Geology and Geophysics (G&G)

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Scientists in the G&G Department seek knowledge of the structure, composition, and dynamics of the earth’s interior, the origin and evolution of the earth’s crust, controls on ocean and climate change on time scales of decades to 100 million years, and processes of mass and energy transfer at the land-sea interface.

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Now showing 1 - 20 of 1752
  • Article
    Hot vent beaneath an icy ocean: the Aurora vend field, Gakkel Ridge, revealed
    (Oceanography Society, 2022-11-08) Ramirez-Llodra, Eva ; Argentino, Claudio ; Baker, Maria ; Boetius, Antje ; Costa, Carolina ; Dahle, Håkon ; Denny, Emily M. ; Dessandier, Pierre-Antoine ; Eilertsen, Mari H. ; Ferre, Benedicte ; German, Christopher R. ; Hand, Kevin ; Hilário, Ana ; Hislop, Lawrence ; Jamieson, John W. ; Kalnitchenko, Dimitri ; Mall, Achim ; Panieri, Giuliana ; Purser, Autun ; Ramalho, Sofia P. ; Reeves, Eoghan P. ; Rolley, Leighton ; Pereira, Samuel I. ; Ribeiro, Pedro A. ; Sert, Muhammed Fatih ; Steen, Ida H. ; Stetzler, Marie ; Stokke, Runar ; Victorero, Lissette ; Vulcano, Francesca ; Vågenes, Stig ; Waghorn, Kate Alyse ; Buenz, Stefan
    Evidence of hydrothermal venting on the ultra-slow spreading Gakkel Ridge in the Central Arctic Ocean has been available since 2001, with first visual evidence of black smokers on the Aurora Vent Field obtained in 2014. But it was not until 2021 that the first ever remotely operated vehicle (ROV) dives to hydrothermal vents under permanent ice cover in the Arctic were conducted, enabling the collection of vent fluids, rocks, microbes, and fauna. In this paper, we present the methods employed for deep-sea ROV operations under drifting ice. We also provide the first description of the Aurora Vent Field, which includes three actively venting black smokers and diffuse flow on the Aurora mound at ~3,888 m depth on the southern part of the Gakkel Ridge (82.5°N). The biological communities are dominated by a new species of cocculinid limpet, two small gastropods, and a melitid amphipod. The ongoing analyses of Aurora Vent Field samples will contribute to positioning the Gakkel Ridge hydrothermal vents in the global biogeographic puzzle of hydrothermal vents.
  • Article
    Significance of short‐wavelength magnetic anomaly low along the East Pacific Rise axis, 9°50′N
    (American Geophysical Union, 2023-05-16) Berrios‐Rivera, Natalia ; Gee, Jeffrey S. ; Parnell‐Turner, Ross ; Maher, Sarah ; Wu, Jyun‐Nai ; Fornari, Daniel ; Tivey, Maurice ; Marjanović, Milena ; Barreyre, Thibaut ; McDermott, Jill
    Magnetic anomaly variations near mid‐ocean ridge spreading centers are sensitive to a variety of crustal accretionary processes as well as geomagnetic field variations when the crust forms. We collected near‐bottom vector magnetic anomaly data during a series of 21 autonomous underwater vehicle Sentry dives near 9°50′N on the East Pacific Rise (EPR) covering ∼26 km along‐axis. These data document the 2–3 km wide axial anomaly high that is commonly observed at fast‐spreading ridges but also reveal the presence of a superimposed ∼800 m full wavelength anomaly low. The anomaly low is continuous for ≥13 km along axis and may extend over the entire survey region. A more detailed survey of hydrothermal vents near 9°50.3′N reveals ∼100 m diameter magnetic lows, which are misaligned relative to active vents and therefore cannot explain the continuous axial low. The axial magnetization low persists in magnetic inversions with variable extrusive source thickness, indicating that to the extent to which layer 2A constitutes the sole magnetic source, variations in its thickness alone cannot account for the axial low. Lava accumulation models illustrate that high geomagnetic intensity over the past ∼2.5 kyr, and decreasing intensity over the past ∼900 years, are both consistent with the broad axial anomaly high and the superimposed shorter wavelength low. The continuity of this axial low, and similar features elsewhere on the EPR suggests, that either crustal accretionary processes responsible for this anomaly are common among fast‐spread ridges, or that the observed magnetization low may partially reflect global geomagnetic intensity fluctuations.
  • Article
    The FORCIS database: a global census of planktonic Foraminifera from ocean waters
    (Nature Research, 2023-06-03) Chaabane, Sonia ; de Garidel-Thoron, Thibault ; Giraud, Xavier ; Schiebel, Ralf ; Beaugrand, Gregory ; Brummer, Geert-Jan ; Casajus, Nicolas ; Greco, Mattia ; Grigoratou, Maria ; Howa, Hélène ; Jonkers, Lukas ; Kucera, Michal ; Kuroyanagi, Azumi ; Meilland, Julie ; Monteiro, Fanny ; Mortyn, Graham ; Almogi-Labin, Ahuva ; Asahi, Hirofumi ; Avnaim-Katav, Simona ; Bassinot, Franck ; Davis, Catherine V. ; Field, David B. ; Hernández-Almeida, Iván ; Herut, Barak ; Hosie, Graham ; Howard, Will ; Jentzen, Anna ; Johns, David G. ; Keigwin, Lloyd ; Kitchener, John ; Kohfeld, Karen E. ; Lessa, Douglas V. O. ; Manno, Clara ; Marchant, Margarita ; Ofstad, Siri ; Ortiz, Joseph D. ; Post, Alexandra ; Rigual-Hernandez, Andres ; Rillo, Marina C. ; Robinson, Karen ; Sagawa, Takuya ; Sierro, Francisco ; Takahashi, Kunio T. ; Torfstein, Adi ; Venancio, Igor ; Yamasaki, Makoto ; Ziveri, Patrizia
    Planktonic Foraminifera are unique paleo-environmental indicators through their excellent fossil record in ocean sediments. Their distribution and diversity are affected by different environmental factors including anthropogenically forced ocean and climate change. Until now, historical changes in their distribution have not been fully assessed at the global scale. Here we present the FORCIS (Foraminifera Response to Climatic Stress) database on foraminiferal species diversity and distribution in the global ocean from 1910 until 2018 including published and unpublished data. The FORCIS database includes data collected using plankton tows, continuous plankton recorder, sediment traps and plankton pump, and contains ~22,000, ~157,000, ~9,000, ~400 subsamples, respectively (one single plankton aliquot collected within a depth range, time interval, size fraction range, at a single location) from each category. Our database provides a perspective of the distribution patterns of planktonic Foraminifera in the global ocean on large spatial (regional to basin scale, and at the vertical scale), and temporal (seasonal to interdecadal) scales over the past century.
  • Article
    Constraints on bend‐faulting and mantle hydration at the Marianas Trench from seismic anisotropy
    (American Geophysical Union, 2023-05-13) Mark, Hannah F. ; Lizarralde, Daniel ; Wiens, Douglas A.
    Subduction zones are a key link between the surface water cycle and the solid Earth, as the incoming plate carries pore water and hydrous minerals into the subsurface. However, water fluxes from surface to subsurface reservoirs over geologic time are highly uncertain because the volume of water carried in hydrous minerals in the slab mantle is poorly constrained. Estimates of slab mantle hydration based on seismic tomography assume bulk serpentinization, representing an upper bound on water volume. We measure azimuthal seismic anisotropy near the Marianas Trench, use spatial variations in anisotropy to constrain the extent and geometry of bend‐related faulting, and place a lower bound on slab mantle water content for the case where serpentinization is confined within fault zones. The seismic observations can be explained by a minimum of ∼0.85 wt% water in the slab mantle, compared to the upper bound of ∼2 wt% obtained from tomography.
  • Article
    Elastic and anelastic adjoint tomography with and full Hessian kernels
    (Oxford University Press, 2023-03-17) Xie, Yujiang ; Rychert, Catherine A. ; Harmon, Nicholas
    The elastic and anelastic structures of the Earth offer fundamental constraints for understanding its physical and chemical properties. Deciphering small variations in the velocity and amplitude of seismic waves can be challenging. Advanced approaches such as full-waveform inversion (FWI) can be useful. We rewrite the anelastic Fréchet kernel expression of Fichtner & van Driel using the displacement–stress formulation. We then derive the full Hessian kernel expression for viscoelastic properties. In these formulations, the anelastic Fréchet kernels are computed by the forward strain and a shift of the adjoint strain. This is complementary to the quality factor Q (i.e., inverse attenuation) Fréchet kernel expressions of Fichtner & van Driel that are explicit for the velocity–stress formulation. To reduce disk space and I/O requirements for computing the full Hessian kernels, the elastic full Hessian kernels are computed on the fly, while the full Hessian kernels for Q are computed by a combination of the on-the-fly approach with the parsimonious storage method. Applications of the Fréchet and full Hessian kernels for adjoint tomography are presented for two synthetic 2-D models, including an idealized model with rectangular anomalies and a model that approximates a subduction zone, and one synthetic 3-D model with an idealized geometry. The calculation of the full Hessian kernel approximately doubles the computationally cost per iteration of the inversion; however, the reduced number of iterations and fewer frequency stages required to achieve the same level of convergence make it overall computationally less expensive than the classical Limited-memory Broyden–Fletcher–Goldfarb–Shanno (L-BFGS) FWI for the 2-D elastic tested models. We find that the use of full Hessian kernels provides comparable results to the L-BFGS inversion using the improved anelastic Fréchet kernels for the 2-D anelastic models tested for the frequency stage up to 0.5 Hz. Given the computational expense of the Q full Hessian kernel calculation, it is not advantageous to use it in Q inversions at this time until further improvements are made. For the 3-D elastic inversion of the tested model, the full Hessian kernel provides similar image quality to the L-BFGS inversion for the frequency stage up to 0.1 Hz. We observe an improved convergence rate for the full Hessian kernel inversion in comparison to L-BFGS at a higher frequency stage, 0.1–0.2 Hz, and we speculate that at higher frequency stages the use of full Hessian kernels may be more computationally advantageous than the classical L-BFGS for the tested models. Finally, we perform 3-D elastic and Q L-BFGS inversions simultaneously using the rederived Q kernels, which can reduce the computational cost of the inversion by about 1/3 when compared to the classical anelastic adjoint tomography using the additionally defined adjoint source. The recovered Q model is smeared when compared to the recovered elastic model at the investigation frequencies up to 0.5 Hz. Q inversion remains challenging and requires further work. The 2-D and 3-D full Hessian kernels may be used for other purposes for instance resolution analysis in addition to the inversions.
  • Article
    Phosphate availability and implications for life on ocean worlds
    (Nature Research, 2023-04-25) Randolph-Flagg, Noah G. ; Ely, Tucker ; Som, Sanjoy M. ; Shock, Everett L. ; German, Christopher R. ; Hoehler, Tori M.
    Several moons in the outer solar system host liquid water oceans. A key next step in assessing the habitability of these ocean worlds is to determine whether life's elemental and energy requirements are also met. Phosphorus is required by all known life and is often limited to biological productivity in Earth's oceans. This raises the possibility that its availability may limit the abundance or productivity of Earth-like life on ocean worlds. To address this potential problem, here we calculate the equilibrium dissolved phosphate concentrations associated with the reaction of water and rocks-a key driver of ocean chemical evolution-across a broad range of compositional inputs and reaction conditions. Equilibrium dissolved phosphate concentrations range from 10-11 to 10-1 mol/kg across the full range of carbonaceous chondrite compositions and reaction conditions considered, but are generally > 10-5 mol/kg for most plausible scenarios. Relative to the phosphate requirements and uptake kinetics of microorganisms in Earth's oceans, such concentrations would be sufficient to support initially rapid cell growth and construction of global ocean cell populations larger than those observed in Earth's deep oceans.
  • Article
    Radiocarbon dating of the historic grand baobab of Mahajanga, Madagascar
    (Studia Chemia, 2023-01-09) Patrut, Adrian ; Patrut, Roxana T. ; Rakosy, Laszlo ; Ratiu, Ileana Andreea ; Danthu, Pascal ; Leong Pock Tsy, Jean-Michel ; Von Reden, Karl F.
    The article reports the AMS (accelerator mass spectrometry) radiocarbon investigation of the historic Grand Baobab of Mahajanga. The largest African baobab of Madagascar exhibits a cluster structure, which consists of 6 fused ordinary stems and of 3 small binding stems. Two samples were collected from the largest stem and from a primary branch, out of which several tiny segments were extracted and dated by radiocarbon. The oldest dated sample segment had a radiocarbon date of 214 [+ or -] 17 BP, which corresponds to a calibrated age of 265 [+ or -] 25 calendar years. The dating results indicate that the Grand Baobab of Mahajanga is 275 [+ or -] 25 years old. Keywords: AMS radiocarbon dating, Adansonia digitata, dendrochronology, Madagascar, age determination, multiple stems.
  • Article
    Benthic foraminiferal Mn/Ca as low‐oxygen proxy in Fjord sediments
    (American Geophysical Union, 2023-05-03) Brinkmann, Inda ; Barras, Christine ; Jilbert, Tom ; Paul, K. Mareike ; Somogyi, Andrea ; Ni, Sha ; Schweizer, Magali ; Bernhard, Joan M. ; Filipsson, Helena L.
    Fjord systems are typically affected by low‐oxygen conditions, which are increasing in extent and severity, forced by ongoing global changes. Fjord sedimentary records can provide high temporal resolution archives to aid our understanding of the underlying mechanisms and impacts of current deoxygenation. However, such archives can only be interpreted with well‐calibrated proxies. Bottom‐water oxygen conditions determine redox regime and availability of redox‐sensitive trace elements such as manganese, which in turn may be recorded by manganese‐to‐calcium ratios (Mn/Ca) in biogenic calcium carbonates (e.g., benthic foraminifera tests). However, biological influences on Mn incorporation (e.g., species‐specific Mn fractionation, ontogeny, living and calcification depths) are still poorly constrained. We analyzed Mn/Ca of living benthic foraminifera (Bulimina marginata, Nonionellina labradorica), sampled at low‐ to well‐oxygenated conditions over a seasonal gradient in Gullmar Fjord, Swedish West coast (71–217 μmol/L oxygen (O2)), by laser‐ablation ICP‐MS. High pore‐water Mn availability in the fjord supported Mn incorporation by foraminifera. B. marginata recorded contrasting Mn redox regimes sensitively and demonstrated potential as proxy for low‐oxygen conditions. Synchrotron‐based scanning X‐ray fluorescence nanoimaging of Mn distributions across B. marginata tests displayed Mn/Ca shifts by chambers, reflecting bottom‐water oxygenation history and/or ontogeny‐driven life strategy preferences. In contrast, Mn/Ca signals of N. labradorica were extremely high and insensitive to environmental variability. We explore potential biologically controlled mechanisms that could potentially explain this species‐specific response. Our data suggest that with the selection of sensitive candidate species, the Mn/Ca proxy has potential to be further developed for quantitative oxygen reconstructions in the low‐oxygen range.
  • Article
    The role of on‐ and off‐axis faults and fissures during eruption cycles and crustal accretion at 9°50′N, East Pacific Rise
    (American Geophysical Union, 2023-04-19) Wu, Jyun‐Nai ; Parnell‐Turner, Ross ; Fornari, Daniel J. ; Berrios‐Rivera, Natalia ; Barreyre, Thibaut ; McDermott, Jill M.
    Fissures and faults provide insight into how plate separation is accommodated by magmatism and brittle deformation during crustal accretion. Although fissure and fault geometry can be used to quantify the spreading process at mid‐ocean ridges, accurate measurements are rare due to insufficiently detailed mapping data. Here, fissures and faults at the fast‐spreading 9°50′N segment of the East Pacific Rise were mapped using bathymetric data collected at 1‐m horizontal resolution by autonomous underwater vehicle Sentry. Fault dip estimates from the bathymetric data were calibrated using co‐registered near‐bottom imagery and depth transects acquired by remotely operated vehicle Jason. Fissures are classified as either eruptive or non‐eruptive (i.e., cracks). Tectonic strain estimated from corrected fault heaves suggests that faulting plays a negligible role in the plate separation on crust younger than 72 kyr (<4 km from the ridge axis). Pre‐ and post‐eruption surveys show that most fissures were reactivated during the eruptions in 2005–2006. Variable eruptive fissure geometry could be explained by the frequency with which each fissure is reactivated and partially infilled. Fissure swarms and lava plateaus in low‐relief areas >2 km from the ridge are spatially associated with off‐axis lower‐crustal magma lenses identified in multichannel seismic data. Deep, closely spaced fissures overlie a relatively shallow portion of the axial magma lens. The width of on‐axis fissures and inferred subsurface dike geometry imply a ∼9‐year long diking recurrence interval to fully accommodate plate spreading, which is broadly consistent with cycle intervals obtained from estimates of melt extraction rates, eruption volumes, and spreading rate.
  • Article
    High‐resolution magnetic‐geochemical mapping of the serpentinized and carbonated Atlin ophiolite, British Columbia: toward establishing magnetometry as a monitoring tool for in situ mineral carbonation
    (American Geophysical Union, 2023-04-10) Tominaga, Masako ; Beinlich, Andreas ; Lima, Eduardo A. ; Pruett, Paiden ; Vento, Noah R. ; Weiss, Benjamin P.
    We address in situ serpentinization and mineral carbonation processes in oceanic lithosphere using integrated field magnetic measurements, rock magnetic analyses, superconducting quantum interference device (SQUID) microscopy, microtextural observations, and energy dispersive spectroscopy phase mapping. A representative suite of ultramafic rock samples were collected, within the Atlin ophiolite, along a 100‐m long transect across a continuous outcrop of mantle harzburgite with several alteration fronts: serpentinite, soapstone (magnesite + talc), and listvenite (magnesite + quartz). Strong correlations between changes in magnetic signal strengths and amount of alteration are shown with distinctive contrasts between serpentinite, transitional soapstone, and listvenite that are linked to the formation and breakdown of magnetite. While previous observations of the Linnajavri ultramafic complex indicated that the breakdown of magnetite occurred during listvenite formation from the precursor soapstone (Tominaga et al., 2017, https://doi.org/10.1038/s41467-017-01610-4), results from our study suggest that magnetite destabilization already occurred during the replacement of serpentinite by soapstone (i.e., at lower fluid CO2 concentrations). This difference is attributed to fracture‐controlled flow of sulfur‐bearing alteration fluid at Atlin, causing reductive magnetite dissolution in thin soapstone zones separating serpentinite from sulfide‐mineralized listvenite. We argue that magnetite growth or breakdown in soapstone provides insight into the mode of fluid flow and the composition, which control the scale and extent of carbonation. This conclusion enables us to use magnetometry as a viable tool for monitoring the reaction progress from serpentinite to carbonate‐bearing assemblages in space and time with a caution that the three‐dimensionality of magnetic sources impacts the scalability of measurements.
  • Article
    Vertical‐slice ocean tomography with seismic waves
    (American Geophysical Union, 2023-04-15) Callies, Jörn ; Wu, Wenbo ; Peng, Shirui ; Zhan, Zhongwen
    Seismically generated sound waves that propagate through the ocean are used to infer temperature anomalies and their vertical structure in the deep East Indian Ocean. These T waves are generated by earthquakes off Sumatra and received by hydrophone stations off Diego Garcia and Cape Leeuwin. Between repeating earthquakes, a T wave's travel time changes in response to temperature anomalies along the wave's path. What part of the water column the travel time is sensitive to depends on the frequency of the wave, so measuring travel time changes at a few low frequencies constrains the vertical structure of the inferred temperature anomalies. These measurements reveal anomalies due to equatorial waves, mesoscale eddies, and decadal warming trends. By providing direct constraints on basin‐scale averages with dense sampling in time, these data complement previous point measurements that alias local and transient temperature anomalies.
  • Article
    A North Pacific meridional section (U.S. GEOTRACES GP15) of helium isotopes and noble gases I: deep water distributions
    (American Geophysical Union, 2023-04-18) Jenkins, William J. ; Doney, Scott C. ; Seltzer, Alan M. ; German, Christopher R. ; Lott, Dempsey E. ; Cahill, Kevin L.
    The noble gas signature of incoming Pacific Bottom Water (PBW), when compared to North Atlantic Deep Water, indicates the addition of 450 ± 70 GT a−1 glacial melt water to form AABW and subsequently PBW. The downstream evolution of this signature between the southern (20°S to equator) and northern (25°–45°N) bottom waters indicates a decrease in sea level pressure around Antarctica over the past two millennia. Vertical profiles of noble gases in the deep Pacific show exponential relationships with depth with scale heights identical to temperature and salinity. Unlike the other noble gases, helium isotopes show evidence of mid‐depth injection of non‐atmospheric helium. Using observed deviations from exponential behavior, we quantify its magnitude and isotope ratio. There is a clear latitude trend in the isotope ratio of this added helium that decreases from a high exceeding 9 RA (atmospheric 3He/4He ratio) in the south to around 8 RA near the equator. North of 30–40°N, it systematically decreases northward to a low of ∼2 RA north of 50°N. This decline results from a combination of northward decline in seafloor spreading, release of radiogenic helium from increased sediment thickness, and the possible emission of radiogenic helium through cold seeps along the Alaskan and North American margins. Finally, we derive an improved method of computing the excess helium isotope concentrations and that the distributions of bottom water 3HeXS/4HeXS are consistent with what is known about bottom water flow patterns and the input of low 3He/4He sedimentary helium.
  • Article
    Refractory dissolved organic matter has similar chemical characteristics but different radiocarbon signatures with depth in the marine water column
    (American Geophysical Union, 2023-04-04) White, Margot E. ; Nguyen, Tran B. ; Koester, Irina ; Lardie Gaylord, Mary C. ; Beman, J. Michael ; Smith, Kenneth L. ; McNichol, Ann P. ; Beaupré, Steven R. ; Aluwihare, Lihini I.
    The >5,000‐year radiocarbon age (14C‐age) of much of the 630 ± 30 Pg C oceanic dissolved organic carbon (DOC) reservoir remains an enigma in the marine carbon cycle. The fact that DOC is significantly older than dissolved inorganic carbon at every depth in the ocean forms the basis of our current framing of the marine DOC cycle, where some component persists over multiple cycles of ocean mixing. As a result, 14C‐depleted, aged DOC is hypothesized to be present as a uniform reservoir with a constant 14C signature and concentration throughout the water column. However, key requirements of this model, including direct observations of DOC with similar 14C signatures in the surface and deep ocean, have never been met. Despite decades of research, the distribution of Δ14C values in marine DOC remains a mystery. Here, we applied a thermal fractionation method to compare operationally defined refractory DOC (RDOC) from different depths in the North Pacific Ocean. We found that RDOC shares chemical characteristics (as recorded by OC bond strength) throughout the water column but does not share the same 14C signature. Our results support one part of the current paradigm—that RDOC is comprised of structurally related components throughout the ocean that form a “background” reservoir. However, in contrast to the current paradigm, our results are consistent with a vertical concentration gradient and a vertical and inter‐ocean Δ14C gradient for RDOC. The observed Δ14C gradient is compatible with the potential addition of pre‐aged DOC to the upper ocean.
  • Article
    Evidence for heterothermic endothermy and reptile-like eggshell mineralization in Troodon, a non-avian maniraptoran theropod
    (National Academy of Sciences, 2023-04-11) Tagliavento, Mattia ; Davies, Amelia J. ; Bernecker, Miguel ; Staudigel, Philip T. ; Dawson, Robin R. ; Dietzel, Martin ; Götschl, Katja ; Guo, Weifu ; Schulp, Anne S. ; Therrien, François ; Zelenitsky, Darla K. ; Gerdes, Axel ; Müller, Wolfgang ; Fiebig, Jens
    The dinosaur–bird transition involved several anatomical, biomechanical, and physiological modifications of the theropod bauplan. Non-avian maniraptoran theropods, such as Troodon, are key to better understand changes in thermophysiology and reproduction occurring during this transition. Here, we applied dual clumped isotope (Δ47 and Δ48) thermometry, a technique that resolves mineralization temperature and other nonthermal information recorded in carbonates, to eggshells from Troodon, modern reptiles, and modern birds. Troodon eggshells show variable temperatures, namely 42 and 29 ± 2 °C, supporting the hypothesis of an endothermic thermophysiology with a heterothermic strategy for this extinct taxon. Dual clumped isotope data also reveal physiological differences in the reproductive systems between Troodon, reptiles, and birds. Troodon and modern reptiles mineralize their eggshells indistinguishable from dual clumped isotope equilibrium, while birds precipitate eggshells characterized by a positive disequilibrium offset in Δ48. Analyses of inorganic calcites suggest that the observed disequilibrium pattern in birds is linked to an amorphous calcium carbonate (ACC) precursor, a carbonate phase known to accelerate eggshell formation in birds. Lack of disequilibrium patterns in reptile and Troodon eggshells implies these vertebrates had not acquired the fast, ACC-based eggshell calcification process characteristic of birds. Observation that Troodon retained a slow reptile-like calcification suggests that it possessed two functional ovaries and was limited in the number of eggs it could produce; thus its large clutches would have been laid by several females. Dual clumped isotope analysis of eggshells of extinct vertebrates sheds light on physiological information otherwise inaccessible in the fossil record.
  • Article
    Through the eDNA looking glass: Responses of fjord benthic foraminiferal communities to contrasting environmental conditions
    (Wiley, 2023-03-27) Brinkmann, Inda ; Schweizer, Magali ; Singer, David ; Quinchard, Sophie ; Barras, Christine ; Bernhard, Joan M. ; Filipsson, Helena L.
    The health of coastal marine environments is severely declining with global changes. Proxies, such as those based on microeukaryote communities, can record biodiversity and ecosystem responses. However, conventional studies rely on microscopic observations of limited taxonomic range and size fraction, missing putatively ecologically informative community components. Here, we tested molecular tools to survey foraminiferal biodiversity in a fjord system (Sweden) on spatial and temporal scales: Alpha and beta diversity responses to natural and anthropogenic environmental trends were assessed and variability of foraminiferal environmental DNA (eDNA) compared to morphology‐based data. The identification of eDNA‐obtained taxonomic units was aided by single‐cell barcoding. Our study revealed wide diversity, including typical morphospecies recognized in the fjords, and so‐far unrecognized taxa. DNA extraction method impacted community composition outputs significantly. DNA extractions of 10 g sediment more reliably represented present diversity than of 0.5‐g samples and, thus, are preferred for environmental assessments in this region. Alpha‐ and beta diversity of 10‐g extracts correlated with bottom‐water salinity similar to morpho‐assemblage diversity changes. Sub‐annual environmental variability resolved only partially, indicating damped sensitivity of foraminiferal communities on short timescales using established metabarcoding techniques. Systematically addressing the current limitations of morphology‐based and metabarcoding studies may strongly improve future biodiversity and environmental assessments.
  • Article
    Transport of melt, pressure and heat through a magma mush
    (Frontiers Media, 2023-03-02) Liao, Yang
    Prior to intrusion, magma migrates through the crustal plumbing system that likely contains layers or columns of crystal mush. To better understand the behavior of the crustal magmatic system during magmatic unrest, it is important to examine the process of melt migration within the crystal mush and the associated evolution in pressure and temperature. In this study I use an analytical model to explore the characteristics of transport of melt, pressure, and heat through an idealized crystal mush layer/column under uniaxial strain condition. The model invokes a thermo-poro-viscoelastic rheology and uses a frequency-domain method to explore two scenarios of magmatic unrest: harmonic perturbation of fluid pressure, and step-rise in fluid pressure at a source location. Several factors influence the transport of melt, pressure and heat, including the thermal-mechanical coupling arising from the mush rheology, the advection of heat by melt flows, the competition between thermal diffusivity and poroelastic diffusivity, and the viscoelastic relaxation of the crystalline framework. One key finding is the development of transport asymmetry: when a background temperature gradient exists, the transport properties become different for propagation along the background thermal gradient and propagation against the background thermal gradient. Analysis on an endmember case shows that the transport asymmetry is associated to the competition between the diffusion and advection of pore pressure, which determines a Peclet number that depends on the temperature difference across the mush and the thermal expansion coefficients. Because the temperature in magma mushes in the crust likely increase with depth, the observed propagation asymmetry suggests some intrinsic difference between a bottom-up vs. a top-down triggering mechanism for magmatic unrest. The results from this study highlight the importance for further exploration for a more complete description of the transport properties in the crystal mush.
  • Article
    Gravity anomalies and implications for shallow mantle processes of the western Cocos‐Nazca spreading center
    (American Geophysical Union, 2023-03-02) Zheng, Tingting ; Lin, Jian ; Schouten, Hans ; Smith, Deborah K. ; Klein, Emily ; Parnell‐Turner, Ross
    This study analyzes up‐to‐date gravity data in the Galapagos triple junction region to understand crustal structure and melt distribution beneath the propagating Cocos‐Nazca spreading center (CNSC). Application of a standard thermal model to the mantle Bouguer gravity anomaly (MBA) does not appear to result in a realistic crustal thickness in this region. The cross‐CNSC MBA profiles flatten and axial values increase from east toward the western end of the CNSC. A simple smoothing filter applied to the standard thermal model with different filter widths can explain the progressive flattening of the MBA and is interpreted as different distribution widths (concentrations) of partial melt in the mantle. The east‐west residual MBA gradient along the CNSC is similar to the east flank of the East Pacific Rise (EPR), suggesting that the along‐CNSC gradient could partly reflect the shallow mantle properties associated with the EPR.
  • Article
    Strategies for and barriers to collaboratively developing anti‐racist policies and resources as described by geoscientists of color participating in the unlearning racism in Geoscience (URGE) program
    (American Geophysical Union, 2023-02-17) Burton, Carlene ; Duran, Gabriel ; Wright, Vashan ; Chmiel, Rebecca
    The Unlearning Racism in Geosciences (URGE) program guides groups of geoscientists as they draft, implement, and assess anti‐racist policies and resources for their workplace. Some participating Geoscientists of Color (GoC) shared concerns about microaggression, tokenism, and power struggles within their groups. These reports led us to collect and analyze data that describe the experiences of GoC in URGE. The data are from five discussion groups and two surveys. Our analyses revealed that participating GoC want to continue working with White colleagues on anti‐racist work. GoC want White colleagues not to shy away from doing anti‐racist work. Instead, GoC want White colleagues (a) to create and adhere to robust behavioral codes of conduct, (b) to focus discussions on anti‐racism, (c) to act on anti‐racism initiatives, (d) not to prompt GoC to educate them or reveal trauma, and (e) to refrain from microaggressions and tokenism. These desired outcomes were achieved in some groups with varying degrees of success. Correcting a history of mistrust relating to racism and anti‐racism action is key to implementing and assessing effective anti‐racist policies and resources. This requires leadership support, following through on anti‐racism action, and deepening relationships between GoC and White colleagues. Future anti‐racist programs should spend a substantial amount of time on and demonstrate the importance of training participants how to discuss racism effectively and how to create and adhere to robust behavioral codes of conduct. Future programs should also explore developing a robust program‐wide code of conduct that includes a policy for reporting offenses.
  • Article
    Near-surface seismic anisotropy in Antarctic glacial snow and ice revealed by high-frequency ambient noise
    (Cambridge University Press, 2022-12-19) Chaput, Julien ; Aster, Rick ; Karplus, Marianne ; Nakata, Nori ; Gerstoft, Peter ; Bromirski, Peter D. ; Nyblade, Andrews A. ; Stephen, Ralph A. ; Wiens, Douglas A.
    Ambient seismic recordings taken at broad locations across Ross Ice Shelf and a dense array near West Antarctic Ice Sheet (WAIS) Divide, Antarctica, show pervasive temporally variable resonance peaks associated with trapped seismic waves in near-surface firn layers. These resonance peaks feature splitting on the horizontal components, here interpreted as frequency-dependent anisotropy in the firn and underlying ice due to several overlapping mechanisms driven by ice flow. Frequency peak splitting magnitudes and fast/slow axes were systematically estimated at single stations using a novel algorithm and compared with good agreement with active source anisotropy measurements at WAIS Divide determined via active sources recorded on a 1 km circular array. The approach was further applied to the broad Ross Ice Shelf (RIS) array, where anisotropy axes were directly compared with visible surface features and ice shelf flow lines. The near-surface firn, depicted by anisotropy above 30 Hz, was shown to exhibit a novel plastic stretching mechanism of anisotropy, whereby the fast direction in snow aligns with accelerating ice shelf flow.
  • Article
    Seismicity properties of the chain transform fault inferred using data from the PI‐LAB experiment
    (American Geophysical Union, 2023-02-23) Leptokaropoulos, Konstantinos ; Rychert, Catherine A. ; Harmon, Nicholas ; Kendall, John Michael
    Oceanic transform faults are intriguing in that they do not produce earthquakes as large as might be expected given their dimensions. We use 1‐year of local seismicity (370 events above MC = 2.3) recorded on an array of ocean bottom seismometers (OBSs) and geophysical data to study the seismotectonic properties of the Chain transform, located in the equatorial Mid‐Atlantic. We extend our analysis back in time by considering stronger earthquakes (MW ≥ 5.0) from global catalogs. We divide Chain into three areas (east, central, and west) based on historical event distribution, morphology, and multidimensional OBS seismicity cluster analysis. Seismic activity recorded by the OBS is the highest at the eastern area of Chain where there is a lozenge‐shaped topographic high, a negative rMBA gravity anomaly, and only a few historical MW ≥ 5.5 events. OBS seismicity rates are lower in the western and central areas. However, these areas accommodate the majority of seismic moment release, as inferred from both OBS and historical data. Higher b‐values are significantly correlated with lower rMBA and with shallower bathymetry, potentially related to thickened crust. Our results suggest high lateral heterogeneity along Chain. Patches with moderate to low OBS seismicity rates that occasionally host MW ≥ 6.0 earthquakes are interrupted by segments with abundant OBS activity but few historical events with 5.5 ≤ MW < 6.0. This segmentation is possibly due to variable fluid circulation and alteration, which may also change in time.