Archive for sea ice

Profiling floats reveal fate of Southern Ocean phytoplankton stocks

Posted by mmaheigan 
· Tuesday, September 1st, 2020 

More observations are needed to constrain the relative roles of physical (advection), biogeochemical (downward export), and ecological (grazing and biological losses) processes in driving the fate of phytoplankton blooms in Southern Ocean waters. In a recent paper published in Nature Communications, authors used seven Biogeochemical Argo (BGC-Argo) floats that vertically profiled the upper ocean every ten days as they drifted for three years across the remote Sea Ice Zone of the Southern Ocean. Using the floats’ biogeochemical sensors (chlorophyll, nitrate, and backscattering) and regional ratios of nitrate consumption:chlorophyll synthesis, the authors developed a new approach to remotely estimate the fate of the phytoplankton stocks, enabling calculations of herbivory and of downward carbon export. The study revealed that the major fate of phytoplankton biomass in this region is grazing, which consumes ~90% of stocks. The remaining 10% is exported to depth. This pattern was consistent throughout the entire sea ice zone where the floats drifted, from 60°-69° South.

Figure Caption: Southern Ocean Chlorophyll a climatology and floats’ trajectories (top panel). Total losses of Chlorophyll a (including grazing and phytodetritus export, left panel). Phytodetritus export (right panel).

 

This study region comprises two of the three major krill growth and development areas—the eastern Weddell and King Haakon VII Seas and Prydz Bay and the Kerguelen Plateau—so the observed grazing was probably due to Antarctic krill, underscoring their pivotal importance in this ecosystem. Building upon the greater understanding of ocean ecosystems via satellite ocean colour development in the 1990s, BGC-Argo floats and this new approach will allow remote monitoring of the different fates of phytoplankton stocks and insights into the status of the ecosystem.

 

Authors:
Sebastien Moreau (Norwegian Polar Institute, Tromsø, Norway)
Philip Boyd (Institute for Marine and Antarctic Studies, Hobart, Australia)
Peter Strutton (Institute for Marine and Antarctic Studies, Hobart, Australia)

The causes of the 90-ppm glacial atmospheric CO2 drawdown still strongly debated

Posted by mmaheigan 
· Tuesday, July 9th, 2019 

Joint feature with GEOTRACES

Figure: Illustration of the two main mechanisms identified by this study to explain lower atmospheric CO2 during glacial periods. Left: present-day conditions; right: conditions around 19,000 years ago during the Last Glacial Maximum. The obvious explanation for lower CO2 during glacial periods – cooler ocean temperatures (darker blue shade) making CO2 more soluble, much as a glass of sparkling wine will remain fizzier for longer when it is colder – has long been dismissed as not being a significant factor. However, previous calculations assumed that the ocean cooled uniformly and was saturated in dissolved CO2. The model, consistent with reconstructions of sea surface temperature, predicts more cooling at mid latitudes compared with polar regions and also accounts for undersaturation. This nearly doubles the effect of temperature change and accounts for almost half the 90 ppm glacial-interglacial atmospheric CO2 difference. Another quarter is explained in this model by increased growth of marine algae (green blobs and inset) in the waters off Antarctica. Algae absorb CO2 from the atmosphere during photosynthesis and “pump” it into the deep ocean when they die and sink. But their growth in the present-day ocean, especially the waters off Antarctica, is limited by the availability of iron, an essential micronutrient primarily supplied by wind-borne dust. In our model an increased supply of iron to the Southern Ocean, likely originating from Patagonia, Australia and New Zealand, enhances their growth and sucks CO2 out of the atmosphere. This “fertilization” effect was greatly underestimated by previous studies. The study also finds that, contrary to the current consensus, a large expansion of sea ice off Antarctica and reconfiguration of ocean circulation may have played only a minor role in glacial-interglacial CO2 changes. Credit: Illustration by Andrew Orkney, University of Oxford.

Using an observationally constrained earth system model, S. Khatiwala and co-workers compare different processes that could lead to the 90-ppm glacial atmospheric CO2 drawdown, with an important improvement on the deep carbon storage quantification (i.e. Biological Carbon Pump efficiency). They demonstrate that circulation and sea ice changes had only a modest net effect on glacial ocean carbon storage and atmospheric CO2, whereas temperature and iron input effects were more important than previously thought due to their effects on disequilibrium carbon storage.

Authors:
Samar Khatiwala (University of Oxford, UK)
Andreas Schmittner and Juan Muglia (Oregon State University)

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