The Big Event: Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM) is perhaps the most important geological event in the Cenozoic Era which has drawn the greatest scientific attention. Scientists consider this event to be the closest analogue of the recent anthropogenic rise of atmospheric carbon dioxide at present, which provides important implications for future projections.

What happened during the PETM?

The Paleocene-Eocene Thermal Maximum, occurred around 55 million years ago, is one of the most abrupt global warming events ever recorded in the last 65 million years of Earth’s history. In less than 10,000 years, global sea surface and deep sea temperatures rose by 5-9^oC. This abrupt oceanic warming was mainly caused by the rapid release of carbon (~2000 x 10⁹ metric tonnes) from deep ocean floor in the form of methane hydrates. This evident release of carbon was supported by a coeval global carbon isotope excursion (CIE), where the marine and terrestrial carbon isotope values (δ¹³C) decreased by 3 to 8 per mil, indicating a large input of carbon into the ocean and the atmosphere. The released methane was rapidly oxidised to carbon dioxide, which has significantly lowered seawater pH and its carbonate ion concentration via dissolution of this CO₂. These ocean chemistry changes have caused a rapid shoaling of the calcite compensation depth (CCD) or the calcite saturation horizon of more than 2km in less than 10,000 years, marked by a prominent transition from carbonate-rich to clay layer in the South Atlantic deep-sea sections (Zachos et al., 2005).

The abrupt transition from carbonate to clay layer at the P-E boundary (Zachos et al., 2005)

What were the impacts?

The PETM has triggered a series of biological responses within the plankton assemblages. The most catastrophic biological response throughout the PETM was the mass extinction of benthic foraminifera, with 30-50% of benthic foraminiferal species became extinct (Gibbs et al 2006). It was likely to be caused by the excessive carbonate undersaturation in the deep ocean at the Paleocene-Eocene (P-E) boundary, which prohibited the calcification of the benthic foraminifera. In addition, benthic foraminifera were used to living under the stable condition of the deep ocean, which lowered their ability to cope with such rapid environmental changes.  Therefore, in actual fact, most of the plankton species, which are more adapted to changing environmental condition in surface water survived throughout the event. Gibbs et al. (2006a) presented some interesting evolutionary changes of calcareous nannoplankton from their paleontological records. Instead of a mass extinction, their findings show a prominent increase in origination and extinction of calcareous nannoplankton during the first 70ky of the PETM, with a rate of 1.6 and 1.7% per 10ky respectively compared to 0.5% and 0.1% per 10ky in the pre-event background interval. In contrast with the destiny of the benthic foraminifera species, the excessive carbonate undersaturation during the PETM did not affect the survivorship of most calcareous nannoplankton taxa. Instead, it was concluded that the rate of environmental changes such as temperature, salinity, nutrient availability was the main driver for this evolutionary turnover pattern.

Recovery stage of the PETM

The ocean carbonate chemistry started to recover within 110 to 210ky after the onset of the PETM. Signs of recovery include the gradual recovery of the CCD i.e. carbonate content and δ¹³C (Gibbs et al., 2006a). The PETM recovery is believed to be attributed to the negative feedback mechanism of the oceanic system, in which increased calcareous phytoplankton productivity with intensified chemical weathering of silicate rocks (Zachos et al., 2005) and nutrient run-off was largely responsible for the sequestration of carbon (Gibbs et al., 2006b).

Source

Whoops! Just realised that I’ve written so much about this big event in this post. In case too much information has already been overwhelming you, I’ll carry on talking about the important implications of this big event in my next post instead. Meanwhile, hang on to your thoughts about this big event to be the closest analogue of today’s anthropogenic era and we shall carry on our discussion next week!