Geochemical Sediment Analysis Procedures

Carbon:

The Woods Hole Field Center (WHFC) has the capability to measure organic, inorganic, and total carbon contact using Coulometer and CHN Analyzers. Total and organic carbon percentages are determined using the Perkin Elmer 2400 Series II CHN Analyzer (Figure 1). From these analyses, one can determine the percent inorganic carbon of a sediment sample by subtracting the percent organic carbon from the percent total carbon:
%total carbon = %inorganic carbon + %organic carbon. Another instrument available for the determination of inorganic carbon content in sediment, is the UIC Coulometrics, Inc. Carbon Dioxide Coulometer with an acidification unit
(Figure 2).

The organic carbon fraction in sediments is important because it serves as a binding site for contaminant metals. The abundance of organic carbon controls many diagenic processes. Normalizing contaminant data by the % organic carbon allows for the distinction of specific anthropogenic sources.

Geochemistry:

This laboratory is equipped to handle wet sediment samples (cores and grabs) as they are returned from field collection. Samples are collected from marine and coastal areas and returned to the laboratory for analysis. Cores are sectioned and sub-sampled using titanium tools to minimize contamination (Figure 3). Samples from cores and grabs are freeze-dried (Figure 4) and sent to contract laboratories for various analyses including toxicity tests, mercury concentrations, foraminifera identification, pollen counting, nitrogen isotope analysis, and organic and inorganic contaminant concentrations.

Core sectioning procedures - A fully illustrated tutorial on the methods used at the Woods Hole Field Center is presented in this publication.

Freeze Drying procedures - A fully illustrated tutorial on the methods used at the Woods Hole Field Center is presented in this publication.

Freeze drying is a process whereby water is removed from frozen materials by converting the frozen water directly into its vapor without the intermediate formation of liquid water. The basis for this sublimation process involves: the use of a vacuum pump to enhance the removal of water vapor from the surface of the sample; the transfer and deposition of water vapor onto the condenser; the removal of heat, due to ice formation, from the condenser by means of a refrigeration system. In essence, the freeze dry process is a balance between the heat absorbed by the sample to vaporize the water and the heat removed from the condenser to convert the water vapor into ice (LABCONCO Instruction Manual).
The vacuum environment is important as it speeds the sublimation process by removing atmospheric pressure which would act to contain the molecules in liquid form.

Figure 3. Sampling a core using a hydraulic extruder.

Figure 4. Freeze Dryer unit

Radiochemistry:

This laboratory is equipped to determine gamma decay emissions from dried sediment samples. This method allows for the determination of the sample's age and the sedimentation rates in sediment cores. Two types of gamma detectors are used: well-type (Figure 5) and planar-type (Figure 6). These high purity germanium (HPGe) detectors are in constant use and are located in a shared laboratory on the Woods Hole Oceanographic campus.

Radioisotopes, such as Pb-210 and Cs-137, can be measured by gamma emission and used in conjunction with a numerical model to estimate sedimentation rates. These measurements are important in modeling chronological changes such as heavy metal pollution. The WHFC conducts studies in the New York Bight (NYB) and in Long Island Sound (LIS) because they are in close proximity to high-density population centers and sewage dumping. Anthropogenic sources have added metals, carbon, bacteria, and organic contaminants to the sea floor. Although these pollutants have been dispersed over time, they are still present in the sedimentary record and can be measured. Cs-137 is a fallout isotope produced from bomb tests in the 1950's and 1960's and since it is not a naturally occurring isotope, it is known that the onset date for this isotope is 1954 and this information can be used to date the sediment cores collected from those areas (Santschi et al, 1999).

Figure 5. Well-type Gamma Detector.

Figure 6. Planar-type Gamma Detector

Biogenic Silica:

The WHFC has the capability to digest sediment samples to be further analyzed by Inductively Coupled Plasma - Emission Spectrometry (ICP-ES) for biogenic silica. Biogenic silica fluxes indicate ecosystem productivity over time. The WHFC does not, at this time, have the instrumentation to analyze the biogenic silica extracts, so contract laboratories are used.

Biogenic silica digestion procedures - A fully illustrated tutorial on the methods used at the Woods Hole Field Center is provided in this publication. The method used is adapted from Mortlock, R.A. et. al (1989).

Equipment for this procedure includes a heated shaker bath (Figure 7) to resuspend the sediment and a centrifuge
(Figure 8) to help remove the supernatent, as well as ovens to dry sediment samples, a mortar and pestle to grind the sediment samples, a balance to weigh samples, a vortexer to swirl the samples, a sonicator to break up sample particles, and the chemicals used in the preparation of the sediment samples.