A seasonality trigger for carbon injection at the Paleocene–Eocene Thermal Maximum

Abstract

The Paleocene-Eocene Thermal Maximum (PETM) represents a ∼ 170 kyr episode of anomalous global warmth ∼ 56 Ma ago. The PETM is associated with rapid and massive injections of 13 C-depleted carbon into the ocean-atmosphere system reflected as a prominent negative carbon isotope excursion (CIE) in sedimentary components. Earth's surface and deep ocean waters warmed by ∼ 5 • C, of which part may have occurred prior to the CIE. However, few records document continental climatic trends and changes in seasonality have not been documented. Here we present the first high-resolution vegetation and paleoclimate reconstructions for the PETM, based on nearest living relative analysis of terrestrially derived spore and pollen assemblages preserved in an expanded section from the central North Sea. Our data indicate reductions in boreal conifers and an increase in mesothermal to megathermal taxa, reflecting a shift towards wetter and warmer climate. We also record an increase in summer temperatures, greater in magnitude than the rise in mean annual temperature changes, and a shift to a summer-wet seasonality. Within the CIE, vegetation varies significantly with initial increases in epiphytic and climbing ferns, and development of extensive wetlands, followed by abundance of Carya spp. indicative of broadleaf forest colonization. Critically, the change in vegetation we report occurs prior to the CIE, and is concomitant with anomalous marine ecological change, as represented by the occurrence of Apectodinium augustum. This suggests that amplifications of seasonal extremes triggered carbon injection.

Late Eocene to middle Miocene (33 to 13 million years ago) vegetation and climate development on the North American Atlantic Coastal Plain (IODP Expedition 313, Site M0027)

“…These changes and their driving factors are mainly understood from marine climate records, while examinations integrating terrestrial proxies remain rare (e.g. Eldrett et al, 2009Eldrett et al, , 2014Pross et al, 2012). This is particularly the case for eastern North America, whereas several integrated studies for Eurasia have been published during the last 2 decades (e.g.…”

“…One of these methods is the "nearest-livingrelative" (NLR)/"co-existence" or "mutual climate range" (MCR) method (e.g. Mosbrugger and Utescher, 1997;Pross et al, 2000;Thompson et al, 2012;Reichgelt et al, 2013;Eldrett et al, 2014): climatic limits of fossil plant taxa are assumed to be similar to those of their nearest living relatives. Therefore, the intervals of climate parameters for a given fossil flora, in which a maximal number of nearest relatives can coexist, can be considered the best possible description of the palaeoclimate under which the fossil flora lived (Thompson et al, 2012).…”

“…To determine the climate envelope that accommodated a majority of taxa from a given fossil assemblage with respect to a given climate parameter, the zone of overlap was calculated using the 10th percentile (as lower limit) and 90th percentile (as upper limit) of the total range for all NLRs represented in that sample (Greenwood et al, 2005;Reichgelt et al, 2013;Eldrett et al, 2014). This calculation removes or down-weights outliers, i.e.…”

“…Heusser and Shackleton, 1979;Eldrett et al, 2009Eldrett et al, , 2014Pross et al, 2012;Contreras et al, 2013). Methods have been developed to calculate quantitative climate data (e.g.…”

“…Each profile contains the maximum and minimum values for a range of climate and related environmental variables with respect to an individual NLR taxon, based on their present-day distribution. For this study, the majority of climate profiles were obtained from Thompson et al (1999Thompson et al ( , 2000Thompson et al ( , 2012 and Fang et al (2011) for trees and shrubs, and the online database of Natural Resources Canada (2012) for non-trees, supplemented by data from sources outlined in Eldrett et al (2014).…”