Michael P. Lamb’s research while affiliated with California Institute of Technology and other places

Flocculation controls mud sedimentation and organic carbon burial rates by increasing mud settling velocity. However, calibration and validation of floc settling velocity models in freshwater are lacking. We used a camera, in situ laser diffraction particle sizing, and suspended sediment concentration–depth profiles to measure flocs in Wax Lake Delta, Louisiana. We developed a new workflow that combines our multiple floc data sources to distinguish between flocs and unflocculated sediment and measure floc attributes that were previously difficult to constrain. Sediment finer than ∼10 to 55 µm was flocculated with median floc diameter of 30 to 90 µm, bulk solid fraction of 0.05 to 0.3, fractal dimension of ∼2.1, and floc settling velocity of ∼0.1 to 1 mm s⁻¹, with little variation along water depth. Results are consistent with a semi-empirical model indicating that sediment concentration and mineralogy, organics, water chemistry, and, above all, turbulence control floc settling velocity. Effective primary particle diameter is ∼2µm, about 2 to 6 times smaller than the median primary particle diameter, and is better described using a fractal theory. Flow through the floc increases settling velocity by an average factor of 2 and up to a factor of 7 and can be described by a modified permeability model that accounts for the effect of many primary particle sizes on flow paths. These findings help explain discrepancies between observations and an explicit settling model based on Stokes' law that depends on floc diameter, permeability, and fractal properties.

The rate of river migration affects the stability of Arctic infrastructure and communities1,2 and regulates the fluxes of carbon3,4, nutrients⁵ and sediment6,7 to the oceans. However, predicting how the pace of river migration will change in a warming Arctic⁸ has so far been stymied by conflicting observations about whether permafrost⁹ primarily acts to slow10,11 or accelerate12,13 river migration. Here we develop new computational methods that enable the detection of riverbank erosion at length scales 5–10 times smaller than the pixel size in satellite imagery, an innovation that unlocks the ability to quantify erosion at the sub-monthly timescales when rivers undergo their largest variations in water temperature and flow. We use these high-frequency observations to constrain the extent to which erosion is limited by the thermal condition of melting the pore ice that cements bank sediment¹⁴, a requirement that will disappear when permafrost thaws, versus the mechanical condition of having sufficient flow to transport the sediment comprising the riverbanks, a condition experienced by all rivers¹⁵. Analysis of high-resolution data from the Koyukuk River, Alaska, shows that the presence of permafrost reduces erosion rates by 47%. Using our observations, we calibrate and validate a numerical model that can be applied to diverse Arctic rivers. The model predicts that full permafrost thaw may lead to a 30–100% increase in the migration rates of Arctic rivers.

Plain Language Summary The bed surface layer of many rivers is a mixture of sand and gravel. This mixture is described by the probability distribution of grain sizes, and in particular by the median size. Consider the long profile of such a river. Surface median size commonly becomes finer downstream, but often changes abruptly from a value above 5 mm to a value below 1 mm over a short reach. The range 1–5 mm is termed “gap sediment.” Here we explain how this abrupt change evolves, even when there is no deficit of gap sediment supplied to the reach, and even though particle abrasion is not included. The grain size distribution autogenically develops two peaks, one in the sand range and one in the gravel range above 5 mm. When abrupt floodplain widening is included, the gravel peak is stronger in the upper reach and the sand peak is stronger in the lower reach, leading to a relatively abrupt gravel‐sand transition. Gap sediment can be diluted both upstream and downstream by a combination of effects due to bedload and suspended load, so that it dominates nowhere in the grain size distribution.

Rapid warming in the Arctic threatens to destabilize mercury (Hg) deposits contained within soils in permafrost regions. Yet current estimates of the amount of Hg in permafrost vary by ∼4 times. Moreover, how Hg will be released to the environment as permafrost thaws remains poorly known, despite threats to water quality, human health, and the environment. Here we present new measurements of total mercury (THg) contents in discontinuous permafrost in the Yukon River Basin in Alaska. We collected riverbank and floodplain sediments from exposed banks and bars near the villages of Huslia and Beaver. Median THg contents were 49⁺¹³/−21 ng THg g sediment⁻¹ and 39⁺¹⁶/−18 ng THg g sediment⁻¹ for Huslia and Beaver, respectively (uncertainties as 15th and 85th percentiles). Corresponding THg:organic carbon ratios were 5.4+2.0/−2.4 Gg THg Pg C⁻¹ and 4.2 +2.4/−2.9 Gg THg Pg C⁻¹. To constrain floodplain THg stocks, we combined measured THg contents with floodplain stratigraphy. Trends of THg increasing with smaller sediment size and calculated stocks in the upper 1 m and 3 m are similar to those suggested for this region by prior pan-Arctic studies. We combined THg stocks and river migration rates derived from remote sensing to estimate particulate THg erosional and depositional fluxes as river channels migrate across the floodplain. Results show similar fluxes within uncertainty into the river from erosion at both sites (95⁺¹²/−47 kg THg yr⁻¹ and 26⁺¹⁵⁴/−13 kg THg yr⁻¹ at Huslia and Beaver, respectively), but different fluxes out of the river via deposition in aggrading bars (60⁺⁴⁰/−29 kg THg yr⁻¹ and 10+5.3/−1.7 kg THg yr⁻¹). Thus, a significant amount of THg is liberated from permafrost during bank erosion, while a variable but generally lesser portion is subsequently redeposited by migrating rivers.

Vegetation is thought to be a main source of riverbank cohesion, enabling meandering and a deposit architecture characterized by sandy channel belts isolated in mudstone. However, early Earth and Mars had meandering rivers without vegetation, implying that other sources of bank strength can allow meandering with potentially different deposit characteristics. Here we studied the Amargosa River in Death Valley, California, USA, as a modern analog of meandering rivers without vegetation. We monitored flow and erosion at two bends and used radiocarbon dating of strandlines to quantify flood frequency. We also sampled cutbank mud and constrained an erosion theory using flume experiments. Cutbank erosion occurred for floods with >2 yr recurrence intervals, and 18 cm occurred for an ∼6 yr reoccurrence, bankfull event. Mud set the rate of meander migration: salt crusts rapidly and completely dissolved during floods, vegetation was absent, and mud entrainment theory matched observed erosion rates. Flood-frequency analysis showed that most bank erosion occurs at flows below bankfull, challenging the threshold channel hypothesis. We used meander migration rates to constrain the time scale of channel-belt formation and compared it to the time scale of avulsion. These calculations, combined with floodplain facies mapping and core sedimentology, indicated a likely deposit architecture of sandy point bar accretion sets intermixed with muddy overbank facies. This deposit architecture is characteristic of vegetated meandering rivers, but due to muddy banks, occurred for the Amargosa River in the absence of plants.

Permafrost influences 25% of land in the Northern Hemisphere, where it stabilizes the ground beneath communities and infrastructure and sequesters carbon. However, the coevolution of permafrost, river dynamics, and vegetation in Arctic environments remains poorly understood. As rivers meander, they erode the floodplain at cutbanks and build new land through bar deposition, creating sequences of landforms with distinct formation ages. Here we mapped these sequences along the Koyukuk River floodplain, Alaska, analyzing permafrost occurrence, and landform and vegetation types. We used radiocarbon and optically stimulated luminescence (OSL) dating to develop a floodplain age map. Deposit ages ranged from modern to 10 ka, with more younger deposits near the modern channel. Permafrost rapidly reached 50% areal extent in all deposits older than 200 years then gradually increased up to ∼85% extent for deposits greater than 4 Kyr old. Permafrost extent correlated with increases in black spruce and wetland abundance, as well as increases in permafrost extent within wetland, and shrub and scrub vegetation classes. We developed an inverse model to constrain permafrost formation rate as a function of air temperature. Permafrost extent initially increased by ∼25% per century, in pace with vegetation succession, before decelerating to <10% per millennia as insulating overbank mud and moss slowly accumulated. Modern permafrost extent on the Koyukuk floodplain therefore reflects a dynamic balance between widespread, time‐varying permafrost formation and rapid, localized degradation due to cutbank erosion that might trigger a rapid loss of permafrost with climatic warming.

Introduction: Sedimentary rocks on Mars preserve a record of past depositional and diagenetic conditions that offers an opportunity to investigate the planet's geologic and climate history beyond the limits of the geomorphic and impact crater records. Since 2019, new rover and orbiter observations and analyses of the Martian sedimentary record, along with sediment transport modeling advances, have provided important insights into the environment, climate, and habitability of early Mars. This review presents: (1) key advances in Martian sedimentology and stratigraphy over the past 5 years, (2) current limitations, and (3) priority areas for future research. Advances: Evolution of Mars surface environments through time. The high spatial resolution and ground-based viewing perspective of rovers with access to 100s of meters of stratigraphy has enabled the recognition of environmental and climatic oscillations at an unprecedented level of detail and complexity. In Gale crater, Curiosity data reveal transitions between lacustrine, fluvial, littoral, and aeolian depositional settings within Aeolis Mons (informally, Mount Sharp) strata [1-7], as well as high frequency wet-dry cycling [8] and evidence of seasonal atmospheric changes [9]. In Jezero crater, Perseverance has observed a time-ordered sequence of alluvial, lacustrine, deltaic, and fluvial sedimentary rocks [10] with fluctuations in lake level recorded in this stratigraphy [11]. Significance of mineral stacking patterns observed in orbiter data. Rover observations reveal that mineralogical transitions and associations observed in orbiter data often record diagenesis rather than primary depositional conditions. For example, the clay-sulfate transition in Gale crater results from an increase in the population of diagenetic sulfate-rich veins and concretions [12, 13] formed at depth [14]. The hematite in Vera Rubin ridge, once considered a candidate for deposition in a redox-stratified lake [15], records a concentration of diagenetic hematite [16]. Observations by Perseverance in Jezero crater reveal that clay minerals detected on the upper surface of the fan are not

Lateral migration of meandering rivers poses erosional risks to human settlements, roads, and infrastructure in alluvial floodplains. While there is a large body of scientific literature on the dominant mechanisms driving river migration, it is still not possible to accurately predict river meander evolution over multiple years. This is in part because we do not fully understand the relative contribution of each mechanism and because deterministic mathematical models are not equipped to account for stochasticity in the system. Besides, uncertainty due to model structure deficits and unknown parameter values remains. For a more reliable assessment of risks, we therefore need probabilistic forecasts. Here, we present a workflow to generate geomorphic risk maps for river migration using probabilistic modeling. We start with a simple geometric model for river migration, where nominal migration rates increase with local and upstream curvature. We then account for model structure deficits using smooth random functions. Probabilistic forecasts for river channel position over time are generated by Monte Carlo runs using a distribution of model parameter values inferred from satellite data. We provide a recipe for parameter inference within the Bayesian framework. We demonstrate that such risk maps are relatively more informative in avoiding false negatives, which can be both detrimental and costly, in the context of assessing erosional hazards due to river migration. Our results show that with longer prediction time horizons, the spatial uncertainty of erosional hazard within the entire channel belt increases – with more geographical area falling within 25 % < probability < 75 %. However, forecasts also become more confident about erosion for regions immediately in the vicinity of the river, especially on its cut-bank side. Probabilistic modeling thus allows us to quantify our degree of confidence – which is spatially and temporally variable – in river migration forecasts. We also note that to increase the reliability of these risk maps, we need to describe the first-order dynamics in our model to a reasonable degree of accuracy, and simple geometric models do not always possess such accuracy.

Mars has many well‐exposed fluvial ridges and fluvio‐deltaic basins; in two of these locations, the Curiosity and Perseverance rovers are currently searching for signs of habitability. The distribution of organic carbon that might persist in ancient fluvial deposits present on Mars is not well understood. In this study, we set out to assess the preservation potential of organic carbon in a hyperarid fluvial environment with observations and analyses of the Amargosa River in Death Valley, California (United States). The lower reaches of the Amargosa River in Badwater Basin are nearly devoid of plants and contain low gradient, meandering channels, making them a valuable terrestrial analog for early martian fluvial systems. We analyzed sediment taken from fluvial deposits exposed in cutbanks of two bends of a meandering channel. We found total organic carbon abundances that were on average 0.15% up to a meter below the surface. X‐ray diffraction and electron microscopy analyses revealed a suite of high redox potential mineral phases (including iron and manganese oxides) mixed with detrital and authigenic silicates, carbonate, and sulfate salts at or close to redox equilibrium with pore fluids in contact with the atmosphere. This finding highlighted that organic carbon can persist in fluvial deposits at low abundance despite oxidizing conditions and saturated sediments and suggested that ancient fluvial deposits on Mars may retain traces of organics in fine‐grained deposits if they are present during deposition.

How will bank erosion rates in Arctic rivers respond to a warming climate? Existing physical models predict that bank erosion rates should increase with water temperature as permafrost thaws more rapidly. However, the same theory predicts much faster erosion than is typically observed. We propose that these models are missing a key component: a layer of thawed sediment on the bank that buffers heat transfer and slows erosion. We developed a 1D model for this thawed layer, which reveals three regimes for permafrost riverbank erosion. Thaw‐limited erosion occurs in the absence of a thawed layer, such that rapid pore‐ice melting sets the pace of erosion, consistent with existing models. Entrainment‐limited erosion occurs when pore‐ice melting outpaces bank erosion, resulting in a thawed layer, and the relatively slow entrainment of sediment sets the pace of erosion similar to non‐permafrost rivers. Third, the intermediate regime occurs when the thawed layer goes through cycles of thickening and failure, leading to a transient thermal buffer that slows thaw rates. Distinguishing between these regimes is important because thaw‐limited erosion is highly sensitive to water temperature, whereas entrainment‐limited erosion is not. Interestingly, the buffered regime produces a thawed layer and relatively slow erosion rates like the entrainment‐limited regime, but erosion rates are temperature sensitive like the thaw‐limited regime. The results suggest the potential for accelerating erosion in a warming Arctic where bank erosion is presently thaw‐limited or buffered. Moreover, rivers can experience all regimes annually and transition between regimes with warming, altering their sensitivity to climate change.