JR Geochemistry Lab: Seeing the Unseen

On the JR located one deck below the Core Lab on the F-deck is the Geochemistry Lab. Here is where four inorganic geochemists and technicians analyze the deep sea cores to determine the chemical properties of the rocks and sediments. Similar to the physical properties team, the geochemistry team conducts a number of measurements to characterize the material recovered from the seafloor. They take samples from the working half of the core sections. This team of 340 scientists analyzes the sediments and the pore water that is located in between the sediment grains. The sediments are analyzed for the minerals present, and the organic carbon and calcium carbonate concentrations. From the pore water samples, the major and minor elements, alkalinity, ammonia and salinity are measured. Geochemist make the often less visible science of chemistry visible by identifying the chemicals that make up the visible rocks and sediments.

 

Who are the 340 Ocean Detectives in the Geochemisty Lab— Benoit Villemant from France, Konduri Subramanyam from India, Martin Palmer from United Kingdom and James McManus from the United States. The technical staff aboard the JR who keep the instruments calibrated and working includes Erik Moortgat, Michael Bertolli and XRD Technician Heather Barnes. Thank you to Konduri Subramanyam and Martin Palmer for contributions to this blog entry.

340 Geochemist, Benoit Villemant examing sediments from core section for analysis (Photo, Etienne Claassen)

By analyzing the chemistry of the sediments and water between the sediment grains geochemist can answer questions about what the ocean’s water chemistry was like in past at different times in the earth’s history, and what the ocean was like when the rocks and sediments were deposited in the sea. In addition, they can also determine the extent to which the chemical composition of the sediments is modified after they are deposited by oxidation of organic carbon in the sediments, alteration of volcanic material and other processes that come under the collective term of sediment diagenesis.

 

340 Geochemist, Konduri Subramanyam preparing an analysis using ICP spectrometry

The geochemists have an entire lab dedicated to looking at the chemistry of the sediments and pore water that flows between the sediment grains. Some of the tools of their science are described below. For measurements of the sediments gas chromatography is used and works like this--- a sample of sediment is gently heated and the composition of the gas that is driven off is measured. For pore water measures ion chromatography is used-- the pore waters are injected into a chromatographic column and the concentrations of the different elements are measured by the extent of electrical conductivity of the solution as it passes a detector. Another way the sediments are measured is by a thermal analysis--- when a sample of the sediment is heated to high temperature and the total amount of carbon and nitrogen that are driven off the sample are measured.

 

340 Geochemist, Martin Palmer pointing to volcano awaiting Core on Deck to collect pore water samples (Photo, Etienne Claassen)

The carbonates in a sediment sample are also measured. Carbonates can be from the ocean or land or volcanoes. To measure carbonates a colorimetric analysis is used--- when acid is added to the sample and the amount of carbon dioxide that is drive off is used as a measure of the total amount of carbonate in the sample. Finally, the minerals that make up the rock or sediment samples are identified using a special machine for X-Ray Diffraction (XRD). Like every person has their own unique set of fingerprints, every mineral has a unique diffraction pattern that can be used for identification. The common minerals that make up most of the oceanic crust are called rock-forming minerals. They come from the upper mantle and are distributed at spreading centers and hotspots. The most common are basalts formed above the surface and gabbros that crystallize beneath the surface.

 

340 XRD Technician, Heather Barnes loading samples (powered sediments) into the XRD difractometer for mineral identification

Teaching Resources from Deep Earth Academy

Subduction Zone Conditions  - Grades 9-12. Students investigate core samples obtained across a subduction zone off the east coast of Japan and compare the rock found in each drill hole.

Mineralogy and Petrology of Oceanic Crust - Grades 9-12/Undergraduate. Students will be able to examine and match whole thin section photos to the cores from which they were sampled by defining textural characteristics.

Density of Oceanic Crust  - Grades 9-12/Undergraduate. Using prior knowledge and the formula for density, students will be able to 1) calculate the density of samples from a single core, 2) determine the relationship between density and depth in a given core; and 3) measure, calculate, and compare continental rock samples