Award: ANT-1543483

Located on the Antarctic continental margin, the waters of the Ross Sea are among the most productive in the Southern Ocean. Here primary production by phytoplankton comprises the foundation of the larger Antarctic ecosystem, and regulates the ocean-atmosphere exchange of carbon dioxide, which holds importance for global climate. As such, there is considerable interest in understanding the factors that control phytoplankton production in the Ross Sea, and how it will change as the climate warms. One of these factors is dissolved iron, an essential micronutrient for phytoplankton, the supply of which is known to limit phytoplankton growth in the Ross Sea during the summer months. Recent research suggests that a major portion of the iron that supports phytoplankton growth at the start of the austral spring growing season is supplied by the deep vertical mixing that occurs during the winter months, transporting iron-rich bottom waters to the surface. However, there remain uncertainties about this process and its sensitivity to ongoing climatic changes, owing to a lack of any iron measurements in the Ross Sea during the late fall and winter; during this period, intense katabatic winds descend from the Antarctic continent to drive surface cooling, sea ice formation, and deep vertical mixing in coastal polynyas (areas of persistently low sea ice cover that are maintained by katabatic winds). This project directly addressed these uncertainties, by supporting participation in a seagoing research expedition to the Ross Sea, Antarctica, during the austral fall-winter. During the project field expedition in April-June 2017, water column samples for iron analysis were collected from two coastal polynyas in the Ross Sea, the Terra Nova Bay (TNB) polynya and the Ross Ice Shelf (RIS) polynya (Figure 1). In the TNB polynya, several katabatic events were observed (Figure 2), with high wind speeds (over 60 knots), cold temperatures (less than -20 degrees celcius) and active sea ice formation driving vertical mixing that extended to around 600 m depth. Analyses of dissolved iron in our water samples showed that this deep mixing was just starting to excavate the dense, iron-rich waters that fill the deeper TNB basin (Figure 3, upper panels), demonstrating that such mixing is indeed a potential source of iron for Ross Sea surface waters. In the RIS polynya, where wind conditions were less extreme, mixing extended to only around 300 m depth, which was not sufficient to access the iron-rich deep waters (Figure 3, lower panels). Taken together, the project results suggest that deep vertical mixing and the vertical resupply of iron progresses slowly and episodically during the winter months, implying that additional late winter observations are needed to fully characterize the seasonal supply of dissolved iron to Ross Sea surface waters, and its potential for future change. Broader impacts of this project included contributions to graduate research work, with the participation of an Old Dominion University MS student in the field program, and the development of educational outreach materials targeting elementary and middle school students, pre-service science teachers, and in-service science teachers, through collaboration with science education professionals at East Carolina University. Last Modified: 06/25/2020 Submitted by: Peter N Sedwick