We all know the tale of the dinosaurs’ demise: some 65 million years ago, a large asteroid impacted the Earth near the modern Yucatan peninsula. The cataclysmic event resulted in one of the largest mass extinctions in Earth history, spelling doom not only for the lizard tyrants, but a majority of marine life as well. Once stable ecosystems became a vacuum, which was filled by the rapid evolution of pioneering species, giving rise to the dominion of diverse mammalian fauna. The world we know today was thus conceived in a fiery collision between two stones.
Since it was originally proposed by Luis and Walter Alvarez, this story has never failed to intrigue the layman and scientist alike. The geological transition from Cretaceous to Paleocene is one of the most studied in the world. But how much do we know about the world since that time? What on Earth happened over the past 65 million years?
Thousands of studies are available to answer these questions—far more than one could learn in a lifetime. So I want to focus on one excellent review paper by Zachos et al. (2001), entitled Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present, which summarizes how marine sediments have revealed the story of Earth’s climate. The authors compiled stable-isotope data from more than 40 deep-ocean cores around the world, all of which record at least 65 million years worth of sedimentation. The layers in these cores have been dated by a variety of methods, including radiocarbon (to estimate recent sedimentation rates), magnetostratigraphy, and direct dating of volcanic ashes. One of the most important dating tools, however, is the use of orbital tuning, because oxygen-isotope values oscillate in accordance with past variations in Earth’s orbit around the sun. Combined, these techniques provide us with a high-resolution portrait of the Cenozoic—the era of ‘new life’.
What do stable isotopes tell us about climate?
Deep ocean sediments are populated with the ‘shells’ (called tests) of ancient foraminifera, a type of microorganism that thrives in both shallow and deep waters. The shells are composed of calcite (calcium, carbon, and oxygen), which mineralized out of the very ocean water in which the foraminifera lived. By determining the ratio of heavy isotopes (like 18O) to light isotopes (16O) in their shells, we can thus reconstruct how isotopes in the ocean have changed over millions of years.
Oxygen isotopes in particular are incredibly useful when discussing climate, because the ratio of heavy-to-light oxygen depends on two main factors: the water temperature when the shell was formed, and the volume of glacial ice on land. When the water is colder, the abundance of heavy oxygen (δ18O) in the shell goes up; when more water is stored on land as ice, the abundance of heavy oxygen (δ18O) also goes up. Conveniently, these factors go hand in hand, because the colder the Earth’s surface, the more ice can be preserved on land in the form of glaciers. If you can remember this relationship, then you can interpret that main plot of the paper: high δ18O values imply a colder climate with bigger glaciers, while low δ18O values imply a warmer climate with few to no glaciers.
Geologists can use an additional proxy, however, to estimate temperature from the same foraminifera: the ratio of magnesium to calcium. Having an independent estimate of water temperature allows paleoceanographers to approximate how much temperature and glacial ice have each affected δ18O values in the past.
If we focus on the left side of Figure 2 from the paper, two major trends appear: first, a broad-scale warming from 68 until ~52 million years ago, after which Earth’s climate has steadily cooled until the present day. In other words, surface temperatures during the early Eocene (52 Ma) were more than 10°C higher than today—not a world in which humans could comfortably thrive. Independent proxies for atmospheric carbon dioxide suggest that CO2 was at least 3–5 time higher than today (1,000–2,000 ppm, compared to 400 ppm in the year 2014 AD). During this time also, there were no ice sheets present anywhere on Earth, including Antarctica, so sea-level would have been much higher. Studying the climate of the early Eocene, therefore, we get a glimpse into the greenhouse effect that potentially awaits our distant descendants. Fortunately for us, massive ice sheets in Antarctica and Greenland still reflect enough of the sun’s energy back to space to buffer and slow down global warming. But they won’t last forever.
So what caused the global cooling trend after ~52 million years ago? In a nutshell: the Himalayas. Only a few million years prior, the Indian subcontinent had finally made contact with Asia. This tectonic collision resulted in rapid uplift of new mountain ranges along the border of these tectonic plates. What does this have to do with climate? Well, if you’ve ever lived in the vicinity of a mountain range, then you’ve likely experienced their ability to change the weather. As air is forced upward into the atmosphere, it rapidly cools, forcing rain and snow to fall on the rocky slopes. The bigger the mountain range, the more rock is exposed to weathering processes, and chemical weathering tends to remove CO2 from the atmosphere. Combining these elements, we can understand how plate tectonics control climate: bigger mountains lead to more weathering, which lowers atmospheric CO2 on multimillion-year timescales and cools the surface of the Earth. Eventually, this fall in CO2 gave rise to grasses less than 10 million years ago. These frequently trodden plants are specially suited to thrive in low-CO2 environments through a unique photosynthetic process (C4) that artificially concentrates carbon dioxide within its cells.
Taking a closer look at the figure, we can observe some more rapid shifts in δ18O—between 34–26 Ma, after 15 Ma, and from ~7 Ma to present—which were not caused by changes in surface temperature alone. Marine sediments help us to constrain, therefore, when major ice sheets began to form on Earth. Based on paleontological evidence, we know that the Antarctic Ice Sheet first began to form around 34 million years ago. Unsurprisingly, this is well reflected in the δ18O value of marine sediments around the world.
The initial growth of Antarctic ice sheets was accelerated by two tectonic events between 30–32 million years ago: separation of the Australian and South American continents from Antarctica, opening the Tasmania-Antarctic seaway and the Drake Passage. This tectonic separation turned Antarctica into a giant, polar island, meaning that oceanic currents were then free to circle its coastline. Even in the face of modern warming, these currents keep Antarctica much colder than its northern counterpart.
When the δ18O record is examined at finer resolution, some magnificent patterns appear. Over the last 34 million years, Earth’s climate has alternated between distinctly warmer and colder epochs at characteristic frequencies. As Zachos et al. phrase it, “climate varies in a quasi-periodic fashion during all intervals characterized by glaciation”. In other words, as long as large ice sheets were present on land, global climate fluctuated predictably (and measurably) like a sine wave, in accordance with variations in Earth’s orbit. But what does glaciation have to do with the orbit of the Earth? The relationship is indirect, due what is called a positive feedback mechanism. Ice sheets function like giant mirrors that reflect much of the sun’s energy back into space, making cold intervals even colder. Without ice sheets, climate cycles still exist, but they are not nearly as pronounced. Once falling CO2 levels cooled the planet to allow ice sheets to form, the ice created a positive feedback that cooled the planet even further.
This review by Zachos et al. thus confirmed what had been known for a long time: regular variations in Earth’s orbit around the sun control the amount of solar energy reaching the surface. Figure 4 tells us, in a nutshell, that the number of cycles recorded in ocean sediments (and the average time between cycles) over the past 34 million years is precisely in accordance with Milankovitch theory.
Figure 4 also tells us which Milankovitch cycles were most important at various intervals during the Cenozoic. For example, ocean temperature and ice volume fluctuated every 41,000 years and every 100,000 years over the past 4 million years, reflecting the obliquity and eccentricity cycles. Interestingly, the eccentricity signature became more pronounced, however, only in the past ~1 million years (Fig. 3a, below). The reason is that as Earth’s surface continued to cool, ice sheets were able to reach greater masses, and large ice sheets can actually prevent Earth’s temperature from getting hot enough to melt that ice in large quantities. As a result, it took significantly more solar energy (which peaks every 100,000 years) to initiate full-scale deglaciation, during which Earth moved from an ice age to the warm period we enjoy today.
Finally, the authors describe three major perturbations to Earth’s climate (including the carbon cycle) during the past 65 million years. These events are unique and more sudden, occurring over the course of ‘only’ tens of thousands of years. Keep in mind that in geological terms, 10,000 years is more or less “instantaneous”.
The first major event is well known to anyone studying modern climate change. Approximately 55 million years ago (the boundary between Paleocene and Eocene), ocean temperatures rose by more than 5°C in less than 10,000 years, resulting in what has been termed the Paleocene-Eocene Thermal Maximum. It took more than 200,000 years for Earth to recover, in terms of climate, but the accompanying widespread extinctions would change life forever. Researchers generally agree that warming was initiated by a pulse of methane to the oceans/atmosphere and disruption of the global carbon cycle. This fact is well reflected in the sudden drop in δ13C that mirrors the shift in δ18O (Fig. 5): methane contains very little ‘heavy carbon’, so when mixed into the oceans, it causes δ13C values everywhere to fall. Since methane is a potent greenhouse, temperature rose sharply in response.
The remaining two aberrations mark the beginning and the end of the Oligocene period. Approximately 34 million years ago, the Antarctic ice sheets began to grow, accelerating the global cooling trend. This cooling event took ~400,000 years to reach its maximum and resulted in a major reorganization of ocean currents and productivity. As the southern pole froze over to become an icy desert, the rest of the planet thrived. In fact, the first appearance of whales, gargantuan grazers of the surface ocean, coincides with this event.
What mean these cores?
Earth history is recorded in fine detail for those who can decipher its cryptic text. Our knowledge of the past is not limited to speculation, eyewitness historiography, or mythic representation. Were we there? No, but we can speak to the stones and creatures who were through a common tongue called geology. Their fascinating tales are limited only by our capacity for inquiry, and they remind us of the intricate series of events that made Earth habitable for—and inhabited by—humanity.
Featured image reproduced from Heritage Daily.