Carbon Cycling and Biogases Sub-Project

Objectives

To describe, quantify and model air-sea exchange processes for climatically and biogeochemically important elements such as CO₂, CH₄, dimethylsulphide and their oxidation products.

To study the physical parameters that underline the mechanisms controlling the gaseous fluxes in the marine atmospheric surface layer.

To estimate the importance of different carbon species to the overall carbon budget at the shelf edge.

Air-Sea Exchange Processes

The exchange of biogases between the ocean and the atmosphere is of major importance for the earth climate as some of them strongly affect the world’s radiation balance, i.e. they act as so called “greenhouse gases”.

About 40% of the CO₂ produced by fossil fuel burning is believed to be taken up by the ocean. Sulphur gases are produced in the upper surface layer of the ocean and escape to the atmosphere, affecting cloud formation and hence the earth's albedo. The biogeochemical processes at the shelf edge control the abundance of these trace gases. Hence, estimating fluxes of biogases at the ocean margin requires high spatial and temporal resolution.

During OMEX I, concentrations of carbon dioxide, dimethylsulphide and its precursors, carbonyl sulphide and methane were measured in both the water column and the atmosphere in order to quantify this.

Gaseous Flux Mechanisms

The net flux of gases between the air and the sea depends upon the partial pressure difference and the resistance at the interface, the latter depending on physical conditions such as wind speed, sea surface temperature and whitecaps (bubbles). There are two approaches to estimate this flux. One involves direct observations of atmospheric fluctuations over the sea surface; the other is to measure the concentration gradient and parameters controlling the exchange velocity. Both techniques were employed during the OMEX I project.