Sediment Core Sectioning and Extraction of Pore Waters under Anoxic Conditions

Summary

A protocol for sectioning sediment cores and extracting pore waters under anoxic conditions in order to permit analysis of redox sensitive species in both solids and fluids is presented.

Abstract

We demonstrate a method for sectioning sediment cores and extracting pore waters while maintaining oxygen-free conditions. A simple, inexpensive system is built and can be transported to a temporary work space close to field sampling site(s) to facilitate rapid analysis. Cores are extruded into a portable glove bag, where they are sectioned and each 1-3 cm thick section (depending on core diameter) is sealed into 50 ml centrifuge tubes. Pore waters are separated with centrifugation outside of the glove bag and then returned to the glove bag for separation from the sediment. These extracted pore water samples can be analyzed immediately. Immediate analyses of redox sensitive species, such as sulfide, iron speciation, and arsenic speciation indicate that oxidation of pore waters is minimal; some samples show approximately 100% of the reduced species, e.g. 100% Fe(II) with no detectable Fe(III). Both sediment and pore water samples can be preserved to maintain chemical species for further analysis upon return to the laboratory.

Introduction

Researchers often wish to study the redox state and geomicrobiology of a sediment-water system. This ideally utilizes data from both sediments and pore waters, as pore waters are often sensitive monitors of the system and are a common source, though not the only source, of ecological exposure to redox-sensitive heavy metals¹ such as arsenic and uranium. Pore water data may be obtained in situ using diffusion equilibrium filters, also known as "peepers," installed into the sediment². Peepers are most commonly used in settings where the field site is known prior to beginning field work and where multiple visits over an extended per....

Protocol

1. Preparation of Equipment

1. Preparation of Core Liners
   1. Calculate thickness of core slice that will be obtained using Volume = πr² x thickness; the final volume must be <50 cm³. With a 10 cm core diameter, 2 cm thick slices may be obtained.
      NOTE: It is not necessary to have the volume be a full 50 ml, but porewater volumes obtained will be proportionately smaller.
   2. Using a jigsaw (or similar) slice one core liner, or a plastic tube of identical diameter, into 2 cm rings (or other thickness if using a different diameter core). Obtain 3-5 rings.
   3. Clean all plastic material that wi....

Results

The type of results obtained depends on analyses performed and on the geochemical setting from which the core was obtained. Dissolved oxygen may be measured in the extracted porewaters, but in many settings this will be zero below the first few cm of the core. Analyses that usually provide more meaningful information include iron speciation (Fe II/Fe III)¹², arsenic speciation (As III/As V)¹³, and sulfide¹⁴. Presence of reduced species such as sulfide indi.......

Discussion

The technique described herein is a flexible one that can be adjusted for a wide range of locations, core sizes, core section thickness, etc. There are three essential components to this system.

First, prepare a core extrusion system of the right dimensions for the core to be analyzed. Instructions here are given assuming an approximately 30" core; much longer cores may require more PVC extender pieces and PVC fittings to extrude fully. Plan the extrusion system and the packing ca.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Disposable glove bag(s)	Sigma-Aldrich	Z106089-1EA	One per two cores to be processed is usually sufficient.
N2 tank	Praxair		Often gas supply companies can deliver these directly to the field laboratory.
Nitrogen gas regulator	VWR	55850-478	Or similar
Several feet of tubing that fits the regulator	VWR	89403-862	Or similar
Safety equipment to secure the tank	VWR	60142-006
Adjustable tubing clamp	VWR	62849-112
Waterproof, good sealing electrical tape	Scotch	Super 33+	Widely available
2-4 short bungee cords			Widely available
Squirt bottles of nanopure water	VWR	16650-082	Any similar bottle is fine; pack an additional supply of nanopure water to refill these.
Large supply of paper towels and Kimwipes			Widely available
50 ml centrifuge tubes	VWR	21008-951	Acid cleaned as described in protocol. At least 2/core section needed.
Several permanent in markers.			Widely available
Several straight razor blades and box cutters.			Widely available
Centrifuge	Beckman-Coulter	Allegra X-22	Faster rotor allows greater separation.
Rotor to accommodate 50 ml tubes	Beckman-Coulter	SX-4250
50 ml plastic syringes without black rubber tip on the barrel	VWR	66064-764	Acid cleaned as described in protocol. At least 1/core section needed, plus 1 for overlying water.
Syringe filters compatible with aqueous solutions.	VWR	28143-310	Either 0.45 μm or 0.20 μm poresizes may be used. Plan on five filters per core section processed.
Plastic (disposable) spoons.			Widely available; Acid cleaned as described in protocol.
Several boxes of disposable gloves.			Widely available
Large plastic beakers or other waste containers to place in the glove bag.	VWR	13890-148
Laboratory balance	VWR	10205-008	An available balance will be fine; high precision not required
Dry shipper, pre-charged with liquid nitrogen	VWR	82005-416	Needed only if samples are being returned to the home laboratory for sensitive analyses.
Laboratory notebooks			Water repellent can be useful
Core liners	Watermark	77280	Available from Forrestry Suppliers
Core caps	Ben Meadows	218105
Core slicers	McMaster Carr	8707K111	Cut this into 9 3 x 3 squares
PVC spacers	McMaster Carr	48925K96	Cut this into short lengths
PVC couplings	McMaster Carr	4880K76	Approximately 12 needed
Dowel			Widely available
Lab stopper	VWR	59580-400	Check to ensure the correct size to fit snugly within the core liners
Plywood for core guidance plate and top of lab jack			Widely available
Lab jack	VWR	89260-826
Clamps			Widely available
Portable oxygen monitor	RKI instruments	OX-07