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The USGS Pacific Coastal and Marine Science Center stores, processes, and analyzes seafloor cores and sediment samples in Santa Cruz, California. For more information about our capabilities, please contact the lab manager, Michael Torresan, firstname.lastname@example.org, 831-460-7425. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
A new core or sample of ocean sediment might go through many different processing and analytical steps, depending on research needs. Below, follow along as Angela Tan, a USGS physical scientist, demonstrates.
Click any photograph to see a larger version in a new window.
Also, watch a time-lapse video of Angela working in the sediment analysis laboratory.
We take most cores and samples straight from the loading dock into a large walk-in refrigerator (about 780 square feet), kept at the international core curation standard of 4° C plus or minus 2° C. Each core and sample must be labeled with an identifier and metadata, which follows the material through processing and analysis.
We store cores on large racks that can hold about 4,500 full sized cores or D-tubes with split cores, up to 1.5 meters long.
These track-mounted racks pack together to save space. Cranking a handle moves the aisle between racks for core access.
On another wall is shelving which can store thousands of samples.
Some researchers use coolers to store groups of samples.
We keep certain samples at -20 °C using freezers running inside the refrigerator. These freezers don’t work very hard, saving energy and money.
The first stop for many cores is the Geotek MSCL-S multi-sensor core logger. The logger automatically measures P-wave velocity, magnetic susceptibility, electrical resistivity, and gamma density at intervals from 1 millimeter to 1 centimeter along cores up to 1.5 meters long. The device can also automatically linescan photograph split cores at 10 micron resolution.
Next stop is the Geotek core splitter, which cuts each core in half lengthwise using oscillating saws and a wire cutter.
Each half of the core is wrapped in plastic to prevent drying and contamination. For long-term storage, we can shrink-wrap one half with a thick film that prevents moisture loss.
We slip split cores into a labeled D-tube, then return the archival half to the refrigerator. USGS and non-USGS scientists often use our core and sample archives for new research. Contact the lab manager for access policies and other details.
Using the working half of the core, an expert lab technican creates a written description, referencing Munsell chart colors and standard phrases.
Each core may be subsampled for further processing and analysis in the Sediment Lab across the hall.
This is our Sediment Lab, where the magic happens for sediment samples and core subsamples.
We store samples in plastic bags to prevent contamination, and track each sample using spreadsheets.
We add about 20 grams from the sample to distilled water for particle size analysis. Then we add strong hydrogen peroxide to break down organic matter that makes clay particles stick together. Digestion takes place overnight.
Then we cook off the remaining hydrogen peroxide on a controlled hot plate next to a slit hood, which removes that “low tide” smell.
We spin down the sample in a centrifuge, and pour off most of the water. This process removes dissolved salts.
Washing the sample through two sieves with distilled water lets us measure the fractions of gravel (bigger than 2 millimeters or -1 phi) and sand (2 millimeters to 63 microns, -1 phi to 4 phi). Smaller sediment passes through the sieves into a standard 1-liter graduated cylinder.
After adding a little sodium hexametaphosphate dispersant, we use a plunger to carefully stir the cylinder then let it settle, to ensure good mixing and standardized suspension of the sediment.
Then we take a sample of the sediment suspended in liquid, for analysis in one of several ways.
The Beckman Coulter LS 13 320 uses laser diffraction to automatically analyze sediment size fractions between 2 millimeters and 0.1 microns (-1 phi to 13 phi).
The Micromeritics Sedigraph III (PDF) uses X-rays to automatically analyze settling times for sediment sizes between 300 microns and 0.1 microns (2 phi to 13 phi).
Or we can use the tried-and-true method of washing samples through finer and finer sieves, then weighing the sediment trapped in each sieve, to determine sediment size fractions.
Three WS Tyler RX-29 Ro-Taps can dry-sieve coarser samples. This machine automatically rotates and taps the stack of sieves, so that smaller sediment falls through to the next sieve. Again, weighing the sediment trapped in each sieve gives us sediment size fractions.
For some projects, scientists want to calculate sediment settling velocity and need to measure particle size using gravity. We use custom-built settling tubes filled with water – three at 2 meters tall (shown) and one 3 meters tall.
Each tube delivers samples to the water column through a brass gate, which opens like a venetian blind.
A pan and microbalance collects and weighs sediment reaching the bottom of the tube. A computer records the cumulative sediment weight over time. The 2-meter tubes can report settling velocities in quarter-phi fractions from -1 phi to 4 phi (2 millimeters to 63 microns.)
The 3-meter tube goes to 11.
In another lab, these three instruments (the UIC CM250) automatically analyze the total carbon, total organic carbon, and total inorganic carbon content of multiple dried sediment samples.
This older UIC CM5230 plus UIC CM5015 analyzes total inorganic carbon content. It’s less automated than the CM250, but often easier to use, and can handle liquid samples.
We also have a small organic geochemistry laboratory for analyzing the carbon-containing chemical components of sediment samples. As of April 2017, we’re building a larger, more complete lab down the hall.
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