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1.
Pore water sampling from lake and estuary sediments using Rhizon samplers   总被引：1，自引：0，他引：1  
Rhizon samplers have been used for several years for pore water extraction from soils (e.g. Knight et al. in Environ Pollut 99:293–298, 1998; Tye et al. in Geochim Cosmochim Acta 67:375–385, 2003; Sigfusson et al. in Sci Total Environ 369:203–219, 2006). With a few exceptions (e.g. Song et al. in Chemosphere 50:711–715, 2003; Seeberg-Elverfeldt et al. in Limnol Oceanogr Methods 3:361–371, 2005), they are rarely used for the extraction of pore water from sediments. The current study highlights the advantages and disadvantages of Rhizon samplers and outlines a simple, but effective method for pore water collection from lacustrine and estuarine sediment cores. This method can be used in the field or laboratory and does not require the use of a glove box to prevent change in pore water redox status.  相似文献   

2.
Sedimentological, mineralogical and compositional analyses performed on short gravity cores and long Kullenberg cores from meromictic Montcortès Lake (Pre-Pyrenean Range, NE Spain) reveal large depositional changes during the last 6,000 cal years. The limnological characteristics of this karstic lake, including its meromictic nature, relatively high surface area/depth ratio (surface area ~0.1 km2; z max = 30 m), and steep margins, facilitated deposition and preservation of finely laminated facies, punctuated by clastic layers corresponding to turbidite events. The robust age model is based on 17 AMS 14C dates. Slope instability caused large gravitational deposits during the middle Holocene, prior to 6 ka BP, and in the late Holocene, prior to 1,600 and 1,000 cal yr BP). Relatively shallower lake conditions prevailed during the middle Holocene (6,000–3,500 cal years BP). Afterwards, deeper environments dominated, with deposition of varves containing preserved calcite laminae. Increased carbonate production and lower clastic input occurred during the Iberian-Roman Period, the Little Ice Age, and the twentieth century. Although modulated by climate variability, changes in sediment delivery to the lake reflect modifications of agricultural practices and population pressure in the watershed. Two episodes of higher clastic input to the lake have been identified: 1) 690–1460 AD, coinciding with an increase in farming activity in the area and the Medieval Climate Anomaly, and 2) 1770–1950 AD, including the last phase of the Little Ice Age and the maximum human occupation in late nineteenth and early twentieth centuries.  相似文献   

3.
We present a Holocene record of climate and environmental change in central New York (USA) inferred using lithologic and stable isotope data from two sediment cores recovered in Cayuga Lake. The record was divided into three intervals: (1) early Holocene (~11.6–8.8 ka), (2) Hypsithermal (~8.8–4.4 ka), and (3) Neoglacial (~4.4 ka to present). The early Holocene began abruptly, with rising lake level and relatively deep water. Between ~10.8 and 9.2 ka, cool and dry conditions prevailed at a time of maximum solar insolation. This anomaly has been referred to as the “post-Younger Dryas climate interval” and lasted ~1,600 years, the approximate length of one “Bond cycle.” The Hypsithermal was the warmest, wettest and most biologically productive interval of the Holocene in central New York. The Hypsithermal was characterized by centennial to multi-centennial-scale variability. The 8.2 ka event is one such variation. The Neoglacial was an interval of generally cooler and dryer conditions, falling lake levels, and several prominent climate anomalies. At approximately 2.4 ka, δ13C of bulk organic matter increased abruptly by 5‰ as lake level declined, and the lake flora was dominated by Chara sp. during the coldest interval of the Neoglacial. Numerous sediment variables display increased variability ~2.0 ka, which continues today. Archaeological data from the literature suggest that Native American populations may have been large enough to impact land cover by about 2.4 ka and we hypothesize that the “Anthropocene” began at about that time in central New York. We also found paleolimnological evidence for the Medieval Warm Period (~1.4–0.5 ka), which was warmer and wetter than today, and for the Little Ice Age (~500–150 years ago), a period with temperatures colder than today.  相似文献   

4.
Lake Shkodra (northern Albania, southern Montenegro) is a large (45 km length, 15 km width) and shallow (5 m mean depth) lake, developed on a mainly karstic carbonate substratum. Its recent sedimentary fill (mixed calcareous/siliciclastic clayey silts) was analysed through short gravity cores representing five centuries of environmental archive. A combination of high resolution texture analysis (particle size), magnetic susceptibility, carbon content, and radionuclide activity profiles (210Pb, 137Cs, 241Am) permitted us to detect and date environmental changes, such as the Little Ice Age and the 1962–1963 catastrophic floods. Anthropogenic influences on the watershed, such as damming of the Drin River in the late 1970s, also appear to be recorded. Impacts of known strong earthquakes (1905, 1979), however, could not be clearly detected in the sediment record. The potential of the Shkodra lacustrine archive for paleoseismic investigations on long time intervals (such as Holocene) is discussed. This paper is a preliminary contribution to Shkodra Lake’s Holocene paleolimnology.  相似文献   

5.
We developed an inference model to infer dissolved organic carbon (DOC) in lakewater from lake sediments using visible-near-infrared spectroscopy (VNIRS). The inference model used surface sediment samples collected from 160 Arctic Canada lakes, covering broad latitudinal (60–83°N), longitudinal (71–138°W) and environmental gradients, with a DOC range of 0.6–39.6 mg L−1. The model was applied to Holocene lake sediment cores from Sweden and Canada and our inferences are compared to results from previous multiproxy paleolimnological investigations at these two sites. The inferred Swedish and Canadian DOC profiles are compared, respectively, to inferences from a Swedish-based VNIRS-total organic carbon (TOC) model and a Canadian-based diatom-inferred (Di-DOC) model from the same sediment records. The 5-component Partial Least Squares (PLS) model yields a cross-validated (CV) RCV2 R_{CV}^{2}  = 0.61 and a root mean squared error of prediction (RMSEP CV ) = 4.4 mg L−1 (11% of DOC gradient). The trends inferred for the two lakes were remarkably similar to the VNIRS-TOC and the Di-DOC inferred profiles and consistent with the other paleolimnological proxies, although absolute values differed. Differences in the calibration set gradients and lack of analogous VNIRS signatures in the modern datasets may explain this discrepancy. Our results corroborate previous geographically independent studies on the potential of using VNIRS to reconstruct past trends in lakewater DOC concentrations rapidly.  相似文献   

6.
The Late-glacial and Holocene environmental history of the Gordano Valley, North Somerset, UK has been reconstructed using pollen, sediment particle size and mineralogical analyses and radiocarbon dating. A Devensian sediment ridge across the valley confined the waters of a small lake, within which the initial sedimentation was minerogenic. Radiocarbon dating of overlying organic-rich deposits suggests that this began late in the Dimlington Stadial c. 18,000–15,000 Cal. BP. Petrographic analyses indicate the minerogenic sediments were partly wind-blown in origin. Climatic amelioration during the Windermere Interstadial c. 15,000 Cal. BP encouraged a shift from minerogenic to biogenic sedimentation. A brief return to minerogenic sedimentation between c. 10,400 and c. 9,520 Cal. BP was followed by uninterrupted fen peat accumulation throughout the Holocene. The later minerogenic horizon appears to represent the Loch Lomond Stadial. Few stratigraphic sequences preserving the complete Devensian Late-glacial–Holocene transition exist in southwest Britain, making the sedimentary archive of the Gordano Valley valuable regionally for reconstructing Late-glacial climate change.  相似文献   

7.
Serpent River Bog lies north of North Channel, 10 m above Lake Huron and 15 m below the Nipissing Great Lake level. A 2.3 m Holocene sequence contains distinct alternating beds of inorganic clastic clay and organic peat that are interpreted as evidence of successive inundation and isolation by highstands and lowstands of the large Huron-Basin lake. Lowstand phases are confirmed by the presence of shallow-water pollen and plant macrofossil remains in peat units. Twelve 14C dates on peat, wood and plant macrofossils combined with previously published 14C ages of lake-level indicators confirm much of the known early Holocene lake-level history with one notable exception. A new Late Mattawa highstand (8,390 [9,400 cal]–8,220 [9,200 cal] BP) evidenced by a sticky blue-grey clay bed is tied to outburst floods of glacial Lake Minong during erosion of the Nadoway drift barrier in the eastern Lake Superior basin. A subsequent Late Mattawa highstand (8,110 [9,040 cal]–8,060 [8,970 cal] BP) is attributed to enhanced meltwater inflows that first had deposited thick varves throughout Superior Basin. Inundation by the Nadoway floods and possibly the last Mattawa flood were likely responsible for termination of the Olson Forest (southern Lake Michigan). A pollen diagram supports the recognized progression of Holocene vegetation, and defines a subzone implying a very dry, cool climate about 7.8–7.5 (8.6–8.3 cal) ka BP based on the Alnus crispa profile during the Late Stanley lowstand. A new date of 9,470 ± 25 (10,680–10,750 cal) BP on basal peat over lacustrine clay at Espanola West Bog supports the previous interpretation of the Early Mattawa highstand at ca. 9,500 (10,740 cal) BP. The organic and clastic sediment units at these two bogs are correlated with other records showing coherent evidence of Holocene repeated inundation and isolation around northern Lake Huron. Taken together the previous and new lake-level data suggest that the Huron and Georgian basin lakes were mainly closed lowstands throughout early Holocene time except for short-lived highstands. Three of the lowstands were exceptionally low, and likely caused three episodes of offshore sediment erosion which had been previously identified as seismo-stratigraphic sequence boundaries.  相似文献   

8.
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9.
Historical phosphorus (P) dynamics were studied using sediment cores from three oligotrophic, acidic lakes in Maine, USA. Long-term oligotrophy of these lakes is consistent with high sediment aluminum (as Al(OH)3) concentrations, as Al inhibits internal P loading, even under reducing conditions. The role of microbially-mediated reactions in controlling redox conditions was evaluated by estimating microbial biomass and relative abundance of specific functional groups. Sediments were fractionated using a sequential chemical extraction technique and all lakes met criteria for P retention based on threshold sediment concentrations of Al, Fe, and P fractions as determined by (Kopáček et al. (2005) Limnol Oceanogr 52: 1147–1155). Sediment NaOH-extractable molybdate-reactive P (rP) and non-reactive P (nrP) represent P associated with non-reducible phases, and organic matter-related P, respectively. Total P (TP) does not decrease with sediment depth, as is typical of eutrophic lake sediments; however, nrP/TP decreases and rP/TP increases for all three lakes, indicating nrP mineralization without any significant upward diffusion and release into the hypolimnion; i.e. diagenesis of P is conservative within the sediment. Two diagenetic models were developed based on nrP and rP concentrations as a function of sediment age. The first model assumes a first-order decay of nrP, the rate coefficient being a function of time, and represents irreversible nrP mineralization, where the produced PO4 is permanently sequestered by the sediment. The second model assumes a first-order reversible transformation between nrP and rP, representing biotic mineralization of organic P followed by incorporation of inorganic P into microbial biomass. Both models reflect preservation of TP with no loss to overlying water. The rate coefficients give us insight into qualities of the sediment that have affected mineralization and sequestration of phosphorus throughout the 210Pb-dateable history of each lake. Similar models could be constructed for other lakes to help reconstruct their trophic histories. Paleolimnological reconstruction of the sediment P record in oligotrophic lakes shows mineralization of nrP to rP, but unlike the case in eutrophic lake sediments, sediment TP is preserved in these sediments.  相似文献   

10.
This study examines the anthropogenic alteration of sedimentation in Frenchman’s Bay in Lake Ontario, using high-resolution particle size analysis in two 200 cm cores. Lithofacies were determined using the particle size data of both the terrigenous sediment and terrigenous sediment + diatom fractions. Terrigenous particle size data from the centre of the lagoon provided the most representative record of anthropogenic impacts. Three distinctive lithofacies were recognized: (1) a Natural Wetland (NW) lithofacies (106–200 cm) had an average mean particle size of 49.4 μm, a mode of 29.4 μm and an average standard deviation of 119.1 μm; (2) an Agricultural and Deforestation (AD) lithofacies (40–105 cm) had a statistically significant lower average mean (30.8 μm), mode (13.5 μm), and standard deviation (48.5) μm; (3) an Urbanized (U) lithofacies (0–40 cm) showed a continued trend towards smaller particle sizes with an average mean of 21.2 μm, a mode of 9.4 μm, and an average standard deviation of 32.7 μm. The lithofacies correlated with previously identified trends in thecamoebian biofacies and magnetic susceptibility data showing post-colonial lagoon eutrophication and increased overland soil erosion. The up-core trend towards finer and less variable particle sizes is attributed to erosion of fine-grained watershed sediments (glacial Lake Iroquois silts and clay) during land-clearance and modification of natural drainage patterns. The influx of silts and clays into the lagoon is also recorded by increased sediment accumulation rates and a reduction in seasonal sediment variability in the wetland. Based on the 210Pb dates, sedimentation rates increased at 1850 ±56 AD (AD lithofacies) and suggest an exponentially increasing trend in accumulation rates. Increasing sedimentation rates can be attributed to the progressive loss of native vegetation and intensified erosion of Lake Iroquois deposits via stream and hillslope erosion. Ecologically, the increased input of fine-grained sediments into the wetland has resulted in reduced water clarity and has altered the wetland substrate contributing to wetland loss in Frenchman’s Bay.  相似文献   

11.
In 1999, the large surface-rupturing earthquakes of Izmit and Duzce completed a 60-year cycle that included a westward migration of nine consecutive large earthquake failures (>50 km surface rupture), which started with the 1939 Erzincan earthquake in eastern Turkey. In this study, we focused on seismic cycles and seismic risk predictability along the North Anatolian Fault (NAF). Toward the west end of the NAF (26°E–32°E, i.e. Bolu), large earthquake frequency is measured from either historic earthquake catalogs, or geologic records from isolated outcrops and marine sediment cores from the Marmara Sea. In comparison, the eastern part of the NAF zone (32°E–42°E) is less well documented by palaeo-seismologic archives. Thus, the sediment records of lake basins located on the eastern NAF zone constitute a unique opportunity for testing a new palaeo-seismologic approach. To this end, we used a diverse array of complementary methods involving: (1) a 600-km transect of fault-related lakes, (2) sedimentologic observations on cores from six lakes, and (3) a comparison between records of catastrophic sediment transfers in lakes (i.e. radionuclide chronomarkers and erosion tracers) and historic earthquake reports. Our study indicates that lakes along the NAF are sensitive geologic recorders of large surface-rupturing earthquakes (surface-wave magnitude (M s) ≥ 6.9); smaller intensities are not recorded. The most responsive lake systems exhibit increases in sediment accumulation by a factor of >40 for a >3-m strike-slip displacement (M s ≥ 7). However, based on results from the 1939 Erzincan earthquake (M s = 7.8) chronostratigraphic marker, large surface-rupturing earthquakes are detected only by certain lake records and not by others. Matching multiple lake records along the NAF provides information both on the location of a surface rupture of a paleo-earthquake as well as its magnitude. Finally, the shallow lake basins along the NAF could potentially document cycles of large seismic events for at least the late Holocene.  相似文献   

12.
Records from lake sediment cores are critical for assessing the relative stability of climate and ecosystems over the Holocene. Duck Lake in south-central Lower Michigan, USA, was the focus of a study that identified how changes in the geochemical variables in lake sediments relate to variations in regional climate and local land use during the Holocene. More than 8.5 m of lacustrine sediment were recovered using Livingston and freeze corers and analyzed for organic carbon, inorganic (carbonate) carbon, total nitrogen, and trace metals. Repeating packages of sediment (1–10 cm thick) that grade from light (inorganic carbon-rich) to dark (organic carbon-rich) were found from the surface to a depth of about 8 m. Variations in the high-resolution gray scale data from core X-radiographs are highly correlated to the relative amount of inorganic carbon. Geochemical analyses of the upper 8.5 m of sediment revealed a wide range of values: 0.05–10.6% for inorganic carbon (i.e. 0.5–89% calcium carbonate) and 1.1–28% for organic carbon (i.e. 2.7–70% organic matter). Organic carbon to nitrogen ratios indicate that most of the sediment organic matter is produced within the lake. A core chronology based on eight AMS radiocarbon dates shows low sediment accumulation rates (0.05 cm/year) from 10,000 to 3,800 cal year BP and higher sediment accumulation rates (0.1–0.3 cm/year) from 3,800 cal year BP to present. We suggest that carbonate accumulates during relatively dry times, whereas organic matter accumulation dominates when nutrient input to the lake is enhanced by wetter climate. The Duck Lake core records a distinct low point in inorganic carbon deposition that may be related to the 8.2 ka cooling event now documented from several sites in North America. Spectral analysis of gray scale values shows significant ~200-year periodicities over the past 8,000 years, hypothesized to result from climate changes induced by solar forcing. Concentrations of trace metals (e.g. lead, iron, copper, zinc) indicate the onset of regional anthropogenic influence about 150 cal year BP.  相似文献   

13.
A 12.87-m-long sediment core was retrieved from closed-basin Lake Daihai in the monsoon–arid transition zone of north-central China. Oxides of major elements and their ratios normalized to Al in the AMS-14C-dated core were employed to evaluate chemical weathering intensity (CWI) in the lake drainage basin, which reflects hydrothermal conditions in the study area. Lower CWI periods occurred prior to 14.5 ka BP, and during the intervals ca. 11.7–10.3, 3.5–3.2, 2.6–1.7 ka BP, and 1.2–0 ka BP, indicating relatively low temperatures and moisture availability. Greater CWI during the intervening periods ca. 14.5–11.7, 10.3–9.0, 3.2–2.6, and 1.7–1.2 ka BP, with the maximum CWI at ca. 6.7–3.5 ka BP, imply ameliorated hydrothermal conditions in the lake basin, i.e. higher temperatures and precipitation. Exceptionally low CWI, associated with high CaO/MgO ratio during ca. 9.0–6.7 ka BP, suggests higher evaporation rates in the area under warmer temperature. Overall, CWI displays in-phase variations with changes in organic matter (TOC, TN), carbonate (CaCO3) and pollen assemblages, all of which are related to variations in monsoon effective precipitation. High CWI indicates strong monsoon-induced precipitation, whereas low CWI reflects a weak precipitation regime. The optimum hydrothermal status, recorded by the strongest CWI and maximum monsoon effective precipitation during ca. 6.7–3.5 ka BP defines the Holocene climate optimum (HCO) in the Lake Daihai region. These results indicate that the HCO prevails after the early Holocene in the monsoon–arid transition zone of north-central China. Temperature and precipitation variations during most of the Holocene, inferred from the lake sediments, are due largely to insolation forcing. Dry but warm conditions ca. 9.0–6.7 ka BP, however, probably reflect the complex interactions between insolation and geography (e.g. altitude and local topography).  相似文献   

14.
Paleohydrology studies at Mathews Pond and Whitehead Lake in northern Maine revealed synchronous changes in lake levels from about 12,000 14C yrs BP to the present. We analyzed gross sediment structure, organic and carbonate content, mineral grain size, and macrofossils of six cores from each of the two lakes, and obtained 72 radiocarbon dates. Interpretation of this paleo-environmental data suggests that the late-glacial and Younger Dryas climate was dry, and lake levels were low. Early Holocene lake levels were considerably higher but declined for an interval from about 8000 to 7200 14C yrs BP. Sediment of both lakes contains evidence of a dry period at ∼7400 14C yrs BP (8200 cal yr). Lake levels of both sites declined abruptly about 4800 14C yrs BP and remained low until 3000 14C yrs BP. Modern lake levels were achieved only within the past 600 years. The west-to-east, time-transgressive nature of lake-level changes from several sites across northeastern North America suggests periodic changes in atmospheric circulation patterns as a driving force behind observed moisture balance changes. Electronic supplementary material to this article is available at and accessible for authorized users.  相似文献   

15.
Both radiocarbon and optically stimulated luminescence (OSL) dating methods were applied to test their suitability for establishing a chronology of arid-zone lacustrine sediments using a 5.88-m-long core drilled from Lake Ulaan, southern Mongolia. Although the radiocarbon and OSL ages agree in some samples, the radiocarbon ages are older than the corresponding OSL ages at the 550-cm depth horizon (late Pleistocene) and in the 100–300-cm interval (early to late Holocene). In the early to late Holocene, radiocarbon ages are consistently older than OSL ages by 4,100–5,800 years, and in the late Pleistocene by 2,700–3,000 years. Grain-size analysis of early to late Holocene sediments and one late Pleistocene sediment sample (550-cm depth) indicates that eolian processes were the dominant sediment-transport mechanism. Also, two late Pleistocene sediments samples (from 400- to 500-cm depths) are interpreted to have been deposited by both eolian and glaciofluvial processes. Accordingly, the radiocarbon ages that were older than the corresponding OSL ages during the Holocene seem to have been a consequence of the influx of 14C-deficient carbon delivered from adjacent soils and Paleozoic carbonate rocks by the westerly winds, a process that is also active today. In addition to the input of old reworked carbon by eolian processes, the late Pleistocene sediments were also influenced by old carbon delivered by deglacial meltwater. The results of this study suggest that when eolian sediment transport is suspected, especially in lakes of arid environments, the OSL dating method is superior to the radiocarbon dating method, as it eliminates a common ‘old-carbon’ error problem.  相似文献   

16.
We inferred late Pleistocene and early Holocene (24–10 ka BP) environmental conditions in and around Lago Petén Itzá, Guatemala from ostracode remains in the lake sediments. Multivariate statistics were run on autecological information for 29 extant ostracode species collected in 63 aquatic ecosystems on the Yucatán Peninsula along a steep, increasing NW–S precipitation gradient and across a large altitudinal range. Conductivity and water depth are the most important factors that shape ostracode communities. Transfer functions were developed and applied to fossil ostracode assemblages in a ~76-m sediment core (PI-6, ~85 ka) taken in 71 m of water from Lago Petén Itzá, to infer past shifts in conductivity and water level. Results suggest climate was cold and wet during the Last Glacial Maximum (LGM). Alternating dry and wet conditions characterized the deglacial. Early Holocene climate was warmer and wetter. The LGM was characterized by low ostracode species richness (4 spp.) and abundance (<940 valves g−1), dominance of benthic over nektobenthic taxa, abundant Physocypria globula, conductivity as low as 190 μS cm−1, and clay-rich sediments with relatively high total organic carbon and low C/N ratios (<14), suggesting relatively deeper water at the core site associated with abundant precipitation. Greatest water depth at the core site during the LGM occurred late in the period and was ~50 m. The deglacial was characterized by drier conditions, higher ostracode species richness (6 spp.) and abundances up to 18,115 valves g−1, dominance of nektobenthic species, and presence of shallow-water and littoral-zone indicators such as Heterocypris punctata and Strandesia intrepida, conductivity up to 550 μS cm−1, C/N ratios as high as 37, and gypsum deposition. Lowest inferred lake depth at the core site during the deglacial was ~20 m. The early Holocene was characterized by high numbers of ostracode remains, up to 25,500 valves g−1, and the presence of L. opesta and P. globula. Cytheridella ilosvayi was absent from late Pleistocene sediments, suggesting it colonized northern Central America during the Holocene.  相似文献   

17.
Rapid urbanization and increased tourism around Nainital Lake in the Kumaun Himalayan region in north India has raised concerns about sediment and water pollution. Lead-210 dated sediment cores from the lake represent ~95 years of accumulation and yield a mean sedimentation rate of ~4.7 mm year−1. Total organic carbon (TOC), percent N and S and their atomic C/N and C/S ratios, stable isotopes (δ13C, δ15N, and δ34S), and specific biomarkers (n-alkanes and pigments) were measured in the core. Organic matter is primarily derived from in-lake algal production and TOC flux varies from 1.0 to 3.5 g m−2 year−1. Sediments are anoxic (Eh −328 to −187 mV) and have low (0.10–0.30 g m−2 year−1) N, but high (0.37–1.0 g m−2 year−1) S flux. Shifts in δ13C, δ15N, and δ34S suggest in-lake microbial processes dominated by denitrification and sulfate reduction. The sediments are dominated by short-chain hydrocarbons with low Carbon Preference Index values. The pigments indicate a gradual shift to cyanobacterial domination of the phytoplankton community in recent years. Despite an increase in external input of nutrients, the trophic state of the lake has remained largely unchanged, and the perceived human-induced impacts are limited.  相似文献   

18.
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.
Zachary SharpEmail:
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19.
Lake Elsinore is the largest natural lake in Southern California. As such, the lake provides a unique opportunity to investigate terrestrial climate on timescales otherwise underrepresented in the region’s terrestrial environment. In November 2003, three ∼10 m drill cores were extracted from the depocenter region of Lake Elsinore. These drill cores, spanning the past 9,500–11,200 calendar years, represent the first complete Holocene record of terrestrial climate from Southern California. In this paper, we focus on two adjacent, depocenter cores (LEGC03-2 and LEGC03-3), which have been correlated to develop a single composite core. Twenty-two AMS 14C dates on bulk organic matter and one cross-correlated exotic pollen age constitute the composite core’s age control. Several methods of analysis, including mass magnetic susceptibility, % total organic matter, % total carbonate, % HCl-extractable Al, and total inorganic P are used to infer climate for the past 9,500 calendar years in Southern California. Together, these data indicate a wet early Holocene followed by a long-term drying trend. Recent lake-level reconstructions from Owens Lake and Tulare Lake support our contention for a wetter-than-today early Holocene. Lacustrine sediments from the Mojave Desert also support our conclusions. We suggest that over the duration of the Holocene changing summer/winter insolation alters the region’s long-term hydrologic balance through its modulation of atmospheric circulation and its associated storm tracks. Minimum early Holocene winter insolation and maximum summer insolation act together to increase the region’s total annual precipitation by increasing the frequency of winter storms as well as enhancing the magnitude and spatial extent of the North American monsoon, the frequency of land-falling tropical cyclones in Southern California, and regional convective storms, respectively. Gradual decreases in summer insolation and increases in winter insolation produce the opposite effect with maximum drying in the late Holocene.  相似文献   

20.
Sulfur has played a central role in the acidification of many lakes in Scandinavia and other regions. As part of the research into sulfur cycling, numerous studies have analyzed the sediment record in order to develop insights into past in-lake cycling of sulfur, particularly in the context of reconstructing past deposition rates. Although many of these studies have shown that it is not easy to interpret the sediment record in terms of past sulfur deposition rates, analyses of sulfur in sediment still provide valuable information on the response of lakes to anthropogenic sulfur deposition. Here, we have analyzed sulfur in top and bottom samples from short surface cores (25–35 cm, representing ≥250 years) as well as bulk cores from ∼110 lakes located throughout Sweden, which were collected during 1986, as well as in more-detailed profiles from six lakes. The lakes with the highest surface sediment concentrations (9–24 mg S g−1 dry mass) and the highest calculated inventories of ‘excess’ sulfur (20–180 g S m−2) are found in southern Sweden and around one industrial area along the northeastern coast where sulfur deposition rates and lake-water concentrations have been highest. For many lakes in the central and northern inland region it is common that the sediment cores exhibit either no enrichment or even a decline in sulfur concentrations in near-surface sediments, which we suggest was the pre-pollution norm for lakes. Although interpreting sulfur sediment profiles is problematic for reconstructing deposition, a more-comprehensive spatial sampling approach shows that there is a good geographic agreement between sulfur deposition, lake-water chemistry and sediment sulfur accumulation.  相似文献   

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