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1.
The variation of major and rare earth elements and yttrium (REY) in the monomictic hardwater Lake Tiberias during the wet and dry seasons of the hydrological year was studied in two profiles. The average volume and Cl concentration of the known and unknown saline inflows of 1.6 × 107 m3 and 1.2 × 109 mol are derived by closing both balances. This brine corresponds to a mixture of 83% of groundwater from Cretaceous aquifers and 17% of very saline deep brine. Taking cycling of calcite in the hypolimnion into account, the settling rate of authigenic calcite is estimated to be 3.3 mol m−2 a−1.In the stratified lake of the dry season dissolved inorganic carbon increases by 490 μM at the thermo-/chemocline due to microbial reduction of SO42−, NO3−, chemical reduction of Fe(III) and MnO2 colloids, and cycling of calcite in the hypolimnion. REY distribution in the stratified water column is dominantly controlled by coprecipitation with calcite, hydrous ferric oxides and MnO2 in the epilimnion and cycling of these compounds in the hypolimnion. The positive Ce anomaly in the hypolimnetic water is produced by cycling of MnO2. The simulation of the increase of REY in the hypolimnion reveals that hydrous ferric and manganese oxides only play a negligible role except Ce. Only about 10% of REY from cycled matter enhance REY in solution. Most of the released REY are adsorbed by particular matter and thus settling on the floor of the lake.Different from Na, U, SO42− and SiO2, the other elements, in particular REY, increase in the mixed water column from the top to the lower third and mostly decrease thereafter toward the bottom in the mixed lake during the wet season. The behavior of REY is caused by some cycling of calcite and pH-dependent re-equilibration of REY bound to hydrous ferric and manganese oxides adsorbed by particular matter.  相似文献   

2.
《Applied Geochemistry》2006,21(10):1799-1817
Release of acid drainage from mine-waste disposal areas is a problem of international scale. Contaminated surface water, derived from mine wastes, orginates both as direct surface runoff and, indirectly, as subsurface groundwater flow. At Camp Lake, a small Canadian Shield lake that is in northern Manitoba and is ice-covered 6 months of the year, direct and indirect release of drainage from an adjacent sulfide-rich tailings impoundment has severely affected the quality of the lake water. Concentrations of the products from sulfide oxidation are extremely high in the pore waters of the tailings impoundment. Groundwater and surface water derived from the impoundment discharge into a semi-isolated shallow bay in Camp Lake. The incorporation of this aqueous effluent has altered the composition of the lake water, which in turn has modified the physical limnology of the lake. Geochemical profiles of the water column indicate that, despite its shallow depth (6 m), the bay is stratified throughout the year. The greatest accumulation of dissolved metals and SO4 is in the lower portion of the water column, with concentrations up to 8500 mg L−1 Fe, 20,000 mg L−1 SO4, 30 mg L−1 Zn, 100 mg L−1 Al, and elevated concentrations of Cu, Cd, Pb and Ni. Meromictic conditions and very high solute concentrations are limited to the bay. Outside the bay, solute concentrations are lower and some stratification of the water column exists. Identification of locations and composition of groundwater discharge relative to lake bathymetry is a fundamental aspect of understanding chemical evolution and physical stability of mine-impacted lakes.  相似文献   

3.
The isotopic composition of water and dissolved Sr as well as other geochemical parameters at the 2516 m deep Outokumpu Deep Drill Hole, Finland were determined. The drill hole is hosted by Palaeoproterozoic turbiditic metasediments, ophiolite-derived altered ultramafic rocks and pegmatitic granitoids. Sodium–Ca–Cl and Ca–Na–Cl-rich waters (total dissolved solids up to ca. 70 g L−1) containing significant amounts of gas, mainly CH4 (up to 32 mmol L−1), N2 (up to 10 mmol L−1), H2 (up to 3.1 mmol L−1) and He (up to 1.1 mmol L−1) discharge from fracture zones into the drill hole. This water is distinct from the shallow fresh groundwater of the area, and has an isotopic composition typical of shield brines that have been modified during long-term water–rock interaction. Based on water stable isotopes and geochemistry, the drill hole water profile can be divided into five water types, each discharging from separate fracture systems and affected by the surrounding rocks. The δ2H varies from −90‰ to −56‰ (VSMOW) and δ18O from −13.5‰ to −10.4‰ (VSMOW), plotting clearly above the Global and Local Meteoric Water Lines on a δ2H vs. δ18O diagram. The 87Sr/86Sr ratios range between 0.72423 and 0.73668. Simple two-component mixing between 2H and 18O rich end-member brine and meteoric water cannot explain the water stable isotopic composition and trends observed. Instead, hydration of silicates by ancient groundwaters recharged under different climatic conditions, warmer than at present, is the most likely mechanism to have caused the variation of the δ2H and δ18O values. Water types correlate with changes in microbial communities implying that different ecosystems occur at different depths. The different water types and microbial populations have remained isolated from each other and from the surface for long periods of time, probably tens of millions of years.  相似文献   

4.
《Chemical Geology》2006,225(1-2):16-29
Pyrite oxidation rates were examined at various concentrations of dissolved oxygen (DO) in the presence of the sulfur and iron oxidizer Acidithiobacillus ferrooxidans. Five different batch experiments were performed at room temperature for 75 days under various DO levels (273, 129, 64.8, 13.2, and ≤ 0.006 μM), containing pyrite grains (particle size 63–250 μm) and a modified 9K nutrient medium at pH 3. The reactors were inoculated with A. ferrooxidans. In all experiments, pH decreased with time and sulfur and iron were released to the solution, indicating pyrite oxidation at all DO levels. Pyrite oxidation rates (ca. 5 × 10− 10 mol m− 2 s− 1 at 273 μM DO) from all experiments showed positive correlation with DO, Fe(III), and bacterial concentration. These rates were significantly slower than rates presented in other published studies, but this is probably due to the significantly greater Fe(III) concentration at lower pH in these previous studies. The results obtained in this study suggest that ferric iron reduction at the pyrite surface is the primarily mechanism for microbial pyrite oxidation in the presence of DO. The results from our study support the indirect mechanism of sulfide oxidation, where A. ferrooxidans oxidizes ferrous iron in the presence of DO, which then oxidizes pyrite.  相似文献   

5.
We report the presence of coenzyme factor 430 (F430), a prosthetic group of methyl coenzyme M reductase for archaeal methanogenesis, in the deep sub-seafloor biosphere. At 106.7 m depth in sediment collected off Shimokita Peninsula, northwestern Pacific, its concentration was estimated to be at least 40 fmol g sediment−1 (i.e. 36 pg g−1 wet sediment). This is about three orders of magnitude lower than typical concentrations of archaeal intact polar lipids in similar sub-seafloor sediments. On the basis of the concentration of F430 in methanogens and conversion to biomass composed of typical sub-seafloor microbial cells, we estimated that ca. 2 × 106 cells g−1 could be methanogens in the deeply buried marine sediment.  相似文献   

6.
Temporal changes in paleoproductivity of Lake Biwa (Japan) over the past 32 kyr have been studied by analyzing bulk organic carbon and photosynthetic pigments (chlorins) in the BIW95-5 core. Primary productivity was estimated on the assumption of C/Norg values of 8 for autochthonous organic matter (OM) and 25 for allochthonous OM and using an equation developed for the marine environment. The estimate indicates that primary productivity ranges from 50 to 90 g C m?2 yr?1 in the Holocene, while it is ～60 g C m?2 yr?1 on average in the last glacial. Pheophytin a and pheophorbide a are the major chlorins. A downcore profile of chlorin concentration normalized to autochthonous organic carbon (OC) shows a decreasing trend. Chlorin productivity was corrected by removal of the effect of post-burial chlorin degradation. The temporal profile of chlorin productivity thereby obtained resembles that from autochthonous OC.The difference in primary productivity between the Holocene and the glacial for the lake is markedly smaller than that for Lake Baikal situated in the boreal zone. This difference between the two lakes is probably caused by the difference in their climatic conditions, such as temperature and precipitation. Precipitation at Lake Biwa is relatively large during the glacial and the Holocene because of the continuous influence of the East Asian monsoon. Lake Baikal precipitation is generally small as a result of control by the continental (Siberia) climate regime. In addition, a significant difference in productivity between the glacial and the Holocene for Lake Baikal may be essentially controlled by the hydrodynamic systems in the lake.Lake Biwa terrigenous OM input events occurred at least five times over the period 11–32 kyr BP, suggesting enhanced monsoon activity. Molecular examination of the layer with a large input of terrigenous OM during the Younger Dryas indicates that concentrations of terrigenous biomarkers such as n-C27–C31 alkanes, lignin phenols, cutin acids, ω-hydroxy acids and C29 sterols are high, suggesting that soil OM with peat-like material entered the lake as a result of flooding. An enhanced sedimentation rate in the last 3000 years might have been partially caused by agricultural activity around the lake.  相似文献   

7.
Boom Clay is studied as a potential host formation for the disposal of high-and intermediate level long-lived radioactive waste in Belgium. In such a geological repository, generation of gases (mainly H2 from anaerobic corrosion) will be unavoidable. In order to make a good evaluation of the balance between gas generation vs. gas dissipation for a particular waste form and/or disposal concept, good estimates for gas diffusion coefficients of dissolved gases are essential. In order to obtain an accurate diffusion coefficient for dissolved hydrogen in saturated Boom Clay, diffusion experiments were performed with a recently developed through-diffusion set-up for dissolved gases. Due to microbial activity in the test set-up, conversion of hydrogen into methane was observed within several experiments. A complex sterilisation procedure was therefore developed in order to eliminate microbiological disturbances. Only by a combination of heat sterilisation, gamma irradiation and the use of a microbial inhibitor, reliable, reproducible and accurate H2(g) diffusion coefficients (measured at 21 °C) for samples oriented parallel (Deff = 7.25 × 10−10 m2/s and Deff = 5.51 × 10−10 m2/s) and perpendicular (Deff = 2.64 × 10−10 m2/s) to the bedding plane were obtained.  相似文献   

8.
The polymetallic Mykonos vein system in the Cyclades, Greece, consists of 15 tension-gashes filled with barite, quartz, pyrite, sphalerite, chalcopyrite and galena in ca. 13.5 Ma, I-type, Mykonos monzogranite. Zones of silica and chlorite–muscovite alteration are associated with the veins and overprint pervasive silicification, phyllic and argillic alteration that affected large parts of the monzogranite. The mineralization cements breccias and consists of an early barite–silica–pyrite–sphalerite–chalcopyrite assemblage followed by later argentiferous galena. A combination of fluid inclusion and stable isotope data suggests that the barite and associated mineralization were deposited from fluids containing 2 to 17 wt.% NaCl equivalent, at temperatures of ~ 225° to 370 °C, under a hydrostatic pressure of ≤ 100 bars. The mineralizing fluids boiled and were saturated in H2S and SO2.Calculated δ18OH2O and δDH2O, initial 87Sr/86Sr isotope compositions and the trace and REEs elements contents are consistent with a model in which the mineralizing fluids were derived during alteration of the Mykonos intrusion and subsequently mixed with Miocene seawater. Heterogeneities in the calculated δ34SSO4− 2 and δ34SH2S compositions of the ore fluids indicate two distinct sources for sulfur, namely of magmatic and seawater origin, and precipitation due to reduction of the SO4− 2 during fluid mixing. The physicochemical conditions of the fluids were pH = 5.0 to 6.2, logfS2 = − 13.8 to − 12.5, logfO2 = − 31.9 to − 30.9, logfH2S(g) = − 1.9 to − 1.7, logfTe2 = − 7.9 and logα(SO4− 2(aq)/H2S(aq)) = + 2.6 to + 5.5. We propose that retrograde mesothermal hydrothermal alteration of the Mykonos monzogranite released barium and silica from the alkali feldspars. Barite was precipitated due to mixing of SO4− 2-rich Miocene seawater with the ascending Ba-rich magmatic fluid venting upwards in the pluton.  相似文献   

9.
Chromophoric dissolved organic matter (CDOM) is an important component in the aquatic environment and plays a key role in light attenuation and in carbon biogeochemical cycles. We examined CDOM production in each of two laboratory experiments in which phytoplankton and macrophyte degradation were monitored using absorption and excitation–emission matrix fluorescence spectroscopy (EEMs). During the incubation period, CDOM was produced from phytoplankton and macrophytes, and partly decomposed by microorganisms. The absorption spectra of the phytoplankton derived and the macrophyte derived CDOM were distinct and characterized by peaks and shoulders in the UV bands. Production of CDOM absorption at 350 nm, a(350), was 0.0125 m2/g per unit of chlorophyll a from phytoplankton CDOM from 0–3 d. Meanwhile a(350) production was 2.708 × 10−4 m2/g per unit of wet biomass from macrophytes CDOM from 1–7 d. Despite the high production of CDOM by phytoplankton and macrophytes, extrapolation of these values to the field indicated that about 15% of total CDOM was produced from phytoplankton during algal blooms in Meiliang Bay in summer and about 8% of total CDOM was produced from macrophytes in the macrophyte dominated littorals. The mean value of the spectral slope (S) describing the exponential decrease of the absorption spectrum, which was strongly correlated to an optical index of molecular size, for the phytoplankton derived CDOM was 10.26 ± 2.05 μm−1, which was significantly lower than the mean S of 14.47 ± 2.88 μm−1 for the macrophyte derived CDOM (t-test, p < 0.001). The mean value of the spectral slope ratio (SR) for the phytoplankton derived CDOM was 1.79 ± 0.52, which was significantly higher than that of 0.35 ± 0.58 for the macrophyte derived CDOM (t-test, p < 0.001). Three fluorescent components were validated in parallel factor analysis (PARAFAC) models calculated separately for phytoplankton derived and macrophyte derived CDOM, each CDOM source resulting in distinct excitation and emission maxima for each component. The significant differences in CDOM absorption spectra, S, SR and PARAFAC fluorescence component characteristics, all showed that phytoplankton derived CDOM was compositionally distinct from macrophyte derived CDOM. Overall both sources were important to the CDOM pool in the shallow temperate lake.  相似文献   

10.
《Quaternary Science Reviews》2007,26(13-14):1871-1883
Multi-proxy palaeoenvironmental studies of nine sediment sequences from four areas in north-western Russia reveal significant changes in climate, lake productivity and vegetation during the Lateglacial and early Holocene that show some degree of correlation with changes reconstructed from sites throughout the North Atlantic region. At Lake Nero in the Rostov-Jaroslavl’ area, which is outside the maximum limit of the Scandinavian Ice Sheet, sedimentation recommenced shortly after 15 cal ka BP in response to increases in temperature and humidity during Greenland Interstadial 1 (GI-1; Bølling-Allerød). However, climatic amelioration during GI-1 was slow to increase lake organic productivity or trigger large-scale changes in much of northwestern Russia. In general, this region was characterised by long-lasting lake-ice cover, low lake productivity, soil erosion, and dwarf shrub and herb tundra until the end of Greenland Stadial 1 (GS-1; Younger Dryas). At some sites, distinct increases in lake organic productivity, mean summer temperatures and humidity and the expansion of forest trees coincide with rapid warming at the beginning of the Holocene and the increasing influence of warm air masses from the North Atlantic. At other sites, particularly on the Karelian Isthmus, but also in Russian Karelia, the delayed response of limnic and terrestrial environments to early Holocene warming is likely related to the cold surface waters of the Baltic Ice Lake, the proximity of the Scandinavian Ice Sheet and associated strengthened easterlies, and/or extensive permafrost and stagnant ice. These multi-proxy studies underscore the importance of local conditions in modifying the response of individual lakes and their catchments.While Lateglacial vegetation was dominated by Betula nana and Salix shrubs and various herbs, pollen and plant macrofossils suggest that Betula pubescens trees became established as early as 14–13 cal ka BP in the Rostov-Jaroslavl’ area. In general, our data sets suggest that trees migrated from the southeast to the west and then spread later to the northeast and northwest, paralleling the direction of ice retreat, with Betula pubescens immigrating first, followed by Pinus sylvestris and Picea abies. However, palaeoecological records from Lake Terebenskoye in the Valdai Highlands suggest that the arrival of Picea abies preceded other trees in that area and that it colonised tundra communities as early as 12 cal ka BP. Since Lateglacial vegetation change in north-western Russia was time-transgressive, independent measures of palaeoclimate (e.g., chironomid-based palaeotemperature estimates) are needed for this region.  相似文献   

11.
Extensive coastal dunes occur in the Great Lakes region of North America, including northwestern Michigan where some are perched on high (~ 100 m) bluffs. This study focuses on such a system at Arcadia Dunes and is the first to systematically generate optical ages from stratigraphic sections containing buried soils. Dune growth began ca. 4.5 ka during the Nipissing high lake stand and continued episodically thereafter, with periods of increased sand supply at ca. 3.5 ka and ca. 1.7 ka. The most volumetrically dominant phase of dune growth began ca. 1.0 ka and continued intermittently for about 500 years. It may have begun due to the combined effects of a high lake phase, potential changes in lake hydrodynamics with final isostatic separation of Lake Superior from Lakes Michigan and Huron, and increased drought and hydrologic variability associated with the Medieval Warm Period. Thus, this latest eolian phase likely reflects multiple processes associated with Great Lakes water level and climate variability that may also explain older eolian depositional events. Comparison of Arcadia ages and calendar corrected 14C ages from previous studies indicate broad chronological agreement between events at all sites, although it appears that dune growth began later at Arcadia.  相似文献   

12.
《Quaternary Science Reviews》1999,18(10-11):1185-1203
A 13 m long core from the lake-swamp at Pobochnoye (53°01′30″ N, 51°50′30″ E) in the Buzuluk pine forest in the middle part of the Volga River basin, Russia was studied for pollen, peat stratigraphy, mollusc, δ18O/δ16O and δ13C/δ12C analyses and radiocarbon dating. For the first time the environment history of the east European Russia has been reconstructed for the last 14,000 years; ca 14,000–13,000 BP cold dry steppes spread across the basin of the Samara River. Isotope data indicate that the main climate shift occurred ca 10,000 BP at the Lateglacial–Holocene transition when climate became warmer and forests expanded. Pinus sylvestris L. expanded 10,000 BP. Ca 9,000 BP Ulmus, Quercus and Corylus appeared in the Buzuluk forest followed at ca. 7,000 BP by Alnus, then Tilia and Acer at 6000 BP. Between 6000 and 4500 BP the climatic conditions were optimal for the forest growth in the Samara River basin. 5500–5000 BP the lake became shallower and was transformed into the eutrophic peat swamp. Between 4500–3500 BP climate became drier and hotter and forest less abundant. Between 3500–2400 BP the forest cover again increased. Between 2400–2000 BP the pine forest area has reduced, apparently due to increased dryness, and around 2000 BP the modern environment in the Buzuluk area has been in existence.  相似文献   

13.
The Late Pleistocene to Middle Holocene African Humid Period (AHP) was characterized by dramatic hydrologic fluctuations in the tropics. A better knowledge of the timing, spatial extent, and magnitude of these hydrological fluctuations is essential to decipher the climate-forcing mechanisms that controlled them. The Suguta Valley (2°N, northern Kenya Rift) has recorded extreme environmental changes during the AHP. Extensive outcrops of lacustrine sediments, ubiquitous wave-cut notches, shorelines, and broad terrace treads along the valley margins are the vestiges of Lake Suguta, which once filled an 80 km long and 20 km wide volcano–tectonic depression. Lake Suguta was deep between 16.5 and 8.5 cal ka BP. During its maximum highstand, it attained a water depth of ca 300 m, a surface area of ca 2150 km2, and a volume of ca 390 km3. The spatial distribution of lake sediments, the elevation of palaeo-shorelines, and other geomorphic evidences suggest that palaeo-Lake Suguta had an overflow towards the Turkana basin to the north. After 8.5 cal ka BP, Lake Suguta abruptly disappeared. A comparison of the Lake Suguta water-level curve with other reconstructed water levels from the northern part of the East African Rift System shows that local insolation, which is dominated by precessional cycles, may have controlled the timing of lake highstands in this region. Our data show that changes of lake levels close to the Equator seem to be driven by fluctuations of spring insolation, while fluctuations north of the Equator are apparently related to variations in summer insolation. However, since these inferred timings of lake-level changes are mostly based on the radiocarbon dating of carbonate shells, which may have been affected by a local age reservoir, alternative dating methods are needed to support this regional synthesis. Between 12.7 and 11.8 cal ka BP, approximately during the Northern Hemisphere high-latitude Younger Dryas, the water level of Lake Suguta fell by ca 50 m, suggesting that remote influences also affected local hydrology.  相似文献   

14.
《Applied Geochemistry》2006,21(1):48-57
Core samples of boreal terrestrial sediments from depths of 0–300 cm at Rikubetsu, Hokkaido, Japan were analyzed for alkaline and acid phosphatase enzymatic activities. Enzymatic activities of alkaline phosphatase (ALP) and acid phosphatase (ACP) were greatest at the surface and decreased with depth; ALP and ACP activities were 25.5 and 22.0 nmol min−1 g−1, respectively, within the top 5 cm. These biological indicators were compared with measurements of microbial cell density and chemical indicators, including total organic C (TOC) and total hydrolyzed amino acids (THAA). The product–moment correlation coefficients (r) for ALP and ACP versus microbial cell density were 0.949 and 0.810, respectively. The coefficients for THAA and TOC versus ALP were 0.997 and 0.995, respectively. Vertical distributions of enzymatic activity are highly consistent with the observed microbial biomass profile and diagenetic organic matter in the sediment. However, the vertical profile of PO4 concentration shows a negative correlation coefficient for ALP and ACP of −0.937 and −0.855, respectively. Consequently, ALP and ACP were shown to be useful biomarkers of microbial activities in the terrestrial sediment over the past 10 ka at Rikubetsu, Hokkaido, Japan.  相似文献   

15.
This study presents isotope geochemical analyses conducted on water column samples and core sediments collected from the Swan Lake Basin. Water analyses include the dissolved methane (CH4) content and the ratio of carbon-13 to carbon-12 (δ13C) in dissolved inorganic carbon (DIC). The core sediments – sandy muds containing inorganic calcite, organic matter, and opal phases ± ostracods – were examined by X-ray diffraction, dated by radiocarbon (14C), analyzed for wt% organic carbon, wt% organic nitrogen, wt% organic matter, wt% calcite, δ13C of bulk-sediment insoluble organic matter (kerogen), 18O:16O ratio (δ18O) and δ13C of bulk and ostracod calcite. Of particular significance is the large enrichment in carbon-13 (δ13C = +4.5 to +20.4‰ V-PDB) in the calcite of these sediments. The 13C-enriched calcite is primarily formed from DIC in the water column of the lake as a result of the following combined processes: (i) the incorporation of 13C enriched residual carbon dioxide (CO2) after partial reduction to CH4 in the sediments and its migration into the water column-DIC pool; (ii) the preferential assimilation of 12C by phytoplankton during photosynthesis; (iii) the removal of 13C-depleted CH4 by ebullition and of organic matter by sedimentation and burial. The 13C enrichment was low between 3624 and 2470 yr BP; high between 2470 and 1299 yr BP; and moderate since 1299 yr BP. Low 13C enrichment was formed under low water-column carbon levels while higher ones were formed under elevated rates of biomass and calcite deposition. These associations seem to imply that biological productivity is the main reason for carbon-13 enrichments.  相似文献   

16.
The Holocene successions of numerous shallow lakes located along the Coorong coastal plain in South Australia attest to the impact of rising sea level and changing climate on their depositional environment. Old Man Lake is one of the smallest perennial alkaline lakes in the region. Its succession comprises a basal lagoonal sand rich in humic organic matter (OM) overlain by a 3.7 m thick upward shoaling lacustrine mudstone. The latter features three discrete sapropel units deposited between 3270 and 4910 cal yr BP, a time of increasing aridity throughout southeastern Australia. A core taken from the lake’s eastern margin yielded sedimentological, mineralogical, geochronological and micropaleontological data. Coring at five other sites across the lake provided sections of the humic and sapropelic facies (n = 20) for total organic carbon and Rock–Eval analysis; isotopic characterization of their micritic carbonate (δ13Ccarb, δ18Ocarb) and co-existing OM (δ13Corg); and GC–MS and GC–irMS analysis of their free aliphatic hydrocarbons. For each ‘sapropel event’ high productivity of diatoms and green algae was the principal driver of the accumulation and preservation of OM in such high concentrations. The precursor algal blooms were likely triggered by the influx of fresh water following winter rainfall. The combination of kerogen hydrogen index and δ13Ccarb–δ13Corg, previously employed to track secular changes in algal productivity and organic preservation, proved useful in identifying synchronous geographic differences in these processes across the lake. Highly branched isoprenoids (HBI: C25:1 ≫ C20:0) are prominent components of the aliphatic hydrocarbons in the sapropels, confirming the significant contribution of diatoms to their OM. The C isotopic signatures of the principal C25:1 HBI isomer and the co-occurring C23–C31 odd carbon numbered n-alkanes further document the non-uniformity of biomass preservation within and between the three sapropel units. The evidence from this study suggests that seasonal algal blooms and meromixis, although not necessarily an anoxic hypoliminion, were required for sapropel formation in the Holocene lakes of the Coorong region. Higher resolution sampling, dating and comparative analysis (microfossil, biomarker and isotopic) of these sapropels is required to clarify their potential significance as palaeoclimate proxies.  相似文献   

17.
《Applied Geochemistry》2005,20(8):1496-1517
Chloride concentrations were as high as 230 mg/L in water from the surface discharge of long-screened production wells in Pleasant Valley, Calif., about 100 km NW of Los Angeles. Wells with the higher Cl− concentrations were near faults that bound the valley. Depending on well construction, high-Cl−water from different sources may enter a well at different depths. For example, Cl− concentration in the upper part of some wells completed in overlying aquifers influenced by irrigation return were as high as 220 mg/L, and Cl− concentrations in water sampled within wells at depths greater than 450 m were as high as 500 mg/L. These high-Cl− waters mix within the well during pumping and produce the water sampled at the surface discharge. Changes in the major ion, minor ion, trace element, and δ34S and δ13C isotopic composition of water in wells with depth were consistent with changes resulting from SO4 reduction, precipitation of calcite, and cation exchange. The chemical and isotopic composition of high-Cl− water from deep wells trends towards the composition of oil-field production water from the study area. Chloride concentrations in oil-field production water present at depths 150 m beneath freshwater aquifers were 2200 mg/L, and Cl− concentrations in underlying marine rock were as high as 4400 mg/L. High-Cl− concentrations in water from deeper parts of wells were associated with dissolved organic C composed primarily of hydrophobic neutral compounds believed to be similar to those associated with petroleum in underlying deposits. These compounds would not be apparent using traditional sampling techniques and would not be detected using analytical methods intended to measure contamination.  相似文献   

18.
Streams and rivers are major exporters of C and other dissolved materials from watersheds to coastal waters. In streams and rivers, substantial amounts of terrigenous organic C is metabolized and degassed as CO2 to the atmosphere. A long-term evaluation of CO2 dynamics in streams is essential for understanding factors controlling CO2 dynamics in streams in response to changes in climate and land-use. Long-term changes in the partial pressure of CO2 (pCO2) were computed in the Anacostia River and the lower Potomac River in the Chesapeake Bay watershed. Long-term estimates were made using routine monitoring data of pH, total alkalinity, and dissolved nutrients from 1985 to 2006 at 14 stations. Longitudinal variability in pCO2 dynamics was also investigated along these rivers downstream of the urban Washington D.C. metropolitan area. Both rivers were supersaturated with CO2 with respect to atmospheric CO2 levels (392 μatm) and the highly urbanized Anacostia waters (202–9694 μatm) were more supersaturated than the Potomac waters (557–3800 μatm). Long-term variability in pCO2 values may be due to changes in river metabolism and organic matter and nutrient loadings. Both rivers exchange significant amounts of CO2 with the atmosphere (i.e., Anacostia at 0.2–72 mmol m−2 d−1 and Potomac at 0.12–24 mmol m−2 d−1), implying that waterways receiving organic matter and nutrient subsidies from urbanized landscapes have the potential to increase river metabolism and atmospheric CO2 fluxes along the freshwater–estuarine continuum.  相似文献   

19.
《Quaternary Science Reviews》2007,26(13-14):1736-1758
This paper presents a high-resolution lake-level record for the Holocene at Lake Accesa (Tuscany, north-central Italy) based on a range of sedimentological techniques validated in previous studies, with a chronology derived from 43 radiocarbon dates and four tephra layers. It gives evidence of centennial-scale fluctuations with major highstands at ca 11 500, 11 100, 10 200, 9400, 8200, 7300, 6200, 5700–5200, 4850, 4200, 3400, 2600, 1200 and 400 cal BP. Except for the Early Holocene until ca 10 500 cal BP, this pattern of hydrological changes appears to be in agreement with the regional pattern established for west-central Europe. Correlations with the Preboreal oscillation and the 8.2 ka event as well as with the atmospheric 14C residual series suggest that lake-level fluctuations developed at Accesa in response to (1) final steps of the deglaciation in the North Atlantic area and (2) variations in solar activity. For the period after 4500 cal BP, correlations with other palaeohydrological records from central Italy indicate that lake-level changes reconstructed at Accesa were mainly driven by climatic variations while anthropic activities and local geomorphological factors only played a secondary role. The Accesa lake-level record also highlights millennial-scale variations with a maximal lowstand at ca 9200–7700 cal BP contemporaneous with Sapropel event 1 in the Mediterranean. It was followed by generally higher lake-level conditions. This appears to be the opposite of that observed in Sicily (southern Italy) where a lake-level maximum developed at ca 9000–8200 cal BP and was followed by a general trend towards aridification. These opposite patterns were interpreted as contrasting hydrological responses to orbitally induced changes in summer insolation. This interpretation has to be tested by further lake-level studies in the central Mediterranean region. Finally, correlations between major lowstands and periods of maximal representation of Quercus ilex point to convergences between climate oscillations and Holocene vegetation history in the Accesa region. However, the maximal representation of Abies during the first half of the Holocene, including a time window where lake level reached a minimal level, suggests a more subtle impact of seasonality processes.  相似文献   

20.
《Applied Geochemistry》2006,21(11):1868-1879
Ultra-clean sampling methods and approaches typically used in pristine environments were applied to quantify concentrations of Hg species in water and microbial biomass from hot springs of Yellowstone National Park, features that are geologically enriched with Hg. Microbial populations of chemically-diverse hot springs were also characterized using modern methods in molecular biology as the initial step toward ongoing work linking Hg speciation with microbial processes. Molecular methods (amplification of environmental DNA using 16S rDNA primers, cloning, denatured gradient gel electrophoresis (DGGE) screening of clone libraries, and sequencing of representative clones) were used to examine the dominant members of microbial communities in hot springs. Total Hg (THg), monomethylated Hg (MeHg), pH, temperature, and other parameters influential to Hg speciation and microbial ecology are reported for hot springs water and associated microbial mats.Several hot springs indicate the presence of MeHg in microbial mats with concentrations ranging from 1 to 10 ng g−1 (dry weight). Concentrations of THg in mats ranged from 4.9 to 120,000 ng g−1 (dry weight). Combined data from surveys of geothermal water, lakes, and streams show that aqueous THg concentrations range from l to 600 ng L−1. Species and concentrations of THg in mats and water vary significantly between hot springs, as do the microorganisms found at each site.  相似文献   

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