IODP Proceedings    Volume contents     Search


Inorganic geochemistry

Pore water dissolved ion and gas samples and data were collected using three different methods to maximize the analytical program, minimize possible sample handling and contaminations sources, and measure ephemeral dissolved gas data. These methods include standard sediment squeezing, pore water extraction using Rhizon samplers, and measurements using oxygen optodes.

Sediment squeezing

Shipboard interstitial water samples were obtained from 10–20 cm long whole-round samples cut on the catwalk. Samples were capped and taken to the laboratory for immediate processing. Sampling resolution ranged from 1–2 per core to 1 per section to obtain a high-resolution data set. When there were too many interstitial water samples to process immediately, the capped whole-round samples were stored temporarily in a nitrogen-filled glove bag at 4°C. Samples sat for no more than 3 h before processing. Sediment processing for interstitial water sampling in the laboratory was carried out in a nitrogen-flushed and pressurized glove bag. After extrusion from the core liner, the outer layer of each whole-round sample was carefully scraped with a spatula to remove potential contamination from drill water (surface seawater). The remaining sediment was placed in a titanium squeezer modified after the standard stainless steel squeezer of Manheim and Sayles (1974). The piston was positioned on top of the squeezer, and the entire unit was removed from the glove bag and placed on the hydraulic press. Pressures as high as 76 MPa were applied in the squeezer, calculated from the measured hydraulic press pressure and the ratio of the piston areas of the hydraulic press and the squeezer.

Interstitial water was passed through a prewashed Whatman Number 1 filter above a titanium screen and subsequently extruded into an acid washed or sterile 50 mL plastic syringe. Fluids were then filtered through a 0.45 µm Gelman polysulfone disposable filter into appropriate sample containers. Aliquots for shore-based trace metal and elemental analyses were placed in HDPE plastic bottles. These bottles were cleaned by soaking them in 10% HCl for 24 h at 60°C and rinsing them repeatedly with 18.2 MΩ water. Some of these aliquots were acidified with 4 mL of subboiled 6N HCl per liter of sample for shore-based trace element analysis. Samples for 14C analysis were placed in evacuated glass serum bottles (60 mL). These bottles contained 20 µL of saturated mercuric chloride that was evaporated before the bottle was filled with sample. Samples for isotopic analysis (H, O, and dissolved inorganic carbon) were stored at 4°C without headspace in glass vials with Teflon-lined screw cap lids.

Squeezed pore water samples were analyzed according to standard procedures (Gieskes et al., 1991). Only two measurements were conducted at sea (pH and alkalinity). The pH was determined by ion-selective electrode. Alkalinity was determined by Gran titration with a Metrohm autotitrator. Shore-based analyses included chlorinity measurements, conducted using a potentiometric titration with silver nitrate and a Mettler Toledo DL25 titrator equipped with a silver ring electrode (Mettler/​Toledo ME 89599) filled with a 2M KNO3 solution. Chlorinity measurements were standardized with International Association for the Physical Sciences of the Ocean (IAPSO) standard seawater. Other shore-based analyses include Na calculated by difference, major and minor ions measured using inductively coupled plasma–optical emission spectroscopy (Na, Mg, Ca, K, Sr, Li, B, Mn, Fe, Li, Si, and Ba), and some trace elements measured using inductively coupled plasma–mass spectroscopy with a sample dilution of 1:75 (U, V, Co, Rb, Mo, Cs, and Ba).

Extraction using Rhizon samplers and dissolved oxygen measurements

Following removal of whole round core and syringe samples on the catwalk for microbiological sampling and pore water squeezing, all remaining core sections were capped and immediately transferred to the ship Hold Deck refrigerated storage area for oxygen profiling and pore water extraction with Rhizon samplers (Rhizosphere Research Products, Wageningen, The Netherlands), in a similar manner as was conducted during IODP Expedition 329 (Expedition 329 Scientists, 2011b). The cold storage area was roughly 6°C throughout the sampling period, and cores were delivered to cold storage typically within 45 min of receiving core on deck. As soon as possible after delivery of core sections to the sampling area, Rhizon samplers were inserted through holes drilled in the core liner to collect pore water for nutrient and isotopic analyses in shore-based laboratories. The first 1 mL of fluid collected was wasted to ensure that any possible contaminants in the Rhizon filter were removed. Up to 10 mL of pore water was collected in each sample for a period of up to 24 h, depending on sediment porosity. Fluids for nutrient analysis were stored frozen (–20°C) in acid-cleaned HDPE vials. These samples were analyzed using standard colorimetric methods (flow injection analysis) for nutrient analysis (ammonium, nitrate, and phosphate). Additional fluids for isotopic analysis were stored at 4°C without headspace in glass vials with Teflon-lined screw cap lids.

After the core sections had equilibrated to cold storage temperatures (as measured with a PreSens PT 1000 temperature sensor inserted into the interior of the core, PreSens, Regensburg, Germany), oxygen concentrations were determined. Core temperatures were as high as 18°C when the cores arrived in the cold room, in response to time spent on the catwalk. Oxygen was measured using needle-type fiber-optic oxygen microsensors (optodes; PreSens, Regensburg, Germany; Fischer et al. 2009) inserted manually into sediments through holes drilled in the core liners. Temperature was measured within 2 cm of the oxygen sample location. Both optode and temperature sensors were connected to a MICROX TX3 single channel fiber-optic oxygen meter (PreSens) graciously loaned by Dr. Lars Ottosen (Danish Technological University, Aarhus, Denmark). Signals were recorded using the OxiView software provided by the manufacturer. Prior to sample measurement, optodes were calibrated using water-saturated air (100% air saturation) and sodium sulfite–saturated solution (0% air saturation), according to manufacturer protocols. Optodes frequently had to be replaced due to wear or breakage. The signal for oxygen measurements was allowed to equilibrate to a steady value, ~3–10 min depending on sediment porosity. Roughly 1–2 oxygen measurements were made per core section delivered to the cold storage area, with higher frequency near the sediment/​water and sediment/​basement interfaces, resulting in a resolution of roughly 20–100 cm. Some sections could not be analyzed in this fashion because of high sediment disturbance or inclusion of rocks within the sediment matrix. Optode measurements were conducted at random depths within a core section to prevent bias.