Location: Ross Sea Polynya

Sampling

Ten stations in the Ross Sea polynya, including stations in in the western Ross Sea and Terra Nova Bay, were sampled during the NBP-1801 expedition aboard the RV/IB N.B. Palmer from February-March 2018. Stations numbers for Mn sampling were less frequent than expedition stations and were numbered sequentially and correspond to expedition stations as described in Table S1 (Stations Mn-1 to Mn-10). Measurements were made for soluble phase Mn speciation using a 12-bottle (X-Niskin bottles) trace metal clean CTD (Conductivity, Temperature and Depth) rosette sampling system equipped with SeaBird equipment for salinity, temperature, dissolved oxygen, conductivity and fluorometry. Sampling depths were selected based on the down-cast profile of salinity, fluorescence and dissolved oxygen. For most stations, 8 depths were selected for soluble Mn speciation. Upon retrieval, Niskin bottles were transported with gloved hands to a trace metal clean van and sampled into acid-washed 1 L PTFE bottles. Bottles were triple rinsed before filling and were overflowed to prevent oxygen contamination. The seawater was immediately filtered in the main laboratory through 0.2 µm PES Millipore filters using acid washed Savillex vacuum-filtration rigs. One 10 mL volume of filtrate was immediately amended with 1 µM hydroxylamine hydrochloride for total MnT (over 10 times excess predicted total dMn concentration), and one volume was immediately analyzed for soluble Mn(II) + Mn(III)-Lw

Shipboard Mn speciation and concentrations

Speciation of soluble Mn(II) and Mn(III)-L was carried out using the spectrophotometric competitive ligand assay first described by Madison et al. (2011) and modified for low-level analysis by Oldham et al. (2017). In brief, a meso-soluble porphyrin ligand α, β, γ, δ-tetrakis(4-carboxyphenyl) porphine (T(4-CP)P) is added to the sample, complexes Mn(II), and rapidly oxidizes to form Mn(III)-T(4-CP)P in the presence of oxygen. This complex is quantified spectrophotometrically at 468 nm. For complete determination of Mn speciation, a separate sample aliquot is completely reduced to Mn(II) by addition of hydroxylamine and quantified similarly. The difference in these two parallel measurements permits determination of total Mn, Mn(II) and strongly bound Mn(III)-L by difference (Oldham et al., 2017).

Filtered samples were immediately analyzed for Mn speciation shipboard using UV/Vis spectrophotometry. One aliquot of sample was amended with 1 µM hydroxylamine and allowed to react overnight at 4 ⁰C before analysis (total dissolved Mn [dMn]). A second aliquot was analyzed immediately; the unreduced fraction represents Mn(II), but as the reaction proceeds for over an hour, it is likely that this fraction also contained some weakly bound Mn(III)-L complexes (Mn(III)-Lw), so our reported Mn(III)-L is a conservative estimate. Samples were added to a solution containing T(4-CP)P, CdCl2 (which complexes T(4-CP)P, opening its ring structure), and imidazole tetraborate buffer (pH=8.2) at a 1:12 dilution factor to avoid chloride interference (Madison et al., 2011). The samples were then heated for 60 minutes in a 90 ⁰C hot water bath, cooled to room temperature, then injected by syringe into the spectrophotometric setup. The analytical setup uses a 100 cm liquid waveguide capillary cell (World Precision Instruments) coupled with an Ocean Optics UV/Vis spectrophotometer in which a mini deuterium halogen light source (DT-Mini-2-GS) is coupled with a USB2000+ fiber optic spectrometer, controlled with SpectraSuite software. The Mn(III)-T(4-CP)P complex is measured at its absorbance maximum against appropriate reagent blanks. The detection limit for this method is 0.5 nM.