ONE Planet DTP - Sunburnt organic matter: the role of photochemical dissolved inorganic carbon production in the marine carbon cycle (OP20291)

Marine dissolved organic carbon (DOC) is one of the largest actively cycled carbon reservoirs on Earth, rivalling global carbon pools in atmosphere and vegetation (IPCC, Chapter 6). DOC also fulfils vital ecosystem roles by acting as food and nutrient source to microbes, and by moderating primary production and UV exposure via its light absorbing, coloured fraction (CDOM). Given the key roles of DOC in marine ecosystems and carbon cycling, we require a sound understanding of DOC cycling. Recent work supports the notion that DOC removal is dominated by photodegradation to dissolved inorganic carbon (DIC) in sunlit marine waters (Aarnos et al. 2018; Stubbins et al. 2006). However, reports of DIC photoproduction in seawater are scant, biased towards coastal areas affected by terrestrial DOC inputs, and reveal high spatial variability. This variability likely reflects differences in the abundance and photoreactivity of DOC from distinct sources (terrestrial, coastal & open ocean). Consequently, estimating DIC photo-production at the global scale is inherently uncertain.

This PhD project will address these uncertainties directly, by (i) assessing DIC photoproduction in marine waters selected to reduce existing bias in spatial coverage, (ii) relating DIC photoproduction to DOC reactivity as reflected by its UV-visible absorbance and fluorescence characteristics, and (iii) deriving regional to global scale estimates of DIC photoproduction from (i) and (ii) and complementary hydrographic data. To address existing bias, the project will involve participation in an ocean going research cruise. Joining the scientific crew will provide you with opportunities to interact with leading marine scientists and postgraduate students outside Newcastle. We have secured a berth on one future Atlantic Meridional Transect (AMT), due to its extensive coverage of open ocean areas (50ºN-50ºS) and links to previous work on seawater photochemistry and CDOM characterisation. Additional coastal sampling will be supported by our research vessel. You will receive training in DIC analysis, photochemical facilities (solar simulator, spectroradiometer), and optical CDOM characterisation. Participation in cruise planning and post cruise data analysis workshops will provide additional networking opportunities.