Modeling the Seasonal Cycle of Iron and Carbon Fluxes in the Amundsen Sea Polynya, Antarctica

Abstract

The Amundsen Sea Polynya (ASP) is distinguished by having the highest net primary production per unit area in the coastal Antarctic. Recent studies have related this high productivity to the presence of fast-melting ice shelves, but the mechanisms involved are not well understood. In this study we describe the first numerical model of the ASP to represent explicitly the ocean-ice interactions, nitrogen and iron cycles, and the coastal circulation at high resolution. The study focuses on the seasonal cycle of iron and carbon, and the results are broadly consistent with field observations collected during the summer of 2010-2011. The simulated biogeochemical cycle is strongly controlled by light availability(dictated by sea ice, phytoplankton self-shading, and variable sunlight). The micronutrient iron exhibits strong seasonality, where scavenging by biogenic particles and remineralization play large compensating roles. Lateral fluxes of iron are also important to the iron budget, and our results confirm the key role played by inputs of dissolved iron from the buoyancy-driven circulation of melting ice shelf cavities (the "meltwater pump"). The model suggests that westward flowing coastal circulation plays two important roles: it provides additional iron to the ASP and it collects particulate organic matter generated by the bloom and transports it to the west of the ASP. As a result, maps of vertical particulate organic matter fluxes show highest fluxes in shelf regions located west of the productive central ASP. Overall, these model results improve our mechanistic understanding of the ASP bloom, while suggesting testable hypotheses for future field efforts.

Keywords: Antarctica; ice shelves; oceanography; phytoplankton; polynyas; sea ice.

Figure 1

Model domain with the main ice shelves labeled. (a) The open boundaries of the model correspond to the edges of the figure. Color shading represents isobaths every 75 m. Features that are labeled are as follows: Dotson Trough (DT), Central Trough (CT), Eastern Trough (ET), Bear Ridge (BR), Amundsen Sea Polynya (ASP, red contour line), Getz Ice Shelf (GIS), Dotson Ice Shelf (DIS), Bear Peninsula (BP), Crosson Ice Shelf (CIS), Thwaites Glacier Tongue (TGT, white contour line), Thwaites Ice Shelf (TIS), B22 is a large grounded (static) iceberg, Thwaites Fast‐ice Tongue (TFT), and Pine Island Glacier (PIG). The ASP represented is a climatological average for the month of January (years 2006–2011, Cavalieri et al., 2014). The green line is the section of Figure 3. (b) Location of ASPIRE stations.

Figure 2

Horizontal distribution of dissolved iron (dFe). Observations from Amundsen Sea Polynya International Research Expedition are represented by colored circles; contoured values are modeled fields (averaged between 15 December 2010 and 15 January 2011) at depths of (a) 0, (b) 150, (c) 300, and (d) 500 m. Black contour lines represent the grounding line and the 500‐ and 1,000‐m isobaths. The gray contour line is the climatological extent of the polynya in January.

Figure 3

Vertical distributions from the Dotson Ice Shelf to the shelf break (see Figure 1 for the location of the section, green). (a) Dissolved iron (dFe), (b) biogenic particulate iron (bpFe), and (c) particulate organic nitrogen (PON). Observations from Amundsen Sea Polynya International Research Expedition are represented by colored circles; contoured values are modeled fields averaged between 15 December 2010 and 15 January 2011. Note the log scale in (b) and (c).

Figure 4

Seasonality of nutrients (dissolved inorganic nitrogen [DIN] and dissolved iron [dFe]) and particulate organic nitrogen (PON, October 2010 to September 2011) at two representative individual stations of the polynya occupied during the ASPIRE cruise. (a–c) Station 29 (center of the polynya; see Figure 1b for locations). (d–f) Station 48. Observations from ASPIRE are represented by colored circles; contoured values are modeled fields. Note the log scale used for PON. Additional stations can be seen in the supporting information.

Figure 5

Vertical carbon flux at Stations 35 and 57 during the Amundsen Sea Polynya International Research Expedition period (austral summer 2010–2011). The modeled flux is smoothed (cutoff frequency of 1/15 days) and shown at four depth horizons. Blue, green, and red crosses are from drifting sediment traps released at the two stations (see Yager et al., 2016), while the cyan crosses are from a moored trap at Station 57 (Ducklow et al., 2015).

Figure 6

Simulated sea ice concentration, surface nutrients (dissolved inorganic nitrogen [DIN] and dissolved iron [dFe]) and surface particulate organic nitrogen (PON) during the summer 2010–2011. The figure illustrates the typical evolution of the simulated summer bloom. A video showing the same fields at daily interval can be downloaded from the supporting information (Movie S1).

Figure 7

Seasonality of modeled distributions at the center of the polynya (Station 29; see Figure 1b for the location of the stations). (a) Dissolved iron (dFe), (b) biogenic particulate iron (bpFe), and (c) particulate organic nitrogen (PON). The second and third rows decompose bpFe and PON into their respective individual components (see section 2). The profiles are averaged over 2006–2013. Note the linear scale in (a) and the log scale in the other figures.

Figure 8

Seasonal iron budget for the control volume. The volume corresponds to the red contour line in Figure 1. (a) Budget of dissolved iron (dFe) for the entire water column. “Net change” is the temporal derivative of dFe inventory. “Residual” represents the budget closure. (b) Budget of biogenic particulate iron (bpFe) for the entire water column. (c) Inventory of dFe, bpFe, and dFe + bpFe in the water column. (d) Same as (c) but in the upper 100 m. The terms are averaged over 2006–2013.

Figure 9

Light and iron availability and sources/losses of living phytoplankton‐iron $\left(F{e}_P\right)$ inside the control volume. The volume corresponds to the red contour line in Figure 1. (a) Availability of light and iron to phytoplankton in the surface mixed layer (equation (1)). The blue shading is the 10th/90th percentile representing the daily variation around the mean. (b) Source and loss terms of F e _P integrated from the surface to the bottom. Biological uptake (blue curve) is a source while the other four terms represent losses. “Sedimentation” is the flux of F e _P at the water‐sediment interface and is generally zero. The terms are averaged over 2006–2013.

Figure 10

Fluxes of dissolved iron (dFe) across the control volume (illustrated in panel b and including the surface to the bottom). (a) Sketch illustrating the different fluxes across the faces of the volume. (b) Dissolved iron concentrations averaged over the upper 300 m (shading) and barotropic circulation on the shelf (streamlines; contour interval 0.2 Sv). Both are annual averages over 2006–2013. The coastal current is highlighted with blue arrows. (c) Net flux across the edge of the polynya (i.e., “coastal current,” blue) or across the ice shelf front (“meltwater pump,” red). Positive values indicate a net gain. (d) Contributions from lateral gradients (dFe_in − dFe_out) and volume transport (Q _in) to the variability of the coastal current's dFe flux. All values are averaged over 2006–2013. ASP = Amundsen Sea Polynya.

Figure 11

Vertically integrated particulate organic carbon concentrations (surface to bottom). The monthly fields (October to December, (a)‐(l)) are averaged over years 2006–2013. Black contour lines represent the grounding line, ice shelf front, and 500‐ and 1,000‐m isobaths. The gray contour line is the climatological extent of the polynya in January.

Figure 12

Average vertical carbon flux at four depth horizons (100m, 300m, 500m and 700m; (a)‐(d)). The fluxes are are annually averaged over 2006–2013. Black contour lines represent the grounding line, ice shelf front, and 500‐ and 1,000‐m isobaths. The gray contour line is the climatological extent of the polynya in January. Note the log scale used in the figure.

Figure 13

Seasonal budget for particulate organic carbon (POC) on the continental shelf of the model (see Figure 1 for its extent). (a) POC inventory integrated from the surface to the bottom and laterally averaged over the continental shelf. (b) Budget of POC for the same area. “Net change” is the temporal derivative of POC inventory. “Residual” is the change in POC that is not accounted by remineralization nor sedimentation. It includes primary production (mostly October to March) and the net lateral transport at the edges of the continental shelf. Values are averaged over 2006–2013.