Collaborative Research: Taking the Reliability of Cenozoic Boron Isotope pH and pCO2 reconstructions to the next level

Lead PI: Dr. Baerbel Hoenisch

Unit Affiliation: Geochemistry, Lamont-Doherty Earth Observatory (LDEO)

April 2017 - March 2021
Project Type: Research

DESCRIPTION: If anthropogenic carbon dioxide (CO2) emissions continue to increase unabated, the continued accumulation of this gas in the atmosphere will warm Earth's climate via the greenhouse effect, while the dissolution of CO2 in the surface ocean makes seawater more acidic. Although historical CO2 levels and global warming are reasonably well documented for the past few decades, scientists' understanding of climate and particularly ecosystem sensitivity to much higher CO2 levels is limited and greatly impedes accurate projections of future changes. Earth history can aide scientists in improving their understanding, by providing much longer geological records of variations in past seawater chemistry and their associated ecological responses, if any. This research aims to provide fundamental constraints on surface ocean acidity and atmospheric CO2 levels throughout the past 65 million years, when a number of climate shifts containing important information about oceanic and atmospheric chemical changes occurred. The research will combine surface ocean acidity reconstructions from geochemical information stored in the fossil shells of plankton organisms, as well as computer model simulations of ocean carbon chemistry. In addition to informing scientists and policy makers about the impact of rising atmospheric CO2 levels on global climate, the outcomes of this study are also of great value to paleoecologists, who want to understand the effect of ocean acidification on marine life. The study will support the research of two graduate students at Columbia University and University of California at Riverside, and will provide training in carbon cycle modeling to interested researchers. Specifically, the project will measure the boron isotopic composition (delta11B) recorded in fossil foraminifera shells to reconstruct past seawater-pH and, by inference, atmospheric CO2 levels. Earlier studies using this approach have only been able to reconstruct relative shifts in seawater-pH and atmospheric CO2, because slight differences in the delta11B recorded by individual foraminifera species complicate the application of this proxy to the distant past, as most ancient foraminifera species are now extinct. In addition, a second parameter (e.g., alkalinity) of the marine carbonate system is needed to accurately calculate atmospheric CO2 levels from surface ocean chemistry records, but researchers have not yet found a reliable geochemical indicator for alkalinity in ocean sediments. This research aims to overcome these limitations, by (1) cross-calibrating modern and extinct planktic foraminifera species, (2) refining Earth system model estimates of surface ocean alkalinity, and (3) deriving a refined and self-consistent reconstruction of Cenozoic pCO2 from boron isotopes and modeled alkalinity. In addition to the low resolution 65-million year paleoreconstruction anticipated by this study, the cross-calibration of vital effects on foraminiferal delta11B and improved alkalinity estimates from geochemical modeling will facilitate future studies of absolute rather than relative pH and pCO2 in the past.

OUTCOMES: This project is still ongoing.