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Anoxic mineralization of biogenic debris in near-shore marine sediments (Gulf of Trieste, northern Adriatic)
The Science of the Total Environment 266 Ž2001. 143᎐152
Anoxic mineralization of biogenic debris in near-shore marine sediments ž Gulf of Trieste, northern Adriatic/ B. Cermelja,U , N. Ogrinc b, J. Faganeli a a
Marine Biological Station, Fornace 41, 6330 Piran, Slo¨ enia Jozef Stefan Institute, Jamo¨ a 39, 1000, Ljubljana, Slo¨ enia
b
Received 17 September 1999; accepted 28 June 2000
Abstract Anoxic degradation of sedimentary biogenic debris using closed incubation experiments was studied at two sampling stations in the Gulf of Trieste Žnorthern Adriatic.. Production rates of dissolved inorganic carbon ŽDIC., 3y NHq and dissolved Si ŽdSi., and reduction rates of SO42y were measured and anoxic mineralization rates were 4 , PO4 modeled using a first order G-model and multi-G approach. The depth profiles of these rates revealed an exponential decrease indicating that the largest fraction of mineralization of biogenic debris and SO42y reduction occurs in the surficial sediment layer and on the sediment surface. Comparing the depth-integrated anoxic mineralization rates at 3y and dSi measured at the in situ temperature in the dark, it both stations with benthic fluxes of DIC, NHq 4 , PO4 appears that the DIC and PO43y fluxes are higher because the mineralization mostly occurs at the sediment᎐water interface, and that besides SO42y reduction, other electron acceptors are involved in the organic matter decomposiq tion pathway in these surficial sediments. The NHq 4 production was higher than the benthic fluxes because of NH 4 oxidation. The production of dSi was in good agreement with benthic fluxes implying that temperature is the main factor of dSi production and benthic fluxes in these sediments. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Mineralization; Sediments; Benthic fluxes; Carbon; Nitrogen; Phosphorus; Silicon; Northern Adriatic
U
Corresponding author. Tel.: q386-5-6745306;fax: q386-5-6746367. E-mail address: [email protected] ŽB. Cermelj.. 0048-9697r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 0 . 0 0 7 4 1 - 5
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1. Introduction
2. Materials and methods
Organic matter degradation and burial in marine sediments represents an important step in the global carbon cycle ŽEmerson and Hedges, 1988.. In continental shelves and coastal areas, a large fraction of depositing organic material escapes degradation within the water column, with the result that easily degradable substrate is available at the sediment᎐water interface ŽHedges and Keil, 1995.. In shallow coastal waters, where the euphotic zone reaches the sea bottom, the benthic macro- and microalgae may be a significant source of easily degradable sedimentary organic matter ŽRizzo, 1990.. An improved knowledge of the biogeochemical processes in coastal marine areas requires better information about the sources, transformations, recycling and burial of biogenic debris in sediments. Oxygen can influence the organic carbon mineralization in sediments if the penetration of dissolved oxygen is deep enough and enhanced by bioturbation activity of benthic fauna. Since oxygen typically penetrates only a few millimeters in coastal sediments, other electron acceptors are also involved in degradation of sedimentary organic matter. Although the euphotic zone reaches the benthos throughout the Gulf of Trieste Žnorthern Adriatic., the benthic O 2 consumption exceeds benthic primary production ŽHerndl et al., 1989., and anaerobic processes probably account for the majority of organic matter decomposition in these coastal sediments ŽHines et al., 1997.. The role of benthic macrofaunal bioturbation and irrigation on benthic nutrient biogeochemistry is important in these sediments, especially in summertime, leading to benthic fluxes which exceed diffusive fluxes by up to 10-fold ŽCermelj et al., 1997.. Previous studies have demonstrated that nutrients released from pore waters could be mostly utilized in benthic microalgal production ŽBertuzzi et al., 1997.. The aim of the present study was: Ž1. to determine the anoxic sedimentary organic matter decomposition pathway in the Gulf of Trieste using closed sediment incubation experiments; and Ž2. to compare these reactions rates with measured benthic fluxes.
2.1. Closed sediment incubations Sediment samples were collected at sampling stations CZ Ž13⬚ 37.998⬘ E, 45⬚ 37.398⬘ N. and F Ž13⬚ 33⬘ E, 45⬚ 32.298⬘ N. in the central and southern part of the Gulf of Trieste, respectively. Virtually undisturbed sediment cores were taken to a depth of 15 cm at station CZ in August 1996 and at station F in May 1996 by scuba divers inserting a Plexiglas tube Ž6 cm i.d.. directly into the sediment. Sediment samples from three sediment layers Ž0᎐2, 4᎐6 and 10᎐12 cm. were gently homogenized, packed into 50-ml polyethylene centrifuge tubes Žusually 8 per layer. and incubated Ž120 days at station F and 200 days at station CZ. in the dark at temperatures of 6, 10 and 20⬚C. No additional water was added. One tube from each series was removed approximately every 25 days and centrifuged at 7000 rev.rmin for 15 min. The supernatant was sampled under N2 , filtered through 0.45-m pore size Millipore HA membrane filters and analyzed for solutes as described below. The concentration changes in the supernatant waters through time were used to obtain the production rates, corrected for porosity ŽCermelj et al., 1997.. Depth-integrated production rates were obtained by vertically integrating rates to a depth of 12 cm Žthis was the deepest layer used in benthic flux experiments in the Gulf.. The closed incubation technique used here is similar to that presented by Aller and Yingst Ž1988., and Mackin and Swider Ž1989.. 2.2. Core incubations In March and August 1996 benthic fluxes were estimated by incubating surficial sediment from station F in the laboratory at the in situ temperatures in the dark. Acrylic box cores Ž25 cm height, 30 cm i.d., 0.6 cm wall thickness . were inserted into the sediment to a depth of approximately 10 cm by a scuba diver. In the laboratory, the water
B. Cermelj et al. r The Science of the Total En¨ ironment 266 (2001) 143᎐152
overlying each core was drained using an aspirator. Sediment in the core was successively filled with the filtered seawater collected in the bottom water layer at the same location. The cores were used as an incubation flux chamber to measure benthic fluxes. Samples were collected periodically and the volume of water withdrawn was balanced by bottom seawater that exchanged with the chamber water through a Tygon tube. Fluxes of solutes across the sediment᎐water interface were evaluated from linear regressions of solute concentrations over time. 2.3. Analyses Alkalinity in water samples was measured by Gran titration ŽEdmond, 1970.. DIC was calculated from alkalinity, the in situ temperature-corrected pH, and salinity ŽMillero, 1995.. Nitrate, ammonium, phosphate and silicate were measured photometrically ŽGrasshoff et al., 1983., and sulfate turbidimetrically ŽTabatabai, 1974.. Dissolved Ca2q and Mg 2q were determined by flame AAS.
3. Results 3.1. Production rates Productionrreduction rates for DIC, NHq 4, PO43y and SO42y ŽFig. 1. during anoxic incubation experiments were calculated as the slope of the solute vs. incubation time using the G-model: Gm ,i Ž t . s Gm ,i ,0 w exp Ž yk i t .x
Ž1.
ŽWestrich and Berner, 1984; Burdige, 1991., where t is time, Gm, i is the concentration of metabolizable organic carbon, nitrogen or phosphorus and k Ž1ryear. is the first-order rate constant. NHq 4 and PO43y production rates were corrected for adsorption to sediment solids. Adsorption coefficients Ž K . after adjusting for porosity were ; 1.3 Ž . for NHq 4 Mackin and Aller, 1984 and ; 1.2 for PO43y ŽKrom and Berner, 1980., respectively. For
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DIC, and SO42y, Ks 0 was assumed for calculation of productionrreduction rates from anoxic incubation results ŽLi and Gregory, 1974.. Using the multiple-G model: Gm ,i Ž t . s Gm ,i ,1 w exp Ž yk i ,1 t .x q Gm ,i ,2 w exp Ž yk i ,2 t .x q GN R
Ž2.
Žsee Westrich and Berner, 1984; Burdige, 1991. it was possible to better explain the differences in the mineralization rates in surficial sediment at station CZ at 20⬚C ŽFig. 1.. The sedimentary organic matter consists of two reactive fractions Ž Gm,1 and Gm,2 . with lower and higher CrN and CrP ratios, respectively. Approximately 80% of the NHq 4 and 30% of the DIC produced originate from the more reactive fraction, while the production of PO43y is affected by adsorption and precipitation. Changes in dSi concentrations ŽFig. 2. were determined using the following equation: dCrdt s k Ž Cs y C .
Ž3.
ŽLerman, 1979., where Cs is the solubility of the dissolving phase, and k Ž1ryear. is the reaction rate constant. If Cs ) C dissolution occurs, and if Cs - C precipitation occurs. By solving Eq. Ž3. and fitting the data to the analytical solution we obtained the values for the dissolution rate constant Ž k . and the solubility Ž Cs .. 3y The production rates of DIC, NHq and 4 , PO4 dSi were highest close to the sediment᎐water interface at both stations and decreased exponentially with depth ŽFig. 2., indicating the depletion of reactive biogenic debris with depth in sediments. The rates calculated at a sediment depth between 10 and 12 cm were low or approximated zero. The CrN and CrP ratios Žatomic. of metabolizable organic matter, determined from the 3y plots of NHq vs. SO42y ŽFig. 3., in4 and PO4 creased with sediment depth, indicating its refractory nature. Vertically integrated production rates of DIC and PO43y were higher at station CZ, 2y while those of NHq reduc4 production and SO4
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3y 2y Fig. 1. Examples of the evolution of DIC, Ca, NHq during closed incubation experiments at station CZ at a 4 , PO4 , dSi and SO4 temperature of 20⬚C. Solid and dashed lines represent the multiple-G model and simple-G model, respectively.
tion rates were lower compared to station F ŽTable 1.. Ca2q consumption appeared only at station CZ at the depth interval of 0᎐2 cm during the closed incubation experiment at 20⬚C. However the production of Ca2q was lower, nearly constant, or consumption even prevailed at a temperature of 20⬚C. The ratio between DIC production and sulfate reduction was lower than the stoichiometry Ž; 2., ranging between 1.7 and 0.8 at station CZ, and 1.4 and 0.2 at station F, respectively, indicating that during intense sulfate re-
duction and the absence of bioturbation in closed incubation experiments the consumption of DIC proceeds. This is due mostly to reverse weathering processes ŽStumm and Morgan, 1996., except in the surficial layer at station CZ at a higher temperature Ž20⬚C.. This is probably due to carbonate precipitation. The impact of temperature ŽFig. 2. on sulfate reduction was significant only in the surficial 0᎐2 cm layer, suggesting that the reactive fraction of sedimentary organic matter degrades mostly in surficial sediment and at the
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3y Fig. 2. Vertical distribution of DIC, Ca2q, NHq and dSi production, and SO42y reduction in sediments from stations F and 4 , PO4 CZ at temperatures of 6, 10 and 20⬚C.
sediment᎐water interface, while at depth the organic matter becomes more refractory with higher atomic CrNrP Ž171:21:1. ratios. The temperature dependence of organic matter decomposition was quantified assuming an Arrhenius rate law in the form: k s AeyE a r RT
Ž4.
where Ea is activation energy, R gas constant, A is a pre-exponential factor and T absolute tem3y perature Ž⬚K.. The slopes of DIC, NHq 4 and PO4 production or reduction rates vs. absolute temperature yield apparent Ea values for processes ranging between 45 and 140 kJrmol. Cs and k were used to calculate Ea ŽEq. Ž4.. and Gibbs free energy Ž ⌬G, Eq. Ž5.. for silica dissolution rate in each case ŽLawson et al., 1978.. Cs s ey⌬ G r RT
Ž5.
The average ⌬G for silica dissolution was 17
kJrmol, indicating that the decomposition of phytoplanktonic assemblages is the primary source of the dSi ŽLawson et al., 1978.. The Ea determined for NHq 4 and dSi production are similar to those found for benthic fluxes in the Gulf of Trieste ŽBertuzzi et al., 1996. and production in other aquatic environments ŽAller and Yingst, 1988; Mackin and Swider, 1989.. 3.2. Benthic fluxes The temporal variations in solute concentrations from incubated flux chambers at station F 2y Ž are presented in Fig. 4. O 2 , NOy not 3 and SO4 presented. decreased significantly during incuba3y tions. DIC, NHq concentrations in 4 and PO4 chamber waters always increased, but a marked increase was noticeable during the anoxic phase. During incubation experiments in March 1996 we observed a decrease of Ca2q concentrations, while in September 1996 Ca2q concentrations in oxic conditions decreased but increased in anoxic con-
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Table 1 3y Anoxic production of DIC, Ca2q, NHq and dSi, and SO42y reduction rates at stations CZ and F, and benthic fluxes of DIC, 4 , PO4 3y Ca2q, NHq and PO at station F in the Gulf of Trieste 4 4 Production rates Žintegrated over 12 cm. Žmmolr m2 day.
Benthic fluxes Žmmolr m2 day.
CZ T Ž⬚C. DIC Ca2q NHq 4 PO43y dSi SO42y
6 4.31 0.49 1.32 0.011 0.27 y2.08
F 10 6.45 0.91 1.07 0.025 0.31 y3.27
20 10.64 0.52 1.86 0.039 0.98 y6.40
6 3.36 14.60 0.55 0.003 0.26 y5.96
10 7.20 6.50 2.18 0.016 0.35 y6.91
ditions. The measured benthic fluxes are presented in Table 1. The benthic fluxes of all other
Oxic 20 8.63 3.77 3.51 0.025 0.59 y8.05
10 5.02 y44.70 0.28 0.02
Anoxic 10 y3.98 1.14 0.06
Oxic
Anoxic
20 10.06 4.08 0.52 0.02
20 5.07 y7.73 2.28 0.04
studied species were higher in September 1996 compared to March 1996.
3y Fig. 3. Plots of DIC, NHq vs. SO42y at station CZ. 4 and PO4
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2q 3y Fig. 4. Examples of the evolution of O 2 , NOy , NHq in the overlying water during sediment core 3 , DIC, Ca 4 and PO4 incubations at station F in August 1996. Concentrations shown are those obtained after corrections were made for input of the ‘refill’ water. Closed and open points represent oxic and anoxic phase of the experiment, respectively.
4. Discussion The anoxic production of DIC is important to understand the anoxic mineralization of sedimentary organic matter in shallow coastal waters, such as in the Gulf of Trieste, where degradation proceeds using various electron acceptors ŽHines et al., 1997.. In these anoxic, high carbonate sediments ŽOgorelec et al., 1991. irrigation by benthic infauna ŽCermelj et al., 1997. can drastically influence the pathways of organic matter decomposition by pumping oxygenated water into
burrows and mixing oxidized surficial sediment into anoxic subsurface layers. Ca2q, monitored to test the impact of carbonate dissolution and precipitation on DIC production, can be affected by carbonate dissolution, which is in accordance with the general undersaturation of pore waters in these sediments with respect to calcite and aragonite ŽOgrinc et al., in preparation., precipitation, reverse weathering and exchange with clay minerals. Ca2q produced from the dissolution of carbonates compensates the sinks of Mg 2q due to exchange reactions with clays ŽFig. 5.. Montmoril-
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lonite especially, present in our sediments ŽOgorelec et al., 1991., has a high cation exchange capacity ŽManheim, 1976; von Breymann et al., 1990; Stumm and Morgan, 1996.. Precipitation observed in the surficial layer at station CZ at a temperature of 20⬚C in concomitance with increasing sulfate reduction Žlower sulfate concentrations . is evident from the correlation between Ca2q and SO42y concentrations ŽFig. 5.. Because quantitative discrimination between different processes affecting Ca2q behavior in our sediments is almost impossible, we did not consider this in the DIC production rates. The summer benthic Ca2q efflux indicates the dissolution of carbonates due to the acid produced during the degradation of sedimentary organic matter, especially by sulfuric acid as an oxidation product of sulfides produced by intensive sulfate reduction at higher temperatures ŽHines et al., 1997.. The estimated contribution of carbonate dissolution from Ca2q efflux to DIC benthic flux amounts to approximately 40%. In anoxic conditions and in winter oxic conditions, the high influx of Ca2q could be due to precipitation of carbonates according to the general supersaturation of supernatant waters at the sediment᎐water interface with respect to calcite and aragonite ŽOgrinc et al., in preparation., adsorption on mineral sur-
faces ŽVan Cappellen et al., 1993. and Ca2q exchange in clay minerals ŽManheim, 1976; von Breymann et al., 1990; Stumm and Morgan, 1996.. Because of difficulties in quantification of the impact of carbonate precipitation and dissolution we used uncorrected DIC fluxes in further comparisons. This is justified by recent measurements of 13 C DIC benthic fluxes revealing that the DIC benthic fluxes originates mainly from the degradation of sedimentary organic matter ŽOgrinc et al., in preparation.. The comparison between anoxic DIC production and DIC benthic fluxes ŽTable 1. revealed lower production, probably because the mineralization proceeds mostly at the sediment᎐water interface, and also because other electron acceptors besides sulfate are involved in this process ŽHines et al., 1997.. Sulfate reduction, estimated from anoxic incubation experiments, revealed rates similar in magnitude to those directly measured previously in surficial sediment Ž0᎐12-cm thick layer. at station F using 35 SO42y ŽHines et al., 1997.. Considering the total reduced S burial flux of approximately 0.01 mol Srm2 year, estimated from the mean content of total reduced S ŽTRS. of approximately 0.05% ŽHines et al., 1997. and sediment accumulation rate ŽFaganeli et al., 1991., and the mean sulfate reduction rate of approximately 7
Fig. 5. Plots of Ca2q vs. SO42y at a depth of 0᎐2 cm, and y⌬SO42y vs. yŽ ⌬Ca q ⌬Mg. at a depth of 10᎐12 cm at station CZ, where ⌬ is the difference between concentration at start Ž t s 0. of the experiments and that at time t.
B. Cermelj et al. r The Science of the Total En¨ ironment 266 (2001) 143᎐152
mmolrm2 day, only approximately 1% of the reduced S species are preserved as solid phase S in sediment. NHq 4 production in the incubation experiments greatly exceeded the benthic fluxes under oxic conditions, while in anoxic conditions the differences were smaller. The ratios between DIC and NHq 4 production in the surficial layer were similar to the Redfield ratio while in deeper layers they were similar to DICr NHq 4 ratios in benthic fluxes in oxic conditions, normally exceeding 20 Žatomic.. These results suggest that NHq 4 produced in sediments is oxidized before it escapes from sediments to the overlying water. The reactions involved are nitrification and denitrification, nitrification and sulfide oxidation, and Mn oxidation and reduction ŽFroelich et al., 1979.. The final production of N2 of 1.5᎐3.0 mmolrm2 day is similar to the values calculated by Bertuzzi et al. Ž1996. at station AA1, situated in the vicinity of station CZ, using Redfield stoichiometry for the oxic degradation of degradable sedimentary organic matter. The phosphate production rates were similar to PO43y benthic fluxes in oxic conditions but lower than those in anoxic conditions, suggesting that the PO43y formed from decomposing organic P in the closed anoxic incubation experiment is probably precipitated and adsorbed onto carbonates ŽCermelj et al., 1997.. In the anoxic phase of the benthic flux experiment a considerable amount of PO43y is mobilized from dissolving Fe phosphates and released to the overlying water. The temperature-dependent production rates of dSi, mostly observed in the surficial layer, showed that, in the colder period of the year, the dissolution of biogenic silica is about three-fold lower than in the warmer part of the year. This is not in accordance with the depth distribution of biogenic Si ŽCermelj et al., 1997., indicating that dSi is also influenced by other silicate minerals present in our sediments. The calculated Ea for dSi production is similar to that obtained for dissolution of various siliceous organisms Ždiatoms, radiolarians. reported by Lawson et al. Ž1978.; however, the concentrations of dSi were always much lower than the solubility of amorphous Si in sea water Ž Cmw .. This demonstrates
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that biogenic Si is not the unique source of dSi in our sediments. The low CsrCmw ratios observed suggest that other Si, probably clay, minerals were involved. Considering the low CsrCmw ratios, we can expect a larger impact of clay minerals on concentrations of dSi at higher temperatures and lower sediment depths. However, comparison of dSi production rates with dSi benthic fluxes in the nearby AA1 station ŽBertuzzi et al., 1996. shows good agreement, indicating that temperature should be the main factor governing the production and benthic flux of dSi in these sediments.
5. Conclusions The depth profiles of production rates of DIC, 3y NHq and dSi, and reduction rates of SO42y 4 , PO4 revealed the highest rates in the surficial sediment layer, decreasing with depth, indicating that the largest fraction of mineralization of sedimentary debris in the Gulf of Trieste occurs in the surficial layer and at the sediment᎐water interface. The measured production of Ca2q indicates that Ca2q is also involved in reverse weathering processes and exchange reactions in clays. The consumption of DIC is probably also influenced by reverse weathering processes and carbonate precipitation. Comparison between anoxic DIC production and benthic fluxes showed higher DIC benthic fluxes, probably because most mineralization proceeds at the sediment᎐water interface, and also because other electron acceptors besides sulfate are involved. Hence, the DIC production in the anoxic incubation experiments cannot quantify the organic matter decomposition in high carbonate and clayey sediments having an oxic sediment᎐water interface. The measurement of the reduction rates of various terminal electron acceptors in respiration is needed to assess the real biological benthic DIC production. Greater NHq 4 production rates compared to oxic benthic fluxes suggest that the majority of NHq 4 is oxidized before escaping to the overlying water. Phosphate production rates, although similar to oxic benthic fluxes, were lower than anoxic benthic fluxes due to the precipitation and adsorption onto carbonates in the closed incubation experi-
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ments. Biogenic Si is not a unique source of dSi in the closed incubation experiments. In deeper sediment layers and at higher temperatures, other Si minerals, especially clays, are involved. References Aller RC, Yingst JY. Relationship between microbial distribution and the anaerobic composition of organic matter in surface sediments of Long Island Sound. Mar Biol 1988;56:29᎐42. Bertuzzi A, Faganeli J, Brambati A. Annual variation of benthic nutrient fluxes in shallow coastal waters ŽGulf of Trieste, northern Adriatic Sea.. PSZN I: Mar Ecol 1996;17:261᎐278. Bertuzzi A, Faganeli J, Welker C, Brambati A. Benthic fluxes of dissolved inorganic carbon, nutrients and oxygen in the Gulf of Trieste Žnorthern Adriatic.. Water Air Soil Pollut 1997;99:305᎐314. Burdige DJ. The kinetics of organic matter mineralization in anoxic marine sediments. J Mar Res 1991;49:727᎐761. Cermelj B, Bertuzzi A, Faganeli J. Modelling of pore water nutrient distribution and benthic fluxes in shallow coastal waters ŽGulf of Trieste, northern Adriatic.. Water Air Soil Pollut 1997;99:435᎐444. Edmond JM. High precision determination of titration alkalinity and total carbon dioxide content of sea water by potentiometric titration. Deep-Sea Res 1970;17:737᎐750. Emerson S, Hedges JI. Processes controlling the organic carbon content of open ocean sediments. Paleoceanography 1988;3:621᎐634. Faganeli J, Planinc R, Pezdic J, Smodis B, Stegnar P, Ogorelec B. Marine geology of the Gulf of Trieste Žnorthern Adriatic.: geochemical aspects. Mar Geol 1991;99:93᎐108. Froelich PN, Klinkhammer GP, Bender ML et al. Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochim Cosmochim Acta 1979;43:1075᎐1090. Grasshoff K, Ehrhardt M, Kremling K. Methods of Seawater Analysis, 2. Weinheim, Deerfield Beach FA, Basel: Verlag Chemie, 1983. 419 pp. Hedges JI, Keil RG. Sedimentary organic matter preservation: an assessment and speculative synthesis. Mar Chem 1995; 49:81᎐115. Herndl GJ, Peduzzi P, Fanuko N. Benthic community metabolism and microbial dynamics in the Gulf of Trieste Žnorthern Adriatic Sea.. Mar Ecol Progr Ser 1989;53: 169᎐178.
Hines ME, Faganeli J, Planinc R. Sedimentary anaerobic microbial biogeochemistry in the Gulf of Trieste, northern Adriatic Sea: influences of bottom water oxygen depletion. Biogeochemistry 1997;39:1᎐22. Krom MD, Berner RA. Adsorption of phosphate in anoxic marine sediments. Limnol Oceanogr 1980;25:797᎐806. Lawson SD, Hurd DC, Stuart Pankratz H. Silica dissolution rates of decomposing phytoplankton assemblages at various temperatures. Am J Sci 1978;278:1373᎐1393. Lerman A. Geochemical Processes: Water and Sediment Environments. New York, Chicester, Brisbane, Toronto: John Wiley & Sons, 1979. 479 pp. Li YH, Gregory S. Diffusion of ions in sea water and in deep-sea sediments. Geochim Cosmochim Acta 1974; 38:703᎐714. Mackin JE, Aller RC. Ammonium adsorption in marine sediments. Limnol Oceanogr 1984;29:250᎐257. Mackin JE, Swider KT. Organic matter decomposition pathways and oxygen consumption in coastal marine sediments. J Mar Res 1989;47:681᎐716. Manheim FT. Interstitial waters of marine sediments. In: Riley JP, Chester R, editors. Chemical Oceanography, 2. London, New York, San Francisco: Academic Press, 1976:115᎐186. Millero FJ. Thermodynamics of the carbon dioxide system in the oceans. Geochim Cosmochim Acta 1995;59:661᎐677. Ogorelec B, Misic M, Faganeli J. Marine geology of the Gulf of Trieste Žnorthern Adriatic.: sedimentological aspects. Mar Geol 1991;99:79᎐92. Rizzo WW. Nutrient exchange between the water column and a subtidal benthic microalgal community. Estuaries 1990;13:219᎐226. Stumm W, Morgan JJ. Aquatic Chemistry, 3. New York, Chicester, Brisbane, Toronto, Singapore: John Wiley & Sons Inc, 1996. 1022 pp. Tabatabai MA. A rapid method for determination of sulfate in water samples. Environ Lett 1974;7:237᎐243. Van Cappellen P, Charlet L, Stumm W, Wersin P. A surface complexation model of the carbonate mineral᎐aqueous solution interface. Geochim Cosmochim Acta 1993; 57:3505᎐3518. von Breymann MT, Collier R, Suess E. Magnesium adsorption and ion exchange in marine sediments: a multi-component model. Geochim Cosmochim Acta 1990;54:3295᎐3313. Westrich JT, Berner RA. The role of sedimentary organic matter in bacterial sulfate reduction: the G model tested. Limnol Oceanogr 1984;29:236᎐249.
Anoxic mineralization of biogenic debris in near-shore marine sediments ž Gulf of Trieste, northern Adriatic/ B. Cermelja,U , N. Ogrinc b, J. Faganeli a a
Marine Biological Station, Fornace 41, 6330 Piran, Slo¨ enia Jozef Stefan Institute, Jamo¨ a 39, 1000, Ljubljana, Slo¨ enia
b
Received 17 September 1999; accepted 28 June 2000
Abstract Anoxic degradation of sedimentary biogenic debris using closed incubation experiments was studied at two sampling stations in the Gulf of Trieste Žnorthern Adriatic.. Production rates of dissolved inorganic carbon ŽDIC., 3y NHq and dissolved Si ŽdSi., and reduction rates of SO42y were measured and anoxic mineralization rates were 4 , PO4 modeled using a first order G-model and multi-G approach. The depth profiles of these rates revealed an exponential decrease indicating that the largest fraction of mineralization of biogenic debris and SO42y reduction occurs in the surficial sediment layer and on the sediment surface. Comparing the depth-integrated anoxic mineralization rates at 3y and dSi measured at the in situ temperature in the dark, it both stations with benthic fluxes of DIC, NHq 4 , PO4 appears that the DIC and PO43y fluxes are higher because the mineralization mostly occurs at the sediment᎐water interface, and that besides SO42y reduction, other electron acceptors are involved in the organic matter decomposiq tion pathway in these surficial sediments. The NHq 4 production was higher than the benthic fluxes because of NH 4 oxidation. The production of dSi was in good agreement with benthic fluxes implying that temperature is the main factor of dSi production and benthic fluxes in these sediments. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Mineralization; Sediments; Benthic fluxes; Carbon; Nitrogen; Phosphorus; Silicon; Northern Adriatic
U
Corresponding author. Tel.: q386-5-6745306;fax: q386-5-6746367. E-mail address: [email protected] ŽB. Cermelj.. 0048-9697r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 0 . 0 0 7 4 1 - 5
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1. Introduction
2. Materials and methods
Organic matter degradation and burial in marine sediments represents an important step in the global carbon cycle ŽEmerson and Hedges, 1988.. In continental shelves and coastal areas, a large fraction of depositing organic material escapes degradation within the water column, with the result that easily degradable substrate is available at the sediment᎐water interface ŽHedges and Keil, 1995.. In shallow coastal waters, where the euphotic zone reaches the sea bottom, the benthic macro- and microalgae may be a significant source of easily degradable sedimentary organic matter ŽRizzo, 1990.. An improved knowledge of the biogeochemical processes in coastal marine areas requires better information about the sources, transformations, recycling and burial of biogenic debris in sediments. Oxygen can influence the organic carbon mineralization in sediments if the penetration of dissolved oxygen is deep enough and enhanced by bioturbation activity of benthic fauna. Since oxygen typically penetrates only a few millimeters in coastal sediments, other electron acceptors are also involved in degradation of sedimentary organic matter. Although the euphotic zone reaches the benthos throughout the Gulf of Trieste Žnorthern Adriatic., the benthic O 2 consumption exceeds benthic primary production ŽHerndl et al., 1989., and anaerobic processes probably account for the majority of organic matter decomposition in these coastal sediments ŽHines et al., 1997.. The role of benthic macrofaunal bioturbation and irrigation on benthic nutrient biogeochemistry is important in these sediments, especially in summertime, leading to benthic fluxes which exceed diffusive fluxes by up to 10-fold ŽCermelj et al., 1997.. Previous studies have demonstrated that nutrients released from pore waters could be mostly utilized in benthic microalgal production ŽBertuzzi et al., 1997.. The aim of the present study was: Ž1. to determine the anoxic sedimentary organic matter decomposition pathway in the Gulf of Trieste using closed sediment incubation experiments; and Ž2. to compare these reactions rates with measured benthic fluxes.
2.1. Closed sediment incubations Sediment samples were collected at sampling stations CZ Ž13⬚ 37.998⬘ E, 45⬚ 37.398⬘ N. and F Ž13⬚ 33⬘ E, 45⬚ 32.298⬘ N. in the central and southern part of the Gulf of Trieste, respectively. Virtually undisturbed sediment cores were taken to a depth of 15 cm at station CZ in August 1996 and at station F in May 1996 by scuba divers inserting a Plexiglas tube Ž6 cm i.d.. directly into the sediment. Sediment samples from three sediment layers Ž0᎐2, 4᎐6 and 10᎐12 cm. were gently homogenized, packed into 50-ml polyethylene centrifuge tubes Žusually 8 per layer. and incubated Ž120 days at station F and 200 days at station CZ. in the dark at temperatures of 6, 10 and 20⬚C. No additional water was added. One tube from each series was removed approximately every 25 days and centrifuged at 7000 rev.rmin for 15 min. The supernatant was sampled under N2 , filtered through 0.45-m pore size Millipore HA membrane filters and analyzed for solutes as described below. The concentration changes in the supernatant waters through time were used to obtain the production rates, corrected for porosity ŽCermelj et al., 1997.. Depth-integrated production rates were obtained by vertically integrating rates to a depth of 12 cm Žthis was the deepest layer used in benthic flux experiments in the Gulf.. The closed incubation technique used here is similar to that presented by Aller and Yingst Ž1988., and Mackin and Swider Ž1989.. 2.2. Core incubations In March and August 1996 benthic fluxes were estimated by incubating surficial sediment from station F in the laboratory at the in situ temperatures in the dark. Acrylic box cores Ž25 cm height, 30 cm i.d., 0.6 cm wall thickness . were inserted into the sediment to a depth of approximately 10 cm by a scuba diver. In the laboratory, the water
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overlying each core was drained using an aspirator. Sediment in the core was successively filled with the filtered seawater collected in the bottom water layer at the same location. The cores were used as an incubation flux chamber to measure benthic fluxes. Samples were collected periodically and the volume of water withdrawn was balanced by bottom seawater that exchanged with the chamber water through a Tygon tube. Fluxes of solutes across the sediment᎐water interface were evaluated from linear regressions of solute concentrations over time. 2.3. Analyses Alkalinity in water samples was measured by Gran titration ŽEdmond, 1970.. DIC was calculated from alkalinity, the in situ temperature-corrected pH, and salinity ŽMillero, 1995.. Nitrate, ammonium, phosphate and silicate were measured photometrically ŽGrasshoff et al., 1983., and sulfate turbidimetrically ŽTabatabai, 1974.. Dissolved Ca2q and Mg 2q were determined by flame AAS.
3. Results 3.1. Production rates Productionrreduction rates for DIC, NHq 4, PO43y and SO42y ŽFig. 1. during anoxic incubation experiments were calculated as the slope of the solute vs. incubation time using the G-model: Gm ,i Ž t . s Gm ,i ,0 w exp Ž yk i t .x
Ž1.
ŽWestrich and Berner, 1984; Burdige, 1991., where t is time, Gm, i is the concentration of metabolizable organic carbon, nitrogen or phosphorus and k Ž1ryear. is the first-order rate constant. NHq 4 and PO43y production rates were corrected for adsorption to sediment solids. Adsorption coefficients Ž K . after adjusting for porosity were ; 1.3 Ž . for NHq 4 Mackin and Aller, 1984 and ; 1.2 for PO43y ŽKrom and Berner, 1980., respectively. For
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DIC, and SO42y, Ks 0 was assumed for calculation of productionrreduction rates from anoxic incubation results ŽLi and Gregory, 1974.. Using the multiple-G model: Gm ,i Ž t . s Gm ,i ,1 w exp Ž yk i ,1 t .x q Gm ,i ,2 w exp Ž yk i ,2 t .x q GN R
Ž2.
Žsee Westrich and Berner, 1984; Burdige, 1991. it was possible to better explain the differences in the mineralization rates in surficial sediment at station CZ at 20⬚C ŽFig. 1.. The sedimentary organic matter consists of two reactive fractions Ž Gm,1 and Gm,2 . with lower and higher CrN and CrP ratios, respectively. Approximately 80% of the NHq 4 and 30% of the DIC produced originate from the more reactive fraction, while the production of PO43y is affected by adsorption and precipitation. Changes in dSi concentrations ŽFig. 2. were determined using the following equation: dCrdt s k Ž Cs y C .
Ž3.
ŽLerman, 1979., where Cs is the solubility of the dissolving phase, and k Ž1ryear. is the reaction rate constant. If Cs ) C dissolution occurs, and if Cs - C precipitation occurs. By solving Eq. Ž3. and fitting the data to the analytical solution we obtained the values for the dissolution rate constant Ž k . and the solubility Ž Cs .. 3y The production rates of DIC, NHq and 4 , PO4 dSi were highest close to the sediment᎐water interface at both stations and decreased exponentially with depth ŽFig. 2., indicating the depletion of reactive biogenic debris with depth in sediments. The rates calculated at a sediment depth between 10 and 12 cm were low or approximated zero. The CrN and CrP ratios Žatomic. of metabolizable organic matter, determined from the 3y plots of NHq vs. SO42y ŽFig. 3., in4 and PO4 creased with sediment depth, indicating its refractory nature. Vertically integrated production rates of DIC and PO43y were higher at station CZ, 2y while those of NHq reduc4 production and SO4
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3y 2y Fig. 1. Examples of the evolution of DIC, Ca, NHq during closed incubation experiments at station CZ at a 4 , PO4 , dSi and SO4 temperature of 20⬚C. Solid and dashed lines represent the multiple-G model and simple-G model, respectively.
tion rates were lower compared to station F ŽTable 1.. Ca2q consumption appeared only at station CZ at the depth interval of 0᎐2 cm during the closed incubation experiment at 20⬚C. However the production of Ca2q was lower, nearly constant, or consumption even prevailed at a temperature of 20⬚C. The ratio between DIC production and sulfate reduction was lower than the stoichiometry Ž; 2., ranging between 1.7 and 0.8 at station CZ, and 1.4 and 0.2 at station F, respectively, indicating that during intense sulfate re-
duction and the absence of bioturbation in closed incubation experiments the consumption of DIC proceeds. This is due mostly to reverse weathering processes ŽStumm and Morgan, 1996., except in the surficial layer at station CZ at a higher temperature Ž20⬚C.. This is probably due to carbonate precipitation. The impact of temperature ŽFig. 2. on sulfate reduction was significant only in the surficial 0᎐2 cm layer, suggesting that the reactive fraction of sedimentary organic matter degrades mostly in surficial sediment and at the
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3y Fig. 2. Vertical distribution of DIC, Ca2q, NHq and dSi production, and SO42y reduction in sediments from stations F and 4 , PO4 CZ at temperatures of 6, 10 and 20⬚C.
sediment᎐water interface, while at depth the organic matter becomes more refractory with higher atomic CrNrP Ž171:21:1. ratios. The temperature dependence of organic matter decomposition was quantified assuming an Arrhenius rate law in the form: k s AeyE a r RT
Ž4.
where Ea is activation energy, R gas constant, A is a pre-exponential factor and T absolute tem3y perature Ž⬚K.. The slopes of DIC, NHq 4 and PO4 production or reduction rates vs. absolute temperature yield apparent Ea values for processes ranging between 45 and 140 kJrmol. Cs and k were used to calculate Ea ŽEq. Ž4.. and Gibbs free energy Ž ⌬G, Eq. Ž5.. for silica dissolution rate in each case ŽLawson et al., 1978.. Cs s ey⌬ G r RT
Ž5.
The average ⌬G for silica dissolution was 17
kJrmol, indicating that the decomposition of phytoplanktonic assemblages is the primary source of the dSi ŽLawson et al., 1978.. The Ea determined for NHq 4 and dSi production are similar to those found for benthic fluxes in the Gulf of Trieste ŽBertuzzi et al., 1996. and production in other aquatic environments ŽAller and Yingst, 1988; Mackin and Swider, 1989.. 3.2. Benthic fluxes The temporal variations in solute concentrations from incubated flux chambers at station F 2y Ž are presented in Fig. 4. O 2 , NOy not 3 and SO4 presented. decreased significantly during incuba3y tions. DIC, NHq concentrations in 4 and PO4 chamber waters always increased, but a marked increase was noticeable during the anoxic phase. During incubation experiments in March 1996 we observed a decrease of Ca2q concentrations, while in September 1996 Ca2q concentrations in oxic conditions decreased but increased in anoxic con-
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Table 1 3y Anoxic production of DIC, Ca2q, NHq and dSi, and SO42y reduction rates at stations CZ and F, and benthic fluxes of DIC, 4 , PO4 3y Ca2q, NHq and PO at station F in the Gulf of Trieste 4 4 Production rates Žintegrated over 12 cm. Žmmolr m2 day.
Benthic fluxes Žmmolr m2 day.
CZ T Ž⬚C. DIC Ca2q NHq 4 PO43y dSi SO42y
6 4.31 0.49 1.32 0.011 0.27 y2.08
F 10 6.45 0.91 1.07 0.025 0.31 y3.27
20 10.64 0.52 1.86 0.039 0.98 y6.40
6 3.36 14.60 0.55 0.003 0.26 y5.96
10 7.20 6.50 2.18 0.016 0.35 y6.91
ditions. The measured benthic fluxes are presented in Table 1. The benthic fluxes of all other
Oxic 20 8.63 3.77 3.51 0.025 0.59 y8.05
10 5.02 y44.70 0.28 0.02
Anoxic 10 y3.98 1.14 0.06
Oxic
Anoxic
20 10.06 4.08 0.52 0.02
20 5.07 y7.73 2.28 0.04
studied species were higher in September 1996 compared to March 1996.
3y Fig. 3. Plots of DIC, NHq vs. SO42y at station CZ. 4 and PO4
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2q 3y Fig. 4. Examples of the evolution of O 2 , NOy , NHq in the overlying water during sediment core 3 , DIC, Ca 4 and PO4 incubations at station F in August 1996. Concentrations shown are those obtained after corrections were made for input of the ‘refill’ water. Closed and open points represent oxic and anoxic phase of the experiment, respectively.
4. Discussion The anoxic production of DIC is important to understand the anoxic mineralization of sedimentary organic matter in shallow coastal waters, such as in the Gulf of Trieste, where degradation proceeds using various electron acceptors ŽHines et al., 1997.. In these anoxic, high carbonate sediments ŽOgorelec et al., 1991. irrigation by benthic infauna ŽCermelj et al., 1997. can drastically influence the pathways of organic matter decomposition by pumping oxygenated water into
burrows and mixing oxidized surficial sediment into anoxic subsurface layers. Ca2q, monitored to test the impact of carbonate dissolution and precipitation on DIC production, can be affected by carbonate dissolution, which is in accordance with the general undersaturation of pore waters in these sediments with respect to calcite and aragonite ŽOgrinc et al., in preparation., precipitation, reverse weathering and exchange with clay minerals. Ca2q produced from the dissolution of carbonates compensates the sinks of Mg 2q due to exchange reactions with clays ŽFig. 5.. Montmoril-
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lonite especially, present in our sediments ŽOgorelec et al., 1991., has a high cation exchange capacity ŽManheim, 1976; von Breymann et al., 1990; Stumm and Morgan, 1996.. Precipitation observed in the surficial layer at station CZ at a temperature of 20⬚C in concomitance with increasing sulfate reduction Žlower sulfate concentrations . is evident from the correlation between Ca2q and SO42y concentrations ŽFig. 5.. Because quantitative discrimination between different processes affecting Ca2q behavior in our sediments is almost impossible, we did not consider this in the DIC production rates. The summer benthic Ca2q efflux indicates the dissolution of carbonates due to the acid produced during the degradation of sedimentary organic matter, especially by sulfuric acid as an oxidation product of sulfides produced by intensive sulfate reduction at higher temperatures ŽHines et al., 1997.. The estimated contribution of carbonate dissolution from Ca2q efflux to DIC benthic flux amounts to approximately 40%. In anoxic conditions and in winter oxic conditions, the high influx of Ca2q could be due to precipitation of carbonates according to the general supersaturation of supernatant waters at the sediment᎐water interface with respect to calcite and aragonite ŽOgrinc et al., in preparation., adsorption on mineral sur-
faces ŽVan Cappellen et al., 1993. and Ca2q exchange in clay minerals ŽManheim, 1976; von Breymann et al., 1990; Stumm and Morgan, 1996.. Because of difficulties in quantification of the impact of carbonate precipitation and dissolution we used uncorrected DIC fluxes in further comparisons. This is justified by recent measurements of 13 C DIC benthic fluxes revealing that the DIC benthic fluxes originates mainly from the degradation of sedimentary organic matter ŽOgrinc et al., in preparation.. The comparison between anoxic DIC production and DIC benthic fluxes ŽTable 1. revealed lower production, probably because the mineralization proceeds mostly at the sediment᎐water interface, and also because other electron acceptors besides sulfate are involved in this process ŽHines et al., 1997.. Sulfate reduction, estimated from anoxic incubation experiments, revealed rates similar in magnitude to those directly measured previously in surficial sediment Ž0᎐12-cm thick layer. at station F using 35 SO42y ŽHines et al., 1997.. Considering the total reduced S burial flux of approximately 0.01 mol Srm2 year, estimated from the mean content of total reduced S ŽTRS. of approximately 0.05% ŽHines et al., 1997. and sediment accumulation rate ŽFaganeli et al., 1991., and the mean sulfate reduction rate of approximately 7
Fig. 5. Plots of Ca2q vs. SO42y at a depth of 0᎐2 cm, and y⌬SO42y vs. yŽ ⌬Ca q ⌬Mg. at a depth of 10᎐12 cm at station CZ, where ⌬ is the difference between concentration at start Ž t s 0. of the experiments and that at time t.
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mmolrm2 day, only approximately 1% of the reduced S species are preserved as solid phase S in sediment. NHq 4 production in the incubation experiments greatly exceeded the benthic fluxes under oxic conditions, while in anoxic conditions the differences were smaller. The ratios between DIC and NHq 4 production in the surficial layer were similar to the Redfield ratio while in deeper layers they were similar to DICr NHq 4 ratios in benthic fluxes in oxic conditions, normally exceeding 20 Žatomic.. These results suggest that NHq 4 produced in sediments is oxidized before it escapes from sediments to the overlying water. The reactions involved are nitrification and denitrification, nitrification and sulfide oxidation, and Mn oxidation and reduction ŽFroelich et al., 1979.. The final production of N2 of 1.5᎐3.0 mmolrm2 day is similar to the values calculated by Bertuzzi et al. Ž1996. at station AA1, situated in the vicinity of station CZ, using Redfield stoichiometry for the oxic degradation of degradable sedimentary organic matter. The phosphate production rates were similar to PO43y benthic fluxes in oxic conditions but lower than those in anoxic conditions, suggesting that the PO43y formed from decomposing organic P in the closed anoxic incubation experiment is probably precipitated and adsorbed onto carbonates ŽCermelj et al., 1997.. In the anoxic phase of the benthic flux experiment a considerable amount of PO43y is mobilized from dissolving Fe phosphates and released to the overlying water. The temperature-dependent production rates of dSi, mostly observed in the surficial layer, showed that, in the colder period of the year, the dissolution of biogenic silica is about three-fold lower than in the warmer part of the year. This is not in accordance with the depth distribution of biogenic Si ŽCermelj et al., 1997., indicating that dSi is also influenced by other silicate minerals present in our sediments. The calculated Ea for dSi production is similar to that obtained for dissolution of various siliceous organisms Ždiatoms, radiolarians. reported by Lawson et al. Ž1978.; however, the concentrations of dSi were always much lower than the solubility of amorphous Si in sea water Ž Cmw .. This demonstrates
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that biogenic Si is not the unique source of dSi in our sediments. The low CsrCmw ratios observed suggest that other Si, probably clay, minerals were involved. Considering the low CsrCmw ratios, we can expect a larger impact of clay minerals on concentrations of dSi at higher temperatures and lower sediment depths. However, comparison of dSi production rates with dSi benthic fluxes in the nearby AA1 station ŽBertuzzi et al., 1996. shows good agreement, indicating that temperature should be the main factor governing the production and benthic flux of dSi in these sediments.
5. Conclusions The depth profiles of production rates of DIC, 3y NHq and dSi, and reduction rates of SO42y 4 , PO4 revealed the highest rates in the surficial sediment layer, decreasing with depth, indicating that the largest fraction of mineralization of sedimentary debris in the Gulf of Trieste occurs in the surficial layer and at the sediment᎐water interface. The measured production of Ca2q indicates that Ca2q is also involved in reverse weathering processes and exchange reactions in clays. The consumption of DIC is probably also influenced by reverse weathering processes and carbonate precipitation. Comparison between anoxic DIC production and benthic fluxes showed higher DIC benthic fluxes, probably because most mineralization proceeds at the sediment᎐water interface, and also because other electron acceptors besides sulfate are involved. Hence, the DIC production in the anoxic incubation experiments cannot quantify the organic matter decomposition in high carbonate and clayey sediments having an oxic sediment᎐water interface. The measurement of the reduction rates of various terminal electron acceptors in respiration is needed to assess the real biological benthic DIC production. Greater NHq 4 production rates compared to oxic benthic fluxes suggest that the majority of NHq 4 is oxidized before escaping to the overlying water. Phosphate production rates, although similar to oxic benthic fluxes, were lower than anoxic benthic fluxes due to the precipitation and adsorption onto carbonates in the closed incubation experi-
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ments. Biogenic Si is not a unique source of dSi in the closed incubation experiments. In deeper sediment layers and at higher temperatures, other Si minerals, especially clays, are involved. References Aller RC, Yingst JY. Relationship between microbial distribution and the anaerobic composition of organic matter in surface sediments of Long Island Sound. Mar Biol 1988;56:29᎐42. Bertuzzi A, Faganeli J, Brambati A. Annual variation of benthic nutrient fluxes in shallow coastal waters ŽGulf of Trieste, northern Adriatic Sea.. PSZN I: Mar Ecol 1996;17:261᎐278. Bertuzzi A, Faganeli J, Welker C, Brambati A. Benthic fluxes of dissolved inorganic carbon, nutrients and oxygen in the Gulf of Trieste Žnorthern Adriatic.. Water Air Soil Pollut 1997;99:305᎐314. Burdige DJ. The kinetics of organic matter mineralization in anoxic marine sediments. J Mar Res 1991;49:727᎐761. Cermelj B, Bertuzzi A, Faganeli J. Modelling of pore water nutrient distribution and benthic fluxes in shallow coastal waters ŽGulf of Trieste, northern Adriatic.. Water Air Soil Pollut 1997;99:435᎐444. Edmond JM. High precision determination of titration alkalinity and total carbon dioxide content of sea water by potentiometric titration. Deep-Sea Res 1970;17:737᎐750. Emerson S, Hedges JI. Processes controlling the organic carbon content of open ocean sediments. Paleoceanography 1988;3:621᎐634. Faganeli J, Planinc R, Pezdic J, Smodis B, Stegnar P, Ogorelec B. Marine geology of the Gulf of Trieste Žnorthern Adriatic.: geochemical aspects. Mar Geol 1991;99:93᎐108. Froelich PN, Klinkhammer GP, Bender ML et al. Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochim Cosmochim Acta 1979;43:1075᎐1090. Grasshoff K, Ehrhardt M, Kremling K. Methods of Seawater Analysis, 2. Weinheim, Deerfield Beach FA, Basel: Verlag Chemie, 1983. 419 pp. Hedges JI, Keil RG. Sedimentary organic matter preservation: an assessment and speculative synthesis. Mar Chem 1995; 49:81᎐115. Herndl GJ, Peduzzi P, Fanuko N. Benthic community metabolism and microbial dynamics in the Gulf of Trieste Žnorthern Adriatic Sea.. Mar Ecol Progr Ser 1989;53: 169᎐178.
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