Southern Ocean GasEx Blog

Dispatches from the Southern Ocean Gas Exchange Experiment

Last Cast

Posted by sogasex on April 7, 2008

By David Ho, LDEO

A couple nights ago, we had our last CTD cast (see below) and are now on our way to Montevideo. We’re scheduled to arrive on the morning of April 10th. While I doubt any of us miss sampling from the CTD, least of all Paul and Matt who had to find the center of the tracer patch at 9 am and 9 pm, it did provide a nice routine to the day. Now, I just see people wondering around the ship aimlessly, overwhelmed by their newfound freedom.

As Pete mentioned in his blog, once the CTD is on deck, it is sampled in order of time sensitivity. Gases go first, in the order of their volatility, and then other things like nutrients and particles. There are varieties and different levels of complexity in people’s sampling methods:

  • My method for 3He is by far the loudest (involving banging the aluminum channels with a dead blow hammer, and then tightening stainless steel clamps with an impact wrench). It’s no doubt one of the reasons why many people are happy to be done with the CTD.
  • Roberta’s other nobel gases takes the most amount of time (read about it here).
  • SF6 is pretty standard, but it’s imperative that Kevin doesn’t get any bubbles or a headspace in the sample.
  • Sara and Roberta sample oxygen in a funny looking bottle, and measure the water temperature during sampling. They add reagents before capping the bottles, and then shake the bottles rigorously.
  • Bob, Geoff, and Paul are responsible for the CO2 parameters (pCO2, DIC, TAlk), and all those samples need to be poisoned to ensure that biological activity doesn’t alter the sample in the bottle.
  • Steve is by far the most stealth sampler. He stands in the background with his bottles ready to sample DMS, and has the bottle numbers written on his hand (or rather, glove; while the rest of us use sample sheets). When it’s his turn, he just shows the bottle to the sample cop, and then samples from the appropriate bottle. Contrast this with the rest of us, who yell out our sample numbers to the cop.
  • Unlike most of us, Carlos doesn’t use a noodle (either Tygon or silicon tubing) for his samples, because they could contaminate his DOC samples.
  • Charlie always shows up right before he’s due to sample (and all of us yell “Charlie!” like they called “Norm” when he came into Cheers), and holds about 5 (small) bottles for nutrients in his hands and samples the bottles rapidly, also sans noodle.
  • Scott seems to have the easiest sampling gig. He only samples one Niskin bottle, and gets the whole Niskin bottle to himself. He connects his noodle to the bottle and drain the entire content into a small drum. He finishes by opening the bottom of the Niskin bottle and draining the rest of the water and suspended solids into a contraption that looks like a beer funnel.
  • Pete, Dave, Veronica, Bob, and Bruce then basically takes all the water that is left for productivity and filtering for chlorophyll and particles.
  • Sara bats clean up, and takes 2 samples for salinity per CTD in bottles that resemble old medicine bottles.

All of this takes about an hour, after which some people start analyzing their samples, while others wait to ship the samples back to the lab for analysis.

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The group poses in front of the last CTD cast before sampling

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Sampling for 3He

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Sampling for oxygen

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Sampling for pCO2

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