Boron isotope evidence for high-latitude ocean control of glacial CO₂

Abstract

High-latitude oceans are crucial components of the global carbon cycle, acting as primary drivers for atmospheric CO₂ changes over glacial cycles due to their connection to the deep ocean and unique biogeochemistry. However, the key processes responsible for glacial CO₂ change in the high-latitude oceans remain poorly constrained, due to lack of surface water CO₂ reconstructions. This thesis addresses this gap by advancing the analysis and application of the boron isotope (δ¹¹B)-pH proxy.

Chapter 2 presents a novel pumped chromatography technique to purify boron from carbonate and seawater matrices, called “peri-columns”, which operates eight-fold faster than the gravity column methods, while achieving significantly lower total procedural blanks (~11 pg boron).

Chapter 3 presents an 800-kyr-long Southern Ocean reconstruction and shows that major pCO₂ decline in the Antarctic Zone (AZ) at glacial inception was pivotal in driving the initial atmospheric CO₂ drawdown. This was mechanistically linked to shifts in ocean circulation—likely driven by changes in sea ice and the westerly winds—which reduced surface carbon supply and suppressed CO₂ outgassing.

Chapter 4 presents the first paired δ¹¹B and Mg/Ca records for the subpolar North Pacific (SNP) spanning the past 150 ka, which highlight its carbon cycle dynamics under different boundary conditions. Deglacial surface SNP pCO₂ change during Termination II was driven by sequential changes in deep convection and wind-driven upwelling. Enhanced Pacific Meridional Overturning Circulation reduced glacial CO₂ and warmed surface waters during Marine Isotope Stage (MIS 4), with subsequent weakening leading to CO₂ rise during early MIS 3.

Chapter 5 details the first evaluation of δ¹¹B analysis using the Sapphire collision-cell multi-collector inductively coupled plasma mass spectrometer, identifying optimal instrumental configurations that yield precision comparable to other established techniques.

Date of Award	3 Jul 2026
Original language	English
Awarding Institution	University of St Andrews
Supervisor	James Rae (Supervisor), Claus-Dieter Hillenbrand (Supervisor) & Andrea Burke (Supervisor)