Posts Tagged ‘Critchfield’s spruce’

The Nonconnah Creek Fossil Site in Memphis, Tennessee

April 28, 2014

Only 1 vertebrate species was excavated from Nonconnah Creek, a tributary of the Mississippi River that flows through Memphis, Tennessee.  In 1977 some kids found a skull complete with tusks and teeth of a male mastodon, later estimated to be between 15-18 years old at the time of its death.  Archaeologists and geologists soon descended upon the site and conducted a wonderful study that is very exciting for the paleoecology geek in me.  They found plant macrofossils and pollen, snail shells, and even insect parts in the sediment mixed with the mastodon specimen–a jackpot of information that can help scientists determined what the environment was like in this region during the late Wisconsinian Ice Age.

Nonconnah Creek is on the right side of the Mall of Memphis parking lot.  The mastodon fossils were uncovered after construction workers dug drainage ditches. 

New Pictures 064

Photo of tusk and teeth from the Nonconnah Creek Mastodon.  This photo is from the below reference.

The mastodon found here died about 20,000 calender years ago during the Last Glacial Maximum when the Laurentide Ice Sheet reached its maximum extent as far south as central Ohio.  The environment around Nonconnah Creek was much different during the LGM from that of today.  A closed canopy forest consisting of spruce and oak dominated the landscape.  The most common species of spruce in this forest was an extinct species known as Critchfield’s spruce (Picea critchfeldii).  (See:https://markgelbart.wordpress.com/2011/09/03/the-extinction-of-critchfields-spruce-picea-critchfieldii/)  This species had longer cones than the extant white spruce which it resembled.  This spruce and oak forest likely extended as far south as Louisiana and as far east as western Georgia.  Summers were much cooler, but winters were just slightly cooler than those of today.  Average temperatures were not as cold as scientists initially thought because they were using the presence of existing species of spruce trees as a proxy for temperature range, not realizing the spruce fossils they were finding were from an extinct species that was probably at home in temperate climates.  Foggy conditions often prevailed in the area then.  Glacial meltwater descending down the Mississippi River struck warm fronts originating from the Gulf of Mexico creating a cool moist environment that favored spruce trees.  The Mississippi River had a braided pattern with many exposed sandbars.  Frequent cold winds, blowing from the northern Ice Sheets,  blew sand dunes from the river into the spruce and oak forests, occasionally burying the trees.  This explains how these plant fossils were preserved.  (See:https://markgelbart.wordpress.com/2011/08/28/the-fossil-rich-region-of-tunica-hills-louisiana/).  Other species of trees occurring in this Ice Age spruce/oak forest included willow, chestnut, black walnut, hickory, paper birch, elm, cherry, sycamore, sugar maple, and red maple.  Mistletoe, a spruce tree parasite, was found here as well. Though modern hardwoods were present, they were far outnumbered by spruce.

The environment during the Farmdalian Interstadial that preceded the Last Glacial Maximum differed markedly from the spruce/oak forest discussed above.  Between 32,000 BP-28,000 BP, open pine and oak woodland with prairie openings prevailed here.  The Farmdalian was a weak interstadial that gradually gave way to cooler temperatures with lower evapotranspiration rates and lower CO2 levels in the atmosphere–conditions that evidentally favored spruce.  Fewer thunderstorms and lightning strikes reduced the incidence of wild fire, probably also explaining why closed canopy spruce/oak became dominant over open pine and oak woodland. 

 

Valvata tricarinata–A freshwater snail  It’s a common freshwater snail of lakes presently located north of where the Laurentide Glacier extended during the Last Glacial Maximum.  It’s uncommon south of that region though a disjunt relic population lives in the Savannah River, Georgia.  Interestingly, this was the most common species of snail found in the Nonconnah Creek fossil site–evidence its range was displaced farther south during the Pleistocene

Most of the Pleistocene-aged snail shells found at this site were from species commonly found in the region today, and they represent denizens of a variety of aquatic habitats from stagnant pools to fast moving streams.  Dam-building beavers and natural forest debris slowed down some parts of the creek while in other parts it was unimpeded.  Scientists identified 2 species of snails that no longer occur in the region. Today, Fossaria reflexa is restricted to the New England region, but during the Ice Age this species evidentally was displaced at least as far south as Tennessee.  Valvata tricarinata is a common snail species today in the region that was covered by glaciers during the Ice Age.  This species was also displaced south of its current range.  Curiously, a few disjunct populations of Valvata are found as far south as the Savannah River, but it is not common in the southern parts of its range.  However, it was the most common species of snail found at Nonconnah Creek during the Ice Age.  Snails are creatures often used to illustrate slowness, but these 2 species were able to keep pace with rapid climate change when they recolonized newly available habitat after the Ice Age.

 

 Dicaelus sculptilis - Dicaelus sculptilis

Dicaelus sculptilis, a ground beetle today found north and west of Nonconnah Creek.  It was the only recognizable insect found as fossil remains, dating to the Ice Age.

Most of the grasshopper and beetle remains associated with the mastodon had been dashed to unrecognizable smithereens.  However, scientists were able to identify 1 species–Dicaelus sculptilis, a ground beetle that currently ranges to the north and west of this site.

Reference:

Brister, Ronals; John Armon and David Dye

“American Mastodon Remains and Late Glacial Conditions at Nonconnah Creek, Memphis, Tennessee”

Memphis State University Anthropological Research Center Occasional Papers 10 1981

 

 

 

The Mysterious Nodoroc Site in Winder, Georgia

January 9, 2013

The geological origin of a mud volcano located in Winder, Georgia is a mystery waiting to be solved.  As far as I know, geologists haven’t ever studied this unexplained watery bog that last erupted circa 1800.  It’s probably similar to mud volcanoes found in Africa and southeast Asia.  About 10 years ago, a mud volcano in Cameroon exploded and killed a whole village by carbon dioxide asphyxiation. The carbon dioxide displaced the oxygen in the atmosphere.  An atmosphere of just 10% carbon dioxide causes people to become comatose; an atmosphere of 30 % CO2 causes people to drop dead immediately.

Winder, Georgia is in Barrow County which is between Athens and Atlanta.  My grandparents used to live there.

Nodoroc is a Creek Indian word meaning gateway to hell.  It’s an odd natural boggy pond that used to release a constant bluish smoke while bubbling.  The Creek Indians built an altar of heavy stones on the edge of the volcano where they executed criminals and then threw the corpses in the bog.  It was their way of sending deserving souls to hell.  They believed the volcano was protected by the wog–a devil dog with red eyes and the head of a bear.  Despite the cultural importance of the site, the Creek Indians sold the pond and the land around it to the English for 14 pounds of beads.  Some early colonist made off with the altar and now it’s lost to archaeologists.

I couldn’t find a photo of Nodoroc on google images that I could confirm was the actual site, but the below link is an aerial photograph.  The brown muddy expanse is labeled and obvious to see.  The link below that is of video of a mud volcano in Yellowstone National Park.  Unlike Nodoroc, it is still active and it probably has a different kind of origin.  The Yellowstone mud volcano is caused by heating and cooling subterranean rock on a fault line, while the Nodoroc mud volcano resulted from decaying organic matter.

http://www.wikimapia.org/#lat=33.973821&lon=-83.6727574&z=19&l=0&m=b

http://www.youtube.com/watch?v=iJ8qfpMkDs0

The first European explorers to visit Nodoroc say that it burned and dissolved everything they threw in it.  Even rainwater evaporated when striking the bog.  They reported that the area attracted lots of animals, but I think the abundance of wildlife resulted from Indians avoiding the place which they only used for executions.

Circa 1800 John Gosset built a cabin nearby and cleared a field around Nodoroc.  One day, Gosset and his wife witnessed the last eruption of the mud volcano.  His wife noticed an unusual amount of fog over the bog that morning.  She called her husband who was busy ploughing to come see it.  A loud explosion was followed by a shower of hot mud particles.  After the eruption the volcano settled several feet and cooled.  Decaying matter likely caused a fermentation reaction, releasing carbon dioxide and methane that had been trapped under the mud.

In the days of free range livestock Nodoroc was known as a cattle mire because cows constantly were  getting stuck in the quicksand-like mud.  Farmers eventually erected a fence to prevent any more losses.  Nodoroc was formerly about 5 acres in extent but circa 1900 John Harris drained part of it and grew several successful crops of corn.  While ploughing, he often came across bones and horns.  They were mostly the bones of recent cattle, but I suspect they may have been mixed in with Pleistocene fossils–a point I will discuss later.

Gary Bolton, a layman, visited the site in 1987 out of curiousity.  He noted that tulip tree saplings had colonized part of the pond, but many fell over, as if the shaky marsh ground couldn’t support deep roots.  Most of Nodoroc was covered with “thousands” of crayfish chimneys.  He mentioned sticking a shovel in the pond and finding that the level of muck was deeper than the shovel itself.

Acidic peat bogs are rare in the piedmont region of southeastern North America.  Peat bogs often contain and preserve pollen, plant macrofossils, animal fossils, and human artifacts.  So far, Nodoroc has only attacted 2 paleobotanists.  Dr. Stephen Jackson and Dr. Donald Whitehead investigated the site in 1981 and published their findings 10 years later.  They took 2 deep piston cores of sediment that they analyzed.  They did find 2 statigraphically datable segments in the core: 1 dated from 26,000 BP-22,00 BP, and the other from 3600 BP to the present.  This study was done before radiocarbon dates were recalibrated.  These dates roughly translate to between 30,000 calender years BP-26,000 calender years BP, and from 4,000 calender years to the present.  The lack of continuous stratigraphy is explained by long periods of time when water level was low and deposition didn’t occur.  The oldest segment dates to the weak interstadial immediately prior to the cold phase that led to the Last Glacial Maximum.

The site itself was probably an open marshy environment during the interstadial.  Plant macrofossils and pollen indicate an abundance of sphagnum peat moss, arrowhead (Sagitteria sp.), sedges, and carnivorous pitcher plants growing directly on the site.  Shrubs such as alder, myrtle, and mountain laurel and/or blueberry (Ericereae genus) grew on the marsh edge.  Pines and oaks dominated the forest surrounding the site.  With the exception of white pine (Pinus strobus) scientists can’t distinguish between species of pine by looking at pollen grains. After looking at the pine pollen and pine needle fossils under a microscope Drs. Jackson and Whitehead were only able to eliminate 3 species–longleaf, slash, and table mountain, none of which would have been expected to be here anyway.  However, the size of the pollen grains suggests both northern and southern species of pines were present.  Northern species of pine tend to have smaller pollen grains, while southern species of pine tend to have larger grains.  Both large and small grains were present.  My educated guess is that jack pine (Pinus banksiana) and shortleaf pine (Pinus echinata) were the dominant pinus species, while white pine was also present.  Macrofossils of jack pine, a species that now no longer occurs farther south than Michigan, have been found in Missouri and north Georgia sites dating to the mid-Wisconsinian interstadial.  Apparently, it was a more widespread species then than it is today.  Shortleaf pine, a southern species, occurs as far north as southern Ohio, and its short needles can endure snowy and icy conditions without much damage.  There’s no way of determining which species of oaks predominated here–all oak pollen looks too similar.

Other important trees growing in this interstadial forest were Critchfield’s spruce (my species assumption), fir, and hickory.  Chestnut, beech, sugar maple, and birch were present but at low levels.  (After the Ice Age chestnut became much more abundant in the region until its unfortunate extirpation 100 years ago.)  Hazelnut was a common bush growing in the understory.  During the Wisconsinian Ice Age hazelnut ranged throughout the south but is practically absent here today.  Noticeably absent or rare then were sweetgum, tupelo, and red maple all of which are common today in this area.

Fir trees thrive in regions with snowy winters.  As I mentioned in last week’s blog entry, the piedmont region of southeastern North America during the Ice Age was an abrupt transition zone between subtropical Gulf Coast Corridor grasslands and boreal forests of the Southern Appalachians.  Humid tropical fronts often hit cold air causing lots of snowfall.  Critchfield’s spruce, firs, and short-needled pines were and are well adapted to the snowy conditions that may have once blanketed the south.  It would have been interesting to see this environment where warm climate fauna frequently wandered to mix with creatures from the cold north.

I’m surprised no Pleistocene fossils habe been discovered or noticed from Nodoroc.  If cows often perished in the mire, I’m sure some of the Pleistocene megafauna did as well.  Perhaps the Indian legend of the wog is based on skeletel material of extinct beasts they discovered.  If I owned the pond I’d have it dragged for fossils and artifacts.

Artist’s rendition of the wog, a creature of Creek Indian legend that supposedly guarded Nodoroc.  Was the legend based on Pleistocene-aged fossils Indians found in the bog?  Cows used to get stuck in the mire and perish.  Pleistocene megafauna must have also.  This site should be prospected for Pleistocene vertebrate fossils which are probably mixed with modern livestock bones.

References:

Bartow County Historical Society

The History of Nodoroc and Tales of the Wog

Jackson, Stephen; and Donald Whitehead

“Pollen and Macrofossils from Wisconsinian Interstadial Sediments in Northeastern Georgia”

Quaternary Research 39 1993

Wilson, Gustavius

The early history of Jackson County, Georgia

W.E. White 1914

Sagponds Contain a Partial Pollen Record of Ice Age Vegetation in Northwest Georgia

February 2, 2012

In the southern Appalachian mountains when groundwater dissolves underlying limestone the ground slumps to a granite bottom, creating an interesting environment known as a sagpond.  These differ from sagponds located in California which are formed from strike-slip faults.  Georgia sagponds form from the same geological principle that creates limesink lakes in Florida.  Dr. Charles Wharton counted as many as 96 sagponds per square mile in Bartow County.  They range in size from a few feet across to several acres.  Some go seasonally dry; others retain water year round.  Scientists recognize 4 types of sagponds.  Dry sagponds drain within an hour after rain stops.  Young sagponds, where the ground hasn’t finished slumping, hold water during winter and spring.  Mature sagponds are partially filled with colluvial silt and retain water year round.  Extinct sagponds have been completely filled with silt but still might be moist enough to support a marsh.

Topographical map of Green Pond.  Sagponds often form on mountains and provide wetland habitats in otherwise dry upland environments.

These low wet areas dotting the otherwise dry upland ridge and valley region host disjunct populations of at least 49 species of plants that are normally found on the coastal plain, including the evergreen laurel oak, gallberry (a type of holly), fetterbush, rock rose,  joint grass, panic grass, beaked rush, and many others.  Common aquatic plants found in sagponds are tupelo, buttonbush, red maple, and sedges.  Buttonbush and tupelo tend to grow in the middle of a sagpond; grass and sedge grow on the edges; than there is a heath zone ringed by a lowland woods consisting of loblolly pine, laurel oak, and red maple.

Page from the book Natural Environments of Georgia with a photo of a dry sagpond in Bartow County.

Photo by Alan Cressler of a sagpond on Keel Mountain in Madison County, Alabama.  I couldn’t find any photos on the web of sagponds in Georgia, though there are supposed to be a lot of them.

Dr. Wharton reports that Bloody Pond in Chickamauga Battlefield Park has a large stand of willow oaks.  (While researching this topic, I discovered that there are at least 2 other Civil War era battlefields with areas known as “Bloody Pond.”)

Scientists looking for ancient pollen and Pleistocene-aged plant macrofossils have excavated and cored several sagponds–Bob Black Pond, Quicksand Pond, Green Pond, Pigeon Mountain Marsh, and Lookout Mountain Marsh.  The pollen record from all these sites combined cover a timespan of between ~34,000 Bp-~10,000 BP in calender years.  Some of the studies date to before scientists realized carbon-dating gave dates too young when compared to tree rings.  The data I give in this discussion mention dates I’ve adjusted.  They’re just rough estimates based on the inaccurate dates from the early studies.

Pollen from Green Pond, believed to date to the Farmdalian Interstadial, suggests an environment in northwest Georgia consisting of dry oak and hickory woodlands with prairie openings.  Dr. W. A. Watts (of Penn State but now retired) conducted this study in 1973 and gave carbon dates of 29,630 BP- 25,000 BP which should be adjusted to about 34 ka BP-30 ka BP.  Pine pollen was scarce.  ~28,000 years ago pine and spruce pollen began to increase, indicating an abrupt change to a cooler drier climate.  Surprisingly, pond cypress was locally present.  This shows that individual plant species ranges didn’t always change in predictable patterns.  Pond cypress is absent in this region today, despite a warmer climate.  Shortly after this date, the pond filled with sediment and the pollen record ends for this site.  Conveniently, this is when the pollen record from sediment cored from Bob Black Pond, which Dr. Watts also studied in the early 1970’s, begins. 

Dr. Stephen Jackson of Wyoming University also studied Bob Black Pond over 20 years later.  He found boreal forest species dating to about 22,000 BP.  In addition to pollen he found plant macrofossils of white pine, red pine, jack pine, white spruce, Critchfield’s spruce, and paper birch.  Critchfield’s spruce is an extinct species thought to be adapted to temperate climates, and white pine still grows on the mountains of north Georgia, but the other 4 species no longer range much further south than Canada.

Jack pine present day range map.  This species lived as far south as northwest Georgia during the LGM.  Today, it lives no further south than northern Michigan.  Red pine is restricted to New England.  White spruce lives across Canada as does paper birch.

There’s no doubt the climate in north Georgia during the Last Glacial Maximum was cooler than that of today, but it wasn’t necessarily as cool as present day southern Canada, despite the presence of Canadian species of trees.  Scientists may have originally misinterpeted this data when they assumed the climate here was comparable to today’s southern Canada.  Pine and grass grow better in atmospheres with lower CO2 than broad-leafed trees.  So moderately lower temperatures combined with lower CO2 levels allowed northern species of conifers to compete better with oaks, maples, and other broad-leafed trees for space in the southern Appalachians.  It’s a good thing for the boreal species too because their present day range was completely under miles of ice then.

Fossil pollen from Pigeon Marsh in Walker County dates to approximately the LGM.  Jack and red pine pollen ranges from 25%-45%, oak pollen ranges from 30%-40%, and spruce pollen ranges from 1%-2%.  Hickory and chestnut were present but not abundant.   After the LGM, as the climate remained cool but became more moist, beech became the dominant tree here until the modern floral composition took over.

During the Pleistocene sagponds would’ve been prime foraging grounds for herds of hungry mastodons.  Sagponds supported the kinds of aquatic plants (especially buttonbush and pond cypress) that we know mastodons ate from analysis of their coprolites.  And indeed, fossil evidence of mastodons has been found in the ridge and valley region of Georgia and Tennessee.

References:

Watts, W. A.

“The Vegetation Record of a Mid-Wisconsinian Interstadial in Northwest Georgia”

Quaternary Research 3 (2) 1973

Wharton, Charles

The Natural Environments of Georgia

Georgia Department of Natural Resources 1980

The Extinction of Critchfield’s Spruce (Picea critchfieldii)

September 3, 2011

Photo of a white spruce (Picea glauca) from google images.  The extinct Critchfield’s spruce was similar enough to this species that scientists originally misidentified it as such.  The size of the cones, cone scales, and needles differ to a statistically significant degree between the two.

Critchfield’s spruce dominated lower Mississippi River valley forests during the Last Glacial Maximum (~28,000 BP- ~15,000 BP) and commonly occurred throughout the southeast elsewhere.   That such a once abundant tree became completely extinct is surprising and a little unsettling.  It means that common and beneficial plant species that we take for granted today could abruptly become extinct in the future.  Since the 19th century Americans have already experienced the catastrophic loss of the American chestnut and are witnessing the perhaps irreversible decline of the hemlock–both due to human-introduced diseases and insects.  These species have existed for millions of years.  Our environment is becoming more and more impoverished.  However, man is probably not the culprit for the extinction of Critchfield’s spruce.

Fossils of Critchfield’s spruce have been excavated from the Tunica Hills region in Louisiana (as discussed last week) and Nonconnah, Tennessee.  In both of these locations it was found in association with mixed hardwoods such as oak, maple, walnut, etc.  In Georgia Critchfield’s spruce fossils were found at Bob Black Pond in Bartow County (northwestern part of the state) and Andersonville Clay Pit (southwestern part of the state).  At the former site Critchfield’s spruce fossils were found in association with fossils of northern species of trees–pitch pine, jack pine, red pine, white pine, and white spruce.  Evidentally, in the northern part of its range, Critchfield’s spruce overlapped with boreal species.  To the east and west of these fossil localities remains of Critchfield’s spruce are unknown, but its presence can be inferred from the pollen record.  Spruce pollen is present but not dominant in pollen studies from central and southeast Georgia, and South Carolina during the LGM.  And there is a 12,000 BP pollen record in north Florida of an unusual forest consisting of beech, hickory, and spruce. The spruce pollen undoubtedly originated from the  Critchfield’s spruce species.   Scientists assume that unlike its relatives it was a temperate species.  All the fossil wood from this species dates from 25,500 BP-16,000 BP, but spruce pollen continued to appear in southeastern pollen records until ~9,000 BP, indicating it survived until then.

This is a page from the below referenced paper that diagnosed Critchfield’s spruce as a distinct species. 

Southeastern pollen records show an inverse relationship between spruce and oak.  During interglacials and interstadials oak pollen increased while spruce pollen decreased.  During stadials pine and spruce pollen increased while oak pollen decreased.  Stadials were colder, drier, and windier, and had lower levels of CO2 in the atmosphere.  Conifers are better able to grow in these conditions.  Their springy limbs bend and sway in windstorms and icy weather; limbs from hardwoods more readily break.  Plants need CO2 for respiration.  The needles on conifers are more efficient at respiration than broad leaves, and they also lose less water during drought.  In warm wet climates with higher CO2 levels broadleafed trees tend to outcompete and shade out conifers.  This explains the cycle.

Data from ODP (Ocean Drilling Project) 1059A (see my May archives for an article about this), the only pollen record in the south from 150,000 BP-50,000BP,  shows that spruce pollen decreased rapidly following the end of the Illinois Ice Age and ranged between 0%-5% during the Sangamonian Interglacial and early Wisconsinian Ice Age.  It didn’t rise significanly again until 70,000 BP when it reached 10%.  Though Critchfield’s spruce must have declined to low numbers during the Sangamonian Interglacial, it didn’t become extinct as it did in the present one which brings to mind the question of why did it become extinct?

The authors of the below referenced study give 3 speculative causes for its extinction.  A pathogen spread as the climate warmed.  Critchfield’s spruce failed to disperse and colonize newly available habitat–something other species of spruce did as the Laurentide Glacier retreated.  And habitat space disappeared when broad-leafed trees outcompeted them.  I would like to add some other speculative causes.  It could have just as easily been an insect infestation as a pathogen in an environment with a longer time for a potential insect pest to reproduce.  Maybe it was a pathogen spread by an insect or a pathogen plus an insect infestation.  During previous interglacials broad-leafed trees very nearly outcompeted Critchfield’s spruce into extinction, but the latter survived in some refuge per chance.  Maybe this time perchance it didn’t find that refuge.  I doubt man played a role in its extinction, but there is a possibility Indian-set fires eliminated these isolated refuges where this tree had previously survived during interglacial periods.

Reference:

Jackson, Stephen; and Chengyu Weng

“”Late Quaternary Extinction of a tree species in eastern North America”

PNAS 96:24 11-23-1999

*********************************************************

This is a vegetation map of the southeast during the LGM that I’ve produced based on my complete study of available pollen records.  Like all vegetation maps, this is a vast oversimplification.  The solid black represents an environment dominated by the extinct Critchfield’s spruce.  This region also included mixed hardwoods, extensive meadows, and some wetlands.  Boreal species grew in the northern part of this region.  Crosshatching represents a pine dominant environment mixed with oaks, other hardwoods, Critchfield’s spruce, grasslands, wetlands, and even patches of desert-like habitat including scrub oak and eolian sand dunes.  The dotted region was likely bur oak and cedar savannah with some Critchfield’s spruce.  To the north below the Laurentide Glacier was a mixture of boreal spruce forests, grassy steppes, and bogs depending on local conditions.  The southern tip of Florida consisted of open pine savannahs and cypress swamps.  It was out of phase with the rest of the continent due to a shift in the Gulf Stream–south Florida had warm wet conditions while the rest of North America had cold arid climate.  Along the coast maritime oak, coastal prairie, and salt marsh prevailed.

Partial list of northern fauna that colonized the southeast during the LGM.

red squirrel

northern flying squirrel

porcupine

fisher

caribou

stag-moose

helmeted musk ox

pine siskin

gray jay

sawhet owl

snowy owl

The Fossil Rich Region of Tunica Hills, Louisiana

August 28, 2011

The Tunica Hills region is located in the geographical corner between southwestern Mississippi and southeastern Louisiana.  Ice Age climate variations instead of tectonic processes created these uplands.  During times of glacial expansion the Laurentide glacier scraped bedrock in the upper midwest, grinding it down into what geologists actually call “rock flour.”  During interstadials meltwater pulses carried this rock flour down the Mississippi River.  The warm climatic phases that caused meltwater pulses always preceded changes to cold arid conditions.  The decrease in precipitation led to a greatly reduced flow in the Mississippi River, and massive amounts of river sediment became exposed to the air in giant sandbars and dunes.  The windy environment of this glacial stage blew this river sediment, which consisted largely of rock flour (also known as loess), to the east where it settled in big hills.  So the Tunica Hills are actually great big piles of midwestern rock dust.  Later, streams (known as bayous in Louisiana) eroded through the hills and cut through fossil deposits of different ages, washing Miocene, Pliocene, and Pleistocene fossils into the water for today’s fossil hunters to find.

Ravine in the Tunica Hills.  The walls are made of midwestern rock dust blown here during the Ice Age.  From google images.

The loess buried and preserved lots of paleobotanical evidence from the last Ice Age–not only pollen but macrofossils such as logs, cones, and leaves.  The age of the pollen and fossil plants gives us a continuous record from 24,500 BP-17,700 BP, the height of the Last Glacial Maximum.  (The Laurentide Glacier reached its southernmost extent 18,000 BP, but I consider the whole time span of ~28,000 BP-~15,000 BP to be the LGM because that’s when Wisconsinian Ice Age climate was the coldest.  However, there were several weak and brief interstadials within this timespan.)  Scientists found evidence of a forest dominated by spruce but with hardwoods such as oak, beech, elm, hickory, walnut, and maple present.  Spruce made up 40%-70% of the pollen, pine 6%-17%, oak 3%-10%, and other hardwoods <2%.  Grass and composites such as aster, daisies, and sunflowers were also well represented, indicating widespread meadows.  Dwarf mistletoe, an interesting parasite that grows on spruce trees, was abundant.

At first scientists misinterpeted this data because they catalogued the spruce fossils as belonging to an extant boreal species–white spruce.  But in 1999 scientists identified the species they were finding as an extinct temperate species they’ve named Critchfield’s spruce.  Formerly, scientist believed that during the LGM the Tunica Hills region was comparable in climate to the modern day Great Lakes region.  Critchfield’s spruce was likely adapted to warmer climatic conditions than white spruce, nullifying this previous assumption.  Instead, climate in Louisiana then was probably only slightly cooler than today.

Conifers and grasses grew better in an Ice Age atmosphere with lower CO2 levels than did broad-leaved trees.   This probably accounts for why Critchfield’s spruce became a dominant tree over hardwoods.   It was lower atmospheric CO2 rather than lower temperatures that facilitated the growth of this forest.   This unique ecotone of dominant spruce with mixed hardwoods and meadows stretched all the way from Tunica Hills east to Kentucky, Tennessee, and western Georgia.  Critchfield’s spruce fossils have also been found in 2 different localites in western Georgia and 1 in Tennessee.  Eastern Georgia and South Carolina had a somewhat different landscape with more pine and oak, though spruce was present.  West of Tunica Hills an open oak (probably bur oak) and cedar savannah prevailed–grasses and composites made up 20%-50% of the pollen, oaks 10%-25%, and spruce 1%-15%.

Today, Tunica Hills supports an unusual forest for the region.  Beech, magnolia, and holly are dominant but many species occur including laurel oak, water oak, osage orange, hackberry, maple, silver bell, paw paw, and river cane.  Cool ravines provide relief from the hot southern sun, so trees with northern affinities grow next to warm climate trees.  A disjunct population of eastern chipmunks resides here.

The Miocene deposit is located geologically in the Pascagoula Formation.  It’s believed to have been an estuary then.  The Miocene (25 million BP-5 million BP) could be known as either the age of the horse or the age of the rhino because they were the 2 most common large mammal species then.

Hipparion–a 3 toed gazelle horse common during the Miocene.  Miocene Louisiana was mostly tropical.

Naturalists excavated specimens of both the hippo-like rhino (Teleoceras) and the hornless rhino (Aphelops) from Tunica Hills.  At least 5 different species of horses galloped the region then–Astrohippus, Cormohipparion, Hipparion, Neohipparion, and Nannihippus aztecus.  They were all 3-toed gazelle-horses.  The cavalcade of Miocene ungulates also included peccaries, tapirs, an extinct species of pronghorn antelope, and the odd synthetocorus

A synthetocorus. What an odd hooved animal.  Note the bone on its nose.

Mammut mathewii roamed what was to become Louisiana then.  It was an early form of mastodon.  Giant tortoise, alligator, snake, and fish remains were collected as well.

Some of these species survived to early in the Pliocene but among the 3-toed horses, only Nannihippus aztecus survived to the end of the Pliocene, sharing the range with the 1 toed modern genera of horses which then included the American zebra.

A list of Pleistocene fossils collected from the Tunica Hills region consists of the familiar animals discussed on this blog–Jefferson’s ground sloth, Harlan’s ground sloth, horse, saber-tooth, giant armadilloes, and grouse.  A glyptodont fossil is the most recent addition to the Tunica Hills fossil list.  This year scientists identified glyptodont scutes and a partial rib from a Tunica Hills creek.  This is the first find of this species between Florida and Texas.

Reconstructed glyptodont displayed at the University of Florida museum.  I don’t know if this is a replica or the real bones.

Scientists refer to fossils found in the bayou as “float” though they don’t literally float but are usually recovered by sifting the bottom.  The age of the fossils in Tunica Hills can’t be determined based on stratigraphy because streams eroded them from different age strata, but scientist have figured out how to used an interesting method to distinguish whether a fossil is late Miocene/early Pliocene or Pleistocene.

Some of the highlighted elements (known as rare earth elements) are the ones that can be used to determine which age a fossil belongs to.  The ratio of these elements varies in the environment over time.

Scientists use knowledge of rare earth elements to determine the relative age of different fossils.  One collegel student, Lindsey Yann, conducted a study for her Masters thesis comparing the ratios of rare earth elements in different fossils found in the Tunica Hills bayous.  Rare earth elements (a bit of a misnomer because they’re not particularly rare) include elements on the periodic table numbered 57-71.  They occur in groundwater in certain fixed ratios.  Animals absorb ground water by ingestion and then for thousands of years after they die their bones continue to become saturated with it.  Eventually, the bone reaches a saturation poiunt and won’t take in any more.  The ratio of rare earth elements in that particular fossil becomes fixed.  However, over millions of years the ratios of rare earth elements in ground water changes.  So an animal that lived 5 million years ago will have a different ratio of rare earth elements than an animal that lived 25,000 years ago.

References:

Jackson, Stephen; and Charles Givens

“Late Wisconsinian Vegetation and Environment of the Tunica Hills Region, Louisiana and Mississippi”

Quaternary Research 41 1994 pp. 306-325

Schiebout, J.; et. al.

“Miocene Vertebrate Fossils Recovered from the Pascagoula Formation in southeastern Louisiana”

Geology Paper

Schiebout, J.

“Fossil Evidence of Glyptotherium C.F. floridanus in the Pleistocene of Lousiana”

GSA paper March 2011

Yann, Lindsey

“Rare Earth Elements as an Investigative Tool into the source, age, and ecology of Late Miocene to Late Pleistocene Fossils from the Tunica Hills, Louisiana”

Masters thesis for LSU

http://etd.lsu.edu/docs/available/etd-06292010-133339/unrestricted/YannThesis.pdf