Extreme zonal and vertical gradients of nutrients and greenhouse gases in the subtropical Eastern South Pacific basin

Between October 12 and November 5, 2015, the Cimar 21 “Ocean Islands” cruise was developed, organized and managed by the Navy's Hydrographic and Oceanographic Service (SHOA). This cruise covered the zonal transect in the subtropical region of the Eastern South Pacific Basin from Caldera (27.00°S; 70.88°W) to near Rapa Nui Island (27.04°S; 109.31°W). This region is characterized by presenting very contrasting trophic systems; from the coastal zone with eutrophic (rich in nutrients), colder and suboxic ([O2] <22 µM); to ultra-oligotrophic (with undetectable nutrient levels Raimbault et al., 2008), warmer and oxygenated waters that belong to South Pacific Subtropical Gyre, which has the clearest waters of the global ocean (Morel et al., 2010). In addition to trophic gradient, the Eastern South Pacific region presents an oxygen minimum zone (OMZ (Fuenzalida et al, 2009) with marked oxygen gradients where various biogeochemical processes can recycle greenhouse gases such as nitrous oxide (N2O) (Trocoso et al., 2018) and even methane (CH4) (Farías et al., 2021). From a total of 19 stations sampled, we present a set of data collected between 0 and 500 m depth using a CTD rosette for physicochemical variables such as temperature, salinity and oxygen (obtained from the CTD) and nutrients (nitrite, nitrate, phosphate, silicate) and greenhouse gases N2O and CH4 (obtained from Niskin bottles mounted in an oceanographic rosette). The gas samples were analyzed by gas chromatography through a gas chromatograph (Schimadzu 17A) using an electron capture detector at 350ºC and connected to an autoanalyzer, while the CH4 samples were analyzed manually in a chromatograph gas with flame ionization detector (Agilent Model 6850 GC-Fid) with a Restek RT QS-Bond column (30 meters 053 mm ID, 20 μm Film) with a temperature of 40ºC and a column flow of 4mL min-1. Meanwhile, nutrient samples with micromolar concentration (≥ 1 µM) were analyzed using standard colorimetric techniques (Grasshoff et al., 1983) in a Seal AA3 segmented flow auto-analyzer, whereas when the nutrient concentration was submicromolar (< 1 µM) for samples of nitrite, nitrate, and phosphate in the gyre, the Seal AA3 segmented flow autoanalyzer was used coupled to two 50 cm Liquid waveridge capillary cells (LWCC, Type II), which allowed to increase the sensitivity of the detection spectrophotometric (Troncoso et al., 2018).