Middle Miocene Climatic Optimum

The Middle Miocene Climatic Optimum (MMCO), sometimes referred to as the Middle Miocene Thermal Maximum (MMTM),

Based on the magnetic susceptibility of Miocene sedimentary stratigraphic sequences in the Huatugou section in the Qaidam Basin, the MMCO lasted from 17.5 to 14.5 Ma; rocks deposited during this interval have a high magnetic susceptibility due to the production of supermaramagnetic and single domain magnetite amidst the warm and humid conditions at the time that define the MMCO.^[3]

Estimates derived from Mg/Ca palaeothermometry in the benthic foraminifer Oridorsalis umbonatus suggest the onset of the MMCO occurred at 16.9 Ma, peak warmth at 15.3 Ma, and the end of the MMCO at 13.8 Ma.^[4]

Climate

The Arctic was ice free and warm enough to host permanent forest cover across much of its extent. Iceland had a humid and subtropical climate.[2]

The mean annual temperature (MAT) of the United Kingdom was 16.9 °C.

The Northern Hemisphere summer location of the Intertropical Convergence Zone (ITCZ) shifted northward; because the ITCZ is the zone of maximal monsoonal rainfall, the precipitation brought by the East Asian Summer Monsoon (EASM) increased over southern China while simultaneously declining over Indochina.^[13] The Tibetan Plateau was overall wetter and warmer.^[3]

Overall, western North America north of 40 °N was wetter than south of 40 °N.^[14] The Mojave region of western North America exhibited a drying trend.^[15] Along the New Jersey shelf, the MMCO did not result in any discernable climatic signal relative to earlier or later climatic intervals of the Miocene; temperatures here may have been kept low by an uplift of the Appalachian Mountains.^[16]

Northern South America developed increased seasonality in its precipitation patterns as a consequence of the ITCZ's northward migration during the MMCO.^[17] The Bolivian Altiplano had a MAT of 21.5-21.7 ± 2.1 °C, in stark contrast to its present MAT of 8-9 °C, while its MMCO precipitation patterns were identical to those of today.^[18]

In Antarctica, average summer temperatures were about 10 °C.^[19] The East Antarctic Ice Sheet (EAIS) was severely reduced in area.^[20][21] However, despite its diminished size and its retreat away from the coastline of Antarctica, the EAIS remained relatively thick.^[22] Additionally, Antarctica's polar ice sheets exhibited high variability and instability throughout this warm period.^[23]

Causes

The global warmth of the MMCO resulted from its elevated atmospheric carbon dioxide concentrations relative to the rest of the Neogene.^[2] Boron-based records indicate pCO₂ varied between 300 and 500 ppm during the MMCO.^[23] A MMCO pCO₂ estimate of 852 ± 86 ppm has been derived from palaeosols in Railroad Canyon, Idaho.^[24] The primary cause of this high pCO₂ is generally accepted to be elevated volcanic activity.^[25][26] Hydrothermal alteration by magmatic dikes and sills of sediments rich in organic carbon further contributed to rising pCO₂.^[27] The activity of the Columbia River Basalt Group (CRBG), a large igneous province in the northwestern United States that emitted 95% of its contents between 16.7 and 15.9 Ma, is believed to be the dominant geological event responsible for the MMCO.^[28] The CRBG has been estimated to have added 4090–5670 Pg of carbon into the atmosphere in total, 3000-4000 Pg of which was discharged during the Grande Ronde Basalt eruptions, explaining much of the MMCO's anomalous warmth. Carbon dioxide was released both directly from volcanic activity as well as cryptic degassing from intrusive magmatic sills that liberated the greenhouse gas from existing sediments. However, CRBG activity and cryptic degassing does not sufficiently explain warming before 16.3 Ma.^[29] Enhanced tectonic activity led to increased volcanic degassing at plate margins, enabling high background warmth to occur and complementing CRBG activity in driving temperatures upwards.^[30]

Albedo decrease from the reduction in Earth's surface area covered by deserts and the expansion of forests was an important positive feedback enhancing the warmth of the MMCO.^[31]

The increase in organic carbon burial on lands submerged by rising sea levels resultant from the increased warmth were an important negative feedback inhibiting further warming.^[32][33] This positive carbon excursion is called the Monterey Carbon Excursion, which is globally recorded but mainly in the circum pacific belt.^{[34][35][36][37]} The Monterey Excursion seems to envelop the MMCO, meaning this carbon excursion started just before the climatic optimum and it ended just after it.

Climate modelling has shown that there remain as-of-yet unknown forcing and feedback mechanisms that had to have existed to account for the observed rise in temperature during the MMCO,^[38] as the amount of carbon dioxide known to have been in the atmosphere during the MMCO along with other known boundary conditions are insufficient in explaining the high temperatures of the Middle Miocene.^[2]

Biotic effects

The world of the MMCO was heavily forested; trees grew across the Arctic and even in parts of Antarctica.

Northern North America was dominated by cool-temperate forests. Western North America was mostly composed of warm-temperate evergeen broadleaf and mixed forest.^[14] In spite of the climatic changes, the niches of Oregonian equids were unchanged throughout the MMCO.^[40] What is now the Mojave Desert was a grassland dominated by C₃ grasses during the MMCO.^[15] Central America was tropical, as it is today.^[14]

In Europe, the MMCO witnessed the northward expansion of thermophilic plants.^[9] Along the northwestern coast of the Central Paratethys, mixed mesophytic forest vegetation predominated.^[41] At the Stetten locality, spruces and firs increased in abundance during transgressive phases of precessionally forced transgressive-regressive cycles, while marshes, many of them saline, dominated by Cyperaceae and swamps dominated by Taxodiaceae prevailed during sea level lowstands.^[12] Because of the dense, humid forests covering central eastern France and northern Germany, the species richness of these areas was high and the mammal community dominated by small taxa, while the more arid Iberian Peninsula had a lower species richness and a relative absence of medium-sized mammals.^[10] Europe also contained an abundance of ectothermic vertebrates due to its much warmer climate in the MMCO compared to the present.^[9]

Northern South America possessed tropical evergreen broadleaf forests. The Atacama Desert already existed along the western coast of central South America and graded into temperate xerophytic shrubland and temperate sclerophyll woodland and shrubland to the south. In eastern South America south of 35 °S, warm-temperate evergreen broadleaf and mixed forest predominated, alongside temperate grassland.^[14] The MMCO played a major role in the partitioning and diversification of South America's land mammal faunas.^[42]

Comparison to present global warming

The MMCO's temperature estimates of 3-4 °C above the preindustrial mean are similar to those projected in the future by mid-range forecasts of

• Palaeocene-Eocene Thermal Maximum
• Early Eocene Climatic Optimum
• Middle Eocene Climatic Optimum

References