The ocean plays a critical role in sequestering CO2 by exporting fixed carbon to the deep ocean through the biological pump. There is a pressing need to understand the systematics of carbon export in the Southern Ocean in the context of global warming because of the sensitivity of this region to climate change, already manifested as significant temperature increases. Numerous studies have indicated that Fe supply is a primary control on phytoplankton biomass and productivity in the Southern Ocean. The results from previous cruises in Feb-Mar 2004 and Jul-Aug 2006 have revealed the major natural Fe fertilization from Fe-rich shelf waters to the Fe-limited high nutrient low chlorophyll (HNLC) Antarctic Circumpolar Current Surface Water (ASW) in the southern Drake Passage, producing a series of phytoplankton blooms. Remaining questions include: How is natural Fe transported to the euphotic zone through small-meso-large scale horizontal-vertical transport and mixing in different HNLC ACC areas? How does plankton community structure evolve in response to a natural Fe addition, how does Fe speciation respond to biogeochemical processes, and how is Fe recycled to determine the longevity of phytoplankton blooms? How does the export of POC evolve as a function of upwelling-mixing, Fe addition-recycling and bacteria-plankton structure? This synthesis proposal will address these fundamental questions using a unique dataset combining multiyear physical, Fe and biogeochemical data collected between 2004 and 2006 from 2 NSF-funded Fe fertilization experiment cruises and 3 Antarctic Marine Living Resource (AMLR) cruises in the southern Drake Passage and southwestern Scotia Sea through collaboration with scientists in the AMLR program and US Southern Ocean GLOBEC projects. All investigators involved in this study are engaged in graduate and undergraduate instruction, and mentoring of postdoctoral researchers. Each P.I. will incorporate key elements of the proposed syntheses in our lectures, problem sets and group projects. The project includes support to convene a 4-5 day international workshop on natural Fe fertilization at Woods Hole Oceanographic Institution. The workshop will include scientists from United Kingdom, France and Germany who have conducted natural Fe fertilization experiments, and Korea and China who are planning to conduct natural Fe fertilization experiments. The participation of graduate students and postdoctoral scholars will be especially encouraged. The results will be published in a Deep-Sea Research II special issue.
The synthesis project resulted in a special issue of Deep Sea Research II (Volume 90). The 12 manuscripts within this issues covered the fields of physical, chemical, biological and modeling expertise analysing data from two cruises to the Bransfield Strait/Elephant Island region of the Drake Passage, Antarctica. Those cruises that had been conducted in both the summer and the winter seasons were designed to understand the causes for the sudden onset of high biological productivity in this region of the ocean where the levels are normally very low, despite the abundance of dissolved nutrients in the surface waters that would normally be expected to fuel high productivity. The conclusions of these synthesis papers that combined our understanding of the physics, chemistry and biology and that utilised the insights provided by modeling was, that the interaction of the Antarctic Circumpolar Current with the sediments of the shelf region around Elephant Island leads to a large input of dissolved Fe to the surface waters of the region. The input of dissolved iron to these waters that are extremely low naturally in dissolved iron permits the rapid growth of biological organisms that require more dissolved Fe than is available. It was also shown that this process is related to the chemical state of the sediments and that it is restricted to regions where acceleration of the water masses as they pass through narrow passages and over shallow banks is sufficient to resuspend sedimentary material. Unlike deliberate Fe fertilisation experiments where dissolved Fe has been added to areas of the ocean to see if biological activity could be stimulated, this is a natural process that presumably also occurs in other places around Antarctica and other regions of the ocean that are Fe poor, but rich in other nutrients. Additionally, since the Fe addition can almost be considered as a point source addition, it would be possible to follow the biological response to Fe addition as a function of time since that addition simply by sampling the water as it moves downstream from the Fe addition point. Thus having identified this place and understanding the processes that are occurring will allow us to design a future sampling scheme that would allow us to characterise the response of the ocean to Fe addition events and to follow the various processes and measure them accurately. With this knowledge we will then be able to understand past processes in the ocean during changed climate conditions and may be able to build models that can accurately predict how this system might function under future changed climate conditions.
