Southern Ocean ecosystem and global carbon cycle responses to iron fertilisation during the last glacial cycle

• Main Author: Saini, Himadri
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: UNSW, Sydney 2023
• Subjects: Marine biogeochemistry; Global carbon cycle; Heinrich Events; Iron fertilisation; anzsrc-for: 3702 Climate change science; Southern Ocean; Weddell; Weddell Sea; Sea ice
• Online Access: http://hdl.handle.net/1959.4/101331; https://unsworks.unsw.edu.au/bitstreams/a644a984-202b-4223-ae50-d9858d615e31/download; https://doi.org/10.26190/unsworks/25038

Rising atmospheric CO2 concentration is one of the major drivers of climate change. To provide effective mitigation policies to curb these emissions, a thorough understanding of past changes in the carbon cycle is required. Decades of research on understanding carbon cycle changes during the last glacial cycle have put forward several processes impacting the concentration of atmospheric CO2. One of these processes is changes in aeolian iron flux into the Southern Ocean. Marine plankton fix dissolved inorganic carbon (DIC) during photosynthesis and transfer the fixed carbon to the deep ocean. DIC removal from the surface lowers the surface ocean partial pressure of CO2, which leads to carbon drawdown from the atmosphere. As the Southern Ocean is a high-nutrient-low-chlorophyll region, the increase in iron input can impact Southern Ocean marine ecosystems, by increasing export production, and therefore decreasing surface DIC. This thesis aims to investigate the responses of Southern Ocean marine ecosystems to changes in iron flux, and their impact on ocean biogeochemistry and atmospheric CO2 during the last glacial period. For this, I use a recently developed complex ecosystem model, which includes four different classes of phytoplankton functional types. Chapter 2 of this thesis is the first study to use this complex ecosystem model and document the competitive dynamics between different plankton species for light and nutrient availability under Last Glacial Maximum (LGM) climate boundary conditions (∼21 thousand years ago, 21 ka). Chapter 2 further assesses the impact of enhanced aeolian iron input on ecosystems. This study shows that lower sea surface temperatures and greater sea ice cover during the LGM causes a 2.4% reduction in Southern Ocean export production. However, a 78% increase in iron supply with a weaker ventilation in the Weddell Sea, increases diatoms and coccolithophores in the Southern Ocean, leading to a 4.4% higher carbon export at the LGM compared to pre-industrial (PI). Proxy records ...