Water erosion shapes landscapes, with groundwater instrumental in creating ‘theatre-shaped’ valleys and canyons.
While groundwater is believed to have shaped seafloors in coastal areas known as continental margins, the link between groundwater erosion and these landscapes has typically been based on onshore studies.
“We need to re-evaluate the evidence of groundwater’s role in offshore geomorphology in general and canyon erosion in particular, especially at the continental margin,” says Aaron Micallef, project coordinator of the EU-funded MARCAN project, supported by the European Research Council.
Continental margins make up almost one fifth of the ocean floor, host offshore resources and geohazards, and are an archive of past climatic and geological activity. Submarine canyons, which cut into 20 % of continental margins, are important biodiversity hotspots, regulating oceanographic processes and offering a record of past hydrocarbon reservoirs.
MARCAN uses a combination of geophysical measurements and borehole data to develop 3D groundwater and landscape models to better understand groundwater changes over time.
“Our insight was that ignoring the impact of sea level and groundwater fluctuations has likely led to overly simplified, possibly incorrect, concepts of continental margin evolution,” explains Micallef. “And our findings about the role groundwater plays in seafloor evolution highlights its importance in assessing hazards such as submarine landslides.”
Sea levels – the secret to more accurate modelling
When groundwater, replenished by rain, flows along the gradients of the landscape, it is known as topographically driven meteoric (TDM) flow.
MARCAN’s modelling was inspired by the realisation that continental margins, past and present, can only be understood when viewed against lower sea levels and TDM groundwater systems that extend offshore.
This is because for 80 % of the last 2.5 million years, sea levels have been lower than today. Lower levels would leave more of the continental shelf exposed, reduce sea water pressure, and create more potential groundwater energy due to higher elevations. Crucially, the larger area available for rain to recharge groundwater also creates the ideal conditions for groundwater to extend beyond present coastlines.
This has profound effects for the local topology. “Our simulations show that at lower sea levels TDM recharge must have created extensive water tables and massive groundwater fluxes. This would have generated high pressures in sediments across the continental shelf and upper slopes, making them more susceptible to collapse,” says Micallef.
Offshore groundwater systems, driven by TDM recharge, are known to create reservoirs of good quality fresh and brackish water with estimated global volumes equivalent to about 500 000 km2.
The team collected its geophysical data from the waters and coastlines of New Zealand and Malta.
The project confirmed that TDM recharge during low sea levels does appear to be the key global mechanism for delivering groundwater to offshore reservoirs. In New Zealand, where the team have assembled one of the best groundwater data sets globally, this is likely to have happened over the course of 300 000 years, during the last three glacial cycles.
The team also found strong evidence of a reservoir of groundwater offshore from Malta, especially significant given that this archipelago is one of the world’s poorest countries for water resources per inhabitant.
Water and the marine environment are priorities for the EU, as demonstrated by the Water and Marine Strategy Directives. MARCAN contributes environmental baseline data, scientific knowledge, observational tools and quantitative models to help decision makers design the best environmental policies, while sustainably tapping valuable offshore freshwater resources.
Sectors relying on seafloor engineering, carbon dioxide sequestration, alongside ore deposit and petroleum exploration, will also be interested in MARCAN’s work as it increases knowledge about past climate and hydrological changes and helps refine hazard assessments. MARCAN could even help with assessments about the habitability of extreme environments, such as the deep seafloor and even other planets.
“We are now looking at how groundwater flow changes seafloor material down to its sediment particles, with encouraging preliminary results. But a step change in knowledge requires hydrogeological drilling for accurate sampling and testing. We have a proposal pending with the International Ocean Discovery Program to start this,” concludes Micallef.