Table S1: Particulate Ba (pBa), particulate Al (pAl) and barium excess (Baxs) profile concentrations for seven stations along the 30ĦE transect study during winter 2017.

Table S2: Winter 30ĦE transect station information; regions - north of the Polar Front (NPF), south of the Polar Front (SPF) and the subtropical zone (STZ), mixed layer depth (MLD) and mesopelagic depth-weighted average Baxs concentrations.

Table S3: Barium excess data used for the Southern Ocean (SO) compilation dataset, split into regions sampled during each study (south of the Polar Front (SPF), north of the Polar Front (NPF) and the subtropical zone (STZ)), the SO basin sampled, estimated POC remineralisation fluxes (POCrem) using the Baxs method, integrated POCrem is the POC flux multiplied by number of days from preceding September up to 1 month prior to sampling, PPintegrated is the integrated remotely sensed PP multiplied by the number of days used for integrated POCrem and % integrated POCrem is the percentage integrated POCrem to PPintegrated. No remotely sensed PP was available for studies preceding September 1997. *Particulate sample digestion methods did not use HF and thus may overestimate Baxs concentrations.

Table S4: Least squares regression statistics for correlations between mesopelagic Baxs stock and temporally integrated remotely sensed PP of the SO compilation data for all tested timescales between the preceding September and 1 month prior to sampling (Sept - T1) where the T-value is the number of months prior to sampling. Sept - T1 regressions considered integrated PP from Sept 2016 to June 2017 and T1 - T2 considered PP from May 2017 to June 2017, for this study (July 2017). For a study conducted in March 1998, Sept - T1 considered PP from September 1997 to February 1998 and T1 - T2 considered PP from January 1998 to February 1998.