Basal Peat Deposits Research Articles

Holocene relative sea-level changes in western Brittany (France) between 7600 and 4000 cal. BP: Reconstitution from basal-peat deposits

With respect to others European oceanic facades, the Holocene relative sea-level (RSL) changes along the French Atlantic coasts have received relatively little attention. Even if it received some attention for geophysical modelling (Van de Plassche, 1991; Lambeck, 1997; Leorri et al., 2012), only two field-based studies really dealt with this subject in Western Brittany, agreeing on an oscillatory RSL pattern with a conspicuous pluri-metre negative variation occurring around 3000 BP (Morzadec-Kerfourn, 1974; Stephan, 2011). This paper is the first step on the way to update Western Brittany Holocene RSL data. It presents a new long-term Holocene RSL reconstruction based on basal peat deposits, compaction-free deposits that have been accumulated on top of the Pleistocene formations during the early-Holocene stages of the post-glacial transgression. Data used for the sea-level reconstruction are both new data and previously published datasets we have reassessed. New cores, extracted along the Finistere peninsula in the framework of a coastal risk assessment research program, allowed us to collect new stratigraphic data. Dates initially published on basal peat deposits in previous studies (Morzadec-Kerfourn, 1974) were re-assessed through the same process to allow their incorporation in a combined dataset. In the absence of microfossils in most of our deposits, an alternate approach was used to determine the salinity regime of the basal peat deposit environments through the use of stable carbon isotope ratios and the observation of macro-botanical remains: brackish peat deposits were used as sea-level index points, while freshwater ones provided only high-limiting constraints. Eight new radiocarbon datings were obtained. A total of six new sea-level index-points and six new limiting points allowed us to derivate the long-term (millennial) changes in RSL in Western Brittany. A comparison of these data with some foraminifera-based regional RSL data shows that both data sets closely match. Altogether, the combined data show the RSL bending ca. 7000 and 3000 cal. BP, indicating progressively decreasing rising rates. Further work is needed to fill the centennial to pluri-centennial gaps that remain to ascertain potential periods of stability or strong decrease in the rising rates of the RSL ca. 6200 ± 500 and 3000 ± 500 BP.

A Standard Method for Evaluating Radiocarbon Dates of Local Deglaciation: Application to the Deglaciation History of Southern Labrador and Adjacent Québec

Sixty-two radiocarbon dates of marine shells, basal lake sediments, and basal peat deposits from the southeastern part of the Labrador-Ungava Peninsula, Canada, were evaluated for their accuracy and precision in estimating local deglaciation time. A procedure for the consistent evaluation of basal dates of lake sediments was developed after consideration of the problems with dating early-postglacial sediments. Of the 62 dates, 42 were selected to construct the regional déglaciation chronology, interpreted in the context of the glacial geology of the area. Late Wisconsin ice stretched to or beyond the present-day coastline throughout the study area, except in southeastern Labrador where the extreme southeastern coast was probably ice-free during Late Wisconsin time. Déglaciation began in southeastern Labrador by 13.OK. It proceeded slowly at first, and between 9.7K and 10.5K the retreating ice formed the Paradise Moraine, which parallels the southeastern Labrador coastline. In contrast, the coast at Sept-îles was not ice-free until about 9.4K. Shortly thereafter the extensive Québec North Shore Moraine System developed, although its precise age is uncertain. The position of the ice margin at 8.0K is uncertain, but may still have been near the Québec North Shore Moraine System. After 8.0K ice retreat was rapid, although extensive ice persisted at 6.5K in western Labrador and adjacent Québec. Final wastage of ice occurred in two localities in the center of the Labrador-Ungava Peninsula just after 6.OK. The precision of the well-dated glacial isochrones is estimated to be about ± 23 km.

Permafrost Distribution in Peatlands of West-Central Canada During the Holocene Warm Period 6000 Years BP

The extent and distribution of permafrost in peatlands 6000 years ago was investigated in the present discontinuous and continuous permafrost zones of west-central Canada. Permafrost peatlands were cored at 161 locations and the floristic composition of the peat was determined from macrofossil analysis. The reconstructed paleoenvironments were used to indicate the presence or absence of permafrost at the time of peat formation. Chronological control was provided by radiocarbon dating of substantial changes in the peat sequences and by dates of basal peat deposits. Peatland formation began after glacial ice disappeared from the land surface of west-central Canada. Macrofossils indicate that most peatlands were fens without permafrost at 6 ka, except in the far north. Permafrost was already present in many areas of the Arctic, and peat accumulation occurred under permafrost conditions. In the southern area, permafrost development in peatlands began about 4 ka as the middle Holocene warm period came to a close. Permafrost development in the fens was associated with the development of a Sphagnum-dominated surface on the fens, caused by the onset of a cooler and moister climate. The insulation provided by the surface peat layer and by the associated tree cover initiated permafrost development in small lenses that coalesced into large permafrost bodies according to the prevailing climatic conditions. At the target date, 6 ka, permafrost was present in some peatlands, but the distribution zones shifted 300 to 500 km to the north, relative to the present zonation. It is estimated that this corresponds to a mean annual temperature that was about 5°C warmer than at present.

The onset of deglaciation of Cumberland Bay and Stromness Bay, South Georgia

Carbon dating of basal peat deposits in Cumberland Bay and Stromness Bay and sediments from a lake in Stromness Bay, South Georgia indicates deglaciation at the very beginning of the Holocene before c. 9500 14C yr BP. This post-dates the deglaciation of one local lake which has been ice-free since at least 15 700 14C yr BP on account of its atypical geomorphological location. The latter indicates the likely presence of floristic refugia on South Georgia during the Last Glacial Maximum from which newly exposed terrestrial and aquatic habitats were rapidly colonized.

Vegetation and climate during the Weichselian Early Glacial and Pleniglacial in the Niederlausitz, eastern Germany — macrofossil and pollen evidence

AbstractCryoturbated organic beds and channel fills, intercalated with sandy and gravelly fluvial units, have been studied in an opencast brown‐coal mine near Nochten (Niederlausitz), eastern Germany. The fluvial–aeolian sequence covers parts of the Early, Pleni‐ and Late‐glacial. The detailed chronology is based on 11 radiocarbon and 12 OSL dates, covering the period between ca. 100 kyr and 11 kyr BP. Basal peat deposits are correlated with an Early Weichselian interstadial. During this period boreal forests were present and minimum mean summer temperatures were > 13°C. Early Pleniglacial deposits are absent. The Middle and Late Pleniglacial environments were treeless and different types of tundra vegetation can be recognised. Minimum mean summer temperatures varied between 10 and 15°C. Vegetation and climate is reconstructed in detail for the periods around 34–38 kyr BP and 24–25 kyr BP. Around 34–38 ka, a mixture between a low shrub tundra and a cottongrass tussock–subshrub tundra was present. The botanical and sedimentological data suggest that from the Middle to the Late Pleniglacial, the climate became more continental, aridity and wind strength increased, and the role of a protecting winter snow cover decreased. A sedge–grass–moss tundra dominated around 24 and 25 kyr BP. Copyright © 2001 John Wiley & Sons, Ltd.

An updated Holocene sea-level curve for the Delaware coast

Abstract We present an updated Holocene sea-level curve for the Delaware coast based on new calibrations of 16 previously published radiocarbon dates ( Kraft, 1976; Belknap and Kraft, 1977 ) and 22 new radiocarbon dates of basal peat deposits. A review of published and unpublished 137 Cs and 210 Pb analyses, and tide gauge data provide the basis for evaluating shorter-term (10 2 yr) sea-level trends. Paleosea-level elevations for the new basal peat samples were determined from the present vertical zonation of marsh plants relative to mean high water along the Delaware coast and the composition of plant fossils and foraminifera. Current trends in tidal range along the Delaware coast were used to reduce elevations from different locations to a common vertical datum of mean high water at Breakwater Harbor, Delaware. The updated curve is similar to Belknap and Kraft's [J. Sediment. Petrol., 47 (1977) 610–629] original sea-level curve from 12,000 to about 2000 yr BP. The updated curve documents a rate of sea-level rise of 0.9 mm/yr from 1250 yr BP to present (based on 11 dates), in good agreement with other recent sea-level curves from the northern and central U.S. Atlantic coast, while the previous curve documents rates of about 1.3 mm/yr (based on 4 dates). The precision of both estimates, however, is very low, so the significance of these differences is uncertain. A review of 210 Pb and 137 Cs analyses from salt marshes of Delaware indicates average marsh accretion rates of 3 mm/yr for the last 100 yr, in good agreement with shorter-term estimates of sea-level rise from tide gauge records.

Peatland Initiation During the Holocene in Continental Western Canada

Today, the southern limit of peatlands in continental western Canada is largely limited by thermal seasonal aridity, although physiographic parameters of substrate texture, topography, and salinity also exsert important controls on the presence and absence of peatlands. Factors that control peatland distribution today also operated in the past, thus the initiation of peatlands during the Holocene was mainly limited by aridity and physiography. Calibrated radiocarbon dates of basal peat deposits from 90 locations across continental western Canada indicate that peat formation began approximately 8,000 to 9,000 years BP in nucleation zones along the upper elevations of the Montane region of Alberta and in northern Alberta uplands after an initial deglacial lag. Predictions of maximum early Holocene summer insolation by climate simulations provide a mechanism for limiting peatland establishment during the early Holocene. From 6,000 to 8,000 years ago, peat formation in continental western Canada expanded eastwards into Manitoba responding to decreases in summer insolation. Peatland expansion during the early Holocene was more extensive in Alberta than in Manitoba in response to a southwesterly shift in the Arctic front. The displacement of the Arctic front allowed for more frequent incursions of moist Pacific air into Alberta while limiting it in Manitoba. After 6,000 years BP, the trend of southeasterly peatland expansion continued. Peatlands are youngest in the southern Boreal Forest and Aspen Parkland Region as well as in the lower elevations of the Peace-Wapiti River drainage basin, forming over the last 3,000 to 4,000 years. Peatlands are also young in the lower elevations of the Hudson Bay Lowlands where peat initiation has been limited by timing of emergence from glacial rebound. The spatial and temporal distribution of peatland initiation during the Holocene is verified by existing pollen records and corroborates some simulated climate models.

Lake Michigan water levels and the development of Holocene beach-ridge complexes at Two Rivers, Wisconsin: Stratigraphic, geomorphic, and radiocarbon evidence

Two beach-ridge complexes formed at Two Rivers, Wisconsin, on the west shore of Lake Michigan during the past 5700 yr. Auger drilling, closely spaced vibracoring, and subsurface geophysical profiling using ground-penetrating radar along two east-west transects across ridges and swales indicate that several facies of beach sediment are capped with dune sand. 14C ages were obtained from basal-peat deposits collected from swales between successive beach ridges. Associated lake levels were interpreted from the elevation of coarse foreshore deposits observed on the east (basinward) side of the next lakeward beach ridge. 14C ages, geomorphic relationships, and stratigraphic interpretations indicate that progradation and water-level drop followed a major transgression. Lake level fluctuated on a scale of 1.7–2 m during the past 3000 yr.

Forest and Moorland in the South Pennine Uplands in the Mid-Flandrian Period.: III. The Spread of Moorland--Local, Regional and National

(1) Pollen analyses from basal peat deposits at a range of sites in different topographic situations in the South Pennine uplands are compared with a long radiocarbondated pollen diagram from one of the sites, and used to reconstruct the patterns of blanket peat initiation on the moorlands. (2) Three distinct phases of peat initiation in water-collecting situations, followed by subsequent spread to water-shedding situations, are recognized: c. 9000 B.P., 7500-7000 B.P. and c. 5500 B.P.. The total timespan of peat spread in the South Pennines was nearly 5000 years. (3) Published pollen analyses and radiocarbon dates of basal peat deposits at other upland sites in England and Wales show a similar protracted period of peat initiation (from c. 9000-1000 B.P. overall). Peat began to form earlier in the two upland regions where Mesolithic activity has been clearly demonstrated (the southern Pennines and the Black Mountain of South Wales). (4) It is concluded that the replacement of the upland forest and scrub of the early Flandrian period by moorland vegetation occurred in response to a complex of environmental changes, including forest clearance, climatic change and soil deterioration, that varied in their balance and timing from region to region.