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Aragonite dissolution kinetics and calcite/aragonite ratios in sinking and suspended particles in the North Pacific

Sat, 03/23/2019 - 19:10

Publication date: 1 June 2019

Source: Earth and Planetary Science Letters, Volume 515

Author(s): Sijia Dong, William M. Berelson, Nick E. Rollins, Adam V. Subhas, John D. Naviaux, Aaron J. Celestian, Xuewu Liu, Nitya Turaga, Nathaniel J. Kemnitz, Robert H. Byrne, Jess F. Adkins

Abstract

The lack of consensus on CaCO3 dissolution rates and calcite to aragonite production and export ratios in the ocean poses a significant barrier for the construction of global carbon budgets. We present here a comparison of aragonite dissolution rates measured in the lab vs. in situ along a transect between Hawaii and Alaska using a 13C labeling technique. Our results show a general agreement of aragonite dissolution rates in the lab versus in the field, and demonstrate that aragonite, like calcite, shows a non-linear response of dissolution rate as a function of saturation state (Ω). Total carbon fluxes along the N. Pacific transect in August 2017, as determined using sediment traps, account for 11∼23 weight % of total mass fluxes in the upper 200 m, with a PIC (particulate inorganic carbon) /POC (particulate organic carbon) mole ratio of 0.2∼0.6. A comparison of fluxes at depths of 100 m and 200 m indicates that 30∼60% PIC dissolves between these depths with 20∼70% attenuation in POC fluxes. The molar ratio of PIC to POC loss is 0.29. The simultaneous loss of PIC and POC in the upper 200 m potentially indicates PIC dissolution driven by organic matter respiration, or metazoan/zooplankton consumption. The calcite/aragonite ratio in trap material is significantly lower in the subtropical gyre than in the subarctic gyre. Aragonite fluxes vary from 0.07 to 0.38 mmol m−2 day−1 at 100 m, and 0.06 to 0.24 mmol m−2 day−1 at 200 m along the North Pacific transect, with no specific trend over latitude. The identification of suspended PIC mineral phases by Raman spectroscopy shows the presence of aragonite below 3000 m in the subtropical gyre, but none in the subpolar gyre. These multiple lines of evidence suggest that predictions based on a strictly thermodynamic view of aragonite dissolution, combined with measured aragonite fluxes, underestimate observed alkalinity excess and measured PIC attenuation in sinking particles. Our measured aragonite flux combined with our inorganic dissolution rate only account for 9% and 0.2% of the excess alkalinity observed in the North Pacific (Feely et al., 2004), assuming aragonite sinking rates of 1 m day−1 and 100 m day−1, respectively. However, respiration-driven dissolution or metazoan/zooplankton consumption, indicated by the simultaneous attenuation of PIC and POC in sediment traps, is able to generate the magnitude of dissolution suggested by observed excess alkalinity.

Upper mantle seismic anisotropy as a constraint for mantle flow and continental dynamics of the North American plate

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Wanying Wang, Thorsten W. Becker

Abstract

The alignment of intrinsically anisotropic olivine crystals under convection is typically invoked as the cause of the bulk of seismic anisotropy inferred from shear-wave splitting (SWS). This provides a means of constraining the interplay between continental dynamics and the deep mantle, in particular for densely instrumented regions such as North America after USArray. There, a comparison of “fast orientations” from SWS with absolute plate motions (APM) suggests that anisotropy is mainly controlled by plate motions. However, large regional misfits and the limited realism of the APM model motivate us to further explore SWS based anisotropy. If SWS is estimated from olivine alignment in mantle circulation instead, plate-driven flow alone produces anisotropy that has large misfits with SWS. The addition of large-scale mantle density anomalies and lateral viscosity variations significantly improves models. Although a strong continental craton is essential, varying its geometry does, however, not improve the plate-scale misfit. Moreover, models based on higher resolution tomography degrade the fit, indicating issues with the flow model assumptions and/or a missing contributions to anisotropy. We thus compute a “lithospheric complement” to achieve a best-fit, joint representation of asthenospheric and frozen-in lithospheric anisotropy. The complement shows coherent structure and regional correlation with independently imaged crustal and upper mantle anisotropy. Dense SWS measurements therefore provide information on depth-dependent anisotropy with implications for tectonics, but much remains to be understood about continental anisotropy and its origin.

Diagenesis of mollusc aragonite and the role of fluid reservoirs

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): C. Pederson, V. Mavromatis, M. Dietzel, C. Rollion-Bard, G. Nehrke, N. Jöns, K.P. Jochum, A. Immenhauser

Abstract

Biogenic carbonate minerals are widely used as archives in paleoenvironmental research, providing substantial information for past depositional and diagenetic regimes. However, nearly all biogenic carbonates undergo post-mortem diagenetic alteration to variable degrees. Diagenetic features are essentially caused by complex fluid-solid interaction including recrystallization and neomorphosis of shell architecture and related geochemical resetting. A common conception is that a given primary shell texture is replaced by a secondary fabric via micro-scale dissolution-reprecipitation reactions that may reach geochemical and/or isotopic equilibrium with the diagenetic fluid. Here we document that the process of petrographic and geochemical alteration of a biogenic carbonate archive progresses stepwise, and the re-equilibration processes can be separated in space and time. More specifically, attached and bound aqueous fluids within the shell organic matter and fluid inclusions likely play a crucial role in the early stages of alteration within a relatively rock-buffered system. Degradation of organics was observed via decreased sulfur concentration and decreased fluorescence. In this early stage of alteration, the conversion of micro-scale aragonite biominerals to metastable diagenetic calcite (meso)crystals is documented by RAMAN spectroscopy, electron backscatter diffraction (EBSD), and a decrease in strontium concentrations. Further stabilization to larger, neomorphosed calcite crystals resulted from re-equilibration with external fluids and was evidenced by EBSD, oxygen isotope data, and further reduction of strontium. Spatial chemical and isotope data were used to decipher the impact of fluid availability and transfer behavior, i.e. carbonate-buffered versus diagenetic-fluid buffered, and isotope and element exchange behavior, and analyzed in context to textural alteration features. These data suggest that during diagenesis, the evolution to more sustainable (diagenetically induced) fabrics is accompanied by individual reaction steps traced by elemental and isotopic signatures. Kinetics, and thus degree of diagenetic alteration of biominerals at a given exposure to physicochemical alteration conditions is initially strongly controlled by the micro- to nano-scale internal architecture governing the availability and transfer of aqueous fluids. Results shown here have significance for those concerned with biogenic carbonate archives and have wider implications for a mechanistic understanding of carbonate diagenesis.

Sources vs processes: Unraveling the compositional heterogeneity of rejuvenated-type Hawaiian magmas

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Paul Béguelin, Michael Bizimis, Eleanor C. McIntosh, Brian Cousens, David A. Clague

Abstract

Spatial isotope variations in oceanic island basalts hold clues to the geochemical structure of mantle plumes. This signal is however susceptible to contributions from non-plume sources and modifications by physical processing in the upper mantle, i.e. source hybridization and segregation through gradients in melting conditions. A comprehensive survey of lava isotope systematics is therefore necessary to reveal spatiotemporal patterns at the scale of a single hotspot and disentangle plume sources from upper mantle physical and chemical contributions. We present Hf and Pb isotope data on 72 previously characterized onland and submarine lavas from the islands of Ni‘ihau and Kaua‘i, and from the North Arch Volcanic Field. These lavas cover the entire ∼ 5 Ma eruptive history of Ni‘ihau and Kaua‘i, allowing for a detailed assessment of the temporal geochemical evolution of Hawaiian volcanism. Furthermore, the broad lateral coverage of volcanic activity offered by these lavas constrains the spatial variability of sources across the volcanic chain, allowing investigation of the role of an upper asthenosphere contribution to plume volcanism. Early, shield-building (tholeiitic) lavas from Kaua‘i and Ni‘ihau overlap in Nd–Hf–Sr–Pb isotopes, contrasting with the across-plume asymmetry seen in younger shield lavas (i.e. the LOA-KEA double chain), and their isotope systematics suggest sampling of both LOA and KEA components. In contrast, late Kaua‘i and Ni‘ihau rejuvenated lavas do not overlap in isotope composition. They share common isotope characteristics with rejuvenated lavas from North Arch, Ka‘ula and O‘ahu, and together form two isotopically distinct, southwestern and northeastern groups. The two groups have steep slopes (>1.7) in a εNd − εHf plot and require distinct depleted components. The high εHf for a given εNd, high 87Sr/86Sr, and trace element systematics of the southwestern group (Ni‘ihau, Ka‘ula and O‘ahu) indicate a depleted component intrinsic to the plume. In contrast, lavas from the northeastern group (Kaua‘i and North Arch) indicate a source component isotopically identical to Pacific MORB. The shift from a spatially broad and homogeneous plume source in the shield stage of Kaua‘i and Ni‘ihau (6–3.5 Ma) to a bilaterally heterogeneous source during the rejuvenated stage (<3.5 Ma) is synchronous with the emergence of the LOA and KEA geochemical double chain in younger Hawaiian shield lavas. Our data reveal a shift to Pacific asthenosphere contribution to Kaua‘i volcanism through time, while Ni‘ihau exclusively samples the plume. These observations are inconsistent with a scenario where plume geochemical structure is the primary control on spatial heterogeneities in lava chemistry, and instead point to a more complex and dynamic interaction between plume, Pacific asthenosphere and lithosphere. Therefore, deep mantle structure might not be the primary control on the spatial distribution of isotope systematics in oceanic island lavas, making a link between the latter and contrasts imaged in the lower mantle through seismic tomography tenuous.

Ice-sheet driven weathering input and water mass mixing in the Nordic Seas during the last 25,000 years

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Torben Struve, Natalie L. Roberts, Martin Frank, Alexander M. Piotrowski, Robert F. Spielhagen, Marcus Gutjahr, Claudia Teschner, Henning A. Bauch

Abstract

Neodymium (Nd) isotopes are a powerful proxy tool for reconstructing past changes in water mass mixing, but reliable application of this proxy requires constraints on past changes of source water compositions. A key region of the global deep water circulation system are the Nordic Seas, which provide dense waters fundamental for the formation of North Atlantic Deep Water (NADW). Yet, the Nd isotope evolution of past deep waters in the Nordic Seas is so far poorly constrained. Here we present the first reconstructions of seawater Nd isotope compositions extracted from marine sediments at two locations in the central and northern Nordic Seas covering the period from the last glacial to the present. Further insights into past changes in sediment provenance, weathering inputs and water mass mixing are provided by complementary seawater and detrital strontium (Sr) and lead (Pb) isotope compositions.

Our new data reveal that changes in source and magnitude of weathering inputs from the Scandinavian and Svalbard–Barents Ice Sheets (SIS and SBIS, respectively) controlled the Nd and Pb isotope composition of the Nordic Seas' deep waters during the last glacial period. During the Last Glacial Maximum (LGM), deep waters showed distinctly unradiogenic Nd and radiogenic Pb isotope signatures most likely driven by weathering inputs of the SBIS. In contrast, the deglaciation was characterized by enhanced SBIS ice dynamics and/or meltwater release delivering sediments from the distal Eurasian shelves to the Norwegian Sea. Pulses of volcanogenic sediment supply changed the deep water Nd isotope composition during Heinrich Stadials 1, 2 and the Bølling period. As such, the glacial–deglacial Nd and Pb isotope evolution was markedly different in the Nordic Seas, the North Atlantic and the Arctic Oceans, respectively. During the Holocene, Pb isotopes indicate synchronized weathering fluxes around the North Atlantic, whereas the influence of local weathering input on the Nd isotope evolution of the deep Nordic Seas ceased. Instead, the Holocene Nd isotope signal has been driven by the strength of the advection/convection of water masses in the study area. These new constraints on changes in the Nordic Seas provide important endmember information for Nd isotope based reconstructions of NADW export downstream in the Atlantic Ocean.

Precise determination of Ar, Kr and Xe isotopic fractionation due to diffusion and dissolution in fresh water

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Alan M. Seltzer, Jessica Ng, Jeffrey P. Severinghaus

Abstract

Dissolved noble gases are ideal conservative tracers of physical processes in the Earth system due to their chemical and biological inertness. Although bulk concentrations of dissolved Ar, Kr, and Xe are commonly measured to constrain physical models of atmosphere, ocean, and terrestrial hydrosphere processes, stable isotope ratios of these gases (e.g. 136Xe/129Xe) are seldom used because of low signal-to-noise ratios. Here we present the first results from a new method of dissolved gas sampling, extraction and analysis that permits measurement of stable Ar, Kr, and Xe isotope ratios at or below ∼5 per meg amu−1 precision (1σ), two orders-of-magnitude below conventional Kr and Xe isotopic measurements. This gain in precision was achieved by quantitative extraction and subsequent purification of dissolved noble gases from 2-L water samples via helium sparging and viscous dual-inlet isotope ratio mass spectrometry. We have determined the solubility fractionation factors (αsol) for stable Ar, Kr, and Xe isotope ratios between ∼2 and 20 °C via laboratory equilibration experiments. We have also conducted temperature-controlled air-water gas exchange experiments to estimate the kinetic fractionation factors (αkin) of these isotope ratios. We find that both αsol and αkin, normalized by isotopic mass difference (Δm), decrease in magnitude with atomic number but are proportional to Δm for isotope ratios of the same element. With the new ability for high precision isotopic measurements, we suggest that dissolved Kr and Xe isotope ratios in groundwater represent a promising, novel geochemical tool with important applications for groundwater modeling, water resource management, and paleoclimate.

Halogens in Atlantis Bank gabbros, SW Indian Ridge: Implications for styles of seafloor alteration

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Mark A. Kendrick

Abstract

The processes controlling halogen (F, Cl, Br, I) abundances in gabbroic ocean crust recovered from the 809-m deep Hole U1473A drilled on the Atlantis Bank during International Ocean Discovery Program (IODP) Expedition 360 were investigated. The aims were to provide new constraints on hydrothermal alteration and the abundances of halogens potentially transported to subduction zones in oceanic crust produced on a slow spreading ridge.

Halogens in 51 gabbros and felsic veins have concentrations of ∼20-260 ppm F, 15-840 ppm Cl, 44-1230 ppb Br and 1-2490 ppb I. On average the gabbros retain a melt-derived magmatic F component of 58 ± 26% but are dominated by ∼96% hydrothermal Cl, Br, I and H2O. The abundances of hydrothermal Cl, Br and I are consistent with alteration at a seawater/rock ratio of <1. However, hydrothermal F is more enriched than expected and some amphiboles have high F/Cl ratios of 10-30 that provide evidence for the minor additional involvement of F-rich magmatic fluids. The abundant late-stage felsic veins that transect the gabbros and account for 1.5 vol.% of Hole U1473A lithologies are suggested as the most likely source of F-rich magmatic fluids.

Downhole variations show F, Cl, Br and H2O are most abundant in amphibole- and clay-rich alteration zones at the top of the hole. The highest I concentrations of 1-2.5 ppm delineate an oxidised CO2-rich alteration zone in which seawater iodate was incorporated into carbonate and Fe-oxyhydroxide alteration. The role of iodate, which is more reactive than other halides, in generating I-rich alteration can be distinguished from alteration by I-rich sedimentary pore waters because the oxidised alteration is characterised by high I/Cl together with low Br/Cl, whereas organic matter in pore waters is enriched in both Br/Cl and I/Cl.

The halogens have inferred compatibilities of F− > IO3− > OH− > Cl− ≥ Br− ∼ ≥ I− in the investigated alteration assemblage. EPMA and SHRIMP analyses indicate amphibole contains 1000-3000 ppm Cl in amphibolite facies alteration at the top of the Hole. Amphibole, chlorite and talc in greenschist facies alteration have much lower concentrations, typically in the range of 20-800 ppm Cl (median ∼100 ppm Cl), and F/Cl ratios of <4. In comparison, low temperature limonite (FeOOH.nH2O) has 160-8500 ppm Cl (median ∼840 ppm Cl). Amphibole is the dominant host of Cl and F in amphibolite facies alteration, but chlorite and limonite are important at lower grades. The samples have fairly constant Br/Cl ratios suggesting that Br excluded from the amphibole lattice is retained in minerals such as chlorite and/or non-structural sites. High I concentrations of 10's of ppm are inferred for some carbonate and limonite. Overall amphibole is a less dominant host of halogens than has been suggested previously, which has important implications for the eventual release and availability of halogens during subduction zone metamorphism.

Deformation of crust and upper mantle in central Tibet caused by the northward subduction and slab tearing of the Indian lithosphere: New evidence based on shear wave splitting measurements

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Chenglong Wu, Xiaobo Tian, Tao Xu, Xiaofeng Liang, Yun Chen, Michael Taylor, José Badal, Zhiming Bai, Yaohui Duan, Guiping Yu, Jiwen Teng

Abstract

Shear-wave splitting provides insight into geodynamic processes such as lithospheric deformation and upper mantle flow. This study presents shear wave splitting parameters determined from XKS (SKS, SKKS and PKS) and Pms phases (from receiver functions) recorded by the 2D seismic array, SANDWICH, deployed in central Tibet from November 2013 to April 2016. The XKS splitting measurements show a generally strong anisotropy with an average of 1.3 s, that even includes 17 stations with delay times no less than 2.0 s. Interestingly, significantly weak splitting is also observed between Nam Tso and Siling Tso near 90°E. Spatial coherence analysis of the splitting parameters indicates an upper asthenospheric source as the primary cause of seismic anisotropy in the study region. The regionally dominant ENE-WSW oriented upper-mantle anisotropy can emerge from the corner flow in the overlying mantle wedge induced by the subduction of the Indian lithospheric slab. The crustal anisotropy obtained through Pms splitting analysis might reflect lattice preferred orientation of anisotropic minerals formed by the ENE oriented middle-lower crustal flow, with a minor contribution to the strong anisotropy. The weak splitting observations in the east are likely caused by the nearly vertical α-axis of olivine induced by upwelling asthenosphere at a slab tear in the Indian lithosphere.

Graphical abstract

Paleogene crustal extension in the eastern segment of the NE Tibetan plateau

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Long-Gang Fan, Qing-Ren Meng, Guo-Li Wu, Hong-Hong Wei, Zhong-Ming Du, Erchie Wang

Abstract

The Tibetan plateau experienced lateral growth in response to the India–Eurasia collision, but uncertainties remain about when and how its present-day margins were formed. It was commonly viewed that crustal shortening in the NE Tibetan plateau had commenced since the Eocene and led to reverse faulting and compressional basins. Our recent investigations of some Cenozoic intermontane basins reveal that synsedimentary normal faults occur widely in the eastern segment of the NE Tibetan plateau. Normal faulting at basin bases obviously controlled basin sedimentation and presumably resulted in footwall uplift and basement exhumation. Small-scale synsedimentary normal faults are very common in basin fill sequences although they decrease in intensity upwards and eventually vanish in the uppermost Paleogene sequences. Measurement of the attitudes of synsedimentary normal faults indicates that crustal stretching was in NW–SE direction. The NW–SE extension was consistent with Paleogene stress field of the whole North China craton, which were thought responsible for the NE–SW-trending rift basins in the interior of the North China craton, such as the Yinchuan, Weihe, and Bohai Bay rift basin systems. A broad unconformity separates Paleogene and Neogene sequences in the eastern segment of the NE Tibetan plateau, contrasting strikingly with conformable Oligocene to Middle Miocene sequences in the neighboring Ningnan and Yinchuan rift basins. It thus follows that the eastern segment of the NE Tibetan plateau had not been formed in Paleogene when it was basically dictated by crustal extension, possibly as a result of far-field effect of subduction of the western Pacific plate. End-Oligocene termination of normal faulting and the presence of a regional unconformity indicate that the eastern segment of the NE Tibetan plateau began rising at the beginning of the Miocene.

Seismic imaging of Santorini: Subsurface constraints on caldera collapse and present-day magma recharge

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): E.E.E. Hooft, B.A. Heath, D.R. Toomey, M. Paulatto, C.B. Papazachos, P. Nomikou, J.V. Morgan, M.R. Warner

Abstract

Volcanic calderas are surface depressions formed by roof collapse following evacuation of magma from an underlying reservoir. The mechanisms of caldera formation are debated and predict differences in the evolution of the caldera floor and distinct styles of magma recharge. Here we use a dense, active source, seismic tomography study to reveal the sub-surface physical properties of the Santorini caldera in order to understand caldera formation. We find a ∼3-km-wide, cylindrical low-velocity anomaly in the upper 3 km beneath the north-central portion of the caldera, that lies directly above the pressure source of the 2011-2012 inflation. We interpret this anomaly as a low-density volume caused by excess porosities of between 4% and 28%, with pore spaces filled with hot seawater. Vents that were formed during the first three phases of the 3.6 ka Late Bronze Age (LBA) eruption are located close to the edge of the imaged structure. The correlation between older volcanic vents and the low-velocity anomaly suggests that this feature may be long-lived. We infer that collapse of a limited area of the caldera floor resulted in a high-porosity, low-density cylindrical volume, which formed by either chaotic collapse along reverse faults, wholesale subsidence and infilling with tuffs and ignimbrites, phreatomagmatic fracturing, or a combination of these processes. Phase 4 eruptive vents are located along the margins of the topographic caldera and the velocity structure indicates that coherent down-drop of the wider topographic caldera followed the more limited collapse in the northern caldera. This progressive collapse sequence is consistent with models for multi-stage formation of nested calderas along conjugate reverse and normal faults. The upper crustal density differences inferred from the seismic velocity model predict differences in subsurface gravitational loading that correlate with the location of 2011-2012 edifice inflation. This result supports the hypothesis that sub-surface density anomalies may influence present-day magma recharge events. We postulate that past collapses and the resulting topographical and density variations at Santorini influence magma focusing between eruptive cycles, a feedback process that may be important in other volcanoes.

Coastal water vapor isotopic composition driven by katabatic wind variability in summer at Dumont d'Urville, coastal East Antarctica

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Camille Bréant, Christophe Leroy Dos Santos, Cécile Agosta, Mathieu Casado, Elise Fourré, Sentia Goursaud, Valérie Masson-Delmotte, Vincent Favier, Olivier Cattani, Frédéric Prié, Benjamin Golly, Anaïs Orsi, Patricia Martinerie, Amaëlle Landais

Abstract

Dumont d'Urville station, located on the East coast of Antarctica in Adélie Land, is in one of the windiest coastal region on Earth, due to katabatic winds downslope from the East Antarctic ice sheet. In summer, the season of interest in this study, coastal weather is characterized by well-marked diel cycles in temperature and wind patterns. Our study aims at exploring the added value of water vapor stable isotopes in coastal Adélie Land to provide new information on the local atmospheric water cycle and climate. An important application is the interpretation of water isotopic profiles in snow and ice cores recently drilled in Adélie Land. We present the first continuous measurements of δ18O and d-excess in water vapor over Adélie Land. During our measurements period (26/12/2016 to 03/02/2017), we observed clear diel cycles in terms of temperature, humidity and isotopic composition. The cycles in isotopic composition are particularly large given the muted variations in temperature when compared to other Antarctic sites where similar monitoring have been performed. Based on data analyses and simulations obtained with the regional MAR model on the coastal Adélie Land, we suggest that the driver for δ18O and d-excess diel variability in summer at Dumont d'Urville is the variation of the strength of the wind coming from the continent: the periods with strong wind are associated with the arrival of relatively dry air with water vapor associated with low δ18O and high d-excess from the Antarctic plateau. Finally, in addition to the interpretation of snow and ice core isotopic profiles in the coastal regions, our study has implications for the evaluation of atmospheric models equipped with water isotopes.

Marine biogeochemical cycling of cadmium and cadmium isotopes in the extreme nutrient-depleted subtropical gyre of the South West Pacific Ocean

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Ejin George, Claudine H. Stirling, Melanie Gault-Ringold, Michael J. Ellwood, Rob Middag

Abstract

The Cd isotopic analysis of waters from an increasing number of oceanic regions has provided a wealth of new information on the oceanic cycling of Cd, revealing the complex interplay of a multitude of different biogeochemical processes. In this study, paired Cd concentration and Cd isotopic measurements were made on samples collected during the GEOTRACES GP-13 zonal section in the South West Pacific Ocean. The South Pacific subtropical gyre is the most oligotrophic gyre in the global ocean and a unique area to study the Cd isotope systematics associated with phytoplankton productivity under ultra-low nutrient concentrations. The dissolved Cd and PO4 concentrations of the study area are well correlated and can be expressed by two different linear relationships, as observed in other oceanic regions. The near quantitative biological uptake of Cd in the upper water column and the mixing of different water masses with different pre-formed Cd/PO4 ratios likely produces the ‘kink’ in the Cd–PO4 relationship. Across the GP-13 zonal section, the Cd isotopic composition of deep waters is relatively constant, as observed in other regions, and is centred around a δ114Cd value of 0.26 ± 0.11‰ (2 SD, n=40). In contrast, across the thermocline depth range, extending from 150 to 1500 m depth, the Cd concentration and δ114Cd values are negatively correlated and best described by Cd isotope fractionation under open-system conditions with continuous replenishment of the Cd source/s and a fractionation factor of 1.0006 ± 0.0002. This contrasts with the closed-system conditions without Cd replenishment that have been used to describe some other open ocean settings. Below 500 m depth, the Cd isotope systematics can largely be explained by three component mixing between key water masses with different pre-formed Cd isotope signatures. However, above 500 m, the Cd isotope systematics appear to be influenced by both water mass mixing and the biological uptake of isotopically light Cd in local and remote surface waters and the regeneration of Cd from sinking organic material deeper in the water column. Additionally, an unusual positive correlation was observed between Cd concentration and Cd isotopic composition in the upper water column of the South Pacific Ocean, from 15 to 150–200 m depth. These systematics can potentially be explained by one, or a combination, of the following processes: (i) a dominant role of supply-limited conditions during Cd uptake, (ii) partitioning of Cd into ligand phases, and/or (iii) atmospheric sources of Cd in these oligotrophic waters. Although subject to uncertainty, flux calculations suggest that atmospheric input could contribute 10–83% of the total Cd input to the surface waters of the subtropical South Pacific gyre.

Corrigendum to “Climate preconditions the Critical Zone: Elucidating the role of subsurface fractures in the evolution of asymmetric topography” [Earth Planet. Sci. Lett. 513 (2019) 197–205]

Sat, 03/23/2019 - 19:10

Publication date: Available online 20 March 2019

Source: Earth and Planetary Science Letters

Author(s): Nicole West, Eric Kirby, Andrew A. Nyblade, Susan L. Brantley

A shorter Archean day-length biases interpretations of the early Earth's climate

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Christopher Spalding, Woodward W. Fischer

Abstract

Earth's earliest sedimentary record contains evidence that surface temperatures were similar to, or perhaps even warmer than modern. In contrast, standard Solar models suggest the Sun was 25% less luminous at this ancient epoch, implying a cold, frozen planet—all else kept equal. This discrepancy, known as the Faint Young Sun Paradox, remains unresolved. Most proposed solutions invoke high concentrations of greenhouse gases in the early atmosphere to offset for the fainter Sun, though current geological constraints are insufficient to verify or falsify these scenarios. In this work, we examined several simple mechanisms that involve the role played by Earth's spin rate, which was significantly faster during Archean time. This faster spin rate enhances the equator-to-pole temperature gradient, facilitating a warm equator, while maintaining cold poles. Results show that such an enhanced meridional gradient augments the meridional gradient in carbonate deposition, which biases the surviving geological record away from the global mean, toward warmer waters. Moreover, using simple atmospheric models, we found that the faster-spinning Earth was less sensitive to ice-albedo feedbacks, facilitating larger meridional temperature gradients before succumbing to global glaciation. We show that within the faster-spinning regime, the greenhouse warming required to generate an ice-free Earth can differ from that required to generate an Earth with permanent ice caps by the equivalent of 1–2 orders of magnitude of pCO2. Accordingly, the resolution of the Faint Young Sun problem depends significantly on whether the early Earth was ever, or even at times, ice-free.

Volcanic ash generation: Effects of componentry, particle size and conduit geometry on size-reduction processes

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Joali Paredes-Mariño, Bettina Scheu, Cristian Montanaro, Alejandra Arciniega-Ceballos, Donald B. Dingwell, Diego Perugini

Abstract

Grain size distributions (GSD) of pyroclastic materials are the product of processes ranging from primary fragmentation efficiency to tephra transport. As such, a detailed description of their physical and chemical state can provide pivotal information regarding such processes. By constraining the GSD of volcanic deposits one can thereby deliver powerful constraints on the energetics and dynamics of eruptions. In order to do so we must distinguish between two primary controls: 1) fragmentation of magma to tephra, and 2) secondary transport-related processes (such as abrasion and comminution), within the conduit and until deposition of the particles. Variations in particle interactions in the conduit together with conduit geometry, may be major factors in modifying the GSD. As in all physicochemical processes, for eruptive dynamics an experimental basis is an essential element of calibration and quantification. Here, we have conducted the first experimental investigation linking fragmentation and ash production via the influence of 1) particle-componentry and particle-size, and 2) the conduit geometry (constricted versus unconstricted) on the GSD. Rapid decompression experiments with loose tephra material from the fall deposit of Pomici Principali eruption (10.3 ka, Campi Flegrei) have been conducted in an optically transparent setup that enables the optical monitoring of particle dynamics with a high-speed camera. The samples employed can be classified into two main groups; 1) pumices, and 2) dense clasts (including crystals, lava clasts and wall rock fragments). Our results indicate that 1) a conduit diameter constriction (simulating obstacles in the conduit walls) is likely to reduce the average diameter of individual clasts and increase the generation of ash (<63 μm, fine ash); 2) the presence of larger pumice particles in the starting sample results in more efficient ash production; 3) the presence of dense particles is linked to a higher efficiency of size-reduction processes (whereby the presence of crystal-rich pumice appears to have a counterproductive effect). These findings correlate well with existing studies, enabling a general assessment of possible secondary size-reduction processes controlling the GSD of a volcanic deposit. Further, our results also suggest that secondary-generated ash should be considered for hazard assessment and modeling, since increased amounts of ash may affect, e.g., the plume dispersion dynamics, and in turn the area affected by ash deposition.

Topographic evolution of the western United States since the early Miocene

Sat, 03/23/2019 - 19:10

Publication date: 15 May 2019

Source: Earth and Planetary Science Letters, Volume 514

Author(s): Quan Zhou, Lijun Liu

Abstract

The origin of the high topography within the western United States has been attributed to either crustal/lithospheric isostasy or dynamic topography, but their relative contributions remain unconstrained. Here we investigate this problem using gravity, residual topography and geodynamic modeling. We first evaluate two end-member scenarios of isostatic balance: crustal isostasy and lithospheric isostasy. Both cases lead to prominent negative mantle residual gravity within the tectonically active western U.S. and unrealistic crustal/lithospheric density structures, requiring the presence of low-density mantle underneath. The negative mantle residual gravity is consistent with both the estimated positive residual topography and calculated dynamic uplift due to the presence of hot asthenospheric mantle underneath. Geodynamic modeling further reveals that this landward migrating dynamic uplift originates from the eastward intrusion of the hot Pacific mantle through tears and edges of the Juan de Fuca slab since middle Miocene. The estimated paleotopography maps by combining dynamic topography and lithosphere isostasy over the western U.S. are consistent with several observational constraints, including episodic uplifts of the Sierra Nevada, post-mid-Miocene uplift of the Idaho batholith, the sustaining subsidence within most of the B&R, and the largely stable topography of central Colorado Plateau since 20 Ma.

Comment on “Particle fluxes in groundwater change subsurface rock chemistry over geologic time”

Sat, 03/23/2019 - 19:10

Publication date: Available online 8 March 2019

Source: Earth and Planetary Science Letters

Author(s): Carleton R. Bern, Tiffany Yesavage

Reply to the comment on “Particle fluxes in groundwater change subsurface shale rock chemistry over geologic time”

Sat, 03/23/2019 - 19:10

Publication date: Available online 26 February 2019

Source: Earth and Planetary Science Letters

Author(s): Hyojin Kim, Xin Gu, Susan L. Brantley

Editorial Board

Thu, 03/21/2019 - 19:10

Publication date: 1 May 2019

Source: Earth and Planetary Science Letters, Volume 513

Author(s):

Climate preconditions the Critical Zone: Elucidating the role of subsurface fractures in the evolution of asymmetric topography

Thu, 03/21/2019 - 19:10

Publication date: 1 May 2019

Source: Earth and Planetary Science Letters, Volume 513

Author(s): Nicole West, Eric Kirby, Andrew A. Nyblade, Susan L. Brantley

Abstract

Although it has been long understood that rock properties strongly modulate the chemical and physical transformation of rock to regolith, recent studies highlight the central role of mechanical fracturing in the shallow subsurface. Competing hypotheses for how fractures co-evolve with surface processes suggest either that topographic stresses enhance or dampen the potential for fracture propagation, or alternatively suggest that fracture growth depends on the coupling between subsurface hydrology and weathering reactions. Both end-member models produce predictable and systematic spatial patterns of subsurface fracture distributions.

To elucidate the processes responsible for the development of asymmetric hillslope topography and critical zone structure, we combine direct observations of subsurface fracture distributions in boreholes, indirect measures of subsurface rock properties inferred from seismic refraction surveys, and geochemical variations from subsurface samples at a small, shale-underlain catchment in central Pennsylvania. Our results reveal that seismic P-wave velocity profiles are consistent with subsurface fracture densities and zones of chemical depletion observed in boreholes, but that these distributions co-vary with hillslope aspect. Simple models of frost cracking show that, while modern climates are not conducive to subsurface segregation ice growth, asymmetric fracture distributions could have been achieved by subtle differences in aspect-related microclimates driving freeze-thaw during past periglacial climates. The spatial co-variance among fracture density, hillslope gradient, regolith depth, and regolith transport efficiency is consistent with chemo-mechanical damage arising from microclimatic conditions governing the long-term architecture of this watershed. Thus, damage driven by climate could explain the asymmetric topography observed today.

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