Climatic variations in China during the quaternary

Climatic variations in China during the Quaternary have long been a question of debate. Considerable data now available support Professor Li Siguang's (J. S. Lee) suggestion that there were four glacial periods. In this later years the views of Professor Li have received support from more recent information demonstrating that a further glacial period and periglacial phenomena preceded the Boyang Period.During the Quaternary 3,500,000 years ago there were a number of climatic fluctuations in China, having an amplitude of over 10°C and a cycle of 10⁴ to 10⁵ years. Each fluctuation lasted from the onset of one glacial period to that of the next, encompassing a glacial period and an interglacial. Tab 1 provides a comparison between the glacial periods in China during the Quaternary and those in other regions of the world.     

U-Pb systematics of garnet: dating the growth of garnet in the late Archean Pikwitonei granulite domain at Cauchon and Natawahunan Lakes,Manitoba, Canada

This study considers the potential of using the U-Pb dating of garnet for determining quantitative P-T-t paths for the late Archean metamorphism in the Pikwitonei granulite domain. Garnets for U-Pb dating were selected mainly from samples that also provide information on pressure and temperature. The garnets used for dating were clear and free of any visible inclusions. Pb concentrations range from 63 ppb to 966 ppb and U from 136 ppb to 1143 ppb. The measured ²⁰⁶Pb/²⁰⁴Pb ratios range from 52.8 to 529.4. The ages are generally discordant with U/Pb ages that may lie above or below concordia. The discordance is caused by a recent disturbance of the U/Pb ratio in the garnets as indicated by replicate analyses on the same garnet separates that reproduce ²⁰⁷Pb/²⁰⁶Pb ages well within analytical uncertainty and in most cases within ±1.5 Ma at 2600–2750 Ma. High grade metamorphism continued over a period of at least one hundred million years, but the garnet-K-feldspar Pb-Pb ages suggest that, during this time, garnet growth has been favored during three distinct periods in the Cauchon Lake area: 2700–2687 Ma 2660–2637 Ma 2605–2591 Ma The ca. 2695 Ma garnet ages from Cauchon Lake date the time of melting and staurolite breakdown during prograde metamorphism, the ca. 2640 Ma ages date the time of extensive migmatization and the last period of metamorphic garnet growth, the ca. 2600 Ma ages date the time of crystallization of igneous garnet in late granitic intrusions. Peak metamorphism occurred around 2640 Ma followed by the intrusions of pegmatites starting at 2629 Ma. The Pb-Pb ages for garnet are similar to the U-Pb ages for zircon that date a leucocratic mobilizate (2695 Ma), a plagioclaseamphibole mobilizate (2637 Ma) and pegmatite (2598 Ma) (Heaman et al. 1986 a; Krogh et al. 1986; this study). Xenocrysts of garnet from 2600 Ma old graphic granites give minimum ages of 2984 Ma and 2741 Ma which are minima for the times of garnet growth in the source of the granites. The agreement of the zircon and garnet ages suggests that the metamorphism may have been punctuated by events that led to the development of melts or encouraged mineral growth at specific times. If so, the prograde and retrograde paths of metamorphism in the area may have contained minor excursions in pressure, temperature or fluid fugacities. In the Natawahunan Lake area some 50 km northwest of Cauchon Lake, garnet growth associated with the prograde breakdown of staurolite occurred at ca. 2744–2734 Ma. This suggests that a similar style of metamorphism may have occurred earlier in the Natawahunan Lake area than at Cauchon Lake area, or higher grades of metamorphism were reached earlier and were of longer duration associated with the somewhat greater depths in the Natawahunan Lake area. These results indicate the these garnets, which are 0.1–1 cm in diameter, have maintained closed system behavior for U and Pb at peak metamorphic conditions, i.e. temperatures up to 800° C and pressures of 7.5 kb.     

Late Pleistocene paleoclimates of North America as inferred from stable isotope studies of speleothems

Some speleothems (CaCO₃ cave deposits) can be demonstrated to have been formed in oxygen isotopic equilibrium with their parent seepage waters and thus a record of relative fluctuations in depositional temperature can be obtained from the δ¹⁸O variations in successive growth layers of such deposits. These temperature fluctuations reflect variations in the average annual air temperature at the surface above the cave, and therefore permit inference of past continental climate changes. Equilibrium deposits have been obtained from caves in San Luis Potosi, Bermuda, Kentucky, West Virginia, Iowa, and Alberta, ranging in age from 200,000 years BP to the present, as determined by ${}^{230}\text{Th}{}^{234}\text{>U}$ dating of the speleothems. The δ¹⁸O time curves for the six sites show the following synchronous climatic fluctuations: warm periods from 190 to 165 and from 120 to 100 Ka, at 60 and 10 Ka, and cold periods from 95 to 65 and from 55 to 20 Ka. The periods of thermal maxima correspond in time to the interglacial periods of the marine foraminiferal isotopic and faunal temperature records and to periods of high sea stand as observed from radiometric dating of raised coral reefs. Maxima and minima in insolation appear to be synchronous with this record as well.     

Solar insolation driven periodicities in southwest monsoon and its impact on NE Arabian Sea paleoceanography

In the northeastern(NE) Arabian Sea,the fluctuation in terrestrial and freshwater runoff directly depends on southwest monsoon(SWM) precipitation as well as the meltwater flux provided by Indus River.Therefore,analysis of multi-proxy spectral signatures was carried out to trace the high-resolution SWM periodicities and their influence on the productivity,regional sea level fluctuations and depositional processes in the NE Arabian Sea.The time series data of stable isotopes of oxygen(δ~(18)O_(G.ruber)).carbon(δ~(13)C_(G.ruber) and δ~(13)C_(org)) and nitrogen(δ~(15)N).Total Organic Carbon(TOC),planktic-benthic foraminiferal ratio(P/B ratio) and63 μm coarse fraction(CF) were used from two coastal sedimentary cores located offshore Saurashtra,NE Arabian Sea(Core SK-240/485 having 88 m water depth;Core GC/SK-240/496 having 174 m water depth).The REDFIT based spectral analysis recorded significant periodicities(90% significance) in δ~(18)0 time series centered at~1609,~667,~525,~296,~290 and~256 years.Further,the significant periodicities recorded in carbon isotopes time series(δ~(13)C_(G.ruber) and δ~(13)C_(org))centered at~681,~512,~471,~452,~438,~360,~292,~275,~269,~245 and~209 years.The significant periodicities in TOC include~471 and-322 years whereas δ~(15)N time series recorded significant periodicity centered at-360 years.The significant periodicities in P/B ratio time series centered at~512,~388,~304,~250,~235,~217,~152,~139 and~135 years while CF recorded~268,~216,~209,~198,~188,~173 and~140 years significant periodicities.The observed periodicities in the multi-proxy record consist of similar cycles(within the radiocarbon dating error) which also natch with previously reported solar insolation influenced SWM and other global and regional cycles.Further,the stationarity of the data has been verified using wavelet analysis and shows similar periodicities as observed in REDFIT analysis.Thereafter,the depositional behaviour was studied using correlation analysis of the common periods of δ~(18)0 time series of both the cores.The result suggests that the depositional behaviour was different for both the core sites during the early Holocene and became similar during the middle Holocene.The correlation analysis of Total Solar Index(TSI) with δ~(18)O time series reveals a significant correlation with the core SK-240/485 whereas an insignificant correlation with the core GC/SK-240/496.These observations suggest that the solar insolation has been a leading factor responsible for the SWM trends during the Holocene which may have further influenced the productivity.regional sea level fluctuations and depositional conditions in the NE Arabian Sea.However,these trends are better preserved in shallow marine sediments as compared to the deeper marine sediments.     

Holocene glacial and tree-line variations in the White River Valley and Skolai Pass,Alaska and Yukon Territory

Complex glacier and tree-line fluctuations in the White River valley on the northern flank of the St. Elias and Wrangell Mountains in southern Alaska and Yukon Territory are recognized by detailed moraine maps and drift stratigraphy, and are dated by dendrochronology, lichenometry, ¹⁴C ages, and stratigraphic relations of drift to the eastern (1230 ¹⁴C yr BP) and northern (1980 ¹⁴C yr BP) lobes of the White River Ash. The results show two major intervals of expansion, one concurrent with the well-known and widespread Little Ice Age and the other dated between 2900 and 2100 ¹⁴C yr BP, with a culmination about 2600 and 2800 ¹⁴C yr BP. Here, the ages of Little Ice Age moraines suggest fluctuating glacier expansion between ad 1500 and the early 20th century. Much of the 20th century has experienced glacier recession, but probably it would be premature to declare the Little Ice Age over. The complex moraine systems of the older expansion interval lie immediately downvalley from Little Ice Age moraines, suggesting that the two expansion intervals represent similar events in the Holocene, and hence that the Little Ice Age is not unique. Another very short-lived advance occurred about 1230 to 1050 ¹⁴C yr BP. Spruce immigrated into the valley to a minimum altitude of 3500 ft (1067 m), about 600 ft (183 m) below the current spruce tree line of 4100 ft (1250 m), at least by 8020 ¹⁴C yr BP. Subsequent intervals of high tree line were in accord with glacier recession; in fact, several spruce-wood deposits above current tree line occur bedded between Holocene tills. High deposits of fossil wood range up to 76 m above present tree line and are dated at about 5250, 3600 to 3000, and 2100 to 1230 ¹⁴C yr BP. St. Elias glacial and tree-line fluctuations, which probably are controlled predominantly by summer temperature and by length of the growing and ablation seasons, correlate closely with a detailed Holocene tree-ring curve from California, suggesting a degree of synchronism of Holocene summer-temperature changes between the two areas. This synchronism is strengthened by comparison with the glacier record from British Columbia and Mt. Rainier, Likewise, broad synchronism of Holocene events exists across the Arctic between the St. Elias Mountains and Swedish Lappland. Finally, two sequences from the Southern Hemisphere show similar records, in so far as dating allows. Hence, we believe that a preliminary case can be made for broad synchronism of Holocene climatic fluctuations in several regions, although further data are needed and several areas, particularly Colorado and Baffin Island, show major differences in the regional pattern.     

14C‐dated fluctuations of the western flank of the Scandinavian Ice Sheet 45–25 kyr BP compared with Bølling–Younger Dryas fluctuations and Dansgaard–Oeschger events in Greenland

Mangerud, J., Gulliksen, S. & Larsen, E. 2009: ¹⁴C‐dated fluctuations of the western flank of the Scandinavian Ice Sheet 45–25 kyr BP compared with Bølling–Younger Dryas fluctuations and Dansgaard–Oeschger events in Greenland. Boreas, 10.1111/j.1502‐3885.2009.00127.x. ISSN 0300‐9483. We present 32 accelerator mass spectrometry (AMS) ¹⁴C dates obtained on well‐preserved bones from caves in western Norway. The resulting ages of 34–28 ¹⁴C kyr BP demonstrate that the coast was ice‐free during the so‐called Ålesund Interstadial. New AMS ¹⁴C dates on shells aged 41–38 ¹⁴C kyr BP are evidence of an earlier (Austnes) ice‐free period. The Ålesund Interstadial correlates with Greenland interstadials 8–7 and the Austnes Interstadial with Greenland interstadials 12–11. Between and after the two interstadials, the ice margin reached onto the continental shelf west of Norway. These events can be closely correlated with the Greenland ice core stratigraphy, partly based on identification of the Laschamp and Mono Lake palaeomagnetic excursions. We found that the pattern of the NGRIP δ¹⁸O curves for the two periods Greenland Interstadial (GI) 8 to Greenland Stadial (GS) 8 and GI 1–GS 1 (Bølling–Younger Dryas) were strikingly similar, which leads us to suggest that the underlying causes of these climate shifts could have been the same. We therefore discuss some aspects of glacial fluctuations during the Bølling–Younger Dryas in order to elucidate processes during Dansgaard–Oeschger events.     

Fluctuations in circum‐pacific volcanic activitSy

Graphs of the yearly number of volcanoes in eruption from 1900 through 1968 for 21 volcanic areas of the circum‐Pacific belt show fluctuations with statistically significant amplitudes in eight of the areas, and some indication of auto‐correlation and periodicity in 14 of them. Thus, in a total of 16 out of the 21 areas studied, these volcanic pulses appear to be statistically significant. They are best defined in the New Guinea‐Solomons area, Santa Cruz‐New Hebrides‐Matthew Island area, and the West Indies where the numbers of volcanoes in eruption have significant fluctuations in amplitude, are auto‐correlated and tend to be periodic.

The periods (around 17 years) of the western Pacific areas are, with few exceptions, much less than those of the eastern Pacific areas suggesting an overall first‐order cause for these volcanic fluctuations which, in each area, seem to be due to the volcanoes’ responding to regional stress pulses. These may be caused by variations in stress owing to sea‐floor spreading on the East Pacific Rise, the dichotomy of periods being due to a different mantle‐flow regime on either side of it, resulting from the proximity of the Rise to the eastern Pacific margin.

An examination of the years for which the maximum or one less than the maximum number of volcanoes are in eruption suggests that large pulses of volcanic activity tend to migrate southwards.

There are six main pulses in the western Pacific and three in the eastern. The latter are better‐defined and more widely spaced in time. The pulse occurring in the first decade of this century is simultaneous on both sides of the Pacific, a reflexion of the global tectonic instability of this period.

The reason for this southward migration of the volcanic pulses is not yet known. It may be a strain‐release phenomenon.     

Radiocarbon Dating of Interdune Paleo‐Wetland Deposits to Constrain the Age of Mid‐to‐Late Holocene Microlithic Artifacts from the Zhongba site,Southwestern Qinghai‐Tibet Plateau

Microlithic artifacts, some found in situ, are abundant in the Zhongba archaeological site in southwestern Tibet. The site environment consists of extant wetlands and paleo‐wetland deposits found in depressions between sand dunes derived from the Yarlung Tsangpo floodplain. Constraining ¹⁴C dates from wetland vegetation and shell from one site fall between ca. 6600–2600 cal. yr B.P., while a second site is dated 3400–1200 cal. yr B.P. A significant and variable ¹⁴C reservoir effect—up to 1400 ¹⁴C yr—limits these ranges to terminus post quem constraints. The in situ artifacts are supplemented by surface collections fully characterizing raw material and typological variability for each site. Raw material found at Zhongba is chert and chalcedony likely sourced from Cretaceous bedrock near the site. Typologically, microblades are nongeometric and are derived from conical and wedge‐shaped cores similar to those identified in the Qinghai Lake Basin and the Chang Tang Nature Reserve of similar or greater age. The later occupation period at Zhongba is broadly contemporaneous with sites on the Qinghai‐Tibet Plateau containing bronze and iron artifacts, indicating microlithic technology remained an important tool‐making strategy in western Tibet late into the protohistoric period.     

Climate of the last 53,000 Years in the eastern Mediterranean, based on soil-sequence Stratigraphy in the coastal plain of Israel

In this paper we present a detailed record of proxy-climatic events in the coastal belt of the eastern Mediterranean during the past 53,000 years. A sequence of alternating palaeosols, aeolianites, and dune sands, which have been dated by luminescence and by ¹⁴C, was studied by the magnetic susceptibility, particle-size distribution, clay mineralogy and soil micromorphology. Thirteen proxy-climatic events, demonstrating fluctuations of relatively dry and wet episodes, were recognized. The soil parent materials, as well as the different soil types, were rated in a semi-quantitative “dry” to “wet” scale. The palaeosol sequence is compared to a proxy-climatic record of oxygen and carbon isotopes in speleothems from a karstic cave in central Israel and to a record of lake levels of Lake Lisan and its successor, which is known as the Dead Sea. A genuine red Mediterranean Soil (Rhodoxeralfs), localy designated as “Hamra Soil” developed during the Last Glacial Stage, from 40 to 12.5 thousand calendar years BP. Climatic fluctuations that were recorded in speleothems and in changing lake levels were not preserved in this soil. During the cold and dry Younger Dryas, ca 12.5 to 11.5 calendar ka BP, a thick bed of loess material, deriving from atmospheric dust of the Sahara and Arabian deserts, covered the entire coastal belt. During this phase Lake Lisan was desiccated and turned into the modern, smaller Dead Sea. During the early Holocene, some 10–7.5 calendar ka BP, a second Red Hamra soil developed in warm and wet environments, associated with a relatively high stand of the Dead Sea level. A depletion of δ¹⁸O and a significant enrichment of δ¹³C in the speleothems were recorded during this episode. This event was in phase with the widespread distribution of freshwater lakes in the Sahara Desert and the accumulation of the S1 Sapropel in the eastern Mediterranean. Several small-scale dry and somewhat wet fluctuations of the Late Holocene, from 7.5 calendar ka BP to the present, were recorded in the coastal belt. Changes in human history, as reflected in archaeological records, are associated with proxy-climatic fluctuations. Periods of desertification and deterioration are coupled with dry episodes; periods of relative human prosperity are coupled with wetter episodes.     

Vegetation change in the coastal-lowland rainforest at Avai'o'vuna Swamp, Vava'u, Kingdom of Tonga

Avai'o'vuna Swamp, a small coastal wetland in Vava'u, Kingdom of Tonga, produced a 4500-year pollen and sediment record. Results are: (1) a mid-Holocene sea level highstand is confirmed for Tonga between about 4500 and 2600 ¹⁴C yr B.P.; marine clay contains pollen from mangroves (Rhizophora mangle), coastal forest trees (Barringtonia asiatica and Cocos nucifera), and rainforest trees (Alphitonia, Rhus, Hedycarya and Calophyllum). (2) Microscopic charcoal first appeared at 2600 ¹⁴C yr B.P., coincident with the arrival of Polynesians. (3) Cocos, Pandanus, Excoecaria, Macaranga, and Elaeocarpaceae pollen reflects the establishment of a mixed coastal-lowland rainforest in the last 2500 years. (4) The loss of Hedycarya, Elaeocarpus, Calophyllum, and Guettarda and the reduction of Terminalia and taxa in the Papilionaceae family by about 1000 years ago may be due to habitat destruction and the loss of dispersal capabilities of some species through the extinction of the two largest pigeons in Tonga.     

The Iberian-Roman Humid Period (2600-1600 cal yr BP) in the Zoñar Lake varve record (Andalucía, southern Spain)

The Iberian-Roman Humid Period (IRHP, 2600-1600 cal yr BP), is the most humid phase of the last 4000 yr in southern Spain as recorded in the sedimentary sequence of Zoñar Lake (37°29′00″N, 4°41′22″ W, 300 m a.s.l.). A varve chronology supported by several AMS ¹⁴C dates allows study of the lake evolution at annual scale in response to human impact and climate changes. There are four climate phases within this period: i) gradual transition (2600-2500 yr ago, 650-550 BC) from a previous arid period; ii) the most humid interval during the Iberian-Early Roman Epoch (2500-2140 yr ago, 550-190 BC); iii) an arid interval during the Roman Empire Epoch (2140-1800 yr ago, 190 BC AD 150); and iv) a humid period synchronous with the decline of the Roman Empire (1800-1600 yr ago, AD 150-350). Varve thickness and geochemical proxies show a multi-decadal cyclicity similar to modern North Atlantic Oscillation (NAO) (60, 20 years) and solar variability cycles (11 yr). The timing and the structure of this humid period is similar to that described in Eastern Mediterranean and northern European sites and supports the same large-scale climate control for northern latitudes and the Mediterranean region.     

Isolation and water-level fluctuations of Lake Kachishayoot, Northern Québec, Canada

This study combines different methods, including grain size, macrofossil, and pollen analyses, to reconstruct paleogeographical and paleoclimatological conditions for periods before, during, and after the isolation of a small lake (Lake Kachishayoot) in northern Québec. After the retreat of the Laurentide Ice Sheet around 8000 ¹⁴C yr B.P., the area was submerged by the Tyrrell Sea. The transition from marine to lacustrine environment occurred about 5400 yr B.P. Two major periods of water-level fluctuations were inferred from organic and mineral sediments: a high water level that occurred after 3200 yr B.P. and a low water level that started before 2200 yr B.P. Our chronological data for the first period are consistent with those from nearby Lac des Pluviers and from other lakes in east central Canada and in the northeastern United States. During the low-water-level period, however, there is no evidence for minor fluctuations, whereas other lakes in northern Québec and east-central Canada underwent several brief lowerings. Long-term changes in atmospheric circulation caused by changing global boundary conditions likely explained long-term water-level fluctuations of Lake Kachishayoot.     

Thermoluminescence and hydroxyl defects associated with broad-band infra red absorption in synthetic quartz

A study has been made of thermoluminescence from synthetic quartz with varying hydroxyl impurity concentrations up to approximately 300 H/10⁶ Si which are associated with a “broad-band” IR absorption in the range 2600–3700 cm⁻¹. These hydroxyl defects are known to be important in the hydrolytic weakening of quartz. We have found only minor differences in the glow curves of unheated crystals but significant intensity increases when “wet” crystals are heated sufficiently to cause bubble formation. It would seem that the electron traps are unaffected by the bubble formation, but the electron/luminescence centre radiative recombination probability is increased.     

Oxygen isotopes of bovid teeth as archives of paleoclimatic variations in archaeological deposits of the Ganga plain, India

Oxygen isotope analysis was performed on enamel phosphate of mammalian teeth from archaeological sites Kalli Pachchhim and Dadupur in the central Ganga plain and Charda in the northern Ganga plain. The bulk oxygen isotopic compositions of enamel phosphate from third molars (M3) of Bos indicus individuals belonging to different cultural periods were used to understand the climatic changes during the past 3600 cal yr B.P. Oxygen isotope ratios indicate humid conditions around 3600 cal yr B.P., followed by a trend toward drier conditions until around 2800 cal yr B.P. Then from 2500 to 1500 cal yr B.P. there is a trend toward higher humidity, followed by the onset of a dry period around 1300 cal yr B.P. The study of intratooth δ¹⁸O variations in teeth from different periods demonstrates that the monsoon seasonality was prominent. Spatial changes in the amount of annual rainfall are also reflected in the δ¹⁸O values. Teeth derived from areas with intense rainfall have lighter isotope ratios compared to teeth from regions receiving less rain, but they show similar seasonal patterns. The long-term paleoclimatic variations reflected by fluctuations in bulk δ¹⁸O_p values from M3 teeth match well with the regional paleoenvironmental records and show a good correlation to the cultural changes that took place during this time span in Ganga plain.     

Variations in the wind‐energy regime of the Taklimakan Desert,central Asia,over the last 700 years as inferred from nebkha sedimentology and chronology

Changes in the wind‐energy environment between AD 1359 and 2010 in the Taklimakan Desert (central Asia) are recorded by the evolution of Chinese tamarisk (Tamarix taklamakanensis) nebkhas. The carbonate component and sedimentological properties of the nebkha excavated during the study, together with AMS ¹⁴C dating control, indicate that significant regional environmental changes have occurred in the central Taklimakan Desert during the last 700 years. The nebkha data presented show that in the periods of AD c. 1480–1560, c. 1640–1690, c. 1760–1820, c. 1860–1930 and c. 1970–1980 the Taklimakan Desert was a relatively high wind‐energy environment. Although changes in the wind‐energy regime in the desert were mainly in phase with fluctuations of the Siberian High, the wind systems and the variations in wind energy exhibit slight differences when compared with the Tarim Basin. Nebkhas that develop in this region originate from the surfaces of mobile dunes or sand sheets, which differs from the origins of nebkhas found in other arid regions of China.     

Hydrologic scenarios applied to the agricultural management of the northwest of the Buenos Aires Province,Argentina

The study area is located in the Pampean region of Argentina, in the northwest of Buenos Aires Province, covering about 50,000 km², within an area of great plain with particular hydrologic characteristics, where vertical water movements predominate over horizontal. The objective of this work is the analysis of the relations between different hydrologic scenarios (characterized by the soil water balance and the water table levels) and the yields of soybeans that have been increasing in recent years. It is concluded that water table fluctuations are strongly related to the elements of the local hydrologic balance. On the other hand, the geographic distribution of soybean yields corresponds directly with the values of the soil water reserve during the critical stages of the growth. After dry periods, the lowering of the water table makes access to water from the root zone difficult, and the crop is susceptible to the effects of drought. However, after wetter periods the extra contribution from the water table situated near the surface is favorable for growth, often preventing its deterioration in conditions of drought. Nevertheless, an excessive rise of the water table can produce disadvantages at seedtime and during harvest. The importance of an evaluation of the different scenarios that may have significant implications for agricultural activity is discussed.     

Late Holocene environmental reconstruction using cave sediments from Belize

Cave sediments collected from Reflection Cave on the Vaca Plateau, Belize show variations in the δ¹³C values of their fulvic acids (FAs), which indicate periods of vegetation change caused by climatic and Maya influences during the late Holocene. The δ¹³C values range from − 27.11‰ to − 21.52‰, a shift of ∼ 5.59‰, which suggests fluctuating contributions of C₃ and C₄ plants throughout the last 2.5 ka, with C₄ plant input reflecting periods of Maya agriculture. Maya activity in the study area occurred at different intensities from ∼ 2600 cal yr BP until ∼ 1500 cal yr BP, after which agricultural practices waned as the Maya depopulated the area. These changes in plant assemblages were in response to changes in available water resources, with increased aridity leading to the eventual abandonment of agricultural areas. The Ix Chel archaeological site, located in the study area, is a highland site that would have been among the first agricultural settlements to be affected during periods of aridity. During these periods, minimal water resources would have been available in this highly karstified, well-drained area, and supplemental groundwater extraction would have been difficult due to the extreme depth of the water table.     

Little Ice Age fluctuations of Glaciar Río Manso in the north Patagonian Andes of Argentina

Little Ice Age (LIA) fluctuations of Glaciar Río Manso, north Patagonian Andes, Argentina are studied using information from previous work and dendrogeomorphological analyses of living and subfossil wood. The most extensive LIA expansion occurred between the late 1700s and the 1830-1840s. Except for a massive older frontal moraine system apparently predating ca. 2240 ¹⁴C yr BP and a small section of a south lateral moraine ridge that is at least 300 yr old, the early nineteenth century advance overrode surficial evidence of any earlier LIA glacier events. Over the past 150 yr the gently sloping, heavily debris-covered lower glacier tongue has thinned significantly, but several short periods of readvance or stasis have been identified and tree-ring dated to the mid-1870s, 1890s, 1900s, 1920s, 1950s, and the mid-1970s. Ice mass loss has increased in recent years due to calving into a rapidly growing proglacial lake. The neighboring debris-free and land-based Glaciar Frías has also retreated markedly in recent years but shows substantial differences in the timing of the peak LIA advance (early 1600s). This indicates that site-specific factors can have a significant impact on the resulting glacier records and should thus be considered carefully in the development and assessment of regional glacier chronologies.     

Norwegian sea‐surface palaeoenvironments of marine oxygen‐isotope stage 3: the paradoxical response of dinoflagellate cysts

High‐resolution marine palynological data have been obtained from two very long sediment cores (MD952009 and MD952010) retrieved from the southern Norwegian Sea. The dinoflagellate cyst assemblages show pronounced fluctuations in composition, which correlate strongly with magnetic susceptibility records and also mimic the δ¹⁸O signal of the GISP2 Greenland ice‐core. If focusing on the period from 48 to 30 cal. kyr BP, this correlation suggests a paradoxical response of the sea‐surface environments to the atmospheric conditions over Greenland: when the Greenland δ¹⁸O signal reflects warm interstadial conditions, the Norwegian Sea depicts cold sea‐surface temperatures with quasi‐perennial sea‐ice cover (based on dinoflagellate cysts). In contrast, when the Greenland δ¹⁸O records cold stadial periods, the Norwegian Sea‐surface temperatures are warm (based on dinoflagellate cysts), probably linked to inflow of the North Atlantic Drift. These results, similar in both cores, are contrary to those of previous studies and shed light on a possible decoupling of Norwegian sea surface‐water conditions and atmospheric conditions over Greenland. This decoupling could be linked to an atmosphere–ocean system behaving similar to that which the Northern Hemisphere is experiencing at present, i.e. strongly variable owing to the North Atlantic Oscillation. Copyright © 2002 John Wiley & Sons, Ltd.     

Floodplain formation processes and archaeological implications at the Grand Banks site,lower Grand River,southern, Ontario

Processes of floodplain development and the record of Princess Point cultural occupation (A.D. 500–1000) were examined at the Grand Banks site in the lower Grand River of southern Ontario. The Princess Point Complex of the early Late Woodland is significant because it represents the first shift to horticulture in this region in which inhabitants made significant use of floodplains. The floodplain of the lower Grand River has been constructed primarily via vertical accretion of sediment in a low energy environment conducive to limited erosion and slow burial of middle and late Holocene sediments. At this site, cultural materials are preferentially preserved in two buried soils each corresponding to relatively stable periods of valley infilling at or before 3200 B.P. and 1500 B.P. (¹⁴C years). Initial formation of the floodplain and subsequent stability of the floodplain surface can be tied to middle Holocene, and later, base-level fluctuations in Lake Erie. Understanding floodplain development is crucial in determining the linkages between settlement pattern and chronology, and, conversely, the archaeological record in floodplain settings provides important contemporary data for modeling floodplain geomorphological processes. © 1997 John Wiley & Sons, Inc.