Posts Tagged ‘stadial’

When Ice Age Georgia Became Desert-like

November 7, 2011

During the Last Glacial Maximum when much of the world’s water became locked in glacial ice sheets, the climate in southeastern North America was much drier.  Small rivers dried up and larger rivers shrank in size and became braided in pattern so that they were like long chains of disconnected channels.  The exposed river sands blew into eolian dunes like those from this picture of a modern Mojave desert dune.  Ironically, during these cold stadials, as the climate became dry in southeastern North America, precipitation increased in the southwest, so that it was quite lush there.  

Dry climate phases have periodically struck southeastern North America many times over the past 5 million years.  Scientists know little about the paleoecological details of most of these phases because there’s not much available data.  But thanks to one study, they do have a relatively clear understanding of the ecological composition of south central Georgia from 30,000 years BP-25,000 years BP.

Scientists took a 17 foot core of sediment from a peat deposit next to Sandy Run Creek on Warner Robins Air Force Base which is located near Macon, Georgia.  They radio-carbon dated the sample and counted the pollen grains to determine what the environment was like during this time period.  This was a time of glacial expansion and as much of the earth’s water became locked in ice, less moisture in the atmosphere was available as precipitation.  Southeastern North America experienced extended droughts causing the water table to drop.  Water flow on major rivers was greatly reduced creating braided river patterns and this turned the rivers into chains of disconnected channels interspersed with islands and sandbars.  In some cases small rivers completely dried up, exposing great quantities of river borne sand.  Atmospheric conditions caused by the glacier to the north spawned frequent westerly and southwesterly winds that blew this river borne sand across the landscape forming huge eolian sand dunes.

Today, vegetation has taken root and holds down the eolian sand dunes that formerly rolled across Georgia’s landscape during cold arid stadials.  Now, scrub turkey oak and longleaf pine covers the Ohoopee sand dune in Georgia.  The sandy soil is of poor quality and not enough litter accumulates to foster fire, allowing scrub oak to become more common than pine. Photo is from google images.

Photo from google images of the Ohoopee River.  This and other small rivers dried out during cold dry climate phases.  Instead, small pools of water appeared sporadically in the river bed.  The scene would have resembled modern African water holes.

The evidence from the pollen composition of the Sandy Run Creek peat core indicates a much different landscape than occurs anywhere in Georgia today.  While eolian sand dunes rolled to the east of reduced or even completely dried rivers, lightly wooded grasslands predominated over much of the environment.  Here and there were groves of pine with some spruce.  (The species of pine isn’t known, but my educated guess is they were a mixture of northern and southern species, probably shortleaf and white.  The species of spruce was likely the extinct Critchfield’s.) Oaks and other deciduous trees clung to the vicinity of shrinking water holes found along the braided rivers.  Pine composed 39%-75% of the pollen, while oak only made up 12%. Grass and coniferous trees require less water than hardwoods, and are less prone to physical damage from frequent wind, explaining why they were more abundant.  Scientists found no charcoal in the part of the core dating from 30,000 BP-25,500 BP–evidence wild fires were a rarity.  This suggests a thinly vegetated environment where combustible material such as dead wood didn’t accumulate.  Moreover, lightning storms that ignite fires were uncommon.  Charcoal is present in the part of the core dating from 25,500 BP- 25,000 BP, perhaps indicating a weak interstadial with more frequent electrical storms.

Grass-eaters such as mammoths, horses, and bison likely predominated in this kind of environment along with the occasional Harlan’s ground sloth which preferred more open environments than its cousin–Jefferson’s ground sloth.  Badgers, thirteen-lined ground squirrels, and perhaps jackrabbits colonized the region then.  Animals that prefered more forested environments were restricted to riverine woods.  Game accumulated around shrinking water holes and this probably contributed to erosion of riverbanks which in turn added sediment to the formation of eolian dunes.  These congregations of herbivores attracted predators such as dire wolves, jaguars, and saber-tooths.

You can buy this illustration of flat-headed peccaries (Platygonus compressus) from the website engraved on the image.  This kind 0f peccary was probably pretty common during dry climate phases in the south.  They ate tough spiny vegetation such as cactuses.  They lived in large herds that were probably aggressively defensive, much like modern white-lipped peccaries.

Photo of a jaguar in Arizona.  Jaguars inhabit many different types of environments such as deserts and rain forests.  They were adaptable enough to probably have been the most common large cat in southeastern North America during stadials and interstadials.

Photo of a hog-nosed skunk.  Today, this species lives in Mexico and the southwestern United States.  Fossils of hog-nosed skunks have been found in Georgia and Florida.  They must have colonized the region during dry climatic phases.

There’s no sediment in the Sandy Run Creek peat core dating from 25,000 BP-13,000 BP.  Scientists call this an erosional unconformity.  They believe the creek changed coarse or flooded and washed away all the sediment accumulated during this time period.  This is consistent with what we know of the environmental changes that occurred during this time period.  About 16,000 years BP the Boling-Alerod interstadial began.  The Laurentide Glacier rapidly commenced melting, putting more moisture in the atmosphere and precipitation increased.  The water table rose and so did river flow.  Rivers no longer consisted of braided patterns, but instead meandered to an even greater degree than they do today, forming scroll-like sandbars.

Satellite view of a meandering river with scroll bars.  During the Boling-Alerod Interstadial beginning about 16,000 years ago, precipitation increased, causing rivers to meander even more than they do today.  This new river pattern formed frequent scroll bars. a kind of sand bar created when the meander of a river continously shifts and leaves ridges parallel to the meander.

Though sand dunes no longer rolled across the landscape during the interstadial, there’s evidence of considerable sandbar formation.  Scroll bar formation suggests a dry season/wet season climate.  Autumn and early winter were mostly dry and river levels fell, exposing sand bars.  But existing atmospheric factors caused heavy precipitation in late winter, spring, and early summer.  Warm tropical fronts collided with cold parabolic winds originating from the still extant glacier.  This spawned great snow, ice, and rain storms that caused massive floods.  Rivers shifted.  Occasional tropical storms compounded this trend.

The pollen record of the Sandy Run Creek peat core, which picks up again about 13,000 BP, demonstrates a much different environment from that of 25,000 BP.  From 13,000 BP-11,000 BP there was a sudden cooling trend known as the Younger Dryas.  The paleobotanical evidence, however, still shows the influence of the previous interstadial warming trend.  A cool, moist, open oak woodland prevailed in south central Georgia during this time period.  Oak pollen doubled from 12% to 24% while pine pollen declined to just 7%.  Critchfield’s spruce and fir were still present but so were hickory and beech–a clue that temperatures were moderate but remained cooler than those of today.  An increase in charcoal is evidence that vegetation was thicker than it had been in the previous time period because now there was more forest litter available as tinder for fires.  And the frequency of lightning storms, which ignites fires, increased.  A northern species of alder, a type of shrubby birch that no longer occurs in Georgia, commonly grew in abandoned, dry, river meanders.  Grasslands still existed to a greater extent than occurs naturally today but had declined in abundance compared with 25,000 years BP.

Scientists use an interesting method to help determine changes in the density of vegetation over time.  They add exotic pollen to cores of sediment.  In this study of the Sandy Run Creek sediment core, they added a known quantity of eucalyptus–a species which didn’t live in North America during the Pleistocene.  The ratio of eucalyptus to native pollen was high during the time period of 30,000 years BP-25,000 years BP–evidence vegetation density was low.  Conversely, the ratio of introduced eucalyptus pollen was low during the time period between 13,000 years BP-11,000 years BP–evidence the vegetation density was higher.

The change to a more moist wooded environment favored higher populations of mastodon, deer, long-nosed peccary, bears, beavers, tree squirrels, and cottontails.

References:

Lamoreaux, Heidi; George Brook, and John Knox

“Late Pleistocene and Holocene Environments of the Southeastern U.S. from the Stratigraphy and Pollen Content of a Peat Deposit on the Georgia Coastal Plain”

Paleogeography, Paleolimnology, and Paleoecology 2009

Leigh, David

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

Geomorphology 2008

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Paleomeander Scars along the Altamaha River

June 23, 2011

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

Ocean Drilling Project 1059A Found a Treasure for Paleoecologists

May 9, 2011

A photo of an ocean drilling project from google images.

In 1997 oceanographers journeyed to a remote location over the Blake Outer Ridge, an extension of the continental shelf that consists of sedimentary drift.  “Sedimentary drift” is just a fancy expression for the sloughing off of eroded land.  Here, the ocean is almost two miles deep.  Nevertheless, they were able to send a drill to the bottom, but it didn’t stop there–it pierced the deep sea mud for the length of a football field and brought back a plug of this carbonate ooze for analysis.  They found no oil, nor gold, but they did find something of indispensible value for paleoecologists and paleoclimatologists–ancient pollen and foraminifera.

Map of Blake Outer Ridge from google images.  The coordinates of where Ocean Drilling Project 1059A took a core is 31 degrees north 40,46 and 75 degrees west 21,13.

Two brilliant scientists from Columbia University conducted a remarkable study of this plug of ocean mud.  Linda Huesser and D. Oppo took samples at 10 cm. intervals to a depth of 40 meters.  They estimated these intervals to be the temporal equivalent of ~400 year intervals, so that for every 10 cm. they were turning the page of an ecological record book that stretched back 400 years a page.  The study covers a period of time from ~140,000 Bp-~50,000 Bp, making this the only “chronostratigraphical” study of pollen from this time period in the southeastern region of North America.

They found a correlation between oxygen isotope ratios in oceanic foraminfera, and the waxing and waning of spruce/pine and oak forest abundance.  Foraminifera are single celled protozoa with shells made of calcium carbonate.  Foraminifera absorb oxygen in their shells from water.  This sea water contains varying amounts of heavy oxygen, or O-18, which is an isotope of normal oxygen.  (An isotope is an element with a different number of neutrons than its parent element.)  During cold climatic stages known as glacials and stadials, there is more heavy oxygen in the ocean because of increased evaporation due to a more arid climate as much of earth’s water becomes locked in ice.

Photos from google images.  Top: Cibicidoides weullerstarfi.  Bottom: Globigerinoides ruber.  Scientists used fossils of both of these species of foraminfera  in this study to determine past temperatures.

Cesar Emiliani formulated a mathmatical chart that estimated past temperatures based on ratios of heavy oxygen to normal oxygen in the shells of foraminifera.  These estimates are considered accurate.  He numbered past stages of climate corresponding to the oxygen isotope ratios.  The stages from 75,000-140,000 years ago are known as 5a, 5b, 5c, 5d, 5e, and 6.  The final stage of the Illinois Ice Age is 6; the Sangamonian Interglacial is knowns as 5e; 5c and 5a are warm interstadials within the Wisconsinian Ice Age; 5b and 5d are stadials, or cold stages within the Wisconsinian.

Pollen graph from the paper referenced below.  The x axis is time represented at 5,000 year intervals from 140,000-50,000 BP.  The y axis is abundance of pollen from oak, pine, spruce, hemlock, and herbs.  Planktonic and benthic foraminfera oxygen isotope ratios are also on the y axis.  There is a correlation between oxygen isotope ratios and the abundance of certain kinds of plant pollen.  Low ratios of heavy oxygen indicate warm climate.  These low ratios correlate closely with high abundance of oak, and vice versa.

Pollen from southeastern North America reached the Blake Outer Ridge via wind and rivers flowing into the ocean.  The pollen graph above shows the correlation between tree pollen and oceanic temperatures estimated using oxygen isotope ratios in foraminfera as proxies.  140,000 years ago pine dominated the forests and spruce was a common component.  The transition ~132,000 years ago from the Illinois Ice Age to the Sangamonian Interglacial was marked by a dramatic increased abundance of oak and a significant decrease in spruce.  Pine remained common throughout, only falling below 50% during one interstadial, though at the beginning of the Sangamonian it fell from 70% to 55%.  This data is interpeted to mean that oak forests and oak and pine savannahs prevailed during warm climatic stages, while pine/spruce dominated the landscape during cold climatic stages.  In the southeast, however, spruce never became as prevalent as they did in the Appalachians and midwest.  Probably, the increase of spruce in the south can be explained for a couple of reasons: 1) Cooler summers allowed for an influx of spruce from the extensive spruce forests of the north, and they took space away from oaks. Pure spruce forests may have existed in north Georgia, but in the piedmont and coastal plain it probably existed as an occasional component of mixed forests. 2) Coniferous trees compete better than oaks in arid atmospheres with lower CO2 levels like those that occurred during stadials.

The graph shows that oaks increased with every interstadial, but unlike the interglacial of today, spruce and hemlock didn’t disappear completely from the coastal plain, indicating summers that were still cool enough at least for one species of spruce as well as the hemlock.

In my March 23rd blog entry I reviewed some of the Natural Environments of Georgia chronicled by the late Charles Wharton.  Here, I hypothesize on some Ice Age environments that likely were common during stadials and interstadials.  Ice Age floral associations should be considered the norm because glacial stages including both usually last 10 times longer than full blown interglacials.  Today, floral associations that we consider normal are actually an aberration.

1. Spruce, hickory, beech–W.A. Watts found a pollen fossil site in north Florida dating to late in the Wisconsinian Ice Age that seems to have been a forest co-dominated by these unlikely “disharmonious” species.  He discovered this site before science knew about the extinct species, Critchfield’s spruce, which apparently was a more temperate type than its northerly cousins.  I suspect spruce, hickory, beech was probably a common type of old growth forest association throughout the south, especially during interstadials.  There is no evidence that any of the modern spruce species penetrated farther south than Bartow County, Georgia.  Most of the spruce pollen from the ODP 1059 study likely came from Critchfield’s spruce.

2. Grassy oak savannahs–Extremely rare today, I suspect this was a common landscape during warm dry interstadials.  A pollen sample from sediment at the vertebrate fossil site, Watkins Quarry, showed evidence of an environment with lots of grass and some oak.  Dry climates inhibited the growth of trees, but the decreased number of thunderstorms meant a lesser frequency of fire, perhaps allowing oaks to outcompete pines.  An environment like this would have been ideal for megafauna and perhaps they maintained it through grazing, trampling, and uprooting trees.  Acorns, grass, and berries provided a lot of food.

3. Open pine savannah–I already discussed this type (still common until European settlement) in an earlier blog entry.

4. Mixed forests of southern and northern pines and oaks–The piedmont was  likely a transition zone between spruce forests in the mountains and pine and oak savannahs in the coastal plain.  The Nodoroc mud volcano in Winder, Georgia yielded pollen from both northern and southern pines as well as oak, hickory, fir, chestnut, and beech.  It was likely a region with mixed stands of white pine (a northern type) and shortleaf pine (a southern type), and oaks interspersed with meadows and bushy thickets in varying stages of forest succession depending upon when the last fire or tornado swept through.  These provided a variety of patch habitats for wildlife.

5. Blue Stem Black Belt Prairie–Black belt soil with blue stem grass prairie was more common in Alabama and Mississippi until European settlement but some occurred in Houston County, Georgia.  The soil in some of these areas inhibits tree growth.  Unchecked fires caused by frequent thunderstorms during warm wet climates, droughts during dry climatic phases, and megafauna grazing would’ve made this type of environment more widespread during the Ice Age.

6. Oak sand scrub–During cold arid stadials, rivers shrank and dried up completely in many places.  Winds blew river sands across the landscape in big eolian dunes.  Blackjack oaks, cedars, and grass took root on some of these, but the poor quality of the soil only allowed for scrubby growth.  Oak thickets, cactus, and sparse grass were likely a favored habitat of the flat-headed peccary and hog-nosed skunk.

7. Mountain pine, spruce, and meadow–This type was probably common in the north Georgia mountains and consisted of white pine, and several kinds of spruce interspersed with large meadows. Appalachian balds may be relics of this type of environment.  

Photo I took of an Appalachian bald in North Carolina.  This type of landscape was probably common in north Georgia during the Ice Age, and may have occurred well into the piedmont.  One of my very first blog entries is all about Appalachian balds which contain disjunct populations of plant species not found anywhere else south of Canada.

8. Hemlock Forests–Cool moist climatic stages allowed for the spread of this kind of environment.  The highest level of hemlock pollen occurred during the late Sangamonian Interglacial, indicating cool moist climate preceded the regrowth of glacial ice.  In fact one of the findings of the below referenced study was that  temperature changes precede changes in growth or dissolution of glacial ice.

Reference:

Heusser, Linda; and D. Oppo

“Millenial and orbital scale climatic variability in southeastern U.S. and in the subtropical Atlantic durin MIS 5: evidence from pollen and isotope in ODP site 1059”

Earth and Planetary Science Letters 214 (2003)

How Unusually Cool Ice Age Summers Probably Shaped Periodical Cicada (Magicicada) Evolution

May 3, 2011

It sounds like everybody’s burglar alarm is blaring in Evans, Georgia.  But the noise doesn’t originate from annoying, malfunctioning security systems.  Instead, the 13 year periodical cicadas have emerged.  For over a decade these insects have lived a foot underground, well below the frost line, but now they’re ready to mate–a frantic affair that takes place within a timespan of 3-4 weeks.  Almost their entire lives, they’ve lived as nymphs, surviving on the xylem fluids of deciduous trees.  The urge to mate causes them to dig tunnels to the surface which they crawl through.  Sometimes they continue to crawl, making it halfway up a wall or a tree trunk before the winged adult bursts through the back of the shell of its thorax.  The holes from which they’ve emerged are visible, their molted shells scattered under foot like discarded shrimp exoskeletons at a Cajun seafood boil.

Photo from google images of a periodical cicada.

The unusually long period between emergences among the 7 species of periodical cicadas in the genus Magicicada puzzles scientists who hypothesize about its evolution.  Magicicada emergences in large numbers are obviously a defense mechanism known as predator satiation.  Like passenger pigeons, they occur in such a high population that they overwhelm the ability of predators to consume them.  Unlike other types of cicadas which are strong, fast fliers, periodical cicadas are clumsy and slow, but so many appear at once that predators are unable to consume most of them before they have a chance to mate and lay eggs.  Some scientists think their emergences every 13 or 17 years (depending on the species) is a way to keep predators from increasing their own populations after exploiting them as a food source, thus avoiding a cycle when predator numbers eventually escalate enough to decimate cicada numbers.  This seems an unlikely explanation to me for 2 reasons: cicadas are only available for a few weeks a year which is not enough time to have a significant long term impact on predator populations.  More bird nestlings may survive at first due to the abundance of cicadas, but then for the rest of the year, they must adapt to the normal supply of food.  Moreover, the average wild bird only lives 2 years–far shorter than 13 or 17 years.  Cicadas could avoid upswings in predator populations with much shorter periods between emergences.  Other scientists believe the odd high prime numbers emergences can be explained by a combination of predator cycle avoidance and interspecific competition among nymphs.  Although several entymologists have derived statistical models supporting this theory, I think the paleoclimate explanation proposed by R.T. Cox, C.E. Carlton, and independently by Yoshimura is more plausible.

Periodical cicadas depend specifically upon deciduous forests.  During the coldest stages of Ice Ages, deciduous forests north of the southern Appalachians were rare relics outnumbered by other environments such as spruce forests and prairies.  The bulk of deciduous forests then occurred south of the Appalachian mountains.  Even here, summer temperatures occasionally were too cold for cicadas in the Magicicada family.  They require temperatures above 68 degrees F for a period of 3-4 weeks for flight and mating.  Drs. Cox and Carlton assumed that during the coldest stadials (which lasted on average 1500 years) 1 in 50 summers failed to reach this temperature, and cicada reproduction failed.  Using a statistical formula, they estimated that over a 1500 year stadial, cicadas emerging every 6 years had a 4% chance of avoiding unusually cool summers; cicadas emerging every 11 years had a 51% chance of avoiding unusually cool summers; but cicadas emerging every 17 years had a 96% chance of avoiding unusually cool summers.  Cicadas emerging after shorter periods were eventually eliminated from the gene pool, while those with genes for longer cycles became dominant.

Map of Magicicada ranges from the below referenced paper.  The distribution of 13 and 17 year periodical cicadas supports the paleoclimatic explanation for their high prime number emergences.

17 year cicada species tend to live north of 13 year cicada species, even though the shorter cycle is a dominant gene.  Summers too cool for breeding would’ve occurred more frequently in the norther parts of their range, so those with 17 year cycles would’ve had a greater chance of avoiding them than those with the 13 year cycle.

Reference:

Cox, R.T. and C.E. Carlton

“Paleoclimatic influence in the evolution of periodical cicadas (Insects: Homiptra:Cicidae: Magicicada spp.)”

The American Midland Naturalist 120: 183-193 1988

Notes on my observations of periodical cicadas

–Periodical cicadas are slow.  I was able to catch one by simply picking it off the ground after if fell in flight and landed on its back.

–Birds are feasting on them.  Crows are catching the nymphs as soon as they crawl to the top of their tunnels.  I also saw a Canadian goose nab one that fell in a small lake.

–Wow! They are loud.

–They must be mole food during their nymph stage.

–Evans has become heavily developed.  Cicada habitat has been greatly reduced and covered with blacktop parking lots from which they can never emerge and escape.  I wonder if this is another creature that survived for millions of years, until the actions of Homo sapiens eventually renders them extinct.

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

If I could live during the Pleistocene (Part one)

September 9, 2010

For many years now I’ve often fantasized what I would see, if I could really travel in time back to the Pleistocene.  A quick trip of a few hours wouldn’t give me enough time to study the plants, animals, and climate of that time.  Instead, it would be necessary to spend many years there.  In fact I would like to spend a lifetime living in that long gone unspoiled wilderness.  Nevertheless, I’m not a big fan of roughing it, and I don’t want to give up the creature comforts of modern civilization.  I just don’t want to live without such things as college football, good books and movies, the internet, certain foods, and modern dental care.  So let’s suppose there’s a kind of time portal or wormhole that connects the modern world in 2010 with a time period, say about 38,000 radiocarbon (or about 41,000 calender) years before present.  I would live in a dwelling that I would construct 41,000 BP, but for example if I get a painful cavity, I would travel through the wormhole to the dentist’s office.  Moreover, I’d have wires running through the wormhole so I could communicate with the present.  I could sit in my 41,000 year BP home and watch modern television and communicate through phone lines and the internet with the modern world of 2010.

Why I would choose 41,000 BP

I would choose a place in what’s now Georgia before people lived in the region.  I don’t fear the megafauna–I can avoid rampaging mammoths and packs of dire wolves–but primitive men do worry me.  They could be cannibalistic or maybe they’d choose to kill me just for the hell of it–who knows what a primitive man’s motivation might be?  Besides, I want to study the ecology before humans had any impact on it at all.  Therefore, 41,000 years ago is probably a safe bet for avoiding humans in southeastern North America.  According to archaeologists, concrete evidence of humans in the region prior to 14,000 calender years ago is scarce.  Before 19,000 calender years ago, it’s nonexistent.   If there were people on the continent 41,000 years ago, which is highly doubtful, they were so few in number I’d doubt we’d cross paths.

Another reason why I’d choose 41,000 calender years ago is climate.  This time period was an interstadial–a somewhat warmer and rainier phase of the Wisconsinian Ice Age than a stadial or glacial phase.  Though temperatures are still cooler than those of today, for about 2,000 years they were warmer than the coldest time of the Ice Age.  When I originally conceived of this fantasy, I thought I might like to live during the last Interglacial, about 120,000 years ago.  However, the heat of the current summers in Georgia are uncomfortable enough, and the Sangamonian interglacial was even warmer.  I decided I’d rather live during a time when Georgia summers were much cooler than those of today.  The following is a link with a graph depicting climate change over the past 40,000 radiocarbon years.  Scientists are able to estimate past average temperatures based on the gas content found in air bubbles encased in a glacier in Greenland.  Scientists take cores of this 100,000 year old glacier and can count the years which correspond to the rings in the ice created by melting in summer and addition of ice in the winter, much like a tree’s age can be calculated by counting the number of rings in the trunk.  The ratio of heavy to light oxygen isotopes correlates to exact average temperatures.  Using this method, scientists know what temperatures were thousands of years ago.  Analysis of carbon dioxide and the amount of dust particles also give evidence of past climate.  Here’s the link:

http://www.mos.org/soti/icecore/studies.html

See the first upward spike in this graph.  That’s the time I would choose to live.  Note the precipitous drop in temperatures after the interstadial ended.  Average annual temperatures must have dropped drastically within a few decades.  Note also how steady average annual temperatures have been over the last 11,000 years (the Holocene) compared to the Pleistocene.  These sudden changes in climate from moderate and rainy to cold and dry must have had a profound effect on the composition of plant species and the distribution of animal species.  In recorded history climate fluctuations that are merely a blip compared to the huge shifts during the Pleistocene have had devastating effects on agriculture.  A period known as the little Ice Age lasted from 1320-1870.  Cold rainy summers ruined crops and caused mass starvation.  Imagine what would happen to modern agriculture, if average temperatures suddenly dropped by more than ten degrees F as happened frequently during the Pleistocene. 

Fluctuations in Pleistocene temperatures correlate with the waxing and waning of the Laurentide glacier which covered most of Canada throughout the last Ice Age.  Interstadials resulted from when warm temps melted the glacier which put more moisture in the atmosphere; stadials occurred when this melting ice caused massive amounts of cold fresh water and icebergs to flood the north Atlantic which shut down the gulf stream, causing a sudden reversal in climate.  More moisture became locked in ice and the atmosphere became more arid and sea level dropped.

Georgia’s summer temperatures were much cooler and more comfortable during the interstadial than thay are in the present, but winter temperatures in the southeast were probably only a little cooler than they are today, thanks to the gulf stream off the Atlantic coast, which scientist believe created a warm thermal enclave in the region, especially near the coast.

During the interstadial I’d expect to find a mix of 75% forest and 25% meadow or small prairie in what’s now east central Georgia.  I’ll have more on the  ecology and landscapes of my chosen place of habitation next week along with a discussion of the geographical location of my Pleistocene homestead, and the adobe house I would build.