Southern Ocean GasEx Blog

Dispatches from the Southern Ocean Gas Exchange Experiment

Archive for April 7th, 2008

Tiny Bubbles

Posted by sogasex on April 7, 2008

By Carlos Del Castillo, The Johns Hopkins University-APL

The loud popping sound was immediately followed by pressurized 4°C seawater being sprayed all over the room. We are working inside the wet lab on board the NOAA Ship Ronald H. Brown (see Richard’s blog entry) and one of the clean seawater lines that feeds our instruments just burst. A high-pressure water line does not just burst and calmly spills water. The line swings left and right, up and down, squirting water on everything and everyone. But no worries, we are in the wet lab. It is supposed to be wet. Before the indoor shower, we had settled into an easy, boring routine for our long transit to the proposed research site, so the burst line was almost a welcomed distraction. Almost welcomed because a busted line means some data will be lost, and the inevitable invasion of air bubbles into our system. We do not like bubbles in the wet lab. Air bubbles dramatically change the optical properties of water and create a lot of noise in our data. Bubbles must be dealt with. Bubbles are the enemy. We battle bubbles along three fronts. The water that flows through our optical instruments enters the boat through an intake that is several meters below the sea surface. There are not many bubbles at this depth unless the weather is bad. Weather is almost always bad in the Southern Ocean. The second line of defense is a “debubbler.” This plastic contraption uses a vortex to trap bubbles and send them back to the ocean – where they belong- while tunneling bubble-free water to our instruments. Bubble-free water is good. In our quest for bubble free water we keep all the lines that feed the instruments submerged in a water bath as our third line of defense. By doing this, we keep the water inside the lines very cold to avoid degassing– or the formation of un-welcomed bubbles that will eventually migrate to our instruments. In this case, the water bath is a large sink where we also keep the instruments to avoid temperature fluctuations. The water in the bath is the same 4°C seawater that flows through the instruments.

In this expedition we encountered our first un-welcomed bubbles in bottled water. As in most countries, bottle water in Chile can be found in two varieties, sparkling water and regular water, or “agua con gas y agua sin gas.” Sparkling water seems to be the most popular and the default offering unless otherwise specified. So, if one does not add the “sin gas” modifiers, one may get bubbles. Agua con gas is not all that bad, we are just not used to it. The wet lab gang prefers to drink our bubbles with beer.

Our instruments in the wet lab measure several parameters. We have two acs’s (absorption, attenuation, spectral) that measure light absorption and attenuation from ~400 through ~700 nm at 4 nm resolution. These are the successors to the veritable WET Labs ac9 (same measurements but at 9 wavelengths). Light attenuation is measured along a fixed path length and represents the loss of incident light due to light absorption by chromophores (i.e. colored dissolved organic matter –CDOM- and photopigments), and losses due to light scattered away from a narrow detection angle. The absorption measurements include incident light losses due only to absorption by chromophores. The measurements are achieved by using two cells, or in this case plastic flow through cylinders. The attenuation tube (c) has an opaque inner wall so that scattered light is absorbed by the tube and counted as light loss. The absorption tube (a) has a highly reflective inner wall so that light scattered forward and away from the direction of the incident light field is not absorbed by the inner walls and reaches the detector. The cell in this case works as a waveguide. Clearly, backscattered light is lost, but most of the light scattering is forward scattering. In addition we have a Turner Designs C-6 fluorometer that measures the fluorescence of CDOM and phytoplankton, and a ctd that measures salinity and water temperature.

The color of a substance is an expression of its chemical characteristics. In the case of seawater, its optical properties – or its color – can give us information about the concentrations of chlorophyll and organic matter in seawater. These measurements are very important to further our understanding of the carbon cycle. Our instruments only measure these parameters along the thin line that is the track of the research vessel. However, several NASA research satellites are equipped with ocean color sensors that provide daily coverage over the globe. The data provided by these satellites are essential to our understanding of the global carbon budget and climate change. Data from these sensors, however, has to be interpreted using complex mathematical algorithms. These algorithms are created and validated using field data like the data provided by our instruments in the wet lab. Curiously enough, satellite ocean color sensors can be affected by bubbles. White caps (or “espuma”) formed in the ocean when winds exceed ~ 14 knots, are nothing more than bubbles at the air-sea interface. White caps change the optical properties of surface waters making it more difficult for the satellites to detect the true color of the ocean. Also, bubbles injected into the water column by large braking waves interfere with satellite color measurements. Again, bubble-free water is good.

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So, here we are in our wet lab, happily bubble free and drinking liquids without gas – at least until the next water line bursts.

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The tangle of hoses that is our underway system

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Scott Freeman (standing on the right) and Carlos Del Castillo (with the funny hat) calibrating one of the acs’s using pure water.

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Carlos Del Castillo cleaning the interior of one of the optical tubes of an acs.

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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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