Holocene environmental changes in the alpine zone,northern San Juan Mountains,Colorado: Evidence from bog stratigraphy and palynology

Cores from five high alpine basins in the northern San Juan Mountains show several fluctuations in lithology. Typically, peats are interbedded with coarser clastic sediments or else woody peats alternate with fibrous peat. Twenty ¹⁴C dates provide radiometric control. Sediment rates averaged about 2.5 cm/100 yr but varied at the different sites between 1.19 and 50 cm/100 yr. Rates were lower during the middle of the Holocene. Basal radiocarbon dates indicate that these high (ca. 3600 m a.s.l.) northeasterly facing cirques were icefree by 9000 BP. There is some evidence in the cores for a short climatic reversal sometime between 8000 and 7000 BP. A major change occurred in the high basins very close to 5000 BP and thereafter there are several intervals of increased clastic sedimentation which may be related to Neoglacial climatic fluctuations. Analysis of a 2.15 m core near Hurricane Basin indicates significant fluctuation of pollen and macrofossils occurred during the 9000 ± year record. The Picea/Pinus ratios are used to delimit changes in the apparent elevation of the site: the ratios indicate that a short drop of “treeline” occurred about 8000 BP and then remained near present level until about ≥1800 BP when the apparent elevation of the site rose. Macrofossils indicate that spruce was present in the Hurricane Basin (and others) at specific periods and confirms the general results of the Picea/Pinus ratios. The San Juan Mountains do not possess a glacial Neoglacial record but the stratigraphy of these high cirque basins can be used to define glacial stades (cf. Jardine, 1972). The interpreted climatic response record on vegetation and sediment flux has both similarities and differences from other records in the western mountains of North America.     

Peatland development at the arctic tree line (Québec, Canada) influenced by flooding and permafrost

In this study, we documented the Holocene history of a peat plateau at the arctic tree line in northern Québec using stratigraphic and macrofossil analyses to highlight the effects of geomorphic setting in peatland development. Paludification of the site began about 6800 cal yr BP. From 6390 to 4120 cal yr BP, the peatland experienced a series of flooding events. The location of the peatland in a depression bounded by two small lakes likely explains its sensitivity to runoff. The proximity of a large hill bordering the peatland to the south possibly favored the inflow of mineral-laden water. The onset of permafrost aggradation in several parts of the peatland occurred after 3670 cal yr BP. Uplifting of the peatland surface caused by permafrost stopped the flooding. According to radiocarbon dating of the uppermost peat layers, permafrost distribution progressed from the east to the west of the peatland, indicating differential timing for the initiation of permafrost throughout the peatland. Most of the peatland was affected by permafrost growth during the Little Ice Age. Picea mariana macroremains at 6450 cal yr BP indicate that the species was present during the early stages of peatland development, which occurred soon after the sea regression.     

Holocene environmental change in two polygonal peatlands, south-central Nunavut, Canada

Holocene histories of two polygonal peatlands in the low arctic of south-central Nunavut, Canada, are reconstructed using plant macrofossil and pollen stratigraphies of four cores. Peat accumulation began in both basins between 7600 and 8000 cal. yr BP, within less than 1000 years after deglaciation. Mid- to late-Holocene vegetation changes recorded in the peat cores may be related to permafrost aggradation, associated with a regional cooling trend inferred from a nearby lake sediment record. However, differences in the timing of changes among the peatland coring sites indicate that local autogenic processes have also played an important role. Peat accumulation rates have decreased considerably in the past 3000 to 5000 years compared to the early Holocene. Our results illustrate the complexity of peatland development and peat accumulation dynamics in areas of permafrost, resulting from the important influences of both internal autogenic factors and external environmental forces such as climatic change.     

Coastal peats from northwest Ireland: implications for late-Holocene relative sea-level change and shoreline evolution

Peat and organic rich sediments at coastal sites in extreme northwest Ireland have accumulated in a wide variety of environments, often strongly influenced by late Holocene changes in relative sea level and by geomorphic processes. A deep peat sequence on the coast of Aranmore Island accumulated initially in a lake and subsequently in a freshwater marsh environment. The long pollen record serves as a template for regional events. It extends over much of the Holocene and shows relatively high levels of Pinus pollen up to just before the disappearance of this taxon at c . 3600 BP. Coastal peat occurrences elsewhere are much thinner and have accumulated over shorter periods; they contain further evidence to show that coastal areas were well-wooded compared with today, and that Pinus was an important woodland component prior to c . 4000 BP. At sites in Gweebarra Bay intertidal peats record the closure of small estuaries by geomorphological events during the past 5000 years. Coastal sites at Ballyness, Clonmass, and Trawenagh display regressive stratigraphies ˜ minerogenic marine sediments are overlain by silty peats capped by highly organic freshwater peats. Basal radiocarbon dates range from 4500 to 3300 BP. The silty peats are interpreted as having formed in salt-marsh environments and contain distinctive pollen spectra, marked by high levels of Pinus and Compositae Liguliflorae pollen. The data suggest that relative sea level attained levels close to that of today by the mid-Holocene in this region. The pattern of relative sea-level change agrees well with that predicted by geophysical modelling.     

A subsurface Middle Wisconsinan interstadial site at Waterloo, Ontario, Canada

Commercial borings to a depth of 36 m encountered organic sediments under till. A continuously cored borehole was subsequently put down to 42 m and revealed a succession of sand and silt, over clayey Maryhill hill, over sandy Catfish Creek Till, over fossiliferous sand. Pollen, plant macrofossils, and a few ostracodes, molluscs, and insects provide a coherent picture of interstadial conditions. Two pollen zones are represented. The older zone is characterized by Picea and ThujalJuniperus pollen, and Dryopteris- type and Sphagnum spores, which suggest a lowland, forested peatland. Reduction of peatland, and transition to a wetland with more open water associated with a stream is indicated by an increase in Cyperaceae pollen, Pediastrum algae, and a diversity of aquatic plant macrofossils during the later zone. Fossil plant assemblages are similar to modern southern boreal associations in Ontario, which implies that the climate was cooler and possibly drier than the present. Phytogeographically, Picea needles and wood, and seeds of Brasenia schreberi and Potamogetan spirillus are noteworthy among the plant macrofossils. The sequence of Late Wisconsinan tills overlying the fossiliferous zone implies a Middle Wisconsinan age for the interstadial. This is supported by an accelerator radiocarbon date on small wood pieces of 40,080 + 1,200 years B.P.     

Holocene palaeoenvironments at the timberline in the French Alps—a multidisciplinary approach

An attempt is made to reconstruct the palaeoenvironmental Holocene history at the timberline on the basis of the analysis of various palaeoecological proxy indicators available on a marshy area and its surroundings in the Taillefer Massif (Isère. France). The multidisciplinary approach involves analyses of pollen assemblages. plant macrofossils, coleoptera remains. subfossil trunks extracted from lakes or peat-bogs. and charcoals derived from the surrounding soils. This enables definition of the respective roles of five forest taxa ( Pinus uncinata Mill., Pinus cembra L., Larix decidua Mill., Abies alba Mill, and Picea abies L. Karsten) in the evolution of high altitude forests during the Holocene. Pinus uncinata was present on the plateau throughout the Holocene. Larix and Pinus cembra were present only during two periods: 7500-5000 BP and 3500-2000 BP. All trees disappeared from the plateau at about 2000 BP, while, at lower altitude. Abies was replaced by Picea. The action of both climate and early human impact can explain these changes.     

Climate and vegetation history from a 14,000-year peatland record, Kenai Peninsula, Alaska

Analysis of pollen, spores, macrofossils, and lithology of an AMS ¹⁴C-dated core from a subarctic fen on the Kenai Peninsula, Alaska reveals changes in vegetation and climate beginning 14,200 cal yr BP. Betula expansion and contraction of herb tundra vegetation characterize the Younger Dryas on the Kenai, suggesting increased winter snowfall concurrent with cool, sunny summers. Remarkable Polypodiaceae (fern) abundance between 11,500 and 8500 cal yr BP implies a significant change in climate. Enhanced peat preservation and the occurrence of wet meadow species suggest high moisture from 11,500 to 10,700 cal yr BP, in contrast to drier conditions in southeastern Alaska; this pattern may indicate an intensification and repositioning of the Aleutian Low (AL). Drier conditions on the Kenai Peninsula from 10,700 to 8500 cal yr BP may signify a weaker AL, but elevated fern abundance may have been sustained by high seasonality with substantial snowfall and enhanced glacial melt. Decreased insolation-induced seasonality resulted in climatic cooling after 8500 cal yr BP, with increased humidity from 8000 to 5000 cal yr BP. A dry interval punctuated by volcanic activity occurred between 5000 and 3500 cal yr BP, followed by cool, moist climate, coincident with Neoglaciation. Tsuga mertensiana expanded after ~ 1500 cal yr BP in response to the shift to cooler conditions.     

Peat mounds of southwest Tasmania: Possible origins

Small mounds of peat rise several metres above the level of the water‐table at Melaleuca Inlet and Louisa Plains on the buttongrass plains in southwest Tasmania. Possible origins of the peat mounds have been explored by pollen analysis and radiocarbon dating of a set of samples taken from a vertical section of one peat mound at Melaleuca. The peat accumulation is entirely of Holocene age although the mound is underlain by sapric peats preserving a cold climate palynoflora of probable Late Pleistocene age. Peats at and near the base of the mound accumulated under a heath sedgeland during the earliest Holocene while after about 7630 a BP the peat‐forming vegetation was shrub‐dominated. The radiocarbon data indicate two main phases of overall peat accumulation, between 7630 and 5340 a BP (Middle Holocene) and between 4450 and 450 a BP (Late Holocene), that were interrupted by a wildfire which burnt into the surface peats. The maintenance of high surface and internal levels of moisture almost certainly was the critical factor behind the low incidence of in situ fires burning into the surface peats on the mound. The perennial influx of groundwater below the mound is a possible origin that fits well with our observations, although the expansion and contraction of soils cannot be discounted as an initiating factor. Enhanced nutrient input from birds may have helped promote growth in the peat‐forming communities. The data do not support the mounds being eroded remnants of a former blanket peat cover or being due to periglacial activity. The peat mounds of southwest Tasmania deserve maximum protection because of their rarity in the Australian landscape and, it seems, elsewhere.     

Geology and paleoecology of a mid-Wisconsin peat from the Queen Charlotte Islands, British Columbia, Canada

A peat bed on east-central Graham Island of the Queen Charlotte Islands occurs within a nonglacial fluvial succession that is both overlain and underlain by glacial deposits. Radiocarbon dates of 27,500 ± 400 and 45,700 ± 970 yr B.P. at the top and base of the peat, respectively, indicate that it was deposited during the mid-Wisconsin nonglacial interval. The peat is the first documented mid-Wisconsin organic deposit in northern coastal areas of British Columbia. Three local pollen zones are represented. The lowest zone (PM-1) is restricted to sandy silt directly underlying the dated peat. Very high Cyperaceae and moderate Poaceae pollen percentages characterize zone PM-1, and a variety of other herbs are common, suggesting an open landscape rather than a forested one. The middle zone (PM-2) is characterized by abundant pollen of Picea, Tsuga mertensiana, and Cyperaceae, and also contains pollen of Abies, a genus now absent from the Queen Charlotte Islands. Graham Island probably had extensive forests at this time, but abundant pollen and macrofossils of Cyperaceae and emergent aquatics such as Hippuris vulgaris, Veronica scutellata, Potentilla palustris, and Menyanthes trifoliata indicate that there also were open wetland areas. Zone PM-3 also contains abundant arboreal pollen. Large amounts of Sphagnum spores and Selaginella selaginoides megaspores indicate succession of the wetland area at the sample site to a peat bog. Paleoecological analysis of the data suggests that subalpine vegetation elements were depressed by at least 400 m, probably due to a cooler climate. Probable modern analogs in southeastern Alaska and the presence of Abies (probably A. amabilis) indicate that precipitation was higher on eastern Graham Island during the mid-Wisconsin than at present.     

Holocene vegetation and climate change in southern New Zealand: linkages between forest composition and quantitative surface moisture reconstructions from an ombrogenous bog

This paper presents a Holocene pollen record from an ombrotrophic bog in Southland, New Zealand, together with multiproxy data (testate amoebae, peat humification and plant macrofossils) from the same core to establish an independent semiquantitative record of peatland surface moisture. Linkages between reconstructed peatland surface moisture and regional forest composition are investigated using redundancy analysis of the forest pollen data constrained with predicted bog water‐table depths. Over 32% of the pollen data variance can be explained by surface moisture changes in the bog, suggesting a common cause of water‐table and regional vegetation change. Water tables were higher during the early to mid‐Holocene when the forest was dominated by podocarp taxa. Water tables lowered after about 3300 cal. yr BP coevally with the expansion of Nothofagus species, culminating with the dominance of Nothofagus subgenus Fuscospora in the past 1200 cal. yr BP. This is in apparent opposition to the warm/dry to cool/wet trend suggested by subjective interpretation of pollen data alone, from this and other studies. We suggest that during the late Holocene, drier summers associated with shifts in solar insolation caused reduced surface wetness and summer humidity, which together with a trend to cooler winters, apparently favoured the regeneration of Nothofagus species. Copyright © 2002 John Wiley & Sons, Ltd.     

Vegetation history of a polygonal peatland, northern, Yukon

Pollen, macrofossils and matrix composition are described for a 221 cm core from a polygonal peatland overlying a late-Wisconsin lakebed. A hydroseral succession of wetland communities occurred at the site, and included a submerged assemblage with Chara , a Carex aquatilis - Eleocharis marsh, a fen, and finally a Sphagnum - Ledum bog. A transition about 9,600 B.P. to a wetter S. balticum - Andromeda carpet is attributed to formation of permaforst and polygonal ice wedges. This community, sometimes with S. compactum and Chamaedaphne , prevailed until about 3,000 B.P. when the polygon became high centered, and peat growth declined. Peat growth rate is determined from 11 ¹⁴C dates, and is used to calculate pollen influx. Apparent periodic oscillations every 2,000 years, in total influx, are not accepted as sufficient evidence of treeline fluctuations.     

Multi-proxy climate and environmental records from a Holocene eutrophic mire,southern taiga subzone,West Siberia

Palaeoenvironmental reconstructions from peat are strongly focused on ombrotrophic mires, but this study demonstrates that eutrophic mires can also be used. A multi-proxy approach was applied to a eutrophic mire on a floodplain terrace in the southern taiga of West Siberia. The results of the reconstruction were considered in the wide geographic context of the surrounding regions, including Siberia and Central Asia. Different palaeoecological proxies (analysis of plant macrofossils, testate amoebae, oribatid mites, molluscs, peat humification, ash content and spectral characteristics of humic acids) were used in this study. The results of different proxies showed a high level of consistency among themselves, which allowed for a robust interpretation of Holocene mire development. Throughout the ~7800 years history of the mire, there was a high level of surface wetness. The presence of mineral matter in the peat between 7800 and 5100 cal. a BP indicates regular flooding caused by the intensive fluvial activity, apparently resulting from increased precipitation. This was followed by a trend towards a gradual decrease in surface wetness from conditions of high surface moisture (stagnant water) between 5100 and 3000 cal. a BP to present day conditions of moderate surface moisture with a water table slightly below the mire surface. This pattern is consistent with the well-documented long-term trend from palaeoecological records throughout the taiga and arctic zones in West Siberia and central arid Asia. Our data further support the idea that the westerlies were the dominant driver of climate for the southern taiga of West Siberia during the Middle to Late Holocene.     

Blanket mire formation at the Cross Lochs, Sutherland, northern Scotland

A brief review of the genesis of blanket peats is presented, together with detailed analyses of basal peats from northern Scotland. Particular attention is paid to local factors affecting peat growth and the problems of sampling and interpretation arising from them. Macrofossil and loss-on-ignition analyses of four peat-mineral interfaces solve many of the terminological problems of such profiles and indicate that interpretations based on field stratigraphy alone are uncertain. Pollen analyses of two profiles from contrasting microtopographic situations show well-defined vegetational change associated with early peat development. Fully organic ombrotrophic peat is present from 6805 ± 50 BP at one site, following anthropogenic burning from c . 7500 BP and partial recovery of the open birch woodland. A change from birch woodland to Calluna heath occurs at approximately the same time on a more freely drained site with much later peat development, following further burning from 4890 ± 65 BP. Anthropogenic activity is clearly associated with changes in soil and vegetation preceding peat formation, and the role of climatic factors remains equivocal.     

A new late glacial to early Holocene palaeobotanical and archaeological record in the Eastern Pre‐Alps: the Palughetto basin (Cansiglio Plateau,Italy)

A late glacial to early Holocene lacustrine and peat succession, rich in conifer remains and including some palaeolithic flint artefacts, has been investigated in the Palughetto intermorainic basin (Venetian Pre‐Alps). The geomorphological and stratigraphical relationships, ¹⁴C dates and pollen analyses allow a reconstruction of the environmental history of the basin and provide significant insights into the reforestation and peopling of the Pre‐Alps. The onset of peat accumulation is dated to 14.4–14.1 kyr cal. BP, coinciding with reforestation at middle altitudes that immediately post‐dates the immigration of Larix decidua and Picea abies subsp. europaea. Plant macrofossils point to the expansion of spruce about 14.3 kyr cal. BP, so far one of the earliest directly dated in the late glacial period of southern Europe. The previous hypothesis of an early Holocene spruce immigration in the Southern Alps from Slovenia needs reconsideration. Organic sedimentation stopped at the end of the Younger Dryas and was followed by the evolution of hydromorphic soils containing lithic artefacts, anthropic structures and wood charcoal. The typological features of the flint implements refer human occupation of the site to the end of the recent Epigravettian. Charcoals yielded dates either consistent with, or younger than, the archaeological chronology, in the early and middle Holocene. Copyright © 2000 John Wiley & Sons, Ltd.     

Holocene changes on a peatland in northwestern Ontario interpreted from testate amoebae (Protozoa) analysis

Radiocarbon-dating and analyses of fossil testate amoebae (Protozoa) have established changes in soil moisture conditions on the developing surface of a Sphagnum -dominated peatland near Emo in northwestern Ontario.
The distribution and composition of modern testate amoebae communities were studied from peatlands in the region of Ontario and Minnesota as a guide to interpreting fossil assemblages. Although the core spans all of the Holocene, fossil testate amoebae were recovered only from the part post-dating 6500 BP. Earliest testate amoebae assemblages associated with bryophytic and cyperaceous-rich fen peat are dominated by species in the genera Cyclopyxis and Centropyxis . By 5000 BP, Amphitrema Jraaum, Assulina muscorum, Heleopera sphugni and Hyalosphenia subjaoa become important species as Sphagnum-rich peat accumulated at the site. Present-day microtopographic differentiation probably developed during historic time when the site became progressively drier, as indicated by a change of Nebela griseola, N. militaris and Trigonopyxis arcula . Although it is possible to derive quantitative estimates of changing soil moisture conditions from testate amoebae, care should be taken in interpreting results, particularly from non-Sphagnum-rich peats, until more is learned about the distribution and ecology of modern faunas.     

A 12.5-kyr history of vegetation dynamics and mire development with evidence of Younger Dryas larch presence in the Verkhoyansk Mountains, East Siberia, Russia

Werner, K., Tarasov, P. E., Andreev, A. A., Müller, S., Kienast, F., Zech, M., Zech, W. & Diekmann, B. 2009: A 12.5‐kyr history of vegetation dynamics and mire development with evidence of Younger Dryas larch presence in the Verkhoyansk Mountains, East Siberia, Russia. Boreas, 10.1111/j.1502‐3885.2009.00116.x. ISSN 0300‐9483. A 415 cm thick permafrost peat section from the Verkhoyansk Mountains was radiocarbon‐dated and studied using palaeobotanical and sedimentological approaches. Accumulation of organic‐rich sediment commenced in a former oxbow lake, detached from a Dyanushka River meander during the Younger Dryas stadial, at ～12.5 kyr BP. Pollen data indicate that larch trees, shrub alder and dwarf birch were abundant in the vegetation at that time. Local presence of larch during the Younger Dryas is documented by well‐preserved and radiocarbon‐dated needles and cones. The early Holocene pollen assemblages reveal high percentages of Artemisia pollen, suggesting the presence of steppe‐like communities around the site, possibly in response to a relatively warm and dry climate ～11.4–11.2 kyr BP. Both pollen and plant macrofossil data demonstrate that larch woods were common in the river valley. Remains of charcoal and pollen of Epilobium indicate fire events and mark a hiatus ～11.0–8.7 kyr BP. Changes in peat properties, C31/C27 alkane ratios and radiocarbon dates suggest that two other hiatuses occurred ～8.2–6.9 and ～6.7–0.6 kyr BP. Prior to 0.6 kyr BP, a major fire destroyed the mire surface. The upper 60 cm of the studied section is composed of aeolian sands modified in the uppermost part by the modern soil formation. For the first time, local growth of larch during the Younger Dryas has been verified in the western foreland of the Verkhoyansk Mountains (～170 km south of the Arctic Circle), thus increasing our understanding of the quick reforestation of northern Eurasia by the early Holocene.     

Scandium in the coals of Northern Asia (Siberia,the Russian Far East,Mongolia, and Kazakhstan)

We present new original data on the geochemistry of scandium in the coals of Asian Russia, Mongolia, and Kazakhstan. In general, the studied coals are enriched in Sc as compared with the average coals worldwide. Coal deposits with abnormally high, up to commercial, Sc contents were detected in different parts of the study area. The factors for the accumulation of Sc in coals have been identified. The Sc contents of the coals depend on the petrologic composition of coal basins (composition of rocks in their framing) and the facies conditions of coal accumulation. We have established the redistribution and partial removal of Sc from a coal seam during coal metamorphism. The distribution of Sc in deposits and coal seams indicates the predominantly hydrogenic mechanism of its anomalous concentration in coals and peats. The accumulation of Sc in the coals and peats is attributed to its leaching out of the coal-bearing rocks and redeposition in a coal (peat) layer with groundwater and underground water enriched in organic acids. The enrichment of coals with Sc requires conditions for the formation of Sc-enriched coal-bearing rocks and conditions for its leaching and transport to the coal seam. Such conditions can be found in the present-day peatland systems of West Siberia and, probably, in ancient basins of peat (coal) accumulation.     

Palynology and sediment slumping in a high arctic Greenland lake

A 103-cm core from a high arctic lake (80°49'N, 15°00'W) on Amdrup Land, NE Greenland has been analysed for pollen and other microfossils, as well as bulk samples from 10 cores for macrofossils. AMS ¹⁴C-dating of aquatic mosses ( Drepanocladus exannulatus and Scorpidiurn scorpioides ) revealed that the lowermost 90 cm of the core was deposited during a 1.5-2 ka interval in the early Holocene, before the appearance of Salix arctica which immigrated to N Greenland c . 7 ka BP. Armeria scabra , now extinct in the area, larval cases and apotomes of Apatcmiu zonella , today not known north of c. 77°N, and other indices point to a climate warmer than that of today. A thin, sandy layer 13 cm below the lake bottom, accompanied by marked changes in the content of pollen and macrofossils, such as the sudden occurrence of large amounts of Salix pollen and leaves, indicates a large hiatus. This is confirmed by the dating to c . 7 ka BP of mosses just below the sand, and of Salix leaves just above it to c . 1 ka BP. The hiatus must be caused by sediment slumping which transported all the sediment from the intervening period to the deeper part of the lake. A similar, sandy layer in the uppermost 2–6 cm may be caused by younger, (sub-)recent slumping, or both may be the result of the same disturbance. The possibility of a reservoir effect causing too old ¹⁴C-ages for the moss is discussed.     

A lateglacial climatic reversion in hokkaido, Northeast Asia, inferred from the Larix pollen record

At present, Larix gmelini is a main component of taiga developed in north Sakhalin, northeast China and east Siberia. During the interval from ∼11.8 to 12.4 ka BP, pollen assemblages from north Hokkaido, Japan, are dominated by Larix. Pollen assemblages of this Lateglacial climatic reversal, known as the “Kenbuchi Stadial”, are similar to pollen assemblages from the Last Glacial Maximum in north Hokkaido. Vegetation of Hokkaido inferred from these pollen assemblages — open taiga composed of Larix gmelini, Pinus pumila and Picea jezoensis and/or Picea glehnii — may have been like that of north Sakhalin today. Comparison of these fossil pollen assemblages from Hokkaido with pollen assemblages from modern surface samples in Sakhalin suggests that ∼ 11.8 to 12.4 ka BP, temperatures relative to today were at least 9°C lower in January and 7°C lower in August, and that annual precipitation was at least 735 mm lower than present.