about a seventh of the globally averaged POC accumulation rate in deep-ocean seafloor. To our knowledge, POC accumulation in hadal trenches has been quantified for the first time, and we highlight that organic matter degradation in deep hadal trenches should not be neglected and may represent a significant component of the global carbon cycle.

Luo, M, Gieskes J, Chen LY, Shi XF, Chen DF.  2017.  Provenances, distribution, and accumulation of organic matter in the southern Mariana Trench rim and slope: Implication for carbon cycle and burial in hadal trenches. Marine Geology. 386:98-106.   10.1016/j.margeo.2017.02.012   AbstractWebsite

It is generally accepted that organic matter content and accumulation rate at the seafloor attenuate with increasing water depth. However, hadal trenches, which represent the deepest portion of the hydrosphere on Earth, do not abide by this universal rule. It has been speculated that hadal trenches would serve as organic matter depocenters, where bacteria-mediated organic matter degradation intensifies. Here we examine the contents of total organic carbon (TOC) and total nitrogen (TN), as well as the 8.13C values of TOC in three gravity cores, two box cores, and three grab samples with water depth from 4900 to 7118 m, in order to reveal the provenances of organic matter, spatial distribution and accumulation rates of particulate organic carbon (POC) in the southern Mariana Trench rim and slope. Although the deepest area was not sampled, trench rim and slope is also of importance in terms of transportation and accumulation processes of organic matter. A vast majority of the sediment samples have bulk TOC/TN molar ratios and delta C-13 values of TOC ranging from 4.2 to 11 and from 21.8 to 18.9%0, respectively, implying that the organic matter was primarily sourced from marine algae. Two exceptions have been found at 101 cm and 201 cm depths of core GC05 with a possible input of terrestrial material suggested by an abrupt increase in TOC contents and TOC/TN molar ratios accompanied by marked decreases in delta C-13 values of TOC. Moreover, TOC contents in surface sediments basically increased with water depth. Based on the published excess 21 Pb data and by assuming a simplified one-dimensional sediment loading despite of frequent occurrences of mass wasting transports in hadal trenches, the sedimentation rate in the southern Challenger Deep (6037 m) was estimated to be 0.02 cm yr(-1). This much higher sedimentation rate compared to the globally averaged values in deep ocean sediments coupled with the trend of increasing in TOC contents with water depth may serve as evidence for the lateral transport of sediment particles induced by the funnel-like topography and localized current dynamics within the trench. The average POC accumulation rate within the southern Challenger Deep roughly amounts to 1.5 x 10(-5) g cm(-2) yr(-1), equivalent to about a seventh of the globally averaged POC accumulation rate in deep-ocean seafloor. To our knowledge, POC accumulation in hadal trenches has been quantified for the first time, and we highlight that organic matter degradation in deep hadal trenches should not be neglected and may represent a significant component of the global carbon cycle. (C) 2017 Elsevier B.V. All rights reserved.

Luo, M, Algeo TJ, Tong H, Gieskes J, Chen L, Shi X, Chen D.  2017.  More reducing bottom-water redox conditions during the Last Glacial Maximum in the southern Challenger Deep (Mariana Trench, western Pacific) driven by enhanced productivity. Deep Sea Research Part II: Topical Studies in Oceanography.   10.1016/j.dsr2.2017.01.006   Abstract

The modern southern Mariana Trench is characterized by oligotrophic surface waters, resulting in low primary productivity and well-oxygenated bottom waters. This study investigates changes in the redox conditions of bottom waters in the southern Mariana Trench during the Last Glacial Maximum (LGM) and their potential causes. We measured major, trace, and rare earth elements (REE) in three gravity cores (GC03, GC04, and GC05) and one box core (BC11) retrieved from the southern Challenger Deep at water depths from 5289 to 7118 m. The upper sediment layers of both GC05 and BC11 are dominated by valve fragments of the giant diatom Ethmodiscus rex, forming laminated diatom mats (LDMs). 14C-AMS dates of bulk organic matter show that the LDMs accumulated between 18.4 and 21.8 kyr B.P., corresponding to the LGM. Modest enrichments of U and Mo along with weak or absent Ce anomalies in the LDM point to suboxic conditions during the LGM. In contrast, non-LDM samples exhibit little to no enrichment of redox-sensitive elements as well as negative Ce anomalies, indicating deposition under oxic bottom-water conditions. The Ce anomalies are considered valid proxies for bottom-water redox conditions because REE signatures were acquired in the early diagenetic environment, as indicated by strong P-REE correlations and middle-REE enrichment associated with early diagenetic cycling of Fe-Mn oxyhydroxides in the sediment column followed by capture of the REE signal by biogenic and/or authigenic apatite. We postulate that the more reducing bottom-water conditions during the LGM were linked to increased primary productivity induced by enhanced Asian dust input. As shown in earlier studies, the increased primary productivity associated with Ethmodiscus rex blooms in the eastern Philippine Sea played a significant role in capturing atmospheric CO2 during the LGM. Consequently, the magnitude of atmospheric CO2 sequestration by giant diatom blooms during the LGM may have been greater than previously envisaged.

2015
Gieskes, JM, Han S, Rathburn A, Rothwell G, Pérez EM, Porrachia M, Barbanti A, Deheyn DD.  2015.  Anthropogenic contaminants in Venice Lagoon sediments and their pore fluids: Results from the SIOSED Project. Marine Chemistry. 174:73-84.   10.1016/j.marchem.2015.05.008   AbstractWebsite

Investigations of sediment geochemistry and interstitial water chemistry during SIOSED (Scripps Institution of Oceanography Sediment Research Project) revealed information about the characteristics and depth range of contamination in sediments associated with dredging operations in the Venice Lagoon, Italy. Results from gravity cores indicate that contamination ranges larger and deeper in sediments associated with Porto Marghera and the Venice Industrial Zone compared with sediments at greater distances from dredged shipping canals or pollution sources. The effects of sediment re-deposition were evaluated from a pore water chemistry study of artificial banks constructed by placing dredged canal sediments on top of background sediments. Rapid decreases in dissolved sulfate associated with increases in alkalinity, sulfide, and nutrients, such as ammonium and phosphate, indicate that sediment dredging led to enhanced bio-chemical diagenesis of organic matter near the surface of the re-deposited sediments. Continued diagenesis of organic matter in re-deposited sediments maintained extrema in alkalinity, dissolved sulfate, sulfide, and ammonium. The artificial banks retained their pore water signatures over the duration of the project. Sediment redistribution can thus cause important changes in pore water profiles, as observed from the chemistry in long cores studied in this program.

Luo, M, Dale AW, Wallmann K, Hensen C, Gieskes J, Yan W, Chen D.  2015.  Estimating the time of pockmark formation in the SW Xisha Uplift (South China Sea) using reaction-transport modeling. Marine Geology. 364:21-31.   10.1016/j.margeo.2015.03.006   Abstract

Carbon cycling and fluid seepage in marine sediments over the late Quaternary were investigated at a now-extinct pockmark located in a mega-pockmark field in the SW Xisha Uplift (NW South China Sea). Measured particulate organic carbon (POC) content, and porewater sulfate (SO42 −), dissolved inorganic carbon (DIC) concentrations and δ34S-SO42 − distributions were used to constrain a non-steady-state reaction-transport model and quantify POC mineralization rates as well as estimate the time when fluid flow ceased at the investigated pockmark. An increase in POC content and δ34S-SO42 − and a decrease in sulfate concentrations in the upper ca. 2 m at the pockmark and a reference core implied an increase in the flux and reactivity of organic matter during the early Holocene around 10 kyr B. P. caused by enhanced primary productivity during the strengthened southwestern summer monsoon. These features were simulated with the model assuming a Holocene increase in POC flux and reactivity. Subsequently, starting from an initial condition reminiscent of a modern active cold seep (Hydrate Ridge), hindcast simulations showed that fluid seepage at the pockmark ceased ca. 39 kyr ago. This corresponds to a relative sea level high-stand, which is believed to be associated with gas hydrate stabilization. The non-steady-state model presented in this contribution can also be used to constrain the time when fluid seepage ceased at other presently extinct cold seeps when suitable sediment and porewater data are available.

2013
Gieskes, JM, Elwany H, Rasmussen L, Han S, Rathburn A, Deheyn DD.  2013.  Salinity variations in the Venice Lagoon, Italy: Results from the SIOSED Project, May 2005–February 2007. Marine Chemistry. 154:77-86.   http://dx.doi.org/10.1016/j.marchem.2013.05.011   AbstractWebsite

As a component of the Scripps Institution of Oceanography Sediment Research Project (SIOSED) in the Venice Lagoon, we studied the variability of the salinity regime with time at three principal locations in the south-central part of the Lagoon. We carried out measurements of salinity near the bottom of the Lagoon at these stations, as well as measurements of the chloride concentrations of the sedimentary pore fluids with depth into the sediments. The principal aim of these combined studies was to examine the relationship between the chloride contents of the overlying waters with those in the pore fluids. Salinity variations were measured several times over month-long periods during the SIOSED program. Occasionally these measurements overlapped with the taking of sediment cores from which pore fluids were extracted for studies of interstitial water chemistry. Generally chloride concentrations in the upper 2–4 cm of the sediments reflected the average chlorides in the overlying water column, estimated from average salinities measured over the sites of the cores, notwithstanding the short-term variability of the salinities as a function of tidal currents. Chloride concentration-depth profiles are interpreted in terms of exchange with overlying waters as well as in terms of potential influences of underlying aquifers. Though cores were taken at a higher frequency than the salinity measurements, we feel justified to use this data set together with the pore fluid data in recording the time change in chlorides of the near-bottom waters of two major SIOSED sites from May 2005 through February 2007. Results provide a better understanding of the salinity dynamics of the Venice Lagoon, and revealed that deep pore water gradients in salinity likely reflect diffusive exchange with underlying freshwater aquifers. These findings highlight the need for additional long cores, and will guide subsequent geochemical investigations of the Lagoon. Indications of the influences of aquifers on pore waters will help prevent misinterpretation of pore water trends as indications of gradual changes in the salinity of the tidal prism over time.

2011
Gieskes, J, Rathburn AE, Martin JB, Perez ME, Mahn C, Bernhard JM, Day S.  2011.  Cold seeps in Monterey Bay, California: Geochemistry of pore waters and relationship to benthic foraminiferal calcite. Applied Geochemistry. 26:738-746.   10.1016/j.apgeochem.2011.01.032   AbstractWebsite

An extensive geochemical and biogeochemical examination of CH(4) seeps in the Clam Flats area of Monterey Bay provides insight into the character of relationships between seep geochemistry and benthic foraminiferal geochemistry. The area is characterized by sulfide-rich fluids. Sulfide increases are associated with large increases in alkalinity, as well as small decreases in dissolved Ca and Mg. In addition, only small increases in NH(4) are observed, but values of delta(13)C of dissolved inorganic C are as low as -60 parts per thousand at shallow depths (< 3 cm). These observations indicate that all these processes are related to the bacterial oxidation of CH(4), which is transported upward by slow seepage of pore fluids. The geochemistry of the pore fluids should be relevant to the geochemistry of the carbonate tests of living and dead foraminifera. However, a profound disequilibrium of approximately an order of magnitude occurs between the delta(13)C values of stained (cytoplasm-containing) foraminiferal carbonate and the C isotope values of ambient pore water dissolved inorganic C. Reasons are unclear for this isotopic disequilibrium, but have important implications for interpretations of foraminiferal carbonate as a paleoenvironmental proxy. Much fine scale work is needed to fully understand the relationships between the biogeochemistry of benthic foraminifera and the geochemistry of the pore waters where they live. (C) 2011 Elsevier Ltd. All rights reserved.

Han, S, Gieskes J, Obraztsova A, Deheyn DD, Tebo BM.  2011.  Relocation effects of dredged marine sediments on mercury geochemistry: Venice lagoon, Italy. Estuarine Coastal and Shelf Science. 93:7-13.   10.1016/j.ecss.2011.03.004   AbstractWebsite

Understanding the biogeochemical process of Hg is critical in the overall evaluation of the ecological impacts resulting from the reuse of Hg-contaminated dredged sediment. Sediment banks (V1 and V2) were constructed with freshly dredged sediments from a navigational channel in Venice Lagoon, Italy, with the goal of clarifying potential differences in the biogeochemistry of Hg between the reused dredged sediments and those from surrounding sites (SS1 and S2). Toward this purpose, Hg and monomethylmercury (MMHg) concentrations, and Hg methylation rates (MMRs) in the surface 2.5 cm sediments were monitored, along with ammonium, iron, sulfate and sulfide concentrations in the pore waters of banks and surrounding sites from November 2005 to February 2007. Pore water analyses indicate that the bank sediments are characterized by lower levels of sulfate and iron, and by higher levels of ammonium and sulfide compared to the surrounding sediments. With respect to Hg speciation, the fractions of MMHg in total Hg (%MMHg/Hg) and the MMRs were significantly lower in the bank V1 compared to those in the reference site SS1, whereas the %MMHg/Hg and the MMRs were similar between V2 and S2. A negative correlation is found between the logarithm of the particle-water partition coefficient of Hg and the MMR, indicating that the reduced MMRs in V1 are caused by the limited concentrations of dissolved Hg. Organic matter appears to play a key role in the control of MMR via the control of Hg solubility. (C) 2011 Elsevier Ltd. All rights reserved.

Han, S, Porrachia M, Volpato E, Gieskes J, Deheyn DD.  2011.  Mercury Concentration and Monomethylmercury Production in Sediment: Effect of Dredged Sediment Reuse on Bioconcentration for Ragworms. Water Air and Soil Pollution. 219:59-68.   10.1007/s11270-010-0683-7   AbstractWebsite

The effect of dredged sediment reuse on the production and bioconcentration of monomethylmercury (MMHg) was investigated by examining sediments and ragworms found in dredge material banks and surrounding sites in the Venice Lagoon, Italy. Total Hg concentrations in the surface 20 cm of sediments were higher in the banks than in the surrounding sites, but MMHg concentrations were similar, which suggests reduced MMHg production in the banks. Monomethylmercury content in ragworms was significantly lower (p < 0.05) in the banks than in the surrounding sites. In pore water, concentrations of both sulfate and Fe decreased with depth in the upper 20 cm of the dredge banks. In contrast, sulfate concentrations were constant with depth and large amounts of dissolved Fe occurred in the upper 20 cm of sediments of surrounding sites. Continuous sulfate reduction and possible precipitation of iron sulfide may decrease the production and bioconcentration of MMHg in the dredge material banks compared to the surrounding sites. Overall, the production of MMHg in sediments and its bioconcentration in benthic organisms were connected to the process of sediment diagenesis of organic matter through the control of pore water and sediment geochemistry involving iron and sulfur.

Kim, M, Han S, Gieskes J, Deheyn DD.  2011.  Importance of organic matter lability for monomethylmercury production in sulfate-rich marine sediments. Science of the Total Environment. 409:778-784.   10.1016/j.scitotenv.2010.10.050   AbstractWebsite

Sediment cores were collected from two shallow sites in the Venice Lagoon, Italy, in order to study the lability of organic matter and the methylation rate of inorganic Hg(II). Measurements were made of concentrations of total Hg and monomethylmercury (MMHg), Hg(II) methylation rates, concentrations of total organic carbon and total nitrogen in the sediments, and dissolved sulfate, sulfide, and alkalinity in sedimentary pore waters. A positive linear relationship was detected between the specific Hg(II) methylation rate constant and the fraction of total Hg comprised of MMHg (%MMHg/Hg), indicating that short-term Hg(II) methylation rate reflects a long-term accumulation of MMHg in sediment. In addition, the %MMHg/Hg and specific Hg(II) methylation rate constant in sediment increased with decreasing ratios of total organic carbon to total nitrogen (C/N), whereas concentrations of dissolved sulfate, sulfide, and alkalinity in pore water remained constant. This result suggests that the Hg(II) methylation rate was affected by lability of organic matter. In particular, surface sediments, which contained large fractions of fresh algal organic material (C/N = 5.8-7.8), showed higher Hg(II) methylation rates than did deeper sediments (C/N > 10). Our results indicate that the Cl N ratio can be used as a proxy for the lability of organic matter that influences Hg(II) methylation rate in sulfate-rich marine sediments. (C) 2010 Elsevier B.V. All rights reserved.

2010
Han, S, Narasingarao P, Obraztsova A, Gieskes J, Hartmann AC, Tebo BM, Allen EE, Deheyn DD.  2010.  Mercury Speciation in Marine Sediments under Sulfate-Limited Conditions. Environmental Science & Technology. 44:3752-3757.   10.1021/es903112m   AbstractWebsite

Sediment profiles of total mercury (Hg) and monomethylmercury (MMHg) were determined from a 30-m drill hole located north of Venice, Italy. While the sediment profile of total Hg concentration was fairly constant between 1 and 10 m, that of the MMHg concentration showed an unexpected peak at a depth of 6 m. Due to the limited sulfate content (<1 mM) at the depth of 6 m, we hypothesized that the methylation of inorganic Hg(II) at this depth is associated with the syntrophic processes occurring between methanogens and sulfidogens. To test this hypothesis, anoxic sediment slurries were prepared using buried Venice Lagoon sediments amended with HgCl(2), and we monitored MMHg concentration in sediment slurries over time under two geochemical conditions: high sulfate (1-16 mM) and limited sulfate concentrations (<100 mu M). After day 52 and onward from the addition of inorganic Hg(II), the MMHg concentrations were higher in sulfate-limited slurries compared to high sulfate slurries, along with methane production in both slurries. On the basis of these results, we argue that active methylation of inorganic Hg(II) occurs under sulfate-limited conditions possibly by syntrophic processes occurring between methanogens and sulfidogens. The environmental significance of syntrophic Hg(II) methylation should be further studied.

2009
Basak, C, Rathburn AE, Perez ME, Martin JB, Kluesner JW, Levin LA, De Deckker P, Gieskes JM, Abriani M.  2009.  Carbon and oxygen isotope geochemistry of live (stained) benthic foraminifera from the Aleutian Margin and the Southern Australian Margin. Marine Micropaleontology. 70:89-101.   10.1016/j.marmicro.2008.11.002   AbstractWebsite

Comparisons of ambient bottom-water geochemistry and stable isotopic values of the tests of living (stained) calcareous benthic foraminifera from the North Pacific (on the Aleutian Margin, water depth 1988 m) and Murray Canyons group in the Southern Indian Ocean (Australian Margin, water depths 2476 m and 1634 m) provide modem environmental analogs to calibrate paleoenvironmental assessments. Consistent with the hypothesis that microhabitat preferences influence foraminiferal isotopic values, benthic foraminifera from both margins were depleted in (13)C with respect to bottom-water dissolved inorganic carbon (DIC). The carbon isotope values of deep infaunal foraminifera (Chilostomella oolina, Globobulimina pacifica) showed greater differences from estimates of those of DIC than shallow benthic foraminifera (Bulimina mexicana, Bolivinita quadrilatera, Pullenia bulloides). This study provides new isotopic and ecological information for B. quadrilatera. The mean Delta delta(13)C value, defined as foraminiferal delta(13)C values minus estimated ambient delta(13)C values from the Aleutian Margin, is 0.97 parts per thousand higher for G. pacifica than the mean from the Murray Canyon. This difference may result either from genetic or biological differences between the populations or from differences in environmental isotopic influences (such as pore water differences) that were not accounted for in the equilibrium calculations. These analyses provide calibration information for the evaluation of bottom water conditions and circulation patterns of ancient oceans based on fossil foraminiferal geochemistry. (C) 2008 Elsevier B.V. All rights reserved.

Rathburn, AE, Levin LA, Tryon M, Gieskes JM, Martin JM, Perez ME, Fodrie FJ, Neira C, Fryer GJ, Mendoza G, McMillan PA, Kluesner J, Adamic J, Ziebis W.  2009.  Geological and biological heterogeneity of the Aleutian margin (1965-4822 m). Progress in Oceanography. 80:22-50.   10.1016/j.pocean.2008.12.002   AbstractWebsite

Geological, biological and biogeochemical characterization of the previously unexplored margin off Unimak Island, Alaska between 1965 and 4822 m water depth was conducted to examine: (1) the geological processes that shaped the margin, (2) the linkages between depth, geomorphology and environmental disturbance in structuring benthic communities of varying size classes and (3) the existence, composition and nutritional sources of methane seep biota on this margin. The study area was mapped and sampled using multibeam sonar, a remotely operated vehicle (ROV) and a towed camera system. Our results provide the first characterization of the Aleutian margin mid and lower slope benthic communities (micro-biota, foraminifera, macrofauna and megafauna), recognizing diverse habitats in a variety of settings. Our investigations also revealed that the geologic feature known as the "Ugamak Slide" is not a slide at all, and could not have resulted from a large 1946 earthquake. However, sediment disturbance appears to be a pervasive feature of this margin. We speculate that the deep-sea occurrence of high densities of Elphidium, typically a shallow-water foraminiferan, results from the influence of sediment redeposition from shallower habitats. Strong representation of cumacean, amphipod and tanaid crustaceans among the Unimak macrofauna may also reflect sediment instability. Although some faunal abundances decline with depth, habitat heterogeneity and disturbance generated by canyons and methane seepage appear to influence abundances of biota in ways that supercede any clear depth gradient in organic matter input. Measures of sediment organic matter and pigment content as well as C and N isotopic signatures were highly heterogeneous, although the availability of organic matter and the abundance of microorganisms in the upper sediment (1-5 cm) were positively correlated. We report the first methane seep on the Aleutian slope in the Unimak region (3263-3285 m), comprised of clam bed, pogonophoran field and carbonate habitats. Seep foraminiferal assemblages were dominated by agglutinated taxa, except for habitats above the seafloor on pogonophoran tubes. Numerous infaunal taxa in clam bed and pogonophoran field sediments and deep-sea "reef' cnidarians (e.g., corals and hydroids) residing on rocks near seepage sites exhibited light organic delta(13)C signatures indicative of chemosynthetic nutritional sources. The extensive geological, biogeochemical and biological heterogeneity as well as disturbance features observed on the Aleutian slope provide an attractive explanation for the exceptionally high biodiversity characteristic of the world's continental margins. (C) 2008 Elsevier Ltd. All rights reserved.

2008
Han, S, Obraztsova A, Pretto P, Deheyn DD, Gieskes J, Tebo BM.  2008.  Sulfide and iron control on mercury speciation in anoxic estuarine sediment slurries. Marine Chemistry. 111:214-220.   10.1016/j.marchem.2008.05.002   AbstractWebsite

In order to understand the role of sulfate and Fe(III) reduction processes in the net production of monomethylmercury (MMHg), we amended anoxic sediment slurries collected from the Venice Lagoon, Italy. with inorganic Hg and either potential electron acceptors or metabolic byproducts of sulfate and Fe(III) reduction processes, gradually changing their concentrations. Addition of sulfide (final concentration: 0.2-6.3 mM) resulted in an exponential decrease in the sulfate reduction rate and MMHg concentration with increasing concentrations of sulfide. Based on this result, we argue that the concentration of dissolved sulfide is a critical factor controlling the sulfate reduction rate, and in turn, the net MMHg production at steady state. Addition of either Fe(II) (added concentration: 0-6.1 mM) or Fe(III) (added concentration: 0-3.5 mM) resulted in similar trends in the MMHg concentration, an increase with low levels of Fe additions and a subsequent decrease with high levels of Fe additions. The limited availability of dissolved Hg, associated with sulfide removal by precipitation of FeS, appears to inhibit the net MMHg production in high levels of Fe additions. There was a noticeable reduction in the net MMHg production in Fe(III)-amended slurries as compared to Fe(II)-amended ones, which could be caused by a decrease in the sulfate reduction rate. This agrees with the results of Hg methylation assays using the enrichment cultures of anaerobic bacteria: whereas the enrichment cultures of sulfate reducers showed significant production of MMHg (4.6% of amended Hg), those of Fe(III), Mn(IV), and nitrate reducers showed no production of MMHg. It appears that enhanced Fe(III)-reduction activities suppress the formation of MMHg in high sulfate estuarine sediments. (c) 2008 Elsevier B.V. All rights reserved.

2007
Gieskes, JM, Mahn C.  2007.  Halide systematics in interstitial waters of ocean drilling sediment cores. Applied Geochemistry. 22:515-533.   10.1016/j.apgeochem.2006.12.003   AbstractWebsite

The distribution of dissolved halides (Cl, Br, I) in interstitial waters of deep drill holes in the ocean is discussed for a number of environments: hemi-pelagic sediments; hydrate containing sediments; sediments affected by halide brines; and also in hydrothermal environments. The importance of the diagenesis of organic C in the production of I- and Br- is discussed, with reference to the observations of dissolved SO42- and NH4+ in the pore fluids. More importantly, in the zone of methano-genesis, below the SO42- reduction zone, and Usually associated with high organic C accumulation rates, increases in I- and Br- are of importance and usually are much higher than the increases observed in the SO42- reduction zone. In pore fluids affected by dissolution of evaporites the Br/Cl is often a strong indicator of the degree of evaporation of seawater demonstrated from results obtained in the Mediterranean Sea. Dissolution of halites on the other hand leads to Br/Cl much lower than those of sea water. In hydrothermal sediments, there is no doubt that I- and Br- contents of the fluids are influenced by hydrothermal alteration of the sediments and their organic C, but signals are not clear. The present study provides an overview of previous DSDP and ODP studies of halide distributions in sedimentary pore waters, in addition to other contributions on halide distributions in sedimentary pore waters in this symposium volume. (c) 2007 Published by Elsevier Ltd.

Han, S, Obraztsova A, Pretto P, Choe KY, Gieskes J, Deheyn DD, Tebo BM.  2007.  Biogeochemical factors affecting mercury methylation in sediments of the Venice Lagoon, Italy. Environmental Toxicology and Chemistry. 26:655-663.   10.1897/06-392r.1   AbstractWebsite

Mercury methylation and sulfate reduction rates, total Hg, and monomethyl Hg in the sediments of the Venice Lagoon (Italy) were measured in June 2005 in order to identify the factors affecting the methylation of inorganic Hg. While the rates of Hg methylation and sulfate reduction were generally higher in the surface layers (0-2.5 cm), the correlation between Hg methylation and sulfate reduction rates was not significant when considering all depths and sites. This discrepancy is discussed considering two factors: the activity of sulfate-reducing bacteria and Hg solubility. The former factor is important in determining the Hg methylation rate in comparable geochemical conditions as evidenced by similar vertical profiles of Hg methylation and sulfate reduction rates in each sediment core. The latter factor was assessed by comparing the Hg methylation rate with the particle-water partition coefficient of Hg. The Hg methylation rates normalized to sulfate reduction rates showed a negative linear correlation with the logarithm of the particle-water partition coefficient of Hg, suggesting that the availability of dissolved Hg is a critical factor affecting Hg methylation. Solid FeS seems to play an important role in controlling the solubility of Hg in Venice Lagoon sediments, where sulfate and iron reductions are the dominant electron-accepting processes. Overall, the production of monomethyl Hg in the Venice Lagoon is controlled by a fine balance between microbial and geochemical processes with key factors being the microbial sulfate reduction rate and the availability of dissolved Hg.

2005
Gieskes, J, Mahn C, Day S, Martin JB, Greinert J, Rathburn T, McAdoo B.  2005.  A study of the chemistry of pore fluids and authigenic carbonates in methane seep environments: Kodiak Trench, Hydrate Ridge, Monterey Bay, and Eel River Basin. Chemical Geology. 220:329-345.   10.1016/j.chemgeo.2005.04.002   AbstractWebsite

Analyses of the chemical and isotopic composition of carbonates rocks recovered from methane seepage areas of the Kodiak Trench, Hydrate Ridge, Monterey Bay Clam Flats, and the Eel River Basin, coupled with the studies of the chemistry of the pore fluids, have shown that these carbonates have grown within the sediment column. Geochemical profiles of pore fluids show that, in deep water seeps (Kodiak Trench-4450 m; Monterey Bay-1000 m; Hydrate Ridge-650 m), delta(13) C (DIC) values are low (isotopically light), whereas in the Eel River area (similar to 350-500 M), delta(13)C (DIC) values are much higher (isotopically heavier). In all cases, the delta(13)C values indicate that processes of methane oxidation, associated with sulfate reduction, are dominant in the shallow sediments. Data on the isotopic composition of authigenic carbonates found at sites in Kodiak Trench, Eel River Basin South, and Eel River Basin North indicate a variable composition and origin in different geochemical environments. Some of the authigenic carbonates from the study sites show a trend in their delta(13)C values similar to those of the pore fluids obtained in their vicinity, suggesting formation at relatively shallow depths, but others indicate formation at greater sediment depths. The latter usually consist of high magnesium calcite or dolomite, which, from their high values of delta(13)C (up to 23 parts per thousand) and delta(18)O (up to 7.5 parts per thousand), suggest formation in the deeper horizons of the sediments, in the zone of methanogenesis. These observations are in agreement with observations by other workers at Hydrate Ridge, in Monterey Bay, and in the Eel River Basin. (c) 2005 Elsevier B.V. All rights reserved.

2004
You, CF, Gieskes JM, Lee T, Yui TF, Chen HW.  2004.  Geochemistry of mud volcano fluids in the Taiwan accretionary prism. Applied Geochemistry. 19:695-707.   10.1016/j.apgeochem.2003.10.004   AbstractWebsite

Taiwan is located at the collision boundary between the Philippine Sea Plate and the Asian Continental Plate and is one of the most active orogenic belts in the world. Fluids sampled from 9 sub-aerial mud volcanoes distributed along two major geological structures in southwestern Taiwan, the Chishan fault and the Gutingkeng anticline, were ana-lyzed to evaluate possible sources of water and the degree of fluid-sediment interaction at depth in an accretionary prism. Overall, the Taiwanese mud volcano fluids are characterized by high Cl contents, up to 347 mM, suggesting a marine origin from actively de-watering sedimentary pore waters along major structures on land. The fluids obtained from the Gutingkeng anticline, as well as from the Coastal Plain area, show high Cl, Na, K, Ca, Mg and NH4, but low SO4 and B concentrations. In contrast, the Chishan fault fluids are much less saline (1/4 seawater value), but show much heavier 0 isotope compositions (delta(18)O = 5.1-6.5 parts per thousand). A simplified scenario of mixing between sedimentary pore fluids and waters affected by clay dehydration released at depth can explain several crucial observations including heavy O isotopes, radiogenic Sr contents (Sr-87/Sr-86=0.71136-0.71283), and relatively low salinities in the Chishan fluids. Gases isolated from the mud volcanoes are predominantly CH4 and CO2, where the CH4-C isotopic compositions show a thermogenic component of delta(13)C = -38 parts per thousand.. These results demonstrate that active mud volcano de-watering in Taiwan is a direct product of intense sediment accretion and plate collision in the region. (C) 2003 Elsevier Ltd. All rights reserved.

Martin, JB, Day SA, Rathburn AE, Perez ME, Mahn C, Gieskes J.  2004.  Relationships between the stable isotopic signatures of living and fossil foraminifera in Monterey Bay, California. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000629   AbstractWebsite

[ 1] Fossil foraminifera are critical to paleoceanographic reconstructions including estimates of past episodes of methane venting. These reconstructions rely on benthic foraminifera incorporating and retaining unaltered the ambient isotopic compositions of pore fluids and bottom waters. Comparisons are made here of isotopic compositions of abundant live and fossil foraminifera (Uvigerina peregrina, Epistominella pacifica, Bulimina mexicana, and Globobulimina pacifica) collected in Monterey Bay, CA from two cold seeps ( Clam Flats and Extrovert Cliffs) and from sediments -5 m outside of the Clam Flats seep. Clam Flats has steep delta(13) CDIC gradients ( to <- 45%), but DIC at Extrovert Cliffs is less enriched in C-12 ( to approximately -22%). Oxygen isotope values of fossil foraminifera at Clam Flats are -1.5% enriched in O-18 over the living foraminifera, as well as those of both live and fossil foraminifera at Extrovert Cliffs, suggesting they may have lived during the last glacial maximum. Statistical comparisons ( Student's t and Kolmogorov- Smirnov tests) of δC-13 and δO-18 values indicate that live and fossil foraminifera come from different populations at both Clam Flats and Extrovert Cliffs. At Clam Flats, the difference appears to result from alteration enriching some fossil foraminifera in C-12 over live foraminifera. At Extrovert Cliffs, the fossil foraminifera are enriched in C-13 over the live foraminifera, suggesting they lived prior to the onset of venting and thus that venting began recently. The short time of venting at Extrovert Cliffs may be responsible for the less alteration there compared with Clam Flats. These results indicate that preservation of foraminifera is likely to be poor within long-lived cold seeps, but that foraminifera living in the surrounding sediment may incorporate and preserve broad basin-wide changes in isotopic compositions of the ambient water.

2003
You, CF, Chan LH, Gieskes JM, Klinkhammer GP.  2003.  Seawater intrusion through the oceanic crust and carbonate sediment in the Equatorial Pacific: Lithium abundance and isotopic evidence. Geophysical Research Letters. 30   10.1029/2003gl018412   AbstractWebsite

1] Large- scale lateral advection of seawater within the oceanic crust is thought to be the cause of low heat flow in the Equatorial Pacific. Until now supportive evidence is limited to reversals of B and delta(11) B, Ca, Mg, SO4-2, Sr and Sr-87/Sr-86 in sediment pore waters. Here we report new evidence from a detailed study of Li and its isotopes in sediment cores from ODP Sites 844 and 851. Carbonates at these sites were significantly recrystallized leading to large variations in Li and delta(7) Li in the sediments and associated pore waters. In addition to diagenetic effects, distinct reversals in lithium concentration and isotopic ratio toward modern seawater composition are observed in waters near the basaltic basement, lending further support to the seawater intrusion hypothesis. Three endmembers are identified in the pore waters: seawater, a diagenetically altered component, and evolved crustal fluid. The new lithium isotopic results underscore the importance of diagenetic artifacts in carbonate sediments.

Kopf, A, Deyhle A, Lavrushin VY, Polyak BG, Gieskes JM, Buachidze GI, Wallmann K, Eisenhauer A.  2003.  Isotopic evidence (He, B, C) for deep fluid and mud mobilization from mud volcanoes in the Caucasus continental collision zone. International Journal of Earth Sciences. 92:407-425.   10.1007/s00531-003-0326-y   AbstractWebsite

The Caucasian orogenic wedge formed as a consequence of the closure of the Tethyan Ocean, and numerous fields of active mud volcanoes pepper the area adjacent to the Black and Caspian Seas. Stable isotope ratios of boron, helium, and carbon have been measured for gas, fluid and sediment samples from active mud volcanoes of Taman Peninsula and Georgia to estimate the sources and mobilization depths of the fluid phase and mud. Boron concentrations in mud volcano fluids were found to be 5-35x higher than seawater. Fluid isotope ratios vary between delta(11)B=22 and 39parts per thousand, while isotope ratios of the smectite- and illite-rich extruded mud are considerably depleted in heavy B-11 (delta(11)B=-8 to +7parts per thousand). B contents of these muds are similar to8x higher than modern marine sediments. This suggests that liquefaction prior to mud volcanism was accompanied by both B enrichment and isotope fractionation, most likely at an intermediate depth mud reservoir at 2-4 km. The hydrocarbon-generating source beds to the mud volcanoes are located at 7 to >10 km depth in the folded Maikop Formation and are of proposed Oligocene-Miocene age. The most likely mechanism is re-hydration of these shales by both hydrocarbons and a geochemically mature fluid from greater depth within the orogenic wedge. Such a deep fluid source is supported by our results from gas analyses, which imply an admixture of minor amounts (less than 1%vol) of He-3 (Georgia), thermogenic C-13 in methane as well as 'ultraheavy' C-13 in CO2 (both Taman and Georgia). The overall results attest active local flow of geochemically different fluids along deep-seated faults penetrating the two study areas in the Caucasian orogenic wedge, with the waters as well as the gases coming from below the Maikop Formation.

Rathburn, AE, Perez ME, Martin JB, Day SA, Mahn C, Gieskes J, Ziebis W, Williams D, Bahls A.  2003.  Relationships between the distribution and stable isotopic composition of living benthic foraminifera and cold methane seep biogeochemistry in Monterey Bay, California. Geochemistry Geophysics Geosystems. 4   10.1029/2003gc000595   AbstractWebsite

As part of an ongoing effort to explore the use of foraminifera as a means to assess modern and ancient methane release, we compared ambient pore water chemistry with the distribution and stable isotopic composition of living (rose Bengal stained) foraminifera in MBARI ROV Ventana tube cores taken from modern seepage areas (about 1000 m water depth) in Monterey Bay, California. Benthic foraminiferal isotopic differences between sites clearly indicate that methane-influenced pore waters affect foraminiferal distributions and carbonate isotope geochemistry. Carbon isotope signatures of living benthic foraminifera did not conform to the very negative (-30 to -48parts per thousand), methane-influenced carbon isotope values of the pore waters they live in. Instead, the influence of methane seep pore waters was reflected in the greater range and carbon isotopic variability of living seep foraminifera compared with published delta(13)C values of foraminifera living in nonseep habitats. It is not clear what relative influences biological, ecological, and physical factors have on the carbon isotopic signatures observed in seep foraminifera. Substantial carbon isotope differences can exist between individuals of the same seep species. For instance, delta(13)C values of living Globobulimina pacifica varied by as much as 2.9% between seeps within 8 km of each other, whereas delta(13)C values of living Uvigerina peregrina varied by as much as 1.95parts per thousand within the same seep. Provided there is no diagenetic alteration of the test carbonate, isotopic results of individual seep foraminifera support the hypothesis that foraminifera can be used to assess past and present methane seepage.

Levin, LA, Ziebis W, Mendoza GF, Growney VA, Tryon MD, Brown KM, Mahn C, Gieskes JM, Rathburn AE.  2003.  Spatial heterogeneity of macrofauna at northern California methane seeps: influence of sulfide concentration and fluid flow. Marine Ecology-Progress Series. 265:123-139.   10.3354/meps265123   AbstractWebsite

Relationships among fluid flow, sulfide concentration, sulfur bacteria and macrofaunal assemblages were examined at methane seeps on the northern California margin, near the mouth of the Eel River (512 to 525 m). Over a 6 mo period, sediments covered with microbial mats exhibited significant but variable outflow of altered fluids, with no flow reversals. This fluid flow was associated with high porewater sulfide concentrations (up to 20 mM) and almost no oxygen penetration of sediments (<0.1 mm). Vesicomya pacifica (clam) bed and non-seep sediments exhibited little net fluid outflow and similar oxygen penetration (3 and 4 mm, respectively); however, sulfide concentrations were higher in subsurface clam-bed sediments (up to 2 mM) than in non-seep sediments (<200 muM). Macrofaunal densities did not differ among the 3 habitats (13 800 to 16 800 ind. m(-2); >300 mum), but biomass and diversity (no. species per core, E(S-100), H') were lower and composition varied in the sulfidic microbial mat sediments relative to clam-bed and non-seep sediments. The community in microbial mat-covered sediments consisted largely (82%) of 6 species in the polychaete family Dorvilleidae, whereas the clam-bed and non-seep microhabitats supported a mixture of annelids, peracarid crustaceans, nemerteans, and mollusks. Vertical microprofiling of sulfide in animal cores indicated that most taxa avoid H2S concentrations >1 mM. However, sulfide-oxidizing filamentous bacteria, dorvilleid polychaetes and bivalves (mainly V. pacifica) exhibited highest densities at sulfide concentrations of 1 to 5 mM sulfide. Horizontal and vertical patterns of sulfide availability have a strong influence on the fine-scale distribution, structure and composition of macrofaunal assemblages inhabiting methane seeps and must be accounted for when characterizing the microbiology and ecology of seep habitats.

2002
Gieskes, JM, Simoneit BRT, Shanks WC, Goodfellow WD, James RH, Baker PA, Ishibashi J.  2002.  Geochemistry of fluid phases and sediments: relevance to hydrothermal circulation in Middle Valley, ODP Legs 139 and 169. Applied Geochemistry. 17:1381-1399.   10.1016/s0883-2927(02)00108-7   AbstractWebsite

Geochemical and isotopic studies of pore fluids and solid phases recovered from the Dead Dog and Bent Hill hydrothermal sites in Middle Valley (Ocean Drilling Program Leg 169) have been compared with similar data obtained previously from these sites during Ocean Drilling Program Leg 139. Although generally the hydrothermal systems reflect non-steady state conditions, the data allow an assessment of the history of the hydrothermal processes. Sediment K/Al ratios as well as the distribution of anhydrite in the sediments suggest that the Dead Dog hydrothermal field has been, and still is, active. In contrast, similar data in the Bent Hill hydrothermal field indicate a waning of hydrothermal activity. Pore fluid and hydrothermal vent data in the Dead Dog hydrothermal field are similar in nature to the data collected during ODP Leg 139. In the area of the Bent Hill sulfide deposit, however, the pore water data indicate that recent wholesale flushing of the sediment column with relatively unaltered seawater has obliterated a previous record of hydrothermal activity in the pore fluids. Data from the deepest part of Hole 1035A in the Bent Hill locality show the presence of hydrothermal fluids at greater depths in this area. This suggests the origin of the hydrothermal fluids found to be emanating from Hole 1035F, which constitutes one of the first man made hydrothermal vents in the Middle Valley hydrothermal system. Similarly, CORKed Hole 858G, because of seal failures, has acted as a hydrothermal vent, with sulfide deposits forming inside the CORK. (C) 2002 Elsevier Science Ltd. All rights reserved.