Using ship-based measurements and profiling floats to estimate net community production and air-sea CO2 flux from the Southern Indian Ocean

Supervisory Team:

Primary supervisor: Zanna Chase

Co-supervisor: Christina Schallenberg

Additional supervisors: Elizabeth Shadwick       

Brief project description:

The Southern Ocean plays an important role in the global carbon cycle (Gruber et al. 2009). This region is responsible for the uptake of about 40% of anthropogenic CO2 emissions (Frolicher et al. 2015), and is believed to play a key role in glacial-interglacial climate change (Sigman et al. 2010).  Large-scale upwelling in the Southern Ocean delivers waters rich in nutrients and natural carbon from the deep ocean to the sunlit surface waters. This upwelled water has the potential to release large amounts of natural carbon dioxide to the atmosphere, and to take up anthropogenic CO2 from the atmosphere (Gruber et al. 2009). While biological productivity can consume CO2 before it is released to the atmosphere, a lack of the micro-nutrient iron generally inhibits biological productivity in the Southern Ocean (Boyd et al. 2000; Coale et al. 2004).

Biological consumption of carbon in the southern-most part of the Southern Ocean, the Antarctic Zone, is particularly important in the carbon cycle. In this region, south of the so-called “biogeochemical divide” (Marinov et al. 2006), upwelled waters flow south after reaching the surface, and are ultimately incorporated into the deepest water mass in the ocean- the Antarctic Bottom water. Biological productivity in this region therefore has the potential to lock carbon away in the deep ocean for up to a thousand years.

Despite the importance of the Antarctic Zone in the global carbon cycle, this region remains poorly studied, due to its remoteness and the inhospitable conditions over winter. The recent development of autonomous profiling floats, instrumented with chemical and optical sensors (biogeochemical Argo floats; BGC-Argo), is changing our view of Southern Ocean carbon cycling: float-based estimates of carbon flux suggest sparse ship-based measurements have underestimated the release of carbon to the atmosphere in the Antarctic Zone (Gray et al. 2018), particularly over autumn and winter.  However, even the float-based study is based on a relatively small number of floats, covering a small fraction of the waters around Antarctica.

This project will estimate the biological productivity and air-sea CO2 flux using data collected from a Jan-Feb 2022 voyage to the Cape Darnley region, East Antarctica. This is one of only 4 regions around the continent where AABW is formed (Ohshima et al. 2013). The area has recently been identified as a strong CO2 sink in summer (Murakami et al. 2020). The voyage will be collecting oceanographic data, including nutrient and carbon measurements, and deploying BGC-Argo floats over the continental slope. The student will work with these data sets and make measurements of particulate organic carbon, biogenic silica and chlorophyll, in order to address the following objectives:

  • Summarise nutrient, carbon and biomass parameters encountered during the voyage
  • Calculate Net Community Production (NCP) from nutrient depletion relative to winter water characteristics, and examine the stoichiometry of nutrient depletion
  • Calculate air-sea CO2 flux from underway pCO2 data
  • Compare the ship-based estimates of NCP and CO2 flux with estimates based on BGC-Argo floats
  • Use the float data through winter to extend observations of the seasonal cycle of NCP and pCO2 flux

Skills students will develop during this research project:

Data analysis and visualisation; lab work.

Authorised by the Executive Director, Institute for Marine and Antarctic Studies
November 23, 2021