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1.
Over the last 12600 years, lake levels in the eastern Lake Erie basin have fluctuated dramatically, causing major changes in drainage patterns, flooding and draining ephemeral Lake Wainfleet several times and widening and narrowing the Niagara Gorge as the erosive effects of Niagara Falls waxed and waned. The control sill for Lake Erie levels was at first the Fort Erie/Buffalo sill, before the Lyell/Johnson sill in Niagara Falls took over due to isostatic rebound. This sill, in time, was eventually eroded by the recession of Niagara Falls and the Fort Erie/Buffalo sill regained control. The environmental picture is complicated by catastrophic outbursts from glacial Lake Agassiz and Lake Barlow-Ojibway, changes in outlet routes, isostatic rebound and climatic changes over the Great Lakes basins. Today, the flow of water into Lake Erie from the streams and rivers surrounding it only accounts for about 13% of the flow out of it, therefore, the importance of flow from the Upper Great Lakes, specifically the flow from Lake Huron, has a great effect on Lake Erie levels. While the changing control sills, Lyell/Johnson and Buffalo/Fort Erie would affect Lake Erie levels, overall they are mostly input driven by the amount of waters received from the Upper Great Lakes. Since Lake Erie's water level changes are so closely tied to Lake Huron's water level changes we have decided to use names assigned to Lake Huron such as the two Mattawa highstands and three Stanley lowstands rather than inflict a whole new set of names on the public. While the duration of each high and lowstand in Erie and Huron may not always be the same, they always happen within the same time frame. The datum elevations used for Lake Huron (175.8 m) and Lake Erie (173.3 m) are historically recorded averages. The Lake Erie levels proposed in this paper reflect Lake Hurons effects on Lake Erie and the levels occuring at the eastern end of the Erie Basin throughout the last 12600 years. All dates in this paper are uncorrected 14 C dates unless the date was obtained from shells, then the date has been corrected for hard-water effects. Also, all heights are given as modern day elevations and are not adjusted for isostatic rebound.  相似文献   

2.
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.  相似文献   

3.
Two distinct episodes of increased water flux imposed on the Great Lakes system by discharge from upstream proglacial lakes during the period from about 11.5 to 8 ka resulted in expanded outflows, raised lake levels and associated climate changes. The interpretation of these major hydrological and climatic effects, previously unrecognized, is mainly based on the evidence of former shorelines, radiocarbon-dated shallow-water sediment sequences, paleohydraulic estimates of discharge, and pollen diagrams of vegetation change within the basins of the present Lakes Superior, Michigan, Huron, Erie and Nipissing. The concept of inflow from glacial Lake Agassiz adjacent to the retreating Laurentide Ice Sheet about 11–10 and 9.5–8.5 ka is generally supported, with inflow possibly augmented during the second period by backflooding of discharge from glacial Lake Barlow-Ojibway.Although greater dating control is needed, six distinct phases can be recognized which characterize the hydrological history of the Upper Great Lakes from about 12 to 5 ka; 1) an early ice-dammed Kirkfield phase until 11.0 ka which drained directly to Ontario basin; 2) an ice-dammed Main Algonquin phase (11.0–10.5 ka) of relatively colder surface temperature with an associated climate reversal caused by greater water flux from glacial Lake Agassiz; 3) a short Post Algonquin phase (about 10.5–10.1 ka) encompassing ice retreat and drawdown of Lake Algonquin; 4) an Ottawa-Marquette low phase (about 10.1–9.6 ka) characterized by drainage via the then isostatically depressed Mattawa-Ottawa Valley and by reduction in Agassiz inflow by the Marquette glacial advance in Superior basin; 5) a Mattawa phase of high and variable levels (about 9.6–8.3 ka) which induced a second climatic cooling in the Upper Great Lakes area. Lakes of the Mattawa phase were supported by large inflows from both Lakes Agassiz and Barlow-Ojibway and were controlled by hydraulic resistance at a common outlet — the Rankin Constriction in Ottawa Valley — with an estimated base-flow discharge in the order of 200000 m3s–1. 6) Lakes of the Nipissing phase (about 8.3–4.7 ka) existed below the base elevation of the previous Lake Mattawa, were nourished by local precipitation and runoff only, and drained by the classic North Bay outlet to Ottawa Valley.Geological Survey of Canada Contribution 42488.This is the twelfth of a series of papers to be published by this journal that was presented in the paleolimnology sessions organized by R. B. Davis and H. Löffler for the XIIth Congress of the International Union for Quaternary Research (INQUA), which took place in Ottawa, Canada in August 1987. Dr. Davis is serving as guest editor of this series.  相似文献   

4.
Lake Agassiz water oxygen isotopic compositions inferred from sediment core organics and pore waters provide some additional insight into the paleohydrology of the Great Lakes and their drainage into the North Atlantic during the late glacial and early Holocene. Isotopically enriched Lake Agassiz water supports the hypothesis that high Huron Basin lake (Mattawa) phases, during the early Holocene (9600–9300 and 9100–8100 years BP) resulted from an influx of Lake Agassiz water and suggests that low lake (Stanley) phases (9800–9600, 9300–9100, 8100–7400 years BP) were influenced more by regional influxes of isotopically depleted glacial melt water. Eastward drainage of enriched early Lake Agassiz water supports an active Port Huron outlet between 11000 and 10500 years BP and also helps to explain the absence of an 18O depleted interval in North Atlantic foram records. This may be the result of a balance between the opposing isotopic effects of depleted Lake Agassiz water and lower sea surface temperatures on carbonate precipitation between 11000 and 10000 years BP.  相似文献   

5.
J.L. Hough in 1962 recognized an erosional unconformity in the upper section of early postglacial lake sediments in northwestern Lake Huron. Low-level Lake Stanley was defined at 70 m below present water surface on the basis of this observation, and was inferred to follow the Main Algonquin highstand and Post-Algonquin lake phases about 10 14C ka, a seminal contribution to the understanding of Great Lakes history. Lake Stanley was thought to have overflowed from the Huron basin through the Georgian Bay basin and the glacio-isostatically depressed North Bay outlet to Ottawa and St. Lawrence rivers. For this overflow to have occurred, Hough assumed that post-Algonquin glacial rebound was delayed until after the Lake Stanley phase. A re-examination of sediment stratigraphy in northwestern Lake Huron using seismic reflection and new core data corroborates the sedimentological evidence of Hough’s Stanley unconformity, but not its inferred chronology or the level of the associated lowstand. Erosion of previously deposited sediment, causing the gap in the sediment sequence down to 70 m present depth, is attributed to wave erosion in the shoreface of the Lake Stanley lowstand. Allowing for non-deposition of muddy sediment in the upper 20 m approximately of water depth as occurs in the present Great Lakes, the inferred water level of the Stanley lowstand is repositioned at 50 m below present in northwestern Lake Huron. The age of this lowstand is about 7.9 ± 0.314C ka, determined from the inferred 14C age of the unconformity by radiocarbon-dated geomagnetic secular variation in six new cores. This relatively young age shows that the lowstand defined by Hough’s Stanley unconformity is the late Lake Stanley phase of the northern Huron basin, youngest of three lowstands following the Algonquin lake phases. Reconstruction of uplift histories for lake level and outlets shows that late Lake Stanley was about 25–30 m below the North Bay outlet, and about 10 m below the sill of the Huron basin. The late Stanley lowstand was hydrologically closed, consistent with independent evidence for dry regional climate at this time. A similar analysis of the Chippewa unconformity shows that the Lake Michigan basin also hosted a hydrologically closed lowstand, late Lake Chippewa. This phase of closed lowstands is new to the geological history of the Great Lakes. This is the ninth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M Lewis were guest editors of this special issue.  相似文献   

6.
The evolution of the early Great Lakes was driven by changing ice sheet geometry, meltwater influx, variable climate, and isostatic rebound. Unfortunately none of these factors are fully understood. Sediment cores from Fenton Lake and other sites in the Lake Superior basin have been used to document constantly falling water levels in glacial Lake Minong between 9,000 and 10,600 cal (8.1–9.5 ka) BP. Over three meters of previously unrecovered sediment from Fenton Lake detail a more complex lake level history than formerly realized, and consists of an early regression, transgression, and final regression. The initial regression is documented by a transition from gray, clayey silt to black sapropelic silt. The transgression is recorded by an abrupt return to gray sand and silt, and dates between 9,000 and 9,500 cal (8.1–8.6 ka) BP. The transgression could be the result of increased discharge from Lake Agassiz overflow or the Laurentide Ice Sheet, and hydraulic damming at the Lake Minong outlet. Alternatively ice advance in northern Ontario may have blocked an unrecognized low level northern outlet to glacial Lake Ojibway, which switched Lake Minong overflow back to the Lake Huron basin and raised lake levels. Multiple sites in the Lake Huron and Michigan basins suggest increased meltwater discharges occurred around the time of the transgression in Lake Minong, suggesting a possible linkage. The final regression in Fenton Lake is documented by a return to black sapropelic silt, which coincides with varve cessation in the Superior basin when Lake Agassiz overflow and glacial meltwater was diverted to glacial Lake Ojibway in northern Ontario.  相似文献   

7.
Sub-bottom profiling and coring were undertaken at eight sub-basins along the lower French River and at five small lakes near North Bay, Ontario, to collect stratigraphical and chronological evidence to investigate whether lakes occupying the Huron–Georgian basins during the early- to mid-Holocene became hydrologically closed. All of the coring sites are located within the route of the North Bay outlet that carried outflow from the upper Great Lakes during this period. Sand beds containing organic detritus are present within five cores from Muskrat, Crombie and Deep bays that otherwise are composed of glaciolacustrine rhythmites or fine-grained lacustrine deposits. These sand beds are interpreted to represent intervals when water levels within the sub-basins were lower than present, based on chronology, sediment texture, and macrofossil assemblages. It is inferred that the water surface in the Huron–Georgian basins fell below the level of the Dalles Rapids sill isolating the lower French River sub-basins from the large lake. A core from Depensier Lake, North Bay, contains an organic-rich sand interval within a thicker sand unit barren of organic materials. Macrofossils within this organic-rich interval are interpreted to be evidence of substantially diminished flow through the North Bay outlet channel. Radiocarbon dates of terrestrial macrofossils provide correlation of the sand beds between the French River cores as well as with the organic-rich sand in the Depensier Lake core. The possibility that the sand beds in the French River cores represent flood deposits rather than evidence of hydrologically closed conditions is considered, but rejected, based on the occurrence of multiple peaty layers and the record of shallow water conditions inferred from macrofossils within the upper sand bed of core MUS1, Muskrat Bay, in combination with the evidence of quiescent depositional conditions from similarly aged macrofossils in the core from Depensier Lake. Eight radiocarbon dates from the French River cores are incorporated into an elevation-age plot of paleo-indicators of water levels in the Huron–Georgian basins, using additional data from the literature. This plot and stratigraphic evidence from the Muskrat Bay cores indicates that separate closed-basin intervals occurred between 9.0 and 8.4, and 9.5 and 9.3 ka cal BP (~ 8.1 and 7.6, and ~ 8.5 and 8.3 ka BP). The occurrence of these two closed-basin intervals between 9.6 and 8.4 ka cal BP (~ 8.7 and 7.6 ka BP) implies that run-off derived exclusively from precipitation within the non-glaciated portions of the upper Great Lakes drainage basins was likely insufficient at this time to support an open-basin lake hydrology during the contemporary climate, which was colder and drier than present, without being supplemented from glacial Lake Agassiz overflow and/or Laurentide Ice Sheet meltwater.  相似文献   

8.
Piston cores from deep-water bottom deposits in Lake Ontario contain shallow-water sediments such as, shell-rich sand and silt, marl, gyttja, and formerly exposed shore deposits including woody detritus, peat, sand and gravel, that are indicative of past periods of significantly lower water levels. These and other water-level indicators such as changes in rates of sedimentation, mollusc shells, pollen, and plant macrofossils were integrated to derive a new water-level history for Lake Ontario basin using an empirical model of isostatic adjustment for the Great Lakes basin to restore dated remnants of former lake levels to their original elevations. The earliest dated low-level feature is the Grimsby-Oakville bar which was constructed in the western end of the lake during a near stillstand at 11–10.4 (12.9–12.3 cal) ka BP when Early Lake Ontario was confluent with the Champlain Sea. Rising Lake Ontario basin outlet sills, a consequence of differential isostatic rebound, severed the connection with Champlain Sea and, in combination with the switch of inflowing Lake Algonquin drainage northward to Ottawa River valley via outlets near North Bay and an early Holocene dry climate with enhanced evaporation, forced Lake Ontario into a basin-wide lowstand between 10.4 and 7.5 (12.3 and 8.3 cal) ka BP. During this time, Lake Ontario operated as a closed basin with no outlets, and sites such as Hamilton Harbour, Bay of Quinte, Henderson Harbor, and a site near Amherst Island existed as small isolated basins above the main lake characterized by shallow-water, lagoonal or marsh deposits and fossils indicative of littoral habitats and newly exposed mudflats. Rising lake levels resulting from increased atmospheric water supply brought Lake Ontario above the outlet sills into an open, overflowing state ending the closed phase of the lake by ~7.5 (8.3 cal) ka BP. Lake levels continued to rise steadily above the Thousand Islands sill through mid-to-late Holocene time culminating at the level of modern Lake Ontario. The early and middle Holocene lake-level changes are supported by temperature and precipitation trends derived from pollen-climate transfer functions applied to Roblin Lake on the north side of Lake Ontario.  相似文献   

9.
New stratigraphic evidence from the Rossendale area, Manitoba, Canada, provides insight into the early postglacial evolution of the southeastern Assiniboine Delta. In this region, much of the upper 13+ m of sediment accumulation is characterized by multiple cycles of sandy rhythmites interbedded with massive to laminated silt. These sediments were deposited rapidly by traction or turbidity currents and record the construction of the Assiniboine fan-delta during the deep-water Lockhart Phase of glacial Lake Agassiz (>10.8 14C ka BP). Shortly before ∼10 14C ka BP, fluvial incision into deltaic deposits occurred locally at the Rossendale Gully site in response to the regression of glacial Lake Agassiz during the Moorhead Phase. Plant macrofossils deposited in the gully by 10 14C ka BP provide the first information on early postglacial plant colonization of the distal Assiniboine delta. These data suggest initial establishment of Scorpidium scorpioides, Potamogeton spp., Scirpus spp., and other wetland plants, followed by colonization of uplands by a Picea-Populus assemblage. Importantly, because the gully is located in a protected depression behind the Campbell beach, evidence of water table rise from aquatic macrophytes suggests that glacial Lake Agassiz transgressed to the Campbell level during the early Emerson Phase (∼10 14C ka BP). Furthermore, no evidence exists for a post-Lockhart rise in Lake Agassiz above the Upper Campbell beach. If Agassiz stood at the Campbell level during the early Emerson Phase, then drainage through the southern outlet may have been possible at this time. This scenario, if true, may suggest that the northwestern outlet was temporarily closed by a glacial advance shortly before 10 14C ka BP. This is the first in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue  相似文献   

10.
Intrusive magmatism is an integral and understudied component in both volcanic and nonvolcanic passive margins. Here, we investigate the thermal effects of widespread (ca. 20 000 km2) intrusive magmatism on the thermal evolution of organic‐rich sedimentary rocks on the nonvolcanic Newfoundland passive margin. ODP 210‐1276 (45.41°N, 44.79°W) intersects two sills: an older, upper sill and a younger, lower sill that are believed to correspond to the high amplitude ‘U‐reflector’ observed across the Newfoundland Basin. A compilation of previous work collectively provides; (1) emplacement depth constraints, (2) vitrinite reflectance data and (3) 40Ar/39Ar dates. Collectively, these data sets provide a unique opportunity to model the conductive cooling of the sills and how they affect thermal maturity of the sedimentary sequence. A finite differences method was used to model the cooling of the sills, with the model outputs then being entered into the EASY%Ro vitrinite reflectance model. The modelled maturation profile for ODP 210‐1276 shows a significant but localized effect on sediment maturity as a result of the intrusions. Our results suggest that even on nonvolcanic margins, intrusive magmatism can significantly influence the thermal evolution in the vicinity of igneous intrusions. In addition, the presence of widespread sills on nonvolcanic passive margins such as offshore Newfoundland may be indicative of regional‐scale thermal perturbations that should be considered in source rock maturation studies.  相似文献   

11.
Paleoshoreline evidence for postglacial tilting in Southern Manitoba   总被引：1，自引：0，他引：1  
Detailed air photo interpretation and four seasons of field mapping and surveying in southern Manitoba have revealed that the once-level paleoshorelines of Lake Winnipegosis and Dauphin Lake and the Burnside shoreline of former Lake Agassiz have been tilted up to the northeast by postglacial differential rebound. Our investigation has also revealed that Lake Winnipegosis has the best preserved paleoshoreline record of any of the large lakes in southern Manitoba, including lakes Winnipeg and Manitoba. This is because northeasterly uptilting shifts the region's lakes to the southwest. Lakes with southern outlets, like Lake Winnipegosis, undergo general regression as the outlet is lowered relative to the rest of the basin. Lakes with northern outlets, like lakes Winnipeg and Manitoba, undergo general transgression as northeasterly uptilting raises the outlet relative to the rest of the basin. Along the northeastern shore of Lake Winnipegosis a staircase of at least 32 abandoned Winnipegosis shorelines exists that is consistent with northeasterly tilting. The Dawson level represents the major mid-Holocene highstand on Lake Winnipegosis. It persisted for about 500 years, peaking at 5290 14C yr B.P. (early Dawson) and then falling about 3 m by 4740 14C yr B.P. (late Dawson). The early Dawson shoreline is tilted at 13.5 cm km-1 in a direction N24.3°E. Three other shorelines informally named shoreline 4, shoreline 3, and shoreline 2 are also tilted up to the northeast. Their radiocarbon ages (and slopes in cm km-1) are 3330 yr B.P. (2.2), 1510 yr B.P. (1.3), and 1080 yr B.P. (0.7), respectively. On Dauphin Lake shoreline IV is the oldest level mapped for this study. It has a 14C age of 7910 yr B.P. and is tilted at 21.7 cm km-1 in a direction N44.4°E. The Id shoreline marks the major mid-Holocene highstand for Dauphin Lake. It peaked at 4640 14C yr B.P. followed by a rapid decline of about 1 m to the Ib shoreline, which is dated at 4320 14C yr B.P. Id is tilted up at 8.8 cm km-1 in a direction N53.4°E. The next major shoreline is Ia3 which has a 14C age of 3020 yr B.P. and is tilted up at 5.3 cm km-1 in a direction N62.3°E. Tilt directions are significantly more easterly for the Dauphin Lake shorelines than those from Lake Winnipegosis or any of the much older Lake Agassiz shorelines. Taken together, the Winnipegosis and Dauphin isobases indicate that the direction of tilt in southern Manitoba is more complex than a simple uni-directional pattern. The observed pattern of tilting for paleoshorelines in southern Manitoba agrees better with predictions derived from the recently revised loading history model ICE-4G than with those from its predecessor ICE-3G. In general, the calculated tilt based on the ICE-3G load tends to exceed the observed tilt, while ICE-4G tends to underestimate it. Both ice load models appear to disagree most with our observed tilts in this region during the interval before about 9000 cal yr B.P., when deglaciation was proceeding rapidly and the large water load associated with Lake Agassiz covered the region. Because both of these ice load models have been estimated mainly from a global data set of relative sea level curves from marine coast sites, it is not unexpected that model tilts derived from them do not agree well with observations in the North American continental interior. The pattern of postglacial crustal deformation for southern Manitoba described in this paper could be used to further refine ice load models for the North American continental interior.  相似文献   

12.
The recognition of ice-marginal deltas constructed during the formation of the Nakina II moraine and a previously unrecognized spillway, in the vicinity of Longlac, northern Ontario, indicates that existing concepts of ancestral lake level history and drainage systems in the Lake Superior–Lake Nipigon region is inadequate. Based on isostatically corrected digital elevation maps, ice-marginal deltas of the Nakina II moraine probably formed at the level of glacial Lake Minong, most likely Minong III, and not glacial Lake Nakina as has been commonly suggested. In addition, the presence of a spillway near Longlac indicates that lake water drained southward through the Mullet Outlet–Pic River system immediately following ice-marginal retreat from the Nakina II moraine and not eastward as previously proposed. Architectural-element analysis of exposures within the spillway indicates hyperconcentrated outbursts of meltwater produced thick channel-fill elements during flood conditions with peak-velocities exceeding 3 m/s. Subsequent retreat of ice from the Pic River valley to the east, may have allowed waters of Lake Agassiz, Lake Barlow–Ojibway, or both, to drain into post-Minong lake levels in the Lake Superior basin. These findings place major constraints on previously proposed concepts of northeastern or eastern outlets of Lake Agassiz.  相似文献   

13.
Exposures along the lower Kaministiquia River (near Thunder Bay, Ontario, Canada) provide insight into early Holocene lake level fluctuations and paleoenvironmental conditions in the northwestern Lake Superior basin. These exposures show at least two large paleochannels which were downcut into offshore sediments, and were later filled with >2 m of sand, ~3 m of rhythmically laminated silt and clay, and ~6 m of interbedded silt and sand. Buried by the rhythmically laminated silty clay unit is a well-preserved organic deposit with abundant plant macrofossils from terrestrial and emergent taxa, including several upright tree trunks. Three AMS radiocarbon ages were obtained on wood and conifer cones from this deposit: 8,135 ± 25 (9,130–9,010 cal), 8,010 ± 25 (9,010–8,780 cal), and 7,990 ± 20 (8,990–8,770 cal) BP. This sequence records an early postglacial high-water phase, followed by the Houghton lowstand, and reflooding of the lower Kaministiquia River Valley. The drop in lake level associated with the Houghton phase forced the ancestral Kaministiquia River to downcut. By ~9,100 cal (~8,100) BP, older channels eroded into subaqueous underflow fan deposits in the Thunder Bay area near Fort William Historical Park (FWHP) were abandoned and colonized by a Picea-Abies-Larix forest. Based on stratigraphic data corrected for differential isostatic rebound, the lake was below the Sault Ste. Marie bedrock sill between at least 9,100 cal (8,100) and 8,900 cal (8,000) BP. Shortly after 8,900 cal BP, the lake quickly rose and buried in situ lowland vegetation at FWHP with varved sediments. We argue that this transgression was due to overflow from glacial Lakes Agassiz or Ojibway associated with the retreat of the Laurentide Ice Sheet from the Nakina moraine and/or the Cochrane surge margins in the Hudson Bay Lowlands. A continued rise in lake level after 6,420 ± 20 (7,400 cal) BP at FWHP may record uplift of the North Bay outlet above the Sault Ste. Marie bedrock sill and the onset of the Nipissing transgression in the Lake Superior basin.  相似文献   

14.
Water levels in the Lake Erie basin are inferred from glacial lake times to present. An era of early to middle Holocene lowstands is defined below outlets by a submerged paleo-beach, and truncated reflectors in glaciolacustrine sediment beneath a mud-covered wave-cut terrace. Also, the glacial clay surface above the paleo-shore level has elevated shear strength because of porewater drainage during subaerial exposure. Below the paleo-shore where exposure did not occur, clay strength remained normal. Sedimentation rates were reduced during the lowstands. The distortion of once-level shore zone indicators by differential glacial rebound was removed by computing original elevations of the indicators using an empirical model of rebound based on observations of upwarped former lake shorelines. Erie water-level history was inferred from a plot of the original elevations of lake-level constraints and outlets versus age. The lake history was validated by reference to ~83 water-level indicators, not used as constraints. During the deglaciation, lake-crossing moraines were likely eroded by fluvial drainage into low-level Lake Ypsilanti and a subsequent unnamed low lake to produce the Lorain Valley and Pennsylvania Channel. Once inflow from the upper Great Lakes basins was directed to Ottawa Valley about 10,400 (12,270 cal BP), Erie water levels descended in a dry, evaporative climate to a closed lowstand during which ostracode δ18O increased ~2‰ above present values. Lake level began to rise 6,000 to 7,000 (6,830 to 7,860 cal) BP in response to increased atmospheric moisture and later, to northern inflow as the Nipissing Transgression returned upper Great Lakes drainage to Lake Erie by about 5,200 (6,000 cal) BP. At that time, the lake overflowed the uplifted Lyell–Johnson Sill north (downstream) of the present Niagara Falls at higher-than-present levels. After recession of the Falls breached this sill about ~3,500 (~3,770 cal) BP, Lake Erie fell 3–4 m to its present Fort Erie–Buffalo Sill. The extended low-water phase with its isolated sub-basins could have restricted migration of aquatic fauna. The early to middle Holocene closed-basin response highlights the sensitivity of Lake Erie to climatic reductions in its water budget.  相似文献   

15.
West Hawk Lake (WHL) is located within the glacial Lake Agassiz basin, 140 km east of Winnipeg, Manitoba. The small lake lies in a deep, steep-sided, meteorite impact crater, which has been partly filled by 60 m of sediment that today forms a flat floor in the central part of the basin below 111 m of water. Four cores, 5–11 m in length, were collected using a Kullenberg piston gravity corer. All sediment is clay, contains no unconformities, and has low organic content in all but the upper meter. Sample analyses include bulk and clay mineralogy, major and minor elements, TOC, stable isotopes of C, N, and O, pollen, charcoal, diatoms, and floral and faunal macrofossils. The sequence is divided into four units based mainly on thickness and style of lamination, diatoms, and pollen. AMS radiocarbon dates do not provide a clear indication of age in the postglacial sequence; possible explanations include contamination by older organic inwash and downward movement of younger organic acids. A chronological framework was established using only selected AMS dates on plant macrofossils, combined with correlations to dated events outside the basin and paleotopographic reconstructions of Lake Agassiz. The 822 1-cm-thick varves in the lower 8 m of the cored WHL sequence were deposited just prior to 10,000 cal years BP (∼8,900 14C years BP), during the glacial Lake Agassiz phase of the lake. The disappearance of dolomite near the top of the varved sequence reflects the reduced influence of Lake Agassiz and the carbonate bedrock and glacial sediment in its catchment. The lowermost varves are barren of organisms, indicating cold and turbid glacial lake waters, but the presence of benthic and planktonic algae in the upper 520 varves indicates warming; this lake phase coincides with a change in clay mineralogy, δ18O and δ13C in cellulose, and in some other parameters. This change may have resulted from a major drawdown in Lake Agassiz when its overflow switched from northwest to east after formation of the Upper Campbell beach of that lake 9,300–9,400 14C years ago. The end of thick varve deposition at ∼10,000 cal years BP is related to the opening of a lower eastern outlet of Lake Agassiz and an accompanying drop in West Hawk Lake level. WHL became independent from Lake Agassiz at this time, sedimentation rates dropped, and only ∼2.5 m of sediment was deposited in the next 10,000 years. During the first two centuries of post-Lake Agassiz history, there were anomalies in the diatom assemblage, stable O and C isotopes, magnetic susceptibility, and other parameters, reflecting an unstable watershed. Modern oligotrophic conditions were soon established; charcoal abundance increased in response to the reduced distance to the shoreline and to warmer conditions. Regional warming after ∼9,500 cal years BP is indicated by pollen and diatoms as well as C and O isotope values. Relatively dry conditions are suggested by a rise in pine and decrease in spruce and other vegetation types between 9,500 and 5,000 cal years BP (∼8,500–4,400 14C years BP), plus a decrease in δ13Ccell values. After this, there was a shift to slightly cooler and wetter conditions. A large increase in organic content and change in elemental concentration in the past several thousand years probably reflects a decline in supply of mineral detritus to the basin and possibly an increase in productivity.  相似文献   

16.
Detailed mapping and elevation measurements of glacial lake shorelines in the Flin Flon region has permitted the reconstruction of 6 well-defined levels of Lake Agassiz formed around 8.3 ka to 7.9 ka 14C BP. The Stonewall, The Pas, Gimli, Grand Rapids, Drunken Point and Ponton paleo-water planes have been tilted upward to the northeast in the Holocene, with gradients decreasing, from the highest to the lowest level, from about 0.34 m km-1 to 0.22 m km-1 in the study area. The Setting level, lower than the Ponton but less well defined, is also documented here for the first time. This mapping conclusively refutes the view, entrenched in the literature from the 1890's to the 1960's, that there has been negligible differential uplift in the region following final drainage of Lake Agassiz. The finding has major consequences regarding correlation of glacial lakes across the mid-continent, the post-glacial history of large lakes in the region, and for interpretation of earth rheology and its implications for ice sheet reconstruction.  相似文献   

17.
Causal links that connect Holocene high stands of Lake Superior with dune building, stream damming and diversion and reservoir impoundment and infilling are inferred from a multidisciplinary investigation of a small watershed along the SE shore of Lake Superior. Radiocarbon ages of wood fragments from in-place stumps and soil O horizons, recovered from the bottom of 300-ha Grand Sable Lake, suggest that the near-shore inland lake was formed during multiple episodes of late Holocene dune damming of ancestral Sable Creek. Forest drownings at 3000, 1530, and 300 cal. years BP are highly correlated with local soil burial events that occurred during high stands of Lake Superior. During these and earlier events, Sable Creek was diverted onto eastward-graded late Pleistocene meltwater terraces. Ground penetrating radar (GPR) reveals the early Holocene valley of Sable Creek (now filled) and its constituent sedimentary structures. Near-planar paleosols, identified with GPR, suggest two repeating modes of landscape evolution mediated by levels of Lake Superior. High lake stands drove stream damming, reservoir impoundment, and eolian infilling of impoundments. Falling Lake Superior levels brought decreased sand supply to dune dams and lowered stream base level. These latter factors promoted stream piracy, breaching of dune dams, and aerial exposure and forestation of infilled lakebeds. The bathymetry of Grand Sable Lake suggests that its shoreline configuration and depth varied in response to events of dune damming and subsequent dam breaching. The interrelated late Holocene events apparent in this study area suggest that variations in lake level have imposed complex hydrologic and geomorphic signatures on upper Great Lakes coasts.  相似文献   

18.
The post-glacial history of the Great Lakes has involved several changes in lake levels throughout the latest Pleistocene and Holocene, resulting from the changing position of the retreating Laurentide ice sheet, outlet incision and isostatic rebound. The final lowering of lake levels occurred at approximately 7600 14C yr BP, after which lake levels began to rise again to the Nipissing highstand at approximately 4700 14C yr BP. During this time of rising lake levels, black bands of iron sulfide were being formed in the sediments of all five of the Great Lakes. These bands signify suboxic to anoxic conditions, at least within the sediments and possibly at the sediment-water interface, during the middle Holocene warm interval. During this interval, the climate was warmer and drier than present, possibly resulting in the occasional absence of seasonal turnover in the lakes. We examined a series of piston cores from northern Lakes Michigan and Huron and found that the black bands are correlatable among cores taken from within the same basin. The observation that the banding can be correlated suggests a basin-wide cause, near-bottom or sub-bottom anoxia in the northern Michigan and northern Huron sediments during the mid-Holocene warm period. The sedimentary and geochemical processes in the Great Lakes during the middle Holocene warm interval are good indicators of possible future scenarios for the lakes as a result of global warming, as 21st-century temperatures are predicted to reach similar levels due to increased concentrations of greenhouse gases.  相似文献   

19.
Variations in the oxygen-isotope composition of paleo-water bodies in the Lake Superior Basin provide information about the timing and pathways of glacial meltwater inflow into and within the Lake Superior Basin. Here, the oxygen-isotope compositions of Lake Superior have been determined using ostracodes from four sediment cores from across the Basin (Duluth, Caribou and Ile Parisienne sub-basins, Thunder Bay trough). The δ18O values indicate that lake water (Lake Minong) at ~10,600–10,400 cal [~9,400–9,250] BP was dominated by glacial meltwater derived from Lake Agassiz and the Laurentide Ice Sheet (LIS). From that time to ~9,000 cal [~8,100] BP, a period associated with formation of thick varves across the Lake Superior Basin, the δ18O values of Lake Minong decreased even further (−24 to −28‰), symptomatic of an increasing influx of glacial meltwater. Its supply was reduced between ~9,000 and ~8,900 cal [~8,100–8,000] BP, and lake water δ18O values grew higher by several per mil during this period. Between ~8,900 and ~8,800 cal [~8,000–7,950] BP, there was a return to δ18O values as low as −29‰ in some parts of the Lake Superior Basin, indicating a renewed influx of glacial meltwater before its final termination at ~8,800–8,700 cal [~7,950–7,900] BP. The sub-basins in the Lake Superior Basin generally displayed very similar patterns of lake water δ18O values, typical of a well-mixed system. The final stage of glacial meltwater input, however, was largely expressed near its input (Thunder Bay trough) and recognizable in dampened form mainly in the Duluth sub-basin to the west. Water in the easternmost Ile Parisienne sub-basin was enriched in 18O relative to the rest of the lake, particularly after ~10,000 cal [~8,900] BP, probably because of a strong influence of local precipitation/runoff, and perhaps also enhanced evaporation. By ~9,200 cal [~8,250] BP, lake water δ18O values in the Ile Parisienne sub-basin were similar to the adjacent Lake Huron Basin, suggesting a strong hydraulic connection between the two water bodies, and common responses to southern Ontario’s shift to warmer and dry climatic conditions after ~9,000 cal [~8,100] BP.  相似文献   

20.
Preliminary results of a multidisciplinary study of cores in southwestern Lake Michigan suggest that the materials in these cores can be interpreted in terms of both isostatically and climatically induced changes in lake level. Ostracodes and mollusks are well preserved in the Holocene sediments, and they provide paleolimnologic and paleoclimatic data, as well as biogenic carbonate for stable-isotope studies and radiocarbon dating. Pollen and diatom preservation in the cores is poor, which prevents comparison with regional vegetation records. New accelerator-mass spectrometer 14C ages, from both carbon and carbonate fractions, provide basin-wide correlations and appear to resolve the longstanding problem of anomalously old ages that result from detrital organic matter in Great Lakes sediments.Several cores contain a distinct unconformity associated with the abrupt fall in lake level that occurred about 10.3 ka when the isostatically depressed North Bay outlet was uncovered by the retreating Laurentide Ice Sheet. Below the unconformity, ostracode assemblages imply deep, cold water with very low total dissolved solids (TDS), and bivalves have 18O (PDB) values as light as — 10 per mil. Samples from just above the unconformity contain littoral to sublittoral ostracode species that imply warmer, higher-TDS (though still dilute) water than that inferred below the unconformity. Above this zone, another interval with 18O values more negative than — 10 occurs. The isotopic data suggest that two influxes of cold, isotopically light meltwater from Laurentide ice entered the lake, one shortly before 10.3 ka and the other about 9 ka. These influxes were separated by a period during which the lake was warmer, shallower, but still very low in dissolved solids. One or both of the meltwater influxes may be related to discharge from Lake Agassiz into the Great Lakes.Sedimentation rates appear to have been constant from about 10 ka to 5 ka. Bivalve shells formed between about 8 and 5 ka have 18O values that range from-2.3 to-3.3 per mil and appear to decrease toward the end of the interval. The ostracode assemblages and the stable isotopes suggest changes that are climatically controlled, including fluctuating water levels and increasing dissolved solids, although the water remained relatively dilute (TDS < 300 mg/l).A dramatic decrease in sedimentation rates occurred at about 5 ka, about the time of the peak of the Nippissing high lake stage. This decrease in sedimentation rate may be associated with a large increase in effective wave base as the lake approached its present size and fetch. A dramatic reduction in ostracode and mollusk abundances during the late Holocene is probably due to this decrease in sedimentation rates, which would result in increased carbonate dissolution. Ostracode productivity may also have declined due to a reduction in bottom-water oxygen caused by increased epilimnion algal productivity.Woods Hole Oceanographic Institute Contribution No. 7492  相似文献   

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