Late Quaternary eolian sand depositional record for southwestern Kansas: Landscape sensitivity to droughts

Eolian deposits and landforms are ubiquitous in western Kansas, particularly south of the Arkansas River with the presence of a stabilized dune field. Stratigraphic studies and associated optical dating reveal a complex depositional history for this dune field spanning the late Quaternary. The oldest eolian deposits from ca. 16 to 12 ka completely or partially bury high terraces with ages of ca. 16 and 33 ka. It is unknown whether these eolian deposits reflect regional aridity and/or a change in sediment availability with the transition of the Arkansas River from a single channel to a braided system. There is pervasive evidence for episodic eolian erosion and deposition ca. 9.8 to 6.3 ka, generally coincident with loess deposition on upland surfaces [Olson, C.G., Nettleton, W.D., Porter, D.A., Brasher, B.R., 1997. Middle Holocene aeolian activity on the High Plains of west-central Kansas. Holocene, 7(3): 255–261], and thus it is inferred to reflect regional aridity. Sites within the dune field show a sequence of eolian sands and weak buried soils, reflecting either dune migration or sand sheet accretion at ca. 1490, 430, 380–320, 180, and 70 yr ago, which correspond well with continental-scale droughts in the tree-ring record. Eolian sand in the Arkansas River dune field may be derived principally with variability in fluvial activity and in climate during the late Pleistocene to the middle Holocene, with substantial reactivation of eolian systems during decadal-scale drought variability in the past 2000 yr.     

Sources and accumulation rates of organic carbon in an equatorial peat bog (Burundi, East Africa) during the Holocene: carbon isotope constraints

The relationship between organic carbon accumulation rates and ¹³C/¹²C ratios of total organic carbon (TOC) was investigated in an highland peat bog core (Ru-3) from Equatorial Africa. This core yielded a sequence spanning the last 14 kyr and was analysed with a 100–300 yr resolution for TOC-δ¹³C values. The Holocene section shows contrasted TOC accumulation regimes and TOC δ¹³C varying between −28.5 and −19.5‰ with a few very short ‘isotopic excursions' (dated at ca. 9.3, 7.5, 4.2 ka B.P.). The organic carbon accumulation rates range from 2 to 20 mg C cm⁻² yr⁻¹. They increase when TOC becomes more depleted in ¹³C, notably between 12 and 9.8 ka B.P., 8.5 and 7.8 ka B.P. and after 1.6 ka B.P. Periods of restricted carbon storage correspond to heavier TOC accumulation at 9.3, and between 7.5 and 1.6 ka B.P. At the study site, the δ-variations can be related to variable C4-plant inputs, and possibly, to changes in the fractionation between CO₂ and the organic carbon in C3 vascular plants. The Ru-3 record indicates restricted carbon storage during the periods of increased contribution from C4 plants and/or of decreased fractionation between CO₂ and organic carbon in C3 plants. Changes in TOC-δ¹³C values in core Ru-3 seem to match fluctuations of East Equatorial African lakes. High lake stands correspond to low δ¹³C intervals and vice versa. This points to indirect climatic forcing of δ¹³C changes in intertropical peats.     

The upper Quaternary sedimentary sequence at the Rieti Basin (central Italy): a record of sedimentation response to climatic changes

Lithostratigraphy, chronology and depositional environments of sixteen Holocene alluvial fill sediment cores from the southern part of the Conca of Rieti (central Italy) was established by sedimentological, palaeontological and ¹⁴C analyses. The study area experienced a lacustrine (or palustrine) conditions and the water level was effectively controlled by a natural damming of the valley due to intermitted travertine deposition at Le Marmore site. Apart from cores S10, S13, S14 and S8 which, at their bottom, cored sediments referable to ?Upper Pliocene and Middle Pleistocene which are not the object of this paper, as a rule the drilled sediments can be referred, bottom to top, to the following four episodes: interval a, corresponding to freshwater lacustrine/palustrine environment affected by alluvial fan progradations from the edges of the valley and active by ?Middle/?Late Pleistocene to early Holocene; in this interval, several hiatuses are supposed but are not evidenced by the bad state of preservation of the sediment cores; interval b, from 361 to 371–376 m a.s.l., recording a decrease of the water level, bracketed between the ¹⁴C ages of 6000 and 2700 yr B.P., which produced the formation of distinct shallow ponds/bogs, rich in aquatic vegetation; interval c, from 381 to 384 m a.s.l., indicating a further rise of the water level, recorded by sediments containing abundant molluscs and ostracods; interval d, the upper part of the cores, records an almost continuous drying up the area as shown by abundant hydrophilic land molluscs; and interval e, the top of all sediment cores, representing the present-day soil. It has been found that the water level fluctuations occurring in the ancient Conca are chronologically consistent with the postglacial climate pattern. It is further noticed that the episodes of water highstand correspond to the Boreal–Atlantic and Subatlantic climatic ameliorations, respectively, and that the water level dropped during the global cooling episodes recurred from ca. 6000 to 2100 yr B.P. The resulting palaeoenvironmental history agrees with that previously reported for the northern reach of the Conca of Rieti, thus suggesting a common setting for the whole area.     

A late pleistocene and holocene vegetation and environmental record from Lake Wangoom, Western Plains of Victoria, Australia

Pollen, microfaunal and sedimentological evidence from the top 20 m of sediment in a closed volcanic crater lake is used to construct a detailed record of vegetation and environmental conditions through the Holocene and a substantial part of the Late Pleistocene. Radiocarbon dating suggests that the sequence covers tha last 51,000 yr or so. High lake-levels and the presence of forest or woodland vegetation indicate that the Holocene and the basal few thousand years of the record experienced wet and warm conditions. The earliest period was succeeded by a long phase of lower but variable moisture levels before more arid conditions resulted in the replacement of forest and woodland by herbaceous vegetation and frequent lake drying. Periods of slight amelioration occurred between about 27,000 and 19,000 and before 15,000 radiocarbon yr ago. Maximum aridity occurred between about 19,000 and 10,000 yr B.P., a period incorporating the height of the last glacial. During the Pleistocene, the lake became gradually more saline but has been fresh during the Holocene, even under low lake-levels.

An increase in charcoal values is recorded, probably around 20,000 years ago, and this could have resulted from increased burning as a result of the activities of Aboriginal people. This may have also caused a change in understorey vegetation from one composed largely of Asteraceae, to grassland.

The record is compared with others from southeastern Australia and there is good correspondence through the last 20,000 or 30,000 yr but, before this, problems of dating and the condensed nature of most sequences prevent detailed correlation.     

Climatic conditions in Southern Africa since the last glacial maximum, inferred from pollen analysis

A review of Late Quaternary palaeoclimatic data derived from several pollen sequences, between ca. 22 and 34°S in Southern Africa, shows a degree of similarity in temperature and moisture variations between the various site. Pollen data from sites such as Wonderkrater and Rietvlei (Transvaal), Tate Vondo (Venda), Elim (Orange Free State), Equus Cave (Southern Kalahari) and Boomplaas (Cape Province) suggest relative dryness during the Last Glacial Maximum, wet conditions during the Late Glacial, and dry conditions during the Early Holocene ca. 8000 yr B.P., followed by progressively moister conditions peaking soon after the development of a temperature optimum ca. 6500 yr. B.P. Problems with radiocarbon dating of polleniferous spring deposits, however, prevent precise correlations of especially Late Pleistocene sequences. Different seasonal patterns in the Late Glacial and Early Holocene may explain palaeobotanical data and are partly compatible with simulations of past climates of Kutzbach and Guetter (1986).     

Late Holocene lake-level fluctuations in Walker Lake, Nevada, USA

Walker Lake, a hydrologically closed, saline, and alkaline lake, is situated along the western margin of the Great Basin in Nevada of the western United States. Analyses of the magnetic susceptibility (χ), total inorganic carbon (TIC), and oxygen isotopic composition (δ¹⁸O) of carbonate sediments including ostracode shells (Limnocythere ceriotuberosa) from Walker Lake allow us to extend the sediment record of lake-level fluctuations back to 2700 years B.P. There are approximately five major stages over the course of the late Holocene hydrologic evolution in Walker Lake: an early lowstand (> 2400 years B.P.), a lake-filling period ( 2400 to  1000 years B.P.), a lake-level lowering period during the Medieval Warm Period (MWP) ( 1000 to  600 years B.P.), a relatively wet period ( 600 to  100 years B.P.), and the anthropogenically induced lake-level lowering period (< 100 years B.P.). The most pronounced lowstand of Walker Lake occurred at  2400 years B.P., as indicated by the relatively high values of δ¹⁸O. This is generally in agreement with the previous lower resolution paleoclimate results from Walker Lake, but contrasts with the sediment records from adjacent Pyramid Lake and Siesta Lake. The pronounced lowstand suggests that the Walker River that fills Walker Lake may have partially diverted into the Carson Sink through the Adrian paleochannel between 2700 to 1400 years B.P.     

Late quaternary pollen records from two Ugandan mires: evidence for environmental changes in the Rukiga Highlands of southwest Uganda

High resolution pollen data from two mires in the Rukiga Highlands, southwest Uganda (Ahakagyezi Swamp and Muchoya Swamp) provide detailed evidence of the environmetal history of the region since ca. 42 000 yr B.P. According to these data, there a was period pre ca. 42 000 yr B.P.>. when climatic conditions were similar to the present. Conditions considerably colder and drier than the present occurred during two periods in the past (pre ca. 42 000 yr B.P. to ca. 32 000 yr B.P., and ca. 21 000 yr B.P. to ca. 12 000 yr B.P.). The most recent period of climatic improvement began ca. 14 000 yr B.P. Moist lower montane forest colonised the Muchoya catchment by ca. 11 100 yr B.P. and the Ahakagyezi catchment by ca. 10 600 yr B.P. indicating the establishment there of climatic conditions broadly similar to the present.

Pollen data from both mires reveal forest disturbances beginning ca. 2200 yr B.P. This disturbances is likely to have been human-induced.     

A 12 000-year record of environmental change in the Lomond Hills, Fife, Scotland: Vegetational and climatic variability

The Lomond Hills of Fife, an isolated upland area rising to over 500 m, provide an opportunity to investigate the effect of altitude on vegetation and climate in an area otherwise dominated by lower-lying land. The West Lomond site contains sediments of the Devensian Late-glacial period; they reveal a well-defined sequence of Bolling-Older Dryas-Allerod-Younger Dryas events, commencing ca. 12 190 radiocarbon years B.P. and a probable Amphi-Atlantic Oscillation between ca. 11 040 and 10 800 B.P. The Holocene record is constrained by low sediment input but does reveal a woodland presence at this altitude, dominated byBetula andCorylus. Size statistics forBetula pollen are presented and the implications of the vegetational and climatic record are discussed. The traditional view of a smooth progress towards more temperate conditions following the Younger Dryas is not supported; between 10 180 and 9120 B.P., three cooler periods are inferred, the earliest of which may belong to a terminal phase of the Younger Dryas. Comparative pollen ‘influx’ data strongly suggest thatQuercus,Ulmus andAlnus were not present locally. As a working hypothesis, it is suggested that the demise of woodland, from ca. 5950 B.P., was a result of exposure. Pollen indicative of human impact was probably derived from areas of lowland agricultural activity from ca. 5330 B.P. onwards.     

A 36,000-yr vegetation history from the Peloncillo Mountains, southeastern Arizona, USA

A 36,200 cal yr B.P. vegetation history was developed from macrofossils and pollen from 55 packrat middens from 1287 to 1442 m elevation in the Peloncillo Mountains of southeasternmost Arizona, USA. Today, these elevations are dominated by semidesert grassland with a mixture of Chihuahuan and Sonoran Desert shrubs, including an eastern disjunct population of jojoba (Simmondsia chinensis). From 36,200 to 15,410 cal yr B.P., rocky areas just above large, pluvial lakes that occupied what are now dry playas supported Pinus edulis, Juniperus osteosperma, Juniperus cf. coahuilensis, Quercus cf. turbinella and a rich understory of summer-flowering C₄ annuals and grasses, indicating abundant summer rains and mild winters. After  15,410 cal yr B.P., P. edulis declined in abundance and disappeared briefly at 13,925 cal yr B.P., coincident with expansion of more xeric species and paleohydrological evidence for regional aridity during the Bølling–Allerød warm interval. P. edulis rebounded briefly during the Younger Dryas at 12,405 cal yr B.P. before disappearing along with other mesic woodland species sometime after 12,100 cal yr B.P. The few middens dating from the early to middle Holocene (10,000–4000 yr B.P.) indicate wetter conditions than today at 7790 cal yr B.P. followed by a general drying trend. The 35 middens from the late Holocene detail the sequential arrival of desertscrub species as vegetation became more modern in character.     

The laminated sediments of Loch Ness, Scotland: Preliminary report on the construction of a chronology of sedimentation and its potential use in assessing Holocene climatic variability

Two sediment cores, ca. 6 m long, have been recovered from the north basin of Loch Ness, Scotland. Each consists of ca. 4.5 m of laminated gyttja, terminating in a basal grey, unlaminated clay. A suite of three ¹⁴C AMS dates have been obtained, and place the base of the gyttja at ca. 9500 yr B.P. Investigations into the structure, composition and formation of the laminae are in progress. Analysis by Backscatter Scanning Electron Microscopy (BSEM) has demonstrated that they consist of couplets comprising dark, clay-rich sediments thought to be deposited from spring to autumn, and pale, silt-rich layers believed to represent sedimentation through winter. The laminae are thus thought to record incidence and intensity of streamflow into the Loch over the past nine millennia, and thus illustrate environmental change over the region for most of the Holocene. X-ray densitometry has been utilised in order to count the laminations and test the hypothesis that they are varves. It has, however, proved difficult to obtain a continuous sequence of countable laminations, although it has been possible to construct a fragmented, floating chronology which indicates that the hypothesis may be correct. Examination of lamination thickness reveals that although the average rate of sedimentation throughout the time periods studied seems to have remained fairly constant, significant variations have occurred. As Loch Ness is located on the northwest oceanic fringe of Europe, any climatic signal which the laminations contain will be closely related to even larger scale events over and within the North Atlantic Ocean, which is a major controller of global climate. Preliminary statistical investigation of sequences of laminae is being carried out in order to search for periodicity of sedimentation which may then be related to appropriate climatic indices.     

Holocene sedimentary history of some coastal plains in Hokkaido,Japan V. Sedimentary history of Kushu Lake and Akkeshi

Diatom assemblages of sediments obtained from Kushu Lake and Akkeshi were analyzed in order to clarify the local Holocene sedimentary history. The results revealed the following: 1) Sometime before about 9000 yr B.P., Kushu Lake was originally a freshwater environment. 2) The first marine diatom zone (MD₁ Zone) was deposited on the bottom of the paleo-Kushu Bay between 8500 and 6000 yr B.P. 3) This site changed to become the bottom of the paleo-Kushu Lagoon around 6000 yr B.P. owing to the formation of a bay-mouth bar across the paleo-Kushu Bay and the first Holocene regression resulted in a freshwater lake at about 4500 yr B.P. 4) At this time, the Akkeshi site changed from a sublittoral shore in a drowned valley to a peat moor. 5) The second Holocene transgression at about 3000 yr B.P. was detected at the Akkeshi site. 6) At Kushu Lake site the occurrence ofMastogloia elliptica between 7000 and 5000 yr B.P. suggested the influx of the Tsushima warm current into the Japan Sea.     

Holocene sedimentary history of some coastal plains in Hokkaido,Japan IV. Diatom assemblages in the sediments from Kushiro moor (2)

Diatom assemblages of sediments obtained from three sites on Kushiro Moor were analyzed to investigate the Holocene sedimentary history. The results showed that: 1) The Takkobu site was originally at the bottom of the paleo-Kushiro Bay, and after-wards the paleo-Takkobu Lagoon developed, became sealed off, and changed to a freshwater lake. The succession to peat moor probably began about 2000 yr B.P. at the Takkobu site. 2) The Tsurui site was originally at the bottom of the paleo-Kushiro Bay, then changed to the paleo-Kushiro Lagoon and became peat moor as a result of the first Holocene regression, which finished about 3600 yr B.P. The site then returned to a brackish lake again, probably due to the second Holocene transgression between 3600 and 3000 yr B.P., thereafter passing through brackish lake and freshwater lake stages, and eventually becaming peat moor at about 2000 yr B.P., 3) At the Chuo site, the second paleo-Kushiro Bay developed again as a result of the second Holocene transgression, which finished about 3000 yr B.P. Thereafter, brackish or freshwater lakes, rivers, and then peat moor developed in the central area of Kushiro Moor. 4) The second marine diatom zone (MD₂ Zone), which indicates the second Holocene transgression, complete by about 3000 yr B.P., is detected only at the Chuo site in the central area of Kushiro Moor.     

Foraminiferal response to Holocene environmental changes of a tidal estuary in Victoria, southeastern Australia

The Holocene strata in the Anderson Inlet area in Victoria can be stratigraphically divided into four units, Unit I, Unit II, Unit III, and Unit IV. Unit I and Unit IV lack fossils and were deposited in non-marine, probably fluvial environments. Unit II and Unit III contain abundant foraminifera with molluscs, ostracods and bryozoans. Foraminiferal analysis suggests that Unit III was formed in a partially sheltered marine environment, while the high plankton content and relatively high diversity of benthic species in Unit II indicate that this unit was deposited in an open bay at water depths possibly less than 5 m. The foraminiferal data are integrated with radiocarbon dates to arrive at the following Holocene palaeoenvironmental history in this area: (1) low alluvial plain stage (10,000–7000 yr B.P.); (2) open bay environment stage (7000–5500 yr B.P.); (3) partially sheltered marine environment stage (5500–4500 yr B.P.); (4) alluvial plain and coastal lagoon environment stage (since about 4500 yr B.P.). The foraminiferal fauna show a clear response to these palaeoenvironmental changes. Globigerina bulloides can be used as an indicator for cold water marine environments. The high concentration of this species in these middle Holocene sediments shows a strong cold water influence on the coastal environments which reduced the effect of regional warm currents during this period. The Holocene palaeoenvironmental changes in the area were controlled by the Holocene sea-level fluctuations associated with the deglaciation history during this period. Similar integrated studies of shallow to marginal marine strata in southern Africa, America and New Zealand will lead to a better understanding of Holocene relative sea-level change and the interplay between Holocene cold and warm water regimes in the Southern Hemisphere.     

根据孢粉分析论青藏高原西部和北部全新世环境变化

通过对青藏高原西部班公湖钻孔和北部中昆仑山３个湖相剖面的孢粉研究,揭示青藏高原西部和北部地区全新世１万年期间植被的演替和气候变化,西部在全新世早期９９００－７８００ｙｒ．Ｂ．Ｐ．植被由荒漠转为草原,气候好转;中期７８００－３５００ｙｒ．Ｂ．Ｐ．,草原发展,气候较适宜,以７２００－６３００ｙｒ．Ｂ．Ｐ．为高温湿期,５５００ｙｒ．Ｂ．Ｐ．和３５００ｙｒ．Ｂ．Ｐ．出现干旱,晚期从３５００ｙｒ．Ｂ．Ｐ．至今植被为荒漠,气候干旱,其中７００ｙｒ．Ｂ．Ｐ．气候恶化。北部地区全新世时期为荒漠植被,当气候温湿时,蒿和禾本科,莎草科成份增加,藜科减少,气候干旱时则相反,北部全新世的气候分期和干湿波动与西部相近,两地在晚期气候朝干旱化发展。     

Optical dating of dune sands in the northeastern deserts of China

Optical dating has been used to obtain the ages of fossil-stabilized sand dunes from four sections in the northeastern deserts of China. Our results indicate that the optically stimulated luminescence ages of the four sections correlate well, even though the samples were collected from different deserts about 600 km apart. Our results also indicate that active dune formation in this region lasted from the Last Glacial Maximum to about 10 ka, and that the warm climate of the Holocene was interrupted by a cold/dry dune formation episode about 3.5-1.7 ka. The Holocene Optimum in this region is between 10 and 3.6 ka, and a later warm/humid dune stabilization phase lasted from at least 1.6 to 1.0 ka. The youngest age on the uppermost sand unit yielded an age of only 40 yr, supporting the previous argument that the existence of modern active eolian sands in the regions with a mean annual precipitation of up to 450 mm is not mainly due to drought, but to extensive land cultivation over historic time. From the luminescence properties of the quartz grains, it is hypothesized that the sands in most of the sections are probably derived from more than one source, with a minor source of quartz having a different thermal history before deposition.     

Variation of upwelling/oceanic conditions during the latest Pleistocene through Holocene off the central Peruvian coast: A diatom record

Diatoms are well preserved in partly laminated Holocene and late Pleistocene sediments on the upper continental slope of central Peru. Accumulation rates of diatom species previously recognized as tropical (oceanic, warm water) and subtropical (neritic, cooler water, upwelling) changed markedly downcore in Kasten cores obtained at 11°15′S, 12°58′S, and 13°37′S latitude. Subtropical species (e.g., Thalassiosira eccentrica “group”, Thalassiothrix frauenfeldii, Chaetoceros spp. resting spores, Delphineis spp.) predominated in late Pleistocene sediments of the two southern cores. A mix of tropical and subtropical diatom assemblages occurred in latest Pleistocene sediments, while tropical species (e.g., Coscinodiscus nodulifer/radiatus, Pseudoeunotia doliolus, Thalassionema nitzschioides var. parva) predominated in the late Holocene sediments. In the northern core latest Pleistocene sediments also contained a mix of tropical and subtropical species. Similarly, late Holocene sediments contained predominately tropical species of diatoms. Neoglacial periods (200–400 yr B.P., 2000–2700 yr B.P.), however, were sometimes characterized by a greater abundance of subtropical species and punctuated periodically with blooms of Skeletonema costatum and Rhizosolenia shrubsolei. It is proposed that during times of Southern Hemisphere cooling, Peruvian coastal waters north of 12°S were subjected to episodic mixing with warmer water masses before being overwhelmed by tropical waters that may have either passed southward across a weakened Equatorial Front or arose from intensified offshore countercurrents. Coastal waters south of 12°S latitude were probably unaffected by the tropical influence. Rather, these waters may have undergone intensified upwelling during the last Glacial epoch.     

Paoay Lake,northern Luzon,the Philippines: a record of Holocene environmental change

The last 7000 years of environmental history for Paoay Lake and its surrounding landscape is examined through the analysis of pollen, diatoms, charcoal, mineral magnetics and AMS dating. Basal sediments contain shells of Cerithiidae and the saline‐tolerant diatom Diploneis indicating that this was an estuarine environment before becoming a freshwater lake after 6000 bp . Pollen analysis shows that submontane forests, characterized by Pinus pollen, underwent a major disturbance around 5000 years ago, recovering to previous levels by 1000 years ago. Charcoal as an indicator of fire is abundant throughout record, although the highest levels occur in the earlier part of the record, between 6500 and 5000 years ago. An aspect of the project was to examine whether there is evidence of land clearance and agricultural development in the region during the late Holocene. While a clear signal of human impact in the record remains equivocal, there appears to be a correspondence between submontane forest decline and mid‐Holocene ocean data that depict warmer and possibly drier conditions for the region. The study highlights the vulnerability of these montane forests to forecasts of a warmer and drier climate in the near future.     

西北干旱区石羊河流域全新世早期植被与环境演化

西北干旱区石羊河流域终闾湖泊边缘三角城剖面湖相沉积物（10.0-6.3ka.B.P.）高分辨率（每样平均40－50a左右）孢粉分析揭示该流域全新世早期植被与气候环境变化过程是：全新世初期（10－9.8ka.B.P.）,温度、湿度开始上升,山上针叶林发育,该期持续较短时间后,温度、湿度下降（9.8-9.2ka.B.P.）,山上森林萎缩,山下荒漠范围扩大;此后是一个持续时间较长、波动的温度、湿度上升、植被发育状况逐步好转的过程（9.2-7.75ka.B.P.）;随后又是短暂的气候冷干、植被亚化阶段(7.75-7.25ka.B.P.）和一个相对持续时间较长,植被发育较好的暖湿期（7.25-6.3ka.B.P.）。石河流域全新世早期气候环境变化具有较强的不稳定性,每个相对暧湿期和冷干期中都有多个次一级的冷干、暖湿波动,植被也相应地随之变化。     

Humptulips revisited: a revised interpretation of Quaternary vegetation and climate of western Washington, USA

New pollen data from a 770-cm core of a mire at Humptulips on the southwestern Olympic Peninsula (47°17′00″N, 123°54′40″W) and from a nearby deposit regarded as interglacial in age on the West Fork of the Humptulips River provide evidence for a reinterpretation of previous accounts of the Quaternary vegetation and climate. Using a new age model, the highly variable record at Humptulips is here correlated with marine oxygen-isotope stages (OIS) 1–5a. Vegetation during OIS 5a was pine-dominated, mixed woodland and tundra. In OIS 4, 3, and 2, oscillations in pollen assemblages imply repeated intervals of mountain hemlock parkland and tundra. Pine, as an indicator of openness, is also abundant in these stages. Interstadial fluctuations in OIS 3 reflect episodes of lowland western hemlock communities. The earliest of the episodes is of infinite radiocarbon age and the latest dates to approximately 30,000 ¹⁴C yr B.P. Parkland in OIS 2, occurring after 24,600 until at least 18,440 ¹⁴C yr B.P., was replaced by transitional pine–alder in OIS 1 (Holocene), which, unlike any of the earlier stages, conveys the development of modern lowland forest dominated by western hemlock. Pollen assemblages over the length of record imply that temperature and humidity at no time were as high as at present; only in OIS 3 do conditions approach those occurring in OIS 1. During OIS 2, 3, and 4, when tree line apparently stood at the location of the site, climate was colder and drier. Temperatures were depressed an estimated ≥5°C with precipitation close to 1000 mm compared with 2000–3000 mm at present. Atmospheric circulation during OIS 2 appears to have been much controlled by the location of the Laurentide ice sheet in the continental interior, whereas under the current climatic regime beginning in the early Holocene, westerly air flow has dominated, regulated by interplay between the North Pacific high in summer and the wintertime Aleutian low.     

Holocene vegetational and coastal environmental changes from the Lago Crispim record in northeastern Pará State, eastern Amazonia

Vegetational and coastal environmental changes have been interpreted from a 600cm long and 764014C yr B.P. old sediment core from Lago Crispim located in the northeastern Pará State in northern Brazil. The radiocarbon dated sediment core was studied by multi-element geochemistry, pollen and charcoal analysis.Holocene Atlantic sea-level rise caused an elevation of local water table, which allowed the formation of organic deposits in a probably former inter-dune valley. Dense, diverse and tall Amazon rain forest, and some restinga (coastal vegetation) covered the study area at the beginning of the record at 764014C yr B.P. Mangrove vegetation developed along rivers close to the core site at that time. Subsequent decrease in less mangrove vegetation near the study site indicates a sea-level regression, beginning since around 700014C yr B.P. Lower sea-levels probably favoured the formation of a local Mauritia/Mauritiella palm swamp at 662014C yr B.P. Oscillations of higher and lower sea-level stands probably changed the size of the local palm swamp area several times between 6620 and 363014C yr B.P. Sea-level transgression at around 363014C yr B.P., caused marked coastal environmental changes: the development of mangroves near the site, the replacement of the local palm swamp by a Cyperaceae swamp, the substitution of the surrounding former Amazon rain forest and some restinga vegetation mainly by salt marshes. High amount carbonised particles suggest a strong human impact by burning on the coastal ecosystems during this late Holocene period.Highest concentrations of NaCl and also Ca, Mg and K in the upper sediment core indicate that the Atlantic was close during the late Holocene period. The core site, which is today 500m from the coastline and only 1-2m above modern sea-level, was apparently never reached by marine excursions during the Holocene.Less representation of mangrove since ca. 184014C yr B.P., may be related due to a slightly lower sea-level or to human impact in the study region. The modern shallow lake seems to be formed recently by road construction, forming an artificial dam.