Pore water geochemistry along continental slopes north of the East Siberian Sea: inference of low methane concentrations

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Clint M. Miller, Gerald R. Dickens, Martin Jakobsson, Carina Johansson, Andrey Koshurnikov, Matt O’Regan,
Francesco Muschitiello, Christian Stranne, and Carl-Magnus Mörth

Abstract. Continental slopes north of the East Siberian Sea potentially hold large amounts of methane (CH4) in sediments as gas hydrate and free gas. Although release of this CH4 to the ocean and atmosphere has become a topic of discussion, the region remains sparingly explored. Here we present pore water chemistry results from 32 sediment cores taken during Leg 2 of the 2014 joint Swedish–Russian–US Arctic Ocean Investigation of Climate–Cryosphere–Carbon Interactions (SWERUS-C3) expedition. The cores come from depth transects across the slope and rise extending between the Mendeleev and the Lomonosov ridges, north of Wrangel Island and the New Siberian Islands, respectively. Upward CH4 flux towards the seafloor, as inferred from profiles of dissolved sulfate (SO2−4), alkalinity, and the δ13C of dissolved inorganic carbon (DIC), is negligible at all stations east of 143◦ E longitude. In the upper
8 m of these cores, downward SO2−4 flux never exceeds 6.2 mol m−2 kyr−1, the upward alkalinity flux never exceeds
6.8 mol m−2 kyr−1, and δ13C composition of DIC (δ13CDIC) only moderately decreases with depth (−3.6 ‰ m−1 on
average). Moreover, upon addition of Zn acetate to pore water samples, ZnS did not precipitate, indicating a lack of dissolved H2S. Phosphate, ammonium, and metal profiles reveal that metal oxide reduction by organic carbon dominates the geochemical environment and supports very low organic carbon turnover rates. A single core on the Lomonosov Ridge differs, as diffusive fluxes for SO2−4 and alkalinity were 13.9 and 11.3 mol m−2 kyr−1, respectively, the δ13C-DIC gradient was 5.6 ‰ m−1, and Mn2+ reduction terminated within 1.3 m of the seafloor. These are among the first pore water results generated from this vast climatically sensitive region, and they imply that abundant CH4, including gas hydrates, do not characterize the East Siberian Sea slope or rise along the
investigated depth transects. This contradicts previous modeling and discussions, which due to the lack of data are almost entirely based on assumption.

Biogeosciences, 14, 2929–2953, 2017
https://doi.org/10.5194/bg-14-2929-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License

EPSL: High pore fluid pressures stabilize faults

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The rupture, localization, and slip of faults in serpentinite were studied under varying pore fluid pressure conditions to understand deformation mechanisms potentially responsible for slow slip in fault zones. Experiments were conducted at a constant effective confining pressure of 10 MPa and under pore fluid pressures from 0 to 120 MPa and at temperatures from 23 to 110◦C. With no fluid pressure, faulting occurs rapidly and audibly, and the duration of failure increases monotonically with increasing fluid pressure and temperature. Although non-dilatant during initial strain hardening, the serpentinite dilates during strain weakening concomitant with fault rupture and slip. Non-dilatant strain hardening occurs by microcracking along serpentine basal planes and grain boundaries and rarely in mode I orientations, consistent with previous studies. Dilatant fault rupture produces a network of transgranular shear fractures in conjugate orientations, generally with one dominant fracture. Structural observations show that as fluid pressure increases, the number of transgranular fractures increases. We propose that when faulting occurs over a distributed zone rather than a pre-existing principal slip surface, dilatant hardening causes deformation to migrate. This process causes an increase in slip weakening distance and fracture energy at elevated fluid pressures that can lead to more stable failure. Further, thermally-activated processes causes deformation at propagating crack tips, which also increases the slip weakening distance and the effective fracture energy with increasing temperature. Given the geologic settings for slow slip, our results indicate that high fluid pressure, distributed deformation, and thermally-activated processes may all contribute to slow fault rupture and slip.

French, Melodie E., and Wenlu Zhu. “Slow fault propagation in serpentinite under conditions of high pore fluid pressure.” Earth and Planetary Science Letters 473 (2017): 131-140. doi: 10.1016/j.epsl.2017.06.009

Bounds on Geologically Current Rates of Motion of Groups of Hotspots

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 Chengzu Wang, Richard G. Gordon*, and Tuo Zhang

It is widely believed that groups of hotspots in different regions of the world are in relative motion at rates of 10 to 30 mm a–1 or more. Here we present a new method for analyzing geologically current motion between groups of hotspots beneath different plates. In an inversion of 56 globally distributed, equally weighted trends of hotspot tracks, the dispersion is dominated by differences in trend between different plates rather than differences within plates. Nonetheless the rate of hotspot motion perpendicular to the direction of absolute plate motion, vperp, differs significantly from zero for only three of ten plates and then by merely 0.3 to 1.4 mm a–1. The global mean upper bound on |vperp| is 3.2 ±2.7 mm a–1. Therefore, hotspots move slowly and can be used to define a global reference frame for plate motions.

Link: http://onlinelibrary.wiley.com/doi/10.1002/2017GL073430/full

DOI: 10.1002/2017GL073430

The rigid-plate and shrinking-plate hypotheses: Implications for the azimuths of transform faults

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Jay Kumar Mishra and Richard G. Gordon*

The rigid-plate hypothesis implies that oceanic lithosphere does not contract horizontally as it cools (hereinafter “rigid plate”). An alternative hypothesis, that vertically averaged tensional thermal stress in the competent lithosphere is fully relieved by horizontal thermal contraction (hereinafter “shrinking plate”), predicts subtly different azimuths for transform faults. The size of the predicted difference is as large as 2.44° with a mean and median of 0.46° and 0.31°, respectively, and changes sign between right-lateral (RL)-slipping and left-lateral (LL)-slipping faults. For the MORVEL transform-fault data set, all six plate pairs with both RL- and LL-slipping faults differ in the predicted sense, with the observed difference averaging 1.4° ± 0.9° (95% confidence limits), which is consistent with the predicted difference of 0.9°. The sum-squared normalized misfit, r, to global transform-fault azimuths is minimized for γ = 0.8 ± 0.4 (95% confidence limits), where γ is the fractional multiple of the predicted difference in azimuth between the shrinking-plate (γ = 1) and rigid-plate (γ = 0) hypotheses. Thus, observed transform azimuths differ significantly between RL-slipping and LL-slipping faults, which is inconsistent with the rigid-plate hypothesis but consistent with the shrinking-plate hypothesis, which indicates horizontal shrinking rates of 2% Ma−1 for newly created lithosphere, 1% Ma−1 for 0.1 Ma old lithosphere, 0.2% Ma−1 for 1 Ma old lithosphere, and 0.02% Ma−1 for 10 Ma old lithosphere, which are orders of magnitude higher than the mean intraplate seismic strain rate of ~10−6 Ma−1 (5 × 10−19 s−1).

 

Link: http://onlinelibrary.wiley.com/doi/10.1002/2015TC003968/full

DOI: 10.1002/2015TC003968

Comparison of full wavefield synthetics with frequency-dependent traveltimes calculated using wavelength-dependent velocity smoothing

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Comparison of full wavefield synthetics with frequency-dependent traveltimes calculated using wavelength-dependent velocity smoothing

Jianxiong Chen and Colin A. Zelt

Journal of Environmental and Engineering Geophysics, 22, 133-141, 2017.

Ray theory-based traveltime calculation that assumes infinitely high frequency wave propagation is likely to be invalid in the near-surface (upper tens of meters) due to the relatively large seismic wavelength compared with the total travel path lengths and the scale of the near -surface velocity heterogeneities. The wavelength-dependent velocity smoothing (WDVS) algorithm calculates a frequency-dependent, first-arrival traveltime by assuming that using a wavelength-smoothed velocity model and conventional ray theory is equivalent to using the original unsmoothed model and a frequency-dependent calculation. This paper presents comparisons of WDVS-calculated traveltimes with band-limited full wave field synthetics including the results from 1) different velocity models, 2) different frequency spectra, 3) different values of a free parameter in the WDVS algorithm, and 4) different levels of added noise to the synthetics. The results show that WDVS calculates frequency-dependent travel times that are generally consistent with the first arrivals from band-limited full wavefield synthetics. Compared to infinite-frequency traveltimes calculated using conventional ray theory, the WDVS frequency-dependent traveltimes are more consistent with the first arrivals picked from full wavefield synthetics in terms of absolute time and trace-to-trace variation. The results support the use of WDVS as the forward modeling component of a tomographic inversion method, or any seismic method that involves modeling first-arrival traveltimes.

 

Detecting a known near-surface target through application of frequency-dependent traveltime tomography and full waveform inversion to P- and SH-wave seismic refraction data

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Detecting a known near-surface target through application of frequency-dependent traveltime tomography and full waveform inversion to P- and SH-wave seismic refraction data

Jianxiong Chen, Colin A. Zelt and Priyank Jaiswal

Geophysics, 82, R1-R17, 2017.

We have applied a combined workflow of frequency-dependent

traveltime tomography (FDTT) and full-waveform inversion

(FWI) to 2D near-surface P- and SH-wave seismic data

to detect a known target consisting of a buried tunnel with concrete

walls and a void space inside. FDTT inverted the P- and

SH-wave picked traveltimes at 250 Hz to provide long-wavelength

background velocity models as the starting models for

FWI. FWI inverted 18–54 Hz P-wave data and 16–50 Hz

SH-wave data to produce velocity models with subwavelengthand

wavelength-scale features allowing for direct interpretation

of the velocity models as is usually carried out in conventional

imaging using seismic reflection data. The P- and SH-wave

models image the top part of the tunnel at the correct location

at a depth of 1.6 m as a high-velocity anomaly. The P-wave

models also image the air in the void space of the tunnel as

a low-velocity anomaly. The inverted models were assessed

by synthetic tests, the consistency of the inverted sources,

and the fit between the predicted and observed data. As a comparison,

conventional ray-theory infinite-frequency traveltime

tomography (IFTT) was also applied in a combined workflow

with FWI. The comparisons of the inverted models favor the use

of FDTT over IFTT because (1) The FDTT models better recover

the magnitude of the velocity anomalies and (2) the FDTT

model serves as a better starting model for FWI, which results in

a more accurate FWI velocity estimation with better recovery of

the magnitude and location of the key features. FDTT will not

provide significant benefits over IFTT in all studies, particularly

those in which ray theory is valid.

IGR: Deep mantle roots and continental emergence: implications for whole-Earth elemental cycling, long-term climate, and the Cambrian explosion

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Deep mantle roots and continental emergence: implications for whole-Earth elemental cycling, long-term climate, and the Cambrian explosion

Cin-Ty A. Lee, Jeremy Caves, Hehe Jiang, Wenrong Cao, Adrian Lenardic, N. Ryan McKenzie, Oliver Shorttle, Qing-zhu Yin & Blake Dyer
International Geology Review 2017, http://dx.doi.org/10.1080/00206814.2017.1340853

Elevations on Earth are dominantly controlled by crustal buoyancy, primarily through variations in crustal thickness: continents ride higher than ocean basins because they are underlain by thicker crust. Mountain building, where crust is magmatically or tectonically thickened, is thus key to making continents. However, most of the continents have long passed their mountain building origins, having since subsided back to near sea level. The elevations of the old, stable continents are lower than that expected for their crustal thicknesses, requiring a subcrustal component of negative buoyancy that develops after mountain building. While initial subsidence is driven by crustal erosion, thermal relaxation through growth of a cold thermal boundary layer provides the negative buoyancy that causes continents to subside further. The maximum thickness of this thermal boundary layer is controlled by the thickness of a chemically and rheologically distinct continental mantle root, formed during large-scale mantle melting billions of years ago. The final resting elevation of a stabilized continent is controlled by the thickness of this thermal boundary layer and the temperature of the Earth’s mantle, such that continents ride higher in a cooler mantle and lower in a hot mantle. Constrained by the thermal history of the Earth, continents are predicted to have been mostly below sea level for most of Earth’s history, with areas of land being confined to narrow strips of active mountain building. Large-scale emergence of stable continents occurred late in Earth’s history (Neoproterozoic) over a 100–300 million year transition, irreversibly altering the surface of the Earth in terms of weathering, climate, biogeochemical cycling and the evolution of life. Climate during the transition would be expected to be unstable, swinging back and forth between icehouse and greenhouse states as higher order fluctuations in mantle dynamics would cause the Earth to fluctuate rapidly between water and terrestrial worlds.

JGR-Planets: Carbon contents in reduced basalts at graphite saturation: Implications for the degassing of Mars, Mercury, and the Moon

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Abstract:

Carbon contents in reduced Martian basalts at graphite saturation were experimentally studied at 1400-1550 °C, 1-2 GPa, and logfO2 of IW-0.4 to IW+1.5 (IW denotes the Fe-FeO buffer). The results show that carbon solubility in Martian basalts, determined by SIMS, is 20 to 1400 ppm, increasing with increasing fO2. Raman and FTIR measurements on the quenched silicate glasses show that the dominant carbon species in Martian basalts is carbonate (CO32-). The experimental data generated here were combined with literature data on similar graphite-saturated C solubility for mafic-ultramafic compositions to develop an empirical model that can be used to predict carbon content of graphite-saturated reduced basalts at vapor-absent conditions:

At IW+1.7 ≥ logfO2 ≥ IW-1:

log(C, ppm) = -3702(±534)/T – 194(±49)P/T – 0.0034(±0.043)logXH2O + 0.61(±0.07)NBO/T + 0.55(±0.02)ΔIW + 3.5(±0.3) (R2=0.89)

At IW-5.3 ≤ logfO2 ≤ IW-1:

log(C, ppm) = 0.96(±0.19)logXH2O – 0.25(±0.04)ΔIW + 2.83(±0.34) (R2=0.6)

in which T is temperature in K, P is pressure in GPa, is mole fraction of water in basalts,  is the oxygen fugacity relative to the IW buffer, and . This model was applied to predict carbon content in graphite-saturated mantle melts of the Mercury, Mars, and the Moon. The results show that graphite may be consumed during the production and extraction of some Martian basalts, and CO2 released by volcanism on Mars cannot be an efficient greenhouse gas in the early Mars. The lunar mantle carbon may be one of the main propellant driving the fire-fountain eruption on the Moon; however, the mantle carbon may not be an important propellant for the explosive eruption on Mercury.

 

Li, Y., Dasgupta, R., Tsuno, K. (2017). Carbon contents in reduced basalts at graphite saturation: Implications for the degassing of Mars, Mercury, and the Moon. Journal of Geophysical Research – Planets 122. doi:10.1002/2017JE005289

Geomorphology: Stepwise morphological evolution of the active Yellow River (Huanghe) delta lobe (1976–2013): Dominant roles of riverine discharge and sediment grain size

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Stepwise morphological evolution of the active Yellow River (Huanghe) delta lobe (1976–2013): Dominant roles of riverine discharge and sediment grain size

Xiao Wu, Naishuang Bi, Jingping Xu, Jeffrey A. Nittrouer, Zuosheng Yang, Yoshiki Saito, Houjie Wang

The presently active Yellow River (Huanghe) delta lobe has been formed since 1976 when the river was artificially diverted. The process and driving forces of morphological evolution of the present delta lobe still remain unclear. Here we examined the stepwise morphological evolution of the active Yellow River delta lobe including both the subaerial and the subaqueous components, and illustrated the critical roles of riverine discharge and sediment grain size in dominating the deltaic evolution. The critical sediment loads for maintaining the delta stability were also calculated from water discharge and sediment load measured at station Lijin, the last gauging station approximately 100 km upstream from the river mouth. The results indicated that the development of active delta lobe including both subaerial and subaqueous components has experienced four sequential stages. During the first stage (1976–1981) after the channel migration, the unchannelized river flow enhanced deposition within the channel and floodplain between Lijin station and the river mouth. Therefore, the critical sediment supply calculated by the river inputs obtained from station Lijin was the highest. However, the actual sediment load at this stage (0.84 Gt/yr) was more than twice of the critical sediment load (~ 0.35 Gt/yr) for sustaining the active subaerial area, which favored a rapid seaward progradation of the Yellow River subaerial delta. During the second stage (1981–1996), the engineering-facilitated channelized river flow and the increase in median grain size of suspended sediment delivered to the sea resulted in the critical sediment load for keeping the delta stability deceasing to 0.29 Gt/yr. The active delta lobe still gradually prograded seaward at an accretion rate of 11.9 km2/yr at this stage as the annual sediment load at Lijin station was 0.55 Gt/yr. From 1996 to 2002, the critical sediment load further decreased to 0.15 Gt/yr with the sediment grain size increased to 22.5 μm; however, the delta suffered net erosion because of the insufficient sediment supply (0.11 Gt/yr). In the most recent stage (2002 − 2013), the intensive scouring of the lower river channel induced by the dam regulation provided relatively coarser sediment, which effectively reduced the critical sediment load to 0.06 Gt/yr, much lower than the corresponding sediment load at Lijin station (~ 0.16 Gt/yr). Consequently, the subaerial Yellow River delta transitioned to a slight accretion phase. Overall, the evolution of the active Yellow River delta is highly correlated to riverine water and sediment discharge. The sediment supply for keeping the subaerial delta stability is inconstant and varying with the river channel morphology and sediment grain size. We conclude that the human-impacted riverine sediment discharge and grain-size composition play dominant roles in the stepwise morphological evolution of the active delta lobe.

Nature Comm: Lithospheric foundering and underthrusting imaged beneath Tibet

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Lithospheric foundering and underthrusting imaged beneath Tibet

Min Chen, Fenglin Niu, Jeroen Tromp, Adrian Lenardic, Cin-Ty A. Lee, Wenrong Cao & Julia Ribeiro

Nature Communications 8, Article number: 15659 (2017), doi:10.1038/ncomms15659

Long-standing debates exist over the timing and mechanism of uplift of the Tibetan Plateau and, more specifically, over the connection between lithospheric evolution and surface expressions of plateau uplift and volcanism. Here we show a T-shaped high wave speed structure in our new tomographic model beneath South-Central Tibet, interpreted as an upper-mantle remnant from earlier lithospheric foundering. Its spatial correlation with ultrapotassic and adakitic magmatism supports the hypothesis of convective removal of thickened Tibetan lithosphere causing major uplift of Southern Tibet during the Oligocene. Lithospheric foundering induces an asthenospheric drag force, which drives continued underthrusting of the Indian continental lithosphere and shortening and thickening of the Northern Tibetan lithosphere. Surface uplift of Northern Tibet is subject to more recent asthenospheric upwelling and thermal erosion of thickened lithosphere, which is spatially consistent with recent potassic volcanism and an imaged narrow low wave speed zone in the uppermost mantle.