The Amundsen Sea Polynya International Research Expedition (ASPIRE): Bioactive trace metal dynamics component.

by Rob Sherrell

Cruise track from the Oden Southern Ocean 2007-08 cruise showing satellite image of phytoplankton bloom in the Amundsen Sea Polynya

The cruise track of the NB Palmer in Dec. 2010 to Jan. 2011 in the Amundsen polynya (black is open water). The colors indicate Chlorophyll concentrations in surface waters

Around the coast of Antarctica, there are thirty-some regions where sea-ice blows away from the ice-covered land, creating a polynya ("pool" in Russian) of open water bordered by the continent and its continental ice shelves on one side and by floating sea-ice on all other sides.  This allows sunlight to enter the surface waters, and if conditions are right, large blooms of phytoplankton grow in the Antarctic summer.

The Amundsen Sea, located in West Antarctica between the Antarctic Peninsula and the Ross Sea, is the location of one such polynya. Remote sensing measurements from satellites indicate that the Amundsen polynya has the highest biological productivity of all Antarctic polynyas (e.g >40 mg/m³ Chl-a in surface waters, Dec. 2010). The Amundsen Sea is bordered immediately to the north by the circulating waters of the open Antarctic Circumpolar Current (ACC), one of the three main regions of the ocean where primary productivity is known to be limited by the supply of bioavailable iron (Fe), rather than the more typical limitation by one of the macro-nutrients nitrate, phosphate, or silicate. The unusual productivity of the Amundsen polynya therefore must result, at least in part, from natural Fe fertilization. However, until 2007, no research ship had reached this remote area to try to determine who is growing there, how fast, and why.

The ASPIRE trace metal team with the new trace metal CTD/rosette ready to deploy. Left to right, graduate student Kat Esswein, Assistant Professor Silke Severmann, and Professor Rob Sherrell.

Following on the preliminary work carried out by Rob Sherrell and co-PIs during the Oden Southern Ocean 2007-08 expedition, the first dedicated investigation of the biogeochemistry of the Amundsen polynya was carried out during the recently completed 2010-11 ASPIRE project (Amundsen Sea Polynya International Research Expedition) co-funded by NSF and the Swedish Research Council. The PIs included Rob Sherrell, Oscar Schofield, and Silke Severmann from IMCS Rutgers, Sharon Stammerjohn from UC Santa Cruz, Hugh Ducklow from the Marine Biological Laboratory, Kevin Arrigo from Stanford University, and lead PI Tish Yager of the University of Georgia. This team includes marine chemists, phytoplankton biologists, microbial ecologists, carbon cycle scientists, sea ice specialists, and physical oceanographers. Many graduate students, postdocs, and undergrads are involved in the effort.

Kuria Ndungu and Maria Lagerstrom of the University of Stockholm analyzing dissolved Fe in the trace metal "bubble" a hand-built clean lab on the Palmer.

One of Rob Sherrell's main goals in ASPIRE is to locate the primary sources of bioavailable Fe, to understand their pathways to the euphotic zone of the polynya in the context of regional mixing and circulation, and to estimate the potential for continued Fe fertilization through the life of the bloom. Candidate sources for bioavailable Fe include atmospheric deposition, upwelling of Modified Circumpolar Deep Water from the ACC to the north, sea ice melting, glacier and iceberg melting, and inputs from shallow sediments. To answer these questions, we measured dissolved and particulate Fe, as well as a number of other biologically active trace metals (Mn, Zn, Cu, Ni, Co), by collecting samples using some brand new equipment. Sherrell lead the effort to design and build a new trace metal sampling system for the US Antarctic Program comprising a special winch, a plastic-coated polymer cable, a custom CTD/rosette, and 12 specially designed Niskin bottles. This trace metal system, required for collection of clean seawater samples while avoiding the ever-present risk of contamination, went on its maiden voyage on the icebreaker Nathaniel B. Palmer during the ASPIRE cruise. Shipboard measurements of dissolved Fe, carried out laboriously by Swedish collaborator Kuria Ndungu, demonstrated that our system was collecting clean samples for Fe - not an easy task on a large ship made of Fe!

Large volume in situ pumps being deployed from the Palmer.

In addition to deploying the Trace Metal CTD many times to completely represent the spatial variability of the polynya, the trace metal team of Sherrell, Severmann, graduate student Kat Esswein and shipboard colleagues deployed a drifting sediment trap array to collect metal-clean samples of sinking particulate matter. These will help them determine metal removal rates and ratios. Large volume in situ pumps were also deployed to various depths to collect suspended matter samples from hundreds of liters of seawater, and were complemented by the collection of many samples of sediment from the bottom. Silke Severmann will analyze these to determine the isotope ratios of Fe, a clue to the sources of the iron. The team also worked with colleagues from Stanford University to carry out shipboard incubation experiments to determine the relative importance of Fe and light in controlling phytoplankton composition and productivity. Finally, 5 liter samples were filtered and collected for analysis of neodymium isotope ratios by Swedish colleague Per Andersson, for tracing continental inputs to the shelf system and to the open ACC.

A pumped filter.

Initial results show little evidence of Fe limitation in the polynya, at least over the Dec-Jan period of the ASPIRE cruise. The distribution of dissolved Fe supports earlier findings based on particulate Fe distributions, that buoyant flow emanating from under melting continental ice shelves is a major source of Fe to the euphotic zone of the polynya. This Fe source may be derived from direct glacial melting or from shallow sedimentary sources, including glacial flour deposits beneath the ice shelves. Resolving that issue will help the team to predict whether continued global warming is likely to affect the Fe supply to the polynya. While melting sea ice may be a local source of bioavailable Fe, it is unlikely to be able to supply Fe throughout the long growing season of the polynya bloom.

Following several months of sample analysis, the whole ASPIRE group will meet in November 2011 to discuss findings and write publications. The result will be the first comprehensive look at the intertwined physical, geological, chemical and biological processes in this remote but important part of the Antarctic marine system.

Sea ice at the edge of the Amundsen polynya, from the upper deck of the Palmer.

Marker buoy for drifting sediment trap, in the very green highly productive water of the central polynya.