Th-234 as a tracer of particulate export and remineralization in the southeastern tropical Pacific

Abstract

Oxygen minimum zones (OMZs) are thought to be regions of decreased carbon attenuation in the upper ocean with a biological pump that could deliver a greater percentage of exported carbon to the mesopelagic relative to surrounding waters. However, much is still unknown about carbon cycling through these zones and the areas of extreme oxygen minima (nM O2) or oxygen deficient zones (ODZs) within the OMZs. Paired sampling for 234Th (t½ ~24.1 days) and particulate organic carbon (POC) was performed along a zonal transect between 77° W and 152° W during the U.S. GEOTRACES Southeastern Tropical Pacific campaign in 2013 in order to constrain the magnitude of carbon export and remineralization through the Peruvian OMZ. POC export varied by an order of magnitude from the coast to 152° W, reflecting a decrease in POC:234Th ratios (>51 μm) with distance offshore and the influence of upwelling at the coast. Modeling indicated that 234Th fluxes could be underestimated at coastal stations by up to 4-fold without adjustment for the impact of upwelling, which in turn would produce much lower carbon export estimates. Low carbon Export:NPP ratios (<0.15) at the base of the euphotic zone (Ez) in the gyre support previous findings of inefficient surface export via the biological pump in the southeastern tropical Pacific. A broad remineralization feature beginning at Ez was observed across >7500 km that resulted in, on average, 3% of the POC exported from the euphotic zone reaching 100 m below Ez. Although the highest percentages (>10%) of total exported POC at 100 m below Ez were observed in the coastal ODZ region, the observed remineralization was also most pronounced these stations. While an average of 75% of the carbon export from the euphotic zone remained at Ez +100 m in the gyre, a range of 10% to 50% was observed at ODZ stations, reflecting increased attenuation. Local subsurface minima in light transmission and maxima in fluorescence were observed in the regions of greatest remineralization at the upper ODZ boundary, suggesting that complex bacterial community dynamics play a role in increased attenuation through these zones. With ODZs and OMZs predicted to grow worldwide with climate change, these areas require further large-scale and seasonal studies to assess the permanency of these attenuation features and the impact of high Gyre and lower ODZ transfer of POC on the overall efficiency of carbon export in the Pacific.