Posts Tagged ‘Sangamonian Interglacial’

The Stagell Interglacial (MIS 11) is an Orbital Analogue for the Holocene

November 12, 2012

Scientists have no concrete way of determining whether changes in the climate are due to natural or manmade causes.  This would require the existence of a control earth in a parallel universe where man never evolved.  Scientists could then compare the climates and temperatures of the 2 worlds to see what the differences would be.  Because a control earth doesn’t exist, scientists must concoct models using knowledge of the known natural variations of earth’s paleoclimate.  The models estimate what average temperatures and CO2 concentrations should be due to natural forcing.  Scientists use these models and compare them with actual observed data.  According to the almost unanimous consensus among climate scientists, earth’s climate is now warming due to both natural forcing and manmade production of greenhouse gases through the burning of fossil fuels.  However, earth is currently in an unusual point of a 400,000 year cycle of orbital geometry that complicates climate scientist’s models and makes it even more difficult to discern the difference between natural and anthropogenic influence on the present day’s climate.

Illustration of earth’s orbital geometry.  During Ice Ages earth’s orbit is more elliptical in shape, reducing the amount of solar radiation it receives.  During Interglacials earth’s orbit is more round.  On a 400,000 year cycle earth goes into a prolonged period of time in a round orbit.  During the Stagell Interglacial (~420,000 BP-~395,000 BP) earth was in a prolonged round orbit.  Earth’s orbital geometry for the past 11,000 years has been similar to the orbit that earth was in during the Stagell Interglacial, suggesting we will remain within an interglacial for tens of thousands of years with or without the influence of anthropogenic greenhouse gases.

Natural variations in earth’s climate are triggered by the Milankovitch cycles.  The gravitational pull of Jupiter and Saturn causes eccentricities in earth’s orbit.  During Ice Ages, earth’s orbit is more elliptical in shape reducing the amount of solar heat reaching earth.  The orbit of earth is more round during interglacials.  This variation in earth’s orbit goes through 100,000 year cycles.  The degree of earth’s tilt also varies but on a 41,000 year cycle.  The gravitational pull of moon and sun causes this variation in earth’s tilt.  The 23,000 year cycle of earth’s precession is a third influence on the amount of solar heat (insolation) reaching earth.  Precession is the time of year when either the southern or northern hemisphere is closest (perihelion) or farthest (apehelion) from the sun.  All of these cycles cause the amount of insolation reaching the earth to vary slightly, but over long periods of time these small differences trigger the alternating cycles of cold Ice Ages and warm Interglacials.

Formerly, scientists thought Interglacial periods lasted on average 10,000 years while Ice Ages lasted for 100,000 years.  More recent evidence suggests Interglacials average between 15,000-20,000 years.  However, every 400,000 years, earth enters a prolonged period of time within a round orbit, resulting in a much longer interglacial.  The Stagell Interglacial, known as the Hoxnian in Europe, occurred from ~420,000 BP-~395,000 BP when earth was in this prolonged round orbit.  Some scientists estimate it lasted even longer from ~423,000 BP-~362,000 BP.  The present day Interglacial, the Holocene, began about 11,000 years ago.  The earth is currently in a similar orbital geometry as it was during the Stagell Interglacial, meaning we are not even halfway through the present day interglacial.  Scientists are studying the Stagell Interglacial to predict what earth’s climate will be like in the future, specifically the next 50,000 years.  Some climate models suggest that because of the current unique orbital geometry, earth won’t shift into an Ice Age again for another 40,000 years.  The models predict a complete melting of the northern Ice Cap 30,000 years in the future, but if modern anthropogenic factors are considered, the northern Ice Cap will collapse in less than 200 years.

Chart of the past 600,000 years showing fluctuating carbon dioxide levels.  Higher carbon dioxide levels are correlated with higher temperatures.  The Stagell Interglacial is scientifically known as Marine Isotope Stage 11 (MIS 11).  Note the length compared to other Interglacials.  Interglacials are odd numbers.  MIS 3 isn’t labeled on the chart and was an interstadial (a briefer warming period) rather than a full blown interglacial. One of the ways scientists determine past climates is by taking cores at the bottom of the ocean.  The core layers are radiometrically dated, and fossils of microscopic organisms known as foraminifera are examined and analyzed.  The various species of foraminifera live within strict temperatures ranges, so a general idea of the temperatures at the time of their deposition can be assumed simply based on species composition.  However, exact average annual temperatures can even be determined based on the chemical isotopic compositions of the fossil foraminifera shells, thus the name–Marine Isotope Stages.

The Gubik geological formation along the coast of northwest Alaska provides evidence of high marine transgressions dating all the way back to the Pliocene.  One of these layers includes a high sea stand from the Stagell Interglacial that was 23 meters above modern sea level.

A species of ostracod.  Fossilized ostracods found in layered sediment on the sea bottom provide paleoclimatic data based on species and chemical isotopic composition.

Devil’s Hole, a cave in southern Nevada.  Calcite veins below the water table here provide radiometrically datable  isotopic evidence of past climates going back 800,000 years.  It was evidence from this site that initially showed the Stagell interglacial was much longer than the average warm period.  Evidence from this site also created a dilemma for scientists’ assumptions about the Milankovitch cycles.  Isaac Winograd determined from data found here that interglacials were twice as long as previously thought. Some scientists think this evidence still supports the theory that Milankovtich cycles cause the alternating cycles between warm and cold stages, while others think the data is not consistent with and therefore debunks the theory.

Some of the best evidence of the Stagell Interglacial comes from the Gubik formation in northwest Alaska where in some places remnants of 6 former marine highstands, including layered fossil shorelines and barrier islands, are found superimposed (conveniently for geologists) from youngest to oldest.  The marine highstand dating to the Stagell Interglacial is 23 meters above sea level, evidence of just how much polar ice must have melted.

Studies of average annual temperatures during the Stagell Interglacial are contradictory.  Some studies suggest it was warmer than other interglacials, while others seem to indicate that though it was longer, it was not as warm as other interglacials.  Evidence found at the bottom of the ocean is stark and dramatic.  The Ice Age preceding the Stagell, known as Marine Isotope Stage 12, was a particularly cold one–it left a record amount of debris carried by icebergs during meltwater pulses.  (Rocks are carried by icebergs out to sea.  Icebergs break off glaciers during minor warm phases within Ice Ages and carry debris in events referred to as “ice rafting.”  Eventually the icebergs melt and drop this debris, known as drop stones,  in visible piles at the bottom of the ocean.)  But during the next 23,000 years during the Stagell Interglacial, ice rafting debris is almost completely absent.  This suggests the glaciers and ice caps in the northern hemisphere were completely or seasonally absent then.

The northern hemisphere ice caps may have completely collapsed during the most recent interglacial previous to the one we live in now.  The Sangamonian Interglacial (~132,000 BP-~118,000 BP) reached modern day temperatures by ~132,000 BP and kept getting warmer until 130,000 BP.  The highest sea level transgression occurred by ~125,000 BP.  Unlike during the Stagell Interglacial, however, ice began to reform and grow back rapidly as earth’s orbit returned to a more elliptical geometry.

During the Stagell Interglacial most of southeastern North America was probably covered in forest, though paleobotanical evidence from this period of time is scare, if it exists at all.  A Late Irvingtonian Land Mammal Age fauna was present.  It was similar in species composition to the later Rancholabrean Land Mammal Age with the exception of bison which hadn’t crossed the Bering land bridge yet.  Mastodon, deer, peccary, llamas, horses,  tapirs, Armbruster’s wolves, and jaguars were likely common.  Ancestral forms of saber-tooths and ground sloths (Smilodon gracilis and Megalonyx whealeyi  respectively) lived in the region then.  Giant tortoises must have been pretty widespread in the mild climate as well.

Carbon dioxide levels during the Sangamonian Interglacial were higher than those of the Holocene until the 20th century when man’s burning of fossil fuels artifically pushed CO2 levels above even Sangamonian levels.  Carbon dioxide doesn’t cause higher temperatures.  Instead it usually lags natural temperature increases by thousands of years.  But it is correlated with temperature increases, and scientists believe it magnifies them in some kind of poorly understood feedback mechanism.  Of course, it isn’t up to me, but I think humans need to stop treating earth’s atmosphere like it’s some kind of gigantic chemistry experiment before mankind creates yet another ecological catastrophe.

References:

Loutre, N.F.

“Clue from MIS 11 to Predict the Future Climate–A Modelling Point of View”

Earth and Planetary Science Letters 212 (2003)

Poore, Richard: et. al.

“Marine Isotope Stage 11 and Associated Terrestrial Records”

U.S. Geological Survey open file report 00-312

Both of these references are available online as pdf.  You can find them easily with a google search.

The Interglacial Invasion of Warm Climate Species into Southeastern North America

January 21, 2012

Humans have been enjoying a relatively stable warm climate phase for roughly 11,000 years now–a period of time known as the Holocene.  We’ve probably been experiencing an interglacial because it’s likely we’re between Ice Ages, although with the extraordinary release of CO2 from industrial activities, there’s no telling when the next Ice Age will occur.  This phase of warm stable climate has allowed agriculture to flourish.  If climate had remained unstable and as cool as it did during the last Ice Age, civilization as we know it may never have come into existence.

The most recent interglacial previous to the present one was the Sangamonian Interglacial which lasted from 132,000 BP-118,000 BP.  Climate during the Sangamonian was even warmer than that of today.  At one point during this interglacial the north polar ice cap completely melted and sea levels were higher than they are now.  Cypress swamps grew as far north as Illinois, alligators swam in rivers flowing through what today is Missouri, and giant tortoises roamed the ridge and valley region of the southern Appalachians.  This wasn’t the warmest era in geological history–it wasn’t even close to as warm as much of the Pliocene, Miocene, Oligocene, etc. ages–but it was unusually warm compared to most of the Pleistocene.  This prolonged warm climate phase allowed many frost sensitive species of vertebrates to colonize much of southeastern North America, at least temporarily.  But because cold phases of climate during the Pleistocene lasted 10 times longer than warm phases, fossils of these tropical and subtropical species are in some cases extremely rare.  There are probably more species than the following pictorial cavalcade illustrates, but these are the ones confirmed by science.

Eremotherium laurillardi, the largest ground sloth to ever live in North America, grew to 18 feet long and weighed up to 3 tons.  Fossils of this species are quite common along Georgia’s coastal fossil sites which mostly date to the Sangamonian and early Wisconsinian.  Cold climate eventually drove them from what is now Georgia, but they persisted in Florida until maybe 30,000 BP when the beginning of the LGM became too cold for them even there.  They did continue to live in South America until 10,000 BP when hunting Indians likely drove them to extinction.  If it wasn’t for man, they may have recolonized the gulf coast of today.  2 species of ground sloths (Jefferson’s and Harlan’s) were able to survive in North America during the Ice Age, but Eremotherium must have been incapable of tolerating frosts.

Evidence that the South American marsh deer (Blastoceras dichotomous) once lived in the southeast comes from 1 mandible found at Saber-tooth Cave in Florida.  It was given the scientific name, Blastoceras extraneous, but was likely the same species populating the present day South American pampas.  Dr. Richard Hulbert expressed doubt in his book, The Fossil Vertebrates of Florida, that this mandible was correctly identified, but that was before he himself indentified the presence of collared peccaries in the Florida Pleistocene–a big surprise.

Collared peccaries were only identified from the Florida Pleistocene within the last few years.  Apparently, they colonized the south during the Sangamonian and probably other interglacials.  2 other species of peccaries–the flat-headed and the long-nosed–did commonly occur in the south during cold stages as well.

1 ocelot specimen from the Florida Pleistocene proves this cat lived in the south.  It seems that this cat should be able to survive in Florida today.  I suspect Indians coveting its spotted coat led to its demise there.

Fossil evidence of a small species of cat resembling the modern day margay comes from Florida and 2 widely separated sites in Georgia–Ladds and the Isle of Hope site.  Scientists are uncertain of the identification–it’s either a margay,  jaguarundi, or a distinct extinct species.  Despite the scientific genus name, Leopardus, it’s not at all closely related to a leopard.  Was it climate or paleo-Indian desire for spotted coats that restricted this species to isolated jungles?

Giant tortoise fossils dating to the Pleistocene were found at Ladds, the northernmost locality, though during the Pliocene, which was mostly warmer than the Pleistocene, they lived as far north as Kansas.  In contradiction to what most scientists think, I suspect giant tortoises were capable of surviving light frosts.  See my reasoning in a blog entry from my April 2011 archives.

In the Sangamonian of Georgia I suspect alligators may have ranged into the Etowah River.  If giant tortoises lived in the area, alligators surely must have been able to live there too.

Many species of South American and Central American birds also extended their range north in Sangamonian times.

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)

Seals and Walruses off Southeastern North America’s Pleistocene Coast

January 28, 2011

Scientists expressing concern about anthropogenic global warming always seem to ignore paleontological evidence.  There is no better example of their alarmist approach than the oft-stated fear that global warming might cause the extinction of arctic marine mammals.  Yet, during the Sangamonian Interglacial (~132,000-~118,000 BP) the north polar ice cap completely melted, and there are no known extinctions of arctic marine mammals when this occurred.  Pleistocene ranges of seals and walruses were greatly expanded compared to the extent of where they live now.  And during both cold and warm climate cycles marine mammals occurred much farther south than they do today.  It’s more likely that their ranges are more limited today due to anthropogenic overhunting rather than changes in climate.  Evidence of my above-stated observation comes from fossils of seals and walruses found off the coast of South Carolina.

Gray Seal–Halichoerus grypus

Photo of gray seal from google images.  This species occurred at least as far south as South Carolina during the Pleistocene.  Now that they’re protected, it’s believed their range will expand farther south than New Jersey once again.

Fossils of this species have been found on Edisto Beach, South Carolina.  Until the Marine Mammal Protection Act of 1972, bounties kept the population of gray seals low, reducing them to 2,000 individuals on the North American side of the north Atlantic.  Since then, the population has expanded south to New Jersey where they frequently resort during winter.  They’re expected to spread even further south and may some day recolonize South Carolina.  They eat fish, lobster, and octopus.

Hooded Seal–Erignathus barbatus

Hooded seal from a picture at google images.  Man are they ugly.  The balloon on the males’ nose is used to attract mates.

Hooded seal flipper bone fossil found off the South Carolina coast.  All the pictures of fossils in this blog entry are from Additions to the Pleistocene Mammal Fossils of South Carolina, North Carolina, and Georgia by Albert Sanders.

The above fossil flipper bone was recovered in Horry County, South Carolina and is believed to be Sangamonian in age.  This solitary species mostly occurs in the arctic but is known to wander widely and has been reported in modern times as far south as the Carribbean.  Perhaps this specimen belonged to one such straggler.

Walurs–Odobenus romarus

Photo of a walrus from google images.

Fragments of walrus tusk fossils  discovered off the coast of South Carolina.

At least 6 fragments of fossil walrus tusks have been recovered from sites near Charleston, South Carolina, proving this species lived in the southeast long ago.  Moreover, one amateur fossil collector reports finding a walrus fossil in Florida.  Other walrus fossils turned up in North Carolina and Virginia, so at one time they must have been frequent visitors to southern beaches.  Most of the walrus fossils are assumed to be from a warm interglacial age because they’re found near the present day coast.  During glacial times, the Atlantic coast was many miles to the east of the present day coast due to the drop in sea level during the Ice Age.  I have no doubt walruses are solely limited to arctic regions today because of the remote geography where they’re difficult for human hunters to access.

Walruses mostly eat marine worms and molluscs which they find on the ocean floor with their whiskers.  Male walruses occasionally kill or scavenge and eat seals.  This behavior is probably related to high testosterone levels, much like male elephants that go on a rampage when they’re in the mood to mate.

Monk Seal–Monachus tropicalis

It’s sad to think that this is the only evidence left of a species once frequenting Georgia’s coast.  It’s a vertebrate of a monk seal.

Man hunted this warm climate seal into extinction by 1952.  It’s last reported sighting was off the coast of Jamaica.  It was last reported off the coast of Texas in the 1930’s.  One fossil collector  found bones of a monk seal on Andrews Island near Brunswick, Georgia, and others have found them at many sites off the coast of South Carolina.  What a shame the species couldn’t make it to 1972 when the Marine Mammal Protection Act passed.

Reference

Sanders, Albert

Additions to the Pleistocene Mammal Faunas of South Carolina, North Carolina, and Georgia

American Philosophical Society 2002