Variations of atmospheric 14C concentrations over the Alleroød-Younger Dryas transition

Highly variable atmospheric radiocarbon concentrations are a distinct feature during the last deglaciation. The synchronisation of two high-resolution AMS ¹⁴C-dated records, Lake Go?ci??, and a floating Late Weichselian glacial varve chronology at the Allerød-Younger Dryas transition allowed us to assess in detail atmospheric Δ¹⁴C changes between late Allerød and early Preboreal. The combined data set shows a drastic rise in Δ¹⁴C during the first 200 years or so of Younger Dryas and the two following about 500 year-long ¹⁴C plateaux. Model experiments which included variations in the geomagnetic field, atmospheric CO₂ variations and a drastic reduction in North Atlantic Deep Water flux at the onset of Younger Dryas allowed to reproduce the distinct rise in Δ¹⁴C during the first 200 years of Younger Dryas fairly well. Also the drop in Δ¹⁴C at the Younger Dryas/Holocene boundary seems reasonably explained by changes in North Atlantic Deep Water circulation. However, the reason behind the anomalous behaviour of the Δ¹⁴C signal in the middle of Younger Dryas remains an open question.     

Younger dryas and holocene glacier fluctuations and equilibrium-line altitude variations in the Jostedalsbre region,western Norway

Reconstructed Younger Dryas (11000-10000 y BP) valley- and cirque glaciers west of the Jostedalsbre ice cap suggest an equilibrium-line altitude (ELA) depression of 450±50 m compared to the present level. The mid-Preboreal (9500±200 y BP) deglaciation was characterized by vertical wastage, indicating that the ELA was above the summit plateaus. During the Erdalen event (9100±200 y BP) marginal moraines were formed up to 1 km beyond the Little Ice Age (LIA) moraines which lie in front of the present valley outlet glaciers of the Jostedalsbre ice cap. The average ELA lowering during this event is calculated to 325 m below the modern level. Lithostratigraphic and paleobotanical studies show that the Hypsithermal (ca. 8000-6000 y BP) ELA was about 450 m higher than at present. As a result, Jostedalsbreen probably disappeared entirely during that period. The glacier reformed about 5300 y BP. The ELA intersected the modern mean equilibrium line altitude five times from ca. 2600 y BP to the present. The outlet valley glaciers reached their maximum Neoglacial extent during the LIA in the mid-18th century, when the ELA was depressed 100–150 m below the present level.Contribution to Clima Locarno — Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate Program     

Rates of change and chronological problems during the late-glacial period

Results of high-resolution AMS ¹⁴C dating of terrestrial plant macrofossils from late-glacial and early-Holocene lake deposits in Switzerland show three periods with constant radiocarbon ages. These plateaux of constant age occur at 12700, 10000, and 9500 y BP. A comparison of this radiocarbon chronology with a varve chronology documents discrepancies between the sidereal and the radiocarbon time-scale for the late-glacial period. The age-plateaux and the time-scale discrepancies have a significant impact on the estimation of rates of change during this period: estimates of rates of change can be very misleading if calculated on the basis of radiocarbon ages. This is illustrated by an example of estimated rates of late-glacial and early Holocene palynological change in Switzerland.Contribution to Clima Locarno — Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate Program     

新仙女木事件及全新世早中期降温事件——来自洱海湖泊沉积的记录

Three cold events (the Younger Dryas, 9.4 ka cal BP, 5.8 ka cal BP) since the 13 ka cal BP in Erhai (EH) Lake catchment, Yunnan Province, were analyzed using the Total Organic Carbon (TOC) series of the EH core. By comparison of the EH core, Qinghai Lake core and Guliya ice core, differences of these cold events were determined. Erhai Lake's responses to the global cold events were lagged in time and weakened in intensity in comparison with Qinghai Lake's. The latitude location of Erhai Lake and the obstruction of Tibetan Plateau may in part explain the differences. However, the remarkable cold event of 8.2 ka cal BP in the Guliya ice core was absent in the records of Erhai Lake and Qinghai Lake. Power spectrum analysis of the TOC proxy series shows that there were three kinds of millennial cycles, i.e. 5 ka, 2.3 ka, and 1.5 ka, in climate changes in Erhai Lake, which reveal the responses of climate to suborbit cycles.     

Paleoclimatic inferences from glacial fluctuations on Svalbard during the last 20 000 years

The climate history of western Spitsbergen, Svalbard is deduced from variations of glaciers during the last 20 000 years. A major depression of the regional equilibrium line altitude (ELA) occurred during the Late Weichselian glacial maximum (18000–13000y ago) when low summer temperatures may have caused year-round snow accumulation on the ground. This rapid expansion of the glaciers also indicates nearby moisture sources, suggesting partly open conditions in the Norwegian Sea during the summers. A rapid glacial retreat around 13 000–12 500 y BP was caused by a sudden warming. During the Younger Dryas the ELA along the extreme western coast of Spitsbergen was not significantly lower than at present. In contrast to Fennoscandia, the British Isles and the Alps, there is no evidence for readvance of local glaciers during Younger Dryas on western Spitsbergen. This difference is attributed to a much dryer climate on Spitsbergen and probably only slight changes in sea surface temperatures. In addition, summer melting in this high arctic area is more sensitive to orbitally increased insolation. Around 10 000 y BP another rapid warming occurred and during early and mid Holocene the summer temperatures were significantly higher than at present. A temperature decline during the late Holocene caused regrowth of the glaciers which reached their maximum Holocene position during the last century.Contribution to Clima Locarno — Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate Program     

Modelling the concentration of atmospheric CO2 during the Younger Dryas climate event

 The Younger Dryas (YD, dated between 12.7–11.6 ky BP in the GRIP ice core, Central Greenland) is a distinct cold period in the North Atlantic region during the last deglaciation. A popular, but controversial hypothesis to explain the cooling is a reduction of the Atlantic thermohaline circulation (THC) and associated northward heat flux as triggered by glacial meltwater. Recently, a CH₄-based synchronization of GRIP δ¹⁸O and Byrd CO₂ records (West Antarctica) indicated that the concentration of atmospheric CO₂ (CO^atm ₂) rose steadily during the YD, suggesting a minor influence of the THC on CO^atm ₂ at that time. Here we show that the CO^atm ₂ change in a zonally averaged, circulation-biogeochemistry ocean model when THC is collapsed by freshwater flux anomaly is consistent with the Byrd record. Cooling in the North Atlantic has a small effect on CO^atm ₂ in this model, because it is spatially limited and compensated by far-field changes such as a warming in the Southern Ocean. The modelled Southern Ocean warming is in agreement with the anti-phase evolution of isotopic temperature records from GRIP (Northern Hemisphere) and from Byrd and Vostok (East Antarctica) during the YD. δ¹³C depletion and PO₄ enrichment are predicted at depth in the North Atlantic, but not in the Southern Ocean. This could explain a part of the controversy about the intensity of the THC during the YD. Potential weaknesses in our interpretation of the Byrd CO₂ record in terms of THC changes are discussed. Received: 27 May 1998 / Accepted: 5 November 1998     

Limits on the ventilation rate for the deep ocean over the last 12000 years

In this paper we present accelerator radiocarbon measurements on hand picked benthic and planktonic foraminifera separated from two deep sea cores raised from the South China Sea. From the benthic-planktonic age differences we are able to place limits on the extent to which the ventilation rate of the deep Pacific Ocean has changed over the last 12000 years. While much work remains to be done before any definitive answers for the global oceans can be given, these results on cores with sedimentation rates suitably high to avoid major corrections for bioturbation effects suggest that the ventilation rate of the deep Pacific Ocean has remained nearly the same throughout Holocene time. Further, there is no suggestion that the rate was slower during the period of major glacial retreat. These results confirm that the changes in atmospheric¹⁴C/C ratio over the last 10000 years owe their origin to radiocarbon production rate changes.     

Decadal- to interannual-scale source water variations in the Caribbean Sea recorded by Puerto Rican coral radiocarbon

Water that forms the Florida Current, and eventually the Gulf Stream, coalesces in the Caribbean from both subtropical and equatorial sources. The equatorial sources are made up of, in part, South Atlantic water moving northward and compensating for southward flow at depth related to meridional overturning circulation. Subtropical surface water contains relatively high amounts of radiocarbon (¹⁴C), whereas equatorial waters are influenced by the upwelling of low ¹⁴C water and have relatively low concentrations of ¹⁴C. We use a 250 year record of Δ¹⁴C in a coral from southwestern Puerto Rico along with previously published coral Δ¹⁴C records as tracers of subtropical and equatorial water mixing in the northern Caribbean. Data generated in this study and from other studies indicate that the influence of either of the two water masses can change considerably on interannual to interdecadal time scales. Variability due to ocean dynamics in this region is large relative to variability caused by atmospheric ¹⁴C changes, thus masking the Suess effect at this site. A mixing model produced using coral Δ¹⁴C illustrates the time varying proportion of equatorial versus subtropical waters in the northern Caribbean between 1963 and 1983. The results of the model are consistent with linkages between multidecadal thermal variability in the North Atlantic and meridional overturning circulation. Ekman transport changes related to tradewind variability are proposed as a possible mechanism to explain the observed switches between relatively low and high Δ¹⁴C values in the coral radiocarbon records.     

Glacial to Holocene implications of the new 27000-year dust record from the EPICA Dome C (East Antarctica) ice core

Insoluble dust concentrations and volume-size distributions have been measured for the new 581 m deep Dome C-EPICA ice core (Antarctica). Over the 27000 years spanned by the record, microparticle measurements from 169 levels, to date, confirm evidence of the drastic decrease in bulk concentration from the Last Glacial Maximum (LGM) to the Holocene (interglacial) by a factor of more than 50 in absolute value and of about 26 in flux. Unique new features revealed by the EPICA profile include a higher dust concentration during the Antarctic Cold Reversal phase (ACR) by a factor of 2 with respect to the Holocene average. This event is followed by a well-marked minimum that appears to be concomitant with the methane peak that marks the end of the Younger Dryas in the Northern Hemisphere. Particle volume-size distributions show a mode close to 2 7m in diameter, with a slight increase from the LGM to the Holocene; the LGM/Holocene concentration ratio appears to be dependent on particle size and for diameters from 2 to 5 7m it changes from 50 to 6. Glacial samples are characterised by well-sorted particles and very uniform distributions, while the interglacial samples display a high degree of variability and dispersion. This suggests that different modes of transport prevailed during the two climatic periods with easier penetration of air masses into Antarctica in the Holocene than during Glacial times. Assuming that southern South America remained the main dust source for East Antarctica over the time period studied, the higher dust content recorded during the ACR which preceded the Younger Dryas period, represents evidence of a change in South America environmental conditions at this time. A wet period and likely mild climate in South America is suggested at circa 11.5-11.7 kyr BP corresponding to the end of the Younger Dryas. The Holocene part of the profile also shows a slight general decrease in concentration, but with increasingly large particles that may reflect gradual changes at the source.     

Possible solar forcing of 400-year wet–dry climate cycles in northwestern China

Here we present a multi-proxy paleolimnological record from a closed-basin lake (Ebinur Lake) in northwestern China to investigate climate change in this arid region during the last 1,500 years. The 120-cm long sediment core was dated by AMS radiocarbon and ²¹⁰Pb methods. The fine-grained clay sediments contain 3–17% organic matter (OM) and 9–31% carbonate, and are interrupted by multiple sand and silt layers. These sand/silt layers, having consistently low OM, were found at 700–800, 1000–1100, 1300–1400, and 1700–1750 a.d., with a time spacing of 300–400 years. We interpret that the low OM sand/silt layers were deposited during higher lake levels caused by increased river inflow from the surrounding mountains during wet climate intervals. This interpretation is supported by concurrent decreases in δ ¹⁸O and δ ¹³C of bulk carbonate and in carbonate content. Wet climate intervals at 700–800 a.d. and at 1700–1750 a.d. also correlate with elevated snow accumulation and low δ ¹⁸O from Guliya ice core on the NW Tibetan Plateau, both regions strongly influenced by the westerlies. This approximate 400-year periodicity of wet–dry climate oscillations appear to correlate with solar activity as shown by atmosphere ¹⁴C concentration and with paleo-moisture records in interior North America. Our results suggest that solar activities might have played a significant role in driving wet–dry climate oscillations at centennial scales in the interior of Eurasian continent.     

Impact of the Atlantic Warm Pool on precipitation and temperature in Florida during North Atlantic cold spells

Recurrent phases of increased pine at Lake Tulane, Florida have previously been related to strong stadials terminated by so-called Heinrich events. The climatic significance of these pine phases has been interpreted in different ways. Using a pollen?Cclimate inference model, we quantified the climate changes and consistently found that mean summer precipitation (P _JJA) increased (0.5?C0.9?mm/day) and mean November temperature increased (2.0?C3.0°C) during pine phases coeval with Heinrich events and the Younger Dryas. Marine sea surface temperature records indicate that potential sources for these moisture and heat anomalies are in the Gulf of Mexico and the western tropical Atlantic. We explain this low latitude warming by an increased Loop Current facilitated by persistence of the Atlantic Warm Pool during summer. This hypothesis is supported by a climate model sensitivity analysis. A positive heat anomaly in the Gulf of Mexico and equatorial Atlantic best approximates the pollen-inferred climate reconstructions from Lake Tulane during the (stadials around) Heinrich events and the Younger Dryas.     

High frequency pulses of East Asian monsoon climate in the last two glaciations: link with the North Atlantic

Three loess sections in the Loess Plateau of China have been studied to characterize the variations of the East Asian monsoon climate in the Late Quaternary period. Paleo-weathering profiles based on two weathering indices reveal a series of spatially correlative intervals of lower weathering intensity (LW), in the last glacial and late penultimate glacial loess, indicating significantly cooler or/and drier conditions. Most of them are identified at or near the transitional boundaries between loess and soil units, suggesting that climatic conditions favorable for LW events tended to occur at or near major climatic boundaries. Twenty-eight radiocarbon and thermoluminescence measurements, combined with Kukla's magnetic susceptibility age model date these events at ∼14, ∼21, ∼29, ∼38, ∼52, ∼71, ∼135, and ∼145 ka BP, with ∼5–10 ka frequency inlaid within the orbitally induced ∼20-ka periodicity. The ages of the first six events in the last glacial period are therefore close to those of the coarse-grained Heinrich layers in the North Atlantic Ocean, which resulted from massive discharges of icebergs. The results indicate that the climate in the area affected by the East Asian monsoon has experienced high-frequency changes more or less synchronous with the Heinrich events. These high-frequency changes have been reported by Porter and An, based on the grain-size time series from the Luochuan loess section. Our data also reveal that similar events also occurred during the penultimate glaciation. The mechanisms linking the Heinrich events and the East Asian monsoon climate may be similar to that driving the Younger Dryas event which has been identified in a large number of records in China and in the surrounding oceans. Received: 16 October 1995 / Accepted: 10 May 1996     

Stable isotope (O and C) and pollen trends in eastern Lake Erie,evidence for a locally-induced climatic reversal of Younger Dryas age in the Great Lakes basin

A cool period from about 11000 to 10 500 BP (11 to 10.5 ka) is recognized in pollen records from the southern Great Lakes area by the return of Picea and Abies dominance and by the persistence of herbs. The area of cooling appears centred on the Upper Great Lakes. A high-resolution record (ca. 9 mm/y) from a borehole in eastern Lake Erie reveals, in the same time interval, this pollen anomaly, isotope evidence of meltwater presence (a — 3 per mil shift in ¹⁸O and a +1.1 per mil shift in ¹³C), increased sand, and reduced detrital calcite content, all suggesting concurrent cooling of Lake Erie. The onset of cooling is mainly attributed to the effect of enhanced meltwater inflow on the relatively large upstream Main Lake Algonquin during the first eastward discharge of glacial Lake Agassiz. Termination of the cooling coincides with drainage of Lake Algonquin, and is attributed to loss of its cooling effectiveness associated with a substantial reduction in its surface area. It is hypothesized that the cold extra inflow effectively prolonged the seasonal presence of lake ice and the period of spring overturn in Lake Algonquin. The deep mixing would have greatly increased the thermal conductive capacity of this extensive lake, causing suppression of summer surface lakewater temperatures and reduction of onshore growing-degree days. Alternatively, a rapid flow of meltwater, buoyed on sediment-charged (denser) lakewater, may have kept the lake surface cold in summer. Other factors such as wind-shifted pollen deposition and possible effects from the Younger Dryas North Atlantic cooling could have contributed to the Great Lakes climatic reversal, but further studies are needed to resolve their relative significance.Contribution to Climo Locarno — Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate ProgramGeological Survey of Canada Contribution 58 890     

A method of automatic solar radiation integration with printed records including an assessment of its reliability

Summary Solar radiation intensity has been recorded on a single trace strip chart potentiometer and integrated automatically. The integral can be printed at frequencies from minutes to hours as required. The performance of the integration equipment has been assessed by (1) comparing chart area expected from a calibration of the printed digital output with the area as measured by planimeter and (2) estimating the scatter in the relation of chart area to printed output per hour on a number of days over periods up to a month. The errors (95% limits) are about ±1 cal cm^–2 for hourly values and ±4 cal cm^–2 for daily values.

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Zusammenfassung Die Intensität der Sonnenstrahlung wurde mit einem selbstabgleichenden Millivoltschreiber registriert und automatisch integriert. Das Integral kann nach Bedarf über Zeiten von der Größenordnung von Minuten bis Stunden gebildet werden. Die Leistungsfähigkeit des Integrators wurde abgeschätzt 1. durch Vergleich der auf Grund der Eichung des Integrators aus seinen Angaben errechneten Fläche unter der Registrierkurve mit der planimetrierten Fläche und 2. durch Abschätzung der Streuung der Werte des Zusammenhanges zwischen den pro Stunde angegebenen Integratorwerten und der Fläche auf dem Registrierstreifen an einer Anzahl von Tagen aus Perioden von einer Dauer bis zu einem Monat. Der Fehler (bei einer Genauigkeitsgrenze von 95%) liegt bet ±1 cal cm^–2 für Stundensummen und bei ±4 cal cm^–2 für Tagessummen.

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Résumé L'intensité du rayonnement solaire fut enregistrée par un millivoltmètre compensateur et intégrée automatiquement. L'intégration peut s'étendre à des temps allant de la minute à plusieurs heures. L'exactitude de l'intégrateur fut estimée 1) en comparant la surface résultant de ses indications à celle que l'on planimètre, et 2) en calculant la dispersion des écarts entre les valeurs données par heure par l'intégrateur et celles qui résultent de la planimétrie, cela pour un certain nombre de jours choisis dans des périodes pouvant atteindre le mois. L'erreur (pour un domaine de validité de 95%) est de ±1 cal cm^–2 pour des sommations horaires, de ±4 cal cm^–2 pour des sommations diurnes.

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With 4 Figures     

Climatic and limnological changes associated with the Younger Dryas in Atlantic Canada

Pollen, diatom and chironomid fossils from the sediments of a core from Brier Island Bog Lake, Nova Scotia were studied in an attempt to relate changes in microfossil composition to a climatic cooling in Atlantic Canada correlative with the European Younger Dryas ca. 10 to 11 ka. Our paleolimnological data were then compared to similar types of data from Splan Pond, New Brunswick to determine if there were any significant differences between a coastal and a more inland site. Nonarboreal pollen was dominant throughout the Brier Island core and the interval 10.0–11.0 ka did not show the typical decline in Picea and increases in tundra-like vegetation characteristic of many sites in Atlantic Canada. However, the limnological indicators did undergo marked changes in taxon composition. The chironomid assemblage was initially dominated by shallow-water, warm-adapted chironomid taxa followed by abundant Sergentia (a cold stenotherm) during 10–11 ka. Sergentia disappeared in the post Younger Dryas interval and the warm-adapted genera resumed dominance. Chironomid-inferred paleotemperature reconstructions revealed that at both Brier Island Bog Lake and Splan Pond, summer surface-water temperatures dropped abruptly to between 13 and 17°C during the 10–11 ka interval, suggesting that a cooler climate was present in Atlantic Canada correlative with the European Younger Dryas. Diatom assemblage changes during the same period corroborate the occurrence of limnological fluctuations.     

Land-ice teleconnections of cold climatic periods during the last Glacial/Interglacial transition

Independent calendar year chronologies are a basic requirement for the establishment of high resolution land-ice teleconnections. The annually laminated Meerfelder Maar record provides both an independent chronology, established by varve counting, and high resolution lithological proxy data for the period of the last Glacial/Interglacial transition. These data reveal a series of four periods of climatic deterioration coinciding with negative isotopic deviations in the GRIP record signal, thus demonstrating the synchronicity of environment changes in Western Germany and temperature shifts in Greenland. The terrestrial data supports a further sub-division of the event stratigraphy based on the GRIP core, by introducing the cold event GI-1c2 between 13 500 and 13 400 calendar years BP. Multiproxy analyses reveal that the environmental response at Meerfelder Maar was not linear throughout the Lateglacial but was modified by local processes. A change in the response of the lake environment to climate deterioration was observed during sub-stage GI-1b (Gerzensee oscillation), the only event with gradual rather than abrupt transitions. The two-fold character of the Younger Dryas as seen in the GRIP record is more pronounced in the Meerfelder Maar record. This lithological signal occurred with a delay of 60 years to the GRIP signal, and has been linked to a shift in the catchment. It is proposed that the trigger for this shift was a trend towards a more humid second half of the Younger Dryas. Received: 21 May 1999 / Accepted: 18 August 1999     

Climatic interpretation of the recently extended Vostok ice records

A new ice core drilled at the Russian station of Vostok in Antarctica reached 2755 m depth in September 1993. At this depth, the glaciological time scale provides an age of 260 ky BP (±25). We refine this estimate using records of dust and deuterium in the ice and of ¹⁸O of O₂ in the entrapped air. ¹⁸O of O₂ is highly correlated with insolation over the last two climatic cycles if one assumes that the EGT chronology overestimates the increase of age with depth by 12% for ages older than 112 ky BP. This modified age-depth scale gives an age of 244 ky BP at 2755 m depth and agrees well with the age-depth scale of Walbroeck et al. (in press) derived by orbital tuning of the Vostok D record. We discuss the temperature interpretation of this latter record accounting for the influence of the origin of the ice and using information derived from deuterium-excess data. We conclude that the warmest period of stage 7 was likely as warm as today in Antarctica. A remarkable feature of the Vostok record is the high level of similarity of proxy temperature records for the last two climatic cycles (stages 6 and 7 versus stages 1–5). This similarity has no equivalent in other paleorecords.     

Regional and Global Tropopause Fold Occurrence and Related Ozone Flux Across the Tropopause

This paper gives a synthesis of three algorithms to detect the presenceof tropopause folds from vertical ozone/radio-sounding profiles and frommeteorological analysis. Also an algorithm to identify injection ofstratospheric air into the lower troposphere fromozone/⁷beryllium time series is presented. Differences in theresults obtained from the algorithms are observed and discussed with respectto the criteria for fold detection and input data used. Spatial gradients inthe obtained folding frequencies are made evident on a global scale from thealgorithm based on meteorological analysis (Q-vector/potential vorticity)and probably also on a regional European scale from algorithms both basedmeteorological analyses and on ozone/PTU soundings. The observed seasonalvariation of folding occurrence is rather flat except during summer whenalso some differences appear between the algorithms. By combining thefolding frequencies with literature estimates of the cross-tropopause ozonetransfer in single folding events, an average stratospheric ozone influxinto the troposphere of 5.7 ± 1.3× 10¹⁰ mol.cm^-2 s^-1 is obtained for the Northern hemisphereand 12± 2.7× 10¹⁰ mol. cm^-2s^-1 for Western Europe. Potential additional contributions dueto other stratosphere-troposphere exchange processes than folds are not yetincluded in these estimates. Finally, the link between statistics fromozone/⁷beryllium data and folding statistics is brieflydiscussed.     

Climate stability and the development of agricultural societies

Although Modern Man had developed long before the migration from Africa began ～ 55,000 years ago no agricultural societies developed until about ～ 10,000 years ago. In the next 5,000 years agricultures developed independently in at least six regions of the world. It is virtually certain that it was not a chance occurrence that so many new agricultures appeared in the same 5,000 years. What inhibited agriculture world wide for 44,000 years and what changed ～ 10,000 years ago? Here we suggest that a major factor influencing the development of agricultural societies was climate stability. From the experience of four cultures we estimate that the development of agriculture needed ～ 2,000 years of climate free from significant climate variations on time scales of a few centuries. Using the Empirical Mode Decomposition technique specifically designed to exhibit the time history of the amplitude of variations in non-stationary time series such as climate proxy records, we find that between 50,000 years ago and the termination of the Younger Dryas ～ 11,600 years ago there was probably no time span as long as 2,000 years that was free of relatively large century scale variations. Furthermore variations on these time scales appear to have been relatively small since the Younger Dryas (YD) ended, supporting our proposition concerning the importance of climate stability in the history of human culture.     

An Experimental Study on the Temperature Dependence for the Gas-Phase Reactions of NO3 Radical with a Series of Aliphatic Aldehydes

The absolute rate constants for the gas-phasereactions of the NO₃ radical with a series ofaldehydes such as acetaldehyde, propanal, butanal,pentanal, hexanal and, heptanal were measured overthe temperature range 298–433 K, using a dischargeflow system and monitoring the NO₃ radical byLaser Induced Fluorescence (LIF).The measured rate constants at 298 K for thereaction of NO₃, in units of 10^–14 cm³molecule^–1 s^–1, were as follows:acetaldehyde 0.32 ± 0.04, propanal 0.60 ± 0.06, butanal 1.46± 0.16, pentanal 1.75 ±0.06, hexanal 1.83 ± 0.36, and heptanal 2.37 ±0.42. The proposed Arrhenius expressions arek₁ = (6.2 ± 7.5) × 10^–11 exp[–(2826 ± 866)/T] (cm³ molecule^–1s^–1),k₂ = (1.7 ± 1.0) × 10^–11 exp[–(2250 ± 192)/T] (cm³ molecule^–1s¹), k₃ =(7.6 ± 9.8) × 10¹¹ exp[–(2466 ± 505)/T] (cm³ molecule^–1s^–1),k₄ = (2.8 ± 1.4) × 10^–11 exp[–(2189 ± 156)/T] (cm³ molecule^–1s^–1), k₅ = (7.0 ± 1.8) ×10^–11 exp [–(2382 ± 998)/T](cm³ molecule^–1 s^–1), andk₆ = (7.8 ± 1.0) × 10^–11 exp[–(2406 ± 481)/T](cm³ molecule^–1 s^–1).Tropospheric lifetimes for these aldehydes werecalculated at night and during the day for typicalNO₃ and OH average concentrations and showed thatboth radicals provide an effective tropospheric sinkfor these compounds and that the night-time reactionwith the NO₃ radical can be an important, if notdominant, loss process for these emitted organics andfor NO₃ radicals.