Dissertation Defense: Niels Hauksson

Title: Disentangling Coastal Carbon Reservoirs using Carbon Isotopes

Abstract: Coastal waters connect the terrestrial and marine carbon cycles and are the location of 85% of the organic carbon (OC) burial in the ocean. Anthropogenic activity, such as dam construction, CO2 emissions, and eutrophication has affected the carbon (C) cycling in these waters. However, the heterogeneity of these systems makes generating global estimates challenging. Thus, detailed information about the mechanisms and long-term trends of C  fluxes and storage are needed to assess past and future changes to the coastal C cycle.

This dissertation examines the mixing of multiple pools of C in coastal waters using C isotopes (13C and 14C).  First, we performed a series of laboratory experiments that examined whether sorption of riverine dissolved OC to sediments could be isotopically selective. We found that compounds with higher ∆14C values and lower δ13C values were selectively sorbed by the sediments. Lignin phenols and black C were suspected as possible sorbed compounds. Second, we report a timeseries of ∆14C and δ13C values from dissolved inorganic C at the Newport Beach Pier, CA. This record showed a steady decrease in the ∆14C and δ13C values over the study period that indicates an increasing quantity of fossil fuel derived CO2 dissolved in the surface waters.  Third, we compare the magnitude and composition of particulate OC and sedimentary OC from the Santa Clara River Estuary during periods of extreme precipitation and extreme drought.  The results mirror the change in the C isotope values of atmospheric CO2 and show that even in low precipitation years, significant quantities of eroded petrogenic OC are exported by this watershed. Recently produced plant and algae derived OC is highly variable in the estuary while petrogenic OC is relatively stable. Together, these studies enhance our understanding of the processes that control C cycling within coastal waters.