Quantification of surface water volume changes in the Mackenzie Delta using satellite multi-mission data

Quantification of surface water storage in extensive floodplains and their dynamics are crucial for a better understanding of global hydrological and biogeochemical cycles. In this study, we present estimates of both surface water extent and storage combining multi-mission remotely sensed observations and their temporal evolution over more than 15 years in the Mackenzie Delta. The Mackenzie Delta is located in the northwest of Canada and is the second largest delta in the Arctic Ocean. The delta is frozen from October to May and the recurrent ice break-up provokes an increase in the river's flows. Thus, this phenomenon causes intensive floods along the delta every year, with dramatic environmental impacts. In this study, the dynamics of surface water extent and volume are analysed from 2000 to 2015 by combining multi-satellite information from MODIS multispectral images at 500 m spatial resolution and river stages derived from ERS-2 (1995–2003), ENVISAT (2002–2010) and SARAL (since 2013) altimetry data. The surface water extent (permanent water and flooded area) peaked in June with an area of 9600 km2 (±200 km2) on average, representing approximately 70 % of the delta's total surface.\ud Altimetry-based water levels exhibit annual amplitudes ranging from 4 m in the downstream part to more than 10 m in the upstream part of the Mackenzie Delta. A high overall correlation between the satellite-derived and in situ water heights (R > 0.84) is found for the three altimetry missions. Finally, using altimetry-based water levels and MODIS-derived surface water extents, maps of interpolated water heights over the surface water extents are produced. Results indicate a high variability of the water height magnitude that can reach 10 m compared to the lowest water height in the upstream part of the delta during the flood peak in June. Furthermore, the total surface water volume is estimated and shows an annual variation of approximately 8.5 km3 during the whole study period, with a maximum of 14.4 km3 observed in 2006. The good agreement between the total surface water volume retrievals and in situ river discharges (R =  0.66) allows for validation of this innovative multi-mission approach and highlights the high potential to study the surface water extent dynamics

Initiation and evolution of knickpoints and their role in cut-and-fill processes in active submarine channels

Submarine channels are the main conduits and intermediate stores for sediment transport into the deep sea, including organics, pollutants, and microplastics. Key drivers of morphological change in channels are upstream-migrating knickpoints whose initiation has typically been linked to episodic processes such as avulsion, bend cutoff, and tectonics. The initiation of knickpoints in submarine channels has never been described, and questions remain about their evolution. Sedimentary and flow processes enabling the maintenance of such features in non-lithified substrates are also poorly documented. Repeated high-resolution multibeam bathymetry between 2012 and 2018 in the Capbreton submarine canyon (southeastern Bay of Biscay, offshore France) demonstrates that knickpoints can initiate autogenically at meander bends over annual to multi-annual time scales. Partial channel clogging at tight bends is shown to predate the development of new knickpoints. We describe this initiation process and show a detailed morphological evolution of knickpoints over time. The gradients of knickpoint headwalls are sustained and can grow over time as they migrate through headward erosion. This morphology, associated plunge pools, and/or development of enhanced downstream erosion are linked herein to the formation and maintenance of hydraulic jumps. These insights of autogenically driven, temporally high-frequency knickpoints reveal that cut-and-fill cycles with depths of multiple meters can be the norm in submarine systems

Quantification of surface water volume changes in the Mackenzie Delta using satellite multi-mission data

International audienceQuantification of surface water storage in extensive floodplains and their dynamics are crucial for a better understanding of global hydrological and biogeochemical cycles. In this study, we present estimates of both surface water extent and storage combining multi-mission remotely sensed observations and their temporal evolution over more than 15 years in the Mackenzie Delta. The Mackenzie Delta is located in the northwest of Canada and is the second largest delta in the Arctic Ocean. The delta is frozen from October to May and the recurrent ice break-up provokes an increase in the river's flows. Thus, this phenomenon causes intensive floods along the delta every year, with dramatic environmental impacts. In this study, the dynamics of surface water extent and volume are analysed from 2000 to 2015 by combining multi-satellite information from MODIS multispectral images at 500 m spatial resolution and river stages derived from ERS-2 (1995-2003), ENVISAT (2002-2010) and SARAL (since 2013) altimetry data. The surface water extent (permanent water and flooded area) peaked in June with an area of 9600 km2 (±200 km2) on average, representing approximately 70 % of the delta's total surface. Altimetry-based water levels exhibit annual amplitudes ranging from 4 m in the downstream part to more than 10 m in the upstream part of the Mackenzie Delta. A high overall correlation between the satellite-derived and in situ water heights (R > 0.84) is found for the three altimetry missions. Finally, using altimetry-based water levels and MODIS-derived surface water extents, maps of interpolated water heights over the surface water extents are produced. Results indicate a high variability of the water height magnitude that can reach 10 m compared to the lowest water height in the upstream part of the delta during the flood peak in June. Furthermore, the total surface water volume is estimated and shows an annual variation of approximately 8.5 km3 during the whole study period, with a maximum of 14.4 km3 observed in 2006. The good agreement between the total surface water volume retrievals and in situ river discharges (R= 0.66) allows for validation of this innovative multi-mission approach and highlights the high potential to study the surface water extent dynamics

Carbonate slope morphology revealing a giant submarine canyon (Little Bahama Bank, Bahamas)

International audienceNew high-quality multibeam data detail the morphology of the giant 135-km-long Great Abaco Canyon (GAC) located between Little Bahama Bank (LBB, Bahamas) and Blake Plateau. Knickpoints, chutes, and plunge pools mark the canyon main axis, which is parallel to the LBB margin. The canyon head covers a large area but does not represent the main source of the modern sediments. The material supplied through the LBB canyon systems originates below this head, which only shows erosive lineaments related to the pathway of currents running along the seafloor and restricted failure scars. Most of the sediment supply originates from the canyon sides. The northern canyon flank incises the Blake Plateau, which comprises contourites on top of a drowned Cretaceous carbonate platform. These deposits are susceptible to translational slides and form dissymmetric debris accumulations along the northern edge of the canyon. A large tributary drains the Blake Plateau. Two large tributaries connecting the southern flank of the GAC directly to the LBB upper slope form additional sources of sediments. Subbottom profiles suggest the presence of a sedimentary levee on the tributary canyon and of sediment gravity flow deposits. The GAC has been a permanent structure since the drowning of the Cretaceous platform, and its size and morphology are comparable to those of canyons in siliciclastic environments. The orientation of the GAC parallel to large-scale regional tectonic structures suggests a structural control. The size of the observed structures, especially plunge pools at the base of gigantic chutes, is unusual on Earth. The presence of deposits downflow of the pools suggests that the GAC results from or at least is maintained by persistent and sustained submarine gravity flows rather than by retrogressive erosion

Into the deep: A coarse-grained carbonate turbidite valley and canyon in ultra-deep carbonate setting

International audienceNew high-resolution multibeam mapping images detail the southern part of Exuma Sound (Southeastern Bahamas), and its unchartered transition area to the deep abyssal plain of the Western North Atlantic, bounded by the Bahama Escarpment extending between San Salvador Island and Samana Cay. The transition area is locally referred to as Exuma Plateau. The newly established map reveals the detailed and complex morphology of a giant valley draining a long-lived carbonate platform from its upper slope down to the abyssal plain. This giant valley extends parallel to the slope of Long Island, Conception Island, and Rum Cay. It starts with a perched system flowing on top of a lower Cretaceous drowned main carbonate platform. The valley shows low sinuosity and is characterized by several bends and flow constrictions related to the presence of the small relict isolated platforms that kept alive longer than the main platform before drowning and merging tributaries. Turbidite levees on either side of the valley witness the pathway of multiple gravity flows, generated by upper slope over steepening around Exuma Sound through carbonate offbank transport, some of them locally >15°, and resulting slumping. In addition, additional periplatform sediments are transported to the main valley through numerous secondary slope gullies and several kilometre-long tributaries, draining the upper slopes of cays and islands surrounding Exuma Plateau. Some of them form knickpoints indicating surincision of the main Exuma Valley which is consistent with an important lateral supply of the main Exuma Valley. Prior to reaching the abyssal plain, the main valley abruptly evolves into a deep canyon, 5 km in width at its origin and as much as 10 km wide when it meets the abyssal plain, through two major knickpoints named “chutes” with outsized height exceeding several hundred of meters in height. Both chutes are associated with plunge pools, as deep as 200-m. In the deepest pools, the flows generate a hydraulic jump and resulting sediment accumulation. When the canyon opens to the San Salvador abyssal plain, the narrow, deep, and strong flows release significant volume of coarse-grained calcareous sediments in numerous turbidite layers interbedded with fine mixed siliciclastic and carbonate sediments transported by the Western Boundary Undercurrent (WBUC) along the Bahama Escarpment. Carbonate gravity flows exiting the canyon decelerate at the abyssal plain level and construct a several-kilometre-wide coarse-grained deep-sea turbidite system with well-developed lobe-shape levees, partially modified by the flow of strong contour-currents along the Bahama Escarpment

Knickpoints and crescentic bedform interactions in submarine channels

Submarine channels deliver globally important volumes of sediments, nutrients, contaminants and organic carbon into the deep sea. Knickpoints are significant topographic features found within numerous submarine channels, which most likely play an important role in channel evolution and the behaviour of the submarine sediment-laden flows (turbidity currents) that traverse them. Although prior research has linked supercritical turbidity currents to the formation of both knickpoints and smaller crescentic bedforms, the relationship between flows and the dynamics of these seafloor features remains poorly constrained at field-scale. This study investigates the distribution, variation and interaction of knickpoints and crescentic bedforms along the 44km long submarine channel system in Bute Inlet, British Columbia. Wavelet analyses on a series of repeated bathymetric surveys reveal that the floor of the submarine channel is composed of a series of knickpoints that have superimposed, higher-frequency, crescentic bedforms. Individual knickpoints are separated by hundreds to thousands of metres, with the smaller superimposed crescentic bedforms varying in wavelengths from ca 16m to ca 128m through the channel system. Knickpoint migration is driven by the passage of frequent turbidity currents, and acts to redistribute and reorganize the crescentic bedforms. Direct measurements of turbidity currents indicate the seafloor reorganization caused by knickpoint migration can modify the flow field and, in turn, control the location and morphometry of crescentic bedforms. A transect of sediment cores obtained across one of the knickpoints show sand–mud laminations of deposits with higher aggradation rates in regions just downstream of the knickpoint. The interactions between flows, knickpoints and bedforms that are documented here are important because they likely dominate the character of preserved submarine channel-bed deposits