Gorda Ridge Research Articles

Peculiarities of Clay Mineral Formation in Pleistocene Sediments Under Specific Tectonomagmatic and Hydrothermal Conditions of the Central Hill (Escanaba Trough, Gorda Ridge, Pacific Ocean): Communication 2. Holes ODP 1038A and 1038H

Peculiarities of Clay Mineral Formation in Pleistocene Sediments Under Specific Tectonomagmatic and Hydrothermal Conditions of the Central Hill (Escanaba Trough, Gorda Ridge, Pacific Ocean): Communication 2. Holes ODP 1038A and 1038H

Authigenic biotite from hydrothermally altered terrigenous sediments of the Central Hill (Escanaba Trough, Gorda Ridge, Pacific Ocean, hole ODP 1038B)

For the first time, authigenic dispersed biotite was discovered in Pleistocene terrigenous sediments of the Central Hill, located in the Escanaba Trough in the southern part of the Gorda Ridge (northeastern sector of the Pacific Ocean), which accounts for almost the entire content of fine fractions 1 μm of some samples from ODP 1038B hole. The authigenic nature of biotite is associated with the metasomatic effect of hydrothermal solution on terrigenous clay minerals after intrusion of basaltic magma in the form of laccolith with a temperature of ~1200°C. The mineral composition of fine fractions of sediments was studied using complex analytical methods, including modeling of their diffraction patterns. It has been established that the dispersed micaceous mineral (biotite) is trioctahedral, high-iron, does not swell when saturated with glycol, but contracts after heating at 550°C. It is shown that in its structure there are no foreign layers, the height and composition of which differ from the micaceous layers. The decrease in the height of micaceous layers upon heating biotite to 550°C is mainly due to a decrease in the height of 2:1 octahedra due to the difference in the Fe2+–O and Fe3+–O bond lengths as a result of the oxidation of Fe2+ cations It has been established a limit value for the coefficient of variation CV, characterizing the absence of mixed-layering in a regular structure, which should not exceed a value of 0.10.

Globally-distributed microbial eukaryotes exhibit endemism at deep-sea hydrothermal vents.

Single-celled microbial eukaryotes inhabit deep-sea hydrothermal vent environments and play critical ecological roles in the vent-associated microbial food web. 18S rRNA amplicon sequencing of diffuse venting fluids from four geographically- and geochemically-distinct hydrothermal vent fields was applied to investigate community diversity patterns among protistan assemblages. The four vent fields include Axial Seamount at the Juan de Fuca Ridge, Sea Cliff and Apollo at the Gorda Ridge, all in the NE Pacific Ocean, and Piccard and Von Damm at the Mid-Cayman Rise in the Caribbean Sea. We describe species diversity patterns with respect to hydrothermal vent field and sample type, identify putative vent endemic microbial eukaryotes, and test how vent fluid geochemistry may influence microbial community diversity. At a semi-global scale, microbial eukaryotic communities at deep-sea vents were composed of similar proportions of dinoflagellates, ciliates, Rhizaria, and stramenopiles. Individual vent fields supported distinct and highly diverse assemblages of protists that included potentially endemic or novel vent-associated strains. These findings represent a census of deep-sea hydrothermal vent protistan communities. Protistan diversity, which is shaped by the hydrothermal vent environment at a local scale, ultimately influences the vent-associated microbial food web and the broader deep-sea carbon cycle.

Stable potassium (K) isotope characteristics at mid-ocean ridge hydrothermal vents and its implications for the global K cycle

Recent discoveries of significant variations in stable K isotope ratios (41K/39K or δ41K) among various terrestrial samples indicate that K isotopes can be a novel tracer for the global K cycle, but a key observation that seawater δ41K is ∼0.6‰ higher than the bulk silicate Earth remains unexplained. An unconstrained component critical to this puzzle is hydrothermal systems that represent both a major K source and sink in the ocean. Here we report δ41K results on mid-ocean ridge (MOR) hydrothermal fluids from the Gorda Ridge and ∼9°N East Pacific Rise (EPR), including time-series samples that recorded major perturbations in fluid chemistry induced by a local volcanic eruption. Fluid δ41K values range from -0.46‰ to -0.15‰, falling between those of fresh basalts and seawater. δ41K values of “time-zero” fluids collected shortly after the volcanic eruption are shifted towards the seawater value, followed by a return to pre-eruption values within ∼2 years. Fluid δ41K variations are largely influenced by water–rock interactions, but they cannot be solely explained by simple mixing of seawater and K leached from basalts at high temperatures. Instead, these data imply small but significant isotope fractionation that enriches heavy K isotopes in basalts, likely caused by low-temperature alteration during the recharge stage of hydrothermal circulation. Our results preclude MOR hydrothermal systems as the cause for the heavy δ41K value of seawater. Using fluid δ41K data and K isotope fractionation constrained here for hydrothermal systems, a K mass-balance model implies a critical role for a marine sedimentary sink, possibly authigenic clay formation, in the global K cycle. Also, applying the K isotope fractionation constrained here to the published δ41K data from ophiolites shows the possibility for significantly lower seawater δ41K during the Ordovician, which can be explained by enhanced reverse weathering in response to distinct climate and tectonics at that time.

Protistan grazing impacts microbial communities and carbon cycling at deep-sea hydrothermal vents

Microbial eukaryotes (or protists) in marine ecosystems are a link between primary producers and all higher trophic levels, and the rate at which heterotrophic protistan grazers consume microbial prey is a key mechanism for carbon transport and recycling in microbial food webs. At deep-sea hydrothermal vents, chemosynthetic bacteria and archaea form the base of a food web that functions in the absence of sunlight, but the role of protistan grazers in these highly productive ecosystems is largely unexplored. Here, we pair grazing experiments with a molecular survey to quantify protistan grazing and to characterize the composition of vent-associated protists in low-temperature diffuse venting fluids from Gorda Ridge in the northeast Pacific Ocean. Results reveal protists exert higher predation pressure at vents compared to the surrounding deep seawater environment and may account for consuming 28 to 62% of the daily stock of prokaryotic biomass within discharging hydrothermal vent fluids. The vent-associated protistan community was more species rich relative to the background deep sea, and patterns in the distribution and co-occurrence of vent microbes provide additional insights into potential predator-prey interactions. Ciliates, followed by dinoflagellates, Syndiniales, rhizaria, and stramenopiles, dominated the vent protistan community and included bacterivorous species, species known to host symbionts, and parasites. Our findings provide an estimate of protistan grazing pressure within hydrothermal vent food webs, highlighting the important role that diverse protistan communities play in deep-sea carbon cycling.

Forward geochemical modeling as a guiding tool during exploration of Sea Cliff hydrothermal field, Gorda Ridge

Maximizing scientific return is critical to the success of space exploration. During the SUBSEA project, which explored the Sea Cliff vent site aboard the E/V Nautilus as an analog for hydrothermal systems on Ocean Worlds, we used forward geochemical modeling to guide decision-making during the process of exploration. Before the expedition, we performed 1670 reaction-path calculations to simulate water-rock interactions during hydrothermal circulation covering wide ranges of reaction conditions to predict the diversity of possible chemical compositions and energy available for chemosynthetic microorganisms at the Sea Cliff vents. Calculation of the information entropy of predicted concentrations of major solutes and pH allowed us to identify dissolved silica as the chemical species capable of yielding the most information about reaction conditions; as such, the measurement of this parameter was implemented aboard ship for our field program. Using telepresence, results of onboard chemical analyses of fluid samples collected during seafloor Dive n were sent to our shore-based scientific team, who processed the data and used the outcomes to inform the design of Dive n ​+ ​2. Combining data processing with forward modeling revealed, within just two dives, that all the observed fluids venting from 10 °C to 300 ​°C most likely resulted from simple conservative mixing between seawater and a common hydrothermal fluid end-member: the result of reaction of seawater with basalt at ≥350 ​°C. Identification of these reaction conditions early within the cruise allowed additional calculations to be performed to quantify the energy available from redox disequilibria as a function of vent-fluid temperatures as they exited the seafloor. These calculations can help inform and optimize real-time microbiological sampling and culture experiments onboard the ship during field expeditions. The success of our approach coupling forward modeling and onboard ship analyses allowed improved efficiency in completing process studies at the Sea Cliff vent site, providing time for further exploration and sampling of a newly discovered vent site: Apollo. This study demonstrates a novel application of forward and real-time modeling for scientific exploration that allows the time required for result-informed decision making to be reduced from years to hours – an essential breakthrough for future space exploration missions.

Fourier transform ion cyclotron resonance mass spectrometric analysis of NSO-compounds generated in hydrothermally altered sediments from the Escanaba Trough, northeastern Pacific Ocean

To date little is known about the functionalized and heteroatomic compounds present in hydrothermal petroleum and how these compounds are affected by high temperature vent fluids. To address this, the maltene fractions of three hydrothermal petroleum samples spanning the shallow depth of a sediment core collected in the Escanaba Trough of the Gorda Ridge in the northeast Pacific were analyzed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in both negative electrospray ionization (ESI) and negative atmospheric pressure photoionization (APPI) ionization modes. These polar compounds were structurally similar to asphaltenes, but did not exceed a nominal mass range of ∼800 Da. They were derived primarily from terrestrially sourced organic matter (OM) with common heteroatom arrangements of CaHbOc, CaHbOcNd, CaHbNd, CaHbOcSe, CaHbOcNdSe, CaHbNdSe, and CaHbSe. The samples included compounds containing up to 19 oxygen heteroatoms and ranged in size from 10 to 53 carbon atoms with double bond equivalent (DBE) values of 1–38. Molecular formulae with N and S heteroatoms were minor. Within this sulfidic environment the formation and preservation of sulfur heterocyclic compounds was very low, which could be due to sulfurization being out-competed by the high concentrations of metals in the vent fluids or as a result of the thermal instability of C–S bonds. The dominance of oxygenated, and not thiolated, compounds was primarily source-dependent with secondary alteration by aromatization in this system proceeding by H abstraction and reduction of sulfate to H2S. We identified differences in the distributions of heteroatomic compounds caused by the expulsion, thermochemical oxidation, and differentiation of petroleum products during the migration of fluids to the sediment–water interface.

Lava Flows Erupted in 1996 on North Gorda Ridge Segment and the Geology of the Nearby Sea Cliff Hydrothermal Vent Field From 1-M Resolution AUV Mapping

Author(s): Clague, DA; Paduan, JB; Caress, DW; McClain, J; Zierenberg, RA | Abstract: The northernmost segment of the Gorda mid-ocean ridge is the site of a small-volume eruption in 1996 and the persistent off-axis Sea Cliff hydrothermal vent field. To better understand the geologic setting and formation of these features, 1-m resolution bathymetric mapping using autonomous underwater vehicles was completed in 2016. The mapped region covers 35 km2 and 15.6 km of the volcanic axis from south of the 1996 lava flows, and a cross section for ∼4.5 km perpendicular to the axis, that extends beyond the Sea Cliff hydrothermal vent field. A proposed 1996 flow ∼7 km south of previously mapped flows is an artifact from a poor pre-eruption survey. The 1996 flows consist of three discrete steep hummocky mounds of pillows and syneruptive talus. The Sea Cliff hydrothermal field is located a few km north of the narrowest, shallowest section of the ridge segment, 2.6 km east of the center of the neovolcanic zone, and ∼370 m above the average depth of the axial graben on the largest offset ridge-parallel fault. No evidence supports the prior hypothesis that the site is located where two fault systems intersect. The axial graben is asymmetrical with larger fault offsets on the east side. The ridge axis below the hydrothermal field and to the south toward the 1996 flows is constructed dominantly of hummocky flows of pillow basalt, many unusually steep-sided, with syneruptive talus at the base of their steep slopes. Three channelized flows ponded between steep hummocky flows, and then partially drained. Some low-eruption-rate hummocky flows and high-eruption-rate channelized flows have nearly identical compositions, supporting the idea that eruption rates on mid-ocean ridges vary because of different dike widths. Four volcanic structures with volumes between 0.18 and 0.25 km3 occur in the axial graben south of the 1996 flows. Two are flat-topped cones, another is a 1.5-km diameter inflated hummocky flow with 7 pit craters that demonstrate that the flow had a molten interior during growth. The fourth voluminous structure is a steep ridge with abundant syneruptive talus on its lower slopes. The North Gorda segment is an end-member, structurally and volcanically, compared with other Pacific intermediate-rate spreading ridges.

Hydrothermal alteration of sediments in the high-temperature reaction zone at Central Hill of Escanaba Trough, Gorda Ridge

In sediments from Holes ODP 1038A and 1038H, drilled near the hot springs at Central Hill, Escanaba Trough (Gorda Ridge), redistribution of major elements occurs during the water-rock interaction. Contents of Si, Fe, Mg, Ca, Na and K have changed in altered sediments. In this process, an increase/decrease in contents of major elements in altered sediments shows a decrease/increase in their contents in fluids. The irregular distribution of major elements in altered sediments resembles a layer pie, which reflects the existence of lateral flows of hydrothermal fluids.

A new deep-sea genus of the family Artotrogidae (Copepoda: Siphonostomatoida) from the Gorda Ridge (North Pacific Ocean)

A new deep-sea genus of the family Artotrogidae (Copepoda: Siphonostomatoida) from the Gorda Ridge (North Pacific Ocean)

Mantle dynamics beneath the discrete and diffuse plate boundaries of the Juan de Fuca plate: Results from Cascadia Initiative body wave tomography

AbstractWe use the delay times of teleseismic S phases recorded by ocean bottom seismometers during the plate‐scale Cascadia Initiative community experiment to constrain the heterogeneity of seismic velocity structure beneath young oceanic lithosphere. Our study area covers the entire Juan de Fuca (JdF) and Gorda plates, from their creation at the JdF and Gorda Ridges to their subduction beneath the North American continent, and the entire length of the Blanco transform fault. The range of the observed Vs anomalies requires variations in the melt fraction of the asthenosphere. The data require that low Vs anomalies extend to depths of at least 200 km, which is within the carbonatite melting regime. In the upper 200 km of the mantle, Vs increases rapidly to the east of the JdF Ridge, while there is no clear relationship with the age of the lithosphere in the Gorda region. The distribution of melt is asymmetric about both the JdF and Gorda Ridges. Dynamic upwelling – due to the buoyancy of the mantle – and accompanying downwelling can explain the rapid decrease in melt fraction to the east of the JdF Ridge, the asymmetry about the JdF Ridge, and the sinuous pattern of upwelling near the Blanco transform fault. Finally, mantle flow beneath the diffuse Gorda and Explorer plate boundaries is distinct from that beneath the discrete plate boundary of the JdF Ridge. In particular, shear between the Pacific and JdF plates appears to dominate mantle deformation over seafloor spreading beneath the Gorda Ridge.

Stalked Crinoids Collected off California with Descriptions of Three New Genera and Two New Species of Hyocrinidae (Echinodermata)

Five species of stalked crinoids collected at depths from 1,693 to 4,135 m off the California coast, chiefly on seamounts, are described. Four were photographed in situ and collected during Monterey Bay Aquarium and Research Institute cruises using ROV Tiburon . Four specimens with intact crowns are attributed to Bathycrinus equatorialis A. H. Clark, 1908 a , previously known from a single stalk with basal ring collected in the central eastern Pacific. Parahyocrinus claguei Roux, n. gen., n. sp., here described from a growth series of six specimens, is closely related to Hyocrinus biscoitoi Roux, 2004 (herein transferred to Parahyocrinus Roux, n. gen.), from the East Pacific Rise, but differs in features of the tegmen, pinnules, and stalk articulations. The single, finely ornamented specimen of Tiburonicrinus ornatus Roux, n. gen., n. sp., displays an original arrangement of characters suggesting affinities with Thalassocrinus alvinae from the Gorda Ridge. Gephyrocrinus messingi Roux & Lambert, 2011, known from off British Columbia and California, is transferred as type species to Lamberticrinus Roux, n. gen., after a reexamination of its pinnule architecture. Specimens of the fifth species, Bathycrinus complanatus A. H. Clark, 1908 b , were trawled off California by R/V New Horizon .

High seismic attenuation at a mid-ocean ridge reveals the distribution of deep melt.

At most mid-ocean ridges, a wide region of decompression melting must be reconciled with a narrow neovolcanic zone and the establishment of full oceanic crustal thickness close to the rift axis. Two competing paradigms have been proposed to explain melt focusing: narrow mantle upwelling due to dynamic effects related to in situ melt or wide mantle upwelling with lateral melt transport in inclined channels. Measurements of seismic attenuation provide a tool for identifying and characterizing the presence of melt and thermal heterogeneity in the upper mantle. We use a unique data set of teleseismic body waves recorded on the Cascadia Initiative's Amphibious Array to simultaneously measure seismic attenuation and velocity across an entire oceanic microplate. We observe maximal differential attenuation and the largest delays ([Formula: see text] s and δTS ~ 2 s) in a narrow zone <50 km from the Juan de Fuca and Gorda ridge axes, with values that are not consistent with laboratory estimates of temperature or water effects. The implied seismic quality factor (Qs ≤ 25) is among the lowest observed worldwide. Models harnessing experimentally derived anelastic scaling relationships require a 150-km-deep subridge region containing up to 2% in situ melt. The low viscosity and low density associated with this deep, narrow melt column provide the conditions for dynamic mantle upwelling, explaining a suite of geophysical observations at ridges, including electrical conductivity and shear velocity anomalies.

Ridge asymmetry and deep aqueous alteration at the trench observed from Rayleigh wave tomography of the Juan de Fuca plate

AbstractUsing Rayleigh wave tomography of noise‐removed ocean bottom seismometer data from the Cascadia Initiative, we illuminate the structure of the upper mantle beneath the Juan de Fuca plate. Beneath the Juan de Fuca ridge, there is strong asymmetry, with a pronounced low‐velocity zone in the 25–65 km depth range. Extending to the west from the spreading axis, this anomaly has velocities low enough to indicate the presence of melt. The asymmetry in velocity structure and the much greater abundance of seamounts on the west flank of the ridge suggest that dynamic, buoyant upwelling is important, perhaps triggered by thermal or compositional anomalies beneath Axial Seamount. In contrast, there is no evidence for asymmetry in the axial zone or lower than expected velocities beneath the Gorda ridge. On the eastern flank of the Juan de Fuca ridge, the shear velocity in the 25–65 depth range is higher than expected; the lithosphere appears to be colder and thicker than predicted by standard plate cooling models, perhaps caused by the downwelling counterpart of the upwelling on the west side of the ridge. Close to the trench, there is a sharp decrease in shear velocity. We interpret this as aqueous alteration caused by hydrothermal circulation through deep normal faults associated with bending of the plate. Beneath the Astoria and Nitinat fans, where abyssal plain sediment is thickest, the velocity decrease is much smaller, which is consistent with a thick sediment cap that prevents hydrothermal alteration of the plate.

A new deep-sea genus and species of the family Ecbathyriontidae (Copepoda: Siphonostomatoida) from the Gorda Ridge (North Pacific Ocean)

Adult females of Bathygordion cliffus new gen. and new sp. belonging to the family Ecbathyriontidae Humes, 1987 are described from the Gorda Ridge of the North Pacific Ocean. The copepods were found at depth of 2701 m on the margin of a hydrothermal vent field (Sea Cliff, GR-14) among invertebrates colonizing wood blocks deployed for 2 years. The new genus differs from the only monotypic genus Ecbathyrion Humes, 1987 reported from hydrothermal vents of the Galapagos Rift and the East Pacific Rise in a number of morphological features (such as three articulating abdominal somites, mandible with 1 palp bearing setae, base of leg 1 with inner seta, distal exopodal segment of leg 4 with four inner setae, leg 5 2-segmented), suggesting long evolutionary history of the deep-sea family endemic to hydrothermal vents of the East Pacific. The derived feature of the new genus is the presence of one terminal spine on the distal endopodal segment of leg 3 (instead of bearing two terminal setae in Ecbathyrion). The diagnosis of the family Ecbathyriontidae is emended.

Molluscan Species Diversity at North Pacific Hydrothermal Vents: What We Know and What it May Mean*

Hydrothermal vents in the deep sea are harsh, temporally unpredictable habitats with what appears to be a distinct fauna. Decades of subsea research with crewed and remote vehicles have generated a list of known species; is the species list complete? Evidence derived from mollusks sampled at hydrothermal vents on the East Pacific Rise (EPR), and Gorda and Juan de Fuca ridges suggests that the answer is yes. A 2006 compilation of hydrothermal vent species based on decades of research is updated and compared to specimens from these three active ridges in collections of the Field Museum of Natural History (FMNH) that resulted from limited collecting activity. Only three dives at each of the two Gorda Ridge vents collected all named species; 90% of the 20 species known from Juan de Fuca Ridge were collected in nine cruises. At the EPR, only 81% of the 43 known species were collected, but differences among the ridges were not significant. The limited FMNH collections increased the known ranges of six species from Juan de Fuca to Gorda Ridge and of nine species on the EPR. In addition, the EPR appears to host more rare species, potentially due to the frequent temporal changes at these vents. Mollusks currently known from each ridge, with their expanded ranges, are listed; the implications of these results for recent discoveries of slow-spreading vent fields are discussed.

Preeruptive flow focussing in dikes feeding historical pillow ridges on the Juan de Fuca and Gorda Ridges

Linear, hummocky pillow mound volcanism dominates at slow and intermediate spreading rate mid‐ocean ridges. Volcanic hummocks are thought to be formed by low effusion rates or as a result of flow focussing during effusive fissure style eruptions in which the initial dike intercepts the seafloor and erupts along its entire length. In this study, high‐resolution autonomous underwater vehicle (AUV) bathymetry is used to accurately map the extents of four historical fissure eruptions of the Juan de Fuca and Gorda ridges: on the North Gorda, North Cleft, and CoAxial ridge segments. The four mapped eruptions take the form of pillow mounds, which are similar in both lithology and dimension to hummocks on the Mid‐Atlantic Ridge. Pillow mounds may be isolated, or coalesce to form composite mounds, aligned as ridges or as clustered groups. In three of the four mapped sites, the eruptions were discontinuous along their lengths, with pillow mounds and composite mounds commonly separated by areas of older seafloor. This style of discontinuous eruption is inconsistent with typical en echelon fissure eruptions and is probably due to a mildly overpressured, fingering dike intersecting the seafloor along parts of its length.

Characterization of a monophylum Echinocletodes, its exclusion from Ancorabolinae (Copepoda, Harpacticoida), and displacement of E. bodini and E. walvisi to Cletodes, including the description of two new species.

The discovery of two new species of Harpacticoida (Crustacea, Copepoda), Echinocletodes voightae sp. nov. (Ancorabolidae Sars, 1909) and Cletodes meyerorum sp. nov. (Cletodidae T. Scott, 1905 sensu Por, 1986), permitted a phylogenetic re-examination of the genus Echinocletodes Lang, 1936, whose membership in Ancorabolidae has been increasingly queried in the past decade. Echinocletodes voightae was discovered in wood-falls in the Gorda Ridge (Northeast Pacific Ocean). It resembles the type species E. armatus T. Scott, 1903, sharing with it seven synapomorphies. In contrast, the other two species in the genus, E. bodini Dinet, 1974 and E. walvisi Dinet, 1974, were originally collected in the deep Angola Basin (SE Atlantic), and do not exhibit any of these synapomorphies. In fact they correspond more closely with Cletodes meyerorum, which is undoubtedly Cletodes Brady, 1972, due to the presence of two apomorphies of that genus. Consequently, both E. bodini and E. walvisi are moved from Echinocletodes to Cletodes. A revision of ancorabolid apomorphies showed ambiguities, being absent from some Ancorabolidae but present in other non-ancorabolid taxa (like e.g. Cletodes). Ancorabolidae were, therefore, considered to be a paraphyletic group that requires extensive re-evaluation, additionally including at least the Cletodidae and Laophontidae T. Scott, 1905. Similarly when comparing Echinocletodes with the two ancorabolid subfamilies, Ancorabolinae Sars, 1909 and Laophontodinae Lang, 1944, it was revealed that many species are apparently distributed chaotically over several supraspecific taxa (at least Ancorabolidae and Cletodidae) instead of forming monophyla.

The composition, origin and fate of complex mixtures in the maltene fractions of hydrothermal petroleum assessed by comprehensive two-dimensional gas chromatography

Sedimentary organic matter in hydrothermal systems can be altered by high temperature fluids to generate petroleum. The saturated and aromatic fractions of these hydrothermal oils are compositionally similar to conventional oil with the exception that they often contain higher concentrations of polycyclic aromatic hydrocarbons (PAH) as well as substantial mixtures of coeluting organic compounds that produce dramatically rising signal on the baseline of gas chromatograms termed unresolved complex mixtures (UCMs). Little is known about the compounds that compose UCMs and why or how they form. This is in part due to an inability to discriminate between in situ and migrated components that characterize the petroleum generated in hydrothermal systems. However, UCMs are also a product of the limitations imbedded in analytical separation techniques. With the advent of comprehensive two-dimensional gas chromatography (GC×GC), a revision of what should constitute molecular complexity needs to be considered. We address these problems by comparing the molecular compositions of the maltene fractions of three previously published hydrothermal petroleum samples using time of flight-mass spectrometry (GC×GC–ToF-MS) and 12 hydrothermal petroleum samples in cores from three locales using comprehensive two-dimensional gas chromatography with flame ionization detection (GC×GC–FID). The sediment cores were collected from Middle Valley, located off the axis of the Juan de Fuca Ridge, and the Escanaba Trough, along the Gorda Ridge, both in the NE Pacific Ocean, as well as from the Guaymas Basin in the Gulf of California. We define a UCM in GC×GC data to be a condition in which ⩾25% of the detected peaks within a chromatographic area coelute in either the first or second dimension. In turn, complex (CM) and simple mixtures (SM) are defined as having 5–24% and <5% coelution, respectively. All CM and UCMs were dominated by an array of configurational isomers, which becomes increasingly aromatic with higher molecular weight. We relate this to a multi-molecular complexity metric (MCM) by quantitatively comparing the difference in total peak variance and peak density for a GC×GC chromatogram. MCM values correlate with biomarker thermal maturity ratios for the Escanaba Trough and Guaymas Basin samples indicating that molecular complexity in these hydrothermal environments is in part a function of burial temperatures. Partial Least Squares (PLS) linear regression was applied to the total number of peak retention times as a proxy for the bulk molecular differences between each hydrothermal oil sample. Differences in the sample regressions correlate with the thermal maturity and the degree of PAH alkylation, indicating that this technique can be used to assess the degree of oxidative weathering due to dehydrogenation and hydrocarbon cracking. Subtracted chromatograms were then used to quantitatively track all of the individual molecular changes within the pyrolytic regime at Escanaba Trough. These subtracted chromatograms indicate that high molecular weight PAHs are highly mobile in hydrothermal fluids and may represent a phase partitioning that is occurring at greater depths. This phase condenses just below the seafloor to form an UCM in the near surface sediments. Saturated hydrocarbon biomarkers, such as hopanes, steranes and biphytanes are less mobile and more prone to being cracked and/or aromatized prior to migration toward the ocean floor. Together these techniques suggest that the molecular complexity of hydrothermal petroleum maximizes during the early stages of thermal maturation. The diversity of compounds forming these UCMs then decreases with increasing dehydrogenation, dealkylation and condensation reactions associated with elevated thermal stress and exposure to oxidants within the hydrothermal fluids.

USArray observations of quasi‐Love surface wave scattering: Orienting anisotropy in the Cascadia plate boundary

Quasi‐Love (QL) surface wave anomalies are observed along multiple great circle paths crossing the Pacific and through the mid‐ocean ridge and subduction zone associated with the Juan de Fuca plate. The long‐period QL waves observed on the USArray component of EarthScope arrive immediately after the fundamental Love wave arrival, suggesting that the QL waves were generated close to USArray and near the plate boundary. The dense USArray network allows the analysis of the azimuthal dependence of the QL surface wave scattering to determine the horizontal anisotropic axes of symmetry in Earth's upper mantle. We focus on the location, 42.5°N and 127.5°W just offshore of the southern Oregon coastline near the Gorda Ridge, and we analyze the QL scattering along 14 different great circle paths crossing through this location. We low‐pass filter the data to suppress overtones and crustal effects. QL amplitude and polarity constrains an anisotropic axis of symmetry that correlates well with the hot spot referenced Juan de Fuca plate motion and regional shear wave splitting. QL observations at multiple periods (50, 75, 100, 150, and 200 s) suggest a maximum Love‐to‐Rayleigh scattering at 100 s period, which is consistent with anisotropic lateral gradients in the asthenosphere beneath the Juan de Fuca and Gorda plates. Our observations do not follow the predictions of slab rollback and suggest the entrainment of asthenosphere with the overriding Juan de Fuca and Gorda plates.