20 research outputs found
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Temperature and Precipitation History of the Arctic
lands that now support only polar desert and tundra. Global oceanic and atmospheric circulation was substantially different between 3 and 2.5 Ma than subsequently. The development of the first continental ice sheets over North America and Eurasia led to changes in the circulation of both the atmosphere and oceans. The onset of continental glaciation is most clearly defined by the first appearance of rock fragments in sediment cores from the central Atlantic Ocean about 2.6 Ma. These rock fragments, often referred to as ice-rafted debris (IRD), are too heavy to have blown or been washed into the central Atlantic; they must have been delivered by large icebergs emanating from continental ice sheets. The first appearance of IRD marks the onset of the Quaternary Period (2.6–0 Ma), generally equated with “ice-age” time, even though for a small fraction (about 10%) of the time the ice sheets were very likely to have been as small as or smaller than their present size. From about 2.7 to about 0.8 Ma, the ice sheets came and went about every 41 k.y., the same timing as cycles in the tilt of Earth’s axis. Ice sheets grew when Earth’s tilt was at a minimum, resulting in less seasonality (cooler summers, warmer winters), and they melted when tilt was at a maximum and seasonality was at its greatest (warmer summers and cooler winters). For the past 600 k.y., ice sheets have grown larger and ice-age times have been longer, lasting about 100 k.y.; those icy intervals have been separated by brief warm periods (interglaciations), when sea level and ice volumes were close to those at present. The duration of interglaciations ranges from about 10 k.y. to perhaps 40 k.y. The cause of the shift from 41 k.y. to 100 k.y. glacial cycles is still being debated. Most explanations center on the continued gradual planetary cooling that may have produced larger ice sheets that were more resistant to melting, or with removal of soft sedimentary cover over bedrock in glaciated regions that, once removed, increased the frictional coupling of the ice sheet to its bed, resulting in steeper ice-sheet profiles and thicker ice sheets, again more resistant to melting. The relatively warm planetary state during which human civilization developed is the most recent of the warm interglaciations, the Holocene (about 11.5–0 kiloannum (thousands of years ago (ka)). During the penultimate warm interval, about 130–120 ka, solar energy in summer in the northern high latitudes was greater than at any time in the current warm interval. As a consequence, the Arctic summer was about 5°C warmer than at present and almost all glaciers melted completely except for the Greenland Ice Sheet, and even it was reduced in size substantially from its present extent. With the increased ice melt, sea level was about 5 meters (m) higher than at present; the extra melt came from both Greenland and Antarctica as well as from small glaciers (Overpeck et al., 2006; Meier et al., 2007). Although sea ice is difficult to reconstruct, the evidence suggests that the central Arctic Ocean retained some permanent ice cover or was periodically ice free, even though the flow of warm Atlantic water into the Arctic Ocean was very likely to have been greater than during the present warm interval. The Last Glacial Maximum (LGM) peaked about 21 ka when mean annual temperatures in parts of the Arctic were as much as 20°C lower than at present. Ice recession was well underway by 16 ka, and most of the Northern Hemisphere ice sheets had melted by 7 ka. Summertime solar energy rose steadily in the Arctic from 21 ka to a maximum (10% higher than at present) about 11 ka and has been decreasing since then, primarily in response to the precession of the equinoxes causing Earth’s distance from the Sun during Northern Hemisphere summer to decrease from 21 to 11 ka and then to increase to the present. The extra energy received in early Holocene summers warmed summers throughout the Arctic about 1°–3°C above 20th century averages, enough to completely melt many small glaciers throughout the Arctic (although the Greenland Ice Sheet was only slightly smaller than present). Summer sea ice limits were substantially smaller than their 20th century average, and the flow of Atlantic water into the Arctic Ocean was substantially greater. As summer solar energy decreased in the second half of the Holocene, glaciers re-established or advanced, sea ice extended, and the flow of warm Atlantic water into the Arctic Ocean diminished. Late Holocene cooling reached its nadir during the Little Ice Age (about 1250–1850 AD), when sun-blocking volcanic eruptions and perhaps other causes added to the orbital cooling, allowing most Arctic glaciers to reach their maximum Holocene extent. During the warming of the past century and a half, glaciers have receded throughout the Arctic, terrestrial ecosystems have advanced northward, and perennial Arctic Ocean sea ice has diminished. Paleoclimate reconstructions indicate that Arctic temperature changes typically have been larger than corresponding hemispheric or globally averaged changes. This behavior is observed with conditions both warmer and colder than recently, indicating that Arctic amplification is a pervasive feature of the climate system
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Basic hydrology, limnology, and meteorology of modern Lake El’gygytgyn, Siberia
A survey of the modern physical setting of Lake El’gygytgyn, northeastern Siberia, is presented here to facilitate interpretation of a 250,000-year climate record derived from sediment cores from the lake bottom. The lake lies inside a meteorite impact crater that is approximately 18 km in diameter, with a total watershed area of 293 km2, 110 km2 of which is lake surface. The only surface water entering the lake comes from the approximately 50 streams draining from within the crater rim; a numbering system for these inlet streams is adopted to facilitate scientific discussion. We created a digital elevation model for the watershed and used it to create hypsometries, channel networks, and drainage area statistics for each of the inlet streams. Many of the streams enter shallow lagoons dammed by gravel berms at the lakeshore; these lagoons may play a significant role in the thermal and biological dynamics of the lake due to their higher water temperatures (\u3e6°C). The lake itself is approximately 12 km wide and 175 m deep, with a volume of 14.1 km3. Water temperature within a column of water near the center of this oligotrophic, monomictic lake never exceeded 4°C over a 2.5 year record, though the shallow shelves (\u3c10\u3em) surrounding the lake can reach 5°C in summer. Though thermally stratified in winter, the water appears completely mixed shortly after lake ice breakup in July. Mean annual air temperature measured about 200 m from the lake was −10.3°C in 2002, and an unshielded rain gage there recorded 70 mm of rain in summer of 2002. End of winter snow water equivalent on the lake was approximately 110 mm in May 2002. Analysis of NCEP reanalysis air temperatures (1948–2002) reveals that the 8 warmest years and 10 warmest winters have occurred since 1989, with the number of days below −30°C dropping from a pre-1989 mean of 35 to near 0 in recent years. The crater region is windy as well as cold, with hourly wind speeds exceeding 13.4 m s−1 (30 mph) typically at least once each month and 17.8 m s−1 (40 mph) in winter months, with only a few calm days per month; wind may also play an important role in controlling the modern shape of the lake. Numerous lines of evidence suggest that the physical hydrology and limnology of the lake has changed substantially over the past 3.6 million years, and some of the implications of these changes on paleoclimate reconstructions are discussed
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Climate Dynamics and Global Environments: A community vision for the next decade in ICDP
Today\u27s provocative transformation of the Earth\u27s climate system provides timely scientific impetus to an array of paleolimnological studies aimed at understaning natural climate variability vs. anthropogenic-induced change on global and regional scales. Continental drilling to acquire long paleoclimate records from a strategic network of sites is essential to documenting regional hydrologic and climatic responses to atmospheric change, providing a record that is key to resolving climate dynamics at fine spatial scales relevant to both climate modeling and societal impacts of climate change. An in-depth scientific assessment of natural climate variability based on lake drilling will also allow us to close huge gaps in our knowledge on the impact of climate change on the continental landscape and its ecosystems, vegetation and other biota, and ultimately the human environment. The scientific returns from lake drilling include, e.g., data needed to assess the environmental context of early human evolution, knowledge of paleoseismicity, natural hazard frequency, paleohydrology, and drought. Core scanning technology and other emerging proxy developments continue to propel international standards for initial core processing and storage ensuring the maximum investment return on studies of past continental and environmental change
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Overview and significance of a 250 ka paleoclimate record from El’gygytgyn Crater Lake, NE Russia
Abstract Sediment piston cores from Lake El’gygytgyn (67N, 172E), a 3.6 million year old meteorite impact crater in northeastern Siberia, have been analyzed to extract a multi-proxy millennial- scale climate record extending to nearly 250 ka, with distinct fluctuations in sedimentological, physical, biochemical, and paleoecological parameters. Five major themes emerge from this research. First the pilot cores and seismic data show that El’gygytygn Crater Lake contains what is expected to be the longest, most continuous terrestrial record of past climate change in the entire Arctic back to the time of impact. Second, processes operating in the El’gygytygn basin lead to changes in the limnogeology and the biogeochemistry that reflect robust changes in the regional climate and paleoecology over a large part of the western Arctic. Third, the magnetic susceptibility and other proxies record numerous rapid change events. The recovered lake sediment contains both the best-resolved record of the last interglacial and the longest terrestrial record of millennial scale climate change in the Arctic, yielding a high fidelity multi-proxy record extending nearly 150,000 years beyond what has been obtained from the Greenland Ice Sheet. Fourth, the potential for evaluating teleconnections under different mean climate states is high. Despite the heterogeneous nature of recent Arctic climate change, millennial scale climate events in the North Atlantic/Greenland region are recorded in the most distal regions of the Arctic under variable boundary conditions. Finally, deep drilling of the complete depositional record in Lake El’gygytgyn will offer new insights and, perhaps, surprises into the late Cenozoic evolution of Arctic climate
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Sediment fabric, clay mineralogy, and grainsize as indicators of climate change since 65 ka at El’gygytgyn Crater Lake, Northeast Siberia
Abstract El’gygytgyn Crater Lake, NE Siberia was investigated for sedimentological proxies for regional climate change with a focus on the past 65 ka. Sedimentological parameters assessed relative to magnetic susceptibility include stratigraphy, grain size, clay mineralogy and crystallinity. Earlier work suggests that intervals of high susceptibility in these sediments are coincident with warmer (interglacial- like) conditions and well-mixed oxygenated bottom waters. In contrast, low susceptibility intervals correlate with cold (glacial-like) conditions when perennial ice-cover resulted in anoxia and the dissolution of magnetic carrier minerals. The core stratigraphy contains both well-laminated to non-laminated sequences. Reduced oxygen and lack of water column mixing preserved laminated sequences in the core. A bioturbation index based upon these laminated and nonlaminated sequences co-varies with total organic carbon (TOC) and magnetic susceptibility. Clay mineral assemblages include illite, highly inter-stratified illite/smectite, and chlorite. Under warm or hydrolyzing conditions on the landscape around the lake, chlorite weathers easily and illite/ smectite abundance increase, which produces an inverse relationship in the relative abundance of these clays. Trends in relative abundance show distinct down-core changes that correlate with shifts in susceptibility. The mean grain-size (6.92 lm) is in the silt-size fraction, with few grains larger than 65 lm. Terrigenous input to the lake comes from over 50 streams that are filtered through storm berms, which limits clastic deposition into the lake system. The sedimentation rate and terrigenous input grain-size is reduced during glacial intervals. Measurements of particle-size distribution indicate that the magnetic susceptibility fluctuations are not related to grain size. Lake El’gygytgyn’s magnetic susceptibility and clay mineralogy preserves regional shifts in climate including many globally recognized events like the Younger Dryas and Bolling/Allerod. The sedimentary deposits reflect the climatic transitions starting with MIS4 through the Holocene transition. This work represents the first extensive sedimentological study of limnic sediment proxies of this age from Chukotka (Fig. 1)
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Diatom stratigraphy of the last 250 ka at Lake El’gygytgyn, northeast Siberia
Diatom species counts were conducted on 171 sediment samples from the 13-m-long core PG1351 from Lake El’gygytgyn, northeast Siberia. The planktonic Cyclotella ocellata-complex dominates the diatom assemblage through most of the core record, persisting through a variety of climate conditions. Periphytic diatoms, although less abundant, have greater diversity and greater down-core assemblage variation. During warm climate modes, longer summer ice-free conditions may have allowed more complex diatom communities to develop in shallow-water habitats, and enhanced circulation may have increased transport of these diatoms to deeper parts of the lake. Zones of low overall diatom abundance further support inferred intervals of low lake productivity during times of extended lake ice and snow cover. More data on the modern spatial and temporal distribution of diatom species in the Lake El’gygytgyn system will improve inferences from core records
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History of Arctic Sea Ice
The volume and areal extent of Arctic sea ice is rapidly declining, and to put that decline into perspective we need to know the history of Arctic sea ice in the geologic past. Sedimentary proxy records from the Arctic Ocean floor and from the surrounding coasts can provide clues. Although incomplete, existing data outline the development of Arctic sea ice during the last several million years. Some data indicate that sea ice consistently covered at least part of the Arctic Ocean for no less than 13–14 million years, and that ice was most widespread during the last approximately 2 million years in relation with Earth’s overall cooler climate. Nevertheless, episodes of considerably reduced ice cover or even a seasonally ice-free Arctic Ocean probably punctuated even this latter period. Ice diminished episodically during warmer climate events associated with changes in Earth’s orbit on the time scale of tens of thousands of years. Ice cover in the Arctic began to diminish in the late 19th century, and this shrinkage has accelerated during the last several decades. Shrinkages that were both similarly large and rapid have not been documented during at least the last few thousand years, although the paleoclimatic record is sufficiently sparse that similar events might have been missed. Orbital changes have made ice melting less likely than during the previous millennia since the end of the last ice age, making the recent changes especially anomalous. Improved reconstructions of sea-ice history would help clarify just how anomalous these recent changes are
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Elemental and Isotopic Constraints on the Late Glacial-Holocene Transgression and Paleoceanography of the Chukchi Sea
Results obtained from analyses of three sediment cores collected from the Chukchi Shelf at 55m, 80m, and 107m water depths show changes in the composition and quantity of sedimentary organic matter delivered to the core locations from the Late-glacial up to near modern times. Two of the cores (80m, and 107m) show an abrupt and substantial increase in the total organic content and a shift in stable carbon and nitrogen isotopic composition of the organic matter at 8000-9000 years BP. We interpret this shift to be indicative of increased marine primary productivity that subsequently led to the onset of denitrification, as observed today in modern Chukchi Shelf sediments. The shift coincides with the probable re-advance of sea ice coverage at 8500 14C yrs BP, as well as a shift in the Trans Polar Drift. Together, these occurrences suggest a major reorganization of Arctic paleoceanography at 8-9ka that has more or less persisted through the Holocene. A small increase in productivity is also observed at ~11ka in the 80m core that could coincide with the first onset of Bering Strait through-flow following the LGM. Additional studies are underway to help constrain sea ice conditions at various times during the Holocene in the Bering and Chukchi Seas in order to elucidate the relationship between former ice coverage regimes and primary production
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Paloenvironmental Conditions in Western Beringia before and during the Last Glacial Maximum
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Executive Summary
Paleoclimate records play a key role in our understanding of Earth’s past and present climate system and in our confidence in predicting future climate changes. Paleoclimate data help to elucidate past and present active mechanisms of climate change by placing the short instrumental record into a longer term context and by permitting models to be tested beyond the limited time that instrumental measurements have been available. Recent observations in the Arctic have identified large ongoing changes and important climate feedback mechanisms that multiply the effects of global-scale climate changes. Ice is especially important in these “Arctic amplification” processes, which also involve the ocean, the atmosphere, and the land surface (vegetation, soils, and water). As discussed in this report, paleoclimate data show that land and sea ice have grown with cooling temperatures and have shrunk with warming ones, amplifying temperature changes while causing and responding to ecosystem shifts and sea-level changes