Harmonizing Miocene alkenone- and boron-based carbonate chemistry reconstructions

Abstract:

The carbon isotopic composition of phytoplankton organic matter and the boron isotopic composition of planktic foraminifera are two key archives for reconstructing past marine dissolved inorganic carbon chemistry. Pairing records of atmospheric CO₂ from phytoplankton δ¹³C and surface ocean pH from planktic foraminifera δ¹¹B can shed light on the temporal evolution of the surface ocean carbonate system. Here we report new constraints on atmospheric CO₂ change through the Miocene using a reformulated alkenone paleobarometer informed by a culture-based model, which quantitatively considers the influence of cell size and irradiance. Our data suggest a dramatic decrease in surface ocean pCO₂ of ~600 μatm from the Early Miocene to the Pleistocene, with a large step-wise drop between ~9 and 8 Ma. These estimates differ from those based on the foraminifera boron isotope pH proxy, which suggest little to no long-term change in pCO₂ during the late Miocene (11.6-5.3 Ma) when calculated using constant alkalinity. We combine our alkenone CO₂ estimates with existing boron isotope pH estimates to speciate the surface ocean carbonate system through the Neogene. We calculate a 50-100% decrease in both dissolved inorganic carbon and surface ocean alkalinity from the early to late Miocene. At face value, this estimate is substantially different from what has been suggested for the Miocene using reconstructions of the carbonate compensation depth, but finds support from recent estimates of pelagic carbonate burial fluxes and changes in volcanic outgassing. Using sensitivity analyses, we explore how the underlying assumptions of each proxy must be altered to bring the two estimates of the carbonate system into agreement.