Volcanic ashes from the Etna eruption (March, 2012) and the extreme cold from the previous winter, created an authentic shower of manna in the Ierapetra Basin (4.430 meters depth abyss) in one of the less productive marine environments of the Eastern Mediterranean Sea.
This is one of the main conclusions of the article published in the journal Geophysical Research Letters by the experts Rut Pedrosa-Pàmies, Anna Sanchez Vidal, Antoni Calafat and Miquel Canals, from the Research Group on Marine Geosciences (GRC), from the Faculty of Earth Sciences of the University of Barcelona, and a team from the Hellenic Centre for Marine Research (HCMR) in Crete (Greece).
Ierapetra Bassin, in the south-east of Crete and part of the Pliny-Strabo trench region is not the deepest place in the Mediterranean Sea -the Calypso Deep in the Ionic Sea covers 4.267 meters- but it is deeper than the ones in Western Europe (3.600 meters). The REDECO project, led by Nikolaos Lampadarious, from the Hellenic Centre for Marine Research, the team studied which processes enable the transport of organic matter and capture of atmospheric carbon in the abyssal depths of the Mediterranean Sea.
To carry this research out, the team of the GRC Marine Geosciences worked on a line set at 4.300 meters depth in Ierapetra -a technological and logistic challenge regarding the depth- with a sediment trap for particles and a current meter. From 2010 to 2013, they registered physical and biogeochemical conditions of Ierapetra Bassin. The study published in Geophysical Research Letters shows unpublished data about the origins, quantity and season variability and interannual flow of the organic matter in the Mediterranean Sea, between the surface and deep marine waters.
Volcanic ashes and extreme cold in the Mediterranean Sea
"The results show the oligotrophic character -low in nutrients- of Western Mediterranean. However, in March 2012, the mix of a cold winter with Etna's volcanic activity in Sicily created a sudden and massive phyloplankton growth (mostly diatom particles), the largest one in the last decades," says Professor Antoni Calafat, from the Department of Earth and Ocean Dynamics of the UB.
"This phenomenon -says Rut Pedrosa- created flows of organic matter higher than 12 milligrams per square meter and day. That is, a shower of manna two orders of higher magnitude than the usual flow in this marine environment which is extremely poor."
Submarine cascades: when cold waters sink to depth water
In some parts of the Mediterranean Sea, surface water masses get cold in the winter, and sink and enable the arrival of organic matter to abyssal areas. During the 2012 winter, which was quite cold in the Mediterranean, there were cases of cascading in the Gulf of Lion and the Adriatic Sea, and convections in the open sea in the area of the Rhodes Gyre. In that area, this intense convection provoked the rise of cold waters rich in nutrients, boosting the phyloplanktonic growth -especially diatom particles. This exceptional blooming was probably strengthened by the arrival of nutrients coming from the movement of the volcanic ashes from Etna's eruption in the spring of 2012. As a result, in April 2012 the organic carbon export increased up to fourteen times compared to April 2011 and 2012, which had a regular exportation in these marine areas.
According to Anna Sanchez-Vidal, "so far, nobody explained that the mix of convections due cold water surfaces and the arrival of nutrients due volcanic ashes is a factor that boosts the flow of organic matter in abyssal depths."
Ocean machinery enabling particle transport to the basins is linked to blooms, which have an essential role in trophic chains of marine ecosystems. "If this process has a sinking of dense waters, which carry organic matter to big depths, efficacy increases. Volcanic ashes, which ballast organic particles and bring them to abyssal depths without affecting their nutrition values, can boost results even more" says Rut Pedrosa.
Nutrient and carbon traps in marine depths
Biodiversity can decrease with the depths of these marine areas. Interestingly, this tendency is different in the unknown abyssal basins, according to previous studies by the scientific team. The hypothesis of the study sees the abyssal trenches as traps for organic matter in ocean depths. Carbon, carried to extreme depths until being isolated from cycles of active exchange with the atmosphere, would remain in these big submarine depressions. Moreover, the arrival of labile matter to bathyal zones represents an important contribution to carbon's remineralisation and oxygen reduction in deep waters.
This phenomenon described in the journal Geophysical Research Letters could happen in other abyssal trenches worldwide, in marine regions with seismic and volcanic activity, and processes of dense water formation. The process of cascading occurs in seas and oceans around the world, and it was described in submarine canyons in North-Western Mediterranean (Gulf of Lion, 2006) in an article of the journal Nature, whose main author was Professor Miquel Canals, Head of the GRC Marine Geosciences of the University of Barcelona.
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