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doi:10.2204/iodp.proc.320321.210.2013

Introduction

During Integrated Ocean Drilling Program (IODP) Expedition 320/321, pore water samples were obtained from eight sites (U1331–U1338; Table T1) in the equatorial Pacific. These expeditions were the first after the extensive refit and laboratory updating for the R/V JOIDES Resolution completed in early 2009. Pore water measurements previously done with long-established techniques using an ultraviolet-visible light spectrophotometer (Gieskes et al., 1991) were to be carried out with newly adapted techniques using a discrete analyzer (see the “Methods” chapter [Expedition 320/321 Scientists, 2010a]). These techniques had not been thoroughly vetted before the expeditions, the module needed for nitrate measurements was not present/working during Expedition 320, and the discrete analyzer failed entirely before Expedition 321. As a result, nitrate measurements were not made shipboard, and silicate measurements were made using two different analytical techniques. To address these gaps, we report the concentrations of dissolved nitrate and silicate measured at a shore-based facility from archived splits of pore water samples obtained by whole-round squeezing and from Rhizon sampling.

The quantification of nitrate concentrations complements shipboard pore water measurements of dissolved manganese, iron, and sulfate (see the “Expedition 320/321 summary” chapter [Pälike et al., 2010b]) in the characterization of diagenetic processes involving organic carbon. Our shore-based measurements of dissolved silicate were prompted by inconsistencies (up to ~200 µM) (Expedition 320/321 Scientists, 2010a) between repeated shipboard analyses of reference samples caused by the use of two different analytical techniques during Expeditions 320 and 321. Expedition 320 used an OI Analytical discrete analyzer (DA3500) spectrophotometer, which failed before Expedition 321, and inductively coupled plasma–atomic emission spectroscopy was used in its place (Expedition 320/321 Scientists, 2010a). For analytical continuity, we measured all archived pore water samples using an automated coulometric technique in a flow injection analysis system (Lachat QuickChem 8000 FIA).

Measurements were made on two archived pore water sample types: whole round and Rhizon. Whole-round samples are hydraulically squeezed from 5–10 cm long whole-round sediment specimens and are a routine part of geochemical measurements on IODP expeditions. Whole-round sample resolution is generally one sample per section for all sites. Rhizon sampling uses plastic syringes to pull pore water through thin tubes inserted directly into core liners. This type of sampling was adapted for measurement of pore water in sediment during IODP Expedition 302 (Dickens et al., 2007) and provides a nondestructive method to obtain higher resolution sampling. Sites U1331, U1334, U1337, and U1338 include Rhizon samples (up to every 10 cm at Sites U1331 and U1338 and every ~1 m at other sites) across zones where pronounced chemical gradients and sediment color changes occur.