UC Santa Cruz

The opening and closure of ocean gateways over geological time have played a critical role in reshaping ocean circulation patterns and closely linked to major climate events during the Neogene period. This dissertation focuses on tropical ocean gateways in the Pacific—the Indonesian Throughflow (ITF) and the Central American Seaway (CAS)—and investigates how changes in these gateways have influenced ocean biogeochemical processes and climate across different geological periods. This dissertation is presented in three chapters.Chapter 1 examines hydroclimate and productivity changes in the Timor Sea, located off Northwest Australia, over the last ~ 2 Myrs. The study particularly focuses on the period from ~1.3 to 2 Ma, during the 40 kyr world of the Pleistocene, and examines how these changes were influenced by the ITF. By presenting multi-proxy records from International Ocean Discovery Program (IODP) Site U1483, we found that northwest Australia underwent a step of increased aridification and that productivity in the Timor Sea declined during the transition from ~ 1.7 to ~ 1.4 Ma. We attribute this aridification to the reduced moisture supply to this region caused by the ITF restriction and warm pool contraction. Our results show a declined productivity in the Timor Sea during the same transition, which reflected a decrease in the nutrient supply of the Pacific source water associated with global nutrient redistribution. At orbital timescale, multiple mechanisms, including sea level changes, monsoon, and the Intertropical Convergence Zone (ITCZ) dynamics, and variations in the ITF and Walker circulation could have controlled variations of productivity and terrigenous input in the Timor Sea during the 40 kyr world.Chapters 2 and 3 examine the effects of the CAS shoaling on long-term marine nitrogen cycling dynamics since the Miocene, with a particular focus on the evolution of oxygen deficient zones (ODZs) in the eastern tropical Pacific (ETP). Chapter 2 presents two new foraminifera-bound nitrogen isotope (δ15N) records from the west tropical and east equatorial Pacific spanning the last ~ 20 Myrs. Our results indicate that ETP ODZs underwent significant development between ~ 12 and 9 Ma, coinciding with a major step in CAS closure and global circulation changes. This suggests that the development of ETP ODZs was triggered by the shoaling of the CAS during the mid-late Miocene. This study provides evidence for CAS shoaling-driven ODZs development, as previously suggested by model-based research. It offers insights into the dynamics of Pacific nitrogen cycling during the Miocene, highlighting the role of tectonic-driven circulation changes in deoxygenation development and global biogeochemical cycling.In Chapter 3, in order to more accurate interpret the δ15N records and disentangle the underlying mechanisms, we develop a new biogeochemical box model of global oceanic nitrogen cycle to simulate basin-resolved δ15N and constrain the rates of nitrogen cycle processes over geological time. Our model results yielded two quantitative equations that describe the relationship between denitrification fluxes, the inter-basin δ15N gradient, and the mean ocean δ15N, which enable us to estimate changes in denitrification rates based on observed δ15N records. Furthermore, by applying our model to the Miocene scenarios, we found that to develop the observed strong δ¹⁵N gradient between ~12 and 9 Ma, water column denitrification must have undergone a significant increase (by at least a factor of two) during this period. Our model supports that the shoaling of the CAS could have played a critical role in in the development of enhanced ODZs during that period. Our model offers inferences about the operation of the ocean nitrogen cycle for future studies, and the Miocene simulations provide new insight into long-term nitrogen dynamics during the past warm periods.