Joseph Resing and Tamara Baumberger

Executive Summary

Southern Ocean (SO) occupies ~26% of the global ocean area and currently accounts for ~40% of total anthropogenic CO₂ uptake by the Ocean. It is widely considered to be the area most suitable to ocean iron fertilization (OIF). Here, CO₂ uptake is facilitated by transport of cool surface waters northward and their subsequent subduction below the surface, as well as by an active biological pump, whereby CO₂ fixed by phytoplankton eventually sinks to depth. The SO is the largest of three high nutrient-low chlorophyll regions of the global ocean where low trace metal concentrations, especially iron (Fe) and manganese (Mn), limit phytoplankton growth. Understanding trace-metal forcing on net primary production is critical for predicting atmospheric CO₂ uptake in the SO. This research explores a large, annually recurring, phytoplankton bloom lying above the Australian Antarctic Ridge, marking this site as a location of natural OIF¹ where Fe and Mn deficiency is relieved by its supply from the AAR, thereby fueling the growth of this bloom.

Broader Impacts

It is societally important to understand the role of trace micronutrients in regulating ocean biology and carbon cycling, and their linkages to future climatic and environmental change. There is a strong societal interest in the capture and disposal/storage of anthropogenic CO₂. One proposed technique is the addition of Fe to areas of the ocean where Fe-deficiency limits ocean productivity. These artificial OIFs (aOIFs) are controversial, however sites like the AAR bloom where natural OIF takes place allow an understanding of the impacts to be assessed and applied to potential aOIF sites.

Background

Primary productivity in the SO is patchy varying by two orders of magnitude between the productive continental shelves and the more oligotrophic Antarctic Circumpolar Current (ACC) region to the north. However, even within the ACC anomalously large phytoplankton blooms are present with high phytoplankton biomass. Some of these are well understood, such as blooms in the Scotia Sea and near the Kerguelen Plateau where phytoplankton growth is stimulated by sediment-derived Fe mixed into surface waters as the ACC flows over shallow topography. The other large phytoplankton bloom in the ACC lies north-west of the Ross Sea above the Australian-Antarctic Ridge (AAR); this bloom has received comparatively little attention. This AAR bloom has an annual Net Primary Production ⁓300% greater than surrounding waters and exhibits a significant surface ocean pCO₂ drawdown from ⁓400 μatm outside the bloom to ⁓200 μatm within it. The bloom is a hot spot for upper trophic level activity, especially for krill and whales. Satellite images back to1998 show that this bloom is a recurring feature, forming each spring in the same location suggesting that it receives a recurring supply of Fe from some consistent but unknown source(s) as evidenced unusually elevated Fe levels just below the bloom.

Processes that add Fe to Southern Ocean surface waters include sediment resuspension, glacier/iceberg melt, release from melting seasonal sea-ice, atmospheric dust deposition, volcanism, and hydrothermal activity. Our initial hypothesis is that glacier/iceberg/sea ice melt input and atmospheric dust deposition, are unlikely sources of Fe to this area of the SO. The bloom is located at the northern edge of the sea ice zone where melting ice could introduce Fe into surface waters and vertical upwelling could bring deep-water Fe closer to the surface, however it is unclear why either of these processes would be restricted, year after year, to this single location. There are two possible sources of Fe would are geographically fixed. The first is the Antarctic shelf wherein Fe-rich shelf water may be entrained into the Ross gyre and carried to the bloom area. The second is the Australian Antarctic Ridge (AAR) with the bloom lying downstream of two ridge segments of the AAR.  Prior EOI efforts have shown these two segments are hydrothermally active², producing plumes of Fe-rich water. The position of the bloom adjacent to the polar front and its associated steeply sloping isopycnals favors the upward transport of hydrothermal Fe. In addition to these two ridge segments, the presence of ⁓20 morphologically young submarine volcanoes with summits lying between 1886 an 586 m provide potential sources of Fe that lie closer to the surface ocean than the ridge crest; the hydrothermal state of these volcanoes is unknown. Our past work suggests that hydrothermal vents are an important source of Fe to the ocean and that ridge crests closest to, or within, the SO are the most important in this regard³.

Research

The annual reoccurrence of the AAR bloom in this location presents a rare opportunity to intensively study the suite of physical and chemical processes that greatly enhance surface productivity at the confluence of the sea ice edge, a major oceanographic front, and an active hydrothermal vent site. Specifically, we will study the fate of hydrothermal Fe, Mn and 3-He along the segments of the AAR where the AAR bloom forms, determine the physical processes responsible for moving Fe from deep waters to the ocean surface.  Hydrographic sections, satellite data, and chemical measurements will be examined in context of a numerical model that will be used to identify the physical processes responsible for bringing subsurface Fe into the euphotic zone and determine why this is restricted to the area of the AAR bloom. The Coastal and Regional Ocean COmmunity (CROCO) model will be used at several resolutions to simulate hydrothermal plume rise and mesoscale and submesoscale eddies and fronts to assess the various processes that lead to vertical dispersal of Fe, Mn and 3-He.  3-He will be used to identify the provenance of the Fe source. The position, timing, and intensity of the bloom will be identified using satellite data, while phytoplankton taxonomic composition will be assessed, and incubation bioassay experiments will be used to discern the response of the different phytoplankton assemblages to Fe and Mn additions. In addition to dissolved and particulate Fe, Fe-binding ligands will be quantified and characterized. Underway pCO2 measurements will allow carbon draw-down to be documented.  Select samples will be collected for inorganic carbon parameters and methane (a green house gas of concern for OIF) may be measured depending on personnel. Existing BGC-Argo floats and floats deployed as a part of this project will be used to help characterize the AAR bloom.

Operational Details

Cruise is scheduled in and out of Christchurch, New Zealand from December 16, 2024 to February 2, 2025.

Funding

National Science Foundation Polar Programs Lead Institution Stanford University, K. Arrigo; L. Thomas; University of Washington: J. Resing; R. Bundy; Oregon State University T. Baumberger

NOAA-PMEL and CICOES Mission

This research will ultimately prepare society to respond to a rapidly changing planet by leading integrated and interdisciplinary ocean, atmosphere, climate, and ecosystem research. The research is aimed at transforming our understanding of how climate change affects carbon, oxygen, nutrient, and marine components of the hydrologic cycle. The role of iron and Mn in increasing primary productivity and carbon export to the deep ocean will contribute to scientific evaluation of the safety and efficacy of artificial ocean iron fertilization for mitigation of climate change.

Possible PMEL Crossover Collaborations

BGC-ARGO data interpretation; inorganic carbon chemistry; tracer/N₂O chemistry; Atmospheric Chemistry (DMS production); genomics; modeling.

1. Bach, L. T. & Boyd, P. W. Seeking natural analogs to fast-forward the assessment of marine CO2 removal. Proc. Natl. Acad. Sci. U. S. A. 118, 1–8 (2021).
2. Hahm, D. et al. First hydrothermal discoveries on the Australian-Antarctic Ridge: Discharge sites, plume chemistry, and vent organisms. Geochemistry, Geophys. Geosystems 16, (2015).
3. Tagliabue, A. & Resing, J. Impact of hydrothermalism on the ocean iron cycle. Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 374, (2016).