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Seabed Imprint of Dense Shelf Water Cascading

Periodic Reporting for period 1 - SIDEW (Seabed Imprint of Dense Shelf Water Cascading)

Reporting period: 2016-04-01 to 2018-03-31

SIDEW studies the past, present and future effects of dense shelf water cascading (DSWC) on the oceans seafloor. The formation of dense water contributes to the deep ocean ventilation, plays a role in the global thermohaline circulation (and hence global climate), and involves the massive transfer of energy and matter (including sedimentary particles, organic carbon and pollutants) from shallow to deep waters. The impact of this process on seafloor and deep-sea ecosystems is unquestionable. But how does DSWC form? DSWC starts when surface waters over the relatively shallow continental shelves become denser than surrounding waters, sink and cascades downslope along the seabed usually at high speeds. DSWC is a widespread phenomenon that occurs intermittently in time and space in European Seas (Arctic, Atlantic and Mediterranean) and around the globe, especially in high latitudes. The main mechanisms leading to the formation of DSW are cooling, evaporation, sea-ice freezing with brine rejection and deep sub-ice shelf melting in Antarctica.

DSWC is highly sensitive to temperature change in both the lower atmosphere and the sea surface, so global warming will likely modify the frequency and intensity of DSWC in the coming decades, which will in turn alter the biogeochemical transfers from shallow to deep and the ventilation of the deep ocean. This could significantly affect the deep-sea ecosystems whose functioning is critically dependent on this process. Overall, dense water formation is expected to decline over both continental shelves and offshore, particularly in polar and subpolar latitudes where sea-ice production is declining.

DSWC has been extensively studied by the physical oceanography community in the last decades. However, the far-reaching effects of these flows on the seafloor relief, the deep-sea ecosystem, the demersal fisheries, and the transfer of the climate signal, organic carbon, chemical pollutants and litter to the deep have been largely unexplored. Although DSWC is frequently invoked as a possible mechanism controlling seafloor morphology on the outer shelf and continental slope, there is a lack of studies that integrate concepts from geomorphology, sedimentology and oceanography. This shortcoming is particularly evident in high-latitude margins, despite the fact that the largest number of DSWC occurrences has been reported there.

In this project we have studied the global seafloor imprint of DSWC, with special emphasis on the sensitivity of this widespread process to changes in climate and in ice-sheet and sea-ice extent. A main contribution of the project is a better understanding of the physical processes involved in the propagation of these dense flows and their capacity to erode, transport and deposit sediment on continental shelves and slopes.
Despite the importance of DSW cascades there were previously no studies that systematically analyse cascade distribution in the context of what are considered to be their main producers in the polar regions (i.e. ice shelves and coastal polynyas) and favourable physiographic conditions (i.e. vast continental shelves). During the life of SIDEW we have addressed this shortcoming by reviewing nearly 50 years of hydrographic observations spanning the circum-Antarctic continental slope and thoroughly analysing their physiographic context.

The results obtained show a clear relationship between the distribution of DSW overflows around Antarctica and the presence of polynyas, which are ice-free zones generated by a combination of strong katabatic winds and physical barriers that block the inflow of sea ice. Conversely our study plays down the importance of large ice shelves or vast continental shelves, both previously considered to be necessary for the formation of large volumes of DSW.

To better understand how DSWC events of varying size and duration interact with variable seafloor topography and erodability, we have adapted a model initially developed to represent turbidity currents (Nixes-Tc model, developed at LGS-IFREMER). Several runs using varied sets of flow parameters, boundary conditions and generic terrain models allowed us to compare the differences in erosion and sediment transport between DSWC and turbidity currents.

In SIDEW we have also collaborated with researchers from Institute of Cosmos Sciences of the University of Barcelona (Spain) to adapt an algorithm (FAPEC), initially developed for Space data communications, to compress multibeam echosounder water column data. This tool will facilitate the efficient browsing, querying, sharing, storage and transfer of the rising volume of water column data being acquired by the modern echosounders. This might help to further study the physics of dense shelf water cascades and other water column phenomena.

We have presented the SIDEW project ideas and results in several conferences in UK, Canada, USA, Colombia, India, Spain and France. We have published them in journals addressed both to the broad and specialized public. Most of the contents and results of the project are now freely accessible through public data repositories. New free-licensed educational media content has been also created and it is now available for download in Wikipedia and Wikimedia Commons.
This project has enhanced the understanding of the formation and export of DSW and it highlights the variability and complexity of ice-ocean systems on high-latitude continental margins. From the results obtained we anticipate more findings of active DSW production in little studied regions, such as the East Antarctica margin.

The few decades of DSW cascading records reviewed in SIDEW show a disturbing reduction of DSW export in some areas in Antarctica that denotes their vulnerability in the face of rapid climate change.

We have also performed a systematic investigation of DSWC hydrodynamics and sediment transport by using a numerical model initially developed for turbidity currents that have been adapted to recreate DSWC in the frame of the project. This has broad our understanding on how DSWC events of varying size and duration interact with variable seafloor topography and erodibility. We are now able to compare the hydrodynamics and morphodynamics associated with these relatively dilute flows with other better-known sediment-laden currents such as turbidity currents.

Finally, we have promoted the development of a lossless and lossy tailored data compression solution for multibeam water column datagrams. This algorithm, which was initially developed for space research, will facilitate the study of multibeam-derived water column data by reducing the huge storage requirements inherent of this technique.

The new data and results obtained in the proposed project will extend European leadership on the study of DSWC and will represent an effective instrument to address the EU challenges in the Horizon 2020 programme.
Poster showing the main ideas behind the SIDEW project