Posts Tagged ‘south Alabama’

Hurricane Ivan Uncovered a 60,000 year old Cypress Forest in the Gulf of Mexico

August 9, 2017

In 2004 Hurricane Ivan spawned 140 mph winds, 90 foot waves, and the fastest sea floor current ever recorded.  That incredible sea floor current removed a sediment layer covering a 60,000 year old cypress forest in the Gulf of Mexico.  The exposed trees formed a natural reef, attracting a concentration of fish and other sea life 60 feet below the ocean surface and 15 miles offshore.  Fishermen noticed the unusual concentration of fish and asked scuba divers to investigate.  The scuba divers discovered the uncovered ancient forest, and scientists are now studying this rare site.

The scientists who visited the flooded forest were impressed with the marine life they encountered–flounder, cardinal fish, red snapper, blennies, sea bass, moray eels, sandbar sharks, hawksbill turtles, octopus, boring worms, anemones, and sponges.  But they were even more impressed with the ancient cypress wood they brought with them to the surface.  They sawed through it in the laboratory and smelled fresh sap.  Nevertheless, they couldn’t use radiocarbon dating because they discovered the wood was over 50,000 years old–too old for that method.  Instead, they found the nearest organic material that could be dated and estimated a 60,000 year old date based on stratigraphic location and assumed rates of deposition.

Image result for map of south Alabama

Map of weather stations in south Alabama.  During Ice Ages dry land extended for miles into the Gulf of Mexico.  Mobile Bay was a valley of forests and grasslands.  Dauphin Island and Fort Morgan were high hills “hundreds of feet above the surrounding landscape.”

When it was alive, this flooded forest stood during a time period classified as Marine Isotope Stage 3.  I am fascinated with MIS3 because the dramatically fluctuating climate cycles had a major impact on natural communities.  MIS3 occurred just before the Last Glacial Maximum (the coldest stage of the last Ice Age), but unlike the LGM, MIS3 experienced warm interstadials alternating with cold phases.  Many geographical regions hosted an admixture of northern flora and fauna with warm climate species of plants and animals because of this climatic instability.  Tree rings on the fossil cypress wood excavated from this locality demonstrate this instability.  The tree rings provide a 489 year record of climate from MIS3.  The cypress tree rings show climate varied with warm wet years and dry cold spells but for the most part they are narrower than tree rings found in modern day cypress trees.  This reflects a cooler drier climate with lower levels of carbon dioxide in the atmosphere.  The trees were especially stressed during the last 50 years of their existence, and they all died at the same time, though the trees were of different ages.  Saltwater intrusion killed the trees.

Sea level rose rapidly here, probably during a warm phase of climate when glaciers were melting.  Cypress wood is resistant to decay, an adaptation for living in aquatic environments, but when exposed to air will eventually rot away.  The dead stand of cypress wood likely stood for decades, perhaps a century, before becoming covered in sand and mud.  Thus sealed off from air, it was preserved for tens of thousands of years.  Now that it is exposed to oxygen again, it will decay into nothing in a few centuries.

Scientists cored into the mud around the trees and took samples of pollen to analyze the type of natural environment that existed here 60,000 years ago.  Cypress, oak, and alder pollen dominated.  The palynologist who analyzed the pollen composition (the data as far as I know is still unpublished) concluded the forest was a rare type that no longer occurs in the region.  The closest modern analogue is classified as an Atlantic Coastal Plain Blackwater Bar/Levee Forest.  This type of forest occurs in small areas near the coasts of North and South Carolina.  Bar/Levee forests grow on soil formed on the inside bend of a river.  Sediment accumulates here through deposition, and the area is seasonally flooded.  (Indeed, this particular forest occurred alongside a river, and the paleomeander scar is still visible at the bottom of the ocean adjacent to the flooded forest.)  Dominant trees in a Bar/Levee forest are cypress, river birch, laurel oak, overcup oak, willow oak, sweetgum, red maple, elm, and loblolly pine.  The understory consists of holly and hop hornbeam along with red maple and ash saplings.  The shrub layer is made up of blueberry, titi, sweetspire, grape, poison ivy, climbing hydrangea, Alabama supplejack, greenbrier, sweet pepperbush, violet, and sedge.  Spanish moss covers the trees.  Bar/levee forests are similar to bottomland hardwood forests but are distinguished by the abundant presence of river birch or water elm (Planara aquatica which is not a true elm).

Macrofossils of Atlantic white cedar and palm have also been found among the dead cypress.  There are small disjunct colonies of Atlantic white cedar scattered throughout the southeast, indicating it was more widespread in the region during the Ice Age.  (See: https://markgelbart.wordpress.com/2012/03/11/the-discontinuous-range-of-the-atlantic-white-cedar-chamaecyparis-thyoides/ ) The presence of palm shows that climate, though cooler than that of today, was still warm enough for that species.  I suspect this was a unique forest that doesn’t exactly match any classified natural community of the present day.

Image result for cypress swamp with oak trees

60,000 years ago, a cypress and oak forest grew at a location 15 miles off the coast of Alabama.  It was rapidly inundated during a sudden rise in sea level, becoming covered in sediment before the cypress trees rotted away.

The pollen evidence suggests alder was a pioneer species here that probably became established when the point bar of the river began depositing sediment.  Cypress and oak became dominant for about 500 years.  Then, after salt water intrusion killed the cypress, grass pollen predominates, suggesting a salt marsh replaced the cypress forest.  Extinct megafauna such as mastodon, tapir, and capybara undoubtedly passed through this environment, but vertebrate fossils have yet to be found.

Below is a documentary about the flooded forest–the source of information for much of this blog entry.

Reference:

Schafale, M; and A. Weakley

“Classifications of the Natural Communities of North Carolina, third approximation”

North Carolina Natural Heritage Program 1990

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