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Ocean Bottom Seismometers Provide Direct Measurements of Pulsed‐Structure and Turbulence of Turbidity Currents Overspilling From a Submarine Channel
Turbidity currents transport vast amounts of sediment, carbon, and heat along submarine channels, yet their overspill onto channel‐levees and abyssal mixing remain poorly constrained due to lack of direct observations. Ocean‐bottom seismometers (OBS) deployed on the Congo Canyon–Channel levees captured the structure and turbulence of overspill during an exceptionally large canyon‐flushing event in 2020. Overspill persisted for 3 weeks and comprised numerous short (20‐min to 2‐hr) pulses focused at outer bends. Spectra during overspill show well‐resolved turbulence inertial subranges, yielding event‐average dissipation rates of 10−6–10−5 m2 s−3, comparable to energetic internal‐tide breaking. Abyssal overspill can therefore be long‐lasting and highly pulsed, providing an episodic but locally important source of deep‐ocean mixing. This new view of levee overspill has important implications for building levees and the interpretation of ancient turbidites. Individual levee deposits may be formed incrementally by many pulses of dilute and fine‐grained flow from a single turbidity current.
Journal Article
Ocean discoveries
\"Imagine a fish without a face, drones exploring the ocean floor, and underwater waterfalls! Readers learn all about these amazing underwater discoveries and many more in these carefully-leveled and engaging books reviewed by Smithsonian experts\"-- Provided by publisher.
Book
Seabed Seismographs Reveal Duration and Structure of Longest Runout Sediment Flows on Earth
Turbidity currents carve the deepest canyons on Earth, deposit its largest sediment accumulations, and break seabed telecommunication cables. Powerful canyon‐flushing turbidity currents break sensors placed in their path, making them notoriously challenging to measure, and thus poorly understood. This study provides the first remote measurements of canyon‐flushing flows, using ocean‐bottom seismographs located outside the flow's destructive path, revolutionizing flow monitoring. We recorded the internal dynamics of the longest sediment flows yet monitored on Earth, which traveled >1,000 km down the Congo Canyon‐Channel at 3.7–7.6 m s−1 and lasted >3 weeks. These observations allow us to test fundamental models for turbidity current behavior and reveal that flows contain dense and fast frontal‐zones up to ∼400 km in length. These frontal‐zones developed near‐uniform durations and speeds for hundreds of kilometres despite substantial seabed erosion, enabling flows to rapidly transport prodigious volumes of organic carbon, sediment, and warm water to the deep‐sea. Plain Language Summary Seafloor avalanches of sediment, called turbidity currents, transport huge volumes of sediment and organic carbon to the deep‐sea, and they break critical seabed telecommunication cables that underpin global data transfer. However, turbidity currents are very difficult to measure directly as they often damage sensors placed in their flow path, so they are poorly understood. Here we show that turbidity currents generate ground vibrations that can be measured using ocean‐bottom seismographs placed outside the flow's destructive path, revolutionizing flow monitoring. These seismographs recorded the longest sediment flows yet measured in action on Earth, which traveled >1,000 km along the submarine Congo Canyon‐Channel offshore West Africa. We use these observations to test fundamental models of turbidity current flow behavior. Our measurements show that the front of the flows contain a fast frontal‐zone with high sediment concentrations, which can be up to ∼400 km long, whilst the whole duration of the flow can last for more than 3 weeks. These frontal‐zones develop near‐uniform durations and speeds, despite extensive seabed erosion that adds sediment into the flow. New information on flow durations shows how turbidity currents rapidly deliver prodigious volumes of organic carbon, sediment, and warm water to the deep‐ocean floor. Key Points Remote seismic monitoring reveals the duration, internal structure, and evolution of powerful canyon‐flushing turbidity currents Flows contain dense and fast frontal‐zones (up to ∼400 km long) that maintain uniform durations and speeds despite huge seabed erosion Canyon‐flushing flow frontal‐zones can bring substantial fluxes of organic carbon, sediment, and warm water to the deep‐sea in <24 hr
Journal Article
Seismic Imaging of Upper Mantle Serpentinization and Ponded Melt at the Boundary of the Lithosphere and Asthenosphere in the Central Mariana Subduction Zone
The Mariana subduction zone, with its simple oceanic structure and water‐rich environment, provides a natural laboratory for studying mantle hydration and the Lithosphere–Asthenosphere Boundary (LAB). Here, we analyze Ocean Bottom Seismometer (OBS) deployed across both the forearc and incoming plate regions to investigate the S‐wave velocity structure beneath the central Mariana subduction zone. Using multi‐frequency receiver functions and surface‐wave dispersion data, we apply a transdimensional Bayesian joint inversion that accounts for water‐layer effects to obtain high‐resolution images of the subsurface structure. Our results confirm significant mantle hydration and reveal a distinct low‐velocity zone at the LAB. Unlike previous studies that depict the LAB as a single boundary, our results indicate a rapid velocity decrease followed by an equally sharp increase, delineating a ∼15 km thick melt‐rich zone. The existence of such a melt‐rich zone may reduce mantle viscosity and facilitate decoupling between the lithosphere and asthenosphere.
Journal Article
Mysteries of the deep : how seafloor drilling expeditions revolutionized our understanding of earth history
\"This book tells the story of how scientific ocean drilling, a crowning achievement of science and engineering in the 20th century, transformed our understanding of Earth's history\"-- Provided by publisher.
Book
Frequency Dependent Microseisms Sources: A Case Study in Oregon
The origin of microseisms—whether from deep‐ocean sources or coastal reflections—has been debated for decades. In this study, we use Distributed Acoustic Sensing (DAS) and Ocean Bottom Seismometer data collected offshore Oregon to investigate microseisms sources across a range of frequency bands. Our results reveal a clear frequency dependence: high‐frequency (0.35–1.5 Hz) microseisms primarily originates near the coastline due to wind ocean waves, with minimal contribution from the deep ocean. In short‐period double frequency (SPDF, 0.2–0.35 Hz) microseisms, the source regions extend farther offshore and are increasingly influenced by deep‐ocean sources. Long‐period double frequency (LPDF, 0.1–0.2 Hz) microseisms are predominantly generated in the deep ocean. Furthermore, we find that microseisms generated by coastal reflections do not propagate into the deep ocean.
Journal Article
Microseisms at the Gakkel Ridge, Arctic Ocean: Results from the JASMInE ocean bottom seismic experiment
To characterize mid-ocean microseisms in the Arctic Ocean and explore potential seismic ambient noise imprint of Arctic warming, this study analyzes seismic records from nine ocean bottom seismometers (OBSs) deployed along the eastern Gakkel Ridge during the Joint Arctic Scientific Middle-ocean ridge Insight Expedition (JASMInE) in August 2021. In the period band of single frequency microseisms (10–20 s), typically produced by ocean waves directly impacting coastlines, no prominent spectral peaks are observed. In the double frequency microseism (DFM) period band (2–10 s), spectral powers are far less energetic than those in the open oceans by approximately 20–40 dB, especially for OBSs deployed off the Gakkel Ridge axis. This dramatically weak DFMs can be attributed to the presence of the perennial sea-ice cover, which hinders atmosphere-ocean interactions and thus obstructs the generation of DFMs. Based on polarization analyses of Rayleigh waves and correlations of DFM power and ocean wave height, the weak DFMs recorded on the seafloor likely originate from the northern Barents Sea and adjacent regions. As an Arctic warming hotspot, the northern Barents Sea is experiencing reduced sea-ice import from the interior Arctic which enhances atmosphere-ocean interactions. In this region, wind-driven waves with highly variable directions as documented by oceanographic data and the presence of the perennial sea-ice cover may promote the formation of wave trains propagating in nearly opposite directions, which nonlinear interactions excite DFMs. These DFMs continuously lose power due to seismic attenuation during propagation, becoming so weak that the OBSs deployed off the ridge axis detect no noticeable spectral peaks in the DFM period band. The OBSs along the ridge axis, by contrast, reveal more energetic power in the short-period DFM band of 2–5 s, which can be attributed to local DFM amplification caused by the thick unconsolidated sediment layer.
Journal Article