Forest Bed Research Articles

Stratigraphy, age, and flora of the Southport forest bed, Southeastern Wisconsin

A mid-Holocene buried organic layer 10 to 60 cm thick is present along the Lake Michigan shoreline in southeastern Wisconsin. Named the Southport forest bed for its location in Kenosha County, the unit has yielded abundant wood specimens, including large logs, branches, twigs, stumps, and root material. Roots of truncated in situ oak ( Quercus) and elm ( Ulmus) trees extend into the underlying till of the late Wisconsin Oak Creek Formation. Nearshore lacustrine sand above the organic layer contains abundant driftwood, overlain by 2 to 3 m of cross-bedded dune sand. More than 50 wood samples have been identified; the assemblage is that of a mixed hardwood forest dominated by oak ( Quercus) and hickory ( Carya), which account for about 60% of the assemblage. Although the site has been mostly concealed for many years, it was beautifully exposed in the 1960s and early 1970s, when it was initially studied by Phil Sander, who discovered the site and documented it with field notes and numerous photographs. Here we report several new (unpublished) radiocarbon dates and details of the stratigraphy and arboreal flora. Radiocarbon dates suggest that the Southport forest probably lived for 900–1000 14C years, possibly longer, reaching its climax at about 5300 B.P. The site is of particular importance because of its proximity to the Nipissing shoreline and provides a significant point on the Nipissing transgression time curve. The unusually large number of identified specimens affords an accurate evaluation of the warm and relatively dry climate that characterized the southern Great Lakes region during the mid-Holocene.

High Rate of N 2 Fixation by East Siberian Cryophilic Soil Bacteria as Determined by Measuring Acetylene Reduction in Nitrogen-Poor Medium Solidified with Gellan Gum

For evaluating N(2) fixation of diazotrophic bacteria, nitrogen-poor liquid media supplemented with at least 0.5% sugar and 0.2% agar are widely used for acetylene reduction assays. In such a soft gel medium, however, many N(2)-fixing soil bacteria generally show only trace acetylene reduction activity. Here, we report that use of a N(2) fixation medium solidified with gellan gum instead of agar promoted growth of some gellan-preferring soil bacteria. In a soft gel medium solidified with 0.3% gellan gum under appropriate culture conditions, bacterial microbiota from boreal forest bed soils and some free-living N(2)-fixing soil bacteria isolated from the microbiota exhibited 10- to 200-fold-higher acetylene reduction than those cultured in 0.2% agar medium. To determine the N(2) fixation-activating mechanism of gellan gum medium, qualitative differences in the colony-forming bacterial components from tested soil microbiota were investigated in plate cultures solidified with either agar or gellan gum for use with modified Winogradsky's medium. On 1.5% agar plates, apparently cryophilic bacterial microbiota showed strictly distinguishable microbiota according to the depth of soil in samples from an eastern Siberian Taiga forest bed. Some pure cultures of proteobacteria, such as Pseudomonas fluorescens and Burkholderia xenovorans, showed remarkable acetylene reduction. On plates solidified with 1.0% gellan gum, some soil bacteria, including Luteibacter sp., Janthinobacterium sp., Paenibacillus sp., and Arthrobacter sp., uniquely grew that had not grown in the presence of the same inoculants on agar plates. In contrast, Pseudomonas spp. and Burkholderia spp. were apparent only as minor colonies on the gellan gum plates. Moreover, only gellan gum plates allowed some bacteria, particularly those isolated from the shallow organic soil layer, to actively swarm. In consequence, gellan gum is a useful gel matrix to bring out growth potential capabilities of many soil diazotrophs and their consortia in communities of soil bacteria.

Feeding behaviour of middle-size deer from the Upper Pliocene site of Saint-Vallier (France) inferred by morphological and micro/mesowear analysis

In the middle Villafranchian (Late Pliocene) fossil deposit of Saint-Vallier (Drôme, France), the international biochronological reference for the MN17 biozone, deer are the most common species. Two middle-sized taxa, “ Metacervoceros” rhenanus [Dubois, E., 1904. On an equivalent of the Cromer Forest Bed in the Netherlands. Koninklijke Akademie van Wetenschappen, 214–222] and Croizetoceros ramosus (Croizet and Jobert, 1828), are particularly abundant. We have performed functional morphology and micro- and mesowear analyses in order to explain if and how the two herbivorous species avoided intra-guild competition. The results of the microwear analysis show a clear difference in the wear pattern of their tooth enamel, demonstrating the two species had different feeding behaviour habits.

Changing ideas on the identity and stratigraphic significance of the Sheep Creek tephra beds in Alaska and the Yukon Territory, northwestern North America

Sheep Creek tephra (SCt) consists of inflated, rhyolitic pumice with abundant plagioclase and hornblende, lesser amounts of ferrian ilmenite and magnetite, and trace quantities of basaltic hornblende, augite, quartz, apatite, and zircon. It has an adakitic composition; the glass has low Y, Yb, and high La/Yb and Sr/Y values. Previously, compositional differences between samples were known to exist and thought to be due to derivation from a compositionally zoned magma involving several closely spaced eruptions. SCt was recognized as a useful stratigraphic marker across central Alaska and western Yukon with an age of ∼190 ka. New OSL dates on a SCt occurrence in western Yukon are ∼80 ka and force a reassessment of the earlier work. SCt is now known to be made up of several stratigraphic units: SCt-F (Fairbanks, Alaska; ∼200 ka), SCt-CC (Canyon Creek, Alaska), and in the western Yukon, SCt-C, SCt-K (∼80 ka), and SCt-A. Beds C, K, and A must be similar in age because of their close stratigraphic association at Ash Bend in Yukon. Compositional and temporal controls, including Sr isotopes, suggest Mount Drum in the Wrangell volcanic field as the source of the SCt beds. Revision of the age of the SCt beds in western Yukon means that the Reid Glaciation of Yukon is likely of MIS 6 age and probably correlates with the Delta Glaciation of Alaska. Further, the buried forest bed just below SCt-K in the Klondike goldfields must be of MIS 5 age, not MIS 7, as previously thought.

The Arboreal Component of The Neogene Forest Bed, West Point Island, Falkland Islands

In 1907 Halle (1911) collected a diverse wood assemblage from the organic clays and lignites of the Forest Bed on West Point Island. This wood assemblage provides a reliable basis for determining the local arboreal component of the vegetation comprising the Forest Bed community. Five wood types have been recognised from Halle's collection as belonging to the Cupressaceae and Podocarpaceae. A reinvestigation of the only dicotyledonous angiosperm wood collected from this bed found that it resembled most closely Weinmannia L. of the Cunoniaceae. Together these remains represent an increased diversity of wood types derived from the arboreal component of thc vegetation, relative to previous studies. Gymnosperms are likely to have comprised the overstorey with angiosperms forming a closed canopy, interpreted as a broadleaf-gymnosperm temperate-type rainforest. The results agree with arecent palynological study of the same deposits, which concluded that the vegetative composition of the Forest Bed is a unique variant of the continuum of the Nothofagus Blume-Podocarpaceae rainforests that existed on middlehigh latitude landmasses across the Southern Hemisphere during the Late Tertiary.

Estimation of erosion rate in a small catchment of Loktak wetland, India, using lead-210 (210Pb) technique

Loktak Lake, one of the largest freshwater wetlands in the northeastern India, is under severe pressure due to many reasons-the main being increased siltation from the degraded catchment area. In the present study Nuclear Technique [Lead-210 (210Pb) Technique] has been applied to estimate soil erosion rates in the catchment of the lake. The erosion rates of the various land uses in the catchment have determined for various land uses, which are; fallow land (1.86 t ha−1 y−1), paddy cultivation (1.69 t ha−1 y−1), mixed forest (1.59 t ha−1 y−1) and river bed (1.35 t ha−1 y−1). The results obtained from the unsupported 210Pb are compared with equivalent results based on the modified Morgan Morgan Finney model and the results are found satisfactory.

Geo (Im) pulse Caprovis savinii (Bovidae, Mammalia) rediscovered: horn core finds of an Early Pleistocene antelope from the North Sea floor

AbstractFor more than 120 years since the name Caprovis savinii was founded on the basis of a bovid horn core from the ‘Forest Bed’ at Overstrand (Norfolk, UK), only one other find has been assigned to this species. Recently, two horn cores and one horn core fragment of the species have been discovered at two sand and gravel sorting centres in the Netherlands. The bone-bearing sediments are exposed on the North Sea floor, just off the coast of Great Yarmouth (Norfolk, UK). By comparing fossil and extant taxa, a classification within the bovid tribe Antilopini is proposed. Based on the accompanying fauna from the specific dredging area and the litho- and chronostratigraphy at Overstrand, C. savinii is placed within the Early Pleistocene.

Age and implications of the Forest Bed, Falkland Islands, southwest Atlantic Ocean: Evidence from fossil pollen and spores

Diverse pollen and spore assemblages (microfloras) are preserved in organic clays and lignites associated with fossil tree remains in the Forest Bed. At present, these microfloras provide the only reliable basis for determining the age, composition, structure and affinity of the Forest Bed community. The Bed is suggested to be Middle? Miocene to Early Pliocene (age range Oligocene–Early Pliocene) based on the time distribution of a number of rare pollen types and the present-day ecology of their nearest living relatives in central–southern America and the southwest Pacific. The wet forest community whose remains form the Forest Bed was a form of broadleaf-gymnosperm temperate rainforest in which Dacrydium, Lagarostrobos and Podocarpus are likely to have formed an overstorey above a closed canopy dominated by Nothofagus ( Nothofagus) spp. The latter subgenus is now endemic to ±uniformly wet regions in southern South America whilst two of the tall gymnosperms ( Dacrydium, Lagarostrobos) are now endemic to the southwest Pacific region. An extinct, probable shrub-sized, conifer ( Podosporites microsaccatus) appears to have formed the ground cover. Ferns and fern allies were very rare, in contrast to most living Nothofagus-gymnosperm communities. Fossil pollen and spore types produced by uncommon subcanopy tree, shrub and fern species potentially provide a means for more accurately dating the deposit once their time of first occurrence and extinction at subantarctic latitudes in the southwest Atlantic region is established (nearest living equivalents given in parentheses) — Clavatipollenites cf glarius ( Hedyosmum), Thymelaepollis sp. ( Ovidia-type) and Cyatheacidites annulatus ( Lophosoria). This will require palynostratigraphic analysis of Late Palaeogene–Neogene sediments recovered from basins surrounding the Falkland Islands, for example from DSDP Site 329 drilled on the Maurice Ewing Bank at the eastern margin of the Falkland Plateau.

Late Pliocene Dawson Cut Forest Bed and new tephrochronological findings in the Gold Hill Loess, east-central Alaska

Research Article| January 01, 2009 Late Pliocene Dawson Cut Forest Bed and new tephrochronological findings in the Gold Hill Loess, east-central Alaska T.L. Péwé; T.L. Péwé 1Department of Geology, Arizona State University, Tempe, Arizona 85287, USA *Deceased. Search for other works by this author on: GSW Google Scholar J.A. Westgate; J.A. Westgate † 2Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada †E-mail: westgate@geology.utoronto.ca. Search for other works by this author on: GSW Google Scholar S.J. Preece; S.J. Preece 2Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada Search for other works by this author on: GSW Google Scholar P.M. Brown; P.M. Brown 3Rocky Mountain Tree-Ring Research, Inc., 2901 Moore Lane, Fort Collins, Colorado 80526, USA Search for other works by this author on: GSW Google Scholar S.W. Leavitt S.W. Leavitt 4Laboratory of Tree-Ring Research, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Author and Article Information T.L. Péwé *Deceased. 1Department of Geology, Arizona State University, Tempe, Arizona 85287, USA J.A. Westgate † 2Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada S.J. Preece 2Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada P.M. Brown 3Rocky Mountain Tree-Ring Research, Inc., 2901 Moore Lane, Fort Collins, Colorado 80526, USA S.W. Leavitt 4Laboratory of Tree-Ring Research, University of Arizona, Tucson, Arizona 85721, USA †E-mail: westgate@geology.utoronto.ca. Publisher: Geological Society of America Received: 05 Sep 2007 Revision Received: 09 Feb 2008 Accepted: 22 Apr 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2008 Geological Society of America GSA Bulletin (2009) 121 (1-2): 294–320. https://doi.org/10.1130/B26323.1 Article history Received: 05 Sep 2007 Revision Received: 09 Feb 2008 Accepted: 22 Apr 2008 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation T.L. Péwé, J.A. Westgate, S.J. Preece, P.M. Brown, S.W. Leavitt; Late Pliocene Dawson Cut Forest Bed and new tephrochronological findings in the Gold Hill Loess, east-central Alaska. GSA Bulletin 2009;; 121 (1-2): 294–320. doi: https://doi.org/10.1130/B26323.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Thick loess deposits in the Fairbanks region of interior Alaska are a rich source of information on past climates and environments during the late Cenozoic, and the numerous distal, silicic tephra beds preserved within them offer the potential for good chronological control. The Dawson Cut Forest Bed lies in the lower part of this loess cover. Plant macrofossils consist of Picea glauca, Picea mariana, Betula papyrifera, and Populus balsamifera but no Pinus. These fossils, together with the abundance and size of spruce remains, pollen, tree-ring characteristics, and δ13C values of spruce wood, demonstrate that the boreal forest represented by the Dawson Cut Forest Bed was similar to the modern boreal forest of central Alaska. Warming conditions during the early part of the Dawson Cut Interglaciation initiated thawing of permafrost and melting of ground ice, as evidenced in the presence of ice-wedge casts and major erosion of the lower Gold Hill Loess. Tephrochronological, magnetostratigraphic, and glass fission-track dating studies in the Fairbanks area and at the Palisades site on the Yukon River in central Alaska suggest an age for the Dawson Cut Forest Bed of ca. 2 Ma. Hence, the northern boreal forest of northwestern North America, as we know it today, has a long history that probably extends back to at least 2 Ma. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Characteristics of mule deer day-bed and forage sites in current-condition and restoration-treated ponderosa pine forest

We characterized microhabitat (structure and microclimate) at 236 mule deer (Odocoileus hemionus) day-beds, 152 diurnal forage sites, and 439 random locations during summers of 1998–2000. Our objectives were to 1) identify important microhabitat characteristics of day-beds and forage sites in untreated forest, 2) compare relative availability of bed and forage sites having these characteristics between forest types, 3) determine whether mule deer selected either forest type disproportionately for these activities, and 4) compare characteristics of day-bed and forage sites between forest types. Mule deer selected bed and forage sites with specific characteristics in untreated forest each year. Site temperature and canopy closure were the most influential attributes in bed-site selection, while forage availability best described forage sites. Availability of characteristic day-bed microhabitat was reduced 50–100%, while forage characteristics increased 31–66% each year in treated forest. Foraging mule deer selected treated forest in 1 year but demonstrated no preference in 2 years. Mule deer continued to use day-beds in both forest types each year as expected based on proportional area of each forest type, but did so in treated forest in 1999 by utilizing a novel suite of microhabitat features. Bed sites in each forest type had similar levels of midstory canopy closure, but treated forest bed sites were warmer in one year, exclusively located under oaks (Quercus gambelii) rather than conifers, and less concealed than untreated forest beds. Small oak stands were the only dense patches retained in the restoration prescription we examined and provided marginal thermal and hiding cover for day-bed use by female mule deer. These differences may increase thermoregulatory stress and the potential for increased disturbance and predation of female mule deer and fawns during summer. We recommend retaining >0.04-ha patches of dense bedding and hiding cover in areas where both forest restoration and mule deer are management concerns.

The Dawson Cut Forest Bed in the Fairbanks area, Alaska, is about two million years old☆

The Dawson Cut Forest Bed lies in the lower part of thick, late Cenozoic loess deposits in the Fairbanks area. It is associated with several distal tephra beds that provide age control and offer the opportunity of its recognition elsewhere in central Alaska. EC tephra (named herein) occurs in the uppermost part of the Dawson Cut Forest Bed and its petrographic and chemical properties point to a co-magmatic relationship with PA tephra, which has not been found in direct association with the forest bed. Both tephra beds are pink and have unusually high Cl in their glass shards, which readily separates them from all other tephra beds in the Fairbanks area. They were produced by discrete eruptions, closely spaced in time. PA tephra has a glass-fission-track age of 2.02 ± 0.14 myr, indicating that the Dawson Cut Forest Bed must be about 2 million years old. The Palisades tephra (named herein) has very similar properties to these two tephra beds, suggesting that the buried forest bed just above it at the Palisades site on the Yukon River, about 250 km west of Fairbanks, correlates with the Dawson Cut Forest Bed.

Late Pliocene to late Pleistocene environments preserved at the Palisades Site, central Yukon River, Alaska

The Palisades Site is an extensive silt-loam bluff complex on the central Yukon River preserving a nearly continuous record of the last 2 myr. Volcanic ash deposits present include the Old Crow (OCt; 140,000 yr), Sheep Creek (SCt; 190,000 yr), PA (2.02 myr), EC (ca. 2 myr), and Mining Camp (ca. 2 myr) tephras. Two new tephras, PAL and PAU, are geochemically similar to the PA and EC tephras and appear to be comagmatic. The PA tephra occurs in ice-wedge casts and solifluction deposits, marking the oldest occurrence of permafrost in central Alaska. Three buried forest horizons are present in association with dated tephras. The uppermost forest bed occurs immediately above the OCt; the middle forest horizon occurs below the SCt. The lowest forest bed occurs between the EC and the PA tephras, and correlates with the Dawson Cut Forest Bed. Plant taxa in all three peats are common elements of moist taiga forest found in lowlands of central Alaska today. Large mammal fossils are all from common late Pleistocene taxa. Those recovered in situ came from a single horizon radiocarbon dated to ca. 27,000 14C yr B.P. The incongruous small mammal assemblage in that horizon reflects a diverse landscape with both wet and mesic environments.

Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone

Zircons from the Black Forest Bed, Petrified Forest Member, Chinle Formation, in Petrified Forest National Park, yield ages that range from Late Triassic to Late Archean. Grains were analyzed by multigrain TIMS (thermal-ionization mass spectrometry), single-crystal TIMS, and SHRIMP (sensitive, high-resolution ion-microprobe). Multiple-grain analysis yielded a discordia trajectory with a lower intercept of 207 6 2 Ma, which because of the nature of multiple-grain sampling of a detrital bed, is not considered conclusive. Analysis of 29 detrital-zircon grains by TIMS yielded UPb ages of 2706 6 6M a to 2066 6 Ma. Eleven of these ages lie between 211 and 216 6 6.8 Ma. Our statistical analysis of these grains indicates that the mean of the ages, 213 6 1.7 Ma, reflects more analytical error than geologic variability in sources of the grains. Grains with ages of ca. 1400 Ma were derived from the widespread plutons of that age exposed throughout the southwestern Cordillera and central United States. Twelve grains analyzed by SHRIMP provide 206 Pb*/ 238 U ages from 214 6 2M a to 200 6 4 Ma. We use these data to infer that cores of inherited material were present in many zircons and that single-crystal analysis provides an accurate estimation of the age of the bed. We further propose that, even if some degree of reworking has occurred, the very strong concentration of ages at ca. 213 Ma provides a maximum age for the Black Forest Bed of 213 6 1.7 Ma. The actual age of the bed may be closer to 209 Ma. Dating continental successions is very difficult when distinct ash beds are not clearly identified, as is the case in the Chinle Formation. Detrital zircons in the Black Forest Bed, however, are dominated by an acicular morphology with preserved delicate terminations. The shape of these crystals and their inferred environment of deposition in slow-water settings suggest that the crystals were not far removed from their site of deposition in space and likely not far in time. Plinian ash clouds derived from explosive eruptions along the early Mesozoic Cordilleran margin provided the crystals to the Chinle basin, where local conditions insured their preservation. In the case of the Black Forest Bed, the products of one major eruption may dominate the volcanic contribution to the unit. Volcanic detritus in the Chinle Formation was derived from multiple, distinct sources. Coarse pebble- to cobble-size material may have originated in eastern California and/or western Arizona, where Triassic plutons are exposed. Fine-grained detritus, in contrast, was carried in ash clouds that derived from caldera eruptions in east-central California or western Nevada.

Last Interglacial age of the Eva Forest Bed, Central Alaska, from thermoluminescence dating of bracketing loess

The Eva Forest Bed in the Yukon-Tanana Upland of east-central Alaska represents a frozen, buried, boreal forest. This bed consists of well-preserved peat lenses, sticks, roots, logs, as well as rooted and unrooted stumps of trees. Where preserved, the Eva Forest Bed is overlain by the Goldstream Formation and underlain by Gold Hill Loess. Almost all exposures have a prominent unconformity at the top of the Gold Hill Loess, even where the forest bed is not preserved. Attempts have been made over the last 50 yr to determine the age of this bed numerically. We report here details of thermoluminescence (TL) dating of 14 samples of loess from above and below this unconformity in the Fairbanks area. Together with knowledge about the climatic indicators from the Eva Forest Bed, these TL age estimates: (1) indicate that the most probable age of the Eva Forest Bed is ∼125 ka, the time of the warmest part of oxygen isotope stage 5; and (2) confirm stratigraphic inferences from tephra beds in the upper Gold Hill Loess that erosion has removed variable thicknesses of the original loess complex “just prior” to the development of the Eva Forest Bed.

Vegetation and paleoclimate of the last interglacial period, central Alaska

The last interglacial period is thought to be the last time global climate was significantly warmer than present. New stratigraphic studies at Eva Creek, near Fairbanks, Alaska indicate a complex last interglacial record wherein periods of loess deposition alternated with periods of soil formation. The Eva Forest Bed appears to have formed about the time of or after deposition of the Old Crow tephra (dated to ∼160 to ∼120 ka), and is therefore correlated with the last interglacial period. Pollen, macrofossils, and soils from the Eva Forest Bed indicate that boreal forest was the dominant vegetation and precipitation may have been greater than present around Fairbanks during the peak of the last interglacial period. A new compilation of last interglacial localities indicates that boreal forest was extensive over interior Alaska and Yukon Territory. Boreal forest also extended beyond its present range onto the Seward and Baldwin Peninsulas, and probably migrated to higher elevations, now occupied by tundra, in the interior. Comparison of last interglacial pollen and macrofossil data with atmospheric general circulation model results shows both agreement and disagreement. Model results of warmer-than-present summers are in agreement with fossil data. However, numerous localities with boreal forest records are in conflict with model reconstructions of an extensive cool steppe in interior Alaska and much of Yukon Territory during the last interglacial.

Eva Interglaciation Forest Bed, Unglaciated East-Central Alaska: Global Warming 125,000 Years Ago

Eva Interglaciation Forest Bed, Unglaciated East-Central Alaska: Global Warming 125,000 Years Ago

An early Holocene oxygen isotope record from the Olson buried forest bed, southern Lake Michigan

Analysis of a 3.5 m vibracore from the Olson buried forest bed in the southern Lake Michigan basin provides new paleolimnological data for the early Holocene. The core records a rise in lake level from the Chippewa low water phase toward the Nipissing high water phase. Deepening of the water level at the core site is suggested by a trend toward decreasing organic carbon content up core that is interpreted as a response to increasing distance between terrestrial debris sources and the core site.Published data from deep water cores from the southern Lake Michigan basin suggest there had been an inflow of isotopically light water from glacial Lake Agassiz into the southern basin between 10.5-11 ka (A1 event). The data also indicate a second flood of isotopically light water between 8-9 ka (A2 event).Three new 14C dates from the Olson site core suggest that most of the sediment was deposited between 8.45 ka and 8.2 ka, an interval roughly coeval with the second pulse of 18O-depleted water (A2) from Lake Agassiz into the southern basin. Oxygen isotope ratio analysis of shell aragonite from the gastropods Probythinella lacustris and Marstonia deceptashows increasingly negative values up core. This trend inσ18O values suggests that 18O - depleted water entered the southern basin about 8.4 ka. The Olson site core thus provides a chronology of events in the southern Lake Michigan basin associated with the draining of glacial Lake Agassiz.

Age verification of the Lake Gribben forest bed and the Younger Dryas Advance of the Laurentide Ice Sheet

Analysis of nine wood samples from the Lake Gribben forest bed near Lake Gribben, Michigan, yielded a combined age of 10 025 ± 100 14C years BP, which confirms and refines prior age estimates for the bed. The stratigraphic position of these samples below a prograding ice-contact fan indicates the time that a glacial margin reached the southern edge of the Lake Superior basin. Geomorphic tracing and correlation of associated deposits indicate that a contemporaneous margin extended almost 1000 km from Duluth, Minnesota, across the Lake Superior basin to North Bay, Ontario. Along the southern shore of Lake Superior ice-margin expansion began during and ended at the close of the Younger Dryas. A surging glacier system would not produce a nearly linear moraine system across both a major basin (Lake Superior) and a major upland (Abitibi Upland). Therefore, we attribute this advance of the Laurentide Ice Sheet to climatic forcing of the Younger Dryas event.

Age verification of the Lake Gribben forest bed and the Younger Dryas Advance of the Laurentide Ice Sheet

Age verification of the Lake Gribben forest bed and the Younger Dryas Advance of the Laurentide Ice Sheet

Palynological and Radiocarbon Evidence for Deglaciation Events in the Green Bay Lobe, Wisconsin

A new and significant site of organic silty sand has been found beneath the Valders till at Valders Quarry in northeastern Wisconsin. This is now the earliest known late-glacial site associated with red till ice advances in the western Great Lakes area. Leaves of terrestrial plants washed into a small depression provide a date of 12,965 ± 200 yr B.P. (WIS-2293), which is significantly older than the Two Creeks Forest Bed (ca. 11,800 yr B.P.). Percentage and concentration pollen diagrams suggest that the site was open and distant from a closedPiceaforest. No wood orPiceaneedles have been found. This date is statistically indistinguishable from 12,550 ± 233 yr B.P., the mean of three dates for the end of inorganic varve sedimentation at Devils Lake, 160 km southwest at the terminus of the Green Bay Lobe. Assuming that the Green Bay lobe vacated its outermost moraine in the interval from 13,000 to 12,500 yr B.P., only a short time was available for retreat of the ice margin over 350 km, drainage of red sediment from Lake Superior into the Lake Michigan basin, readvance of over 250 km, retreat of at least 80 km, and advance to this site. The time for these events appears to have been too short to resolve by current radiocarbon technique. This extremely rapid collapse of the Green Bay lobe has a calibrated age of about 15,000 cal yr B.P., about that of the dramatic warming seen in the Greenland ice cores.