Analyzing Sediment Samples from Maupiti and Mo‘orea: First week!

This first week (with the help and instruction of VIMS grad student Danielle Tarpley) I was able to sieve and pipette 10 samples of sediments collected by W&M Professor Dr. Kahn from the island of Maupiti, all taken from different depths in an approximately 1.5 m deep test pit at MAU-5 (see abstract for more information about MAU-5 and my previous post for details about the methods I am using).

As with any project, there were several issues that arose during his first week of work. To start, my samples are very clumpy – even treating with calgon and sonicating did not break down all the aggregates. This meant that we had to manually break down many of the clumps with metal scoops before sieving. In the future, we will probably soak the samples in water and calgon overnight before sonicating. Hopefully this will cut down on the clumping issues!

Another interesting issue that arose was the mineralogy of the samples. Maupiti is a volcanic island composed of basalt and hawaiite (an olivine basalt), with dykes of mugearite and benmoreite (rocks of similar composition to basalt, but with different varieties of feldspar) (Blais et al 2002). These rocks contain minerals like pyroxenes which have a relatively high density. When we were sieving, we noticed a dark-colored portion that was heavier than the rest of the sediment. Since the Rapid Sediment Analyzer (RSA) calculates grain size based on settling time, and these particles will settle much faster than quartz (which is the density on which the RSA bases its settling times), their presence might affect the results from the RSA. A possible solution to this problem would be to separate the heavier minerals by panning, and then running the lighter portion through the RSA.

A third question that came up surrounded the method for determining organic content of these samples (loss on ignition). I will be weighing the 1 mm, 63 μm, 4/8phi combined, and 8phi samples, then heating them to 550°C in order to burn off any organic content, then reweighing to determine percentage of organic content. However, I will also be using the 1 mm portion to make point counting slides (see Methods post). We worried that the high heat of the muffle furnace would destroy the shell, and thus I would have to re-sieve the samples to  have enough sediment for point counting. At the suggestion of Grace Cartwright (program manager for CHSD) I took some shell from another sample and placed it in the muffle furnace for an hour at 550°C. Afterwards, the shell (even the small fragments) were still present in the sample and did not seem negatively impacted by the furnace.

 

REFERENCES CITED

Blais, S., G. Guille, H. Guillou, C. Chauvel, RC Maury, G. Pernet, and J. Cotton 2002, The island of Maupiti: the oldest emergent volcano in the Society hot spot chain (French Polynesia). Bulletin de la Societe Geologique de France 173(1):45-55.