Geology Articles Published Online Ahead of Print 29 March

Geological Society of America

Boulder, Colo., USA: Article topics and locations include the life and death of a subglacial lake in West Antarctica; evidence for Caribbean plate subduction in southern Costa Rica; and "Did nutrient-rich oceans fuel Earth's oxygenation?" These Geology articles are online at .

Structural control of Cambrian paleotopography and patterns of transgression in western Laurentia

Paul M. Myrow; Michael Hasson; John F. Taylor; Lidya G. Tarhan; Gerardo Ramirez ...

The Cambrian transgression across the Great Unconformity produced one of the largest expansions of shallow marine habitats and associated diversification of marine invertebrate faunas in Earth history. However, identification of the underlying controls on the pattern of transgression of Cambrian seas has been hampered by imprecise or inaccurate age assignments for many formations. Recovery of an Ehmaniella Zone trilobite fauna from the Lodore Formation in northwestern Colorado (United States) revises the age of this unit to be significantly older, specifically middle Miaolingian (upper Wuliuan). This expands the established distribution of thick Miaolingian deposits of the northern Rocky Mountains to within 90 km of a broad region of central Colorado where Miaolingian strata are missing and Furongian successions rest directly on basement. The boundary between these two regions marks the position of an ~200 km east-west offset within the generally north-south–trending Cambrian paleoshoreline of western North America. The offset is co-located with a Precambrian continental suture zone (Cheyenne belt) at the northern Yavapai terrane margin and is directly east of an offset of similar magnitude and latitude in the early to middle Paleozoic shelf edge in Nevada. We thus posit that Precambrian deep-seated crustal-scale features controlled the Cambrian paleotopography of western Laurentia, strongly influencing the patterns of Cambrian transgression and structure of the shallow marine ecosystem established during continental submergence. These continental-scale structural elements remained a major control on marine paleogeography for >200 m.y. into the late Paleozoic.

Phosphorus deficit in continental crust induced by recycling of apatite-bearing cumulates

Ronghua Cai; Jingao Liu; Yao Sun; Ruohan Gao

Trace-element anomalies in the continental crust (e.g., Nb-Ta-Pb) are useful to decipher the formation and evolution of continents. We found that the trace-element patterns of upper, middle, and lower crust are all marked by strongly negative phosphorus anomalies, which were not likely to have been generated directly by mantle melting. In order to address this "crust composition paradox," we compiled major- and trace-element data from global arc magmas, igneous minerals, and deep arc cumulates and found that the phosphorus deficit in continental crust is coupled with an elevated Th/La ratio. This feature can be best explained by the fractional crystallization of apatite during arc magma evolution rather than the influence of a subducted slab. Accumulation of apatite-bearing mafic cumulates in deep arcs followed by foundering into the upper mantle causes the mismatch in the phosphorus systematics between the mantle and the continental crust. This process plays a critical role in shaping the chemical composition of continental crust.

Laurentide Ice Sheet persistence during Pleistocene interglacials

Danielle E. LeBlanc; Jeremy D. Shakun; Lee B. Corbett; Paul R. Bierman; Marc W. Caffee ...

While there are no ice sheets in the Northern Hemisphere outside of Greenland today, it is uncertain whether this was also the case during most other Quaternary interglacials. We show, using in situ cosmogenic nuclides in ice-rafted debris, that the Laurentide Ice Sheet was likely more persistent during Quaternary interglacials than often thought. Low 26Al/10Be ratios (indicative of burial of the source area) in marine core sediment suggest sediment source areas experienced only brief (on the order of thousands of years) and/or infrequent ice-free interglacials over the past million years. These results imply that complete Laurentide deglaciation may have only occurred when climate forcings reached levels comparable to those of the early Holocene, making our current interglacial unusual relative to others of the mid-to-late Pleistocene.

Steady decline in mean annual air temperatures in the first 30 k.y. after the Cretaceous-Paleogene boundary

Lauren K. O'Connor; Emily Dearing Crampton-Flood; Rhodri M. Jerrett; Gregory D. Price; B. David A. Naafs ...

The Cretaceous-Paleogene (K-Pg) boundary marks one of the five major mass extinctions of the Phanerozoic. The ways in which the climate system responded to a bolide impact and extensive volcanism at this time over different time scales are highly debated. We used the distribution of branched tetraether lipids (brGDGT) from fossil peats at two sites in Saskatchewan, Canada (paleolatitude ~55°N), to generate a high-resolution (millennial) record of mean annual air temperature (MAAT) spanning the last ~4 k.y. of the Cretaceous and the first ~30 k.y. of the Paleogene. Our study shows that MAATs ranged from 16 to 29 °C, with the highest value in the first millennia of the Paleogene. The earliest Paleogene averaged ~25 °C—maintaining or enhancing warmth from the latest Cretaceous—followed by a general cooling to ~20 °C over the following ~30 k.y. No abrupt postboundary cooling (e.g., an "impact winter") or abrupt warming is evident in our data, implying that if such phenomena occurred, their duration was relatively short-lived (i.e., sub-millennial-scale). Further, no long-term impact- or volcanism-driven warming is evident. The range of temperature change observed is considerably greater than that derived from marine proxy records over the same time interval. Our findings therefore more properly place bounds on the magnitude and duration of temperature change on land during this critical interval—the main setting for the demise of nonavian dinosaurs and the rise of mammals.

Deep-water circulation in the northeast Atlantic during the mid- and Late Cretaceous

Shan Liu; F. Javier Hernández-Molina; Sara Rodrigues; David Van Rooij

The Cretaceous ocean was significantly different from its modern counterpart due to its ice-free condition. Deep waters were primarily sourced by evaporation at the ocean surface, although their circulation pattern and bottom-current dynamics have been largely unknown. Here we present a study of deeply buried contourite drifts in the southern proto–Bay of Biscay to unravel the circulation pattern of the mid- and Late Cretaceous deep waters across the northeast Atlantic. The generation of plastered drifts (120 Ma to 100–90 Ma) and a mounded drift (100–90 Ma to 65 Ma) suggests that a significant change in deep-water source regions from the Tethys to the high-latitude region occurred at ca. 100–90 Ma. These contourite drifts were buried after ca. 65 Ma when the Cretaceous ocean circulation transitioned to the Cenozoic style in the northeast Atlantic. Tectonic configurations of the Pyrenean and the Equatorial Atlantic gateways were moreover tied to significant changes in northeast Atlantic deep-water circulation at ca. 100–90 and ca. 65 Ma. Northeastern Spain and France might be potential sites for the detection of Late Cretaceous contourite outcrops. These outcrops could have fundamental implications for the sedimentary facies and sequence model of contourites as well as shed light on the paleoceanography and paleoclimate of Cretaceous Earth.

Melt inclusion evidence for limestone assimilation, calc-silicate melts, and "magmatic skarn"

Xinyue Xu; Xiaochun Xu; Marko Szmihelsky; Jun Yan; Qiaoqin Xie ...

Chemical exchange between silicate magmas and carbonate rocks has major implications for igneous fractionation, atmospheric CO2 flux, and formation of mineral deposits. However, this process is only partly understood, and long-standing questions of whether, where, and how carbonate rocks can be digested by silicate melts remain controversial. We describe evidence for pervasive chemical exchange between silicate melt and carbonate rock in a shallow porphyry setting driven by limestone assimilation. Melt inclusions in endoskarn from the Chating Cu-Au deposit in eastern China reveal that the calc-silicate assemblage (diopside + andradite ± wollastonite ± epidote) was molten at the time of skarn formation and coexisted with CO2 vapor as well as sulfate- and chloride-salt melts. Hence, we argue that endoskarn at Chating formed by crystallization of an immiscible calc-silicate melt produced by assimilation of carbonate rock, aided by the presence of sulfate and other fluxes, which in turn promoted desilication of the intruding magma and drove vigorous CO2 release.

A water transport system across the mantle transition zone beneath western North America as imaged by electrical conductivity data

Shiwen Li; Yabin Li; Junhao Guo; Zikun Zhou; Aihua Weng

The stability field of hydrous phases carried by subducting slabs regulates water transport along the subduction pathway. The ultimate vertical distribution of this water at different depths in Earth's mantle is governed by the thermal state of the slab. A warm slab is considered to lose water mainly in the upper mantle. However, whether a warm slab can carry water into the lower mantle (LM) is uncertain because of the scarcity of geophysical observations. We report an electrical conductivity model of the mantle transition zone (MTZ) and uppermost LM beneath North America to confirm the water-transporting ability of a warm slab. A high-conductivity anomaly was identified beneath the western United States. The LM portion of this anomaly is interpreted as a hydrous region containing rehydrated stishovite, whereas the lower MTZ portion of the anomaly is ascribed to the presence of water (~0.8 wt%) released by the hydrous stishovite. We speculate that warm slabs, such as the Farallon slab, can cause the breakdown of large amounts of dense hydrous magnesium silicates, releasing water mainly into the upper mantle. This water rehydrates stishovite when percolating through the slab, enabling the slab to continue to transport water into the LM. The identified high-conductivity anomaly, together with previously recognized electrical features in the upper mantle, demonstrates the existence of a trans-MTZ water transport system associated with the warm subduction of the Farallon slab.

Evolutionary history of the groundwater system in the Pearl River Delta (China) during the Holocene

Shengchao Yu; Jiu Jimmy Jiao; Xin Luo; Hailong Li; Xuejing Wang ...

Coastal groundwater reservoirs are sensitive to the complicated evolution of marine transgressions and regressions in river delta regions. We integrated hydrogeological investigation and hydrogeochemical data with numerical modeling to assess the evolution of the groundwater system in the Pearl River Delta's aquifer system (in southeastern China). We studied the effects of flow dynamics and redox conditions on the biogeochemical processes of nutrients in the regional groundwater flow systems in response to the transient states related to variable paleoprecipitation and seawater salinity decline from the late Pleistocene to the Holocene. The results from paleo-hydrogeological reconstruction of the aquifer-aquitard system showed that the saline groundwater formed by paleo-seawater intrusion was still present in the old marine aquitard and affected groundwater salinity and chemicals in the adjacent aquifers, while most of the groundwater in the shallow young marine aquitard has been freshened by infiltrated old/fresh rainwater. Consequently, total ammonium and carbon stored in the Pearl River Delta were estimated to be (1.91 ± 1.13) × 107 mol m –1 and (5.74 ± 4.05) × 107 mol m–1, respectively, and the ammonium and bicarbonate fluxes derived from groundwater discharge to the sea were calculated as (90.6 ± 55.9) mol m–1 yr–1 and (301.02 ± 196.23) mol m–1 yr –1, respectively. If the buried ammonium in the delta is released to the sea, it would be equivalent to nearly 205 ± 123 yr of Pearl River fluvial loading. These findings suggest that the chemicals trapped in the deltaic aquifer system during the Holocene could contribute to future ocean eutrophication and acidification.

The early opening of the Equatorial Atlantic gateway and the evolution of Cretaceous peak warming

Wolf Dummann; Peter Hofmann; Jens O. Herrle; Martin Frank; Thomas Wagner

The Cretaceous opening of the Equatorial Atlantic gateway (EAG) is considered a driver of major changes in global oceanography, carbon cycling, and climate. However, the early stages of EAG opening are poorly understood. We present seawater Nd-isotope, bulk geochemical, and micropaleontological data from two South Atlantic drill cores that constrain the onset of shallow (2000 m) was enabled by at least ca. 100 Ma, as much as 10 m.y. earlier than previously estimated. In response to EAG opening, deep-water ventilation in the South Atlantic, North Atlantic, and Tethys basins intensified, thereby triggering basin-scale reductions in organic carbon burial. We propose that the consequent drop in carbon sequestration in concert with increased atmospheric CO2 fluxes from subduction zones acted as major amplifiers of global warming that culminated in peak greenhouse conditions during the mid-Cretaceous.

Catastrophic craton destruction via wholesale lithosphere delamination

Hao Chen; Ming Tang; Shuguang Song

The nuclei of continents, manifested as cratons, are the most long-lived parts of Earth's lithosphere. However, ancient cratons in some areas can be substantially destroyed through mechanisms that are not fully understood. We used experimentally calibrated geobarometers to calculate the equilibrium pressures of mafic magmas in the North China craton, which directly constrain the evolving depth of the lithosphere-asthenosphere boundary beneath the craton through time. We show that the lithospheric thickness of the eastern part of the craton decreased from ~200 km to ~35 km in the Early Cretaceous. This intense destruction took place within a short time interval of ~10 m.y., at least locally. Following this destruction, the lithosphere gradually rethickened and stabilized as the upwelling asthenosphere cooled and formed a juvenile lithosphere. We suggest that this catastrophic lithosphere thinning resulted from wholesale lithosphere delamination. As a consequence of this catastrophic loss of thick mantle roots, the eastern part of the North China craton may have undergone rapid crustal rebound and surface uplift, as recorded by the regional unconformities formed between 130 and 120 Ma in the destructed area.

The garnet effect on hafnium isotope compositions of granitoids during crustal anatexis

Long Chen; Chris Yakymchuk; Kai Zhao; Zifu Zhao; Dongyong Li ...

Radiogenic Hf isotope disequilibrium during crustal anatexis complicates petrogenetic studies that link sources to sinks in granitoid systems and hinders the applications of Hf isotopes to evaluating long-term crustal growth and evolution. Garnet can be a dominant host of radiogenic Hf in crustal rocks, and its behavior in granitoid sources may play a crucial role in isotopic decoupling between residue and melt. We document covariation between (Gd/Lu)N ratios and εHf(t) in post-collisional granitoids from the Dabie orogen (central China). This covariation reflects different garnet modal contents in the residue during anatexis. Quantitative modeling further confirms the dominant role of mixing between melts derived from garnet-rich and garnet-poor residua in producing the observed covariation patterns, but results are inconsistent with the entrainment of garnet rich in radiogenic Hf in the melt. Our results demonstrate that the garnet effect on Hf isotope ratios during crustal anatexis is a crucial factor in elucidating the granitoid source and complicates interpretations of crustal growth from the global zircon archive.

Shearing-enhanced deep fluid circulation induces seismic anisotropy in the lower crust at slow-spreading oceanic ridges

Baojun Zhou; Junlai Liu; Jiaxin Yan; Chunru Hou; Xiaoyu Chen ...

Although long-lived detachment faulting plays an important role in fluid circulation and in accommodating tectonic extension at slow-spreading oceanic ridges, it is still unclear how the fluid-enriched faults contribute to the observed seismic anisotropy in the lower crust. We investigated sheared and altered gabbros along the detachment fault zones from the Xigaze ophiolite in the southern Tibetan Plateau. Results demonstrate that the positive feedback between fluid circulation and shearing, linked by dissolution-precipitation creep of amphibole, resulted in fluid enrichment during strain localization along the fault zones. Based on this shearing-enhanced fluid circulation model, our calculations of the seismic properties show that amphiboles (de)formed by dissolution-precipitation creep along the fault zones largely contribute to the seismic anisotropy (P and S waves) and S-wave delay time in the lower crust at slow-spreading ridges, with the polarization directions of fast shear waves being subparallel to the ridges. The strength of resulting seismic anisotropy is largely a function of crustal thickness, fault zone attitude, and metasomatism intensity. This study provides a novel explanation for the origin of seismic anisotropy in the lower oceanic crust at slow-spreading ridges. The conclusion may also have implications for the origin of seismic anisotropy at fast-spreading ridges where there are high melt supplies.

Climate control on the relationship between erosion rate and fluvial topography

Eyal Marder; Sean F. Gallen

Conceptual and theoretical models for landscape evolution suggest that fluvial topography is sensitive to climate. However, it remains challenging to demonstrate a compelling link between fluvial topography and climate in natural landscapes. One possible reason is that many studies compare erosion rates to climate data, although theoretical studies show that, at steady state, climate is encoded in the relationship between erosion rate and topography rather than erosion rate alone. We use an existing global compilation of 10Be basin-averaged erosion rates to isolate the climate signal in topography as a function of erosion rate for morphologically steady-state, fluvially dominated basins underlain by crystalline bedrock. Our results show that the relationship between erosion rate (a proxy for rock uplift rate) and the normalized river channel steepness index (a proxy for fluvial relief) becomes increasingly nonlinear with increasing mean annual precipitation and decreasing aridity. This result indicates that erosional efficiency increases in wetter and more humid climates, lowering fluvial relief for a given erosion rate. When interpreted in the context of detachment-limited bedrock incision models that account for incision thresholds and stochastic flood distributions, this systematic pattern can be explained by a decrease in discharge variability in wetter and more humid landscapes, assuming incision thresholds are important on a global scale.

Dolomite recrystallization revealed by Δ47/U-Pb thermochronometry in the Upper Jurassic Arab Formation, United Arab Emirates

M. Gasparrini; D. Morad; X. Mangenot; M. Bonifacie; S. Morad ...

The process of recrystallization affecting dolomitic successions remains a longstanding enigma in carbonate research. Recrystallization influences the accuracy of genetic dolomitization models as well as the prediction of porosity and permeability distribution within dolomitic reservoirs. We investigate early-formed dolomites of the Upper Jurassic Arab Formation reservoir (Arabian Platform, United Arab Emirates), where recrystallization is not easily ascertained based on petrographic and O-C-Sr isotope analyses. Conversely, the application of Δ47/U-Pb thermochronometry revealed the occurrence of burial recrystallization over a temperature-time interval of ~45 °C/45 m.y. during the Early and Late Cretaceous. The process was initially driven by Late Jurassic mixed marine-meteoric fluids, which evolved during burial in a closed hydrologic system and remained in thermal equilibrium with the host rocks. Recrystallization was a stepwise process affecting the succession heterogeneously, so that samples only few meters apart presently record different temperature-time stages of the process that stopped when hydrocarbons migrated into the reservoir. Our results illustrate how Δ 47/U-Pb thermochronometry may provide a novel approach to unravel dolomite recrystallization and to precisely determine the timing and physicochemical conditions (temperature and δ18Ow ) that characterized the process. Therefore, this study paves the way for better appraisal of recrystallization in dolomitic reservoirs.

Pyroxenite melting at subduction zones

Emilie E. Bowman; Mihai N. Ducea

Arc magmatism is thought to be driven by peridotite melting in the mantle wedge. Yet pyroxenites are ubiquitous in the melting region beneath magmatic arcs. Because they typically have lower solidi temperatures and higher melt productivities compared to peridotite, pyroxenites likely play a significant role in magma generation. Here, we use the Zn/Fe ratios of a global database of Pliocene–Holocene primitive arc magmas to show that, as the crustal thickness of the overlying plate increases, so does the proportion of pyroxenite-derived melts relative to peridotite-derived melts. In fact, at arcs with crustal thicknesses >40 km, the majority of magmas are sourced from pyroxenite. Major and trace element geochemistry of pyroxenite melts is consistent with derivation from mafic magmas frozen in the mantle en route to the surface. We hypothesize that, as the thickness of the continental crust increases, the mantle wedge is displaced toward higher pressures and cooler temperatures, thereby lowering the extent of peridotite melting and allowing magmas sourced from the pyroxenite-veined mantle to dominate the arc budget.

Forming and preserving aragonite in shear zones: First report of blueschist facies metamorphism in the Jabal Akhdar Dome, Oman Mountains

C. Zuccari; G. Vignaroli; I. Callegari; F. Nestola; D. Novella ...

We report the first occurrence of high-pressure metamorphic aragonite in Precambrian carbonates of the Jabal Akhdar Dome in the Oman Mountains (northern Oman). We propose a model for both its formation at blueschist facies conditions and its subsequent preservation to the surface within the tectonic framework of the Late Cretaceous obduction of the Semail Ophiolite. Aragonite formed at temperature ~350 °C and pressure ≥0.9 GPa and is preserved within mylonitic shear zones and in stretched-fiber dilational veins where the necessary conditions for its formation and preservation, such as plastic strain accommodation, fluid-enhanced mineralogical reactions, and an anisotropic permeability structure, were preferentially met with respect to the surrounding rock. High-strain structural domains are ideal sites to look for and study prograde and retrograde high-pressure metamorphic histories in deeply subducted and exhumed terrains.

Dating rare earth element enrichment in deep-sea sediments using U-Pb geochronology of bioapatite

Dengfeng Li; Jinzhou Peng; David Chew; Yongjia Liang; Pete Hollings ...

Deep-sea sediments rich in rare earth elements and yttrium (REY) are promising mineral resources that are believed to be associated with the burial of fish debris. However, the nature of the REY enrichment is poorly understood, in part due to a lack of robust age constraints. We report bioapatite U-Pb ages from an Ocean Drilling Program (Leg 199, Hole 1218A) core and a REY-rich sedimentary core from the Pacific Ocean, which yielded U-Pb ages ranging from 22.8 to 18.2 Ma and 6.5 to 2.2 Ma, respectively. The U-Pb fish teeth ages from the 1218A core are consistent with biostratigraphic constraints, shed light on the application of the U-Pb bioapatite chronometer, and yield an absolute time scale for stratigraphy, especially for sequences deposited below the calcite compensation depth (CCD), where there is an absence of fossil carbonate. The successful measurement of U-Pb ages from REY-enriched fish teeth in the REY-rich sediment core suggests the mineralization occurred no later than the Miocene in the western Pacific Ocean. Uranium is positively correlated with REY, suggesting that the U and REY were incorporated into the fish teeth lattice simultaneously, making the bioapatite U-Pb chronometer suitable for constraining the timing of REY mineralization. When combined with published data, our study suggests that the Miocene REY accumulation event in the western Pacific Ocean was influenced by high P2O5 and MnO2 contents correlated with oxic bottom water.

The life and death of a subglacial lake in West Antarctica

M.R. Siegfried; R.A. Venturelli; M.O. Patterson; W. Arnuk; T.D. Campbell ...

Over the past 50 years, the discovery and initial investigation of subglacial lakes in Antarctica have highlighted the paleoglaciological information that may be recorded in sediments at their beds. In December 2018, we accessed Mercer Subglacial Lake, West Antarctica, and recovered the first in situ subglacial lake-sediment record—120 mm of finely laminated mud. We combined geophysical observations, image analysis, and quantitative stratigraphy techniques to estimate long-term mean lake sedimentation rates (SRs) between 0.49 ± 0.12 mm a–1 and 2.3 ± 0.2 mm a–1, with a most likely SR of 0.68 ± 0.08 mm a –1. These estimates suggest that this lake formed between 53 and 260 a before core recovery (BCR), with a most likely age of 180 ± 20 a BCR—coincident with the stagnation of the nearby Kamb Ice Stream. Our work demonstrates that interconnected subglacial lake systems are fundamentally linked to larger-scale ice dynamics and highlights that subglacial sediment archives contain powerful, century-scale records of ice history and provide a modern processbased analogue for interpreting paleo–subglacial lake facies.

Hydrothermal sulfate surges promote rare earth element transport and mineralization

Ye Wan; I-Ming Chou; Xiaolin Wang; Ruoheng Wang; Xiaochun Li

The generation of sulfate-rich hydrothermal fluids is of great significance to investigate because it is closely associated with the formation of many important ore deposits, such as hydrothermal rare earth element (REE) deposits. However, the transport of REEs in sulfate-rich hydrothermal fluids is complicated by the retrograde solubility of common sulfate minerals depicted in current thermodynamic models. We present in situ and ex situ hydrothermal experimental evidence suggesting that the solubility of alkali sulfate changes from retrograde at low pressures to prograde at elevated pressures. Accordingly, we propose a sulfate surge temperature and pressure (T-P) window (250 °C, 90 MPa), above which the solubility of alkali sulfate increases significantly with increasing P and T. Although REE sulfates are weakly soluble in water, sulfate-rich hydrothermal fluids can transport high contents of REEs under the T-P conditions above the sulfate-surge window. Our results indicate that depressurization, cooling, and alkali loss are key factors controlling REE mineralization, which agrees well with geological observations.

Did nutrient-rich oceans fuel Earth's oxygenation?

Birger Rasmussen; Janet R. Muhling; Nicholas J. Tosca; Woodward W. Fischer

Phosphorus (P) availability exerts a strong influence on primary productivity in global oceans. However, its abundance and role as a limiting nutrient prior to the start of the Great Oxygenation Event (GOE) 2.45–2.32 Ga is unclear. Low concentrations of seawater P have been proposed to explain the apparent delay between the early appearance of oxygen-producing Cyanobacteria and the onset of atmospheric oxygenation. We report evidence for seawater precipitation of Ca-phosphate nanoparticles in 2.46–2.40 Ga iron formations deposited on a marine shelf, including shallow-water facies, immediately prior to the onset of the GOE. Our modeling shows that the co-precipitation of Ca-phosphate and ferrous silicate (greenalite) required ferruginous seawater with dissolved P concentrations many orders of magnitude higher than in today's photic zone. If correct, it follows that P availability is unlikely to have suppressed the expansion of Cyanobacteria prior to the GOE. A reservoir of P-rich surface water shortly before 2.40 Ga could ultimately have triggered a rapid rise in atmospheric oxygen by fueling a sharp increase in primary productivity and organic-carbon burial. We speculate that the enigmatic Lomagundi positive carbon-isotope excursion, recorded in 2.32–2.06 Ga shallow-water carbonates, may mark a key step in the transition toward a modern biosphere of high biological productivity controlled by nutrient availability.

Insights into magma dynamics at Etna (Sicily) from SO2 and HCl fluxes during the 2008–2009 eruption

A. La Spina; M. Burton; G. Salerno; T. Caltabiano

Magma convection, where low-viscosity, gas-rich magma ascends, degasses, and crystallizes before sinking down the same conduit in either annular or side-by-side flows, has been proposed for active basaltic volcanoes, where excess gas fluxes relative to erupted lava volume can be observed. Experimental studies show that convection is produced by buoyant ascending gas-rich magma and descending degassed magmas following density difference contrast, while geophysical studies point to the endogenous growth of active volcanoes through magma accumulation in plutons. However, many aspects of the convection process remain unclear, in particular, the depth to which magma ascends before overturning. Models have been proposed where overturn occurs near the surface and also at depths greater than 2 km from the top of the magma-filled conduit. The long-term monitoring of volcanic gas compositions may reveal new insights into the convection process, as each gas has a unique solubility-pressure profile. We report measurements of SO2 and HCl gas fluxes from Etna between October 2007 and May 2011, in which an ~90% collapse in halogen flux was observed together with an effusive eruption. This observation indicates that the halogen fluxes, during quiescent periods on Etna, require both magma supply to the shallowest levels and a period of residence. The lava effusion has the effect of reducing the shallow residence time, drastically reducing the halogen flux. These results provide a new interpretative framework for the degassing process and gas composition monitoring to explain subtle variations in magma supply and residence times in basaltic volcanism.

Discriminating carbon dioxide sources during volcanic unrest: The case of Campi Flegrei caldera (Italy)

Gianmarco Buono; Stefano Caliro; Antonio Paonita; Lucia Pappalardo; Giovanni Chiodini

Large calderas are among the main emitters of volcanic CO2, which is mainly supplied by the deep degassing of magmatic fluids. However, other sources of non-magmatic CO2 can also occur due to the intense interaction among magmatic fluids, wide hydrothermal systems, and their host rocks. In particular, massive amounts of CO2 are released by calderas during unrest phases and have been often detected before eruptions. An accurate assessment of CO2 sources is thus fundamental to properly understand gas monitoring signals during volcanic crises. We focused on the restless Campi Flegrei caldera, in southern Italy, where CO2 fluxes at the Solfatara-Pisciarelli hydrothermal site have been progressively increasing up to 4000–5000 t/d during the ongoing unrest that started in 2005. Theoretical models of magma degassing have been able to reproduce the CO2-N2-He variations at the Solfatara fumaroles. However, a time-dependent deviation between measured and modeled N2/CO2 and He/CO 2, well correlated with the temporal evolution of ground uplift and temperature of the hydrothermal system, has been observed since 2005. We show that these variations are controlled by intense physical-chemical perturbation of the hydrothermal system, which is driving the decarbonation of hydrothermal calcite stored in reservoir rocks. This process is providing large volumes of non-magmatic CO2 during the current unrest, contributing up to 20%–40% of the total fumarolic CO2.

Evidence for Caribbean plate subduction in southern Costa Rica

James R. Bourke; Vadim Levin; Ivonne G. Arroyo; Lepolt Linkimer

Nestled between the Cocos, Nazca, Caribbean, and South American plates, the Panama microplate represents an area of rapidly evolving tectonics throughout the past ~10 m.y. Past and current studies have observed a notable amount of seismicity throughout this region, in particular the Caribbean coast of Costa Rica, which experienced a Mw 7.7 earthquake in 1991 CE. We investigated the crust and upper mantle structure of this region using the receiver function methodology and report two results: (1) first-order lateral constraints on the position of the Panama microplate boundary near the intersection between the Central Costa Rica Deformed Belt (onshore) and North Panama Deformed Belt (offshore), and (2) an impedance contrast south and east of these belts, supporting that the Caribbean plate currently subducts beneath the Panama microplate. Observed local seismicity is a consequence of the recently (ca. 14 Ma) initiated Caribbean plate subduction beneath the overlying Panama microplate. Our results are also consistent with a doubly convergent subduction margin dominating southern Costa Rica tectonics, uplifting the Talamanca Cordillera, and causing the cessation of southern Costa Rica volcanism over the past ~10 m.y.

Dating submarine landslides using the transient response of gas hydrate stability

Alexey Portnov; Kehua You; Peter B. Flemings; Ann E. Cook; Mahdi Heidari ...

Submarine landslides are prevalent on the modern-day seafloor, yet an elusive problem is constraining the timing of past slope failure. We present a novel age-dating technique based on perturbations to underlying gas hydrate stability caused by slide-impacted seafloor changes. Using three-dimensional (3-D) seismic data, we mapped an irregular bottom simulating reflection (BSR) underneath a submarine landslide in the Orca Basin, Gulf of Mexico. The irregular BSR mimics the pre-slide seafloor geometry rather than the modern bathymetry. Therefore, we suggest that the gas hydrate stability zone (GHSZ) is still adjusting to the post-slide sediment temperature. We applied transient conductive heat-flow modeling to constrain the response of the GHSZ to the slope failure, which yielded a most likely age of ca. 8 ka, demonstrating that gas hydrate can respond to landslides even on multimillennial time scales. We further provide a generalized analytical solution that can be used to remotely date submarine slides in the absence of traditional dating techniques.

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