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Unraveling Secrets of the Ocean

posted Jul 12, 2012, 9:24 AM by Shannon Bresnahan   [ updated Jul 12, 2012, 9:45 AM ]
Wow, what a cruise. During the past ten days, I have pulled multiple all-nighters, completed 38 CTD casts, fallen asleep in strange places, and analyzed over 700 water samples. This has been one of the most challenging field expeditions that I have been a part of, and as I stared ghostly at my computer screen, or squatted next to the rosette collecting 4oC seawater at four in the morning, I found myself asking this question more than once. Why am I putting myself through all of this?

I think there are a few reasons, such as professional responsibility and the desire not to let the team down. But by far the biggest motivation is the excitement that comes with unraveling secrets of the ocean. The more you stare at the data you collected, the more information you can get out of it. And that is awesome, and that is what helped me push through times of mental and physical exhaustion. So in this blurb, I want to share a little bit of what I did on this cruise.

I measured the inorganic carbon parameters, or basically, the amount of carbon dioxide (CO2) in the seawater samples we collected on this cruise. To fully quantify the inorganic carbon system in seawater you have to measure at least two parameters; I chose pH and Dissolved Inorganic Carbon (DIC). pH tells you information about the level of CO2 in seawater, because when CO2 dissolves into seawater, it becomes a compound called carbonic acid, and lowers the pH. You may have heard of something called ocean acidification, where the world’s oceans are becoming more acidic due to climate change. This is because increased levels of CO2 in the atmosphere are entering the ocean, becoming carbonic acid, and lowering the pH of the ocean. DIC is the total amount of CO2 in the water, including the dissolved gas form, carbonic acid, bicarbonate, and carbonate ions.

Since we are measuring gases in the seawater, the biggest source of contamination is the atmosphere. As soon as the rosette comes onto deck, it is a race against time. Using a sampling tube, most commonly known to me as the “Schnoodle,”we collect the seawater from the niskin bottles and take them straight to the analyzer.

Photo: Mike Navarro models a "schnoodle,"
the sampling tube used to siphon seawater from the CTD.

Photo: Dye is injected into seawater to test for pH

To measure pH, we use the same concept as the litmus paper, but a little more sophisticated. We add a little bit of dye (m-cresol purple) to the sample, and depending on the pH the dye can become yellow, purple, and every color in between. Once the dye is added, we measure the color of the sample with an instrument called the spectrophotometer.

In order to measure DIC, we first have to change all of the carbonic acid, bicarbonate, and carbonate ions into CO2 gas. We do this by adding acid to the sample; when seawater becomes very acidic, all of carbonic acid/bicarbonate/carbonate becomes CO2 gas. Then we extract this gas by bubbling nitrogen through the sample and measuring the amount of CO2 that comes out of the sample by using an infrared detector.

Unfortunately, the massive amounts of data that we collected on this cruise have yet to be processed, so I cannot present any discoveries in this post. But rest assured, as soon as we get back on shore, we will get straight to work. We oceanographers will continue to collect data, one bottle at a time, to unravel the best kept secrets of the ocean.

-- Yui Takeshita, Scripps Graduate Student