from the Deep Blue Sea
But some scientists did think that methane (though not specifically hydrate methane) could have a serious impact on Earth's climate. In attempting to explain why circumpolar environments of the Early Eocene (roughly from 55 to 50 million years ago) seemed considerably warmer and quite hospitable to mammals, reptiles and even deciduous forests, Lisa Sloan and her colleagues proposed that the cause could have been methane from peatlands. During the Early Eocene, peatlands may have covered more than three times the area they cover today. A slight warming and drying of these lands could have released substantial quantities of methane. Such a release, reasoned Sloan and colleagues, could have produced polar clouds that would have trapped outgoing long-wave radiation and therefore the heat needed to explain the then warmer environment (Sloan, 1992).
In 1995 a paper by Gerald Dickens and three co-authors proposed that the cause of Eocene warming was more likely methane from oceanic hydrate, not methane from peatlands. Dickens hypothesized that there was a major release of methane from hydrate at about 55 million years ago, a release that produced the significant negative carbon isotopic excursion noted in the rocks and fossils from that time. With proper scientific restraint, Dickens stated that the "fate of CH¸4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes [overall conditions] that operated [at the time]" (Dickens, 1995).
Fifty-five million years ago marked the event known as the Late Paleocene Thermal Maximum, or LPTM (Zachos, 1993). This event is also referred to as the Latest Paleocene Thermal Maximum, because of uncertainty as to whether it occurred at the exact end of the Paleocene, or merely close to the end. But the terms are frequently used interchangeably, even in the same scientific paper (for example, Dickens, 2000). (The terminology issue has continued. More recently, this event has been referred to as the Paleocene-Eocene Thermal Maximum, or PETM: Zachos, 2003, and as the initial Eocene Thermal Maximum, or IETM: Dickens, 2004, and Svensen, 2004. Despite the changing terminology, the event referred to is the same. Here the original name, Late Paleocene Thermal Maximum, or LPTM, will be used.)
There is also some serious dispute as to when the LPTM (and therefore the end of the Paleocene) actually did take place. Some scientists think it occurred up to five million years earlier than its usually accepted date of about 55 million years ago, which would make it contemporaneous with the first pulse of volcanism in the North Atlantic Igneous Province (Jolley, 2002). Others strongly contest such an early date (Wei, 2003; Thomas, 2003; Srivastava, 2003), in one case objecting that this proposed timeline would cause the Paleocene epoch to be unacceptably shortened from about 10 million to only 3.5 million years (Aubry, 2003).
In any case, the Paleocene was the first temporal subdivision (epoch) of the Tertiary Period, which followed the end-Cretaceous catastrophe. It lasted about ten million years, and was followed by the Eocene epoch. The "thermal maximum" was a time of exceptional warmth, when global temperatures were several degrees higher than at any time since. The warmth is recorded in carbonate found in several, widely separated locations, thus confirming that the warmth was global in extent. The oxygen isotopes in these carbonates provide evidence of the temperatures of the time. Reading this "oxygen thermometer" is not easy, as a number of factors, such as the salinity of the ocean, can affect the mix of oxygen isotopes. But scientists are aware of these confounding factors, and do include their possible effects in estimating ancient temperatures.
The warming was considerable: as much as 4°C (7.2°F) in the deep ocean, and 8°C (14.4°F) in high-latitude (near polar) surface water (Katz, 1999). Tropical sea surface temperatures rose by as much as 4 to 5°C (Zachos, 2003). The deep water temperature, initially about 11°C (about 52°F), rose to about 15°C (about 59°F)(Kennett and Stott, 1991; Zachos, 1993; Zachos, 1994; Thomas and Shackleton, 1995). These unusually high temperatures provided the label, "thermal maximum."