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.
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)