Widespread gas hydrate instability on the upper U.S. Beaufort margin

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 119 (2014): 8594–8609, doi:10.1002/2014JB011290.The most climate-sensitive methane hydrate deposits occur on upper continental slopes at depths close to the minimum pressure and maximum temperature for gas hydrate stability. At these water depths, small perturbations in intermediate ocean water temperatures can lead to gas hydrate dissociation. The Arctic Ocean has experienced more dramatic warming than lower latitudes, but observational data have not been used to study the interplay between upper slope gas hydrates and warming ocean waters. Here we use (a) legacy seismic data that constrain upper slope gas hydrate distributions on the U.S. Beaufort Sea margin, (b) Alaskan North Slope borehole data and offshore thermal gradients determined from gas hydrate stability zone thickness to infer regional heat flow, and (c) 1088 direct measurements to characterize multidecadal intermediate ocean warming in the U.S. Beaufort Sea. Combining these data with a three-dimensional thermal model shows that the observed gas hydrate stability zone is too deep by 100 to 250 m. The disparity can be partially attributed to several processes, but the most important is the reequilibration (thinning) of gas hydrates in response to significant (~0.5°C at 2σ certainty) warming of intermediate ocean temperatures over 39 years in a depth range that brackets the upper slope extent of the gas hydrate stability zone. Even in the absence of additional ocean warming, 0.44 to 2.2 Gt of methane could be released from reequilibrating gas hydrates into the sediments underlying an area of ~5–7.5 × 103 km2 on the U.S. Beaufort Sea upper slope during the next century.This work was supported by the U.S. Department of Energy (DOE), grant DE-FE0010180 to SMU and a USGS-DOE interagency agreement DE-FE0005806.2015-06-0

Oceanic sediment accumulation rates predicted via machine learning algorithm: towards sediment characterization on a global scale

AbstractObserved vertical sediment accumulation rates (n = 1031) were gathered from ~ 55 years of peer reviewed literature. Original methods of rate calculation include long-term isotope geochronology (14C,210Pb, and137Cs), pollen analysis, horizon markers, and box coring. These observations are used to create a database of global, contemporary vertical sediment accumulation rates. Rates were converted to cm year−1, paired with the observation's longitude and latitude, and placed into a machine learning–based Global Predictive Seabed Model (GPSM). GPSM finds correlations between the data and established global "predictors" (quantities known or estimable everywhere, e.g., distance from coastline and river mouths). The result, using a k-nearest neighbor (k-NN) algorithm, is a 5-arc-minute global map of predicted benthic vertical sediment accumulation rates. The map generated provides a global reference for vertical sedimentation from coastal to abyssal depths. Areas of highest sedimentation, ~ 3–8 cm year−1, are generally river mouth proximal coastal zones draining relatively large areas with high maximum elevations and with wide, shallow continental shelves (e.g., the Gulf of Mexico and the Amazon Delta), with rates falling exponentially towards the deepest parts of the oceans. The exception is Oceania, which displays significant vertical sedimentation over a large area without draining the large drainage basins seen in other regions. Coastal zones with relatively small drainage basins and steep shelves display vertical sedimentation of ~ 1 cm year−1, which is limited to the near shore when compared with shallow, wide margins (e.g., the western coasts of North and South America). Abyssal depth rates are functionally zero at the time scale examined (~ 10−4 cm year−1) and increase one order of magnitude near the Mid-Atlantic Ridge and at the Galapagos Triple Junction

A machine-learning derived model of seafloor sediment accumulation

Abstract Previous studies regarding the depositional pattern and quantity of accumulated seafloor sediment tend to be regional, limited in scope and involving costly and time-consuming geologic field campaigns and laboratory work. Presented herein is a global map of predicted modern (postindustrial, 20th and 21st century) oceanic mass accumulation rates of 5-arc-minute pitch and in log10-space, trained on observed marine mass accumulation rates from 43 peer reviewed sources (n = 1744) and predicted using a k-nearest neighbor geospatial algorithm. The resultant model predicts ~3.3 × 104 Mt. yr−1 of sediment accumulating onto the sea floor (R2 = 0.88). Most sediment accumulates proximal to major river outlets and deltas. Continental regions with the highest sediment accumulation are Asia and Oceania. This model is the first of its kind to predict the rate and quantity of sediment accumulating on to the ocean floor, globally, using decades of regional real-world observations. The generated global map of modern, benthic mass accumulation rates also serves to highlight areas of interest for future study in related fields, such as sediment dynamics and seafloor stability

Heat Flow on the U.S. Beaufort Margin, Arctic Ocean: Implications for Ocean Warming, Methane Hydrate Stability, and Regional Tectonics

Abstract Results from the first focused heat flow study on the U.S. Beaufort Margin provide insight into decadal‐scale Arctic Ocean temperature change and raise new questions regarding Beaufort Margin evolution. This study measured heat flow using a 3.5‐m Lister probe at 103 sites oriented along four north‐south transects perpendicular to the ~700‐km long U.S. Beaufort Margin. The new heat flow measurements, corrected both for seasonal ocean temperature fluctuations and bathymetric effects, reveal low average heat flow values (~35 mW/m2) at seafloor depths of 300–900 m below sea level (mbsl) and anomalously high (~80 mW/m2) values at seafloor depths of >1,000 mbsl, near the predicted continent‐ocean transition. Anomalously low heat flow values measured on the upper margin are consistent with previous studies suggesting decadal‐scale ocean temperature warming to ~500 mbsl. Our results, however, indicate this ocean warming likely extends to depths as great at 900 mbsl—400 m deeper than previous studies suggest—implying widespread, ongoing, methane hydrate destabilization across much of the U.S. Beaufort Margin. The cause of the anomalously high heat flow values observed at seafloor depths >1,000 at the continent‐ocean transition is unclear. We suggest three candidate processes: (1) higher heat production and lower thermal conductivity on the margin edge due to the thickest sedimentary cover at the ocean‐continent transition, (2) seaward migrating subsurface advection, and (3) possible fault‐reactivation at the northern boundary of the Alaskan Microplate

A Machine Learning Approach Using Legacy Geophysical Datasets To Modeling Quaternary Marine Paleotopography

High-resolution subsurface marine mapping tools, including chirp and 3D seismic, enable the reconstruction of ancient landscapes that have been buried and subsequently submerged by marine transgression. However, the established methods for paleotopographic reconstruction require time consuming field and data interpretation efforts. Here we present a novel methodology using machine learning to estimate Marine Isotope Stage 2 (MIS2) paleotopography over a large (22 000 km2) area of the Northern Gulf of Mexico with meter-scale accuracy (2.7 m mean prediction error, 4.3 m 1-σ mean uncertainty). A relatively small area (3300 km2) of high-resolution (30 × 30 m) interpreted paleotopography is used as training and validation data, while modern bathymetry and MIS2 paleovalley location (binary deep/shallow paleotopography) are used as predictors. This approach merges the high-resolution of modern mapping techniques and the broad coverage of low-resolution legacy geophysical data. Machine learning-modeled paleotopography is not a substitute for precise high-resolution paleotopography reconstruction techniques, but it can be used to reasonably approximate paleotopography over large areas with greatly reduced expense and expertise

A machine learning approach using legacy geophysical datasets to model Quaternary marine paleotopography

High-resolution subsurface marine mapping tools, including chirp and 3D seismic, enable the reconstruction of ancient landscapes that have been buried and subsequently submerged by marine transgression. However, the established methods for paleotopographic reconstruction require time consuming field and data interpretation efforts. Here we present a novel methodology using machine learning to estimate Marine Isotope Stage 2 (MIS2) paleotopography over a large (22 000 km2) area of the Northern Gulf of Mexico with meter-scale accuracy (2.7 m mean prediction error, 4.3 m 1-σ mean uncertainty). A relatively small area (3300 km2) of high-resolution (30 × 30 m) interpreted paleotopography is used as training and validation data, while modern bathymetry and MIS2 paleovalley location (binary deep/shallow paleotopography) are used as predictors. This approach merges the high-resolution of modern mapping techniques and the broad coverage of low-resolution legacy geophysical data. Machine learning-modeled paleotopography is not a substitute for precise high-resolution paleotopography reconstruction techniques, but it can be used to reasonably approximate paleotopography over large areas with greatly reduced expense and expertise

Deep Fault‐Controlled Fluid Flow Driving Shallow Stratigraphically Constrained Gas Hydrate Formation: Urutī Basin, Hikurangi Margin, New Zealand

The Hikurangi Margin east of New Zealand's North Island hosts an extensive gas hydrate province with numerous gas hydrate accumulations related to the faulted structure of the accretionary wedge. One such hydrate feature occurs in a small perched upper‐slope basin known as Urutī Basin. We investigated this hydrate accumulation by combining a long‐offset seismic line (10‐km‐long receiver array) with a grid of high‐resolution seismic lines acquired with a 600‐m‐long hydrophone streamer. The long‐offset data enable quantitative velocity analysis, while the high‐resolution data constrain the three‐dimensional geometry of the hydrate accumulation. The sediments in Urutī Basin dip landward due to ongoing deformation of the accretionary wedge. These strata are clearly imaged in seismic data where they cross a distinct bottom simulating reflection (BSR) that dips counterintuitively in the opposite direction to the regional dip of the seafloor. BSR‐derived heat flow estimates reveal a distinct heat flow anomaly that coincides spatially with the upper extent of a landward‐verging thrust fault. We present a conceptual model of this gas hydrate system that highlights the roles of fault‐controlled fluid flow at depth merging into strata‐controlled fluid flow into the hydrate stability zone. The result is a layer‐constrained accumulation of concentrated gas hydrate in the dipping strata. Our study provides new insight into the interplay between deep faulting, fluid flow and gas hydrate formation within an active accretionary margin. Plain Language Summary Gas hydrates are ice‐like substances in which natural gas molecules are trapped in a cage of water molecules. They exist where the pressure is high, temperature is cold, and enough methane is present. These conditions exist in the marine environment at water depths greater than 300–500 m near sediment‐rich continental margins and in polar regions. It is important to study gas hydrates because they represent a significant part of the Earth's carbon budget and influence the flow of methane into the oceans and atmosphere. In this study, we use the seismic reflection method to generate images of gas‐hydrate‐bearing marine sediments east of New Zealand. Our data reveal an intriguing relationship between deep‐sourced fluid flow upward along a tectonic fault, and shallower flow through dipping sediments. This complex fluid flow pattern has led to disruption of the gas hydrate system and the formation of concentrated gas hydrate deposits within the dipping sediments. Our study highlights the relationships between relatively deep tectonic processes (faulting and fluid flow) and the shallow process of gas hydrate formation in an active subduction zone. Key Points A distinct gas‐hydrate to free‐gas transition is mapped using high‐ and low‐frequency seismic data Gas and hydrate accumulations in the Urutī Basin are controlled by the structural setting, ongoing deep‐sourced fluid flow, and near surface stratigraphy Regions of high modeled heat flow can be directly related to accumulations of gas and gas hydrate

The Hidden History of the South‐Central Cascadia Subduction Zone Recorded on the Juan de Fuca Plate Offshore Southwest Oregon

Abstract New seismic reflection data collected and processed as part of early career scientist training at sea and in classroom projects fill gaps in seismic coverage of the Cascadia subduction zone and provide new insights into anomalous subduction behavior and mass wasting along the south‐central Cascadia Subduction Zone (CSZ) between 42°20’N and 44°15’N. The data reveal at least six distinct buried horizons of folded and faulted sediments similar to strata recently interpreted to result from in situ deformation induced by the load imposed by a large blocky mass transport deposit known as the 44°N slide. Although our results support prior studies indicating that the south‐central CSZ has experienced large slope instabilities, they indicate that the slides were more frequent but volumetrically smaller than previously thought. Similar strata have not been observed elsewhere beneath the abyssal plain adjacent to the Cascadia subduction zone. The structure of the deformation front along this segment is also indistinct, in contrast to the clear frontal faults outboard of folded trench strata observed immediately to the north and south (and generally throughout the rest of Cascadia). We attribute the anomalous nature of this segment of the margin to past subduction of shallow and rough seafloor, which resulted in greater uplift of the forearc than elsewhere along the margin. A consequence of this postulated history would be the shedding of older, more consolidated blocks onto the Juan de Fuca plate, resulting in the observed distinctive stratigraphy offshore southern Oregon