During the Last Glacial Maximum when much of the world’s water became locked in glacial ice sheets, the climate in southeastern North America was much drier. Small rivers dried up and larger rivers shrank in size and became braided in pattern so that they were like long chains of disconnected channels. The exposed river sands blew into eolian dunes like those from this picture of a modern Mojave desert dune. Ironically, during these cold stadials, as the climate became dry in southeastern North America, precipitation increased in the southwest, so that it was quite lush there.
Dry climate phases have periodically struck southeastern North America many times over the past 5 million years. Scientists know little about the paleoecological details of most of these phases because there’s not much available data. But thanks to one study, they do have a relatively clear understanding of the ecological composition of south central Georgia from 30,000 years BP-25,000 years BP.
Scientists took a 17 foot core of sediment from a peat deposit next to Sandy Run Creek on Warner Robins Air Force Base which is located near Macon, Georgia. They radio-carbon dated the sample and counted the pollen grains to determine what the environment was like during this time period. This was a time of glacial expansion and as much of the earth’s water became locked in ice, less moisture in the atmosphere was available as precipitation. Southeastern North America experienced extended droughts causing the water table to drop. Water flow on major rivers was greatly reduced creating braided river patterns and this turned the rivers into chains of disconnected channels interspersed with islands and sandbars. In some cases small rivers completely dried up, exposing great quantities of river borne sand. Atmospheric conditions caused by the glacier to the north spawned frequent westerly and southwesterly winds that blew this river borne sand across the landscape forming huge eolian sand dunes.
Today, vegetation has taken root and holds down the eolian sand dunes that formerly rolled across Georgia’s landscape during cold arid stadials. Now, scrub turkey oak and longleaf pine covers the Ohoopee sand dune in Georgia. The sandy soil is of poor quality and not enough litter accumulates to foster fire, allowing scrub oak to become more common than pine. Photo is from google images.
Photo from google images of the Ohoopee River. This and other small rivers dried out during cold dry climate phases. Instead, small pools of water appeared sporadically in the river bed. The scene would have resembled modern African water holes.
The evidence from the pollen composition of the Sandy Run Creek peat core indicates a much different landscape than occurs anywhere in Georgia today. While eolian sand dunes rolled to the east of reduced or even completely dried rivers, lightly wooded grasslands predominated over much of the environment. Here and there were groves of pine with some spruce. (The species of pine isn’t known, but my educated guess is they were a mixture of northern and southern species, probably shortleaf and white. The species of spruce was likely the extinct Critchfield’s.) Oaks and other deciduous trees clung to the vicinity of shrinking water holes found along the braided rivers. Pine composed 39%-75% of the pollen, while oak only made up 12%. Grass and coniferous trees require less water than hardwoods, and are less prone to physical damage from frequent wind, explaining why they were more abundant. Scientists found no charcoal in the part of the core dating from 30,000 BP-25,500 BP–evidence wild fires were a rarity. This suggests a thinly vegetated environment where combustible material such as dead wood didn’t accumulate. Moreover, lightning storms that ignite fires were uncommon. Charcoal is present in the part of the core dating from 25,500 BP- 25,000 BP, perhaps indicating a weak interstadial with more frequent electrical storms.
Grass-eaters such as mammoths, horses, and bison likely predominated in this kind of environment along with the occasional Harlan’s ground sloth which preferred more open environments than its cousin–Jefferson’s ground sloth. Badgers, thirteen-lined ground squirrels, and perhaps jackrabbits colonized the region then. Animals that prefered more forested environments were restricted to riverine woods. Game accumulated around shrinking water holes and this probably contributed to erosion of riverbanks which in turn added sediment to the formation of eolian dunes. These congregations of herbivores attracted predators such as dire wolves, jaguars, and saber-tooths.
You can buy this illustration of flat-headed peccaries (Platygonus compressus) from the website engraved on the image. This kind 0f peccary was probably pretty common during dry climate phases in the south. They ate tough spiny vegetation such as cactuses. They lived in large herds that were probably aggressively defensive, much like modern white-lipped peccaries.
Photo of a jaguar in Arizona. Jaguars inhabit many different types of environments such as deserts and rain forests. They were adaptable enough to probably have been the most common large cat in southeastern North America during stadials and interstadials.
Photo of a hog-nosed skunk. Today, this species lives in Mexico and the southwestern United States. Fossils of hog-nosed skunks have been found in Georgia and Florida. They must have colonized the region during dry climatic phases.
There’s no sediment in the Sandy Run Creek peat core dating from 25,000 BP-13,000 BP. Scientists call this an erosional unconformity. They believe the creek changed coarse or flooded and washed away all the sediment accumulated during this time period. This is consistent with what we know of the environmental changes that occurred during this time period. About 16,000 years BP the Boling-Alerod interstadial began. The Laurentide Glacier rapidly commenced melting, putting more moisture in the atmosphere and precipitation increased. The water table rose and so did river flow. Rivers no longer consisted of braided patterns, but instead meandered to an even greater degree than they do today, forming scroll-like sandbars.
Satellite view of a meandering river with scroll bars. During the Boling-Alerod Interstadial beginning about 16,000 years ago, precipitation increased, causing rivers to meander even more than they do today. This new river pattern formed frequent scroll bars. a kind of sand bar created when the meander of a river continously shifts and leaves ridges parallel to the meander.
Though sand dunes no longer rolled across the landscape during the interstadial, there’s evidence of considerable sandbar formation. Scroll bar formation suggests a dry season/wet season climate. Autumn and early winter were mostly dry and river levels fell, exposing sand bars. But existing atmospheric factors caused heavy precipitation in late winter, spring, and early summer. Warm tropical fronts collided with cold parabolic winds originating from the still extant glacier. This spawned great snow, ice, and rain storms that caused massive floods. Rivers shifted. Occasional tropical storms compounded this trend.
The pollen record of the Sandy Run Creek peat core, which picks up again about 13,000 BP, demonstrates a much different environment from that of 25,000 BP. From 13,000 BP-11,000 BP there was a sudden cooling trend known as the Younger Dryas. The paleobotanical evidence, however, still shows the influence of the previous interstadial warming trend. A cool, moist, open oak woodland prevailed in south central Georgia during this time period. Oak pollen doubled from 12% to 24% while pine pollen declined to just 7%. Critchfield’s spruce and fir were still present but so were hickory and beech–a clue that temperatures were moderate but remained cooler than those of today. An increase in charcoal is evidence that vegetation was thicker than it had been in the previous time period because now there was more forest litter available as tinder for fires. And the frequency of lightning storms, which ignites fires, increased. A northern species of alder, a type of shrubby birch that no longer occurs in Georgia, commonly grew in abandoned, dry, river meanders. Grasslands still existed to a greater extent than occurs naturally today but had declined in abundance compared with 25,000 years BP.
Scientists use an interesting method to help determine changes in the density of vegetation over time. They add exotic pollen to cores of sediment. In this study of the Sandy Run Creek sediment core, they added a known quantity of eucalyptus–a species which didn’t live in North America during the Pleistocene. The ratio of eucalyptus to native pollen was high during the time period of 30,000 years BP-25,000 years BP–evidence vegetation density was low. Conversely, the ratio of introduced eucalyptus pollen was low during the time period between 13,000 years BP-11,000 years BP–evidence the vegetation density was higher.
The change to a more moist wooded environment favored higher populations of mastodon, deer, long-nosed peccary, bears, beavers, tree squirrels, and cottontails.
References:
Lamoreaux, Heidi; George Brook, and John Knox
“Late Pleistocene and Holocene Environments of the Southeastern U.S. from the Stratigraphy and Pollen Content of a Peat Deposit on the Georgia Coastal Plain”
Paleogeography, Paleolimnology, and Paleoecology 2009
Leigh, David
“Late Quaternary Climates and River Channels of the Atlantic Coastal Plain, Southeastern USA”
Geomorphology 2008
GeorgiaBeforePeople 