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THESIS STATEMENT. Thehyperthermal Paleocene-Eocene Thermal Maximum (PETM) cannot be explained solely by the release of carbon from the oxidation of seafloor methane hydrates; an additional trigger for warming and an additional large source of carbon is required.

BODY OF ARGUMENT

Why is seafloor methane hydrates considered a likelycause of warming?

- Rapid climate warming (5-8°C) combined with a sharp decrease in 13C/12Cratio of sediments suggest that the PETM was related to a rapid injection of 13C depleted CO2 and/or CH4 (Sluijs,Bowen,Brinkhuis,Lourens &Thomas 2007)

- Methane hydrates beneath the seafloor form a large reservoir of light carbon. However, the maximum size of the methane hydrate reservoir seems to be insufficient to generate the observed climate change (Higgins & Schrag 2006)

-Carbon release by the oxidation of methane hydrates is expected to followa 4-5°C warming of deep watervia changes in ocean circulation (Triparti & Elderfield 2005)

-However, recent evidence suggests that warming occurred in both surface and deep waters prior to therelease of a pulse of13C-poor carbon into the atmosphere (Dickens, Castillo & Walker 1997; Higgins & Schrag 2006)

What is the magnitude of the required carbon pulse?

-The maximum carbon that could have been stored in seafloor methane hydrates during the Eocene is close to 2000GtC. The observed negative carbon excursion event (CIE) was ~2.5‰ in deep sea sediment layers(Pagani, Caldeira, Archer & Zachos 2006).

- Additional warming of the already warm Paleocene climate would require either massive carbon inputs or extremely high carbon sensitivity (Pagani et al. 2006)

- A recent study of planktic foraminifera and terrestrial plants suggests a larger CIE than that observed in deep sea sediments: approx. 3.3 to 5‰ (Handley, Pearson, McMillian & Pancost 2008)

- Recent climate models have indicated that a light carbon pulse of at least 6800 GtC was required during the PETM. (Panchuck, Ridgwell & Kump 2008)

What was the order of events occurring at the onset of the PETM?

- Changes in benthic foraminifera species assemblages in the late Paleocene indicate the beginning of a shift in seafloor environmental conditions prior to the injection of 13C-depleted carbon (Guisberti, Coccioni, Sprovieri & Tateo 2009)

- Thedinoflagellate taxaApectodiniumreaches its acme prior to the CIE and then shows little change during the initial CIE event. Thisshows that environmental changewas well underway before the carbon pulse actually occurred (Sluijs et al. 2007)

- The paleothermometer TEX86 indicates that the pulse of 13C-light carbon occurred after climate warming had already been initiated, and did not cause the onset of the PETM. Whatever event was responsible for the initial stages of climate warming occurred without noticeably impacting the δ13C value. (Sluijs et al. 2007)

What caused the initial phase of climate warming at the onset of the PETM?

- The beginning of the PETM closely correlates with the onset of massive flood basalt volcanism and the initiation of continental breakup, both related to the North Atlantic Igneous Province (NAIP) emplacement (Storey, Duncan & Swisher 2007)

- Mantle-derived carbon has a δ13C of -5 to -7%, so volcanic carbon would have minimal impact on the δ13C signature of the ocean-atmosphere system (Sluijs et al. 2007)

- Major and trace element abundances over 56-51 Ma indicate that subaerial volcanism during the emplacement of the NAIP injected mantle carbon directly into the atmosphere (Thomas & Bralower 2005)

What was the source of the 13C-light carbon pulse after the onset of warming?

- A lack of evidence for significant hydrothermalism suggests that NAIP emplacement is unlikely to be a significant source of organic (δ13C light) (Thomas & Bralower 2005)

- Another positive feedback from initial warming may be the increase in methanotrophic bacterial taxa in the fossil record, indicating a corresponding increase in terrestrial methane production and release (Pancost et al. 2007)

- The TEX86 paleothermometer indicates that the warmest Eocene temperatures werein the 35-40°C range, with temperatures prior to the PETM only ~5°C less.An initial warming event may have induced massive tropical plant dieback, adding thousands of GtC to the atmosphere (Huber 2008)

- Soot and black carbon deposits suggest that wildfires burning living biomass were common during the PETM (Moore & Kurtz 2008)

REFERENCES

Dickens, G. R., O'Neil, J. R., Rea, D. K., & Owen, R. M. (1995). Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene.Paleoceanography, 10(6), 965-971.

Giusberti, L., Coccioni, R., Sprovieri, M., & Tateo, F. (2009). Perturbation at the sea floor during the paleocene-eocene thermal maximum: Evidence from benthic foraminifera at contessa road, italy. Marine Micropaleontology, 70(3-4), 102-119.

Handley, L., Pearson, P. N., McMillan, I. K., & Pancost, R. D. (2008). Large terrestrial and marine carbon and hydrogen isotope excursions in a new Paleocene/Eocene boundary section from tanzania. Earth and Planetary Science Letters, 275(1-2), 17-25.

Higgins, J. A., & Schrag, D. P. (2006). Beyond methane: Towards a theory for the paleocene-eocene thermal maximum. Earth and Planetary Science Letters, 245(3-4), 523-537.

Huber, M. (2008). A hotter greenhouse? Science, 321(5887), 353-354.

Moore, E. A., & Kurtz, A. C. (2008). Black carbon in paleocene-eocene boundary sediments: A test of biomass combustion as the PETM trigger. Palaeogeography, Palaeoclimatology, Palaeoecology, 267(1-2), 147-152.

Pancost, R. D., Steart, D. S., Handley, L., Collinson, M. E., Hooker, J. J., Scott, A. C., et al. (2007). Increased terrestrial methane cycling at the palaeocene-eocene thermal maximum. Nature, 449(7160), 332-335.

Panchuk, K., Ridgwell, A., & Kump, L. R. (2008). Sedimentary response to paleocene-eocene thermal maximum carbon release: A model-data comparison. Geology, 36(4), 315-318.

Pagani, M., Caldeira, K., Archer, D., & Zachos, J. C. (2006). An ancient carbon mystery. Science, 314(5805), 1556-1557.

Sluijs, A., Bowen, G. J., Brinkhuis, H., Lourens, L. J., & Thomas, E. (2007). The palaeocene-eocene thermal maximum super greenhouse: Biotic and geochemical signatures, age models and mechanisms of global change. Geological Society Special Publication, pp. 323-349

Sluijs, A., Brinkhuis, H., Schouten, S., Bohaty, S. M., John, C. M., Zachos, J. C., et al. (2007). Environmental precursors to rapid light carbon injection at the Palaeocene/Eocene boundary. Nature, 450(7173), 1218-1221.

Storey, M., Duncan, R. A., & Swisher III, C. C. (2007). Paleocene-eocene thermal maximum and the opening of the northeast atlantic. Science, 316(5824), 587-589.

Thomas, D. J., & Bralower, T. J. (2005). Sedimentary trace element constraints on the role of north atlantic igneous province volcanism in late paleocene-early eocene environmental change. Marine Geology, 217(3-4), 233-254.

Tripati, A., & Elderfield, H. (2005). Paleoclimate: Deep-sea temperature and circulation changes at the paleocene-eocene thermal maximum.Science, 308(5730), 1894-1898.

Zachos, J. C., Dickens, G. R., & Zeebe, R. E. (2008). An early cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature, 451(7176), 279-283.

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