John A. Goff

I present evidence, through an empirical pre‐whitening analysis, that periodic signals are embedded in otherwise aperiodic abyssal hills across paleo‐spreading rates ranging from 1 to 9 cm/yr (half rate). Data are comprised of archival trackline and multibeam bathymetry profiles in north and south Atlantic Oceans, Pacific‐Antarctic Ridge flanks, and north and south East Pacific Rise flanks. Detected periods vary widely, from the low tens of thousands to millions of years, and do not cluster at specific values. Up to half spreading rates of ∼4 cm/yr, periodicities are negatively correlated with spreading rate at high confidence, despite the strong scatter in results; there is no correlation at higher spreading rates. Both the strong scatter and spreading rate dependence indicate that periodicities are intrinsically generated. I hypothesize that periodic signals are controlled by volcanic processes, whereas aperiodic morphology is controlled by faulting.

Sea-level is expected to continue to rise in the next century, and as society prepares to deal with this hazard it is critically important to understand how coastal systems will respond, especially in regions with rapid rates of coastal erosion and relative sea-level rise like the Gulf of Mexico Texas coast. Tide gauge records in Galveston Bay, Texas, indicate that local sea level rise rates are more than twice the global average, raising important questions about the long-term stability of the barrier islands protecting the bay and how the estuary and coastline will respond to sea-level rise. However, tide gauge records only go back to the beginning of the last century, and longer records are needed to provide insight into dynamic coastal response to sea-level fluctuations. Here, we combine geophysical (chirp sub-bottom profiler) surveys and sediment cores (providing sedimentological and micropaleontological data constrained by radiocarbon dating) to characterize paleoenvironmental change in the Holocene estuary system offshore modern Galveston Bay over the last ~10 kyr; with the first 4 kyr of this time span undergoing a period of rapid sea level rise more than twice the modern rate. Our foraminiferal analysis provides ecological context on the stability of these paleoenvironments and the timing of coastal change over the last ~10 kyr. We provide a model of Holocene shoreline change differing from existing interpretations of rapid landward shifts with asymmetric coastal geometry to one composed of more gradual transitions matching modern coastal geometry and argue for an overall stable paleoestuarine environment throughout the middle Holocene (~6.9 ka – 8.8 ka). Subsequent shoreline shifts occurred after global sea level rise slowed below modern rates, indicating hydroclimate impacts on sediment flux likely had a greater influence on the earlier stability of the estuarine system and later shoreline retreat than rates of sea-level rise.

The Gulf of Mexico shorelines of Texas are in a largely regressive state due to an amalgamation of factors including but not limited to: sea level rise, subsidence, littoral drift, and riverine supply disruptions. In some areas the erosion rate is several meters annually, leading to significant loss of coastal barrier island area. The bulk of the coastal barrier systems in Texas is undeveloped. However, barrier island loss is no longer able to go unnoticed as there is ever-growing recognition of their intrinsic value to coastal habitat and economies and their role in coastal storm resilience. Projects with significant merit and sediment needs are being identified for funding, which could result in a firstcome- first-serve extraction of the easily available and less costly sediment resources. Without proper management, the complexity and cost of future projects will naturally escalate, leading to a reduction in investments due to lower returns. The following questions need to be addressed: how can lower returns be avoided to incentivize investment and maximize coastal restoration efforts in Texas? And what can be done to galvanize support for beneficial use (BU) placement of beach-quality dredge material to minimize its removal from the littoral system? What can be done now to inform and leverage the existing and future knowledge so that management decisions lead to better outcomes for the environment and Texas constituents? A well thought out and executed plan maximizing returns and coastal resilience will enable other regions to more easily replicate the success.

Abyssal hills are potential proxy records of mid‐ocean ridge faulting and volcanism, as well as the spreading rates and mantle properties that influence these processes. Satellite gravity‐based global prediction of abyssal hill root‐mean‐square (RMS) heights could provide such a proxy, but are broadly influenced off‐axis by pelagic sediment cover, which can lower their values by preferentially filling lows. Here I formulate a sediment‐corrected RMS height prediction by estimating an empirical relationship between mean RMS and sediment thickness as a function of spreading rate. I utilize these values to investigate regional variations in the spreading‐rate dependence of mean RMS across eight regions world‐wide, focusing on half spreading rates <40 mm/yr where spreading rate dependence is strongest. I find that regional variations in this relationship are significant, likely indicating heterogeneity in mantle properties between the different regions.

Many of the world's mega‐river deltas are currently degrading due to a shortage of sediment supply feeding their construction. Here, using bathymetric and seismic data, the geomorphological/sedimentary evolution on and near a recently abandoned (1976 CE) Huanghe delta lobe is investigated. The delta front (DF) exhibits a lobe switching‐controlled pattern of progradation followed by mass wasting on the upper front, and redeposition at the seaward toe of the front; the latter process appears to be associated with sediment failure and contributes to delta degradation. During degradation, mass wasting‐driven deepening of the upper DF was initially up to ∼0.86 m/yr from 1978 to 1985 CE, slowed to ∼0.12 m/yr from 1985 to 2000 CE, and has been negligible since. In either prograding or degrading, gravity‐driven sedimentation results in a downlapping seismic unit superimposed on the bottomset of an earlier converging clinoform, with numerous buried/exposed submarine channels at its seaward terminus. These units exhibit a shore‐parallel elongated and continuous geometry, suggesting lateral spreading, which we attribute to alongshore hydrodynamic (especially tidal) remolding. Our findings underscore the significance of sediment gravity flows in the evolution of a degrading delta front, particularly as a principal mechanism of downslope mass transport that gentled (halving the gradient) and reshaped the DF over time. Gravity‐driven accumulations sourced from upper DF collapse can account for ∼1/3 of the previous DF and laterally extend over an unexpectedly large area (∼60‐km long) through hydrodynamic redistribution. Both processes should be considered for modeling other subaqueous deltas.

Subsurface fluvial deposits in coastline‐proximal settings record the spatiotemporal evolution of the coastal landscape and may be viable repositories of sediment for future coastal restoration projects. However, quantitative linkages between the geomorphic form and stratigraphic expression of coastal plain fluvial elements remain lacking, complicating coastal stratigraphic interpretations and subsurface resource assessment. This study explores the expression and preservation of fluvial coastal plain geomorphic features through outcrop‐scale seismic stratigraphic analysis of ultra‐high‐resolution chirp acoustic reflection data from the north‐western Gulf of Mexico inner continental shelf, offshore the Brazos River, Texas, USA. The chirp data exhibit decimetre‐scale vertical resolution within the upper 35 m of the shelf, evincing the preservation of two distinct types of coexisting fluvial channel‐forms: Type 1, a 1 km wide and 9 m deep single‐storey channel belt filled with sand‐rich lateral accretion deposits and channel fills; and Type 2, numerous 10 to 800 m wide and 1 to 20 m deep incisional floodplain channels filled with extensive drapes of overbank‐derived sediments. The chirp data resolve fluvial geomorphic elements including lateral accretion surfaces, levées, floodplain deposits and abandoned channel fills. Rollover of lateral accretion surfaces, widespread draping of seismic reflectors within channel fills, and quantitative comparison between the interpreted stratigraphic forms and analogous features on the nearby Texas Coastal Plain suggest near‐complete stratigraphic preservation of the geomorphic form of both channel belt and floodplain channel elements. Rapid aggradation of the coastal plain by the Brazos River in this region during the Holocene transgression is hypothesized as a mechanism for the high degree of preservation achieved. This study proposes the first recognition criteria for the seismic stratigraphic expression of coastal floodplain channels and provides direct linkages between the geomorphic and stratigraphic expression of a coastal plain fluvial landscape, promoting improved coastal stratigraphic interpretations and assessment of subsurface lithofacies distributions.

Radiation and breaking of internal lee waves are thought to play a significant role in the energy and heat budget of the Southern Ocean. Open questions remain, however, regarding the amount of energy converted from the deep flows of the Antarctic Circumpolar Current (ACC) into lee waves and how much of this energy dissipates locally. This study estimated the linear lee‐wave energy radiation using a unique 4‐year time series of stratification and near‐bottom currents from an array of Current and Pressure measuring Inverted Echo Sounders (CPIES) spanning Drake Passage. Lee‐wave energy was calculated from two 2D anisotropic and one 1D isotropic abyssal hill topographies. Lee‐wave energy radiation from all three topographies was largest in the Polar Front Zone associated with strong deep meandering of the ACC fronts. Both baroclinic and barotropic instabilities appeared to modulate the conversion to lee waves in the Polar Front Zone. Fine structure temperature, salinity, and velocity profiles at the CPIES locations were used to estimate turbulent dissipation due to breaking internal waves by employing a finescale parameterization. High dissipation near the bottom was consistent with upward‐propagating, high‐frequency lee waves as found by earlier studies. In contrast to idealized numerical predictions of 50% local dissipation of lee‐wave energy, this study found less than 10% dissipated locally similar to some other studies. Improving the representation of the abyssal hills by accounting for anisotropy did not reduce the discrepancy between radiated lee‐wave energy and local dissipation. Instead, alternative fates must be considered for the excess radiated lee‐wave energy.

Modern lowland river systems show transitions in flow characteristics near coastlines that lead to systematic changes in sediment deposition and stratigraphic architecture. Sensitivity of fluvial morphodynamics to base-level has important implications for the prediction and interpretation of fluvial stratigraphy, particularly in deposits formed during periods of relative sea-level rise such as the early Holocene. Improving our understanding of how fluvial stratigraphy is created and preserved in such environments is crucial to paleoenvironmental reconstruction, sand resource estimation, and mapping subsurface facies distributions. A significant challenge has been capturing the spatiotemporal evolution of such systems on long timescales (centuries to millennia), often due to poor data coverage and resolution. Here we investigate the offshore stratigraphic architecture of the Trinity coastal river system, Texas. The paleo-valley is interpreted to have formed during the last lowstand and filled by backstepping fluvial-deltaic and estuarine sediments over the Holocene. However, the nature of this transition and resulting stratigraphy is unclear. We present unprecedented imaging of the incised valley and fluvial stratigraphy using a combination of over 500km2 of 3D seismic and nearly 700km of full waveform chirp data with 250m line spacing, combined with sediment cores and geotechnical borings. Our chirp processing technique creates images of strata on the decimeter scale, allowing for near outcrop scale mapping and interpretation. Archival industry 3D seismic data show that the basal valley fill is comprised of a highly-amalgamated fluvial channel belt with numerous avulsions and loop cutoffs. The chirp data show the transition of individual fluvial channels from sand rich, laterally migrating systems to muddy channels that depict high rates of vertical aggradation and little lateral movement. We observe the interaction of these channels with the paleo-floodplain and show that aggradation and infilling of the incised valley is dominated by fluvial processes before a transition to bay and estuarine conditions. Our work illustrates that traditional models of incised valley filling and fluvial response to transgression fail to fully capture the morphodynamics of coastal river systems.

To date, ∼20% of the ocean floor has been surveyed by ships at a spatial resolution of 400 m or better. The remaining 80% has depth predicted from satellite altimeter‐derived gravity measurements at a relatively low resolution. There are many remote ocean areas in the southern hemisphere that will not be completely mapped at 400 m resolution during this decade. This study is focused on the development of synthetic bathymetry to fill the gaps. There are two types of seafloor features that are not typically well resolved by satellite gravity; abyssal hills and small seamounts (<2.5 km tall). We generate synthetic realizations of abyssal hills by combining the measured statistical properties of mapped abyssal hills with regional geology including fossil spreading rate/orientation, rms height from satellite gravity, and sediment thickness. With recent improvements in accuracy and resolution, it is now possible to detect all seamounts taller than about 800 m in satellite‐derived gravity and their location can be determined to an accuracy of better than 1 km. However, the width of the gravity anomaly is much greater than the actual width of the seamount so the seamount predicted from gravity will underestimate the true seamount height and overestimate its base dimension. In this study, we use the amplitude of the vertical gravity gradient (VGG) to estimate the mass of the seamount and then use their characteristic shape, based on well‐surveyed seamounts, to replace the smooth‐predicted seamount with a seamount having a more realistic shape.