UC San Diego

The marine carbon cycle is a major driver of global climate. Reliable marine carbon cycle proxies are required to study the relationship between life in the ocean and climate in the past and to predict how the marine carbon cycle may change in the future. Pelagic barite in marine sediments is a valuable marine carbon cycle proxy. However, many aspects of pelagic barite dynamics are poorly constrained. Important questions remain regarding how and where pelagic barite forms, what factors influence pelagic barite dissolution in the water column, and which aspects of the marine carbon cycle the pelagic barite proxy captures. This dissertation presents observational studies and laboratory experiments that shed light on these questions and thus advance the ability of the pelagic barite proxy to provide insights into earth’s history. Chapter 1 presents a quantitative visual analysis of 5481 barite microcrystals from the Eastern Pacific water column. This extensive dataset provides new evidence for the role of organic matter aggregates in barite formation and suggests that barite records are influenced by water column dissolution and spatially heterogeneous formation. Chapter 2 uses laboratory experiments to constrain physical and chemical conditions during pelagic barite formation. These findings suggest that pelagic barite forms in marine organic matter aggregate microenvironments that are short-lived, moderately supersaturated with respect to barite, and rich in soy phospholipids. Chapter 3 uses laboratory incubations to constrain plausible water column pelagic barite dissolution rates under a range of seawater conditions. These experiments suggest that organic matter aggregates play a vital role in shielding pelagic barite from dissolution, which implies that the pelagic barite proxy captures the arrival of organic matter to the sediment-water interface. Chapter 4 presents preliminary findings regarding how pelagic barite size, abundance, and morphology vary within sediment core tops and across the sediment water interface over an offshore gradient off the coast of Southern California. This early work suggests differential dissolution of very small barite crystals at the sediment-water interface, and the data and samples acquired for this investigation are available for potential collaborators interested in pursuing the topic further. Together, these chapters increase our confidence in the barite proxy by shedding light on pelagic barite formation, dissolution, and preservation. This work advances the accurate and reliable interpretation of barite records to gain insight into past ocean conditions.