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The uppermost mantle seismic velocity structure of West Antarctica from Rayleigh wave tomography: Insights into tectonic structure and geothermal heat flow

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): J.P. O'Donnell, G.W. Stuart, A.M. Brisbourne, K. Selway, Y. Yang, G.A. Nield, P.L. Whitehouse, A.A. Nyblade, D.A. Wiens, R.C. Aster, S. Anandakrishnan, A.D. Huerta, T. Wilson, J.P. Winberry

Abstract

We present a shear wave model of the West Antarctic upper mantle to ∼200 km depth with enhanced regional resolution from the 2016-2018 UK Antarctic Seismic Network. The model is constructed from the combination of fundamental mode Rayleigh wave phase velocities extracted from ambient noise (periods 8-25 s) and earthquake data by two-plane wave analysis (periods 20-143 s). We seek to (i) image and interpret structures against the tectonic evolution of West Antarctica, and (ii) extract information from the seismic model that can serve as boundary conditions in ice sheet and glacial isostatic adjustment modelling efforts. The distribution of low velocity anomalies in the uppermost mantle suggests that recent tectonism in the West Antarctic Rift System (WARS) is mainly concentrated beneath the rift margins and largely confined to the uppermost mantle (<180 km). On the northern margin of the WARS, a pronounced low velocity anomaly extends eastward from beneath the Marie Byrd Land dome toward Pine Island Bay, underlying Thwaites Glacier, but not Pine Island Glacier. If of plume-related thermal origin, the velocity contrast of ∼5% between this anomaly and the inner WARS translates to a temperature difference of ∼125-200 C∘. However, the strike of the anomaly parallels the paleo-Pacific convergent margin of Gondwana, so it may reflect subduction-related melt and volatiles rather than anomalously elevated temperatures, or a combination thereof. Motivated by xenolith analyses, we speculate that high velocity zones imaged south of the Marie Byrd Land dome and in the eastern Ross Sea Embayment might reflect the compositional signature of ancient continental fragments. A pronounced low velocity anomaly underlying the southern Transantarctic Mountains (TAM) is consistent with a published lithospheric foundering hypothesis. Taken together with a magnetotelluric study advocating flexural support of the central TAM by thick, stable lithosphere, this points to along-strike variation in the tectonic history of the TAM. A high velocity anomaly located in the southern Weddell Sea Rift System might reflect depleted mantle lithosphere following the extraction of voluminous melt related to Gondwana fragmentation. Lithospheric thickness estimates extracted from 1D shear wave velocity profiles representative of tectonic domains in West Antarctica indicate an average lithospheric thickness of ∼85 km for the WARS, Marie Byrd Land, and Thurston Island block. This increases to ∼96 km in the Ellsworth Mountains. A surface heat flow of ∼60mW/m2 and attendant geotherm best explains lithospheric mantle shear wave velocities in the central WARS and in the Thurston Island block adjacent to Pine Island Glacier; a ∼50mW/m2 geotherm best explains the velocities in the Ellsworth Mountains, and a ∼60mW/m2 geotherm best explains a less well-constrained velocity profile on the southern Antarctic Peninsula. We emphasise that these are regional average (many hundreds of km) heat flow estimates constrained by seismic data with limited sensitivity to upper crustal composition.

Graphical abstract

Constraints on terrestrial planet formation timescales and equilibration processes in the Grand Tack scenario from Hf-W isotopic evolution

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Nicholas G. Zube, Francis Nimmo, Rebecca A. Fischer, Seth A. Jacobson

Abstract

We examine 141 N-body simulations of terrestrial planet late-stage accretion that use the Grand Tack scenario, coupling the collisional results with a hafnium-tungsten (Hf-W) isotopic evolution model. Accretion in the Grand Tack scenario results in faster planet formation than classical accretion models because of higher planetesimal surface density induced by a migrating Jupiter. Planetary embryos that grow rapidly experience radiogenic ingrowth of mantle 182W that is inconsistent with the measured terrestrial composition, unless much of the tungsten is removed by an impactor core that mixes thoroughly with the target mantle. For physically Earth-like surviving planets, we find that the fraction of equilibrating impactor core kcore≥ 0.6 is required to produce results agreeing with observed terrestrial tungsten anomalies (assuming equilibration with relatively large volumes of target mantle material; smaller equilibrating mantle volumes would require even larger kcore). This requirement of substantial core re-equilibration may be difficult to reconcile with fluid dynamical predictions and hydrocode simulations of mixing during large impacts, and hence this result does not favor the rapid planet building that results from Grand Tack accretion.

Element and Sr–O isotope redistribution across a plate boundary-scale crustal serpentinite mélange shear zone, and implications for the slab-mantle interface

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): James M. Scott, Steven A.F. Smith, Matthew S. Tarling, Petrus J. le Roux, Chris Harris, J. Elis Hoffmann, Sophie Scherzer, Chris J. Tulley

Abstract

Fluid flow along crustal plate boundary-scale serpentinite shear zones may provide insights into element transfer at one of Earth's most important but least accessible lithological boundaries: the slab-mantle wedge plate interface. For ∼55 km in the Southern Alps of New Zealand, a 10 to 100s m wide block-in-matrix serpentinite mélange shear zone – the Livingstone Fault – separates the harzburgitic base of the Dun Mountain Ophiolite Belt from arc-derived metasedimentary and minor metabasaltic rocks. Transects across the shear zone reveal systematic chemical and isotopic changes over several hundred meters, from harzburgite (<1 wt% H2O, 87Sr/86Sr = 0.7031–0.7045, δ18O=∼+5.3‰) to massive and then foliated mélange serpentinite (up to 13.7 wt% H2O, 87Sr/86Sr = 0.7084, δ18O=+7.8‰). These changes require hydration of peridotite by fluids derived from, or that interacted with, the metasediment-dominated wallrock (87Sr/86Sr = 0.7073–0.7093, δ18O≥+9.6‰). Tremolite-veined serpentinite and metasomatized greyschist blocks (87Sr/86Sr = 0.7063–0.7071; δ18O=+8.7) within the serpentinite mélange also require ultramafic- and metasediment–derived element mixing. Halogens (Br and I) were enriched in serpentinized ultramafic rocks. By analogy with the crustal Livingstone Fault, the slab-mantle plate interface may have a strongly anisotropic basal serpentinite mélange shear zone that grades into a thicker zone of massive and variably serpentinized peridotite with a progressively decreasing “slab” element and isotope signature. Permeability in subduction-related shear zones is predicted to be mainly parallel to the slab but channelized fluid mobility across the Livingstone Fault was aided by transient fracturing. Oxidation of harzburgite during serpentinization appears to have been accompanied by mobilization of Si, Ca, Mg, Sr, Br, I, Ga, Mo, Ta, W, V and Y. As many of these elements occur in the sources of arc magmas, serpentinization may play an important role in mobilizing them. The Livingstone Fault lithologies and their element and isotope transfer records means this shear zone may represent a field-accessible analogue for the shallow slab-mantle plate interface.

Bedrock fracture density controls on hillslope erodibility in steep, rocky landscapes with patchy soil cover, southern California, USA

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Alexander B. Neely, Roman A. DiBiase, Lee B. Corbett, Paul R. Bierman, Marc W. Caffee

Abstract

Although many steep landscapes comprise a patchwork of soil-mantled and bare-bedrock hillslopes, models typically assume hillslopes are entirely soil-mantled or bare-bedrock, making it challenging to predict how rock properties influence hillslope erodibility and landscape evolution. Here, we study headwater catchments across the San Gabriel Mountains (SGM) and Northern San Jacinto Mountains (NSJM) in southern California; two steep landscapes with similar climate and lithology, but with distinctly different bedrock fracture densities, ∼5× higher in the SGM. We combine new and published detrital in-situ cosmogenic 10Be-derived erosion rates with analysis of high resolution imagery and topography to quantify how the morphology and abundance of bare-bedrock and soil-mantled hillslopes vary with erosion rate within and between the two landscapes. For similar mean hillslope angles (35-46°), catchments in the NSJM erode at rates of 0.1-0.6 m kyr−1, compared to 0.2-2.2 m kyr−1 in the SGM. In both landscapes, bare-bedrock hillslopes increase in abundance with increasing erosion rate; however, more and steeper bedrock is exposed in the NSJM, indicating that wider bedrock fracture spacing reduces soil production efficiency and supports steeper cliffs. Additionally, higher erosion rates in the SGM require a 3× higher soil transport efficiency, reflecting an indirect control of bedrock fracture density on the size of sediment armoring hillslopes. Our data highlight how hillslope morphodynamics in steep landscapes depend on the strength of soil and bedrock and the efficiency of soil production and transport, all of which are variably sensitive to rock properties and influence the partitioning of soil and bare-bedrock on hillslopes.

Influence of glacial isostatic adjustment on river evolution along the U.S. mid-Atlantic coast

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): T. Pico, J.X. Mitrovica, J.T. Perron, K.L. Ferrier, J. Braun

Abstract

Long-term river evolution depends partly on crustal deformation, which shapes the topography crossed by rivers. On glacial timescales, ice-sheet growth and decay can produce crustal vertical motion of ∼10 mm/yr resulting from the solid Earth's adjustment to variations in ice and water loads, comparable to tectonically-driven rates in the most rapidly uplifting mountains on Earth. This process of glacial isostatic adjustment (GIA) can influence river courses and drainage basins substantially, particularly near former ice margins. We explore the extent to which GIA influenced the evolution of rivers along the United States east coast during the last glacial cycle. We compute gravitationally self-consistent GIA responses that incorporate recent constraints on the Laurentide Ice Sheet history through the last glacial build-up phase, and we connect the predicted variations in topography to abrupt changes in river dynamics recorded in the Hudson, Delaware, Susquehanna, and Potomac Rivers from 40 ka to present. To the extent that increases in sediment transport capacity imply increases in river incision rate, the GIA-driven changes in slope and drainage area are consistent with episodes of erosion and sedimentation observed in the Hudson, Delaware, and Potomac Rivers, but inconsistent with the observed accelerated river incision in the Susquehanna River at 30-14 ka. These analyses add to a growing body of evidence showing that GIA strongly influences river evolution over millennial timescales.

Bulk chondrite variability in mass independent magnesium isotope compositions – Implications for initial solar system <sup>26</sup>Al/<sup>27</sup>Al and the timing of terrestrial accretion

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Tu-Han Luu, Remco C. Hin, Christopher D. Coath, Tim Elliott

Abstract

We have determined Δ′26MgDSM-3, the mass-independent variations in 26Mg/24Mg, of primitive, bulk meteorites to precisions better than ±3 ppm (2se). Our measurements of samples from 10 different chondrite groups show Δ′26MgDSM-3 that vary from −5 to 22 ppm. Our data define an array with a positive slope in a plot of Δ′26MgDSM-3 against 27Al/24Mg, which can be used to determine (26Al/27Al)0, i.e. initial 26Al/27Al, and (Δ′26MgDSM-3)0, i.e. initial Δ′26MgDSM-3. On such an isochron plot, the best fit of our new measurements combined with literature data implies (26Al/27Al)0 of (4.67±0.78)×10−5 and (Δ′26MgDSM-3)0 of −31.6 ± 5.7 ppm (2se) for ordinary and carbonaceous chondrites, other than CR chondrites, which have anomalously low Δ′26MgDSM-3. These parameters are within uncertainty of those defined by previous measurements of bulk calcium-, aluminium-rich inclusions (CAIs) that set canonical (26Al/27Al)∼05×10−5. The most straightforward interpretation of all these observations is that differences in the Al/Mg of bulk ordinary and carbonaceous chondrites are dominantly controlled by variable contributions of early-formed refractory and major silicate components derived from a common, canonical reservoir. The Δ′26MgDSM-3 of enstatite chondrites are slightly more radiogenic (∼3 ppm) at similar Al/Mg to the ordinary chondrites. We speculate that this is related to the timing of removal of a refractory component from the source reservoirs of these different meteorite groups; the higher Δ′26MgDSM-3 of the enstatite chondrites suggests later (∼0.5 Ma post CAIs) condensation and loss of this refractory component. Despite inferred consistency of (26Al/27Al)0 and (Δ′26MgDSM-3)0 across most chondrite groups, some nebular heterogeneity is required to account for the compositions of CR chondrites. Our preferred interpretation is that the CR source region has lower (Δ′26MgDSM-3)0. As the most appropriate isotopic reference for the Earth, our new mean enstatite chondrite composition allows us to assess possible ingrowth of 26Mg from live 26Al during accretion of the Earth. The Earth has Δ′26MgDSM-3 within uncertainty of enstatite chondrites, despite its higher Al/Mg. This requires that the terrestrial increase in Al/Mg, which we attribute to vapour loss during accretion, must have happened >1.5 Ma post CAI formation, in an instantaneous fractionation model.

Paleomagnetic evidence for cold emplacement of eruption-fed density current deposits beneath an ancient summit glacier, Tongariro volcano, New Zealand

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): R.P. Cole, C. Ohneiser, J.D.L. White, D.B. Townsend, G.S. Leonard

Abstract

The thermal structures of eruption-fed, subglacial volcaniclastic deposits are poorly understood because their emplacement is hazardous to observe or obscured by the glacier. Determining deposit emplacement temperature, however, supports improved understanding of the flow dynamics and emplacement processes of volcaniclastic material beneath a glacier. Understanding the emplacement temperatures of ancient volcanic deposits is also important because they can be used, in combination with field studies, to infer the eruptive environment. Here, we use paleomagnetic techniques to quantify the emplacement temperatures of two ancient, proximal, eruption-fed density current deposits at Tongariro volcano, New Zealand. Stepwise thermal demagnetisation of lithic and recycled juvenile block-sized clasts reveal randomly orientated directions of magnetisation, suggesting that the clasts were rotated within the flow but not heated. Additional data from thermomagnetic, hysteresis, and isothermal remanent magnetisation tests indicate that the principal carrier of magnetic remanence is magnetite, and that the magnetisation directions are a primary remanence rather than post-depositional chemical remanent magnetisations. Following systematic removal of any viscous remanent magnetisation, the post-emplacement equilibrium temperatures for the deposits can be estimated at <150 °C. The paleomagnetic data support field evidence for rapid cooling of clasts and waterlain deposition. The deposit-forming eruptions took place beneath a summit glacier where the freshly erupted tephra was efficiently cooled by mixing with meltwater. Lithic blocks and recycled juvenile bombs were entrained and remobilised within the cool currents that drained along meltwater channels beneath the ice. This is the first study in which paleomagnetic data have been used to determine the equilibrium temperatures of subglacial density current deposits. The data provide new insight into volcaniclastic flow dynamics beneath a glacier and current-meltwater interactions.

Xenon and iodine behaviour in magmas

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): C. Leroy, H. Bureau, C. Sanloup, C. Raepsaet, K. Glazirin, P. Munsch, M. Harmand, G. Prouteau, H. Khodja

Abstract

Iodine (I) and xenon (Xe) are two key elements that trace Earth's differentiation (e.g. atmosphere formation) and dynamics (e.g. volcanism and recycling at subduction zones). Iodine and Xe abundances are linked through the decay of the extinct 129I that produced 129Xe, which is today depleted in the Earth's atmosphere compared to the composition of the solar system (i.e. chondrites). Iodine and Xe cycles and storage in the deep Earth are almost unknown, which is in large part due to the fact that their behaviour in magmas and fluids, key agents of mass transfer through planetary envelopes, are poorly known. Here, the solubility of Xe and I in melts is measured under high pressure (P) and temperature (T) conditions using large volume presses, and Xe and I behaviour in melts and fluids is monitored in situ under high P-T conditions using resistive heating diamond anvil cells combined with synchrotron x-ray fluorescence (XRF) and Raman spectroscopy. Xenon, I and H (H2O) contents were measured in quenched glasses by particle x-ray Emission (PIXE) and Elastic Recoil Detection Analysis (ERDA). Solubility, speciation and degassing processes are investigated for two different compositions: haplogranitic melt (HPG analogue for crustal melts) and basaltic melts (MORB and IAB). Experimentally measured solubilities for both elements are much higher than their natural abundances in terrestrial magmas. Xenon solubility at 3.5 GPa reaches 4.00 wt.% in HPG and 0.40 wt.% in basalts. Iodine solubility is 0.46 wt.% at 0.4 GPa on average in HPG, and reaches 1.42 wt.% in basalts at 2 GPa. The in situ Raman spectroscopic study shows that I forms I-I bonds in hydrous high P fluids/melts unlike Xe that was previously shown to oxidize in high P melts. The XRF monitoring of I and Xe partitioning between aqueous fluids and silicate melts during decompression (i.e. water degassing) shows that Xe degassing is strongly P-T dependent and can be retained in the melt at deep crust conditions, while I is totally washed out from the silicate melt by the aqueous phase. Xenon and I degassing processes are based on different mechanisms, which implies that the atmospheric isotopic signature of Xe cannot be inherited from a process involving volcanic water degassing. Instead, 129Xe depletion may originate from a separation of both elements at depth, by deep fluids, a proposition that agrees with a deep storage of Xe in minerals.

Aftershock deficiency of induced earthquake sequences during rapid mitigation efforts in Oklahoma

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): T.H.W. Goebel, Z. Rosson, E.E. Brodsky, J.I. Walter

Abstract

Induced seismicity provides a rare opportunity to study earthquake triggering and underlying stress perturbations. Triggering can be a direct result of induced stress changes or indirect due to elastic stress transfer from preceding events leading to aftershocks. Both of these processes are observable in areas with larger magnitude induced events, such as Oklahoma. We study aftershock sequences of M2.5 to M5.8 earthquakes and examine the impact of targeted injection rate reductions. In comparing aftershock productivity between California and Oklahoma, we find similar exponential scaling statistics between mainshock magnitude and average number of aftershocks. For events with M≥4.5 Oklahoma exhibits several mainshocks with total number of aftershocks significantly below the average scaling behavior. The sequences with deficient aftershock numbers also experienced rapid, strong mitigation and reduced injection rates, whereas two events with M4.8 and M5.0 with weak mitigation exhibit normal aftershock productivity. The timing of when aftershock activity is reduced correlates with drops in injection rates with a lag time of several days. Large mainshocks with significantly reduced aftershocks may explain decreasing seismicity rates while seismic moment release was still increasing in Oklahoma in 2016. We investigate the expected poroelastic stress perturbations due to injection rate changes within a layered axisymmetric model and find that stresses are lowered by 10s to 100s kPa within the injection-affected zone. For earthquakes induced by poroelastic stress-increase at several kilometers from wells, the rapid shut-in of wells may lead to elastic stress reductions sufficiently high to arrest unfolding aftershock sequences within days after mitigation starts.

Eastern North American climate in phase with fall insolation throughout the last three glacial-interglacial cycles

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Hai Cheng, Gregory S. Springer, Ashish Sinha, Benjamin F. Hardt, Liang Yi, Hanying Li, Ye Tian, Xianglei Li, Harold D. Rowe, Gayatri Kathayat, Youfeng Ning, R. Lawrence Edwards

Abstract

The nature and controls of orbital-scale climate variability in North America (NA) are subjects of ongoing debates. On the basis of previous cave records from Southwestern United States, two mutually incompatible hypotheses have been proposed. One links NA orbital-scale climate variability to Northern Hemisphere (NH) summer insolation forcing in a manner analogous to low-latitude monsoon systems, while the other suggests that it is not causally tied to either changes in global ice-volumes or NH summer insolation. Here we report new cave oxygen isotope (δ18O) records from Buckeye Creek Cave (BCC), West Virginia, east central North America, covering most of the past three glacial-interglacial periods (∼335 to 45 kyr ago). The BCC δ18O record exhibits a strong precession-band cycle, which is in-phase with changes in global ice-volumes (i.e., sea level), sea surface temperatures in the NE Gulf of Mexico and is consistent with the results from published cave records from Nevada and Devils Hole. As with global ice-volume, the BCC records lag summer insolation at 65°N by ∼5000 yr, which stands in contrast with records of low-latitude monsoon variability in South America and Asia, which are in phase and out-of-phase with changes in summer insolation and sea level, respectively. Provided the degree of lag to summer insolation provides a measure of competing forcing from global ice-volume and summer insolation, our data suggest that NA orbital-scale climate variability is dominantly driven by ice-volume forcing. In addition, the sea surface temperatures in the NE Gulf of Mexico and changes in northern high-latitude cryosphere may be also important in explaining the unusually low δ18O values at times of the intermediate ice-volume periods in BCC and other NA cave records.

Magma extraction pressures and the architecture of volcanic plumbing systems

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Guilherme A.R. Gualda, Darren M. Gravley, Chad D. Deering, Mark S. Ghiorso

Abstract

The deposits of large volcanic eruptions provide the sole record of the architecture of magmatic plumbing systems in the moments when large pools of crystal-poor, eruptible magma are present in the crust. It is widely accepted that silicic magmas form by segregation of melt-rich, crystal-poor magma from a crystal-rich source; however, the depths at which segregation takes place and the distribution of the magma within the crust are not well constrained. We present a new approach to calculate pressures at which crystal-poor, eruptible magma is extracted from a crystal-rich source (i.e. mush). We apply the approach to a sequence of large (>50 km3) eruptions from the Taupo Volcanic Zone (TVZ), New Zealand, which were part of a volcanic flare-up. We compare the calculated extraction pressures with pre-eruptive storage pressures for the same units. Our results show that storage and extraction pressures do not always coincide. Instead, eruptible magma can be completely segregated from the crystal-rich source, and stored at shallower levels in the crust prior to eruption. In the case of the TVZ flare-up, repeated input of material and heat – probably coupled with tectonic extension – gradually conditioned the crust and allowed extraction of eruptible magma over a growing range of pressures with time. Our approach has the potential to reveal important information on the structure and distribution of magmatic systems within the shallow crust.

Porosity of metamorphic rocks and fluid migration within subduction interfaces

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): A.C. Ganzhorn, H. Pilorgé, B. Reynard

Abstract

Large earthquakes break the subduction interface to depths of 60 to 80 km. Current models hold that seismic rupture occurs when fluid overpressure builds in link with porosity cycles, an assumption still to be experimentally validated at high pressures. Porosities of subduction zone rocks are experimentally determined under pressures equivalent to depths of up to 90 km with a novel experimental approach that uses Raman deuterium-hydrogen mapping. Natural rocks (blueschists, antigorite serpentinites, and chlorite-schists) representing a typical cross-section of the subduction interface corresponding to the deep seismogenic zone are investigated. In serpentinite, and to a smaller extent blueschist, porosity increases with deformation, whereas chlorite-rich schists remain impermeable regardless of their deformation history. Such a contrasting behavior explains the observation of over-pressurized oceanic crust and the limited hydration of the forearc mantle wedge. These results provide quantitative evidence that serpentinite, and likely blueschist, may undergo porosity cycles making possible the downdip propagation of large seismic rupture to great depths.

Planktonic adaptive evolution to the sea surface temperature in the Neoproterozoic inferred from ancestral NDK of marine cyanobacteria

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Mariko Harada, Aki Nagano, Sota Yagi, Ryutaro Furukawa, Shin-ichi Yokobori, Akihiko Yamagishi

Abstract

The optimum growth temperature of ancestral cyanobacteria inhabiting the sea surface in the Neoproterozoic was estimated based on the thermal stability of experimentally reconstructed ancestral NDK enzymes. Ancestral NDKs of cyanobacteria that diversified ∼1.7, ∼1.0, ∼0.9, ∼0.7, ∼0.6, and ∼0.5 billion years ago were reconstructed and analyzed, and the unfolding midpoint temperatures (Tms) ranged from ∼65 °C to ∼70 °C. Among the host of analyzed NDKs, the ancestors of marine, planktonic α-cyanobacteria diversified ≤ ∼1.0 Ga are highly likely to have inhabited marine environments during the Neoproterozoic, while ancestral cyanobacteria diversified ∼1.7 billion years ago were possibly marine but the habitat is less constrained compared to the others. According to the calibration curves derived from extant organisms, the obtained Tms of α-cyanobacteria diversified ≤ ∼1.0 Ga correspond to the range of optimum growth temperatures of around ∼33–48 °C. The temperature range agrees well with the long-term sea temperature trend during Neoproterozoic suggested by δ18O and δ30Si records from marine cherts. Adaptation to the low temperature during the snowball glaciations in the late Neoproterozoic was not observed, implying that adaptation of optimum growth conditions to the episodic low temperature may not have been necessary. Therefore, ancestral marine plankton must have consistently adapted to the interglacial sea surface temperature in the Neoproterozoic, which was approximately 5–20 °C higher than that is today. They may have survived the glaciations by acquiring cold tolerance and/or by suppressing growth rate.

Coupled climate and subarctic Pacific nutrient upwelling over the last 850,000 years

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Savannah Worne, Sev Kender, George E.A. Swann, Melanie J. Leng, Ana Christina Ravelo

Abstract

High latitude deep water upwelling has the potential to control global climate over glacial timescales through the biological pump and ocean-atmosphere CO2 exchange. However, there is currently a lack of continuous long nutrient upwelling records with which to assess this mechanism. Here we present geochemical proxy records for nutrient upwelling and glacial North Pacific Intermediate Water (GNPIW) formation in the Bering Sea over the past 850 kyr, which demonstrates that glacial periods were characterised by reduced nutrient upwelling, when global atmospheric CO2 and temperature were also lowered. We suggest that glacial expansion of sea ice in the Bering Sea, and the simultaneous expansion of low nutrient GNPIW, inhibited vertical mixing and nutrient supply across the subarctic Pacific Ocean. Our findings lend support to the suggestion that high latitude sea ice and the resultant intermediate water formation, modulated deep water upwelling and ocean-atmosphere CO2 exchange on glacial-interglacial timescales.

Light Mg isotopes in mantle-derived lavas caused by chromite crystallization, instead of carbonatite metasomatism

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Ben-Xun Su, Yan Hu, Fang-Zhen Teng, Yan Xiao, Hong-Fu Zhang, Yang Sun, Yang Bai, Bin Zhu, Xin-Hua Zhou, Ji-Feng Ying

Abstract

Carbonatite metasomatism plays an important role in modifying the composition of Earth's mantle, however, its effect on mantle Mg isotopic composition is poorly constrained. Here, we report high-precision mineral Mg isotope data for three suites of mantle peridotite xenoliths that experienced variable degrees of carbonatite metasomatism. The δ26Mg values of minerals in these xenoliths are variable and range from −0.32 to −0.11‰ in olivine, from −0.28 to −0.09‰ in orthopyroxene, from −0.27 to −0.05‰ in clinopyroxene, from 0.06 to 0.44‰ in spinel and from −0.61 to −0.37‰ in garnet. Calculated bulk-rock δ26Mg values of the peridotites vary from −0.27 to −0.10‰, falling within and slightly higher than the normal mantle range (−0.25 ± 0.07‰). The coexisting minerals are in isotopic equilibrium, with clinopyroxene δ26Mg values correlated with the carbonatite metasomatic indices such as MgO and Na2O in orthopyroxene. These results suggest that carbonatite metasomatism does not produce light Mg isotopic signature in mantle peridotites as previously suggested, instead it might slightly elevate their δ26Mg values. Therefore, carbonatite-metasomatized peridotites in the mantle cannot be the primary source rocks of low-δ26Mg mantle-derived magmas. Instead, fractional crystallization and accumulation of chromite during ascent of the basaltic magmas may explain the isotopically light basalts, as supported by the covariations of δ26Mg with chemical indices of chromite crystallization (e.g., Cr, V, Fe and Ti). Consequently, chromite crystallization may significantly influence the physiochemical processes on the genesis of basalts, which would require comprehensive evaluation in future studies.

Oscillations of global sea-level elevation during the Paleogene correspond to 1.2-Myr amplitude modulation of orbital obliquity cycles

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Yang Liu, Chunju Huang, James G. Ogg, Thomas J. Algeo, David B. Kemp, Wenlong Shen

Abstract

The mechanisms involved in the formation of 1- to 2-Myr-long third-order stratigraphic sequences through the Phanerozoic have been extensively debated, but the main underlying forcings remain uncertain. In this study, we investigated third-order sequences of Paleocene-Eocene age in the East China Sea Shelf Basin, identifying a prominent ∼1.2-Myr periodicity in the 405-kyr-tuned gamma ray profile of the WP-1 borehole. We infer that this ∼1.2-Myr cycle, which corresponds to long-period amplitude modulation of obliquity, was the main driver of third-order sea-level changes and sequence development. This modulation entails variation in the range of obliquity amplitude between a maximum of 2.4° (i.e. 24.5°–22.1°) and a minimum of 0.3° (i.e. 23.3°–23.0°) as a result of interactions among the three primary parameters of Earth's orbit (i.e., eccentricity, obliquity, and precession). We anchored our floating astronomical time scale (ATS) to the T42 seismic horizon, a major sequence boundary between the Lin Feng and Mingyuefeng formations that corresponds to the top of Nannofossil Zone NP8 (reported age of 57.7 Ma). This yields a fixed ATS for the late Paleocene-to-middle Eocene interval spanning 60.84 to 48.71 Ma. Finally, we evaluated our findings relative to the third-order sequence stratigraphy of the East China Sea Shelf Basin and to records of early Cenozoic eustasy and global events.

Graphical abstract

Effect of water and rock composition on re-strengthening of cohesive faults during the deceleration phase of seismic slip pulses

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): M. Violay, F. Passelegue, E. Spagnuolo, G. Di Toro, C. Cornelio

Abstract

The elastic strain energy release rate and seismic waves emitted during earthquakes are controlled by the on-fault temporal evolution of the shear stress during rupture propagation. High velocity friction experiments highlighted that shear stress on the fault surface evolves rapidly during seismic slip pulses. This temporal evolution of shear stress is controlled by both fault weakening at seismic slip initiation and re-strengthening rate towards the end of slip. While numerous studies focused on fault weakening, less attention was given to co-seismic re-strengthening processes. Here we performed 53 friction experiments (normal stress ≤30 MPa, slip-rate ≤6.5 m s−1) imposing constant slip acceleration and deceleration (7.8 m s−2), on cohesive Carrara marble (99% calcite) and micro-gabbro (silicate-built rock) under dry, vacuum and water pressurized conditions. Microstructural observations showed that micro-gabbro accommodated seismic slip by bulk melting of the sliding surfaces, whereas Carrara marble by coupled decarbonation and grain-size dependent crystal plastic processes. Under room humidity conditions and low imposed power density (i.e., product of normal stress per slip rate), re-strengthening rate during the deceleration stage was up to ∼ 17 times faster in marble than in micrograbbro. In the latter, the re-strengthening rate increased slightly with the power density. The presence of water enhanced further this trend. On the contrary, in marbles the re-strengthening rate decreased drastically with power density and in the presence of water. Our experimental observations highlighted the first order importance of the mineralogy and rheology of the slip zone materials and, to a second order, of the presence of water in controlling co-seismic re-strengthening of faults during seismic slip deceleration.

Methane in the Precambrian atmosphere

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Thomas A. Laakso, Daniel P. Schrag

Abstract

Biological methane production occurs in anoxic terrestrial wetlands and in sulfate depleted marine sediments. It has been suggested that methanogens may have flourished in the anoxic and sulfate-poor environments of the Precambrian ocean, generating large amounts of methane that would have accumulated in the atmosphere and warmed the early Earth. However, modern ferruginous and sulfate-poor environments such as Lake Matano are not very efficient at generating methane, converting only about 5% of organic carbon to CH4 despite the lack of alternative remineralization pathways. We apply this restriction to a simple model of marine carbon cycling in order to generate new estimates of methane concentrations in the Precambrian atmosphere. Our results suggest that, if the ancient biosphere were similarly inefficient, atmospheric methane concentrations did not exceed 1 ppm, or about twice the pre-industrial value, at any time during the Proterozoic. Following the evolution of oxygenic photosynthesis, the maximum methane concentration in the Archean atmosphere was order 100 ppm, but no more than 1 ppm if steady state oxygen concentrations were greater than 10−8 present atmospheric levels. Substantially larger methane concentrations are only possible before the evolution of oxygenic photosynthesis.

Reassessment of pre-eruptive water content of lunar volcanic glass based on new data of water diffusivity

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Li Zhang, Xuan Guo, Wan-Cai Li, Jiale Ding, Dongyuan Zhou, Lingmin Zhang, Huaiwei Ni

Abstract

Lunar volcanic glasses associated with mare basalt magmatism experienced significant degree of degassing, and to retrieve their initial water contents requires data of water diffusivity. We carried out diffusion experiments at 0.5 GPa and 1703–1903 K in a piston cylinder apparatus for two synthesized lunar basaltic melts with compositions corresponding to Apollo green glass and yellow glass. The water diffusion profiles measured by FTIR spectroscopy yield water diffusivities 0.5–1 order of magnitude greater than those of terrestrial basaltic melts, which is attributed to the difference in melt polymerization and modest contribution from H2 diffusion. However, hydroxyl (OH) is not only the dominant H species but is also inferred to be the major carrier of H in our experiments at oxygen fugacity estimated IW ± 1 (IW: iron-wüstite buffer). Modeling of previously reported profiles of volatile loss in an Apollo green glass bead using the new water diffusivity indicates an average cooling rate of 1–2 °C/s and an initial water content of 120–260 μg/g. With the assumption of limited degassing before magma fragmentation, the lunar mantle source is inferred to contain 6–22 μg/g H2O. The lunar interior appears to be less hydrous the Earth's interior but still contains a considerable amount of water.

Detection of channel-hillslope coupling along a tectonic gradient

Wed, 07/17/2019 - 19:10

Publication date: 15 September 2019

Source: Earth and Planetary Science Letters, Volume 522

Author(s): Martin D. Hurst, Stuart W.D. Grieve, Fiona J. Clubb, Simon M. Mudd

Abstract

Landscape morphology reflects the spatial and temporal history of erosion. Erosion in turn embodies the competition between tectonic and climatic processes. Quantitative analysis of topography can therefore reveal the driving tectonic conditions that have influenced landscape development, when combined with theoretical understanding of erosion processes. Recent developments in the automated analysis of high-resolution (<10 m) topographic data mean that integrated analysis of hillslope and channel topographic metrics can provide understanding of the transient response of landscapes to changing boundary conditions. We perform high-resolution topographic analysis of hillslopes and channels in small (<3 km2) catchments spanning an inferred uplift gradient along the Bolinas Ridge, California, USA, revealing tight coupling between channel steepness and hillslope metrics thought to be proxies for erosion rates. We find that the concavity of channel longitudinal profiles varies inversely with uplift rates, although drainage density increases with uplift rates. Both of these results can be explained by the contribution of mass wasting processes to valley formation in steeper (high uplift rate) landscapes. At the catchment scale, hillslope and channel metrics for erosion are correlated, hillslopes and channels steepen in concert, and hilltops (ridges) get sharper with increased uplift rate. This broad agreement suggests that hillslopes are responding to erosion rates in the channel network, which implies that landscape uplift is relatively stable and prolonged. Hillslope morphology deviates systematically from the steady-state predictions of established geomorphic transport laws, suggesting that hillslope adjustment is ongoing and that relief is growing.

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