Lake Agassiz Research Articles

Glacial Lake Agassiz: Its northwest maximum extent and outlet in Saskatchewan (Emerson Phase)

Six different lines of evidence support the hypothesis that glacial Lake Agassiz expanded an additional 70,000 km2 over that previously mapped in northwestern Saskatchewan and that the lake discharged out the northwestern (Clearwater) outlet, then through glacial Lake McConnell and Mackenzie River to the Arctic Ocean. Elevations of formerly unmapped (1) strandlines and (2) glaciolacustrine sediments between the previously mapped northwest limit of Lake Agassiz and the Clearwater-lower Athabasca spillway indicate that water extended 170 km farther northwest. Recently, mapped strandlines at elevations up to 60 m above the previously mapped extent of Lake Agassiz can be traced along (3) isobases to the mouth of the spillway. Based upon (4) six radiocarbon dates recovered from spillway flood deposits in the Athabasca River valley and its late Pleistocene delta, the (5) Clearwater spillway was cut at 9.9 ka BP. This date is synchronous with the initiation of the Emerson Phase (9.9 ka BP) in the southern Lake Agassiz basin and (6) coincides with the position of the Laurentide Ice Sheet at the Cree Lake Moraine (10 ka BP) along the northern margin of the lake.Following closure of the eastern outlets at the onset of the Emerson Phase, Lake Agassiz transgressed toward the northwest into the deglaciated and isostatically depressed glacial foreland in the Churchill River valley to an elevation of 490 m, the pre-flood elevation of the Churchill-Mackenzie drainage divide at the head of the Clearwater-lower Athabasca spillway. The Beaver River Moraine (an earthen drainage divide) was breached, resulting in lowering Lake Agassiz 52 m to a stable elevation at 438 m. The lake discharged 21,000 km3 of water into the Arctic Ocean that raised global sea level by 6 cm.

History of late glacial runoff along the southwestern margin of the Laurentide Ice Sheet

The routing of runoff from the retreating Laurentide Ice Sheet was controlled by a combination of ice-marginal position and topography; isostatic depression also played an important role in the evolution of the ice-marginal landscape. A major redirection of drainage occurred after ice retreated north of the Missouri Escarpment about 12 ka BP. Runoff that had previously flowed south to the Missouri River could then be routed eastward to Lake Agassiz, which in turn drained to the upper Mississippi River valley.Changes in drainage routing in the region west of glacial Lake Agassiz typically were abrupt, as new and lower paths along the retreating ice margin became available. Each new basin was larger than its predecessor and its headwaters extended farther west. Four distinct, sequential phases of ice-marginal drainage occurred between 12 and 10.7 ka BP: the James, Sheyenne, Souris-Pembina and Qu'Appelle-Assiniboine. Starting with the Qu'Appelle-Assiniboine phase about 11.2 ka BP, Lake Agassiz collected runoff from as far west as the Rocky Mountains. By about 10.7 ka BP, ice had retreated north of the lower reaches of the Saskatchewan River, allowing meltwater to by-pass the Qu'Appelle-Assiniboine basin. At about the same time, the eastern outlets of Lake Agassiz opened, initiating drainage to the North Atlantic Ocean from this vast region.Drainage routes evolved by a common sequence of events, beginning with the impoundment of proglacial lakes along the ice margin. Most of these lakes drained catastrophically, resulting in the formation of spillways with distincitve geomorphic features that include broad, scoured and streamlined subupland channels along with subsequently incised narrow, deep inner channels. Characteristic deposits of these outbursts include boulder mantles on subupland channel floors, boulder-gravel bars within the inner channels, and large, primarily subaqueous fans in lake basins that received the flood bursts. These outbursts progressed from basin to basin, causing the drainage of other proglacial lakes, until they reached Lake Agassiz. Spillways associated with the next younger drainage phase were commonly incised across the abandoned lake floors.

Two episodes of meltwater influx from glacial Lake Agassiz into the Lake Michigan basin and their climatic contrasts

Two episodes of meltwater influx from glacial Lake Agassiz are recorded as prominent sedimentologic, isotopic, magnetic, and faunal signatures in southern Lake Michigan profundal sediments. As a tributary to the main path of eastward Lake Agassiz flow, southern Lake Michigan recorded only the largest, catastrophic discharges. The distinctive Wilmette Bed, a massive gray mud that interrupts laminated red glaciolacustrine clays, marks the first episode, which occurred near the beginning of the Younger Dryas cooling event. The associated discharge may have played a role in the inception or severity of the Younger Dryas event. An oxygen isotope excursion in biogenic carbonate and changes in ostracode assemblages mark the second episode, which appears to have had at least two pulses, dated by accelerator mass spectrometer 14C ages on biogenic carbonate at about 8.9 and 8.6 ka. The second episode occurred during the early Holocene peak in global meltwater discharge and apparently had little wide- spread climatic or oceanographic effect. The contrast between the effects associated with these two episodes of meltwater discharge emphasizes the complexity of the ice sheet-ocean-climate system.

The impact of glacial lake runoff on the goldthwait and champlain seas: The relationship between glacial lake agassiz runoff and the younger dryas

The freshwater flux to the North Atlantic Ocean during the last deglaciation included runoff (meltwater plus precipitation) from glacial Lake St Lawrence (ca. 11.6 ka BP), from glacial Lake Agassiz (10.9–9.9 and 9.5–8.0 ka BP) and from glacial Lake Barlow-Ojibway (9.5–8.0 ka BP). Runoff from the glacial lakes does not appear to have mixed with the deep water of the Goldthwait Sea in the Gulf of St Lawrence and was part of the surface outflow to the North Atlantic Ocean. Radiocarbon-dated invertebrate faunal assemblages show that the major impact of Lake Agassiz runoff to the North Atlantic Ocean during the 10.9 to 9.9 ka BP interval occurred after 10.5 ka BP, resulting in the freshening of the Champlain Sea. The Younger Dryas cold episode, on the other hand, began about 11.0 ka BP. The discordance between the major impact of Lake Agassiz runoff on the Champlain Sea and the beginning of the cold episode indicates that Lake Agassiz runoff did not trigger the Younger Dryas cooling. However, the runoff from glacial Lake Agassiz may have sustained the cold climate during the latter part of the Younger Dryas cold episode.

Lakes of the Huron basin: their record of runoff from the laurentide ice sheet

The 189,000 km2 Huron basin is central in the catchment area of the present Laurentian Great Lakes that now drain via the St. Lawrence River to the North Atlantic Ocean. During deglaciation from 21-7.5 ka BP, and owing to the interactions of ice margin positions, crustal rebound and regional topography, this basin was much more widely connected hydrologically, draining by various routes to the Gulf of Mexico and Atlantic Ocean, and receiving overflows from lakes impounded north and west of the Great Lakes-Hudson Bay drainage divide.Early ice-marginal lakes formed by impoundment between the Laurentide Ice Sheet and the southern margin of the basin during recessions to interstadial positions at 15.5 and 13.2 ka BP. In each of these recessions, lake drainage was initially southward to the Mississippi River and Gulf of Mexico. In the first recession, drainage subsequently switched eastward along the ice margin to the North Atlantic Ocean. In the second recession, drainage continued southward through the Michigan basin, and later, eastward via the Ontario basin and Mohawk River valley to the North Atlantic Ocean. During the final retreat of ice in the Huron basin from 13 to 10 ka BP, proglacial lake drainage switched twice from the Michigan basin and the Mississippi River system to the North Atlantic via the Ontario basin and Mohawk River valley, finally diverting to the Champlain Sea in the St. Lawrence River valley at about 11.6 ka BP.New seismo- and litho-stratigraphic information with ostracode data from the offshore lacustrine sediments were integrated with the traditional data of shorelines, uplift histories of outlets, and radio-carbon-dated shallow-water evidence of transgressions and regressions to reconstruct the water level history and paleolimnological record for the northern Huron basin for the 11-7 ka BP period. Negative excursions in the δ18O isotopic composition of ostracodes and bivalves in southern Lake Michigan, southwestern Lake Huron and eastern Lake Erie indicate an influx of water from ice-marginal Lake Agassiz in central North America about 11 ka BP. A major decline in water levels of the Huron basin after 10.5 ka BP followed the high-level Main Lake Algonquin phase as ice receded and drainage was established through the North Bay area to Ottawa River valley. During the subsequent Mattawa-Stanley phase, the lake level history was dominated by fluctuations of tens of meters. Highstands of the earliest oscillations, whose origin is not clear, might be related to some of the well known Post Algonquin shorelines. After 9.6 ka BP, it is suggested that large inflows from Lake Agassiz and hydraulic damming in downstream outlets were the likely cause of the Lake Mattawa highstands. A lowstand at 9.3–9.1 ka BP occurred when these inflows were diverted, or impeded by an ice advance in the Nipigon basin area, while undiluted meltwater continued to enter the Huron basin. Assemblages and isotopic composition of the ostracode fauna indicate very dilute meltwater during the lowstands as late as 7.5 ka BP, and precipitation runoff with comparatively higher dissolved solids during the highstands. We speculate that the water composition of the Lake Mattawa highstands was dominated by the Agassiz inflows; by that time, much of Lake Agassiz was remote from ice-marginal environments, and the inflows were drawn from surface water of the southern sector of the lake, which was largely supplied by runoff and dissolved solids from the exposed land area of western Canada. Major inflows apparently ended about 8 ka BP, but northern proglacial lakes apparently continued as meltwater persisted in the Huron basin until about 7.5 ka BP. The cessation of major inflows initiated the final lowstand in the Huron basin and the present hydrological regime of local runoff.

Glacial lake McConnell: Paleogeography, age, duration, and associated river deltas, mackenzie river basin, western Canada

Glacial Lake McConnell lasted from 11.8 to 8.3 ka BP while occupying parts of the Great Bear, Great Slave and Athabasca Lake basin. The retreating Laurentide ice-front formed the eastern margin, whereas low rolling hills formed the north, west and south shorelines. Three major deltas were deposited at the mouths of the Laird, Peace and Athabasca rivers. The total extent of all phases of the lake was 240,000 km2, while the largest extent was 210,000 km2 at 10.5 ka BP. Downwarping of the basin by glacial ice was the main cause of the lake, whereas sediment blockage between Jean Marie River and Fort Simpson was secondary.Initially, glacial Lake McConnell occupied the northwestern corner (Smith Arm) of the Great Bear Lake basin and discharged through the Hare Indian River outlet. By 11.5 ka BP the enlarged water body flowed out the Great Bear River, but only for a short period of time. The Mackenzie River formed the third outlet near Jean Marie River at 11 ka BP and flow in the Great Bear River ceased until 9 ka BP. At 9.9 ka BP glacial Lake McConnell was impacted by a major flood from glacial Lake Agassiz with a peak discharge of 2–7 × 106 m3/sec. Flood water discharged from glacial Lake McConnell, peaking at 0.35–0.57 × 106 m3/sec and receding flow continued for 30 months. The massive influx of floodwater into glacial Lake McConnell caused an abrupt increase of discharge, which enlarged the outlet channel to between 6 and 13 km wide between Fort Simpson and Jean Marie River. At 8.3 ka BP, isostatic rebound ended the 3500-year-old extensive lake by dividing it into the Great Slave Lake and Lake Athabasca.

Lake-Level History of Lake Michigan for the Past 12,000 Years: The Record From Deep Lacustrine Sediments

Collection and analysis of an extensive set of seismic-reflection profiles and cores from southern Lake Michigan have provided new data that document the history of the lake basin for the past 12,000 years. Analyses of the seismic data, together with radiocarbon dating, magnetic, sedimentologic, isotopic, and paleontologic studies of core samples, have allowed us to reconstruct lake-level changes during this recent part of the lake's history. The post-glacial history of lake-level changes in the Lake Michigan basin begins about 11.2 ka with the fall from the high Calumet level, caused by the retreat of the Two Rivers glacier, which had blocked the northern outlet of the lake. This lake-level fall was temporarily reversed by a major influx of water from glacial Lake Agassiz (about 10.6 ka), during which deposition of the distinctive gray Wilmette Bed of the Lake Michigan Formation interrupted deposition of red glaciolacustrine sediment. Lake level then continued to fall, culminating in the opening of the North Bay outlet at about 10.3 ka. During the resulting Chippewa low phase, lake level was about 80 m lower than it is today in the southern basin of Lake Michigan. The rise of the early Holocene lake level, controlled primarily by isostatic rebound of the North Bay outlet, resulted in a prominent, planar, transgressive unconformity that eroded most of the shoreline features below present lake level. Superimposed on this overall rise in lake level, a second influx of water from Lake Agassiz temporarily raised lake levels an unknown amount about 9.1 ka. At about 7 ka, lake level may have fallen below the level of the outlet because of sharply drier climate. Sometime between 6 and 5 ka, the character of the lake changed dramatically, probably due mostly to climatic causes, becoming highly undersaturated with respect to calcium carbonate and returning primary control of lake level to the isostatically rising North Bay outlet. Post-Nipissing (about 5 ka) lake level has fallen about 6 m due to erosion of the Port Huron outlet, a trend around which occurred relatively small (± ∼2 m), short-term fluctuations controlled mainly by climatic changes. These cyclic fluctuations are reflected in the sed-imentological and sediment-magnetic properties of the sediments.

Isostatic rebound and its effects on fish colonization and distribution in the western Lake Superior basin

Ichthyofaunal surveys of the Huron Mountains and Isle Royale, Michigan, and the Sibley Peninsula, Ontario, allow for both a comparative study of colonization events and the effects of sequential isostatic rebound within a large portion of the western Lake Superior basin. The distribution of some fish species in these areas is the result of catastrophic events related to glacial retreat. The highest Huron Mountain lakes were colonized during channel events occurring shortly after the Marquette readvance began its retreat. Some species present on the Sibley Peninsula were likely carried by overflows from Lake Agassiz. Most lakes within these areas, however, were colonized well after 9700 BP, when large numbers of species had gained entrance to Lake Superior, mainly from Mississippi basin refugia. Several species, presumably because of earlier warming periods, had a wider distribution than they exhibit today. Some colonization of Isle Royale was probably through the straying of a few individuals from these populations. Lake Superior remains a formidable barrier to many species, restricting them to favourable areas within the western basin.

Deglaciation, lake levels, and meltwater discharge in the Lake Michigan basin

The deglacial history of the Lake Michigan basin, including discharge and routing of meltwater, is complex because of the interaction among (1) glacial retreats and re-advances in the basin (2) the timing of occupation and the isostatic adjustment of lake outlets and (3) the depositional and erosional processes that left evidence of past lake levels. In the southern part of the basin, a restricted area little affected by differential isostasy, new studies of onshore and offshore areas allow refinement of a lake-level history that has evolved over 100 years. Important new data include the recognition of two periods of influx of meltwater from Lake Agassiz into the basin and details of the highstands gleaned from sedimentological evidence. Major disagreements still persist concerning the exact timing and lake-level changes associated with the Algonquin phase, approximately 11,000 BP. A wide variety of independent data suggests that the Lake Michigan Lobe was thin, unstable, and subject to rapid advances and retreats. Consequently, lake-level changes were commonly abrupt and stable shorelines were short-lived. The long-held beliefs that the southern part of the basin was stable and separated from deformed northern areas by a hinge-line discontinuity are becoming difficult to maintain. Numerical modeling of the ice-earth system and empirical modeling of shoreline deformation are both consistent with observed shoreline tilting in the north and with the amount and pattern of modern deformation shown by lake-level gauges. New studies of subaerial lacustrine features suggest the presence of deformed shorelines higher than those originally ascribed to the supposed horizontal Glenwood level. Finally, the Lake Michigan region as a whole appears to behave in a similar manner to other areas, both local (other Great Lakes) and regional (U.S. east coast), that have experienced major isostatic changes. Detailed sedimentological and dating studies of field sites and additional development of geophysical models offer hope for reconciling the field data with our understanding of earth rheology.

Paleohydrology and paleochemistry of Lake Manitoba, Canada: the isotope and ostracode records

Lake Manitoba, the largest lake in the Prairie region of North America, contains a fine-grained sequence of late Pleistocene and Holocene sediment that documents a complex postglacial history. This record indicates that differential isostatic rebound and changing climate have interacted with varying drainage basin size and hydrologic budget to create significant variations in lake level and limnological conditions. During the initial depositional period in the basin, the Lake Agassiz phase (∼12–9 ka), δ18O of ostracodes ranged from −16‰ to −5‰ (PDB), implying the lake was variously dominated by cold, dilute glacial meltwater and warm to cold, slightly saline water.Candona subtriangulata, which prefers cold, dilute water, dominates the most negative δ18O intervals, when the basin was part of proglacial Lake Agassiz. At times during this early phase, the δ18O of the lake abruptly shifted to higher values; euryhaline taxa such asC. rawsoni orLimnocythere ceriotuberosa, and halobiont taxa such asL. staplini orL. sappaensis are dominant in these intervals. This positive covariance of isotope and ostracode records implies that the lake level episodically fell, isolating the Lake Manitoba basin from the main glacial lake.

The orientation of buried iceberg scours and other linear phenomena

The average orientations of surficial iceberg scours can be obtained from sidescan sonograms. Using certain assumptions, the average orientation of similar buried phenomena can be estimated from a pattern of intersecting seismic profiles. Two possible solutions are generated at each intersection of two seismic profiles, each of which is oriented at an angle α i from the profile which encounters the fewer scours, where α i = tan −1 [ sin ☎i (ϱ r ± cos ☎i) ] , and ϱ r is the ratio of the number of scours observed on the two seismic lines. Several pairs of solutions are required to determine which solution of each pair is significant. Iceberg scours observed on aerial photographs of the floor of glacial lake Agassiz demonstrate that the estimates provided by this technique correspond with the average orientation of the population. Applying this method to buried iceberg scours in Emerald Basin, Scotian Shelf, gives orientations which are parallel to paleo-bathymetric contours.

Late Quaternary paleoceanography of deep and intermediate water masses off Gaspé Peninsula, Gulf of St. Lawrence: foraminiferal evidence

Radiocarbon-dated benthonic foraminiferal zones in three cores provide new information on the evolution of the deep and intermediate water masses off Gaspé Peninsula. The deglacial phase in the deep Laurentian Channel began before 14 000 BP and was characterized by low-salinity (<20‰) or alternating low-salinity and saline (~35‰) water. This was followed by a cold saline phase, which ended ca. 13 500 BP, and a salinity minimum (30–33.5‰), which began ca. 12 100 BP. Between 8700 and 7900 BP, the temperature and salinity of the deep water mass increased, resulting in the modern deep water mass (temperature 4–6 °C, salinity 34.5–34.9‰) at the end of the Goldthwait Sea episode. The salinity of the deep water was apparently controlled by the meltwater flux from the ice front during the deglacial phase. After the deglacial phase the characteristics of the deep water mass were determined by the composition of offshore water entering the Laurentian Channel. Runoff from the Lake Agassiz – Great Lakes system does not appear to have mixed with the deep water of the Goldthwait Sea. The deglacial phase in Chaleur Trough, which is within the intermediate water mass, began before 12 200 BP. The temperature of the intermediate water mass has remained close to 0 °C after deglaciation; however, the salinity has increased from 25–30‰ at 12 200 BP to about 33.5‰ by 5900 BP.

Glacial Lake Agassiz: The northwestern outlet and paleoflood

Valley morphology and sediment in the Fort McMurray region of Alberta indicate that a catastrophic flood discharged down the lower Clearwater and Athabasca river valleys 9900 yr B.P. Geomorphic and chronologic evidence suggests that glacial Lake Agassiz (Emerson phase) was the probable water source. As the flood incised a drainage divide located near the Alberta-Saskatchewan border, the level of glacial Lake Agassiz decreased by 46 m, discharged 2.4 x 106 m3/s for at least 78 days, and stabilized at 438 m above sea level in the Lake Wasekamio area. At that time water entered the Arctic Ocean via glacial Lake McConnell and the Mackenzie River, rather than the Gulf of Mexico via the Mississippi River, as previously thought. Such a large influx of fresh water (8.6 km3/h) into the Arctic at the close of the last glaciation may have had an abrupt, major influence on northern climate.

Exploration for Pleistocene aggregate resources using process-depositional models in the Fort Mcmurray region, Ne Alberta, Canada

At present, the Fort McMurray region of Alberta, Canada, is experiencing an aggregate supply shortage. Both Syncrude and Suncor oil sand mines have exploited most of their local reserves, and haul costs are rapidly increasing from distant new sources. Previous aggregate studies in this region have focused on mapping known deposits rather than explaining the distribution of gravel by using process-depositional models as exploration tools and exploring for sources closer to the aggregate users. Aggregate deposits in the McMurray region are associated with two major Pleistocene geomorphic events: (1) glacial Lake McMurray; and (2) a catastrophic flood from glacial Lake Agassiz. Aggregate resources associated with Late Pleistocene glacial Lake McMurray are subaqueous fan deposits, beaches and deltas. Several large aggregate deposits are found along the benchlands of the lower Clearwater and Athabasca valleys. These valleys are a former spillway system eroded by a catastrophic flood from glacial Lake Agassiz 9900 years before present (BP). Discovery of other aggregate resources in the McMurray region will be best accomplished by understanding the flood soillway depositional patterns and regional deglacial history.

Glacial-Lake Outburst Erosion of the Grand Valley, Michigan, and Impacts on Glacial Lakes in the Lake Michigan Basin

Geomorphic and sedimentologic evidence in the Grand Valley, which drained the retreating Saginaw Lobe of the Laurentide Ice Sheet and later acted as a spillway between lakes in the Huron and Erie basins and in the Michigan basin, suggests that at least one drainage event from glacial Lake Saginaw to glacial Lake Chicago was a catastrophic outburst that deeply incised the valley. Analysis of shoreline and outlet geomorphology at the Chicago outlet supports J H Bretz's hypothesis of episodic incision and lake-level change. Shoreline features of each lake level converge to separate outlet sills that decrease in elevation from the oldest to youngest lake phases. This evidence, coupled with the presence of boulder lags and other features consistent with outburst origin, suggests that the outlets were deepened by catastrophic outbursts at least twice. The first incision event is correlated with a linked series of floods that progressed from Huron and Erie basin lakes to glacial Lake Saginaw to glacial Lake Chicago and then to the Mississippi. The second downcutting event occurred after the Two Rivers Advance of the Lake Michigan Lobe. Outbursts from the eastern outlets of glacial Lake Agassiz to glacial Lake Algonquin are a possible cause for this period of downcutting at the Chicago outlets.

Hydraulic reversals and episodic methane emissions during drought cycles in mires

Research Article| March 01, 1993 Hydraulic reversals and episodic methane emissions during drought cycles in mires Edwin A. Romanowicz; Edwin A. Romanowicz 1Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070 Search for other works by this author on: GSW Google Scholar Donald I. Siegel; Donald I. Siegel 1Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070 Search for other works by this author on: GSW Google Scholar Paul H. Glaser Paul H. Glaser 2Limnological Research Center, University of Minnesota, Minneapolis, Minnesota 55455 Search for other works by this author on: GSW Google Scholar Author and Article Information Edwin A. Romanowicz 1Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070 Donald I. Siegel 1Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070 Paul H. Glaser 2Limnological Research Center, University of Minnesota, Minneapolis, Minnesota 55455 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (3): 231–234. https://doi.org/10.1130/0091-7613(1993)021<0231:HRAEME>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Edwin A. Romanowicz, Donald I. Siegel, Paul H. Glaser; Hydraulic reversals and episodic methane emissions during drought cycles in mires. Geology 1993;; 21 (3): 231–234. doi: https://doi.org/10.1130/0091-7613(1993)021<0231:HRAEME>2.3.CO;2 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 SocietyGeology Search Advanced Search Abstract Reversals in hydraulic head and extensive methane losses were identified during a drought cycle from changes in hydraulic-head and dissolved-methane profiles in peat, Glacial Lake Agassiz Peatlands, northern Minnesota, United States. During the summer 1990 drought, regional ground water discharged from the mineral soil underlying the peat to raised bogs. Concentrations of dissolved CH4 in pore water showed that much of the peat column was supersaturated with respect to a reference standard of I atm partial pressure CH4. By the summer of 1991, the severity of the drought lessened, and the upper peat column was resaturated with precipitation-derived water. Ground-water flow was then controlled by local, precipitation-driven, recharge flow systems, and the regional ground water that was discharged affected only the peat immediately above the mineral soil-peat interface. In contrast, during the summer of 1991, concentrations of dissolved CH4 in pore water were all undersaturated with respect to 1 atm partial pressure CH4. The decrease in the amount of dissolved CH4, in the pore water from the summer of 1990 to the summer of 1991 probably was caused by changes in the CH4 flux and degassing to the unsaturated zone and to a lesser extent by changes in the partial pressure of gaseous CH4 in the peat as the peat volume increased with expansion as it resaturated. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Variation in the preoperculomandibular canal of the johnny darter, Etheostoma nigrum, with associated zoogeographical considerations

A total of 1267 specimens (from 87 stations) of the johnny darter, Etheostoma nigrum, were studied to examine the geographic variation in the numbers of pores on the preoperculomandibular canal. The pore count is bimodal for the total sample. These modes correspond to distinct geographic regions. Fishes from northern Ontario (west and north of Lake Nipigon), Manitoba, Saskatchewan, Minnesota, Wisconsin, Iowa, and the Upper Peninsula of Michigan usually have 7 or fewer pores (mode = 6). Populations from northern Ontario (east and south of Lake Nipigon), southern Ontario, Quebec, Illinois, Indiana, Kentucky, Tennessee, Maryland, Ohio, New York, Pennsylvania, West Virginia, Virginia, North Carolina, South Carolina, and the Lower Peninsula of Michigan have 8 or more pores (mode = 9). The differentiation between morphs predates their present distribution and the morphs probably occupied distinct geographic areas within the Mississippi refugium during the last glaciation. Etheostoma nigrum dispersed north, following two postglacial routes: (i) via the Mississippi River to Lake Agassiz (12 800 years BP) then eastward to the Hudson Bay and James Bay drainages via Lake Barlow–Ojibway (9500 years BP), and (ii) via a northeastern spread from the Great Lakes and Ohio River drainages to the St. Lawrence River and Ottawa River drainages (12 000 years BP).

Temperature Dependence of Soil Water Potential

Abstract To understand the process of coupled heat and water transport, the relationship between temperature and soil water potential must be known. Two clays, Avonlea bentonite and Lake Agassiz clay, are being considered as the clay-based sealing materials for the Canadian nuclear fuel waste disposal vault. Avonlea bentonite is distinguished from Lake Agassiz clay by its high sealing potential in water. A series of experiments was performed in which the two clays were mixed with equal amounts of sand and were compacted to a dry density of 1.67 Mg/m3 under various moisture contents and temperatures. A psychrometer was placed within the compacted clay-sand to measure the soil water potential based on the electromotive force measured by the psychrometer. The results indicate that the soil water potential at a particular temperature is higher for both clay-sand mixtures than predicted by the change in the surface tension of water; this effect is much more prominent in the Avonlea bentonite and at low moisture contents. The paper presents empirical equations relating the soil water potential with the moisture content and temperature of the two clay-sand mixtures.

Evidence for high water levels in the Erie basin during the Younger Dryas chronozone

A high water phase in the Lake Erie basin is identified from a variety of evidence for the period 11.0 ka to 10.5 ka. It is believed to correspond to the first Agassiz inflow to the upper Great Lakes (Main Lake Algonquin phase) when Agassiz waters discharged in both catastrophic and equilibrium modes to Lake Superior. After allowing for differential isostatic rebound, a computational model is used to estimate the lake levels in the Erie basin needed to generate Agassiz-equivalent discharges out of the basin into Lake Ontario. Computations suggest that Lake Tonawanda spillways would be re-activated by the high lake levels needed to sustain Agassiz-equivalent discharges. Existing published evidence from the Erie basin, Niagara River, and western New York (including 14C dates), is consistent with this interpretation. Additional evidence from the Niagara Peninsula (pollen spectra and geomorphology) supports the inference that extensive flooding of the southern Niagara Peninsula (Lake Wainfleet) occurred due to high water levels in the Erie basin. In the Niagara Peninsula, very shallow ‘washover’ spillways would only operate when standard hydrologic variations of lake level in the Erie basin coincided with short term high levels driven by catastrophic inflows to the Great Lakes from Lake Agassiz. We support the view of Lewis & Anderson (1992) that a meltwater flux from Agassiz inflows reached Lake Erie.

Stable isotope (O and C) and pollen trends in eastern Lake Erie, evidence for a locally-induced climatic reversal of Younger Dryas age in the Great Lakes basin

A cool period from about 11000 to 10 500 BP (11 to 10.5 ka) is recognized in pollen records from the southern Great Lakes area by the return of Picea and Abies dominance and by the persistence of herbs. The area of cooling appears centred on the Upper Great Lakes. A high-resolution record (ca. 9 mm/y) from a borehole in eastern Lake Erie reveals, in the same time interval, this pollen anomaly, isotope evidence of meltwater presence (a — 3 per mil shift in σ18O and a +1.1 per mil shift in σ13C), increased sand, and reduced detrital calcite content, all suggesting concurrent cooling of Lake Erie. The onset of cooling is mainly attributed to the effect of enhanced meltwater inflow on the relatively large upstream Main Lake Algonquin during the first eastward discharge of glacial Lake Agassiz. Termination of the cooling coincides with drainage of Lake Algonquin, and is attributed to loss of its cooling effectiveness associated with a substantial reduction in its surface area. It is hypothesized that the cold extra inflow effectively prolonged the seasonal presence of lake ice and the period of spring overturn in Lake Algonquin. The deep mixing would have greatly increased the thermal conductive capacity of this extensive lake, causing suppression of summer surface lakewater temperatures and reduction of onshore growing-degree days. Alternatively, a rapid flow of meltwater, buoyed on sediment-charged (denser) lakewater, may have kept the lake surface cold in summer. Other factors such as wind-shifted pollen deposition and possible effects from the Younger Dryas North Atlantic cooling could have contributed to the Great Lakes climatic reversal, but further studies are needed to resolve their relative significance.