Periodic Reporting for period 5 - MARCAN (Topographically-driven meteoric groundwater – An important geomorphic agent)
Reporting period: 2022-01-01 to 2022-12-31
a. What is the problem/issue being addressed?
Groundwater has been implicated as a factor in landscape development both onshore and offshore. However, this association has been the subject of considerable debate. Experiments and numerical models studying groundwater erosion have been based on simple assumptions, whilst observations of groundwater processes in the field and the laboratory are lacking. As a result, we do not have a good understanding of how groundwater can erode sediments and rocks. This is particularly the case for seafloor settings.
For 80% of the last 2.6 million years, sea levels have been lower than today, and groundwater systems extended beyond today’s coastlines. Remnants of these systems have been documented in many parts of the world’s seafloor. We know very little about the characteristics of this offshore groundwater because most of the information we have is derived from incidental and sporadic discoveries during drilling campaigns. Since offshore groundwater systems are rarely in equilibrium with sea levels, our understanding of seafloor evolution by groundwater processes can only be understood in terms of lower sea levels and more extensive groundwater systems than we have today.
b. What are the overall objectives?
MARCAN addresses the hypothesis that groundwater plays a key role in the seafloor evolution. The objectives of MARCAN are to:
(i) Quantify the characteristics of offshore groundwater systems.
a. Define the 3D geometry, extent and characteristics of offshore groundwater systems and their evolution in response to changing sea levels through time.
b. Determine how offshore groundwater systems and their characteristics are controlled by seafloor and sub-seafloor structures.
(ii) Demonstrate that offshore groundwater is an important factor in seafloor evolution.
a. Identify and quantify the processes through which groundwater erodes sediments and rocks.
b. Evaluate if, and under which conditions, offshore groundwater can modify seafloor landscapes.
c. Why is it important for society?
Offshore groundwater systems are clearly of interest as a source of drinking water, especially in islands and densely populated coastal regions. MARCAN has contributed new tools and knowledge to make the EU a leader in the assessment and environmentally-sustainable exploitation of these valuable offshore resources. Sectors involving seafloor engineering, carbon dioxide sequestration, and ore deposit and petroleum exploration will also have a direct interest in MARCAN because it places better constraints on how fluids have flowed in the past and improve risk assessments. MARCAN also contributes essential environmental baseline data, scientific knowledge, observational tools and quantitative models that will help decision-makers on their design of environmental policy and management of water resources.
Groundwater has been implicated as a factor in landscape development both onshore and offshore. However, this association has been the subject of considerable debate. Experiments and numerical models studying groundwater erosion have been based on simple assumptions, whilst observations of groundwater processes in the field and the laboratory are lacking. As a result, we do not have a good understanding of how groundwater can erode sediments and rocks. This is particularly the case for seafloor settings.
For 80% of the last 2.6 million years, sea levels have been lower than today, and groundwater systems extended beyond today’s coastlines. Remnants of these systems have been documented in many parts of the world’s seafloor. We know very little about the characteristics of this offshore groundwater because most of the information we have is derived from incidental and sporadic discoveries during drilling campaigns. Since offshore groundwater systems are rarely in equilibrium with sea levels, our understanding of seafloor evolution by groundwater processes can only be understood in terms of lower sea levels and more extensive groundwater systems than we have today.
b. What are the overall objectives?
MARCAN addresses the hypothesis that groundwater plays a key role in the seafloor evolution. The objectives of MARCAN are to:
(i) Quantify the characteristics of offshore groundwater systems.
a. Define the 3D geometry, extent and characteristics of offshore groundwater systems and their evolution in response to changing sea levels through time.
b. Determine how offshore groundwater systems and their characteristics are controlled by seafloor and sub-seafloor structures.
(ii) Demonstrate that offshore groundwater is an important factor in seafloor evolution.
a. Identify and quantify the processes through which groundwater erodes sediments and rocks.
b. Evaluate if, and under which conditions, offshore groundwater can modify seafloor landscapes.
c. Why is it important for society?
Offshore groundwater systems are clearly of interest as a source of drinking water, especially in islands and densely populated coastal regions. MARCAN has contributed new tools and knowledge to make the EU a leader in the assessment and environmentally-sustainable exploitation of these valuable offshore resources. Sectors involving seafloor engineering, carbon dioxide sequestration, and ore deposit and petroleum exploration will also have a direct interest in MARCAN because it places better constraints on how fluids have flowed in the past and improve risk assessments. MARCAN also contributes essential environmental baseline data, scientific knowledge, observational tools and quantitative models that will help decision-makers on their design of environmental policy and management of water resources.
The first work package entailed collection of data during a number of field campaigns in New Zealand and the Maltese Islands. These included three oceanographic expeditions and five onshore expeditions, during which a range of geological, geophysical and geochemical data were acquired.
The second work package mapped offshore groundwater systems. Numerical models were constructed to estimate the characteristics and evolution of the offshore groundwater systems, and determine the influence of sub-seafloor structures.
During the third work package we carried out laboratory experiments to simulate the flow of groundwater through rocks and sediments, and its impact on their geotechnical properties. We also carried out field visits to observe changes in landscape associated with groundwater seepage.
In the fourth and final work package, 2D and 3D landscape evolution models were developed to assess whether offshore groundwater flow can generate mechanical instabilities in continental margins, and to define the characteristics and controls of associated seafloor landforms.
The second work package mapped offshore groundwater systems. Numerical models were constructed to estimate the characteristics and evolution of the offshore groundwater systems, and determine the influence of sub-seafloor structures.
During the third work package we carried out laboratory experiments to simulate the flow of groundwater through rocks and sediments, and its impact on their geotechnical properties. We also carried out field visits to observe changes in landscape associated with groundwater seepage.
In the fourth and final work package, 2D and 3D landscape evolution models were developed to assess whether offshore groundwater flow can generate mechanical instabilities in continental margins, and to define the characteristics and controls of associated seafloor landforms.
Below are the main results achieved during the MARCAN project.
1. Offshore freshened groundwater (OFG) review: OFG has a global volume of 1 million km3; it predominantly occurs within 55 km of the coast and down to a water depth of 100 m. OFG is mainly hosted within siliciclastic aquifers on passive margins and recharged by meteoric water during sea-levels lowstands.
2. OFG offshore New Zealand: We document an extensive OFG system with along-shelf variability in salinity offshore Canterbury. The majority of the OFG appears to have been emplaced via topographically driven flow during sea-level lowstands in the last 300 ka.
3. OFG offshore Malta: We identify a distinct resistivity anomaly offshore the SE coast of Malta, which is indicative of pore fluid freshening. Hydrogeological modelling suggests that emplacement of OFG took place during sea-level lowstands. Fluid is preserved in low permeability units.
4. Evolution of offshore groundwater system in the Canterbury Bight, New Zealand: Consideration of continental margin evolution provides a more accurate assessment of OFG resources.
5. Evolution of onshore-offshore groundwater system in the Maltese Islands: The volume of freshened groundwater offshore the Maltese Islands is estimated at 1 km3, which could potentially provide an alternative supply of potable water to the Maltese Islands for 75 years. A 30% decrease in recharge predicted in the coming 100 years will diminish OFG extent by 38% and onshore groundwater volume by 16%.
6. Impact of OFG flow at the fine scale: We document a ~50% decrease in undrained cohesive strength of seafloor sediment after flushing by OFG. Salt leaching can trigger slope failure when the thickness of the flushed layer is >3.5 m or when the slope gradient is >3°.
7. OFG and mechanical instabilities at continental margin scale: Conceptual model results show that mechanical instabilities by OFG flow are most likely to occur in the outer shelf to upper slope, at or shortly before the Last Glacial Maximum sea-level lowstand. Models with evolving stratigraphy show that OFG flow is a key driver of pore pressure development and instability in carbonate margins.
8. Seafloor erosional landforms formed by OFG flow: In siliciclastic margins, flow and seepage of OFG can generate landforms in the presence of buried channels. Depth of the shelf-break controls the type, location and timing of landform formation. In carbonate escarpments, box canyons can initiate and retrogressively evolve by fluid seeping via joints, which causes a reduction in rock strength due to fluid pressure and dissolution, resulting in periodic block failure.
10. Onshore analogues of groundwater flow and valley formation
Unconsolidated sediments: Gully formation is an episodic process associated to groundwater flow that occurs once every 227 d on average, when rainfall intensities exceed 40 mm d−1. Gullies can form at rates of up to 30 m d−1 via two processes – alcove and tunnel formation, followed by slope failure due to excess pore pressure development. The location of gullies is determined by the occurrence of hydraulically conductive zones.
Bedrock: Groundwater seepage can be the main driver of theatre-headed valleys (THV) formation in limestone. The inferred erosion mechanisms entail (1) widening of joints and fractures by fluid pressure and dissolution and (2) creeping of an underlying clay layer, which lead to slope failure at the valley head and its upslope retreat.
1. Offshore freshened groundwater (OFG) review: OFG has a global volume of 1 million km3; it predominantly occurs within 55 km of the coast and down to a water depth of 100 m. OFG is mainly hosted within siliciclastic aquifers on passive margins and recharged by meteoric water during sea-levels lowstands.
2. OFG offshore New Zealand: We document an extensive OFG system with along-shelf variability in salinity offshore Canterbury. The majority of the OFG appears to have been emplaced via topographically driven flow during sea-level lowstands in the last 300 ka.
3. OFG offshore Malta: We identify a distinct resistivity anomaly offshore the SE coast of Malta, which is indicative of pore fluid freshening. Hydrogeological modelling suggests that emplacement of OFG took place during sea-level lowstands. Fluid is preserved in low permeability units.
4. Evolution of offshore groundwater system in the Canterbury Bight, New Zealand: Consideration of continental margin evolution provides a more accurate assessment of OFG resources.
5. Evolution of onshore-offshore groundwater system in the Maltese Islands: The volume of freshened groundwater offshore the Maltese Islands is estimated at 1 km3, which could potentially provide an alternative supply of potable water to the Maltese Islands for 75 years. A 30% decrease in recharge predicted in the coming 100 years will diminish OFG extent by 38% and onshore groundwater volume by 16%.
6. Impact of OFG flow at the fine scale: We document a ~50% decrease in undrained cohesive strength of seafloor sediment after flushing by OFG. Salt leaching can trigger slope failure when the thickness of the flushed layer is >3.5 m or when the slope gradient is >3°.
7. OFG and mechanical instabilities at continental margin scale: Conceptual model results show that mechanical instabilities by OFG flow are most likely to occur in the outer shelf to upper slope, at or shortly before the Last Glacial Maximum sea-level lowstand. Models with evolving stratigraphy show that OFG flow is a key driver of pore pressure development and instability in carbonate margins.
8. Seafloor erosional landforms formed by OFG flow: In siliciclastic margins, flow and seepage of OFG can generate landforms in the presence of buried channels. Depth of the shelf-break controls the type, location and timing of landform formation. In carbonate escarpments, box canyons can initiate and retrogressively evolve by fluid seeping via joints, which causes a reduction in rock strength due to fluid pressure and dissolution, resulting in periodic block failure.
10. Onshore analogues of groundwater flow and valley formation
Unconsolidated sediments: Gully formation is an episodic process associated to groundwater flow that occurs once every 227 d on average, when rainfall intensities exceed 40 mm d−1. Gullies can form at rates of up to 30 m d−1 via two processes – alcove and tunnel formation, followed by slope failure due to excess pore pressure development. The location of gullies is determined by the occurrence of hydraulically conductive zones.
Bedrock: Groundwater seepage can be the main driver of theatre-headed valleys (THV) formation in limestone. The inferred erosion mechanisms entail (1) widening of joints and fractures by fluid pressure and dissolution and (2) creeping of an underlying clay layer, which lead to slope failure at the valley head and its upslope retreat.