Eocene Thermal Maximum 2
Encyclopedia
Eocene Thermal Maximum 2 (ETM-2), also called H-1 or the Elmo event, was a transient period of global warming
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

 that occurred approximately 53.7 million years ago (Ma). It appears to be the second major "hyperthermal" that punctuated the long-term warming trend from the Late Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

 through the early Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 (58 to 50 Ma).

The hyperthermals were geologically brief time intervals (<200,000 years) of global warming and massive carbon input. The most extreme and best-studied event, the Paleocene-Eocene Thermal Maximum
Paleocene-Eocene Thermal Maximum
The most extreme change in Earth surface conditions during the Cenozoic Era began at the temporal boundary between the Paleocene and Eocene epochs . This event, the Paleocene–Eocene Thermal Maximum , was associated with rapid global...

 (PETM or ETM-1), occurred about two million years before ETM-2, at approximately 55.5 Ma. Other hyperthermals likely followed ETM-2 at nominally 53.6 Ma (H-2), 53.3 (I-1), 53.2 (I-2) and 52.8 Ma (informally called K, X or ETM-3). The number, nomenclature, absolute ages and relative global impact of the Eocene hyperthermals are the source of much current research. In any case, the hyperthermals appear to have ushered in the Early Eocene Climatic Optimum, the warmest interval of the Cenozoic Era. They also definitely precede the Azolla event
Azolla event
The Azolla event occurred in the middle Eocene period, around , when blooms of the freshwater fern Azolla are thought to have happened in the Arctic Ocean...

 about 49 Ma.

ETM-2 is clearly recognized in sediment sequences by analyzing the stable carbon isotope composition of carbon-bearing material. The 13C
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...

/12C
Carbon-12
Carbon-12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons....

 ratio of calcium carbonate or organic matter drops significantly across the event. This is similar to what happens when one examines sediment across the PETM, although the magnitude of the negative carbon isotope excursion is not as large. The timing of Earth system perturbations during ETM-2 and the PETM also appear different. Specifically, the onset of ETM-2 may have been longer (perhaps 30,000 years) while the recovery seems to have been shorter (perhaps <50,000 years). (Note, however, that the timing of short-term perturbations during both events remains difficult to constrain).

A thin clay-rich horizon marks ETM-2 in marine sediment from widely separated locations. In sections recovered from the deep-sea (for example those recovered by Ocean Drilling Program
Ocean Drilling Program
The Ocean Drilling Program was an international cooperative effort to explore and study the composition and structure of the Earth's ocean basins. ODP, which began in 1985, was the direct successor to the highly successful Deep Sea Drilling Project initiated in 1968 by the United States...

 Leg 208 on Walvis Ridge
Walvis Ridge
Walvis Ridge is an ocean ridge in the southern Atlantic Ocean, extending for thousands of miles, off the coast of southwest Africa. Both it and the Rio Grande Rise originated from hotspot volcanism now occurring at the islands of Tristan da Cunha , 300 kilometres east of the crest of the...

), this layer is caused by dissolution of calcium carbonate. However, in sections deposited along continental margins (for example those now exposed along the Clarence River, New Zealand
Clarence River, New Zealand
Clarence River is located on South Island of New Zealand. It is 160 kilometres long.For its first 50 kilometres, the river runs in a generally southeastern direction. It then turns northeast, running down a long straight valley between the Inland and Seaward Kaikoura Ranges...

), the clay-rich horizon represents dilution by excess accumulation of terrestrial material entering into the ocean. Similar changes in sediment accumulation are found across the PETM. In sediment from Lomonosov Ridge
Lomonosov Ridge
The Lomonosov Ridge is an unusual underwater ridge of continental crust in the Arctic Ocean. It spans 1800 km from the New Siberian Islands, as it is part of Eurasia, over the central part of the ocean to Ellesmere Island of the Canadian Arctic Archipelago. The width of the Lomonosov Ridge varies...

 in the Arctic Ocean
Arctic Ocean
The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...

, intervals across both ETM-2 and the PETM shows signs of higher temperature, lower salinity and lower dissolved oxygen.

The PETM and ETM-2 are thought to have a similar generic origin, although this idea is at the edge of current research. During both events, a tremendous amount of 13C-depleted carbon rapidly entered the ocean and atmosphere. This decreased the 13C/12C ratio of carbon-bearing sedimentary components, and dissolved carbonate in the deep ocean. Somehow the carbon input was coupled to an increase in Earth surface temperature and a greater seasonality, which also explains the excess terrestrial sediment discharge along continental margins. Possible explanations for changes during ETM-2 are the same as those for the PETM, and are discussed under the latter entry.

The H-2 event appears to be a "minor" hyperthermal that follows ETM-2 (H-1) by about 100,000 years. This has led to speculation that the two events are somehow coupled and paced by changes in orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

.
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