Southeastern Lake Superior Research Articles

Micrometeorological measurement of hexachlorobenzene and polychlorinated biphenyl compound air-water gas exchange in Lake Superior and comparison to model predictions

Abstract. Air-water exchange fluxes of persistent, bioaccumulative and toxic (PBT) substances are frequently estimated using the Whitman two-film (W2F) method, but micrometeorological flux measurements of these compounds over water are rarely attempted. We measured air-water exchange fluxes of hexachlorobenzene (HCB) and polychlorinated biphenyls (PCBs) on 14 July 2006 in Lake Superior using the modified Bowen ratio (MBR) method. Measured fluxes were compared to estimates using the W2F method, and to estimates from an Internal Boundary Layer Transport and Exchange (IBLTE) model that implements the NOAA COARE bulk flux algorithm and gas transfer model. We reveal an inaccuracy in the estimate of water vapor transfer velocity that is commonly used with the W2F method for PBT flux estimation, and demonstrate the effect of use of an improved estimation method. Flux measurements were conducted at three stations with increasing fetch in offshore flow (15, 30, and 60 km) in southeastern Lake Superior. This sampling strategy enabled comparison of measured and predicted flux, as well as modification in near-surface atmospheric concentration with fetch, using the IBLTE model. Fluxes estimated using the W2F model were compared to fluxes measured by MBR. In five of seven cases in which the MBR flux was significantly greater than zero, concentration increased with fetch at 1-m height, which is qualitatively consistent with the measured volatilization flux. As far as we are aware, these are the first reported ship-based micrometeorological air-water exchange flux measurements of PCBs.

Late Holocene Lake-level Variation in Southeastern Lake Superior: Tahquamenon Bay, Michigan

Internal architecture and ages of 71 beach ridges in the Tahquamenon Bay embayment along the southeastern shore of Lake Superior on the Upper Peninsula of Michigan were studied to generate a late Holocene relative lake-level curve. Establishing a long-term framework is important to examine the context of historic events and help predict potential future changes critical for effective water resource management. Ridges in the embayment formed between about 4,200 and 2,100 calendar years before 1950 (cal. yrs. B.P.) and were created and preserved every 28 ± 4.8 years on average. Groups of three to six beach ridges coupled with inflections in the lake-level curve indicate a history of lake levels fluctuations and outlet changes. A rapid lake-level drop (approximately 4 m) from about 4,100 to 3,800 cal. yrs. B.P. was associated with a fall from the Nipissing II high-water-level phase. A change from a gradual fall to a slight rise was associated with an outlet change from Port Huron, Michigan/Sarnia, Ontario to Sault Ste. Marie, Michigan/Ontario. A complete outlet change occurred after the Algoma high-water-level phase (ca. 2,400 cal. yrs. B.P.). Preliminary rates of vertical ground movement calculated from the strandplain are much greater than rates calculated from historical and geologic data. High rates of vertical ground movement could have caused tectonism in the Whitefish Bay area, modifying the strandplain during the past 2,400 years. A tectonic event at or near the Sault outlet also may have been a factor in the outlet change from Port Huron/Sarnia to Sault Ste. Marie.

Mesoscale variability of physical and biological fields in southeastern Lake Superior>

A cruise was conducted in southeastern Lake Superior east of Marquette, Michigan from 15 to 20 July 1999. A towed, vertically undulating instrument package containing a conductivity‐temperature‐depth (CTD) profiler, a fluorometer, and an optical plankton counter was used for data collection. Results indicated that the concurrent, high‐resolution survey of both physical and biological variables provided the fundamental basis for illustrating the correlation between mesoscale physical and biological fields and for investigating mesoscale physical and biological processes. It was revealed that the high chlorophyll a biomass beneath the thermocline co‐occurs with up‐tilting of the thermocline. This tilting of the thermocline was associated with the thermally driven coastal current. We hypothesize that the up‐tilting of the thermocline lifted deep water into the euphotic zone, leading to high productivity of phytoplankton beneath the thermocline. The presence of both warm water and food provided a favorable physical and biological environment for zooplankton. The abundance of zooplankton reached 10 x 103 individuals m−3 in the warm surface water and was down to 3 × 103 individuals m−3 at a depth of 40 m.

Structural interpretation of the Midcontinent Rift in eastern Lake Superior from seismic reflection and potential-field studies

Integrated interpretations of potential-field and GLIMPCE and industry seismic reflection data in eastern Lake Superior reveal the structural and stratigraphic complexity of the Midcontinent Rift in this region. Projection of the Keweenaw fault into southeastern Lake Superior suggested by early potential-field studies is confirmed by seismic reflection data. Analysis of seismic data in conjunction with aeromagnetic anomalies and regional gravity data also reveals a continuous section of basalt in the footwall of the Keweenaw fault. The lateral dimensions of this section vary along the strike of the rift from the center of the basin towards the southern flank. Spatially extensive anticlinal and synclinal features, reverse faults and related drag folds imaged by the reflection and enhanced potential-field data attest to the influence of a late-stage compressional event in this region. East-northeast trending gradients and displacements associated with observed potential-field anomalies and fault traces mapped at the surface also indicate a degree of accommodation perpendicular to the strike of the rift. These trends parallel the prevalent tectonic grain in the adjacent Archean basement rocks, perhaps suggesting that structures within the rift were in part controlled by preexisting crustal features.

Deep sediment mixing by burbot(Lota lota), Caribou Island Basin, lake superior, USA

Video sediment‐profile images and cores were collected from a site 300 m deep in southeastern Lake Superior (47° 12.03'N, 86° 04.80'W) using a submersible. These observations revealed additional large linear trenches in bottom sediments made by the burbot Lota lota, first seen in 1985, and facilitated accurate measurement of their dimensions. Trench complexes have a U‐ to V‐shaped profile and are 3 to 5 m long, 15 cm wide for much of their length, and 10 to 25 cm deep. A V‐shaped incision observed in one trench cut as much as 8 to 12 cm below the base of the trench, into stiff light gray clay, and was filled with fluid dark‐gray surface muds. Burbot are thus capable of mixing surface sediments to a depth at least 30 cm below the sediment‐water interface. This datum is the deepest biogenic sediment mixing measured in the Great Lakes. Assuming average sedimentation rates of 0.01 to 0.03 cm/yr in this region of Lake Superior, we estimate that the last 3,000 yr of the postglacial sedimentary record may be loc...

Plant and insect fossils from Nipissing sediments along the Goulais River, southeastern Lake Superior

Fossiliferous sediments have long been known to occur along the banks of the Goulais River. Radiocarbon dating has established their time of deposition as late in the Nipissing transgression. Sedimentary environments and fluvial conditions in the ancestral river channel shaped fossil assemblages. Pollen reflects regional upland forests of Pinus and Picea, with other trees and shrubs such as Quercus, Fraxinus, Acer, Ulmus, and Alnus. Forested peatlands, fens, and emergent and submerged aquatic communities were common in lowland valleys and along the floodplain of the river. These reconstructions are supported by aquatic insect fossils and hygrophilous taxa such as Micropeplus sculptus and Elaphrus olivaceus and also are confirmed by plant macrofossils such as Myrica gale, Triadenum virginicum, Dulichium arundinaceum, Typha, Brasenia schreberi, Potamogeton spirillus, and possibly Zizania palustris. There is no evidence for regional climatic change during this portion of Holocene time.

Observations of Hydra from a Submersible at Two Deepwater Sites in Lake Superior

During a series of submersible dives at depths of 300 m and 400 m in southeastern Lake Superior, Hydra were observed attached to a variety of hard substrates including individual rocks, the edges of large boulders, and the undersurfaces of sandstone ledges. Although widespread, populations were generally found in distinct clumps. Based strictly on morphological features, the species resembled H. littoralis or H. carnea. The finding of Hydra at depths of 400 m in Lake Superior, the deepest sounding in the Great Lakes, would indicate that Hydra are not limited by depth.

Sex and Year Comparisons of Lake Superior Pink Salmon ( Oncorhynchus Gorbuscha) Adipose Fins

The adipose fins of pink salmon ( Oncorhynchus gorbuscha Walbaum) from the Carp River, a southeastern Lake Superior tributary, are sexually dimorphic. The adipose fins of males are consistently longer than those of females for all sizes of fish measured, despite sex and year differences in fish size. Greater relative adipose fin size in males may help reduce caudal peduncle wear resulting from agonistic interactions among competing males on the spawning grounds. Differences in relative adipose fin size between years were not found, consistent with the lack of segregation between even- and odd-year stocks which characterizes Great Lakes populations. Adipose fin size increased in a positive allometric fashion with increasing fish length in both sexes. Compared to British Columbia pink salmon, adipose fin sizes are less distinct sexually but similarly variable. This finding holds promise for use as a technique for sexing older Great Lakes pinks externally, and hence may have management implications.

Sedimentation Rates and Depositional Processes in Lake Superior from 210Pb Geochronology

Sedimentation rates range from 0.01 to 0.32 cm/yr in 17 sediment box cores from Lake Superior, as determined by 210Pb geochronology. Shoreline erosion and resuspension of nearshore sediments causes moderate to high (0.05–0.11 cm/yr) sedimentation rates in the western arm of Lake Superior. Sedimentation rates are very high (> 0.15 cm/yr) in marginal bays adjoining Lake Superior; and moderate to very high (0.07–0.19 cm/yr) in open lake regions adjacent to marginal bays. Resuspension of nearshore and shoal top sediments in southern and southeastern Lake Superior by storms is responsible for depositional anomalies in 210Pb profiles corresponding to 1905, 1916–1918, and 1940 storms. Sedimentation rates are very low (0.01–0.03 cm/yr) in the central basins due to isolation from sediment sources. These data indicate that sedimentation rates and processes vary significantly in different regions of Lake Superior. The sedimentation rates provided by this study, in conjunction with previously-reported sedimentation rates, yield a better understanding of the Lake Superior depositional environment.

Sediment Distribution and Bottom Topography of Southeastern Lake Superior

Sediment distribution in the complex region of troughs and shoals in southeastern Lake Superior was studied by means of echo sounding, coring and surface sediment sampling. The primary factor governing sediment type is water depth with sediment source, water movement, trough morphology and lake history superimposed upon it. Thick accumulations of fine-grained sediment are confined to the trough bottoms whereas coarser sediments occur on the sides and crests. In the southern part of the region subaqueous erosion of red tills leads to the deposition of brown sandy sediments whereas farther offshore dark gray fine-grained sediments show the influence of eroding varved clays. Current action along the south shore and along the trough bottoms produces sediment sorting even in deep water. On slopes greater than one op two degrees, fine-grained postglacial sediment is very thin or absent and lag deposits overlie glacial sediments. Evidence of ice scour indicates that modifications to sediment distribution may have occurred during the low level Houghton stage about 8000 B.P. Stratigraphic evidence suggests that the most probable origin of the complex bottom topography is erosion by moving ice.

The deglaciation history of the Lake Superior region and its climatic implications

A zone of synchronous end moraines has been recognized in the Lake Superior region across northern Ontario and Michigan. The moraines were formed between 11,000 and 10,100 y.a. as cold climate resulted in successive halts in the general ice retreat. The cold climate is also indicated by the presence of tundra near Lake Superior until about 10,000 y.a. This episode is here referred to as the Algonquin Stadial. It was preceded and followed by rapid deglaciation. The Algonquin Stadial is comparable in age with the Younger Dryas Stadial of Europe, and indicates a reversal in the continuous trend toward a warmer climate during Late-Wisconsin (an) time. The apparent conflict between the present result (based on geologic evidence) and earlier pollen stratigraphical studies with no reversal is discussed. Glacial Lake Duluth formed in the western Lake Superior basin before 11,000 BP, followed by a series of Post-Duluth lakes between approximately 11,000 and over 10,100 BP. The Main Lake Algonquin stage in the Huron and Michigan basins terminated approximately 11,000 BP. The subsequent high-level post-Main Algonquin lakes, which were contemporaneous with the Post-Duluth lakes, existed in the southeastern Lake Superior basin. When the ice margin was along the north shore 9500 BP Lake Minong occupied the whole Lake Superior basin. By 9000 BP the ice had retreated north of Lake Superior-Hudson Bay divide.