Alkenone and boron-based Pliocene pCO2 records

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Abstract

The Pliocene period is the most recent time when the Earth was globally significantly (∼ 3 °C) warmer than today. However, the existing pCO2 data for the Pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo-pCO2. This disagreement, coupled with the general low temporal resolution of the published records, does not allow a robust assessment of the role of declining pCO2 in the intensification of the Northern Hemisphere Glaciation (INHG) and a direct comparison to other proxy records are lacking. For the first time, we use a combination of foraminiferal (δ11B) and organic biomarker (alkenone-derived carbon isotopes) proxies to determine the concentration of atmospheric CO2 over the past 5 Ma. Both proxy records show that during the warm Pliocene pCO2 was between 330 and 400 ppm, i.e. similar to today. The decrease to values similar to pre-industrial times (275–285 ppm) occurred between 3.2 Ma and 2.8 Ma — coincident with the INHG and affirming the link between global climate, the cryosphere and pCO2.

Introduction

The current increase in the atmospheric concentration of the greenhouse gas carbon dioxide, from 275 to 285 ppm in pre-industrial times to > 380 ppm today, is unprecedented in recent Earth history (Solomon et al., 2007), with present levels exceeding the natural range of at least the last 800 kyr (Siegenthaler et al., 2005b, Lüthi et al., 2008). Understanding the relationship between pCO2 and climate is therefore central for the accurate prediction of future climate change (Solomon et al., 2007). Past responses to pCO2 change are important components in resolving these relationships, and the most informative palaeoclimate analogues will be in the recent geological past, when geographical configurations, ocean currents and marine and terrestrial ecosystems were similar to today. The Pliocene (5.3 to 2.6 Ma), the most recent potential analogue, is characterised by mean global temperatures ∼ 3 °C warmer than today, comparable to those predicted for the second half of the 21st century (Haywood et al., 2002, Dowsett, 2007). This interval immediately preceded the Late Pliocene intensification of Northern Hemisphere Glaciation (3.2 to 2.7 Ma) and, as a result, Pliocene sea levels were 15 to 25 m higher, indicating smaller continental ice sheets than today (Shackleton et al., 1995).

Our understanding of Pliocene climate depends on the accuracy and resolution of reconstructed pre-Pleistocene (pre-ice core record) pCO2. It has been suggested that a pCO2 decline during the Late Pliocene is the most likely cause for global cooling and the intensification of continental glaciation in the Northern Hemisphere (Raymo et al., 1988, Maslin et al., 1998, Lunt et al., 2008). However, current Pliocene pCO2 estimates are inadequate to examine such fundamental issues. These records suggest that Pliocene pCO2 was between 200 and 400 ppm, based on a few, very low resolution studies of leaf stomatal density (Kürschner et al., 1996), low resolution boron isotope measurements (Pearson and Palmer, 2000) and sedimentary bulk organic matter δ13C values (Raymo et al., 1996). Developments of all of these proxies do challenge these initial estimates; for example, previous calculations of pCO2 from δ11B in part utilised undetermined mixed planktic species, which could have biased the record given the different δ11B recorded by different species (Ni et al., 2007, Foster, 2008). Recent isotopic analyses of alkenones (Pagani et al., 2010) have effectively confirmed the higher end pCO2 estimates of Raymo et al. (1996) while better constraining potential biases in phytoplankton ecology, but a direct comparison to other proxy records at a single site is lacking. Crucially, only this most recent record, unverified by other proxies, has the temporal resolution and accuracy to allow us to assess the changes in pCO2 associated with the Pliocene warmth and the Late Pliocene growth of the Northern Hemisphere ice sheets. Here we use a multiproxy approach using both sedimentary alkenones and the δ11B of foraminifera, both informed by methodological advances over the past decade, to determine atmospheric pCO2 from the Early Pliocene to the Pleistocene.

Section snippets

Material and sample selection

We developed continuous, relatively high temporal resolution (∼ 100 kyr/sample) records of alkenone-based εp values and planktic foraminiferal δ11B values and B/Ca ratios (for Globigerinoides sacculifer, 300–355 μm, and G. ruber, 300–355 μm), for the last 5.3 Myr from ODP Leg 165, Site 999 Holes A and B, in the Caribbean Sea (12°44.639′N, 78°44.360′W, 2838 m water depth). Today, the region is close to being in equilibrium with the atmosphere (Takahashi et al., 2009) and has been so during much of the

Alkenone-based estimates of pCO2

δ13C values of di-unsaturated alkenone (Fig. 1a) and calcite tests of planktic foraminifers (Fig. 1b and Table 1 in Appendix A) have been used to calculate εp37:2 values (Fig. 1c). At Site 999 εp37:2 values ranged from 10.3 to 13.0‰ over the past 5.3 Myr. In general, εp37:2 values are highest in the Pliocene, and lowest in the Pleistocene, with a stepwise decrease occurring at ca. 3 Ma. These changes are driven predominantly by shifts in alkenone δ13C values. Site 1241 in the Equatorial East

Multiproxy Plio-Pleistocene pCO2 records

Although all of our proxy records show generally similar trends and elevated Pliocene pCO2 levels, it is important to acknowledge that all our records require the assumption that the atmosphere is in equilibrium with surface seawater. This was probably not true for the eastern equatorial Pacific (Site 1241), which is influenced by upwelling throughout this time period. Near atmosphere–ocean equilibrium does characterise the Caribbean today and through much of the last 130 kyr (see Section 3.1.).

Conclusions

We demonstrate here for the first time that alkenone δ13C and boron-based pCO2 estimates agree well when carried out on the same core material. The inherent uncertainties for both approaches are very different and this agreement provides a high level of confidence in the accuracy of the generated pCO2 records. Our multi proxy reconstruction indicates that pCO2 was 50–120 ppm higher during the Pliocene compared to pre-industrial (280 ppm) times but that it was similar to today (∼ 384 ppm). Absolute

Acknowledgements

This research used samples and data provided by the Ocean Drilling Program, which is sponsored by the U.S. National Science Foundation and participating countries under management of Joint Oceanographic Institutions. OS acknowledges the postdoctoral fellowship supported by the Japan Society for the Promotion of Science. GLF and DNS are funded by NERC and Royal Society University Research Fellowships, respectively. KK thanks the support by the Ministry of Education, Culture, Sports, Science and

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    Now at: School of Ocean and Earth Science, National Oceanography Centre, Southampton, University of Southampton, Waterfront Campus, Southampton SO14 3ZH, UK.

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