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1.
依据近期发表的古丝绸之路沿线若干地区(点)的温度重建序列,结合干湿变化等代用记录,分析了过去千年古丝绸之路沿线温度变化的基本特征,以及这些地区(点)在“中世纪气候异常期”(MCA,约950-1250年)和“小冰期”(LIA,约1450-1850年)的干湿特征异同。主要结论为：①过去2000年古丝绸之路的温度变化经历了1-3世纪温暖、4-7世纪前期寒冷、7世纪后期-11世纪初温暖、11世纪中期-12世纪初偏冷、12世纪中期-13世纪中期温暖、13世纪末-19世纪中期寒冷和20世纪快速增暖的百年际波动过程;但不同区域间的年代至百年尺度变化位相不完全同步,波动幅度也存在差异。②各地干湿特征在MCA和LIA也存在一定差异：中国的关中平原及河西走廊在MCA间的干湿变率较LIA大;中亚干旱区MCA期间气候偏干,LIA期间偏湿;欧洲中北部以及斯堪的纳维亚半岛南部等地在MCA间气候较LIA偏干,且中部地区LIA间的干湿变率较MCA大;芬兰和斯堪的纳维亚半岛北部以及俄罗斯等地MCA间的气候较LIA更湿润。  相似文献   

2.
Instrumental climate records from the central Canadian treeline zone display a pattern of variation similar to general Northern Hemisphere temperature trends. To examine whether this general correspondence extends back beyond the instrumental record, we obtained a sediment core from Lake S41, a small lake in the Northwest Territories of Canada at 63°43.11′ N, 109°19.07′ W. A radiocarbon-based chronology was developed for the core. The sediments were analyzed for organic-matter content by loss-on-ignition (LOI), biogenic-silica content (BSi), and chironomid community composition to reconstruct July air temperature and summer water temperature. The paleolimnological records were compared with records of atmospheric CO2 concentration, solar variability, and hemispheric temperature variations over the past 2000 years. The results of the analyses suggest that widely-documented long-term variations in Northern Hemisphere temperature associated with radiative forcing, namely the cooling following the medieval period during the Little Ice Age (LIA), and twentieth century warming, are represented in the central Canadian treeline zone. There is also evidence of a brief episode of warming during the eighteenth century. As evidenced by LOI and BSi, the twentieth century warming is typified by increased lake productivity relative to the LIA. Depending upon the measure, the increased productivity of the twentieth century nearly equals or exceeds that of any other period in the past 2000 years. In contrast, the rate of chironomid head capsule accumulation decreased and remained low during the twentieth century. Although the chironomid-inferred temperature reconstructions indicate cooling during the LIA, they present no evidence of greatly increased temperatures during the twentieth century. Warming during the twentieth century might have enhanced lake stratification, and the response of the chironomid fauna to warming was attenuated by decreased oxygen and lower temperatures in the hypolimnion of the more stratification-prone lake.
Glen M. MacDonaldEmail:
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3.
A new lichen dating method and new moraine observations enabled us to improve the chronology of glacier advances in the Cordillera Blanca (Peru) during the Little Ice Age (LIA). Our results reveal that an early LIA glacial advance occurred around AD 1330 ± 29. However, a second major glacial advance at the beginning of the 17th century overlapped the earlier stage for most glaciers. Hence, this second glacial stage, dated from AD 1630 ± 27, is considered as the LIA maximum glacial advance in the Cordillera Blanca. During the 17th–18th centuries, at least three glacial advances were recorded synchronously for the different glaciers (AD 1670 ± 24, 1730 ± 21, and 1760 ± 19). The moraines corresponding to the two first stages are close to the one in 1630 suggesting a slow recession of about 18% in the total length of the glacier. From the LIA maximum extent to the beginning of the 20th century, the 24 glaciers have retreated a distance of about 1000 m, corresponding to a reduction of 30% in their length. This rate is comparable to that observed during the 20th century. Estimates of palaeo-Equilibrium Line Altitudes show an increase in altitude of about 100 m from the LIA maximum glacial extension at the beginning of the 17th century to the beginning of the 20th century. Because long time series are not available for precipitation and temperature, this glacial retreat is difficult to explain by past climate changes. However, there is a fair correspondence between changes in glacier length and the δ18O recorded in the Quelccaya ice core at a century timescale. Our current knowledge of tropical glaciers and isotope variations leads us to suggest that this common tropical signal reflects a change from a wet LIA to the drier conditions of today. Finally, a remarkable synchronicity is observed with glacial variations in Bolivia, suggesting a common regional climatic pattern during the LIA.  相似文献   

4.
Present climate warming strongly affects limnological and ecological properties of lakes and may cause regime shifts that alter structure and function in the water bodies. Such effects are especially pronounced in climatologically extreme areas, e.g. at high altitudes. We examined a sediment core from Lake Oberer Landschitzsee, Austrian Alps, which spans the period from the Little Ice Age (LIA) to present. We investigated whether post-LIA climate warming altered aquatic invertebrate communities and limnological status in this sensitive high Alpine lake. Fossil Cladocera (Crustacea) and Chironomidae (Diptera) and organic matter in the core were analyzed. Chironomids were used to assess the lake??s benthic quality (i.e. oxygen availability). An instrumental Alpine temperature record was used to assess whether changes in the biotic assemblages correspond to post-LIA temperature trends. The planktonic and macro- and microbenthic invertebrate communities exhibit almost complete and simultaneous species turnover after the LIA, from about AD 1850 onward, when Sergentia coracina-type replaced oxyphilous Micropsectra contracta-type as the dominant macrobenthic taxon, and phytophilous Acroperus harpae outcompeted Alona affinis and Alona quadrangularis in the microbenthos. These directional community shifts corresponded with a period of reduced benthic quality, higher sediment organic content, and progressive climate warming, superimposed on Alpine land-use changes, until the early twentieth century. Detected changes suggest increased productivity and lower benthic oxygen availability. Faunal shifts were even more pronounced during the late twentieth century, simultaneous with enhanced warming. A new planktonic Cladocera species, Bosmina longirostris, typically absent from high Alpine lakes, colonized the lake and gradually became dominant toward the core top. Results show that post-LIA climate warming, coupled with increasing benthic and planktonic production, substantially altered the limnological and ecological status of this remote Alpine lake. Observed faunal turnovers provide evidence that temperature-driven ecological thresholds, whether associated directly or indirectly with greater human activity, have been crossed. Species abundances and distributions have changed in response to post-LIA and late twentieth century climate warming.  相似文献   

5.
A multi-proxy study of short sediment cores recovered in small, karstic Lake Estanya (42°02?? N, 0°32?? E, 670 m.a.s.l.) in the Pre-Pyrenean Ranges (NE Spain) provides a detailed record of the complex environmental, hydrological and anthropogenic interactions occurring in the area since medieval times. The integration of sedimentary facies, elemental and isotopic geochemistry, and biological proxies (diatoms, chironomids and pollen), together with a robust chronological control, provided by AMS radiocarbon dating and 210Pb and 137Cs radiometric techniques, enabled precise reconstruction of the main phases of environmental change, associated with the Medieval Warm Period (MWP), the Little Ice Age (LIA) and the industrial era. Shallow lake levels and saline conditions with poor development of littoral environments prevailed during medieval times (1150?C1300 AD). Generally higher water levels and more dilute waters occurred during the LIA (1300?C1850 AD), although this period shows a complex internal paleohydrological structure and is contemporaneous with a gradual increase of farming activity. Maximum lake levels and flooding of the current littoral shelf occurred during the nineteenth century, coinciding with the maximum expansion of agriculture in the area and prior to the last cold phase of the LIA. Finally, declining lake levels during the twentieth century, coinciding with a decrease in human pressure, are associated with warmer climate conditions. A strong link with solar irradiance is suggested by the coherence between periods of more positive water balance and phases of reduced solar activity. Changes in winter precipitation and dominance of NAO negative phases would be responsible for wet LIA conditions in western Mediterranean regions. The main environmental stages recorded in Lake Estanya are consistent with Western Mediterranean continental records, and show similarities with both Central and NE Iberian reconstructions, reflecting a strong climatic control of the hydrological and anthropogenic changes during the last 800 years.  相似文献   

6.
This study assesses Little Ice Age (LIA) lake sediment morphological and geochemical records and moraine chronologies in the upper Fraser River watershed, British Columbia, Canada, to resolve differences in paleoenvironmental interpretation and to clarify sediment production and sediment delivery processes within alpine geomorphic systems. Moose Lake (13.9 km2), situated at 1032 m a.s.l., contains a partially varved record indicating variable rates of accumulation during the last millennium that, in general, coincide with previously documented LIA glacial advances in the region and locally. Dendrochronological assessment of forefield surfaces in the headwaters of the catchment (Reef Icefield) shows that periods of moraine construction occurred just prior to ad 1770, ad 1839 and ad 1883, and some time before ad 1570. Taken collectively, increases in varve thickness within eight Moose Lake sediment cores coincide with documented glacier advances over the twelfth through fourteenth centuries, the eighteenth century, and nineteenth through twentieth centuries. Glacial activity during the sixteenth century is also indicated. While varve thickness variations in proximal and distal sediments clearly reflect glacial activity upstream of Moose Lake, the intermediate varve record is relatively insensitive to these decadal and longer‐term catchment processes. Variations in Ca and related elements derived from glaciated carbonate terrain within the Moose River sub‐catchment (including Reef Icefield) indicate gradually increasing delivery from these sources from the twelfth through twentieth centuries even where the varve thickness record is unresponsive. Elevated carbonate concentrations confirm glacial activity c. ad 1200, ad 1500, ad 1750, and ad 1900.  相似文献   

7.
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   

8.
We analyzed sediments of the past 2000 years from Ongoke Lake, southwest Alaska, for organic carbon, organic nitrogen, biogenic silica (BSi), and diatom assemblages at decadal to centennial resolution to infer limnological changes that may be related to climatic variation in southwestern Alaska. The chronology is based on a 210Pb profile from bulk sediments and nine AMS 14C ages from terrestrial plant macrofossils. Four of the 14C ages span a core depth interval of 60.5 cm but are statistically indistinguishable from one another with a mean of ~1300 AD, which compromises the determination of temporal trends at Ongoke Lake and comparison with other paleoclimate records. The diatom record suggests changes in the duration of ice cover and strength of thermal stratification that are probably related to temperature variation. This variation includes a cold interval around the first millennium cooling (FMC) and a warm interval spanning the medieval climate anomaly (MCA). However, the lake-sediment record shows no clear signals of temperature variation for the period of the Little Ice Age (LIA) or the twentieth century. Climatic changes during these periods may have been manifested through effective-moisture (precipitation minus evapotranspiration) variation in the Ongoke Lake area. We estimate water depths and infer effective-moisture fluctuations by applying a regional transfer function to our diatom record. Together with inferences from diatom autecologies, this water-depth reconstruction suggests that effective moisture increased steadily from 50 BC to 350 AD, which was followed by relatively dry conditions between 550 and 750 AD and relatively wet conditions between 750 and 1450 AD. Effective moisture was low from ~1450 to 1850 AD, coinciding with the LIA; an alternative age model places this interval between ~1315 and 1850 AD. During the past 150 years, effective moisture increased, with estimated water depths reaching peak values in the second half of the twentieth century. This study offers the first paleolimnological record for inferring centennial-scale climatic variation over the past two millennia from southwestern Alaska.
Feng Sheng HuEmail:
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9.
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10.
ABSTRACT. A Scots pine ( Pinus sylvestris L.) tree-ring width chronology from Jämtland, in the central Scandinavian Mountains, built from living and sub-fossil wood, covering the period 1632 BC to AD 2002, with a minor gap during AD 887–907, is presented. This is the first multi-millennial tree-ring chronology from the central parts of Fennoscandia. Pine growth in this tree line environment is mainly limited by summer temperatures, and hence the record can be viewed as a temperature proxy. Using the regional curve standardization (RCS) technique, pine-growth variability on short and long time scales was retained and subsequently summer (June–August) temperatures were reconstructed yielding information on temperature variability during the last 3600 years. Several periods with anomalously warm or cold summers were found: 450–550 BC (warm), AD 300–400 (cold), AD 900–1000 (the Medieval Warm Period, warm) and AD 1550–1900 (Little Ice Age, cold). The coldest period was encountered in the fourth century AD and the warmest period 450 to 550 BC. However, the magnitude of these anomalies is uncertain since the replication of trees in the Jämtland record is low during those periods. The twentieth century warming does not stand out as an anomalous feature in the last 3600 years. Two multi-millennial tree-ring chronologies from Swedish and Finnish Lapland, which have previously been used as summer temperature proxies, agree well with the Jämtland record, indicating that the latter is a good proxy of local, but also regional, summer temperature variability.  相似文献   

11.
This paper reviews the evidence and history of glacier fluctuations during the Little Ice Age (LIA) in the Canadian Rockies. Episodes of synchronous glacier advance occurred in the 12th–13th, early 18th and throughout the 19th centuries. Regional ice cover was probably greatest in the mid-19th century, although in places the early 18th century advance was more extensive. Glaciers have lost over 25% of their area in the 20th century. Selective preservation of the glacier record furnishes an incomplete chronology of events through the 14th–17th centuries. In contrast, varve sequences provide continuous, annually resolved records of sediments for at least the last millennium in some highly glacierized catchments. Such records have been used to infer glacier fluctuations. Evaluation of recent proxy climate reconstructions derived from tree-rings provides independent evidence of climate fluctuations over the last millennium. Most regional glacier advances follow periods of reduced summer temperatures, reconstructed from tree rings particularly ca. 1190–1250, 1280–1340, 1690s and the 1800s. Reconstructed periods of higher precipitation at Banff, Alberta since 1500 are 1515–1550, 1585–1610, 1660–1680 and the 1880s. Glacier advances in the early 1700s, the late 1800s and, in places, the 1950–1970s reflect both increased precipitation and reduced summer temperatures. Negative glacier mass balances from 1976 to 1995 were caused by decreased winter balances. The glacier fluctuation record does not contain a simple climate signal: it is a complex response to several interacting factors that operate at different timescales. Evaluation of climate proxies over the last millennium indicates continuous variability at several superimposed timescales, dominated by decade–century patterns. Only the 19th century shows a long interval of sustained cold summers. This suggests that simplistic concepts of climate over this period should be abandoned and replaced with more realistic records based on continuous proxy data series. The use of the term LIA should be restricted to describing a period of extended glacier cover rather than being used to define a period with specific climate conditions.  相似文献   

12.
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.
We present a multiproxy paleoclimate record using leaf wax hydrogen isotopes (??2Hwax) and varve thickness from Arctic proglacial lake sediments. We also provide one of the first evaluations of the utility of ??2Hwax as a paleoclimate proxy in Arctic proglacial lakes. We compare varve thickness and ??2Hwax at sub-decadal resolution from 1948 to 2004 AD, and at sub-centennial resolution from 1450 to 2004 AD. Varve thickness and ??2Hwax both contain large interannual variability and are anti-correlated during the late twentieth century, suggesting that both proxies respond rapidly, but by different mechanisms, to catchment-scale forcings. At longer time scales, varve thickness exhibits a strong response to Little Ice Age cooling (1661?C1827 AD in this record) but does not show evidence for twentieth century warming recorded throughout the Arctic. ??2Hwax does record regional-scale temperature changes, with more 2H-depleted values during the Little Ice Age and an abrupt change to more 2H-enriched values in the twentieth century. This corresponds well with a recent Arctic-wide temperature reconstruction in which the seventeenth century is the coldest interval, and the twentieth century is the warmest interval. Our results suggest that ??2Hwax is a promising proxy that can be applied at high resolution in proglacial Arctic lakes.  相似文献   

14.
We present a study of two short sediment cores recovered from Lago Enol, in the Picos de Europa National Park, Cantabrian Mountains, northern Iberia. We inferred past climate conditions and anthropogenic impacts using geochemical and biological (pollen and diatoms) variables in the dated sequences, in conjunction with temperature and precipitation data collected since 1871 at meteorological stations in the region. The record provides evidence of environmental changes during the last 200 years. At the end of the Little Ice Age (~1800?C1875 AD) the region was characterized by an open landscape. Long-term use of the area for mixed livestock grazing in the mountains, and cultivation of rye during the nineteenth century, contributed to the expansion of grassland at the expense of forest. Warmer temperatures since the end of the nineteenth century are inferred from a change in diatom assemblages and development of the local forest. Socioeconomic transformation during the twentieth century, such as livestock changes related to dairy specialization, planting of non-native trees, mining activities, and management of the national park since its creation in 1918, caused profound changes in the catchment and in the lake ecology. The last several decades (~1970?C2007 AD) of the Lago Enol sediment record are strikingly different from previous periods, indicating lower runoff and increasing lake productivity, particularly since AD 2000. Today, the large number of tourists who visit the area cause substantial impacts on this ecosystem.  相似文献   

15.
The recently observed recession of glaciers on King George Island is associated with decades of climate warming in the Antarctic Peninsula region. However, with only 60 years of glaciological observations in the study area ages of the oldest moraines are still uncertain. The goal of the study was to estimate ages of lichen colonization on the oldest moraines of the Ecology and White Eagle Glaciers on King George Island and on the Principal Cone of Penguin Island volcano. The first lichenometric studies on these islands from the late 1970s used rates that had about four to five times slower Rhizocarpon growth rates. We re‐examined the sites and measured 996 thalli diameters to establish the surface ages. To estimate the age we used (1) long‐term Rhizocarpon lichen group growth rates established by authors using data from a previous lichenometric study on King George Island, and (2) previous data of lichen growth rates from other sub‐Antarctic islands. Our results suggest growth rates between 0.5 and 0.8 mm yr–1. According to these rates the ages of the oldest moraine ridges are of the Little Ice Age and were colonized at the beginning of the twentieth century. The mid‐twentieth century age of lichen colonization on the historically active Penguin Island volcano might support the date of the last eruption reported by whalers in the end of the nineteenth and the beginning of the twentieth century.  相似文献   

16.
Little Ice Age (LIA) moraines along the margins of Skálafellsjökull and Heinabergsjökull, two neighbouring outlet glaciers flowing from the Vatnajökull ice‐cap, have been re‐dated to test the reliability of different lichenometric approaches. During 2003, 12 000 lichens were measured on 40 moraine fragments at Skálafellsjökull and Heinabergsjökull to provide surface age proxies. The results are revealing. Depending on the chosen method of analysis, Skálafellsjökull either reached its LIA maximum in the early 19th century (population gradient) or the late 19th century (average of five largest lichens), whereas the LIA maximum of Heinabergsjökull occurred by the mid‐19th century (population gradient) or late‐19th century (average of 5 largest lichens). Discrepancies (c. 80 years for Skálafellsjökull and c. 40 years for Heinabergsjökull) suggest that the previously cited AD 1887 LIA maxima for both glaciers should be reassessed. Dates predicted by the lichen population gradient method appear to be the most appropriate, as mounting evidence from other geochronological reconstructions and sea‐ice records throughout Iceland tends to support an earlier LIA glacier maximum (late 18th to mid‐19th century) and probably reflects changes in the North Atlantic Oscillation. These revised chronologies shed further light on the precise timing of the Icelandic LIA glacier maximum, whilst improving our understanding of glacier‐climate interactions in the North Atlantic.  相似文献   

17.
ABSTRACT. We have studied a 33.7 m deep ice core from a small polythermal Scandinavian ice cap to determine whether it is possible to recover pre-20th century climatic information from the glacier. Ice structural studies show a significant change from clear ice above 11 m depth (superimposed ice indicating refreezing) to bubbly ice below 11 m depth, indicating this is the transition between Little Ice Age (LIA) and 20th century ice. Calculations with a Nye-age model, along with a mass balance reconstruction, show that this structural boundary likely formed in the last part of the LIA, which in this region ended about 1910. The ice below this boundary was sampled and analysed for stable isotopic composition and ionic content, which both show significant variations with depth. The stable isotope record likely contains cycles of annual duration during the LIA. The chemistry in the ice core indicates that the information is useful, and can be used to interpret climatic and environmental variables during the LIA. A comparison of Riukojietna ion chemistry and oxygen isotope records with similar records from other glaciers in this region reveals a clear continental-maritime gradient. Changes in this gradient with time may be possible to resolve using such ice core records. Results from this study demonstrate that ice cores from glaciers in this climatic environment can be useful in revealing environmental conditions from climatically colder periods and yield pre-industrial benchmark values for chemical loading and oxygen isotopes, but that hiatuses complicate the depth-age relationship.  相似文献   

18.
Data on accumulation and concentration of chemical compounds recorded in an essentially unexplored area(Dome Argus)of the Indian Ocean sector of eastern Antarctica during the past 2,680 years(680 B.C. to 1999 A.D.) are presented. During the first 1, 700 years(680 B. C. to 1000 A. D.), the accumulation data shows a slightly decreasing trend, while chemical ions appear to be stable, representing a stable climatic condition. An intensive increasing trend of the accumulation occurred during the 12th to 14th century. The period from 15th to 19th century was characterized by a rapid reducing accumulation and concentrations of volatile compounds suffering post-depositional loss linked to sparse precipitation amount,which was temporally consistent with the Little Ice Age(LIA) episode. Comparison between observed accumulation rates with other eastern Antarctic ice cores show a consistent decreasing trend during LIA, while sea salt and dust-originated ions increased due to sea ice extent and intensified atmospheric transportation. Distribution of volcanic originated sulfate over the Antarctic continent show a significant change during the 15th century, coincident with the onset of the LIA. These results are important for the assessment of Antarctic continent mass balance and associated interpretation of the Dome A deep ice core records.  相似文献   

19.
Evidence from lake sediments and glacier forefields from two hydrologically isolated lake basins is used to reconstruct Holocene glacier and climate history at Hallet and Greyling Lakes in the central Chugach Mountains of south-central Alaska. Glacial landform mapping, lichenometry, and equilibrium-line altitude reconstructions, along with changes in sedimentary biogenic-silica content, bulk density, and grain-size distribution indicate a dynamic history of Holocene climate variability. The evidence suggests a warm early Holocene from 10 to 6 ka, followed by the onset of Neoglaciation in the two drainage basins, beginning between 4.5 and 4.0 ka. During the past 2 ka, the glacial landforms and lacustrine sediments from the two valleys record a remarkably similar history of glaciation, with two primary advances, one during the first millennium AD, from ~500 to 800 AD, and the second during the Little Ice Age (LIA) from ~1400 to 1900 AD. During the LIA, the reconstructed equilibrium-line altitude in the region was no more than 83 ± 44 m (n = 21) lower than the modern, which is based on the extent of glaciers during 1978. Differences between the summer temperature inferred from the biogenic-silica content and the evidence for glacial advances and retreats suggest a period of increased winter precipitation from 1300 to 1500 AD, and reduced winter precipitation from 1800 to 1900 AD, likely associated with variability in the strength of the Aleutian Low.
Darrell S. KaufmanEmail:
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20.
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.
Broxton W. BirdEmail:
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