Analyzing Sediment Samples from Maupiti and Mo‘orea: Methods

Last week I started my summer research at VIMS, performing grain size analysis on sediment samples from the islands of Maupiti and Mo‘orea in Central East Polynesia (take a look at my abstract for more details on the specifics of the project). When I wrote that abstract at the beginning of the summer, I was planning on analyzing samples from just Maupiti. However, after taking a look at the samples that Dr. Kahn collected in 2014, we decided that in addition to analyzing the two sites on Maupiti, we would be able to look at several samples from the nearby island of Mo‘orea as well.

Maupiti and Mo'oreaImage modified from Google Earth (click to enlarge)

The Society Islands archipelago (see image above) is a chain of hotspot volcanoes and the age of the islands decreases to the southeast. Maupiti is the oldest island, while Mo‘orea is relatively young (Guillou et al 2005). While Maupiti emerged about 4 million years ago, Mo‘orea emerged closer to 2 million years ago (Guillou et al 2005). As a result, Maupiti is a much smaller island than Mo’orea, with a larger lagoon between the main landmass and the coral motus surrounding it. On the island of Mo’orea, the site that I will be looking at is called ScMo-350. It is located on the southeastern part of the island and has a history of occupation stretching back 800 years (Kahn 2014). It will be interesting to see if this difference in age has any effect on the sediments that I am studying on the different islands.

In this post I also wanted to talk a little bit about the methods that I’m going to be using to analyze the samples from Maupiti and Mo‘orea.

In order to determine grain size distribution of my samples I will be wet sieving and pipetting them. This method is used by the Coastal Hydrodynamics and Sediment Dynamics (CHSD) lab at VIMS. 10 grams of sample are first treated with calgon (sodium metaphosphate and sodium bicarbonate) and then placed in the sonicater for one hour in order to disaggregate clumps of sediment. My samples are then pushed with deionized water through two sieves, one with 1 mm openings and one with 63 μm openings. The sediment caught on these screens is placed in a beaker, dried, weighed, then heated to 550°C and reweighed to determine organic content (which burns off in the oven). The sediment captured on the 63 μm screen will be put through the Rapid Sediment Analyzer (RSA), which will provide the grain size distribution of this portion based on the settling time of the sediment in a column of water. Anything that is too small to be captured by the 1 mm or 63 μm screens is placed in a 1000 mL graduated cylinder, along with all the water used to sieve it (up to 1000 mL).

After waiting for the water in the graduated cylinder to reach room temperature (between 22°C and 24°C), the samples are stirred for 20 seconds to re-suspend the sediment in the water column. Then 20 mL are drawn from the cylinder at 20 cm down and placed in an aluminum weighing tray – this represents the total mud in the sample, a combination of 4phi and 8phi particles. The sediment is left to settle for around two hours, although this time varies depending on the size of particles that one wants to capture (4phi and 8phi, in my case) and the room temperature. Specifically, the time is calculated using Stoke’s Law, which takes into account the density of the particle, the temperature, and the viscosity of the settling medium. Once the sediment settles, 20 mL is drawn from 10 cm down in the cylinder and placed in an aluminum weighing tray in order to capture particles that are 8phi in size. As with the 1 mm and 63 μm samples, the trays are dried, weighed, then muffled and reweighed to determine organic content.

In addition to grain size distribution, I will be looking at the lithology using a method called point counting. I will perform the point counting on the 1 mm (0phi) portion that I collected in the sieving. These grains will be affixed to a microscope slide and then I will count 100 grains on the microscope slide, keeping track of percentages of different kinds of material (lithics, shells, charcoal, etc.). These microscope slides will also be useful for looking at the angularity, roundedness, and sorting/uniformity of these samples.

I am looking forward to completing my first full week of research, and hopefully getting some interesting results from the first few samples I sieved this past week!

 

REFERENCES CITED

Guillou, Herve, Rene C. Maury, Sylvain Blais, Joseph Cotten, Christelle Legendre, Gerard Guille, and Martial Caroff

2005 Age progression along the Society hotspot chain (French Polynesia) based on new unspiked K-Ar ages. Bulletin de la Societe Geologique de France 176(2):135-150.

Kahn, Jennifer G. 2014 Annual Report of Archaeological Research Activities Between May 30 to August 1, 2014 Maupiti and Mo‘orea Islands, French Polynesia.