Significance The importance of high-magnitude, short-lived events in controlling the evolution of landscapes is not well understood. This matters because during such events, erosion processes can ...surpass thresholds and cause abrupt landscape changes that have a long-lasting legacy for landscape morphology. We show that extreme flood events, during which the flow depth exceeds the threshold for erosion through plucking rather than abrasion, are the dominant control on the evolution of a large bedrock canyon in Iceland. The erosive signature of these events is maintained within a dynamic landscape over millennial timescales, emphasizing the importance of episodic extreme events in shaping landscapes.
Extreme flood events have the potential to cause catastrophic landscape change in short periods of time (10 ⁰ to 10 ³ h). However, their impacts are rarely considered in studies of long-term landscape evolution (>10 ³ y), because the mechanisms of erosion during such floods are poorly constrained. Here we use topographic analysis and cosmogenic ³He surface exposure dating of fluvially sculpted surfaces to determine the impact of extreme flood events within the Jëökulsëöárgljëöáúfur canyon (northeast Iceland) and to constrain the mechanisms of bedrock erosion during these events. Surface exposure ages allow identification of three periods of intense canyon cutting about 9 ka ago, 5 ka ago, and 2 ka ago during which multiple large knickpoints retreated large distances (>2 km). During these events, a threshold flow depth was exceeded, leading to the toppling and transportation of basalt lava columns. Despite continuing and comparatively large-scale (500 m ³/s) discharge of sediment-rich glacial meltwater, there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive event because the vertical knickpoints have not diffused over time. We provide a model for the evolution of the Jëöáúökulsëöáúöárgljëöáúöáúfur canyon through the reconstruction of the river profile and canyon morphology at different stages over the last 9 ka and highlight the dominant role played by extreme flood events in the shaping of this landscape during the Holocene.
Fluids play a key role in modifying the chemical and physical properties of fault zones, which may prime them for repeated rupture by the generation of high pore fluid pressures and precipitation of ...commonly weak, secondary minerals. Fluid flow paths, sources and fluxes, and the permeability evolution of fault zones throughout their seismic cycles remain poorly constrained, despite their importance to understanding fault zone behaviour. Here we use geochemical tracers of fluid–rock exchange to determine budgets for meteoric, metamorphic and mantle fluids on a major compressional tectonic plate boundary.
The Alpine Fault marks the transpressional Pacific–Australian plate boundary through South Island, New Zealand and appears to fail in regular (329±68 yrs) large earthquakes (Mw∼8) with the most recent event in 1717 AD. Significant convergent motion has formed the Southern Alps and elevated geothermal gradients in the hangingwall, which drive crustal fluid flow. Along the Alpine Fault the Alpine Schist of the Pacific Plate is thrust over radiogenic metasedimentary rocks on the Australian plate. The absence of highly radiogenic (87Sr/86Sr > 0.7200) strontium isotope ratios of hangingwall hot springs and hydrothermal minerals formed at a range of depths in the Alpine Fault damage zone indicates that the fluid flow is restricted to the hangingwall by a cross-fault fluid flow barrier throughout the seismogenic crust. Helium isotope ratios measured in hot springs near to the Alpine Fault (0.15–0.81 RA) indicate the fault is a crustal-scale feature that acts as a conduit for fluids from the mantle. Rock-exchanged oxygen, but meteoric water-like hydrogen isotope signatures of hydrothermal veins indicate that partially rock-exchanged meteoric fluids dominate down to the top of the brittle to ductile transition zone at ∼6 km. Geochemical tracer transport modelling suggests only ∼0.02 to 0.05% of total rainfall west of the Main Divide penetrates to depth, yet this recharge flux is sufficient to overwhelm other fluid contributions. Calculated mantle fluid fluxes of CO2 and H2O (0.2 and 3 to 13 mol/m2/yr respectively) and metamorphic H2O fluxes (4 to 750 mol/m2/yr) are considerably lower than the focused meteoric water discharge flux up the Alpine Fault (4 × 103 to 7 × 104 mol/m2/yr), driven by the >3000 m hydrologic head of the Southern Alps. Meteoric waters are primarily responsible for modifying fault zone permeability during fluid–rock interactions and may facilitate the generation of high pore fluid pressures that could assist episodic earthquake rupture.
•Helium isotopes indicate the Alpine Fault is a conduit for fluids from the mantle.•The Alpine Fault is a barrier to cross-fault flow throughout the seismogenic crust.•This barrier and the Southern Alps high topography focuses meteoric fluid discharge.•Meteoric fluids dominate with minor volumes of metamorphic and mantle fluids.•Focused fluid flow through the Alpine Fault zone may promote fault weakening.
We present a study of the regional distribution pattern (gas composition and isotope ratios of CO2 and He) of 25 degassing locations (mineral springs and mofettes) in the East Eifel (EE) and 2 in the ...South Eifel (SE). In the EE, the CO2 concentrations in the free gas phase range from >99vol.% in the surroundings of the Laacher See to about 2.8vol.% at the southern periphery (Bad Kreuznach). At the locations with more than 99vol.% CO2, δ13C values range between −6.3 and −3.0‰ and 3He/4He ratios are between 1.3 and 5.6 Ra. At the two locations sampled in the SE, the gas contains also more than 99% CO2, with a δ13C signature of −2.0‰ and a 3He/4He ratio of ~4 Ra.
Detailed investigations of the gas composition and the isotope compositions of C, N, He, Ne, and Ar were carried out at least yearly at two locations in the SE and two locations in the EE area for five and ten years, respectively. As a result, these locations are characterised by robust average values of their most significant fluid signatures, which we use to argue for the existence of at least two different magmatic reservoirs beneath the Eifel area. The fluid signatures of the monitored Eifel locations are compared with that of Bublák (NW Bohemia, Czech Republic), where ongoing magmatic activity was recognized by long-term studies during the last 20years. Based on the stable isotope signature of the major magmatic component CO2 in the free gas phase and on the geological background, we propose that at the SE locations and at Bublák mantle-derived volatiles are supplied from magmatic reservoirs with a similar melt composition. In general, the results of our time series studies have provided robust data for the reservoir discussion and underscore the potential of such a multi-isotope approach of free gases for the evaluation of the geodynamic situation as a whole.
•New gas composition and isotopic data from cold and hydrothermal vents from the Eifel (Germany)•3He/4He≥4 Ra is associated with high gas flow, nearly pure CO2, δ13C from −2 to −5‰.•The gas data indicate a predominantly mantle-derived origin of the volatiles.•Judging from the comprehensive isotope data, at least two magmatic reservoirs may exist beneath the Eifel area.
The Kula volcanic field in Western Turkey comprises about 80 cinder cones and associated basaltic lava flows of Quaternary age. Based on geomorphological criteria and K-Ar dating, three eruption ...phases, β2–β4, were distinguished in previous studies. Human footprints in ash deposits document that the early inhabitants of Anatolia were affected by the volcanic eruptions, but the age of the footprints has been poorly constrained. Here we use 3He and 10Be exposure dating of olivine phenocrysts and quartz-bearing xenoliths to determine the age of the youngest lava flows and cinder cones. In the western part of the volcanic field, two basalt samples from a 15-km-long block lava flow yielded 3He ages of 1.5 ± 0.3 ka and 2.5 ± 0.4 ka, respectively, with the latter being in good agreement with a 10Be age of 2.4 ± 0.3 ka for an augen gneiss xenolith from the same flow. A few kilometers farther north, a metasedimentary xenolith from the top of the cinder cone Çakallar Tepe gave a 10Be age of 11.2 ± 1.1 ka, which dates the last eruption of this cone and also the human footprints in the related ash deposits. In the center of the volcanic field, a basalt sample and a metasedimentary xenolith from another cinder cone gave consistent 3He and 10Be ages of 2.6 ± 0.4 ka and 2.6 ± 0.3 ka, respectively. Two β4 lava flows in the central and eastern part of the volcanic province yielded 3He ages of 3.3 ± 0.4 ka and 0.9 ± 0.2 ka, respectively. Finally, a relatively well-preserved β3 flow gave a 3He age of ∼13 ka. Taken together, our results demonstrate that the penultimate eruption phase β3 in the Kula volcanic field continued until ∼11 ka, whereas the youngest phase β4 started less than four thousand years ago and may continue in the future.
•First 3He and 10Be exposure ages for the Kula volcanic field, western Turkey.•Fossil human footprints in volcanic ash at Çakallar Tepe were dated to ∼11 ka.•Youngest phase of volcanic activity at the Kula volcanic field started <4 ka ago.•Cosmogenic nuclide 10Be and 3He exposure ages are consistent.
Mofettes are gas emission sites where high concentrations of CO2 ascend through conduits from as deep as the mantle to the Earth’s surface and as such provide direct windows to processes at depth. ...The Hartoušov mofette, located at the western margin of the Eger Graben, is a key site to study interactions between fluids and swarm earthquakes. The mofette field (10 mofettes within an area of 100 m × 500 m and three wells of 28, 108, and 239 m depth) is characterized by high CO2 emission rates (up to 100 t/d) and helium signatures with (3He/4He)c up to 5.8 Ra, indicating mantle origin. We compiled geological, geophysical, geochemical, and isotopic data to describe the mofette system. Fluids in the Cheb basin are mixtures between shallow groundwater and brine (>40 g/L at a depth of 235 m) located at the deepest parts of the basin fillings. Overpressured CO2-rich mineral waters are trapped below the mudstones and clays of the sealing Cypris formation. Drilling through this sealing layer led to blow-outs in different compartments of the basin. Pressure transients were observed related to natural disturbances as well as human activities. External (rain) and internal (earthquakes) events can cause pressure transients in the fluid system within hours or several days, lasting from days to years and leading to changes in gas flux rates. The 2014 earthquake swarm triggered an estimated excess release of 175,000 tons of CO2 during the following four years. Pressure oscillations were observed at a wellhead lasting 24 h with increasing amplitudes (from 10 to 40 kPa) and increasing frequencies reaching five cycles per hour. These oscillations are described for the first time as a potential natural analog to a two-phase pipe–relief valve system known from industrial applications.
We derive a slip rate for a thrust at the central Qilian Shan mountain front by combining structural investigations, satellite imagery, topographic profiling, luminescence dating, and 10Be exposure ...dating. The seismically active Zhangye thrust transects late Pleistocene alluvial fan deposits and forms a prominent north facing scarp. The fault consists of two segments that differ in orientation, scarp height, and age. A series of loess‐covered terraces records the uplift history of the western thrust segment. Loess accumulation on all terraces started at 8.5 ± 1.5 kyr and postdates terrace formation. Gravels from the highest terrace yielded a 10Be exposure age of 90 ± 11 kyr, which dates the onset of faulting. With a displacement of 55–60 m derived from fault scarp profiles, this yields a vertical slip rate of 0.64 ± 0.08 mm yr−1. Along the eastern thrust segment, three 10Be ages from the uplifted alluvial fan constrain that faulting started at ∼31 ± 5 kyr. Together with a displacement of 25–30 m this leads to a vertical faulting rate of 0.88 ± 0.16 mm yr−1. A dip estimate of 40° to 60° for the fault plane combined with lower and upper limits of ∼0.6 and ∼0.9 mm yr−1 for the vertical slip rate gives minimum and maximum horizontal shortening rates of 0.4 and 1.1 mm yr−1 across the Zhangye thrust. Our results are consistent with geologic and GPS constraints, which suggest that NNE directed shortening across the northeastern Tibetan Plateau is distributed on several active faults with a total shortening rate of 4 to 10 mm yr−1.
Comprehensive studies of CO2‐rich fluids close to the swarm earthquake region Nový Kostel at the Czech‐German border have been started 15 years ago and have in particular included two extended ...chemical and isotope monitoring studies lasting for several years each. The regional surface distribution patterns of the fluid signatures including the identification of the origin of fluid components are the focus of the detailed studies. Three degassing centers (Cheb basin, Mariánské Lázně, and Karlovy Vary) with high CO2 flux and the same level of δ13C values, but different levels of 3He/4He ratios, have been identified. The studies have located the CO2 source and have investigated seismically induced changes in fluid characteristics on the basis of unique weekly sampling campaigns at selected locations. A seismically triggered release of crustal helium was confirmed by both monitoring campaigns. Finally, indications for a presently active magmatic process beneath the Cheb basin have been found. In contrast to volcanically active regions, magma accumulation in the study area takes place at the crust‐mantle boundary and is not yet accompanied by heat transfer to the surface. Likewise, reactive magma‐derived components are absent in the degassing fluids. The area of investigation has the potential to be a natural laboratory for fundamental studies of active geodynamic processes. The results of our fluid monitoring, including the stunning observation of mantle‐derived free fluids marked by 3He/4He ratios within the subcontinental mantle range, are supported by geophysical findings from seismic studies and geologic indications.
The timing and impact of deglaciation and Holocene readvances on the terrestrial continental margins of the Antarctic Peninsula (AP) have been well-studied but are still debated. Potter Peninsula on ...King George Island (KGI) (Isla 25 de Mayo), South Shetland Islands (SSI), NW Antarctic Peninsula, has a detailed assemblage of glacial landforms and stratigraphic exposures for constraining deglacial landscape development and glacier readvances. We undertook new morphostratigraphic mapping of the deglaciated foreland of the Warszawa Icefield, an outlet of the Bellingshausen (Collins) Ice Cap on Potter Peninsula, using satellite imagery and new lithofacies recognition and interpretations, combined with new chronostratigraphic analysis of stratigraphic sections, lake sediments, and moraine deposits. Results show that the deglaciation on Potter Peninsula began before c. 8.2 ka. Around c. 7.0 ka, the Warszawa Icefield and the marine-facing Fourcade Glacier readvanced across Potter Peninsula and to the outer part of Potter Cove. Evidence of further readvances on Potter Peninsula was absent until the Warszawa Icefield margin was landward of its present position on three occasions: c. 1.7–1.4 ka, after c. 0.7 ka (most likely c. 0.5–0.1 ka), and by 1956 CE. The timing of Holocene deglaciation and glacier fluctuations on Potter Peninsula are broadly coeval with other glacier- and ice-free areas on the SSI and the northern AP and likely driven by interactions between millennial–centennial-scale changes in solar insolation and irradiance, the southern westerlies, and the Southern Annular Mode.
The Cheb Basin (Czech Republic) is characterized by emanations of magma-derived gases and repeated occurrences of mid-crustal earthquake swarms with small to intermediate magnitudes (M
L
< 4.5). ...Associated intense mantle degassing occurs at the Hartoušov Mofette, a representative site for the Cheb Basin. Here, we performed 14 sampling campaigns between June 2019 and March 2020. Gas samples of fluids ascending in two boreholes (F1, ∼28 m depth and F2, ∼108 m depth) and from a nearby natural mofette were analyzed for their chemical (CO
2
, N
2
, O
2
, Ar, He, CH
4
, and H
2
) and isotope compositions (noble gases and CO
2
). CO
2
concentrations were above 99.1% in most samples, while O
2
and N
2
were below 0.6%. He ranged from 19 to 34 μmol/mol and CH
4
was mostly below 12 μmol/mol. Isotope compositions of helium and carbon in CO
2
ranged from 5.39 to 5.86 R
A
and from −2.4 to −1.3 ‰ versus VPDB, respectively. Solubility differences of the investigated gases resulted in fluctuations of their chemical compositions. These differences were accompanied by observed changes of gas fluxes in the field and at the monitoring station for F1. Variations in solubilities and fluxes also impacted the chemical concentration of the gases and the δ
13
C values that were also likely influenced by Fischer-Tropsch type reactions. The combination of (a) the Bernard ratio, (b) CH
4
/
3
He distributions, (c) P-T conditions, (d) heat flow, and (e) the sedimentary regime led to the hypothesis that CH
4
may be of mixed biogenic and volcanic/geothermal origin with a noticeable atmospheric contribution. The drilling of a third borehole (F3) with a depth of ∼238 m in August 2019 has been crucial for providing insights into the complex system of Hartoušov Mofette.
The formally named SP lava flow is a quartz-, olivine- and pyroxene-bearing basalt flow that is preserved in the desert climate of northern Arizona, USA. The flow has an 40Ar/39Ar age of 72 ± 4 ka ...(2σ) and has undergone negligible erosion and/or burial, making its surface an ideal site for direct calibration of cosmogenic nuclide production rates. Production rates for cosmogenic 3He (3Hec) and 21Ne (21Nec) have been determined from SP flow olivine and pyroxene in this study. The error-weighted mean, sea-level, high latitude (SLHL) total reference production rates of 3He in olivine and pyroxene have identical values of 135 ± 8 at/g/yr (2σx‾; standard error) using time-independent Lal (1991)/Stone (2000) (St) scaling factors. These production rates decrease to identical values of 130 ± 8 at/g/yr (2σx‾; standard error) when 3He measurements are standardized to the CRONUS-P pyroxene standard. The St-scaled, error-weighted mean, total reference production rates of 21Ne in olivine and pyroxene are 48.4 ± 2.9 at/g/yr and 26.5 ± 1.7 at/g/yr (2σx‾; standard error), respectively, increasing to 49.3 ± 3.0 at/g/yr and 27.0 ± 1.7 at/g/yr (2σx‾; standard error), respectively, when standardized to the CREU-1 quartz standard. 3He and 21Ne production rates (St) overlap within 2σ uncertainty with other St-scaled production rates in the literature. SLHL 3He and 21Ne production rates in SP flow olivine and pyroxene are nominally lower if time-dependentLm and Sa scaling factors are used. Olivine and pyroxene both have identical, error-weighted mean SLHL production rates of 127 ± 8 at/g/yr (2σx‾; standard error) using Lm scaling factors and CRONUS-P standardized 3He measurements. These production rates decrease to identical values of 110 ± 7 at/g/yr (2σx‾; standard error) for olivine and pyroxene when using Sa scaling factors. The Lm-scaled, error-weighted mean, total reference production rates of 21Ne in olivine and pyroxene are 48.1 ± 2.8 at/g/yr and 26.4 ± 1.7 at/g/yr (2σx‾; standard error), respectively, when standardized to the CREU-1 quartz standard. The error weighted mean, local 21Ne/3He production rate ratio in olivine is 0.358 ± 0.009 (2σx‾; standard error), which increases to 0.378 ± 0.012 when using CREU-1 standardized 21Ne production rates and CRONUS-P standardized 3He production rates. The error weighted mean, local 21Ne/3He production rate ratio in pyroxene is 0.197 ± 0.006, or 0.208 ± 0.008 when 21Ne and 3He are standardized to CREU-1 and CRONUS-P, respectively. The updated, CREU-1 standardized 21Nec rate (St) in SPICE quartz is 16.5 ± 1.1 at/g/yr. Production of 21Ne in coexisting SPICE olivine (ol), pyroxene (px), and quartz (qz) (standardized to CREU-1; Fenton et al., 2019; this study) yields error-weighted mean, local production rate ratios of 3.00 ± 0.13 (2σx‾) and 1.64 ± 0.08 (2σx‾) for 21Neol/21Neqz and 21Nepx/21Neqz, respectively. This study suggests that production rates of 3He and 21Ne in SPICE olivine and pyroxene agree well with St- and Lm-scaled global mean production rates in the literature. It also indicates that CRONUS-P and CREU-1 standardizations yield production rates in even stronger agreement with these global mean rates.
•3Hec and 21Nec production rates (St) in ol are 135 ± 8 and 48.4 ± 2.9 at/g/yr.•3Hec and 21Nec production rates (St) in px are 135 ± 8 and 26.5 ± 1.7 at/g/yr.•CRONUS-P standardized 3Hec rate (St) in both ol and px is 130 ± 8 at/g/yr.•CREU-1 standardized ol and px21Nec rates (St) are 49.3 ± 3.0 and 27.0 ± 1.7 at/g/yr.•CREU-1 standardized 21Nec rate (St) in quartz is 16.5 ± 1.1 at/g/yr.
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