Earth and Planetary Science Letters最新文献

Trivalent cations such as Al³⁺ and Fe³⁺ can be incorporated into the crystal structure of bridgmanite either by the charge-coupled mechanism forming the FeFeO₃, AlAlO₃, and FeAlO₃ components or by the oxygen vacancy mechanism forming the MgAlO_2.5 and MgFeO_2.5 components. They may affect the physical properties of bridgmanite and thus affect lower mantle dynamics. In this study, we investigated the effects of Al and Fe³⁺ on the grain growth kinetics of bridgmanite at a pressure of 27 GPa and temperatures of 2000 – 2300 K by multi anvil experiments. The experimental results indicate that the FeFeO₃, AlAlO₃, and FeAlO₃ components enhance the growth rate of bridgmanite, while the MgAlO_2.5 and MgFeO_2.5 components have negligible effects. However, due to the relatively low Fe³⁺ and Al³⁺ contents of bridgmanite in the lower mantle, none of them affect the lower mantle rheology significantly. In particular, the mid-mantle viscosity jump interpreted from geoid analysis is unlikely to be caused by the decreasing of MgAlO_2.5 and MgFeO_2.5 concentrations with increasing depth.

Submarine channels are conduits for sediment-laden flows called turbidity currents, which play a globally significant role in the offshore transport of sediment and organic carbon and pose a hazard to critical seafloor infrastructure. Time-lapse repeat surveys of active submarine channels have recently shown that upstream-migrating knickpoints can dominate channel evolution. This finding contrasts with many studies of ancient outcrops and subsurface geophysical data that inferred channel bends migrate laterally, as occurs in meandering rivers. Here, we aim to test these two contrasting views by analysing two high-resolution repeat seafloor surveys acquired 13 years apart across the entirety of an active submarine channel in Knight Inlet, British Columbia. We find that two main mechanisms control channel evolution, with the normalised channel radius of curvature (specifically, R* - channel radius of curvature normalised to channel width) explaining which of these mechanisms dominate. Pronounced outer bend migration only occurs at tight bends (R*<1.5). In contrast, at broader bends and straighter sections (R*>1.5), erosion is focused within the channel axis, where upstream-migrating knickpoints dominate. High centrifugal accelerations at tight bends promote super-elevation of flows on the outer channel flank, thus, enhancing outer bend erosion. At R*>1.5, flow is focused within the channel axis, promoting knickpoints that migrate upstream at an order of magnitude faster than the rate of outer bend erosion at tight bends. Despite the dominance of knickpoints in eroding the channel axis, their stratigraphic preservation is very low. In contrast, the lateral migration of channel bends results in much higher preservation via lateral accretion of deposits on the inner bend. We conclude that multiple mechanisms can control evolution at different channel reaches and that the role of knickpoints has been underestimated from past studies that focused on deposits due to their low preservation potential.

Evaporation can fractionate elements and their isotopes between the condensed and gas phases. The fractionation of zinc isotopes during impact-induced evaporation can be used to effectively determine the extent of volatile loss. A robust understanding of the Zn isotope system in assessing the volatile loss, however, relies on well-constrained empirical isotopic fractionation factors (α) during evaporation under a range of pressure and temperature conditions. In this study, Zn isotopic data for well-documented impact glasses from six sites (Darwin, Australia; Zhamanshin, Kazakhstan; El'gygytgyn, Russia; Boltysh, Ukraine; Lonar, India; and Ries, Germany) are reported to investigate the extent of Zn isotopic fractionation under conditions of impact-induced evaporation on Earth. Our findings suggest that the initial Zn isotopic composition in terrestrial impact glasses is comparable to that of continental crustal rocks, but this composition becomes progressively heavier as more isotopically light Zn is lost from the impact melt, reaching a maximum δ⁶⁶Zn value of +1.1 ‰. The investigated samples show a statistically significant negative correlation between δ⁶⁶Zn values and Zn contents, especially those from the Darwin crater (R² = 0.90). These samples define an α value of 0.99971 ± 0.00005 (1SE). This α value is consistent with those previously estimated for melt glasses and fused sands (α = 0.9997 to 0.9998) from the Trinity nuclear detonation site, slightly higher than the value estimated from tektites (α = ∼0.998), and notably higher than that theoretically expected for evaporation into a vacuum (α = 0.985 to 0.993). This result highlights the limited fractionation of Zn isotopes during terrestrial impact processes. Moreover, the modelling suggests that the range of α values from 0.9997 to 0.9998 aligns with the observed compositions in lunar mare basalts and products from nuclear detonation, supporting α values close to but not exactly unity for Zn isotopic fractionation during various high-energy impact events. Utilizing the modelled fractionation factor (α = 0.9997), it is possible to reproduce the Zn concentration and isotopic composition of the lunar mare basalts, indicating a loss of about 98 % of the Moon's initial Zn inventory. Terrestrial impact glasses demonstrate that, under natural impact conditions, stable Zn isotopes can undergo evaporative fractionation to a degree comparable to lunar mare basalts and melted fallout glass and fused sands from nuclear detonation, suggesting an important contribution from impact to the volatile depletion of terrestrial planets.

The middle Cretaceous greenhouse period experienced profound environmental change including episodes of enhanced global burial of organic carbon marked by carbon isotopic excursions (CIEs). However, the role and response of polar regions like the newly formed, partially enclosed Arctic Ocean Basin during middle Cretaceous carbon burial remains enigmatic. We present the first Arctic deepwater CIE record that characterizes conditions offshore of the Alaska margin north of 75°N paleolatitude. Organic carbon isotopes (δ¹³C_org) and 103–82 Ma ash zircon U-Pb dates from the distal Hue Shale record multiple Albian–Campanian CIEs during slow ∼3–15 m/Myr sediment accumulation rates. Average total organic carbon (TOC) increased substantially during large 2–3 ‰ CIEs of the ∼101 Ma Albian-Cenomanian boundary event (from 7 to 18 % TOC) and ∼94 Ma Cenomanian-Turonian boundary event (5 to 10 % TOC). Turonian TOC remained elevated (8–13 %) during high global sea levels and temperatures of the Cretaceous Thermal Maximum, followed by an increase from 7 to 11 % TOC during the ∼90 Ma late Turonian event 1.5 ‰ CIE. Average TOC subsequently decreased in the Coniacian–Campanian, but relative maxima occurred during subtle 0.5–1 ‰ CIEs interpreted as the ∼87 Ma late Coniacian event (increase from 4 to 7 % TOC), ∼85 Ma Horseshoe Bay event (3.5 to 4.5 % TOC), and ∼84 Ma Santonian-Campanian boundary event (3.5 to 5 % TOC). Increases in hydrogen index and productivity proxies (P, Ba, Nd) that accompanied each CIE episode with enhanced TOC suggest a strong link between marine productivity and organic carbon burial at short-term CIE timescales. However, long-term (>5–8 Myr) changes in trace metal redox (Mo, Fe, V) and salinity (B/Ga) proxies suggest shifts in prevailing environmental conditions at timescales longer than the CIEs. Late Albian–middle Turonian marine salinity occurred during euxinic (103–98 Ma) and suboxic (98–90 Ma) conditions with deposition interpreted to have occurred within and beneath an oxygen minimum zone, respectively. In contrast, late Turonian–early Campanian (90–82 Ma) freshening and restricted euxinic basin conditions may signal the start of widespread restriction known to characterize the Paleogene Arctic. Overall, these results highlight that middle Cretaceous Arctic deepwater remained a productive marine carbon sink coupled to the global carbon cycle despite evolving Arctic greenhouse conditions.

Constraining the absolute time and duration of geologic processes is one of the great challenges and goals in Earth sciences. Increasingly, the integration of geochronologic constraints with petrologic information is being qualitatively applied to understanding the timescales of metamorphic, igneous, tectonic, and fluid-related processes. Many rocks and geochronometers preserve relative age constraints such as compositional zoning or cross cutting relationships. This prior information can be formalized in a Bayesian statistical framework to generate a probabilistic posterior chronology. As we show here, these “age-sequence” models can enhance precision on geochronologic dates and rates and insight into tectonic models. Bayesian modeling of complex, concentrically, zoned monazite from the northern Appalachian orogen was used to develop a detailed temperature-time history through the Acadian (∼405–395 Ma) and Neoacadian (∼380–350 Ma) orogenies with significantly reduced uncertainties (40–70 %). Modeling of zoned monazite from a southern Trans-Hudson orogen granulite yielded durations of 0.5⁺⁹/_-0.4 Ma and 20⁺⁵/_-8 Ma for biotite-dehydration melting and suprasolidus conditions, respectively. The relatively short intervals of heating and peak conditions are consistent with a back-arc tectonic setting. A complementary approach, Bayesian change point detection, provides a framework to constrain the timing of compositional changes that can be linked with metamorphic reactions. Applying this approach in the northern Appalachian orogen demonstrates contrasting durations of low-Y monazite crystallization (∼10 vs ∼30 myr) in regions with different pressure-temperature histories. Compositionally distinct monazite domains can be linked with garnet stability, which provides a key constraint on tectonic models. Bayesian statistical analysis represent a powerful tool that can be widely applied to refine the absolute time and duration of geologic processes. A more objective and reproducible set of interpretations are produced by this more formal, although not necessarily complex, statistical analysis.

The 1975–1984 Krafla Fires in northeast Iceland was the first plate-boundary rifting episode to be tracked using seismic and geodetic monitoring. Geophysical observations from this episode have inspired conceptual models of magma transport during plate spreading, but a lack of complementary petrologic insights has hindered a holistic understanding of the events. To address this knowledge gap, we studied the petrochemistry of all nine Krafla Fires basaltic eruptions. Our large dataset of new whole-rock, matrix glass and mineral analyses from samples collected during or shortly after each eruption reveal a clear compositional bimodality in the erupted magmas that persisted across the episode, with evolved quartz tholeiite (MgO = 5.7–6.4 wt.%) erupted inside Krafla caldera, and more primitive (usually olivine-normative) tholeiite (MgO = 6.4–8.7 wt%) erupted north of the caldera margin. Barometric calculations indicate tapping of these magmas from distinct reservoirs: a primitive lower-crustal reservoir at a most probable depth of ∼14–19 km, and a more evolved, shallower reservoir at a most probable depth of ∼7–9 km beneath the caldera. These reservoirs were tapped simultaneously in several of the nine eruptions, and in three events the two magma types mixed near the northern caldera margin. Varying levels of trace element depletion in the deep-sourced primitive melts reflect incomplete mixing of diverse mantle-derived melts at depth; the most enriched of these melts could be parental to evolved inside-caldera magma via fractional crystallization. Clinopyroxene rims on gabbroic nodules from primitive September 1984 lavas record lower crustal pressures, while diffusion models suggest that these rims grew up to within a few months before eruption. Ascent of the primitive magma from the lower crust thus occurred over timescales much shorter than eruptive repose periods, without prolonged stalling at shallow depths. These observations are inconsistent with the view that the eruptions were entirely fed by lateral magma outflow from the shallow reservoir. They instead require some decoupling of the flow paths of the two magma types: the primitive magma either bypassed the sub-caldera reservoir laterally or ascended vertically beneath the northern vents. The two reservoirs nonetheless shared a hydraulic connection and jointly responded to rifting. Comparison of the Krafla Fires with other rifting events and eruptions highlights the complexity and diversity of magma transport during plate boundary rifting events, which is not yet captured by a generalizable model. Integration of petrologic, geochemical and geophysical data is essential to provide a holistic view of future rifting events in Iceland and at other spreading centres.

Orogenic plateaus are known for their high and flat interiors, but the mechanism by which the high-elevation, low-relief topography is formed remains controversial. The Tibetan Plateau, which may have had a broad central valley along the Bangong suture zone during the Eocene, is a valuable research target to test potential driving mechanisms. In this study, we report both stable and clumped isotopes of lacustrine carbonates of the Lunpola Basin to better constrain the paleoelevation history of the Bangong suture zone in Central Tibet. Covariations of the stable oxygen and carbon isotopes indicate increasing evaporative enrichment of lake water reflecting enhanced aridity from the middle Eocene to early Miocene. After removing altered samples using multiple diagenesis screening methods and the potential influence of global cooling, our clumped isotope temperatures indicate consistent paleoelevations of 2.2 ± 1.1 km at 40–20 Ma, followed by an abrupt 1.4 ± 0.8 km surface uplift at 20–19 Ma and an additional 1.0 ± 0.7 km surface uplift between 16 Ma and the present to achieve the current high elevation of ∼4.6 km. These new paleoelevation results suggest that the Neogene was the primary period of topographic growth for the Lunpola Basin, which contradicts previous inferences emphasizing Paleogene growth. We argue that the Bangong suture zone in Central Tibet was probably a broad low-elevation valley throughout the Paleogene and uplifted mainly during the Neogene by multiple subsurface geodynamic processes, including convective removal of the lower lithosphere and middle–lower crustal flow. A comparison with the north-central Tibet Hoh Xil Basin and South American Altiplano indicates that these subsurface geodynamic processes may be common for uplifting and flattening orogenic plateaus.

Continental rifts are thought to transition from a power-law fault length distribution during the juvenile stages of extension, to an exponential distribution during break-up and oceanic spreading. However, fault scaling relationships have rarely been quantified in natural incipient rifts, particularly in the absence of magmatic influence. We address this knowledge-gap in the incipient Okavango-Makgadikgadi Rift Zone, Northern Botswana, consisting of the amagmatic Okavango Rift Basin (ORB) and Makgadikgadi Rift Basin (MRB). We utilize high-resolution satellite topography data (12.5 m TanDEM-X and 30 m SRTM) and aeromagnetic data to image and map faults across the rift zones and generate a new robust fault map for the region. Further, we analyze the length-frequency distribution of faults using a two-sample Kolmogorov-Smirnov test. The results show that the Makgadikgadi Rift Basin exhibits an exponential distribution associated with a nucleating rift as it exhibits diffuse faulting and lacks a border-fault, whereas the Okavango Rift Basin exhibits a power-law distribution that is consistent with its relatively more evolved rift structure with established border faults. Thus, we propose that continental divergent plate boundaries commonly nucleate with an initial exponential distribution of fault lengths which subsequently transition into a power-law distribution as the rift evolves into its stretching phase, and may again transition into an exponential distribution as break-up initializes.

On January 15, 2022, the eruption of the Hunga-Tonga volcano unleashed a tsunami that rapidly traversed the Pacific Ocean, soon reaching the Sulzberger Ice Shelf (SIS), West Antarctica. In the aftermath, the West Sulzberger Ice Shelf (WSIS) experienced a significant calving event, shedding 106 km² of ice and reducing its size to the smallest recorded since 1948. To investigate the potential association between this calving event and the tsunami, and to elucidate the long-term evolution of the WSIS, this study employed tide gage records and remote sensing data for an extensive cross-cycle observation spanning 74 years. The results indicate that post-2014, the WSIS exhibited heightened instability, manifesting as an increased frequency of calving events, a shift from an expanding to a contracting shelf area, and accelerated ice flow velocities (Video S1). The critical rift, instrumental in the 2022 calving, also displayed an accelerated widening rate post-2014, culminating in rapid expansion upon the tsunami's arrival, completing the final 2% of the iceberg's detachment boundary. Modeling results quantify the tsunami induced flexural strains on the ice shelf, with peak values occurring near the region of the subsequent calving event. Notably, multiple calving events during the study period coincided with local minima in landfast sea ice extent, underscoring its protective role for the ice shelf. Furthermore, a significant decline in landfast sea ice near the SIS had been observed over the past two decades, suggesting a weakening of this protective effect. A combination of modeling results and comparative analysis with other similar calving events suggests that tsunamis tend to expedite ice shelf calving predominantly when the shelf is already teetering on instability, which casts a shadow on the SIS's future resilience against natural hazards.