Posted by sogasex on April 3, 2008
By Byron Blomquist, University of Hawaii
Oceans and forests are the lungs of our planet. Oxygen that makes life possible for animals (like us) was originally produced by the first microscopic plants in ancient oceans. We rely on green plants to sustain us. And as they exhale oxygen they inhale carbon dioxide, converting it to wood, leaves and the carbonate shells of marine plankton. Some of this carbon is returned to the atmosphere as CO2 through respiration when bacteria, fungi and animals feed on plants and organic matter. A small amount settles into long term storage as coal, oil and chalk deposits. This in brief is the system we call the carbon cycle, and the ocean surface is part of the planetary lung, like the lungs in our bodies, that carbon transits during its cycle.
It has been our goal over the past few weeks to examine a patch of our planet’s lung and observe the details of gas exchange between the ocean and atmosphere, to better understand how our planet “breathes”. Ultimately, we would like to accurately predict when, where, and how much CO2 (or dimethylsulfide, DMS) passes through the ocean surface, since this information is critical to understanding how the climate system functions and to predicting how it may change in the future. But gas exchange is controlled or influenced by numerous physical processes like wind stress, ocean currents, temperature and, in the case of CO2 and DMS, by biological activity in the surface ocean, which itself is modulated by nutrients, seasonal cycles, sunlight, ocean currents, population dynamics, etc. Unravelling the mystery is more than any one of us can hope to achieve alone or more than any one group of scientists can achieve in a single study, but it keeps us focused to have the big picture in mind as we labor in the trenches of our sub-disciplines.
Those of us involved in observing atmospheric flux – the rate at which gases are going into and out of the ocean – have managed to keep our feet dry and our hands warm so far. Our daily routine, between eating and sleeping, consists of monitoring our sometimes finicky instruments and coping with an avalanche of data streaming at 10-20 samples a second per channel, 24 hours a day. We sift through the accumulating gigabytes, identifying and removing the bad data (typically occurring when wind blows from behind, sending the ship’s exhaust and vapors from the galley’s deep frier to our instruments on the bow). Then, from small turbulent variations in gas concentration and wind velocity, aided by considerable mathematical manipulation, we can observe the rate of gas exchange.
The graph above, called a covariance spectrum, summarizes an hour of DMS and wind data. It shows us the flux of DMS is upward – that is, it comes out of the ocean – because the points on the curve are positive. And the sweep of the jagged curve reveals the flux is carried on turbulent eddies at frequencies from 0.002 to 2 Hz (or eddies from roughly 5 meters to more than a kilometer in size) and the dominant frequency is about 0.1 Hz (100 meters). The area under the curve is the flux – in this case about 370 micrograms of DMS per square meter per day. We have seen the ocean “breathe”, and in collaboration with our friends and colleagues on SO GasEx we may be able to discover some details of how this “breathing” actually works.