Quaatro continuous flow analyzer

Samples were pretreated according to specifications for marine monitoring (National Standards of People’s Republic of China, GB 17378.5, 2007) prior to chemical parameter analysis[11 (link)]. PO₄^3– was measured using a QuAAtro continuous flow analyzer (SEAL Analytical, Hamburg, Germany). Temperature, dissolved oxygen (DO), and pH were measured using a YSI handheld meter. Chemical oxygen demand (COD) was measured using the alkaline potassium permanganate. NO₃⁻ and NH₄⁺ levels were measured using a spectrophotometer (752 UV/visible spectrophotometer, Shanghai-Hengping, China). Salinity was calculated by summarizing the concentrations of major ions.

Porewater samples were extracted from selected depths throughout the recovered AMOR cores using 0.2 µm Rhizon filters, and the concentrations of ammonium, nitrate, and nitrite were determined onboard by photometric methods using a 4-channel Quaatro Continuous Flow Analyzer (Seal Analytical Ltd, Southampton, UK), with a detection limit of 0.1 µM for all species. Ammonium was determined using the indophenol method [38 ]. Nitrite was measured as a pink complex after reacting with N-1-naphthylethylenediamine dihydrochloride and sulfanilamide (i.e., Griess Reagent method). The sum of porewater nitrate and nitrite was measured using the same method, after converting nitrate to nitrite by a Cu−Cd reduction coil [39 ]. The difference between these two measurements was taken as the nitrate concentration. The pH measurements for the AMOR sediment cores were performed onboard using a Metrohm 826 pH mobile pH meter in a closed system to avoid outgassing of CO₂. Calibration of the system was done daily using Metrohm disposable buffers (pH 4, 7, and 9). The pH in core U1383E of North Pond was determined onboard by an ion-selective electrode and reported in [40 ].