Material fluxes associated with fluid expulsion at cold seeps and their contribution to oceanographic budgets have not been accurately constrained. Here we present evidence that the barium released at cold seeps along the San Clemente Fault zone may significantly impact the geochemical budget of barium within the basin. Barium fluxes at seep localities on the fault scarp, measured with benthic chambers, reach values as high as 5 mmol m−2 day−1. This is the largest dissolved barium flux measured to date at a cold seep. The discharge of barium-rich fluids results in formation of massive barite deposits along the escarpment wall. The deposits are young (approximately 8 yr) and appear to grow at a minimum rate of 0.2 cm yr−1. This rapid growth rate requires a barium efflux rate that is about two orders of magnitude higher than the measured dissolved flux. We believe that the discrepancy reflects a highly localized seepage system and that chambers positioned as close as possible to the growing chimneys did not sample the foci of fluid discharge. Transport of fine barite particles from the seeps may be responsible for excess rates of barium accumulation throughout the San Clemente Basin, relative to other basins in the California Margin. Based on a preliminary budget, we estimate that cold-seep barite is accumulating at the basin floor in San Clemente at a rate of 2 μmol m−2 day−1, a value that is comparable to the total barium accumulation rates driven by detrital and biogenic components in neighboring basins. Remobilization of cold-seep barite on the basin floor adds to that driven by the biogenic barium flux and results in benthic barium recycling rates (effluxes) within the San Clemente Basin that are as much as seven times higher than the effluxes from surrounding borderland basins. Our estimates imply that processes associated with fluid seepage along the San Clemente Fault significantly contribute to the basin’s barium cycle. The strontium isotopic composition of the seep barite is significantly different from marine ‘biogenic’ barite, which is known to accurately record seawater composition. In addition, the seep deposits are depleted in 226Ra relative to their modern biogenic counterparts, and are likely to be a source of radium-depleted particulate barium to the basin. Thus the impact of barite transport from seeps on the San Clemente escarpment to the basin floor might also have implications for the geochemistry of elements other than barium. 相似文献
Sediment cores retrieved in the Benguela coastal upwelling system off Namibia show very distinct enrichments of solid phase barium at the sulfate/methane transition (SMT). These barium peaks represent diagenetic barite (BaSO4) fronts which form by the reaction of upwardly diffusing barium with interstitial sulfate. Calculated times needed to produce these barium enrichments indicate a formation time of about 14,000 yr. Barium spikes a few meters below the SMT were observed at one of the investigated sites (GeoB 8455). Although this sulfate-depleted zone is undersaturated with respect to barite, the dominant mineral phase of these buried barium enrichments was identified as barite by scanning electron microscopy (SEM). This is the first study which reports the occurrence/preservation of pronounced barite enrichments in sulfate-depleted sediments buried a few meters below the SMT. At site GeoB 8455 high concentrations of dissolved barium in pore water as well as barium in the solid phase were observed. Modeling the measured barium concentrations at site GeoB 8455 applying the numerical model CoTReM reveals that the dissolution rate of barite directly below the SMT is about one order of magnitude higher than at the barium enrichments deeper in the sediment core. This indicates that the dissolution of barite at these deeper buried fronts must be retarded. Thus, the occurrence of the enrichments in solid phase barium at site GeoB 8455 could be explained by decreased dissolution rates of barite due to the changes in the concentration of barite in the sediment, as well as changes in the saturation state of fluids. Furthermore, the alteration of barite into witherite (BaCO3) via the transient phase barium sulfide could lead to the preservation of a former barite front as BaCO3. The calculations and modeling indicate that a relocation of the barite front to a shallower depth occurred between the last glacial maxium (LGM) and the Pleistocene/Holocene transition. We suggest that an upward shift of the SMT occurred at that time, most likely as a result of an increase in the methanogenesis rates due to the burial of high amounts of organic matter below the SMT. 相似文献
Profiles of226Ra and dissolved210Pb have been measured at several stations in the Red Sea. At one station in the central Red Sea an expanded profile was measured including226Ra and dissolved and particulate210Pb and210Po. These profiles show several distinct features: (1)226Ra displays a mid-depth maximum of about 13 dpm/100 kg at about 500 m; (2) dissolved210Pb concentrations are uniformly low at about 2 dpm/100 kg with little lateral or vertical variation; (3) the surface-water210Pb excess which is commonly observed in low-latitude open ocean regions is entirely lacking; (4)210Pb and210Po activities are essentially identical to each other in both particulate and dissolved phases although210Po activities appear somewhat lower; (5) about 20% of the210Pb and210Po in the water column residues on particulate matter.Assuming the atmospheric210Pb flux to be in the dissolved form and at the lower level of the normal range i.e. 0.5 dpm/cm2 yr, the residence time of the dissolved Pb is about 1.5 years. However, if the same atmospheric flux is entirely in particulate form, then the residence time of the dissolved Pb is about 5 years. The residence time of Pb in the particulate phase is less than 0.4 years if all the Pb is removed only by sinking particles. 相似文献
We report here on particulate and dissolved210Pb profiles at 16 stations, and on total210Pb profiles at 3 stations, all occupied during the Pacific GEOSECS expedition. Comparison with measurements at Yale on GEOSECS library samples indicates that during separation of particulate lead from dissolved lead, our filtered water samples suffered some loss of210Pb in the filtration system; this effect appears to have reduced the dissolved210Pb activities by ～ 20% in stations where the water was filtered. However, for these first Pacific data on the210Pb distribution between the two phases, this effect does not significantly interfere with our recognition of the major features of both particulate and dissolved210Pb distributions.The dissolved210Pb profiles in general vary geographically, following the226Ra profiles. In deep water,226Ra increases northward and eastward from the southwest Pacific, from ～ 22dpm/100kg, to over 40 dpm/100 kg in the northeast Pacific. Our dissolved210Pb profiles show a similar increase in deep water, varying from about 10 to 20 dpm/100 kg along this line, and are commonly characterized by a mid-depth maximum. This210Pb maximum reflects the mid-depth226Ra maximum of the Pacific Deep Water observed along the western boundary current.In surface water at low latitudes there is a significant210Pb flux from the atmosphere, which produces a210Pb/226Ra activity ratio generally greater than unity. This flux penetrates as deep as 600 m, as indicated by an “induced”210Pb minimum caused by the surface maximum. The surface water210Pb excess decreases toward high southern latitudes and vanishes in the Circumpolar region.The particulate210Pb profiles show a general increase with depth, from ～ 0.3dpm/100kg in subsurface water to ～ 1.5dpm/100kg in bottom water, with or without a mid-depth maximum that reflects the226Ra or dissolved210Pb maximum. The particulate210Pb normally comprises about 2% of the total210Pb in subsurface water, and this fraction increases to about 10% near the bottom. As the filtration loss is not taken into account, the fraction of particulate210Pb quoted here is an upper limit. Since the particulate matter concentrations are quite uniform in the water column below a few hundred meters, the210Pb activity of the particulate matter also increases with depth. The particulate matter has a210Pb concentration of ～ 100dpm/g in subsurface water, but the concentration increases to ～ 500dpm/g or more toward the bottom. This indicates that there is a cumulative adsorption of Pb onto the suspended particles as they are sinking through the water column. 相似文献
We present 9 bottom222Rn profiles measured from the western and southern Indian Ocean during the 1977–1978 GEOSECS expedition. These profiles can be grouped into three cypes: one-layer, two-layer, and irregular types. The one-layer profiles with quasi-exponential distributions allow one to estimate the apparent vertical eddy diffusivity,Kv, with a simple model. The two-layer profiles show that there is a benthic boundary layer of the order of 50–100 m in which the excess222Rn distribution shows a vertical gradient much smaller than that of the layer immediately above. Within the boundary layer, the STD potential temperature (θ) and density(σ4) profiles are practically constant, and theKv values are of the order of 1000 cm2/s. The STD profiles for the water column above the boundary layer show gradients of increasing stability, and theKv values are of the order of 100 cm2/s. Modeling of the Rn data in the water column above the boundary layer indicates that there is a transition layer which effectively reduces the penetration of excess Rn from the benthic boundary layer into the upper layer.Sarmiento et al. [10] have shown that the buoyancy gradient or stability is inversely correlated with the apparent vertical eddy diffusivity, and the resulting buoyancy flux is fairly uniform, ranging from 1 to 14 × 10?6 cm2/s3 in the Atlantic and Pacific Oceans. However, Sarmiento et al. [11] show that a much higher buoyancy flux is associated with an intensified flow of the bottom water through a passage. In the Indian Ocean basins, we have found that the buoyancy flux has a comparable range (3–14 × 10?6 cm2/s3), except for a couple of stations where both stability and apparent vertical diffusivity are higher, resulting in a much higher buoyancy flux, probably indicative of rapid bottom water flow. 相似文献
Four vertical profiles of230Th and228Th were determined using large volume water samples in the western North Pacific. An almost linear increase of230Th with depth was observed for all of the profiles for which the unidirectional first order scavenging model was difficult to explain. We developed a model which included a dissolved-particulate transformation as well as parameters of the scavenging model. Application of the model to the vertical distributions of total and the GEOSECS particulate Th isotopes (230Th and234Th) yielded the residence time of dissolved Th with respect to adsorption to particles and the turnover time of particulate Th to be 235 days and 57 days, respectively. The Th isotopes appeared to be carried down the water column by fine particles with a mean settling velocity of 1 m/day which continually release Th into sea water as well as pick up Th from the water along their journey to the bottom.For228Th, a large excess over232Th was observed throughout the water column with pronounced high concentrations in surface and bottom waters, suggesting that the228Th was derived from228Ra diffused out of sediments. The vertical distributions of228Th seemed to be significantly influenced by lateral mixing along isopycnals. 相似文献
Vertical eddy diffusivities (Kv's) have been estimated at fourteen widely separated locations from fourteen222Rn profiles and two228Ra profiles measured near the ocean floor as part of the Atlantic and Pacific GEOSECS programs. They show an inverse proportionality to the local buoyancy gradient [(g/?)(??pot/?z)] calculated from hydrographic measurements. The negative of the constant of proportionality is the buoyancy flux [?Kv(g/?)(??pot/?z)] which has a mean of ?4 × 10?6 cm2/sec3. Our results suggest that the buoyancy flux varies very little near the ocean floor. Kv's for the interior of the deep Pacific calculated from the relationship Kv = (4 × 10?6cm2/sec3)/[(g/?)(??pot/?z)] agree well with published estimates. Kv's calculated for the pycnocline are one to two orders of magnitude smaller than upper limits estimated from tritium and7Be distributions.Heat fluxes calculated with the model Kv's obtained from the222Rn profiles average 31 μcal cm?2 sec?1 in the Atlantic Ocean and 8 μcal cm?2 sec?1 in the Pacific Ocean. 相似文献
Total phosphorus and its main forms: dissolved mineral, dissolved organic, particulate organic and particulate mineral in the vertical water column of three subalpine lakes of various types in Italy, has been estimated during the winter-vernal season. The range of variation in the phosphorus content in these waters was as follows: total phosphorus 16 ± 2860 μg/1 PO4, dissolved mineral phosphorus 4 ± 1040 μg/l PO4, dissolved organic phosphorus 1 ± 160μg/l PO4, particulate organic phosphorus 0 ± 290 μg/l PO4 and particulate mineral phosphorus 1 ± 100 μg/l PO4, Generally the content of total phosphorus and dissolved mineral phosphorus (phosphates) increased with the degree of eutrophy with the depth and with the progress of the vernal season towards the summer stagnation time. The amount of phosphates increased in water with the depletion of oxygen, both in the verical water column and with the progress of stagnation time. The amounts of dissolved organic phosphorus decressed with the depth of the vertical water column whereas the dissolved mineral phosphorus increased. The development of the particulate organic phosphorus stratification in the vertical water column was clearly visible in the eutrophic lake. The quantities of total phosphorus and its main component, dissolved mineral phosphorus, decreased evidently from January to May in all three lakes, mostly in the eutrophic lake. The reason of this decrease is sorption by lake sediments and to a certain degree sedimentation of phosphorus sorbed by ferric hydroxide. The increase of dissolved mineral phosphorus and that of total phosphorus in the vertical water column and with the progress of summer stagnation had as a reason the liberation of phosphorus from sediments, and not so much decomposition of sedimentating plankton or dissolved organic phosphorus. The share of single (mean) values of phosphorus forms in the total phosphorus was as follows: In the oligotrophic lake the share of particulate mineral phosphorus was extremely high in March (21% of the total), probably because of the inflow of the melting waters from the drainage area. The development of vertical stratification in waters of three subalpine Italian lakes at the end of the vernal season (May) indicates the quantitative prevailing of dissolved mineral phosphorus with its increase with the depth and domination of dissolved organic phosphorus in the trophogenic zone. 相似文献
Groundwater resources with high salinity content are found in some parts of the lower Shire River valley (Malawi). This paper discusses the geochemistry of minor elements with regards to the prevailing salinity. Hierarchical clustering and principal component analyses were used to identify factors which relate to both minor elements and samples and were interpreted as reflecting the influence of prevailing saline/brackish groundwater. Concentrations of lead (Pb), boron (B), strontium (Sr) and chromium (Cr) were associated with groundwater with high content of total dissolved solids (TDS). Speciation calculations indicated that dissolved Sr, barium (Ba) and lithium (Li) were mainly in the form of free aqueous ions whereas hydrolysed species were significant for aluminium (Al) and Cr, and carbonate complexes for Pb. Chloride complexes were prevalent for silver (Ag). Solubility of cerussite (PbCO3) and barite (BaSO4) was shown to act as a control on the levels of Pb and Ba, respectively. Thus, Ba concentrations were very low in saline groundwater owing to their high sulphate content. A relatively variable B concentration in the groundwater samples was explained using a binary mixing model of saline and fresh groundwater. The mixing of fresh groundwater with saline groundwater was concomitant with high Na+/Ca2+ ratios and enrichment of B, probably by desorption from clays. The WHO drinking water guidelines for Ba, B, Cr and Pb were exceeded in 6.5%, 9.7%, 16.1% and 64.5% of all the samples, respectively. However, all samples were below the Malawian specification of B in borehole and shallow well water quality (MS 733:2005) of 5.0 mg/L. 相似文献
An area of massive barite precipitations was studied at a tectonic horst in 1500 m water depth in the Derugin Basin, Sea of Okhotsk. Seafloor observations and dredge samples showed irregular, block- to column-shaped barite build-ups up to 10 m high which were scattered over the seafloor along an observation track 3.5 km long. High methane concentrations in the water column show that methane expulsion and probably carbonate precipitation is a recently active process. Small fields of chemoautotrophic clams (Calyptogena sp., Acharax sp.) at the seafloor provide additional evidence for active fluid venting. The white to yellow barites show a very porous and often layered internal fabric, and are typically covered by dark-brown Mn-rich sediment; electron microprobe spectroscopy measurements of barite sub-samples show a Ba substitution of up to 10.5 mol% of Sr. Rare idiomorphic pyrite crystals (∼1%) in the barite fabric imply the presence of H2S. This was confirmed by clusters of living chemoautotrophic tube worms (1 mm in diameter) found in pores and channels within the barite. Microscopic examination showed that micritic aragonite and Mg-calcite aggregates or crusts are common authigenic precipitations within the barite fabric. Equivalent micritic carbonates and barite carbonate cemented worm tubes were recovered from sediment cores taken in the vicinity of the barite build-up area. Negative δ13C values of these carbonates (>−43.5‰ PDB) indicate methane as major carbon source; δ18O values between 4.04 and 5.88‰ PDB correspond to formation temperatures, which are certainly below 5°C. One core also contained shells of Calyptogena sp. at different core depths with 14C-ages ranging from 20?680 to >49?080 yr. Pore water analyses revealed that fluids also contain high amounts of Ba; they also show decreasing SO42- concentrations and a parallel increase of H2S with depth. Additionally, S and O isotope data of barite sulfate (δ34S: 21.0-38.6‰ CDT; δ18O: 9.0-17.6‰ SMOW) strongly point to biological sulfate reduction processes. The isotope ranges of both S and O can be exclusively explained as the result of a mixture of residual sulfate after a biological sulfate reduction and isotopic fractionation with ‘normal’ seawater sulfate. While massive barite deposits are commonly assumed to be of hydrothermal origin, the assemblage of cheomautotrophic clams, methane-derived carbonates, and non-thermally equilibrated barite sulfate strongly implies that these barites have formed at ambient bottom water temperatures and form the features of a Giant Cold Seep setting that has been active for at least 49?000 yr. 相似文献
The distribution of210Po and210Po in dissolved (<0.4 μm) and particulate (>0.4 μm) phases has been measured at ten stations in the tropical and eastern North Atlantic and at two stations in the Pacific. Both radionuclides occur principally in the dissolved phase. Unsupported210Pb activities, maintained by flux from the atmosphere, are present in the surface mixed layer and penetrate into the thermocline to depths of about 500 m. Dissolved210Po is ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter is enriched in210Po, with210Po/210Pb activity ratios greater than 1.0, similar to those reported for phytoplankton. Box-model calculations yield a 2.5-year residence time for210Pb and a 0.6-year residence time for210Po in the mixed layer. These residence times are considerably longer than the time calculated for turnover of particles in the mixed layer (about 0.1 year). At depths of 100–300 m,210Po maxima occur and unsupported210Po is frequently present. Calculations indicate that at least 50% of the210Po removed from the mixed layer is recycled within the thermocline. Similar calculations for210Pb suggest much lower recycling efficiencies.Comparison of the210Pb distribution with the reported distribution of226Ra at nearby GEOSECS stations has confirmed the widespread existence of a210Pb/226Ra disequilibrium in the deep sea. Vertical profiles of particulate210Pb were used to test the hypothesis that210Pb is removed from deep water by in-situ scavenging. With the exception of one profile taken near the Mid-Atlantic Ridge, significant vertical gradients in particulate210Pb concentration were not observed, and it is necessary to invoke exceptionally high particle sinking velocities to account for the inferred210Pb flux. It is proposed instead that an additional sink for210Pb in the deep sea must be sought. Estimates of the dissolved210Pb/226Ra activity ratio at depths greater than 1000 m range from 0.2 to 0.8 and reveal a systematic increase, in both vertical and horizontal directions, with increasing distance from the sea floor. This observation implies rapid scavenging of210Pb at the sediment-water interface and is consistent with a horizontal eddy diffusivity of 3?6 × 107 cm2/sec. The more reactive element Po, on the other hand, shows evidence of rapid in-situ scavenging. In filtered seawater,210Po is deficient, on the average, by ca. 10% relative to210Pb; a corresponding enrichment is found in the particulate phase. Total inventories of210Pb and210Po over the entire water column, however, show no significant departure from secular equilibrium. 相似文献
The distribution of nutrient elements and dissolved oxygen in the deep Indian Ocean suggests that the Bay of Bengal Fan sediment serves as a major nutrient element source and oxygen sink. The distribution of nutrient element excesses and oxygen deficiences away from the fan is consistent with diffusion along isopycnal surfaces with circumpolar deep water acting as a sink for nutrient elements and source for dissolved oxygen. In the course of the entire GEOSECS program only at the Bengal Fan station were nutrient element excesses and oxygen deficiencies (relative to the overlying water column) observed in the benthic mixed layer. The unique aspect of this station is likely the result of very high rates of respiration and particle dissolution coupled with a high stability of the water column overlying the benthic mixed layer.The chemical data show that for each mole of organic carbon oxidized 1.5 moles of CaCO3 must dissolve and that for each mole of CaCO3 dissolved about one mole of silica go into solution. The NO3 and PO4 excesses are about two thirds those predicted from the Redfield ratios (i.e., O2 : NO3 : PO4 = 135 : 15 : 1). 相似文献
Sea floor hydrothermal activity in the Guaymas Basin, Gulf of California, is quite different from that associated with ridge crest spreading centers. Injection of hydrothermal fluids occurs in the bottom of a semi-enclosed basin and water column anomalies produced by this activity increase to much higher values than in the open ocean. In the Guaymas Basin the hydrothermal venting generates large clouds of fine suspended particulate matter (SPM) 100–300 m above active mounds and chimneys. These hydrothermal clouds have potential temperature anomalies of about 0.010–0.020°C, are enriched in dissolved silica, particulate manganese, and depleted in dissolved oxygen relative to areas away from the vents. The particulate manganese values increase from about 3 nmol/kg at ～ 1000 m, well above the enclosing topography of the subsill basin, to 100–150 nmol/kg in the clouds of SPM and in the bottom nepheloid layer. The particulate Mn in the hydrothermal clouds appears to originate from both direct precipitation of dissolved Mn2+ injected by the vents and entrainment of Mn-rich SPM in the rising hydrothermal plumes. Injection of silica-rich vent fluids into the basin bottom waters produces a silica anomaly of 10–15 μmol relative to the other deep basins of the Gulf of California. Spillover of Guaymas Basin deep water produces a silica plume just above the basin sill depth which is detectable to the mouth of the Gulf. A simple two-endmember mixing model indicates that the deep waters of the Guaymas Basin contain approximately 0.1% hydrothermal fluid. Oxygen anomalies associated with the hydrothermal clouds are on the order of 5 μmol relative to regions away from active vents. The basin as a whole shows a depletion in oxygen of about 13 μmol relative to the other deep basins of the Gulf. The mixing model shows that this oxygen consumption can be explained by the oxidation of dissolved sulfide and methane injected by the hydrothermal vents. Box models of the deep basins of the southern Gulf of California indicate that the Guaymas Basin has a significantly higher source term for dissolved silica and sink term for dissolved oxygen than the other basins. The calculated flux of hydrothermal fluids into the Guaymas Basin is 10–12 m3/s. 相似文献
Tritium data from the GEOSECS eastern and western track stations sampled in the thermocline of the North Equatorial Counter Current (NECC) showed high values, which are indicative of the contribution of the North Equatorial Current (NEC) to the equatorial region. A tritium front at 6°N is evidence of the penetration of northern latitude water into the NECC. The results of a two-? model show no net flux between NEC and NECC and a lower bound estimate of five years for an exchange time scale between the NEC and NECC in the thermocline; this is consistent with a horizontal diffusivity of 5 × 107 cm2/s. 相似文献
Electron spin resonance (ESR) dating has been applied to barite from hydrothermal vents. Barite in hydrothermal vents cools down rapidly after formation so little attention was paid to the thermal stability of the ESR signal of barite for dating. To discern multiple episodes of fluid flow or to date barite in other geological settings, it is essential to know how the ESR intensity decays with heat, the characteristic decay time and the closure temperature. In this study, we demonstrate that the decay of the ESR intensity of the SO3− radical in barite is a second-order reaction. The characteristic decay time for the ESR intensity to drop by half at 100 °C, 200 °C, 300 °C and 500 °C is of the order of magnitude of 106 years, 10 years, 10 h and 1 s respectively. The closure temperature of barite is generally between 190 and 340 °C. These results provide essential information on environments where the ESR intensity in barite is stable and conditions under which ESR ages can be erased. 相似文献
210Pb and226Ra profiles have been measured at five GEOSECS stations in the Circumpolar region. These profiles show that226Ra is quite uniformly distributed throughout the Circumpolar region, with slightly lower activities in surface waters, while210Pb varies with depth as well as location or area. There is a subsurface210Pb maximum which matches the oxygen minimum in depth and roughly correlates with the temperature and salinity maxima. This210Pb maximum has its highest concentrations in the Atlantic sector and appears to originate near the South Sandwich Islands northeast of the Weddell Sea. Concentrations in this maximum decrease toward the Indian Ocean sector and then become fairly constant along the easterly Circumpolar Current.Relative to226Ra, the activity of210Pb is deficient in the entire water column of the Circumpolar waters. The deficiency increases from the depth of the210Pb maximum toward the bottom, and the210Pb/226Ra activity ratio is lowest in the Antarctic Bottom Water, indicating a rapid removal of Pb by particulate scavenging in the bottom layer and/or a short mean residence time of the Antarctic Bottom Water in the Circumpolar region.226Ra is essentially linearly correlated with silica and barium in the Circumpolar waters. However, close examination of the vertical profiles reveals that Ba and Si are more variable than226Ra in this region. 相似文献
Seasonal changes in water quality were measured in samples taken at various distances from shallow water across mudflat to mangroves during flood period and from mangroves across mudflat to shallow water during ebb period in a subtropical mangrove estuary (Zhangjiang Estuary, Fujian, China). The TN (total dissolved nitrogen), TP (total dissolved phosphorus), COD (chemical oxygen demand), and DOC (dissolved organic carbon) contents during the flood period were significantly higher than those during the ebb period. In contrast, the opposite was true for the POC (particulate organic carbon) content and transparency. The mangroves at Zhangjiang Estuary may trap nutrients at rates of 90.5 g N/m2/yr, 2.2 g TP/m2/yr, and 13.7 g C/m2/yr in the form of DOC, and export POC at a rate of 81.8 g/m2/yr. Our results support the hypothesis that the maintenance of estuarine water quality by mangroves occurs during flood periods. 相似文献
We have used in-situ pumps which filter large volumes of sea water through a 1 μm cartridge prefilter and two MnO2-coated cartridges to obtain information on dissolved and particulate radionuclide distributions in the oceans. Two sites in the northwest Atlantic show subsurface maxima of the fallout radionuclides137Cs,239,240Pu and241Am. Although the processes of scavenging onto sinking particles and release at depth may contribute to the tracer distributions, comparison of predicted and measured water column inventories suggests that at least 35–50% of the Pu and241Am are supplied to the deep water by advection.The depth distributions of the naturally occurring radionuclides232Th,228Th and230Th reflect their sources to the oceans.232Th shows high dissolved concentrations in surface waters, presumably as a result of atmospheric or riverine supply. Activities of232Th decrease with depth to values 0.01 dpm/1000 l.228Th shows high activities in near surface and near bottom water, due to the distribution of its parent,228Ra. Dissolved230Th, produced throughout the water column from234U decay, increases with depth to 3000 m. Values in the deep water (> 3000 m) are nearly constant ( 0.6–0.7 dpm/1000 l), and the distribution of this tracer (and perhaps other long-lived particle-reactive tracers as well) may be affected by the advection inferred from Pu and241Am data.The ratio of particulate to dissolved activity for both230Th and228Th is 0.15–0.20. This similarity precludes the calculation of sorption rate constants using a simple model of reversible sorption equilibrium. Moreover, in mid-depths228Th tends to have a higher particulate/dissolved ratio than230Th, suggesting uptake and release of230Th and228Th by different processes. This could occur if228Th, produced in surface water, were incorporated into biogenic particles formed there and released as those particles dissolved or decomposed during sinking.230Th, produced throughout the water column, may more closely approach a sorption equilibrium at all depths.230Th,241Am and239,240Pu are partitioned onto particles in the sequence Th > Am > Pu with 15% of the230Th on particles compared with 7% for Am and 1% for Pu. Distribution coefficients (Kd) are 1.3–1.6 × 107 for Th, 5–6 × 106 for Am and 7–10 × 105 for Pu. The lower reactivity for Pu is consistent with analyses of Pu oxidation states which show 85% oxidized (V + VI) Pu. However, theKd value for Pu may be an upper limit because Pu, like228Th, may be incorporated into particles in surface waters and released at depth only by destruction of the carrier phase. 相似文献
