Supplement of Seasonal dispersal of fjord meltwaters as an important source of iron and manganese to coastal Antarctic phytoplankton

Abstract. Glacial meltwater from the western Antarctic Ice Sheet is
hypothesized to be an important source of cryospheric iron, fertilizing the
Southern Ocean, yet its trace-metal composition and factors that control its
dispersal remain poorly constrained. Here we characterize meltwater iron
sources in a heavily glaciated western Antarctic Peninsula (WAP) fjord.
Using dissolved and particulate ratios of manganese to iron in meltwaters,
porewaters, and seawater, we show that surface glacial melt and subglacial
plumes contribute to the seasonal cycle of iron and manganese within a fjord
still relatively unaffected by climate-change-induced glacial retreat.
Organic ligands derived from the phytoplankton bloom and the glaciers bind
dissolved iron and facilitate the solubilization of particulate iron
downstream. Using a numerical model, we show that buoyant plumes generated
by outflow from the subglacial hydrologic system, enriched in labile
particulate trace metals derived from a chemically modified crustal source,
can supply iron to the fjord euphotic zone through vertical mixing. We also show that
prolonged katabatic wind events enhance export of meltwater out of the
fjord. Thus, we identify an important atmosphere–ice–ocean coupling
intimately tied to coastal iron biogeochemistry and primary productivity
along the WAP.


Supplement to "Seasonal dispersal of fjord meltwaters as an important source of iron and manganese to coastal Antarctic phytoplankton"

Supplemental Methods: Estimating particulate matter crustal and authigenic fractions
To estimate the fractional contribution of crustal, biogenic, and authigenic particulate matter in our samples using 5 equation 4, we first identify the geochemical composition of the weathered source bedrock surrounding Andvord Bay. It is known that there is widespread volcanism and metamorphism (Jordan, Riley and Siddoway, 2020), and thus, ratios (Me:Al, where Me is either Fe or Mn) should reflect basaltic and andesitic crusts. However, uncertainty of the source of weathered particulate matter leads us to use average upper continental crust values (Table 1), although any contribution of a volcanic source would lead to some enrichment of TpFe and TpMn relative to TpAl and a greater estimate of the crustal contribution. 10 Equation 4 allows for the calculation of the crustal contribution: After accounting for a biological contribution based on Me:P quotas for Fe-replete diatom cultures (0% for all samples, data 15 not shown), we then assume the remaining particulate fraction to be authigenic.

Supplemental Methods: Limitations of surface meltwater dye experiment
When we examine the time series derived from the model, we find the model consistently underestimates the contribution of meltwater to the surface (Fig. S8). The MWf does not exceed 0.0013 at either S3 or GS stations, and its seasonal maximum of 0.0046 is found at IBB in early February. Since processes like melting of drifting icebergs and sea ice cannot be captured 20 in the model, the applied meltwater flux is based on a simplified representation of all new freshwater sources except for precipitation in Andvord Bay. These sources include, for example, surface runoff and local melt of glacial ice exposed to the atmosphere. The flux which best recreates observed salinity and temperature profiles in Andvord Bay was achieved by a meltwater input of 0.15 GT over 4 months (Hahn-Woernle et al., 2020).
The overall low modeled meltwater fraction is likely a consequence of multiple factors of which we discuss three. First, the 25 meltwater was tracked only for the field season. The generally low salinity in the upper layer at the beginning of the season and the presence of meltwater dye at the end of the summer season (fjord average of 0.0003 MWf in upper 20m) suggested that meltwater can reside for multiple years in the fjord and cannot be fully captured by our meltwater dye. Second, local melt of glacial ice, e.g. floating icebergs, caused by a summertime surface heat flux, can have a strong impact on the MWf in the surface layer and is likely to be underestimated and not well-represented with the parameterization of the modeled 30 meltwater input. Third, only meltwater from the inner Andvord Bay is tracked and other sources are neglected. Based on other modeled meltwater dyes that track sources just outside Andvord Bay, the impact of the external sources is minor (maximum of 0.0003 MWf in early February) compared to the local sources, but they still contribute to the seasonal increase in MWf.

35
Supplemental Methods: Estimating total surface meltwater export from WAP fjords To estimate the meltwater export resulting from a single katabatic wind event along the WAP, we first identify two fjord types: 1) fjords where waters are below the freezing temperature (cold-water); and 2) fjords where intrusions of modified UCDW reach the glacier terminus (warm-water). This distinction leads to different MWf production rates. We use data collected from Andvord Bay as a basis for export occurring in cold-water fjords. In this instance, a maximum MWf of 40 0.025 was observed, which corresponded to an export of 38x10 7 m 3 glacial meltwater and is based on the glacial meltwater dye export across the mouth of Andvord Bay integrated over the duration of a week-long katabatic wind event.
Meltwater runoff from glaciers due to warm atmospheric temperatures is parameterized as a function of number of days above a temperature threshold. The area of the glacier in contact with the atmosphere predicts how much meltwater is generated. We use this simple relationship with surface area and relate it to the MWf we observe, allowing us to estimate the 45 fractional contribution from each glacier in Andvord Bay. As an example, Bagshawe Glacier has an area of 250 km 2 , which is 48% of the total glacier area for this fjord, and so would be responsible for producing 48% of the surface glacial meltwater (~18.4 x 10 7 m 3 ). By dividing the total surface glacial meltwater export for a single katabatic wind event by the total area of glaciers in Andvord Bay, we calculate the export rate of meltwater in Andvord Bay glaciers to be 7.4 x 10 5 m 3 km -2 assuming glaciers have an equal rate of meltwater production per unit area. We use this rate as representative for cold-water 50 type glaciers.
Since warm atmospheric temperatures in contact with the glacier surface cause production of meltwater, which enters the ocean as surface runoff, this seems a reasonable assumption. Additionally, intrusions of modified UCDW can reach the glacier terminus, causing slightly higher fractions of meltwater at the surface (~0.06 in Barilari Bay). Our general model results showed exchange with water outside of the fjord occurred during katabatic wind events, including inflow of 55 water masses at depth located from outside of the fjord. Thus, these events are likely to enhance delivery of modified UCDW to the glacier terminus (Jackson, Straneo and Sutherland, 2014). We scale the meltwater export to the meltwater fraction since both Barilari and Andvord Bays had similar mixed layer depths. Also, ~40% export of meltwater during katabatic wind events in our model is reasonable compared to estimates for Arctic fjords (10-50%, Jackson et al., 2014). Based on the area of glaciers in Barilari, we calculate an export rate of meltwater for representative warm-water glaciers to be 10.2 x 10 5 m 3 60 km -2 . We extrapolate these rough estimates for all glaciers on the western coast of the WAP identified by Cook et al. (2016).
All glaciers to the south of Andvord Bay are considered warm-water, while those to the north are cold-water (Fig. S9). The area of each of the glaciers used here is published in Cook et al. (2016).
Summing the entire volume export of surface glacial meltwater, we find that if all surface waters along the western coast of the WAP experienced a single katabatic wind event, reminiscent of the one recorded in Andvord Bay, a total of 3.6 65 x 10 10 m 3 (36 km 3 ) of surface glacial meltwater is exported towards the continental shelf (5 km 3 from cold-water glaciers; 31 km 3 from warm-water glaciers). This latitudinal difference is consistent with greater meltwater fractions found on the continental shelf in the southern lines of the PAL LTER grid (Annett et al., 2017). Based on a recent compilation of TDFe content in icebergs from Antarctica (Hopwood et al., 2019), and including two measurements from our study, we use a median concentration of 544 nM (n = 57). We then assume a rough estimate for 10% of TDFe as the dissolved phase, which 70 yields a dFe content of glacial meltwater to be 54.4 nM. This is close to our average dFe measured for three glacial ice pieces in this study (71±121 nM). To our knowledge, there are no other measurements of dMn in glacial ice, so we use our mean for three glacial ice pieces from this study (49±82 nM). We estimate a single wind event lasting one week on the western coast of the WAP corresponds to an export of 2.0 x 10 6 mol dFe and 1.8 x 10 6 mol dMn.
We realize this analysis does not take in to account the impact of shallow sills in fjords that might be important for 75 restricting UCDW from entering the fjord mouth and interacting with glaciers. Invigorated upwelling due to buoyant plumes originating at the glacier face is expected to have a positive feedback on the melting of the glacier terminus by increasing the delivery of modified UCDW to glaciers and enhancing melt (Cape et al., 2019). This may be driven by warm ocean temperatures, directly melting the face of the glaciers, or atmospheric warming could increase drainage of surface melt to the base of the glacier, resulting in subglacial discharge and buoyant plumes driving circulation. Directionality of the katabatic 80 winds is an important parameter for wind forcing in fjords surrounded by steep topographic features (Lundesgaard et al., 2019). We have explored the possibility when one katabatic wind event per year occurs in the along-fjord direction (seaward) for the entire western coast of the WAP. These mechanisms are fjord specific and deserve further attention due to the complex interactions between the ice, ocean, and atmosphere. We also concede that areal extent of glaciers may not be the most representative measure for meltwater production, when in fact glacier flow velocities might better correlate with 85 meltwater production rates, and thus, meltwater export rates. However, the interplay between surface melt and the subglacial hydrological system, and thus flow rates could mean this is a sufficient, albeit rough assumption. Finally, large uncertainties exist for the average glacial ice content of dFe and the degree to which TDFe may be solubilized and made bioavailable. This analysis does not take into account the large quantities of solid ice (i.e., icebergs) exported via this mechanism.

Figure S2. Porewater dissolved metal concentrations for Fe (red), manganese (blue), and oxygen (green) for Mega Core 8 (left) and
Table S1. Seawater samples: Fe, Mn determined for the dissolved (dTM, 0.2 µm) and the total dissolvable (TDTM) determined by FIA and ICPMS methods, and collected during LMG1510 and NBP1604. Additional information covers sampling date, location 100 (station), and latitude and longitude.