CO2 Uptake by the Ocean

As we come to our last days at sea on the I09N cruise, it’s time to highlight some of the CO₂ analyses being done at sea.  Carbon dioxide (CO₂) is a greenhouse gas – which effectively insulates the Earth by trapping some of the outgoing radiation in our atmosphere - but it’s also involved in photosynthesis, respiration, and metabolism – life-sustaining processes for plants and animals.  Opposite to us, plants take in CO₂ gas and respire out O₂, and if you remember from previous posts - marine plants provide ½ of all the O₂ in the atmosphere!  So the uptake and transformations of CO₂ in the ocean plays a major role in setting our climate. 

Nature works towards balance, and gases like CO₂, O₂, and CFCs are distributed between the ocean, atmosphere, and land to find that balance.  However, over the last few hundred years (i.e., the Industrial Revolution), fossil fuel burning has released CO₂ into the atmosphere at an unprecedented pace.  In the figure below, air space in ice cores from Antarctica reveal how temperature and atmospheric CO₂ have risen and fallen over the last 350 thousand years (glacial-interglacial transitions).  Modern values (shown in red) can be attributable to anthropogenic (man-made) emissions.  Today’s global average atmospheric concentration of CO₂ is over 400ppm – off the geologic charts! 

[Inferred temperature change and atmospheric CO₂ concentration from the Vostok Ice Core in East Antarctica, over the last 350 thousand years (data taken from Petit, et al., 1999, Nature).  Modern CO₂ values shown in red can be attributable to anthropogenic emissions.  More at http://cdiac.ornl.gov/trends/co2/ice_core_co2.html]

 

Now we are investigating how the Earth’s reservoirs (ocean, atmosphere, land) will respond to the massive release of CO₂ from fossil fuel burning.  The figure below details the global flows of carbon between the different reservoirs.  The arrows show rates of transfer, and the boxes show total concentrations of carbon in each reservoir.  While the burning of coal, oil, and gas sends carbon only in one direction (source), the ocean, atmosphere, and plants/soil exchange carbon, acting as sources and sinks.  Note how the surface ocean takes up slightly more carbon than it releases (>100 PgC/y) – the ocean is an overall sink for CO₂.  However, the deep ocean and ocean sediments hold the greatest proportion of carbon (40,000 PgC).  Getting carbon from the surface to the deep ocean, then, is key to removing CO₂ from the atmosphere, and combating the effects of climate change.  This is dependent on the inorganic and biologic transformations of carbon in the ocean, and the movement of water masses in global ocean circulation.

[From: http://cdiac.ornl.gov/carbon_cycle.html]

Currently, the ocean absorbs ¼ of all anthropogenic carbon.  In the ocean, CO₂ reacts with water and this increases the acidity – a phenomenon termed Ocean Acidification.  Actually, due to the various salts dissolved in seawater, the ocean is slightly alkaline (average pH of surface ocean is 8.1, while a pH<7 is acidic, and pH>7 is alkaline).  However, the oceans will continue to absorb the excess anthropogenic carbon for many years to come.  Ocean acidification has been shown to impact marine life in various ways, from effecting physiological functions to reducing growth rates and survival, and has rapid impacts for calcifying organisms like coral and some plankton.

A number of scientists on-board this cruise are taking different approaches to measure CO₂ in the Indian Ocean.  Ellen Briggs, David Cervantes, and Stephanie Mumma measure the total alkalinity (TA) and pH of seawater.  Ellen, below, is a PhD student in her final year from UCSD-SIO.  She is also collecting samples to test out a sensor that can measure alkalinity autonomously, which she developed for her PhD thesis.

[Ellen Briggs (UCSD-SIO) measures the alkalinity of seawater in the Main Lab.]

Bob Castle (NOAA-AOML) and Morgan Ostendorf (NOAA-PMEL) measure the concentration of total dissolved inorganic carbon (DIC) on the ship.  Bob has been a sea-going chemical oceanographer for nearly 25 years!  He has sailed the ocean to remote locations and is an expert in making reliable measurements of carbon in the ocean.   

[Bob Castle (NOAA-AOML) prepares a moisture stripper for measuring DIC.]

The four carbonate parameters (TA, pH, DIC, and pCO₂ - which is measured with an autonomous system on-board) are needed to fully characterize the carbonate system in the ocean. 

All of the analyses of this cruise provide data that are essential for understanding the ocean’s potential for uptake, transfer, and storage of CO₂.  The US GO-SHIP repeat hydrography program affords us the ability to sail the seas and collect the much needed data for understanding the current state of the ocean and the ocean’s response to a changing climate.  Thanks to all the scientists, engineers, and crew members for working so hard to make this cruise a success!!  Till next time J