H. E. Wright

Piston Corers for Peat and Lake Sediments

Two pistons corers are described which are successful in cuting the wood or undecomposed fibrous peat that characterizes many peat deposits. These modified corers are equipped with a serrated cutting edge, along with modifications to permit the core tube to be rotated back and forth, so that undecomposed fibers and roots can be cut. Complete sections of peat and even the underlying silt or sand can be acquired to a depth of several meters. The 10-cm diameter corer provides sufficient material for close-interval macrofossil and chemical analysis and for radiocarbon dating. The 5-cm corer is more portable and more convenient.

A square-rod piston sampler for lake sediments

ABSTRACT The Livingstone-Vallentyne piston sampler has been further modified by replacing the spring catch with splines on a square rod, in order to strengthen the critical points of greatest stress and permit pounding when necessary.

Late quaternary environments of the Soviet Union

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Simulation of the climate of 18,000 years BP: Results for the North American/North Atlantic/European sector and comparison with the geologic record of North America

Abstract The large ice sheets in North America and Europe and the extensive sea-ice cover in the North Atlantic at the time of the last glacial maximum must have greatly modified the surface temperature patterns and, in turn, the location and intensity of the surface winds and jet streams. A general circulation model was used to simulate the January and July patterns of temperature, precipitation, and wind for 18 ka BP. Boundary conditions for the model, consisting of ice-sheet location and height, sea-ice location, and sea-surface temperature were prescribed from CLIMAP (1981). The model results are illustrated and described for the North American/North Atlantic/European sector. The jet stream splits around the North American ice-sheet, and the southern branch strengthens considerably (compared to present) over the southern portion of the United States, the sea-ice margin of the North Atlantic, and the southern edge of the European ice-sheet. Geologic evidence, principally from North America, of wind, temperature and moisture conditions is assessed from sand dune and loess records, estimates of snowline depression, pollen records and lake-level studies. The geologic evidence is generally compatible with the model simulation.

Proposed changes in seasonality of climate during the Lateglacial and Holocene at Lake Zeribar, Iran

Calcareous sediments spanning the last 13 ka from Lake Zeribar, western Iran, were sampled for stable-isotope analysis as a means of augmenting earlier climatic interpretations based on pollen and macrofossils. The Lateglacial period was cold and dry, supporting semi-desert vegetation and highly concentrated lake water. Low evaporation rates due to the cold resulted in long residence times and high lake stands. Durinig the early Holocene, the upland vegetation changed to a pistachio-oak savanna as low temperatures and aridity ameliorated. Lake level dropped in response to increased evapotranspiration. The low oxygen-isotope values of lacustrine calcite during this period are interpreted as a relative increase in the contributioni of winiter moisture rather than overall changes in effective moisture. A gradual increase in oak pollen at ∼7 ka BP signalled the increase in effective moisture to levels similar to those of today. At the same time the δ18O values increased. reflecting a greater percentage of 18O-enriched spring rains. Modern values and seasonal distribution of precipitation were established by 6 ka BP. A short-lived return to winter-only precipitation occuiTed at ∼4.5 ka BP and resulted in a temporary increase in aridity. The covariance among δ13C, carbonate deposition and δ18O values suggests that lake productivity is linked to these seasonal climatic changes.

A possible younger dryas record in Southeastern Alaska

A stratigraphic record of climatic cooling equal in timing and severity to the Younger Dryas event of the North Atlantic region has been obtained from lacustrine sediments in the Glacier Bay area of southeastern Alaska. Fossil pollen show that a late Wisconsin pine parkland was replaced about 10,800 years ago by shrub- and herb-dominated tundra, which lasted until about 9,800 years ago. This vegetational change is matched by geochemical evidence for loss of organic matter from catchment soils and increased mineral erosion. If this event represents the Younger Dryas, then an explanation for a hemisphere-wide propagation of a North Atlantic climatic perturbation must be sought.

Abrupt changes in North American climate during early Holocene times

Recent studies of the Greenland ice cores have offered many insights into Holocene climatic dynamics at decadal to century timescales. Despite the abundance of continental records of Holocene climate, few have sufficient chronological control and sampling resolution to compare with the Greenland findings. Butannually laminated sediments (varves) from lakes can provide high-resolution continental palaeoclimate data with secure chronologies. Here we present analyses of varved sediments from Deep Lake in Minnesota, USA. Trends in the stable oxygen-isotope composition of the sedimentary carbonate indicate a pronounced climate cooling from 8.9 to 8.3 kyr before present, probably characterized by increased outbreaks of polar air, decreased precipitation temperatures, and a higher fraction of the annual precipitation falling as snow. The abrupt onset of this climate reversal, over several decades, was probably caused by a reorganization of atmospheric circulation and cooling of the Arctic airmass in summer that resulted from the final collapse of the Laurentide ice near Hudson Bay and the discharge of icebergs from the Quebec and Keewatin centres into the Tyrell Sea. The timing and duration of this climate reversal suggest that it is distinct from the prominent widespread cold snap that occurred 8,200 years ago in Greenland and other regions,,. No shifts in the oxygen-isotope composition of sediment carbonate occurred at 8.2 kyr before present at Deep Lake, but varve thickness increased dramatically, probably as a result of increased deposition of aeolian dust. Taken together, our data suggest that two separate regional-scale climate reversals occurred between 9,000 and 8,000 years ago, and that they were driven by different mechanisms.

Introduction to Decision-making

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The use of surface samples in quaternary pollen analysis

Abstract Much of the uncertainty in the interpretation of Late Quaternary pollen assemblages can be removed by the judicious use of pollen surface samples from a variety of vegetational formations. The technique is especially useful in new regions of investigation, where not much is known of the pattern of the pollen sequence, and it can even be used in regions where much of the vegetation is disturbed. Two scales of study can be applied: 1. (1) For a regional study in a forested area, samples are taken from the centers of relatively large lakes, so that the regional pollen rain will not be distorted by local pollen rain. The regional pollen rain may then be compared with the regional upland vegetation, as surveyed by standard phytosociological methods, including air-photo study. A very large area must be surveyed, especially if the upland vegetation consists of a species-rich mosaic of forest types in an area of diversified topography. 2. (2) A second type of surface-sample study provides a different basis for historical reconstructions. In this case, transects of surface samples are taken in each of the main forest types, extending from the centers of small ponds or bogs to the adjacent hill slopes. Detailed vegetational analysis must be made of all the plant associations crossed by the transects from lowland to upland, and in the pollen analysis particular attention must be paid to the herbaceous pollen types. In this way the pollen counts can be assigned to either local, extralocal, or regional vegetation. Characteristic local or extralocal pollen percentages may then be identified in a pollen diagram for a long core from a small pond or bog, in order to work out the local vegetational succession that either accompanied or was independent of changes in the climatically controlled regional vegetation.

Bog development and landform dynamics in central Sweden and south-eastern Labrador, Canada

(1) Stratigraphic analysis, detailed surveying, and radiocarbon dating were used to document the development of Hammarmossen, a raised mire in central Sweden, and to contrast the structure, development, and dynamics of landforms there with those on Gilbert bog in south-eastern Labrador, Canada. (2) A series of basal radiocarbon dates from transects of peat cores across Hammarmossen shows that peat started to accumulate approximately 5500 years B.P. and spread radially across a glacial outwash plain throughout the late Holocene. The implications of this pattern of development for models of bog hydrology and growth and for interpretations of the process of paludification, e.g. by Malmstrom, are discussed. (3) On both Swedish and Labrador mires the distribution, shape and development of open-water pools are closely controlled by the topography of the peat surface. The largest and deepest pools occur on the flattest surfaces, where water outflow is slow. Pool development results from hydrological controls on relative rates of peat accumulation in hummocks and hollows. (4) Pools were initiated throughout the last 4000 years of bog development. On Hammarmossen a deep layer of algal gyttja partly fills the pools, whereas on Labrador bogs algal sediment is absent, and the pool floors are degrading peat and peat detritus. This contrast in stratigraphy of bog pools in Sweden and Labrador follows a similar pattern for pools on minerotrophic mires (fens) in the two areas. (5) Once formed, pools undergo similar dynamics in the two regions. Pool depth increases as peat accumulation on hummocks exceeds sedimentation in pools. Lateral expansion of pools occurs through marginal flooding, controlled by differential rates of accumulation and by the breakdown of peat ridges separating adjacent pools. Pools may be drained through surface erosion or through subsurface piping. Erosion of the bog from the mire margin towards the centre may gradually fragment the peat mass. Under very wet conditions raised bogs are inherently unstable systems on which water-dominated landforms increase through time until stream erosion gradually dissects the mire.