The influence of particle concentration and composition on the fractionation of 210Po and 210Pb along the North Atlantic GEOTRACES transect GA03

The disequilibrium between 210Po and 210Pb has been used as a proxy for the particle flux from the upper ocean. The particle concentration and composition effect on the partitioning behavior of 210Po and 210Pb is, however, still unclear. Here, we investigate this association by comparing dissolved (< 0.45 µm) and particulate (small: 1-51 µm; large: > 51 µm) 210Po and 210Pb activity with size-fractionated major particle concentration and composition data from the US GEOTRACES GA03 zonal transect cruises. We observed inverse relationships between partition coefficients (Kd) for the radionuclides and the concentration of suspended particulate matter (SPM) in the water column, known as the “particle concentration effect.” We examined the relationships between 210Po, 210Pb, and particle composition in the top 500 m by using Pearson pairwise correlations for individual phases and principal components analysis (PCA) for variations among multiple phases. In addition to these analyses, an end-member mixing model was developed to estimate Kd for 210Po and 210Pb in the small particulate size fraction from the compositional phases. The model predicted the range of observed Kd(Pb) well, but was unable to predict the observed Kd(Po) as consistently, possibly because of the bio-reactive nature of 210Po. Despite this, we found a strong relationship between 210Po and both CaCO3 and POM, as well as between 210Pb and both opal and lithogenic phases. All of our analyses demonstrated that the fractionation of 210Po and 210Pb differed between the margins and open ocean along the GA03 transect.