Pleistocene Fossils Found on the Georgia Bight

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

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4 Responses to “Pleistocene Fossils Found on the Georgia Bight”

  1. James Robert Smith Says:

    The Earth is certainly a dynamic place, as such studies as you cite have proven. One thing I’d like to see is an animate time-lapse illustration of the ebb and flow of sea level around North America over the past 100K years or so, along with the waxing and waning of forest types over the continent. It would make for an interesting perspective.

  2. markgelbart Says:

    I’ve long wanted to see something like that…not necessarily anime but maps.

    I remember making that request to Natural History Magazine over 20 years ago.

    The only problem is that pollen records are inadequate to determine what the forests were exactly like.

  3. The Mystery of the Silver Bluff Shoreline | GeorgiaBeforePeople Says:

    […] See also https://markgelbart.wordpress.com/2013/03/25/pleistocene-fossils-found-on-the-georgia-bight/ […]

  4. Pleistocene Gray Whale (Eschichtius robustus) Calving Grounds off the Georgia Coast | GeorgiaBeforePeople Says:

    […] also https://markgelbart.wordpress.com/2013/03/25/pleistocene-fossils-found-on-the-georgia-bight/ and […]

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