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1.
Total organic carbon (TOC) content, total nitrogen (TN) content, stable nitrogen isotope (δ15N) and stable organic carbon isotope (δ13Corg) ratios were continuously analysed on a high resolution sediment profile from Lake Sihailongwan (SHL), covering the time span between 16,500 and 9,500 years BP. Strong variations of the investigated proxy parameters are attributed to great climatic fluctuations during the investigated time period. Variations in organic carbon isotope ratios and the ratio of TOC/TN (C/N ratio) are discussed with respect to changing proportions of different organic matter (OM) sources to bulk sedimentary OM. Phases of high TOC content, high TN content, depleted δ13Corg values and high δ15N values are interpreted as times with increased productivity of lacustrine algae in relation to input of terrigenous organic matter. Two distinct phases of enriched nitrogen isotope ratios from 14,200 to 13,700 and 11,550 to 11,050 years BP point towards a reduced phytoplankton discrimination against 15N due to a diminished dissolved inorganic nitrogen pool. The combination of geochemical (TOC, TN, C/N ratio) and isotopic (δ13Corg, δ15N) proxy parameters points to a division of climate development into four stages. A cold and dry stage before 14,200 years BP, a warm optimum stage with high phytoplankton productivity from 14,200 to 12,450 BP, a colder and drier stage from 12,450 to 11,600 BP and a stage of climatic amelioration with high variability in TOC and TN contents after 11,600 BP. These results are discussed in relation to monsoon variability and Northern Hemisphere climate development of the late glacial.  相似文献   

2.
Carbon stable isotope ratios were determined in dominant biotic components of pelagic and littoral systems in Alchichica crater-lake. Results showed that carbon signatures were significantly different between both systems. The pelagic environment was more depleted (−26.15 to −15.14 per mille) than the littoral zone (−21.03 to −17.91 per mille). The potential source end-point in the simplified pelagic community was established to be diatomaceous phytoplankton; its predicted value was −21.7 per mille. There is a clear evidence thatNodularia does not sustain the pelagic food chain. In contrast, the highly diverse littoral community was sustained by epiphytes. No allochthonous sources seemed to influence this food web.13C enrichment was observed along the components of both systems with fractionations of 0.8 to 1.4 per mille. The contribution of the seagrassRuppia maritima is probably associated with the detritus pathway. Carbon source partitioning between both systems was not recorded. The δ13C in Alchichica crater-lake was more enriched than in other saline lakes and could be attributed to different salinity and CO2 concentrations among lakes.  相似文献   

3.
Stable isotope analyses on cladoceran subfossil exoskeletons retrieved from sediment cores could allow the reconstruction of past changes in lake food webs provided the δ13C and δ15N values of the exoskeletons reflect those of the organisms’ whole body. The relationships between the C and N stable isotope compositions of the exoskeletons and those of the whole body were investigated for two freshwater cladoceran taxa (Bosmina sp. and Daphnia sp.) from modern samples. The C and N stable isotope compositions of the exoskeleton and those of the whole body were strongly correlated. Exoskeleton δ13C was similar to the whole body δ13C for both taxa. Daphnia exoskeletons were strongly depleted in 15N (−7.9‰) compared to the whole body. Stable isotope analyses were thereafter performed on cladoceran remains from five downcore samples from Lake Annecy, France. Results showed that Bosmina δ15N values increased by more than 4‰, between the early twentieth and twenty first centuries. Such changes might be the result of changes in nitrogen sources or cycling in the lake and/or of major shifts in Bosmina trophic position within the lake food web. This study sets up the potential of stable isotope analyses performed on cladoceran subfossil remains for paleo-ecological purposes.  相似文献   

4.
We used elemental carbon, nitrogen, phosphorus and hydrogen ratios (C/N, N/P and H/C) with total organic carbon (TOC) and total phosphorus (TP) as well as stable carbon and nitrogen isotopes (δ13C and δ15N) to investigate the source and depositional conditions of organic matter in sediments from Zeekoevlei, the largest freshwater lake in South Africa. Typical C/N (10–12), H/C ratios (≥1.7) and δ13Corganic values (−22 to −19‰) together with the increase in TOC concentration indicate elevated primary productivity in lower middle (18–22 cm) and top (0–8 cm) sections of the sediment cores. Seepage of nutrients from a nearby waste water treatment plant, rapid urbanization and heavily fertilized farming in the catchments are responsible for the increased productivity. Consistent with this, measured δ15Norganic values (∼11‰) indicate increased raw sewage input towards the top-section of the core. Although cyanobacterial blooms are evident from the low δ15N values (∼3‰) in mid-section of the core, they did not outnumber the phytoplankton population. Low N/P ratio (∼0) and high TP (100–2,200 mg l−1) support cyanobacterial growth under N limited condition, and insignificant input of macrophytes towards the organic matter pool. Dredging in 1983, caused sub-aerial exposure of the suspended and surface sediments, and affected organic matter preservation in the upper mid-section (12–14 cm) of the core.  相似文献   

5.
We used a series of experiments to determine whether stable carbon isotope analysis of modern and fossil larval head capsules of chironomids allowed identification of their dietary carbon source. Our main focus was to assess whether carbon from naturally 13C-depleted methane-oxidizing bacteria (MOB) can be traced in chironomid cuticles using stable carbon isotope analysis. We first showed that a minimum sample weight of ~20 μg was required for our equipment to determine head capsule δ13C with a standard deviation of 0.5‰. Such a small minimum sample weight allows taxon-specific δ13C analyses at a precision sufficient to differentiate whether head capsules consist mainly of carbon derived from MOB or from other food sources commonly encountered in lake ecosystems. We then tested the effect of different chemical pre-treatments that are commonly used for sediment processing on δ13C measurements on head capsules. Processing with 10% KOH (2 h), 10% HCl (2 h), or 40% HF (18 h) showed no detectable effect on δ13C, whereas a combination of boiling, accelerated solvent extraction and heavy chemical oxidation resulted in a small (0.2‰) but statistically significant decrease in δ13C values. Using culturing experiments with MOB grown on 13C-labelled methane, we demonstrated that methanogenic carbon is transferred not only into the larval tissue, but also into chironomid head capsules. Taxon-specific δ13C of fossil chironomid head capsules from different lake sediments was analyzed. δ13C of head capsules generally ranged from −28 to −25.8‰, but in some instances we observed δ13C values as low as −36.9 to −31.5‰, suggesting that carbon from MOB is traceable in fossil and subfossil chironomid remains. We demonstrate that stable carbon isotope analyses of fossil chironomid head capsules can give insights into dietary links and carbon cycling in benthic food webs in the past and that the method has the potential to reconstruct the importance of MOB in the palaeo-diet of chironomid larvae and, indirectly, to infer past changes in methane flux at the sediment water interface in lakes.  相似文献   

6.
Total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) were determined in combination with stable isotope ratios of carbon and nitrogen (δ13COrg, δ15N) in a 63 cm sediment core from Longgan Lake, located in the middle reaches of the Yangtze River, China. These geochemical and isotopic records provide a continuous history of lake productivity and trophic state of Longgan Lake since 1890. Variations of δ13COrg, TOC, TN and TP indicate that primary productivity of Longgan Lake increased continuously during the last century and that the trophic state of the lake shifted from oligotrophic to mestrotrophic conditions accordingly. Anthropogenic sources of organic carbon (OC), nitrogen (N) and phosphorus (P) were distinguished from their natural background in the sediments using mass accumulation rates. Element mass accumulation rates suggested increased human activities in the lake’s catchment since 1950s, were especially the utilization of artificial fertilizers amplified the anthropogenic input of N and P into the lake. In the course of the improved availability of dissolved nutrients also primary productivity of Longgan Lake increased, resulting in an increase of the Suess-effect corrected organic carbon isotope ratios. δ15N of bulk sediments show a marked shift towards lower values around 1950 that has been attributed to the input of nitrogen from chemical fertilizers characterized by relatively depleted isotopic signatures into the lake.  相似文献   

7.
A sediment core from Lake Koucha (eastern Tibetan Plateau) was investigated using organic biomarkers and their stable carbon isotope signatures. The correlation between TOC content, total amount of aquatic macrophyte-derived n-alkanes (e.g. nC23) and δ13C values of TOC and nC23 indicates that Lake Koucha was macrophyte-dominated before 8 cal ka BP. Shortly after the lake turned from a saline to a freshwater system at 7.2 cal ka BP, a variety of algal and bacterial markers such as hopanoids and isoprenoids emerged, of which phytane, pentamethylicosene (PMI), moretene and diploptene are particularly abundant. Phytane and PMI show different isotopic signals (≈−18 and ≈−28‰, respectively), which indicates that they originated from different sources. Phytane may have been derived from cyanobacteria, while methanogenic archaea may be the source of PMI. The isotopic depletion of diploptene and moretene (≈−60‰) indicates the presence of methanotrophs. After 6.1 cal ka BP, the saturated C20 highly branched isoprenoid (HBI) became the dominant constituent of the aliphatic hydrocarbon fraction. Such dominance has rarely been reported in lacustrine environments, and indicates a strong presence of algae (most likely diatoms) or cyanobacteria. At 4.7 cal ka BP, the appearance of an unsaturated C25 HBI, which is a specific biomarker for diatoms, was noted. Furthermore, the level of nC17-alkane was observed to increase in abundance in the uppermost two samples. These results suggest that the lake was phytoplankton-dominated during the last 6.1 ka. Relatively low biomarker concentrations and δ13C values at 6.0, 3.1 and 1.8 cal ka BP indicate the occurrence of cool periods, which is in agreement with inferences from other locations on the Tibetan Plateau. The δ13C values of nC23 range from −23.5 to −12.6‰, with high values at the peak of macrophyte abundance at ca. 11 cal ka BP and at the phytoplankton maximum between ca 6.1 and 2.8 cal ka BP. Thus, aquatic macrophyte-derived mid-chain n-alkanes have been found to be excellent indicators of carbon-limiting conditions, which lead to the assimilation of isotopically-enriched carbon species. The limitation of carbon sources could be a localized phenomenon occurring in dense plant stands (as in the older section of the core), or it may be induced by high primary productivity (as in the younger section). Since the δ13C value of the inorganic carbon source may vary, the offset between the δ13C values of nC23 and TIC could serve as a more precise proxy for carbon-limiting conditions in lacustrine environments, which could in turn be interpreted with respect to lacustrine paleo-productivity.  相似文献   

8.
Close correspondence between stable carbon isotope ratios ( 13 C), pollen, and charcoal profiles in sediment cores from Laguna Zoncho and Machita swamp, Costa Rica, shows that prehistoric forest clearance and crop cultivation can be detected in the stable carbon isotope ratios of total organic carbon ( 13C TOC ). Analyses of δ 13C TOC complement evidence from pollen, charcoal, and phytoliths and provide a proxy that is sensitive to the intensity and/or proximity to core sites of prehistoric forest clearance and agriculture in watersheds. Stable carbon isotope analyses are particularly useful in situations in which other evidence of forest clearance and agriculture is limited.  相似文献   

9.
The apparent isotope enrichment factor εmacrophyte of submerged plants (εmacrophyte–DIC = δ13Cmacrophyte − δ13CDIC) is indicative of dissolved inorganic carbon (DIC) supply in neutral to alkaline waters and is related to variations in aquatic productivity (Papadimitriou et al. in Limnol Oceanogr 50:1084–1095, 2005). This paper aims to evaluate the usage of εmacrophyte inferred from isotopic analyses of submerged plant fossils in addition to analyses of lake carbonate as a palaeolimnological proxy for former HCO3 − concentrations. Stable carbon isotopic analysis of modern Potamogeton pectinatus leaves and its host water DIC from the Tibetan Plateau and Central Yakutia (Russia) yielded values between −23.3 and +0.4‰ and between +14.0 and +6.5‰, respectively. Values of ε Potamogeton–DIC (range −15.4 to +1.1‰) from these lakes are significantly correlated with host water HCO3 − concentration (range 78–2,200 mg/l) (r = −0.86; P < 0.001), thus allowing for the development of a transfer function. Palaeo-ε Potamogeton–ostracods values from Luanhaizi Lake on the NE Tibetan Plateau, as inferred from the stable carbon isotope measurement of fossil Potamogeton pectinatus seeds (range −24 to +2.8‰) and ostracods (range −7.8 to +7.5%) range between −14.8 and 1.6‰. Phases of assumed disequilibrium between δ13CDIC and δ13Costracods known to occur in charophyte swards (as indicated by the deposition of charophyte fossils) were excluded from the analysis of palaeo-ε. The application of the ε Potamogeton–DIC-HCO3 − transfer function yielded a median palaeo-HCO3 − -concentration of 290 mg/l. Variations in the dissolved organic carbon supply compare well with aquatic plant productivity changes and lake level variability as inferred from a multiproxy study of the same record including analyses of plant macrofossils, ostracods, carbonate and organic content.  相似文献   

10.
This study presents detailed lithostratigraphy and stable carbon and nitrogen isotopic variations in a 520-cm-long sediment core from a cirque basin in the Labsky důl Valley, Krkonoše Mountains, Czech Republic. Detailed study of the core reveals five major periods of sedimentation during the last 7600 years: silt and sand deposition during ~7.6–5.1 ka cal BP, Sphagnum peat accumulation during ~5.1–4.0 ka cal BP, sandy silt and sand during ~4.0–2.8 ka cal BP, raised peat bog during ~2.8–2.0 ka cal BP (Sphagnum peat), and sedimentation of sandy silt since ~2.0 ka cal BP. The δ13C values of the organic matter in the core vary in the range typical for C3 plants, from −24.35 to −27.68‰, whereas the δ15N values vary from −2.65 to +4.35‰. Core sections having ash contents ≥70% have δ15N > 1‰ and δ13C < −26‰, whereas those having ≤70% ash content have δ15N < 1‰ and δ13C > −26‰. Strong linear correlations are observed between δ13C and δ15N values as well as between C:N ratios and δ15N values in the horizons with ash content >10%, primarily for sand and silt horizons. On the other hand, poor correlations between δ13C and C:N ratio, as well as δ15N and C:N ratio, were observed in Sphagnum peat layers (45–125 and 185–265 cm). We conclude that the primary stable isotope variations are not preserved in the layers where significant correlation between δ15N and C:N ratio is observed. The relatively small δ13C variation in the uppermost Sphagnum peat layer suggests stable temperature during ~2.8–2.0 ka cal BP.  相似文献   

11.
Stable isotope analysis of ostracod shells is used routinely for palaeoenvironmental studies of ostracod-bearing records. Sample treatment usually involves the disaggregation of sediments and sieving; before the sieving residues were washed with water onto petri dishes and oven-dried. In our study, we compared δ18O and δ13C values of shells that were oven-dried from water and from ethanol alternatively. Large isotopic differences of up to 3‰ were determined for δ18O values, whereas differences in δ13C values were less pronounced with differences of up to 1.6‰. Stable isotope values of shells dried from water were lower for both oxygen and carbon as a result of calcite crystals precipitated on the shell surfaces during the drying process. Therefore, ostracod shells for stable isotope analysis should not be prepared by drying from water. Instead, shells should be dried from ethanol to obtain reliable stable isotope data; likewise freeze-drying is expected to provide trustworthy results.  相似文献   

12.
A 7.6-m lake sediment core from a marl lake, Lough Inchiquin, records variation in landscape evolution from 16,800 cal yrs B.P. to 5,540 cal yrs B.P. We observe significant variations (up to 12‰) in δ 13Corg and δ 13Ccalcite values that are interpreted to reflect secular changes in lake water δ 13CDIC values that result from a regional landscape transition from barren limestone bedrock to a forested ecosystem. Lake water δ 13CDIC values are therefore influenced by two isotopically distinct sources of carbon: terrestrial organic material (−27.1 to −31.2‰VPDB) via oxidized soil organic matter and weathered limestone bedrock (+3.4‰VPDB). Isotope excursions in lacustrine sediment records are forced not only by changes in productivity but also by changes in the terrestrial environment. This has profound implications for the interpretation of paleoclimate records derived from lacustrine sediment and suggests that selection of appropriate lakes can provide records of terrestrial change where other related records are not available.  相似文献   

13.
中国亚热带地区造林对土壤碳周转的影响   总被引：5，自引：1，他引：4  
Afforestation in China’s subtropics plays an important role in sequestering CO2 from the atmosphere and in storage of soil carbon (C). Compared with natural forests,plantation forests have lower soil organic carbon (SOC) content and great potential to store more C. To better evaluate the effects of afforestation on soil C turnover,we investigated SOC and its stable C isotope (δ13C) composition in three planted forests at Qianyanzhou Ecological Experimental Station in southern China. Litter and soil samples were collected and analyzed for total organic C,δ13C and total nitrogen. Similarly to the vertical distribution of SOC in natural forests,SOC concentrations decrease exponentially with depth. The land cover type (grassland) before plantation had a significant influence on the vertical distribution of SOC. The SOC ?13C composition of the upper soil layer of two plantation forests has been mainly affected by the grass biomass 13C composition. Soil profiles with a change in photosynthetic pathway had a more complex 13C isotope composition distribution. During the 20 years after plantation establishment,the soil organic matter sources influenced both the δ13C distribution with depth,and C replacement. The upper soil layer SOC turnover in masson pine (a mean 34% of replacement in the 10 cm after 20 years) was more than twice as fast as that of slash pine (16% of replacement) under subtropical conditions. The results demonstrate that masson pine and slash pine plantations cannot rapidly sequester SOC into long-term storage pools in subtropical China.  相似文献   

14.
An 11.6 m long continuous succession of annually laminated sediments from Lake Korttajärvi in central Finland was investigated for the isotopic composition of carbon and hydrogen in organic matter. The sequence covers a time period of 9590 years, and the varve chronology has been thoroughly described in earlier studies. From 7100 to 4400 BC the lake was part of the Ancient Lake Päijänne, but in 4400 BC it became separated and formed the present independent lake system. Two organic fractions were investigated. One fraction obtained by HCl-treatment was analyzed for δ13C and another HCl-HF-digested organic fraction was analyzed for both δ13C and δD. The isotopic data were compared to atomic C/N ratios, carbon contents, diatom-inferred pH values and other environmental parameters. The diatom-inferred pH values and organic carbon contents provide evidence for a long-term change towards more acidic conditions and lower productivity in Lake Korttajärvi. The inferred pH values decrease from 7.0 to 6.1, followed by a slight increase during the last millennia. Variations in pH are accompanied by an increase in the δ13CHCl-HF values of organic matter from ?31.6 to $-29.2\permilleAn 11.6 m long continuous succession of annually laminated sediments from Lake Korttaj?rvi in central Finland was investigated for the isotopic composition of carbon and hydrogen in organic matter. The sequence covers a time period of 9590 years, and the varve chronology has been thoroughly described in earlier studies. From 7100 to 4400 BC the lake was part of the Ancient Lake P?ij?nne, but in 4400 BC it became separated and formed the present independent lake system. Two organic fractions were investigated. One fraction obtained by HCl-treatment was analyzed for δ13C and another HCl-HF-digested organic fraction was analyzed for both δ13C and δD. The isotopic data were compared to atomic C/N ratios, carbon contents, diatom-inferred pH values and other environmental parameters. The diatom-inferred pH values and organic carbon contents provide evidence for a long-term change towards more acidic conditions and lower productivity in Lake Korttaj?rvi. The inferred pH values decrease from 7.0 to 6.1, followed by a slight increase during the last millennia. Variations in pH are accompanied by an increase in the δ13CHCl-HF values of organic matter from −31.6 to , followed by a subtle decrease to . The changes in pH and δ13CHCl-HF are closely related (r = − 0.91, P < 0.01), and apparently reflect changing environmental conditions in the lake and in its catchment area. δD values show a marked shift to higher values during the early Holocene, which may be partly related to a climatic amelioration leading to the Holocene Climatic Optimum in 6000–2500 BC. The Medieval Warm Period in AD 980–1250 is associated with a local maximum in δD, lending support for a significant warming during that time.  相似文献   

15.
西江河口段溶解无机碳稳定同位素组成的时空变化   总被引：3，自引：1，他引：2  
For researching the spatio-temporal variation of the stable isotopic composition of the riverine dissolved inorganic carbon（DIC）,we had carried out a survey throughout the hydrologic year during which theδ^13CDIC of the surface water and its physicochemical parameter were examined along the Xijiang River Inner Estuarine waterway from September 2006 to June 2007.There was a striking seasonal variation on the averageδ^13CDIC,as the averageδ^13CDIC in summer（-13.91‰）or autumn（-13.09‰）was much less than those in spring（-11.71‰）or winter（-12.26‰）.The riverineδ13C DIC was controlled by decomposed condition of the riverine organic matter linking the seasonal variation of the physicochemical parameter in the surface water according to the correlation analysis which indicated notable relations betweenδ^13CDIC and water temperature（p=0.000;r=-0.569）or betweenδ^13CDIC and oxide-reduction potential（p=0.000;r=0.646）.The striking positive correlation between δ^13CDIC and the sampling distance happened in the summer rainy season,while striking negative correlation happened in the spring dry season,indicating that river-sea interaction influenced water physicochemical parameters and controlled the riverine DIC property in the survey waterway.In view of the riverineδ^13CDIC decreasing for the decomposition of the terrestrial organic matter in the rainy season in summer and increasing for the briny invaded zone extending in the spring dry season along the waterway from the Makou gauging station to the Modaomen outlet,theδ^13CDIC spatio-temporal variation was closely related to the geographical environment of the Xijiang drainage basin.  相似文献   

16.
Walker Lake, a hydrologically closed, saline, alkaline lake located along the western margin of the Great Basin of western United States, has experienced a 77% reduction in volume and commitment drop in lake level as a result of anthropogenic perturbations and climatic fluctuations over the last century. The history of lake-level change in Walker Lake has been recorded instrumentally since 1860. A high-resolution multi-proxy sediment core record from Walker Lake has been generated through analysis of total inorganic carbon (TIC), total organic carbon (TOC), and oxygen and carbon isotope ratios (δ18O and δ13 C) of both downcore bulk TIC and ostracods over the last 200 yr. This allows us to examine how these sediment indices respond to actual changes in this lake’s hydrologic balance at interannual to decadal timescales. In Walker Lake sediments, changes in %TIC, %TOC, and δ13C and δ18O of TIC and ostracods are all associated to varying degrees with changes in the lake’s hydrologic balance, with δ18O of the TIC fraction (δ18OTIC) being the most highly correlated and the most effective hydrologic indicator in this closed-basin lake. The δ18OTIC record from Walker Lake nearly parallels the instrumental lake-level record back to 1860. However, comparison with sporadic lake-water δ18O and dissolved inorganic carbon δ13C (δ13CDIC) results spanning the last several decades suggests that the isotopic values of downcore carbonate sediments may not be readily translated into absolute or even relative values of corresponding lake-water δ18O and δ13CDIC. Changes in the lake’s hydrologic balance usually lead to changes in isotopic composition of lake waters and downcore sediments, but not all the variations in downcore isotopic composition are necessarily caused by hydrologic changes.  相似文献   

17.
A ~106-cm sediment core from the eastern basin of Lake Erie was examined to investigate biogeochemical processes in this large lake during its cultural eutrophication over the last century. We measured stable carbon isotopes of total organic carbon and calcium carbonate (δ13CTOC and d13 \textC\textCaCO 3 \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} ) as well as the concentrations of total organic carbon (TOC) and calcium carbonate (CaCO3). δ13CTOC and TOC show a strong positive correlation throughout the core and record changes in phytoplankton productivity and nutrient loading. CaCO3 and TOC concentrations display a negative correlation throughout the core, suggesting that CaCO3 concentrations are controlled primarily by decomposition of TOC in the hypolimnion and the sediments, although temperature and invasive mussels are also potential controlling factors. d13 \textC\textCaCO 3 \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} values show a positive correlation with δ13CTOC between 1909 and 1969, indicating phytoplankton productivity was the primary control for d13 \textC\textCaCO 3 \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} values during eutrophication. However, a negative correlation between d13 \textC\textCaCO 3 \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} and δ13CTOC from 1970 to 2002 suggests that these two proxies tracked different aspects of the carbon cycle in the lake in more recent times. The cause for the negative correlation is not yet known, but it is perhaps associated with temperature variations and seasonal differences in productivity.  相似文献   

18.
Continuous terrestrial records of paleoclimate and paleovegetation that extend to the late Pleistocene are rare for the circum-Caribbean uplands. In this study we analyzed the bulk and compound-specific carbon isotope composition of lake sediments spanning this period from Lago de las Morrenas 1 (LM1), a glacial lake in the highlands of southern Costa Rica, for evidence of climate and vegetation changes that may not have been apparent in previous analyses. The stable carbon isotope ratios of n-alkanes typically derived from terrestrial plants (δ13CC27–C33) indicate an increased abundance of C4 plant taxa during the late Pleistocene and earliest Holocene that may be related to decreased atmospheric carbon dioxide concentrations, increased aridity, or habitat availability. These n-alkane isotope ratios also provide evidence of more arid conditions during the early and late Holocene, and more mesic conditions during the middle Holocene, a pattern prevalent in other paleoclimate records from the region that is thought to be related to millennial-scale dynamics of the intertropical convergence zone (ITCZ). The sensitivity of the LM1 paleorecord to trade wind dynamics provides further support for the role of millennial-scale shifts in ITCZ dynamics in driving neotropical environmental change, and indicates that the effects of ITCZ migration were not limited to the lowlands.  相似文献   

19.
The utility of ostracod-based palaeoenvironmental reconstruction was evaluated using instrumental data for Lake Qarun, Egypt. The euryhaline ostracod Cyprideis torosa was the only species found in the lake’s recent sediment record. This species is known to tolerate salinity levels and water solute compositions that may prevent colonisation by other species. Oxygen and carbon isotope ratios of ostracod carbonate from lake sediments covary with changes in instrumental values for lake level and salinity for the period 1890–1974. δ13C-values correlate negatively with lake water salinity (r 2 = 0.87) and δ18O-values correlate negatively with measured lake level changes (r 2 = 0.41). Other ostracod proxy data provide qualitative information on lake level trends. Fossil assemblage data (juvenile/adult and valve/carapace ratios and valve preservation) provide information on wave energy. Ecophenotypic variation of C. torosa valves provided some useful palaeolimnological information. Sr/Ca and Mg/Ca ratios in ostracods were not found to reflect water composition, due to the uncoupling of these ratios with salinity in Lake Qarun. Overall, our results highlight the need to calibrate ostracod proxy data in modern systems prior to their use for palaeoenvironmental reconstruction.  相似文献   

20.
Lake sediments contain archives of past environmental conditions in and around water bodies and stable isotope analyses (δ13C and δ15N) of sediment cores have been used to infer past environmental changes in aquatic ecosystems. In this study, we analyzed organic matter (OM), carbon (C), nitrogen (N), phosphorus (P), and δ13C and δ15N values in sediment cores from three subtropical lakes that span a broad range of trophic state. Our principal objectives were to: (1) evaluate whether nutrient concentrations and stable isotope values in surface deposits reflect modern trophic state conditions in the lakes, and (2) assess whether stratigraphic changes in the measured variables yield information about shifts in trophic status through time, or alternatively, diagenetic changes in sediment OM. Three Florida (USA) lakes of very different trophic status were selected for this study. Results showed that both δ13C and δ15N values in surface sediments of the oligo-mesotrophic lake were relatively low compared to values in surface sediments of the other lakes, and were progressively lower with depth in the sediment core. Sediments of the eutrophic lake had δ13C values that declined upcore, whereas δ15N values increased toward the sediment surface. The eutrophic lake displayed δ13C values intermediate between those in the oligo-mesotrophic and hypereutrophic lakes. Sediments of the hypereutrophic lake had relatively higher δ13C and δ15N values. In general, we found greater δ13C and δ15N values with increasing lake trophic state.  相似文献   

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