Paleocene-Eocene Thermal Maximum: Part II

As promised last week, I’ll be moving on to discuss the implications of the Paleocene-Eocene Thermal Maximum. But before I move on, I would like to thank Daniel and Emily for their comments and interests for my last entry, especially Emily, who would like to know about the cause of the methane hydrates dissociation which gives rise to the PETM at the first place! To be honest, I’ve never thought about that myself either! So thank you for suggesting this good blog topic for me. Now, I’m going to share with you of what I found about this ‘methane hydrate dissociation hypothesis’. Hope that answers your query!

In fact, the cause of the carbon isotope excursion (CIE) i.e. the dramatic decrease in global δ¹³C during the PETM is still not very clear. Although the ‘methane hydrate dissociation hypothesis’ is still the most dominant explanation as the cause of the PETM, it is still subject to a great debate among the scientific community, often due to model uncertainties and inadequate data resolution. It is believed that the massive methane release was caused by a change in deepwater source regions, which increased water temperatures rapidly enough to trigger a massive thermal dissociation of the methane hydrate gas reservoir underneath the seafloor. Katz et al. (2001) modelled changes in heat flow on gas hydrate reservoir stability through time, together with seismic data and comparison with other published isotopic records, have neither confirmed nor refuted thermal dissociation as the trigger for the PETM methane release. According to the published isotopic records, rapid δ¹⁸O decrease (indicating rapid warming) did not precede a rapid δ¹³C decrease (indicating CH₄ release) by at least 2000-4000 years, which is their modelled minimum time lag required for such a large amount of methane release (~2000Gt of C) via ocean mixing and the change of deepwater source region in order to trigger a global CIE i.e. PETM. However, new recent high-resolution stable isotopic records based on the planktonic and benthic foraminiferal shells analysis revealed that the onset of the CIE was geologically instantaneous and was preceded by a brief period of gradual surface-water warming (Thomas et al., 2011), which supports the thermal dissociation hypothesis of methane hydrates.  

In addition, neither isotopic comparisons nor their heat flow model indicated a sufficient change in deepwater source region to trigger such a rapid thermal dissociation (Katz et al., 2001). However, Tripati and Elderfield (2005) proved that change in ocean circulation has indeed triggered the destabilisation of methane hydrates in deep sea sediments, based on their seawater temperature and salinity reconstruction from benthic foraminifera (δ¹⁸O record) to infer changes in deepwater source regions from the Late Paleocene (>55.60Ma) to Early Eocene (<55.25Ma). A warming of immediate waters before the CIE is detected (Figure 1), triggered by the downwelling in North Pacific and reduced Southern Ocean convection. This further supports the thermal dissociation of methane hydrates being the driver for the onset of the PETM.

Figure 1 Benthic foraminifera Mg/Ca record indicates a warming of immediate waters across the PETM (Tripati and Elderfield, 2005) 

  

Katz et al. (2001) have also argued that the paleobathymetric distribution of the methane release sites is inconsistent with the broader depth range as predicted for thermal dissociation. Therefore, they proposed an alternative hypothesis that mechanical dissociation i.e. continental slope failures and seafloor erosion would also be responsible for the destabilisation of methane hydrates reservoir in the seafloor. However, if this alternative hypothesis stands, the methane release may have caused, rather than the result of the transient warming during the PETM; the methane releases from the seafloor will be oxidised in the ocean and then escaped to the atmosphere. Therefore, it implies that this increased atmospheric CH₄ and/or CO₂ was the cause of the warming. Higgins and Schrag (2006) further supported that the methane hydrates hypothesis alone is not sufficient to account for such a vast amount of carbon release; an oxidation of at least 5000GtC of organic carbon is a more plausible reason to account for the observed climatic changes of the PETM.

Nevertheless, to a large extent, methane hydrate dissociation is responsible for the onset of the PETM. However, significant levels of uncertainties still exist in the available proxy records, for example, the lack of benthic individuals available for analysis during the decline and extinction of benthic foraminifera (Thomas et al., 2011). This hinders the ability to resolve the initial sequence, timing and duration of the events, which is very important to verify methane hydrates dissociation (thermal or mechanical) as a cause of the PETM and other hypotheses.