High‐Latitude Controls on the Silicon Isotope Composition of Silicic Acid in the South Pacific

We present measurements of silicon isotopes in silicic acid, δ 30 Si, from GEOTRACES GP17 OCE between 20°S and 60°S along 152°W. The section sampled key water masses, allowing assessment of controls on large‐scale δ 30 Si features. Observational data paralleled results of a data‐constrained Si isotope model and are consistent with nutrient trapping and partial nutrient consumption in polar surface waters leading to the accumulation of light silicon isotopes in the deep Southern Ocean (SO) and transport of the residually heavy fraction northward within Sub‐Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). Deep‐water δ 30 Si values were nearly constant along isopycnals in the South Pacific, with vertical gradients driven by an increasing fractional contribution of isotopically light regenerated silicic acid with depth. Isotope values in deep waters from the SO to Tahiti exhibited a greater range than previously observed in the South Pacific or predicted by models, with δ 30 Si falling between 1.19‰ and 1.45‰. SAMW and AAIW exhibited relatively heavy, uniform δ 30 Si. Model results suggest that these mode waters are composed of preformed silicic acid subducted in the outcropping region and regenerated silicic acid sourced in roughly equal measure from polar waters and northern distal sources. Distal input is mainly through entrainment of underlying Upper Circumpolar Deep Water into AAIW.