Sediments transported from source terrains to depositional sinks carry environmental signals, which may or may not be preserved in stratigraphy. Recently developed theory suggests storage thresholds for environmental signals are set by the internal dynamics of sediment transport systems. For the first time, we explore this theory by testing whether changes in relative sea level (RSL) of various scales produce detectable signals stored in field scale stratigraphy. This field test builds on results from physical experiments where identifiable stratigraphic signals of RSL change were only produced from RSL cycles with magnitudes and/or periodicities greater than the spatial and temporal scales of the internal dynamics of deltas. Published long term sedimentation rates and sea level reconstructions suggest that the Mississippi River Delta (MRD) should be a good place to study sea level signal storage thresholds. We use publicly available seismic volumes from NAMSS-USGS to study how and if signals of paleo-sea level change are stored in strata of the MRD, comparing strata of the late Miocene (LM) and early Quaternary (EQ). Comparison of the amplitude and period of cycles in these two time periods, constrained by micropaleontological data, predicts storage of RSL signals in EQ strata, but not in the LM strata. This is confirmed as we show that signals of larger amplitude EQ RSL cycles are preserved in the MRD, but smaller amplitude LM signals are not detectable. This study adds field scale observations that quantify the intermingling of stratigraphic products of internal dynamics with products of RSL change over geological timescales.

Tectonic, climatic, and anthropogenic forcing generate sediment flux signals that propagate across the Earth’s surface. Some of these signals get stored in strata but autogenic processes in operation at the Earth's surface can shred (i.e. degrade) and obscure many signals of environmental change prior to stratigraphic storage. Here we advance on earlier seminal work and use a physical rice pile to identify critical autogenic timescales and establish autogenic thresholds that can be used to understand of how a signal of a given period or magnitude is manifested in an efflux time series and whether or not a signal is shredded or obscured by autogenic noise. Time-permitting there will be a more general discussion on how this applies to landscapes and strata.

The interplay between seafloor sediment laden density-driven flows, turbidity currents, and topography helps to shape continental margins. However, these interactions are poorly understood, especially those within enclosed depressions termed minibasins. In this study, novel experiments quantify the three-dimensional (3-D) dynamics of turbidity currents interacting with a range of minibasin geometries that, for the first time, scale within the parameter space of natural systems. Controls on flow dynamics are quantified by measuring the evolving velocity and sediment transport fields, in addition to maps of bathymetry. This study focuses on three aspects of turbidity current interactions with minibasins. First, the results suggest that sediment transport and deposition in minibasins is likely dominated by evolving flow conditions. Contrary to earlier studies in two-dimensional (2-D) flumes, this study supports a time-to-flow equilibrium in mini basins that scales with the time to replace ambient fluid with turbid influx, and this replacement time likely takes days to achieve in many field-scale minibasins. Second, in all experiments, horizontal flow circulation is observed, which is critical for distributing sediment throughout minibasins. However, the strength of the horizontal circulation reduces as the ratio of minibasin length to width increases, which leads to stagnant or even upstream-directed flow near the bed, elevated height of the velocity maximum in flows, the lowering of near-bed shear stresses, and more homogeneous deposits through a reduction in bed reworking. Finally, the results indicate that fluid detrainment from minibasins significantly reduces sediment fall velocities, severely lowering the sediment trapping efficiency for small or light particles. This reduction in effective fall velocities of sediments suggests a mechanism that fractionates fine particulates (e.g., clays), nutrients (e.g., organic carbon), and pollutants (e.g., microplastics) along transport paths down topographically complex margins.

The bathymetry of the northern Gulf of Mexico is strongly influenced by diapirism of subsurface salt. A competition between salt dynamics and the depositional mechanics of sediment laden density flows over geological timescales controls the scale of seafloor depressions, which are the dominant bathymetric features of the margin. Salt domes create topographic highs, and salt removal to the domes creates topographic lows, with sediment deposition driving the gravitational dynamics. The strength of bathymetric self-organization into depressions is inferred through analysis of a vast bathymetric data set made public by the U.S. Bureau of Ocean and Energy Management. Depression geometric scales follow Pareto distributions, and their tail indexes aid inference of the strength of bathymetric self-organization, with lower tail indexes linked to greater self-organization. A comparison is made of margin subregions defined by pseudo-flow drainage density maps, which inversely relates to the pre-deformation thickness of subsurface salt. Tail indexes of distributions decrease with the thickness of the underlying salt. This is linked to the merger of depressions, which is enhanced when depressions can grow wider and deeper, as occurs over thick salt fields, and the development of salt structure. The manner of self-organization results in most of the margin's ponded sediment accommodation residing in relatively few depressions that have reliefs exceeding 100 m. This relief is sufficient to induce sedimentation from even the thickest turbidity currents, which can further drive gravitational dynamics. The bathymetric complexity of depressions is also greatest over regions with the thickest salt, further supporting enhanced self-organization.

Natural depressions on continental margins termed minibasins trap turbidity currents, a class of sediment-laden seafloor density driven flow. These currents are the primary downslope vectors for clastic sediment, particulate organic carbon, and microplastics. Here, we establish a method that facilitates long-distance self-suspension of dilute sediment-laden flows, enabling study of turbidity currents with appropriately scaled natural topography. We show that flow dynamics in three-dimensional minibasins are dominated by circulation cell structures. While fluid rotation is mainly along a horizontal plane, inwards spiraling flow results in strong upwelling jets that reduce the ability of minibasins to trap particulate organic carbon, microplastics, and fine-grained clastic sediment. Circulation cells are the prime mechanism for distributing particulates in minibasins and set the geometry of deposits, which are often intricate and below the resolution of geophysical surveys. Fluid and sediment are delivered to circulation cells by turbidity currents that runup the distal wall of minibasins. The magnitude of runup increases with the discharge rate of currents entering minibasins, which influences the amount of sediment that is either trapped in minibasins or spills to downslope environs and determines the height that deposits onlap against minibasin walls.

Spectral analysis is a central tool regularly used by the scientific community to identify the presence of periodic processes within a time series of information, as spectral peaks at an imposed periodicity can be differentiated from internal (autogenic) variance. In scientific disciplines, such as seismology, the time series of information is of high temporal resolution. Hence, although temporal gaps are present, they do not impact the overall noise structure, meaning that the full spectrum of autogenic variance can be reconstructed. However, power spectra generated from stratigraphic information are affected by temporal incompleteness due to varying episodes of erosion and geomorphic stasis, which generate gaps over a range of scales. This removes information related to the natural and autogenic variability present within sediment-transport systems, which makes it challenging to accurately reconstruct the structure and strength of paleo-surface processes, which defines the detectability of past environmental signals. We explore how incompleteness impacts the temporal structure of autogenic noise within power spectra, and how this influences the detectability of spectral spikes related to environmental signals. We utilize a sediment flux time series from a physical rice pile and progressively degrade these data to mimic varying degrees of stratigraphic incompleteness. We find that incompleteness strongly influences the timescales and spectral structure of autogenic noise evident, and can render signals over all periodicities undetectable within a highly incomplete time series. This offers the ability to confidently justify the interpretation of subtle environmental signals from field measurements and understand the records that may best preserve paleoenvironmental variability. Information about past environmental conditions is recorded in layers of ancient sediment. This enables scientists to identify specific environmental events in Earth's past, in order to understand the impact of current and future environmental events. However, layers of sediment do not represent a complete record of time and environmental signals generated by these events can be mixed up with natural environmental fluctuations (noise). This means that the detection of the original environmental signal can be uncertain. To reduce this uncertainty, we must quantify the duration and magnitude of noise, allowing a threshold to be constructed above which we can be confident that the signal is real. Constructing this for stratigraphy is difficult as strata contain temporal gaps over a wide range of scales, which makes the detection of past environmental signals challenging to the extent where many real signals could be deemed undetectable. We studied how the incompleteness of stratigraphy influences the structure of noise preserved in stratigraphy and how accurately environmental signals can be detected. We show that incompleteness is a major factor as to why a time series of stratigraphic information rarely records the full structure of noise, which consequently affects the detection and reconstruction of environmental signals. Temporal gaps within a time series of stratigraphic measurables severely modify the apparent temporal structure of autogenic processes This has practical implications for how we detect, interpret and reconstruct environmental signals from strata Using an estimate of completeness, we can predict the detectability of an environmental signal and reconstruct signal properties

Time-elevation plots and chronostratigraphic diagrams are valuable for understanding and analyzing stratigraphy when time-elevation data, or some approximation of them, are available, for example in flume experiments, numerical models, and three-dimensional seismic reflection surveys. We developed a Python module called stratigraph, aimed at the reproducible analysis and visualization of stratigraphy, and we use it here to explore data from forward stratigraphic models of meandering channels, the eXperimental EarthScape (XES) facility XES-02 experiment, and two experiments that were conducted at the Tulane University Sediment Dynamics and Stratigraphy Laboratory. We use these tools to generate and visualize three-dimensional chronostratigraphic diagrams, compute maps of stratigraphic completeness and other stratigraphic attributes, and explore the nature of the erosional surfaces. We show that, using a 3D Wheeler diagram, it is possible to create maps of important stratigraphic attributes, in addition to the conventional thickness maps. There are six fundamental stratigraphic attributes that are direct consequences of a quantitative chronostratigraphic approach, as follows. (1) Sediments that were preserved after deposition have a thickness and (2) a duration; normalized by the total time, this duration of preserved deposition is called stratigraphic completeness. (3) The duration of deposition of sediment that was eroded later (called vacuity); (4) the thickness of these sediments is the eroded thickness. (5) At any given geographic location, erosion occurs some of the time, and the duration of these erosive periods is the fifth quantity. (6) Finally, it is quite common that neither significant deposition nor erosion takes place for some time and the duration of this stasis can be considered at every location. These maps give an overview of where erosion or deposition dominate in a source-to-sink system, and for how long; and they make it possible to quickly identify sites with both a high degree of stratigraphic completeness and a significant thickness.

When interpreting environmental signals in the deep marine sedimentary archive, separating the record of local flow and sediment dynamics from that of the terrestrial transport system that feeds it can be challenging. We used a physical experiment to study the dynamics of flow and sedimentation on a prograding, hyperpycnal flow-dominated delta, shelf and submarine slope subject to slow rates of base-level rise (pseudo-subsidence). Our experiments are most relevant to shelf margins where sediment-rich deltaic systems can prograde towards the shelf-edge under relatively mild rates of relative sea-level rise, e.g. recent millennia (~7 ky). Our results offer interesting insight into linked dynamics of terrestrial and submarine transport systems; they apply to time-scales that range from days to millennia, and may be relevant to problems as diverse as delivery of dissolved and particulate anthropogenic pollutants to deep ocean ecosystems and terrestrial paleoenvironmental reconstructions from marine sedimentary records.We asked 3 questions: (1) Are delta channel dynamics reflected in flow and sedimentation on the continental slope? (2) how effectively do shelf and slope systems transfer information from upstream? (3) how does delta growth and progradation to the shelf-edge impact sedimentation on the continental slope? We found that: (1) Changes in flow partitioning through delta-top channels and associated hyperpycnal plume dynamics are recorded in flow and sedimentation on the slope. Channelized delta-top flow resulted in higher localized water discharge and sediment concentrations, and thick, fast-moving, and laterally continuous, turbidity currents on the slope; sheet flow on the delta top, on the other hand, produced thin, slow-moving and laterally discontinuous turbidity currents on the slope. (2) Patterns in flow and sedimentation correlate over longer distances on the advection-settling-dominated subaqueous continental slope than on the transport-limited shelf and delta-top. (3) Delta progradation played an important role in defining the scales of depositional topography and sedimentation dynamics on the slope. Before the delta arrived at the shelf edge, slow-growing, small wave-length depositional topography on the slope remained temporally persistent, and was associated with small cross-stream wavelengths associated with depositional topography and low variance in sedimentation patterns; once the delta reached the shelf-edge, the growth, progradation and lateral stacking of mouth bars on the proximal parts of the continental slope caused an abrupt switch to dynamic depositional topography with large cross-stream wave-lengths and high variance in sedimentation rates.

Autogenic processes contribute noise to sediment transport systems that can degrade or mask externally derived environmental signals and hinder our ability to reconstruct past environmental signals from landscapes and strata. To explore this further, we measure efflux from a physical rice pile to ascertain the temporal structure of autogenic noise, and how this influences the degradation and detection of environmental signals. Our results reveal a tripartite temporal spectral structure segmented at two key autogenic time scales. The shorter autogenic time scale set limits on environmental signal degradation, while the longer autogenic time scale sets limits on environmental signal detection. This work establishes a framework that can be used to explore how autogenic processes interact with external environmental signals in field-scale systems to influence their detectability. We anticipate that the temporal structure and associated time scales identified will arise from autogenic processes in numerous sediment transport systems.

We investigate the interaction of fluvial and non-fluvial sedimentation on the channel morphology and kinematics of an experimental river delta. We compare two deltas: one that evolved with a proxy for non-fluvial (“marsh”) sedimentation (treatment experiment) and one that evolved without the proxy (control). We show that the addition of the non-fluvial sediment proxy alters the delta's channel morphology and kinematics. Notably, the flow outside the channels is significantly reduced in the treatment experiment, and the channels are deeper (as a function of radial distance from the source) and longer. We also find that both the control and treatment channels narrow as they approach the shoreline, though the narrowing is more pronounced in the control compared to the treatment. Interestingly, the channel beds in the treatment experiment often exist below sea level in the terrestrial portion of the delta top, creating a ∼ 0.7 m reach of steady, non-uniform backwater flow. However, in the control experiment, the channel beds generally exist at or above relative sea level, creating channel movement resembling morphodynamic backwater kinematics and topographic flow expansions. Differences between channel and far-field aggradation produce a longer channel in-filling timescale for the treatment compared to the control, suggesting that the channel avulsions triggered by a peak in channel sedimentation occur less frequently in the treatment experiment. Despite this difference, the basin-wide timescale of lateral channel mobility remains similar. Ultimately, non-fluvial sedimentation on the delta top plays a key role in the channel morphology and kinematics of an experimental river delta, producing channels which are more analogous to channels in global river deltas and which cannot be produced solely by increasing cohesion in an experimental river delta.

Tectonic, climatic, and anthropogenic forcing generate sediment flux signals that propagate across the Earth’s surface. Some of these signals get stored in strata but autogenic processes present in the Earth surface active layer can shred (i.e. degrade) and obscure many signals of environmental change prior to stratigraphic storage. In a landmark paper, Jerolmack and Paola (2010) use a numerical rice pile to show that autogenic events in the system saturate at a timescale Tx, which is noted to scale as L2/qinand corresponds to a red-to-white noise transition. The conceptual utility of this is that those environmental signals with periods less than Tx will experience shredding (unless signal magnitude overwhelms autogenic processes), while signals with periods greater than Tx would be detectable in the output. However, the relationships between signal shredding, preservation and detection are currently not established using physical experiments. Advancing on this work, we use a physical rice pile and find that power spectra generated from efflux time-series exhibit a tripartite geometry defined by red, white and blue noise. The transition between each regime defines two key autogenic timescales: Trwand Twb. Trw is defined by the red-to-white noise transition, setting upper bounds on signal degradation, and represents Tx on the power spectra of Jerolmack and Paola (2010), but does not scale with qin. Whereas signals greater than Twb, which scales with qin,are unobscured by autogenic noise and show enhanced detectability in the power spectra. We emphasize that while signals greater than Trw do not experience degradation, they can still be obscured by autogenic noise, unless signal period is greater than Twb. This framework can be used to predict the severity of shredding as signals propagate through the Earth surface active layer, and establish robust confidence limits of signal detectability in landscapes and strata.

Minibasins on continental margins trap turbidity currents transporting material downslope, but little is known about the inherently three-dimensional (3-D) mechanics of these confined flows. Utilizing new methodology, experimental results quantify flow dynamics in minibasins for the first time. It is shown that dynamics are dominated by 3-D circulation cell structures, across the fill-to-strip-to-spill transition that are controlled by flow discharge. Measurements of velocity throughout circulation cells indicate vorticity dominates strain rate with fluid rotating into the center of cells where it upwells: this influences minibasin sediment trapping potential and deposit heterogeneity. Flow properties link to depositional patterns on minibasin slopes. Specifically, higher input discharges are correlated with higher fluxes into the center of minibasins and reduced deposit tapering on minibasin slopes. This geometry is linked to the amount of sediment rich flow runup on the distal minibasin wall, where flow and sediment is delivered to circulation cells.

An appealing strategy for reconstructing the timing and tempo of paleoenvironmental change from sedimentary strata is to linearly interpolate between marker beds of known age. This method requires significant assumptions, but more advanced age modeling methods are usually not feasible. We used experiments to explore how changes in sedimentary processes invalidate these assumptions and affect estimates of time from the strata. When sedimentary processes changed to favor widespread deposition, we found that measuring time linearly systematically overestimated time duration from the resulting strata (time dilation) and misestimated the beginning and end of geologic intervals (phase shifting). When simple age models must be used for sedimentary strata, geologic evidence for transient changes in spatial sediment dispersal may help identify sections of dilated and shifted time, and better resolve time in sedimentary strata.

We present the first investigation of subsidence due to sediment compaction and consolidation in two laboratory‐scale river delta experiments. Spatial and temporal trends in subsidence rates in the experimental setting may elucidate behavior which cannot be directly observed at sufficiently long timescales, except for in reduced scale models such as the ones studied. We compare subsidence between a control experiment using steady boundary conditions, and an otherwise identical experiment which has been treated with a proxy for highly compressible marsh deposits. Both experiments have non‐negligible compactional subsidence rates across the delta‐top, comparable in magnitude to our boundary condition relative sea level rise rate of 250 μm/hr. Subsidence in the control experiment (on average 54 μm/hr) is concentrated in the lowest elevation (<10 mm above sea level) areas near the coast and is likely related to creep induced by a rising water table near the shoreface. The treatment experiment exhibits larger (on average 126 μm/hr) and more spatially variable subsidence rates controlled mostly by compaction of recent marsh deposits within one channel depth (∼10 mm) of the sediment surface. These rates compare favorably with field and modeling based subsidence measurements both in relative magnitude and location. We find that subsidence “hot spots” may be relatively ephemeral on longer timescales, but average subsidence across the entire delta can be variable even at our shortest measurement window. This suggests that subsidence rates over a short time frame may exceed thresholds for marsh platform drowning, even if the long term trend does not.

Basin-wide accommodation production and associated sediment mass deposition exert fundamental controls on stratigraphic architecture, but the details of this relationship are not fully understood. This is because it is unknown how accommodation production directly influences morphodynamics both in terms of channel process (i.e., channel migration, channel avulsion) and floodplain process, both of which are themselves coupled dynamically and are critical to the nature of stratigraphic architecture. To address this, we expand upon existing theory that links sediment mass balance and resultant stratigraphic architecture. We use two fan-delta experiments that each experience different rates of accommodation production to measure key surface morphometrics and subsurface sedimentary characteristics. Importantly, sediment was transported in bedload and suspension in these experiments, allowing for construction of strata characterized by channel bodies surrounded by overbank strata deposited from suspension fallout. From these data we use three key timescales to capture the overall behavior of the system when placed into mass balance space; avulsion setup timescales (TA) and channel mobility timescales (TV) that define short-term surface autogenics, and an accretion timescale (TC) that incorporates longer term deposition. We find that the ratio of both TC/TA and TC/TV are independent of accommodation production rate in mass-balance space, which supports a self-organized response of channel dynamics to environmental boundary conditions. The fraction of strata generated from key depositional environments largely supports this behavior, particularly for channel sand bodies that resulted in deposition from bedload transport. As such, our results suggest that channel body density is independent of accommodation production rate in a mass-balance space. We found that, although contributing to a significant fraction of the basin strata, far-field overbank deposition rates are insensitive to accommodation production and that differences in autogenic timescales between experiments largely resulted from differences in channel deposition rates, highlighting the close coupling between channel dynamics and accommodation generation. More generally the observed self-organized response of surface morphodynamics to accommodation production in mass-balance space provides a process-based framework to explain the utility of balancing mass for the prediction of down-system sediment size fractionation and sedimentary architecture.

Observations of active turbidity currents at field scale offers a limited scope which challenges the development of theory that links flow dynamics to the morphology of submarine fans. Here we offer a framework for predicting submarine fan morphologies by simplifying critical environmental forcings such as regional slopes and properties of sediments, through densimetric Froude (ratio of inertial to gravitational forces) and Rouse numbers (ratio of settling velocity of sediments to shear velocity) of turbidity currents. We leverage a depth-average process-based numerical model to simulate an array of submarine fans and measure rugosity as a proxy for their morphological complexity. We show a systematic increase in rugosity by either increasing the densimetric Froude number or decreasing the Rouse number of turbidity currents. These trends reflect gradients in the dynamics of channel migration on the fan surface and help discriminate submarine fans that effectively sequester organic carbon rich mud in deep ocean strata.

&lt;p&gt;River deltas and the vast marshes that they host are impossible to separate. However, the sediment dynamics of rivers (channel- and lobe-based deposition) and marshes (elevation-based deposition) have not been investigated as a coupled system. We investigate this coupling by comparing a laboratory delta experiment with proxy marsh accumulation to a proxy-less control. The proxy adds just 8% mass to the system but clearly influences delta slopes and mass partitioning. Slopes in the marsh window (elevations around sea level where marsh accumulates) are reduced by 40%, as marsh deposition away from channels smooths topography. While riverine sedimentation continues to be 85% of the deposit in the marsh window, the reduced slopes increase the area in this zone such that 1.3 times more clastic volume is deposited in this window. The area above the marsh window and the mass fraction deposited at these high elevations is correspondingly reduced as if the marshes are &amp;#8220;stealing&amp;#8221; riverine sediment from upstream. Comparing experimental elevation distributions to the field, we show that large deltas might also exhibit this signature. Given that coastal risk is tied to elevation, these findings show that the coupling between marshes and deltas significantly impacts how they should be managed.&lt;/p&gt;

Rising sea levels, subsidence, and decreased fluvial sediment load threaten river deltas and their wetlands. However, the feedbacks between fluvial and non‐fluvial (marsh) deposition remain weakly constrained. We investigate how non‐riverine, elevation‐controlled deposition typified by marshes impacts sediment partitioning between a delta's topset, coastal zone, and foreset by comparing a delta experiment with proxy marsh accumulation to a control. Marsh accumulation alters fluvial sediment distribution by decreasing the slope in the marsh window by ∼50%, creating a 78% larger marsh zone. Fluvial incursions into the marsh window trap 1.3 times more clastic volume. The volume exported to deep water remains unchanged. Marsh deposition shifts elevation distributions toward sea level, which produces a hypsometry akin to field‐scale deltas. The elevation‐lowering effect of marshes on an equilibrium delta shown here constitutes an unexplored feedback and an important aspect of coastal sustainability.