Posts Tagged ‘Gray’s Reef’

Pleistocene Fossils Found on the Georgia Bight

March 25, 2013

Geologists refer to the continental shelf off the Georgia coast as the South Atlantic Bight.  It’s a bend of the continental shelf that stretches from off the North Carolina shore to Cape Canaveral, Florida.  Today, the South Atlantic Bight (SAB) is inundated with sea water, but over the past 2 million years much of it has been above sea level during at least 10 different Ice Ages.  Most recently, the SAB has emerged above sea level from ~38,000 BP-~8,000 BP.  Fossils and pollen from about this time span accumulated near 2 now underwater structures studied by scientists: Gray’s Reef and J Reef.  Both structures are about 60 feet underwater.

Map of Gray’s Reef and J Reef.  Gray’s Reef is a natural sandstone outcropping.  J Reef is composed mostly of scallop shells.  Between 38,000 BP-8,000 BP both were near sea level during interstadials and well above sea level during stadials.  J Reef was inundated by sudden rising sea levels ~8,000 BP about the same time Glacial Lake Agassiz in Canada suddenly drained when an ice dam melted.  The sudden inundation prevented speedy erosion of the reef.

Canebreaks 007

Maps of the Georgia Bight and the location of J Reef and Gray’s Reef during 2 different time periods.  Top:  from 45,000 BP when both sites were at or near the coast.  This is an interstadial period. Bottom: from 13,000 BP when both sites were well above sea level.  Click to enlarge.  Map is from the below reference authored by Ervan Garrison and others.

Gray’s Reef is a dolomitic sandstone outcropping, originating 2-3 million years ago.  Buried sand and seashells became cemented together, then erosion exposed the rock.  Gray’s Reef has been a protected marine sanctuary administered by NOAA since 1981.  It’s located 16 miles east of Sapelo Island and is 22 square miles in extent.  Scientists took cores of sediment from numerous locations within Gray’s Reef, and they analyzed and dated the pollen.

J Reef, 8 miles north of Gray’s Reef, is composed of an entirely different structure.  It’s a shell bed adjacent to a submerged river valley, perhaps a former bluff.  This ledge acted as an impediment that accumulated seashells and bones carried by ocean currents.  Over time, the seashells became cemented together in a lag deposit.  Shells of the giant scallop (Placopecten magellanicus) dominate the lag deposit.  This cold water species no longer occurs south of offshore North Carolina.  The scallop shells that form J Reef date to between ~40,000 BP-~32,000 BP when this area was close or right on the shoreline.  The abundant presence of the giant scallop is evidence the ocean waters off the Georgia coast during this time period were cooler on average than they are today.  In recent decades humans dropped so much junk on the ledge to attract fish that some scientists refer to it as an artificial reef.

I think both of these are shells of the giant scallop (Placopecten magellanicus).  The one on the left turned gray because it has been dead for awhile. Giant scallops are the primary shell that composes J Reef off the Georgia coast.  Today, this species occurs off the continental shelf from Canada to North Carolina.  Between 45,000 BP and 8,000 BP this species lived near inshore waters off the Georgia coast which extended many miles east from where it is today.  The presence of this cold water species suggests cooler waters were prevalent off Georgia’s coast during the Ice Age.

Photo of a live scallop.  Fresh scallops are very delicious.  Frozen scallops sold in mass market grocery stores are treated with preservatives that make them taste terrible.

Scuba divers found the lower jaw bone of a gray whale (Estrichius robustus) embedded in the scallop shell bed, and with difficulty they extracted it a few years ago.  It dates to 36,000 BP, making it the oldest gray whale fossil on the American side of the Atlantic.  (Older specimens have been found off the coast of Europe, Japan, and California–some dating to the Pliocene.)  Whalers hunted gray whales to extinction in the Atlantic by 1695, but they are making a comeback in the Pacific.  Gray whales are spectacular animals, weighing 36 tons (that’s 72,000 pounds!), growing to 45 feet long, and living up to 60 years old.  They used to be called the “devilfish” because they aggressively attacked whalers when they were hunted.  Gray whales use their lower jaws to scrape through bottom sediment.  The baleen in their mouth filters food from the mud, including small crustaceans (mostly amphipods), plankton, squid, and fish.  They use their tongues to dislodge the food from their baleen before swallowing.  They fatten on these organisms, developing a 10 inch layer of blubber that provides sustenence for 3-5 months because they seldom feed in warmer waters where they breed and give birth.  Usually, gray whales are covered in barnacles and whale lice.  Fossils of the still abundant bottlenose dolphin (Tursiops truncatus) have also been found in the shell bed.

36,000 year old gray whale mandible found embedded in J Reef.  It was excavated with difficulty by teams of scuba divers.  Whalers extirpated gray whales from the Atlantic Ocean by 1695.  This is the oldest fossil specimen of this species found on this side of the Atlantic.

Gray whale surrounded by seals.  A live specimen is always more impressive than a fossil.

Scuba divers are finding land mammal fossils in the paleochannel next to J Reef as well.  When the site was above sea level, mammal fossils washed into the river, and now that it’s submerged, they can be found without the need to dredge or remove vegetation.  So far, over 100 specimens have been recovered, though most are disarticulated and fragmentary.  Mammoth, bison, horse, and llama fossils have been brought to the surface by scuba divers.  Scientists have yet to publish a paper detailing the finds.  Local news outlets incorrectly reported a mammoth rib as belonging to a woolly mammoth,  I doubt woolly mammoths ranged this far south.  Instead, the rib is more likely from a Columbian mammoth which ranged throughout the continent.

Archaeologists found a projectile point, an antler/bone tool, and 2 other stone artifacts near J Reef.  The lithic design is classified as archaic Indian, a culture that dates to between 8,000 BP-5,000 BP.  Sea level suddenly rose 12 feet here about 8,000 years ago, inundating Gray’s and J Reef.  This preserved J Reef from rapid erosion, though ocean currents are gradually eating it away.  The massive flood originated from the collapse of an ice dam that created an enormous glacial lake in Canada.  This event is probably the basis for many worldwide flood myths.

A few pieces of fossil wood in cored sediments surprised scientists who were expecting to find just pollen.  They identified red cedar, beech, and pepperleaf sweetwood (Licaria triandea); all dating to about 41,000 BP.  The former 2 still occur on the South Atlantic coastal plain, but pepperleaf sweetwood is restricted to 2 areas within the city limits of Miami, Florida as well as Puerto Rico and South America.  Pepperleaf sweetwood is a tropical plant in the bay family.  It probably ranged on a narrow strip along the coast which remained relatively frost free until the Last Glacial Maximum.  It may have been a relic from an Interglacial expansion that occurred when climate was warmer than that of today.  At one time it may have been widespread throughout the south, but as the Ice Age advanced, its range gradually became more restricted until the climate got too cold, even on the coast.

Evergreen scrub with wax myrtle and pepperleaf sweetwood grew on high dunes behind beaches about 41,000 BP along the Georgia coast, and that habitat probably looked much like this photo.  Pepperleaf sweetwood is a tropical plant that has not yet recolonized the southeast since the Last Glacial Maximum when it became extirpated in the region.  If the planet continues to warm, it may grow here once again.

Pollen evidence from Gray’s Reef sediment cores paint a picture of a dynamic landscape sculpted by frequent fire, sudden dramatic climate changes, and megafauna foraging.  Fire adapted species such as pine, oak, grass, herbs, ragweed, and ferns predominated.  The South Atlantic Coastal plain consisted of a mixture of woodland, grassland, and wetland. The period known as MIS 3 (60,000 BP-25,000 BP) is noted for having highly unstable climate as Ice Age glaciers alternatedly waxed and waned.  Scientists have discerned 5 major changes in floral composition during this stage, and they correlate with Heinrich events–the sudden release of melted glacial water into the north Atlantic that shut down thermohaline circulation and caused average annual temperatures to plummet.  Within MIS 3 average annual temperatures increased during warm swings and decreased during cold swings by as much as 14-28 degrees Fahrenheit in less than a decade, stressing plant and animal life alike.  During cold stages pine increased at the expense of oak because pine is better adapted to wind, drought, and lower levels of CO2.  During warm stages oak increased while pine decreased because under these conditions they shade out conifers.  Up until about 29,000 BP forests on the South Atlantic coastal plain remained diverse with birch, beech, chestnut, and even hemlock growing with pine and oak.  But conditions deteriorated, and South Atlantic forests became species poor, dominated completely by pine and oak.  The forests remained of low diversity until about 15,000 BP when climatic conditions improved.

References:

Garrison, Ervan; et. al.

“Late Quaternary Paleoecology and Heinrich Events at Gray’s Reef National Marine Sanctuary, SAB, Ga.”

Southeastern Geology 48 (4) Feb 2012

http://graysreef.noaa.gov/science/publications/pdfs/garrison_et_al_segeo_feb2012.pdf

Noakes, Scot; Ervan Garrison, and Greg McFall

“Underwater Paleontology: Recovery of a Prehistoric Whale Mandible Offshore Georgia”

Diving for Science 2009

http://graysreef.noaa.gov/science/publications/pdfs/i-25.pdf

The Vanishing Chinkapin (Castania pumila)

April 20, 2011

Photo from google images of chinkapin nuts in a burr.

The chinkapin, a shrubby relative of the American chestnut and not to be confused with the similarly named chinkapin oak (Quercus muhlenberger), used to be locally common, growing on the tops of rocky hills in the piedmont region of the southeast and in the undergrowth of open pine savannahs on the coastal plain.  The early explorer, John Lawson, reported the trees as so common that hogs fattened on the nuts.  He described the nuts as smaller, rounder, and sweeter than those of its relative, the chestnut.  Most sources state that it was the better tasting of the two.  William Bartram found chinkapin growing in association with chestnuts and chestnut oaks (Quercus prinus) on the tops of rocky piedmont hills, a forest type that contrasted with that of the surrounding area which was mostly an oak forest but in the valleys between the rocky hills a much richer forest of black walnut, beech, hackberry, tulip, and sycamore grew.  Moist creek bottoms and richer soils kept the latter area from burning, but the thin dry soils at the tops of rocky hills endured frequent fires.  Oak and chinkapin thrive in fire prone sites because they’re shade intolerant and need open areas to grow.

Most of the jobs I’ve had in the Augusta, Georgia area have taken me to just about every neighborhood in Richmond and Columbia Counties.  I used to survey lawns for Orkin Lawn Care, and I worked for many years as a route manager for the Augusta Chronicle. While working I, of course, took note of the vegetation (ecology has always held a great interest for me), and I’ve never seen a chinkapin.  Botanists warn the chinkapin is in decline for a number of reasons: fire suppression, chestnut blight, and suburban development.  Without fire, shade tolerant trees begin to dominate, and chinkapin can’t grow in the shade.  The chestnut blight completely destroyed the once common chestnut forests.  The chinkapin is also susceptible but is better able to survive because it is a shrub that resprouts and can produce a crop of nuts before it dies back again from the disease.  Still, the blight reduces overall nut production.

The chestnut blight was a disaster for the ecosystem.  Chestnuts and chinkapins were important sources of food for wildlife.  Now, trees such as tulip, which produce no mast, have replaced chestnuts.  They may be beautiful trees but animals can’t eat beauty.  I think the lack of chestnuts explains why I saw almost no wildlife on my trip to the Smoky Mountains National Park last summer (see my blog entry “Gatlinburg, Tennessee: Tale of a Tourist Trap Nightmare” which is I believe in the June 2010 archives).

The chinkapin has two interesting adaptations that help it survive as a species.  It germinates quickly in the fall.  The nut ripens from September to November, and they produce heavily–up to 1500 nuts per bush, beginning when they’re just six years old.  Squirrels disperse the species by burying the highly valued food, but the chinkapins foil the squirrels when they germinate immediately.  After they’ve become a seedling, the squirrel can’t utilize them.  Fall germination prevents animals from destroying the entire progeny, but by producing a nut with high food value, they motivate the squirrels to disperse them.  The other adaptive characteristic is its ability to resprout vigorously.  Fire may kill the main trunk, but chinkapin will resprout and form thickets.  Deer also find chinkapin a favored food and will browse down the main trunk, causing the shrub to resprout and create thickets.  Their thickets provide great cover and food for turkey and grouse.

Fossil evidence shows that turkey and grouse were quite common in upland Georgia during the Pleistocene–both left abundant specimens at Ladds and Kingston Saltpeter Cave in Bartow County.  Two studies of sediment cores in Georgia found that chestnut/chinkapin made up about 2%  of the pollen spectrum during the Pleistocene.  Both sites (Nodoroc and Grays Reef) date to about 30,000 BP.  Chinkapin surely was a common component of the open oak and pine savannahs so prevalent then.  Its ability to resprout and fall germinate is an ancient adaptation to survive fire and megafauna foraging.  The more such animals as mastodon, horse, llama, and deer browsed, the more this shrub would bounce back and form thickets ideal for bird life.

The Indians used to cook chinkapin and hickory nuts with their venison in well-rounded stews.  Chinkapins are a nice starchy substitute for bread or potatoes; hickory nuts provided a nice oily substitute for butter.  Chestnuts, unlike most other nuts, are primarily a carbohydrate based food, rather than a fatty form of sustenance.  They’re sweet and bready and act as a laxative.  I hate to buy expensive imported European chestnuts when I think how abundant and cheap American chestnuts and chinkapins used to be.

Pleistocene Passenger Pigeon Populations

August 27, 2010

(Please forgive the excessive alliteration in the title.)

It’s hard to imagine the massive number of passenger pigeons (Ectopistes migratorius) that used to live in North America as recently as the mid to late 19th century.  For a description of their numbers, I’ve dug up an account J.J. Audubon gave in his Ornithological Biography.  Before I reprint this passage I want to comment on his writing style.  I enjoy his prose, but he does have a bad habit of writing in the passive voice, a style Stephen King in his book, On Writing, referred to as farting in an elevator.  Also, English was his second language because he was born in France.  Nevertheless, I think this makes for a fascinating description of a nature scene that no longer exists.

The multitudes of wild pigeons in our woods are astonishing.  Indeed, after having viewed them so often, and under so many circumstances, I even now feel inclined to pause, and assure myself that what I am going to relate is fact.  Yet I have seen it all, and that too in the company of persons, who, like myself, were struck with amazement.

In the autumn of 1813, I left my house at Henderson, on the banks of the Ohio, on my way to Louisville.  In passing over the barrens a few miles beyond Hardensburgh, I observed the pigeons flying from north-east to south-west, in greater numbers than I thought I had ever seen them before, and feeling an inclination to count the flocks that might pass within the reach of my eye in one hour, I dismounted, seated myself on an eminence, and began to mark with my pencil, making a dot for every flock that passed.  In a short time finding the task which I had undertaken impractical, as the birds poured in countless multitudes, I rose, and counting the dots put down, found that 163 had been made in 21 minutes.  I traveled on, and still met more the farther I proceeded.  The air was literally filled with pigeons; the light of the noon-day was obscured as by an eclipse; the dung fell in spots, not unlike melting flakes of snow; and the continuing buzz of wings had a tendency to lull my senses to repose…

“It may not, perhaps, be out of place to attempt an estimate of the number of pigeons contained in one of those mighty flocks, and of the quantity of food consumed by its members.  The inquiry will tend to shew the astonishing bounty of the great Author of Nature in providing for the wants of his creatures.  Let us take a column of one mile in breadth, which is far below average size, and suppose it passing over us without interruption for three hours, at the rate mentioned above as one mile in a minute.  This will give us a parallelogram of 180 miles by 1,  covering 180 square miles.  Allowing two pigeons to the square yard, we have one billion, one hundred and fifteen million, one-hundred and thirty-six thousand pigeons in one flock.  As every pigeon daily consumed fully half a pint of food, the quantity necessary for supplying this vast multitude must be eight million seven hundred twelve thousand bushels per day.”

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Some archaeologists believe the massive population of passenger pigeons that colonists in North America reported from 1700-1870 was a temporary phenomenon.  Thomas Neuman has written at least two journal articles suggesting passenger pigeon populations exploded following the decimation of Indians after their first contact with European diseases such as smallpox, influenza, and measles.  Before this, Dr. Neuman believes this species was not particularly common.  Supposedly, when Indian populations were reduced, there was more forest mast available for birds because humans weren’t gathering the nuts and acorns they fed upon.  In his book 1491 Charles Mann discusses this theory and notes that archaeologists find few passenger pigeon bones at sites of large Indian settlements.

I disagree with this theory because it makes little sense for several reasons which I shall enumerate.

1. Passenger pigeons could not survive as a species unless they existed in extremely large populations.  The survival strategy of this species was to reproduce rapidly and roost and nest in enormous colonies so that they overwhelmed predation.  Predators ate many individuals, but there was a limit to how much their stomachs could hold.  If, as these archaeologists suggest, the passenger pigeon was just an occasional bird, the species would’ve become extinct long before the white man arrived on the continent because their defense mechanisms revolved around living in large colonies.

2. Even if human populations were at the high end of what archaeologists believe, they would’ve made little impact on the amount of forest mast available.  Pre-Columbian forests were extensive, and there was always plenty of forest mast for both humans and huge pigeon colonies.

3. Archaeologists don’t find many passenger pigeon bones in sites of large Indian settlements because Indians probably went to their roosting grounds and feasted on them there and simply didn’t bring the bones back to their villages.

4. Jacques Cartier, an early explorer, reported large pigeon colonies on Prince Edward Island in 1534…before Indian populations were reduced by disease.

5. Pigeon fossils are abundant in an early Holocene fossil site in Western Canada (Charlie Lake, British Columbia).  They are also a common fossil in late Pleistocene avifaunas including Bell Cave, Alabama, Cheek Bend Cave, Tennessee, and Ladds and Kingston Saltpeter Cave in Georgia.

At Ladds fossils of only 4 bird species were discovered.  Passenger pigeons were 1 of the 4.  This may be coincidence, but it’s believed that passenger pigeon biomass made up 25% of all bird populations in North America during the early part of the 19th century.  Many more species of birds were found in the deposit at Kingston Saltpeter Cave.  I compared the minimum number of individual passenger pigeon fossils from KSC to the total bird remains of all other species found there.  Ruffed grouse were the most common species, making up 30% of bird remains, but passenger pigeons made up 6%, despite being a highly migratory bird.  Assuming they spent 6 months of the year around KSC, that means at times, they may have made up to 12% of the bird population in the area.  If they stayed in the area around the cave for only 2 months of the year, they quite possibly made up 36% of the bird population at certain times of the year there.

I must mention, however, that estimating ancient bird populations based on the number of bird fossils found in cave deposits is a rather dubious method.  Nevertheless, habitat for passenger pigeons in southeastern North America during the Pleistocene must have been ideal for this species.

During colonial times this bird nested throughout the midwest, but spent September-February in southeastern states.  For much of the duration of the Wisconsin Ice Age, most of the area where they later nested was under glacial ice, so it’s likely their nesting areas shifted south.  They probably were year round residents everywhere south of the Laurentide ice sheet, except during nesting season when they nested in southern river bottomlands where beech and oak trees remained plentiful, even during cold arid climatic phases.

Today, beech trees are a rare relic in much of the southeast, but during certain climatic phases of the Wisconsin Ice Age, they were even a dominant tree on some lands, according to records of fossil pollen in Alabama, and South Carolina.  From 14,000-11,000 years BP beech was a common tree, while pine, which dominated southern forests during the LGM, temporarily declined drastically.  Beech is well adapted to pigeon and squirrel foraging because this tree spreads through sucker roots, and if animals eat the tree nuts, this species can still propagate.  Beech tree pollen is also present in the Nodoroc fossil site in central Georgia near Winder and at the Gray’s Reef site off Sapelo Island, which was above sea level 30,000 years ago.  The latter site yielded evidence of a forest consisting of a strange mixture of cool temperate and warm weather species of plants.  The south’s Ice Age ecosystem was a mixture of woodlands and grasslands, and it provided excellent habitat for passenger pigeons.  I think the expansion of southern beech tree forests, as the Ice Age waned, is evidence the population of pigeons may have spiked about 14,000 years BP, creating the nucleus that later colonized the midwest after the glacier melted and broadleaf trees re-established themselves there.

References.

Driver, J.C.; and K.A. Hobson

“A 10,500 year sequence of bird remains from the southern boreal forest region of western Canada”

Arctic 45 (2) 1992

Ellsworth, Joshua; and Brenda McComb

“Potential effects of Passenger Pigeon flocks on the structure and composition of pre-settlement forests of eastern North America”

Conservation Biology 17 (6) pp. 1548-1558 2003

Mann, Charles

1491

Knopf 2005

www.paleodb.org