Paleoceanography of surface waters in the Gulf of Mexico during the late Quaternary

Late Quaternary paleotemperatures and paleosalinities of surface waters of the Gulf of Mexico were estimated using a multivariate statistical analysis of census data of planktonic foraminifera. Two climatic extremes were selected for detailed basinwide study, the climatic optimum 125,000 yr ago and the glacial maximum 18,000 yr ago. In addition, patterns of climatic change were examined in seven piston cores from 127,000 yr ago to the present day. During the climatic optimum 125,000 yr ago temperature distributions in surface waters were similar to those of the present. The 22°C winter isotherm trended northeastward across the central basin and paleotemperatures decreased northward. Summer distributions were nearly homogeneous and ranged between 28° and 29°C. Winter salinities were 1‰ fresher than present values in the northmost Gulf and 0.4‰ fresher in the central basin. Summer salinities were similar during both times. In contrast, during the last glacial maximum temperatures were 1° to 2°C cooler in winter and 1°C cooler in summer, and isotherms formed a circular pattern in the Gulf during both seasons. Salinity was 0.3‰ fresher in winter than at present but 0.6‰ saltier in summer. Conditions deteriorated from the climatic optimum to the glacial maximum. In the Mexico Basin, winter temperatures were 2°C cooler from 75,000 to 45,000 yr ago (Y6 to Y3 Subzones), summer temperatures reached a minimum (3°C cooler) 32,000 yr ago (Y2–Y3 boundary), and seasonality reached minimal values (5°C) from 45,000 to 15,000 yr ago. All three parameters became similar in value to those in the Straits of Florida from 45,000 to 15,000 yr ago, suggesting that the exchange of surface waters was enhanced at this time between the two regions. Summer salinities remained similar to present conditions in the Mexico Basin, whereas, winter salinities increased 2‰ by 32,000 yr ago and then fell 0.5‰ until the glacial maximum ended. The Westerlies may have migrated southward over the Mexico Basin in winter from 32,000 to 15,000 yr ago.     

Palaeoceanographic change in the northeastern South China Sea during the last 15000 years

The sedimentary succession of piston core RC26-16, dated by ¹⁴C accelerator mass spectrometry, provides a nearly continuous palaeoceanographic record of the northeastern South China Sea for the last 15000 yr. Planktic foraminiferal assemblages indicate that winter sea-surface temperatures (SSTs) rose from 18°C to about 24°C from the last glacial to the Holocene. A short-lived cooling of 1°C in winter temperature centred at about 11000 ¹⁴C yr ago may reflect the Younger Dryas cooling event in this area. Summer SSTs have remained between 27°C and 29°C throughout the record. The temperature difference between summer and winter was about ca. 9°C during the last glacial, much higher than the Holocene value of ca. 5°C. During the late Holocene a short-lived cooling event occurred at about 4000 ¹⁴C yr ago. Oxygen and carbon isotopic gradients between surface (0–50 m) and subsurface (50–100 m) waters were smaller during the last glacial than those in the Holocene. The fluctuation in the isotopic gradients are caused most likely by changes in upwelling intensity. Smaller gradients indicate stronger upwelling during the glacial winter monsoon. The fauna-derived estimates of nutrient content of the surface waters indicate that the upwelling induced higher fertility and biological productivity during the glacial. The winter monsoon became weaker during the Holocene. The carbonate compensation depth and foraminiferal lysocline were shallower during the Holocene, except for a short-lived deepening at about 5000 ¹⁴C yr ago. A preservation peak of planktic foraminifera and calcium carbonate occurred between 13400 and 12000 ¹⁴C yr ago, synchronous to the global preservation event of Termination I.     

Paleoclimatology and paleo-oceanography of the Norwegian and Greenland seas: glacial-interglacial contrasts

Synoptically mapped faunal abundance and faunal composition data, derived from a suite of 24 Norwegian Sea cores, were used to derive sea-surface temperatures for the last glacial maximum (18,000 B.P.), the last interglacial (120,000 B.P.), and isotope stage 5a (82,000 B.P.). Surface circulation and ice cover reconstructions for these three times, deduced from the sea-surface temperatures, suggest the following conclusions: (1) During glacial periods, Norwegian Sea surface circulation formed a single, sluggish, counterclockwise gyre that was caused by wind drag on the ubiquitous sea ice cover; (2) the last interglacial was characterized by a circulation pattern similar to that of today except that the two counterclockwise gyres were displaced toward the east and were more vigorous than they are today. This circulation pattern forced the Norwegian Current into a position close to the coast of Norway and permitted formation of a strong east-west temperature gradient close to the Scandinavian landmass; (3) interglacial periods prior to 120,000 B.P. had similar climatic conditions to the 82,000 B.P. level and were characterized by a weak two-gyre circulation pattern. The southern gyre, driven by wind stress in summer months, was ice covered in winters. The northern gyre had little open water even in summers and was primarily formed by wind drag on sea ice. Atmospheric modifications resulting from these circulation patterns and sea ice conditions produced varying climatic conditions in Scandinavia during interglacials prior to the Holocene. The climate was probably warmer and moister during the last interglacial (Eemian) than it is today. Other interglacials during the last 450,000 years, but prior to the Eemian, were probably colder and drier as the Norwegian Sea was not an important source of heat and moisture.     

Pollen and radiolarian records from deep-sea core RC14-103: Climatic reconstructions of northeast Japan and Northwest Pacific for the last 90,000 years

Using modern pollen and radiolarian distributions in sediments from the northwest Pacific and seas adjacent to Japan to interpret floral and faunal changes in core RC14-103 (44°02′N, 152°56′E), we recognize two major responses of the biota of eastern Hokkaido and the northwest Pacific to climatic changes since the last interglaciation. Relatively stable glacial environments (～80,000–20,000 yr B.P.) were basically cold and wet (<4°C and ～1000 mm mean annual temperature and precipitation, respectively) with boreal conijers and tundra/park-tundra on Hokkaido, and cool (<16°C) summer and cold (<1.0°C) winter surface temperatures offshore. Contrasting nonglacial environments (～10,000–4000 yr B.P.) were warm and humid (>8°C and >1200 mm mean annual temperature and precipitation, respectively), supporting climax broadleaf deciduous forest with Quercus and Ulmus/Zelkova, with surface waters in the northwest Pacific characterized by warm (>1.5°C) winter and cold (10.4°–14.3°C) summer temperatures. Climatic evidence from RC14-103 shows a high degree of local and regional variation within the context of global climatic change. Correlative ocean and land records provide the detailed input necessary to assess local/regional responses to variations in other key elements (i.e., solar radiation, monsoonal variations) of the northeast Asian climate system.     

Late Quaternary sapropel sediments in the eastern Mediterranean Sea: Faunal variations and chronology

Distinctive planktonic foraminiferal assemblages which characterize particular late Quaternary sapropel layers in deep basin sediments from the eastern Mediterranean Sea have been identified using cluster analysis. Three distinct clusters allow for identification and intercore correlation of the nine sapropels deposited during the last 250,000 yr. Cluster 1, representing sapropel layers S1 and S9, exhibits low abundances of Neogloboquadrina dutertrei and high abundances of Globigerinoides ruber; Cluster 2, which groups S3, S5, and S7, contains high abundances of G. ruber, N. dutertrei, and Globigerina bulloides, and Cluster 3, which includes samples from S4, S6, and S8, is marked by extremely abundant N. dutertrei and G. bulloides, and rare G. ruber. Analysis of sedimentation rates in 14 cores reveals the following approximate ages for the sapropel layers: S2 = 52,000 yr B.P.; S3 = 81,000–78,000 yr B.P.; S4 = 100,000–98,000 yr B.P.; and S5 = 125,000–116,000 yr B.P. As previously suggested, sedimentation rates on the Mediterranean Ridge were determined to be relatively constant during the last 127,000 yr. In contrast, basin sedimentation rates have fluctuated markedly from lower rates during interglacial stage 5 to higher rates during the last glacial episode. These glacial/interglacial differences are most pronounced in the northern Ionian Basin, because of increased terrigenous sediment deposition during glacial episodes. Unusually high biogenic sedimentation rates occurred in an arc south of Crete during the deposition of sapropel S5, probably due to higher productivity in this region.     

Multiple deglaciations of the Hudson Bay Lowlands,Canada, since deposition of the Missinaibi (Last-integlacial?) formation

The stratigraphic record in the James and Hudson Bay Lowlands indicates that the sequence of glacial events at the geographical center of the 12.6 × 10⁶ km² Laurentide Ice Sheet may have been more complex than hitherto imagined. Isoleucine epimerization ratios of in situ and transported shells recovered from till and associated marine and fluvial sediments cluster into at least 4 discrete groups. Two alternative explanations of the data are offered, of which we strongly favor the first. Hypothesis 1: Setting the age of the “last interglacial” marine incursion, the Bell Sea, at 130,000 yr B.P. results in a long-term average diagenetic temperature for the lowlands of +0.6°C. Using this temperature enables us to predict the age of shells intermediate in age between the “last interglaciation” and the incursion of the Tyrrell Sea 8000 yr ago. Between these two interglacial marine inundations, Hudson Bay is predicted to have been free of ice along its southern shore about 35,000, 75,000, and 105,000 yr ago based on amino acid ratios from shells occurring as erratics in several superimposed tills and fluvial sediments. These results suggest (1) that traditional concepts of ice-sheet build-up and decay must be reexamined; (2) that “high” sea levels may have occurred during the Wisconsin Glaciation; and (3) that a critical reappraisal is required of the open ocean δ¹⁸O record as a simple indicator of global ice volume. An alternative, Hypothesis 2, is also examined. It is based on the assumption that the 35,000-yr-old deposits calculated on the basis of Hypothesis 1 date from the “last interglaciation”; this, in effect, indicates that the Missinaibi Formation, commonly accepted as sediments of the “last interglaciation,” are about 500,000 yr old and that the effective diagenetic temperature in the lowlands during approximately the last 130,000 yr has been close to ?6°C. We argue for rejection of this alternative hypothesis.     

Quaternary Antarctic Bottom-Water history: Deep-sea benthonic foraminiferal evidence from the southeast Indian Ocean

Distinct assemblages of Recent deep-sea benthonic foraminifera from the southeast Indian Ocean have been shown to be associated with Antarctic Bottom Water (AABW) and Indian Bottom Water (IBW). The AABW assemblage is divided into two groups. One is dominated by Epistominella umbonifera and is associated with AABW having temperatures between ?0.2° and 0.4°C. The second group is dominated by Globocassidulina subglobosa and is associated with AABW having temperatures between 0.6° and 0.8°C. The IBW assemblage is marked by the strong dominance of Uvigerina spp. and Epistominella exigua. The faunal-water-mass relationships have been used to infer the history of bottom-water circulation over the last 500,000 yr in this region using faunal data from four Eltanin cores. One core was taken from the Southeast Indian Ridge in association with IBW, and three were taken from the flank of the ridge associated with AABW flowing within a western boundary contour current in the South Australian Basin. Little faunal variation exists in the core beneath IBW (E48-22), indicating that IBW was present on the Southeast Indian Ridge during the last 300,000 yr. A record of the intensity of AABW circulation during the last 500,000 yr is inferred from the benthonic foraminiferal data in the three cores located within the western boundary contour current. Marked oscillations in the relative proportions of AABW and IBW faunal assemblages are found in one core, E48-03. The faunal variations are inferred to have resulted from variation in intensity of AABW circulation between 500,000 and 195,000 yr B.P. In E48-03, the AABW assemblage was present most of the time between 500,000 and 195,000 yr B.P., with low intensity of AABW circulation occurring primarily during the equivalent of stages 8 and 7 (t = 305,000 to 195,000 yr B.P.). The intensity of AABW circulation varied, with a maximum occurring during the equivalent of stage 11 (t = 420,000 yr B.P.). Two additional cores, E45-27 and E45–74, show relatively constant intensity of AABW circulation from 195,000 yr B.P. to the present. The intensity of AABW circulation at the present appears to be intermediate between a maximum during the equivalent of stage 11 (t = 420,000 yr B.P.) and the minimum during the equivalent of stage 8 (t = 275,000 yr B.P.). AABW production has occurred during both glacial and interglacial episodes. Bottom-water productivity has been suggested to play an important role in glacial/interglacial oscillations during the late Quaternary (Weyl, 1968; Newell, 1974). In this study, the relationship between bottom-water circulation and climatic fluctuations appears to be more complex than had been previously suggested, since a simple relationship between Quaternary bottom-water circulation and paleoclimatic fluctuations is not shown.     

Diversity of recent planktonic foraminifera in the southern Indian Ocean and Late Pleistocene paleotemperatures

Planktonic foraminiferal assemblages have been examined in 25 trigger core top samples and 51 piston core top samples collected between latitudes 28° S and 55° S and longitudes 79° E and 120° E from the southern Indian Ocean during cruises of the U.S.N.S. Eltanin. Samples taken from water depths exceeding 4000 m and/or showing evidence of calcium carbonate dissolution were eliminated from further analysis. The final piston core data set consists of 34 samples; the trigger core data set containing 21 samples. A close relationship exists between changes in the planktonic foraminiferal assemblages in the surface sediments and surface water temperatures. Species diversity values were computed for each of the core top assemblages using the Shannon-Wiener Index and the Brillouin Index, each of which takes into consideration the number of species and the proportionment of individuals among the species. The Shannon and Brillouin diversity values for all samples are positively correlated (correlation coefficient (r) = +.999). Regression analysis of latitude versus Shannon diversity values in the trigger core samples clearly shows a decrease in diversity with increasing latitude (r = ?.979). Furthermore, a strong correlation (r = +.977) exists between decreasing species diversity (Shannon) and decreasing average summer-winter temperature of the overlying surface waters. A paleotemperature equation derived from the relationship of diversity in trigger core samples and surface water temperature was used to generate paleotemperature curves for five trigger cores and a 6 m piston core of Late Pleistocene age, located beneath the present position of the Subtropical Convergence. A 7–8° C temperature range is suggested between the interglacial and glacial episodes in this Late Pleistocene sequence, and probably reflects latitudinal shifts of the Subtropical Convergence and Australasian Front during the Late Pleistocene.     

白令海北部陆坡100ka来的古海洋学记录及海冰的扩张历史

白令海北部陆坡B2-9柱状样中生源组分的研究显示, 自MIS5.3期以来表层生产力指标的粗组分和蛋白石含量呈阶梯状增加, 反映表层生产力阶段式的增长.全新世表层生产力达到最高, 并且MIS3.2~2期高, 比MIS5.3~3.3期最低.高有机碳含量对应于高C/N比值, 显示有机碳混合来源, 不能作为表层生产力的指标.MIS5.1, 3.3~3.2期和全新世高的有机碳含量和C/N比值反映间冰期陆源有机物质输入量的增加.MIS5.3期至中全新世, 不断增加的陆源砂级和粉砂级颗粒组分说明随着气候的逐渐变冷, 陆架海冰在不断扩张.伐冰碎屑和碳屑颗粒冰期、间冰段和末次冰消期升高, 而间冰期降低, 反映冰期白令海陆架海冰扩张和间冰期海冰消融的过程.冰期海冰扩张与北美大陆气候的相互关联, 揭示了晚第四纪冰期旋回中白令海海冰扩张及其对全球气候变化的响应.      

Eastern Mediterranean sea during the last glacial maximum; an 18,000-years B.P. reconstruction

An ecological transfer function based on the distribution of planktonic foraminifera in 66 Mediterranean and 8 North Atlantic surface-sediment samples is used to estimate sea-surface temperatures and salinities for the eastern Mediterranean during the last glacial maximum (18,000 yr B.P.). The present-day distribution of planktonic foraminifera can be explained by four faunal assemblages, each of which has diagnostic environmental preferences. Factor 1 is a tropical-subtropical assemblage; factor 2 is a transitional assemblage; factor 3 is a low-salinity assemblage; and factor 4 is a subpolar assemblage. The geographic distribution of these faunal assemblages reflect the variation in overlying hydrographic conditions. The 18,000-yr B.P. samples were selected based on total faunal stratigraphy, oxygen-isotope stratigraphy, and previously determined radiometric dates for eastern Mediterranean volcanic ash layers. Estimated temperature and salinity patterns show that the greatest change between present-day and 18,000-yr B.P. sea-surface conditions existed in the Aegean Sea and immediately south of Crete. The winter temperature anomaly (18,000 yr B.P.-present) within the Aegean Sea is 6°C cooler than present. In contrast to this, the maximum summer temperature anomaly exists to the south of Crete, where sea-surface temperatures were 4°C cooler than present. Estimated sea-surface salinities also show that the greatest change took place within the Aegean Sea, being 5‰ less saline than present. The estimated temperature and salinity patterns seem to reflect changing drainage patterns during glacial times and the diversion of cool, low-salinity water into the Aegean Sea. The source of this glacial runoff appears to be large freshwater lakes that existed during this time over parts of eastern Europe and western Siberia.     

Abrupt vegetation changes during the last glacial to Holocene transition in mid‐latitude South America

A pollen record from the Huelmo site (ca. 41°30′S) shows that vegetation and climate changed at millennial time‐scales during the last glacial to Holocene transition in the mid‐latitude region of western South America. The record shows that a Nothofagus parkland dominated the landscape between 16 400 and 14 600 ¹⁴C yr BP, along with Magellanic Moorland and cupressaceous conifers. Evergreen North Patagonian rainforest taxa expanded in pulses at 14 200 and 13 000 ¹⁴C yr BP, following a prominent rise in Nothofagus at 14 600 ¹⁴C yr BP. Highly diverse, closed canopy rainforests dominated the lowlands between 13 000 and 12 500 ¹⁴C yr BP, followed by the expansion of cold‐resistant podocarps and Nothofagus at ca. 12 500 and 11 500 ¹⁴C yr BP. Local disturbance by fire favoured the expansion of shade‐intolerant opportunistic taxa between 10 900 and 10 200 ¹⁴C yr BP. Subsequent warming pulses at 10 200 and 9100 ¹⁴C yr BP led to the expansion of thermophilous, summer‐drought resistant Valdivian rainforest trees until 6900 ¹⁴C yr BP. Our results suggest that cold and hyperhumid conditions characterised the final phase of the Last Glacial Maximum (LGM), between 16 400 and 14 600 ¹⁴C yr BP. The last ice age Termination commenced with a prominent warming event that led to a rapid expansion of North Patagonian trees and the abrupt withdrawal of Andean ice lobes from their LGM positon at ca. 147 000 ¹⁴C yr BP. Hyperhumid conditions prevailed between 16 400 and 13 000 ¹⁴C yr BP, what we term the ‘extreme glacial mode’ of westerly activity. This condition was brought about by a northward shift and/or intensification of the southern westerlies. The warmest/driest conditions of the last glacial–interglacial transition occurred between 9100 and 6900 ¹⁴C yr BP. During this period, the westerlies shifted to an ‘extreme interglacial mode’ of activity, via a poleward migration of stormtracks. Our results indicate that a highly variable climatic interval lasting 5500 ¹⁴C years separate the opposite extremes of vegetation and climate during the last glacial‐interglacial cycle, i.e. the end of the LGM and the onset of the early Holocene warm and dry period. Copyright © 2003 John Wiley & Sons, Ltd.     

Climatic bisection of the last interglacial warm period in the Polar North Atlantic

New multiproxy marine data of the Eemian interglacial (MIS5e) from the Norwegian Sea manifest a cold event with near-glacial surface ocean summer temperatures (3–4 °C). This mid-Eemian cooling divided the otherwise relatively warm interglacial climate and was associated with widespread expansions of winter sea-ice and polar water masses due to changes in atmospheric circulation and ocean stability. While the data also verify a late rather than early last interglacial warm peak, which is in general disharmony with northern hemisphere insolation maximum and the regional climatic progression of the early Holocene, the cold event itself was likely instrumental for delaying the last interglacial climate development in the Polar North when compared with regions farther south. Such a ‘climatic decoupling’ of the Polar region may bear profound implications for the employment of Eemian conditions to help evaluate the present and future state of the Arctic cryosphere during a warming interglacial.     

Constraints on hydrological and paleotemperature variations in the Eastern Mediterranean region in the last 140 ka given by the δD values of speleothem fluid inclusions

Speleothem fluid inclusions potentially provide vital data on the paleohydrological conditions in glacial and interglacial periods. We show here that δD analyses of fluid inclusions from speleothems in three caves in the Eastern Mediterranean region (Israel) provide a basis for understanding hydrological and temperature variations in the last 140 ka. Using measured δD fluid inclusion values and temperature ranges constrained by the present-day Mediterranean Meteoric Water Line (MMWL) and the global MWL, we show a strong compatibility between the land and marine temperature records, with the highest land temperatures occurring at 120–130 ka (17–22°C) and the present (18°C) and the lowest temperatures during the LGM, about 10°C lower than present. Interglacial waters are constrained to follow the MMWL and most glacial waters also plot close to the MMWL, with a slight shift towards the MWL. However, during two remarkable brief periods at the termination of the LGM, the waters plot on the MWL, suggesting that both sharp cooling and an increase in relative humidity above the EM Sea reduced d-excess values to those of the global system.     

Inferences about winter temperatures and summer rains from the late Quaternary record of C4 perennial grasses and C3 desert shrubs in the northern Chihuahuan Desert

Late Quaternary histories of two North American desert biomes—C₄ grasslands and C₃ shrublands—are poorly known despite their sensitivity and potential value in reconstructing summer rains and winter temperatures. Plant macrofossil assemblages from packrat midden series in the northern Chihuahuan Desert show that C₄ grasses and annuals typical of desert grassland persisted near their present northern limits throughout the last glacial–interglacial cycle. By contrast, key C₃ desert shrubs appeared somewhat abruptly after 5000 cal. yr BP. Bioclimatic envelopes for select C₄ and C₃ species are mapped to interpret the glacial–interglacial persistence of desert grassland and the mid‐to‐late Holocene expansion of desert shrublands. The envelopes suggest relatively warm Pleistocene temperatures with moist summers allowed for persistence of C₄ grasses, whereas winters were probably too cold (or too wet) for C₃ desert shrubs. Contrary to climate model results, core processes associated with the North American Monsoon and moisture transport to the northern Chihuahuan Desert remained intact throughout the last glacial–interglacial cycle. Mid‐latitude effects, however, truncated midsummer (July–August) moisture transport north of 35° N. The sudden expansion of desert shrublands after 5000 cal. yr BP may be a threshold response to warmer winters associated with increasing boreal winter insolation, and enhanced El Niño–Southern Oscillation variability. Published in 2006 by John Wiley & Sons, Ltd.     

Climate change in lowland Central America during the late deglacial and early Holocene

The transition from arid glacial to moist early Holocene conditions represented a profound change in northern lowland Neotropical climate. Here we report a detailed record of changes in moisture availability during the latter part of this transition (～11 250 to 7500 cal. yr BP) inferred from sediment cores retrieved in Lake Petén Itzá, northern Guatemala. Pollen assemblages demonstrate that a mesic forest had been largely established by ～11 250 cal. yr BP, but sediment properties indicate that lake level was more than 35 m below modern stage. From 11 250 to 10 350 cal. yr BP, during the Preboreal period, lithologic changes in sediments from deep‐water cores (>50 m below modern water level) indicate several wet–dry cycles that suggest distinct changes in effective moisture. Four dry events (designated PBE1‐4) occurred centred at 11 200, 10 900, 10 700 and 10 400 cal. yr BP and correlate with similar variability observed in the Cariaco Basin titanium record and glacial meltwater pulses into the Gulf of Mexico. After 10 350 cal. yr BP, multiple sediment proxies suggest a shift to a more persistently moist early Holocene climate. Comparison of results from Lake Petén Itzá with other records from the circum‐Caribbean demonstrates a coherent climate response during the entire span of our record. Furthermore, lowland Neotropical climate during the late deglacial and early Holocene period appears to be tightly linked to climate change in the high‐latitude North Atlantic. We speculate that the observed changes in lowland Neotropical precipitation were related to the intensity of the annual cycle and associated displacements in the mean latitudinal position of the Intertropical Convergence Zone and Azores–Bermuda high‐pressure system. This mechanism operated on millennial‐to‐submillennial timescales and may have responded to changes in solar radiation, glacial meltwater, North Atlantic sea ice, and the Atlantic meridional overturning circulation (MOC). Copyright © 2005 John Wiley & Sons, Ltd.     

Paleoclimate reconstruction based on the timing of speleothem growth and oxygen and carbon isotope composition in a cave located in the rain shadow in Israel

High-resolution ²³⁰Th/²³⁴U ages and δ¹⁸O and δ¹³C compositions of speleothems in Ma’ale Efrayim Cave located to the east of the central mountain ridge of Israel enable us to examine the nature of the rain shadow aridity during glacial and interglacial intervals. Speleothem growth occurred during marine glacial isotopic periods, with no growth during the two last marine isotope interglacial intervals and during the peak of the Last Glacial Maximum. This contrasts with speleothem growth in caves located on the western flank of the central mountain ridge, in the Eastern Mediterranean semiarid climatic zone, which continued throughout the last 240,000 yr. Thus, during glacial periods water reached both sides of the central mountain ridge. A comparison of the present-day rain and cave water isotopic compositions and amounts at the Ma’ale Efrayim Cave site with those on the western flank shows that evaporation and higher temperatures on the eastern flank are major influences on isotopic composition and the lack of rainfall. The δ¹⁸O and δ¹³C profiles of the speleothems deposited between 67,000 and 25,000 yr B.P. match the general trends of the isotopic profiles of Soreq Cave speleothems, suggesting a similar source (eastern Mediterranean Sea) and similar climatic conditions. Thus, during glacial periods the desert boundary effectively migrated further south or east from its present-day location on the eastern flank, whereas interglacial periods appear to have been similar to the present, with the desert boundary at the same position. The decrease in overall temperature and a consequent reduction in the evaporation to precipitation ratios on the eastern flank are viewed as the major factors controlling the decay of the rain shadow effect during glacial periods.     

Impacts of tectonic and orbital forcing on East African climate: a comparison based on global climate model simulations

A global atmosphere–ocean model has been forced with topographic and orbital scenarios in order to evaluate the relative role of both factors for the past climate of East Africa. Forcing the model with a significantly reduced topography in Eastern and Southern Africa leads to a distinct increase in moisture transport from the Indian Ocean into the eastern part of the continent and increased precipitation in Eastern Africa. Simulations with step-wise reduced height show that this climate change occurs continuously with the change in topography, i.e., an abrupt change of local climatic features with a critical height is not found. Simulations of the last interglacial (at 125,000 years before present, i.e., the Eemian interglacial) and the last glacial inception (at 115,000 years before present) are used as examples for the role of orbital-induced changes in insolation. Here, changes in meridional temperature gradients lead to modifications in moisture transport of similar order of magnitude, but with different spatial and seasonal structure. For the Eemian interglacial, a distinct increase in summer moisture transport from the Atlantic deep into the continent at around 20°N is simulated.     

Paleoceanographic changes of surface and deep water based on oxygen and carbon isotope records during the last 130 kyr identified in MD179 cores,off Joetsu,Japan Sea

We reconstructed the paleoenvironmental history of surface and deep water over the last 130 kyr from oxygen and carbon isotope ratios of planktonic and benthic foraminifera in two cores (MD179-3312 and MD179-3304) from the Joetsu Basin, eastern margin of the Japan Sea. Our data showed that paleoceanographic changes such as influx of surface currents and vertical circulation were associated with global glacial–interglacial sea level change. Surface water conditions were influenced by the influx of Tsushima Current, East China Sea coastal or off-shore waters through the Tsushima Strait during interglacial or interstadial stages, and strongly affected by freshwater input during the glacial maximum. During interglacial maximums such as Marine Isotope Stages 1 and 5e, development of well-oxygenated bottom water was indicated. A density-stratified ocean with weak ventilation was inferred from the isotopic records of benthic foraminifera during the Last Glacial Maximum. Local negative excursions in carbon isotopes during deglacial or interglacial periods may suggest the dissolution of gas hydrates or methane seep activities.     

Late-quaternary vegetational dynamics and community stability reconsidered

Defining the spatial and temporal limits of vegetational processes such as migration and invasion of established communities is a prerequisite to evaluating the degree of stability in plant communities through the late Quaternary. The interpretation of changes in boundaries of major vegetation types over the past 20,000 yr offers a complementary view to that provided by migration maps for particular plant taxa. North of approximately 43°N in eastern North America, continual vegetational disequilibrium has resulted from climatic change, soil development, and species migrations during postglacial times. Between 33° and 39°N, stable full-glacial vegetation was replaced by a relatively unstable vegetation during late-glacial climatic amelioration; stable interglacial vegetation developed there after about 9000 yr B.P. Late-Quaternary vegetation has been in dynamic equilibrium, with a relatively constant flora, south of 33°N on upland interfluves along the northern Gulf Coastal Plain, peninsular Florida, and west-central Mexico.     

Stratigraphic and paleoclimatic analysis of late Pleistocene tropical southeast Pacific cores (with an appendix by N. J. Shackleton)

Factor analysis of dissolution-resistant species of planktonic foraminifera in the >250 μm fraction of seabed and core samples from the tropical southeast Pacific yields five mappable varimax assemblages. The distribution of these assemblages in core tops is more related to the subsurface thermal structure than to the surface temperature. A transfer function relating the assemblages in core tops to the temperature at 200 m is applied to five cores. Cycles of estimated temperature in three cores are approximately in phase with climatic cycles in Atlantic cores spanning the past 200,000 yr, and display amplitudes on the order of 5°C. Two zones of increased calcium carbonate dissolution occur in these three cores. These zones occur in the transition from interglacial to glacial intervals. Temperature variations inferred are ascribed to fluctuations of the main thermocline, and the inference is made that during cold intervals the thermocline was displaced upward as the central water mass was areally diminished. A model of circulation dynamics suggests that such areal diminution would be associated with an intensified circulation. No significant fluctuations in the estimated 200 m temperature are inferred for a core in the Galápagos area during the last glacial, but a pronounced peak of high-latitude species Globorotalia inflata does occur. This peak is judged to reflect the removal of a low-oxygen ecological barrier.