ORIGINAL PAPER

Early Holocene drought in the Laurentian Great Lakesbasin caused hydrologic closure of Georgian Bay

Francine McCarthy • John McAndrews

Received: 16 December 2008 / Accepted: 29 January 2010 / Published online: 16 February 2010

� Springer Science+Business Media B.V. 2010

Abstract Multiple proxies record aridity in the

northern Great Lakes basin *8,800–8,000 cal

(8,000–7,200) BP when water levels fell below

outlets in the Michigan, Huron and Georgian Bay

basins. Pollen-climate transfer function calculations

on radiocarbon-dated pollen profiles from small

lakes from Minnesota to eastern Ontario show that a

drier climate was sufficient to lower the Great

Lakes, in particular Georgian Bay, to closed basins.

The best modern climate analog for the early

Holocene late Lake Hough stage in the Georgian

Bay basin is Black Bass Lake near Brainerd MN.

Modern annual precipitation at Brainerd is *35%

lower than at Huntsville ON, in the Georgian Bay

catchment; warmer summers and colder, less snowy

winters make Brainerd drier than the Georgian Bay

snow belt. These values parallel transfer function

reconstructions for the early Holocene from pollen

records at five small lakes in the Georgian Bay

drainage basin. Higher evaporation and evapotrans-

piration due to greater seasonality during the early

Holocene produced a deficit in effective moisture in

Georgian Bay that is recorded by the jack/red pine

pollen zone that spanned *8,800–8,200 cal (8,000–

7,500) BP. This deficit drove late Lake Hough *5 m

below Lake Stanley in the Huron basin, following

diversion of Laurentide Ice sheet meltwater from the

Great Lakes basin. The level of Georgian Bay largely

depends not on fluvial input from its own drainage

basin, but rather from Lake Superior, where the

early Holocene moisture deficit was greater. Recon-

struction of paleoclimates in Minnesota, northwest-

ern Ontario and Wisconsin produced a closed lake

in the Superior basin, which removed the main

water input to Georgian Bay. Once the inflow

through the St. Marys River was reduced and inflow

from other tributary streams was adjusted for

isostatic and climatic differences, input was \5%

of modern values. Consequent high evaporation

rates produced a significant fall in lake level in the

Georgian Bay basin and a negative water budget.

This reduction in basin supply, together with the

high conductivity of stagnant water in late Lake

Hough inferred from microfossils in lowstand sed-

iments, peaked at the end of the jack/red pine zone,

*8,300–8,200 (7,450 ± 90) BP. These major

hydrologic changes resulting from climate change

in the recent geologic past draw attention to possible

declines of the Great Lakes under future climates.

Keywords Great lakes � Lake level �Paleohydrology � Paleoclimate � Pollen-climate

transfer functions

F. McCarthy (&)

Brock University, St. Catharines, ON, Canada

e-mail: FMcCarthy@Brocku.ca

J. McAndrews

University of Toronto, Toronto, ON, Canada

123

J Paleolimnol (2012) 47:411–428

DOI 10.1007/s10933-010-9410-z

Introduction

Explaining hydrologic closure of Georgian Bay

The geologic record (seismic, geomorphological,

sedimentological, and paleontological) shows that

water levels in the upper Great Lakes were lower

during the early Holocene (Lewis et al. 2007; Lewis

2008a, b). Following ice retreat, low-level Lake

Hough developed in the Georgian Bay basin (Fig. 1)

when the upper Great Lakes drainage diverted through

the North Bay outlet and glacial meltwater had

bypassed the Great Lakes basin. Short-lived Mattawa

highstands interrupted Lake Hough, creating three

separate lowstands. The last (and possibly the earlier)

Hough lowstands were hydrologically closed, with the

level of late Lake Hough falling *30 m below the

North Bay outlet (Lewis 2008a, b; McCarthy et al.

2010; Fig. 1). These lowstands cannot be explained

by ice damming or isostatic rebound, leaving climate

change as the only plausible explanation.

The close relation between climate and postglacial

Great Lakes levels has been noted by many workers

(e.g. McCarthy and McAndrews 1988; Hartman 1990;

Fraser et al. 1990; Wolin 1996; Pengelly et al. 1997;

Booth et al. 2002). To date, modelling attempts show

that much drier climates than at present are required to

force the Great Lakes into hydrologic closure (Croley

and Lewis 2006; www.glerl.noaa.gov/Programs/glscf/

hydrology.html). However, hydrologic modelling

has yet to incorporate the variable paleogeography,

or higher insolation and windspeeds of the early

Holocene.

The water level in Georgian Bay largely depends

not only on input from its own drainage basin, but also

from Lake Superior where the moisture deficit was

greater, meeting the criteria of Croley and Lewis

(2006) to produce a closed lake in the Superior basin

during the early Holocene. Streamflow in the St. Marys

River at Sault Ste. Marie is 2,140 m3/s, far exceeding

all of the rivers discharging into North Channel and

Georgian Bay (*606 m3/s; http://www.wsc.ec.gc.ca).

We review proxy climate data throughout the Great

Lakes basin and quantitatively reconstruct early

Holocene climates to see if aridity can explain the

closure of late Lake Hough (Sarvis et al. 1999;

McCarthy et al. 2010).

Georgian Bay: hydrology and paleohydrology

Considered together, the North American Great

Lakes (Fig. 2) are one of the largest reservoirs of

fresh water on Earth; they contain more than

23,000 km3 of water, cover 246,000 km2 and drain

766,000 km2 (Shiklomanov 1999). The lakes owe

their origin to the multiple Quaternary glaciations

that scoured their basins from pre-glacial river

Fig. 1 Early Holocene lake level in the Georgian Bay and

main Lake Huron basins (modified from Lewis et al. 2008a).

Except for short Mattawa highstands, lake level fell to or below

the North Bay outlet (NB) following retreat of Laurentide ice,

which produced closed lakes in the basins of the Laurentian

Great Lakes. Thick black lines on the map at right illustrate

probable shorelines of the closed lakes, Late Lake Stanley in

the main basin of Lake Huron, and Late Lake Hough in the

Georgian Bay basin. The lowest lowstand was the late Lake

Hough phase in the Georgian Bay basin (*8,900 to 8,200 cal

BP, highlighted with stipple). Lake level fell *30 m below the

sills in the North Bay outlet, approximately 5 m below the

level of late Lake Stanley in the main basin of Lake Huron

412 J Paleolimnol (2012) 47:411–428

123

valleys. These currently interconnected lakes drain

into the Atlantic Ocean via the St. Lawrence River,

but the modern drainage pattern only developed

during the mid-Holocene, when postglacial isostatic

rebound transferred drainage from the upper Great

Lakes to the lower Great Lakes during the Nipissing

Great Lakes phase (Lewis et al. 2008a). Variations in

the ice front and in the elevation of the land early in

postglacial time produced the complex hydrology of

the early Great Lakes, with its remarkable lake level

fluctuations (Rea et al. 1994; Lewis et al. 2008a).

Georgian Bay (Fig. 3) is the northeastern arm of

Lake Huron, joined by shallow channels north

(*3 m deep, Canadian Hydrographic Service

chart 2200) and south (30–35 m deep, Blasco 2001)

of Manitoulin Island (Fig. 1). The Bay has a maxi-

mum depth of 171 m north of the Bruce Peninsula

(Blasco 2001; Fig. 1). Its drainage basin of

24,185 km2, and that of the North Channel, has

rivers (the Mississagi, Spanish, French, Magnetawan,

and Muskoka) entering from the Canadian Shield of

Precambrian metamorphic bedrock with thin, discon-

tinuous glacial drift cover, the Nottawasaga River

from thicker drift on more easily eroded shale and

shaley limestone of Cambrian-Ordovician age, and

the Severn River draining parts of both bedrock

types. Most input to Georgian Bay is from Lake

Superior via the St. Marys River (discharge at Sault

Ste. Marie *2,140 m3/s- http://www.wsc.ec.gc.ca)

through the North Channel of Lake Huron, between

Manitoulin Island and the north shore (Fig. 3).

Georgian Bay lies within the Great Lakes-St.

Lawrence Forest Region (‘‘L’’) of Rowe (1972), with

the boundary between subregions L.1- Huron-Ontario

and L.4d- Georgian Bay corresponding to the geolog-

ical boundary between the Canadian Shield, with its

thin, patchy soil cover, and the thicker soils overlying

the Lower Paleozoic sedimentary rocks (Fig. 3). This

mixed forest region is dominated by mesic species

(Maycock 1963), including sugar maple, beech, white

and red ash, yellow birch, red maple, basswood and

eastern hemlock (Rowe 1972). The maple-beech-

hemlock pollen zone 3 of McAndrews (1994) reflects

the vegetation and climate of this region prior to

deforestation and the growth of weedy herbs associ-

ated with European settlement (Liu 1990).

Seasonal frequency of the Arctic, Pacific and

Maritime Tropical air masses over the Great Lakes

Fig. 2 The boundaries between biomes (domains) in the

northern United States and Canada (McAndrews and Manville

1987) closely correspond with the mean positions of the three

air masses that Bryson and Hare (1974) linked to North

American climate. The Laurentian Great Lakes lie primarily

within the Great Lakes-St. Lawrence biome, with the northern

shore of Lake Superior in the Continental Boreal biome, and

southern Lake Michigan, all of Lake Erie and western Lake

Ontario in the Deciduous Forest biome. Site locations (1–15)

used to evaluate early Holocene aridity in this study are

clustered around the basins of the Laurentian Great Lakes

J Paleolimnol (2012) 47:411–428 413

123

watershed drives the climate and net water supply

(Fig. 2). The moist Maritime Tropical air mass

dominates the Georgian Bay basin for 6 months per

year, with the dry Arctic and Pacific air masses each

dominant for 3 months (Bryson and Hare 1974). This

makes xeric species, such as jack pine and oak, a

minor component of the vegetation. Lake effect

precipitation (Fig. 4) peaks east of Georgian Bay

from moisture evaporated from open water in Lake

Huron and Georgian Bay by prevailing westerlies.

Much of the precipitation currently falls in the

Georgian Bay drainage basin as snow between

November and April, when mean temperatures

average -4.1�C. Spring snowmelt from this snow

belt region enhances Georgian Bay’s positive water

budget. The western Great Lakes, in contrast, receive

little snowfall, being summer-wet (Fig. 4).

The sensitivity of the Great Lakes to climate

change is evident. Over the last few decades, GCMs

(Global Circulation Models) show how future global

climate change may affect the Great Lakes, notably

the 2XCO2 scenario (Cohen 1986; Sanderson and

Wong 1987; Cohen and Allsopp 1988). Despite

problems with GCMs for regional hydrologic analy-

sis, many results predict a lowering of both lake level

and outflows with projected warming. An important

Fig. 3 The modern hydrologic input to Georgian Bay is

dominated by discharge from the St. Marys River to the North

Channel, measured *2,140 m3/s at Sault Ste. Marie. The

discharge of the smaller rivers in the watershed (boundary

shown using thick dashed line) accounts for an additional

*606 m3/s. This fluvial input, together with precipitation

directly onto Georgian Bay, currently exceeds net losses

through evaporation and outflow from southern Lake Huron at

Port Huron/Sarnia into the St. Clair River. Small lake core sites

in the Georgian Bay drainage basin are indicated by triangles

and nearby climate stations by circles. The pollen diagram for

Edward Lake is in McAndrews and Manville (1987) and that

for Axe Lake is in McCarthy et al. (2007). The subregions of

the Great Lakes- St. Lawrence Forest bordering Georgian Bay

and the North Channel are shown, following Rowe (1972): L.1-

Huron-Ontario; L.4d- Georgian Bay, and L.10-Algoma. The

boundary between vegetation zones L.1 and L.4d (thick blackline) corresponds to the contact between the Canadian Shield

(with its thin, patchy acidic soils overlying crystalline

metamorphic rocks) and the thicker soils overlying the Lower

Paleozoic sedimentary rocks

414 J Paleolimnol (2012) 47:411–428

123

prediction of the 2XCO2 scenario is decreased mean

winter snowfall of 40–50% in the snow belt areas of

Lake Huron and Georgian Bay, *60–70% along the

north shore of Lake Ontario and through the Niagara

Peninsula, and 60–90% in southwestern Ontario

(Crowe 1985). The modelled decline in snowfall is

a key factor that produced rapid declines in water

levels (Wall et al. 1985). They examined the impli-

cations for tourism in Ontario of this predicted

warming and calculated snow cover suitability for

skiing (defined as the percent probability of a day

with snow cover of at least 5 cm, no measurable

liquid precipitation, and a maximum temperature

\4.5�C). Under the GISS scenario, they found

southern Georgian Bay decreased from 70 days of

marginally reliable snow cover today to 0 days and at

Thunder Bay on Lake Superior from 131 days of

snow cover today to 80 days.

Climate impacts Great Lakes hydrology, even

under modern open-basin conditions (McCarthy and

McAndrews 1988; Hartman 1990; Pengelly et al.

1997; Mortsch et al. 2000). Although the climate and

hydrology during the early Holocene differed from

the modelled warmer planet predicted for the coming

century, rapid (centuries-scale) changes occurred in

the volume and quality of water in the Great Lakes.

The dominance of centropyxid thecamoebians

records slightly brackish bottom waters during the

late Lake Hough lowstand implying that reduced

fluvial input and enhanced evaporation increased the

Fig. 4 Small lake core sites (triangles) and nearby climate

stations (circles) range from the prairie/mixed forest in the

west, to the mixed forest/deciduous forest in the east and north

to the boreal forest (Fig. 1). The contours show mean modern

values of climate parameters, from Steinhauser (1979). The

climate varies from summer-wet in the west to low seasonality

in the east. This is explained by the varying dominance of the

three air masses that Bryson and Hare (1974) held responsible

for the central North American climate, with variations in the

strength of westerlies explaining precipitation anomalies

(Booth et al. 2006). The snow belt in the Georgian Bay

drainage basin is lake-effect precipitation, which falls when

Lake Huron/Georgian Bay is ice-free

J Paleolimnol (2012) 47:411–428 415

123

concentrations of potassium and sulfate ions (McCarthy

et al. 2010). In this paper, we investigate how slight

changes in atmospheric circulation and boundary

conditions produced such significant hydrologic

changes in Georgian Bay.

Early Holocene drought in the Great Lakes basin

Various authors document early Holocene relative

aridity throughout mid-latitude North America east of

the Rockies, citing various climate proxies. Low lake

levels record aridity during the pine zone in New

England (Webb et al. 2004). In the Great Lakes

region, stable isotope data from southern Ontario

lakes record low precipitation and effective humidity

until *8,300 cal (7,600) BP (Edwards et al. 1996).

Between 7,940±410 and 6,670±65 (*8,870–

7,500 cal) BP, Willoughby Bog replaced the former

deep Lake Tonawanda, located between Lake Erie

and western Lake Ontario (Sarvis 2000; Neville et al.

2008). In addition, an intensely oxidized layer over-

lying an organic soil in the nearby Crown Site channel

bog south of western Lake Ontario (Tinkler et al.

1992) radiocarbon dated at 7,740±80 (*8,530 cal)

BP supports increasing aridity in the Niagara region

during the early Holocene (Neville 2007). In Hamil-

ton Harbour, microfossils indicate a shallow, moder-

ately alkaline pond fringed by an extensive wetland

until water levels rose *7,800 cal (7,000) BP (Duthie

et al. 1996). Booth et al. (2002) showed that Mud Lake

MI, near the southern shore of Lake Superior, dried

and became a wetland between 8,600 and 6,600 cal

(7,800 and 5,800) BP, and dune activity was prevalent

in the Plains and Prairies (Keen and Shane 1990;

Wolfe et al. 2006).

The most intense early Holocene drought lay south

and west of Lake Superior (Fig. 1), making the Lake

Superior and Lake Huron drainage basins particularly

dry during the early Holocene (Baker et al. 1992).

Pollen data record eastward expansion of oak-

savanna and prairie beginning * 8,300 cal (7,500)

BP (McAndrews 1966; Webb et al. 1983; Baker et al.

1992; Webb et al. 1993a, b; Dean et al. 1996, 2002;

Shuman et al. 2002; Nelson and Hu 2008). Ostracod

fossils from Elk Lake MN also indicate aridity during

the early Holocene (Forester et al. 1987). Varves in

Elk Lake indicate a shift from strongly stratified to

well-mixed water at 8,200 cal (7,500) BP, accompa-

nied by a shift in diatom flora (Dean et al. 2002).

They attribute this to a replacement of boreal forest,

which would normally shield the lake from winds, by

more open prairie savanna that extended at least

100 km eastward of its modern boundary. Dust

deposition (as measured by Al, Si, pollen and varve

thickness) also increased, likely entrained by north-

westerly winds sweeping the dry floor of Lake

Agassiz. A dry episode from 8,900 to 4,500 cal

(8,000–4,000) BP had annual precipitation 100–

200 mm lower than present and lower lake levels.

Like Elk Lake, Lake Ann (Fig. 4) had a dry interval

with strong winds with sparse vegetation, water table

decline, drying of the soil and peak eolian flux at

8,200, 6,600, and 5,600 cal (7,500, 5,800, and 4,900)

BP that initiated formation of dune fields (Keen and

Shane 1990). With dune building, prairie continued to

expand at the expense of trees, and temperature

(especially winter temperature) increased. An Ambro-

sia peak dated *8,200 cal (7,500) BP, accompanied

high sand influx. On cores from Moon Lake ND,

(Fig. 1) Valero-Garces and Laird (1997) examined

seismic/sediment stratigraphy together with pollen,

diatom and isotope analysis and found low moisture

between *10,900 and 7,900 cal (*9,600–7,100) BP,

with maximum dryness *7,900 cal (7,100) BP. An

Ambrosia peak at 8,900 cal BP, followed by a rise in

Iva annua indicates higher temperature, consistent

with diatom populations that indicate shallow and

highly saline water. Similar results in Minnesota

based on ostracod analysis define a high-aridity

‘‘prairie period’’ *8,500–4,500 cal (7,700–4,000)

BP (Schwalb et al. 1995).

Reorganization of atmospheric circulation

increased Holocene aridity during rapid retreat of

the Laurentide ice sheet (Hu et al. 1999; Dean et al.

2002; Shuman et al. 2002). Hu et al. (1999) infer

decreased precipitation and pronounced cooling

between 8,900 and 8,300 cal (8,000 and 7,500) BP

from stable isotope data from Deep Lake MN, which

they attribute to increased outbreaks of polar air. In

addition, prairie expansion began about 1,000 years

earlier in western Minnesota and South Dakota than

in eastern Minnesota, according to pollen profiles

from 26 lakes (Dean et al. 1996). Yu and Wright

(2001) interpreted climate proxies during deglacia-

tion in the Great Lakes region to record dominance

by cold, dry anticyclonic winds with frequent south-

ward incursions of cold, dry Arctic air along the front

of the wasting ice sheet and rapid drainage of lakes

416 J Paleolimnol (2012) 47:411–428

123

Agassiz and Ojibway. Yu (2003) points out that the

major shift *7,500 (8,200 cal) BP from coniferous

forest to mixed coniferous and deciduous forest (the

pollen zone 2/3 boundary) corresponds to a major

shift in climate regimes from deglacial to full

postglacial climates, associated with the collapse of

the Laurentide ice sheet. Collapse of the Laurentide

ice sheet may explain the onset of the cold, dry,

windy 8,200 cal BP event, originally described from

the Greenland Ice Sheet Project II (GISP2) core

(Alley 1997; Barber et al. 1999; Alley and Agusts-

dottir 2005). The possible relationship between early

Holocene aridity in central North America and the 8.2

ka event is beyond the scope of this paper.

Methods

We investigated whether the widely documented early

Holocene aridity could have produced the closure of

late Lake Hough *8,800–8,000 cal (8,000–7,200)

BP, lowering lake level *25 m below the lowest

outlet (the Dalles Sill) and *5 m below the level of

the coeval late Lake Stanley in the main basin of Lake

Huron. We reconstructed paleoclimates primarily

using pollen-climate transfer functions, supplemented

by the identification of the best modern analog to the

drainage basin of late Lake Hough. Where similar

trends are reconstructed in various cores, particularly

using more than one approach, as we have taken here,

paleoclimate estimates can be considered reliable.

This mirrors the approach of Bartlein and Whitlock

(1993) who found that the paleoclimate reconstructions

obtained using three different numerical approaches

(transfer functions, response surfaces and modern

analogs) were very similar, differing only in detail.

One useful measure of the validity of the transfer

function reconstruction is to assess how closely

modern conditions are reconstructed from the pollen

assemblages in core-top samples.

Transfer functions have been a powerful tool in

environmental reconstruction since the pioneering

study on marine foraminifera of Imbrie and Kipp

(1971). Researchers have since developed equations

to reconstruct climatic parameters from downcore

pollen assemblages (Bartlein and Webb 1985; Bart-

lein and Whitlock 1993; Webb et al. 1993a, b;

Whitmore et al. 2005). Like all other paleoenviron-

mental reconstruction techniques, transfer functions

have great potential but also limitations (Birks 1998).

One of the chief limitations is the absence of modern

analogs for some pollen records, particularly for the

late glacial (Overpeck et al. 1992). Jackson et al.

(1997) found that modern analogs for Holocene

pollen assemblages in eastern North America reflect

modern climate gradients and thus the paleoclimatic

reconstructions made here are considered reliable.

We have thus restricted our reconstructions to the

Holocene record, even though most of the cores

extend into the late Wisconsinan. In addition, recon-

structions of mean January temperature tend to be

quite erratic (McAndrews 1994), responding strongly

to minor variations in pollen spectra, so little reliance

was placed on these reconstructions.

Pollen-climate transfer functions were applied to

dated core data from five small lakes in the Georgian

Bay drainage basin and the results were compared with

modern climate stations (Fig. 4). Paleoclimate recon-

structions are based on pollen data from small lakes,

not from the Great Lakes themselves, because in large

basins taphonomic processes enrich sediments in

oxidation-resistant taxa adapted to long-distance

transport by wind and water (McAndrews and Power

1973; McCarthy et al. 2007). In addition, because the

hydrology of Georgian Bay largely depends on input

from Lake Superior via the St. Marys River at Sault

Ste. Marie, we also examined transfer function recon-

structions from four lakes in Minnesota, Wisconsin

and northwestern Ontario (Fig. 4). The North Amer-

ican Pollen Database (Gajewski 2008, http://www.

paleosciencedata.net/pollen/search) provided data

from lakes in the Great Lakes region. We used transfer

function equations of Bartlein and Whitlock (1993) for

the calibration region 45–55�N, 85–105�W to derive

numerical estimates of mean temperatures for July and

January and mean annual precipitation (Table 1).

Downcore reconstructions of annual precipitation

were plotted against calendar years (Fig. 5) using

CANPLOT (Campbell and McAndrews 1992);

radiocarbon dates were calibrated using Oxcal soft-

ware and the Intcal04 calibration curve (Reimer et al.

2004). The more reliable reconstructions are thought

to be those where modern values are closely approxi-

mated by transfer function reconstructions in core-top

samples, although core top samples were not available

for Lake Ann and Hayes Lake.

Fossil pollen percentages can be converted to

biomass (growing stock volume) with corrections

J Paleolimnol (2012) 47:411–428 417

123

using the r-value model of Davis (1963). Biomass

was reconstructed using CANPLOT (Campbell and

McAndrews 1992) from the pollen data from Lake

Minnie MN and Porqui Pond ON (Fig. 6) to illustrate

vegetation changes that produced the pollen profiles

accompanying the early Holocene drought. Down-

core variations in mean July and January temperature

as well as mean annual precipitation reconstructed

from the pollen spectra highlight differences between

the western and eastern portions of the study area.

The vegetation reconstructed using the biomass

feature allowed us to identify a modern analog for the

early Holocene Georgian Bay drainage basin (Fig. 7).

The climatic parameters in modern Brainerd MN

were then used to evaluate the hydrology of late

Lake Hough independently of the transfer function

reconstructions.

The direct comparison of a modern analog

attempts to capture the complexity of early Holocene

climate change, recognizing that biomes have

changed in distribution, composition, and structure

through time. Williams et al. (2004) note that the

late-glacial to early Holocene, 16,000–8,000 cal

(13,000–7,200) BP were times of rapid shifts in plant

taxon distributions, including east–west shifts in

distribution in addition to the well documented

northward redistribution of most taxa. Various

researchers have concluded that major transitions in

pollen profiles that are synchronous across large

continents result from major reorganizations of

atmospheric circulation (Gajewski et al. 2006; Viau

et al. 2002). The nature of large-scale climate change

associated with aridity in the mid-continent is diffi-

cult to reconstruct, as aridity is a complicated product

of synoptic and dynamic factors (see Harrison et al.

2003; Shinker et al. 2006; Booth et al. 2006). It is

well known that evaporation and depletion of soil

moisture are important factors in producing drought,

in addition to reduced precipitation. Although mean

annual precipitation is the only parameter that the

transfer functions of Bartlein and Whitlock (1993)

directly reconstruct, research has shown that soil

moisture deficits resulting from subtle variations in

the number and timing of rainfall events in northern

Table 1 Modern climate data from stations within the study area compared with peak early Holocene drought (*8,200 BP) values

reconstructed from small lake cores using pollen-climate transfer functions

Climate

station-

modern

Modern

precipitation

(cm/yr)

Precipitation

8,200 cal BP

(cm/yr)

Precipitation

8,200 cal BP

compared to

modern (%)

Modern

mean temp.

July/Jan.

(�C)

8,200 cal BP

mean temp.

July Jan. (�C)

Core

sites

Latitude

longitude

Pollen

analyst

Bemidji 63 42 67 19.9/-14.5 21.0–13.8 LakeMinnie

47�150N95�010W

McAndrews

Brainerd 66 50 76 20.3/-14.7 21.3–15.8 Lake

Ann

45�260N93�410W

Shane

Dryden A 69 64 93 19.2/-17.6 19.9–18.0 Hayes

Lake

49�390N93�440W

McAndrews

Baraboo 86 62 72 20.6/-10.1 20.0–12.1 Devil’s

Lake

43�250N89�440W

Maher

Huntsville

WPCP

99 65 66 19.3/210.5 17.3–13.5 FawnLake

45�250N;

79�230WYu

Huntsville

WPCP

99 68 69 19.7/-10.5 19.3–8.8 Found

Lake

45�300N78�300W

Boyko

Huntsville

WPCP

99 69 70 19.7/210.5 19.2–9.0 AxeLake

45�230N79�310W

McAndrews

Huntsville

WPCP

99 70 71 19.7/-10.5 20.3–8.0 Porqui

Pond

44�560N79�470W

McAndrews

Chatsworth 111 90 81 18.8/-7.7 20.0–8.0 Edward

Lake

44�220N80�150W

McAndrews

Cores with core-top transfer function reconstructions closely approximating modern values at nearby climate stations are shown in

bold, as they are thought to be most reliable. Note that the reconstructions for 8,200 cal BP at these sites cluster tightly between 66

and 71% of modern values

418 J Paleolimnol (2012) 47:411–428

123

Ontario has a substantial impact on plant growth

(Laporte et al. 2002).

We illustrate the general trends in early Holocene

atmosphere circulation leading to the development of

the modern situation using the well-known three air

mass model of Bryson and Hare (1974), realizing that

this is only a first and highly generalized approach at

reconstructing the conditions that produced the

hydrologic deficit in the Great Lakes, and Georgian

Bay in particular.

Results

Throughout the Great Lakes region reconstructed

mean annual precipitation values were lower during

the early Holocene than today. The transfer functions

from Lake Minnie and Lake Ann MN, Hayes Lake

ON, Devil’s Lake WI, Axe Lake, Fawn Lake, Found

Lake, Porqui Pond, and Edward Lake ON (Fig. 5,

Table 1), reconstruct minimum mean annual precip-

itation for centuries around 8,200 cal BP, ranging

from 42 cm/yr at Lake Minnie MN to 90 cm/yr at

Edward Lake ON. Paleoprecipitation shows a steep

east–west decline, and compared with nearby climate

stations (Table 1), the departure from the modern at

the peak of the early Holocene drought in the

Huntsville region east of Georgian Bay was as

intense as in north-central Minnesota: precipitation

near Huntsville ranged 66–71% of modern values,

while westward precipitation near Bemidji and Bra-

inerd were 67 and 76% of modern values, respec-

tively. Summer paleotemperatures were roughly

similar to modern values, with July reconstructions

within 10% of modern values at all sites, although

reconstructed temperatures for January range 22%

below to 25% above modern values (Table 1). Mid-

Holocene climate values approximate modern values,

earlier in the Georgian Bay region, *7,000 cal

(6,200) BP, than in Minnesota, where drought

persisted into the mid Holocene.

In the Georgian Bay watershed, peak Holocene

aridity produced the pine zone, which is pollen zone 2

of McAndrews (1994), particularly subzone 2a (the

red/jack pine zone) *9,900–8,200 cal (*8,800–

7,500) BP; a similar vegetational response to early

Holocene aridity was noted in New England lakes

(Webb et al. 2004). Because pine is overrepresented

Fig. 5 Transfer function reconstructions of annual precipita-

tion vs. calendar years at eight sites from west to east in our

study area. Modern measurements of mean annual precipitation

at the closest climate stations are indicated using heavy tickmarks on the horizontal axes (Bemidji and Brainerd, Minnesota

for Lake Minnie and Lake Ann respectively; Dryden, Ontario

for Hayes Lake; Baraboo, Wisconsin for Devil’s Lake;

Huntsville, Ontario for Axe Lake, Fawn Lake, Found Lake

and Porqui Pond; and Chatsworth, Ontario for Edward Lake).

The reconstructions from Hayes Lake and Lake Minnie do not

extend to the top of the age scale because the cores did not

recover the most recent sediments. Where transfer function

reconstructions at the top of the core approximate modern

conditions (e.g. Lake Minnie, Fawn Lake, Axe Lake, and

Porqui Pond), paleoclimatic reconstructions are probably more

reliable. Peak drought conditions (shaded) existed between

8,800 and 8,000 cal BP throughout the study area, and a sharp

increase in annual precipitation to near modern values occurred

between 8,000 and 6,000 cal BP, generally earlier in Ontario

than in the US Midwest

J Paleolimnol (2012) 47:411–428 419

123

in pollen records (Davis 1969), a biomass recon-

struction of the pollen record from Porqui Pond (see

McCarthy et al. 2010) provides better insight into the

early Holocene vegetation of eastern North America.

Poplar, for instance, was a common component of the

Aspen Parkland vegetation in the late Lake Hough

drainage basin (Figs. 6, 7). Poplar pollen is greatly

underrepresented relative to biomass due to its thin

exine (see Fig. 4 in McCarthy et al. 2010), while pine

pollen is highly overrepresented (Cushing 1967;

Havinga 1967; Davis 1969). The increase in organic

matter in Porqui Pond through the Aspen Parkland

zone records the isolation of Porqui Pond and

subsequent decline in lake level during the drought

that culminated in abundant xeric taxa like pine and

wormwood (Artemisia) at the expense of mesic trees

like sugar maple (Acer saccharum) around 8,000 cal

(7,200) BP.

Based on the biomass reconstruction, the closest

modern analog for the pollen assemblage in the

drainage basin of late Lake Hough is Black Bass

Lake near Brainerd MN (Fig. 7), with *25% of the

pollen assemblage jack/red pine, 45–65% white pine

and 24–29% hardwoods characteristic of oak park-

land (oak, poplar, elm and birch). The climate of

Brainerd MN today (Table 1) is more continental

Fig. 6 Downcore biomass (paleovegetation) reconstructed

from the pollen record of Porqui Pond ON (top). The

deposition of gyttja toward the end of the interval characterized

by Aspen Parkland records organic accumulation in a small

lake isolated by declining Georgian Bay water level. Decreased

organic matter records an increase in lake level that accom-

panied succession to the modern maple-beech-hemlock mixed

forest (Great Lakes-St. Lawrence Forest). Marl in an offshore

core taken in *850 cm water depth from Lake Minnie MN

(bottom) contains the pollen assemblage of oak savanna

vegetation between *8,200 and 5,000 years ago. Later

transgression of the shallow portion of the Lake Minnie basin

coincides with the establishment of deciduous forest and

ultimately the modern pine-dominated mixed forest. This

vegetation succession reflects increased mean annual precip-

itation and decreased temperature (and hence decreased

evaporation) reconstructed by the transfer functions (see also

Fig. 5)

420 J Paleolimnol (2012) 47:411–428

123

than that around modern Georgian Bay (Fig. 4), and

it resembles transfer function reconstructions for

*8,800–8,000 cal (8,000–7,200) BP from small

lakes in the Georgian Bay drainage basin, supporting

Brainerd as a modern analog for late Lake Hough.

Comparison between Brainerd and Huntsville or

Chatsworth ON (Table 1) suggests that early Holo-

cene January temperatures were 4–8.5�C colder than

today in the Georgian Bay drainage basin (Fig. 4).

The decrease in annual precipitation of 30–40 cm

was primarily from decreased snowfall linked to

colder air in the watershed and less lake effect

precipitation due to the lower surface area of Lake

Hough (and Lake Stanley) and the longer duration of

ice cover. Summer conditions during the early

Holocene pine zone differed less than the winter

conditions, based on a comparison with Brainerd and

by examining the transfer function reconstructions of

mean July temperature.

Lake Minnie (Fig. 6) represents the more arid west

where oak savanna replaced a pine forest around

8,200 cal (7,500) BP, and a peak in herbs (ragweed,

wormwood, grasses, other herbs) marks the peak

early Holocene drought (cf. the ‘‘Ambrosia peak

zone’’ of McAndrews and Asaduzzaman 2006).

Strong and Hills (2005) also identified parkland

vegetation in their reconstruction for 8,000 years

ago—Aspen Parkland in northwestern Minnesota and

Oak Parkland southward. The transfer function

reconstruction indicates increased aridity associated

with the oak savanna, with mean annual precipitation

between 42 and 55 cm/yr compared with modern

measurements at Bemidji of 63 cm/yr (Table 1),

while reconstructed mean July temperatures were

slightly warmer (21�C, compared with modern mea-

surements of 19.9�C), suggesting more evaporative

summers. When oak savanna prevailed, Lake Minnie

deposited shallow-water marl in what is now the deep

part of the lake (8.5 m) Today, marl accumulates in

shallow water (*1.5 m), thus marl in the deep-water

core records at least a 5 m decline in water level that

accompanied prairie expansion.

Did early Holocene drought cause hydrologic

closure of late Lake Hough?

During the pine zone, both the reconstructions of

annual precipitation and seasonal temperatures using

pollen-climate transfer functions and the selection of

Black Bass Lake MN as the modern analog for small

lakes in the late Lake Hough drainage basin show that

the Georgian Bay region was much drier, with

seasonal extremes. The greater seasonality reflects

the higher summer and lower winter insolation at this

Fig. 7 Summary of abundances of jack/red pine, white pine

and oak parkland pollen (oak, poplar, elm and birch) in pre-

European sediments from Black Bass Lake (46�08 N,

93�42 W), near Brainerd MN. They compare well with pollen

abundances in sediments deposited during the late Lake Hough

phase *8,600–7,800 (7,800–7,000) BP in Porqui Pond. The

relatively high seasonality at Brainerd, which is summer-wet

with cold, dry winters is the best modern analog for the late

Lake Hough drainage basin

J Paleolimnol (2012) 47:411–428 421

123

latitude during the early Holocene. The early Holo-

cene vegetation reconstruction of Strong and Hills

(2005) identifies more arid conditions than today,

particularly in the western Great Lakes basin with the

eastward expansion of the prairie (grassland). The

greater aridity and seasonality reconstructed for the

Lake Hough drainage basins is consistent with our

selection of Brainerd MN as a modern analog

(Fig. 7). The orbital (Milankovitch) parameters

favoured greater summer insolation and less January

insolation in the northern hemisphere during the early

Holocene (Webb et al. 2004). The resulting greater

summer evaporation and reduced snowfall would

decrease effective moisture in the Lake Hough basin.

The transfer functions reconstruct a one- to two-

century peak in aridity in the Georgian Bay drainage

basin around 8,300–8,200 cal (7,450 ± 90) BP,

when thecamoebians record slightly brackish water

(McCarthy et al. 2010). This is just below the pollen

zone 2a/2b boundary, marked by the replacement of

red/jack pine by white pine, which was a response to

the sudden increase in annual precipitation and

January temperature seen in the transfer function

reconstructions (Fig. 5). The combination of warmer

January temperature and higher annual precipitation

perhaps produced increased snowfall in the Georgian

Bay basin beginning *8,200 cal (7,500) BP, more

similar to modern conditions in Chatsworth and

Huntsville ON, rather than in modern Brainerd MN.

The pollen record suggests that peak aridity in central

Minnesota and southern Wisconsin lagged behind that

in Ontario, but persisted longer, from *8,300 (7,500)

cal BP until *5,000 cal (4,400) BP. The most wide-

spread aridity, spanning the entire Great Lakes basin,

thus appears to have peaked *8,300 to 8,200 cal BP,

perhaps part of the widely recognized 8.2 k event

(Alley et al. 1997; Alley and Agustsdottir 2005).

Changes in atmospheric circulation during the

Holocene have been suggested to explain vegeta-

tional and stable isotope records in the Great Lakes

region of North America (Hu et al. 1999; Yu and

Wright 2001, Yu et al. 2002). Continued presence of

a large mass of continental ice maintained the glacial

anticyclonic circulation until after 8000 (8,900 cal)

BP, when the Laurentide ice sheet retreated from the

region, releasing meltwater from glacial lakes Ojib-

way and Agassiz (Fig. 8). The cold, dry anticyclonic

winds with frequent southward incursions of cold, dry

Arctic air along the front of the wasting ice sheet

explain the aridity in the Laurentian Great Lakes,

even allowing the development of boreal parkland

vegetation 9,000 (10,100 cal) BP at several sites

northeast of Georgian Bay according to the compi-

lation of Dyke et al. (2004), and Forest Tundra

further upwind. The local transport of moisture from

glacial Lake Ojibway to the north shore of lake

Superior by these anticyclonic winds is evident in

their map of paleovegetation 8,000 (8,900 cal) year

BP (Fig. 8), which shows the development of mesic

mixed forest equivalent to the Great Lakes-St.

Lawrence Forest of Rowe (1972; Figs. 2 & 3) along

the north shore of Lake Superior. This region is

presently characterised by colder, drier boreal forest

(Continental Boreal in Fig. 2).

Until *8,900 cal (8,000) BP meltwater was the

major input to the Upper Great Lakes (Teller et al.

2002). The cold, dry winters and relatively warm

summers that had persisted through pollen zones 1

and 2a in southern Ontario produced a hydrologic

deficit in Georgian Bay once the meltwater input to

the Great Lakes basin ceased. Peak aridity

*8,200 cal (7,500) BP is evident in all paleovegeta-

tion reconstructions in the Great Lakes region. But

was this deficit sufficient to lower the level of late

Lake Hough *30 m below the level of the North Bay

outlet and 5 m below the level of the coeval late Lake

Stanley in the main basin of Lake Huron? The transfer

functions reconstruct precipitation 20–35% lower

than today in the Georgian Bay drainage basin; the

comparison between Brainerd MN and climate sta-

tions east of Georgian Bay similarly suggests that the

late Lake Hough drainage basin was *35% drier than

that of Georgian Bay (Table 1). The estimate of

paleoprecipitation values only *2/3 of modern values

is consistent with the most reliable transfer function

reconstructions for 8,200 cal (7,500) BP, that is from

cores where modern precipitation values were most

closely approximated by core-top reconstructions (see

Fig. 5), shown in bold in Table 1. These values of

paleoprecipitation cluster tightly between 66 and 71%

of modern values. The estimates of paleopreciptation

derived using both the transfer function reconstruc-

tions and the modern analog approach are lower than

the precipitation values of *75 cm/yr that Schertzer

et al. (1979) suggested are required for a healthy water

budget in Georgian Bay today.

More frequent incursions of the warm/moist

Maritime Tropical air mass of Bryson and Hare

422 J Paleolimnol (2012) 47:411–428

123

(1974) from the Gulf of Mexico beginning

*8,200 cal (7,500) BP ultimately resulted in the

succession to the mesic maple-beech hemlock mixed

forest around 7,200 cal (6,300) BP in the Georgian

Bay drainage basin. This is supported by the increase

in mean annual precipitation and in mean January

temperature following the pine zone, suggesting the

establishment of the modern snow belt east of

Georgian Bay and a positive water budget by

7,000 cal (6,100) BP (Figs. 5, 6; Table 1). Isostatic

rebound of the North Bay outlet contributed to rising

lake levels, ultimately producing the Nipissing Great

Lakes in the Huron-Michigan basin (Lewis et al.

2008a). Peak aridity shifted west of the Great Lakes

basin after *8,000 cal (*7,200) BP, and a modern

climate was established at Hayes Lake northwest of

Lake Superior by 7,800 cal (7,000) BP (Fig. 5).

Transfer functions reconstruct much drier condi-

tions in Minnesota (42 cm/yr at Lake Minnie and

50 cm/yr at Lake Ann), northwestern Ontario (64 cm/

yr at Hayes Lake) and Wisconsin (62 cm/yr at

Devil’s Lake). By comparison with modern

Fig. 8 Anticyclonic winds

(depicted using arrows)

associated with the wasting

Laurentide ice sheet

continued to dominate

atmospheric circulation in

the Georgian Bay basin

during the early Holocene.

The paleovegetation

reconstructions, modified

from Dyke et al. (2004),

illustrate arid conditions

associated with these dry

prevailing winds: the mesic

mixed forest (equivalent to

the Great Lakes-St.

Lawrence Forest of Rowe

1972 in Figs. 2 and 3) was

restricted to southernmost

Georgian Bay until after

8,000 (8,800 cal) BP, and a

small region of Boreal

Parkland northeast of

Georgian Bay records

conditions similar to

modern Brainerd *9,000

(10,000 cal) BP, with even

greater aridity recorded by

Forest Tundra vegetation

further upwind

J Paleolimnol (2012) 47:411–428 423

123

conditions, the east to west aridity increase, seen in

the transfer function reconstructions, probably

reflects greater influence in the west of the Pacific

airmass of Bryson and Hare (1974) that loses most of

its moisture over the west coast mountain ranges.

Booth et al. (2006) confirm that increased frequency

of dry Pacific air carried by strong July westerlies

produce decreased precipitation in a belt from the

Rocky Mountains to the Midwest. This is associated

with decreased transport of moist Maritime Tropical

air up the Mississippi Valley. Also, a dynamic

linkage of Midwest aridity to enhancement of the

summer monsoon in the American Southwest, as

suggested by Harrison et al. (2003), may be reflected

by the longer persistence of drought conditions in

Minnesota and Wisconsin than in Ontario.

A comparison of paleoprecipitation 8,200 cal

(7,500) BP in north-central Minnesota, northwestern

Ontario, and Wisconsin illustrates reduced precipita-

tion relative to today of 67–93% (Table 1; Fig. 5) or

precipitation deficits of 7–33%. Increased evapora-

tion resulting from higher summer insolation is

recorded by the slightly warmer reconstructed tem-

peratures for July (?0.5–1�C) and for January (?1–

2�C) at most of these western sites and by the strong

winds evident in the development of dunes at Lake

Ann MN, peaking *7,400 cal (6,500) year BP (Keen

and Shane 1990). Thus the criteria of Croley and

Lewis (2006) to produce a closed lake in the Superior

basin (drop of 35–40% in mean annual precipitation

and a 1–2�C temperature increase) were likely met

during the early Holocene.

The closure of the Superior basin eliminated the

input via the St Marys River to the Michigan/Huron/

Georgian Bay basin (discharge measured today at

Sault Ste. Marie 2,140 m3/s, Fig. 3). Because of

isostatic depression, the early gradient of the French

River had been to the east (away from Georgian Bay)

and was nearly level at the time of late Lake Hough

(Brooks et al. 2010). Thus much of the modern inflow

from the French River would not have existed at this

time, so in our model we removed all of the modern

inflow from the French River (177 m3/s) to calculate

the paleohydrology of late Lake Hough (Fig. 9). All

Fig. 9 Late Lake Hough

received no inflow from the

St. Marys River due to

hydrologic closure of the

Superior basin. The

isostatically depressed

French River was the outlet

of the Upper Great Lakes

during the Mattawa

highstands, but not when

the level of Lake Hough fell

below the sills in the French

River-North Bay outlet.

Streamflow into late Lake

Hough was reduced by

*35%, consistent with the

paleovegetation

reconstructions. The

resulting drawdown of late

Lake Hough, and increase

in ionic concentration due

to the negative water

balance and possible

groundwater inflow,

produced the brackish

conditions inferred from the

microfossil record of late

Lake Hough (McCarthy

et al. 2010)

424 J Paleolimnol (2012) 47:411–428

123

input from the North Channel was probably cut off

from late Lake Hough during drawdown of Huron

basin water to the late Lake Stanley level. The drier,

more evaporative climate would reduce discharge

from the other rivers flowing into the main Georgian

Bay basin. Today the Magnetawan, Muskoka, Severn

and Nottawasaga, have a combined discharge of

186 m3/s. Reducing this to 70% of modern values

yields 130 m3/s, only *5% of the modern input

(Fig. 9). This large decline (95%) in river input

combined with the drop in precipitation in the

watershed to 66–71% of modern values (Table 1)

caused the water level to fall below the basin outlet

into hydrologic closure and the late Lake Hough

lowstand.

The concentration of ions in late Lake Hough

increased through enhanced evaporation, and possibly

groundwater seepage, allowing the establishment of a

brackish water thecamoebian assemblage 7,450 ± 90

(8,300–8,200 cal) BP (McCarthy et al. 2010). Slight

climatic changes a few centuries later, particularly

north of Lake Superior, where transfer functions from

the Hayes Lake pollen record reconstruct a rapid

increase to near modern mean annual precipitation,

caused lake levels to rise and terminate the late Lake

Hough phase of hydrologic closure. Independent

evidence of increased precipitation and soil moisture

is evident from the reconstruction of mesic mixed

forest north of Lake Superior (Dyke et al. 2004).

Thecamoebian assemblages characteristic of fresh-

water lakes were quickly established throughout the

Georgian Bay basin following the increase in precip-

itation, with just a slight reversal noted during the

hemlock minimum (McCarthy et al. 2010).

Conclusions

Both pollen transfer function reconstructions of mean

annual precipitation, July and January temperature,

and climatic parameters at Brainerd MN, the best

modern analog for the Georgian Bay drainage basin

during the red/jack pine zone (*8,800–7,500 BP;

*9,900–8,200 cal BP), indicate that the early Holo-

cene climate was *35% drier and more evaporative

than today. During the earliest part of the early

Holocene, the presence of continental ice to the north

maintained an anticyclonic atmospheric circulation of

cold, dry winds which brought aridity, drier in the

west, to the northern basins of the Laurentian Great

Lakes. The more arid conditions reconstructed for the

early Holocene western Great Lakes region were

sufficient to hydrologically close the lake in Superior

basin in the absence of meltwater inflow, thereby

cutting off the dominant input to the Michigan-Huron

basin. Diversion of meltwater from the Great Lakes

*8,900 cal (8,000) BP, combined with high summer

insolation and the loss of inflow from Superior basin,

produced a negative water budget that forced late

Lake Hough into hydrologic closure (water level

below basin outlet). Recession of the ice sheet and its

anticyclonic circulation led to more frequent incur-

sions of warm and moist air from the Gulf of Mexico

beginning about 8,200 cal (7,500) BP. Precipitation

from this moist air mass led to increased water supply

and rising lakes in the Great Lakes basins which

terminated the phase of hydrologic closure and ended

the late Lake Hough lowstand in the Georgian Bay

basin.

Acknowledgments We thank M. Lozon for drafting, and S.

Blasco and C.F.M. Lewis whose interest in Great Lakes water

levels stimulated this research. The comments and suggestions

of two anonymous reviewers and of the editor, C.F.M. Lewis,

resulted in substantial improvements to our coverage of the

pertinent literature and to the discussion of our paleoclimatic/

hydrologic model. This study was supported in part by NSERC

funds granted to F. McCarthy.

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