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1.
We present three new benthic foraminiferal δ13C, δ18O, and total organic carbon time series from the eastern Atlantic sector of the Southern Ocean between 41°S and 47°S. The measured glacial δ13C values belong to the lowest hitherto reported. We demonstrate a coincidence between depleted late Holocene (LH) δ13C values and positions of sites relative to ocean surface productivity. A correction of +0.3 to +0.4 [‰ VPDB] for a productivity-induced depletion of Last Glacial Maximum (LGM) benthic δ13C values of these cores is suggested. The new data are compiled with published data from 13 sediment cores from the eastern Atlantic Ocean between 19°S and 47°S, and the regional deep and bottom water circulation is reconstructed for LH (4–0 ka) and LGM (22–16 ka) times. This extends earlier eastern Atlantic-wide synoptic reconstructions which suffered from the lack of data south of 20°S. A conceptual model of LGM deep-water circulation is discussed that, after correction of southernmost cores below the Antarctic Circumpolar Current (ACC) for a productivity-induced artifact, suggests a reduced formation of both North Atlantic Deep Water in the northern Atlantic and bottom water in the southwestern Weddell Sea. This reduction was compensated for by the formation of deep water in the zone of extended winter sea-ice coverage at the northern rim of the Weddell Sea, where air–sea gas exchange was reduced. This shift from LGM deep-water formation in the region south of the ACC to Holocene bottom water formation in the southwestern Weddell Sea, can explain lower preformed δ13CDIC values of glacial circumantarctic deep water of approximately 0.3‰ to 0.4‰. Our reconstruction brings Atlantic and Southern Ocean δ13C and Cd/Ca data into better agreement, but is in conflict, however, with a scenario of an essentially unchanged thermohaline deep circulation on a global scale. Benthic δ18O-derived LGM bottom water temperatures, by 1.9°C and 0.3°C lower than during the LH at deepest southern and shallowest northern sites, respectively, agree with the here proposed reconstruction of deep-water circulation in the eastern South Atlantic Ocean.  相似文献   

2.
Paleoceanographic changes since the Late Weichselian have been studied in three sediment cores raised from shelf depressions along a north–south transect across the central Barents Sea. AMS radiocarbon dating offers a resolution of several hundred years for the Holocene. The results of lithological and micropaleontological study reveal the response of the Barents Sea to global climatic changes and Atlantic water inflow. Four evolutionary stages were distinguished. The older sediments are moraine deposits. The destruction of the Barents Sea ice sheet during the beginning of the deglaciation in response to climate warming and sea level rise resulted in proximal glaciomarine sedimentation. Then, the retreat of the glacier front to archipelagoes during the main phase of deglaciation caused meltwater discharge and restricted iceberg calving. Fine-grained distal glaciomarine sediments were deposited from periodic near-bottom nepheloid flows and the area was almost permanently covered with sea ice. The dramatic change in paleoenvironment occurred near the Pleistocene/Holocene boundary when normal marine conditions ultimately established resulting in a sharp increase of biological productivity. This event was diachronous and started prior to 10 14C ka BP in the southern and about 9.2 14C ka in the northern Barents Sea. Variations in sediment supply, paleoproductivity, sea-ice conditions, and Atlantic water inflow controlled paleoenvironmental changes during the Holocene.  相似文献   

3.
We obtained the high-resolution record of terrestrial biomarkers (C29 and C31 n-alkanes) for the last 26,000 years from Oki Ridge in the south Japan Sea that enabled us to discuss millennial scale climate changes. Our sampling resolution for the biomarker during the major deglaciation period (10–19.5 cal ka BP) is 300 years and for the elemental analyses (total organic carbon and total nitrogen) is as good as ca 200 years. The estimated mass accumulation rate of these molecules during the last glacial period is substantially higher than during the Holocene. They also exhibited two distinct peaks at 17.6 cal ka BP and 11.4 cal ka BP, which are coincident with Heinrich Event 1 and the latest stage of the Younger Dryas, respectively. The unique oceanographic setting of the Japan Sea tends to preferentially preserve organic material of aeolian origin. The nature of our biomarker record in fact suggests a strong aeolian signal, and hence their flux to the Japan Sea potentially reflects the climate conditions of the dust source regions and transport intensity. Our results are consistent with previously reported monsoon variations based on other proxies that is indicative of a strong linkage between North Atlantic climate and Asian monsoon intensity.  相似文献   

4.
We estimate the intensity of Late-glacial and Holocene methane emissions from peatlands based on their paleo net primary production (PNPP). The PNPP is derived from the carbon accumulation rates of the studied bog profile (Etang de la Gruère, Switzerland), which are corrected for the degree of peat degradation. The obtained PNPP curve is taken as a proxy for methane emissions. It shows relatively high values (90 g C m− 2 yr− 1) early in the Bolling/Allerod and drops to low values (40 g C m− 2 yr− 1) during the Younger Dryas cold period. With the onset of the Holocene the PNPP increases strongly up to 150 g C m− 2 yr− 1 around ca. 10,000 Cal. yr bp. This is followed by a decline to minimum values (30 to 40 g C m− 2 yr− 1) between 6500 and 4000 Cal. yr bp. Thereafter, the PNPP starts to increase again to reach its highest value (175 g C m− 2 yr− 1) around 1000 Cal. yr bp.The PNPP curve correlates well with the evolution of the atmospheric methane concentrations as derived from Greenland ice-cores. For example, minima in atmospheric methane reported during the Younger Dryas and around 5200 Cal. yr bp are coinciding with the lowest values of PNPP and the negative atmospheric methane peak at 8200 Cal. yr bp corresponds to a marked decrease in PNPP.Our PNPP curve suggests that the methane emissions from northern peatlands evolved similar to those of low latitude wetlands and together they largely determined the evolution of atmospheric methane throughout the Late-glacial and the Holocene. The abruptness of the rise of atmospheric methane at the end of the Younger Dryas probably points to an additional source (e.g. marine gas hydrates), but very early in the Holocene the peatlands have likely become the dominant source of atmospheric methane.  相似文献   

5.
The prairie-forest transition in midcontinental North America is a major physiognomic boundary, and its shifts during the Holocene are a classic example of climate-driven ecotonal dynamics. Recent work suggests asymmetrical Holocene behavior, with a relatively rapid early Holocene deforestation and more gradual reforestation later in the Holocene. This paper presents a new synthesis of the Holocene history of the Great Plains prairie-forest ecotone in the north-central US and central Canada that updates prior mapping efforts and systematically assesses rates of change. Changes in percent woody cover (%WC) are inferred from fossil pollen records, using the modern analog technique and surface-sediment pollen samples cross-referenced against remotely sensed observations. For contemporary pollen samples from the Great Plains, %WC linearly correlates to percent arboreal pollen (%AP), but regression parameters vary interregionally. At present, %AP is consistently higher than %WC, because of high background levels of arboreal pollen. Holocene maps of the eastern prairie-forest ecotone agree with prior maps, showing a rapid decrease in %WC and eastward prairie advance between 10,000 and 8000 ka (1 ka = 1000 calibrated years before present), a maximum eastward position of the ecotone from 7 to 6 ka, and increased %WC and westward prairie retreat after 6 ka. Ecotone position is ambiguous in Iowa and southeastern Minnesota, due to a scarcity of modern analogs for early-Holocene samples with high Ulmus abundances and for samples from alluvial sediments. The northern prairie-forest ecotone was positioned in central Saskatchewan between 12 and 10 ka, stabilized from 10 to 6 ka despite decreases in %WC at some sites, then moved south after 6 ka. In both east and north, ecotonal movements are consistent with a dry early Holocene and increasing moisture availability after 6 ka. Sites near the ecotone consistently show an asymmetric pattern of abrupt early Holocene deforestation (< 300 years) and gradual reforestation after 6 ka. Early Holocene decreases in %WC are faster than the corresponding drops in %AP, because the analog-based %WC reconstructions correct for the high background levels of arboreal pollen types that blur temporal variations in %AP. For example, at Elk Lake, the %AP decline lasts 1000 years, whereas the %WC decline occurs between adjacent pollen samples, approximately 300 years apart. Thus, early Holocene deforestation may have been even more abrupt than previously recognized. Rapid deforestation likely was promoted both by rapid climate changes around 8.2 ka and positive fire-vegetation feedbacks. Non-linear vegetational responses to hydrological variability are consistent with 1) other paleorecords showing rapid die-offs of some eastern tree species in response to aridity and 2) observations of threshold-type ecological responses to recent climate events. The 21st-century trajectory for the Great Plains prairie-forest ecotone is uncertain, because climate models differ over the direction of regional precipitation trends, but future drying would be more likely to trigger threshold-type shifts in ecotone position.  相似文献   

6.
New paleovegetation and paleoclimatic reconstructions from the Sierra Madre Occidental (SMO) in northwestern Mexico are presented. This work involves climate and biome reconstruction using Plant Functional Types (PFT) assigned to pollen taxa. We used fossil pollen data from four Holocene peat bogs located at different altitudes (1500‑2000 m) at the border region of Sonora and Chihuahua at around 28° N latitude (Ortega-Rosas, C.I. 2003. Palinología de la Ciénega de Camilo: datos para la historia de la vegetación y el clima del Holoceno medio y superior en el NW de la Sierra Madre Occidental, Sonora, Mexico. Master Thesis, Universidad Nacional Autónoma de México, México D.F.; Ortega-Rosas, C.I., Peñalba, M.C., Guiot, J. Holocene altitudinal shifts in vegetation belts and environmental changes in the Sierra Madre Occidental, Northwestern Mexico. Submitted for publication of Palaeobotany and Palynology). The closest modern pollen data come from pollen analysis across an altitudinal transect from the Sonoran Desert towards the highlands of the temperate SMO at the same latitude (Ortega-Rosas, C.I. 2003. Palinología de la Ciénega de Camilo: datos para la historia de la vegetación y el clima del Holoceno medio y superior en el NW de la Sierra Madre Occidental, Sonora, Mexico. Master Thesis, Universidad Nacional Autónoma de México, México D.F.). An additional modern pollen dataset of 400 sites across NW Mexico and the SW United States was compiled from different sources (Davis, O.K., 1995. Climate and vegetation pattern in surface samples from arid western U.S.A.: application to Holocene climatic reconstruction. Palynology 19, 95–119, North American Pollen Database, Latin-American Pollen Database, personal data, and different scientific papers). For the biomization method (Prentice, I.C., Guiot, J., Huntley, B., Jolly, D., Cheddadi, R., 1996. Reconstructing biomes from paleoecological data: a general method and its application to European pollen data at 0 and 6 ka. Climate Dynamics 12, 185–194), we modified the pollen-PFT and PFT-biomes assignation of Thompson and Anderson (Thompson, R.S., Anderson, K.H., 2000. Biomes of western North America at 18,000; 6000 and 0 14C yr BP reconstructed from pollen and packrat midden data. Journal of Biogeography 27, 555–584) for a better representation of the modern vegetation of NW Mexico. The biome reconstruction method was validated with the modern pollen sites and applied to the fossil sites. Our results show that, during the early Holocene, a cool conifer forest extended at least down to 1700 m, while today this biome is present above 2000 m in the Chihuahua state. The Younger Dryas event was recorded in one site with cold and dry conditions. The reconstructed annual temperature for this period was 3°–6 °C colder than today, and annual precipitation was 250 mm lower than at present (900 mm/yr). The middle Holocene after 9200 cal yr BP was marked by a warming trend, reaching temperatures 2 °C warmer than today at 7000 cal yr BP, and by the installation of a warm mixed forest, the present day biome, at 1700 m elevation, while at higher elevations (1900 m) the cool conifer forest was still present. Summer precipitation was 200 mm/yr above the early Holocene values, suggesting that monsoon-like conditions strengthened since 9200 cal yr BP at this region. During the last 4000 yr, the same warm mixed forest was reconstructed below 1700 m and a conifer forest above 1700 m. A great variability of vegetation and climate patterns was recorded for the last 3000 yr particularly at high elevation sites, where warming and cooling trends would be coeval of the Medieval warm period and Little Ice Age, likely related to ENSO variability.  相似文献   

7.
Rapid climate changes at the onset of the last deglaciation and during Heinrich Event H4 were studied in detail at IMAGES cores MD95-2039 and MD95-2040 from the Western Iberian margin. A major reorganisation of surface water hydrography, benthic foraminiferal community structure, and deepwater isotopic composition commenced already 540 years before the Last Isotopic Maximum (LIM) at 17.43 cal. ka and within 670 years affected all environments. Changes were initiated by meltwater spill in the Nordic Seas and northern North Atlantic that commenced 100 years before concomitant changes were felt off western Iberia. Benthic foraminiferal associations record the drawdown of deepwater oxygenation during meltwater and subsequent Heinrich Events H1 and H4 with a bloom of dysoxic species. At a water depth of 3380 m, benthic oxygen isotopes depict the influence of brines from sea ice formation during ice-rafting pulses and meltwater spill. The brines conceivably were a source of ventilation and provided oxygen to the deeper water masses. Some if not most of the lower deep water came from the South Atlantic. Benthic foraminiferal assemblages display a multi-centennial, approximately 300-year periodicity of oxygen supply at 2470-m water depth. This pattern suggests a probable influence of atmospheric oscillations on the thermohaline convection with frequencies similar to Holocene climate variations. For Heinrich Events H1 and H4, response times of surface water properties off western Iberia to meltwater injection to the Nordic Seas were extremely short, in the range of a few decades only. The ensuing reduction of deepwater ventilation commenced within 500–600 years after the first onset of meltwater spill. These fast temporal responses lend credence to numerical simulations that indicate ocean–climate responses on similar and even faster time scales.  相似文献   

8.
A sediment layer (43 cm thick) and surface sediments (5 cm thick) in a submarine limestone cave (31 m water depth) on the fore-reef slope of Ie Island, off Okinawa mainland, Japan, were examined by visual, mineralogical and geochemical means. Oxygen isotope analysis was performed on the cavernicolous micro-bivalve Carditella iejimensis from both cored sediments and surface sediments, and the water temperature within the cave was recorded for nearly one year. These data show that: (1) water temperature within the cave is equal to that at 30 m deep in the open sea; (2) the biotic and non-biotic environments within the cave have persisted for the past 2000 years; (3) mud-size carbonate detritus is a major constituent of the submarine-cave deposit, and may have come mainly from the suspended carbonate mud produced on the emergent Holocene reef flat over the past two millennia; (4) the δ18O-derived temperature (Tδ18O) of C. iejimensis suggests that the species grows between April and July; (5) the Tδ18O of C. iejimensis from cored sediments implies that there were two warmer intervals, at AD 340 ± 40 and AD 1000 ± 40, which correspond to the Roman Warm Period and Medieval Warm Period, respectively. These suggest that submarine-cave sediments provide unique information for Holocene reef development. In addition, oxygen isotope records of cavernicolous C. iejimensis are a useful tool to reconstruct century-scale climatic variability for the Okinawa Islands during the Holocene.  相似文献   

9.
We correlate Upper Pleistocene glacial and fluvial deposits of the Cinca and Gállego River valleys (south central Pyrenees and Ebro basin, Spain) using geomorphic position, luminescence dates, and time-related trends in soil development. The ages obtained from glacial deposits indicate glacial periods at 85 ± 5 ka, 64 ± 11 ka, and 36 ± 3 ka (from glacial till) and 20 ± 3 ka (from loess). The fluvial drainage system, fed by glaciers in the headwaters, developed extensive terrace systems in the Cinca River valley at 178 ± 21 ka, 97 ± 16 ka, 61 ± 4 ka, 47 ± 4 ka, and 11 ± 1 ka, and in the Gállego River valley at 151 ± 11 ka, 68 ± 7 ka, and 45 ± 3 ka. The times of maximum geomorphic activity related to cold phases coincide with Late Pleistocene marine isotope stages and Heinrich events. The maximum extent of glaciers during the last glacial occurred at 64 ± 11 ka, and the terraces correlated with this glacial phase are the most extensive in both the Cinca (61 ± 4 ka) and Gállego (68 ± 7 ka) valleys, indicating a strong increase in fluvial discharge and availability of sediments related to the transition to deglaciation. The global Last Glacial Maximum is scarcely represented in the south central Pyrenees owing to dominantly dry conditions at that time. Precipitation must be controlled by the position of the Iberian Peninsula with respect to the North Atlantic atmospheric circulation system. The glacial systems and the associated fluvial dynamic seem sensitive to 1) global climate changes controlled by insolation, 2) North Atlantic thermohaline circulation influenced by freshwater pulses into the North Atlantic, and 3) anomalies in atmospheric circulation in the North Atlantic controlling precipitation on the Iberian Peninsula. Our scenario of glacial and fluvial evolution during the Late Pleistocene in northern Spain could be extrapolated to other glaciated mountainous areas in southern Europe.  相似文献   

10.
We measured the δ18O values of the whole shells of the cavernicolous micro-bivalvia Carditella iejimensis obtained from sediments within a submarine cave (31 m water depth) at Ie Island (Okinawa Island, Japan) in the subtropical Northwest Pacific. Our results show no significant millennial-scale trend in the δ18O record, implying that both springtime temperature and the δ18O of sea water at 30 m depth around the Okinawa Islands have been stable for the past 3000 years at values similar to those of today. Moreover, we found one exceptionally light δ18O value from specimens spanning the past 250 years. The δ18O-derived temperature represents a departure of 2.1 °C from the average value for the past 250 years, being equal to the departure recorded during unusually high temperatures in the spring of 1998. This finding may imply that such high springtime sea surface temperature has been a rare event over the past 3000 years.  相似文献   

11.
Recent studies have drawn attention to differences in the seasonal impact of the 8.2 ka event, with longer cooler summers and shorter cooler/drier winters. However, there are no data available on the simultaneity or the rate of onset of the seasonal changes in Europe. Based on the microfacies and geochemical analyses of seasonally laminated varved sediments from Holzmaar, we present evidence of differences in duration and onset time of changes in summer temperature and winter rainfall during the 8.2 ka event. Since both summer and winter climate signals are co-registered within a single varve, there can be no ambiguity about the phasing and duration of the signals. Our data show that the onset and withdrawal of the 8.2 ka summer cooling occurred within a year, and that summer rains were reduced or absent during the investigated period. The onset of cooler summers preceded the onset of winter dryness by ca. 28 years. In view of the differences in nature and duration of the impact of the 8.2 ka event we suggest that a clearer definition of the 8.2 ka event (summer cooling or winter cooling/dryness) needs to be developed. Based on regional comparison and available modelling studies we also discuss the roles of solar variability, changes in North Atlantic Thermohaline circulation, and North Atlantic Circulation (NAO) during the period under consideration. Wavelet analyses of seasonal laminae indicates that the longer NAO cycles, linked to changes in the N. Atlantic temperatures, were more frequent during the drier periods.  相似文献   

12.
Snow algae in a 45.97-m-long ice core from the Tyndall Glacier (50°59′05″S, 73°31′12″W, 1756 m a.s.l.) in the Southern Patagonian Icefield were examined for potential use in ice core dating and estimation of the net accumulation rate. The core was subjected to visual stratigraphic observation and bulk density measurements in the field, and later to analyses of snow algal biomass, water isotopes (18O, D), and major dissolved ions. The ice core contained many algal cells that belonged to two species of snow algae growing in the snow near the surface: Chloromonas sp. and an unknown green algal species. Algal biomass and major dissolved ions (Na+, K+, Mg2+, Ca2+, Cl−, SO42−) exhibited rapid decreases in the upper 3 m, probably owing to melt water elution and/or decomposition of algal cells. However, seasonal cycles were still found for the snow algal biomass, 18O, D-excess, and major ions, although the amplitudes of the cycles decreased with depth. Supposing that the layers with almost no snow algae were the winter layers without the melt water essential to algal growth, we estimated that the net accumulation rate at this location was 12.9 m a− 1 from winter 1998 to winter 1999, and 5.1 m from the beginning of winter to December 1999. These estimates are similar to the values estimated from the peaks of 18O (17.8 m a− 1 from summer 1998 to summer 1999 and 11.0 m from summer to December 1999) and those of D-excess (14.7 m a− 1 from fall 1998 to fall 1999 and 8.6 m a− 1 from fall to December 1999). These values are much higher than those obtained by past ice core studies in Patagonia, but are of the same order of magnitude as those predicted from various observations at ablation areas of Patagonian glaciers.  相似文献   

13.
The dynamic climate in the Northern Hemisphere during the early Holocene could be expected to have impacted on the global carbon cycle. Ice core studies however, show little variability in atmospheric CO2. Resolving any possible centennial to decadal CO2 changes is limited by gas diffusion through the firn layer during bubble enclosure. Here we apply the inverse relationship between stomatal index (measured on sub-fossil leaves) and atmospheric CO2 to complement ice core records between 11,230 and 10,330 cal. yr BP. High-resolution sampling and radiocarbon dating of lake sediments from the Faroe Islands reconstruct a distinct CO2 decrease centred on ca. 11,050 cal. yr BP, a consistent and steady decline between ca. 10,900 and 10,600 cal. yr BP and an increased instability after ca. 10,550 cal. yr BP. The earliest decline lasting ca. 150 yr is probably associated with the Preboreal Oscillation, an abrupt climatic cooling affecting much of the Northern Hemisphere a few hundred years after the end of the Younger Dryas. In the absence of known global climatic instability, the decline to ca. 10,600 cal. yr BP is possibly due to expanding vegetation in the Northern Hemisphere. The increasing instability in CO2 after 10,600 cal. yr BP occurs during a period of increasing cooling of surface waters in the North Atlantic and some increased variability in proxy climate indicators in the region.The reconstructed CO2 changes also show a distinct similarity to indicators of changing solar activity. This may suggest that at least the Northern Hemisphere was particularly sensitive to changes in solar activity during this time and that atmospheric CO2 concentrations fluctuated via rapid responses in climate.  相似文献   

14.
A digital 3D-reconstruction of the Baltic Ice Lake's (BIL) configuration during the termination of the Younger Dryas cold phase (ca. 11 700 cal. yr BP) was compiled using a combined bathymetric–topographic Digital Terrain Model (DTM), Scandinavian ice sheet limits, Baltic Sea Holocene bottom sediment thickness information, and a paleoshoreline database maintained at the Lund University. The bathymetric–topographic DTM, assembled from publicly available data sets, has a resolution of 500 × 500 m on Lambert Azimuthal Equal Area projection allowing area and volume calculations of the BIL to be made with an unprecedented accuracy. When the damming Scandinavian ice sheet margin eventually retreated north of Mount Billingen, the high point in terrain of Southern central Sweden bordering to lower terrain further to the north, the BIL was catastrophically drained resulting in a 25 m drop of the lake level. With our digital reconstruction, we estimate that approximately 7800 km3 of water drained during this event and that the ice dammed lake area was reduced by ca. 18%. Building on previous results suggesting drainage over 1 to 2 years, our lake volume calculations imply that the freshwater flux to the contemporaneous sea in the west was between about 0.12 and 0.25 Sv. The BIL reconstruction provides new detailed information on the paleogeography in the area of southern Scandinavia, both before and after the drainage event, with implications for interpretations of geological records concerning the post-glacial environmental development.  相似文献   

15.
Marine and fluvial terrace sequences near the Waitakere Ranges on the North Island of New Zealand have been surveyed, yielding an inventory of 13 fluvial and 12 marine terrace levels. Based on sparse tephra age control and correlation with the global palaeoclimatic record, rates of regional Quaternary uplift have been reconstructed. Between 1000 ka and 345 ka the time-averaged uplift rate was 0.072 mm a− 1, between 345 ka and 50 ka it increased to 0.278 mm a− 1, accelerating to 0.42 mm a− 1 since 50 ka. The fluvial terrace sequence did not yield clear sedimentary records or other datable material. However, although others have disputed the existence of marine terraces in this study region, a pattern of accelerating regional uplift, superimposed onto glacio-eustatic sea-level changes, is substantiated as the only possible mechanism for maintaining the considerable relief and the active denudation processes inland. The observed uplift is similar to that in other regions where the uplift has been attributed to coupling between surface processes and lower-crustal flow, making this a likely mechanism in the North Island of New Zealand. Regarding the fluvial terrace sequence, the proposed general model is of an actively incising river, carving out on average one strath terrace every ~ 16,000 years. The incision phases are reactivated by sea-level lowering and interrupted by net aggradation events due to landslides triggered by cyclones and/or fires within the catchment; volcanic ash falls also cause transient increases in sediment supply.  相似文献   

16.
Mean-sea-level data from coastal tide gauges in the north Indian Ocean were used to show that low-frequency variability is consistent among the stations in the basin. Statistically significant trends obtained from records longer than 40 years yielded sea-level-rise estimates between 1.06–1.75 mm yr− 1, with a regional average of 1.29 mm yr− 1, when corrected for global isostatic adjustment (GIA) using model data. These estimates are consistent with the 1–2 mm yr− 1 global sea-level-rise estimates reported by the Intergovernmental Panel on Climate Change.  相似文献   

17.
Late Glacial to Holocene ice retreat was investigated along a 120 km long fjord system, reaching from Gran Campo Nevado (GCN) to Seno Skyring in the southernmost Andes (53°S). The aim was to improve the knowledge on regional and global control on glacier recession with special emphasis on latitudinal shifting of the westerlies. The timing of ice retreat was derived from peat and sediment cores, using mineralogical and chemical characteristics, and pollen as proxies. Stratigraphy was based on 14C-AMS ages and tephrochronology. The ice retreat of the Seno Skyring Glacier lobe is marked by an ice rafted debris layer which was formed around 18,300 to 17,500 cal. yr B.P. Subsequently, fast glacier retreat occurred until around 15,000 to 14,000 cal. yr B.P. during which around 84% of Skyring Glacier were lost. This fast recession was probably also triggered by an increase of the Equilibrium Line Altitude (ELA) from 200 to 300 m. Subsequently, the ice surface was lowered below the ELA in an area that previously made up more than 50% of the accumulation area. Much slower retreat and glacier fluctuations of limited extent in the fjord channel system northeast of GCN occurred between around 14,000 to 11,000 cal. yr B.P. during both the Antarctic Cold Reversal and the Younger Dryas. This slow down of retreat indicates a decline in the general warming trend and/or increased precipitation, due to a southward migration of the westerlies. After around 11,000 cal. yr B.P. pollen distribution shows evolved Magellanic Rainforest and similar climate as at present, which lasted throughout most of the Holocene. Only Late Neoglacial moraine systems were formed in the period 1220–1460 AD, and subsequently in the 1620s AD, and between 1870 and 1910 AD. The results indicate that the Gran Campo Nevado ice cap has reacted more sensitive and partly distinct to climate change, compared to the Patagonian Ice Field.  相似文献   

18.
Quaternary uplift of northern England   总被引：3，自引：3，他引：0  
Upland flats, attributable to erosion, have long been recognised in the landscape of the Lake District region of NW England, at altitudes of up to ~ 800 m O.D. Extrapolation using uplift rates derived from dated Pleistocene sites (karstic caves and other features) in the adjacent Pennine uplands suggests that if this succession of flats formed close to sea-level they date from the Middle Pliocene onwards, indicating a subsequent time-averaged uplift rate of almost 0.3 mm a− 1. Numerical modelling indicates that erosion of surrounding areas at a typical rate of 0.2 mm a− 1 since 3.1 Ma could have caused this uplift, as well as constraining the local effective viscosity of the lower crust as ~ 4 × 1018 Pa s and the typical local Moho temperature as ~ 650 °C. It is thus feasible that most of the topography of northern England has developed since the Middle Pliocene, as a consequence of coupling between erosion and the resulting induced flow in the lower continental crust. The much faster vertical crustal motions indicated in this part of northern England, compared with SE England, are thus mainly a consequence of much greater mobility of the lower crust in the north, due to its younger thermal age and the heating effect of radioactive Palaeozoic granites. Uplift of this magnitude, which has previously gone unrecognised, may have affected post-Pliocene global climate.  相似文献   

19.
Accumulation of organic matter (OM) was studied in four ombrotrophic peat bogs in Finland: Harjavalta (vicinity of a Cu–Ni smelter), Outokumpu (near a closed Cu–Ni mine), Alkkia (Ni-treated site) and Hietajärvi (a pristine site). At each sampling site, two peat cores (15 × 15 × 100 cm) were taken. Age-dating of peat was determined using 210Pb method (CRS model). The local annual temperature sum and precipitation for the past 125 years were modeled. The objective was to compare recent net accumulation rates of heavy metal polluted ombrotrophic peat bogs with those of a pristine bog, and to study the relationship between weather and net accumulation rates. Based on 210Pb age-dating, the upper 16-cm peat layer at Harjavalta, 35 cm at Outokumpu and 25 cm at Hietajärvi represents 125 years of peat formation, yielding the following average peat accumulation rates: Harjavalta 1.3 mm year− 1, Outokumpu 2.8 mm year− 1 and Hietajärvi 2.0 mm year− 1. At the Alkkia site, the Ni treatment in 1962 had completely stopped the peat accumulation. Net accumulation rates were related to precipitation at Outokumpu, Harjavalta and Hietajärvi sites. In addition, emissions released from the nearby located Cu–Ni smelter could have affected negatively net OM accumulation rate at Harjavalta site.  相似文献   

20.
Climate changes and recent glacier behaviour in the Chilean Lake District   总被引：1，自引：1，他引：0  
Atmospheric temperatures measured at the Chilean Lake District (38°–42°S) showed contrasting trends during the second half of the 20th century. The surface cooling detected at several meteorological stations ranged from − 0.014 to − 0.021 °C a− 1, whilst upper troposphere (850–300 gpm) records at radiosonde of Puerto Montt (41°26′S/73°07′W) revealed warming between 0.019 and 0.031 °C a− 1. Regional rainfall data collected from 1961 to 2000 showed the overall decrease with a maximum rate of − 15 mm a− 2 at Valdivia st. (39°38′S/73°05′W). These ongoing climatic changes, especially the precipitation reduction, seem to be related to El Niño–Southern Oscillation (ENSO) phenomena which has been more frequent after 1976. Glaciers within the Chilean Lake District have significantly retreated during recent decades, in an apparent out-of-phase response to the regional surface cooling. Moreover, very little is known about upper troposphere changes and how they can enhance the glacier responses. In order to analyse their behaviour in the context of the observed climate changes, Casa Pangue glacier (41°08′S/71°52′W) has been selected and studied by comparing Digital Elevation Models (DEMs) computed at three different dates throughout the last four decades. This approach allowed the determination of ice elevation changes between 1961 and 1998, yielding a mean thinning rate of − 2.3 ± 0.6 m a− 1. Strikingly, when ice thinning is computed for the period between 1981 and 1998, the resulting rate is 50% higher (− 3.6 ± 0.6 m a− 1). This enhanced trend and the related area loss and frontal retreat suggests that Casa Pangue might currently be suffering negative mass balances in response to the upper troposphere warming and decreased precipitation of the last 25–30 yr, as well as debris cover would not prevent the glacier from a fast reaction to climate forcing. Most of recent glaciological studies regarding Andean glaciers have concentrated on low altitude changes, namely frontal variations, however, in order to better understand the regional glacier changes, new data are necessary, especially from the accumulation areas.  相似文献   

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