Alkenone and boron-based Pliocene pCO2 records
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
References (85)
Seawater pH, pCO2 and [CO23−] variations in the Caribbean Sea over the last 130 kyr: a boron isotope and B/Ca study of planktic foraminifera
Earth Planet. Sci. Lett.
(2008)- et al.
Magnitude of climate variability during middle Pliocene warmth: a palaeoclimate modelling study
Palaeogeogr. Palaeoclimatol. Palaeoecol.
(2002) - et al.
Chemical evolution of seawater during the Phanerozoic: implications from the record of marine evaporites
Geochim. Cosmochim. Acta
(2002) - et al.
Trends in Caribbean Paleoproductivity related to the Neogene closure of the Central American Seaway
Mar. Micropaleontol.
(2007) - et al.
Oak leaves as biosensors of late Neogene and early Pleistocene paleoatmospheric CO2 concentrations
Mar. Micropaleontol.
(1996) - et al.
Boron isotope systematics in large rivers: implications for the marine boron budget and paleo-pH reconstruction over the Cenozoic
Chem. Geol.
(2002) - et al.
The contribution of orbital forcing to the progressive intensification of Northern Hemisphere glaciation
Quatern. Sci. Rev.
(1998) - et al.
Carbon isotope fractionation between dissolved bicarbonate and gaseous carbon dioxide
Earth Planet. Sci. Lett.
(1974) Effect of phytoplankton cell geometry on carbon isotopic fractionation
Geochim. Cosmochim. Acta
(1998)- et al.
Mid-Pliocene warmth: stronger greenhouse and stronger conveyor
Mar. Micropaleontol.
(1996)
Carbon isotopic fractionation in synthetic aragonite and calcite: effects of temperature and precipitation rate
Geochim. Cosmochim. Acta
Core-top calibration of the alkenone index vs. sea surface temperature in the Indian Ocean
Deep Sea Res. II
Climatological mean and decadal changes in surface ocean pCO2, and net sea-air CO2 flux over the global oceans
Deep-Sea Res. Part II - Top. Stud. Oceanogr.
Late Cenozoic history of the Polar North Atlantic: results from ocean drilling
Quatern. Sci. Rev.
History of carbonate ion concentration over the last 100 million years
Geochim. Cosmochim. Acta
Carbon dioxide in water and seawater: the solubility of a non-ideal gas
Mar. Chem.
A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry
Geochem. Geophys. Geosyst.
Expeditions into the past: paleoceanographic studies in the South Atlantic
Late Pliocene to Holocene (2.6–0) western Equatorial Atlantic deep-water circulation: inferences from benthic stable isotopes
Consistent fractionation of C-13 in nature and in the laboratory: growth-rate effects in some haptophyte algae
Global Biogeochem. Cycles
Early Pliocene deep water circulation in the western equatorial Atlantic: implications for high-latitude climate change
Paleoceanography
Link between oceanic heat transport, thermohaline circulation, and the Intertropical Convergence Zone in the early Pliocene Atlantic
Geology
Applications of biomarkers for delineating marine paleoclimatic fluctuations during the Pleistocene
Lowering of glacial atmospheric CO[2] in response to changes in oceanic circulation and marine biogeochemistry
Paleoceanography
Closing of the Indonesian seaway as a precursor to east African aridification around 3–4 million years ago
Nature
Neogene planktonic foraminifer biostratigraphy at Site 999, Western Caribbean Sea
Changes in upper water-column structure at Site 925, Late Miocene–Pleistocene: Planktonic foraminifer assemblage and isotope evidence
Comparison of proxy records of climate change and solar forcing
Geophys. Res. Lett.
Thresholds for Cenozoic bipolar glaciation
Nature
The PRISM palaeoclimate reconstruction and Pliocene sea-surface temperature
A short circuit in thermohaline circulation: a cause for northern hemisphere glaciation?
Science
Late Neogene sedimentation patterns in the eastern equatorial Pacific Ocean
Data report: tropical and equatorial calcareous nannofossil pleistocene biostrigraphy, ODP Leg 202
World Ocean Atlas 2005
Effect of the formation of the Isthmus of Panama on Atlantic Ocean thermohaline circulation
Nature
Onset of permanent stratification in the subarctic Pacific Ocean
Nature
Modern Planktonic Foraminifera
Refining ancient carbon dioxide estimates: Significance of coccolithophore cell size for alkenone-based pCO2 records
Paleoceanography
Late Neogene history of deepwater ventilation in the Southern Ocean
Geochem. Geophys. Geosyst.
Ground-truthing the boron isotope-paleo-pH proxy in planktonic foraminifera shells: Partial dissolution and shell size effects
Paleoceanography
Atmospheric carbon dioxide concentration across the mid-Pleistocene transition
Science
Pliocene–Pleistocene ice rafting history and cyclicity in the Nordic Seas during the last 3.5 Myr
Paleoceanography
Cited by (0)
- 1
Now at: School of Ocean and Earth Science, National Oceanography Centre, Southampton, University of Southampton, Waterfront Campus, Southampton SO14 3ZH, UK.