The Silicon Isotopic Composition of the North Pacific: Insights From US GEOTRACES Section GP15 and Modeling

Abstract Silicon isotopes within silicic acid, δ 30 Si, were measured on US GEOTRACES section GP15 from Alaska to Tahiti along 152°W. The distribution of silicic acid, Si(OH) 4 , exhibited a double Si(OH) 4 maximum dominated by a midwater maximum that extended from 55°N latitude to the equator together with a bottom maximum north of 40°N latitude. The midwater maximum is dominated by North Pacific Deep Water and is consistent with the existence of a poorly ventilated North Pacific Shadow Zone between 1,500 and 3,000 m with an ideal mean age in excess of 1,300 years where regenerated Si(OH) 4 accumulates. A data‐constrained model of the marine silicon cycle is used to assess the contribution of the Southern Ocean, North Pacific, and central latitudes to the preformed and regenerated forms of both Si(OH) 4 and δ 30 Si along GP15. Regenerated Si(OH) 4 from the North Pacific and Southern Ocean are the dominant inputs to the midwater maximum. Silicic acid within the midwater maximum becomes lighter toward the north with a δ 30 Si minimum at about 50°N. The model indicates that this feature is shaped by ready access to successive fractionations in the productive high‐latitude oceans of both hemispheres. Silicon trapping within the North Pacific accounts for the upper part the δ 30 Si minimum, while the Shadow Zone's interhemispheric connectivity with the Southern Ocean accounts for the deeper part. Isotope values in the bottom Si(OH) 4 maximum are slightly elevated relative to adjacent waters consistent with a sediment source, although this isotopic constraint lies within statistical uncertainty.