首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 125 毫秒
1.
A chert-phosphorite unit from the Sugetbrak section in the Tarim Basin was analyzed for rare earth elements (Ce, Eu), redox sensitive proxies, and carbon isotopic compositions (δ13Ccarb and δ 13Corg) in the lower Yurtus Formation of the Early Cambrian period. Redox sensitive element ratios (Th/U, V/Cr, Ni/Co, and V/Sc) were employed to determine the palaeoenvironmental conditions during this period. The ratios indicated that the depositional environment of the chert-phosphorite-black shale unit ranged from suboxic to anoxic. Negative Ce and positive Eu anomalies in the chert-phosphorite assemblages of the studied Yurtus Formation indicated the existence of a redox-stratified ocean, similar to that of South China. Overturn or upward expansion of the deep water-mass probably reached the shallow marine zone after the formation of the Yurtus phosphorites. The characteristics of the negative Ce anomaly may be due to phosphoritic inheritance from the Ce-depleted signature of the overlying water column. Subsequent hydrothermal inputs and reduced detrital supplies of the deep water caused by the upwelling affected certain redox sensitive elements in the sedimentary basin. δ 13Ccarb and δ13Corg negative excursions in the Yurtus chert-phosphorite unit may be related to a transgression phase when episodic basinal upwelling moved 12C- and P-rich waters from the pelagic basin floor to the continental shelf. Although carbon isotopic compositions in the Yurtus chert-phosphorite assemblages may have suffered from diagenetic alteration, they can be used to probe diagenetic conditions. Multi-proxy geochemical studies indicated that the δ 13Ccarb values of the Yurtus chert-phosphorite assemblages might be considered reflections of a predominantly suboxic environment that was subsequently affected by hydrothermal inputs due to the upwelling.  相似文献   

2.
Closely spaced samples (285 in number) of varved sediments from the Upper Permian in Delaware Basin, Texas, have been analyzed for δ13Ccarb, δ13Corg, δ18Ocarb, Corg, Ccarb, and calcite/dolomite. δ13C records a dramatic rise from ?2.8 to +5.7‰ in only 4400 years, detected in three sections across the basin, extrapolating smoothly through a 600-year interruption by a local (west side of the basin) fresh-water inflow evidenced by low δ18O. This continuity and low Corg within the basin, both indicate that the excess net deposition of Corg, necessary to generate the rise in δ13C, took place in the ocean external to the Delaware Basin. Correlation with similar records from the Zechstein Basin suggest that the event was world-wide, although this poses obvious difficulties for the carbon cycle. The rate of rise of δ13C, and its sustained high level, must imply conversions of oxidized carbon to reduced carbon that are very large depending on which reservoirs were involved.  相似文献   

3.
Immediately before the extinction of the end‐Guadalupian (Middle Permian; ca 260 Ma), a significant change to the global carbon cycle occurred in the superocean Panthalassa, as indicated by a prominent positive δ13C excursion called the Kamura event. However, the causes of this event and its connection to the major extinction of marine invertebrates remain unclear. To understand the mutual relationships between these changes, we analyzed the sulfur isotope ratio of the carbonate‐associated sulfate (CAS) and HCl‐insoluble residue, as well as the carbon isotope ratio of bulk organic matter, for the Middle‐Upper Permian carbonates of an accreted mid‐oceanic paleo‐atoll complex from Japan, where the Kamura event was first documented. We detected the following unique aspects of the stable carbon and sulfur isotope records. First, the extremely high δ13C values of carbonate (δ13Ccarb) over +5 ‰ during the Capitanian (late Guadalupian) were associated with large isotopic differences between carbonate and organic matter (Δ13C = δ13Ccarb ? δ13Corg). We infer that the Capitanian Kamura event reflected an unusually large amount of dissolved organic matter in the expanded oxygen minimum zone at mid‐depth. Second, the δ34S values of CAS (δ34SCAS) were inversely correlated with the δ13Ccarb values during the Capitanian to early Wuchiapingian (early Late Permian) interval. The Capitanian trend may have appeared under increased oceanic sulfate conditions, which were accelerated by intense volcanic outgassing. Bacterial sulfate reduction with increased sulfate concentrations in seawater may have stimulated the production of pyrite that may have incorporated iron in pre‐existing iron hydroxide/oxide. This stimulated phosphorus release, which enhanced organic matter production and resulted in high δ13Ccarb. Low δ34SCAS values under high sulfate concentrations were maintained and the continuous supply of sulfate cannot by explained only by the volcanic eruption of the Emeishan Trap, which has been proposed as a cause of the extinction. The Wuchiapingian δ34SCAS–δ13Ccarb correlation, likely related to low sulfate concentration, may have been caused by the removal of oceanic sulfate through the massive evaporite deposition.  相似文献   

4.
Organic carbon isotope(δ13Corg) data from two well-preserved sections across a shallow-to-deep water transect of the late Ediacaran-Early Cambrian Yangtze Platform in South China show significant temporal and spatial variations. In the shallow-water Jiulongwan-Jijiapo section, δ13Corg values of the late Ediacaran Dengying Formation range from -29‰ to -24‰. In the deep-water Longbizui section, δ13Corg values from time-equivalent strata of the Dengying Formation are mostly between –35‰ and -32‰. These new data, in combination with δ13Corg data reported from other sections in South China, reveal a 6‰–8‰ shallow-to-deep water δ13Corg gradient. High δ13Corg values(-30‰) occur mostly in shallow-water carbonate rocks, whereas low δ13Corg values(-32‰) dominate the deep-water black shale and chert. The large temporal and spatial δ13Corg variations imply limited buffering effect from a large dissolved organic carbon(DOC) reservoir that was inferred to have existed in Ediacaran-Early Cambrian oceans. Instead, δ13Corg variations between platform and basin sections are more likely caused by differential microbial biomass contribution to total organic matter. High δ13Corg values(-30‰) documented from shallow-water carbonates are within the range of typical Phanerozoic δ13Corg data and may record the isotope signature of organic matter from primary(photosynthetic) production. In contrast, low δ13Corg values(-32‰) from deep-water sections may have resulted from higher chemoautotrophic or methanotrophic biomass contribution to bulk organic matter in anoxic environments. The δ13Corg data provide indirect evidence for ocean stratification and episodic chemocline fluctuations in the Ediacaran-Early Cambrian Yangtze Platform.  相似文献   

5.
After the severest mass extinction event in the Phanerozoic, biotic recovery from the extinction at the Permian–Triassic boundary required approximately 5 my, which covers the entire Early Triassic. It is important to obtain information on the superocean Panthalassa, which occupied most of the world ocean, to explore paleoenvironmental changes during the Early Triassic at the global scale. In order to establish the continuous lithostratigraphy of pelagic sediments in Panthalassa during the Early Triassic, high‐resolution reconstruction of the Lower Triassic pelagic sequence in Japan was conducted for the first time based on detailed field mapping and lithostratigraphic correlation in the Inuyama area, central Japan. The reconstructed Early Triassic sequence is approximately 9.5 m thick, consists of five rock types, and is divided into eight lithological units. For the reconstructed continuous sequence, measurement of carbon isotopic composition of sedimentary organic matter (δ13Corg) was carried out. Stratigraphic variation of the δ13Corg value shows large‐amplitude fluctuations between ?34.4 and ?21.0‰ throughout the sequence. In order to establish a higher resolution age model for the reconstructed Lower Triassic pelagic sequence, we correlated δ13Corg records in the Inuyama area with high‐resolution isotopic profiles of carbonate carbon (δ13Ccarb) from shallow‐marine carbonate sequences in southern China based on the similarity in general variation patterns with age constraints by radiolarian and conodont biostratigraphy. The result provides a high‐resolution time scale for the pelagic sequence of Panthalassa during the Smithian and Spathian. The age model suggests a drastic increase in sedimentation rate during the late Smithian, which should have been caused by the increase in terrigenous input to this site.  相似文献   

6.
Stable isotope analyses in sections across a shelf to basinal transect of the Ediacaran Doushantuo basin show substantial isotope variabilities. In Songlin section where sediments were deposited in an intrashelf basin, δ 13C values are persistently negative (_3‰ to _5‰, VPDB) through the entire Doushantuo Formation, similar to those obtained from the slope section in Wuhe (_5‰ to _10‰, VPDB). Shallow water sections in Weng'an and Duoding show two broad δ 13C anomalies overprinted with significant meter-scale variations, but none of the curves has similar absolute δ 13C values compared to the Yangtze Gorges areas in South China and other sections globally. Such isotope variations, if partially recording ancient seawater signature, imply spatial and temporal chemocline instability in the Doushantuo basin. In combination with available δ 13C records from other Ediacaran successions globally, the data from the Doushantuo basin are consistent, in first order, with the existence and oxidation of a large dissolved organic carbon (DOC) reservoir in Ediacaran oceans, but imply local environmental controls on Neoproterozoic isotope values and call attentions for using δ 13C anomalies as time lines in stratigraphic correlation.  相似文献   

7.
We present a time series of carbon and oxygen stable isotope records of the last 30?000 14C years throughout the last glacial-postglacial cycle from western Qinghai-Xizhang (Tibet) Plateau. A 20-m core drilled in the south basin of Zabuye Salt Lake was analyzed for inorganic and organic carbon and total sulfur contents, δ13C and δ18O values of carbonates. Our results indicate that climatic changes have led to a drastic negative shift of stable isotope ratios at the transition between the Last Full Glacial and the postglacial phase during Later Pleistocene times (∼16.2 kyr BP), and a rapid positive shift at the transition from Pleistocene to Holocene (∼10.6 kyr BP). The first shift is marked by the drop of δ18Ocarb values of about 10‰ (from +2 to −8‰) and δ13Ccarb values of about 3‰ (from 5 to 2‰). The second shift which occurred at the transition from Pleistocene to Holocene was of similar magnitude but in the opposite direction. Isotope data, combined with total organic and inorganic carbon contents and the lithological composition of the core, suggest this lake was an alluvial pre-lake environment prior to ca. 28 14C kyr BP. During ca. 28-16.2 14C kyr BP, Zabuye Lake was likely a moderately deep lake with limited outflow. The cool and arid glacial climate led the lake level to drop drastically. Extended residence time overwhelmed the lower temperature and caused a steady increase of δ13Ccarb and δ18Ocarb values and total inorganic carbon content in the sediments. During ca. 16.2-10.6 14C kyr BP, this lake probably overflowed and received abundant recharge from melting glaciers when the deglaciation was in its full speed. A spike of markedly enhanced δ13Ccarb and δ18Ocarb is seen at ∼11.5 kyr BP, probably due to the isotopic effects left behind by the short but severe Younger Dryas (YD) event. After ca. 10.6 14C kyr BP, Zabuye Lake probably closed its surface outflow, due to strong desiccation and drastic climate warming. The Early and Middle Holocene were characterized by unstable climatic conditions with alternating warmer/cooler episodes as indicated by the severe fluctuations of total organic carbon, δ13C and δ18O values. A hypersaline salt lake environment was finally formed at Zabuye after ∼5 14C kyr BP when the mirabilite and halite concentrations steadily increased and became the dominant minerals in the sediments. Severe imbalance of inflow/outflow resulted in the drastic increase of total sulfur, δ13Ccarb and δ18Ocarb values and dominance of halite in the lake since ca. 3.8 kyr BP to present.  相似文献   

8.
Organic carbon in estuarine sediments can have many different sources. Terrestrial, riverine, estuarine and marine C pools may all contribute to and influence the organic C (Corg) inventory of the estuarine sediments and the differing stable isotope signatures of the sources are reflected in the sediment's overall 13C content. Ecological interpretations of sedimentary isotope data may, however, be limited by the fact the total Corg inventory of a sediment may not be an accurate representation of the fraction that is labile and being actively turned over by the sedimentary community. To gain a better understanding of sedimentary Corg dynamics in estuaries and the relationship between the sedimentary C pool and the Corg undergoing mineralisation, we studied three components of an estuarine system: (1) the sedimentary Corg inventory on a transect from the mouth to the upper end of the estuary, (2) temporal changes of sedimentary Corg at one station throughout a year, and (3) the δ13C of respired CO2 compared to the δ13C of available source material and sedimentary Corg in a novel application of methods developed for soil science. Our experiments demonstrated that material of marine origin dominated the studied estuary. At the time-series station, material of marine origin dominated the sedimentary Corg throughout the 1-yr study period. δ13C values of CO2 released from the sediment differed significantly from the sedimentary Corg inventory at all study sites, but also clearly reflected differences between the main sections of the estuary. These results suggest that δ13C measurements of respired CO2 are promising as a tool to advance our understanding of C cycling in estuaries, and highlight that the sedimentary Corg pool alone may not be a satisfactory indicator of OM utilisation in estuarine sediments.  相似文献   

9.
New analyses reveal two intervals of distinctly lower δ13C values in the terrestrial organic matter of Permian–Triassic sequences in northern Xinjiang, China. The younger negative δ13Corg spike can be correlated to the conspicuous and sharp δ13C drops both in carbonate carbon and organic carbon near the Permian–Triassic event boundary (PTEB) in the marine section at Meishan. The geochemical correlation criteria are accompanied by a magnetic susceptibility pulse and higher abundances of distinctive, chain-like organic fossil remains of Reduviasporonites.The older negative δ13Corg spike originates within a latest Permian regression. Significant changes in organic geochemical proxies are recorded in the equivalent interval of the marine section at Meishan. These include relatively higher concentrations of total organic carbon, isorenieratane, C14–C30 aryl isoprenoids and lower ratios of pristane/phytane that, together, indicate the onset of anoxic, euxinic and restricted environments within the photic zone. The massive and widespread oxidation of buried organic matter that induced these euxinic conditions in the ocean would also result in increased concentrations of 13C-depleted atmospheric CO2. The latest Permian environmental stress marked by the older negative δ13Corg episode can be correlated with the distinct changeover of ostracod assemblages and the occurrences of morphological abnormalities of pollen grains. These observations imply that biogeochemical disturbance was manifested on the land at the end of the Permian and that terrestrial organisms responded to it before the main extinction of the marine fauna.  相似文献   

10.
China’s widespread marine carbonate rock series are mostly characterized by intensive thermal evolu- tion and low abundance of organic matter, especially the Lower Paleozoic carbonate rocks have experienced multi-episodes of tectonics and prolonged history of thermal evolution, thus making it more complicatedethe development and distribution of hydrocar- bon-source rocks reflected in the sedimentary, bio- logical and geochemical facies. Consequently, it seems much less powerful to assess the …  相似文献   

11.
Abstract Carbon isotope fluctuations of sedimentary organic matter along the two geological traverses in the Yezo Group, Hokkaido, northern Japan, elucidate a detailed chemostratigraphy for the Cenomanian Stage on the northwestern Pacific margin. Visual characterization of the kerogen from mudstone samples shows that the major constituents of sedimentary organic matter originated as terrestrial higher plants. The atomic hydrogen/carbon ratios of the kerogen suggest that the original δ13C values of terrestrial organic matter (TOM) have not been affected significantly by thermal diagenesis. The patterns in two δ13CTOM curves are similar and independent of changes in lithology and total organic carbon contents, which suggests that TOM was mixed sufficiently before the deposition in the Yezo forearc basin for the δ13C composition having been homogenized. In addition, this implies that the Hokkaido δ13CTOM profiles represent the averaged temporal δ13C variations of terrestrial higher‐plant vegetation in the hinterlands of northeast Asia during Cenomanian time. Three shorter‐term (ca. 0.1 my duration) positive‐and‐negative δ13CTOM fluctuations of ∼1‰ are present in the Lower to Middle Cenomanian interval in the Yezo Group. On the basis of the age‐diagnostic taxa (ammonoids, inoceramids and planktic foraminifers), these discrete δ13CTOM events are interpreted to be correlated with those in the δ13C curves of pelagic carbonates from European basins. The correlation of δ13C events between the European and Yezo Group sections suggests that the shorter‐term δ13C fluctuations in Cenomanian ocean‐atmosphere carbon reservoirs are useful for global chemostratigraphic correlation of marine strata. In particular, the correlation of δ13C fluctuations of the so‐called ‘Mid‐Cenomanian event’ (MCE) implies: (i) the δ13C variations of global carbon reservoir during the MCE are precisely recorded in the δ13CTOM records; and (ii) the MCE δ13CTOM event is an efficient chronostratigraphic index for the Lower/Middle Cenomanian boundary of the Mid‐Cretaceous sequences.  相似文献   

12.
The Ediacaran Doushantuo Formation with well-preserved fossil record in South China provides a rare window for our understanding of biological evolution,global carbon cycle,and oceanic redox states.Prominent negativeδ~(13)C anomalies(i.e.,the Shuram excursion)in Ediacaran successions worldwide fundamentally challenge the traditional models of isotopic mass balance.Additionally,conflicting opinions of both oxic and anoxic conditions have been proposed for the deep waters during this period.Here,we present a detailed study of pyrite morphology and carbonate carbon isotope data documented from drill core samples at Songtao County,northeastern Guizhou.Framboid aggregates are the dominant pyrite form in black shale and they can transfer to euhedral crystals through continuous growth of the constituent microcrystals.A positive correlation between microcrystal sizes(d)and framboid diameters(D)is observed,while the different D/d ratios of framboids in argillaceous dolostone and black shale reflect different substrate availability.Electron microprobe analyses reveal no consistent compositional patterns between framboidal and euhedral pyrites.Framboid size distributions of the investigated drill core,in combination with previously published redox data from the intra-shelf Jiulongwan section,shelf margin Zhongling section,and lower slope Wuhe section,suggest that three episodes of marine euxinia have been established throughout the deposition of the Doushantuo Formation.The time lag between the uppermost euxinic interval and the Shuram excursion may arise from the depression of sulfate reduction maintained by other oxidants.  相似文献   

13.
Marble has a great potential to understand a history of various geological events occurring during tectonic processes. In order to decode metamorphic–metasomatic records on C–O isotope compositions of marble at mid-crustal conditions, we conducted a C–O–Sr isotope study on upper amphibolite-facies marbles and a carbonate–silicate rock from the Hida Belt, which was once a part of the crustal basement of the East Asian continental margin. Carbon and oxygen isotope analyses of calcite from marbles (Kamioka area) and a carbonate–silicate rock (Wadagawa area) show a large variation of δ13C [VPDB] and δ18O [VSMOW] values (from −4.4 to +4.2 ‰ and +1.6 to +20.8 ‰, respectively). The low δ13C values of calcites from the carbonate–silicate rock (from −4.4 to −2.9 ‰) can be explained by decarbonation (CO2 releasing) reactions; carbon–oxygen isotope modeling suggests that a decrease of δ13C strongly depends on the amount of silicate reacting with carbonates. The occurrence of metamorphic clinopyroxene in marbles indicates that all samples have been affected by decarbonation reactions. All δ18O values of calcites are remarkably lower than the marine-carbonate values. The large δ18O variation can be explained by the isotope exchange via interactions between marble, external fluids, and/or silicates. Remarkably low δ18O values of marbles that are lower than mantle value (~+5 ‰) suggest the interaction with meteoric water at a later stage. Sr isotope ratios (87Sr/86Sr = 0.707255–0.708220) might be close to their protolith values. One zircon associated with wollastonite in a marble thin-section yields a U–Pb age of 222 ± 3 Ma, which represents the timing of the recrystallization of marble, triggered by H2O-rich fluid infiltration at a relatively high-temperature condition. Our isotope study implies that the upper amphibolite-facies condition, like the Hida Belt, might be appropriate to cause decarbonation reactions which can modify original isotope compositions of marble if carbonates react with silicates.  相似文献   

14.
Carbon isotope stratigraphy of the Late Jurassic and earliest Cretaceous was revealed from Torinosu‐type limestone, which was deposited in a shallow‐marine setting in the western Paleo‐Pacific, in Japan. Two sections were examined; the Nakanosawa section of the late Kimmeridgian to early Tithonian age (Fukushima Prefecture, Northeast Japan), and the Furuichi section of the late Kimmeridgian to early Berriasian age (Ehime Prefecture, Southwest Japan). The age‐model was established using Sr isotope ratio and fossil occurrence. The limestone samples have a low Mn/Sr ratio (mostly <0.5) and lack a distinct correlation between δ13C and δ18O, indicating a low degree of diagenetic alteration. Our composite δ13C profile from the two limestone sections shows three stratigraphic correlation points that can be correlated with the profiles of relevant ages from the Alpine Tethyan region: a large‐amplitude fluctuation (the lower upper Kimmeridgian, ～152 Ma), a positive anomaly (above the Kimmeridgian/Tithonian boundary, ～150 Ma), and a negative anomaly (the upper lower Tithonian, ～148 Ma). In addition, we found that δ13C values of the Torinosu‐type limestone are ～1‰ lower than the Tethyan values in the late Kimmeridgian. This inter‐regional difference in δ13C values is likely to have resulted from a higher productivity and/or an organic burial in the Tethyan region. The difference gradually reduces and disappears in the late Tithonian, where the Tethyan and our δ13C records show similar stable values of 1.5–2.0‰. This isotopic homogenization is probably due to changes in the continental distribution and the global ocean circulation, which propagated the 13C‐depleted signature from the larger Paleo‐Pacific to the smaller Tethys Ocean during this time.  相似文献   

15.
Lower Cretaceous C-isotope records show intermittent negative/positive spikes, and consistent patterns of coeval chemostratigraphic curves thus document shifts that signal simultaneous responses of temporal changes in the global carbon reservoir. The standard pattern registered by the δ 13Corg and δ 13Ccarb in Lower Aptian sediments includes distinct isotope segments C1 to C8 (Menegatti et al., 1998). In the El Pui section, Organyà Basin, Spain, C-isotope segment C2 is the longest interval preceding segments C3–C6 associated with oceanic anoxic event 1a (OAE 1a), and reveals a distinct negative shift of ~1.8‰ to ~2.23‰ defining the C-isotope pattern within that interval. Total inorganic carbon (TIC), total organic carbon (TOC), δ 13Corg, microfacies, n-alkanes show no difference before, during, or after the negative inflection. The biomarkers indicate that organic matter (OM) mainly originates from algal/microbial sources because short-chain length homologues (≤nC19) dominate. nC20 through nC25 indicate some contribution from aquatic vegetation, but little from higher plants (>nC25), as also suggested by the terrestrial/aquatic ratio of n-alkanes or (TAR) = [(nC27+nC29+nC31)/(nC15+nC17+nC19)] (averages 0.085). We suggest that conjoint pulses of contemporaneous LIPs (Ontong Java) and massive explosive volcanism in northeast Asia, the Songliao Basin (SB-V), best conform to plausible causes of the negative intra-C2 carbon isotopic excursion (CIE) at that time. Because of its apparent common occurrence the intra-C2 inflection could be a useful marker harbinger to the more pronounced CIE C3, the hallmark of OAE1a.  相似文献   

16.
Aquatic plants are essential for maintaining the diversity and stability of a lake ecosystem. Stable carbon isotopes (δ13C) of macrophytes have been widely used as a powerful tool to study ecological processes and paleoenvironmental evolution in lakes. Varying results are obtained when using the δ13C of macrophytes to study the changes in the lake environment at different spatio-temporal scales. Thus, sample preparation and subsequent laboratory analyses are crucial for studying environmental changes using the isotopic signal retained in the macrophytes, and are essential for the interpretation of isotope-environment relationships. This study analyzed the δ13C of different tissue components of macrophytes in three lakes of the lower Yangtze River basin, and a correlation analysis was performed on aquatic environments influencing the δ13C values in the different tissue components of macrophytes. The test results showed the difference between the δ13C values of the whole sample and cellulose. Relative analyses indicated that the major factors contributing to the δ13C variability in macrophytes were pH and the concentration of dissolved inorganic carbon (DIC). The δ13C of α-cellulose (δ13CAC) is more sensitive to environmental variables than that of the whole sample (δ13CW) and holocellulose (δ13CHC). The results of this study imply that extraction of α-cellulose is a prerequisite for research on the changes in lake environment using δ13C of macrophytes. This study aims to provide theoretical and data basis for further research on the environmental and ecological change using stable carbon isotopes of aquatic plants.  相似文献   

17.
We evaluated changes in siliceous export production and the source of organic matter preserved in sediment core MD07-3109H recovered from the Gulf of Ancud, Chiloé Inner Sea (42°S, 72°W, water column depth: 328 m), southern Chile. We analyzed the abundance of siliceous microfossils (diatoms, silicoflagellates, sponge spicules, Chrysophyte cysts, phytoliths), geochemical proxies (weight percent silicon %SiOPAL, organic carbon, total nitrogen, C/N molar), and sediment stable isotopes (δ13Corg, δ15N). Chronology based on 210Pb and 14C provided an accumulated age of 144 years at the base of the core.Sediments of core MD07-3109H are predominantly marine in origin, averaging δ13Corg=–20.75‰±0.82, δ15N=8.7±0.35‰, and C/N=8.76±0.36. Marine diatoms compose 94% of the total assemblage of siliceous microfossils. Our record of productivity based on the mass accumulation rates of organic carbon, total nitrogen, SiOPAL, and total diatoms showed high values between 1863 and 1869 AD followed by a declining trend until 1921 AD, a transition period from 1921 to 1959 AD with fluctuating values, and a clear decreasing pattern from 1960 AD to the present. This marked reduction in productivity was associated with decreased precipitation and Puelo River streamflow (41°S), as well as a warmer and more stratified water column, especially since the 1980s.  相似文献   

18.
ABSTRACT

This study aims to differentiate the potential recharge areas and flow mechanisms in the North-eastern Basin, Palestine. The results differentiate the recharge into three main groups. The first is related to springs and some of the deep wells close to the Anabta Anticline, through the Upper Aquifer (Turonian) formation, with depleted δ18O and δ2H. The second is through the Upper Cenomanian formation surrounding the Rujeib Monocline in the southeast, where the lineament of the Faria Fault plays an important role, with relatively enriched δ13CDIC values of about ?4‰ (VPDB). The third is the Jenin Sub-series, which shows higher δ13CDIC values, with enriched δ18O and δ2H and more saline content. The deep wells from the Nablus area in the south of the basin indicate low δ13CDIC values due to their proximity to freshwater infiltrating faults. The deep wells located to the northwest of the basin have δ13CDIC values from ?8 to ?9‰ (VPDB), with enriched δ18O signatures, indicating slow recharge through thick soil.  相似文献   

19.
Stable isotopic signatures (δ13C and δ15N) and C/N ratios of suspended particulate organic matter (POM) were investigated from the surface water of Daya Bay during summer and winter of 2015. The relatively high δ13CPOM values suggested the input of 13C-depleted terrigenous organic matter was low in Daya Bay. There were significant correlations between δ13CPOM values and chlorophyll a concentrations both during summer and winter, suggesting the δ13CPOM values were mainly controlled by the phytoplankton biomass in the surface water. The distribution of δ15NPOM values was more complicated than that of δ13CPOM and displayed low values in the outer bay and the Dan'ao River estuary. 15N-depleted ammonia originating from industrial wastewater might have strongly influenced the water quality and stable isotopic signatures of POM near the Dan'ao River estuary. The δ13CPOM and δ15NPOM values strongly reflect the influences of anthropogenic activity and eutrophication in Daya Bay.  相似文献   

20.
Dissolved inorganic carbon isotope (δ13CDIC) is an important tool to reveal the carbon cycle in lake systems. However, there are only few studies focusing on the spatial variation of δ13CDIC of closed lakes. Here we analyze the characteristics of δ13CDIC of 24 sampled lakes (mainly closed lakes) across the Qiangtang Plateau (QTP) and identify the driving factors for its spatial variation. The δ13CDIC value of these observed lakes varies in the range of ? 15·0 to 3·2‰, with an average value of ? 1·2‰. The δ13CDIC value of closed lakes is close to the atmospheric isotopic equilibrium value, much higher than that in rivers and freshwater lakes reported before. The high δ13CDIC value of closed lakes is mainly attributed to the significant contribution of carbonate weathering in the catchment and the evasion of dissolved CO2 induced by the strong evaporation of lake water. The δ13CDIC value of closed lakes has a logarithmic correlation with water chemistry (TDS, DIC and pCO2), also suggesting that the evapo‐concentration of lake water can influence the δ13CDIC value. The δ13CDIC value shows two opposite logarithmic correlations with lake size depending on the δ13CDIC range. This study suggests that the δ13C in carbonates in lacustrine sediments can be taken as an indicator of lake volume variation in closed lakes on QTP. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号