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Relationship Between Rupture Length and Magnitude of Oceanic Transform Fault Earthquakes
No full textThe rupture behavior of large oceanic strike-slip earthquakes remains largely unresolved using seismic signals recorded thousands of kilometers away from the source area. Large submarine earthquakes, however, generate hydroacoustic T-waves propagating through the ocean over long distances. Here, we show that these T-waves recorded at regional distances on the Ascension hydrophone array of the International Monitoring System can provide critical information on the earthquake location and rupture behavior. We use recordings from 47 events in oceanic transform faults, ranging in magnitude from 5.6 ≤ Mw ≤ 7.1, to investigate the rupture processes. We find that most strike-slip earthquakes show unilateral rupture behavior, while a few larger events were more complex. Furthermore, earthquakes in oceanic transforms have longer ruptures than events of the same magnitude in continental faults. We argue that differences in the scaling relation of oceanic and continental strike-slip earthquakes support a lower rigidity in the oceanic lithosphere caused by hydration.
Key Points
Hydroacoustic T-waves from 47 oceanic transform fault earthquakes with Mw 5.6–7.1 were recorded by the IMS Ascension hydrophone array
Our results show that hydroacoustic energy provides precise constraints on the rupture of strong earthquakes in oceanic transform faults
Ruptures of oceanic strike-slip earthquakes are longer than for continental events of the same magnitude, indicating lower rigidity
Plain Language Summary
Oceanic transform faults are strike-slip faults where one plate moves past another laterally, with new seafloor created at adjacent mid-ocean ridge segments on either side of the transform. At the transforms, plate motion generates strong earthquakes, causing seismic waves to propagate for thousands of kilometers. Nevertheless, due to their remoteness the rupture behavior of oceanic earthquakes and their scaling relationship between magnitude and rupture length is poorly constrained and understood. Here, we use hydroacoustic signals, so-called T-waves, which are excited by seismic deformation of the seafloor. T-waves are readily observable at the International Monitoring System hydrophone triplet near Ascension Island, Atlantic Ocean. We present a new relationship between the magnitude of equatorial Atlantic strike-slip earthquakes and the rupture length from 47 events (MW 5.6–7.1). We found that oceanic earthquakes differ from continental ones, showing longer ruptures for the same magnitude, suggesting that oceanic transform faults are weak
Westerlies migrations and volcanic records over the past 4000 years from the Azores lacustrine sequences. Exploring correlations and impacts on Western Europe
No full textHighlights
• The Azores region is a pathway for precipitation fronts traversing the N Atlantic.
• NAO influenced position of westerlies and in the ash dispersal from Azores to Europe.
• wenty Holocene sedimentary records from Azorean lakes have been analysed.
• We tracked changes in the westerlies' latitudinal position over the last 4200 years.
• We create a database to recognize cryptotephras in Europe from Azores eruptions.
Abstract
The Azores region plays a crucial role as a pathway for precipitation fronts traversing the North Atlantic from west to east, driven by the prevailing westerly winds. Variations in the strength of the Azores High affect the dynamics of the North Atlantic Oscillation (NAO), leading to latitudinal shifts in the trajectory of the westerlies and jet stream current over time.
Throughout the Holocene and Late Pleistocene, the Azores islands experienced numerous highly explosive eruptions. Volcanic ash from these events was primarily dispersed to the east, carried by the North Atlantic Jet Stream, with cryptotephras being found across the British Isles and Northern-Central Europe.
To investigate how NAO variations influenced the latitudinal position of the westerlies and in the ash dispersal towards Europe during the Late Holocene, we analysed the stratigraphy and sedimentology from 20 lake sediment sequences across five islands of Azores and revise highstand/lowstand periods in several lakes in Europe. Our facies analysis of Azorean lakes revelated three long-term phases highstand at 0–0.6, 2.6–1.5 and > 4.2–3.4 cal ka BP and two lowstand phases at 1.5–0.6 and 2.6–3.3 cal ka BP which are ultimately related to paleo-NAO intensity and signal variations. By modelling spatial and temporal climate variability between Azores and Europe, we tracked changes in the westerlies' latitudinal position over the last 4200 years.
Additionally, we characterised tephra deposits in Azorean lakes, creating a preliminary database to support future tephrostratigraphic and tephrochronological research. This framework can also be useful for recognising distal cryptotephra layers in Europe and North Africa
4. Wochenbericht M207
No full text04.01.-11.02.2025
Belém – Mindelo
4. Wochenbericht
20.01.-26.91.202
Phanerozoic emergence of global continental collision and onset of massive crustal eclogitization
No full textPost-Archean secular changes in continental crust composition, which provide key evidence for the evolution of plate tectonics, remain uncertain, particularly regarding the lower crust. Here, by digitizing 18,000 km of seismic profiles, we demonstrate a change in bulk crustal composition at the Proterozoic−Phanerozoic transition. We document that a mafic crustal layer is preserved in Proterozoic orogens but generally absent in Phanerozoic orogens. We explain this fundamental shift by a change in the global subduction style, where continental collision became important in the Phanerozoic. Densification of the lower crust by widespread eclogitization, triggered by continental collision and subduction, led to massive recycling of mafic lower crust into the mantle, leaving behind buoyant felsic crust and promoting the rise of continents, which led to the emergence of large continental areas above sea level and the related Neoproterozoic oxidation event, followed by the explosion of life in the Phanerozoic
Foraminiferal denitrification and deep bioirrigation influence benthic biogeochemical cycling in a seasonally hypoxic fjord
No full textBenthic macro- and micro-biota often play significant roles in controlling the biogeochemical dynamics in sediments. Their activity can be influenced by oxygen availability and impacted by the rise in global hypoxia in coastal regions over the last decades. To understand how these organisms interact with coastal hypoxia and influence sediment biogeochemistry, we undertook a study of early diagenesis in Bedford Basin, a seasonally hypoxic fjord on the West Atlantic coast in Nova Scotia, Canada, using a combination of observations and reaction-transport modeling. We observed that the seafloor was a source of ammonium and sink of nitrate with average fluxes of 2.2 ± 1.8 and −0.9 ± 0.7 mmol m−2 d−1 respectively. The diffusive oxygen uptake was 14 ± 4.6 mmol m−2 d−1 and the total organic carbon content in collected sediment cores was 5–7 % with a C/N ratio of ∼10. The pyrite content increased steadily from 0.5 wt% Fe at surface to ∼2 wt% Fe at 20 cm depth. Hydrogen sulfide was negligible down to 25 cm depth most of the time. The sediment was inhabited by tube-forming polychaete Spiochaetopterus sp. that formed tubes up to ∼30 cm in length. The living foraminiferal assemblage in the top 5 cm sediment was found to be dominated (>85 %) by nitrate-storing and denitrifying benthic foraminifera Stainforthia fusiformis. These observations were used to develop and constrain a biogeochemical reaction-transport model. The model results suggest that the observed decrease in porewater concentrations of ammonium and dissolved inorganic carbon below 5 cm depth, was due to deep bioirrigation by tubeworms, accounting for almost 50 % of the benthic efflux. The model further revealed that the deep bioirrigation along with bioturbation and iron cycling prevented accumulation of free sulfide in the top 25 cm sediment despite oxygen penetration depths of ∼1 mm. Modelled organic carbon and nitrogen deposition was 25.2 and 2.9 mmol m−2 d−1 with burial efficiencies of 23 % and 17 %, respectively. The model indicated a total denitrification rate of 1.3 mmol N m−2 d−1 that was largely (∼70 %) driven by benthic foraminifera. This study reports the first evidence of foraminiferal denitrification in western Atlantic coastal sediments, and suggests that eukaryote mediated denitrification is an important driver of sediment N-loss in seasonally hypoxic environments, a process that has been traditionally assumed to be carried out by prokaryotic microbes
The impact of Pangean subducted oceans on mantle dynamics: Passive piles and the positioning of deep mantle plumes
No full textHighlights
• We present global mantle convection models with and without dense mantle heterogeneities near the core-mantle boundary.
• We compare the locations of our model deep mantle plumes with available geophysical and geological data.
• Our model results illustrate the power of sinking ocean plates to stir mantle flow and control the it’s thermal evolution.
Abstract
Seismic imaging of the Earth’s interior reveals plumes originating from relatively hot regions of the lowermost mantle, surrounded by cooler material thought to be remnants of ancient subducted oceans. Currently, there is no clear consensus on the internal composition of the hot regions, with end-member conditions being that they are thermo-chemical in nature or purely thermal plume clusters. Previous modelling studies have shown a range of scenarios where deep chemical heterogenities or purely thermal anomalies are essential in developing appropriate present-day mantle dynamics. Here, we add to this discussion by quantifying the location of rising mantle plumes using numerical 3-D global mantle convection models constrained by 410 million years of palaeo-ocean evolution (encompassing the formation and breakup of supercontinent Pangea). Our study compares numerical simulations with purely thermal convection to those where a deep thermo-chemical anomaly is laterally mobile. The results show that models both with and without large-scale chemical heterogeneities can generate appropriate present-day plume dynamics, which illustrate the power of sinking ocean plates to stir mantle ow and control the thermal evolution of the mantle. Our models add to the discussion on bottom-up and top-down mantle dynamics, indicating the difficulty in unravelling the processes using numerical models alone
Seasonal cycles in a seaweed holobiont: A multiyear time series reveals repetitive microbial shifts and core taxa
No full textSeasonality is an important natural feature that drives cyclic environmental changes. Seaweed holobionts, inhabiting shallow waters such as rocky shores and mud flats, are subject to seasonal changes in particular, but little is known about the influence of seasonality on their microbial communities. In this study, we conducted a three-year time series, sampling at two-month intervals, to assess the seasonality of microbial epibiota in the seaweed holobiont Gracilaria vermiculophylla. Our results reveal pronounced seasonal shifts that are both taxonomic and functional, oscillating between late winter and early summer across consecutive years. While epibiota varied taxonomically between populations, they were functionally similar, indicating that seasonal variability drives functional changes, while spatial variability is more redundant. We also identified seasonal core microbiota that consistently (re)associated with the host at specific times, alongside a permanent core that is present year-round, independent of season or geography. These findings highlight the dynamic yet resilient nature of seaweed holobionts and demonstrate that their epibiota undergo predictable changes. Therewith, this research offers important insights into the temporal dynamics of seaweed-associated microbiota and demonstrates that the relationship between seaweed host and its epibiota is not static but naturally subject to an ongoing seasonal succession process
1. Wochenbericht M207
No full text04.01.-11.02.2025
Belém – Mindelo
1. Wochenbericht 04.01.-05.01.202
Geochemical processes related to mined, milled, or natural metal deposits in a rapidly changing global environment
No full textThe demand for metals and raw materials, such as nickel and copper, has been projected to expand in the coming decades, driven by the global energy transition, the need for green technologies, and expanding infrastructure. Consequently, the increasing extraction and production of mining waste can have adverse impacts on surrounding environments and human health. The aim of this thematic collection is to fill critical knowledge gaps in the present-day cycles of metal(loid)s from source to larger sinks, and the effect of environmental management, anthropogenic development, and climate change. Altogether, the studies have been conducted in different natural settings around the world and comprise investigations in laterites, a soil-medicinal plant system, watersheds, and banded iron formations, among others. The geochemical applications in tracing mineralization, its secondary products, and/or potential impact on the immediate environment are highly diverse with applied tools ranging from isotope tracers to major and trace element systematics. Particularly the use of rare earth elements, their patterns and anomalies are methods employed by several studies in this collection. We summarize the findings to offer a potential future direction for the use of geochemical tracing techniques in resource exploration in the context of climate change and environmental challenges