Carbon - trace metal interactions in the oceanic twilight zone

• Main Author: Ainsworth, Joanna, Jane
• Format: Thesis
• Language: English
• Published: University of Southampton 2022
• Subjects: Southern Ocean
• Online Access: https://eprints.soton.ac.uk/467733/; https://eprints.soton.ac.uk/467733/1/Carbon_trace_metal_interactions_in_the_oceanic_twilight_zone_Ainsworth_Final.pdf; https://eprints.soton.ac.uk/467733/2/Permission_to_deposit_Ainsworth.pdf

Marine microbes are an important control on carbon (C) sequestration depth and biogeochemical cycling of nutrients and trace metals in the global ocean. The biological carbon pump (BCP) is the set of processes by which inorganic carbon (CO 2 ) (along with nutrients and trace metals) is fixed into organic matter via photosynthesis by autotrophic phytoplankton and the C, nutrients and trace metals sequestered away from the atmosphere generally by transport into the deep ocean. Most (~80 %) of the organic C produced by autotrophic phytoplankton is remineralised (returned to the dissolved inorganic inventory from the particulate organic form) in the surface ocean and the inorganic CO 2 is available for release back into the atmosphere. The depth at which remineralisation occurs is important, as the deeper the remineralisation depth of the C the increased likelihood of long term storage in the deep water and sediment. The sequestration of C is primarily dependent on flux attenuation and remineralisation of organic matter in the mesopelagic or ‘twilight’ zone (100-1000 m), where much of the downward particle flux is attenuated via zooplankton and bacterial respiration, replenishing dissolved nutrients and trace metals back into the water column. Understanding the controls on the BCP in the twilight zone is important to understand the transfer efficiency of C sequestration and the regulation of atmospheric CO 2 . Oceanic regions such as the Southern Ocean have inefficient BCPs as the phytoplankton are unable to fully utilise available nutrients, restricting their growth and drawdown of C due to limited access to micronutrients such as iron (Fe). Iron is a scare resource in these regions and low concentrations of bioavailable Fe exert significant controls on global phytoplankton productivity, species composition and therefore ecosystem structure and the C cycle. Iron is not only an important micronutrient for phytoplankton growth but also for heterotrophic bacteria, limiting bacterial secondary production and abundance. ...