Carbon Isotopes ratios (δ 13C) for tracking ocean carbon cycling
Oceanic fronts, analogous to atmospheric fronts, form where different water masses meet. They are characterised as zones of enhanced biological activity. Thus, waters tend to be rich in organic matter, with implications at all scales of the food chain, including fisheries. One such example, the Almeria-Oran front, is found in the south-western Mediterranean Sea where surface waters of Atlantic origin meet more saline Mediterranean water. This front exhibits complex permanent hydrological structures, essentially represented at the surface by a main current along the front (a jet) that is associated with an anticyclonic gyre of Atlantic waters.
Figure 1 Study area of Almofront project and schematic representation of the different hydrodynamic structures of the Almeria-Oran front (Mediterranean Sea). SeaWIFS satellite imagery of surface chlorophyll a, adapted from JRC of E.C., IMW Unit, IES Institute, Frédéric Mélin (blue to red: low to high chlorophyll a concentrations).
Marine biogeochemical studies of particulate matter are fundamental for understanding carbon cycling, and assessing the sources and fate of the organic matter. The lipid biogeochemistry of suspended and sinking particles collected in the Almeria-Oran frontal zone was investigated in the IAEA Marine Environmental Studies Laboratory to determine the sources and transformation processes of organic matter in relation to the physical forcing of the front. This was a joint project with French colleagues from the Laboratoire d'Océanographie de Villefranche-sur-mer.
Techniques used included the recently developed Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometry (GC-C-IRMS) for the determination of the δ 13C of specific lipid biomarkers. The results can be used to distinguish different sources of carbon (e.g., marine, terrestrial, bacterial), and elucidate the transport and degradation processes of organic material through the water column. In this case, data illustrated the influence
of the currents on the composition and transport of particles. A minor terrestrial contribution was also observed, which remained after degradation of the more labile marine organic matter.
Figure 2 The GC-C-IRMS instrument to measure the stable carbon isotope ratios (δ 13C) of individual organic compounds.
The molecular abundance and carbon isotopic composition of lipid biomarkers allowed the characterization of three different areas in the Almeria-Oran front:
i. The Mediterranean zone dominated by moderately degraded phytoplanktonic communities exhibited low zooplanktonic biomass and comparatively high bacterial activity.
ii. The upstream side of the gyre and the jet were characterized by relatively degraded phytoplankton material with high bacterial activity and an enhanced zooplankton biomass.
iii. The downstream side of the gyre was characterized by a phytoplanktonic bloom subject to rather low zooplankton grazing and low bacterial activity.
This study contributed to better understanding the cycling of carbon in complex marine ecosystems, showing the fertilising effect that the frontal zones exhibit compared with its adjacent waters.
References
- Imma Tolosa, Isabelle Vescovali, Nathalie LeBlond, Jean-Claude Marty, Stephen de Mora & Louis Prieur (2004) Distribution of pigments and fatty acid biomarkers in particulate matter from the frontal structure of the Alboran Sea (SW Mediterranean Sea). Marine Chemistry 88, 103-125.
- Imma Tolosa, Nathalie LeBlond, Claire Copin-Montégut, Jean-Claude Marty, Stephen de Mora & Louis Prieur (2003) Distribution of sterol and fatty alcohol biomarkers in particulate matter from the frontal structure of the Alboran Sea (SW Mediterranean Sea). Marine Chemistry 82, 161-183.
