The Paleoenvironment of the Georgia Bight when it was Above Sea Level (~80,000 BP-~7,000 BP)

A wonderful study led by Scott Harris of the College of Charleston was published in Geomorphology last year.   The authors of this study mapped and analyzed various underwater features on the continental shelf of an area known as the Georgia (or South Atlantic) Bight.  I realized Dr. Harris, an expert on the geology of this region, might have an answer to the mystery of the Silver Bluff Shoreline (See: https://markgelbart.wordpress.com/2014/06/01/the-mystery-of-the-silver-bluff-shoreline/).  This shoreline is thought by some to date to about 38,000 BP, a time when most of Canada was under glacial ice, and therefore the shoreline should have been many miles to the east.  Yet, the Silver Bluff Shoreline is near the modern shoreline.  I asked Dr. Harris about this anomaly.  He informed me that he doesn’t like the nomenclature used to delineate this paleo-shoreline.  More importantly, he’s dated this shoreline using optically stimulated luminescense, and he consistently gets dates of 80,000 BP.  These dates are before sea level fell significantly in response to glacial expansion.  In my opinion this mystery is solved: his dates make more sense, and what is known as the Silver Bluff Shoreline is much older than previous researchers estimated.  There is no anomaly.

Map of the Georgia Bight.  The area shaded in light gray was above sea level between ~80,000 BP-~7,000 BP.  It hosted a variety of dry land environments.

The study I mentioned above and reference below has re-ignited my curiosity about the environments on the continental shelf when it was above sea level.  It was a vast extension of the coastal plain–dry land occurred as much as 90 miles east of the modern shoreline.  Dramatic climatic fluctuations influenced the composition of the various environments that existed here.  The time period between ~80,000 BP-~60,000 BP (Marine Isotope Stage 4) was a climate phase of rapid glacial expansion, causing the sudden fall in sea level that exposed the continental shelf.  The period between ~60,000 BP-~28,000 BP (Marine Isotope Stage 3) was an interstadial–perhaps the most intersting climate phase because of an alternating feedback cycle that occurred.  Sudden warming events known as Dansgaard-Oeschger Events caused average annual temperatures to increase by as much as 14 degees F within a decade.  There was increased seasonality as summers averaged 5-9 degrees F warmer than during stadials.  And precipitation increased because the warm summer temperatures melted glacial ice.  After a few thousand years, icebergs and great quantities of glacial meltwater would flood into the North Atlantic.  This is known as a Heinrich Event.  Eventually, all this cold freshwater would shut down the Gulf Stream, causing temperatures to plummet.  Ice would again accumulate into glaciers.  MIS-3 saw at least 5 sudden warm ups followed by 5 cold downturns.  Between ~28,000 BP-~15,000 BP (Marine Isotope Stage 2), temperatures mostly remained in the cold climate phase. After ~15,000 BP sea levels rose until they reached modern levels about 6,000 years ago.  I am interested in how these climatic fluctuations influenced the flora and fauna of the continental shelf.  Below is a review of the types of environments probably found on the continental shelf and how they responded to climate change.

Prairies

I believe prairies were the most common landscapes on the continental shelf during cold arid stadials.  The presence of 13-lined ground squirrels (Spermophilos tridecemlineatus) in the local fossil record supports my hypothesis.   13-lined ground squirrels prefer treeless plains and are found today in the tall grass and short grass prairie regions but are presently absent in the southeast.  The abundant fossils of mammoths, bison, and horses found near the coast and underwater all suggest the existence of extensive grasslands here.  Pine savannahs, open deciduous woods, and even closed canopy forests replaced most but not all of these prairies during warmer wetter interstadials.

13-lined ground squirrels.  The presence of this species in the local fossil record suggests the existence of extensive grasslands on the continental shelf.

Pine Savannahs

Savannahs are dependent upon lightning-ignited fires which increased in frequency during interstadials.  Scrub oak thickets with some pine replaced savannahs when fire intervals were reduced.

Open Oak Woodlands and Forests

A woodland is defined as a natural community that has 50%-80% canopy cover, while a forest has >80% canopy cover.  The arid climate of stadials relegated these environments to river bottomlands and other locally moist areas, but following Dansgaard-Oeschgher Events they expanded across the continental shelf.  Pollen records dating to interstadials show oak, hickory, pine, and grass dominated this region then.  Open woodlands, consisting of large mature trees growing far enough apart to allow for a grassy understory, predominated.  Fire and megafauna foraging shaped this landscape.  However, the pollen evidence indicates the low level presence of trees that are not fire resistent and have northern affinities–hemlock, spruce, beech, and basswood. (Though this pollen may have been blown in from northern latitudes.)  Some areas of the continental shelf may have had low incidence of fires and a cool moist climate during some phases. Deer, long-nosed peccary, tapir, and bears favored these habitats.

Oak Scrub with some Pine

Much of the continental shelf had sandy soils more suitable for desert-like vegetation.  Low incidence of fire during stadials increased the extent of this scrub habitat, and it likely replaced savannah.  It would have provided favorable habitat for flat-headed peccaries and rabbits.

Cypress Swamp

Stadials reduced cypress swamps to relic status.  Low poorly drained areas likely held these relic stands, providing the seed population that allowed cypress swamps to expand following Dansgaard-Oeschger Events.  Submerged cypress swamps have been found off the South Carolina coast.  Rising sea levels at the end of the Ice Age inundated these swamps.

Sand Dune Fields

Sand dunes originating from dry river beds rolled across the landscape during both MIS-2 and MIS-4.  Scrub vegetation covered and held them down during interstadials when they became good habitat for burrowing tortoises.

Carolina Bays and Freshwater Marshes

Wetlands occurred in some low lying areas but were scarce during stadials.  Many wetlands were ephemeral and may have frequently dried out before they could succeed to cypress swamps.  This is where mastodons and giant beavers congregated.

Braided Rivers

Paleomeanders prove that large rivers such as the Savannah and the Altamaha flowed through the continental shelf all the way to the ocean.  These paleomeander scars are still visible for about 60 miles of the continental shelf but strangely disappear on the last 30 miles of the outer shelf.  Dr. Harris believes rivers that flowed over the outer shelf were shallow and had a braided pattern.  When sea level rose, sediment filled the shallower incisions, explaining why these parts of the river scars are not visible.  It has occurred to me that these shallow intermittent parts of the rivers may have impeded some fish migrations.  They may have acted as natural fish traps that paleo-indians could have taken advantage of.

Paleomeander scar 60 feet below the ocean surface off the coast of South Carolina from a sonar image taken by College of Charleston researchers.  This is evidence of rivers flowing through the continental shelf when it was above sea level.

The Narrow Coastal Zone

Sandy beaches with some rocky outcrops, salt marshes, and tidal rivers provided varied habitat for marine life.  Some arctic sea birds along with seals and walruses inhabited this zone.  It was much narrower than the modern coastal zone.  Heinrich Events brought cold currents chilled by icebergs that may have drifted as far south as Florida. Evidence of iceberg draglines on the ocean bottom is abundant off the coast of South Carolina.

In a past blog entry about Pleistocene bears of southeastern North America, I didn’t list polar bears as a species that occurred in the region, but I may have been wrong.  Predators follow their prey anywhere.  In 1534 Jacques Cartier encountered a polar bear that swam 30 miles to feast upon the sea birds nesting upon Funk Island located off the Newfoundland coast.  Strange as it may seem, that means it’s quite possible polar bear stragglers did reach the continental shelf of what’s now Georgia during the Ice Age.  Bears have an amazing sense of smell.  Though Funk Island is named for the strong smell of the tens of thousands of birds that nest there, it’s still surprising that a bear could smell them from 30 miles away.

Colony of Murres on Funk Island off the coast of New Foundland.  It’s named for the funky smell of guano from tens of thousands of sea birds.  Colonies of sea birds like this may have existed on rocky outcrop islands near the shelf edge off the coast of the Carolinas and Georgia during the Wisconsinian Ice Age.

Bulls Scarp, a rocky promontory off the coast of South Carolina, probably looked something like this when this region was above sea level.  This is Greenly Island located off the coast of Labrador, Canada.

Reference:

Harris, Scott; et. al.

“Continental Shelf Landscape of the Southeastern United States since the Last Interglacial”

Geomorphology 2013

Tags: continental shelf environments, mystery of Silver Bluff Shoreline solved, polar bear on Funk Island