Posts Tagged ‘Eolian Sand Dunes’

Inner Coastal Plain Deserts of the Ice Ages

October 4, 2017

A new study reinforces evidence, indicating some regions of southeastern North America were harsh environments during climatic phases when the ice sheets that covered Canada were expanding.  The scientists who wrote this paper took cores of sediment from 2 Carolina Bays (Jones and Singletary Lakes) located in Bladen County, North Carolina. Carolina Bays are elliptical depressions found on the Atlantic Coastal Plain that were formed during Ice Ages.  They were created by a combination of peat fires, and wind and water erosion.  The peat fires lowered the elevation, wind blew out the dried unconsolidated sediment, and wind-driven water shaped them into elliptical formations.  Jones and Singletary Lakes were also studied in the early 1950s in 1 of the first paleoecological studies of late Pleistocene environments of the south.  The new study analyzed pollen composition, charcoal abundance, and biomass; and the authors compared their results to the earlier study.  The data was dated using radio-carbon dating.

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Location of Bladen County, North Carolina.  This is the site of the study areas discussed in this blog entry.

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Photo of Singletary Lake, a Carolina Bay.  Scientists took a sediment core at the bottom of this lake and analyzed pollen, charcoal, and biomass abundance over the past 50,000 years.

Between ~60,000 years BP-~30,000 years BP climate fluctuated drastically between warm wet interstadials and cold arid stadials.  The glaciers covering Canada advanced then retreated then advanced again in fits and starts.  During glacial expansion more of earth’s atmospheric moisture became locked in glacial ice, causing prolonged droughts, but this moisture was released when glaciers were in a meltwater phase.  Oak and grass pollen increased during meltwater phases, and so did charcoal abundance.  An increase in vegetation meant there was more biomass to ignite and burn during electrical storms.  Oak and grass were fairly abundant from ~43,000 years BP-~32,000 years BP.  The environment mostly consisted of woodland and grassland during interstadials,  but about 30,000 years BP the situation deteriorated.

Ice sheets maintained a steady expansion from ~30,000 years BP-~21,000 years BP.  The initial drought that struck the region during this phase killed vegetation and caused a temporary spike of charcoal because the dead biomass was so flammable.  But after this initial spike, fire was rare to non-existent here.  Sand dunes rolled across the landscape because much of the region was sparsely vegetated.  I believe scrub oak thickets with thorny plants adapted to arid climates covered much of the landscape, but this type of environment doesn’t produce much pollen.  Thus, the amount of vegetation on the landscape then is understated in the pollen record.  For this reason I don’t believe the landscape was as bare as the authors of this study concluded when they wrote it was a “windswept sandy desert with riparian communities of pine and oak.”  Nevertheless, it was an harsh environment of thorny thickets interspersed with areas of bare soil and long distances between water and wetland environments where some trees and grass still grew.  Some tough species of mammals that could survive in this type of environment included horse, flat-headed peccary, helmeted musk-ox, and hog-nosed skunk.  Bison evolved into a smaller species more capable of living in a drier natural community. Overall, wildlife populations probably declined during this climatic phase.

About 21,000 years ago, the ice sheets began retreating and precipitation increased.  Oak and grass gradually increased in abundance, and eventually mesic species such as cypress, basswood, hemlock, and beech invaded the resulting wetter habitats.  ~12,000 years ago, man colonized the region and overhunted megafauna into extinction.  Human-set fires combined with an increase in biomass not being consumed by megaherbivores caused a great increase in fire frequency.

I’m skeptical of 1 claim made by this paper.  The authors estimated the average annual temperature and precipitation levels based on plant composition assumed from the pollen record.  During the Last Glacial Maximum they estimated the average January temperature at these sites was 20 degrees F, while the average July temperature was 68 degrees F.  However, they use 2 dubious assumptions.  They believe the pollen grains from northern species of pine can be distinguished from those of shortleaf pine, a southern species.  This is a doubtful assumption that I will examine more thoroughly in my next blog entry.  Moreover, the spruce pollen probably originated from an extinct species of temperate tree known as Critchfield’s spruce.  I don’t think they can estimate average annual temperatures based on pollen composition, unless the exact species are known with more certainty.

The outer coastal plain and the continental shelf, which was above sea level from ~80,000 years BP-~7,000 years BP, likely hosted richer environments than the inner coastal plain during stadials.  Sea breezes and weather fronts spawned in the Atlantic Ocean brought more moisture to the coast, allowing this region to maintain a mosaic of woodland, grassland, and wetland; while the inner coastal plain suffered greater aridity.  These fronts usually dissipated before they reached the inner coastal plain.  The coastal region likely served as a refuge for plants and animals that later re-colonized the inner coastal plain when climatic conditions improved.

Reference:

Spencer, Jessica; et. al.

“Late Quaternary Records of Vegetation and Fire in Southeastern North Carolina from Jones Lake and Singletary Lake”

Quaternary Science Review 174 October 2017

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Sand Dunes Rolling Across the South

May 25, 2017

Ice Age environmental conditions influenced the present day landscapes of many geographical localities, including the Carolina Sandhills.  A recent study determined Ice Age winds shaped the topography of this region.  The sand originated from loose sediment and eroded sandstone within a Cretaceous-age formation located near the surface.  Arid climatic conditions caused by glacial expansion exposed part of this formation.  Frequent droughts reduced vegetative cover, so there were bare patches of soil without tree and grass roots holding the sand in place.  I think overgrazing by megafauna during droughts played a role in denuding the landscape as well.  Cold winds blew the exposed sand into massive eolian dunes that rolled across the land.

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The Sand Hill Region is encircled in red on this map.  At least parts of it originated during Ice Ages when cold winds blew exposed sand across the landscape here.

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Vegetation stabilizes sand dunes in the region today.  Local sand dunes have been inactive for at least 6,000 years.  They were most active during the coldest driest phases of Ice Ages.

The authors of the below referenced study took core samples of sand hill sediment in Chesterfield County, South Carolina.  They found an unconsolidated layer of sand measuring from ~1 foot to about 30 feet deep.  Optically stimulated luminescence dates (See: http://www.usu.edu/geo/luminlab/whatis.html ) suggest dunes were active from 75,000 years BP-37,000 years BP when glaciers were expanding and from 28,000 years BP-18,000 years BP during the Last Glacial Maximum and again from 12,000 years BP-6,000 years BP.  The beginning of the final period of dune activity corresponds with the Younger Dryas cold reversal when average global temperatures suddenly plummeted following a warm climate cycle.  Scientists found no evidence of dune activity between 37,000 years BP-28,000 years BP and 18,000 years BP-12,000 years BP.  These dates correspond with warmer wetter interstadials when plant growth stabilized sand sheets and dunes, holding them in place.  There has been no dune activity in the Carolina Sandhills for at least 6,000 years because higher precipitation levels foster thicker vegetative growth.

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The Great Kobuk sand dune in Alaska.  During the Ice Age landscapes in the sand hill region may have resembled this, though short leaf pines, grass, and scrub oak as well as spruce were the characteristic vegetation between the dunes instead of just spruce.

Sand dunes probably advanced the most on cold windy Ice Age nights.  Experiments show air temperature is a factor in sand particle transport.  Decreasing temperatures increase air density and lower water vapor thus reducing drag on the sand particles.  Larger particles can get picked up and transported by wind more easily when temperatures drop.  The region may have experienced brutally low temperatures compared to present day averages.

Sand dune origin in the Carolina Sandhill region differed a little from sand dune formation in Georgia.  The source of sand in the Carolina Sandhills is a geological formation near the surface, but many dunes in Georgia originated from dry river beds where sand was exposed because water tables dropped.  (See: https://markgelbart.wordpress.com/2012/04/09/the-ohoopee-sand-dunes/ ).

Pollen evidence indicates the dominant flora growing between the sand dunes during the Ice Age consisted of pine, spruce, scrub oak, grass, and asters (sunflowers, daisies, etc.).  It was probably an open woodland type of environment, though scrub oaks may have formed denser thickets.  I believe shortleaf pine (Pinus echinata) and possibly Virginia pine (P. virginiana) were the dominant species of pine here then.  It was too cold and windy for longleaf pine (P. palustris), commonly found here today.  I’m no longer convinced jack pine (P. banksiana) occurred this far south.  Some pollen studies list jack pine as a species present in this region during the Ice Age.  However:

1)The present day range of jack pine is too far away from the Carolina Sandhills. It seems unlikely its range would have retracted so drastically without leaving a relic population anywhere in the south.

2)  There are no definitive macrofossils of jack pine in the south.

3) Evidence jack pine was present in the south is based on identification of pine pollen, but 1 researcher makes the compelling case that jack pine pollen is indistinguishable from shortleaf pine pollen.  (See: https://www.osti.gov/scitech/biblio/564104 ).  Jack pine pollen is distinguished from other species of pine pollen based on the size of its grains but they overlap in size with shorleaf pine pollen grains.  Because shortleaf pine occurs in the region today, it seems more likely pollen evidence represents this species, not jack pine.

The species of spruce tree present in the Carolina Sandhills during Ice Ages was probably the extinct Critchfield’s spruce.  Macrofossils of this species have been found associated with temperate species of hardwoods.  The average annual temperatures of the Carolina Sandhills during Ice Ages was probably similar to present day southern Ohio or Kentucky, not like southern Canada as some researchers estimate based on their mistaken assumptions of tree species composition.  The Carolina Sandhills are located in what was a sharp transition zone of climate during the Ice Age.  A thermal refuge existed to the southeast where warm waters off the Atlantic coast pooled because thermohaline circulation (the tropically heated water that presently flows off the coast of New England) shut down.  But temperatures sharply dropped to the north and west of this thermal refuge, not unlike conditions experienced by mountain climbers rapidly ascending a mountain.

The Carolina Sandhills likely supported significant populations of megafauna, despite the vast expanses of bare sand.  Open grassy land fed mammoths, caribou, horses, bison, llamas, and Harlan’s ground sloth.  Scrub oak thickets attracted herds of flat-headed peccaries.  Giant lions and dire wolves roamed the interdunes, seeking out megaherbivores.

Reference:

Swezey, Christopher; et. al.

“The Carolina Sandhills: Quaternary Eolian Sand Sheets and Dunes along the Updip Margin of the Atlantic Coastal Plain Province, Southeastern United States”

Quaternary Research 86 (2016)

The Windy Ice Ages

December 18, 2013

I love a good brisk wind.  Cool windy days seem to put an extra bounce in my step when I do my near daily 3 or 4 mile run up and down the street in front of my house.  And I love hearing the sound of the wind blowing against the house on a winter’s night while I’m snug under the comforter, dreaming of big-breasted women.  Wind is another reason I wish I could jump in a time machine and take a trip back to the Ice Ages of the Pleistocene because when massive glaciers covered most of what today is Canada, the earth heated even more unevenly than it does today, resulting in windier conditions everywhere.

Scientists measure wind in meters per second.  In modern day Georgia and South Carolina, winds average between 1.3-2.2 meters pers second, but during the Last Glacial Maximum winds averaged 4-6 m/s–more than double those of today.  As I’ve written in previous blog entries (See https://markgelbart.wordpress.com/2011/11/07/when-ice-age-georgia-became-desert-like/ and See https://markgelbart.wordpress.com/2013/03/10/when-sand-dunes-buried-herds-of-flat-headed-peccaries/), Ice Age winds often pushed eolian sand dunes across the landscape.  The climate was much drier then because the expanding glaciers locked up so much of earth’s moisture.  The modern precipitation/evapotranspiration ratio in southeastern North America is 1.23.  (Evapotranspiration is the sum of evaporation and plant transpiration.)  But during the LGM it was less than .3 or about 25% that of today.  Precipitation averaged about 33% less than that of today as well.  Moreover, the ocean receded and rivers had to flow longer distances to reach the sea.  As a result, rivers shrank in size, and riverine sand became exposed.  Strong winds pushed these sands into dunes.  Unlike modern day winds which directionally vary according to the seasons, Ice Age winds were consistently west/southwesterly and pushed these dunes to the northeast sides of rivers.  This is the origin of most of the sandhills found in the southeast today.  Modern day winds are not strong enough to  push sands into dunes or as geologists say they could not maintain a “sustained eolian mobilization.” 

The precipitation/evapotranspiration ratio was much lower during the Ice Age than it is today, resulting in a more sparsely vegetated landscape.

Geologists believe these eolian sand dunes were once u or v-shaped in front. The dunes were unvegetated but had partly vegetated parallel arms upwind from the dune fronts.  One such dune is located east of the Savannah River in Jasper County, South Carolina.

Map of South Carolina highlighting Jasper County

Jasper County, South Carolina

Full-size image (84 K)

Exposed Savannah River Dune.  There are many dunes like this located on the northeast side of southeastern rivers.

The Savannah River dune formed between 40,000 BP-19,000 BP.  It alternated between active and inactive stages in concert with variations in precipitation rates.  The surrounding area around the dune was not barren but consisted of grasslands and open woodlands, composed  mostly of pine and oak with less floral species diversity than modern day forests.  Vegetation was less dense than it is today.

At the end of the Ice Age, glaciers began melting, thus releasing moisture into the atmosphere.  Plants took root on the dunes and started holding them down, rendering them inactive.  During the Younger Dryas about 13,000 BP, climate suddenly became as cold and dry as it was during the LGM but just for a few hundred years.  Several smaller dunes formed again, but since then, all of the sand dunes in southeastern North America have become inactive due to the increase in precipitation and the decrease in wind velocities.

Reference:

Sweezey, Christopher; et. al.

“Quaternary Eolian Dunes in the Savannah River Valley, Jasper County, South Carolina”

Quaternary Research (80) 2013

The Paleoenvironment of the Ice Free Corridor

November 11, 2013

The Wisconsinian Ice Age was an epoch when 2 massive Ice Sheets expanded over Canada.  The Cordilleran Ice Sheet expanded over the western Canadian Rocky Mountains south of Alaska, while the Laurentide Glacier covered all of eastern Canada and even extended over New England, Ohio, and other midwestern states.  (Ironically, most of Alaska stayed ice free during this time and consisted of a barren grassy environment known as the mammoth steppe.)  About 30,000 years ago, the Cordilleran Ice Sheet and the Laurentide Ice Sheet conjoined into 1 massive glacial slab that blocked all human and animal migratory routes between Alaska and the rest of America.  Formerly, scientists thought this barrier of ice existed from ~30,000 BP-~11,000 BP, but more recent studies suggest the 2 Ice Sheets began to separate as early as ~15,000 BP, creating an Ice Free Corridor that men and animals could have migrated through.

Map of North America circa 15,000 calender years BP.

A quite interesting environment existed in the Ice Free Corridor as the 2 Ice Sheets began to recede.  Initially, the area between the Ice Sheets was covered by bare soil, rock, and glacial outwash sands.  Wind blew the sands into deep eolian  dunes, some measuring 15 feet high and a mile long.  Scientists used optically stimulated luminescense dating of these sands to determine that deglaciation occurred earlier than previously thought.  Chunks of ice left behind by retreating glaciers and buried by sand became kettle lakes.  Meltwater streams carried chunks of ice into the corridor and these also became kettle lakes.  Glaciers blocked some meltwater streams, and the backflow created massive glacial lakes.  All these bodies of water attracted large flocks of ducks, geese, swans, and cranes.

Black duck (Anas rubripes).  Waterfowl such as ducks, geese, swans, and cranes nested in the abundant kettle lakes that formed in the Ice Free Corridor following the end of the Last Glacial Maximum.  This source of food attracted Paleo-Indians and enticed them to colonize the newly inhabitable region.

Lichens grew on the bare exposed rock, and caribou were the first large mammal able to survive in the corridor because they can subsist on this fungi/algae symbiote.

Caribou likely were the first large mammal species to colonize the Ice Free Corridor.  They can survive on lichens that  grew on rocks centuries before grass returned to the region.

The melting ice and snow made the soils rich in nitrogen, and glacier-pulverized rock added pottasium and phosphorus.  Within centuries, wind blown grass seeds from north and south of the corridor colonized the area, converting it into a grassy mecca that attracted camels, horses, bison, and elk.  The wolf (Canis lupus) was probably the most common large carnivore to colonize the corridor.  Dire wolves (Canis dirus) never made it this far north.  However, this clad of gray wolves is not ancestral to today’s wolves of Canada and Alaska.  Mysteriously, this lineage died out and was replaced by another line of gray wolves.  Lions may have followed prey into the corridor as well, but the lions living to the north were a different species than the lions living to the south, and the 2 species apparently didn’t interbreed.

The extinct yesterday’s camel (Camelops hesternus). It was well adapted for living in the environment of the Ice Free Corridor.  It would have been one of the first large mammals to colonize the arid grasslands that formed on the rich newly deglaciated soils.

The timing of the closure and opening of the Ice Free Corridor is important for archaeologists speculating about when man first entered North America.  Pre-Clovis cultures were present in North America before the Ice Free Corridor became passable, even if the new dates are taken into consideration.  Therefore, many archaeologists believe paleo-Indians took a Pacific coastal route to arrive in North America via boat.  This may be true, but I’m not so quick to dismiss the possibility that man first came to America through an overland route within the corridor before it closed.  The timing of the initial closure is uncertain, and it may not have occurred until ~21,000 BP.  I suspect a population of humans made it to North America before the Ice Free Corridor closed, but they were too few in number to be visible in the archaeological record.

The deglaciation of the Ice Free Corridor attracted humans from north and south.  Archaeologists have discovered a mixture of artifacts here from cultures that originated separately on both sides of the Ice Sheet.  Populations that had been separated for thousands of years came into contact in the corridor while following herds of game and flocks of waterfowls.  It must have been a challenging environment.  Though the mix of grasslands, wetlands, and bare soil provided plenty of protein, it was a melancholy barren world.  The katabatic winds blowing between the 2 Ice Sheets made for some long cold nights.  There were no trees for firewood or boat construction.  Glacial lakes and frigid meltwater streams made the corridor once again impassable during summer, though they did freeze solid for overland travel during winter.  Nevertheless, man did survive in this unique natural conduit for thousands of years.

Reference:

Pinson, Ariane

“The Ice Free Corridor and the Peopling of the Americas”

Mammoth Trumpet 28 (4) October 2013

A Young Carolina Bay in North Carolina

April 30, 2012

Thousands of oval-shaped depressions, known as Carolina Bays, pockmark the coastal plain from Virginia to Georgia.  Originally, there were about 500,000 Carolina Bays on the southeastern coastal plain, but farmers fully or partially drained most of them.  They range in size from a small fraction of an acre to tens of thousands of acres.  Some stay wet year round, while others may only hold water for a few weeks of the year.  Small bays devoid of egg and tadpole-eating fish are ideal breeding grounds for amphibians.  The wetland habitats, even if seasonal, provide critical environments for aquatic species, including rare flora, such as pitcher plants.

Aerial photograph of some Carolina Bays in South Carolina.  Most geologists believe they are formed from a combination of wind and water erosion as well as peat fires which lower the elevation of the depressions. 

A minority of scientists believe meteor or comet impacts created Carolina Bays, but convincing evidence debunks this theory.  There is a complete absence of the kinds of rock that are associated with extraterrestrial impact.  Moreover, the ages of Carolina Bays vary greatly.  Not all have been dated, but some are tens of thousands of years old, while others may be as young as 6500 years.  Instead, Carolina Bays were likely formed from a combination of wind and water erosion and peat fires.

Stilted trees growing in a dried out Carolina Bay.  A fire just burned the peat causing the elevation to decrease about 4 feet.  The tree roots appear as stilts.

All Carolina Bays are oval in shape and are oriented perpendicular to the prevailing winds of the Pleistocene.  Much of the loess, or sand constituting dunes in the southeast, may have even originated from Carolina Bays.  The two geological anomalies are closely interrelated.  During dry climate phases, peat swamps became dessicated.  Lightning strikes ignited fires that burned off all the peat, thus lowering the elevation by as much as 4-5 feet.  Wind blew the exposed soil to the northeast, explaining why a sandy lip can be found on the northeast side of a Carolina Bay.  Later, after the rains returned, the water table rose and ponded water driven by wind also eroded land along the same axis.

Most Carolina Bays formed during stadials of the Pleistocene, especially the Last Glacial Maximum (~28,000 BP-~15,000 BP), but Lake Mattamuskeet, an enormous Carolina Bay near the North Carolina coast just a few miles from Pamlico Sound, is a young one with origins dating to about 6500 BP.  Scientists knew the lake had to be younger than 80,000 years old because a high stand of the Atlantic ocean inundated this area until then.  When they took cores down to that aged level, they found a layer of marine sediment, including saltwater species of clam and snail shells.  Above this layer they found peat.  Charcoal and estuarine silt (also known as loess) was mixed in with the peat.  The former is evidence of peat and forest fires; the latter was windblown from what is now Pamlico Sound, but was then high and dry land because during the Ice Age the Atlantic Ocean receded many miles to the east, leaving exposed marine-derived soil.

The scientists used ground penetrating radar, vibracores (http://www.vibracoring.com/), and radiocarbon dating to determine that the present day site of Lake Mattamuskeet was a heavily vegetated forest from about 12,000 BP-8500 BP.  Peat swamps grew in 3 areas within this site.  Periodically, they would burn, and silt from dry land or what is now Pamlico Sound would be deposited here via wind.  About 7,000 BP the ocean level rose and filled in Pamlico Sound.  Lake Mattamuskeet began forming about 6500 years BP during dry spells when peat fires left depressions of dry earth further scooped out by wind.  These peat fires still occasionally occur as the above photo shows and they can last for over a year as they slowly smolder.  An Indian legend even supports the scientific study of how Lake Mattamuskeet formed.  Supposedly, the Indian legend claims a great fire burned here for 13 moons, creating a depression that later filled with water.  According to scientists, the 3 bays stopped forming ~5,000 years ago, and the water table rose, joining the 3 lakes into 1.

Lake Mattamuskeet in North Carolina is a Carolina Bay within a few miles of the sea shore.  It’s a shallow freshwater lake though a manmade canal was constructed in 1850 in a failed attempt to drain the water into the saltwater Pamlico Sound.  In this photo it looks as deep as a reservoir, but the depth is only 2-3 feet deep.  Reportedly, there is good fishing, crabbing, and duck hunting here.  Scientists studied the geological history of this Lake and determined it’s just ~6500 years old. 

Reference:

Rodriguez, Antonio; Matthew Waters, Milas Pehler

“Burning Peat and Reworking Loess Contributes to the Formation and Evolution of a Large Carolina Bay Basin”

Quaternary Research 77 (1) Jan. 2012