Plio-Pleistocene pCO2 - a Multiproxy Approach Using Alkenone and Boron Based Carbonate System Proxies

The recent rapid rise in atmospheric CO2 is unprecedented in Earths history, and the current level (385 ppm) is higher than previously experienced for at least the last 650 kyr. Therefore in order to better understand the link between climate and CO2 it is desirable to examine times in the past that experienced similar or higher levels of CO2 compared to today. The Pliocene (2.6 to 5.3 Ma) is the most recent warm period and hence offers such an opportunity. Detailed reconstructions show that global temperatures were 2- 3°C higher ([1], 6°C at high latitudes) and the polar ice sheets were considerably smaller (sea level was 15-20 m higher; [2]) yet other boundary conditions, such as continental configuration, were similar at this time. Earlier studies, typically with low temporal resolution, have shown that at c.3 Ma pCO2 concentrations were in the range 300-400 ppm. A common assumption is that pCO2 dropped from this high value to pre-industrial values at a time coincident with the intensification of northern hemisphere glaciation (NHG) that occurred at c.2.7 Ma, although this has yet to be demonstrated. Here we present a continuous pCO2 record recovered from ocean sediments using a multiproxy approach based on the boron and alkenone carbonate system proxies. This new data allows both a determination of the magnitude of Pliocene pCO2 and for the fist time the Plio-Pleistocene evolution of pCO2 that accompanied the intensification of NHG. We developed continuous records of alkenone based ɛp values and foraminiferal δ11B and B/Ca ratios from ODP Sites 999 and 1000 in the Caribbean Sea and Site 1241 on the western side of the Panama Isthmus in the Eastern Equatorial Pacific spanning the last 5.3 Ma. Following a correction of the alkenone records for coccolith size, and accounting for changing growth rate where appropriate, the alkenone based ɛp record can be used to estimate [CO2]aq and hence pCO2 at the two sites. Similarly, using the core top calibration of [3] the B/Ca and δ11B measurements of the mixed layer foram species G. ruber can be used to estimate [CO32-] and pH and so solve for [CO2]aq and also provide an estimate pCO2. The pCO2 records for both techniques and for both Sites are similar and suggest, in agreement with previous studies, that pCO2 during the Mid-Pliocene was 340-400 ppm. The decline from this high to values similar to the pre-industrial Holocene was relatively rapid and occurred between 2.7 and 3 Ma coincident with the intensification of NHG. This contribution serves to highlight the link between atmospheric CO2 and climate and further we suggest that the Mid-Pliocene is potentially a good analogue for Earths future climate state. [1] N. J. Shackleton, M. A. Hall, and D. Pate, Proceedings of the Ocean Drilling Program, Scientific results, Vol. 138, pp. 337 (1995). [2] H. J. Dowsett, in Deep-time perspectives on climate change: Marrying the signal from computer models and biological proxies, The Micropalaeontological Society Special Publication, London, pp. 459 (2007). [3] G. L. Foster, Earth and Planetary Science Letters 271 (1-4), 254 (2008).