We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week about 200 million years ago, with the opening of the Atlantic Ocean. Europe and North America split apart and started moving away from each other, at about the same rate as our fingernails grow.
With the exception of a hot spot that New England drifted over during this time, there was little volcanic or tectonic activity between then and now. North America slowly drifted poleward from the equator, and the climate of Maine changed accordingly. The Appalachian Mountains eroded, sending a huge sediment load oceanward, forming the coastal plane we see today on the east coast south of Long Island. The continents, freed from the bondage of Pangea, moved slowly over the surface of the Earth, coming closer and closer to their present day positions. And it is this continental movement and positioning that may have set in motion what happened next.
Up until this point, we have been referring to a timescale of 10s to 100’s of millions of years; big and fairly imprecise chunks of time, inferred from a geologic record of highly metamorphosed rocks, thousands of feet of sediment, and broad brush continuities on a global scale. For this next part of the story, we need to hone our gaze and zoom in quite a bit, this next chapter covers only 3 million years at best. We can call this chapter: The Ice Age, and technically, because there are still ice caps on Greenland and Antarctica, we’re still in it.
Ice ages have occurred throughout Earth’s history, current thinking puts the number at 5. The causes of the ice ages are not well understood, though there are many scientists working on this question, as the understanding of what is called “climate forcing” is directly related to current investigations of climate change. Causes likely include changes in oceanic circulation due to continental drift (as the continents move the ocean basins change shape, and currents can be redirected to higher or lower latitude), changes in atmospheric composition (including key greenhouse gasses like carbon dioxide and methane), fluxuation of solar out put, changes in planetary orbits (called orbital forcing) and changes in atmospheric circulation due to tectonic uplift. Its pretty complex stuff, and the scientists working on this are trying to reconstruct all of these factors and let them run in fantastically complicated computer models, hoping that what the models predict should have happened corresponds with the known geologic record.
So for this last ice age, which stretches back about 2.5 million years to the beginning of the Quaternary Period, one event that coincided with the start of global cooling was the cutting off of the Atlantic from the Pacific when the isthmus of Panama fully formed. This changed circulation patterns in both oceans, especially the Atlantic, and it drove the current we now call the Gulf Stream further north. The Gulf Stream is a warm water current, and you may be wondering how a warm water current moving further north triggers a global ice age. This is a reasonable question, however the concept is that warm air is able to hold more moisture than cold air, and warm water evaporates more readily than cold water. Both of these factors put more moisture into the atmosphere in higher, cooler latitudes, which can then be precipitated out. That increase in precipitation could have yielded the continental glaciers that covered much of the northern hemisphere during periods of the past two and a half million years. Is that exactly what happened? We don’t know, yet. Remember, science is about noticing patterns and then trying to explain them with additional evidence. In the Western scientific tradition, we’ve been noticing and trying to explain glaciers for the past 200 years, and much of the current research is simply about trying to discern the observable patterns at higher and higher resolution. Questions about the current patterns of climate change are driving us to want to know more and more about significant climate change events in the past, but most of these questions are still wide open, which makes the research that much more exciting.