Searching for dense water cascades in the Arctic Ocean

The initiative is part of the HERMIONE project (Hotspot Ecosystem Research and Man's Impact on European Seas), organized under the Seventh EU Framework Programme, which analyses deep-sea ecosystems and the impact of human activity on the ocean floor.

Dense water cascades, formed as surface waters cool or evaporate, quickly transfer matter and energy to the ocean floor in a recurring process that carries oxygen and nutrients to deep-sea areas. However, if surface waters do not cool sufficiently, due to global warming or other factors, the process may stall, affecting the fragile equilibrium of deep-see ecosystems. "Thousands of metres below the surface, the cascading mechanism is yet more proof of the creeping influence of climate change," explains Miquel Canals, head of the UB's Marine Geosciences Research Group and leading author of an article that describes this phenomenon as observed in the north-western Mediterranean (Nature, 2006).

Mooring lines in the ocean depths

The Arctic Ocean is a strategic location for the study of cascading. Aboard the RV Jan Mayen, a specialist research ship operated by the University of Tromsø (Norway), the geologists Antoni Calafat, Anna Sànchez-Vidal and Ruth Duran from the Department of Stratigraphy, Paleontology and Marine Geosciences have installed a series of sophisticated instruments on the ocean floor to record data on dense water cascades and assess their overall impact on the marine ecosystem and deep-sea areas. "Our aim is to understand the dynamics of cascading in polar latitudes and to study environmental changes that the phenomenon could bring about on the ocean floor. To obtain data, we have installed four mooring lines with current meters and sediment traps at depths of 1000, 1250, 1500 and 2000 metres," says oceanographer Anna Sànchez-Vidal.

The equipment will record oceanographic and geochemical data at regular intervals and is scheduled to be retrieved at the end of the summer in 2011. Sànchez-Vidal explains that, "the data will give us a time series of measurements showing the properties of water masses at different times (speed and direction of current, temperature, salinity, turbudity, etc.) and the sediment transport profile." For the first time in an Arctic study, data will be complemented by specific examination of microorganisms, which are reliable indicators of environmental changes in deep-sea ecosystems.

The Mediterranean versus the Arctic

The process of cascading has been widely observed by scientists in the Mediterranean. However, the Arctic presents a different series of conditions. As Antoni Calafat explains, "The surface of the Arctic Ocean is divided between one part that remains frozen throughout the year and another, much larger part that freezes during the winter, which leads to a different pattern of cascading. Ice is a good thermal insulator. In addition, in the Arctic we also find polynyas, which are areas of open water surrounded by surface ice, where the wind cools surface water masses and accelerates the formation of dense water. However, this process is dependent on seasonal conditions and can vary from year to year. The relief of the ocean floor is also different in the Arctic to that of the Mediterranean Basin, and the cascading process could drag large quantities of organic matter to deeper areas."

The relief of the ocean floor also affects current dynamics during cascading. As oceanographer Ruth Duran explains, "The morphological parameters of the Svalbard Islands are very different to those of the Mediterranean Basin. We know that morphology, as in the case of Cap de Creus, determines the intensity and direction of currents in the Mediterranean. So during the expedition we produced detailed maps of the ocean floor in the study area -- covering some 2600 km² - that had not been fully charted until then, and this enabled us to determine the precise locations to install the mooring lines."

Cascading is a process that has a particular impact in polar areas and high latitudes. Experts studying the Mediterranean have identified a relationship between cascading and the biological productivity of the red shrimp (Aristeus antennatus). Could cascading have a similar biological effect in other areas of the planet? Research carried out by the Marine Geosciences group, which has worked extensively in the Antarctic, will reveal new information about dense water cascades, climate change and its impact on deep-sea ecosystems. The oceanography study carried out on board the RV Jan Mayen during the summer of 2010 was directed by the expert Jurgen Mienert (University of Tromsø) and contributed to by teams led by Roberto Danovaro (CoNISMa, Polytechnic University of Marche, Italy), Serge Heussner (CEFREM-University of Perpignan, France), Joan Grimalt (IDAEA, CSIC, Spain) and Leonardo Langone (ISMAR-Bologna, Italy), as well as other noted experts.