Early to mid-Holocene Atlantic water influx and deglacial meltwater events, Beaufort Sea slope, Arctic Ocean

Holocene high-resolution cores from the margin of the Arctic Ocean are rare. Core P189AR-P45 collected in 405-m water depth on the Beaufort Sea slope, west of the Mackenzie River delta (70°33.03′N and 141°52.08′W), is in close vertical proximity to the present-day upper limit of modified Atlantic water. The 5.11-m core spans the interval between ∼6800 and 10,400 ¹⁴C yr B.P. (with an 800-yr ocean reservoir correction). The sediment is primarily silty clay with an average grain-size of 9 φ. The chronology is constrained by seven radiocarbon dates. The rate of sediment accumulation averaged 1.35 mm/yr. Stable isotopic data (δ¹⁸O and δ¹³C) were obtained on the polar planktonic foraminifera Neogloboquadrina pachyderma (s) and the benthic infaunal species Cassidulina neoteretis. A distinct low-δ¹⁸O event is captured in both the benthic and planktonic data at ∼10,000 ¹⁴C yr B.P.—probably recording the glacial Lake Agassiz outburst flood associated with the North Atlantic preboreal cold event. The benthic foraminifera are dominated in the earliest Holocene by C. neoteretis, a species associated with modified Atlantic water masses. This species decreases toward the core top with a marked environmental reversal occurring ∼7800 ¹⁴C yr B.P., possibly coincident with the northern hemisphere 8200 cal yr B.P. cold event.     

Optically stimulated luminescence dating of late Holocene raised strandplain sequences adjacent to Lakes Michigan and Superior, Upper Peninsula, Michigan, USA

This study evaluates the accuracy of optically stimulated luminescence to date well-preserved strandline sequences at Manistique/Thompson bay (Lake Michigan), and Tahquamenon and Grand Traverse Bays (Lake Superior) that span the past ∼4500 yr. The single aliquot regeneration (SAR) method is applied to produce absolute ages for littoral and eolian sediments. SAR ages are compared against AMS and conventional ¹⁴C ages on swale organics. Modern littoral and eolian sediments yield SAR ages <100 yr indicating near, if not complete, solar resetting of luminescence prior to deposition. Beach ridges that yield SAR ages <2000 yr show general agreement with corresponding ¹⁴C ages on swale organics. Significant variability in ¹⁴C ages >2000 cal yr B.P. complicates comparison to SAR ages at all sites. However, a SAR age of 4280 ± 390 yr (UIC913) on ridge77 at Tahquamenon Bay is consistent with regional regression from the high lake level of the Nipissing II phase ca. 4500 cal yr B.P. SAR ages indicate a decrease in ridge formation rate after ∼1500 yr ago, likely reflecting separation of Lake Superior from lakes Huron and Michigan. This study shows that SAR is a credible alternative to ¹⁴C methods for dating littoral and eolian landforms in Great Lakes and other coastal strandplains where ¹⁴C methods prove problematic.     

Holocene variations in the Asian monsoon inferred from the geochemistry of lake sediments in central Tibet

We present a record of monsoon variations for the early and middle Holocene that is inferred from the geochemistry of sediment cores from Ahung Co, a lake in central Tibet. The resolution of this record is better than 50 yr and the age model is derived from radiocarbon ages of terrestrial charcoal, which eliminates errors associated with the lake hard-water effect. We made down-core geochemical measurements of % carbonate, % organic carbon, C/N and δ¹³C of bulk organic matter, δ¹³C and δ¹⁸O of carbonate, and % dolomite. Proxy calibration and modern water-balance reconstruction show that these are proxies for lake depth and the amount of monsoon precipitation. We find that lake level and monsoon precipitation have been decreasing at Ahung Co since the early Holocene (∼7500 cal yr B.P.). Superimposed on this trend are rapid declines in monsoon rainfall at 7000-7500 and 4700 cal yr B.P. and seven century-scale wet-dry oscillations. The cores do not contain sediment from the last ∼4000 yr. Surface sediments from the lake accumulated during the 20th century, however. From this, we argue that lake levels have risen again recently following a late Holocene dry period.     

A 23,000-yr pollen record from Lake Euramoo, Wet Tropics of NE Queensland, Australia

A new extended pollen and charcoal record is presented from Lake Euramoo, Wet Tropics World Heritage rainforest of northeast Queensland, Australia. The 8.4-m sediment core taken from the center of Lake Euramoo incorporates a complete record of vegetation change and fire history spanning the period from 23,000 cal yr B.P. to present. The pollen record is divided into five significant zones; 23,000–16,800 cal yr B.P., dry sclerophyll woodland; 16,800–8600 cal yr B.P., wet sclerophyll woodland with marginal rainforest in protected pockets; 8600–5000 cal yr B.P., warm temperate rainforest; 5000–70 cal yr B.P., dry subtropical rainforest; 70 cal yr B.P.–AD 1999, degraded dry subtropical rainforest with increasing influence of invasive species and fire.The process of rainforest development appears to be at least partly controlled by orbital forcing (precession), though more local environmental variables and human activity are also significant factors. This new record provides the opportunity to explore the relationship between fire, drought and rainforest dynamics in a significant World Heritage rainforest region.     

Holocene Climate in the Northern Great Plains Inferred from Sediment Stratigraphy, Stable Isotopes, Carbonate Geochemistry, Diatoms, and Pollen at Moon Lake, North Dakota

Seismic stratigraphy, sedimentary facies, pollen stratigraphy, diatom-inferred salinity, stable isotope (δ¹⁸O and δ¹³C), and chemical composition (Sr/Ca and Mg/Ca) of authigenic carbonates from Moon Lake cores provide a congruent Holocene record of effective moisture for the eastern Northern Great Plains. Between 11,700 and 9500¹⁴C yr B.P., the climate was cool and moist. A gradual decrease in effective moisture occurred between 9500 and 7100¹⁴C yr B.P. A change at about 7100¹⁴C yr B.P. inaugurated the most arid period during the Holocene. Between 7100 and 4000¹⁴C yr B.P., three arid phases occurred at 6600–6200¹⁴C yr B.P., 5400–5200¹⁴C yr B.P., and 4800–4600¹⁴C yr B.P. Effective moisture generally increased after 4000¹⁴C yr B.P., but periods of low effective moisture occurred between 2900–2800¹⁴C yr B.P. and 1200–800¹⁴C yr B.P. The data also suggest high climatic variability during the last few centuries. Despite the overall congruence, the biological (diatom), sedimentological, isotopic, and chemical proxies were occassionally out of phase. At these times the evaporative process was not the only control of lake-water chemical and isotopic composition.     

23,000 yr of vegetation history of the Upper Lerma, a tropical high-altitude basin in Central Mexico

Pollen analysis on a 9.54-m sediment core from lake Chignahuapan in the upper Lerma basin, the highest intermontane basin in Central Mexico (2570 m asl), documents vegetation and limnological changes over the past ∼23,000 ¹⁴C yr. The core was drilled near the archaeological site of Santa Cruz Atizapán, a site with a long history of human occupation, abandoned at the end of the Epiclassic period (ca. 900 AD). Six radiocarbon AMS dates and two well-dated volcanic events, the Upper Toluca Pumice with an age of 11,600 ¹⁴C yr B.P. and the Tres Cruces Tephra of 8500 ¹⁴C yr B.P., provide the chronological framework for the lacustrine sequence. From ca. 23,000 ¹⁴C yr B.P. to ca. 11,600 ¹⁴C yr B.P. the plant communities were woodlands and grasslands based on the pollen data. The glacial advances MII-1 and MII-2 correlate with abundant non-arboreal pollen, mainly grasses, from ca. 21,000 to 16,000 ¹⁴C yr B.P., and at ca. 12,600 ¹⁴C yr B.P. During the late Pleistocene, lake Chignahuapan was a shallow freshwater lake with a phase of lower level between 19,000 and 16,000 ¹⁴C yr B.P. After 10,000 ¹⁴C yr B.P., tree cover in the area increased, and a more variable lake level is documented. Late Holocene (ca. 3100 ¹⁴C yr B.P.) deforestation was concurrent with human population expansion at the beginning of the Formative period (1500 B.C.). Agriculture and manipulation of the lacustrine environment by human lakeshore populations appear at 1200 ¹⁴C yr B.P. (550 A.D.) with the appearance of Zea mays pollen and abundant charcoal particles.     

Holocene climate phases from buried soils in Tigray (northern Ethiopia): comparison with lake level fluctuations in the Main Ethiopian Rift

Stratigraphic analysis of alluvial/colluvial sequences and ¹⁴C dating have been used as proxies for Holocene climate changes in the highlands of Tigray (northern Ethiopia). The studied records show alternations of buried soils and peaty–clayey sediments, pointing to wet, stabilization phases, and organic-free colluvium layers resulting from the abrupt occurrence of dry-climate episodes. The ¹⁴C dates, mostly unpublished, cluster in the 11,090–9915, 9465–9135, 8450–7330, 6720–3635, 2710–2345, and 1265–790 cal yr B.P. time spans. Evidence of subsequent pedogenesis is lacking in the area, apart from a buried humified horizon dated at 300 ± 60 ¹⁴C yr B.P. (460–295 cal yr B.P.). Both the timing and the pattern of Tigray paleoclimatic events fit the corresponding framework, based on lake level changes, previously implemented for the Main Rift Valley. These findings give further support for arguing that the forcing mechanisms of the wet/dry fluctuations during the Holocene were effective over a large scale.     

The GISP2 δO Climate Record of the Past 16,500 Years and the Role of the Sun, Ocean, and Volcanoes

Measured ¹⁸O/¹⁶O ratios from the Greenland Ice Sheet Project 2 (GISP2) ice core extending back to 16,500 cal yr B.P. provide a continuous record of climate change since the last glaciation. High-resolution annual ¹⁸O/¹⁶O results were obtained for most of the current millennium (A.D. 818-1985) and record the Medieval Warm Period, the Little Ice Age, and a distinct 11-yr ¹⁸O/¹⁶O cycle. Volcanic aerosols depress central Greenland annual temperature (1.5°C maximally) and annual ¹⁸O/¹⁶O for about 4 yr after each major eruptive event. On a bidecadal to millennial time scale, the contribution of solar variability to Holocene Greenlandic temperature change is 0.4°C. The role of thermohaline circulation change on climate, problematic during the Holocene, is more distinct for the 16,500-10,000 cal yr B.P. interval. (Analogous to ¹⁴C age calibration terminology, we express time in calibrated (cal) yr B.P. (A.D. 1950 = 0 cal yr B.P.)). The Oldest Dryas/Bølling/Older Dryas/Allerød/Younger Dryas sequence appears in great detail. Bidecadal variance in ¹⁸O/¹⁶O, but not necessarily in temperature, is enhanced during the last phase of lateglacial time and the Younger Dryas interval, suggesting switches of air mass transport between jet stream branches. The branched system is nearly instantaneously replaced at the beginning of the Bølling and Holocene (at 14,670 and 11,650 cal yr B.P., respectively) by an atmospheric circulation system in which ¹⁸O/¹⁶O and annual accumulation initially track each other closely. Thermodynamic considerations of the accumulation rate-temperature relationship can be used to evaluate the ¹⁸ O/¹⁶O-temperature relationship. The GISP2 ice-layer-count years of major GISP2 climate transitions also support the use of coral ¹⁴C ages for age calibration.     

Integrating Stalagmite, Vertebrate, and Pollen Sequences to Investigate Holocene Vegetation and Climate Change in the Southern Midwestern United States

Speleothem carbon and oxygen isotopic records from Onondaga Cave, south-central Missouri, and Beckham Creek Cave, north-central Arkansas, are compared with the Cupola Pond and Oldfield Swamp pollen series from southeastern Missouri and the Rodgers Shelter and Modoc Shelter vertebrate biostratigraphic sequences from central Missouri and southwestern Illinois. Similar, and roughly contemporaneous, shifts between deciduous forest and steppe indicators throughout the Holocene are revealed in each database. These independent proxies record steppe conditions between approximately 9000 and 1500 cal yr B.P. A shift toward lighter speleothem carbon may reflect a change from warm and dry to cool and dry conditions between 4500 and 3000 yr B.P. The sensitive response of speleothem δ¹³C to changes in vegetation emphasizes their importance as paleoclimate records in an area containing few other millenial-scale climate proxies.     

Phytolith evidence for C4-dominated grassland since the early Holocene at Long Pocket, northeast Queensland, Australia

Preliminary phytolith analysis of ephemeral lake fill sediment at Long Pocket, near Toomba, northeast Queensland, Australia, indicates that a C4-dominated grassland with a minor woody component has been present in the region since ca. 8000 cal yr B.P. Based on the modern distribution of C4 and C3 native grasses in Australia, this suggests that mean summer temperatures of at least 14°C (ca. 10°C cooler than present) were maintained since the early Holocene. This interpretation is comparable with previous studies, which together imply that the establishment of C4-dominated grasses in central and northeast Australia occurred between the last glacial maximum (most likely after ca. 16,000 ¹⁴C yr B.P.) and ca. 7200 ¹⁴C yr B.P. (ca. 8000 cal yr B.P.). Taxonomic composition of the grassland appears relatively consistent since the early Holocene at Long Pocket and includes phytoliths comparable with those from modern Arundinoideae, Panicoideae, and Chloridoideae. Rare non-grass phytoliths are also present. A gradual decrease in abundance of saddle phytolith forms (attributed to Chloridoideae grasses) from the base of the record at ca. 6500-7000 cal yr B.P. suggests decreasing aridity throughout the Holocene. This trend could reflect a locally drawn out effect of the end of the postglacial arid period due to the well-drained basalt flow catchment maintaining a local arid habitat for the Chloridoideae grasses.     

Evidence for high-frequency late Glacial to mid-Holocene (16,800 to 5500 cal yr B.P.) climate variability from oxygen isotope values of Lough Inchiquin, Ireland

A 7.6-m core recovered from Lough Inchiquin, western Ireland provides evidence for rapid and long-term climate change from the Late Glacial period to the Mid-Holocene. We determined percentage of carbonate, total organic matter, mineralogy, and δ¹⁸O_calcite values to provide the first high-resolution record of climate variability for this period in Ireland. Following deglaciation, rapid climate amelioration preceded large increases in GISP2 δ¹⁸O_ice values by ∼2300 yr. The Oldest Dryas (15,100 to 14,500 cal yr B.P.) Late Glacial event is documented in this record as a decrease in δ¹⁸O_calcite values. Brief warming at ∼12,700 cal yr B.P. was followed by characteristic Younger Dryas cold and dry climate conditions. A rapid increase in δ¹⁸O_calcite values at ∼10,500 cal yr B.P. marked the onset of Boreal warming in western Ireland. The 8200 cal yr B.P. event is represented by a brief cooling in our record. Prior to general warming, a larger and previously undescribed climate anomaly between 7300 and 6700 cal yr B.P. is characterized by low δ¹⁸O_calcite values with high-frequency variability.     

Geoarchaeology of the Boca Negra Wash Area,Albuquerque Basin,New Mexico,USA

Dozens of Paleoindian sites, including the Boca Negra Wash (BNW) Folsom site (LA 124474), are scattered across a basalt plateau (the West Mesa) on the western side of the Albuquerque Basin, and adjacent uplands. The BNW site, like many others in the area, is located near a small (˜60 × 90 m) playa basin that formed in a depression on the basalt surface and was subsequently covered by an eolian sand sheet (Unit 1) dated by OSL to ˜23,000 yr B.P. Most of the basin fill is ˜2 m of playa mud (Units 2 and 3) dating ˜13,970 ¹⁴C yr B.P. (17,160–16,140 cal yr B.P.) at the sand–mud interface to ˜2810 ¹⁴C yr B.P. (˜2960–2860 cal yr B.P.) at the top. C/N ratios suggest that the BNW playa basin probably held water more often during the Folsom occupation; stable carbon isotope values indicate C3 vegetation was more common as well, but C4 grasses became dominant in the Holocene. Cores extracted from four playa basins nearby revealed a similar stratigraphy and geochronology, documenting presence of wetlands on playa floors during the Paleoindian occupation of the area. © 2006 Wiley Periodicals, Inc.     

A multiproxy climate record from a raised bog in County Fermanagh,Northern Ireland: a critical examination of the link between bog surface wetness and solar variability

A proxy climate record from a raised bog in County Fermanagh, Northern Ireland, is presented. The record spans the interval between 2850 cal. yr BC and cal. yr AD 1000 and chronological control is achieved through the use of tephrochronology and ¹⁴C dating, including a wiggle‐match on one section of the record. Palaeoclimatic inferences are based on a combination of a testate amoebae‐derived water table reconstruction, peat humification and plant macrofossil analyses. This multiproxy approach enables proxy‐specific effects to be identified. Major wet shifts are registered in the proxies at ca. 1510 cal. yr BC, 750 cal. yr BC and cal. yr AD 470. Smaller magnitude shifts to wetter conditions are also recorded at ca. 380 cal. yr BC, 150 cal. yr BC, cal. yr AD 180, and cal. yr AD 690. It is hypothesised that the wet shifts are not merely local events as they appear to be linked to wider climate deteriorations in northwest Europe. Harmonic analysis of the proxies illustrates statistically significant periodicities of 580, 423–373, 307 and 265 years that may be related to wider Holocene climate cycles. This paper illustrates how the timing of climate changes registered in peat profiles records can be precisely constrained using tephrochronology to examine possible climatic responses to solar forcing. Relying on interpolated chronologies with considerable dating uncertainty must be avoided if the climatic responses to forcing mechanisms are to be fully understood. Copyright © 2007 John Wiley & Sons, Ltd.     

Uranium-Series Dating of Carbonate (Tufa) Deposits Associated with Quaternary Fluctuations of Pyramid Lake, Nevada

Uranium-series dating of dense tufa deposited in a small cave, at former lake margins, and in large tufa mounds clarifies the timing of lake-level variation during the past 400,000 yr in the Pyramid Lake basin. A moderate-sized lake occasionally overflowed the Emerson Pass sill at elevation of 1207 m between ca. 400,000 and 170,000 and from ca. 60,000 to 20,000 yr B.P., as shown by²³⁰Th/²³⁴U ages of the cave samples,²³⁰Th-excess ages of tubular tufas, and average isochron-plot ages of shoreline-deposited tufas. (By comparison, modern Pyramid Lake is 50 m below this sill). There is a lack of tufa record during the intervening period from ca. 170,000 to 60,000 yr B.P. After ca. 20,000 yr, Pyramid Lake underwent abrupt changes in level and, based on previous¹⁴C ages, reached its highest elevation (ca 1335 m) at ca. 14,000 yr B.P. The youngest uranium-series ages are comparable with previously reported¹⁴C ages.     

Vegetation and climate change since 14,810 C yr B.P. in southeastern Uruguay and implications for the rise of early Formative societies

This article presents a combined pollen and phytolith record of a 1.70-m sediment core from the wetlands of India Muerta (33° 42′ S, 53° 57′ W) in the lowland Pampa (grasslands) of southeastern Uruguay. Six ¹⁴C dates and the pollen and phytolith content of the samples permitted the recognition of four distinct climatic periods between 14,850 ¹⁴C yr B.P. and the present. The Late Pleistocene period (between ca. 14,810 and ca. 10,000 ¹⁴C yr B.P.) was characterized by drier and cooler conditions indicated by the presence of a C3-dominated grassland. These conditions prevailed until the onset of the warmer and more humid climate of the Holocene around 9450 ¹⁴C yr B.P. The early Holocene (between around 10,000 and 6620 ¹⁴C yr B.P.) was characterized by the establishment of wetlands in the region as evidenced by the formation of black peat, the increase in wetland taxa, and the replacement of C3 Pooideae by C4 Panicoideae grasses. During the mid-Holocene, around 6620 ¹⁴C yr B.P., began a period of environmental change characterized by drier climatic conditions, which resulted in the expansion of halophytic communities in the flat, low-lying areas of the wetlands of India Muerta. About 4020 ¹⁴C yr B.P. a massive spike of Amaranthaceae/Chenopodiaceae coupled with a radical drop in wetland species indicates another major and more severe period of dryness. After ca. 4000 ¹⁴C yr B.P., a decrease of halophytic species indicates the onset of more humid and stable climatic conditions, which characterized the late Holocene.The findings reported in this article substantially improve our knowledge of the late Glacial and Holocene climate and vegetation in the region. The data provide a detailed record of the timing and severity of mid-Holocene environmental changes in southeastern South America. Significantly, the mid-Holocene drying trend coincided with major organizational changes in settlement, subsistence, and technology of the pre-Hispanic populations in the region, which gave rise to early Formative societies. This study also represents the first combined pollen and phytolith record for southeastern South America reinforcing the utility of phytoliths as significant indicators of long-term grassland dynamics.     

Late quaternary floodplain sedimentation along the Pomme de Terre River,southern Missouri

New cross sections and dates from along the Pomme de Terre River clarify the complex local history of valley development and floodplain sedimentation. The observed history begins with a series of ancient bedrock strath terraces that record past bedrock valley positions at 15.5 to more than 58 m above the modern bedrock floor. Each strath is capped by 1–2 m of channel gravel and sand permeated by red clay. Sometime previous to ca. 140,000 yr B.P., a much lower bedrock valley only about 5–6 m above the modern level was excavated. By 140,000 yr B.P., accumulation of red and gray mottled silty clay had commenced, and had reached to 8.5 m above the modern floodplain before 48,900 ± 900 ¹⁴C yr B.P. Sometime between ca. 49,000 and 45,000 ¹⁴C yr B.P., erosion caused abandonment of an oxbow meander, and lowered the bedrock valley to about its present depth. Younger yellowish-red and gray mottled silty clay alluvium then began accumulating. This mid-Wisconsinan fill reached to 2.5 m above the modern floodplain sometime before 31,800 ± 1340 ¹⁴C yr B.P., at which time another erosional phase was in progress. A late Wisconsinan olive clay accumulated between 27,480 ± 1950 and ca. 23,000 ¹⁴C yr B.P., followed by approximate stability until 13,550 ± 400 ¹⁴C yr B.P. After stability, an erosional episode began, but by 10,200 ± 330 ¹⁴C yr B.P., deposition of a distinctive brown clayey silt was underway. This early Holocene fill reached to about the same level as the mid-Wisconsinan fill by 8100 ± 140 ¹⁴C yr B.P. Erosion occurred between this date and 7490 ± 170 ¹⁴C yr B.P., but the former floodplain level was rapidly reattained, and was apparently stable until ca. 5000 ¹⁴C yr B.P. Finally, erosional unconformities and 17 dates from the brown clayey silt, and from younger grayish-brown silty sand underlying the modern floodplain, record subsequent episodes of floodplain erosion at ca. 5000, 2900, 1500 and 350 ¹⁴C yr B.P. The timing of Pomme de Terre floodplain sedimentary regimes, characterized by net aggradation, erosion, or stability, may have been controlled by climate. In particular, both periods of stability appear to have been coeval to times of strongly zonal upper atmospheric circulation. Intensified zonal circulation would have resulted in less frequent large floods and an increased dominance by floods of small to moderate size. In contrast, there are no obvious parallels to be drawn between this local alluvial history and sea level or glacial outwash induced baselevel changes.     

Middle and Late Holocene Sea-Level Changes in Eastern Maine Reconstructed from Foraminiferal Saltmarsh Stratigraphy and AMS C Dates on Basal Peat

A relative sea-level history is reconstructed for Machiasport, Maine, spanning the past 6000 calendar years and combining two different methods. The first method establishes the long-term (10³ yr) trend of sea-level rise by dating the base of the Holocene saltmarsh peat overlying a Pleistocene substrate. The second method uses detailed analyses of the foraminiferal stratigraphy of two saltmarsh peat cores to quantify fluctuations superimposed on the long-term trend. The indicative meaning of the peat (the height at which the peat was deposited relative to mean tide level) is calculated by a transfer function based on vertical distributions of modern foraminiferal assemblages. The chronology is determined from AMS ¹⁴C dates on saltmarsh plant fragments embedded in the peat. The combination of the two different approaches produces a high-resolution, replicable sea-level record, which takes into account the autocompaction of the peat sequence. Long-term mean rates of sea-level rise, corrected for changes in tidal range, are 0.75 mm/yr between 6000 and 1500 cal yr B.P. and 0.43 mm/yr during the past 1500 years. The foraminiferal stratigraphy reveals several low-amplitude fluctuations during a relatively stable period between 1100 and 400 cal yr B.P., and a sea-level rise of 0.5 m during the past 300 years.     

Variations in Sediment Yield from the Upper Doubs River Carbonate Watershed (Jura, France) since the Late-Glacial Period

The Upper Doubs River Valley is a 910-km²watershed feeding into Lake Chaillexon. The lake was formed by a natural rockfall at the end of the Bølling Chronozone (around 14,250 cal yr B.P.) and since then has trapped material eroded from the watershed. The filling process and variations in sediment yield have been investigated by mechanical coring, seismic surveys, and electric soundings. The detrital sediment yield of the upstream watershed can be calculated by quantifying the sedimentary stocks for each climatic stage of the Late-Glacial period and Holocene Epoch and estimating the lake's entrapment capacity. This enables us to determine the intensity of the erosion processes in relation to climate and environmental factors. The Bølling–Allerød Interstade produced the greatest yields with mean values of 19,500 metric tons per calendar year (t/yr). The Younger Dryas Chronozone saw a sharp fall (8900 t/yr) that continued into the Preboreal (2100 t/yr). Clastic supply increased during the Boreal (4500 t/yr) before declining again in the Early Atlantic (2400 t/yr). Since then, yields have risen from 4500 t/yr in the Late Atlantic to 6800 t/yr in the Subboreal and 11,100 t/yr in the Subatlantic. Comparison of quantitative data with the qualitative analysis of the deposits and with the paleohydrologic curve of the watershed based on level fluctuations in lakes around Chaillexon shows that climate was the controlling factor of sediment yield until the Late Atlantic. From the Late Atlantic–Subboreal around 5400 cal yr B.P. (4700¹⁴C yr B.P.) and especially from the end of the Subboreal Chronozone and during the Subatlantic Chronozone (2770 cal yr B.P./2700¹⁴C yr B.P.–present) climatic constraints have been compounded by human activity related to forest clearing and land use.     

Eolian quartz flux variations in Cheju Island, Korea, during the last 6500 yr and a possible Sun-monsoon linkage

We have obtained a high-resolution sedimentary record covering the last 6500 yr from a maar in Cheju Island, Korea, in order to reconstruct the history of variations in the eolian quartz flux (EQF) and hence Asian dust. The long-term variation of EQF reveals three intervals: a period of high EQF (4000-2000 cal yr B.P.) and two periods of low EQF (6500-4000 cal yr B.P. and 2000 cal yr B.P. to present), which have been affected by the East Asian monsoon due to insolation change and the cold air activity in high latitudes correlated with polar high-pressure systems. This long-term variation is superimposed by millennial- and centennial-scale fluctuations with periodicities of 1137, 739, 214, 162, 137, 127, and 111 yr, implying drier conditions in the source areas in China. The detrended EQF record correlates visually and statistically (cross-spectral analysis) with the atmospheric Δ¹⁴C record (solar proxy). The centennial-scale variability in EQF may be affected by the solar activity through the Sun-East Asian monsoon linkage.     

Environmental history since 11,000 C yr B.P. of the northeastern Pampas, Argentina, from alluvial sequences of the Luján River

Sedimentological, malacological, and pollen analyses from ¹⁴C-dated alluvial sections from the Luján River provide a detailed record of environmental changes during the Holocene in the northeastern Pampas of Argentina. From 11,200 to 9000 ¹⁴C yr B.P., both sedimentary and biological components suggest that the depositional environment was eutrophic, alkaline, and freshwater to brackish shallow water bodies without significant water circulation. During this time, bioclastic sedimentation was dominant and the shallow water bodies reached maximum development as the climate became more humid, suggesting an increase in precipitation. Short-term fluctuations in climate during the last stage of this interval may have been sufficient to initiate changes in the water bodies, as reduction of the volume alternated with periods of flooding. The beginning of the evolution of shallow swamps in the wide floodplain or huge wetlands was contemporaneous with a sea level lower than the present one. From 9000 and 7000 ¹⁴C yr B.P., mesotrophic, alkaline, brackish, probably anoxic swamps existed. Between 7000 and 3000 ¹⁴C yr B.P., anoxic calcareous swamps were formed, with subaerial exposure and development of the Puesto Berrondo Soil (3500-2900 ¹⁴C yr B.P.). A trend to a reduction of water bodies is recorded from 9000 to ca. 3000 ¹⁴C yr B.P., with a significant reduction after ca. 7000 ¹⁴C yr B.P. A shift to subhumid-dry climate after 7000 ¹⁴C yr B.P. appears to be the main cause. During this time, an additional external forcing toward higher groundwater levels was caused by Holocene marine transgression causing changes in the water bodies levels. The climate became drier during the late Holocene (ca. 3000 yr B.P.), when clastic sedimentation increased, under subhumid-dry conditions. Flood events increased in frequency during this time. From ca. A.D. 1790 to present, the pollen record reflects widespread disturbance of the vegetation during the European settlement.