Posts Tagged ‘Heinrich Events’

The Vice-Versa Climate Phases of South Florida during the Late Pleistocene

September 3, 2018

Drastic climatic fluctuations occurred during Ice Ages.  Sudden warm spikes in average annual temperatures followed by rapid onsets of much colder climate phases altered the quantitative composition of plant species in the environment.  In most of North America oaks and other broad-leafed trees increased in abundance during warm wet interstadials but waned during arid cold stadials when coniferous parkland forests consisting of pine and spruce expanded in response to the changing climate.  These fluctuations were part of a feedback loop.  The warmer temperature phases gradually caused giant ice dams to weaken and break.  Torrents of cold freshwater from glacial lakes with floating icebergs flooded the North Atlantic, shutting down the Gulf Stream which during present day conditions moderate temperatures.  These deluges of iceberg studded meltwater are known as Heinrich Events, and they caused average annual temperatures to drop by 15-20 degrees F within decades.  In response to the drop in temperatures the Ice Sheets expanded for thousands of years until the next warm phase.  Climate also became drier because moisture for potential precipitation became locked in glacial ice.  However, evidence from a lake in south Florida suggest climate there was out of sync with the rest of North America north of the Rio Grande.

Graph depicting Heinrich Events and the subsequent fall in temperature.  Pollen evidence from sediment in Lake Tulane, Florida reveals the floral response to these climatic shifts, including the most recent 6 Heinrich Events.

Location of Avon Park in Highlands County, Florida.

Lake Tulane is located near Avon Park, Florida.

Scientists have taken numerous cores of sediment from Lake Tulane, located in south central Florida.  Lake Tulane is a very old body of water and has probably existed for almost 5 million years since this region emerged above sea level.  The sedimentary record goes beyond the limits of radio-carbon dating (50,000 years). The pollen composition in these cores shows the local environment’s response to Heinrich Events, but curiously it is the opposite from that of the rest of North America.  When the rest of North America experienced a warm wet interstadial, south Florida became cooler and drier (though mostly not sub-freezing).  The landscape transformed into an environment dominated by scrub oak, Florida hickory, red cedar, ragweed, grass, asters, staggerbush (Lyonia sp.), and rosemary (Ceratiola).  Scrub oak thickets surrounded by open spaces prevailed.  These are all drought-tolerant but shade-intolerant species, and red cedar is fire-intolerant, indicating the rarity of lightning-induced fires.  Conversely, when the rest of North America suffered cold dry stadials south Florida was warmer and wetter and pine forests spread across the land because lightning-induced fire frequency increased, and pine is fire tolerant.  Scientists find macrofossils of aquatic plants in the sediment representing dry phases because the lake was shallow enough to support emergent marsh vegetation, but these are absent during the wetter phases.

A brand new study introduced a new line of evidence that supports assumptions based on the earlier pollen evidence.  The authors of this study looked at variations in carbon and oxygen isotopes from Lake Tulane plant leaf waxes.  Scientists can understand the historical precipitation characteristics by studying the isotopic composition of plant leaf waxes.  (Plants synthesize organic compounds by using hydrogen atoms they absorb from water molecules.  The isotopes vary depending upon their source.)  They discovered that during climatic phases when scrub oak and ragweed dominated, average annual precipitation dropped by 22%.  The source of precipitation was different too.  Oak phase precipitation mostly came from storm fronts, but pine phase precipitation originated from tropical oceanic storms.

The vice-versa climate of south Florida was tied to the shifting Gulf Stream.  Under non-Ice Age conditions the Gulf Stream carries tropically-heated water to the North Atlantic as far as the coast of Canada, moderating temperatures.  When it shut down following Heinrich Events, the warm water stayed near the coast of south Florida, keeping it warm and wet while the rest of North America suffered dry cold conditions.  The Gulf Stream eventually restarted, bringing warmer wetter temperatures north, but this caused cooling and aridity in south Florida.

I hypothesize the Gulf Stream periodically began to restart within stadials, then shut down with new influxes of meltwater.  These partial changes likely influenced temperatures near the coasts of Georgia and South Carolina.  The climate may have temporarily been warmer in this region even during colder climate stages, and the composition of species here may have varied as well, though it involved different species than the Florida endemics.  Eventually, when the Gulf Stream restarted for longer periods, coastal Georgia may have experienced warmer climates centuries before northern latitudes of North America did.

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Common ragweed.  It was much more common in south Florida during the late Pleistocene than it is today.

There is no modern analogue for the abundance of ragweed (Ambrosia artemisifolia) in south Florida during the late Pleistocene.  Ragweed prefers cooler nights than occur in present day Florida for germination, and its abundant presence during the late Pleistocene indicates cooler nights prevailed, even during the out of phase warmer climate stages.  Ragweed is a tough plant that grows on disturbed sites such as abandoned agricultural fields and vacant lots.  It produces up to 32,000 seeds per plant, so it is able to survive heavy foraging by herbivores.  The seeds persist indefinitely and can wait for centuries before germinating when an environment transforms into a sunny one.  This means it can lay dormant through several stages of forest succession, and then colonize the habitat when conditions become favorable.  This may explain why it was so successful during the late Pleistocene when both sudden climatic fluctuations and megafauna foraging greatly disturbed and altered the landscape.  Many animals consume ragweed.  Rabbits and meadow voles eat the leaves while birds including juncos, cowbirds, quail, purple finch, mourning dove, goldfinch, and red-bellied woodpeckers eat the seeds.  Rabbits and meadow voles were common in south Florida during the Pleistocene, though the latter is presently restricted to 1 county in the state.  Farmers report livestock prefer giant ragweed (Ambrosia trifida) over clover.  It’s unclear if this species lived in south Florida during the late Pleistocene.  It is found in a couple counties, and this may represent relic populations, but they also may be invasive.  Livestock will also eat common ragweed, though they don’t like it as much.  Pleistocene megafauna such as horses, bison, and mammoth likely ate ragweed and grass, and they probably occurred in large herds during both climatic phases.  Other species probably common during the oak phase were Harlan’s ground sloth, northern pampathere, flat-headed peccary, and giant tortoise.  Flat-headed peccaries preferred thorny thickets, and the other 3 liked open environments.  Predators such as saber-tooths, lions, jaguars, and dire wolves fed upon the herbivores.  Ragweed survived megafauna foraging by producing large numbers of seeds, but 2 other plant species in this environment survived because their leaves were toxic–rosemary and staggerbush.  I think both climatic phases in Florida supported approximately similar populations of megafauna.

References:

Arnold, T. Elliott; et. al.

“Climate Response of the Florida Peninsula to Heinrich Events in the North Atlantic”

Quaternary Science Reviews 194 2018

Grimm, Eric; et. al.

“A 50,000 year old record of Climate Oscillation from Florida and its Temporal Correlation with Heinrich Events”

Science 9 July 1993

Grimm, Eric: et. al.

“Evidence for Warm Wet Heinrich Events in Florida”

Quaternary Science Review 25 Sept 2006

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Pleistocene Squid

February 11, 2018

The cephalopods were the most intelligent creatures on earth for hundreds of millions of years.  Nectocurus pteryx, a squid-like ancestor of all cephalopods, lived 500 million years ago during the Cambrian Age.  Fossil specimens of this species are found in the famous Burgess Shales.  Cephalopods–a group that includes squid, octopus, cuttlefish, nautiloids, and the extinct ammonites–evolved arms they can use to manipulate objects, and squid, through convergent evolution, evolved eyes quite similar to the human eye, so they can see the world like we do.  This explains how they evolved intelligence much greater than that of other invertebrates.

This blog article, like my entry about Pleistocene spiders, is entirely speculative because cephalopods have soft bodies that are also rarely preserved.  During Ice Ages sea levels receded and dry land extended across the continental shelf, today inundated by ocean water.  It seems likely deep water species of squid inhabited waters adjacent to the shore because steep drop-offs existed much closer to land during these climatic stages.  Giant squid (Architeuthis dux), reaching lengths of 43 feet, and colossal squid (Mesonychoteuthis hamiltoni), almost as long, probably lurked near the coast, whereas today they are normally restricted to deeper waters far out to sea.

The Gulf Stream current that keeps land temperatures moderate in the northern hemisphere often shut down or was greatly reduced during episodes of glacial meltwater influxes known as Heinrich Events.  These must have had an impact on squid migration.  Many species of squid migrate long distances to spawning grounds, and Heinrich Events must have altered their paths of movement, species abundance, and species composition.  Large die-offs probably occurred in some species, while others may have benefitted from the chaos.

Squid are an important food source for marine mammals, and deep sea species of whales likely ventured closer to shore in search of squid during Ice Ages.  Seals then living on the shores of the Atlantic Coastal Plain fed on squid.

The composition and species abundance of squid during various stages of the Pleistocene will forever remain a mystery.  There are over 300 known species of squid in the world today, but scientists know little about squid species abundance of the present day, let alone of the distant past.  One study of squid off the eastern coast of Florida determined eye flash squid (Abralia cf veranyi), flying squid (Ommastrephidae sp.), and shortfin squid (Illex sp.) were the 3 most abundant genera or families.

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Eye flash squid are 1 of the most common species found off the coast of eastern Florida.

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Shortfin squid–another common species.

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Flying squid shoot out of the water to escape predators.  

sperm-whale-and-giant-squid_smalldynamiclead_dynamic_lead_hero_image

Sperm whales feed mostly on squid.  Individuals can be distinguished by scars from battles with giant squid.

I’m not impressed with the flavor of calamari.  I’ve had it in a Vietnamese pho soup.  The soup itself was delicious, but the calamari was rubbery and tasteless.  I’ve tried fried calamari but this too had no flavor.  The best squid I’ve ever eaten was a canned Korean product.  The squid, packed like sardines, were seasoned with soy sauce and sugar.  The seasoning would’ve made anything taste good.  However, the squid were not cleaned, and I had to be careful chewing so I wouldn’t break a tooth on the hard beaks.

Reference:

Erickson, Carrie; Clyde Roper and Michael Vecchione

“Variability of Paralarval-Squid Occurrence in Meter-net Tows from East of Florida, USA”

Southeastern Naturalist 16 (4) 2017

 

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

January 6, 2011

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.

When Icebergs Drifted off the Coast of South Carolina

December 29, 2010

12,900 BP

A giant ice dam cracks in hundreds of places causing deafening roars like thunder.  Chunks of shattered ice slide off the rim, plunging into a growing river of water.  Behind the ice dam, a great lake swirls furiously, bearing enormous pressure against the frozen barrier, like a wild beast scratching against the wall of a cage.  Ravens, ducks, and geese flee the scene.  A sudden thunderstorm blackens the sky, bringing more thunder…and rain, the final blow.  The precipitation heightens the water level which begins to flow between a submerged crack as well.  Angry bubbles blow the fissure open.  And the ancient lake rushes to escape its prison.  The water explodes, now a waterfall not unlike Niagara.  It’s a biblical deluge of icy water roughly following the Nelson River but overflowing its banks for miles.  Big chunks of ice collapse into the deluge, carrying boulders, rocks, and topsoil.  The water strips trees out of the ground, captures and drowns herds of bison and other poor beasts caught in its stormy path.  The rush of water reaches the sea, an army of icebergs and an enormous quantity of fresh, cold, cold water hits the North Atlantic.  This frigid water meets the warm gulf stream, and the cold water wins, forcing the warm salty water to sink.  It shuts down this conveyor belt, the all important thermohaline circulation that keeps the northern hemisphere climate mild and warm.  Earth plunges back into the depths of an Ice Age for another two-thousand years.  Once again, the climate has suddenly become cold and arid, and glaciers advance rather than retreat as they had been doing for two-thousand years.

Side scan sonar image of iceberg keel scours off the coast of South Carolina.  This image is from the paper referenced at the bottom of this blog entry.

My dramatization above is what scientists refer to as a Heinrich Event, also known as a meltwater pulse.  Such events have occurred in regular cycles over the past 11,000 years, but on a much smaller scale than those of the Wisconsinian Ice Age.  During Ice Ages, glaciers expand and dam rivers, creating enormous glacial lakes.  Warm climate cycles melt these ice dams in a process that takes thousands of years.  Interstadials result as warm rainy climate prevails when more and more water is transferred from ice to the atmosphere.  Eventually, these ice dams would collapse and the frozen torrent flowed into the North Atlantic, shutting down thermohaline circulation, causing a reversal in climate phases.  This explains why the Wisconsinian Ice Age (and all Pleistocene Ice Ages) had rapidly alternating interstadial and stadial phases (though orbital perturbations, the rise of the Himalayas, and other tectonic processes are the ultimate factors behind it all).  Today, these cycles are not as dramatic because the north polar ice cap is much smaller and glaciers don’t stretch over all of Canada, like they formerly did.

Evidence of these astonishing Heinrich Events lie on the bottom of the North Atlantic.  “Ice-rafted debris” (actually called Heinrich Layers) consisting of drop stones and boulders, as well as sediment, exist deep under the ocean.  The rocks originated in what today is mid-western North America and could have only arrived at the bottom of the ocean here, if encased in icebergs because water can’t move heavy boulders, but it can float ice impregnated with these heavy rocks.  Numerous furrows also line the North Atlantic floor.  Geologists call these furrows keel scours, and they were made when the toes of icebergs scraped along the bottom of the ocean as the flow of water carried them south.  The toes of icebergs flipped over rocks and boulders that were in their way, and these are visible as well.

Jenna Hill and five other scientists discovered keel scours off the coast of South Carolina about 660 miles south of where the ice margin was during the last glacial maximum (~29,000-15,000 BP).  The scour marks are 500-660 feet under water, so the scientists had to use side scan sonar images to see them.  They took these images from the Nancy Foster, a NOAA research ship.  The scour marks range in width from 30 feet to about the length of a football field, and they’re about 30 feet deep.  Some are as long as 6 or 7 miles, and they’re orented west/southwest which is in the opposite direction of where the gulf stream flows today.  Near the end of these furrows are a series of circular pits.  Apparently, as the icebergs hit shallow ground, they got stuck.  Periodically, they melted, floated a while longer, and got stuck again in a mode of motion resembling a pogo stick.

Imagine vacationing on Myrtle Beach and seeing icebergs and smaller pieces of ice drift by, with seals and walruses sprawled on the latter.  Of course, what today is Myrtle Beach was too far inland then for a person to view the ocean.  Still, it’s amazing to think how much the world has changed since icebergs floated off the coast of South Carolina.

Reference:

Hill, Jenna; et. al

“Iceberg scours along the southern U.S. Atlantic Margin”

Geology June 2008