DOGEE-SOLAS: The UK SOLAS Deep Ocean Gas Exchange Experiment Matt Salter

Overview Why is gas exchange important? What determines gas exchange? What can we measure? What controls k w ? The problem This project- the solution? What will we do? The tracer release Surfactants Further aims

Why is gas exchange important? Gas exchange is an important term in many biogeochemical cycles but remains a major uncertainty Many important issues require accurate estimates of gas exchange rates CO 2 uptake by oceans Marine source of major greenhouse gases such as nitrous oxide and methane Climate forcing involving DMS and iodocarbons

What determines gas exchange? Gas exchange is determined by: 1. the concentration difference between the atmosphere and the surface ocean. This drives the flux of gases 2. a kinetic transport term known as the gas transfer velocity, k w which is a function of the interfacial turbulence ΔCΔC

So what can we measure? Measuring the concentration driven gas flux is relatively easy However k w can only be estimated indirectly hence quantifying it is problematic K w is a function of turbulence resulting from complex interactions between several forcings…

The controls Wave geometry Sea surface roughness Wave breaking Bubbles RainfallSurfactants As many of the controls appear largely wind driven it was logical to derive simple empirical relationships for k w based upon wind speed…

But...

The problem Non-linear nature Strong divergence between predicted k w s, especially at high wind speeds No agreement if any of the relationships are correct It would seem that using wind speed alone to predict k w is impossible U 10 Wind speed ( m s -1 ) K 600 (cm h -1 ) WM 99 W92 N2000 LM86 Figure 1. k w vs wind speed parameterisations where k 600 is a normalised value of k w

So...

We will measure k w We will utilise a dual tracer method to obtain indirect estimates of k w at sea during two cruises in the North Atlantic in December 06 and May 07

The tracer release Involves a simultaneous release of two inert, non-toxic gaseous tracers SF 6 and 3 He The two gases are dissolved in a gas-tight tank of water through a headspace SF 6 3 He The ratio of SF 6 concentration to 3 He is measured

The tracer release The tracers are then displaced by pumping seawater into the tank and are deployed at the release site Seawater

The tracer release The tracers then become diluted due to horizontal and vertical mixing and loss through air-water exchange SF 6 and 3 He mix horizontally and vertically However 3 He diffuses more rapidly than SF 6 across the air-water interface Thus values for k w can be calculated from the change in ratio of the two tracers over time

The effect of surfactants On the second cruise we plan to release two tracer patches in close proximity, one of which will be labeled with a surfactant (a surrogate for natural sea surface surfactants) Tracer patch without surfactantTracer patch labeled with surfactant This will mimic the role of natural surfactants in modifying gas transfer We will compare the gas exchange data for the two patches- the first time this will have been attempted in the field

The effect of surfactants I would also like to quantify and characterise natural surfactants sampled from the microlayer as much as possible during the cruise I hope to achieve this using polarography, an electrochemical method

A.C. Polarography Surfactants reduce the capacitance C formed by the Hg surface and the electrolytic double layer A relationship exists between the surfactant concentration c in the electrolyte and the density of the deposited material, and therefore also between c and C This is expressed by an adsorption isotherm, E vs C

Summary and further aims We will measure k w indirectly during two cruises in the North Atlantic We will investigate the role of surfactants on k w in the field During the same cruises, other groups will be investigating the role of other controlling variables on k w In the end, we hope to combine all this data in order to gain a clearer picture of the controls on k w

Acknowledgments NERC PML for use of their facilities Professor Rob Goddard Dr Phil Nightingale, PML Dr Jo Dixon,PML Dr Alex Baker, UEA