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1.
We used statistical analyses to determine which subset of 36 environmental variables best explained variations in surface
sediment δ13C and δ15N from 50 lakes in western Ireland that span a human-impact gradient. The factors controlling lake sediment δ13C and δ15N depended on whether organics in the lake sediment were mostly derived from the lake catchment (allochthonous) or from productivity
within the lake (autochthonous). Lake sediments with a dominantly allochthonous organic source (high C:N ratio sediments)
produced δ13C and δ15N measurements similar to values from catchment vegetation. δ13C and δ15N measurements from lake sediments with a dominantly autochthonous organic source (low C:N ratio sediments) were influenced
by fractionation in the lake and catchment leading up to assimilation of carbon and nitrogen by lacustrine biota. δ13C values from lake sediment samples in agricultural catchments were more negative than δ13C values from lake sediment samples in non-impacted, bogland catchments. Hypolimnetic oxygen concentrations and methane production
had a greater influence on δ13C values than fractionation due to algal productivity. δ15N from lake sediment samples in agricultural catchments were more positive than δ15N in non-impacted bogland catchments. Lower δ15N values from non-impacted lake catchments reflected δ15N values of catchment vegetation, while higher δ15N values in agricultural catchments reflected the high δ15N values of cattle manure and inorganic fertilisers. The influence of changing nitrogen sources and lake/catchment fractionation
processes were more important than early diagenesis for lake sediment δ15N values in this dataset. The results from this study suggest a possible influence of bound inorganic nitrogen on the bulk
sediment δ15N values. We recommend using a suitable method to control for bound inorganic nitrogen in lake sediments, especially when
working with clay-rich sediments. This study confirms the usefulness of δ13C and δ15N from bulk lake sediments, as long as we are mindful of the multiple factors that can influence these values. This study
also highlights how stable isotope datasets from lake surface sediments can complement site-specific isotope source/process
studies and help identify key processes controlling lake sediment δ13C and δ15N in a study area. 相似文献
2.
Assessment of nutrient sources and paleoproductivity during the past century in Longgan Lake,middle reaches of the Yangtze River,China 总被引:3,自引:0,他引:3
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
3.
Isabela C. Torres Patrick W. Inglett Mark Brenner William F. Kenney K. Ramesh Reddy 《Journal of Paleolimnology》2012,47(4):693-706
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
4.
Biplob Das Anita Narwani Blake Matthews Rick Nordin Asit Mazumder 《Journal of Paleolimnology》2009,42(2):167-181
We investigated how the history of local disturbances in a watershed can influence the regional coherence of ecosystem properties
in lakes that have similar morphometry and climatic conditions. We measured sedimentary δ13C, δ15N, C:N and %BSiO2 in Sooke Lake Reservoir (SOL) and Shawnigan Lake (SHL), which are located within 4 km of each other on Vancouver Island,
Canada. SOL is an impounded lake whose watershed has been fully protected over the last century, although the lake level has
been raised 3 times via impoundment during this period. SHL has a similar limnological regime, but the surrounding watershed
has been developed extensively for residential uses. We investigated how a pulse disturbance regime in SOL (i.e. repeated
dam raising) and a press disturbance regime in SHL (i.e. persistent development) influenced the variability of paleoindicators
in each system over time. We found that these contrasting disturbance regimes reduced the regional temporal coherence of aquatic
productivity between the two lakes (indicated by %BSiO2), but did not influence the regional coherence of nutrient status or the main carbon sources of the lakes (indicated by %C,
%N and δ13C). In contrast, an indicator of the sources and cycling of nitrogen (δ15N) showed increased coherence. Local disturbances also affected the variability of the paleoindicators within each system
over time. In SOL, impoundments led to both declines (%N, δ15N) and increases (δ13C) in the variability of paleoindicators. In SHL, persistent watershed development led to lower variability of two paleoindicators
(%N, %BSiO2). Overall, our data suggest that local disturbances can influence the %BSiO2 and C:N ratio of lake sediments, but are less likely to alter the regional coherence of %C, %N and δ13C between lakes.
Biplob Das and Anita Narwani contributed equally to this work. 相似文献
5.
Yongqiang Xiong Fengchang Wu Jidun Fang Lifang Wang Yun Li Haiqing Liao 《Journal of Paleolimnology》2010,44(1):217-231
In order to investigate the natural ecosystem of Lake Dianchi and to assess its anthropogenic impacts, a stratigraphic study
of bulk and molecular compositions of organic matter was conducted using a 63-cm long sediment core. The results show that
two apparent environmental changes occurred during the evolution of Lake Dianchi: (1) the first change occurred in the 43–63 cm
sediment depth, and was revealed by the amount and the composition of organic matter in the stage. Natural changes were possibly
major factors responsible for triggering the environmental change, but the influence of human activities could not be excluded.
Subsequently, the lake entered into a relatively stable and oligotrophic stage, which maintained until 20-cm sediment depth.
(2) Eutrophication started in the upper 20 cm depth. Human activities became a major factor influencing environmental changes
in this stage. Vertical profiles of various organic geochemical variables in the upper 20-cm sediments show evidence that
primary productivity of the lake increased progressively and that the lake started eutrophic. Especially in the uppermost
10 cm, notable excursions to less negative δ13Corg and δ15Ntotal and high TOC concentrations have recorded an abrupt change in the lacustrine environment, suggesting that the lake entered
a hypereutrophic stage. In addition, enhancement of αβ-hopanes reflects the contribution of fossil fuels to the lake sediments. 相似文献
6.
Supriyo Kumar Das Joyanto Routh Alakendra N. Roychoudhury J. Val Klump 《Journal of Paleolimnology》2008,39(3):349-360
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
7.
Biomarker evidence of macrophyte and plankton community changes in Zeekoevlei,a shallow lake in South Africa 总被引:1,自引:0,他引:1
Supriyo Kumar Das Joyanto Routh Alakendra N. Roychoudhury 《Journal of Paleolimnology》2009,41(3):507-521
Zeekoevlei is the largest freshwater lake in South Africa and has a century-long history of anthropogenic impact that caused
hyper-eutrophic conditions. We used biomarkers (alkanes and pigments), stable isotopes (δ13C and δ15N), rates of primary palaeoproduction and total inorganic carbon (TIC) accumulation rates in the lake sediments to investigate
changes in plankton and macrophyte communities in response to anthropogenic activities in this shallow lake. Specific alkanes
(ΣC15,17,19, pristane, phytane and n-C29/n-C17 ratio) and pigment (chlorophyll a, β,β-carotene, echinenone, fucoxanthin and zeaxanthin) concentrations in lake waters indicated the present-day hyper-eutrophic
condition and seasonal fluctuations of cyanobacteria, zooplankton and diatom populations. Eutrophic conditions were initiated
in the lake with the start of recreational activities and construction of a sewage treatment plant in the early 1920s. The
lake transformed from a eutrophic to a hyper-eutrophic waterbody following damming, pondweed eradication and accelerated catchment-derived
nutrient input. The change in lake trophic state was recorded by a sharp decline in the terrestrial to aquatic ratio (TAR)
of specific n-alkanes, low carbon preference index (CPI) and increased δ13C values in the sediment core. In addition, the aquatic macrophyte n-alkane proxy (Paq) values (~1) indicated a slow takeover by floating macrophytes after the eradication of submerged pondweeds in 1951. Elevated
n-alkane (ΣC15,17,19), total alkane and pigment (chlorophyll a, β,β-carotene, zeaxanthin and zeaxanthin to β,β-carotene ratio) concentrations, low δ15N values and low TIC accumulation rates in the upper middle section of the core indicated the beginning of intense cyanobacterial
blooms after the dredging in 1983. Although the cyanobacterial population has decreased in recent years, hyper-eutrophic conditions
are reflected by low CPI <0.04 and TAR <1 values at the top of the sediment core. 相似文献
8.
Catastrophic hurricane history revealed by organic geochemical proxies in coastal lake sediments: a case study of Lake Shelby,Alabama (USA) 总被引:2,自引:0,他引:2
We developed a new method for reconstructing millennia-long hurricane records from coastal environments that uses Organic
Geochemical Proxies (OGPs) of organic carbon and nitrogen concentrations and their δ13C and δ15N compositions. The new method is independent of presence/absence of sand layers and improves significantly the severe-storm
history resolution. The subject of this investigation is a 1.5 m long sediment core raised at 2.8 m water depth from the center
of Lake Shelby, Alabama, a freshwater lake located approximately 250 m from the Gulf of Mexico, from which an overwash sand-layer
based record was previously derived. The core contains two distinct sediment units; an upper 62 cm thick, fine-grained, organic-rich
lacustrine sapropel (gyttja) that shows no visible structures except one sand lamina at 23.7 cm depth, and an underlying 90 cm
thick, organic-poor lagoon/estuary clay unit. The sapropel unit was deposited over a 682 ± 30 cal year time interval (1320–2002 A.D.)
with a mean sedimentation rate of 0.79 ± 0.04 mm/year. Lake Shelby’s water column exhibits two contrasting states based on
water chemistry surveys (i) an “isolated”, stratified, mode under calm weather conditions with a relatively low trophic state,
and (ii) a “flooded” mode occurring during storm surges when nutrient-rich seawater floods the lake. Statistically significant
δ13C and δ15N positive excursions in organic matter, up to maximum values of −25 (‰ PDB) and 4 (‰ Air N2), respectively, are interpreted as geochemical responses to the marine intrusions that fertilize the lake, increase light
availability, and cause eutrophication spikes. Detailed OGPs analyses crossing a sand layer that offers visual evidence of
a catastrophic hurricane overwash event at 1717 A.D. exhibit large δ13C and δ15N positive shifts bounded by rapid returns to base values, thus confirming the validity of the hurricane identification by
the OGPs model. Our data indicate that 11 catastrophic hurricanes hit the Alabama coast over the past 682 years with a rough
recurrence interval of one in 62 years. 相似文献
9.
Organic-rich sediment from Lake Louise, a dystrophic sinkhole lake in south Georgia, displays variations in C, N, P, C/N,
δ13C, δ15N, biogenic silica (BSi) and diatom flora that document changes in trophic state over the past ~9,500 years. The lake initially
was oligotrophic and moderately productive, but by the middle Holocene a rising regional water table, driven by eustatic sea
level rise, caused expansion of wetlands around the lake and a shift to humic waters. Low rates of sediment accumulation,
low C contents, rising C/N, and light δ13C and δ15N indicate this was a time of low productivity, more anoxic bottom waters and extensive recycling of littoral organic matter.
These conditions persisted until ~1800 AD when a physical disturbance to the watershed, probably the Great Hurricane of 1780,
resulted in a dramatic increase in productivity that has continued to the present day. We attribute this shift, recorded by
a >tenfold increase in sediment accumulation rate, higher C, P, and δ15N, and lower BSi, to establishment of an inflow stream that increased nutrient delivery to the lake, raised water level, and
expanded the wetland area around the lake. Since ~1930, logging, farming, and highway construction have impacted the lake,
further accelerating biological productivity as well as the delivery of terrigenous sediment. Results of this study illustrate
the potential of a single, catastrophic event to permanently alter the hydrology and chemistry of a lacustrine system and
confirm that dystrophic lakes can be highly productive and therefore promising targets for paleolimnological study. 相似文献
10.
Wu Jinglu Huang Chengmin Zeng Haiao Gerhard H. Schleser Rick Battarbee 《Journal of Paleolimnology》2007,38(1):13-23
Environmental change in Lake Taihu and its catchment since the early to middle part of the twentieth century has left a clear
geochemical record in the lake sediments. The human activities in the lake and its catchment responsible for the change include
agriculture, fishery, urbanisation, sewage and industrial waster disposal. Sediment cores were collected from Meilian Bay
of northern Lake Taihu to investigate the record of anthropogenic impacts on the lake’s ecosystem and to assess its natural,
pre-eutrophication baseline state. Two marked stratigraphic sediment units were identified on the basis of total phosphorus
concentration (TP), pigments, total organic carbon (TOC)/total nitrogen (TN), δ13C and δ15N corresponding to stages in the lake history dominated by phytoplankton, and by aquatic macrophytes. Results show that as
TP loading increased from the early 1950s the lake produced sediments with increasing amounts of organic matter derived from
phytoplankton. In the early 1950s, the first evidence for eutrophication at the Meilian Bay site is recorded by an increase
in C/N values and in sediment accumulation rate, but there is little change in phosphorus concentrations, pigments, δ13C and δ15N at this time. After 1990 a more rapid increase in trophic status took place indicated by increased levels of phosphorus,
pigments, δ15N and by decreased δ13C and TOC/TN values in the lake sediments. The first increase in trophic status of the early 1950s results mainly from agricultural
development in the catchment. In contrast, the acceleration from ca. 1990 originates from the recent development of fisheries
and the urbanisation and industrialisation of the catchment. 相似文献
11.
Evaluating sedimentary geochemical lake-level tracers
in Walker Lake, Nevada, over the last 200 years 总被引:1,自引:0,他引:1
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
12.
Katherine Griffiths Neal Michelutti Jules M. Blais Lynda E. Kimpe John P. Smol 《Journal of Paleolimnology》2010,44(2):405-412
The mass transport of nutrients by migratory animals can markedly alter the biogeochemistry and ecology of recipient ecosystems,
particularly in nutrient-poor regions such as the Arctic. However, the role of biovectors in the global cycling of nutrients
is often overlooked. Here we investigate nitrogen dynamics in two seabird-affected ponds in the Canadian High Arctic. The
ponds lie at the base of a large seabird colony and have been greatly enriched in nutrients due to the input of guano and
other wastes. Using sediment cores that span the last ~200 years, we measured stable isotopes of nitrogen (δ15N) in bulk sediments as well from the subfossil remains of chironomid (Diptera) head capsules and Daphnia ephippia. The bulk-sediment samples from our seabird-affected ponds had elevated δ15N values relative to seabird-free sites elsewhere in the Arctic. In general, the chironomid δ15N profiles roughly paralleled those of bulk sediments in both study ponds, while the Daphnia profile remained relatively stable in contrast to the considerable variation recorded in the bulk sediments and chironomids.
Interestingly, no apparent pattern emerged among δ15N values recorded in the bulk sediments, chironomids, and Daphnia between the two study ponds. The stability recorded in the δ15N profiles from bulk sediments relative to the more variable invertebrate profiles point towards the complexity of nitrogen
uptake by chironomids and Daphnia at these sites. These data suggest that the bulk sediments are integrating the different fractions of the overall δ15N pool and thus may be most appropriate for reconstructing overall trends in lake trophic status. 相似文献
13.
Xiaodong Liu Liguang Sun Gangjian Wei Yuhong Wang Hong Yan Kexin Liu Xiaohong Wu 《Journal of Paleolimnology》2008,40(4):987-1002
Abundant ostracode valves (Cyprinotus cingalensis) and caryopses of Urochloa paspaloides, a terrestrial grass, were well preserved in the lacustrine sediments of the Cattle Pond on Dongdao Island, South China Sea.
Oxygen and carbon isotopes, and elements (Ca, Mg) were analyzed on ostracode valves and plant caryopses in this study. The
δ18Oostracode and δ13Costracode exhibited a positive and statistically significant correlation, and showed a decreasing trend toward the top of sediment
sequence with some fluctuations, indicating a gradual increase in effective moisture. The Mg/Ca ratios in the ostracode shells,
generally used as a proxy for salinity changes in lake water, showed a trend toward lower values in the upper samples, reflecting
a gradual decrease of salinity in the lake. The δ13Corg values in the plant caryopses of the upper 14 cm of sediment have lower values than those in the bottom sediments, suggesting
less water and salinity stress. These results indicate increasing effective moisture and rainfall intensity over the past
∼1,100 years on Dongdao Island. 相似文献
14.
A multiple stable isotope record of Late Quaternary limnological changes and chironomid paleoecology from northeastern Iceland 总被引:1,自引:0,他引:1
We present results from multiple stable isotope analyses (δ18O of chironomid larval head capsules, chironomid adult thoraxes and other insect remains and δD, δ13C, δ15N of total organic matter—TOM) of a lake sediment core (04-SVID-03) taken from Stora Vidarvatn in northeastern Iceland to
reconstruct past environmental, limnological and δ18O of past lake water changes during the Holocene. Core 04-SVID-03 represents a ∼12,000 cal. yrs BP to present record. Large
magnitude changes in δ18O occurred during the Holocene at the site. Downcore shifts in δ18O of chironomids did not correlate with measurements of the δ13C and δ15N of chironomid head capsules, implying that the δ18O changes were not primarily driven by changes in chironomid diet during the Holocene. The δD of TOM provided a proxy of relative
lake-water δD changes at the site and also showed large magnitude changes during the record. This approach was supported by
analyses of a modern training set where δDTOM analyses were conducted using surface sediments from a suite of freshwater lakes over a large latitudinal gradient. The magnitude
of changes in both the δ18O and δD and the relatively negative δ18O values throughout much of the core suggest that the proxies represent more paleoenvironmental information than solely temperature.
Additional possible influences on lake-water isotopic composition are discussed, including changes in the seasonality of precipitation,
in the patterns of air masses supplying precipitation to Iceland and in the dominant mode of the North Atlantic Oscillation. 相似文献
15.
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
16.
Fasong Yuan Yongwei Sheng Tandong Yao Chaojun Fan Junli Li Hui Zhao Yanbin Lei 《Journal of Paleolimnology》2011,46(2):291-307
Stable isotopes (δ18O and δD) are useful tracers for investigating hydrologic and climatic variability on a variety of temporal and spatial scales.
Since the early isotopic studies on mountainous glaciers in the late 1960s, a great deal of information has been generated
on the isotopic composition of rainfall, snow, ice, surface waters, and lake carbonate sediments across the Tibetan Plateau.
However, measurements of δ18O and δD values of lake water are scarce. Here we present a new dataset of δ18O and δD values of lake waters collected from 27 lakes across the plateau during a reconnaissance survey in summer 2009. δ18O and δD values of lake water range from −19.9 to 6.6‰ and from −153 to −16‰, respectively. The average values of δ18O and δD are −6.4 and −72‰, considerably greater than those of precipitation observed in this region. The derived Tibetan
lake water line, δD = 5.2δ18O − 38.9, is significantly different from the global meteoric water line. Most of the lakes, including some freshwater lakes,
contain water with negative values of d-excess (d). There is a negative correlation between d and total dissolved solids (TDS). Each of these findings indicates that evaporation-induced isotopic enrichment prevails
in Tibetan lakes. Moreover, we develop an isotope modeling scheme to calculate E/P ratios for Tibetan lakes, using a combination
of existing isotopic fractionation equations and the Rayleigh distillation model. We use the intersection of the local evaporation
line and GMWL as a first approximation of δ18O and δD values of lake water inputs to infer an E/P ratio for each lake. Our modeling calculations reveal that although variable
from lake to lake, the water budget across the plateau is positive, with an average E/P of 0.52. This is in good agreement
with other observational and model data that show varying degrees of increases in lake size from satellite imagery and significant
decreases in lake salinity in many lakes on the plateau over the last several decades. Together with the new isotopic dataset,
the proposed modeling framework can be used to examine and quantify past changes in a lake’s hydrologic balance from the isotopic
record of downcore carbonate sediments in the region. 相似文献
17.
Mark Brenner David A. Hodell Barbara W. Leyden Jason H. Curtis William F. Kenney Binhe Gu Jana M. Newman 《Journal of Paleolimnology》2006,35(1):129-148
We studied the role that submersed aquatic vegetation (SAV) plays in the sedimentation of organic matter (OM) and phosphorus
(P) in Lake Panasoffkee, Florida (USA), a shallow, hard-water, macrophyte-dominated water body. Carbon/Nitrogen ratios (C/N)
and stable isotope signatures (δ13C and δ15N) in algae, higher plants, and surface sediments were measured to identify sources of OM to the lake mud. Pollen, plant macrofossils,
and geochemistry in sediment cores indicated that primary productivity and SAV abundance in Lake Panasoffkee increased in
the late 1800s, probably as a response to increased P loading from human settlement and forest clearance. SAV and associated
periphyton served as temporary sinks for soluble P, maintaining relatively clear-water, low-nutrient conditions in the lake.
P accumulation in Lake Panasoffkee sediments increased together with indicators for greater SAV presence. This suggests that
SAV and associated epiphytes promote P burial and retention in sediments. Although it might be assumed that rooted submersed
macrophytes are directly responsible for P uptake from water and transfer to sediments, C/N and stable carbon isotope results
argue for the importance of other macrophyte growth forms, and perhaps epiphytic algae, in permanent OM and P sequestration.
For instance, high rates of photosynthesis by epiphytes in hard-water systems consume CO2 and promote CaCO3 precipitation. Sloughing of accumulated carbonates from macrophyte leaves transfers epiphytes and associated P to the sediment.
Our paleolimnological findings are relevant to restoration efforts in the Florida Everglades and support the claim that constructed
SAV wetlands remove P from waters effectively. 相似文献
18.
Michael R. Talbot Niels Bo Jensen Tine Lærdal Maria Letizia Filippi 《Journal of Paleolimnology》2006,35(3):467-489
Bulk organic matter (OM) in sediments that accumulated in Lakes Victoria, Albert and Tanganyika during the major transgression
that transformed these basins from a desiccated or lowstand condition at the time of the Last Glacial Maximum to highstand
with overflow in the terminal Pleistocene and early Holocene, show some striking compositional trends. Sediment OM content
(TOC) and Rock-Eval Hydrogen Index (HI) both rise markedly, reflecting increasing primary productivity during the transgression
and enhanced conditions for the preservation of labile OM, In Lake Victoria rising sedimentary sulphur content tracks the
change in HI. Over the same time interval in all three lakes, δ13COM shows a falling trend of 5–8‰ . Nitrogen isotopic and C/N trends are, in contrast, more variable. Relatively high δ15N values at an early stage in the transgression may reflect isotopic effects associated with ammonia loss from an alkaline
waterbody, while high δ15N later in the transgression was probably due to periods when stratification-related denitrification had a particularly strong
impact upon the lakes' N cycle. The observed changes are attributed to the influence of the landscape drowned during the transgression
upon the nutrient budget of the lake, a rise in river-supplied nutrients as the climate became wetter, and an increasing tendency
to stable chemical stratification as the water column deepened. In the case of δ13C, additional contributory factors were changes in the amount and isotopic composition of river-supplied dissolved inorganic
carbon (DIC) as runoff and soil–moisture turnover rates increased, and the replacement of a C4-grassland dominated flora by
mixed woodland-grassland and forest with a significant presence of C3 plants. Distinct geochemical changes are also apparent
in Lakes Victoria and Albert sediments at the end of the transgression which are consistent with a reduction in nutrient loading.
In the case of N this resulted in biological N fixation becoming a major part of the lakes' N cycle, Some of the observed
geochemical variations may be generally applicable as proxy indicators of major transgressions in large lakes.
IDEAL Contribution No. 152. 相似文献
19.
Caleb J. Schiff Darrell S. Kaufman Alexander P. Wolfe Justin Dodd Zachary Sharp 《Journal of Paleolimnology》2009,41(1):189-208
The oxygen isotope ratios of diatoms (δ18Odiatom), and the oxygen and hydrogen isotope ratios of lake water (δW) of lakes in south Alaska provide insight into past changes
in atmospheric circulation. Lake water was collected from 31 lakes along an elevation transect and diatoms were isolated from
lake sediment from one lake (Mica Lake) in south Alaska. In general, δW values from coastal lakes overlap the global meteoric
water line (GMWL). δW values from interior lakes do not lie on the GMWL; they fall on a local evaporation line trajectory
suggesting source isotopes are depleted with respect to maritime lakes. Sediment cores were recovered from 58 m depth in Mica
Lake (60.96° N, 148.15° W; 100 m asl), an evaporation-insensitive lake in the western Prince William Sound. Thirteen calibrated
14C ages on terrestrial macrofossil samples were used to construct an age-depth model for core MC-2, which spans 9910 cal years.
Diatoms from 46, 0.5-cm-thick samples were isolated and analyzed for their oxygen isotope ratios. The analyses employed a
newly designed, stepwise fluorination technique, which uses a CO2 laser-ablation system, coupled to a mass spectrometer, and has an external reproducibility of ±0.2‰. δ18Odiatom values from Mica Lake sediment range between 25.2 and 29.8‰. δ18Odiatom values are relatively uniform between 9.6 and 2.6 ka, but exhibit a four-fold increase in variability since 2.6 ka. High-resolution
sampling and analyses of the top 100 cm of our lake cores suggest large climate variability during the last 2000 years. The
20th century shows a +4.0‰ increase of δ18Odiatom values. Shifts of δ18Odiatom values are likely not related to changes in diatom taxa or dissolution effects. Late Holocene excursions to lower δ18Odiatom values suggest a reduction of south-to-north storm trajectories delivered by meridional flow, which likely corresponds to
prolonged intervals when the Aleutian Low pressure system weakened. Comparisons with isotope records of precipitation (δP)
from the region support the storm-track hypothesis, and add to evidence for variability in North Pacific atmospheric circulation
during the Holocene.
相似文献
Zachary SharpEmail: |
20.
Limnology of Two Antarctic Epishelf Lakes and their Potential to Record Periods of Ice Shelf Loss 总被引:1,自引:0,他引:1
James A. Smith Dominic A. Hodgson Michael J. Bentley Elie Verleyen Melanie J. Leng Steven J. Roberts 《Journal of Paleolimnology》2006,35(2):373-394
George VI Ice Shelf is the largest ice shelf on the western side of the Antarctic Peninsula and its northern margin marks
the southern most latitudinal limit of recent ice shelf retreat. As part of a project to reconstruct the long-term (Holocene)
history of George VI Ice Shelf we studied two epishelf lakes impounded by the ice shelf at Ablation Point, on the east coast
of Alexander Island. These lakes, Moutonnée and Ablation, are stratified water bodies with a lower marine layer and an upper
freshwater layer. To determine if their sediment records could be used to detect past changes in the presence or absence of
the ice shelf it was necessary to describe their present-day limnology and sedimentology. We measured water column chemistry
and sampled the water column and sediments of the lakes along vertical and horizontal transects. We analysed these samples
for diatoms, stable isotopes (δ18O, δ2H, δ13CDIC, δ13Corg), geochemistry (TOC, TN, C/N ratios) and physical sedimentology (grain-size). This was supplemented by chemical and biological
reference data from the catchments. Results showed that the water columns of both lakes are nutrient limited and deficient
in phytoplankton. Benthic productivity is low and decreases with depth. Comparison of water column chemistry with an earlier
survey shows a net increase in the thickness of the freshwater layer in Moutonnée Lake between 1973 and 2001, which could
indicate that George VI Ice Shelf has thinned during this period. However, a similar trend was not observed in Ablation Lake
(5 km to the north) and an alternative explanation is that the changes are a seasonal phenomena. Data from the surface sediment
transects identified a number of proxies that respond to the present day stratification of the water column including diatom
species composition, stable isotopes and geochemistry, particularly in Moutonnée Lake. Collectively these data have been used
to develop a conceptual model for determining past ice shelf configuration in epishelf lakes. Specifically, periods of past
ice shelf loss, and the removal of the ice dam, would see the present stratified epishelf lake replaced by a marine embayment.
It is suggested that this change would leave a clear signature in the lake sediment record, notably the deposition of an exclusively
marine biological assemblage, increased ice rafted debris and δ13Corg values that are indicative of marine derived organic matter.
These authors contributed equally to this work 相似文献