Sources and distributions of a suite of underexplored particulate trace elements on the Southwest African shelf and slope

Particulate trace elements (pTEs) play key roles in marine biogeochemical cycles through their involvement in particle-seawater exchange processes and potential roles as micronutrients for the growth and functioning of phytoplankton. Here, we report on the distributions of labile and refractory particulate gallium (Ga), germanium (Ge), beryllium (Be), tellurium (Te), tin (Sn), antimony (Sb), strontium (Sr), and thallium (Tl) along the GEOTRACES GA08 transect on the southwest African shelf and off shore into the South Atlantic Ocean. Particulate trace element concentrations ranged between 10-1–103 pmol L-1, with distinct pTE and particulate fraction spatial patterns. We observed surface enrichment of pGe, pTe, pSr and pTl, bottom accumulation of refractory pGa, pTe and pSb, and water-column enrichment of pBe and pSn. Average labile particulate fractions (labile/total pTE × 100%; total pTE = labile pTE + refractory pTE) > 90% for pSr, 65% – 75% for pGe, pBe, pTe, pSn and pTl, and < 65% for pGa and pSb. The Congo River dominated inputs of labile pGa, pGe, pSn, pTe, and pTl, as well as refractory pSb and pTl, primarily influencing northern shelf surface waters. Atmospheric deposition had limited impact, and sediment resuspension supplied both labile and refractory pTE to near-bottom waters. Depth-dependent correlations with particulate phosphorus indicated strong biological uptake for labile pGa, pGe, pSn, pSb, and pTe at depths < 500 m, an increasing influence of adsorption and remineralization between 500 and 2000 m, and coupling with regenerated organic matter below 2000 m. These results provide an integrated framework for the cycling of poorly studied pTEs, demonstrating how source mixing and particle reactivity shape their distributions and offering transferable constraints for other oceanic regions.