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

• Published in: Marine chemistry 2018-04, Vol.201, p.35-50
• Main Authors: Black, Erin E., Buesseler, Ken O., Pike, Steven M., Lam, Phoebe J.
• Format: Article
• Language: English
• Subjects: Export; GEOTRACES; GP-16; Oxygen minimum layer; Particulate organic carbon; Remineralization; Thorium
• ISSN: 0304-4203; 1872-7581
• DOI: 10.1016/j.marchem.2017.06.009

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 ~24.1days) and particulate organic carbon (POC) was performed along a zonal transect between 77 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 (>51μm) 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 (7500km that resulted in, on average, 3% of the POC exported from the euphotic zone reaching 100m below the Ez. Although the highest percentages (>10%) of total exported POC at 100m 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+100m 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. •The SE Pacific OMZ has one of the weakest biolo