DOGEE-SOLAS: The UK SOLAS Deep Ocean Gas Exchange

ExperimentMatt Salter

Overview

Why is gas exchange important?What determines gas exchange?What can we measure?What controls kw?The problemThis project- the solution?What will we do?

The tracer releaseSurfactants

Further aims

Why is gas exchange important?

Gas exchange is an important term in many biogeochemical cycles but remains a major uncertaintyMany important issues require accurate estimates of gas exchange rates

CO2 uptake by oceans

Marine source of major greenhouse gases such as nitrous oxide and methaneClimate 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, kw which is a function of the interfacial turbulence

ΔC

So what can we measure?

Measuring the concentration driven gas flux is relatively easyHowever kw can only be estimated indirectly hence quantifying it is problematic Kw is a function of turbulence resulting from complex interactions between several forcings…

The controls

Wave geometrySea surface roughnessWave breakingBubbles

RainfallSurfactants

As many of the controls appear largely wind driven it was logical to derive simple empirical relationships for kw based upon wind speed…

But...

The problem

Non-linear nature Strong divergence between predicted kw’s, especially at high wind speedsNo agreement if any of the relationships are correctIt would seem that using wind speed alone to predict kw is impossible

U10 Wind speed ( m s-1)K

600 (

cm h

-1)

WM99

W92N2000

LM86

Figure 1. kw vs wind speed parameterisations where k600 is a normalised value of kw

So...

We will measure kw

We will utilise a dual tracer method to obtain indirect estimates of kw 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 SF6 and 3HeThe two gases are dissolved in a gas-tight tank of water through a headspace

SF63He

The ratio of SF6 concentration to 3He 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

SF6 and 3He mix horizontally and vertically

However 3He diffuses more rapidly than SF6 across the air-water interface

Thus values for kw 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 surfactant

Tracer patch labeled with surfactant

This will mimic the role of natural surfactants in modifying gas transferWe will compare the gas exchange data for the two patches- the first time this will have been attempted in the field

The effect of surfactantsI would also like to quantify and characterise natural surfactants sampled from the microlayer as much as possible during the cruiseI 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 CThis is expressed by an adsorption isotherm, E vs C

Summary and further aims

We will measure kw indirectly during two cruises in the North AtlanticWe will investigate the role of surfactants on kw in the field

During the same cruises, other groups will be investigating the role of other controlling variables on kw

In the end, we hope to combine all this data in order to gain a clearer picture of the controls on kw

Acknowledgments

NERC

PML for use of their facilitiesProfessor Rob GoddardDr Phil Nightingale, PMLDr Jo Dixon,PMLDr Alex Baker, UEA