Magazine article Oceanus

From the Greenhouse to the Icehouse: A Southern Ocean Perspective of Paleogene Climate

Magazine article Oceanus

From the Greenhouse to the Icehouse: A Southern Ocean Perspective of Paleogene Climate

Article excerpt

Glomar Challenger's retirement in 1983 marked the end of a highly successful 15-year international scientific drilling program that radically altered our understanding of the geologic and climatic evolution of the oceans. Among the many achievements was a new understanding of the early Cenozoic period of climate change known as the Paleogene era, which is further subdivided into three epochs, the Paleocene (57 to 65 million years ago), Eocene (35 to 57 million years ago), and Oligocene (25 to 35 million years ago). Paleontological and geochemical investigations of deep sea cores revealed that the Paleogene was a time of dramatic earth climate transition from warm, equable conditions of the "hothouse" or "greenhouse" mode, to cooler, glacial-like conditions of the "icehouse mode." Although the "greenhouse" mode prevailed during much of the Paleocene and early Eocene, the warmest conditions existed during the early Eocene, some 55 million years ago.

Temperatures of the deep sea at that time were some 10 |degrees~ C warmer than the present, as were temperatures of higher latitude surface waters, which were inhabited mainly by warm-water species of marine plankton. The warmer conditions found in marine environments seemed to conform with reconstructions of climate on the continents, where high-latitude regions were inhabited by temperate to subtropical species of vertebrates and plants, such as alligators and palms. This episode of early Eocene global warmth lasted for several million years before the onset of cooling and a 20-to-30-million-year gradual transition to the "icehouse" mode. By Oligocene time, polar regions had cooled substantially, although it remained unclear whether or not ice sheets had existed.

As the details of this global-climate transformation emerged in the late 1970s, it began to draw the attention of paleoclimatologists who wondered why Earth's climate changed as it did. Was the early Eocene warmer and the Oligocene cooler because of a decline in the concentration of atmospheric carbon dioxide, a greenhouse gas, or were other factors responsible, such as rearrangement of oceanic passages and currents by slowly drifting continents? These questions grew in importance, especially with concern increasing over the future climatic impact of recent high carbon-dioxide levels. However, despite the great interest, the questions remained unanswered, partly because many critical details about the character of the Paleogene climate were still vague. In particular, the absence of sediment cores from the climatically sensitive high latitudes had left a crucial gap in the paleoclimatic record. Attempts to obtain deep sea sediments from polar regions during the initial drilling program were limited by persistent harsh, icy weather. As a result, little was learned about the pre-Pleistocene climate history of the high-latitude oceans.

A New Perspective From the Bottom: Southern Ocean Paleoceanography

In 1985, with the initiation of JOIDES Resolution and the second phase of scientific drilling, scientists gained the capacity to drill in some of the more remote and inhospitable reaches of the world oceans, including the polar oceans. One immediate regional target was the Southern Ocean, where nearly 10 kilometers of sediment were recovered at more than 25 sites during four legs of drilling (Legs 113, 114, 119, and 120). In the years since, shore-based investigations of these cores have provided new insight into the Paleogene climate. Some of these findings are beginning to have profound effects on our understanding of the forces that altered Paleogene climate, as well as on climate-change dynamics in general.

Long- and Short-Term Warming in the Eocene

One of the more unexpected findings from high-latitude drilling resulted from high-resolution geochemical and paleontologic investigations of cores recovered from atop Maud Rise, and later Kerguelen Plateau, which revealed that the long-term climatic transitions were much more complicated than previously recognized. …

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