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1.
The Chilean Lake District (38–42°S) is strongly influenced by Southern westerlies-driven precipitations. At 40°S Lago Puyehue
provides high resolution sedimentation rates (∼1–2 mm/yr) suitable for annual climate reconstruction. Several short and long
sediment cores were collected in this lake. Their analysis aim at a better understanding of climate mechanisms related to
ENSO in this part of the world. The recognition of ENSO related periodicities and their stability is studied through the analysis
of two short varved cores collected from underflow and interflow key sites. According to varve chronology controlled by 137Cs and 210Pb profiles and chronostratigraphical markers, the short core from underflow site (PU-I) spans 294 ± 18 years and the core
in the interflow site (PU-II) covers 592 ± 9 years. Several methods of spectral analysis were applied on the total varve thickness
to identify potential periodicities in the signal. Blackman–Tuckey, Maximum Entropy, Multi-Taper Methods (MTM) and singular
spectrum analysis were applied on the whole record. In addition, evolutive MTM and wavelet analyses allow to identify temporal
influence of some periodicities. In the PU-I studied interval (AD 1700–2000), a period at ∼3.0 years appears in a large part
of the interval, mostly in the recent part. Periods at ∼5.2 and ∼23 years also show up. PU-II record (AD 1400–2000) displays
the most robust periodicities at around 15, 9, 4.4, 3.2 and 2.4 years. These periodicities are in good agreement with the
sub-decadal periods identified by Dean and Kemp (2004) and linked to the El Nino Southern Oscillation and the Pacific Decadal
Oscillation. Differences in the recorded periodicities between PU-I and PU-II sites are consistent with different sedimentation
processes in the lake. According to climate instrumental data for the last 20 years, varves in PU-I site are mostly related
to fluvial dynamics and regional climate factors, i.e., precipitation, temperature and wind. In PU-II site, varves increment
is related to both regional and global climate forcing factors, i.e., El Nino Southern Oscillation. The evolutive MTM analysis
and the wavelet analysis suggest a striking break in the periodicities at around AD 1820. Finally relationships between El
Nino and longer term climate phase like the Little Ice Age (LIA) are also assessed.
This is the seventh in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of
Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M.-F. Loutre
and E. Chapron. 相似文献
2.
G. Petterson I. Renberg P. Geladi A. Lindberg F. Lindgren 《Journal of Paleolimnology》1993,9(3):195-208
As part of an investigation aimed at assessing the potential of northern Swedish varved lake sediments for fine-resolution reconstruction of past climatic conditions, the following questions were addressed; how representative is a single core, does the appearance of varves change as the sediment ages, and how can varve thickness and within-varve structures be digitized? Analyses of replicate cores from Kassjön, using sample sequences of seasonal, annual and centennial time resolution, show that spatial variability of sediment accumulation in the varved sediment is very low. Comparisons of intensity curves from image analysis of freeze cores of recent sediments from Lake Nylandssjön, sampled in 1980 and 1985, indicate that the varves acquire their appearance at the sediment-water interface during sedimentation and that the varve structures are preserved during diagenetic processes. Measurement of varve thicknesses with a tree-ring microscope and with image analysis gave similar results. However, with image analysis, within-varve structures such as colour variations and thicknesses of seasonal layers, can also be recorded, increasing the possibilities for palaeolimnological and palaeoclimatic inferences. 相似文献
3.
A late Holocene varved sediment record of environmental change from northern Ellesmere Island, Canada 总被引:4,自引:0,他引:4
A composite record of varve sedimentation is presented from high arctic meromictic Lake C2. The combination of a short runoff and sediment transport season with the strong density stratification of the lake lead to the formation of annual sediment couplets. This conclusion was confirmed by 210Pb determinations. High intra-lake correlation of the varves allowed the construction of a composite record of varve sedimentation from overlapping segments of multiple sediment cores. Cross-dating between core segments isolated counting errors in individual cores, that could be attributed to minor sediment disturbances and vague structures. Resolving counting errors by cross-dating reduced the chronological error of the composite series to an estimated ±57 years.The Lake C2 series is the first non-ice cap, high resolution late-Holocene environmental record from the Canadian high arctic. The composite varve series compares favorably with other high resolution proxies from the arctic, in particular with the ice core records from Devon Island and Camp Century, Greenland. A general correspondence between the varve record and other North American proxies for the little Ice Age period (1400–1900 AD) suggests that the Lake C2 record is sensitive to large-scale synoptic changes.This is the tenth in a series of papers published in this issue on the Taconite Inlet Lakes Project. These papers were collected by Dr R. S. Bradley. 相似文献
4.
Solar and anthropogenic imprints on Lake Masoko (southern Tanzania) during the last 500 years 总被引:1,自引:0,他引:1
Yannick Garcin David Williamson Laurent Bergonzini Olivier Radakovitch Annie Vincens Guillaume Buchet Joël Guiot Simon Brewer Pierre-Etienne Mathé Amos Majule 《Journal of Paleolimnology》2007,37(4):475-490
The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes covering the last 500 years.
Multi-proxy studies were performed on a 52 cm sediment core retrieved from the deepest part of the lake. Magnetic, organic
carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550–1850), confirming
that the LIA in eastern Africa resulted in marked and synchronous hydrological changes. However, the direction of response
varies between different African lakes (low versus high lake-levels), indicating strong regional contrasts that prevent the
clear identification of climate trends over eastern Africa at this time. Inferred changes in Masoko lake-levels closely resemble
the record of solar activity cycles, indicating a possible control of solar activity on the climate in this area. This observation
supports previous results from East African lakes, and extends this relationship southward. Finally, anthropogenic impact
is observed in the Masoko sediments during the last 60 years, suggesting that human disturbance significantly affected this
remote basin during colonial and post-colonial times. 相似文献
5.
Genki I. Matsumoto Yukinori Tani Koji Seto Tomoko Tazawa Masumi Yamamuro Takahiro Watanabe Toshio Nakamura Tetsuo Takemura Satoshi Imura Hiroshi Kanda 《Journal of Paleolimnology》2010,44(2):677-693
Antarctic climate changes influence environmental changes at both regional and local scales. Here we report Holocene paleolimnological
changes in lake sediment core Sk4C-02 (length 378.0 cm) from Lake Skallen Oike in the Soya Kaigan region of East Antarctica
inferred from analyses of sedimentary facies, a range of organic components, isotope ratios of organic carbon and nitrogen,
and carbon-14 dating by Tandetron accelerator mass spectrometry. The sediment core was composed of clayish mud (378.0–152.5 cm)
overlain by organic sediments (152.5 cm-surface). The age of the surface and the core bottom were 150 (AD1950-1640) and ca.
7,030 ± 73 calibrated years before present (cal BP), respectively, and the mean sedimentation rate was estimated to be 0.55 mm/year.
Multi-proxy analyses revealed that the principal environmental change in the core is a transition from marine to lacustrine
environments which occurred at a depth of 152.5 cm (ca. 3,590 cal BP). This was caused by relative sea level change brought
about by ongoing retreat of glaciers during the mid-Holocene warming of Antarctica, and ongoing isostatic uplift which outpaced
changes in global (eustatic) sea level. The mean isostatic uplift rate was calculated to be 2.8 mm/year. The coastal marine
period (378.0–152.5 cm, ca. 7,030–3,590 cal BP) was characterized by low biological production with the predominance of diatoms.
During the transition period from marine to freshwater conditions (152.5-approximately 135 cm, ca. 3,590–3,290 cal BP) the
lake was stratified with marine water overlain by freshwater, with a chemocline and an anoxic (sulfidic) layer in the bottom
of the photic zone. Green sulfur bacteria and Cryptophyta were the major photosynthetic organisms. The Cryptophyta appeared
to be tolerant of the moderate salinity and stratified water conditions. The lacustrine period (approximately 135 cm-surface,
ca. 3,290 cal BP-present) was characterized by high biological production by green algae (e.g. Comarium clepsydra and Oedegonium spp.) with some contributions from cyanobacteria and diatoms. Biological production during this period was 8.7 times higher
than during the coastal marine period. 相似文献
6.
The recent (1950–1996) varve record from the proximal sediments in Nicolay Lake, Cornwall Island, Nunavut, Canada (77°46′ N,
94°40′ W) contains distinct subannual rhythmites. Deposition of these subannual rhythmites is due primarily to nival snow
melt, with additional sedimentary units resulting from major summer precipitation and subaqueous mass wasting events. In order
to evaluate the potential hydroclimatic signal contained in the varves from the unglacierized catchment, the nival deposition
record was estimated by delineating the initial subannual rhythmite within each varve. When the record is split into temporal
segments based on two phases that exhibit different sediment deposition patterns in the lake, the nival rhythmites are significantly
correlated to annual cumulative melting degree days (MDD) from the nearest weather station Isachsen (78°47′ N, 103°32′ W)
(1950–1962 AD and 1963–1977 AD with r = 0.55 and r = 0.82, respectively). A similar analysis with data from Resolute (74°43′ N,
94°59′ W) yields slightly weaker correlations (1950–1962 AD, r = 0.60; 1963–1994, r = 0.59). The strong positive correlation
with both the Isachsen and Resolute thermal records suggests that the paleoclimatic signal in the sediments reflects regional
climate conditions. Notably, the signal is strongest when the entire melt season is considered; weaker correlations with instrumental
weather records are associated with comparisons limited to the peak melt or early season melt periods. We attribute this to
the ongoing supply of snowmelt through the season in this polar region and the availability of sediment for transport throughout
the melt season. These results indicate that a high resolution hydroclimatic signal is present in the sediments from Nicolay
Lake and can be used for paleoclimate reconstruction provided sedimentary depositional controls are taken into account. 相似文献
7.
Broxton W. Bird Mark B. Abbott Bruce P. Finney Barbara Kutchko 《Journal of Paleolimnology》2009,41(1):25-41
Varved minerogenic sediments from glacial-fed Blue Lake, northern Alaska, are used to investigate late Holocene climate variability.
Varve-thickness measurements track summer temperature recorded at Atigun Pass, located 41 km east at a similar elevation (r
2 = 0.31, P = 0.08). Results indicate that climate in the Brooks Range from 10 to 730 AD (varve year) was warm with precipitation inferred
to be higher than during the twentieth century. The varve-temperature relationship for this period was likely compromised
and not used in our temperature reconstruction because the glacier was greatly reduced, or absent, exposing sub-glacial sediments
to erosion from enhanced precipitation. Varve-inferred summer temperatures and precipitation decreased after 730 AD, averaging
0.4°C above the last millennial average (LMA = 4.2°C) from 730 to 850 AD, and 0.1°C above the LMA from 850 to 980 AD. Cooling
culminated between 980 and 1030 AD with temperatures 0.7°C below the LMA. Varve-inferred summer temperatures increased between
1030 and 1620 AD to the LMA, though the period between 1260 and 1350 AD was 0.2°C below the LMA. Although there is no equivalent
to the European Medieval Warm Period in the Blue Lake record, two warm intervals occurred from 1350 to 1450 AD and 1500 to
1620 AD (0.4 and 0.3°C above the LMA, respectively). During the Little Ice Age (LIA; 1620 to 1880 AD), inferred summer temperature
averaged 0.2°C below the LMA. After 1880 AD, inferred summer temperature increased to 0.8°C above the LMA, glaciers retreated,
but aridity persisted based on a number of regional paleoclimate records. Despite warming and glacial retreat, varve thicknesses
have not achieved pre-730 AD levels. This reflects limited sediment availability and transport due to a less extensive retreat
compared to the first millennium, and continued relative aridity. Overall, the Blue Lake record is similar to varve records
from the eastern Canadian Arctic that document a cool LIA and twentieth century warming. However, the occurrence and timing
of events, such as the LIA and Medieval Warm Period, varies considerably among records, suggesting heterogeneous climatic
patterns across the North American Arctic.
相似文献
Broxton W. BirdEmail: |
8.
Timothy L. Cook Raymond S. Bradley Joseph S. Stoner Pierre Francus 《Journal of Paleolimnology》2009,41(1):77-94
Sediments in Lower Murray Lake, northern Ellesmere Island, Nunavut Canada (81°21′ N, 69°32′ W) contain annual laminations
(varves) that provide a record of sediment accumulation through the past 5000+ years. Annual mass accumulation was estimated
based on measurements of varve thickness and sediment bulk density. Comparison of Lower Murray Lake mass accumulation with
instrumental climate data, long-term records of climatic forcing mechanisms and other regional paleoclimate records suggests
that lake sedimentation is positively correlated with regional melt season temperatures driven by radiative forcing. The temperature
reconstruction suggests that recent temperatures are ~2.6°C higher than minimum temperatures observed during the Little Ice
Age, maximum temperatures during the past 5200 years exceeded modern values by ~0.6°C, and that minimum temperatures observed
approximately 2900 varve years BC were ~3.5°C colder than recent conditions. Recent temperatures were the warmest since the
fourteenth century, but similar conditions existed intermittently during the period spanning ~4000–1000 varve years ago. A
highly stable pattern of sedimentation throughout the period of record supports the use of annual mass accumulation in Lower
Murray Lake as a reliable proxy indicator of local climatic conditions in the past.
相似文献
Pierre FrancusEmail: |
9.
E. Cuna E. Zawisza M. Caballero A. C. Ruiz-Fernández S. Lozano-García J. Alcocer 《Journal of Paleolimnology》2014,51(1):1-14
The Little Ice Age (LIA), AD 1350–1850, represents one of the most recent, persistent global climate oscillations. In Mexico, it has been associated with temperature decreases of 1.5–2 °C and mountain glacier advances, which are not accurately dated. We present new information about the nature of the LIA in central Mexico based on a decadal-resolution sediment sequence from high-altitude, tropical Lake La Luna, in the Nevado de Toluca volcano. We inferred past climatic and environmental changes using magnetic susceptibility, charcoal particles, palynomorphs, diatoms, cladoceran remains and multivariate statistics. The onset of the LIA corresponds with the beginning of a long-term trend to colder and drier climate ca. AD 1360–1910. The coolest and driest episode, ~AD 1660–1760, which corresponds with the Maunder Minimum in solar activity, was characterized by a cladoceran assemblage that showed the greatest dissimilarity to the modern one (no modern analogue), with the presence of cold-water species and Daphnia ephippia. The beginning of a warming trend ca. AD 1760, was identified by a diatom assemblage dominated by species with affinities for higher pH values (>6) and the greatest dissimilarity to the modern assemblage. This less cold, but still dry period, corresponds with historical reports of cattle and crop losses that predated the Mexican wars of Independence (AD 1810–1821) and Revolution (1910–1924). Modern conditions, established around AD 1910, resemble those during the Medieval Climate Anomaly (ca. AD 1200). No clear evidence of modern, human-induced environmental change was recorded, indicating that Lake La Luna is an ideal site in Mexico to monitor future impacts of global change. 相似文献
10.
Clastic varved sediments from Donard Lake, in the Cape Dyer region of Baffin Island, provide a 1250 yr record of decadal-to-centennial scale climate variability. Donard Lake experiences strong seasonal fluctuations in runoff and sediment fluxes due to the summer melting of the Caribou Glacier, which presently dominates its catchment. The seasonal variation in sediment supply results in the annual deposition of laminae couplets. A radiocarbon date measured on moss fragments, with a calibrated age of 860 ± 80 yrs before present (BP), is in close agreement with the age based on paired-layer counts. Together with the fabric of the laminae determined from microscope analysis, the age agreement demonstrates that the laminae couplets are annually deposited varves. Comparisons of varve thickness and average summer temperature from nearby Cape Dyer show a significant positive correlation (r= 0.57 for annual records, r = 0.82 for 3-yr averages), indicating that varve thickness reflects changes in average summer temperature. Varve thickness was used to reconstruct average summer temperatures for the past 1250 yrs, and shows abrupt shifts and large amplitude decadal-to-centennial scale variability throughout the record. The most prominent feature of the record is a period of elevated summer temperatures from 1200-1375 AD, followed by cooler conditions from 1375-1820 AD, coincident with the Little Ice Age. 相似文献
11.
Iceberg Lake, a glacier-dammed proglacial lake in southern Alaska, contains a 1,500+ year varve record complicated by a history of episodic lake-level changes associated with fluctuations in ice-dam thickness and position. To better understand the basinwide glaciolacustrine response to late Holocene climate variability, we collected five cores from two areas in the lake, including a previously unexamined deepwater area distal from inlet streams. Based on eight AMS 14C dates, and correlations among our cores and previously documented outcrops, we describe ~1,000?years of stratigraphy from each area. Deposition at both areas was dominated by fine-grained varves, but cores from the distal area uniquely contain coarser deposits, including rhythmites and graded sand beds, that we attribute to deposition of a subaqueous outwash fan-delta between ~1250 and 1650 AD. We attribute this event to thickening of the impounding glacier and consequent incursion of the glacier margin, and an associated lateral moraine, into the lake. This result suggests an early onset of the Little Ice Age (LIA) glacial advance in this region. Changes in basinwide circulation and sedimentation associated with this event probably caused minor thickening of varves used previously to reconstruct summer temperatures, reducing sensitivity of that record to early LIA cooling. The basinwide impact of this event illustrates the potentially significant spatial and temporal variability of lacustrine sedimentary processes in dynamic glacial landscapes. 相似文献
12.
Tomi P. Luoto Liisa Nevalainen Kaarina Sarmaja-Korjonen 《Journal of Paleolimnology》2008,40(4):1097-1113
We studied a short sediment core from Lake Hampträsk, southern Finland, for evidence of the ‘Little Ice Age’ (LIA) in aquatic invertebrate communities. Subfossil chironomids, cladocerans, and chydorid ephippia were investigated, together with detrended correspondence analyses (DCAs) and loss-on-ignition (LOI). Our results show two cooler periods. The first cooling, indicated by increased numbers of chydorid ephippia and cold-water chironomid taxa, occurred ca. 1400 AD and the second, more drastic cooling, during the seventeenth century, when cold-water chironomids began to increase. Our data suggest that the cooling culminated around 1700 AD, when cold-stenothermic chironomids and chydorid ephippia attained maximal values and the LOI and diversity of invertebrates decreased to minimal values. After the LIA, the aquatic fauna appeared to respond to rising trophic state caused by enhanced land use in the catchment. 相似文献
13.
Juan Pablo Corella Ana Moreno Mario Morellón Valentí Rull Santiago Giralt María Teresa Rico Ana Pérez-Sanz Blas Lorenzo Valero-Garcés 《Journal of Paleolimnology》2011,46(3):351-367
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
14.
Quantitative reconstructions of mean July temperatures (T
jul) based on new and previously published pollen-stratigraphical data covering the last 2000 years from 11 lakes in northern
Fennoscandia and the Kola Peninsula are presented. T
jul values are based on a previously published pollen-climate transfer function for the region with a root-mean-square error
of prediction (RMSEP) of 0.99°C. The most obvious trend in the inferred temperatures from all sites is the general decrease
in T
jul during the last 2000 years. Pollen-inferred T
jul values on average 0.18 ± 0.56°C (n = 91) higher than present (where “present” refers to the last three decades based on pollen-inferred T
jul in core-top samples) are indicated between 0 and 1100 AD (2000–850 cal year BP), and temperatures −0.2 ± 0.47°C (n = 78) below present are inferred between 1100 and 1900 AD (850–50 cal year BP). No consistent temperature peak is observed
during the ‘Medieval Warm Period’, ca. 900–1200 AD (1100–750 cal year BP), but the cooler period between 1100 and 1900 AD
(850–50 cal year BP) corresponds in general with the ‘Little Ice Age’ (LIA). Consistently with independent stable isotopic
data, the composite pollen-based record suggests that the coldest periods of the LIA date to 1500–1600 AD (450–350 cal year
BP) and 1800–1850 AD (150–100 cal year BP). An abrupt warming occurred at about 1900 AD and the twentieth century is the warmest
century since about 1000 AD (950 cal year BP).
相似文献
A. E. BjuneEmail: |
15.
Timothy M. Shanahan Jonathan T. Overpeck J. Warren Beck C. Winston Wheeler John A. Peck John W. King Christopher A. Scholz 《Journal of Paleolimnology》2008,40(1):339-355
The sediments from Lake Bosumtwi, Ghana contain a unique record of fine-scale (mm to sub-mm) laminations, which will provide
a valuable annual chronometer for reconstructing paleoenvironmental changes in West Africa covering much of the last 1 Ma.
Comparisons of laminae counts to independent 210Pb dates and the rise in anthropogenic “bomb” radiocarbon support the interpretation of the laminations in the uppermost sediments
as registering annual events. Radiocarbon dates on in-situ fish-bone collagen are in agreement with varve counts, further
supporting the annual nature of our varve chronology. Over the instrumental period (1925–1999), dark-varve thickness measurements
are correlated with local rainfall (r = 0.54) and appear able to resolve decadal-scale changes in precipitation. The relationship between varve thickness and rainfall
provides support for our interpretation that dark-colored varve thickness records catchment runoff during the rainy season
rather than dust flux during the dry season. Dark laminae alternate with organic and carbonate-rich light laminae formed during
the fall period of enhanced productivity. Downcore, varves undergo significant microstratigraphic and geochemical variations,
but retain the same pattern of alternating clastic and organic-rich laminae, providing support that the laminae may represent
annual time markers for reconstructions of the deeper part of the record. 相似文献
16.
We inferred the temperature and environmental conditions of Smreczynski Staw Lake in the Tatra Mountains, southern Poland,
from a sediment record covering the last 1,500 years. Paleobiological methods (cladocera, chironomid, and diatom analyses)
were used together with sedimentological analysis and dating. These studies provide new information about the timing and character
of climate fluctuations during the Little Ice Age (LIA). The Medieval Warm Period ended in the Tatra region at the beginning
of the thirteenth century, followed by the first episode of the LIA. The LIA was a relatively long but unstable period. The
first part of the LIA was cold in the Tatra Mountains, without evidence of increasing precipitation, while the second part,
after AD 1540, was cold and humid. The LIA terminated in the Tatra Mountains at the beginning of the twentieth century, although
some aspects of its climatic and sedimentological regime continued until the 1920s. We also found some evidence of warming
and acidification during the twentieth century. 相似文献
17.
Image analysis as a method to quantify sediment components 总被引:1,自引:0,他引:1
Varved lake sediments, with their annual to seasonal resolution, have a high potential for inferring past environmental and climatic conditions. To fully utilize the information present in varved records, high-resolution analyses, which often are time-consuming and difficult to perform, are desirable. The investigation reported here aims at (i) developing image analysis as a method for estimating annual accumulation rates of sediment components such as minerogenic matter, organic matter and biogenic silica, and (ii) assessing the relative importance of these components for changes in varve thickness. Image analysis was used to digitize the grey-scale variations and to measure the varve thickness of 540 varves (476-1015 AD) from Lake Kassjön in northern Sweden. From the 35 cm long digitized sediment sequence, 108 consecutive five-year samples were cut out quantitatively, and relationships between grey-scale variations and sediment dry mass and individual sediment components were assessed. There is a strong correlation between corrected grey-scale (i.e. the product of grey-scale and varve thickness) and the dry mass accumulation rate (r = 0.90, p < 0.001). With a stepwise multiple regression a significant model (R2 = 0.81) between corrected grey-scale and the accumulation rates of minerogenic matter (r = 0.90, p < 0.001) and biogenic silica (r 0.26, p < 0.012) was obtained. Considering the minor contribution and weak significance of biogenic silica, image analysis can be used as a fast and non-destructive method to infer past annual accumulation rates of dry mass and minerogenic matter in Kassibn. The model of the relationship between changes in varve thickness, and water content and accumulation rates of sediment components has little predictive power (R2 = 0.45). The result shows that the varve thickness in Kassjön, at least during the period 476-1015 AD, is not determined by a single sediment component but partly depends on interactions between the major sediment components. 相似文献
18.
Tomonori Naya Yoshihiro Tanimura Yutaka Kanai Fujio Kumon Kazuo Amano 《Journal of Paleolimnology》2007,37(4):547-563
In order to assess the recent anthropogenic environmental changes in Lake Kitaura, central Japan, changes during the past
few centuries were reconstructed from results of radiometric and tephrochlonological age determination, magnetic susceptibility
measurements, total organic carbon analyses, total nitrogen analyses and fossil diatom analyses on a sediment core from the
lake. A total of six major and sub-zones are recognized according to the diatom fossil assemblages, and we discuss aquatic
environmental change in Lake Kitaura mainly based on these diatom assemblage change. Zone Ia and Zone Ib (older than AD 1707)
are marine to brackish. In Zone IIa (AD␣1707–AD 1836), most of the brackish diatoms disappeared, and were replaced by freshwater
species indicating a decrease in salinity. We interpret the salinity decrease in Zone I–IIa as a sea-level fall during the
Little Ice Age. The salinity of the lake decreased to near freshwater conditions in Zone IIb (AD 1836–AD 1970), which could
arise from alteration in River Tone or development of a sandspit in the mouth of River Tone in addition to sea-level change.
In Zone IIIa (AD 1970–AD 1987), the diatom assemblage indicates a freshwater environment, and sedimentation rates increase
rapidly. These changes reflect sedimentary environment change and an ecosystem transition due to the construction of the tide
gate. In Zone IIIb (AD 1987–AD 2002), the diatom flux (valves cm−2 y−1) increased and species composition changed. The changes in Zone IIIb show a good agreement with limnological monitoring data
gathered from the lake. These paleolimnological data suggest that the recent human-induced changes of the aquatic environment
of the lake after the 1970s exceed rates during the period concerned in this study. 相似文献
19.
The timing of the last deglaciation in southern Chile is re-evaluated from a calendar varve chronology (Lago Puyehue, 40° S).
The climate shifts are analysed by continuous annual varve-thickness measurements through the ∼17,100 cal. year to 10,800 cal.
year BP time window (∼3.5 m sediment core). The varve years are determined by the alternation of light (phytoplankton-rich)
and dark (terrigenous and organic-rich) layers forming graded annual couplets (∼0.2 to 0.8 mm/year). The varve chronology
is constructed by conventional varve-counting methods on thin sections after correction for instantaneous volcanic and/or
seismic events detected in the thin sections. The calibrated varve-age model derived from the manual varve counting is constrained
by high-resolution grey-scale (GS) semi-automatic counts of the annual light phytoplankton-rich layers (∼120 μm to 300 μm
thick). Due to physical sediment properties the GS constitutes a proxy record for the phytoplankton/terrigenous varve-thickness
variations through the sediment record. The varve couplets are thicker/thinner during humid/dry phases and darker/lighter
(negative/positive annual grey-scale index) during cold/warm phases. Our results show that at 40° S the last deglaciation
took place in two phases between ∼17,100 cal. year and ∼15,500 cal. year BP. We note a climate instability between ∼15,500 cal.
year and 13,300 cal. year BP and a significant dry phase between ∼15,000 and 14,500 cal. year BP. We evidence a cold event
in two phases between ∼13,300 and 12,200 cal. year BP interrupted by a dry event between ∼12,800 and 12,600 cal. year BP.
The onset of a significant warmer period is observed after ∼11,500 cal. year BP. Our results provide new evidence of a Younger
Dryas cool reversal in southern Chile, i.e., the Huelmo/Mascardi event Hajdas et al. (2003) associated with an abrupt dry
pulse at ∼12,800–12,600 cal. year BP. The high-resolution grey-scale measurements performed on the biogenic varves from Lago
Puyehue provide a reliable calibrated chronology of the regional environmental and climate shifts during the last deglaciation.
This is eighth in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of
Lago Puyehue, Chilean Lake District. The papers in this issue were collected by M. De Batist, N. Fagel, M.-F. Loutre and E.
Chapron. 相似文献
20.
Distinguishing climatic from direct anthropogenic influences during the past 400 years in varved sediments from Lake Holzmaar (Eifel,Germany) 总被引:3,自引:0,他引:3
A 336-year floating varve chronology from Lake Holzmaar (Eifel, Western-Germany) covering the recent period has been established by microfacies analysis of thin sections. This sequence terminates 23 cm below the core top. In the top 23 cm, the varves are disturbed. By means of linear regression, the varve sequence was dated to the period AD 1607–1942. The influences of climatic variability and anthropogenic activities in the lakes catchment (e.g., forestry, agriculture) on lithology, fabric, and microfossil content of the varve sublaminae could be discriminated by applying statistical analyses (ordination and clustering) to the combination of the sublaminae in the varves and their thickness. Four clusters are obtained. Cluster 1 indicates cold springs, and shorter, cooler summers reflected primarily in below-average varve thickness (VT) for two stable phases: from AD 1650–1700 (during the Maunder Minimum) and from AD 1750–1785. Cluster 2 indicates years with conditions transitional to that indicated by cluster 1, characterized by vigorous and prolonged spring circulation with massive blooms of the nordic-alpine Aulacoseira subarctica. The samples assigned to Cluster 3 and Cluster 4 show the imprint of anthropogenic influences. Cluster 3 (AD 1795–1815 and AD 1825–1885) is characterized by above-average VT due to high detritus input throughout the year. The increased soil erosion can be linked to anthropogenic deforestation as a consequence of the production increase of the Eifelian iron industry at the end of the 18th century. This input dampens the climatic signal of a colder Dalton Minimum, which is reflected in a short drop in VT centered around AD 1810. At about AD 1885, Cluster 4 conditions, characterized by increased nutrient concentrations, low detritus input, and longer periods of stable summer stratification, become the stable state in Lake Holzmaar. They indicate the response of the lake to natural reforestation and the use of artificial fertilizers in the catchment, which began, according to historical records, in the 1850s in the Eifel region. The prolonged, stable summer stratification periods may be the first indication of the modern warming trend. A drop in VT centered around AD 1890 and recurring cluster-1 conditions may indicate the Damon Minimum. 相似文献