Galicia has one of the most complex coastal environments in the Iberian Peninsula and the peculiar characteristics of its estuaries, valleys flooded approximately 18,000 years ago, arouse great scientific interest, being the subject of numerous studies. In this context, Alexandra María Ramírez’s doctoral thesis fits, a detailed study of the gas fields in the sediments of the Vigo estuary that puts on the table a reality unknown until now and that suggests that the dynamics of oxidation-reduction of sediments containing methane gas is not as simple as previously thought, but that there are more complex cycles in which elements such as sulfur, iron and manganese intervene in the oxidation of methane in the absence of oxygen, thus preventing this methane from being released into the hydrosphere or atmosphere and contributing to global warming of the planet.
Organic matter deposited in the bottoms contributes to the formation of methane
The Vigo estuary is characterized by being a large submerged funnel-shaped valley, with an area of 176 square kilometers, a maximum length of 33 km and a variable width that narrows from the Cíes Islands to the Rande Strait. Characterized by the high productivity of its waters, this place has one of the largest concentrations of mussel beds, which causes a high deposition of fine-grained materials with high organic matter contents on its bottoms. “The degradation of this organic matter together with the high sedimentation rates consumes dissolved oxygen and generates the development of anoxic conditions -oxygen deficiency- which are very favorable for the formation of methane that accumulates or escapes from the sediments towards the hydrosphere and the atmosphere, contributing to the greenhouse effect”, explains the researcher, who has a degree in Environmental Sciences from the University of Córdoba and holds a Master’s degree in Terrestrial Ecosystems, Sustainable Use and Environmental Implications taught at the Faculty of Sciences on the Ourense campus.
Under these anoxia conditions, a series of oxidation-reduction reactions take place in the sedimentary column in which elements such as sulfur, iron and manganese intervene and can affect the dynamics of methane. “Iron and manganese are elements extremely sensitive to oxidation-reduction processes and have the capacity to act both in the degradation of organic matter and in the oxidation of methane,” emphasizes Ramírez.
Journals of high scientific prestige have already echoed this study
In previous work carried out by the Basan research group of the Marine Research Centre of the University of Vigo (CIM-UVIGO), it had already been observed that the sediments of the Vigo estuary harbor methane and different oceanographic campaigns allowed the extent of these gas fields to be delimited, estimating that there were gas fields with areas exceeding 20 square kilometres. “Since methane is a powerful greenhouse gas, 25 times greater than carbon dioxide according to the IPCC (2001), it is necessary to know both the processes of formation and oxidation of this compound and to this end an oceanographic campaign was carried out in 2012 in which the sediment samples that led to the completion of my thesis were collected”, explains the author. In this work, led by the researchers of the CIM-UVIGO, Esther de Blas and Soledad García-Gil, sediments were collected from the seabed to study the behaviour of methane and the influence of sulphur and iron on the dynamics of methane. “This allowed me to obtain valuable information about the production mechanisms and, above all, the oxidation of methane in marine sediments,” emphasizes Ramirez.
Until a few years ago, most studies indicated that there was a vertical zoning of the oxidation-reduction processes of these elements in sediments. However, in this study, already published in highly prestigious journals, and contrary to what was thought, it was observed that in the sediment column of the Vigo estuary this vertical classification is not so strict and different oxidation-reduction processes can coexist in the same place.
Source: DUVI
