Images

Tips of Brazilian waterweed (Egeria densa) break the surface at low tide in Lindsey Slough in the northern Sacramento-San Joaquin River Delta. More commonly, this invasive plant is completely submerged.

On March 21, 2017, the sediment trap from this instrument package (deployed the previous October into Monterey Canyon) is gone and the mounting frame is mangled, having been exposed to several significant turbidity currents in one deployment. 

Scientists deploy a Chirp seismic instrument from the beach. Seismic data provide view of sub-seafloor geology, which records depositional and erosional events and reveals geologic controls on sediment supply.

In the cold storage room at the USGS Pacific Coastal and Marine Science Center, 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.

The back door of the refrigerator connects to our core and sample processing labs.

Each half of a split sediment 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, and both are stored on specialized core racks in a walk-in sample 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.

Lab technicians create written descriptions of sediment cores, referencing Munsell chart colors and standard phrases.

Sediment cores may be subsampled for further processing and analysis in other labs, like the Sediment Lab which is across the hall from the Core Lab.

The first stop for many sediment cores is the Geotek MSCL or 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.

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.

In the core lab, the Geotek core splitter cuts sediment cores in half lengthwise using oscillating saws and a wire cutter.

This is the Pacific Coastal and Marine Science Center's Sediment Prep Lab, where sediment samples and core subsamples are prepared for analyses.

We add about 20 grams of sediment from a 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.

We spin down a sample of sediment that's mixed with distilled water in a centrifuge. This separates the sediment from the water at the bottom of the sample bottle, and we pour off most of the water. This process removes dissolved salts.

Washing a sediment 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.

Lab technician Angela Tan takes a sample of the sediment suspended in liquid, for analysis in one of several ways.

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.

At the USGS Pacific Coastal and Marine Science Center, we have 3 WS Tyler RX-29 Ro-Taps that 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. Weighing the sediment trapped in each sieve gives us sediment size fractions.