Before I start, let me explain something about the state of geology as a science in the 1960s. It was a descriptive science as opposed to an experimental science. Physics, chemistry, and biology are experimental sciences; you can perform experiments and observe their outcomes. Astronomy and geology are descriptive sciences; we can make observations, but we can’t make experiments on the object we are observing. Of course, there are some small areas where one can perform experiments—in the study of igneous rocks one can make melts of different materials and let them crystallize to see what minerals form and in what order. And with sediments, one can experiment with wave tanks and flumes to see bow bedding features are produced.
When I got my Ph.D. paleontologists were called ‘stamp collectors’ by their colleagues. We collected fossils, identified them, named them if they were new species, and we used them to determine the age of the rocks that contained them. As best I can remember, only paleobotanists, the guys dealing with fossil plants, attempted to interpret the environments in which they had lived.
Our tools had always been simple; dental tools and scrapers to prepare the larger fossils, a magnifying glass or hand lens, and the trusty binocular microscope for microfossils. For the nannofossils we needed as high a magnification as one could get with an optical microscope. I was very lucky when I joined the faculty at the University of Illinois; Ken Towe, who later moved on the U.S. National Museum in Washington, was an expert electron microscopist and helped me get started with that new technique. Then in the later 1960s another major improvement in technique became available, the Scanning Electron Microscope. My fellow paleontologist in the Illinois Geology Department, Phil Sandberg, and I helped the University acquire one of the first of these new instruments in the United Sates. It revolutionized the way we could look at our microfossils. We realized that there were a number of new things we could do with the fossils, but the main interest remained their use in determining age.
All of that changed in the mid-1960s but the revolution in using fossils to infer environmental conditions went almost unnoticed in the much larger revolution of plate tectonics and sea-floor spreading.
In the early part of the 20th century Thomas Chrowder Chamberlin at the University of Chicago had proposed two major ways by which the Earth’s climate would have been different in the past. The first was, following Svante Arrhenius’ ideas, that the concentration of CO 2 in the atmosphere might have varied through time. He thought that might explain the alternation between glacials and interglacials. The second was that there might have been a reversal of the deep sea circulation. At present, the deep waters of the ocean are cold, reflecting the fact that they sink in the polar regions. They return to the surface in the low latitudes. That the cold deep waters must originate in the polar regions had been known for over a hundred years. Chamberlin suggested that when the Earth was warmer, as during the Cretaceous, the saltier waters of the tropics might have sunk into the ocean interior and returned to the surface in the polar regions. They were interesting ideas, but few geologists had given them any credence.
It was on Leg 4 of the Deep Sea Drilling Project in1969 that I began to seriously worry about the climate of the Cretaceous Period ( 145.5–65.5 million years ago). It had been known for almost a century that polar temperatures were warm enough to support tropical vegetation. Cretaceous rocks in North America were famous for their dinosaur fossils. You have seen marvelous animations of them in the film ‘Jurassic Park.’ The dinosaurs in the film are almost all of Cretaceous, not Jurassic age.
On Leg 4 I realized that the fluctuations of the calcium carbonate compensation depth might reflect changes in ocean chemistry. Although I didn’t realize it, it was a revolutionary idea. It was published in the Summary and Conclusions section of the DSDP Initial Report for Leg 4, in1970. Nobody read it. The consensus at the time was that the chemistry and salinity of the oceans had remained constant for billions of years. The chemistry of the oceans was thought to be purely the result of inorganic reactions between water and minerals. The classic paper on this topic, by Lars Gunnar Sillén had been published in 1967.
But then, in1972, Wally Broecker published his great paper A kinetic model for the composition of seawater. Although you might not guess it from the title, the paper shows how marine organisms control the concentrations of many of the ions in seawater. It removed the straightjacket that had constrained our thinking.
What did we actually know about the Cretaceous? In Miami we worked on reconstructing the paleogeography, at least the positions of the continents. We didn’t know much about the distribution of mountains, but made some guesses. It was obvious that sea-level had been higher in the Cretaceous. A large area of the continents had been flooded, but we didn’t know much about the general elevations of the continental blocks at that time. We made a lot of guesses. A Visiting Professor from Oregon, Bill Holser, introduced us to the idea that the salinity of the ocean had not been constant in the past, but must have changed. Over the Phanerozoic lot of salt had been removed from the oceans and stored on land as evaporite deposits—salt, gypsum and anhydrite. And we were just discovering through the DSDP that there was a lot more salt stored beneath the floors of marginal seas, like the Gulf of Mexico and Mediterranean. We were discovering that large areas of the oceans had become anoxic, devoid of oxygen. This simply cannot happen today because even if somehow every living thing in the ocean were suddenly to die, the decomposition of the dead organic matter would consume only half the oxygen in the ocean. In 1960 geologists thought they knew so much about the Earth and its history that our science was concerned with making a few minor revisions to the story, sort of mopping up. By 1970 we were becoming aware of just how much we did not know.
The puzzle of a warm Earth with tropical temperatures in the polar regions gave rise to several questions. Overall, the temperature contrasts on the Earth were much smaller than today. The climate was said to be more ‘equable.’ First, how could the polar regions become so warm. The implication was that the heat that accumulated in the equatorial region was being transported poleward more efficiently. But the driving mechanism for the winds is the temperature difference from one place to another. Temperature controls the density of the air, and it is the density differences between different air masses that produce the pressure gradients that power the winds. With warmed polar regions, the winds should have been slower, and the transport of heat less efficient. This became known and the heat transport conundrum. But could it be that the ocean circulation was more vigorous, and it was the oceans that had carried the the heat poleward? Unfortunately, the surface currents of the ocean are driven by the wind. They should have slowed too.
Could the Earth’s albedo have been different? In Leningrad Mikhail Budyko had begun to explore the climatic effects of changing Earth’s albedo and published several important papers on the topic in 1968 and1969. The original publications were in Russian, but the ideas began to percolate through the iron curtain. One of my colleagues at Scripps did some calculations on the effect of removing the present reflective polar ice, and taking into account the larger area of dark ocean. It worked; simple calculations yielded something like a Cretaceous climate. Only one small problem, his Earth had no atmosphere, no clouds, and they are 60 % of today’s albedo. Back to the drawing board.
When I started my joint arrangement between Illinois and Miami, I was able to take advantage of the expertise in physical oceanography and atmospheric science in Miami. Claes Rooth was a physical oceanographer who loved to speculate about how the ocean circulation might respond to different forcing factors, such as changing the speed of the winds and the evaporation-precipitation balance. Eric Kraus was an equally adventuresome atmospheric scientist who liked to think about how different atmospheric gas compositions might affect its structure and circulation. We were also able to organize short courses with well known geochemists such as Dick Holland of Harvard and Bob Garrels of Northwestern as instructors. They told us about the chemical reactions involved in weathering rocks, and how life interacted with the inorganic world. These short-courses were largely funded by tuition charged the industry participants, with academics attending for free.
Our frustration in trying to understand the enigma of the ‘warm Earth’ grew until finally, as recounted in Chap. 14, we sent one of our best students, Eric Barron, off to the National Center for Atmospheric Research in Boulder to do some numerical climate modeling. It was in the late 1970s that a consensus began to form that the culprit in the ‘warm Earth’ drama was atmospheric CO2.It was the only thing that answered all the questions. I remember well that I still didn’t understand why just changing the content of CO2in the atmosphere would preferentially warm the polar regions and result in an ’equable’ global climate. Warren Washington at NCAR explained to me that the Earth already has an even more important greenhouse gas, water vapor, but its concentration depends on temperature. So the Earth always has a greenhouse effect in the tropics. Figure 19.4shows this effect. But if the poles are cold, there is no water vapor in the air. There, it is the concentration of CO2, the second most important greenhouse gas, that is important. If its concentration is low, the poles are cold; it its concentration is high, the poles are warm. Simple as that.
But then, once you begin to think about how the atmosphere and ocean might circulate on a warm Earth you realize that many of the things we take for granted are no longer true. My ideas on this are given in more detail in Sect. 22.6 of, but here is a preview:
Without polar ice Earth becomes a very different planet. Today both polar regions are cold, in winter and summer, because of the high albedo of the ice. But if you replace the ice with water or land, the polar regions will experience seasonal reversals of temperature. In the Cretaceous this is complicated by the fact that the Arctic is water, the Antarctic land. This means that the reversals are in the opposite sense in each hemisphere. The Arctic had a high pressure in summer, low in winter, while the Antarctic had a low in summer and high in winter. The same geographic situation exists today.
Not only today’s atmospheric circulation, but that of the ocean as well, depend on the stability forced by polar ice. Without the ice as a stabilizing factor, the winds would shift latitude and become stronger or weaker with the seasons. If the winds shift, the ocean currents become unstable. The polar sites of deep water formation can shut down and be replaced by tropical sites. The entire vertical structure of the ocean of the ocean changes. Today the ocean is highly structured, with a number of distinct layers. My best guess is that the Cretaceous ocean was completely different—very unstructured. Unfortunately, after 40 years of work we are still in the process of sorting things out and Mother Nature is getting ahead of us.