Rapid Sea Level Rise on the Georgia Coast

Glaciers expanded and sea levels fell during Ice Ages.  20,000 years ago, the Georgia coast was located 90 miles to the east of the present day shoreline.  The continental shelf off the Georgia coast (known as the Georgia bight) was above sea level between 80,000 BP-7,000 BP.  Near the end of the Ice Age, glacial ice melted and sea level rose rapidly but in stages.  The partial dissolution of glacial Lake Agassiz in Canada 12,900 years ago caused a sudden rise in sea level.  The Atlantic Ocean likely advanced toward the present day shoreline many miles in a few short years.  But an ice dam reformed, containing the rest of Lake Agassiz’s water until 8200 BP when it completely collapsed, releasing the rest of this great lake’s water.  (Lake Agassiz contained more water than all of the modern Great Lakes combined.)  This caused another sudden rise in sea level along the Georgia coast.  By 4500 BP the Georgia coast consisted of just 2 barrier islands separated by the southeasterly flowing Altamaha River.  The Ogeechee, Satilla, and St. Mary’s Rivers were tributaries of the Altamaha and didn’t directly drain into the Atlantic Ocean as they do today.  The Altamaha drained into a huge sound located to the east of where Little Cumberland Island stands today.  Sea level rose again, flooding and eroding the areas where the Ogeechee, Satilla, and St. Mary’s flowed into the Altamaha and splitting the 2 barrier islands into 10 smaller ones.  All of these rivers formerly flowed southeasterly, but the high marine transgression caused them to straighten to a more easterly direction.

Map of the  modern day Georgia coast.  About 4500 years ago, there were only 2 barrier islands between the Savannah River and the Florida border, and they were bisected by the Altamaha River which then emptied into a huge sound located at the same latitude as modern day Little Cumberland Island.  The other rivers in this region that today flow into the ocean were tributaries of the Altamaha.

Aerial view of salt marsh in Glynn County, Georgia.  There are 4500 year old logs from a rapidly flooded forest underneath the surface.

Wood storks (Mycteria Americana) on a salt marsh behind Sapelo Island.  There are now 10 major barrier islands off the Georgia coast, but before 4500 years ago, there were just 2, separated by the Altamaha River.  A person using an excavator to dig here would probably find 4500-2000 year old logs in less than an hour.

The Holocene Climatic Optimum between 9000 BP-5000 BP was a warm climatic phase, especially at higher latitudes, resulting from both the northern hemispheric tilt of 24 degrees and the timing of the perihelion.  The earth was closest to the sun during the boreal summer.  Summers were much warmer but winters were cooler.  The effects of the Holocene Climatic Optimum were felt in different regions at different times.  The sea level rise on the Georgia coast occurred toward the end of this climatic phase.  The ocean’s response to the Holocene Climatic Optimum may be a factor explaining this marine transgression.  Tectonic processes (the shifting of the earth’s crust) and isostatic adjustment (the rebound and subsidence of the earth’s crust in response to the weight of glaciers) also probably played a role in this sea level rise.

Scientists studying the rapid rise of sea level on the Georgia coast find fossil trees beneath salt marshes, and they date to between 4400 BP-2000 BP.  St. Simon’s and Jekyll Islands were connected until 2000 BP.  The youngest fossil trees are found here.  This was one of the last areas of the coast to get inundated, though Ossabaw Sound was breached about the same time. There are abandoned river channels beneath salt marshes as well. Sea level is still rising but not at an unprecedented rate as global warming alarmists and political pundits claim.  Sea levels haven’t even reached the marine transgression high stand of the last interglacial known as the Sangamonian (~132,000 BP-~118,000 BP).

References:

Chowns, Timothy

“Drainage Changes at Ossabaw, St. Catherine’s, and Sapelo Islands and their Influence on Island Morphology and Spit building on St. Catherine’s Island”

American Museum of Natural History Anthropology Papers 94

Chowns, Timothy; and S. Hannah Hill

“Mid-Holocene Sea Level Rise on the Georgia Coast”

Georgia Journal of Science Abstracts 2014

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Paleomeander Scars along the Altamaha River

I’ve been a member of the Nature Conservancy for 22 years but have never visited one of their preserves.  I had a sudden urge to see one, and the Moody Natural Area excited my interest.  The Moody Natural Area is 8200 acres of sand scrub, open pine savannah, bluff forest, river bottomland forest, and cypress/tupelo swamp along the Altamaha River southeast of Vidalia, Georgia.  I’ll have to wait til fall though because summer is too !#$!  hot here in Georgia, and I don’t want to drag my poor wife and child out on another long field trip.  To appease my eagerness to see this remnant wonder of nature, I studied satellite photos of the site and found some interesting geology.  Below is a link to the satellite photo, but it won’t link directly to the area my sketch is based on.  To find it, pan to the right and enlarge.

http://maps.google.com/maps?hl=en&sugexp=ldymls&pq=satellite+view+of+appling+county,+georgia&xhr=t&q=Satellite+view+of+Baxley,+Georgia&cp=24&qe=U2F0ZWxsaXRlIHZpZXcgb2YgQmF4bGV5LCBHZW9yZ2lh&qesig=EOXfNfk3bgyRW1kycKKlYQ&pkc=AFgZ2tnGr_kwof9pAkEcEjqYCDUZ04s45pudfCHI7kM4NB0vUl9WZK6ZeoRYVvB9k7wrbFJc1ZikjlI9Te0c21MNqT5JMqKVew&rlz=1R2ADBF_enUS332&bav=on.2,or.r_gc.r_pw.&biw=995&bih=506&wrapid=tljp1308755019906021&um=1&ie=UTF-8&hq=&hnear=0x88f072d6b3107457:0x709010719d231b0,Baxley,+GA&gl=us&t=h&ei=bwQCTry2L4rEgAfHrLmgDQ&sa=X&oi=geocode_result&ct=title&resnum=1&sqi=2&ved=0CFAQ8gEwAA

Sketch of a satellite photo of Altamaha River adjacent to the Moody Nature Preserve.  Note the oxbow lakes and paleomeanders.  Both types of formations were former bends of the river that got cut off from the main channel.  The paleomeanders mostly dried up and filled with vegetation.  Both possibly formed as the Ice Age ended and precipitation increased making the river meander to a greater degree than it does presently.

Paleomeander scars are evident in satellite photos of the Altamaha River.   The Altamaha River used to flow through these spots, creating the visible incisions.  Later, the river shifted to its present location and as always is still slowly altering its position.  I don’t know whether geologists have ever studied the part of the Altamaha adjacent to the Moody Natural Area, but they have examined other areas along this mighty stream.  They’ve excavated braided sands in terraces that date to the Last Glacial Maximum (~30,000 BP-~15,000 BP).  I’ll explain what this means below.  But first, I’d like to suggest that the visible paleomeander scars in my sketch based on this location of the Altamaha may date to the end of the Ice Age (15,000 BP-10,000 BP) when the river meandered to a much greater degree than it does now.  Note that some of these scars still fill with water on occasion.

Geological History of Georgia’s Rivers

36,000 BP-30,000 BP

Prior to this time period, there is only a limited amount of data studied, so I begin here.  It’s likely, however, that time periods and climate phases discussed are repeated cycles that have occurred before.  Climate fluctated rapidly before the LGM, alternating between stadial and interstadial.  Stadials were periods of glacial expansion characterized by cold arid climate.  Though glaciers never came close to what’s now Georgia, the change in climate had a significant impact on Georgia’s rivers.  Long droughts lowered the water table and in many cases completely dried up tributaries.  The major rivers became clogged with huge sand bars and islands.  A decrease in vegetation along the rivers meant even more sandy sediment could accumulate.  The surrounding landscape consisted mostly of pine and oak savannahs.  Grasslands and sand scub grew in some places right up to the water’s edge.  Interstadials were periods of glacial retreats characterized by cool moist climate.  As the glacier to the north melted, moisture levels in the atmosphere increased as did precipitation.  Though average annual temperatures increased, they were still lower than those of today.  Interstadials likely had wet and dry seasons–late winter, spring, and early summer were wet; late summer, fall, and early winter were dry.  Floods during the wet season accumulated river sediment; winds during the dry season converted this accumulation of sand into dunes.  Because climate fluctuated during the this time period, Georgia’s rivers included a combination of meandering and braiding patterns.

30,000 BP-15,000 BP

Temperatures and precipitation rates fell dramatically during the Last Glacial Maximum as the northern glaciers expanded as far south as what today is central Ohio.  This caused water tables to drop creating braided river patterns.

The Platte River in Nebraska is an example of a present day braided river pattern.  Note the prevalent sparsely vegetated sand bars and islands.  Georgia’s rivers looked much like this during the Ice Age rather than their present day meandering patterns.

The climate was so dry that in some places eolian sand dunes born from riverine sand deposits blew across the landscape.  Thinly vegetated grassland grew to the water’s edge in some areas.  Geologists find braided river sands in terraces 0-15 feet above the floodplain adjacent to the rivers.  Some sand dunes formed during this era are quite large but today are covered with vegetation and more recent sediment.  Others have eroded away.

15,000 BP-5,000 BP

Temperatures rose rapidly at the end of the Ice Age, albeit the rise was interrupted by a precipitous fall in in average annual temperatures during the Younger Dryas cold phase.  As the Laurentide Glacier melted, there was a sudden increase in precipitation which caused massive storms and floods.  Georgia’s rivers broke out of their braided pattern and began to meander to a greater extent than they do today.  Flooding was more extensive as well.  For the first 5,000 years of this time period scroll bars were prevalent but these decreased in abundance later.  Scroll bars form when large meanders migrate and create ridges between old meander incisions.  Meanders peaked in intensity about the time the great glacial Lake Agassiz in Canada broke through the ice dams and released most of its water.  (See “Temporal Correlations between Lake Agassiz, the Okefenokee Swamp, and Ancient Flood Myths” from my January archives.)  Rivers still meander but to a lesser extent since the atmosphere has stabilized.

Reference:

Leigh, David

“Late Quaternary Climates and River Channels of the Atlantic Coastal Plain”

Geomorphology 2008

Temporal Correlations between Lake Agassiz, the Okefenokee Swamp, and Ancient Flood Myths

In last week’s blog entry I discussed the cyclical impact of breaches in glacier ice dams on Ice Age climate.  The biggest glacial lake in North America formed from rapid melting of glaciers at the end of the last Ice Age is known as Lake Agassiz

 

Map of ancient Lake Agassiz.  A glacier ice dam formed this lake which existed from ~13,500-~8200 BP.

As the map shows, Lake Agassiz was bigger than all the Great Lakes combined and at the time of its existence was the largest freshwater lake in the world by far.  About 12,900 years ago a major breach of the ice dam that formed this lake occurred, flooding the North Atlantic with cold freshwater and shutting down thermohaline circulation (as I described in last week’s blog entry).  This event (known as a Heinrich event) triggered a sudden cold snap referred to as the Younger Dryas, named after a species of flower that flourished in Europe during this arid cold phase of climate.  The Younger Dryas lasted for 500 years as earth’s temperatures in the northern hemisphere plummetted back to levels equal those of the last glacial maximum (~29,000-~15,000 BP).

As the climate cooled another ice dam formed stopping the outflow of water, so that Lake Agassiz continued to exist.  But climate gradually warmed again and the final dissolution of the lake occurred about 8200 BP.

Map showing final outflow of Lake Agassiz.  After glacier ice dams broke, the water escaped through tributaries leading to the St. Lawrence River, the Mississippi River, Hudson Bay, and western rivers as well.

The final dissolution of Lake Agassiz also shut down thermohaline circulation, causing a shift in climate to arid and cold, though it wasn’t nearly as severe as the Younger Dryas.  Nevertheless, this time, the dissolution of Lake Agassiz was complete, and scientists believe sea levels rose by as much as 1 meter in less than a year.  It occurred to me that this closely correlates with the period of time when the Okefenokee basin filled with water and became a swamp.  Scientists carbon dating the peat there discovered that the modern version of the Okefenokee became swamp about 7,000 years ago.  This is also roughly the time of modern barrier island formation off the Atlantic coast.  Between ~36,000 and ~7,000 years BP, the Okefenokee basin consisted of open pine savannah and scrub oak interspersed with small scale streams and freshwater marshes, but an increase in atmospheric moisture and the rise of the water table led to swamp development.  It’s likely that swamps periodically developed in the basin and periodically dried out following fluctuations in the water table throughout the Pleistocene.

It’s also been noted that the final dissolution of Lake Agassiz and other glacial lakes all over the world may correlate with worldwide flood myths.

The final dissolution of Lake Agassiz caused sea level to rise by 1 meter in just 1 year.  This would have flooded coastal regions.

The literal story of Noah’s flood is impossible, but it may be based on some truth.  A flood that covered the entire world is geologically impossible, and there is no scientific evidence to support this belief.  But it is likely that the dissolution of Lake Agassiz and glacial lakes in Europe did cause major localized flooding in many areas of the world on a scale far surpassing anything from recent recorded history.

One clue regarding changes in climate patterns resulting from this event comes from Genesis 7:12 in the bible.  “It rained for 40 days and 40 nights.”  The Little Ice Age, a relatively minor climatic event but one that had a major impact on European agriculture for 500 years (1314-1850 AD) began with cold rainy summers–lots and lots of rain.  It’s plausible that the initial flood of freshwater into the North Atlantic caused a low pressure system that drew unusually prolonged spells of cold rain in summer, though, of course, not exactly 40 days in a row.  But it wasn’t rain that caused the flooding.  Instead, it was the sudden rise in sea level that destroyed coastal villages.  Whether or not the villages were morally wicked was coincidental.

Today, Lake Winnipeg and a few other Canadien lakes are remnants of Lake Agassiz.  The Great Lakes formed from melted glaciers in ancient river valleys scoured out by glaciers.  The Great Lakes  formed and reformed many times throughout the Pleistocene, and it’s probable a glacial lake has repeatedly re-occurred on the site of Lake Agassiz as well.