Marine phytoplankton CO2 records since the Miocene - magnitudes of change inplied by cellular process models

Abstract:

CO₂ proxy records from the Mio-Pliocene rely heavily on data from the carbon isotopic fractionation of marine phytoplankton during photosynthesis (e_p). However, fractionation is also sensitive to cell size, growth rate, and the degree of active C concentration at the site of photosynthesis. The ACTI-CO cell model provides one venue for exploring the implications of these multiple factors for the CO₂ value consistent with a given e_p determination. Using our recent alkenone e_p record from the last 14 Ma, we explore in ACTI-CO mechanisms for using constraints of coccolith size for cell geometry, and coccolith calcification and isotopic fractionation coccoliths to constrain changes in carbon acquisition and the consequences for CO₂ estimates.

In addition, we present a new e_p  reconstruction for the past 13 Ma from the Equatorial Pacific based on diatom-bound organic matter . The isolation of pennate diatoms assures a similar cell geometry for the entire period, so that unlike the alkenone record, e_p is not sensitive to temporal changes in cell size. The similar trends in CO₂ from the size-corrected alkenone e_p record and the diatom record suggest that there is a common global CO₂ trend, and that size correction of the alkenone record reliably accounts for cell geometry effects. We employ the ACTI-CO model to evaluate the potential influence of changes in active carbon uptake on the magnitude of CO₂ decrease since the Miocene.