The tertiary period (ca. 66.4 to 1.8 million years ago [Ma]) was an interval of enormous geologic, climatic, oceanographic, and biologic change. It spans the transition from a globally warm world of relatively high sea levels to a world of lower sea levels, polar glaciations, and sharply differentiated climate zones. Over the past decade, however, it has become increasingly clear that Tertiary climatic history was not a simple unidirectional cooling driven by a single cause but a much more complicated pattern of change controlled by a complex and dynamic linkage between changes in atmospheric CO2 levels and ocean circulation, both probably ultimately driven by tectonic evolution of ocean-continent geometry. Although satisfactory explanations for many aspects of Tertiary climate history are available, many areas remain incompletely understood.
The early Tertiary (Paleocene and most of the Eocene epochs, ca. 66-50 Ma) was characterized by a continuation of Cretaceous warm equable climates extending from pole to pole. Global temperatures may have been as much as 18-22 degrees F (10-12 degrees C) higher than present, and pole-to-equator temperature gradients were about 9 degrees F (5 degrees C) during the Paleocene, as compared with about 45 degrees F (25 degrees C) today.
The Paleocene-Eocene boundary (about 54 Ma) was marked by a geologically brief episode of global warming known as the Paleocene-Eocene thermal maximum (PETM), characterized by an increase in sea surface temperatures of 9-11 degrees F (5-6 degrees C), in conjunction with ocean acidification, a decline in productivity, and a large and abrupt decrease in the proportion of isotopically heavy terrestrial sedimentary carbon in the oceans. The PETM is thought to have lasted only about 170,000 to 220,000 years, with most of the temperature and isotopic change occurring in the first 10,000 to 20,000 years. Its causes remain unclear, but it was probably associated with dissolution of methane hydrates on the ocean floor, which would then have caused greenhouse warming. Possible triggers for this hydrate release include an increase in volcanism, leading to an increase in atmospheric CO2 and consequent sudden initiation of greenhouse warming; a change in ocean circulation; or massive regional submarine slope collapse.
Global temperatures warmed still further during the early Eocene, reaching their highest levels of the past 65 million years during an interval sometimes called the early Eocene climatic optimum (52-50 Ma). Global cooling began during the early middle Eocene (ca. 50 Ma) and accelerated rapidly across the Eocene-Oligocene boundary (ca. 34 Ma), at which time Antarctic continental glaciation began. This shift is frequently referred to as a change from a greenhouse to an icehouse climate regime, and it was one of the most fundamental reorganizations of global climate known in the geological record.
Initiation of Antarctic glaciation has long been attributed to the tectonic opening of Southern Ocean gateways, especially the Drake Passage between South America and the Antarctic Peninsula, which allowed establishment of the Antarctic Circumpolar
Current and the consequent isolation of the southern continent from warmer low-latitude waters. This has been questioned recently, however, as a result of the redating of the formation of these gateways, as well as modeling results that point to a greater role for reduced atmospheric CO2.
Most estimates of early Cenozoic atmospheric pCO2 range between two and five times the present values in the middle to late Eocene and then decline rapidly during the Oligocene to reach approximately present levels in the latest Oligocene. This decline in CO2 may, in turn, have been at least partly a result of the tectonic uplift of the Tibetan plateau, beginning around 40 Ma, leading to increased rates of chemical weathering. Levels of CO2 remained relatively constant throughout the Miocene, suggesting that the substantial climate changes during this time were driven by other factors, including changes in weathering or ocean circulation.
Global temperatures warmed again in the late Oligocene, followed by a brief (ca. 200,000 years) but deep glacial interval at the Oligocene-Miocene boundary (ca. 24 Ma). Temperatures then stabilized or slightly increased (punctuated by several more brief glacials), leading to what is sometimes referred to as the mid-Miocene climatic optimum around 17 to 15 Ma, during which time deep water and high-latitude sea surface temperatures were 11-18 degrees F (6-10 degrees C) warmer than at present. The causes of this warming are not clear, but they may have been related to increased northward oceanic heat transport in the North Pacific brought via intensified currents primarily triggered by narrowing of the Indonesian Seaway in the western equatorial Pacific.
Another major cooling occurred between 14.2 and 13.7 Ma and is associated with increased production of cold Antarctic deep waters and a growth spurt of the East Antarctic Ice Sheet, leading to an increased latitudinal temperature gradient and drying in midlat-itudes. A further episode of aridity occurred between 8 and 4 Ma. This cooling trend continued into the Quaternary period, with a short warming interval in the early to mid-Pliocene (ca. 5-3.2 Ma), characterized by warmer sea and air temperatures across at least much of the North Atlantic region.
Northern Hemisphere ice sheets first expanded about 3.5 Ma, with a major pulse of growth occurring 2.5 Ma, at which time the Earth is usually said to have passed over a thermal threshold initiating the latest so-called Ice Age, in which mode the planet is still today. The initiation of Northern Hemisphere glaciation has been attributed to completion of the formation of the Central American Isthmus at around 3.5 Ma, which deflected warm low-latitude currents flowing westward from Africa northward into the Gulf of Mexico and through the Florida Straits to join the Gulf Stream. This strengthened Gulf Stream then transported more moisture to high latitudes, where it supplied an increase in snowfall, leading to increased albedo and temperature decline.
