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

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 the Ez was observed across > 7500 km that resulted in, on average, 3% of the POC exported from the euphotic zone reaching 100 m below the Ez. Although the highest percentages (> 10%) of total exported POC at 100 m below the 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.