Deltaic deposits mapped along the Martian crustal dichotomy boundary scarp have been suggested to delineate an ancient ocean in the northern lowlands of Mars. Using recently acquired orbital data, we ...have expanded the dichotomy delta inventory and performed an updated analysis of delta front elevations, a proxy for paleo‐water levels. Our analysis focused near Gale crater, home of the Curiosity rover. We found that delta front elevations vary by approximately 2,400 m, but these elevation variations do not correspond to modeled deformation from true polar wander or Tharsis. Locally, delta front elevations vary by ≤60 m, and using present‐day topography, they correspond to distinct enclosed basins. We infer that these deltas formed in paleo‐lakes up to approximately 13,000 km2 and approximately 0.4 km deep, perhaps coeval with paleo‐lakes in Gale. Our results suggest that a northern ocean is not needed to explain the deltaic deposits in the Gale crater region.
Plain Language Summary
The climate of early Mars is debated. Climate modeling suggests that it was always cold and dry, while geologic evidence suggests that it was much wetter in the past, possibly supporting a northern ocean. Evidence in support of a northern ocean includes deltaic deposits along the margins of the northern lowlands. However, the elevations of these deltas vary greatly, calling into question whether they were all deposited into the same body of water. To test this hypothesis, we used high‐resolution satellite imagery and topography to investigate a large number of lowland deltas in the vicinity of Gale crater, home to the Curiosity rover. We have found that the variability in delta elevations is not due to postformational deformation, rather most deltas in this region formed into lakes, similar in size to the Great Lakes on Earth. These findings show that a northern Martian ocean is not needed to explain the deltas in the vicinity of Gale crater.
Key Points
Twenty‐two candidate deltas occur along the dichotomy boundary in the Gale crater region with delta front elevations spanning approximately 2,400 m
Front elevation variations do not correspond to modeled deformation from true polar wander or Tharsis emplacement
Based on modern topography, most deltas correspond to enclosed basins, which we infer to be paleo‐lakes, not a northern ocean
Permafrost extent sets drainage density in the Arctic Del Vecchio, Joanmarie; Palucis, Marisa C; Meyer, Colin R
Proceedings of the National Academy of Sciences - PNAS,
02/2024, Volume:
121, Issue:
6
Journal Article
Peer reviewed
Open access
Amplified warming of high latitudes and rapid thaw of frozen ground threaten permafrost carbon stocks. The presence of permafrost modulates water infiltration and flow, as well as sediment transport, ...on soil-mantled slopes, influencing the balance of advective fluvial processes to diffusive processes on hillslopes in ways that are different from temperate settings. These processes that shape permafrost landscapes also impact the carbon stored on soil-mantled hillslopes via temperature, saturation, and slope stability such that carbon stocks and landscape morphometry should be closely linked. We studied Formula: see text69,000 headwater basins between 25° and 90 °N to determine whether the thermal state of the soil sets the balance between hillslope (diffusive) and fluvial (advective) erosion processes, as evidenced by the density of the channel networks (i.e., drainage density) and the proportion of convex to concave topography (hillslopes and river valleys, respectively). Watersheds within permafrost regions have lower drainage densities than regions without permafrost, regardless of watershed glacial history, mean annual precipitation, and relief. We find evidence that advective fluvial processes are inhibited in permafrost landscapes compared to their temperate counterparts. Frozen soils likely inhibit channel development, and we predict that climate warming will lower incision thresholds to promote growth of the channel network in permafrost landscapes. By demonstrating how the balance of advective versus diffusive processes might shift with future warming, we gain insight into the mechanisms that shift these landscapes from sequestering to exporting carbon.
Abstract Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant that is accumulating through anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ...ecosystems. The fate of Hg in global soils remains uncertain, however, particularly to what degree Hg is re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we use fallout radionuclide (FRN) chronometry to directly measure Hg accumulation rates in soils. By comparing these rates with measured atmospheric fluxes in a mass balance approach, we show that representative Arctic, boreal, temperate, and tropical soils are quantitatively efficient at retaining anthropogenic Hg. Potential for significant GEM re-emission appears limited to a minority of coniferous soils, calling into question global models that assume strong re-emission of legacy Hg from soils. FRN chronometry poses a powerful tool to reconstruct terrestrial Hg accumulation across larger spatial scales than previously possible, while offering insights into the susceptibility of Hg mobilization from different soil environments.
Debris flows are typically a saturated mixture of poorly sorted particles and interstitial fluid, whose density and flow properties depend strongly on the presence of suspended fine sediment. Recent ...research suggests that grain size distribution (GSD) influences excess pore pressures (i.e., pressure in excess of predicted hydrostatic pressure), which in turn plays a governing role in debris flow behaviors. We report a series of controlled laboratory experiments in a 4 m diameter vertically rotating drum where the coarse particle size distribution and the content of fine particles were varied independently. We measured basal pore fluid pressures, pore fluid pressure profiles (using novel sensor probes), velocity profiles, and longitudinal profiles of the flow height. Excess pore fluid pressure was significant for mixtures with high fines fraction. Such flows exhibited lower values for their bulk flow resistance (as measured by surface slope of the flow), had damped fluctuations of normalized fluid pressure and normal stress, and had velocity profiles where the shear was concentrated at the base of the flow. These effects were most pronounced in flows with a wide coarse GSD distribution. Sustained excess fluid pressure occurred during flow and after cessation of motion. Various mechanisms may cause dilation and contraction of the flows, and we propose that the sustained excess fluid pressures during flow and once the flow has stopped may arise from hindered particle settling and yield strength of the fluid, resulting in transfer of particle weight to the fluid. Thus, debris flow behavior may be strongly influenced by sustained excess fluid pressures controlled by particle settling rates.
Key Points
Coarse grain size distribution and fluid density affects debris flow mobility
Abundant fines can lead to sustained fluid pressures in excess of hydrostatic
Sustained excess fluid pressures may arise from reduced particle settling
Intense Granular Sheetflow in Steep Streams Palucis, Marisa C.; Ulizio, Tom; Fuller, Brian ...
Geophysical research letters,
16 June 2018, Volume:
45, Issue:
11
Journal Article
Peer reviewed
Open access
Quantifying sediment transport rates in mountainous streams is important for hazard prediction, stream restoration, and landscape evolution. While much of the channel network has steep bed slopes, ...little is known about the mechanisms of sediment transport for bed slopes between 10% < S < 30%, where both fluvial transport and debris flows occur. To explore these slopes, we performed experiments in a 12‐m‐long sediment recirculating flume with a nearly uniform gravel bed. At 20% and 30% bed gradients, we observed a 4‐to‐10 particle‐diameter thick, highly concentrated sheetflow layer between the static bed below and the more dilute bedload layer above. Sheetflow thickness increased with steeper bed slopes, and particle velocities increased with bed shear velocity. Sheetflows occurred at Shields stresses close to the predicted bedload‐to‐debris flow transition, suggesting a change of behavior from bedload to sheetflow to debris flow as the bed steepens.
Plain Language Summary
Sediment transport within mountain rivers controls their shape, supplies sediment downstream for aquatic habitat, and can be a major hazard to life and infrastructure. Sediment tends to move by river processes where channel slopes are relatively gentle and by debris flows at very steep gradients; however, the mode of transport is not understood for the range of bed slopes in between. To address this knowledge gap, we performed laboratory experiments in a steep, 12‐m‐long flume to determine when and where river processes transition to debris flows. Surprisingly, we found a distinct third mode of transport developed called sheetflow. Sheetflows are dense granular slurries that are a hybrid between traditional river transport and debris flows. They occur on lower gradient sandy beds under high shear stresses but have not been previously documented in steep mountain streams. We map the parameter space where sheetflows occur, and quantify controls on their thickness and particle velocities. Our results indicate that there is a continuum of behavior as a channel bed steepens, from river to sheetflow to debris flow processes, which has significant implications for predicting sediment fluxes and channel bed morphology in mountain streams.
Key Points
We conducted flume experiments with bed gradients of 10–30% under high Shields stresses where models predict riverbeds should fail
Instead of bed failure, we observed development of a highly concentrated sheetflow layer beneath a dilute bedload layer
Sheetflows were thicker on steeper slopes and had nonlinear velocity profiles that scale with bed shear velocity
The landing site for the Curiosity rover is located at the distal end of the Peace Vallis fan in Gale Crater. Peace Vallis fan covers 80 km2 and is fed by a 730 km2 catchment, which drains an upland ...plains area through a 15 km wide gap in the crater rim. Valley incision into accumulated debris delivered sediment through a relatively low density valley network to a main stem channel to the fan. An estimated total fan volume of 0.9 km3 matches the calculated volume of removal due to valley incision (0.8 km3) and indicates a mean thickness of 9 m. The fan profile is weakly concave up with a mean slope of 1.5% for the lower portion. Numerous inverted channels outcrop on the western surface of the fan, but on the eastern portion such channels are rare suggesting a change in process from distributary channel domination on the west to sheet flow on the eastern portion of the fan. Runoff (discharge/watershed area) to produce the fan is estimated to be more than 600 m, perhaps as much as 6000 m, indicating a hydrologic cycle that likely lasted at least thousands of years. Atmospheric precipitation (possibly snow) not seepage produced the runoff. Based on topographic data, Peace Vallis fan likely onlapped Bradbury Rise and spilled into a topographic low to the east of the rise. This argues that the light‐toned fractured terrain within this topographic low corresponds to the distal deposits of Peace Vallis fan, and in such a setting, lacustrine deposits are expected.
Key Points
Distal end of Peace Vallis fan was near MSL landing
Sediment in the fan was derived from valley incision into colluvium
Fan morphology consistent with fluvial processes
Quantifying flow resistance and sediment transport rates in steep streams is important for flood and debris flow prediction, habitat restoration, and predicting how mountainous landscapes evolve. ...However, most studies have focused on low gradient rivers and the application of this work is uncertain for steep mountain streams where surface flows are shallow and rough, subsurface flows are not negligible, and there is form-drag from bed- and channel-forms that differs from those in low gradient rivers. To evaluate flow resistance relations and sediment transport rates for steep channel beds, experiments were conducted using a range of water discharges and sediment transport rates in a 12 m long recirculating flume with bed slopes of 10%, 20%, and 30%, and a bed of nearly uniform natural gravel. Flow resistance for planar beds and beds that developed bedforms match empirical models that account for bedload-dependent roughness. Some bedforms were atypical for natural rivers at these bed slopes, such as stepped alternate bars and upstream migrating alternate bars. Total flow resistance increased with decreasing particle submergence and energetic sediment transport and drag on bedforms. Using linear stress partitioning to calculate bed stresses due to grain resistance alone, sediment flux relations developed for lower gradient rivers perform well overall, but they overestimate fluxes at 20% and 30% gradients. Based on previous theory, mass failure of the bed, which did not occur, was predicted for the highest Shields stresses investigated at 20% and 30% bed slopes; instead a concentrated layer, four to ten particle diameters deep, of highly concentrated granular sheetflow was observed.
•Flow resistance increased with shallow flow, bedforms, and high sediment transport intensity.•Downstream and upstream migrating alternate bars developed on bed slopes up to 20%.•Transition to upper plane bed coincided with development of concentrated sheetflow.
The quantification of lake levels in Gale crater is important to define the hydrologic and climatic history experienced by the sedimentary deposits found by Curiosity. We propose that there were at ...least three major lake stands within Gale, each persisted >1000 years, and all occurred after Mount Sharp reached close to its current topographic form. Deltaic deposits off the southern rim of Gale, derived from incision of Farah Vallis, and corresponding deposits off the southern flank of Mount Sharp define the highest lake level, which had a mean depth of 700 m. Canyons similar in form to Farah Vallis enter into craters and/or the crustal dichotomy near Gale from the south, suggesting that the highest lake was supplied by a large‐scale flow system. The next lake level, established after a period of drying and rewetting, is defined by four deltaic features, three sourced from Mount Sharp and one from the western rim of Gale, as well as the termination of gullies around the northern rim of Gale. This second lake level had a mean depth of 300 m. The presence of the gullies suggests more locally sourced water. Lake levels then rose another 100 m, as evidenced by two deltaic deposits derived from the rim of Gale and the termination of a second set of gullies. Post‐lake, reduced hydrologic activity continued, evidenced by a time of fan building (including Peace Vallis). The sequence of events suggests an episodic shift through time from relatively wet regional conditions to a drier environment with local runoff.
Key Points
Delta deposits within Gale suggest a series of lake stands post‐Mount Sharp development
Incisional and depositional processes record a sequence of falling then rising lake levels
Water supply for lakes likely shifted from a regional to more local source over time
•Impact crater processes observed during MSL’s first 360 Sols were studied.•Possible impactites are scarce, but include shatter cones and disrupted bedrock.•Loose igneous and sedimentary blocks ...identified near the rover may be distal ejecta.•The smallest observed crater is 0.6m in diam., consistent with atmospheric filtering.•Crater counts constrain the aeolian erosion rate to ~10mm per million years.•The erosion rate is consistent with the observed erosion of the Peace Vallis fan.
Impact processes at all scales have been involved in the formation and subsequent evolution of Gale crater. Small impact craters in the vicinity of the Curiosity MSL landing site and rover traverse during the 364 Sols after landing have been studied both from orbit and the surface. Evidence for the effect of impacts on basement outcrops may include loose blocks of sandstone and conglomerate, and disrupted (fractured) sedimentary layers, which are not obviously displaced by erosion. Impact ejecta blankets are likely to be present, but in the absence of distinct glass or impact melt phases are difficult to distinguish from sedimentary/volcaniclastic breccia and conglomerate deposits. The occurrence of individual blocks with diverse petrological characteristics, including igneous textures, have been identified across the surface of Bradbury Rise, and some of these blocks may represent distal ejecta from larger craters in the vicinity of Gale. Distal ejecta may also occur in the form of impact spherules identified in the sediments and drift material. Possible examples of impactites in the form of shatter cones, shocked rocks, and ropy textured fragments of materials that may have been molten have been observed, but cannot be uniquely confirmed. Modification by aeolian processes of craters smaller than 40m in diameter observed in this study, are indicated by erosion of crater rims, and infill of craters with aeolian and airfall dust deposits. Estimates for resurfacing suggest that craters less than 15m in diameter may represent steady state between production and destruction. The smallest candidate impact crater observed is ∼0.6m in diameter. The observed crater record and other data are consistent with a resurfacing rate of the order of 10mm/Myr; considerably greater than the rate from impact cratering alone, but remarkably lower than terrestrial erosion rates.
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