bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemical and Ultrastructural Features of the Lichen-volcanic/Sedimentary Rock Interface in a Semiarid Region (Almería, Spain)
Authors: Souza-Egipsy Virginia , Wierzchos Jacek , García-Ramos Jose Vicente , Ascaso Carmen
Published: September 2002

The chemical and ultrastructural features of the interface formed by different biotypes of saxicolous lichen species with their rock substrata were investigated in the semiarid habitat of the SE Iberian Peninsula and the relationships between the bioweathering patterns observed and lichen colonization selectivity towards the different rock substrata evaluated. Xanthoria parietina was able to fix to the rock substratum by the adherence of single cell walls from the lower cortex. Neofuscelia pulla used rhizines and loose groups of hyphae for attachment of the thallus to the rock. Colonization by the foliose lichen species was confined to the rock surface, while Diploschistes diacapsis was also able grow below the surface showing two types of hyphal growth. Minerals in close contact with cell walls were biochemically and biophysically weathered, but hyphae showing calcium oxalate crystals did not appear to be directly involved in the patterns observed. The textural characteristics of the substratum seemed to be related to the type of microorganism colonization: sedimentary rocks were more deeply colonized by lichens and other chasmolithic microorganisms than volcanic material. Calcium oxalate crystals were found in the medulla of N. pulla but not at the lichen-substratum interface. Crustose lichens such as D. diacapsis showed calcium oxalate crystals in the upper cortex and over the outside of fungal medullary hyphae but not in direct contact with the rock surface. Calcium oxalate precipitation seems to be related to the different metabolic activities of the mycobiont within the lichen thallus and to different species. D. diacapsis inhibits the growth of other microorganisms in close proximity to the thallus, whereas foliose species were associated with several communities of microorganisms.
Long-Term Monitoring in the Netherlands Suggests that Lichens Respond to Global Warming
Authors: Herk C. M. van , Aptroot A. , Dobben H. F. van
Published: September 2002

There is evidence to suggest that part of the recent changes in the lichen flora of the Netherlands is attributable to an increase in temperature. Changes which have occurred over the last 22 years were studied in detail, and were subjected to a statistical treatment by comparing the change of species to their latitudinal distribution and to ecological determinants.

All 329 epiphytic and terrestrial lichen species occurring in the Netherlands were considered in relation to their world distribution. Arctic-alpine/boreo-montane species appear to be declining, while (sub)tropical species are invading. The proportion of increasing species is by far the largest among the wide-tropical lichens (83%), and smallest among the arctic-alpine/boreo-montane lichens (14%). None of the wide-tropical species was found to decrease, while 50% of the arctic-alpine/boreomontane species show a decline.

Long-term monitoring of the epiphytic lichen flora in the province of Utrecht from 1979 onwards shows that the total number of taxa present increased from 95 in 1979 to 172 in 2001, while the average number of taxa per site increased from 7·5 to 18·9. The rate of increase was greatest by far between 1989 and 1995. The majority of the species (152 taxa or 85%) show a gross increase, only 17 species (10%) show a decrease.

A detailed analysis of these data using multiple regression suggests global warming as an additional cause for recent changes, next to decreasing SO⊂2 and increasing NH⊂3. Changes appear to be correlated initially (1979-1995) only with toxitolerance and nutrient demand. Changes between 1995 and 2001, however, appear positively correlated to both temperature and nutrient demand, indicating a recent and significant shift towards species preferring warm circumstances, independent from, and concurrent with changes due to nutrient availability.

This is the first paper reporting long-term floristic changes for lichens that appear to be correlated significantly with increasing temperatures. We suggest that future lichen monitoring programmes also pay attention to effects of climatic change, instead of focusing on air pollution effects only.
New Species of the Lichen Family Thelotremataceae in SE Asia
Authors: Homchantara N. , Coppins B. J.
Published: September 2002

The lichen family Thelotremataceae was studied in Thailand and Malaysia (including one specimen from Indonesia). Of the 30 new species recognized, 26 are recorded from Thailand: Myriotrema grandisporum, M. khaoyaianum, M. khuntanense, M. rongklaense, M. subanamaliense, M. subgranulosum, M. subminutum, M. thailandicum, M. whalleyanum, Ocellularia brunneospora, O. diospyrosis, O. flavescens, O. inthanonensis, O. kansriae, O. krathingensis, O. melanophthalma, O. neoleucina, O. peremergens, O. pluriporoides, O. rhicnoporoides, O. subleucina, O. wolseleyana, Thelotrema laemense, T. mongkolsukii, T. phliuense and T. rhododiscum; and 4 from Sarawak: M. coppinsii, M. muluense, O. neoperforata, and O. roseotecta.Additional notes are provided on Ocellularia palaeoamplior Aptroot & Sipman and Thelotrema isidiophorum (Kremp.) Zahlbr.
Chronology of Taylor Glacier Advances in Arena Valley, Antarctica, Using in Situ Cosmogenic 3He and 10Be
Authors: Brook Edward J. , Kurz Mark D. , Ackert Robert P. , Denton George H. , Brown Erik T. , Raisbeck Grant M. , Yiou Francoise
Published: September 2002

In situ produced cosmogenic nuclides provide a new technique for constraining exposure ages of glacial deposits. In situ3He and 10Be in quartz sandstone boulders from Arena Valley, southern Victoria Land, Antarctica, provide chronological constraints for a sequence of moraines ("Taylor II-IVb" moraines) related to expansions of Taylor Glacier and the East Antarctic Ice Sheet. Mean 3He ages are 113,000 ± 45,000 yr, 208,000 ± 67,000 yr, 335,000 ± 187,000 yr, and 1.2 ± 0.2 myr, for Taylor II,III,IVa, and IVb moraines, respectively (mean ± 1σ). Corresponding mean 10Be ages for Taylor II and IVb moraines are 117,000 ± 51,000 yr and 2.1 ± 0.1 myr. For the older deposits the 3He ages are probably lower limits due to diffusive loss. Although the exposure ages appear consistent with the few previous age estimates, particularly with an isotope stage 5 age for Taylor II, each moraine exhibits a broad age distribution. The distribution probably results from a variety of factors, which may include prior exposure to cosmic rays, 3He loss, erosion, postdepositional boulder movement, and radiogenic production of 3He. Nonetheless, the exposure ages provide direct chronological constraints for the moraine sequence, and suggest a maximum thickening of Taylor Glacier relative to the present ice surface of ∼500 m since the late Pliocene-early Pleistocene.
Factors Affecting14C Ages of Lacustrine Carbonates: Timing and Duration of the Last Highstand Lake in the Lahontan Basin
Authors: Benson Larry
Published: September 2002

Two processes contribute to inaccurate14C age estimates of carbonates precipitated within the Lahontan basin, NevadaCalifornia: low initial14C/C ratios in lake water (reservoir effect) and addition of modern carbon to calcium carbonate after its precipitation. The mast reliable set of14C ages on carbonates from elevations > 1310 m in the Pyramid and Walker Lake subbasins indicate that lakes in all seven Lahontan subbasins coalesced ∼14,20014C yr B.P. forming Lake Lahontan. Lake Lahontan achieved its 1330-m highstand elevation by ∼13,80014C yr B.P. and receded to 1310 m by ∼13,70014C yr B.P. Calculations, based on measured carbonate-accumulation rates, of the amount of time Lake Lahontan exceeded 1310 and 1330 m (500 and 50 yr) are consistent with this chronology. The timing of the Lake Lahontan highstand is of interest because of the linkage of highstand climates with proximity to the polar jet stream. The brevity of the Lahontan highstand is interpreted to indicate that the core of the southern branch of the polar jet stream remained only briefly over the Lahontan basin.
Lake Evolution during the Last 3000 Years in China and Its Implications for Environmental Change
Authors: Fang Jin-Qi
Published: September 2002

Taking China as a whole, lake expansion occurred 500 B.C. (?)-0 A.D., 650-950 A.D., and 1250-1650 A.D. over the past 3000 yr. The expansions were mainly due to climatic fluctuations and correspond to frequent flood reports. Three regional characteristics of lake development have been recognized. (1) Fluctuations of near-coastal lakes, and those in other regions as well, were out of phase during 500 B.C.-500 A.D., which possibly resulted from out-of-phase fluctuations of climate and sea level on 10- to 100-yr time scale. (2) As the lower Yellow River channel migrated southward, the lakes in the south of the North China Plain formed and reached their largest sizes much later than those in the north. (3) For the lakes in the middle and lower Yangtze River valley, the higher a lake basin lies above a neighboring river and the further the lake is from the river's mouth, in general, the younger the lake is. Through land reclamation, the ancient Chinese have changed most of the lakes greatly. However, human activities were also constrained in certain ways by the natural changes of the lakes. Reclamation usually was banned and farmland was abandoned to repair reservoirs while lakes were expanding.
A 1000-Year Record of Temperature and Precipitation in the Sierra Nevada
Authors: Graumlich Lisa J.
Published: September 2002

Tree-ring data from subalpine conifers in the southern Sierra Nevada were used to reconstruct temperature and precipitation back to A.D. 800. Tree growth of foxtail pine (Pinus balfouriana) and western juniper (Juniperus occidentalis ssp. australis) is influenced by nonlinear interactions between summer temperature and winter precipitation. Reconstruction of the separate histories of temperature and precipitation is feasible by explicitly modeling species and site differences in climatic response using response surfaces. The summer temperature reconstruction shows fluctuations on centennial and longer time scales including a period with temperatures exceeding late 20th-century values from ca. 1100 to 1375 A.D., corresponding to the Medieval Warm Period identified in other proxy data sources, and a period of cold temperatures from ca. 1450 to 1850, corresponding to the Little Ice Age. Precipitation variation is dominated by shorter period, decadal-scale oscillations. The long-term record presented here indicates that the 20th century is anomalous with respect to precipitation variation. A tabulation of 20- and 50-yr means indicates that precipitation equaling or exceeding 20th-century levels occurred infrequently in the 1000+-yr record.
AMS Radiocarbon Age of the Upper Mississippi Valley Roxana Silt
Authors: Leigh David S. , Knox James C.
Published: September 2002

Accelerator mass spectrometer radiocarbon ages of the Roxana Silt (loess) along the Upper Mississippi Valley of Wisconsin and Minnesota indicate that loess sedimentation of the Roxana Silt occurred between about 55,000 and 27,000 14 C yr B.P. However, due to local environmental controls, the basal age at any given site may range from 55,000 to 35,000 14C yr B.P. The radiocarbon ages presented here are in agreement with previous radiocarbon ages for the Roxana Silt in its type area of west-central Illinois, but indicate that long-term sedimentation rates along the bluffline of the Upper Mississippi Valley were very slow (4-8 cm/1000 yr) compared to long-term sedimentation rates along the bluffline of the type area (40-70 cm/1000 yr). Comparison of radiocarbon ages for midcontinent middle Wisconsinan loess deposits indicates that sedimentation along the Mississippi River valley may have preceded loess sedimentation along the Missouri River valley by as much as 20,000 yr or that basal ages for middle Wisconsinan loess along the Missouri Valley are erroneously young. The bracketing ages for the Upper Mississippi Valley Roxana Silt indicate that the Mississippi River valley was receiving outwash sedimentation between 55,000 and 27,000 14C yr B.P.
Macrofossil and Tree-Ring Evidence for a Long-Term Forest Succession and Mid-Holocene Hemlock Decline
Authors: Filion Louise , Quinty François
Published: September 2002

Paludified dune fields in southern Québec provided a unique opportunity to analyze a mid-Holocene forest succession, ranging from xero-mesophilous forests to treed peatlands, which registered the hemlock decline, a major biostratigraphic event in eastern North America. Our fossil-tree chronology established from 66 radiocarbon dates of tree stems indicates that white pine and hemlock were present at the study site about 6000-5700 yr B.P. Hemlock reached a maximum 4900-4800 yr B.P. and then declined between 4600 and 3800 yr B.P.; the species recovered around 3700 yr B.P. and then decreased when white pine and eastern latch dominated the site after 3000 yr B.P. and between 2600 and 2200 yr B.P., respectively. Tree-ring signatures of fossil hemlocks suggest the continuous influence of biotic factors, possibly insect defoliators, throughout the record, as shown by recurrent suppressed-growth pattern. Microscopic examination of the hemlock fossil stems disclosed no evidence of fungal disease. The hemlock decline was probably caused by regional posthypsithermal climatic changes, toward moister conditions.
Postglacial Vegetation and Fire History in the Chirripó Páramo of Costa Rica
Authors: Horn Sally P.
Published: September 2002

Pollen and charcoal analysis of a 5.6-m sediment core from Lago de las Morrenas (9°29′N, 83°29′W; 3480 m) provides evidence of postglacial vegetation and fire history in the highlands of the Cordillera de Talamanca, Costa Rica. The site is presently surrounded by treeless páramo vegetation and apparently has been so since deglaciation about 10,000 yr B.P. Pollen spectra suggest no pronounced changes in vegetation since ice retreat. Fires set by people or lightning have burned the páramo repeatedly, with fire activity probably highest during the late Holocene, but these fires have not carved páramo from forest. Pollen percentages for Gramineae and other páramo taxa decline upward, whereas percentages for certain subalpine, lower montane, and lowland forest taxa increase slightly; these changes may reflect the impact of prehistoric human activity or slight upslope migrations of forest taxa owing to climatic warming. There is no clear evidence of higher timberlines during the mid-Holocene.
Late Quaternary Pollen Record from Cheyenne Bottoms, Kansas
Authors: Fredlund Glen G.
Published: September 2002

A sediment and pollen record from Cheyenne Bottoms, a large (166 km2) enclosed basin in central Kansas, provides evidence for local and regional vegetation and climate change during the late Quaternary (ca. 30,000 yr.). Although radiocarbon dating of the carbonate-rich lacustrine sediments remains problematic, a basic chronological framework for the section is established. Two major litho- and biostratigraphic units, a Farmdalian zone (ca. 30,000 to 24,000 yr B.P.) and a Holocene zone (ca. 11,000 yr B.P. to present), are separated by a major unconformity spanning the Woodfordian (ca. 24,000 to 11,000 yr B.P.). Pollen and sedimentary data indicate a period of basin-wide drying preceding this unconformity. The sustained absence of sediment accumulation within this playa-like basin suggests that early Woodfordian conditions were increasingly arid with strengthened surface winds. Before this, persistent shallow water marshland dominated the local basin-bottom vegetation. Regional upland vegetation was an open grassland-sage steppe throughout the Farmdalian with limited populations of spruce, juniper, aspen, birch, and boxelder in riparian settings and escarpments. Throughout the Holocene, water levels within the basin fluctuated. Changes in wetland vegetation resulting from water level fluctuation have increased during the last 3,000 yr indicating that periodic episodes of wetland loss and rebound are not unique to postsettlement conditions but are an ongoing phenomenon at Cheyenne Bottoms.
Chironomid Evidence for Late-Glacial Climatic Reversals in Maine
Authors: Cwynar Les C. , Levesque André J.
Published: September 2002

Previously published pollen studies from Maine have not identified any late-glacial reversals despite evidence for them from adjacent areas. The distribution and abundance of midge flies are strongly correlated with summer surface-water temperatures. We analyzed fossils of larval midge flies from the sediments of two ponds (Trout and Tilley) from Maine. Each site reveals a major and a minor oscillation during which inferred summer surface-water temperatures fell by 7-13°C and 2.1-2.6°C, respectively. We tentatively correlate these events with the Younger Dryas cooling and Killarney Oscillation reported from adjacent Maritime Canada, where they have been AMS14C-dated at 10,770-10,000 and 11,160-10,910 yr B.P., respectively. A third oscillation occurs at the northernmost site, Tilley Pond, and may represent the effects of a local ice advance in northern Maine.
Lake Michigan Beach-Ridge and Dune Development, Lake Level, and Variability in Regional Water Balance
Authors: Lichter John
Published: September 2002

A sequence of northern Lake Michigan beach ridges records lake-level fluctuations that are probably related to changes in late Holocene climate. Historically, episodes of falling and low lake level associated with regional drought led to the formation of dune-capped beach ridges. The timing of prehistoric ridge formation, estimated by radiocarbon dating of plant macrofossils from early-successional dune species, shows that return periods of inferred drought, averaged for time intervals of 100 to 480 yr, ranged between 17 and 135 yr per drought during the last 2400 yr. In five of ten of these time intervals, the average return period ranged between 17 and 22 yr per drought. These intervals of frequent ridge formation and drought were associated with the development of parabolic dunes, which is indicative of high lake level and moist climate. This seeming paradox suggests that unusually moist decades alternated with unusually dry decades during these time intervals. Regional water balance probably varied less during the time intervals when ridges formed less often and the lake produced no evidence of high level.
Late Pleistocene and Holocene History at Mubwindi Swamp, Southwest Uganda
Authors: Marchant Robert , Taylor David , Hamilton Alan
Published: September 2002

Deposits beneath Mubwindi Swamp provide a partial record of vegetation history since at least 43,000 yr ago. We studied pollen from two cores and obtained nine radiocarbon ages from one of these cores and three radiocarbon ages from the other. Pollen deposited before and soon after the last glacial maximum represents vegetation very different from the modern vegetation of the Mubwindi Swamp catchment. Although species now associated with higher altitudes were dominant some elements of moist lower montane forest persisted, possibly because of favorable soils or topography. The pollen data provides evidence for a late glacial montane forest refuge near Mubwindi Swamp. Moist lower montane forest became much more widespread soon after the glacial maximum. The only irrefutably Holocene sediments from Mubwindi Swamp date to the past 2500 yr. During this time a combination of climatic and human-induced changes in vegetation can be seen in the pollen records.
Potential for14C Dating of Biogenic Carbonate in Hackberry (Celtis) Endocarps
Authors: Wang Yang , Jahren A.Hope , Amundson Ronald
Published: September 2002

Hackberry endocarp (Celtissp.) contains significant amounts (up to 70 wt%) of biogenic carbonate that is nearly pure aragonite (CaCO3). Because of their high mineral content, hackberry endocarps are found abundantly in Tertiary and Quaternary sediments and are very common in many North American archaeological sites. We analyzed the14C content of different components of modern hackberries including the biogenic carbonate in hackberry endocarps collected at known times over the past century. The14C content of the endocarp carbonate accurately records the14C content of the atmosphere.14C dates of fossil endocarp carbonates compared favorably with dates obtained by other means at archaeological and geological sites ranging in age from the late Pleistocene through the early Holocene. We therefore suggest that hackberry endocarp is a suitable substrate for14C dating provided that its morphological and mineralogical integrity is preserved.
Evidence for Cirque Glaciation in the Colorado Front Range during the Younger Dryas Chronozone
Authors: Menounos Brian , Reasoner Mel A.
Published: September 2002

Late Pleistocene glacial chronologies developed for the Front Range Mountains of Colorado include two or more cirque glacier advances, locally known as the Satanta Peak Advances. Sediment cores were recovered from Sky Pond, an alpine lake (3320 m) located less than 100 m downvalley from a moraine that exhibits late Pleistocene to early Holocene relative age features and appears to correlate to the Satanta Peak deposits. One of the cores penetrated 0.5 m of basal diamict and recovered 3.3 m of overlying sediments that are predominantly gyttja. An accelerator mass spectrometry (AMS) age of 12,040 ± 60 yr14C B.P. was obtained from directly above the basal diamict and is similar to other reported ages for cirque deglaciation in the Front Range Mountains. The lower portion of the gyttja contains an interval of clastic sediments that show characteristics consistent with glacial activity in alpine catchments. Radiocarbon ages obtained from below and near the upper contact of this clastic interval are 11,070 ± 50 and 9970 ± 8014C yr B.P., respectively. An additional AMS age of 10,410 ± 9014C yr B.P. was obtained from within the clastic interval in a second core. The most likely source for this interval of clastic sediments is a moraine situated directly upvalley from Sky Pond, and consequently, it appears that the deposition of this moraine was coeval with the European Younger Dryas event (11,000–10,00014C yr B.P.). Similarities in soil development, weathering features, and altitude between this moraine and the type Satanta Peak moraines suggest that the moraines are correlative. These findings are in agreement with a growing body of evidence that suggests a relatively minor advance of alpine glaciers occurred in the North American Rockies during the Younger Dryas Chron.
AMS Radiocarbon Dating of Lake Michigan Beach-Ridge and Dune Development
Authors: Lichter John
Published: September 2002

Strandplains of shore-parallel beach ridges bordering the Great Lakes are valuable for reconstructing histories of climate-related lake-level fluctuations. However, imprecise radiocarbon dates of ridge formation have frustrated development of dependable chronologies from which information about variation in the frequency of ridge formation and inferred climate fluctuations can be obtained. The resolution and precision of radiocarbon chronologies can be improved with AMS 14C dates of roots and rhizomes of plant species associated with the formation and growth of the sand-dune caps of breach ridges. These dates reliably estimate the timing of shore progradation when the base of the previously established beach ridge becomes inundated by the water table. An AMS radiocarbon chronology of beach-ridge formation in northern Lake Michigan shows that information about variation in the frequency of ridge formation is important for paleoclimatic interpretation.
Cultural and Climatic History of Cobá, a Lowland Maya City in Quintana Roo, Mexico
Authors: Leyden Barbara W. , Brenner Mark , Dahlin Bruce H.
Published: September 2002

Lake Cobá, within the archaeological site of Cobá, provides evidence bearing on lowland Maya development. Palynological and geochemical data record multidecadal precipitation cycles from a 8.80-m, >8370-yr lake-sediment sequence terminating on bedrock. Late Classic sedimentation rates are rapid, but an anthropogenically derived colluvium layer is lacking. Initial vegetation was medium semi-deciduous and swamp forest. Forest clearance began 1650 B.C. (Early Preclassic) and maize first occurred at 850 B.C. (Middle Preclassic). Lakeside milpas existed until A.D. 720 (Late Classic) and then were moved from the city center as urbanization intensified and Lake Cobá was diked as a reservoir. Cobá was at most briefly vacated during the Classic Collapse and was abandoned after A.D. 1240, although some habitation persisted. The paleoecological record matches the archaeological history for Cobá, but pervasive disturbance muted the climatic signal, as the Late Classic drought is barely evident. The question whether economic trees were maintained within the city is unresolved. Maize cultivation allowed the Maya to develop a complex society and support a large population, but dependence on maize was ultimately doomed by variable rainfall. Precipitation in extreme years was insufficient to support crops, while native vegetation was not directly affected by drought that devastated Maya agriculture.
Differences in Radiocarbon Age between Shell and Charcoal from a Holocene Shellmound in Northern California
Authors: Ingram B. Lynn
Published: September 2002

The West Berkeley shellmound, the oldest well-dated archaeological site in the San Francisco Bay region, contains shell and charcoal ranging in age from ca. 1200 to 5700 cal yr B.P. Radiocarbon ages of marine shell and charcoal collected from fifteen stratigraphic levels in the West Berkeley shellmound suggest changes in the14C content of San Francisco Bay surface waters relative to the atmosphere (the oceanic reservoir age) over the past 5000 yr. The reservoir age of San Francisco Bay waters fluctuated between 870 and −17014C yr over the past 5000 yr, with the lowest values occurring 2900 to 3800 cal yr B.P. and the highest values between 1200 and 2000 cal yr B.P. Changes in the radiocarbon reservoir age may be due to changes in the strength of seasonal wind-driven upwelling off coastal California, where upwelling brings14C-depleted waters to the surface. The period of lowest ΔRvalues (at 3500 to 3900 cal yr B.P.) is coincident with relatively low salinity in San Francisco Bay (indicating high freshwater inflow) and wet climate in California based on lake level records. The period of high ΔRvalues (1200–2000 cal yr B.P.) is coincident with one of the driest periods in California during the late Holocene. These data suggest a link between coastal upwelling and precipitation over central California. The age of the top of West Berkeley mound and several other mounds in the San Francisco Bay region (1100 to 1300 cal yr B.P.) coincides with a prolonged dry period in California and low river inflow to San Francisco Bay. Perhaps the sites were abandoned because of the drought.
Lake Levels since about 40,000 Years Ago at Lake Chalco, near Mexico City
Authors: Caballero Margarita , Guerrero Beatriz Ortega
Published: September 2002

Diatoms, magnetic susceptibility, organic content, and14C ages of sediments from a 26-m core suggest that Lake Chalco, in the southern part of the basin of Mexico, went through a series of major fluctuations during the late Pleistocene and the Holocene. Before ca. 39,00014C yr B.P. the lake was very deep (about 8–10 m), alkaline, and saline. It then became shallow (<2 m) for most of the time between ca. 39,000 and 22,500 yr B.P. Chalco deepened to about 4–5 m about the time of a major eruption of nearby Popocatepetl volcano ca. 22,000 yr B.P. The lake remained relatively deep and fresh until ca. 18,500 yr B.P., when lower levels and alternating acidic to freshwater conditions were established. After 14,500 yr B.P. lake level rose slightly, but by ca. 10,000 yr B.P. Chalco became very shallow (<2 m), remaining as a low, alkaline saline marsh until ca. 6000 yr B.P. This period corresponds with the Playa cultural phase, during which the earliest human settlements in the basin were established. After ca. 6000 yr B.P. Chalco became a fresh to slightly alkaline shallow lake a few meters deep.
Paleomagnetic Investigation of the Bonneville Alloformation, Lake Bonneville, Utah
Authors: Liddicoat Joseph C. , Coe Robert S.
Published: September 2002

Paleomagnetic secular variation in a portion of the Bonneville Alloformation is compared with secular variation in lacustrine sediments in the Mono Basin, California, and with secular variation in Lake Lahontan sediments in the northwestern Great Basin. The comparison places an age of about 18,000 yr B.P., and a span of 1000 to 3000 yr, on part of a transgressive stage of Lake Bonneville near Delta, Utah, that is coeval with a wet period in the Lahontan Basin.
High-Resolution Marine Record of Climatic Change in Mid-latitude Chile during the Last 28,000 Years Based on Terrigenous Sediment Parameters
Authors: Lamy Frank , Hebbeln Dierk , Wefer Gerold
Published: September 2002

Marine sediment cores from the continental slope off mid-latitude Chile (33°S) were studied with regard to grain-size distributions and clay mineral composition. The data provide a 28,000-yr14C accelerator mass spectrometry-dated record of variations in the terrigenous sediment supply reflecting modifications of weathering conditions and sediment source areas in the continental hinterland. These variations can be interpreted in terms of the paleoclimatic evolution of mid-latitude Chile and are compared to existing terrestrial records. Glacial climates (28,000–18,000 cal yr B.P.) were generally cold–humid with a cold–semiarid interval between 26,000 and 22,000 cal yr B.P. The deglaciation was characterized by a trend toward more arid conditions. During the middle Holocene (8000–4000 cal yr B.P.), comparatively stable climatic conditions prevailed with increased aridity in the Coastal Range. The late Holocene (4000–0 cal yr B.P.) was marked by more variable paleoclimates with generally more humid conditions. Variations of rainfall in mid-latitude Chile are most likely controlled by shifts of the latitudinal position of the Southern Westerlies. Compared to the Holocene, the southern westerly wind belt was located significantly farther north during the last glacial maximum. Less important variations of the latitudinal position of the Southern Westerlies also occurred on shorter time scales.
Late Quaternary Geology and Geochronology of Diring Yuriakh, An Early Paleolithic Site in Central Siberia
Authors: Waters Michael R. , Forman Steven L. , Pierson James M.
Published: September 2002

Diring Yuriakh, an archaeological site on the highest terrace of the Lena River in subarctic eastern Siberia, provides evidence for the oldest and northern-most Early Paleolithic occupation in Asia. Stratigraphic and sedimentological studies at the site show that artifacts occur on a single eolian deflation surface that is underlain by fluvial sediments with inset cryogenic sand wedges and overlain by eolian deposits. Thermoluminescence ages on the fine-grained extracts from the eolian sediments and sand wedges that bound the artifact level indicate that the occupation occurred >260,000 yr B.P. and may possibly date between 270,000 and 370,000 yr B.P. This study documents that the artifacts from Diring Yuriakh are an order of magnitude older than artifacts from any previously reported site from Siberia. The antiquity and subarctic location of Diring Yuriakh indicates that people developed a subsistence strategy capable of surviving rigorous conditions in Siberia by ≥260,000 yr B.P.
A 10,300 14C yr Record of Climate and Vegetation Change from Haiti
Authors: Higuera-Gundy Antonia , Brenner Mark , Hodell David A. , Curtis Jason H. , Leyden Barbara W. , Binford Michael W.
Published: September 2002

Pleistocene and Holocene vegetation dynamics in the American tropics are inferred largely from pollen in continental lake sediments. Maritime influences may have moderated climate and vegetation changes on Caribbean islands. Stable isotope (δ18O) study of a 7.6-m core from Lake Miragoane, Haiti, provided a high-resolution record of changing evaporation/precipitation (E/P) since ∼10,300 14C yr B.P. The Miragoane pollen record documents climate influences and human impacts on vegetation in Hispaniola. The δ18O and pollen data near the base of the core indicate cool, dry conditions before ∼10,000 14C yr B.P. Lake Miragoane filled with water in the early Holocene as E/P declined and the freshwater aquifer rose. Despite increasing early Holocene moisture, shrubby, xeric vegetation persisted. Forest expanded ∼7000 14C yr B.P. in response to greater effective moisture and warming. The middle Holocene (∼7000–3200 14C yr B.P.) was characterized by high lake levels and greatest relative abundance of pollen from moist forest taxa. Climatic drying that began ∼3200 14C yr B.P. may have driven some mesophilic animal species to extinction. The pollen record of the last millennium reflects pre-Columbian (Taino) and European deforestation. Long-term, Holocene vegetation trends in southern Haiti are comparable to trends from continental, lowland circum-Caribbean sites, suggesting a common response to regional climate change.
Modeling North American Freshwater Runoff through the Last Glacial Cycle
Authors: Marshall Shawn J. , Clarke Garry K.C.
Published: September 2002

The Northern Hemisphere ice sheets decayed rapidly during deglacial phases of the ice-age cycle, producing meltwater fluxes that may have been of sufficient magnitude to perturb oceanic circulation. The continental record of ice-sheet history is more obscured during the growth and advance of the last great ice sheets, ca. 120,000–20,000 yr B.P., but ice cores tell of high-amplitude, millennial-scale climate fluctuations that prevailed throughout this period. These climatic excursions would have provoked significant fluctuation of ice-sheet margins and runoff variability whenever ice sheets extended to mid-latitudes, giving a complex pattern of freshwater delivery to the oceans. A model of continental surface hydrology is coupled with an ice-dynamics model simulating the last glacial cycle in North America. Meltwater discharged from ice sheets is either channeled down continental drainage pathways or stored temporarily in large systems of proglacial lakes that border the retreating ice-sheet margin. The coupled treatment provides quantitative estimates of the spatial and temporal patterns of freshwater flux to the continental margins. Results imply an intensified surface hydrological environment when ice sheets are present, despite a net decrease in precipitation during glacial periods. Diminished continental evaporation and high levels of meltwater production combine to give mid-latitude runoff values that are highly variable through the glacial cycle, but are two to three times in excess of modern river fluxes; drainage to the North Atlantic via the St. Lawrence, Hudson, and Mississippi River catchments averages 0.356 Sv for the period 60,000–10,000 yr B.P., compared to 0.122 Sv for the past 10,000 yr. High-amplitude meltwater pulses to the Gulf of Mexico, North Atlantic, and North Pacific occur throughout the glacial period, with ice-sheet geometry controlling intricate patterns of freshwater routing variability. Runoff from North America is staged in the final deglaciation, with a stepped sequence of pulses through the Mississippi, St. Lawrence, Arctic, and Hudson Strait drainages.
Planktonic Foraminiferal and Alkenone Records of the Last Deglaciation from the Eastern Arabian Sea
Authors: Cayre Olivia , Bard Edouard
Published: September 2002

The oxygen isotope record of planktonic foraminifera from tropical core MD77194 (Eastern Arabian Sea) exhibits a clear two-step deglaciation with a brief δ18O transient event. In the tropics, this δ18O maximum could correspond to a cooling or to a change in the δ18O content of sea water. In this study, past sea-surface temperature (SST) and primary production (PP) are reconstructed from foraminiferal transfer functions and compared to values estimated from alkenone measurements. SST and PP records from both proxies indicate a 1.5–2.5°C deglacial warming, coupled with a PP decrease, and a 0.5–1°C cooling during the Younger Dryas (YD). A detailed comparison between independent micropaleontological and geochemical proxies helps us identify potential biases and thus strengthen the paleo-reconstructions.
Implications for the Dating of Wisconsinan (Weichselian) Late-Glacial Events of Systematic Radiocarbon Age Differences between Terrestrial Plant Macrofossils from a Site in SW Ireland
Authors: Turney Chris S.M. , Coope G. Russell , Harkness Doug D. , Lowe J. John , Walker Michael J.C.
Published: September 2002

AMS radiocarbon dates were obtained from Salix herbacea leaves, Carex seeds, and bulk organic detritus from a lake sediment profile of Wisconsinan (Weichselian) Lateglacial age in SW Ireland. There is a systematic age difference between the dated series from the two types of macrofossils, with ages obtained from Salix herbacea leaves being 900 to 1500 14C years younger than those obtained from Carex seeds. The latter tend to be more in accord with dates from the total organic detritus in the lake sediment, although the bulk organic fraction invariably registered the older ages. Intact survival of the fragile Salix leaves indicates that they are unlikely to have been physically transferred within the sediment matrix and/or otherwise reworked from the surrounding catchment. Hence, these macrofossils are the more likely to be contemporaneous with the time of deposition. However, there is no significant correlation between measured 14C age and depth in the Salix values, which scatter over a range of 700 14C years. In contrast, the age/depth relationship for Carex shows a significant reversal, possibly reflecting the redeposition of these macrofossils, and therefore giving radiocarbon ages that are anomalously old. The data have important implications for the dating of lake sediment sequences by AMS radiocarbon measurement of terrestrial plant macrofossils.
Late Pleistocene Climates of Beringia, Based on Analysis of Fossil Beetles
Authors: Elias Scott A.
Published: September 2002

Thirty-one fossil beetle assemblages from central and eastern Beringia (Alaska, the Yukon Territory, and the now-submerged Bering Land Bridge) have yielded seasonal temperature estimates for the interval 43,550–9250 14C yr before present (yr B.P.). Estimates of the mean temperature of the warmest (TMAX) and coldest (TMIN) months were derived by the Mutual Climatic Range method. Assemblages from northern and western sites show a mid-Wisconsin interstadial TMAX warming from 35,000–30,000 yr B.P.; this warming is less pronounced in assemblages for interior regions. There is little or no beetle evidence for the spread of coniferous forest in eastern Beringia during this interstade, except for in parts of the Yukon Territory. During the last glacial maximum TMAX values were depressed by about 2°–2.5°C in Arctic regions of Beringia, and by about 4°C in the interior; TMIN values were about 8°C colder in both regions. TMAX and TMIN values rose rapidly at northern sites after 12,000 yr B.P. Seasonal temperatures peaked at 11,000 yr B.P., just as the Bering Land Bridge was inundated. This was followed by a sharp climatic cooling between 11,000 and 10,000 yr B.P., the equivalent of a Younger Dryas cooling in eastern Beringia.
Sharp Decrease in Summer Monsoon Strength 4000–3500 cal yr B.P. in the Central Higher Himalaya of India Based on Pollen Evidence from Alpine Peat
Authors: Phadtare Netajirao R.
Published: September 2002

Age-constrained pollen data and magnetic susceptibility of an alpine peat profile from the Garhwal Higher Himalaya display a continuous record of climate and monsoon trends for the past 7800 yr. About 7800 cal yr B.P., dominance of evergreen oak (Quercus semecarpifolia), alder (Alnus), and grasses in the pollen record reflect a cold, wet climate with moderate monsoon precipitation. From 7800 to 5000 cal yr B.P., vegetation was progressively dominated by conifers, indicating ameliorated climate with a stronger monsoon. A warm, humid climate, with highest monsoon intensity, from 6000–4500 cal yr B.P. represents the mid-Holocene climatic optimum. Between 4000 and 3500 cal yr B.P., the abundance of conifers sharply decreased, with the greatest increase in evergreen oak. This trend suggests progressive cooling, with a decrease in the monsoon to its minimum about 3500 cal yr B.P. Two relatively minor cold/dry events at ca. 3000 and 2000 cal yr B.P. marked step-wise strengthening of the monsoon until ca. 1000 cal yr B.P. After a cold/dry episode that culminated ca. 800 cal yr B.P., the monsoon again strengthened and continued until today. A sharp decrease in temperature and rainfall at 4000–3500 cal yr B.P. represents the weakest monsoon event of the Holocene record. This cold/dry event correlates with proxy data from other localities of the Indian subcontinent, Arabian Sea, and western Tibet.
Lake-Level Reconstruction and Paleohydrology of Birch Lake, Central Alaska, Based on Seismic Reflection Profiles and Core Transects
Authors: Abbott Mark B. , Finney Bruce P. , Edwards Mary E. , Kelts Kerry R.
Published: September 2002

Lake-level history for Birch Lake, Alaska, was reconstructed using seismic profiles and multiproxy sedimentary analyses including sedimentology, geochemistry, magnetic susceptibility, and palynology. Twenty-two seismic profiles (18 km total) and eight sediment cores taken from the lake margin to its depocenter at 13.5 m provide evidence for low lake stands during the late Pleistocene and Holocene. Thirty-one AMS radiocarbon dates of macrofossils and pollen provide a century-scale chronology. Prior to 12,70014C yr B.P., the lake, which now overflows, was either seasonally dry or desiccated for prolonged periods, indicating a severe period of aridity. Lake level rose more than 18 m between 12,700 and 12,20014C yr B.P. before falling to 17 m below the level of overflow. Between 11,600 and 10,60014C yr B.P. the water remained between 14 and 17 m below the overflow level. Onlap sedimentary sequences were formed during a transgression phase between 10,600 and 10,00014C yr B.P. Between 10,000 and about 880014C yr B.P. the lake was between 6 and 9 m below the overflow level. Lake level again rose, approaching the overflow level, between 8800 and 800014C yr B.P. Seismic and core evidence of minor erosional events suggest lowstands of 2–6 m until 480014C yr B.P. There have been no prolonged periods of lake-level depression since that time. The major restructuring of the climate system during deglaciation evidently generated a complex set of fluctuations in effective moisture in interior Alaska, which likely affected eolian processes and vegetation development, as well as lake levels.
Holocene Environmental Changes from the Rio Curuá Record in the Caxiuanã Region, Eastern Amazon Basin
Authors: Behling Hermann , da Costa Marcondes Lima
Published: September 2002

Holocene environments have been reconstructed by multiproxy studies of an 850-cm-long core from Rio Curuá dating to >800014C yr B.P. The low-energy river lies in the eastern Amazon rain forest in the Caxiuanã National Forest Reserve, 350 km west of Belem in northern Brazil. Sedimentological, mineralogical, and geochemical dates demonstrate that the deposits correspond to two different environments, sediments of an active river before 800014C yr B.P. and later a passive river system. The pollen analytical results indicate four different local and regional Holocene paleoenvironmental periods: (1) a transition to a passive fluvial system and a well-drained terra firme (unflooded upland) Amazon rain forest with very limited development of inundated forests (várzea and igapó) (>7990–703014C yr B.P.); (2) a sluggish river with a localMauritiapalm-swamp and similar regional vegetation, as before (7030–597014C yr B.P.); (3) a passive river, forming shallow lake conditions and with still-abundant terra firme forest in the study region (5970–247014C yr B.P.); and (4) a blocked river with high water levels and marked increase of inundated forests during the last 247014C yr B.P. Increased charcoal during this last period suggests the first strong presence of humans in this region. The Atlantic sea level rise was probably the major factor in paleoenvironmental changes, but high water stands might also be due to greater annual rainfall during the late Holocene.
Holocene Treeline History and Climate Change Across Northern Eurasia
Authors: MacDonald Glen M. , Velichko Andrei A. , Kremenetski Constantine V. , Borisova Olga K. , Goleva Aleksandra A. , Andreev Andrei A. , Cwynar Les C. , Riding Richard T. , Forman Steven L. , Edwards Tom W.D. , Aravena Ramon , Hammarlund Dan , Szeicz Julian M. , Gattaulin Valery N.
Published: September 2002

Radiocarbon-dated macrofossils are used to document Holocene treeline history across northern Russia (including Siberia). Boreal forest development in this region commenced by 10,000 yr B.P. Over most of Russia, forest advanced to or near the current arctic coastline between 9000 and 7000 yr B.P. and retreated to its present position by between 4000 and 3000 yr B.P. Forest establishment and retreat was roughly synchronous across most of northern Russia. Treeline advance on the Kola Peninsula, however, appears to have occurred later than in other regions. During the period of maximum forest extension, the mean July temperatures along the northern coastline of Russia may have been 2.5° to 7.0°C warmer than modern. The development of forest and expansion of treeline likely reflects a number of complimentary environmental conditions, including heightened summer insolation, the demise of Eurasian ice sheets, reduced sea-ice cover, greater continentality with eustatically lower sea level, and extreme Arctic penetration of warm North Atlantic waters. The late Holocene retreat of Eurasian treeline coincides with declining summer insolation, cooling arctic waters, and neoglaciation.
Holocene Paleohydrology and Paleoclimate at Treeline, North-Central Russia, Inferred from Oxygen Isotope Records in Lake Sediment Cellulose
Authors: Wolfe Brent B. , Edwards Thomas W. D. , Aravena Ramon , Forman Steven L. , Warner Barry G. , Velichko Andrei A. , MacDonald Glen M.
Published: September 2002

Lake-water oxygen-isotope histories for three lakes in northern Russia, derived from the cellulose oxygen-isotope stratigraphies of sediment cores, provide the basis for preliminary reconstruction of Holocene paleohydrology in two regions along the boreal treeline. Deconvolution of shifting precipitation δ18O from secondary evaporative isotopic enrichment is aided by knowledge of the distribution of isotopes in modern precipitation, the isotopic composition of paleo-waters preserved in frozen peat deposits, as well as other supporting paleoclimatic information. These data indicate that during the early Holocene, when the boreal treeline advanced to the current arctic coastline, conditions in the lower Yenisey River region were moist compared to the present, whereas greater aridity prevailed to the east near the lower Lena River. This longitudinal moisture gradient is consistent with the suggestion that oceanic forcing (increased sea-surface temperatures in the Nordic Seas and reduced sea-ice cover) was a major contributor to the development of a more maritime climate in western Eurasia, in addition to increased summer insolation. East of the Taimyr Peninsula, large tracts of the continental shelf exposed by glacial sea-level drawdown may have suppressed maritime climatic influence in what are now coastal areas. In contrast, during the late Holocene the two regions have apparently experienced coherent shifts in effective moisture. The similarity of the records may primarily reflect reduced North Atlantic influence in the Nordic Seas and southward retreat of coastline in eastern Siberia, coupled with declining summer insolation.