Terrestrial Plant Response to the Paleocene-Eocene Thermal Maximum

About 55 million years ago, the planet experienced the most abrupt global climate event of the Cenozoic Era. Delineating the Paleocene/Eocene Epoch boundary, this climatic event - the Paleocene-Eocene Thermal Maximum (PETM) - is characterized by a rapid, short-lived warming of the ocean/atmosphere system and a massive perturbation of the global carbon cycle. PETM temperature records indicate a 3-8C and up to 5C increases in deep-sea temperature and high latitude sea-surface temperature, respectively (Kennett and Stott, 1991; Zachos et al., 2001; new tex86 stuff). This rapid rise in ocean temperature occurs in concert with aberrations in the hydrologic cycle (Bowen et al., 2004; Pagani et al., 2006a) and changes in marine and terrestrial biology (Greenwood and Wing, 1995; Thomas, 1998; Thomas et al., 1999; Wing et al., 2000; Sluijs et al., 2005). Further, the PETM features a distinctive negative carbon isotope excursion (CIE) in both marine and terrestrial records; however, the magnitude of the CIE varies between the marine and terrestrial realms (Kennett and Stott, 1991; Koch et al., 1992; Corfield, 1994; Beerling and Jolley, 1998; Bowen et al., 2004; Pagani et al., 2006a). Understanding these differences is important to determine the mass and source of the carbon responsible for the isotopic excursion and remain currently unresolved; nonetheless, the CIE implies a massive and abrupt addition of isotopically-depleted carbon to the ocean/atmosphere system. This research developed a record of compound-specific hydrogen and carbon isotopic measurements of leaf wax lipids from six PETM sections that span a paleo-latitudinal and climatic gradient. The data gathered from modern transects will provide a unique data set that will be compared to a latitudinally similar paleotransect during a well-dated, short interval of extreme warmth and high CO2: the PETM. The PETM and Early Eocene can be described as a “super C3 world” with its warm temperatures, lush climate, and high CO2. To explore these issues, I sample five European PETM sections spanning a paleolatitudinal gradient from 35N to 50N and performed hydrogen and carbon CSIA on extracted n-alkanes. These isotopic data will then be compared against n-alkane δD and δ^13C values from modern C3 species to assess plant isotopic variation under extreme climate. Under PETM conditions C3 plants would perform well, however, it is unclear how C3 plants behave isotopically under such luxuriant conditions. In modern C3 plants, carbon isotopic variability is a function of the external source and environment parameters and with the majority of these environmental stresses removed or diminished during the PETM, we expect plants under these conditions to be isotopically depleted in ^13C, however possibly with a different range and median δ^13C values than modern C3 plants. Additionally, a latitudinal paleotransect will provide a dataset that will test if plants are equally depleted in ^13C across a large range of latitude. In relation to my previous work, PETM C3 plant isotope records would provide a C3 isotopic end-member for intervals such as the Eocene and Early Oligocene, when CO2 was at level much higher than today. Additionally, by looking at the plants δD response to the PETM in a latitudinal context, I will be able to garner information about changes in the hydrological cycle. Do the mid-latitudes become more humid during the PETM as Bowen et al (2004) concludes? By coupling δD/δ^13C isotope ratios, I will constrain water-stress (or lack there of) and changes in evapotranspiration and humidity, testing the idea that plants increased their discrimination against ^13C during the PETM.

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