Midcontinent North America Research Articles

Geotechnical model for great plains hummocky moraine formed by till deformation below stagnant ice

Hummocky or “disintegration” moraine is a widespread late Wisconsin glacial terrain type across the glaciated Great Plains of mid-continent North America. This terrain consists of chaotic hummocks and enclosed “kettle” hollows that are widely underlain by clay-rich till. The prevailing interpretation of this terrain is one of supraglacial deposition from debris-covered stagnant ice and is based largely on geomorphologic comparisons with modern valley glacier margins. Recently, in contrast, it has been argued that hummocky moraine can form subglacially by the “pressing” of fine-grained deformable till under the overburden pressure of stagnant ice. In this paper, we test the postulated subglacial origin of hummocky moraine using a geotechnical model of a deforming soil (till). The model confirms that hummocky topography can be the result of failure and deformation of fine-grained till by differential loading below stagnant ice. Differential pressures at the ice/bed contact arise from the complete or partial perforation of thin stagnant ice sheet lobes by numerous water-filled, doline-like depressions. The growth and decay of ponds as ice down-wastes result in repeated cycles of subglacial till failure and landform development resulting in the observed random, linear, circular, elongate, and other patterns of hummocks, ridges, and hollows. Formerly streamlined, preferentially oriented subglacial landforms, such as drumlins and Rogen moraines, can also be overprinted forming “washboard moraine”.

Cretaceous-Tertiary palynofloral perturbations and extinctions within the Aquilapollenites Phytogeographic Province

A combination of detailed, intermediate and large scale stratigraphic perspectives is necessary to understand the driving mechanisms for floral change across the Cretaceous-Tertiary (K–T) boundary, because short-term physical and biotic events occur within longer term trends. Most Maastrichtian palynofloral extinctions have been identified within the northern circumpolar, supra-continental, Aquilapollenites Province. In mid-continental North America, the presence of a physically defined K–T boundary provides opportunities to examine contemporaneous terrestrial palynofloras over a latitudinal (also approximately paleolatitudinal) distance of 3200 km. Seventy-five percent of last appearances occur before the K–T boundary at high latitudes and must have been caused by ongoing terrestrial processes. Palynofloral events directly associated with the K–T boundary include both local last appearances, and inferred extinctions, and synchronous and diachronous changes in relative abundances. Two different styles of factors are reflected in these events. The first, possibly attributable to chemical or other profound atmospheric perturbations associated with the worldwide distribution of the "fireball" layer of the K–T claystone, resulted in extinctions of angiosperm with a zoophilous pollination strategy. The second factor, possibly reflecting energy released during the deposition of ballistically transported impact debris or hackly (ejecta) layer of the boundary claystone, resulted in the selective, northward-attenuating removal of the canopy vegetation, best documented for southwestern Canada, which was succeeded by a fern-dominated vegetation in mid-continental North America. These consequences of the boundary event were mediated by independent, preexisting circumstances: climate (regional and through time), local depositional environment, and ongoing background extinctions and origins.

Practice Makes Within 5% of Perfect: Visual Perception, Motor Skills, and Memory in Artifact Variation

Practice Makes Within 5% of Perfect: Visual Perception, Motor Skills, and Memory in Artifact Variation

Clay mineral authigenesis along a mid-continental scale fluid conduit in Palaeozoic sedimentary rocks from southern Ontario, Canada

AbstractRocks above and below the Precambrian-Palaeozoic unconformity at the base of many Palaeozoic sedimentary basins of mid-continental North America are distinguished by secondary K-feldspar, chlorite and illite. Thrusting of the Appalachian foreland is generally considered responsible for the fluid migrations which caused this alteration. We have examined secondary chlorite and illite from Cambro-Ordovician rocks which overlie the unconformity in southern Ontario, Canada. This area is situated along the interface between the Michigan and Appalachian basins. Water involved in chloritic alteration at ~150°C has an O isotope composition compatible with a basinal brine evolved from seawater. Brine migration was probably triggered by the Taconic Orogeny to the east. In comparison, the secondary illite (365–321 Ma) largely postdates the Acadian Orogeny, and the illite-forming fluids have stable isotopic compositions typical of tepid (~40–55°C) meteoric water. We suggest that basement arches beneath southern Ontario were reactivated by this orogeny, which facilitated introduction of meteoric water. Similar alteration in underlying Precambrian rocks shows that fluid flow was focused along the unconformity during the regional Ordovician brine migration, as well as during the localized Mississippian introduction of fresh water.

Geochemical Variations in Peoria Loess of Western Iowa Indicate Paleowinds of Midcontinental North America during Last Glaciation

Peoria Loess deposited in western Iowa during the last glacial maximum (LGM) shows distinct geochemical and particle-size variations as a function of both depth and distance east of the Missouri River. Geochemical and particle-size data indicate that Peoria Loess in western Iowa probably had two sources: the Missouri River valley, and a source that lay to the west of the Missouri River. Both sources indicate that LGM paleowinds in western Iowa had a strong westerly component, similar to interpretations of previous workers. A compilation of loess studies in Iowa and elsewhere indicates that westerly winds were dominant during loess transport over much of the midcontinent south of the Laurentide ice sheet, which is not in agreement with paleowinds simulated by atmospheric general circulation models (AGCMs). AGCMs consistently generate a glacial anticyclone with easterly or northeasterly winds over the Laurentide ice sheet and the area to the south of it. Loess deposition in the midcontinent during the LGM may be a function of infrequent northwesterly winds that were unrelated to the presence of the glacial anticyclone.

Mass dynamics of the spleen and other organs in geese: measures of immune relationships to helminths?

The spleen is an important organ of avian immune systems. We examined whether helminth loads were related to spleen mass in the lesser snow goose, Chen caerulescens caerulescens. On 27 collecting occasions, 744 geese were obtained at 13 different locations in a south-north gradient in midcontinental North America. The masses of the spleen, caecum, small intestine, large intestine, pancreas, heart, and crop of all geese were determined, and intestinal and caecal helminths were counted. Seventy-eight percent of geese harbored at least one helminth species. For analyses, helminths were grouped as cestodes (26% prevalence), trematodes (19% prevalence), and nematodes (70% prevalence). After sample location and time, host age, host sex, and host body size were controlled for in a multivariate analysis of covariance, nematodes were the only helminth group associated with variation in organ masses. Greater nematode loads were weakly associated with lower spleen, higher caecum, lower large intestine, and lower heart masses. When uninfected individuals were excluded from the analysis, greater nematode loads were no longer associated with variation in spleen size but were associated with higher crop mass, and greater cestode loads were associated with higher heart mass. In neither of these analyses were any other cestode-organ or trematode-organ associations significant. Geese carrying two or more helminth groups had lower spleen masses than did geese infected with no or one helminth group. When we interchanged response and explanatory variables from the preceding analyses and retained the same covariates, the same organ mass - helminth associations tended to remain significant. Nonetheless, the small variation in helminth loads explained by variation in spleen mass (or vice versa) provided only weak support for the hypothesis that intraspecifically, wild individuals with lower investment in immunity are more susceptible to nematode infections.

Mass dynamics of the spleen and other organs in geese: measures of immune relationships to helminths?

The spleen is an important organ of avian immune systems. We examined whether helminth loads were related to spleen mass in the lesser snow goose, Chen caerulescens caerulescens . On 27 collecting occasions, 744 geese were obtained at 13 different locations in a south-north gradient in midcontinental North America. The masses of the spleen, caecum, small intestine, large intestine, pancreas, heart, and crop of all geese were determined, and intestinal and caecal helminths were counted. Seventy-eight percent of geese harbored at least one helminth species. For analyses, helminths were grouped as cestodes (26% prevalence), trematodes (19% prevalence), and nematodes (70% prevalence). After sample location and time, host age, host sex, and host body size were controlled for in a multivariate analysis of covariance, nematodes were the only helminth group associated with variation in organ masses. Greater nematode loads were weakly associated with lower spleen, higher caecum, lower large intestine, and lower heart masses. When uninfected individuals were excluded from the analysis, greater nematode loads were no longer associated with variation in spleen size but were associated with higher crop mass, and greater cestode loads were associated with higher heart mass. In neither of these analyses were any other cestode-organ or trematode-organ associations significant. Geese carrying two or more helminth groups had lower spleen masses than did geese infected with no or one helminth group. When we interchanged response and explanatory variables from the preceding analyses and retained the same covariates, the same organ mass - helminth associations tended to remain significant. Nonetheless, the small variation in helminth loads explained by variation in spleen mass (or vice versa) provided only weak support for the hypothesis that intraspecifically, wild individuals with lower investment in immunity are more susceptible to nematode infections. 359

Filtration models, guilds, and biofacies: Crinoid paleoecology of the Stanton Formation (Upper Pennsylvanian), midcontinent, North America

Abstract Filtration models make several predictions concerning the distribution of crinoids among benthic habitats: (1) generally, higher energy shoreward settings should contain crinoid assemblages dominated by taxa with dense mesh filtration fans; (2) generally, lower energy offshore settings should be dominated by crinoids with open mesh filtration fans; (3) diversity should be highest in nearshore settings, which generally contain low energy microhabitats within high energy settings where both dense fan and open fan forms can co-occur, whereas offshore assemblages should be dominated by open fan taxa; thus, open fan forms should be more eurytopic than dense fan forms. Also, previous empirical models for crinoid distribution patterns note that nearshore assemblages are dominated by large forms whereas offshore assemblages are dominated by small forms. Filtration fan morphology and body size interact significantly to determine how and where crinoids feed and thus are used here as criteria for distinguishing late Paleozoic crinoid guilds. The distribution patterns of crinoid taxa and guilds among the benthic paleoenvironments of the Upper Pennsylvanian (Missourian) Stanton Formation were used to test the predictions of the filtration models. The results of quantitative analyses show that large, dense fan crinoids are significantly more abundant in shoreward assemblages than smaller, open fan crinoids, which are more abundant in offshore assemblages; these results are consistent with the aforementioned models. However, diversity patterns are not consistent with the filtration models, indicating that mid-depth and aerobic offshore assemblages are more taxonomically and trophically diverse than nearshore and dysaerobic offshore assemblages, ostensibly due to the overlapping ranges of characteristically nearshore and offshore taxa. Analysis of guild distribution patterns also indicates some contradictions to predicted patterns of stenotopy and eurytopy, indicating additional complexities to late Paleozoic crinoid paleoecology. Five recurrent crinoid assemblages, or biofacies, have been identified from the Stanton Formation of midcontinent North America. The guild structure and diversity of these biofacies are characterized here as: (1) a moderate diversity nearshore biofacies dominated by large calyx, dense fan guilds; (2) a high diversity midshelf biofacies characterized by equitable contributions of many taxa and guilds; (3) a moderate diversity offshore biofacies dominated by non-pinnulate and small calyx, open fan pinnulate guilds; (4) a low diversity offshore dysaerobic biofacies dominated by small calyx pinnulate guilds; (5) a depauperate offshore biofacies dominated by a unique stalkless species, Paragassizocrinus tarri. Except for Biofacies 5, the number of guilds appears to increase offshore; however, the equitability of taxa among and within the guilds generally decreases offshore. This pattern indicates that the heterogeneity of resources, or the recognition by taxa of different classes of resources, as reflected by the variety of functional types, increased from nearshore to offshore, but that the abundance or accessibility of these resources, and/or the relaxation of competition within guilds, increased from offshore to nearshore.

Complex Response of a Midcontinent North America Drainage System to Late Wisconsinan Sedimentation

ABSTRACT The geomorphic evolution of Mud Creek basin in eastern Iowa, U.S.A. serves to illustrate how geomorphic influences such as sediment supply, valley gradient, climate, and vegetation are recorded in the alluvial stratigraphic record. Sediment supply to the fluvial system increased significantly during the late Wisconsinan through a combination of periglacial erosion and loess accumulation. Subsequent evolution of the Holocene alluvial stratigraphic record reflects long-term routing of the late Wisconsinan sediment through the drainage basin in a series of cut-and-fill cycles whose timing was influenced by hydrologic response to change in climate and vegetation. When viewed in a regional context, the alluvial stratigraphic record appears to reflect a long-term complex response of the flu ial system to increased sediment supply during the late Wisconsinan. Hydrologic and sediment-supply changes accompanying the spread of Euroamerican agriculture to the basin in the 1800s dramatically upset trends in sedimentation and channel behavior established during the Holocene.

Crinoid biofacies in Upper Carboniferous cyclothems, midcontinent North America: faunal tracking and the role of regional processes in biofacies recurrence

Abstract Coordinated stasis is a pattern of long-term, multispecies evolutionary and ecological stability bounded by rapid turnover events. One class of models proposed to explain the paleoecological pattern of biofacies stability, typified by ecological locking, states that biotas within habitats become integrated into ecological units that, through population and community-level processes, rebound and persist through short-term disturbances. This high frequency community resilience allows the units to persist through geologic time, tracking their preferred habitats during transgressions and regressions. Because species are tightly woven into the fabric of the ecosystem, there is little opportunity for speciation; this also buffers them against extinction. This ecological locking by ecosystem homeostatic processes produces long intervals of regionally stable biofacies in the fossil record. In these models, the primary structuring mechanisms are at the local, community level, controlled by species autecology, species interactions, and local disturbance regime. An opposing class of models states that species are fundamentally independent and that species interactions are not strong nor spatio-temporally persistent enough to forge communities into integrated units with long-term persistence. Community recurrence would be expected as long as the various habitat types recur in time and space, species autecologies remain stable, the pool of available species remains intact and the recruitment of species into local habitats is not interrupted. The focus of these models is dominantly at the regional level, controlled by biogeographic history and dispersal among, as well as within, communities. The results of this study of crinoids from the Upper Carboniferous Lansing Group of midcontinent North America show that they are distributed among five recurrent paleocommunity types, or biofacies, which are generally arrayed along an onshore-offshore gradient. Two of these biofacies are restricted to the offshore, dysaerobic horizons of the most extensive cyclothem in the group. The other three biofacies recur consistently among the cycles. However, this repeated pattern includes a regionally distributed offshore biofacies, unique to late transgressive deposits, that is not found in analogous regressive deposits. While the distribution of the two shoreward biofacies is consistent with faunal tracking, the recurrence of a biofacies unique to late transgressive horizons is not consistent with an integrated paleocommunity that tracks shifting habitats through transgression and regression. These results favor the alternative models, namely that biofacies develop in place via dispersal from a persistent species pool in response to prevailing environmental conditions. The recurrence of apparently ephemeral biofacies through time underscores the importance of regional processes in controlling local paleocommunity composition. The establishment of refuges, or “source” populations, during environmental bottlenecks (i.e. lowstands) is critical to maintaining the regional species pool through time for continued recruitment into local “sink” paleocommunities during more amenable times (i.e. highstands). Metapopulation dynamics among habitats are equally important in maintaining species within a region through time. Conflicting observations of stochastic (individualistic) and deterministic (integrated) behaviors of the same ecological systems probably result from differences in the scales of analyses and therefore the processes being studied.

Anatomically preserved vojnovskyalean seed plants in Upper Pennsylvanian (Stephanian) marine shales of North America

Upper Pennsylvanian dysoxic marine shales of midcontinent North America yield permineralized remains of apparently extrabasinal vegetation. A large percentage of the plants are surprisingly unlike the well known swamp, fluvial and lacustrian floras of the late Paleozoic paleotropics, revealing numerous aspects of the morphology, anatomy and reproductive biology of plants that may have been ancestral to the dominant taxa of the Mesozoic. Included among the assemblages are ovulate coniferophyte remains that demonstrate the occurrence of vojnovskyalean seed plants in equatorial Euramerica. Specimens consist of simple ovulate cones that are more-or-less clustered along eustelic stems in the axils of helically arranged, strap-shaped leaf bases, and are described as Sergeia neuburgii new genus and species. Individual cones are up to approximately 2 cm long and 1.5 cm in maximum diameter, with helically arranged scales and sporophylls that diverge from a eustelic axis. Scales occur in the basal region of the cone, and are laminar and pointed. Sporophylls are borne distally. One specimen shows about 45 sporophylls, each of which terminates as one erect ovule. Most of the other cones have abraded apices, and terminate in relatively terete foliar appendages that are interpreted to be sporophyll bases. Ovules are flattened and winged, approaching 180° rotational symmetry. Integument histology, vascular tissue distribution and pollen chamber structure are similar to those of cordaiteans and callistophytalean seed ferns. Sergeia adds to the number of late Paleozoic conifer-like plants that do not conform to the Pinopsida as traditionally circumscribed, and poses additional questions to assumptions of monophyly for coniferophytes and for conifers sensu lato.

Secondary K-feldspar at the Precambrian–Paleozoic unconformity, southwestern Ontario

The Precambrian–Paleozoic boundary in the subsurface of southwestern Ontario commonly is characterized by secondary K-feldspar. In the weathered and altered Precambrian granitoid gneisses at the unconformity, secondary K-feldspar has replaced preexisting minerals, and also occurs as discrete crystals of adularia, overgrowths on altered minerals, and microcrystalline veinlets. The K-feldspar is chemically pure (Or99–100) and has high δ18O values (+18.9 to +21.4‰ Vienna Standard Mean Ocean Water), features that indicate crystallization at low temperatures. Secondary K-feldspar also occurs in Cambro-Ordovician clastic and carbonate rocks that immediately overlie the Precambrian basement. K/Ar (453 ± 9 to 412 ± 8 Ma) and Rb/Sr (440 ± 50 Ma) dates obtained for secondary K-feldspar from the Precambrian host rocks suggest that its crystallization is unrelated to Precambrian weathering or early diagenesis of the immediately overlying Cambro-Ordovician strata. Estimated crystallization temperatures for the secondary K-feldspar (≥100 °C) exceed presumed burial temperatures for the Precambrian–Paleozoic boundary in southwestern Ontario during Late Ordovician–Silurian time. We infer that secondary K-feldspar formed from a hot brine that moved preferentially along the Precambrian–Paleozoic unconformity. The wide distribution of secondary K-feldspar of Late Ordovician–Silurian age throughout mid-continental North America at the Precambrian–Paleozoic boundary records the regional extent of this process. Some Cambro-Ordovician rocks elsewhere in the mid-continent also contain secondary K-feldspar and illitic clay of Late Pennsylvanian–Early Permian age, suggesting more than one episode of fluid movement. Major pulses of orogenic activity may have initiated brine migration. We speculate that the brine originated as connate (sea) water trapped in lower Paleozoic strata, and was modified by rock–water interaction at elevated temperatures, and by mixing with meteoric water.

Hemphillian (Late Miocene) Snakes from Nebraska, with Comments on Arikareean Through Blancan Snakes of Midcontinental North America

ABSTRACT Middle and late Hemphillian (late Miocene) snakes from Nebraska were identified and interpreted. The mid-Hemphillian (ca. 7 Ma) Lemoyne Quarry of Keith Co., SW Nebraska yielded at least two boid, thirteen colubrid and two viperid snakes. The late Hemphillian (ca. 5 Ma) Devils Nest (Knox Co.), Santee (Knox Co.), and Mailbox Prospect (Antelope Co.) sites of NE Nebraska together yielded at least nine colubrids and one viperid snake. Twenty-four percent of the 16 mid-Hemphillian snake genera and 67% of the 12 species are extinct. Of the 9 genera and 7 species of late Hemphillian snakes, only Paleoheterodon tiheni is extinct. A snake from Lemoyne Quarry is described as new (Arizona voorhiesi). With this new information, the fossil record of Arikareean through Blancan (Miocene through Pliocene) snake genera from midcontinental North America suggests erycinine boids became extinct across the region by late Hemphillian time, and colubrid snakes became generically modern by early Blancan time. These event...

The isotopic record of ocean chemistry and diagenesis preserved in non-luminescent brachiopods from Mississippian carbonate rocks, Illinois and Missouri

Fossil brachiopod shells are commonly used as a faithful record of the chemical and isotopic composition of the ancient ocean water in which they grew, owing to their stable mineralogy and their presence in marine strata throughout Phanerozoic time. The utility of these fossils for preserving ocean-water compositions is evaluated through analyses of multiple specimens that grew in contemporaneous sea water at individual stratigraphic horizons in the Mississippian-age Burlington-Keokuk Formation of mid-continent North America. Significant variations in Sr, C, and O isotopic compositions are observed in contemporaneous, non-luminescent shells, indicating that apparently pristine brachiopod shell material has been altered by post-depositional processes of diagenesis. The least-altered compositions at each horizon are used to estimate secular variations of 87 Sr/ 86 Sr in Early Mississippian ocean water. The estimated rates of change of ocean-water 87 Sr/ 86 Sr are comparable in magnitude to the rapid Sr isotopic changes documented for Cenozoic ocean water, for which high-frequency glacial cycles are inferred as a driving mechanism. The rigorous assessment of diagenetic effects on the isotopic signatures of marine phases is requisite for the advancement of high-resolution reconstructions of paleo-ocean chemistry.

Paleoindians in Beringia: Evidence from Arctic Alaska

Excavations at the Mesa site in arctic Alaska provide evidence for a Paleoindian occupation of Beringia, the region adjacent to the Bering Strait. Eleven carbon-14 dates on hearths associated with Paleoindian projectile points place humans at the site between 9,730 and 11,660 radiocarbon years before present (years B.P.). The presence of Paleoindians in Beringia at these times challenges the notion that Paleoindian cultures arose exclusively in mid-continental North America. The age span of Paleoindians at the Mesa site overlaps with dates from two other cultural complexes in interior Alaska. A hiatus in the record of human occupation occurs between 10,300 and 11,000 years B.P. Late Glacial climatic fluctuations may have made northern Alaska temporarily unfavorable for humans and spurred their southward dispersal.

Thermoluminescence chronology of Toronto-area Quaternary sediments and implications for the extent of the midcontinent ice sheet(s)

We report direct dating, by thermoluminescence (TL) methods, of key Quaternary stratigraphic horizons in the Toronto area. TL ages indicate that (1) most of the exposed sediments of the Don Formation at the sampled site were probably deposited during the closing phase of the Sangamon Interglacial (∼80 ka; oxygen isotope stage 5a), not the climatic optimum substage 5e (∼120∼130 ka),as generally supposed; (2) overlying deltaic strata of the Scarborough Formation, which accumulated in an ice-dammed ancestral Lake Ontario, were deposited between 60 and 50 ka (early Wisconsin, oxygen isotope stage 4); (3) the Sunnybrook diamict, overlying the Scarborough Formation, was deposited in the middle Wisconsin at ∼40-45 ka (oxygen isotope stage 3), likely by glaciolacustrine processes, rather than before 65 ka, as previously supposed; (4) sediments of the Upper Thorncliffe Formation lying stratigraphically just beneath the last-glacial-maximum Hallton Till were deposited at ∼23 ka, consistent with reliable 14 C dating. Thus, the earliest Wisconsin incursion of the (proto) Laurentide Ice Sheet into the Lake Ontario basin, as recorded by the Scarborough Formation, probably did not take place until at least 60 ka, and did not extend south and southwest of the Toronto area until after ∼25 ka. Ice-free conditions probably persisted over large areas of midcontinent North America from the end of the Illinoian glaciation at ∼130 ka to the late Wisconsin glacial maximum at ∼20 ka.

Large increases in flood magnitude in response to modest changes in climate

RECENT examinations of the possible hydrological response to global warming have emphasized changes in average conditions, rather than individual flooding events1–5. Historical accounts suggest, however, that such events may have had a considerable regional impact6–9 even in the face of any relatively modest climate change8. Here I present a 7,000-year geological record of overbank floods for upper Mississippi river tributaries in mid-continent North America, which provides concrete evidence for a high sensitivity of flood occurrence to changing climate. During a warmer, drier period between about 3,300 and 5,000 years ago, the largest, extremely rare floods were relatively small—the size of floods that now occur about once every fifty years. After ~3,300 years ago, when the climate became cooler and wetter, an abrupt shift in flood behaviour occurred, with frequent floods of a size that now recurs only once every 500 years or more. Still larger floods occurred between about AD 1250 and 1450, during the transition from the medieval warm interval to the cooler Little Ice Age. All of these changes were apparently associated with changes in mean annual temperature of only about 1–2 °C and changes in mean annual precipitation of ⩽10–20%.

Frogs in Hemphillian Deposits of Nebraska, with the Description of a New Species of Bufo

-Frog remains from a mid-Hemphillian site in SW Nebraska include at least three bufonids, two ranids, and a hylid. Frog remains from three late Hemphillian sites of NE Nebraska include at least two bufonids, two pelobatids, two ranids, and two hylids. All of the mid-Hemphillian and late Hemphillian genera are extant, but extinct species occur in both faunas. Bufo holmani, described here, is a large, robust, late Hemphillian bufonid distinct from other fossil and living species, but of uncertain group relationship. The temporal ranges of Bufo valentinensis and Bufo pliocompactilis are now known to extend from the Clarendonian into the mid-Hemphillian, and Scaphiopus wardorum from the Barstovian into the late Hemphillian. While Miocene (24 to 5 Ma) fossil beds of midcontinental North America have documented major faunal changes among at least the snakes (Holman, 1979, 1981), knowledge of Miocene amphibians from this region is limited. Frogs from rocks of Hemphillian mammal age (late Miocene; 9-4.5 Ma) are particularly poorly known, and have received little attention since Tihen (1962) discussed a few bufonid species from Great Plains Hemphillian sites. I provide here a report on important new frog fossils collected from Hemphillian sites in Nebraska, which include at least five bufonids with one described as new, two pelobatids, two ranids, and two hylids. These frog remains considerably increase our knowledge of the Hemphillian frog fauna of midcontinental North America. The fossils were collected by University of Nebraska State Museum (UNSM) and Michigan State University Museum (MSUVP) field parties from 4 sites, one of which is mid-Hemphillian, and the other three late Hemphillian.

Increasing Rates of Atmospheric Mercury Deposition in Midcontinental North America

Mercury contamination of remote lakes has been attributed to increasing deposition of atmospheric mercury, yet historic deposition rates and inputs from terrestrial sources are essentially unknown. Sediments of seven headwater lakes in Minnesota and Wisconsin were used to reconstruct regional modern and preindustrial deposition rates of mercury. Whole-basin mercury fluxes, determined from lake-wide arrays of dated cores, indicate that the annual deposition of atmospheric mercury has increased from 3.7 to 12.5 micrograms per square meter since 1850 and that 25 percent of atmospheric mercury deposition to the terrestrial catchment is exported to the lake. The deposition increase is similar among sites, implying regional or global sources for the mercury entering these lakes.

Deep crustal structure of the Precambrian basement beneath northern Lake Michigan, midcontinent North America

A deep seismic-reflection profile in northern Lake Michigan, midcontinent North America, provides a cross section of the crust across the 1850 Ma Penokean orogen, in which an Early Proterozoic island-arc complex was deformed between two converging Archean continental masses. The island-arc crust is about 40 km thick and has a few kilometres of intensely reflective rocks near its base, above which it is variably reflective to transparent. The Archean terranes have thicker crust, as much as 50 km, the lower 20-25 km of which is strongly reflective. Abrupt offsets of Moho near terrane boundaries may have been preserved since accretion during the Penokean orogeny.