A 250,000-year climatic record from great basin vein calcite: implications for milankovitch theory

A continuous record of oxygen-18 (delta(18)O) variations in the continental hydrosphere during the middle-to-late Pleistocene has been obtained from a uranium-series dated calcitic vein in the southern Great Basin. The vein was deposited from ground water that moved through Devils Hole-an open fault zone at Ash Meadows, Nevada-between 50 and 310 ka (thousand years ago). The configuration of the delta(18)O versus time curve closely resembles the marine and Antarctic ice core (Vostok) delta(18)O curves; however, the U-Th dates indicate that the last interglacial stage (marine oxygen isotope stage 5) began before 147 +/- 3 ka, at least 17,000 years earlier than indicated by the marine delta(18)O record and 7,000 years earlier than indicated by the less well dated Antarctic delta(18)O record. This discrepancy and other differences in the timing of key climatic events suggest that the indirectly dated marine delta(18)O chronology may need revision and that orbital forcing may not be the principal cause of the Pleistocene ice ages.     

The last interglacial: paleotemperatures and chronology

The O(18)/O(16) analysis and Th(230)/Pa(231) dating of deep-sea cores showed that the last interglacial age, with an early major temperature maximum followed by two smaller ones, extended from 100,000 to 70,000 years ago and was preceded by a glacial age extending from 120,000 to 100,000 years ago. The O(18)/O(16) analysis and Th(230)/U(234) dating of speleothems confirm and refine these ages.     

Milankovitch forcing of the last interglacial sea level

During the last interglacial, sea level was as high as present, 4000 to 6000 years before peak Northern Hemisphere insolation receipt 126,000 years ago. The sea-level results are shown to be consistent with climate models, which simulate a 3 degrees to 4 degrees C July temperature increase from 140,000 to 130,000 years ago in high latitudes, with all Northern Hemisphere land areas being warmer than present by 130,000 years ago. The early warming occurs because obliquity peaked earlier than precession and because precession values were greater than present before peak precessional forcing occurred. These results indicate that a fuller understanding of the Milankovitch-climate connection requires consideration of fields other than just insolation forcing at 65 degrees N.     

The timing of high sea levels over the past 200,000 years

The (230)Th ages and (234)U/(238)U ratios were determined for Barbados corals that grew during periods of high sea level within the last 200,000 years. The similarity of the initial (234)U/(238)U ratios of some of the corals to the modern marine value suggests that these samples are pristine and that the marine (234)U/(238)U ratio 83,000 and 200,000 years ago was within 2 per mil of the modern value. The accuracies of the (230)Th ages are evaluated on the basis of the (234)U/(238)U values and a model of the behavior of uranium and thorium isotopes during diagenesis. For the last three interglacial and two intervening interstadial periods, sea level peaked at or after peaks in summer insolation in the Northern Hemisphere. This overall pattern supports the idea that glacial-interglacial cycles are caused by changes in Earth's orbital geometry. The sea-level drop at the end of the penultimate interglacial, the last interglacial, and a subsequent interstadial period lagged behind the decrease in insolation by 5,000 to 10,000 years.     

Precise timing of the last interglacial period from mass spectrometric determination of thorium-230 in corals

The development of mass spectrometric techniques for determination of (230)Th abundance has made it possible to reduce analytical errors in (238)U-(234)U-(230)Th dating of corals even with very small samples. Samples of 6 x 10(8) atoms of (230)Th can be measured to an accuracy of +/-3 percent (2sigma) and 3 x 10(10) atoms of (230)Th can be measured to an accuracy of +/-0.2 percent. The time range over which useful age data on corals can be obtained now ranges from about 50 to about 500,000 years. For young corals, this approach may be preferable to (14)C dating. The precision with which the age of a coral can now be determined should make it possible to critically test the Milankovitch hypothesis concerning Pleistocene climate fluctuations. Analyses of a number of corals that grew during the last interglacial period yield ages of 122,000 to 130,000 years. The ages coincide with, or slightly postdate, the summer solar insolation high at 65 degrees N latitude which occurred 128,000 years ago. This supports the idea that changes in Pleistocene climate can be the result of variations in the distribution of solar insolation caused by changes in the geometry of the earth's orbit and rotation axis.     

Plio-Pleistocene ice volume, Antarctic climate, and the global delta18O record

We propose that from approximately 3 to 1 million years ago, ice volume changes occurred in both the Northern and Southern Hemispheres, each controlled by local summer insolation. Because Earth's orbital precession is out of phase between hemispheres, 23,000-year changes in ice volume in each hemisphere cancel out in globally integrated proxies such as ocean delta18O or sea level, leaving the in-phase obliquity (41,000 years) component of insolation to dominate those records. Only a modest ice mass change in Antarctica is required to effectively cancel out a much larger northern ice volume signal. At the mid-Pleistocene transition, we propose that marine-based ice sheet margins replaced terrestrial ice margins around the perimeter of East Antarctica, resulting in a shift to in-phase behavior of northern and southern ice sheets as well as the strengthening of 23,000-year cyclicity in the marine delta18O record.     

Temperature and precipitation estimates through the last glacial cycle from clear lake, california, pollen data

Modern pollen surface samples from six lake and marsh sites in the northern California Coast Ranges establish a linear relation between elevation and the oakl(oak + pine) pollen ratio. Modern temperature and precipitation lapse rates were used to convert variations in the pollen ratio into temperature and precipitation changes. Pollen data from two cores from Clear Lake, Lake County, California, spanning the past 40,000 and 130,000 years were used to estimate temperature and precipitation changes through the last full glacial cycle. The maximum glacial cooling is estimated to be 7 degrees to 8 degrees C; the last full interglacial period was about 1.5 degrees C warmer than the Holocene, and a mid-Holocene interval was warmer than the present. The estimated precipitation changes are probably less reliable than the estimated temperature changes.     

Rapid fluctuations in sea level recorded at huon peninsula during the penultimate deglaciation

About 140,000 years ago, the breakup of large continental ice sheets initiated the Last Interglacial period. Sea level rose and peaked around 135,000 years ago about 14 meters below present levels. A record of Last Interglacial sea levels between 116,000 years to 136, 000 years ago is preserved at reef VII of the uplifted coral terraces of Huon Peninsula in Papua New Guinea. However, corals from a cave situated about 90 meters below the crest of reef VII are 130, 000 +/- 2000 years old and appear to have grown in conditions that were 6 degreesC cooler than those at present. These observations imply a drop in sea level of 60 to 80 meters. After 130,000 years, sea level began rising again in response to the major insolation maximum at 126,000 to 128,000 years ago. The early (about 140,000 years ago) start of the penultimate deglaciation, well before the peak in insolation, is consistent with the Devils Hole chronology.     

Oxygen isotope ratios in trees reflect mean annual temperature and humidity

Values of the oxygen isotope ratios (delta(18)O) in tree-ring cellulose closely reflect the delta(18)O values in atmospheric precipitation and hence mean annual temperature. The change in delta(18)O in cellulose is 0.41 per mil per degree Celsius for selected near-coastal stations. The values of delta(18)O in precipitation and cellulose also change with altitude, as demonstrated for Mount Rainier, Washington. A temperature lapse rate of 5.2 degrees +/- 0.5 degrees C per 1000 meters calculated from cellulose delta(18)O values agrees with the accepted mean annual lapse rate of 5 degrees C per 1000 meters for this region. Cellulose delta(18)O values and delta(18)O values of carbon dioxide equilibrated with leaf water differ by a fixed 16 per mil.     

Patagonian glacier response during the late glacial-Holocene transition

Whether cooling occurred in the Southern Hemisphere during the Younger Dryas (YD) is key to understanding mechanisms of millennial climate change. Although Southern Hemisphere records do not reveal a distinct climate reversal during the late glacial period, many mountain glaciers readvanced. We show that the Puerto Bandera moraine (50 degrees S), which records a readvance of the Southern Patagonian Icefield (SPI), formed at, or shortly after, the end of the YD. The exposure age (10.8 +/- 0.5 thousand years ago) is contemporaneous with the highest shoreline of Lago Cardiel (49 degrees S), which records peak precipitation east of the Andes since 13 thousand years ago. Absent similar moraines west of the Andes, these data indicate an SPI response to increased amounts of easterly-sourced precipitation-reflecting changes in the Southern Westerly circulation-rather than regional cooling.     

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.     

1956～2011年华北地区气温和降水变化特征

利用1956～2011年的气候资料,对近56年来华北地区的气温和降水变化特征进行分析。结果表明,近56年来华北地区年平均气温显著升高,升幅为0.314℃/10a,降水量显著减少,减幅为16.329 mm/10a。气温、降水量均呈东北—西南走向分布,其变化趋势具有全区性,华北大部分地区气温呈升高趋势,降水量呈减少趋势。经M-K检验,华北地区气温变化在1988年存在一个突变点,1988年后华北地区平均气温增幅达1.1℃;降水量在1981年发生突变,华北进入降水偏少时期。     

155,000 years of West African monsoon and ocean thermal evolution

A detailed reconstruction of West African monsoon hydrology over the past 155,000 years suggests a close linkage to northern high-latitude climate oscillations. Ba/Ca ratio and oxygen isotope composition of planktonic foraminifera in a marine sediment core from the Gulf of Guinea, in the eastern equatorial Atlantic (EEA), reveal centennial-scale variations of riverine freshwater input that are synchronous with northern high-latitude stadials and interstadials of the penultimate interglacial and the last deglaciation. EEA Mg/Ca-based sea surface temperatures (SSTs) were decoupled from northern high-latitude millennial-scale fluctuation and primarily responded to changes in atmospheric greenhouse gases and low-latitude solar insolation. The onset of enhanced monsoon precipitation lags behind the changes in EEA SSTs by up to 7000 years during glacial-interglacial transitions. This study demonstrates that the stadial-interstadial and deglacial climate instability of the northern high latitudes exerts dominant control on the West African monsoon dynamics through an atmospheric linkage.     

Evolution of ocean temperature and ice volume through the mid-Pleistocene climate transition

Earth's climate underwent a fundamental change between 1250 and 700 thousand years ago, the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum.     

Quaternary climates and sea levels of the u.s. Atlantic coastal plain

Uranium-series dating of corals from marine deposits of the U.S. Atlantic Coastal Plain coupled with paleoclimatic reconstructions based on ostracode (marine) and pollen (continent) data document at least five relatively warm intervals during the last 500,000 years. On the basis of multiple paleoenvironmental criteria, we determined relative sea level positions during the warm intervals, relative to present mean sea level, were 7 +/- 5 meters at 188,000 years ago, 7.5 +/- 1.5 meters at 120,000 years ago, 6.5 +/- 3.5 meters at 94,000 years ago, and 7 +/- 3 meters at 72,000 years ago. The composite sea level chronology for the Atlantic Coastal Plain is inconsistent with independent estimates of eustatic sea level positions during interglacial intervals of the last 200,000 years. Hydroisostatic adjustment from glacial-interglacial sea level fluctuations, lithospheric flexure, and isostatic uplift from sediment unloading due to erosion provide possible mechanisms to account for the discrepancies. Alternatively, current eustatic sea level estimates for the middle and late Quaternary may require revision.     

Rates of late cenozoic uplift, baldwin hills, los angeles, california

Radiocarbon ages for the marine late Pleistocene stratigraphic units of the Baldwin Hills are 36,200 +/- 2,750 years and 28,450 +/- 2,600 years, respectively, defining the termination of marine deposition in this area of the Los Angeles Basin at less than 28,000 years ago. Faunas of the older sample suggest that water depths were about 100 meters at the time of deposition. Shoaling of waters by deposition resulted in very shallow marine to nonmarine conditions about 28,000 years ago. The average rates of uplift for the past 36,000 years have been between 0.5 and 0.8 meter per 100 years.     

Orbital forcing of the marine isotope stage 9 interglacial

Milankovitch orbital forcing theory has been used to assign time scales to many paleoclimate records. However, the validity of this theory remains uncertain, and independent sea-level chronologies used to test its applicability have been restricted largely to the past approximately 135,000 years. Here, we report U-series ages for coral reefs formed on Henderson Island during sea-level high-stands occurring at approximately 630,000 and approximately 330,000 years ago. These data are consistent with the hypothesis that interglacial climates are forced by Northern Hemisphere summer solar insolation centered at 65 degrees N latitude, as predicted by Milankovitch theory.     

40Ar/39Ar Age of the Lathrop Wells Volcanic Center, Yucca Mountain, Nevada

Paleomagnetic and (40)Ar/(39)Ar analyses from the Lathrop Wells volcanic center, Nevada, indicate that two eruptive events have occurred there. The ages (136 +/- 8 and 141 +/- 9 thousand years ago) for these two events are analytically indistinguishable. The small angular difference (4.7 degrees ) between the paleomagnetic directions from these two events suggests they differ in age by only about 100 years. These ages are consistent with the chronology of the surficial geological units in the Yucca Mountain area. These results contradict earlier interpretations of the cinder-cone geomorphology and soil-profile data that suggest that at least five temporally discrete eruptive events occurred at Lathrop Wells approximately 20,000 years ago.     

High-resolution Greenland ice core data show abrupt climate change happens in few years

The last two abrupt warmings at the onset of our present warm interglacial period, interrupted by the Younger Dryas cooling event, were investigated at high temporal resolution from the North Greenland Ice Core Project ice core. The deuterium excess, a proxy of Greenland precipitation moisture source, switched mode within 1 to 3 years over these transitions and initiated a more gradual change (over 50 years) of the Greenland air temperature, as recorded by stable water isotopes. The onsets of both abrupt Greenland warmings were slightly preceded by decreasing Greenland dust deposition, reflecting the wetting of Asian deserts. A northern shift of the Intertropical Convergence Zone could be the trigger of these abrupt shifts of Northern Hemisphere atmospheric circulation, resulting in changes of 2 to 4 kelvin in Greenland moisture source temperature from one year to the next.     

Loss of carbon from the deep sea since the Last Glacial Maximum

Deep-ocean carbonate ion concentrations ([CO(3)(2-)]) and carbon isotopic ratios (δ(13)C) place important constraints on past redistributions of carbon in the ocean-land-atmosphere system and hence provide clues to the causes of atmospheric CO(2) concentration changes. However, existing deep-sea [CO(3)(2-)] reconstructions conflict with one another, complicating paleoceanographic interpretations. Here, we present deep-sea [CO(3)(2-)] for five cores from the three major oceans quantified using benthic foraminiferal boron/calcium ratios since the last glacial period. Combined benthic δ(13)C and [CO(3)(2-)] results indicate that deep-sea-released CO(2) during the early deglacial period (17.5 to 14.5 thousand years ago) was preferentially stored in the atmosphere, whereas during the late deglacial period (14 to 10 thousand years ago), besides contributing to the contemporary atmospheric CO(2) rise, a substantial portion of CO(2) released from oceans was absorbed by the terrestrial biosphere.