The Ocean, pH, and CO₂

August 22, 2013

This is my final blog post, written as we transit from Station 70 into port in Maderia. The original plan for Leg 1 was to complete 66 stations. Due to everyone's hard work, crew and scientists, we were able to complete four more stations than planned. I have just collected the last set of aerosol samples from this leg of the cruise. Our last CTD cast was yesterday at 4 pm, at Station 69.  The last cast of Leg 1 happened at 1 am this morning, so there are some very tired people on board at the moment.

Yesterday, while our CTD was in the water, I took some pictures of the salts/CO₂ lab. In this lab people are making high-precision measurements of salinity, total alkalinity, pH (not just as simple as putting a pH probe in a bucket of seawater!), total CO₂ , (or dissolved inorganic carbon - actually these guys are out on the afterdeck in a lab van just like the trace metal group) and partial pressure of CO₂  . SCUBA divers or anyone who's taken high school physics should know all about partial pressure and Boyle's Law - the amount of gas that is dissolved in a liquid is inversely proportional to depth and temperature. Since we know that atmospheric CO₂ concentrations are increasing (we've just passed 400 parts per million - higher than any time in the last 800,000 years), and we know that the ocean is a net sink for CO₂  (this means that the ocean absorbs more CO₂ than it releases), we are interested in determining how the increase in atmospheric CO₂ is, or will, affect the chemistry of the ocean.

The ocean is a giant buffer system which, on geological timescales (i.e. 1,000s of years) is in equilibrium. Our blood is also a buffering system. Our bodies can only function within a very narrow pH range. If our blood pH becomes too high (towards basic) or too low (too acidic), all sorts of bad things happen, such as proteins denaturing, red blood cells bursting or collapsing, enzymes are unable to work. So, in order to prevent organ failure and, untimely death, our blood pH is maintained within the optimal pH range. This process is called homeostasis.

The oceans work in much the same way. In seawater at pH 8.2 (the average pH of the global ocean) carbon dioxide exists predominantly in three forms (species): CO₂, HCO^3- and CO^2-₃. The dissolved carbonate species in seawater provide an efficient chemical buffer to various processes that change the properties of seawater. For instance, the addition of a strong acid such as hydrochloric acid (naturally added to the ocean by volcanism), is strongly buffered by the seawater carbonate system. Thus, the pH of seawater stays relatively constant. Of major concern is that scientists have noticed a decrease in oceanic pH in the last 100 years of about 0.1 pH unit (a 30% increase in hydrogen ions/protons). Although the ocean remains a basic medium (> pH 7), this phenomenon has been called 'Ocean Acidification' - or 'The other CO₂ problem'. As life in the ocean has evolved to live at an optimum pH, deviation from this range has serious implications for life - especially for organisms that build calcium carbonate shells/skeletons, such as corals... which begin to dissolve as pH decreases.

I took some pictures of Kevin Sullivan (University of Miami - CIMAS) at work determining the partial pressure of CO₂ in seawater, or more specifically, the fugacity of the gas (accurate calculations of chemical equilibrium for gases require the use of fugacity rather than the pressure). Don't tell anyone I told you to, but take a look at the Wikipedia page for a more complete explanation of fugacity of a gas: http://en.wikipedia.org/wiki/Fugacity.  Kevin says that he is not expecting to observe a change in the fugacity of CO₂ over the last 10 years, despite an increase in atmospheric CO₂ . This is reassuring and is testament to the efficiency of the ocean buffer system. The problem is that no one knows how much CO₂  the ocean can absorb before the ocean can no longer buffer its pH efficiently. So even though on geological timescales we can expect the ocean pH to be effectively buffered, on the timescale of marine life cycles, a much less rosy picture is beginning to emerge.

(The following pics were all taken by me, Rachel Shelley)

Kevin Sullivan (University of Miami - CIMAS) at work determining
the fugacity of CO₂ in water column samples - a novel use of a cooler!

A closer look at Kevin Sullivan's cooler. Despite the home-made look of Kevin's equipment,
it is a very sensitive piece of equipment, used for determining a fundamental property
of seawater and the global carbon cycle.

 

Pam (top) and me (bottom) don our survival suits - a perfect fit!!

 

Sunset off the port side...

... and moonrise off the starboard side.

Goodbye from me - the next post will be from Pete Morton (also a post-doc in the Landing lab at FSU).

 

(EOAS wants to thank Rachel for blogging about her experiences aboard the RV Ron Brown and for explaining and documenting the science being conducted.)