Spontaneously exsolved free gas during major storms as an ephemeral gas source for pockmark formation

Abrupt fluid emissions from shallow marine sediments pose a threat to seafloor installations like wind farms and offshore cables. Quantifying such fluid emissions and linking pockmarks, the seafloor manifestations of fluid escape, to flow in the sub-seafloor remains notoriously difficult due to an incomplete understanding of the underlying physical processes. Here, using a compositional multi-phase flow model, we test plausible gas sources for pockmarks in the south-eastern North Sea, which recent observations suggest have formed in response to major storms. We find that the mobilization of pre-existing gas pockets is unlikely because free gas, due to its high compressibility, damps the propagation of storm-induced pressure changes deeper into the subsurface. Rather, our results point to spontaneous appearance of a free gas phase via storm-induced gas exsolution from pore fluids. This mechanism is primarily driven by the pressure-sensitivity of gas solubility, and the appearance of free gas is largely confined to sediments in the vicinity of the seafloor. We show that in highly permeable sediments containing gas-rich pore fluids, wave-induced pressure changes result in the appearance of a persistent gas phase. This suggests that seafloor fluid escape structures are not always proxies for overpressured shallow gas and that periodic seafloor pressure changes can induce persistent free gas phase to spontaneously appear. Key Points - Storm-induced pressure changes can lead to spontaneous appearance of free gas phase near the seafloor - This process is driven by pressure-sensitive phase instabilities - This mechanism could help explain elusive gas sources in recently observed pockmarks in the North Se

Deep electrical resistivity structure of northwestern Costa Rica

First long-period magnetotelluric investigations were conducted in early 2008 in northwestern Costa Rica, along a profile that extends from the coast of the Pacific Ocean, traverses the volcanic arc and ends currently at the Nicaraguan border. The aim of this study is to gain insight into the electrical resistivity structure and thus fluid distribution at the continental margin where the Cocos plate subducts beneath the Caribbean plate. Preliminary two-dimensional models map the only moderately resistive mafic/ultramafic complexes of the Nicoya Peninsula (resistivity of a few hundred Ωm), the conductive forearc and the backarc basins (several Ωm). Beneath the backarc basin the data image a poor conductor in the basement with a clear termination in the south, which may tentatively be interpreted as the Santa Elena Suture. The volcanic arc shows no pronounced anomaly at depth, but a moderate conductor underlies the backarc with a possible connection to the upper mantle. A conductor at deep-crustal levels in the forearc may reflect fluid release from the downgoing slab

Fluid release from the subducted Cocos plate and partial melting of the crust deduced from magnetotelluric studies in southern Mexico: implications for the generation of volcanism and subduction dynamics

In order to study electrical conductivity phenomena that are associated with subduction related fluid release and melt production, magnetotelluric (MT) measurements were carried out in southern Mexico along two coast to coast profiles. The conductivity-depth distribution was obtained by simultaneous two-dimensional inversion of the transverse magnetic and transverse electric modes of the magnetotelluric transfer functions. The MT models demonstrate that the plate southern profile shows enhanced conductivity in the deep crust. The northern profile is dominated by an elongated conductive zone extending >250 km below the Trans-Mexican Volcanic Belt (TMVB). The isolated conductivity anomalies in the southern profile are interpreted as slab fluids stored in the overlying deep continental crust. These fluids were released by progressive metamorphic dehydration of the basaltic oceanic crust. The conductivity anomalies may be related to the main dehydration reactions at the zeolite → blueschist → eclogite facies transitions and the breakdown of chlorite. This relation allows the estimation of a geothermal gradient of ∼8.5°C/km for the top of the subducting plate. The same dehydration reactions may be recognized along the northern profile at the same position relative to the depth of the plate, but more inland due to a shallower dip, and merge near the volcanic front due to steep downbending of the plate. When the oceanic crust reaches a depth of 80–90 km, ascending fluids produce basaltic melts in the intervening hot subcontinental mantle wedge that give rise to the volcanic belt. Water-rich basalts may intrude into the lower continental crust leading to partial melting. The elongated highly conductive zone below the TMVB may therefore be caused by partial melts and fluids of various origins, ongoing migmatization, ascending basaltic and granitic melts, growing plutons as well as residual metamorphic fluids. Zones of extremely high conductance (>8000 S) in the continental crust on either MT profile might indicate extinct magmatism

Simulated land ice: latitudinal changes (0-2.6 Ma) and importance of CO2-glaciation divergence during times of decreasing obliquity (0-800 ka)

Following Milankovitch's theory the incoming insolation or summer energy at 65°N is typically analysed to predict the waxing or waning of land ice. We here use a model-based deconvolution of the LR04 benthic-d18O stack into land ice distribution (de Boer et al., 2014, Köhler et al., 2015) to verify if the latitudinal focal point of land ice dynamics has changed over the last 2 Myr or whether this choice of 65°N in orbital data is indeed well justified. We find that the 5°-latitudinal band which contributes most to land ice albedo radiative forcing (ΔR_[LI]) is 70-75°N between 2.0-1.5 Myr, which is then until 1.0 Myr gradually substituted by 65-70°N. During the last 1 Myr both 60-65°N and 65-70°N dominate ΔR_[LI] and contribute approximately the same amount, while the relative importance of 70-75°N is shrinking. Our analyses illustrates that the choice of 65°N seems for the last 1 Myr to be well justified, while for earlier parts of the last 2 Myr the dominant land ice changes seems to happen up to 10° further to the north. Focusing on the last 800 kyr (the time for which precise data on atmospheric CO2 concentration exists) we furthermore find that the multi-millennial land ice growth and proxy-based reconstruction of global cooling (= the glaciation) appear synchronously to each other and to decreasing obliquity, but diverge from CO2. This suggests that the global cooling associated with Earth's way into an ice age as deduced in the reconstructions has to be mainly caused by the land ice albedo feedback, and is not dominated by the CO2 greenhouse forcing. One way of perceiving this CO2-glaciation divergence in reconstructions is that the reduced incoming insolation at high latitudes causes land ice growth and cooling, while there is a coexisting process that keeps CO2 at a relatively constant level. Solid Earth modeling experiments have indicated that falling sea level might lead to enhanced magma and CO2 production at mid-ocean ridges. Hasenclever et al. (2017) suggested that the combination of marine volcanism at mid-ocean ridges and at hot spot island volcanoes might react to decreasing sea level and be a potential cause for this CO2-glaciation divergence. This CO2-glaciation divergence needs to be considered, when using paleo data to quantify paleoclimate sensitivity: periods with diverging CO2 and global temperature change should be filtered out when approximating the relationship between global temperature rise and CO2 concentrations (Köhler et al., 2018). References: de Boer et al. (2014). https://doi.org/10.1038/ncomms3999. Köhler et al. (2015). https://doi.org/10.5194/cp-11-1801-2015. Hasenclever et al. (2017). https://doi.org/10.1038/ncomms15867. Köhler et al. (2018). https://doi.org/10.1029/2018GL077717

Lower plate structure and upper plate deformational segmentation at the Sunda-Banda arc transition, Indonesia

The Sunda‐Banda arc transition at the eastern termination of the Sunda margin (Indonesia) represents a unique natural laboratory to study the effects of lower plate variability on upper plate deformational segmentation. Neighboring margin segments display a high degree of structural diversity of the incoming plate (transition from an oceanic to a continental lower plate, presence/absence of an oceanic plateau, variability of subducting seafloor morphology) as well as a wide range of corresponding fore‐arc structures, including a large sedimentary basin and an accretionary prism/outer arc high of variable size and shape. Here, we present results of a combined analysis of seismic wide‐angle refraction, multichannel streamer and gravity data recorded in two trench normal corridors located offshore the islands of Lombok (116°E) and Sumba (119°E). On the incoming plate, the results reveal a 8.6–9.0 km thick oceanic crust, which is progressively faulted and altered when approaching the trench, where upper mantle velocities are reduced to ∼7.5 km/s. The outer arc high, located between the trench and the fore‐arc basin, is characterized by sedimentary‐type velocities (Vp < 5.5 km/s) down to the top of the subducting slab (∼13 km depth). The oceanic slab can be traced over 70–100 km distance beneath the fore arc. A shallow serpentinized mantle wedge at ∼16 km depth offshore Lombok is absent offshore Sumba, where our models reveal the transition to the collisional regime farther to the east and to the Sumba block in the north. Our results allow a detailed view into the complex structure of both the deeper and shallower portions of the eastern Sunda margin

Nature of crustal terranes and the Moho in northern Costa Rica from receiver function analysis

The Central American subduction zone in northern Costa Rica shows along‐strike variations in both the incoming and overriding plates. By analyzing the subducting oceanic Moho (M1) and the upper plate Moho (M2) with receiver functions, we investigate the variability in the hydration state of the subducting Cocos Plate and the nature of crustal terranes within the overriding Caribbean Plate. We calculate high‐quality P and PP wave receiver functions using broadband data of the Global Seismology Network; Geoscope Project; and the CRSEIZE, Pocosol, and Corisubmod experiments. In addition, we estimate the depth (H) and vertically averaged Vp/Vs (κ) to Moho and present a sensitivity study to explore the effects of a dipping interface on receiver functions and the H and κ estimates. Our results are consistent with a drier oceanic mantle subducting beneath the southernmost part of the Nicoya Peninsula, as compared to a serpentinized oceanic mantle subducting beneath the northern part. In the Caribbean Plate, we describe the nature of the Mesquito, Nicoya, and Chorotega terranes by integrating new and published Vp/Vs estimates. Both the Nicoya and Chorotega terranes display high Vp/Vs (1.80–1.92) consistent with their oceanic character. In contrast, the oceanic Mesquito Terrane mostly displays lower Vp/Vs (1.62–1.80) more compatible with continental crust, which may indicate that subduction zone magmatism is modifying the crust to display continental character. Our estimates show that the deepest M2 (∼42 km) is observed in the volcanic arc region whereas the shallowest M2 (∼27–33 km) is observed in parts of the fore‐arc and back‐arc regions.National Sanitation Foundation[EAR0510966]/NFS/Estados UnidosNational Sanitation Foundation[EAR0506463]/NFS/Estados UnidosUCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela Centroamericana de Geologí

Receiver function study in northern Sumatra and the Malaysian peninsula

International audienceIn this receiver function study, we investigate the structure of the crust beneath six seismic broadband stations close to the Sunda Arc formed by subduction of the Indo-Australian under the Sunda plate. We apply three different methods to analyse receiver functions at single stations. A recently developed algorithm determines absolute shear-wave velocities from observed frequency-dependent apparent incidence angles of P waves. Using waveform inversion of receiver functions and a modified Zhu and Kanamori algorithm, properties of discontinuities such as depth, velocity contrast, and sharpness are determined. The combination of the methods leads to robust results. The approach is validated by synthetic tests. Stations located on Malaysia show high-shear-wave velocities () near the surface in the range of 3.4-3.6 km s attributed to crystalline rocks and 3.6-4.0 km s in the lower crust. Upper and lower crust are clearly separated, the Moho is found at normal depths of 30-34 km where it forms a sharp discontinuity at station KUM or a gradient at stations IPM and KOM. For stations close to the subduction zone (BSI, GSI and PSI) complexity within the crust is high. Near the surface low of 2.6-2.9 km s indicate sediment layers. High of 4.2 km s are found at depth greater than 6 and 2 km at BSI and PSI, respectively. There, the Moho is located at 37 and 40 km depth. At station GSI, situated closest to the trench, the subducting slab is imaged as a north-east dipping structure separated from the sediment layer by a 10 km wide gradient in between 10 and 20 km depth. Within the subducting slab ≈ 4.7 km s. At station BSI, the subducting slab is found at depth between 90 and 110 km dipping 20° ± 8° in approximately N 60° E. A velocity increase in similar depth is indicated at station PSI, however no evidence for a dipping layer is found

Beyond Priesthood: Religious Entrepreneurs and Innovators in the Roman Empire

Above all, this volume assesses critically convenient terminological usage and offers a unique insight into a rich gamut of ancient Mediterranean religious specialists