A diatom record of CO2 decline since the late Miocene

Extratropical sea surface temperature records from alkenones record a dramatic cooling of up to 17 degrees C over the last similar to 14 Ma, but the relationship between this cooling and greenhouse gas forcing has been elusive due to sparse and contrasting reconstructions of atmospheric CO2 for the time period. Alkenone carbon isotopic fractionation during photosynthesis has previously been used to estimate changes in pCO(2) over this interval, but is complicated by significant changes in cell size of the alkenone-producing coccolithophorids over this time period. In this study, we reconstruct carbon isotopic fractionation during photosynthesis (epsilon(p)) using organic compounds trapped within the frustules of pennate diatoms in sediments from the Eastern Equatorial Pacific Ocean at Ocean Drilling Program Site 846 over the last 13 Ma. Physical separation of pennate diatoms prior to measuring carbon isotopic fractionation enables us to obtain a record with constant cell geometry, eliminating this factor of uncertainty in our pCO(2) reconstruction. In the past 11 Ma, epsilon(p) declines from 15.5 to 10.3700. Using the classic diffusive model and taking into account variations in opal content, alkenone concentration and coccolith Sr/Ca as indicators of past productivity and growth rate, and sea surface temperature records from the site, we estimate a decline in pCO(2) from 454 (+/-41) to 250 (+/-15) ppmv between similar to 11 and 6 Ma. Models accounting for changing the significance of active carbon uptake for photosynthesis, which likely produce more accurate CO2 estimates, suggest a significant larger pCO(2) decline of up to twice that shown by the classic diffusive model (in average from 794 (+/-233) ppmv at similar to 11 Ma to 288 (+/-25) ppmv at similar to 6 Ma, considering growth rates varying between 0.5 and 1.7 day(-1)). Large uncertainties in the pCO(2) estimated between similar to 8 and 11 Ma using the active uptake model are related to the growth rate used for calculations. Together, these results suggest CO2 forcing for this period of steep decline in temperatures. (C) 2017 Elsevier B.V. All rights reserved.