Posts Tagged ‘oxygen isotopes’

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)

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.