John H. McAndrews

Middle Holocene dry climate caused by change in atmospheric circulation patterns: Evidence from lake levels and stable isotopes

Sediment hiatuses, detritus layers, and inwashed terrestrial moss layers in five cores at Crawford Lake, Ontario, Canada, show that the lake level was low between ca. 4.8 and 2 ka (1 ka = 1000 14 C yr B.P.). This low level is attributed to a dry and warm climate, which has also been documented at other sites in southern Ontario, southern Michigan, and southern Wisconsin. Oxygen isotope (δ 18 O) values from authigenic marl show a negative shift of 2.4‰ from −9‰ to −11.4‰ (Vienna Peedee belemnite) between ca. 5 and 2 ka. Enhanced evaporation under dry and likely closed-basin conditions would lead to more enrichment in 18 O, so we suggest that the trend to depleted 18 O indicates a significant change in the δ 18 O value of the source meteoric water. The major moisture source for the Great Lakes region is the Gulf of Mexico, from which the amount and seasonality of precipitation are affected by the interplay of air masses from the Gulf of Mexico and North Pacific, probably controlled by jet stream positions and storm tracks. In the late middle Holocene, the isotopically heavy moisture from the gulf might have contributed less precipitation and/or a higher proportion in winter months, probably caused by more frequent eastward extension of dry Pacific air depleted in 18 O. This hypothesis implies that the δ 18 O values of paleoprecipitation in the middle Holocene reflected moisture-source history more strongly than paleotemperature.

Human disturbance of North American forests and grasslands: The fossil pollen record

The population of North America, now about 360 million, has grown from 30 million in the 15th century immediately prior to European contact (Driver, 1969). During this population expansion, much of the natural vegetation was destroyed or modified. Mature forests were logged and replaced by early-successional species, farmland and settlements. Grasslands were destroyed by plowing or modified by cattle grazing (Curtis, 1956). Weeds, both native and exotic, flourished. Cultivated plants, both native and introduced, became increasingly important as food, fiber and timber and for their aesthetic value. Introduced pathogenic fungi also decimated native Castanea and Ulmus populations.

A Comparison of Postglacial Arcellacean (Thecamoebian) and Pollen Succession in Atlantic Canada, Illustrating the Potential of Arcellaceans for Paleoclimatic Reconstruction

Cores dating back to deglaciation were taken from three lakes in Atlantic Canada and analyzed for arcellaceans and pollen. Paleotemperatures and paleo-precipitation were calculated from the pollen data using transfer functions. A sudden warming is recorded by the pollen around 10,000 years B.P., followed by a general warming to the mid Holocene Hypsithermal, then by a decrease in temperature and increase in effective precipitation to the present. The three lakes, two in western Newfoundland and one in eastern Nova Scotia, contain similar late glacial (13-10 ka), early Holocene (10-8 ka), mid Holocene (8-4 ka), and late Holocene (4-0 ka) arcellacean assemblages. Immediately following retreat of the ice sheets, Centropyxis aculeata, Centropyxis constricta, Difflugia oblonga, Difflugia urceolata, and Difflugia corona were common. The latter part of the late glacial is characterized by sparse assemblages dominated by C. aculeata. The arcellacean record thus suggests a climatic reversal in Atlantic Canada between 11,500 and 10,000 years B.P., analogous to the Younger Dryas, although this is not recorded by the pollen. Species diversity increased sharply at the beginning of the Holocene, and D. oblonga is the dominant taxon in early Holocene sediments. Difflugia oblonga remained common through the mid Holocene, but percentages of C. aculeata were very low, and Pontigulasia compressa and Difflugia bacillifera peaked in abundance during the Hypsithermal. The late Holocene is characterized by a resurgence in C. aculeata at the expense of other taxa. The increase in Heleopera sphagni and Nebella collaris since 5,000 years B.P. at the two sites in southwestern Newfoundland reflects paludification in response to increased precipitation since the Hypsithermal. Because the changes in arcellacean assemblages are regionally synchronous in all three lakes and coincide with climatically driven vegetational successions indicated by the pollen record, arcellaceans appear to respond to climatic change, and thus may be useful paleoecological and paleolimnological indicators. With their quicker generation time, these protists may be better suited than pollen to recording short-lived phenomena, like the mid-Holocene Hypsithermal and the Younger Dryas reversal.

Prehistorical record of cultural eutrophication from Crawford Lake, Canada

Cultural eutrophication—the process by which human activities increase nutrient input rates to aquatic ecosystems and thereby cause undesirable changes in surface-water quality—is generally thought to have begun with the start of the industrial era. The prehistoric dimension of human impacts on aquatic ecosystems remains relatively undescribed, particularly in North America. Here we present fossil plankton data (diatoms and rotifers), organic and inorganic carbon accumulations, and carbon isotope ratios from a 1000 yr sediment core record from Crawford Lake, Ontario, Canada. The data document increased nutrient input to Crawford Lake caused by Iroquoian horticultural activity from A.D. 1268 to 1486 and show how this increased nutrient input elevated lake productivity, caused bottom-water anoxia, and irreversibly altered diatom community structure within just a few years. Iroquoian settlement in the region declined in the fifteenth century, yet diatom communities and lake circulation never recovered to the predisturbance state. A second phase of cultural eutrophication starting in A.D. 1867, initiated by Canadian agricultural disturbance, increased lake productivity but had comparatively less impact on diatom assemblages and carbon-storage pathways than the initial Iroquoian disturbance. This study deepens our understanding of the impact of cultural eutrophication on lake systems, highlights the lasting influence of initial environmental perturbation, and contributes to the debate on the ecological impacts of density and agricultural practices of native North American inhabitants.

Stable isotope, fossil Coleoptera and pollen stratigraphy in late Quaternary sediments from Ontario and New York State

Stable oxygen isotope contents of marl deposits and molluscs from lakes and ponds in northern New York State (U.S.A.) and southwestern Ontario (Canada) are remarkably uniform for carbonates formed between about 12,600 yr B.P. and 2000 yr B.P.. This suggests relatively stable climatic conditions with slowly increasing temperatures throughout this time span, an observation already indicated by fossil beetle data; and re-confirmed by additional data presented in this paper. The variations seen in pollen diagrams probably are related to plant colonization, and climatic parameters such as changing humidities. This contrasts sharply with observations made in Europe where variations in pollen diagrams are paralleled by changing 18O contents in lacustrine marls and molluscs which clearly reflect climatic/temperature changes during late glacial and early postglacial times (13,500–9000 yr B.P.). 13C contents in marls and molluscs are not directly dependent on climatic change but reflect changing environments with varying aquatic and terrestrial carbon contributions.

Pollen analysis of the 1973 ice core from Devon Island glacier, Canada

Abstract Meltwater from a 299-m-long ice core was filtered and analyzed for fossil pollen and spores. Pollen concentration was higher in the late Holocene and interglacial intervals (ca. 7 liter −1 ) than in the early Holocene and Wisconsinan (ca. 1–2 liter −1 ) ones. The late Holocene and interglacial assemblages were dominated by Alnus (alder), whereas the early Holocene and Wisconsinan ones were dominated by Betula (birch) and Artemisia (sage). During the Holocene and probably the last interglaciation, most of the pollen and spores were blown a minimum of 1000 km from low arctic shrub tundra and adjacent subarctic Picea (spruce) forest; these areas were dominated by the arctic air mass during the summer pollinating season. During the Wisconsinan-early Holocene, glacier ice and arctic air were more widespread and pollen sources were more distant; thus, at this time relatively little pollen was incorporated into the ice. The Devon ice-core data suggest that there should have been pollen in the continental ice sheet of Wisconsin time. When the ice sheet retreated this pollen would be carried by meltwater and redeposited with silt and clay together with contemporary pollen, producing an ecologically anomalous assemblage.

Holocene water levels at Rice Lake, Ontario, Canada: sediment, pollen and plant-macrofossil evidence

Four cores taken along a transect from the western basin and four cores from the middle basin of Rice Lake, Ontario, provide evidence for shoreline transgression during the early Holocene, for low water levels during the mid-Holocene, and for abrupt rise of the lake levels due to dam building in AD 1838. The transition from detritus mud to the overlying marl, spanning from ca. 10000 to 8600 BP, indicates flooding of a wetland by a lake; this flooding is supported by plant-macrofossil succession from Larix, Scirpus, and Carex to Najas flexilis. The transgression was due to isostatic tilt after deglaciation, which raised the eastern outlet sill of the lake and caused the lake water to rise and flood westward. A sediment hiatus between the marl and the overlying gyttja (between 6000 and 4000-3000 BP) across the lake basin, supported by the bracketing radiocarbon dates and missing regional pollen zones, indicates low water level caused by a dry/warm climate. Regional palaeoclimatic estimates from pollen-climate transfer functions indicate that the mid-Holocene mean July temperatures were about 1°C higher and annual precipitations about 10% less than before or after. Subsequent rise of the lake level after the hiatus was a combination of cooling climate and continued isostatic tilt.

Dry Climate Disconnected the Laurentian Great Lakes

Recent studies have produced a new understanding of the hydrological history of North Americas Great Lakes, showing that water levels fell several meters below lake basin outlets during an early postglacial dry climate in the Holocene (younger than 10,000 radiocarbon years, or about 11,500 calibrated or calendar years before present (B.P.)). Water levels in the Huron basin, for example, fell more than 20 meters below the basin overflow outlet between about 7900 and 7500 radiocarbon (about 8770–8290 calibrated) years B.P. Outlet rivers, including the Niagara River, presently falling 99 meters from Lake Erie to Lake Ontario (and hence Niagara Falls), ran dry. This newly recognized phase of low lake levels in a dry climate provides a case study for evaluating the sensitivity of the Great Lakes to current and future climate change.

Glacial Lake Levels and Eastern Great Lakes Palaeo-Indians

This article investigates changing lake levels in the late Pleistocene eastern Great Lakes inorder to gain insights into the Early Palaeo-Indian occupations. Significant new informationbearing on lake level history is provided, notably the first well-documented deposits of a highwater level above modern in the ca. 11,000–10,300 B.P. period in the southern Lake Huronbasin. The lake level information, along with paleoenvironmental and site data, reinforcessite age estimates to the 11th millennium B.P.; suggests significant numbers of sites have beeninundated by rising water levels; provides specific informationonthe settingofarchaeologicalsites such as placing the Parkhill site adjacent to a large lake estuary; indicates reasons forthe attractiveness of shorelines to Palaeo-Indians including persistence of more open areasconducive to higher game productivity; and points to ideal areas for future archaeologicalsite survey, particularly in the Lake Erie drainage. 2000 John Wiley & Sons, Inc.

Dinoflagellate cysts from post-glacial lake muds, Minnesota (U.S.A.)

Abstract Dinoflagellate cysts have been recovered in abundance from latest post-Glacial muds from Glatsch Lake, Minnesota. These cysts are not recycled and were probably produced by motile dinoflagellates living in the lake waters. Four cyst types are described which are characterized by unique archeopyle types and which probably have affinities with species of Peridinium . Separate systems of classification and nomenclature for cysts and for thecas are advocated. Developmental stages of one cyst type preserved in the muds are described. These stages are characterised by the lack of an inner wall and by the granular contents of the cell, and probably represent an arrested stage of development. Maximum frequency of these arrested cyst stages is correlated with abundance of another cyst type which, however, is fully and perfectly developed. The cysts achieved maximum abundance about 300 years B.P. as estimated from vegetational successions indicated by pollen diagrams. This abundance of cysts is apparently not related to climatic change but may be the result of local environmental change. All three cyst types resemble in gross structure many Mesozoic and Cenozoic dinoflagellates usually interpreted as marine. This casts doubt on the correctness of some of the environmental interpretations of the fossil types. The archeopyle may prove to be a feature on which more refined paleoecologic interpretations can be based.