Early Eocene Climatic Optimum

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The Early Eocene Climatic Optimum (EECO), also referred to as the Early Eocene Thermal Maximum (EETM), [1] was a period of extremely warm greenhouse climatic conditions during the Eocene epoch. The EECO represented the hottest sustained interval of the Cenozoic era and one of the hottest periods in all of Earth's history. [2]

Contents

Duration

The EECO lasted from about 54 to 49 Ma. [1] The EECO's onset is signified by a major geochemical enrichment in isotopically light carbon, commonly known as a negative δ13C excursion, that demarcates the hyperthermal Eocene Thermal Maximum 3 (ETM3). [3]

Climate

Following some climate models, the EECO was marked by an extremely high global mean surface temperature, [1] which has been estimated to be anywhere between 23.2 and 29.7 °C, with the mean estimate being around 27.0 °C. [4] In North America, the mean annual temperature was 23.0 °C, while the continent's overall mean annual precipitation (MAP) was about 1500 mm. [2] The mean annual temperature range (MATR) of North America may have been as low as 47 °C or as high as 61 °C, while the MATR of Asia was anywhere from 51 to 60 °C. [5] The Okanagan Highlands had a moist mesothermal climate, with bioclimatic analysis of the region yielding estimates of a mean annual temperature (MAT) of 12.7-16.6 °C, a cold month mean temperature (CMMT) of 3.5-7.9 °C, and a MAP of 103-157 cm. [6] Clumped isotope measurements from the Green River Basin confirm a high seasonality of temperature, contradicting climatological predictions of an equable climate under greenhouse conditions. [7] Lake temperatures in the Green River Formation ranged from 28 °C to 35 °C. [8] Sediments from San Diego County, California record a MAP of 1100 ± 299 mm, notably drier than the region was during the Palaeocene-Eocene Thermal Maximum. [9] Sea surface temperatures (SSTs) off of Seymour Island were ~15 °C. [10] The high elevation areas of Asia, Africa, and Antarctica saw elevation dependent warming (EDW), while those in North America and India saw elevation dependent cooling (EDC). [11]

The latitudinal climate gradient is generally believed to have been smaller, which was mainly the result of a decrease in albedo differences across Earth's surface. [12] Although SSTs are often believed to have had a shallow latitudinal temperature gradient, this is likely to be an artefact of burial-induced oxygen isotope reequilibration in fossilised benthic foraminifera. [13]

Climate modelling simulations point to a carbon dioxide concentration in the atmosphere of about 1,680 ppm to reproduce the observed hothouse conditions of the EECO, [14] although geochemical proxies suggest only 700-900 ppm. [15] Stomatal density in Gingko leaves suggests pCO2 was over twice that of preindustrial levels. [16] Additionally, methane concentrations in the Early Eocene may have been significantly higher than in the present day. [17]

The nature of the hydrological cycle during the EECO is controversial. Evidence from German peat bogs suggests that it was highly variable, with alternations between aridity and humidity. [18] Hydroclimatic variability in the Gonjo Basin was predominantly controlled by orbital eccentricity cycles. [19] Evidence from North America, in contrast, suggests that the hydrological cycle was enhanced during the EECO, although it remained relatively stable, unlike during the earlier hyperthermals, and that the stable hydroclimate may ultimately have ended the EECO by enabling high rates of organic carbon burial in lacustrine settings. [20]

Causes

The EECO was preceded by a major long-term warming trend in the Late Palaeocene and Early Eocene. [21] It was initiated by a series of intense hyperthermal events in the Early Eocene, including Eocene Thermal Maximum 2 (ETM2) and ETM3. [22]

The emplacement of the Pana Formation, a volcanic rock formation in southern Tibet that may represent the product of a supereruption, has also been proposed as a source of excess carbon flux into the atmosphere that drove the EECO. [23] Other research attributes the elevated greenhouse gas levels to increased generation of petroleum in sedimentary basins and enhanced ventilation of marine carbon. [24]

Biotic effects

The final phase of the Angiosperm Terrestrial Revolution occurred during the EECO. [25] The supergreenhouse climate of the EECO fostered extensive floral diversification and increased habitat complexity in North American terrestrial biomes. [2] The hot, humid conditions of the EECO may have facilitated the evolution of epiphytic bryophytes, with the oldest member of Lejeuneaceae being described from fossils from the Cambay amber dating back to the EECO. [26] The Okanagan Highlands in British Columbia and Washington became a biodiversity hotspot from which newly evolved lineages of temperate-adapted plants radiated from following the end of the EECO. [27]

The climate was warm enough to allow palms and palm beetles to inhabit upland regions of British Columbia and Washington. [28] Ellesmere Island became inhabited by basal primatomorphs. [29]

Northern Yakutia was covered in mangroves. [30] Mongolia witnessed a humidification event that transformed it from a shrubland into a forest and significantly reducing local wildfire incidence. [31]

In South America, the EECO coincided with the Itaboraian South American Land Mammal Age. [32] Cingulates diversified over the course of the EECO. [33]

The northern margins of the Australo-Antarctic Gulf, then located at 60-65 °S, were covered in wet-tropical lowland vegetation. [34]

At Shatsky Rise, the planktonic foraminifera Morozovella and Chiloguembelina declined in abundance. Acarinina became the dominant planktonic foraminifer in this locality. [35] Morozovella underwent a switch from dextral to sinistral coiling across the EECO. [36] The euryhaline dinoflagellate Homotryblium became superabundant at the site of Waipara in New Zealand during the early and middle EECO, reflecting the occurrence of significant stratification of surficial waters as well as increased salinity. [37]

Geologic effects

The EECO caused an increase in chert deposition by way of basin–basin fractionation by deep-sea circulation, causing increased silica concentrations in the North Atlantic which in turn resulted in direct precipitation of silica as well as its absorption by clay minerals. [38] The Equatorial Pacific displays extensive chert deposits laid down during the EECO. [39] The EECO was also marked by enhanced glauconite deposition. [40]

Comparison to present global warming

Because the pCO2 values of the EECO could potentially be reached if anthropogenic greenhouse gas emissions continue unabated for three centuries, the EECO has been used as an analogue for high-end projections of the Earth's future climate that would result from humanity's burning of fossil fuels. [41] Based on the Representative Concentration Pathway 8.5 (RCP8.5) emission scenario, by 2150 CE, the climates across much of the world would resemble conditions during the EECO. [42] One scenario of Lee et. al. (2021) suggests that conditions comparable to EECO could occur by 2300 CE. [43]

See also

Related Research Articles

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The Cretaceous is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of the entire Phanerozoic. The name is derived from the Latin creta, 'chalk', which is abundant in the latter half of the period. It is usually abbreviated K, for its German translation Kreide.

<span class="mw-page-title-main">Cenozoic</span> Third era of the Phanerozoic Eon

The Cenozoic is Earth's current geological era, representing the last 66 million years of Earth's history. It is characterized by the dominance of mammals, birds, conifers, and angiosperms. It is the latest of three geological eras, preceded by the Mesozoic and Paleozoic. The Cenozoic started with the Cretaceous–Paleogene extinction event, when many species, including the non-avian dinosaurs, became extinct in an event attributed by most experts to the impact of a large asteroid or other celestial body, the Chicxulub impactor.

<span class="mw-page-title-main">Eocene</span> Second epoch of the Paleogene Period

The Eocene is a geological epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name Eocene comes from the Ancient Greek Ἠώς and καινός and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.

<span class="mw-page-title-main">Holocene</span> Current geological epoch, covering the last 11,700 years

The Holocene is the current geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene together form the Quaternary period. The Holocene is an interglacial period within the ongoing glacial cycles of the Quaternary, and is equivalent to Marine Isotope Stage 1.

<span class="mw-page-title-main">Miocene</span> First epoch of the Neogene Period

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<span class="mw-page-title-main">Oligocene</span> Third epoch of the Paleogene Period

The Oligocene is a geologic epoch of the Paleogene Period that extends from about 33.9 million to 23 million years before the present. As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich from his studies of marine beds in Belgium and Germany. The name comes from Ancient Greek ὀλίγος (olígos) 'few' and καινός (kainós) 'new', and refers to the sparsity of extant forms of molluscs. The Oligocene is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period.

<span class="mw-page-title-main">Paleogene</span> First period of the Cenozoic Era (66–23 million years ago)

The Paleogene Period is a geologic period and system that spans 43 million years from the end of the Cretaceous Period 66 Ma to the beginning of the Neogene Period 23.03 Ma. It is the first period of the Cenozoic Era and is divided into the Paleocene, Eocene, and Oligocene epochs. The earlier term Tertiary Period was used to define the time now covered by the Paleogene Period and subsequent Neogene Period; despite no longer being recognized as a formal stratigraphic term, "Tertiary" still sometimes remains in informal use. Paleogene is often abbreviated "Pg", although the United States Geological Survey uses the abbreviation "Pe" for the Paleogene on the Survey's geologic maps.

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<span class="mw-page-title-main">Ypresian</span> First age of the Eocene Epoch

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<span class="mw-page-title-main">Eocene–Oligocene extinction event</span> Mass extinction event 33.9 million years ago

The Eocene–Oligocene extinction event, also called the Eocene-Oligocene transition (EOT) or Grande Coupure, is the transition between the end of the Eocene and the beginning of the Oligocene, an extinction event and faunal turnover occurring between 33.9 and 33.4 million years ago. It was marked by large-scale extinction and floral and faunal turnover, although it was relatively minor in comparison to the largest mass extinctions.

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<span class="mw-page-title-main">Mid-Piacenzian Warm Period</span>

The Mid-Piacenzian Warm Period (mPWP), or the Pliocene Thermal Maximum, was an interval of warm climate during the Pliocene epoch that lasted from 3.3 to 3.0 million years ago (Ma).

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<span class="mw-page-title-main">Medieval Warm Period</span> Time of warm climate in the North Atlantic region lasting from c. 950 to c. 1250

The Medieval Warm Period (MWP), also known as the Medieval Climate Optimum or the Medieval Climatic Anomaly, was a time of warm climate in the North Atlantic region that lasted from c. 950 to c. 1250. Climate proxy records show peak warmth occurred at different times for different regions, which indicate that the MWP was not a globally uniform event. Some refer to the MWP as the Medieval Climatic Anomaly to emphasize that climatic effects other than temperature were also important.

A hyperthermal event corresponds to a sudden warming of the planet on a geologic time scale.

The Middle Miocene Climatic Optimum (MMCO), sometimes referred to as the Middle Miocene Thermal Maximum (MMTM), was an interval of warm climate during the Miocene epoch, specifically the Burdigalian and Langhian stages.

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