Biogenic particle flux and benthic remineralization in the Eastern Tropical South Pacific

Abstract We studied biogenic rain to the ocean interior and sea floor in the Eastern Tropical South Pacific (ETSP), a region at the intersection of three oceanic regimes; coastal upwelling, equatorial divergence and the South Pacific oligotrophic gyre. Sediment cores from ocean depths >4000 m were collected, pore water was expressed using both whole core squeezing and rhizon techniques, and profiles of nitrate and dissolved Si were modeled to estimate remineralization fluxes. Nitrate modeling was interpreted as representative of C org remineralization assuming the oxic transformation of ammonium to nitrate. A broad range of TCO 2 fluxes were determined: 0.008–0.34 mmol C m −2  d −1 . The range in biogenic silica (bSi) remineralization flux was also large: 0.007–0.15 mmol Si m −2  d −1 . The pattern of TCO 2 flux showed higher particulate organic carbon (POC) inputs at sites closest to coastal and equatorial upwelling and lowest fluxes at the most oligotrophic site. Moored sediment traps, suspended at ~3700 m at 10°S, 100°W and 20°S, 100°W captured the annual pattern of mass and biogenic rain. The annual average mass flux was over five times greater at a 10°S site (30.8 mg m −2  d −1 ) compared to a 20°S site (5.5 mg m −2  d −1 ). The relative wt% of POC, PIC and bSi at these two stations were 4.9, 8.0, 15.5 and 6.6, 8.3, 2.7, respectively. The deep trap POC and bSi annual rain rates were within 0.5–4 times the benthic fluxes estimated from pore water models. The annual averaged surface ocean chlorophyll concentration estimated from satellites is a good predictor of POC rain to the ocean interior at the ETSP sites studied, as is 14 C primary production (PP). However, the POC rain into the deep ocean at ETSP sites per unit chlorophyll or per 14 C PP is significantly less than values obtained from the equatorial Pacific at 140°W or subtropical gyre station HOT. It appears that the ETSP, although underlain by an intense oxygen minimum zone, is inefficient at transferring C org production to deeply sinking POC rain.