Distributions of Dissolved Iron along the 2018 U.S. GEOTRACES GP15 Pacific Meridional Transect

Gabrielle Weiss1, Mariko Hatta1, Chris I Measures1, Jessica N Fitzsimmons2, Nathan Timothy Lanning3, Tim M Conway4 and Matthias Sieber5, (1)University of Hawaii at Manoa, Oceanography, Honolulu, HI, United States, (2)Texas A&M University, Oceanography, College Station, TX, United States, (3)Texas A&M University College Station, College Station, TX, United States, (4)University of South Florida, College of Marine Science, St. Petersburg, United States, (5)University of South Florida, College of Marine Science, St. Petersburg, FL, United States
Dissolved iron (dFe) was determined using a shipboard flow injection (FIA) platform during the Fall 2018 U.S. Pacific Meridional GEOTRACES GP15 section. The cruise began in Alaskan Shelf waters (56.1 °N) and followed 152 °W due south to Papeete, Tahiti (20 °S). Water column samples were collected using a GEOTRACES trace metal clean rosette and were filtered through a 0.2 mm Acropak. Intercalibration of dFe samples between FIA and ICP-MS showed good agreement. On the Alaskan shelf (56.1 °N to 55.6 °N) elevated dFe (~1.5 nM) was seen in surface concentrations, continuing to increase up to 2.5 nM at 80 m. Along the shelf slope (55.6 °N to 54.7 °N) between 50 m to 500 m, an area of elevated dFe (up to 8 nM) was observed, and a diluted dFe signal was observed as far south as 49.5 °N at the edge of the Alaskan Gyre (AG) along the 27 kg/m3 neutral density contour. Surface waters in the AG centered at 47 °N had high concentrations of nitrate from upwelling but were depleted with respect to dFe, consistent with the historical characterization of the AG as a high nutrient low chlorophyll (HNLC) region. Surface water concentrations of dFe were observed to increase from <0.03 nM in the AG to ~0.3 nM in the North Pacific Subtropical Gyre (NPSG). This gradient in dFe is observed in the convergent zone between the two gyres in an area known as the North Pacific Transition Zone. Moving south of the NPSG, surface dFe concentrations decrease to <0.1 nM between 14 °N to 10 °N and then increase to an average of 0.3 nM from 10 °N to 10 °S in equatorial waters.

At depth, elevated dFe concentrations were observed between 1100-1300 m at stations 18 and 19 and were associated with elevated Helium-3, suggesting a hydrothermal source. Samples taken at 10 °S, a crossover station with the GP16, also showed dFe slightly elevated above the background concentration at 3,000m depth consistent with an advected hydrothermal plume from the East Pacific Rise.