Drivers and Variability of Ocean Carbonate Chemistry near Hawai‘i

Abstract

Ocean carbon uptake, cycling and sequestration are variable on all time scales, and modulated by an interplay of complex physical and biogeochemical drivers, including anthropogenic CO2 increase and associated ocean acidification. This dissertation disentangles the contributions of and feedbacks between these drivers to assess past changes and potential future shifts in biological production, carbon sequestration and the interplay between ocean physics and carbon speciation. The variability of the CO2-carbonate system is investigated from seasonal to interannual time scales at two different locations in the North Pacific that represent crucial environments to study in the face of global change. The first two studies investigate dynamics in the oligotrophic North Pacific Subtropical Gyre near Hawai‘i, which is the largest ecosystem on earth, and a significant sink for anthropogenic carbon from the atmosphere. A seasonal mixed layer carbon budget stresses the importance of the (relatively steady) carbon supply from horizontal transport balanced by the (spring to summer) biological drawdown over a year. Long-term changes over 35 years are then explored in the mixed layer, as well as subsurface water masses, at the same location. A substantial enhancement of ocean acidification is detected in several subsurface layers, driven by multiple combinations of source water changes from atmospheric forcing (with a focus on freshwater forcing), and/or biological productivity, as well as increasing ingrowth of respired carbon and alkalinity during subduction. The influence of salinity changes in addition to biological and temperature changes on modulation of ocean acidification is then further examined in a coastal coral reef environment on the shores of O‘ahu, Hawai‘i, where an increase in seawater carbon dioxide has not led to a measurable decline in calcium carbonate saturation state. Here, global temperature and regional salinity changes exert opposing influences on coastal acidification. This is additionally exacerbated by local high respiration, but also buffered by dissolution of calcium carbonate in sediments and/or the water column. Insights from all three chapters shed a light on the complex interplay of physical, geochemical, and biological drivers of marine CO2-carbonate chemistry in these locations, and on the implications for global carbon cycling and the fate of anthropogenic CO2.