Midcontinent North America Research Articles

Pennsylvanian (Desmoinesian–Virgilian) ammonoid zonation for Midcontinent North America

The most complete, abundant and diverse Middle–Upper Pennsylvanian ammonoid succession in the world occurs in the North American Midcontinent, where more than 40,000 ammonoids representing 52 genera from at least 42 successive stratigraphic levels have been recovered. The Desmoinesian–Virgilian succession comprises a series of seven substage-scale ammonoid genus zones, in ascending order: Wellerites, Eothalassoceras, Pennoceras, Preshumardites, Pseudaktubites, Shumardites and Subperrinites (new), which are subdivided into 14 ammonoid species- or genus-based subzones. The base of the Eothalassoceras Zone is near the current base of the global Kasimovian Stage, the base of the Pennoceras Zone coincides with the base of the regional Missourian Stage (and likely will be close to the base of a revised Kasimovian Stage when its official base is formally selected), the base of the Pseudaktubites stainbrooki Subzone nearly coincides with the base of the regional Virgilian Stage, and the base of the Shumardites Zone coincides with the base of the global Gzhelian Stage, illustrating the value of this zonation for regional and global correlation.

CO2 uptake and ecophysiological parameters of the grain crops of midcontinent North America: Estimates from flux tower measurements

We analyzed net CO2 exchange data from 13 flux tower sites with 27 site-years of measurements over maize and wheat fields across midcontinent North America. A numerically robust “light-soil temperature-VPD”-based method was used to partition the data into photosynthetic assimilation and ecosystem respiration components. Year-round ecosystem-scale ecophysiological parameters of apparent quantum yield, photosynthetic capacity, convexity of the light response, respiration rate parameters, ecological light-use efficiency, and the curvature of the VPD-response of photosynthesis for maize and wheat crops were numerically identified and interpolated/extrapolated. This allowed us to gap-fill CO2 exchange components and calculate annual totals and budgets. VPD-limitation of photosynthesis was systematically observed in grain crops of the region (occurring from 20 to 120 days during the growing season, depending on site and year), determined by the VPD regime and the numerical value of the curvature parameter of the photosynthesis-VPD-response, σVPD. In 78% of the 27 site-years of observations, annual gross photosynthesis in these crops significantly exceeded ecosystem respiration, resulting in a net ecosystem production of up to 2100g CO2m−2year−1. The measurement-based photosynthesis, respiration, and net ecosystem production data, as well as the estimates of the ecophysiological parameters, provide an empirical basis for parameterization and validation of mechanistic models of grain crop production in this economically and ecologically important region of North America.

Rock drumlins and megaflutes of the Niagara Escarpment, Ontario, Canada: a hard bed landform assemblage cut by the Saginaw–Huron Ice Stream

The late Wisconsin (c. 18,000 ybp) Saginaw–Huron Ice Stream (SHIS) of the Laurentide Ice Sheet was as much as 800 km long and 250 km wide in the Great Lakes basin of mid-continent North America. In its onset zone in the upper Lake Huron basin of Ontario, Canada it flowed south from the high standing Canadian Shield to move over gently dipping Ordovician and Silurian dolostones devoid of glacial sediment cover. The onset zone is recorded by a distinct ‘hard bed landform assemblage’ across ∼3000 km2 of north-facing escarpments (Niagara, Fossil Hill, and Kagawong) and dip slope pavements on the Bruce Peninsula and Manitoulin Island, and the adjacent floor of Lake Huron. A wide range of glacially-streamlined rock landforms were carved into dolostones below fast flowing ice. The largest are 30 km-wide bullet-shaped escarpment ‘promontories’ that face upglacier. Superposed on these are swarms of rock drumlins up to 5 km wide with downglacier lengths of as much as 10 km. Promontories and drumlins record streaming of dirty basal ice around escarpment highs and create a distinct ‘zig-zag’ planform to the escarpments. In turn, down-dip, trailing-edge dolostone pavements are corrugated by kilometre-long megagrooves and megaflutes cut by basal ice flowing around resistant high-standing bioherm mounds on dolostone bedding planes. The geomorphology of the Niagara Escarpment does not primarily reflect a lengthy history of preglacial Cenozoic fluvial erosion as classically argued, but geologically-brief episodes of accelerated abrasion and quarrying below ice streams within successive Pleistocene ice sheets.

Petrology of the Eureka Quartzite (Middle and Late Ordovician), Utah and Nevada, U.S.A.

The Eureka Quartzite is a sheet-like quartzarenite up to 200-m thick that was deposited on the eastern shelf of the Cordilleran miogeocline from Canada to California. It is the only sandstone lithosome from the Middle Cambrian through Devonian succession in the Great Basin and is remarkable in its purity of detrital and authigenic quartz, scarcity of bedding, and heterogeneity of both grain packing and quartz cement abundance. Sand sources ranged from the Peace River Arch in Canada to the Transcontinental Arch in mid-continental North America. The Eureka represents a third-order regressive–transgressive stratigraphic sequence, although whether the regression formed in response to eustasy or epirogenic uplift of western North America is unresolved. The near absence of detrital clay, body fossils, and subaerial features in addition to the presence of herringbone cross-beds indicate that the Eureka was deposited in intertidal and shallow subtidal environments except for minor eolian deposits. Bioturbation destroyed most primary stratification, although discrete burrows are rare. Textural features of quartz (superb roundness, bean grain shape, crescentic impact scars) indicate a prolonged episode of eolian abrasion prior to marine deposition. Regionally the detrital composition is 99.5 percent detrital monocrystalline quartz, 0.5 percent K-feldspar and carbonate allochems, and a trace of heavy minerals. From 2 percent to 4 percent feldspar and carbonate allochems that were initially present have been replaced by quartz during burial. The chief authigenic phases are quartz overgrowths with minor calcite (now dolomite) and illite. Spheroidal and amoeboid calcite-cemented concretions up to 3 cm in diameter formed at shallow burial depths, but all carbonate in the concretions has been leached in outcrop. The heterogeneity of grain compaction and amount of quartz cement resulted in beds that range from semifriable to sedimentary quartzites in the same outcrop. Compaction by the combination of grain rearrangement, pressure dissolution, and grain fracturing generated anomalously low intergranular volumes that average 14 percent in Nevada and 21 percent in Utah. The normal evolution of microquartz overgrowths ( 10 μm) during cementation was retarded; consequently, microquartz and mesoquartz cement (80 percent) dominate over macroquartz (20 percent), and they are the only cements in the least-cemented beds and laminations. Illite co-precipitated with microquartz and impeded quartz cementation by coating quartz crystal faces. Despite reaching temperatures >135° C for ∼100 million years, much of the Eureka, especially in Utah, remains incompletely cemented and retains porosity of ∼2 percent. The chief cause of cement heterogeneity appears to be authigenic illite abundance. Pressure dissolution of quartz at shale beds and clay drapes that formed stylolites was the most likely major source of silica for quartz cement. Invasion by hydrocarbons and hydrogen sulfide (H2S) resulted in the reduction of iron in hematite grain coats on quartz grains, a remnant of their eolian dune formation, the bleaching of beds, and the generation of pyrite (now hematite). The abundance of iron that now resides in pyrite/hematite suggests that red beds were once widespread. Outcrop and near-outcrop processes generated Liesegang bands of iron oxide, desert varnish, “pockmarks” where carbonate cement in centimeter-scale concretions dissolved, hematite pseudomorphs of pyrite, and minor opal cement.

Impacts of Mute Swans (Cygnus olor) on Submerged Aquatic Vegetation in Illinois River Valley Backwaters

Wetland loss in North America has been considerable and well documented, and the establishment of exotic species in remaining wetlands can further reduce their ability to support native flora and fauna. In the Chesapeake Bay and Great Lakes ecosystems, exotic mute swans (Cygnus olor) have been found to negatively impact wetlands through degradation of submerged aquatic vegetation (SAV) communities. Mute swan populations have expanded into many areas of mid-continental North America outside the Great Lakes ecosystem, but the environmental impact of these populations is not well known. Mid-continental wetlands in North America differ in physical characteristics (e.g., size, depth, and permanency) and aquatic vegetation species composition compared to wetlands in other areas where mute swans have been studied and, thus, may be more or less susceptible to degradation from swan herbivory. To investigate the impact of mute swan herbivory on SAV communities in mid-continent wetlands, we used exclosures to prevent swans from foraging in 2 wetland complexes in central Illinois. Above-ground biomass of vegetation did not differ between exclosures and controls; however, mean below-ground biomass was greater in exclosures (52.0 g/m2, SE = 6.0) than in controls (34.4 g/m2 SE = 4.0). Thus, although swan densities were lower in our study region compared to that of previous studies, we observed potentially detrimental impacts of swan herbivory on below-ground biomass of SAV. Our results indicate that both above-ground and below-ground impacts of herbivory should be monitored, and below-ground biomass may be most sensitive to swan foraging.

Associations between body composition and helminths of lesser snow geese during winter and spring migration

Costs of parasitism are predicted to be higher with greater parasite intensities and higher inter-parasite competition (diversity). We tested whether greater helminth intensities and diversity were associated with poorer body composition (whole-body fat, protein, mineral and true body mass) in lesser snow geese, Chen caerulescens caerulescens. As part of a larger study on nutritional ecology, 828 wintering or migrating geese were shot between January and May 1983 in 27 different date–locations (samples) during their northward migration through mid-continental North America. A large proportion of overall variation in body composition and parasite communities was among samples, so we analyzed data within each of the 27 samples, controlling for structural body size (the first principal component of 10 body size measurements), sex and the age of geese. There was no compelling evidence that cestodes, trematodes or helminth diversity were associated with variation in body composition but nematodes had several negative associations with fat reserves. However, negative associations between fat reserves and nematodes occurred most often in geese collected between March and May when nematode prevalences and intensities were relatively low. This suggests several possibilities: that the most common nematodes (Heterakis dispar and Trichostrongylus tenuis) were more virulent at this time, that infected individuals had been chronically infected and suffered cumulative nutrient deficits that lasted until late in the spring migration, or that geese became more vulnerable to the effects of parasites at this time of year, possibly because they redirected resources away from immunity toward fat storage in preparation for reproduction.

Drought drove forest decline and dune building in eastern upper Michigan, USA, as the upper Great Lakes became closed basins

Current models of landscape response to Holocene climate change in midcontinent North America largely reconcile Earth orbital and atmospheric climate forcing with pollen-based forest histories on the east and eolian chronologies in Great Plains grasslands on the west. However, thousands of sand dunes spread across 12,000 km 2 in eastern upper Michigan (EUM), more than 500 km east of the present forest-prairie ecotone, present a challenge to such models. We use 65 optically stimulated luminescence (OSL) ages on quartz sand deposited in silt caps (n = 8) and dunes (n = 57) to document eolian activity in EUM. Dune building was widespread ca. 10–8 ka, indicating a sharp, sustained decline in forest cover during that period. This decline was roughly coincident with hydrologic closure of the upper Great Lakes, but temporally inconsistent with most pollen-based models that imply canopy closure throughout the Holocene. Early Holocene forest openings are rarely recognized in pollen sums from EUM because faint signatures of non-arboreal pollen are largely obscured by abundant and highly mobile pine pollen. Early Holocene spikes in nonarboreal pollen are recorded in cores from small ponds, but suggest only a modest extent of forest openings. OSL dating of dune emplacement provides a direct, spatially explicit archive of greatly diminished forest cover during a very dry climate in eastern midcontinent North America ca. 10–8 ka.

Subsurface Facies Analysis of the Late Cambrian Mt. Simon Sandstone in Western Ohio (Midcontinent North America)

The Cambrian Mt. Simon Sandstone (MSS) is a possible unconventional gas reservoir in the Illinois, Michigan, and Appalachian Basins, but comparatively little is known about the unit. This study used core and well logs from two deep exploratory wells to interpret the depositional environment of the MSS under western Ohio, where the MSS is about 120 m thick and found 1060 m below ground surface. In western Ohio, the MSS unconformably overlies the Precambrian Middle Run Formation, is conformably overlain by the Cambrian Eau Claire Formation, and has a distinctive gamma-ray log-signature. In well DGS-2627, the MSS consists of tan, friable, moderately sorted, rounded, coarse- to very coarse-grained siliceous quartz arenite with minor heterolithic sandstone-mudstone couplets (rhythmites) and quartz granule conglomerate. Features indicative of tidally-influenced, shallow marine settings include tidal rhythmites, lenticular-, flaser-, and wavy-bedding, herringbone cross-bedding, mud-drapes, tidal bundles, reactivation surfaces, intraclasts, and bioturbation. The unit generally coarsens- and thickens-upward, and is interpreted as migration of a tidally-influenced transgressive barrier sequence. A subsurface facies model for the MSS is developed by interpreting geophysical logs and cores from DGS-2627l, and this model is semi-quantitatively tested by first interpreting well BP-4 using geophysical logs alone, then confirming the results using core.

Absolute gravity calibration of GPS velocities and glacial isostatic adjustment in mid-continent North America

[1] We examine absolute gravity (AG) and vertical Global Positioning System (GPS) time series between 1995 and 2010 at eight collocated sites in mid-continent North America. The comparison of AG and GPS rates aligned to ITRF2005 yields a gravity/uplift ratio of −0.17 ± 0.01μGal mm−1 (1 μGal = 10 nm s−2) and an intercept of −0.1 ± 0.5 mm yr−1. In contrast, aligning the GPS velocities to ITRF2000 results in a gravity/uplift intercept of −1.3 ± 0.5 mm yr−1. The near-zero gravity/uplift offset for the ITRF2005 (or ITRF2008) results shows a good alignment of the GPS vertical velocities to Earth's center of mass, and confirms that GPS velocities in this reference frame can be compared to predictions of geodynamic processes such as glacial isostatic adjustment (GIA) or sea-level rise. The observed gravity/uplift ratio is consistent with GIA model predictions. The ratio remains constant in regions of fast and slow uplift, indicating that GIA is the primary driving process and that additional processes such as local hydrology have a limited impact on a decadal time-scale. Combining AG and GPS measurements can provide significant constraints for geodetic, geodynamic, and hydrological studies.

GENUS-LEVEL TAPHONOMIC VARIATION WITHIN CLADID CRINOIDS FROM THE UPPER PENNSYLVANIAN BARNSDALL FORMATION, NORTHEASTERN OKLAHOMA

Previous comparative taphonomic studies have convincingly demonstrated that the taphonomic state of crinoid fossils is controlled largely by paleoenvironmental processes and constructional morphology. While taphonomic variability among depositional facies has a long history of investigation, the degree to which preservational heterogeneity is controlled by crinoid morphology has only recently been addressed and only at relatively coarse levels. Most studies to date have focused on taphonomic variability at the subclass or ordinal level, with little documentation of lower level taphonomic variation within a single crinoid subclass. A remarkably diverse, abundant, and well-preserved crinoid fauna, recovered from a single mudstone interval within the Upper Pennsylvanian (Missourian) Barnsdall Formation in northeastern Oklahoma, midcontinent North America, provides a unique opportunity to examine taphonomic trends among poteriocrine cladid crinoids at refined taxonomic levels. Genus-level variations in specimen completeness, axis of compaction, arm position, and features attributed to decay and scavenging are observed within this assemblage and taphonomic trends related primarily to the size of individuals are detected. These results indicate that taphonomic variability extends at least to genus level within the subclass Cladida. Understanding this variability is important in interpreting the genesis and nature of crinoid-bearing units, as minor variations in morphology, ethology, and scavenger preferences impart unexpected biostratinomic heterogeneity to Copan crinoid fauna that would otherwise be difficult to explain. Taphonomic variability at low taxonomic levels and the influence of preferential scavenging should be accounted for in future crinoid taphonomic grade studies, particularly in Pennsylvanian and younger deposits, in order to avoid taphonomic assumptions that may be overly broad.

Evidence for periodic, Holocene loess deposition in kettles in a sandy, interlobate landscape, Michigan, USA

Abstract Loess deposits are common in mid-continental North America, but are rare in Michigan, and most loess found in this region is of last-glacial age. We report on evidence for Holocene-age, silt-dominated deposits found in kettle bottoms, which we interpret as loess. These silty deposits contrast with the broader interlobate landscape, which is composed of glaciofluvial materials dominated by medium and fine sands (125–500 μm). The abrupt lateral edges of the silty deposits, and their unique textural properties relative to the surrounding landscape, suggest that the silts were not washed from kettle backslopes. Rather, we suggest that the silts originated as loess that was episodically deposited in kettle bottoms across the Upland. Later, loess that may have been deposited on backslopes was occasionally redeposited into the centers of vegetated kettles, along with some background sands, by wind and water. Evidence in support of our conclusions includes (1) the fine-silty textural characteristics of the sediments, set within an otherwise sand-dominated landscape, (2) depositional sequences of charcoal-rich paleosols, intercalated within the otherwise “clean” kettle bottom silts, pointing to episodes of loess deposition interspersed with periods of slope stability and pedogenesis, and (3) increased silt contents within the upper meter of sandy soils on nearby stable uplands. Radiocarbon ages on bulk charcoal from nine paleosols within the kettle-bottom silt deposits fall mainly within the early Holocene. These deposits and 14C ages provide the first evidence of Holocene loess in the Great Lakes region, some of which probably originated from the nearby Muskegon River floodplain.

Holocene precipitation seasonality captured by a dual hydrogen and oxygen isotope approach at Steel Lake, Minnesota

Middle-Holocene (8 to 4 ka BP) warmth and aridity are well recorded in sediment archives from midcontinental North America. However, neither the climatic driver nor the seasonal character of precipitation during this period is well understood because of the limitations of available proxy indicators. For example, an important challenge is to distinguish among the interacting effects of evaporation, temperature, or precipitation seasonality in existing δ 18O records from the region. Here we combine hydrogen isotopes of palmitic acid and oxygen isotopes of carbonate to derive lake-water isotopic values during the Holocene at Steel Lake in north-central Minnesota. In combination, these data enable us to separate variations in evaporation from variations in the isotopic composition of input-waters to lake. Variations in evaporation are used as a proxy for aridity and lake-water input isotopic values are used as a proxy for the isotopic values of meteoric precipitation. Our results suggest that lake-water input isotopic values were more negative during the middle Holocene than at present. To test whether these more negative values are related to temperature or precipitation seasonality, we compare pollen-inferred temperatures and the expected isotopic value of precipitation resulting from these temperatures to the reconstructed precipitation isotopic values. Results suggest that middle Holocene warmth and aridity were associated with increased evaporation rates and decreased summer precipitation. These inferences are consistent with climate simulations that highlight the role of seasonal insolation and sea surface temperatures in driving variations in precipitation seasonality during the Holocene. Results also suggest that changes in Holocene precipitation seasonality may have influenced the expansion of the prairie-forest border in Minnesota as well as regional variations in grassland community composition. This study demonstrates the efficacy of the dual hydrogen and oxygen isotope approach for distinguishing changes in evaporation and precipitation seasonality in the paleolimnological record.

New York–Alabama lineament: A buried right-slip fault bordering the Appalachians and mid-continent North America

The New York–Alabama (NY-AL) lineament, recognized in 1978, is a magnetic anomaly that delineates a fundamental though historically enigmatic crustal boundary in eastern North America that is deeply buried beneath the Appalachian basin. Data not in the original aeromagnetic data set, particularly the lack of any information available at the time to constrain the southern continuation of the anomaly southwest of Tennessee, left the source of the lineament open to conjecture. We use modern digital aeromagnetic maps to fill in these data gaps and, for the first time, constrain the southern termination of the NY-AL lineament. Our analysis indicates that the lineament reflects a crustal-scale, right-lateral strike-slip fault that has displaced anomalies attributed to Grenville orogenesis by ∼220 km. Palinspastic restoration of this displacement rearranges the trace of the Grenville belt in southern Rodinia and implies only passive influence on later-formed Appalachian structures. The precise timing of dextral movement on the NY-AL structure is not resolvable from the existing data set, but it must have occurred during one of, or combinations of, the following events: (1) a late, postcontractional (post-Ottawan) stage of the Grenville orogeny; (2) late Neoproterozoic to Cambrian rifting of Laurentia; or (3) right-slip reactivation during the late Neoproterozoic–Cambrian rifting of Laurentia, or during Appalachian movements. Our palinspastic reconstruction also implies that the host rocks for modern earthquakes in the Eastern Tennessee Seismic Zone are metasedimentary gneisses, and it provides an explanation for the spatial location and size of the seismic zone.

Testing the climate sensitivity of peat-based paleoclimate reconstructions in mid-continental North America

The sensitivity of peatland water-table depth reconstructions to past hydroclimate variability was assessed by comparing 210Pb-dated, testate amoeba-based reconstructions of water-table depth with instrumental records of temperature, precipitation, and Palmer Drought Severity Index (PDSI). Comparisons were made for the past century at five sites in the western Great Lakes region of North America. The primary objectives were (1) to assess the relative influence of temperature and precipitation on proxy records from these systems, (2) compare an integrated index of past moisture variability, PDSI, to records of peatland surface-moisture, and (3) compare the relative climate sensitivity of paleohydrological records from a raised bog and two types of kettle peatland, including those with and without floating peat mats. Results indicated that the raised bog surface-moisture reconstructions were most strongly correlated with PDSI, with precipitation variability more important than temperature variability. Proxy records from two non-floating kettle peatlands were also correlated with PDSI, although the importance of precipitation and temperature varied between the two sites. Records from floating mats were not correlated with instrumental climate records, although patterns suggest that additional research is needed to assess the sensitivity of these systems to high-magnitude or prolonged drought events. Although the relative importance of precipitation and temperature in controlling peatland surface-moisture conditions may vary in space and time, consistent correlations with PDSI indicate that peatland archives are well suited for studies of past drought variability and should be particularly valuable in humid regions at mid-to-high latitudes where moisture-sensitive records are scarce.

Post-Clovis survival of American Mastodon in the southern Great Lakes Region of North America

The end of the Pleistocene in North America was marked by a wave of extinctions of large mammals, with the last known appearances of many species falling between ca. 11,000–10,000 14C yr BP. Temporally, this period overlaps with the Clovis Paleoindian cultural complex (11,190–10,530 14C yr BP) and with sudden climatic changes that define the beginning of the Younger Dryas chronozone (ca. 11,000–10,000 14C yr BP), both of which have been considered as potential proximal causes of this extinction event. Radiocarbon dating of enamel and filtered bone collagen from an extinct American Mastodon ( Mammut americanum) from northern Indiana, USA, by accelerator mass spectrometer yielded direct dates of 10,055 ± 40 14C yr BP and 10,032 ± 40 14C yr BP, indicating that the animal survived beyond the Clovis time period and into the late Younger Dryas. Although the late survival of this species in mid-continental North America does not remove either humans or climatic change as contributing causes for the late Pleistocene extinctions, neither Clovis hunters nor the climatic perturbations initiating the Younger Dryas chronozone were immediately responsible for driving mastodons to extinction.

Ouachita, Appalachian, and Ancestral Rockies deformations recorded in mesoscale structures on the foreland Ozark plateaus

Mesoscale structures in Paleozoic rocks of the Ozark plateaus reveal four Pennsylvanian deformation episodes in midcontinent North America. The two earliest episodes can be assigned to progressive northwestward docking of the Ouachita terrane with North America. Early extensional structures (Event 1) indicate a northwest/southeast maximum horizontal stress (Hmax) during Early Pennsylvanian Ouachita terrane advance. Event 2 extensional and strike-slip structures indicate Hmax across the Ozark plateaus that varies systematically from north-northwest/south-southeast in the south to northeast/southwest in the north. This suggests development of a slip-line deformation field in response to minor northeastward lateral escape of lithospheric blocks away from the northwestward-moving Ouachita terrane's leading edge, which acted as an indenter in western Arkansas, southeastern Oklahoma, and Texas. Younger contractional and strike-slip structures of Event 3 indicate northeast/southwest Hmax across the entire Ozark plateaus, and deformation orientation and intensity are not readily assigned to Ouachita foreland deformation and may be related to Middle Pennsylvanian Ancestral Rockies contractional deformation. Finally, Event 4 contractional structures indicate northwest/southeast Hmax consistent with southern Appalachian late stage convergence. Deformation episodes are localized along basement fault zones, particularly at major bends, suggesting minor restraining-bend uplifts along strike-slip faults. Geometries of conjugate normal fault and hybrid shear joint arrays indicate localized areas of high differential stress consistent with basement block uplift at these bends. High-angle faults reactivated in a reverse sense and bedding-parallel veins suggest tensile minimum stresses and pore fluid pressures exceeding lithostatic stress, consistent with brine pulses driven into the midcontinent during Late Paleozoic orogeny (as proposed by other authors).

Rapid responses of the prairie-forest ecotone to early Holocene aridity in mid-continental North America

The prairie-forest transition in midcontinental North America is a major physiognomic boundary, and its shifts during the Holocene are a classic example of climate-driven ecotonal dynamics. Recent work suggests asymmetrical Holocene behavior, with a relatively rapid early Holocene deforestation and more gradual reforestation later in the Holocene. This paper presents a new synthesis of the Holocene history of the Great Plains prairie-forest ecotone in the north-central US and central Canada that updates prior mapping efforts and systematically assesses rates of change. Changes in percent woody cover (%WC) are inferred from fossil pollen records, using the modern analog technique and surface-sediment pollen samples cross-referenced against remotely sensed observations. For contemporary pollen samples from the Great Plains, %WC linearly correlates to percent arboreal pollen (%AP), but regression parameters vary interregionally. At present, %AP is consistently higher than %WC, because of high background levels of arboreal pollen. Holocene maps of the eastern prairie-forest ecotone agree with prior maps, showing a rapid decrease in %WC and eastward prairie advance between 10,000 and 8000 ka (1 ka = 1000 calibrated years before present), a maximum eastward position of the ecotone from 7 to 6 ka, and increased %WC and westward prairie retreat after 6 ka. Ecotone position is ambiguous in Iowa and southeastern Minnesota, due to a scarcity of modern analogs for early-Holocene samples with high Ulmus abundances and for samples from alluvial sediments. The northern prairie-forest ecotone was positioned in central Saskatchewan between 12 and 10 ka, stabilized from 10 to 6 ka despite decreases in %WC at some sites, then moved south after 6 ka. In both east and north, ecotonal movements are consistent with a dry early Holocene and increasing moisture availability after 6 ka. Sites near the ecotone consistently show an asymmetric pattern of abrupt early Holocene deforestation (< 300 years) and gradual reforestation after 6 ka. Early Holocene decreases in %WC are faster than the corresponding drops in %AP, because the analog-based %WC reconstructions correct for the high background levels of arboreal pollen types that blur temporal variations in %AP. For example, at Elk Lake, the %AP decline lasts 1000 years, whereas the %WC decline occurs between adjacent pollen samples, approximately 300 years apart. Thus, early Holocene deforestation may have been even more abrupt than previously recognized. Rapid deforestation likely was promoted both by rapid climate changes around 8.2 ka and positive fire-vegetation feedbacks. Non-linear vegetational responses to hydrological variability are consistent with 1) other paleorecords showing rapid die-offs of some eastern tree species in response to aridity and 2) observations of threshold-type ecological responses to recent climate events. The 21st-century trajectory for the Great Plains prairie-forest ecotone is uncertain, because climate models differ over the direction of regional precipitation trends, but future drying would be more likely to trigger threshold-type shifts in ecotone position.

Origin and paleoclimatic significance of late Quaternary loess in Nebraska: Evidence from stratigraphy, chronology, sedimentology, and geochemistry

Muhs, D. R., Bettis III, E. A., Aleinikoff, J. N., McGeehin, J. P., Beann, J., Skipp, G., Marshall, B. D., Roberts, H. M., Johnson, W. C., Benton, R. (2008). Origin and paleoclimatic significance of late Quaternary loess in Nebraska: Evidence from stratigraphy, chronology, sedimentology, and geochemistry. GSA Bulletin, 120(11-12), 1378-1407.

Choice of conodont Idiognathodus simulator (sensu stricto) as the event marker for the base of the global Gzhelian Stage (Upper Pennsylvanian Series, Carboniferous System)

We propose that the level at which the conodont species Idiognathodus simulator (Ellison 1941) (sensu stricto) first appears be selected to mark the base of the Gzhelian Stage, because we believe that this is the optimal level by which this boundary can be correlated. This taxon has a short range and a wide distribution, as shown by correlation of glacial-eustatic cyclothems across the Kasimovian-Gzhelian boundary interval among Midcontinent North America and the Moscow and Donets basins of eastern Europe, based on scale of the cyclothems along with several aspects of biostratigraphy. Outside of these areas, I. simulator (sensu stricto) is known also from other parts of the U.S., and is reported from the southern Urals and south-central China in its expected position between other widespread taxa. Its first appearance is consistent with the current ammonoid placement of the boundary (first appearance of Shumardites cuyleri), and it is also compatible with certain aspects of the distribution of Eurasian fusulinid faunas (e.g., lectotype of Rauserites rossicus).

Evaluation of coupled ocean–atmosphere simulations of the mid-Holocene using palaeovegetation data from the northern hemisphere extratropics

We have used the BIOME4 biogeography–biochemistry model and comparison with palaeovegetation data to evaluate the response of six ocean–atmosphere general circulation models to mid-Holocene changes in orbital forcing in the mid- to high-latitudes of the northern hemisphere. All the models produce: (a) a northward shift of the northern limit of boreal forest, in response to simulated summer warming in high-latitudes. The northward shift is markedly asymmetric, with larger shifts in Eurasia than in North America; (b) an expansion of xerophytic vegetation in mid-continental North America and Eurasia, in response to increased temperatures during the growing season; (c) a northward expansion of temperate forests in eastern North America, in response to simulated winter warming. The northward shift of the northern limit of boreal forest and the northward expansion of temperate forests in North America are supported by palaeovegetation data. The expansion of xerophytic vegetation in mid-continental North America is consistent with palaeodata, although the extent may be over-estimated. The simulated expansion of xerophytic vegetation in Eurasia is not supported by the data. Analysis of an asynchronous coupling of one model to an equilibrium-vegetation model suggests vegetation feedback exacerbates this mid-continental drying and produces conditions more unlike the observations. Not all features of the simulations are robust: some models produce winter warming over Europe while others produce winter cooling. As a result, some models show a northward shift of temperate forests (consistent with, though less marked than, the expansion shown by data) and others produce a reduction in temperate forests. Elucidation of the cause of such differences is a focus of the current phase of the Palaeoclimate Modelling Intercomparison Project.