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1.
Reconnaissance 18O,, D, and 87Sr data for fifteen lakes in the Western Lakes Region of the Sand Hills of Nebraska indicate dynamic hydrologic systems. The rather narrow range of 87Sr from lake water (1.1 to 2.1) and groundwater (0.9 to 1.7) indicates that the groundwater is generally unradiogenic. Groundwater residence times and relatively unradiogenic volcanic ash within the dune sediments control the 87Sr values. Based on the mutual variations of 18O and D, the lakes can be divided into three groups. In Group 1, both 18O and D values increase from spring to fall. The 18O and D values in Group 2 decreased from spring to fall. Group 3 are ephemeral lakes that went dry some time during 1992. The data and isotopic modeling show that variations in the ratio of evaporation relative to groundwater inflow, local humidity conditions, and the a has substantial influence on the isotopic composition. In addition, isotopic behavior in ephemeral lakes can be rather unusual because of the changing activities of water and mineral precipitation and redissolution. The annual and interannual isotopic variability of these lakes which is reflected in the paleonvironmental indicators may be the rule rather than the exception in these types of systems.  相似文献   

2.
The stable isotopic records of ostracode valves deposited during the last interglaciation in Raymond Basin, Illinois, have 13C and 18O values as high as +16.5 and +9.2 respectively, the highest values yet reported from continental ostracodal calcite. Located in south-central Illinois, Raymond, Pittsburg, Bald Knob, and Hopwood Farm basins collectively have yielded important long pollen and ostracode records that date from about 130000 years ago to the present. Although fossils from the present-day interglaciation are not well preserved, these records constitute the only described, conformable, fossiliferous successions of this age from the interior of glaciated North America.The high 13C values from Raymond Basin are attributed to the residual effects of methane loss either by ebullition or by emission through the stems of senescent emergent aquatic vegetation. A mass balance model suggests that an increase in 13C of dissolved inorganic carbon on the order of +15 is possible within a few hours given modest rates of methanogenesis of about 0.02 mol m-2 d-1. The 13C records from other studies of ostracode valves have values approaching, but not exceeding about +14 suggesting a limiting value to 13C enrichment due to simultaneous inputs and outputs of dissolved inorganic carbon.Values of 18O in ostracodal calcite are quite variable (–4 to +9) in sediment from the late Sangamon subepisode. A model of isotopic enrichment in a desiccating water body implies that a reduction in reservoir volume of 20% could produce this range of isotopic values. High humidity and evaporation probably account for most of the 18O variability.  相似文献   

3.
Evaporation dominates the removal of water from Lake Tanganyika, and therefore the oxygen isotope composition of lake water has become very positive in comparison to the waters entering the lake. The surface water in Lake Tanganyika has remained relatively unchanged over the last 30 years with a seasonal range of +3.2 to +3.5 VSMOW. Water from small rivers entering the lake seems to have a 18O value between –3.5 and –4.0, based on scattered measurements. The two largest catchments emptying into the lake deliver water that has a 18O value between these two extremes. This large contrast is the basis of a model presented here that attempts to reconstruct the history of runoff intensity based on the 18O of carbonate shells from Lake Tanganyika cores. In order to use biogenic carbonates to monitor changes in the 18O of mixing-zone water, however, the oxygen isotope fractionation between water and shell carbonate must be well understood. The relatively invariant environmental conditions of the lake allow us to constrain the fractionation of both oxygen and carbon isotope ratios. Although molluskan aragonitic shell 18O values are in agreement with published mineral-water fractionations, ostracode calcite is 1.2 more positive than that of inorganic calcite precipitated under similar conditions. Ostracode shell 18O data from two cores from central Lake Tanganyika suggest that runoff decreased in the first half of this millennium and has increased in the last century. This conclusion is poorly constrained, however, and much more work needs to be done on stable isotope variation in both the waters and carbonates of Lake Tanganyika. We also compared the 13C of shells against predicted values based solely on the 13C of lake water dissolved inorganic carbon (DIC). The ostracode Mecynocypria opaca is the only ostracode or mollusk that falls within the predicted range. This suggests that M. opaca has potential for reconstructing the carbon isotope ratio of DIC in Lake Tanganyika, and may be a useful tool in the study of the history of the lakes productivity and carbon cycle.  相似文献   

4.
A fingernail clam (Sphaerium simile, Sphaeriidae) from Science Lake, a small watershed located in Allegany State Park, New York, USA and a zebra mussel (Dreissena polymorpha, Dreissenidae) from Keuka Lake, New York, the third largest Finger Lake of central New York, were selected to evaluate the applicability of using 18O(CaCO3) and 13O(CaCO3) values for sub-weekly climate records. Seasonal variation in 18O(CaCO3) values was compared with predicted equilibrium values to test the hypothesis that lacustrine molluscs produce shell aragonite according to environmental variables. For the purpose of comparison, aragonite temperature-fractionation equations determined by Grossman& Ku (1986) and Patterson et al. (1993) were used. Sphaerium simile appears to produce 18O(CaCO3) values predicted by Patterson et al. (1993), while Dreissena polymorpha produces 18O(CaCO3) values in agreement with Grossman & Ku (1986). We attribute the difference to family-specific temperature-fractionation relationships. Because both types of mollusc record climate variables with a high degree of integrity, they should each serve as excellent paleoclimate proxies.The fingernail clam collected from a small watershed exhibits higher variation about the seasonal pattern than did the zebra mussel collected from a large watershed. This is attributed to the increased sensitivity of the small watershed to storm perturbation. Analysis of fossil molluscs from such watersheds might be useful in discerning paleo-storminess.  相似文献   

5.
Oxygen isotopes and geochemistry from lake sediments are commonly used as proxies of past hydrologic and climatic conditions, but the importance of present-day hydrologic processes in controlling these proxies are sometimes not well established and understood. Here we use present-day hydrochemical data from 13 lakes in a hydrologically connected lake chain in the northern Great Plains (NGP) to investigate isotopic and solute evolution along a hydrologic gradient. The 18O and 2H of water from the chain of lakes, when plotted in 2H - 18O space, form a line with a slope of 5.9, indicating that these waters fall on an evaporation trend. However, 10 of the 13 lakes are isotopically similar (18O = –6 ± 1 VSMOW) and show no correlation with salinity (which ranges from 1 to 65). The lack of correlation implies that the isotopic composition of various source waters rather than in-lake evaporation is the main control of the 18O of the lakes. Groundwater, an important input in the water budget of this chain of lakes, has a lower 18O value (–16.7 in 1998) than that of mean annual precipitation (–11) owing to selective recharge from snow melt. For the lakes in this chain with salinity < 15, the water Mg/Ca ratios are strongly correlated with salinity, whereas Sr/Ca is not. The poor correlation between Sr/Ca and salinity results from uptake of Sr by endogenic aragonite. These new results indicate that 18O records may not be interpreted simply in term of climate in the NGP, and that local hydrology needs to be adequately investigated before a meaningful interpretation of sedimentary records can be reached.  相似文献   

6.
We explored the use of carbon and nitrogen isotopes (13C and 15N) in sedimented organic matter (OM) as proxy indicators of trophic state change in Florida lakes. Stable isotope data from four 210Pb-dated sediment cores were compared stratigraphically with established proxies for historical trophic state (diatom-inferred limnetic total phosphorus, sediment C/N ratio) and indicators of cultural disturbance (sediment total P and 226Ra activity). Diatom-based limnetic total P inferences indicate a transition from oligo-mesotrophy to meso-eutrophy in Clear Lake, and from eutrophy to hypereutrophy in Lakes Parker, Hollingsworth and Griffin. In cores from all four lakes, the carbon isotopic signature of accumulated OM generally tracks trophic state inferences and cultural impact assessments based on other variables. Oldest sediments in the records yield lower diatom-inferred total limnetic P concentrations and display relatively low 13C values. In the Clear, Hollingsworth and Parker records, diatom-inferred nutrient concentrations increase after ca. AD 1900, and are associated stratigraphically with higher 13C values in sediment OM. In the Lake Griffin core, both proxies display slight increases before ~1900, but highest values occur over the last ~100 years. As Lakes Clear, Hollingsworth and Parker became increasingly nutrient-enriched over the past century, the 15N of sedimented organic matter decreased. This reflects, in part, the increasing relative contribution of nitrogen-fixing cyanobacteria to sedimented organic matter as primary productivity increased in these waterbodies. The Lake Griffin core displays a narrow range of both 13C and 15N values. Despite the complexity of carbon and nitrogen cycles in lakes, stratigraphic agreement between diatom-inferred changes in limnetic total P and the stable isotope signatures of sedimented OM suggests that 13C and 15N reflect shifts in historic lake trophic state.  相似文献   

7.
Geochemical anomalies and stable isotope ratios (18O, 13C) in authigenic carbonates and organic matter (13C) from a 660-year sediment core from Lake Chenghai, southern China, provide a continuous history of recent lake eutrophication. The multi-proxy geochemical and isotopic record can be divided into a three-part history of contrasting limnological development, including: (1) a clear-water, oligotrophic open lake system (1340 and 1690 AD); (2) an environmentally unstable, hydrologically closed, oligotrophic lake system (1690–1940 AD); and (3) an increasingly eutrophic, closed lake system marked by higher organic matter, nitrogen, CaCO3, and pigment concentrations, and lower 18O and 13C values in authigenic calcite (1940–1999 AD). The unanticipated lowering of 18O and 13C of authigenic calcite during eutrophication is thought to be the result of disequilibrium water–carbonate fractionation of oxygen and carbon isotopes during periods of elevated primary production, pH, and [CO3 2–] activities in the water column. The recent eutrophication of Lake Chenghai indicated by these geochemical proxies is essentially simultaneous with large-scale human migration and the application of agricultural fertilizers in the catchment area during the 20th century.  相似文献   

8.
Three piston cores from Lake Victoria (East Africa) have been analysed for organic carbon (TOC) and nitrogen (TN) content, stable isotopes (13C and 15N), and Hydrogen Index (HI). These data are combined with published biogenic silica and water content analyses to produce a detailed palaeolimnological history of the lake over the past ca. 17.5 ka. Late Pleistocene desiccation produced a lake-wide discontinuity marked by a vertisol. Sediments below the discontinuity are characterised by relatively low TOC and HI values, and high C/N, 13C and 15N, reflecting the combined influence of abundant terrestrial plant material and generally unfavourable conditions for organic matter preservation. A thin muddy interval with lower 13C and higher HI and water content indicates that dry conditions were interrupted by a humid period of a few hundred years duration when the lake was at least 35 m deep. The climate changed to significantly more humid conditions around 15.2 ka when the dry lake floor was rapidly flooded. Abundant macrophytic plant debris and high TOC and 13C values at the upper vertisol surface probably reflect a marginal swamp. 13C values decrease abruptly and HI begins to increase around 15 ka BP, marking a shift to deeper-water conditions and algal-dominated lake production. C/N values are relatively low during this period, suggesting a generally adequate supply of nitrogen, but increasing 15N values reflect intense utilisation of the lake's DIN reservoir, probably due to a dramatic rise in productivity as nutrients were released to the lake from the flooded land surface.An abrupt drop in 13C and 15N values around 13.8-13.6 ka reflects a period of deep mixing. Productivity increased due to more efficient nutrient recycling, and 13C values fell as 12C-rich CO2 released by bacterial decomposition of the organic material was brought into the epilimnion. A weak drop in HI values suggests greater oxygen supply to the hypolimnion at this time. Better mixing was probably due to increased wind intensity and may mark the onset of the Younger Dryas in the region.After the period of deep mixing, the water column became more stable. TOC, C/N, 13C and HI values were at a maximum during the period between 10 and 4 ka, when the lake probably had a stratified water column with anoxic bottom waters. A gradual decrease in values over the last 4000 yrs suggest a change to a more seasonal climate, with periodic mixing of the water column. Rising sediment accumulation rates and a trend to more uniform surface water conditions over the last 2000 yrs are probably a result of increased anthropogenic impact on the lake and its catchment.Following a maximum at the time of the rapid lake-level rise during the terminal Pleistocene, 15N has remained relatively low and displays a gradual but consistent trend to lower values from the end of the Pleistocene to the present. TN values have risen during the same period. The lack of correlation between 13C and 15N, and the absence of any evidence for isotopic reservoir effects despite the rise in TN, suggests that the atmosphere, rather than the lake's dissolved nitrogen pool has been the principal source of nitrogen throughout the Holocene. The importance of atmospheric N fixation to Lake Victoria's nitrogen cycle thus predates by a very considerable margin any possible anthropogenic eutrophication of the lake.  相似文献   

9.
Paleoclimate research based on the stable isotopic composition of lake sediments is often hampered by the lack of preservation of suitable material for isotopic analysis. We examined organic material as a proxy for past water isotopic composition in a series of experiments. First, we cultured aquatic moss under constant illumination, temperature, and water 18O, and show that new cellulose records source water 18O precisely (r2 = 0.9997). Second, we analyzed paired lakewater and vegetation samples collected from sites spanning strong climatic gradients. In field conditions, the relationship between organic 18O and water 18O is more variable, though it is still controlled by environmental water isotopic composition. However, terrestrial mosses in the arctic are often significantly enriched in 18O relative to aquatic mosses in nearby lakes due to their use of different water sources. Third, we measured 18O of cellulose extracted from disseminated sedimentary organic material. In the majority of the middle- to high-arctic lakes in this study, the 18O of disseminated sediment cellulose is greatly enriched relative to the expected values based on lakewater 18O, suggesting a significant component of terrestrial cellulose. This interpretation is supported by radiocarbon dates from a Holocene sediment core in which 14C ages of sediment cellulose are 700-5000 yrs older than the enclosing sediments. We conclude that aquatic cellulose can be used as a reliable tracer of lakewater isotope ratios, but terrestrial cellulose often dominates the sedimentary cellulose pool in places such as Baffin Island where sedimentation rates are low enough to allow the degradation of aquatic cellulose. Care must be taken when interpreting sediment cellulose 18O records where diagenesis has played a role, because terrestrial cellulose is more resistant to degradation, and therefore can predominate in environments with low organic carbon burial.  相似文献   

10.
Analysis of 18Ocellulose, 13Corganic matter, and 13Ccellulose at about 100 year intervals from organic matter deposited in Toronto Lake, Northwest Territories, Canada, revealed an 8000-year history of rapid, post-glacial hydrologic change at the treeline zone. Several mid-Holocene phases of enriched 13Corg and 13Ccell, caused by elevated lake productivity, declining [CO2(aq)], and closed basin conditions, were abruptly terminated by intervals of open hydrology recorded by sharply depleted 18Ocell. Two of these events, at 5000 and 4500 BP, are correlated with increased total organic content and Picea mariana pollen concentration, which indicate that high levels of productivity were also accompanied by northern treeline advances. A third treeline advance at about 2500 BP is also marked by an apparent outflow event from Toronto Lake, but this was not associated with 13Corg/cell enrichment in the sediment record because rapid and substantial lake water renewal probably prevented productivity-driven enrichment of the dissolved inorganic carbon and replenished the CO2(aq) supply to thriving phytoplankton. However, high sediment organic content during this period suggests increased productivity. Increases in the inflow:evaporation ratio at about 6500 and 3500 BP were also sufficient to cause Toronto Lake to overflow but the prevailing climate during these periods apparently did not favour appreciable northward treeline migration or changes in lake productivity.This is the 14th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers  相似文献   

11.
Sediments of the marl lake Malham Tarn located in NW Englandpreserve an environmental record since 12 Ka. Eight Holocene pollen zones wereidentified, and the 13C of total organic carbon (TOC) showsthree stratigraphic divisions. The basal clay unit and overlayingsand/clay/marl unit have 13C of –24which decreases at the base of the principal marl unit to a mean value around–30, whilst the topmost black marl unit 13Cincreases to –28 at the surface. Representative samples of theseunits were selected for analysis of n-alkanes andn-fatty acids and their 13C.Samples of modern Chara and peat were analysed forcomparison. The clay unit has a minor contribution of redeposited matureorganic matter and autochthonous algae, the marl unit a high contribution ofChara, and the dark marl unit has a high contribution fromhigher plants. Compound-specific 13C revealssystematic differences between alkanes and fatty acids of different chainlength. The major shift in 13C in the short and medium chainfatty acids are probably due to the decreasing influence of carbonate rockflour as source of DIC. The major shift in 13C in the longchain n-fatty acids andn-alkanes could reflect the lower atmosphericCO2 concentration at Last Glacial. The negative shift of short chainfatty acids in organic rich dark marls reflects introduction of detrital peatinto the lake. The 13C results show a dramatic change fromdominance of autochthonous plus eroded sources up to Pollen Zone IV, then slowcolonisation of the hinterland by higher plants, followed by constantChara contributions throughout the deposition of the marl,and a further increase of higher plant material after the rise in water levelin 1791.  相似文献   

12.
Lake Manitoba, the largest lake in the Prairie region of North America, contains a fine-grained sequence of late Pleistocene and Holocene sediment that documents a complex postglacial history. This record indicates that differential isostatic rebound and changing climate have interacted with varying drainage basin size and hydrologic budget to create significant variations in lake level and limnological conditions. During the initial depositional period in the basin, the Lake Agassiz phase (12–9 ka), 18O of ostracodes ranged from –16 to –5 (PDB), implying the lake was variously dominated by cold, dilute glacial meltwater and warm to cold, slightly saline water.Candona subtriangulata, which prefers cold, dilute water, dominates the most negative 18O intervals, when the basin was part of proglacial Lake Agassiz. At times during this early phase, the 18O of the lake abruptly shifted to higher values; euryhaline taxa such asC. rawsoni orLimnocythere ceriotuberosa, and halobiont taxa such asL. staplini orL. sappaensis are dominant in these intervals. This positive covariance of isotope and ostracode records implies that the lake level episodically fell, isolating the Lake Manitoba basin from the main glacial lake.18O values from inorganic endogenic Mg-calcite in the post-Agassiz phase of Lake Manitoba trend from –4 at 8 ka to –11 at 4.5 ka. We interpret that this trend indicates a gradually increasing influence of isotopically low (–20 SMOW) Paleozoic groundwater inflow, although periods of increased evaporation during this time may account for zones of less negative isotopic values. The 18O of this inorganic calcite abruptly shifts to higher values (–6) after 4.5 ka due to the combined effects of increased evaporative enrichment in a closed basin lake and the increased contribution of isotopically high surface water inflow on the hydrologic budget. After 2 ka, the 18O of the Mg-calcite fluctuates between –13 and –7, implying short-term variability in the lake's hydrologic budget, with values indicating the lake varied from outflow-dominated to evaporation-dominated. The 13C values of Mg-calcite remain nearly constant from 8 to 4.5 ka and then trend to higher values upward in the section. This pattern suggests primary productivity in the lake was initially constant but gradually increased after 4.5 ka.This is the sixth in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers.  相似文献   

13.
Systematic variability occurs between the oxygen isotopic composition of lake water sampled in mid-summer 1993 and cellulose extracted from surficial sediments of a suite of lakes spanning the forest-tundra transition near Noril'sk, Russia. Some tundra and all forest-tundra lakes show greater deviation from expected cellulose-water isotopic separation than forest lakes, apparently because of greater sensitivity to 18O-depleted snowmelt contributions. Cellulose derived from aquatic plants naturally integrates fluctuations in lake water 18O, providing a signal that is inherently more representative of average thaw season lake water 18O than the measure of instantaneous 18O obtained from an individual sample of lake water. Thus, indiscriminate use of empirical cellulose-water relations derived from calibration samples could lead to erroneous assessment of paleohydrology from the oxygen-isotope stratigraphy of sediment cores from arctic lakes. However, deviation from the expected cellulose-water fractionation is a source of lake-specific hydrologic information useful for qualifying paleoenvironmental interpretations and possibly constraining non-isotopic methods that rely on surface-sediment calibrations.  相似文献   

14.
Profiles of percent carbon and nitrogen, carbon/nitrogen (C/N) ratios and stable carbon (13C), and nitrogen (15N) isotopic ratios in organic matter from an 11.6 m core were used to reconstruct environments of deposition in the Swan Lake basin during the past 5300 YBP. The upper 6.5 m consisted of gyttja containing variable amounts of reddish brown-colored fine organic matter and calcium carbonate. It was followed by a 0.5 m sandy silt, which was followed by a 3.6 m reduced layer characterized by large quantities of black organic plant remains, sapropel, and then by another sapropel layer consisting mainly of well-sorted sapropelic sand with relatively low organic matter content. The C- and N-contents in the organic matter in the sediment profile ranged from 0.5 to 23% and from 0.02 to 2%, respectively. Carbon content were positively correlated to both N and clay content while carbon content was negatively correlated to sand content. Two major environmental phases in Swan Lake were apparent from large differences in the C and N data of the sediment organic matter. These include the sapropel (marsh) stage that stretched from approximately 5330 to 3930 YBP, and the following gyttja (open water stage). During the sapropel marsh plants identified in a previous pollen study as cattails and sedges proliferated and produced copious amounts of well-preserved organic matter. C/N ratios, 13C values, and 15N values in the sapropel were significantly different from those that characterized organic matter in the gyttja. During the gyttja 13C values indicated that deep primary producers have dominated lake biomass. By utilizing bicarbonate as their C-source, the accumulating biomass became relatively enriched 13C values. The presence of high sediment CaCO3 contents indicated more alkaline and deeper water conditions prevailed during the gyttja. Further refinement of the data suggested that each major phase initially contained an identifiable transition stage. During the sapropelic (initial marsh stage) which occurred before 5330 YBP, sand content gradually decreased as organic matter increased. As reflected by high C/N ratios and slightly enriched 13C values, these sands appear to have contained sufficient permeability to promote partial mineralization of accumulated organic-N containing compounds. A short initial gyttja transition period from about 3930–3830 YBP occurred in which the sediment silt content was anomalously high relative that measured in the surrounding layers. The silt content suggests that this turbid transition layer can not be completely explained by sediment mixing via bioturbation. The silts appeared to have been associated with the sharp climate change that resulted in higher water-table conditions during the gyttja stage.  相似文献   

15.
The paleohydrological evolution of several high altitude, saline lakes located in the southernmost Altiplano (El Peinado and San Francisco basins, Catamarca province, NW Argentina) was reconstructed applying sedimentological, geochemical and isotopic techniques. Several playa lakes from the San Francisco basin (26° 56 S; 68° 08 W, 3800-3900 m a.s.l.) show evidence of a recent raise in the watertable that led to modern deposition of carbonate and diatomaceous muds. A 2 m - long core from El Peinado Lake (26° 29 59 S, 68°05 32 W, 3820 m a.s.l.) consists of calcitic crusts (unit 3), overlaid by an alternation of macrophyte-rich and travertine clast- rich, laminated muds (unit 2), and topped by travertine facies (unit 1). This sedimentary sequence illustrates a paleohydrological evolution from a subaerial exposure (unit 3) to a high lake stand (unit 2), and a subsequent smaller decrease in lake level (unit 1). The 13Corganic matterrecord also reflects the lake transgression between units 3 and 2. Although there is a general positive correlation between 18Ocarbonate and salinity proxies (Na, Li and B content), the large data dispersion indicates that other factors besides evaporation effects control chemical and isotopic composition of lakewater. Consequently, the oxygen isotopic composition cannot be interpreted exclusively as an indicator of salinity or evaporation ratio. The degassing of CO2 during groundwater discharge can explain the enriched 13C values for primary carbonates precipitated. The carbon budget in these high altitude, saline lakes seems to be controlled by physical rather than biological processes.The Altiplano saline lakes contain records of environmental and climatic change, although accurate 14C dating of these lacustrine sediments is hindered by the scarcity of terrestrial organic material, and the large reservoir effects. Sedimentologic evidence, a 210Pb-based chronology, and a preliminary U/Th chronology indicate a very large reservoir effect in El Peinado, likely as a result of old groundwaters and large contributions of volcanic and geothermal 14C-free CO2 to the lake system. Alternative chronologies are needed to place these paleorecords in a reliable chronological framework. A period of increased water balance in the San Francisco basin ended at about 1660 ± 82 yr B.P. (calendar yr U/Th age), and would correlates with the humid phase between 3000 and 1800 yr B.P detected in other sites of the southern Altiplano. Both, 210Pb and preliminary U/Th dating favor a younger age for the paleohydrological changes in El Peinado. The arid period reflected by subaerial exposure and low lake levels in unit 3 would have ended with a large increase in effective moisture during the late 17th century. The increased lake level during deposition of unit 2 would represent the period between AD1650 - 1900, synchronous to the Little Ice Age. This chronological framework is coherent with other regional records that show an abrupt transition from more arid to more humid conditions in the early 17th century, and a change to modern conditions in the late 19th century. Although there are local differences, the Little Ice Age stands as a significant climatic event in the Andean Altiplano.  相似文献   

16.
A series of water samples (precipitation, surface water and ground water) from the Devils Lake drainage basin in central North Dakota have been analyzed for their 18O and D and 87Sr/86Sr. The 18O and D of snow and most ground water samples fall near the meteoric water line, with the ground water being isotopically heavier than the snow, indicating that a portion of the recharge must come from non-winter precipitation events. One ground water and all the surface water samples fall below the meteoric water line, demonstrating extensive evaporation. The 87Sr/86Sr ratio of the snow is more radiogenic than those of the ground water samples, indicating that the ground water has increased its 86Sr content by the dissolution of mafic rock in the aquifer. Stump Lake has a much more radiogenic Sr isotopic ratio than the waters of Devils Lake suggesting a source of water different from that of Devils Lake. The East Stump Lake water, on the other hand, appears to be either a mix of Devils Lake and Stump Lake waters, or Devils Lake water diluted with precipitation.  相似文献   

17.
The geochemistry of ostracode shells and bulk carbonates in a 19-meter sediment core documents at century-scale resolution the evolution of water chemistry in Coldwater Lake, North Dakota, providing a continuous paleohydrologic record of Holocene climate change in the northern Great Plains. A combination of 18O, 13C, Mg/Ca and Sr/Ca in ostracode calcite aided by Sr/Ca in bulk carbonates are used to constrain the paleoclimatic reconstructions. A fresh-water phase in the early Holocene, indicated by the absence of Candona rawsoni and low concentrations of Sr/Ca in bulk carbonate, was followed by a sharp increase in salinity between 10800 and 8900 yr B.P. The climate was predominately dry during the late part of the early Holocene and most of the middle Holocene (8900–5000 yr B.P.), when the lake was very sensitive and recorded a series of dry and wet oscillations. Maximum salinity occurred around 5500 yr B.P. and was followed by a gradual decrease between 5000 and 2400 yr B.P. From 2400 yr B.P. the 18O, Mg/Ca, and Sr/Ca in the ostracodes indicate generally wet conditions interrupted by a series of lesser salinity and temperature oscillations lasting until 600 yr B.P. Ostracode geochemistry indicates that a warm and dry climate returned at about the time of the Little Ice Age (600–150 yr B.P.). Ostracode 13C shows a ong-term increasing trend during the Holocene, which suggests that lake productivity and atmospheric CO2 exchange made greater contributions to the hypolimnetic carbon pool as the lake became shallower with time.  相似文献   

18.
The Konya plain in south central Anatolia, Turkey, which is now largely dry, was occupied around the time of the Last Glacial Maximum by a fresh-oligosaline lake covering more than 4000 km2. Sediment cores from three residual water bodies (Pinarbai, Akgöl and Süleymanhaci) within the larger Pleistocene lake basin, have been analysed using a multidisciplinary approach. The sediment sequences are dated as spanning the last 50 Ka years, although breaks in sedimentation mean that there is only partial chronological overlap between them. Carbon and oxygen isotope analyses on lacustrine carbonate from the three cores give contrasting isotope profiles which reflect the different ages and independent hydrological behaviour of different sub-basins through the late Quaternary. Distinguishing changes that are regional from local effects is aided by modern isotope hydrology studies and by comparing the carbonate 13C and 18O values to diatom and other analyses undertaken on the same cores.  相似文献   

19.
Optical and geochemical techniques were applied to sedimentary organic matter from the profundal area of the Eocene Lake Prinz von Hessen, which formed in a pull-apart basin on the Sprendlinger Horst, near Darmstadt, Germany. Variations in total sulphur content (S tot) and total organic carbon content (TOC), hydrogen index (HI), oxygen index (OI) and 13C values of the organic matter were used to reconstruct the lakes filling history. Following an initial rapid deepening phase, open lake conditions developed with HI reaching more than 500 mg HC/g TOC and TOC values up to 40%. The productivity of the lake was probably high and organic matter preservation was enhanced by a stratified water column. As the lake began to fill with sediment and became shallower, TOC and HI values declined, as the lake water was better oxygenated and preservation conditions declined. 13C values between –31 and –27 are controlled by the mixing of aquatic (algae and microbial mats) and terrigenous organic matter (wood, spores, pollen and cuticles). Following a rapid drop in lake level, shallow lake conditions alternated with swamp deposits (lignites) in the basin center. The organic matter preserved during this stage is strictly terrigenous in nature and experienced oxic degradation (HI 100 mg HC/g TOC). 13C values between –26 and –24 are typical for Eocene terrigenous matter. The inferred lake level fluctuations are interpreted to have been controlled by tectonic as well as climatic processes.  相似文献   

20.
New sediment core data from a unique slow-sedimentation rate site in Lake Tanganyika contain a much longer and continuous record of limnological response to climate change than have been previously observed in equatorial regions of central Africa. The new core site was first located through an extensive seismic reflection survey over the Kavala Island Ridge (KIR), a sedimented basement high that separates the Kigoma and Kalemie Basins in Lake Tanganyika.Proxy analyses of paleoclimate response carried out on core T97-52V include paleomagnetic and index properties, TOC and isotopic analyses of organic carbon, and diatom and biogenic silica analyses. A robust age model based on 11 radiocarbon (AMS) dates indicates a linear, continuous sedimentation rate nearly an order of magnitude slower here compared to other core sites around the lake. This age model indicates continuous sedimentation over the past 79 k yr, and a basal age in excess of 100 k yr.The results of the proxy analyses for the past 20 k yr are comparable to previous studies focused on that interval in Lake Tanganyika, and show that the lake was about 350 m lower than present at the Last Glacial Maximum (LGM). Repetitive peaks in TOC and corresponding drops in 13C over the past 79 k yr indicate periods of high productivity and mixing above the T97-52V core site, probably due to cooler and perhaps windier conditions. From 80 through 58 k yr the 13C values are relatively negative (–26 to –28 l) suggesting predominance of algal contributions to bottom sediments at this site during this time. Following this interval there is a shift to higher values of 13C, indicating a possible shift to C-4 pathway-dominated grassland-type vegetation in the catchment, and indicating cooler, dryer conditions from 55 k yr through the LGM. Two seismic sequence boundaries are observed at shallow stratigraphic levels in the seismic reflection data, and the upper boundary correlates to a major discontinuity near the base of T97-52V. We interpret these discontinuities to reflect major, prolonged drops in lake level below the core site (393 m), with the lower boundary correlating to marine oxygen isotope Stage 6. This suggests that the previous glacial period was considerably cooler and more arid in the equatorial tropics than was the last glacial period.  相似文献   

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