Collecting Sediment

Labmate Kristin has finally twisted my arm into writing a blog entry on the sediment sampling and analyses that I have been doing at Midway for the past three summers.  So here goes…

In 2008 and 2009 (map at left), I collected nearly 200 sediment samples from across the atoll, leaving only a few regions that need better coverage this summer.  This past week I have enjoyed taking out Seiji and Crystal for shallow sites that can be done free-diving with snorkel gear (10 – 25 ft depths), plus 10 SCUBA dives at deeper sites in the central channel (up to 70 ft).  I hope to collect another 15 – 20 samples at deep diving sites and another 12 – 15 from shallow sites near Eastern Island and the Sand Island piers.

Sampling consists of dropping a quadrat, photographing it in all four directions, pushing a 500 ml jar into the sandy bottom to collect a mini core (pictured right and below), and observing/photographing the area (e.g., habitat, species observed, sediment characteristics, oceanographic conditions).  On land, I wash each sediment sample in fresh water and spread it out to dry on plastic wrapped trays for 3 – 4 days.  We collect six pieces of representative rubble (pictured below) at each site as well.

Once back at Santa Cruz, I sieve a portion of each sediment sample to determine the relative percentage of different grain sizes (useful for describing atoll circulation patterns and the wave/wind energy acting upon bottom sediments).  A second fraction is cast in resin and thin-sectioned for identification purposes under a petrographic microscope.  I identify sediment grains on a 1 x 1 mm grid (coral, coralline algae, Halimeda algae, echinoderm, mollusk, foram, intraclast), which allows me to relate composition to source area and oceanographic factors within the atoll.

I am putting thoughts of this more tedious labwork out of my mind while I savor my time out on the water, collecting samples.  I enjoy exploring the atoll and seeing so many different spots!!  We’ve seen lots of seagrass and big patch reefs with Porites compressa (finger coral) this year, as well as many different species of fish and even a few sharks!  Stay tuned for results from analyses later this fall.

- Anne