2025 Theses Doctoral

Reconstructing atmospheric carbon dioxide and surface ocean carbon chemistry over the last 28 million years using cross-calibrations of boron isotopes in fossil planktic foraminifera

Anderson, Lloyd B.

As fossil fuel emissions continue and associated climate change impacts will accelerate in the future, it is important to establish robust characterizations of warmer, higher-CO₂ climates from earlier in Earth history. The boron isotope (11B) proxy for past seawater pH, analyzed on calcite microfossil shells (planktic foraminifera) from ocean sediment cores, is a promising method to reconstruct atmospheric CO₂ in deep time.

However, recent work has noted that contemporaneous records of 11B-based atmospheric CO₂ estimates unexpectedly diverge from each other during the late Neogene. This divergence may be due to changes in the biology of one or both of the different planktic foraminifera species studied, Trilobatus sacculifer and Globigerinoides ruber. Furthermore, uncertainty remains over how to treat species-specific calibrations that link foraminiferal 11B to pH estimates prior to 22 million years ago, no 11B-based reconstructions of atmospheric CO₂ exist for wide swaths of the Oligocene epoch (from 33 – 23 million years ago), and large variability in CO₂ reconstructions during the Oligocene based on other proxy evidence leaves climate evolution during this period relatively unconstrained.

Apart from atmospheric CO₂, the size of past carbon reservoirs (including the total dissolved inorganic carbon concentration ([DIC]) of the surface and deep ocean) is still poorly quantified. However, the B/Ca ratio in shells of planktic foraminifera is sensitive to the ratio of dissolved borate ion to dissolved inorganic carbon concentrations ([B(OH)4-/DIC]) in seawater and may provide an avenue to reconstruct surface ocean [DIC].To address these sources of uncertainty and help fill gaps in our knowledge of past CO₂ and climate, I utilize the 11B proxy to reconstruct atmospheric CO₂ and surface ocean carbon chemistry over the late Cenozoic, across a time span stretching from approximately 28 million years ago (Ma) to recent. In particular, my reconstructions focus on careful evaluation of 11B calibrations for different planktic foraminifera, including modern and extinct species. Throughout my reconstructions, I employ a cross-calibration approach, which is based on comparison of 11B, Mg/Ca, B/Ca, 18O, and 13C data between co-occurring surface-dwelling planktic foraminifera species that lived within common evolutionary time windows and can be found in the same marine sediment samples.

In Chapter 1, I perform paired 11B and Mg/Ca analyses on co-located specimens from a suite of deep-sea sediment samples to assess potential controls on the evolution of their respective recorded 11B and pH estimates through time. These paired analyses confirm progressively diverging 11B values between G. ruber and T. trilobus over the past 12 million years, i.e., 11BG.ruber values increased relative to 11BT. trilobus from 12 Ma to the present. Habitat-specific Mg/Ca differences between the two species showed only small changes over the same time interval, suggesting that the difference in 11B is not predominantly caused by either species’ shift in habitat depth. Based on documented genetic species diversity in the modern ocean, I suggest that G. ruber may have evolved different 11B vital effects since the Miocene, leading to pH and CO₂ estimates deviating from modern calibrations prior to the Pleistocene.

In Chapter 2, to add to our understanding of Oligocene and early Miocene climate, I generate new atmospheric CO₂ estimates from new 11B data from fossil shells of surface-dwelling planktic foraminifera, including the extinct species Globoturborotalita pseudopraebulloides and Ciperoella angulisuturalis, from the mid-Oligocene to early Miocene (~ 28 – 18 Ma). I estimate atmospheric CO₂ of ~ 680 ppm for the mid-Oligocene, which then evolves to fluctuate between ~ 500 – 570 ppm during the late Oligocene and between ~ 420 – 700 ppm in the early Miocene. These estimates tend to trend higher than Oligo-Miocene CO₂ estimates from other proxies, although there is good proxy agreement in the late Oligocene. Reconstructions of CO₂ fall lower than estimates from paleoclimate model simulations in the early Miocene and mid Oligocene, which indicates that more proxy and/or model refinement is needed for these periods. My species cross-calibrations, assessing 11B, Mg/Ca, 18O, and 13C, are able to pinpoint and evaluate small and consistent differences in the geochemistry of surface-dwelling planktic foraminifera, lending confidence to paleoceanographers applying this approach even further back in time.

In Chapter 3, I apply a new approach to calculate [DIC] from B/Ca of multiple species of planktic foraminifera, spanning the mid-Oligocene (~ 28 Ma) to present. After application of newly developed [B(OH)4-/DIC] calibrations that incorporate changing seawater Mg/Ca composition and account for extinct species’ B/Ca vital effects through cross-calibration, my reconstructions exhibit an overall increase in [DIC] from the early Miocene to recent that I compare to independent estimates from modeling approaches.