FROM THE FIELD
Sampling the Barren Sea

Day 2: What Am I Doing Here, Anyway?

by |December 21, 2015
The planned cruise track of the FS Sonne. The stars are stations that will be sampled during the UltraPac cruise. In color is the bathymetry of the seafloor.

The planned cruise track of the FS Sonne. The stars are stations that will be sampled during the UltraPac cruise. In color is the bathymetry of the seafloor.

By Frankie Pavia

Still stuck in Antofagasta, the scientists are becomingly increasingly antsy. Every day we are stuck at the port is a day of sampling we won’t be able to do at sea. Every time we want to take a sample from the bottom of the ocean, at around 5,000 meters depth (16,404 feet), it will take us four hours to lower the line, several hours to do sampling (fill bottles, pumps, etc), and four hours to pull it back up. There are several of these casts at each of eight stations. Every hour we have at sea is precious for returning valuable samples.

What am I doing here, anyway? I am an oceanographer and an isotope geochemist. Originally, I only planned to measure naturally occurring radionuclides thorium and protactinium dissolved in seawater and stuck onto ocean particles. But slowly, more scientists found out there was a chance to get seawater from this part of the ocean and asked us to take samples for them.

The South Pacific Gyre is the most oligotrophic (nutrient-poor) region in the ocean. This makes it largely barren of life and matter—the waters are the clearest in the ocean. The sediments accumulate below the water at rates as low as 0.1 millimeter per thousand years. So, 10 centimeters of seafloor are equivalent to one million years of material deposition in the South Pacific.

The scarcity of particles and lack of eukaryotic life are two major reasons the South Pacific is fascinating to a chemical oceanographer.

Surface biology and dust deposition are the two main factors regulating the flux of particles through the ocean interior. Being so far from land and upwind of major dust sources, almost no atmospheric material makes its way to the South Pacific. Since there is no dust, and no eukaryotes, the particles must largely be made up of tiny bacteria, of which there are millions in each milliliter of seawater.

Jellyfish floating near the boat. Photo: Frankie Pavia

Jellyfish floating near the boat. Photo: Frankie Pavia

Much of what we are setting out to do is simply the chemical characterization of the region. We are exploring the ocean using chemistry. We can’t see the scarce sinking particles, but trusty old thorium and protactinium can. They are extremely insoluble. Every time they encounter a particle, they stick to it. We exploit this simple characteristic to provide rare accounts of rates in the ocean. Just by measuring protactinium and thorium, we can calculate how fast particles are sinking through the water, how much dust is entering the water column, how fast different elements are being removed from the water at the seafloor, and more. It’s almost incomprehensible that two obscure elements can teach us so much.

These isotopes are the oceanographer’s equivalent to the Hubble telescope.
They help us see where we cannot. We measure thorium and protactinium to tell us input and removal rates. We measure helium isotopes to trace hydrothermal plumes in the deep ocean. We measure radium and actinium isotopes to determine the mixing rates of waters in the deep ocean. None of these processes are discernable by eye, yet all are crucial for understanding the chemical and physical state of the entire ocean.

So we continue to wait to make our measurements and do our science until we can depart. The void is filled by lighthearted scientific arguments, whether or not we could make a jetpack for one of the massive hordes of dead jellyfish floating around the boat. The idea is that you could throw a bit of dry ice underneath the jellyfish, which would then sublimate, expand, and rise out of the water, taking the jellyfish with it.

Ultimately, no one ever tried it. Who wants to do an experiment where you can just see the answer with your own eyes?

Frankie Pavia is a second-year graduate student studying oceanography and geochemistry at Lamont-Doherty Earth Observatory.

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