New radiocarbon and size-fractionation approaches for examining organic matter source and reactivity in the sea

This dissertation comprises two oceanographic studies:;Open Ocean Study (Chapters 1,2): Marine dissolved organic matter (DOM) represents one of Earth's largest actively cycling carbon reservoirs. Radiocarbon (14C) measurements area key tool for understanding DOM composition and reactivity, however, variable cycling rates of individual DOM components, sources of "pre-aged" DOM in the deep ocean, and links between molecular size and reactivity have all emerged as possible factors complicating the interpretation of Delta14C. In order to investigate the relationship between DOM size, 14C-age and reactivity, a new ultrafiltration (UF) based approach was developed for isolating DOM molecular size fractions from the Central North Pacific Ocean (CNP). Results from this approach reconcile all previously published high molecular weight (HMW) DOM Delta 14C values (>200‰ offset) from the CNP. Our models also indicate that DOM molecular size is directly linked to 14C-age, and that this can be used to accurately predict measured DOM Delta14 C values. Together our results suggest that molecular size acts a "master variable" in determining DOM 14C-age, and thus its cycling in the ocean.;Coastal Ocean Study (Chapters 3,4): Coastal upwelling regions are responsible for >10% of global new production. Sea-to-air degassing of upwelled dissolved inorganic carbon (DIC) is likely attenuated by biological uptake and exported as DOM and particulate organic matter (POM). However, the variability in organic matter (OM) composition and production from upwelling systems is largely unknown. Here we assess on the Big Sur coast: 1) variability in abundance and elemental/isotopic composition POM and DOM 2) explore the use of Delta14C as a highly specific tracer for upwelling-derived OM and 3) examine the size--age--composition relationships between all major exportable OM pools. Our results indicate this upwelling region is a source of N-rich material, with low POM abundance and significant DOM production. Seasonal Delta14C offsets in DIC and POM from this upwelling center are large (>40‰) suggesting Delta14 C can be used to trace upwelling-derived OM, although pre-existing source mixtures must be separated before these estimates can be rigorously quantified. Despite the presence of mixtures, robust size-age-composition trends were found across the entire OM size spectrum.