Biogeochemical and photochemical dynamics of oceanic chromophoric dissolved organic matter: A basin-scale approach

Absorption by open-ocean chromophoric dissolved organic matter (CDOM), the optically-active fraction of the marine DOM pool (<0.2 mum), was previously below instrumental detection limits and regarded as relatively insignificant in open-ocean systems. The advent of long pathlength spectrophotometers has since canonized exploration of the optical nature of DOM in the ocean interior and its relevance to global carbon cycling. Over 500 profiles of spectroscopic CDOM absorption in the global ocean were collected along U.S. CO2/CLIVAR Repeat Hydrography Survey transects. These field observations supplement the synoptic view of CDOM in the surface from satellite, promoting a more comprehensive understanding of CDOM cycling and revealing potential utility for CDOM as a biogeochemical proxy and tracer of ventilated water masses. In this dissertation, a descriptive synthesis of CDOM distribution related to large-scale water mass circulation in the Pacific Ocean is contrasted with observations from the Atlantic to illustrate how CDOM abundance may diagnose the relative strengths of bioremineralization and advection within ocean basins.To use CDOM as a tracer of upper ocean circulation, its cycling pathways must be quantitatively constrained, particularly photolysis, the dominant process affecting the surface distribution of CDOM. Defined as the cleaving of chromophoric bonds by solar photons, photolysis is the primary sink for CDOM absorption and rates of this process are best quantified using the apparent quantum yield. Herein, quantum yields are determined for the Pacific, Atlantic, Indian and Southern Oceans using simulated solar irradiance. Results reveal that quantum yield is largely explained by initial spectral absorption thus the inherent solar exposure history of CDOM.As ultraviolet (UV) wavelengths are predominantly responsible for photochemistry, UV absorption must be extrapolated from ocean color retrievals in the visible in order to apply quantum yield to global time-series data for photolysis rate estimation. Herein we utilize the global survey dataset of CDOM spectral slopes to (a) model UV absorption from the satellite-derived colored dissolved and detrital absorption coefficient at 443nm (m-1), resulting in 80% agreement with in situ CDOM measurements, and (b) determine a new parameter, the spectral slope anomaly, S*, to investigate second-order controls on CDOM cycling.