Boron based geochemical reconstructions of ocean pH and atmospheric CO₂ in the geological record

Abstract

The boron isotope proxy is a valuable tool for reconstructing ocean pH and atmospheric CO₂ in Earth’s history. Its use on foraminifera in deep sea sediment cores has helped shape our understanding of Cenozoic carbon cycling and climate change, yet most of the extreme examples of biotic and climatic change are in the geological record, where the proxy has seen limited use. The boron isotope proxy’s widespread usage is also hindered by the challenges involved in traditional methods for measuring it: gravity microcolumns require exacting laboratory conditions and are time intensive. Older samples may have more variable matrices which can also be a problem for a gravity column. In order to expand the boron isotope proxy into a wider variety of matrices, Chapter 3 presents a new batch method for the purification of boron from its sample matrix. Together with methodological improvements for measurement by MC-ICPMS discussed in Chapter 2, these developments allow for measurement of a wide variety of samples with uncertainties roughly half the size, procedural blanks an order of magnitude lower, and sample throughput ~3 times faster than previous methods. These developments are applied to the geological record in Chapters 4 and 5 for two of the most significant biotic and climatic changes in Earth’s history. Chapter 4 presents boron isotope data from the Triassic-Jurassic boundary, indicating a ≥ 0.34 unit drop in ocean pH coincident with the ‘main’ carbon isotope excursion, suggesting that the carbon isotope excursions associated with the end-Triassic mass extinction event were likely also associated with pulses of ocean acidification. Chapter 5 presents a boron isotope record through the Ordovician into the early Silurian, showing a substantial rise in ocean pH and likely fall in atmospheric CO₂ accompanying the large cooling trend and major radiation in biodiversity though the Ordovician. CO₂-driven cooling thus sets the stage for the glaciation thought to trigger the end-Ordovician extinction event, while rising CO₂ in the early Silurian would have driven climatic recovery. Together these results highlight the success of the application of the boron isotope proxy to carbon cycle perturbations in the rock record and reveal, for the first time, the changes in the CO₂ system associated with some of the most important events in Earth’s history. In summary, this thesis allows for more accurate and precise boron isotope analysis and a deeper understanding of some of the major environmental events of the Phanerozoic.