User:Anneflorineyapo/Paleocene-Eocene thermal maximum

The Paleocene-Eocene Thermal Maximum (PETM) is a geologically rapid and warming event during what the global temperature increased by 5 to 8°C during thousand years. The age is uncertain, but we know that it occurred at the boundary of the Paleocene and Eocene epochs (approximately 56 Mya). This event led to global ocean acidification, increased halocline stratification and deep-sea anoxia [1].Some studies talk about the fact that the PETM carbon release was a response to internal carbon-cycle instabilities in a warmer world, such as the release of methane from the deep ocean or the oxidation of organic carbon in melting permafrost [2].This event is usually associated to a pronounced carbon excursion (CIE) which has led to profound changes in the atmosphere (climate cycling), the hydrosphere (geochemical cycling) and the biosphere (biotic evolution). In the deep sea, 30-50% of benthic foraminiferal species and planktonic biota went extinct or at least were affected and the oceans rapidly acidified. Global warming also may have led to a pulse of speciation or migration among mammalian groups [3]The CIE of 2 to 8 ‰ has also a phase of severe carbonate dissolution on the ocean floor. The combination of both (CIE and carbonate dissolution) is evidence that a large amount of carbon was injected in the atmosphere-ocean system, resulting in strong perturbations of the global carbon cycle. This also suggest a causal relationship between carbon injections and the PETM. In the Pacific surface-ocean, the dissolved organic carbon increased by + 1.010 micro-mol per kilogram [2]. The PETM and the CIE are often thought as the best geologic analog for the current anthropogenic rise in pCO2 [2].

Age and preservation of the PETM and the CIE
PETM is well preserved in many shallow-marine siliciclastic and hemipelagic/pelagic sections on some modern and ancient carbonate platforms. For example, there is a good preservation in the Forada section in the northern Italy, which has been deposited in a hemipelagic, near continental setting [3]. Compared to pelagic sediments, shallow-marine carbonates deposited on carbonate platforms/ramps were formed at higher sedimentation rates and can avoid the effect of carbonate dissolution. Compared to shallow-marine siliciclastic sediments, carbonates formed during the CIE in the depositional environment that is less likely by terrestrial input (for example, river runoff), which can facilitate the preservation of primary signals of carbon isotope variations in the ocean. The CIE is mainly preserved in limestone deposited on tropical carbonate ramp during the PETM. In pelagic sequences of the CIE, there is a pronounced dissolution layer of condensed interval and a recovery by a lithologically uniform carbonate-rich interval. An exceptionally well preserved and complete PETM section can be found on the island of Fur in northern Denmark. This section is composed of marine clays interbedded with ash layers. Estimates of the absolute age and the duration of the PETM vary between 54.88 and 55.50 My, and 100 and 250 ka respectively [3] The CIE age is 55.8 Mya. Deep sea bulk carbonate records show that the CIE began with an abrupt initial carbon 13 decrease of approximately 1% followed by a more gradual decrease of similar magnitude.

Depositional features
PETM/CIE evidence are generally found on carbonate platforms, ancient and modern. Studies on some modern and ancient carbonate platforms suggest that seawater carbon 13 in shallow epicontinental seas can be disturbed by some local processes such as evaporation, sea level changes but also oxidation of organic matter. It these cases, it would lead to the decoupling of the carbon cycling in shallow epicontinental seas and in the open surface ocean. Generally, the CIE is expressed as a clay layer of less than a centimeter. The northern Salisbury Embayment is a good place to study that. The clay layer on this site is part of the Marlboro Clay unit, which contain distinct and rhythmic bedding of silty, kaolinitic clays through an entire section containing the CIE.

Magnitude
Many other anomalies can be compared to this event, but the PETM is unique in terms of magnitude and duration of warming [1]. The event started with a rapid carbon decrease (approximately 1‰), followed by other similar decrease phases. The first phase lasted approximately 30 000 years. In northern Salisbury Embayment, an overall magnitude of - 5.5‰ has been recorded, which a magnitude of 3.5‰ for the initial phase.

Stratigraphic features
Rocks in the PETM-CIE interval are mainly composed of calcareous marl and marly nodular limestone. For example, the Forada section in northern Italy is composed of sharply interrupted succession of limestone-marl couplets. The interruption is approximately 3.3 m thick unit of clays and marls representing the “clay layer” of the PETM [3]. In the five locations below (table), we can observe the same sedimentary cyclicity. Many studies on the PETM reconstructions found differences in temperatures between the bottom-water and the sea surface, showing large variations in temperature increase depending on depth, latitude, seawater chemistry and the choice of proxy and calibration [1].

Proposed mechanism(s)
The source mechanism of the PETM-CIE are still the focus of much debate. The causal mechanisms for the PETM remains unidentified because of the large uncertainties in the duration of the CIE’s onset. The potential sources have all distinct carbon signatures and the choice of the “good” source leads to large differences in the amount of carbon released [2]. However, we think that mechanisms such as volcanic emissions and the associated carbon released caused by the emplacement of the North Atlantic Igneous Province (NAIP) can be a good explanation. Both require massive addition of depleted carbon 13 to the ocean-atmosphere system in a geologically short interval of time. The NAIP is composed of extrusive and intrusive rocks around the modern Northeast Atlantic margins, emplaced between 52-63 Ma with the most voluminous activity during the opening of the North Atlantic [1]. A reconstructed increase in the oceanic DIC reservoir of +14.900 PgC imply a substantial contribution of a magma-derived CO2 emissions to the peak-PETM oceanic CIE [2]. A recent study suggests that up to 10.000 PgC could have been released from oxidation of remobilized fossil carbon during the peak-PETM interval, which has probably led to the temporal extension of the CIE [2]Other proposed mechanisms include the input of methane into the atmosphere-ocean system from the dissociation of methane wildfires burning peatlands, desiccation of a large epicontinental sea, decomposition of terrestrial permafrost and bolide impact. It is also possible that more than one source is implied here because the dissolved inorganic carbon increase requires a dominant contribution of an isotopically heavy carbon source such as magmatic CO2.

Local or diagenetic
Reduced pelagic calcification caused by environmental changes may also have played a role in increasing alkalinity during peak acidification.