New Geology Articles Published Online Ahead of Print in January

Boulder, Colo., USA: Article topics and locations include a two-pronged kill mechanism at the end-Triassic mass extinction; volcanic lightning at Taal volcano, Philippines; the Smackover Formation, Gulf of Mexico; “narrow is normal”; and Etna volcano, Italy. These Geology articles are online at https://geology.geoscienceworld.org/content/early/recent .

Rapid retreat of the southwestern Laurentide Ice Sheet during the Bølling-Allerød interval
Sophie L. Norris; Lev Tarasov; Alistair J. Monteath; John C. Gosse; Alan J. Hidy ...
Abstract: The timing of Laurentide Ice Sheet deglaciation along its southwestern margin controlled the evolution of large glacial lakes and has implications for human migration into the Americas. Accurate reconstruction of the ice sheet’s retreat also constrains glacial isostatic adjustment models and is important for understanding ice-sheet sensitivity to climate forcing. Despite its significance, retreat of the southwestern Laurentide Ice Sheet (SWLIS) is poorly constrained by minimum-limiting ¹⁴C data. We present 26 new cosmogenic ¹⁰Be exposure ages spanning the western Interior Plains, Canada. Using a Bayesian framework, we combine these data with geomorphic mapping, ¹⁰Be, and high-quality minimum-limiting ¹⁴C ages to provide an updated chronology. This dataset presents an internally consistent retreat record and indicates that the initial detachment of the SWLIS from its convergence with the Cordilleran Ice Sheet began by ca. 15.0 ka, concurrent with or slightly prior to the onset of the Bølling-Allerød interval (14.7–12.9 ka) and retreated >1200 km to its Younger Dryas (YD) position in ∼2500 yr. Ice-sheet stabilization at the Cree Lake Moraine facilitated a meltwater drainage route to the Arctic from glacial Lake Agassiz within the YD, but not necessarily at the beginning. Our record of deglaciation and new YD constraints demonstrate deglaciation of the Interior Plains was ∼60% faster than suggested by minimum ¹⁴C constraints alone. Numerical modeling of this rapid retreat estimates a loss of ∼3.7 m of sea-level equivalent from the SWLIS during the Bølling-Allerød interval.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49493.1/610234/Rapid-retreat-of-the-southwestern-Laurentide-Ice

Middle Oxfordian carbon cycle perturbation expressed in the Smackover Formation, Gulf of Mexico
James S. Eldrett
Abstract: The Middle–Late Jurassic opening of the central Atlantic and proto-Caribbean seaways connecting the Pacific with the Tethys Ocean combined with rising sea level led to a major oceanographic and climatic reorganization conducive to the development of widespread carbonate platforms and diverse reefs. The timing of this shift in carbonate production and opening of the Tethys Ocean is well-constrained from outcrops in Europe and western Asia and marked by a notable positive carbon isotope (δ¹³C) excursion in the middle Oxfordian Gregoryceras transversarium ammonite Zone, termed “MOxE”. However, the temporal constraints from the western arm of the circum-global seaway through the proto-Caribbean and Gulf of Mexico are not well resolved. Presented here are the first paired organic and carbonate carbon-oxygen isotope records from the Smackover Formation, Gulf of Mexico, that record a positive δ¹³C isotope excursion that is correlated to the middle Oxfordian transversarium Zone. These data are the first record of the MOxE in the Western Hemisphere and provide temporal constraints on the opening of the Gulf of Mexico. Furthermore, the existence of similar δ ¹³C profiles from European basins strongly suggests a coeval response of the carbon cycle to the opening of the central Atlantic and proto-Caribbean seaways connecting the Pacific with the Tethys Ocean.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49662.1/610896/Middle-Oxfordian-carbon-cycle-perturbation

Eruption dynamics leading to a volcanic thunderstorm—The January 2020 eruption of Taal volcano, Philippines
Alexa R. Van Eaton; Cassandra M. Smith; Michael Pavolonis; Ryan Said
Abstract: Advances in global lightning detection have provided novel ways to characterize explosive volcanism. However, researchers are still at the early stages of understanding how volcanic plumes become electrified on different spatial and temporal scales. We deconstructed the phreatomagmatic eruption of Taal volcano (Philippines) on 12 January 2020 to investigate the origin of its powerful volcanic thunderstorm. Satellite analysis indicated that the water-rich plume rose >10 km high before creating lightning detected by Vaisala’s global lightning data set (GLD360). Flash rates increased with plume heights and cloud expansion over time, producing >70 flashes min^-1. Photographs revealed a highly electrified region at the base of the umbrella cloud, where we infer strong convective updrafts and icy collisions enhanced the electrical activity. These findings inform a conceptual model with overlapping regimes of charge generation in wet eruptions—initially due to ash particle collisions near the vent, followed by thunderstorm-like electrification in icy regions of the upper plume. Despite the wide reach of Taal’s ash cloud, most of the lightning occurred within 20–30 km of the volcano, producing thousands of hazardous cloud-to-ground flashes over a densely populated area. The eruption demonstrates that volcanic lightning can pose a hazard in its own right, embedded within the broader hazards of explosive volcanism in an urban setting.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49490.1/610898/Eruption-dynamics-leading-to-a-volcanic

Narrow is normal: Exploring the extent and significance of flooded marine shelves in icehouse, transitional, and greenhouse climate settings
Peter M. Burgess; Jinyu Zhang; Ronald Steel
Abstract: Marine shelves are a ubiquitous feature of modern Earth, developed across a wide range of scales in many sedimentary basins and representing the flooded portion of basin-margin clinoform topsets. Analysis of 80 clinoforms from 10 basins spanning Cenozoic and Mesozoic icehouse, transitional, and greenhouse climate settings indicates that normalized mean greenhouse marine shelf width is 33% of normalized mean total measured clinoform topset length. The equivalent value for transitional settings is 43%, and 72% for icehouse marine shelves. These values demonstrate that greenhouse marine shelves were substantially narrower than icehouse equivalents, suggesting that narrower shelves with persistent shelf-edge deltas were a consequence of lower rates of accommodation change in greenhouse climate intervals that lacked the large ice sheets required to drive high-amplitude high-frequency glacio-eustasy. Because greenhouse climates have been the dominant mode through Earth history, narrow shelves have probably been the dominant form, and conceptual models based on modern relatively wide shelves may be poor predictors of paleogeography, sediment routing, and sediment partitioning throughout much of Earth history.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49468.1/610899/Narrow-is-normal-Exploring-the-extent-and

A carbon-rich lithospheric mantle as a source for the large CO ₂ emissions of Etna volcano (Italy)
Alessandro Bragagni; Filippo Mastroianni; Carsten Münker; Sandro Conticelli; Riccardo Avanzinelli
Abstract: Etna volcano in Italy releases an exceptional amount of CO₂ (9083 t/day) and contributes to 10% of global volcanic emission. The reasons for its extreme CO₂ degassing are not yet understood. Using high-precision high field strength element (HFSE) concentrations in magmas from volcanoes in southern Italy, we show that the high Nb/Ta of Etna (up to 26) reveals a mantle source affected by carbonatite metasomatism, which is likely responsible for the large CO₂ fluxes. As observed at Etna, carbon-rich mantle domains influence CO ₂ degassing also outside of continental rifts and therefore play a fundamental role in explaining volcanic CO₂ fluxes in different geodynamic settings. Collectively, our study demonstrates that HFSE ratios in magmatic rocks are viable tracers for volcanic carbon degassing that can be used to study present-day settings and, possibly, past emissions.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49510.1/610900/A-carbon-rich-lithospheric-mantle-as-a-source-for

Portable optically stimulated luminescence age map of a paleoseismic exposure
Christopher B. DuRoss; Ryan D. Gold; Harrison J. Gray; Sylvia R. Nicovich
Abstract: The quality and quantity of geochronologic data used to constrain the history of major earthquakes in a region exerts a first-order control on the accuracy of seismic hazard assessments that affect millions of people. However, evaluations of geochronological data are limited by uncertainties related to inherently complex depositional processes that may vary spatially and temporally. To improve confidence in models of earthquake timing, we use a high-density suite of radiocarbon and optically stimulated luminescence (OSL) ages with a grid of 342 portable OSL samples to explore spatiotemporal trends in geochronological data across an exemplary normal fault colluvial wedge exposure. The data reveal a two-dimensional age map of the paleoseismic exposure and demonstrate how vertical and horizontal trends in age relate to dominant sedimentary facies and soil characteristics at the site. Portable OSL data provide critical context for the interpretation of ¹⁴C and OSL ages, show that geochronologic age boundaries between pre- and post-earthquake deposits do not match stratigraphic contacts, and provide the basis for selecting alternate Bayesian models of earthquake timing. Our results demonstrate the potential to use emergent, portable OSL methods to dramatically improve paleoseismic constraints on earthquake timing.
View article: https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G49472.1/610770/Portable-optically-stimulated-luminescence-age-map

Magma recharge patterns control eruption styles and magnitudes at Popocatépetl volcano (Mexico)
Martin F. Mangler; Chiara Maria Petrone; Julie Prytulak
Abstract: Diffusion chronometry has produced petrological evidence that magma recharge in mafic to intermediate systems can trigger volcanic eruptions within weeks to months. However, less is known about longer-term recharge frequencies and durations priming magma reservoirs for eruptions. We use Fe-Mg diffusion modeling in orthopyroxene to show that the duration, frequency, and timing of pre-eruptive recharge at Popocatépetl volcano (Mexico) vary systematically with eruption style and magnitude. Effusive eruptions are preceded by 9–13 yr of increased recharge activity, compared to 15–100 yr for explosive eruptions. Explosive eruptions also record a higher number of individual recharge episodes priming the plumbing system. The largest explosive eruptions are further distinguished by an ~1 yr recharge hiatus directly prior to eruption. Our results offer valuable context for the interpretation of ongoing activity at Popocatépetl, and seeking similar correlations at other arc volcanoes may advance eruption forecasting by including constraints on potential eruption size and style.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49365.1/610674/Magma-recharge-patterns-control-eruption-styles

Multi-proxy evidence of Caribbean-sourced marine incursions in the Neogene of Western Amazonia, Brazil
Lilian M. Leandro; Ana P. Linhares; Marcelo A. De Lira Mota; Gerson Fauth; Alessandra Santos ...
Abstract: The timing of continental-scale marine flooding events in Western Amazonia during the Neogene is still an unsolved question. Despite broad proxy-based evidence of such events, the pathways and duration of late Miocene marine incursions remain controversial. We provide coupled calcareous and organic microfossil and geochemical data from six onshore cores from Neogene sequences of the Solimões Basin, Brazil. Our records support minor marine influence in the early Miocene (23.0, 21.1, 18.6, and 16.3 Ma), middle Miocene (14.9, 13.7, and 12.9 Ma) and early Pliocene (4.7, 4.2–4.1, and 3.8 Ma), and conspicuous marine incursions in the late Miocene (11.1–8.8 Ma) suggested by the consistent presence of salinity-indicative microfossils and geochemical data. Our findings challenge the view of major marine incursions in the early and middle Miocene in the studied area. We propose for the first time a new late Miocene incursion (LMI) event as the main marine flooding event in Western Amazonia during the Neogene. These onshore records are compared with three offshore cores from the Atlantic and Pacific Oceans. The similarity between microfossil assemblages of the Solimões Basin and the Caribbean Sea, and evidence of increased runoff from the Orinoco river drainage system, strongly suggest the Caribbean Sea as the primary source area of the marine incursions, supporting a Venezuelan seaway. We further show for the first time the potential linkage between Neogene marine incursions (mainly the LMI) into the Solimões Basin and major disturbances in the global carbon cycle.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49544.1/610675/Multi-proxy-evidence-of-Caribbean-sourced-marine

Are fluid inclusions in gypsum reliable paleoenvironmental indicators? An assessment of the evidence from the Messinian evaporites
D. Bigi; S. Lugli; V. Manzi; M. Roveri
Abstract: The paleosalinity of water from which the gypsum precipitated during the Messinian salinity crisis is a controversial issue. Recent microthermometry studies on primary fluid inclusions in gypsum provided very low salinity values not compatible with precipitation from seawater, and suggested strong mixing between seawater and nonmarine waters enriched in calcium sulfate. We applied a new microthermometric protocol on gypsum crystals from nine Mediterranean sections that were experimentally stretched to measure a larger population of fluid inclusions. The results show salinities ranging from 9 to 238 wt‰ NaCl equivalent, largely falling within the evaporation path of normal seawater. The data from previous studies were obtained mostly from those fluid inclusions capable of nucleating a stable bubble after a weak stretching, which probably correspond to those having a lower salinity acquired through post-depositional crack-and-seal processes. Our data suggest instead that the primary gypsum precipitated from a marine brine, later modified by post-trapping processes during tectonics and exhumation.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49475.1/610676/Are-fluid-inclusions-in-gypsum-reliable

Experimental evidence on the origin of Ca-rich carbonated melts formed by interaction between sedimentary limestones and mantle-derived ultrabasic magmas
Michele Lustrino; Natascia Luciani; Vincenzo Stagno; Silvia Narzisi; Matteo Masotta ...
Abstract: In this experimental study, we documented the formation of strongly ultrabasic and ultracalcic melts through the interaction of melilititic and basanitic melts with calcite. Three strongly to moderately SiO₂ -undersaturated volcanic rocks from the Bohemian Massif (central Europe) were mixed with 10, 30, and 50 wt% CaCO₃ and melted at 1100, 1200, and 1300 °C at 2 kbar to evaluate the maximum amount of carbonate that can be assimilated by natural ultrabasic melts at shallow depths. Experiments revealed a surprisingly complete dissolution of the CaCO ₃, only rarely reaching carbonate saturation, with typical liquidus phases represented by olivine, spinel, melilite, and clinopyroxene. Only in the runs with the most SiO₂ -undersaturated compositions did abundant monticellite form instead of clinopyroxene. For all starting mixtures, strongly ultrabasic (SiO ₂ down to 15.6 wt%), lime-rich (CaO up to 43.6 wt%), ultracalcic (CaO/Al₂O₃ up to ~27) melt compositions were produced at 1200 and 1300 °C, with up to ~25 wt% dissolved CO₂. When present, quenched olivine showed much higher forsterite content (Fo _95–97) than olivine in the natural samples (Fo_79–85). The two major results of this study are (1) silicate-carbonatite melt compositions do not necessarily imply the existence of carbonatitic components in the mantle, because they are also produced during limestone assimilation, and (2) Fo-rich olivines cannot be used to infer any primitive character of the melt nor high potential temperature (T _p).
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49621.1/610678/Experimental-evidence-on-the-origin-of-Ca-rich

A red bole zircon record of cryptic silicic volcanism in the Deccan Traps, India
Liam O’Connor; Dawid Szymanowski; Michael P. Eddy; Kyle M. Samperton; Blair Schoene
Abstract: Silicic magmas within large igneous provinces (LIPs) are understudied relative to volumetrically dominant mafic magmas despite their prevalence and possible contribution to LIP-induced environmental degradation. In the 66 Ma Deccan LIP (India), evolved magmatism is documented, but its geographic distribution, duration, and significance remain poorly understood. Zircons deposited in weathered Deccan lava flow tops (“red boles”) offer a means of indirectly studying potentially widespread, silicic, explosive volcanism spanning the entire period of flood basalt eruptions. We explored this record through analysis of trace elements and Hf isotopes in zircon crystals previously dated by U–Pb geochronology. Our results show that zircon populations within individual red boles fingerprint distinct volcanic sources that likely developed in an intraplate setting on cratonic Indian lithosphere. However, our red bole zircon geochemical and isotopic characteristics do not match those from previously studied silicic magmatic centers, indicating that they must derive from yet undiscovered or understudied volcanic centers associated with the Deccan LIP.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49613.1/610679/A-red-bole-zircon-record-of-cryptic-silicic

Two-pronged kill mechanism at the end-Triassic mass extinction
Calum P. Fox; Jessica H. Whiteside; Paul E. Olsen; Xingqian Cui; Roger E. Summons ...
Abstract: High-resolution biomarker and compound-specific isotope distributions coupled with the degradation of calcareous fossil remnants reveal that intensive euxinia and decalcification (acidification) driven by Central Atlantic magmatic province (CAMP) activity formed a two-pronged kill mechanism at the end-Triassic mass extinction. In a newly proposed extinction interval for the basal Blue Lias Formation (Bristol Channel Basin, UK), biomarker distributions reveal an episode of persistent photic zone euxinia (PZE) that extended further upward into the surface waters. In the same interval, shelly taxa almost completely disappear. Beginning in the basal paper shales of the Blue Lias Formation, a Lilliput assemblage is preserved consisting of only rare calcitic oysters (Liostrea) and ghost fossils of decalcified aragonitic bivalves. The stressors of PZE and decalcification parsimoniously explain the extinction event and inform possible combined causes of other biotic crises linked to emplacement of large igneous provinces, notably the end-Permian mass extinction, when PZE occurred on a broad and perhaps global scale.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49560.1/610680/Two-pronged-kill-mechanism-at-the-end-Triassic

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