Probing the accumulation history of the voluminous Toba magma

The age and compositional zonation in crystals from the Youngest Toba Tuff record the prelude to Earth's largest Quaternary eruption. We used allanite crystals to date and decipher this zoning and found that the crystals retain a record of at least 150,000 years of magma storage and evolution. The dominant subvolcanic magma was relatively homogeneous and thermally stagnant for approximately 110,000 years. In the 35,000 years before eruption, the diversity of melts increased substantially as the system grew in size before erupting 75,000 years ago.     

Past glacial activity in the canadian high arctic

Field observations on northeast Ellesmere Island indicate that the maximum advance of the northwest Greenland Ice Sheet was about 100 kilometers beyond its present margin. This occurred before the outermost Ellesmere Island ice advance, which took place more than 30,000 years before present (B.P.). Recession from the Ellesmere Island ice margin began at least 28,000 to 30,000 and possibly more than 35,000 years B.P. During this sequence of glacial events, significant land areas remained free of ice. The late Wisconsin ice extent along both northeast Ellesmere Island and northwest Greenland was extremely limited, leaving an ice-free corridor along Kennedy and Robeson channels. Recession from these ice margins is indicated by initial postglacial emergence around 8100 to 8400 years B.P. The relatively minor extent of late Wisconsin ice in the High Arctic probably reflects a period of extreme aridity occasioned by the buildup of the Laurentide Ice Sheet to the south.     

西瓜大棚栽培技术

通过采用大棚设施栽培西瓜可实现春节前上市,并多次结果,连续采收半年以上,每667 m2产量5000 kg左右。该文总结了多年来西瓜大棚栽培的成功经验和技术细节。     

North atlantic ice-rafting: a major change at 75,000 years before the present

During the last interglacial-to-glacial climatic cycle [127,000 to 10,000 years before the present (B.P.)], the fundamental geographic shift in the main axis of ice-rafting deposition occurred at 75,000 years B.P. An earlier meridional depositional maximum along the Greenland-Newfoundland coasts was superseded by a nearly zonal and much stronger axis some 1500 kilometers to the south along 40 degrees N to 50 degrees N. Both depositional patterns are best explained by cyclonic flow in the subpolor gyre, with the depositional shift related to the retreat of warm, ice-melting North Atlantic drift water from the northwestern half of the gyre. Similar shifts must have characterized preceding interglacial-glacial cycles.     

West antarctic ice sheet: present-day thinning and holocene retreat of the margins

Retreat of the margins of the West Antarctic ice sheet associated with rising sea level during the last 15,000 years is the main cause for the thinning of the ice sheet by approximately 300 meters. The West Antarctic ice sheet during the late Wisconsin was at least 30 percent wider than it is today, and Holocene retreat of its margins has added about 6 meters to the world sea level.     

Glacial age marsh, lafayette park, washington, d.C

Organic sediments beneath historic Lafayette Park mark the site of a freshwater marsh which bordered the Potomac River when it was 15 meters above its present level. Plant microfossils and ice-rafted boulders indicate a climate much colder than now. The carbon-14 age of more than 45,000 years and palynological studies suggest an early Wisconsin age.     

Late pleistocene vegetation and degree of pluvial climatic change in the chihuahuan desert

Eight Pleistocene wood rat middens at elevations of 1200, 880, and 600 meters in the Chihuahuan Desert contain abundant macrofossils of pinyon pine, juniper, shrubby liveoak, and Opuntia, together with smaller quantities of Agave lecheguilla and other xerophytes of existing desert vegetation, which in dicate a xerophilous woodland vegetation in the lowlands, as much as 800 meters below existing woodland, during the Wisconsin pluvial. Ten radiocarbon dates show ages that range from 11,560 to 14,800 and 16,250 to 20,000 years, and to more than 40,000 years. Absence of most mesophytic montane species in deposits as high as 1200 meters indicates a lack of equivalent downward dis placement for the ponderosa pine zone or other zones of montane vegetation. Uneven stocking of isolated peaks in the Chihuahuan Desert province with montane species suggests that long-distance transport of propagules, rather than former continuity, may account for the disjunct distributions of many species.     

Late wisconsin climate of northern Florida and the origin of species-rich deciduous forest

Species-rich mesic forest covered northern Florida as early as 14,000 radiocarbon years before present. It probably originated in deciduous tree populations already present locally in conifer forest between 24,000 and 18,600 years before present. The cold, dry Late Wisconsin climate ended before 14,600 years before present. A transitional warm, dry phase preceded a precipitation increase at 14,000 years before present.     

North american glacial history extended to 75,000 years ago

By concentrating carbon-14 through thermal diffusion, it is possible to extend the range of carbon-14 dating to 75,000 years ago. Samples with very low contamination levels have been encountered, and a reliable chronology appears possible. A Pacific Northwest climatic curve has been derived from palynological studies. The Pacific Northwest curve and the Great Lakes glacial history are age-calibrated by radiocarbon dating. The climatic patterns in the Pacific Northwest and Northwest Europe are similar in the early part of the last glaciation, with interstades near 60,000, 65,000, and 70,000 years ago. An age of 74,700 years for the St. Pierre interstade indicates a possible correlation with the previous interglacial.     

Cosmogenic chlorine-36 chronology for glacial deposits at bloody canyon, eastern sierra nevada

Deposits from mountain glaciers provide an important record of Quaternary climatic fluctuations but have proved difficult to date directly. A chronology has been obtained for glacial deposits at Bloody Canyon, California, by measurement ofthe accumulation of chlorine-36 produced by cosmic rays in boulders exposed on moraine crests. The accumulation ofchlorine-36 indicates that episodes of glaciation occurred at about 21, 24, 65, 115, 145, and 200 ka (thousand years ago). Although the timing of the glaciations correlates well with peaks of global ice volume inferred from the marine oxygen isotope record, the relative magnitudes differ markedly. The lengths of the moraines dating from 115 ka and 65 ka show that the early glacial episodes were more extensive than those during the later Wisconsin and indicate that the transition from interglacial to full glacial conditions was rapid.     

Late pleistocene paleotemperatures at tongue of the ocean, bahamas

Estimates of paleotemperatures, based upon paleoecological analysis of planktonic foraminiferal thanatocoenoses of three piston cores from Tongue of the Ocean, Bahamas, indicate a maximum mean variation between Late Pleistocene glacial and nonglacial stages of 3.6 degrees C. The data also indicate that the Early Wisconsin glacial stage was 0.7 degrees C warmer than the Late Wisconsin glacial stage.     

An Aboriginal Australian genome reveals separate human dispersals into Asia

We present an Aboriginal Australian genomic sequence obtained from a 100-year-old lock of hair donated by an Aboriginal man from southern Western Australia in the early 20th century. We detect no evidence of European admixture and estimate contamination levels to be below 0.5%. We show that Aboriginal Australians are descendants of an early human dispersal into eastern Asia, possibly 62,000 to 75,000 years ago. This dispersal is separate from the one that gave rise to modern Asians 25,000 to 38,000 years ago. We also find evidence of gene flow between populations of the two dispersal waves prior to the divergence of Native Americans from modern Asian ancestors. Our findings support the hypothesis that present-day Aboriginal Australians descend from the earliest humans to occupy Australia, likely representing one of the oldest continuous populations outside Africa.     

Pollen sequence from late quaternary sediments in yellowstone park

A radiocarbon-dated pollen diagram from a depression near the end of the southeast arm of Yellowstone Lake, Wyoming, records the vegetation sequence from the retreat of Late Wisconsin (Pinedale) ice to the present time. The tundra Picea-Abies-Pinus albicaulis (spruce-fir-whitebark pine) parkland vegetation inferred during late glacial time changed rapidly to a Pinus contorta (lodgepole pine) forest shortly before 11,550 carbon-14 years ago, suggesting a warming trend at that time. The Pinus contorta forest persisted with minor modification throughout postglacial time.     

Glomalean fungi from the Ordovician

Fossilized fungal hyphae and spores from the Ordovician of Wisconsin (with an age of about 460 million years) strongly resemble modern arbuscular mycorrhizal fungi (Glomales, Zygomycetes). These fossils indicate that Glomales-like fungi were present at a time when the land flora most likely only consisted of plants on the bryophytic level. Thus, these fungi may have played a crucial role in facilitating the colonization of land by plants, and the fossils support molecular estimates of fungal phylogeny that place the origin of the major groups of terrestrial fungi (Ascomycota, Basidiomycota, and Glomales) around 600 million years ago.     

Laurentide ice sheet meltwater recorded in gulf of Mexico deep-sea cores

Oxygen isotopic measurements in three Late Quaternary deep-sea cores from the Gulf of Mexico record a major anomaly between about 15,000 and 12,000 years ago superimposed on a more characteristic oceanic oxygen isotopic curve. This resulted from major influx of isotopically light glacial meltwater via the Mississippi River from the disintegrating Late Wisconsin Laurentide Ice Sheet 2000 kilometers to the north.     

Baffin island refugia older than 54,000 years

Two radiocarbon determinations on marine shells from the east coast of Baffin Island give ages exceeding 50,000 years. These findings indicate the existence of unglaciated areas ( refugia) between fiords occupied by outlet glaciers flowing toward Baffin Bay, from the central part of the Wisconsin ice sheet, over the Foxe Basin-Hudson Bay area.     

Erratum

Two errors of affiliation were made in the article about Albert Szent-Gy?rgyi in the issue of 9 February (News and Comment, page 522). Harold Swartz is with the Medical College of Wisconsin, not the University of Wisconsin. Gabor Fodor is at the University of West Virginia, not the University of Wisconsin.     

Status of the Pleistocene Wisconsin Stage in Central North America

A brief review of the history of the Wisconsin Stage in Pleistocene stratigraphy and of research since 1950 shows that post-Sangoman glacial drift older than the Wisconsin drift reported in the older literature is present in central North America. Known and possible stratigraphic positions of the relevant units are shown.     

Teotihuacan: completion of map of giant ancient city in the valley of Mexico

The detailed archeological map of Teotihuacán, near Mexico City, demonstrates what the prehistoric city was like from its densely crowded center to its more sparsely settled peripheries. The city's population lived in crowded one-story apartment compounds, grouped into neighborhoods based at least partly on occupation. At its height the city had a minimum population of 75,000, a probable population of 125,000, and a possible population of more than 200,000. Those involved in craft production and associated activities may have numbered in the tens of thousands. The scope and intensity of urbanization at Teotihuacán is not paralleled in other contemporary New World centers. The growth potential of the obsidian and other industries, the rise of Teotihuacán as a market and trade center, and its attraction as a religious center may have combined in a self-generating process that led to the creation of Teotihuacán's unique urban society.     

A possible younger dryas record in southeastern alaska

A stratigraphic record of climatic cooling equal in timing and severity to the Younger Dryas event of the North Atlantic region has been obtained from lacustrine sediments in the Glacier Bay area of southeastern Alaska. Fossil pollen show that a late Wisconsin pine parkland was replaced about 10,800 years ago by shrub- and herb-dominated tundra, which lasted until about 9,800 years ago. This vegetational change is matched by geochemical evidence for loss of organic matter from catchment soils and increased mineral erosion. If this event represents the Younger Dryas, then an explanation for a hemisphere-wide propagation of a North Atlantic climatic perturbation must be sought.