Offshore California Research Articles

Multi-Site Wind Speed Prediction Based on Graph Embedding and Cyclic Graph Isomorphism Network (GIN-GRU)

Accurate and reliable wind speed prediction is conducive to improving the power generation efficiency of electrical systems. Due to the lack of adequate consideration of spatial feature extraction, the existing wind speed prediction models have certain limitations in capturing the rich neighborhood information of multiple sites. To address the previously mentioned constraints, our study introduces a graph isomorphism-based gated recurrent unit (GIN-GRU). Initially, the model utilizes a hybrid mechanism of random forest and principal component analysis (PCA-RF) to discuss the feature data from different sites. This process not only preserves the primary features but also extracts critical information by performing dimensionality reduction on the residual features. Subsequently, the model constructs graph networks by integrating graph embedding techniques with the Mahalanobis distance metric to synthesize the correlation information among features from multiple sites. This approach effectively consolidates the interrelated feature data and captures the complex interactions across multiple sites. Ultimately, the graph isomorphism network (GIN) delves into the intrinsic relationships within the graph networks and the gated recurrent unit (GRU) integrates these relationships with temporal correlations to address the challenges of wind speed prediction effectively. The experiments conducted on wind farm datasets for offshore California in 2019 have demonstrated that the proposed model has higher prediction accuracy compared to the comparative model such as CNN-LSTM and GAT-LSTM. Specifically, by modifying the network layers, we achieved higher precision, with the mean square error (MSE) and root mean square error (RMSE) of wind speed at a height of 10 m being 0.8457 m/s and 0.9196 m/s, respectively.

Slip Rate for the Rose Canyon Fault through San Diego, California, Based on Analysis of GPS Data: Evidence for a Potential Rose Canyon–San Miguel-Vallecitos Fault Connection?

ABSTRACT The Rose Canyon fault is the southern extension of the larger Newport–Inglewood–Rose Canyon fault system, which represents a major structural boundary in the Inner Continental Borderland (ICB) offshore of southern California. Ten to fifteen percent of total plate boundary motion in southern California is thought to be accommodated by the faults of the ICB, but the exact distribution of slip is uncertain. With an onshore segment, the Rose Canyon fault offers an opportunity to measure the slip rate using traditional geodetic methods. In this study, we use Global Positioning System (GPS) surface velocities from a combined campaign and continuous GPS network to constrain elastic models of the Rose Canyon fault. We then compare the observed surface velocities with proposed conceptual models of regional fault connections that facilitate the transfer of slip into the Rose Canyon fault to assess how well the observations are explained by the models. The results of elastic half-space models suggest that the Rose Canyon fault may be slipping toward the higher end of geologic estimates, with the preferred model indicating a slip rate of 2.4 ± 0.5 mm/yr. Although limited in terms of near-fault benchmarks, we find an improved model fit using an asymmetrical elastic half-space model and a higher slip rate, suggesting a potential rheological contrast across the Rose Canyon fault, similar to observations from the northern Newport–Inglewood fault segments. Observed GPS surface velocities, background seismicity, and gravity anomalies south of San Diego Bay point toward a more easterly trace for the Rose Canyon fault, suggesting a possible connection with the San Miguel–Vallecitos fault system. Such a connection could increase the potential rupture lengths of future earthquakes and have important consequences for regional seismic hazards.

Measurement-based carbon intensity of US offshore oil and gas production

The United States (US) produces oil and gas from six offshore regions: the North Slope of Alaska, Cook Inlet in Alaska, offshore California, and three Gulf of Mexico (GOM) sub-regions: state shallow, federal shallow, and deep waters. Measurement-based assessment of direct greenhouse gas emissions from this production can provide real-world information on carbon emissions to inform decisions on current and future production. In evaluating the climate impact of production, the carbon intensity (CI, the ratio of greenhouse gases emitted compared to the energy of fuels produced) is often used, though it is rarely quantified with measurements. Here, we complete an observational evaluation of the US offshore sector and present the largest current set of measurement-based CIs. We collected airborne measurements of methane, carbon dioxide, and nitrogen oxides from the North Slope, Cook Inlet, and California and combined with prior GOM results. For Alaska and California, we found emissions agree with facility-level inventories, however, the inventories miss some facilities. The US offshore CI, on a 100 year GWP basis, is 5.7 g CO2e/MJ[4.5, 6.8, 95% confidence interval]. This is greater than double the CI based on the national US inventory, with the discrepancy attributed primarily to methane emissions from GOM shallow waters, with a methane dominated CI of 16[12, 22] for GOM federal shallow waters and 43[25–65] for state shallow waters. Regional intensities vary, with carbon dioxide emissions largely responsible for CI on the North Slope 11[7.5, 15], in Cook Inlet 22[13, 34], offshore California 7.2[3.2, 13], and in GOM deep waters 1.1[1.0, 1.1]. These observations indicate offshore operations outside of the GOM in the US have modest methane emissions, but the CI can still be elevated due to direct carbon dioxide emissions. Accurate assessment of different offshore basins, with differing characteristics and practices, is important for the climate considerations of expanded production.

Oligocene and Miocene blowtorch volcanism within the California Borderland and its influence on the plate-boundary transition

Abstract The California Borderland is a uniquely broad and complex region of the North American–Pacific transform margin. Oligocene to Miocene structural reorganization, including large-magnitude extension and vertical-axis rotation of crustal blocks, formed a network of submerged fault-bounded basins, ridges, and islands offshore of southern California during its evolution from a convergent to transform boundary. Here, we report new 40Ar/39Ar incremental heating age determinations and geochemical analyses of volcanic rocks in the California Borderland, as well as a reconciled regional tectonic model. The California Borderland volcanic rocks are calc-alkaline in composition and mostly silica-oversaturated and represent relatively large-degree partial melts of the asthenosphere that incorporated hydrated forearc lithospheric components. A temporal reconciliation of these records shows that subduction of an active spreading center starting in the Oligocene and again in the Miocene was the critical control on the observed volcanism at ca. 31–30 Ma and 19–13 Ma. We posit that prolonged trench-ridge interaction generated a blowtorch effect, which resulted in widespread forearc volcanism and weakening of the crust to facilitate the unique deformation styles documented in the borderland that did not occur elsewhere along the North American–Pacific plate boundary.

Offshore high chlorophyll eddies: Separating upwelling from nearshore migration from ocean color remote sensing

Offshore eddies are often associated with high amounts of phytoplankton (represented by the chlorophyll-a concentration (Chla)), or, phytoplankton blooms, which can be detected from ocean color satellites. The phytoplankton “blooms” in these eddies are commonly explained as a result of enhanced nutrients - local growth - brought up from deeper waters by these eddies, although potentially they could simply be a migration of high Chla waters from nearshore regions. To better understand the interactions between physical forcing and phytoplankton dynamics, it is necessary to separate these “blooms” between local growth and migration. In this study, we first updated the multiple pigment inversion model using a synthetic dataset, for retrieving the absorption coefficient and absorption Gaussian peaks of phytoplankton from remote sensing reflectance in the broad aquatic environments. On this basis, a two-dimensional spatial model was developed to identify the sources of phytoplankton associated with offshore eddies. The model was based on the absorption coefficient of phytoplankton at 443 nm (aph(443)) and the ratio of two Gaussian peaks at 519 nm and 435 nm, where these two peaks represent different contributions of phytoplankton pigments to aph. This two-dimensional spatial model was applied to images collected by the Ocean and Land Color Instrument in the California offshore region to demonstrate that the scheme effectively separated offshore upwelling waters from those migrating from nearshore waters. Such separations provide independent sources for identifying offshore upwelling water that will be important for studying offshore circulation processes.

Identifying Meteorological Drivers for Errors in Modeled Winds along the Northern California Coast

Abstract An accurate wind resource dataset is required for assessing the potential energy yield of floating offshore wind farms that are expected along the California outer continental shelf. The National Renewable Energy Laboratory has developed and disseminated an updated wind resource dataset offshore of California, using the Weather Research and Forecasting Model, referred to as the CA20 dataset. As compared to buoy lidar measurements that have become available recently, the CA20 dataset showed significant positive biases for 100-m wind speeds along Northern California wind energy lease areas. To investigate the meteorological drivers for the model errors, we first consider two 1-yr simulations run with two different planetary boundary layer (PBL) parameterizations: the Mellor–Yamada–Nakanishi–Niino (MYNN) PBL scheme (the chosen configuration in the CA20 dataset) and the Yonsei University PBL scheme (which significantly reduces the bias in modeled winds). By comparing the 1-yr simulations to the concurrent lidar buoy observations, we find that errors are larger with the MYNN PBL scheme in warm seasons. We then dive deeper into the analysis by running simulations for short-term (3-day) case studies to evaluate the sensitivity of initial/boundary condition forcings on model results. By analyzing the short-term simulations, we find that during synoptic-scale northerly flows driven by the North Pacific high and inland thermal low, a coastal warm bias in the MYNN simulation is mainly responsible for the modeled wind speed bias by altering the boundary layer thermodynamics. The results of our analysis will help guide the creation of an updated version of the CA20 dataset.

Overview of the tectonic and seismicity in Continental Borderland (offshore) Baja California, México

The seismic activity in the Continental Borderland, offshore Baja California, presented a unique opportunity to study a region that is not typically known for high seismic activity, in comparison to the nearby Mexicali Valley. In 2022 and the beginning of 2023, this region experienced unusual activity. Some events caused concern among the population, specifically, two earthquakes occurred near the city of Ensenada on April 15th and July 25th, 2022, with magnitudes of Mw 4.2 and 4.5, respectively. A third event, with Mw6.1, occurred near San Quintin, Baja California, on November 22nd, 2022, and is associated with the San Isidro Fault. This event, surpassing the typical magnitude observed in the region based on historical seismicity, was felt across a significant stretch of the Pacific coast, spanning from Guerrero Negro to Tijuana, México. In June of the same year, a magnitude 4.2 event occurred in the same fault. This fault is a relay for the Santa Catalina Fault in its southeastern sector. The hypocentral locations of the earthquakes varied in depth from 5 to 20 km, mostly occurring between 5 and 20 km. Magnitudes ranged from 1.5 to 6.1. Additionally, continuous offshore seismic activity was observed between Punta Colonet and the town of San Quintin in November–December 2022 and May 2023. The hypocentral locations of these events ranged from depths of 2–18 km, with the majority occurring between 3 and 10 km. Magnitudes ranged from 1.5 to 4.5. This activity appears to be in the form of swarms, as previously reported by other authors. However, the seismic activity observed in May 2023 displays a northeast-southeast trend, with focal mechanisms exhibiting similar strike angles. This leads us to suggest that this seismic activity is associated with the Colonet Fault, which can be observed in the continental region of the coast. Focal mechanisms of major earthquakes indicated strike-slip motion, aligning with the dominant deformation pattern controlled by major faults in the region. A concentration of activity near the coastline becomes apparent upon analyzing seismic activity recorded in various catalogs from agencies such as ISC, RESNOM, USGS, NCEDC, and SCEC. While no reports of moderate inverse mechanism earthquakes have been documented in the Baja California offshore region, they occur in California. Notably, no seismic activity was reported in the area where the Mw6.1 earthquake occurred between 1930 and 2021, suggesting the reactivation of old faults along the continental border, similar to what occurs in the sector of California. One possibility is that the recurrence intervals of these earthquakes are very long, which would explain the lack of historical records. Previous authors have highlighted the potential for large complex ruptures involving combinations of strike-slip and dip-slip faults, particularly exemplified by the 2010 Mw7.1 El Mayor- Cucapah event in northern Baja California. Seismic activity in the region concentrates along the El Descanso, Coronado-Banks, Santa Catalina, Bahía Soledad, and San Isidro faults. This observation is supported by gravimetric and magnetic anomalies found in various structures that have not been previously interpreted, including the newly reported La Chorera fault, based on bathymetry data. Most of the mentioned faults occur in the gravimetric and magnetic anomalies, however, many other structures have not yet been described.

Performance and dynamics of California offshore wind alongside Western US onshore wind and solar power

We use statistical methods to evaluate the coincidence, variability, alignment, and predictability of California offshore wind power production with electricity demand and onshore renewable resources in the United States Western Interconnection region. We consider previously reported benefits, and evaluate possible advantages of deploying offshore wind as compared to onshore wind or solar power generation, such as coincidence with peak power demand and complementarity with other renewable power resources. Additionally, we use the Fast Fourier Transform and linear regression to assess the predictability of power with onshore wind and solar. The Pearson correlation reveals that onshore wind and solar are the most complementary renewable power generation resources, while there is no consistent correlation or anti-correlation between the offshore wind sites and the demand, solar, or onshore wind. Using a demand-based value metric, we find that the six potential offshore wind sites are more valuable than all other renewable resources during peak hours in the summer in California. Finally, the Fourier analysis reveals that offshore wind may be a highly variable resource.

An initial spatial conflict analysis for potential deep-sea mining of marine minerals in U.S. Federal Waters

Demand for critical minerals is projected to double or even triple by 2030 as the global energy and transportation sectors transition away from fossil fuels toward renewable energy. This anticipated demand raises interest in new critical mineral resources, including marine minerals, to supplement future supplies. While the environmental benefits of transitioning to green energy are many, there are inherent environmental costs. In this study, we compiled broad-scale datasets of prominent environmental features and human uses within four deep ocean regions of the U.S. Exclusive Economic Zone where marine minerals are predicted to occur, called prospective regions: they are the Blake Plateau, the Hawaiian Islands, Offshore California, and the Gulf of Alaska. We generated a normalized metric to score regions based on the co-occurrence of either environmental or human use variables within hexagonal grid cells, allowing us to assess the occurrence intensity and spatial patterns of environmental and/or human use variables within each region. Driven by the presence of threatened species and total species richness, the Blake Plateau and the Hawaiian Islands have the highest environmental occurrence scores, followed by Offshore California and the Gulf of Alaska. Offshore California is the busiest shipping region, followed by the Blake Plateau, the Gulf of Alaska and Hawaiian Islands, and subsea cables are densest in Offshore California, followed by Gulf of Alaska and Hawaiian Islands. The Gulf of Alaska has the largest reported fisheries landings. The sensitivities of biota and human uses to mining are highest for benthic or sessile species and infrastructure on or near ferromanganese crusts or manganese nodules that would be directly and physically altered through crust extraction or nodule removal. This study illustrates the usefulness of comprehensive, spatially explicit risk assessments to inform deep sea mining management and minimize ecological harm and human use conflicts. We assert that the relatively straightforward GIS-based methodology tested here can be applied and iterated upon elsewhere.

Organic compounds in aquatic sediments analyzed by pyrolysis–GC–MS with tetramethylammonium hydroxide (TMAH) and alkaline CuO oxidation methods

Thermally assisted hydrolysis and methylation with tetramethylammonium hydroxide (TMAH) can analyze various organic compounds, e.g., lignin phenols, fatty acids, sterols, etc., thus, suitable for profiling organic matter in sediments. Although the method uses pyrofoils with different reaction temperatures, changes in the yield of organic compounds in sediment samples at different temperatures are unclear and requires clarification. This study characterizes the compounds released during the TMAH reaction by testing the effect of reaction temperatures and comparing the results with a conventional method (alkaline CuO oxidation). The reaction temperatures (315 °C, 358 °C, 445 °C, 500 °C, 590 °C, and 670 °C) effects via thermochemolysis with TMAH using a Curie point pyrolyzer–GC–MS instrument (TMAH method) on lignin phenols, alkanoic acids, cutin acids, alcohols, sterols, and phytol in marine (offshore California) and lake (Lake Baikal) sediments were determined. Comparison between two different sediment locations provided a variety of compounds derived from marine and terrestrial environments. Lower temperatures (≤ 358 °C) were suitable for n-C14–C18 alkanoic acids, unsaturated n-C16:1 and C18:1 acids, cutin acids, n-C16–C28 alcohols, sterols, and phytol, whereas higher temperatures (from 445 °C to 500 °C or 590 °C) were appropriate for lignin phenols, n-C20–C30 alkanoic acids, ω-hydroxyalkanoic acids, and n-alkanedioic acids. However, relatively high concentrations determined under high temperature conditions may be due to the apparent high concentrations obtained by the decomposition of the internal standard (n-C19 alkanoic acid). Vertical distribution of lignin phenols, alkanoic acids, and cutin acids collected from lake sediments using the TMAH method at 358 °C and 590 °C were compared with those obtained by alkaline CuO oxidation (CuO method). Significant linear relationships were observed for lignin phenols and alkanoic acids, except for n-alkanedioic acids. Differences were found in some yield amounts between the methods; the yields of cinnamyl phenols (at 358 °C and 590 °C) and n-C14–C18 alkanoic acids (at 358 °C) obtained by the TMAH method significantly exceeding those obtained by the CuO method. Additionally, cutin acid yields obtained by the TMAH method were substantially lower than those obtained from the CuO method. We recommend using lower temperatures for the TMAH reaction to optimize this analytical method for different compound classes.

Projected cross-shore changes in upwelling induced by offshore wind farm development along the California coast

In California offshore waters, sustained northwesterly winds have been identified as a key resource that can contribute substantially to renewable energy goals. However, the development of large-scale offshore wind farms can reduce the wind stress at the sea surface, which could affect wind-driven upwelling, nutrient delivery, and ecosystem dynamics. Here we examine changes to upwelling using atmospheric and ocean circulation numerical models together with a hypothetical upper bound buildout scenario of 877 turbines spread across three areas of interest. Wind speed changes are found to reduce upwelling on the inshore side of windfarms and increase upwelling on the offshore side. These changes, when expressed in terms of widely used metrics for upwelling volume transport and nutrient delivery, show that while the net upwelling in a wide coastal band changes relatively little, the spatial structure of upwelling within this coastal region can be shifted outside the bounds of natural variability.

Decadal cyclical geological atmospheric emissions for a major marine seep field, offshore Coal Oil Point, Southern California

The greenhouse gas, methane, budget has significant uncertainty for many sources, including natural geological emissions. A major uncertainty of geological methane emissions, including onshore and offshore hydrocarbon seepage from subsurface hydrocarbon reservoirs is the gas emissions’ temporal variability. Current atmospheric methane budget models assume seepage is constant; nevertheless, available data and seepage conceptual models suggest gas seepage can vary considerably on timescales from second to century. The assumption of steady-seepage is used because long-term datasets to characterize these variabilities are lacking. A 30-year air quality dataset downwind of the Coal Oil Point seep field, offshore California found methane, CH4, concentrations downwind of the seep field increased from a 1995 minimum to a 2008 peak, decreasing exponentially afterward with a 10.2-year timescale (R2 = 0.91). Atmospheric emissions, EA, were derived by a time-resolved Gaussian plume inversion model of the concentration anomaly using observed winds and gridded sonar source location maps. EA increased from 27,200 to 161,000 m3 day−1 (corresponding to 6.5–38 Gg CH4 year−1 for 91% CH4 content) for 1995–2009, respectively, with 15% uncertainty, then decreased exponentially from 2009 to 2015 before rising above the trend. 2015 corresponded to the cessation of oil and gas production, which affects the western seep field. EA varied sinusoidally with a 26.3-year period (R2 = 0.89) that largely tracked the Pacific Decadal Oscillation (PDO), which is driven on these timescales by an 18.6-year earth-tidal cycle (27.9-year beat). A similar controlling factor may underlie both, specifically varying compressional stresses on migration pathways. This also suggests the seep atmospheric budget may exhibit multi-decadal trends.

The co-benefits of California offshore wind electricity

California has set forth an ambitious goal of generating all its electricity from carbon-free technologies by 2045. Offshore wind (OSW) presents several attractive system, economic, and environmental attributes to help the state achieve these goals. Inclusion of OSW into the clean electricity generation portfolio could contribute significantly to total resource cost savings. In addition, OSW offers several major co-benefits. Its high and consistent capacity factor and generation time profile complements that of solar and helps enhance renewable electricity generation reliability. OSW could also be instrumental in early retirement of costly and pollution-heavy natural gas plants and lead to substantial job creations. Moreover, California could reap additional economic co-benefits from the development of a local wind energy industry. Additionally, OSW has the potential to advance environmental justice through reduction of ordinary air pollutants in urban areas and by bringing economic opportunities to lagging areas. At the same time, there are multiple challenges that must be addressed for OSW to reach its full potential. Our analysis is intended also to serve as a template for studies elsewhere by providing a comprehensive framework for estimating co-benefits, taking account of important local conditions, and identification of challenges and how they might be overcome.

Measuring Floating Thick Seep Oil from the Coal Oil Point Marine Hydrocarbon Seep Field by Quantitative Thermal Oil Slick Remote Sensing

Remote sensing techniques offer significant potential for generating accurate thick oil slick maps critical for marine oil spill response. However, field validation and methodology assessment challenges remain. Here, we report on an approach to leveraging oil emissions from the Coal Oil Point (COP) natural marine hydrocarbon seepage offshore of southern California, where prolific oil seepage produces thick oil slicks stretching many kilometers. Specifically, we demonstrate and validate a remote sensing approach as part of the Seep Assessment Study (SAS). Thick oil is sufficient for effective mitigation strategies and is set at 0.15 mm. The brightness temperature of thick oil, TBO, is warmer than oil-free seawater, TBW, allowing segregation of oil from seawater. High spatial-resolution airborne thermal and visible slick imagery were acquired as part of the SAS; including along-slick “streamer” surveys and cross-slick calibration surveys. Several cross-slick survey-imaged short oil slick segments that were collected by a customized harbor oil skimmer; termed “collects”. The brightness temperature contrast, ΔTB (TBO − TBW), for oil pixels (based on a semi-supervised classification of oil pixels) and oil thickness, h, from collected oil for each collect provided the empirical calibration of ΔTB(h). The TB probability distributions provided TBO and TBW, whereas a spatial model of TBW provided ΔTB for the streamer analysis. Complicating TBW was the fact that streamers were located at current shears where two water masses intersect, leading to a TB discontinuity at the slick. This current shear arose from a persistent eddy down current of the COP that provides critical steering of oil slicks from the Coal Oil Point. The total floating thick oil in a streamer observed on 23 May and a streamer observed on 25 May 2016 was estimated at 311 (2.3 bbl) and 2671 kg (20 bbl) with mean linear floating oil 0.14 and 2.4 kg m−1 with uncertainties by Monte Carlo simulations of 25% and 7%, respectively. Based on typical currents, the average of these two streamers corresponds to 265 g s−1 (~200 bbl day−1) in a range of 60–340 bbl day−1, with significant short-term temporal variability that suggests slug flow for the seep oil emissions. Given that there are typically four or five streamers, these data are consistent with field emissions that are higher than the literature estimates.

Effect of Floating Offshore Wind Turbines on Atmospheric Circulation in California

In California offshore waters, sustained northwesterly winds have been identified as a key energy resource that could contribute substantially to California’s renewable energy mandate. It is these winds that drive upwelling, which is responsible for much of the primary productivity that sustains one of the richest ecosystems on the planet. The goal of this study is to quantify changes in wind fields at the sea surface as the result of offshore wind turbine deployments by use of an atmospheric model. Modeled wind fields from this study will drive an ocean circulation model. The Weather Research and Forecasting model was implemented on a regional scale along the U.S. west coast, with a higher resolution nest along the California continental shelf. Simulated arrays of offshore wind turbines were placed within call areas for wind farm development offshore of Central and Northern California. At full build-out, it was found that wind speeds at 10 m height are reduced by approximately 5%, with wakes extending approximately 200 km downwind of the nominated lease block areas. The length scale of wind speed reductions was found to be several times the internal Rossby radius of deformation, the spatial scale at which rotationally-influenced ocean circulation processes such as upwelling occur.

Long-term atmospheric emissions for the Coal Oil Point natural marine hydrocarbon seep field, offshore California

Abstract. In this study, we present a novel approach for assessing nearshore seepage atmospheric emissions through modeling of air quality station data, specifically a Gaussian plume inversion model. A total of 3 decades of air quality station meteorology and total hydrocarbon concentration, THC, data were analyzed to study emissions from the Coal Oil Point marine seep field offshore California. THC in the seep field directions was significantly elevated and Gaussian with respect to wind direction, θ. An inversion model of the seep field, θ-resolved anomaly, THC′(θ)-derived atmospheric emissions is given. The model inversion is for the far field, which was satisfied by gridding the sonar seepage and treating each grid cell as a separate Gaussian plume. This assumption was validated by offshore in situ data that showed major seep area plumes were Gaussian. Plume total carbon, TC (TC = THC + carbon dioxide, CO2, + carbon monoxide), 18 % was CO2 and 82 % was THC; 85 % of THC was CH4. These compositions were similar to the seabed composition, demonstrating efficient vertical plume transport of dissolved seep gases. Air samples also measured atmospheric alkane plume composition. The inversion model used observed winds and derived the 3-decade-average (1990–2021) field-wide atmospheric emissions of 83 400 ± 12 000 m3 THC d−1 (27 Gg THC yr−1 based on 19.6 g mol−1 for THC). Based on a 50 : 50 air-to-seawater partitioning, this implies seabed emissions of 167 000 m3 THC d−1. Based on atmospheric plume composition, C1–C6 alkane emissions were 19, 1.3, 2.5, 2.2, 1.1, and 0.15 Gg yr−1, respectively. The spatially averaged CH4 emissions over the ∼ 6.3 km2 of 25 × 25 m2 bins with sonar values above noise were 5.7 µM m−2 s−1. The approach can be extended to derive emissions from other dispersed sources such as landfills, industrial sites, or terrestrial seepage if source locations are constrained spatially.

Water masses and larval fish habitats in the Pacific tropical-subtropical convergence off Mexico

In early summer (June 2010), we used zooplankton and hydrographic sampling to analyze relationships between larval fish habitats and converging water masses in the Pacific tropical-subtropical region off Mexico. Results showed an equatorward jet parallel to the Baja California Peninsula (the Tropical Branch of California Current) which was located northward of 25oC surface isotherm. In this area, a larval fish habitat indicated by Triphoturus mexicanus larvae was identified. The habitat had low larval abundance (17 larvae per 10 m2) with major larval concentration below the thermocline (∼ 20 Co and oligotrophic conditions, < 0.3 mg m-3). Two larval fish habitats were defined south of the 25oC surface isotherm. One larval fish habitat was located in a coastal jet and cyclonic field immersed in the Transitional Waters, with the highest larval fish abundance (49 larvae 10 per m2). The indicator species were Vinciguerria lucetia and Diogenichthys laternatus. Both species had the greatest abundance in the warm (∼25 oC) and mesotrophic (>1 mg m-3) waters of the shallow thermocline (∼30 m depth). The other larval fish habitat occupied an area of anticyclonic circulation in the offshore California Current Water, where the thermocline was depressed ∼ 70 m depth. Oligotrophic conditions (0.3 mg m-3) corresponded with the lowest mean larval abundance (14 larvae per 10 m2) of the study. The distribution of the larval fish habitats and their indicator species correspond with the water masses distributions in the Tropical-Subtropical convergence, which are modulated by mesoscale structures affecting the thermocline depth and the larval life-history.

Preconditioning by sediment accumulation can produce powerful turbidity currents without major external triggers

Turbidity currents dominate sediment transfer into the deep ocean, and can damage critical seabed infrastructure. It is commonly inferred that powerful turbidity currents are triggered by major external events, such as storms, river floods, or earthquakes. However, basic models for turbidity current triggering remain poorly tested, with few studies accurately recording precise flow timing. Here, we analyse the most detailed series of measurements yet made of powerful (up to 7.2 ms−1) turbidity currents, within Monterey Canyon, offshore California. During 18-months of instrument deployment, fourteen turbidity currents were directly monitored. No consistent triggering mechanism was observed, though flows did cluster around enhanced seasonal sediment supply. We compare turbidity current timing at Monterey Canyon (a sandy canyon-head fed by longshore drift) to the only other systems where numerous (>10-100) flows have been measured precisely via direct monitoring; the Squamish Delta (a sandy fjord-head delta), and the Congo Canyon (connected to the mud-dominated mouth of the Congo River). A common seasonal pattern emerges, leading to a new model for preconditioning and triggering of turbidity currents initiating through slope failure in areas of sediment accumulation, such as canyon heads or river mouths. In this model, rapid or sustained sediment supply alone can produce elevated pore pressures, which may persist, thereby predisposing slopes to fail. Once preconditioned, a range of minor external perturbations, such as moderate storm-waves, result in local pore pressure variation, and thus become effective triggers. Major external triggers are therefore not always a prerequisite for triggering of powerful turbidity currents.

The Role of Oil and Gas Conductors as Fish Habitat at Two Southern California Offshore Platforms

Worldwide, all oil and gas offshore platforms contain conductors, a series of closely spaced vertical pipes, that carry oil and gas from the wells. During decommissioning, these conductors are usually removed. However, worldwide, there has been no research on the relationship between conductor arrays and fish densities. During summer and fall of 2017 and 2018, at Platforms Holly and A, in the Santa Barbara Channel, California, we conducted visual scuba surveys of the fish densities associated with 1) conductors, 2) crossbeams, and 3) blue (open) water under these platforms. A minimum of 14 species were observed at Platform Holly and 14 species at Platform A. Substantially more fishes, and higher densities, were observed at Holly (17,533, 636 per 100 m–2, respectively) than at A (1,185, 171 per 100 m2). Although there were similarities, the dominant species at the two platforms were distinct. Rockfishes (genus Sebastes), particularly Sebastes hopkinsi, dominated the Holly assemblage, while the damselfish, Chromis punctipinnis, was the most abundant at A. Overall, at both platforms, there was no statistical difference between fish densities at conductors and at crossbeams and fish densities were higher at both conductors and crossbeams when compared to those in blue water. The species assemblage among three habitats were similar, although there were two survey occasions where no fish were observed in blue water. Our research strongly suggests that, to the extent possible, retaining conductors during partial removal will likely have a positive influence on both fish diversity and fish abundance, during at least some years.

Modulation of seismic noise near the San Jacinto fault in southern California: origin and observations of the cyclical time dependence and associated crustal properties

SUMMARYWe examine the cyclic amplitude variation of seismic noise recorded by continuous three-component broad-band seismic data with durations spanning 91–713 d (2008–2011) from three different networks: Anza seismic network, IDA network and the transportable seismic array. These stations surround the San Jacinto Fault Zone (SJFZ) in southern California. We find the seismic noise amplitudes exhibit a cyclical variation between 0.3 and 7.2 Hz. The high-frequency (≥0.9 Hz) noise variations can be linked to human activity and are not a concern. Our primary interest is signals in the low frequencies (0.3–0.9 Hz), where the seismic noise is modulated by semi-diurnal tidal mode M2. These long-period (low-frequency) variations of seismic noise can be attributed to a temporal change of the ocean waves breaking at the shoreline, driven by ocean tidal loading. We focus on the M2 variation of seismic noise at f = 0.6 Hz, travelling distances of ∼92 km through the crust from offshore California to the inland Anza, California, region. Relative to the shoreline station, data from the inland stations show a phase lag of ∼ –12°, which we attribute to the cyclic change in M2 that can alter crustal seismic attenuation. We also find that for mode M2 at 0.6 Hz, the amplitude variations of the seismic quality factor (Q) depend on azimuth and varies from 0.22 per cent (southeast to northwest) to 1.28 per cent (northeast to southwest) with Q = 25 for Rayleigh waves. We propose the direction dependence of the Q variation at 0.6 Hz reflects the preferred orientation of subfaults parallel to the main faulting defined by the primarily N45°W strike of the SJFZ.