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Eocene Thermal Maximum 2 (ETM-2), also called H-1 or Elmo (Eocene Layer of Mysterious Origin), was a transient period of global warming that occurred around 54 Ma.^[1]^[2] ^[3]^[4]^[5] It was the second major hyperthermal that punctuated long-term warming from the Late Paleocene through the Early Eocene (58 to 50 Ma).^[6]

The hyperthermals were geologically brief time intervals (<200,000 years) of global warming and massive input of isotopically light carbon into the ocean and atmosphere.^[7]^[8] The most extreme and best-studied event, the Paleocene-Eocene Thermal Maximum (PETM or ETM-1), occurred about 1.8 million years before ETM-2, at approximately 55.8 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.^[9]^[10] In any case, the hyperthermals appear to have ushered in the Early Eocene Climatic Optimum (EECO), the warmest sustained interval of the Cenozoic Era.^[11] They also definitely precede the Azolla event at about 49 Ma.

Timing

ETM-2 is clearly recognized in sediment sequences by analyzing the stable carbon isotope composition of carbon-bearing material.^[3]^[9]^[10] The ¹³C/¹²C ratio of calcium carbonate or organic matter drops significantly across the event.^[12] 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 PETM also appear different.^[5] 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).^[5] (Note, however, that the timing of short-term carbon cycle 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 Leg 208 on Walvis Ridge), this layer is caused by dissolution of calcium carbonate.^[5] However, in sections deposited along continental margins (for example those now exposed along the Waiau Toa / Clarence River, New Zealand), the clay-rich horizon represents dilution by excess accumulation of terrestrial material entering the ocean.^[4] Similar changes in sediment accumulation are found across the PETM.^[4] In sediment from Lomonosov Ridge in the Arctic Ocean, intervals across both ETM-2 and PETM show signs of higher temperature, lower salinity and lower dissolved oxygen.^[8]

Causes

The PETM and ETM-2 are thought to have a similar generic origin,^[4]^[8]^[5] although this idea remains 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. The source of this ¹³C-depleted carbon during ETM2 is believed to be organic carbon.^[13] Somehow carbon input was coupled to an increase in Earth surface temperature and a greater seasonality in precipitation, which explains excess terrestrial sediment discharge marking both events in continental margin sections. Explanations for changes during ETM-2 are the same as those for the PETM, and are discussed in that article.

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.^[4]^[5]

Sea surface temperatures (SSTs) climbed by 2–4 °C and salinity by ~1–2 ppt^{[clarification needed]} in subtropical waters during ETM-2.^[14]

Effects

Ocean acidification did occur during ETM2 as it did in the PETM, but the magnitude of the drop in pH was significantly lower.^[15] Along the Atlantic Coastal Plain, changes in local hydrology and nutrient supply were minimal, unlike during the PETM.^[16] In the Tethys Ocean, an increase in surface water eutrophication is recorded.^[17]

The marine ecological recovery from the PETM was significantly inhibited by ETM2.^[18] As in the case of the PETM, reversible dwarfing of mammals has been noted to have occurred during the ETM-2.^[19]^[20] Unlike during the PETM, there was no change in the photosymbiont associations of the planktonic foraminifer Acarinina soldadoensis, possibly because the PETM had already selected for adaptations enabling them to withstand extreme hyperthermals or because of the lesser magnitude of ETM2.^[21] In the Tethys, planktonic foraminifer test size decreased by 40%, while calcareous nannoplankton community sizes dropped as reflected by increased abundance of small placoliths.^[22]