Tracing Carbon Dynamics in the Southern Ocean and Great Barrier Reef Using Strontium and Calcium

Abstract

This study examines carbon cycling in the Southern Ocean and the Great Barrier Reef using strontium (Sr) and calcium (Ca) tracers. Traditionally, major cations like Ca and Sr were considered to have constant concentrations due to their long residence times. However, recent studies reveal that when normalised to salinity, their distribution is heterogeneous, suggesting a significant influence of biological processes. Sr and Ca are closely linked to the carbon cycle, which regulates air-sea CO2 exchange and deep-ocean carbon storage. Calcifying organisms, such as corals and pelagic plankton, reply on CaCO3 formation for their skeletons, but climate change threatens this process, potentially disrupting the carbon cycle. By analysing Sr and Ca, this research aims to improve understanding of how biological processes influence the carbon cycle and CO2 regulation, which is essential for predicting the impacts of ocean acidification on global carbon cycles.

The thesis covers Sr and Ca cycling in both the open ocean and coral reefs, followed by a description of the high-precision Ca and Sr analysis method. The main chapters explore three objectives: examining Ca and Sr cycling in the Southern Ocean (Chapters 3 and 4) and the Great Barrier Reef (Chapter 5).

Chapter 3 investigates CaCO3 cycling in the Southern Ocean using alkalinity (TA) and Ca tracers. The Southern Ocean is key for carbon sequestration and climate regulation. This study finds that nutrient-corrected, salinity-normalized TA and Ca align, suggesting that calcification and dissolution predominantly control their cycling. The analysis of dissolved Ca concentrations shows significant contributions to particulate inorganic carbon (PIC) export, especially in the Subantarctic Zone, highlighting the value of dissolved Ca as a tool to estimate PIC fluxes and refine the carbon budget.

Chapter 4 explores the role of Acantharia, marine protists with star-shaped SrSO4 skeletons, in Sr cycling and POC export in the Southern Ocean. Despite initial assumptions that Acantharia would quickly dissolve in undersaturated seawater, emerging evidence shows they descend to deep ocean depths, contributing to POC export. This chapter shows that Acantharia influence dissolved Sr distribution and contribute to seasonal variations in particulate Sr fluxes, especially during summer, supporting the need for further research on their role in the carbon cycle.

Chapter 5 shifts to One Tree Island in the Great Barrier Reef, where both inorganic (calcification/dissolution) and organic (photosynthesis/respiration) processes shape reef metabolism. Crustose coralline algae (CCA), particularly through high-Mg calcite production, is crucial for reef stability. The study shows that CCA also buffer the carbonate system by dissolving at night, maintaining a supersaturated state favourable for reef growth. This process enhances reef resilience to ocean acidification, supporting long-term reef health. Show more