DR. TIM WHITE: Paleoclimatology is the study of ancient climates. In our next lesson, we'll consider aspects of paleoclimatology that involve computer modeling, and you'll meet one of my colleagues who uses computer models to understand ancient climates.
But now I'd like to show you some of the geologic evidence for past climate change that serves as the backdrop from which modelers ply their trade. Earth has undergone many drastic shifts in climate outside those experienced by humanity. For example, here in middle-latitude Pennsylvania, we experience a moist, continental climate that sustains our expansive deciduous forests and agricultural pursuits.
However, Pennsylvania has not always experienced an equable climate. 360 to 370 million years ago, during the Devonian Period, portions of Pennsylvania were covered by a broad, coastal plain next to a large inland sea that existed to the west in the interior of North America. North America was positioned in the Southern Hemisphere at this time.
Notice the gray to green resistant layers behind me, and these reddish recessive layers right here at my feet. The resistant layers are sandstones that were deposited in river channels. Whereas these recessive layers were deposited in the ancient floodplain, where soils formed during that time.
Notice these nodules within the reddish recessive layers. These nodules are calcite formed in soils in the ancient floodplain of the Devonian. These nodules help us to identify these paleosols as calcic Aridisols formed in a much dryer climate than Pennsylvania experiences today-- perhaps similar to West Texas or northwestern Argentina now.
If you need to, return to the lesson on soils and review the modern Aridisol distribution map. You'll find that Aridisols are developed within and along the fringes of many of our modern deserts. And so this ancient setting was much dryer than the climate we experience today, and, in part, can be described by North America's position south of the equator during this time.
We're in the so-called barrens just northwest of State College, and we're trying to make our way over to that little exposure there, which is in an old bauxite mining pit from the 1870s.
In contrast to the red Devonian calcic Aridisols that we just viewed, these paleosols behind me are oxisols formed sometime between about 15 and 50 million years ago in a very warm and wet tropical setting. Notice the light gray to white color of the soil matrix. The tropical weathering conditions were intense enough to leech most of the elements and nutrients from the soil profile, hence the dull or plain color of the matrix.
You can also see that the profile's much thicker than Devonian paleosols, or any of the soils that we visited in the Introduction to Soils video. This thickness is a tribute to the intense, tropical weathering conditions that existed here deep in the past.
During the time when these paleosols formed, Pennsylvania was located approximately in the same position as it is today. So the presence of these oxisols here indicate that tropical weathering conditions were much more widespread during this deep-time episode of global warmth.