Earth and Planetary Science Letters

Syndicate content
ScienceDirect RSS
Updated: 1 day 4 hours ago

Editorial Board

Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s):

End-Permian (252 Mya) deforestation, wildfires and flooding—An ancient biotic crisis with lessons for the present

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Vivi Vajda, Stephen McLoughlin, Chris Mays, Tracy D. Frank, Christopher R. Fielding, Allen Tevyaw, Veiko Lehsten, Malcolm Bocking, Robert S. Nicoll

Abstract

Current large-scale deforestation poses a threat to ecosystems globally, and imposes substantial and prolonged changes on the hydrological and carbon cycles. The tropical forests of the Amazon and Indonesia are currently undergoing deforestation with catastrophic ecological consequences but widespread deforestation events have occurred several times in Earth's history and these provide lessons for the future. The end-Permian mass-extinction event (EPE; ∼252 Ma) provides a global, deep-time analogue for modern deforestation and diversity loss. We undertook centimeter-resolution palynological, sedimentological, carbon stable-isotope and paleobotanical investigations of strata spanning the end-Permian event at the Frazer Beach and Snapper Point localities, in the Sydney Basin, Australia. We show that the typical Permian temperate, coal-forming, forest communities disappeared abruptly, followed by the accumulation of a 1-m-thick mudstone poor in organic matter that, in effect, represents a ‘dead zone’ hosting degraded wood fragments, charcoal and fungal spores. This signals a catastrophic scenario of vegetation die-off and extinction in southern high-latitude terrestrial settings. Lake systems, expressed by laterally extensive but generally less than a few-metres-thick laminated siltstones, generally lacking bioturbation, hosting assemblages of algal cysts and freshwater acritarchs, developed soon after the vegetation die-off. The first traces of vascular plant recovery occur ∼1.6 m above the extinction horizon. Based on analogies with modern deforestation, we propose that the global fungal and acritarch events of the Permo-Triassic transition resulted directly from inundation of basinal areas following water-table rise as a response to the abrupt disappearance of complex vegetation from the landscape. The δ13Corg values reveal a significant excursion toward low isotopic values, down to −31‰ (a shift of ∼4‰), across the end-Permian event. The magnitude of the shift at that time records a combination of changes in the global carbon cycle that were enhanced by the local increase in microbial activity, possibly also involving cyanobacterial proliferation. We envisage that elevated levels of organic and mineral nutrients delivered from inundated dead forests, enhanced weathering and erosion of extra-basinal areas, together with local contributions of volcanic ash, led to eutrophication and increased salinity of basinal lacustrine–lagoonal environments. We propose that the change in acritarch communities recorded globally in nearshore marine settings across the end-Permian event is to a great extent a consequence of the influx of freshwater algae and nutrients from the continents. Although this event coincides with the Siberian trap volcanic activity, we note that felsic–intermediate volcanism was extensively developed along the convergent Panthalassan margin of Pangea at that time and might also have contributed to environmental perturbations at the close of the Permian.

Graphical abstract

Uranium isotopes as a proxy for primary depositional redox conditions in organic-rich marine systems

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Michelle L. Abshire, Stephen J. Romaniello, Amy M. Kuzminov, Jessica Cofrancesco, Silke Severmann, Natascha Riedinger

Abstract

In marine sediments, authigenic uranium (U) enrichments and U isotope compositions are important tools for interpreting changes in redox conditions, however, their use as paleoproxies requires a comprehensive understanding of the dominant processes that contribute to sediments becoming enriched or depleted. This study focuses on the U content and 238U/235U ratio of organic-rich surface sediments from the Namibian continental margin, where high productivity results in an expanded oxygen minimum zone (OMZ). The investigated core sample sites are located on the shelf, shelf break, and slope where bottom water redox conditions vary from anoxic to suboxic to oxic, respectively. While all cores have relatively high total organic carbon (TOC) contents (up to 12 wt.%), each location displays a unique U to TOC relationship. Shelf sediment exhibit a fair correlation between U and TOC, while the shelf break and slope sediments show a pronounced decoupling of U and TOC. On the Namibia continental margin, particle-rich nepheloid layers transport organic-rich deposits from within the OMZ, through oxic water, to be redeposited on the slope. Due to the sensitivity of U to changes in redox conditions, this lateral movement results in the release of the reduced U phases back into the water column through oxidation while transporting the partially remineralized organic carbon to the slope. Oxidation of U during transport does not alter the average primary 238U/235U isotopic signature in redeposited sediment, and the combination of high TOC, low U content and high δ238U values may become a useful tool for the identification of the boundaries of ancient OMZs.

Sediment efflux of silicon on the Greenland margin and implications for the marine silicon cycle

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Hong Chin Ng, Lucie Cassarino, Rebecca A. Pickering, E. Malcolm S. Woodward, Samantha J. Hammond, Katharine R. Hendry

Abstract

The polar region is experiencing one of the most rapid environmental changes driven by atmospheric warming, and feedbacks within the cryosphere. Under such a setting, it is crucial to understand the biogeochemical cycling of the nutrient silicon (Si) in the high latitudes, which is regulating the nutrient supply to polar ecosystems, and is linked to the global carbon cycle via diatom production. However, these efforts have been hindered by a lack of understanding of the benthic Si cycle, particularly the quantification of the sediment efflux of Si, and identification of the responsible mechanistic processes during early diagenesis. Here, we address these issues using new pore water profiles and incubation experiments on sediment cores collected from the Greenland margin and Labrador Sea, combined with Si isotope analysis and a mass balance model. Benthic Si flux at our study sites is found to be greatly heightened from values sustained by pore water molecular diffusion. The remainder of the flux is likely accountable with early dissolution of reactive biogenic silica phases at the upper sediments, and advective transport of pore waters. Our results highlight an active benthic Si cycle at a northern high-latitude continental margin, which could play a key role in recycling significant amounts of biologically available dissolved Si to the overlying water, and influencing the growth of benthic and planktonic communities in the polar region.

A new approach to thermal history modelling with detrital low temperature thermochronological data

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Kerry Gallagher, Mauricio Parra

Abstract

We present an inverse modelling strategy to infer thermal history information from detrital low temperature thermochronological data from modern sediment sampling the outlet of a single catchment. As presented, the method relies on the assumption that the geological timescale thermal history was the same across the catchment. The detrital sample is assumed to represent a mixture of grains originating from a potentially unknown sampling of the present elevation range in the catchment. The approach also implements a method to infer a function describing the topographic sampling represented in the detrital sample. In practice, this may reflect variations in erosion with elevation but also lithological differences in the catchment (fertility) and the nature of erosion/transport processes in the catchment. A combination of detrital and in-situ bedrock data are recommended to improve the resolution of the topographic sampling function. We demonstrate the application of the approach to a set of fission track data from the Fundación catchment in the Sierra Nevada de Santa Marta in northern Colombia. The inferred thermal history suggest a period of rapid cooling initiated around 50-30 Ma, followed by slower cooling to the present day, consistent with the regional geological history. The topographic sampling function estimates suggest that the hypsometric distribution is not appropriate in terms of the contributions from different elevations to the detrital sample. Rather, the data imply a higher proportion of sampling from lower elevations close to the location of the outlet where the detrital sample was collected.

A lower to middle Eocene astrochronology for the Mentelle Basin (Australia) and its implications for the geologic time scale

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Maximilian Vahlenkamp, David De Vleeschouwer, Sietske J. Batenburg, Kirsty M. Edgar, Emma Hanson, Mathieu Martinez, Heiko Pälike, Kenneth G. MacLeod, Yong-Xiang Li, Carl Richter, Kara Bogus, Richard W. Hobbs, Brian T. Huber, Expedition 369 Scientific Participants

Abstract

The geologic time scale for the Cenozoic Era has been notably improved over the last decades by virtue of integrated stratigraphy, combining high-resolution astrochronologies, biostratigraphy and magnetostratigraphy with high-precision radioisotopic dates. However, the middle Eocene remains a weak link. The so-called “Eocene time scale gap” reflects the scarcity of suitable study sections with clear astronomically-forced variations in carbonate content, primarily because large parts of the oceans were starved of carbonate during the Eocene greenhouse. International Ocean Discovery Program (IODP) Expedition 369 cored a carbonate-rich sedimentary sequence of Eocene age in the Mentelle Basin (Site U1514, offshore southwest Australia). The sequence consists of nannofossil chalk and exhibits rhythmic clay content variability. Here, we show that IODP Site U1514 allows for the extraction of an astronomical signal and the construction of an Eocene astrochronology, using 3-cm resolution X-Ray fluorescence (XRF) core scans. The XRF-derived ratio between calcium and iron content (Ca/Fe) tracks the lithologic variability and serves as the basis for our U1514 astrochronology. We present a 16 million-year-long (40-56 Ma) nearly continuous history of Eocene sedimentation with variations paced by eccentricity and obliquity. We supplement the high-resolution XRF data with low-resolution bulk carbon and oxygen isotopes, recording the long-term cooling trend from the Paleocene-Eocene Thermal Maximum (PETM – ca. 56 Ma) into the middle Eocene (ca. 40 Ma). Our early Eocene astrochronology corroborates existing chronologies based on deep-sea sites and Italian land sections. For the middle Eocene, the sedimentological record at U1514 provides a single-site geochemical backbone and thus offers a further step towards a fully integrated Cenozoic geologic time scale at orbital resolution.

Testing the occurrence of Late Jurassic true polar wander using the La Negra volcanics of northern Chile

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Roger R. Fu, Dennis V. Kent, Sidney R. Hemming, Pablo Gutiérrez, Jessica R. Creveling

Abstract

True polar wander (TPW) is the reorientation of the crust-mantle system driven by the redistribution of masses in the mantle and on the Earth's surface. In the ideal case, characterization of TPW requires paleomagnetic constraints on the motion of all major plates and independent reconstructions of relative plate positions. While such complete datasets are absent for pre-Mesozoic TPW inferences due to the absence of oceanic plates, they are available for the Late Jurassic (165-145 Ma) “monster shift”, a ∼30° amplitude proposed TPW event. Here we perform paleomagnetic sampling and Ar-Ar geochronology on the La Negra volcanics of Northern Chile, producing two new paleomagnetic poles with ages 165.8 ± 1.8 Ma (1σ; 84.3°N 0.9°E; α95=7.6°; N=28) and 152.8 ± 0.8 Ma (84.5°N 256.4°E; α95=10.8°; N=18). By combining these data with other recently published results, we compute a net lithospheric rotation of 25.3° ± 7.3° (1σ) at a mean rate of 1.21° ± 0.35° My−1 between 170 and 145 Ma with a peak rate of 1.46° ± 0.65° My−1 between 160 and 145 Ma. These rates are consistent with inferences from the Pacific Plate, implying true whole lithosphere rotation. Given coherent motion involving the entire lithosphere, we conclude that the Earth underwent rapid TPW between approximately 165 and 145 Ma, potentially driven by the cessation of subduction along the western North American margin.

Dy, Er, and Yb isotope compositions of meteorites and their components: Constraints on presolar carriers of the rare earth elements

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Quinn R. Shollenberger, Gregory A. Brennecka

Abstract

One way to study the original building blocks of the Solar System is to investigate primitive meteorites and their components. Specifically, isolating these meteorites' individual components via sequential acid leaching can reveal isotopically diverse material present in the early Solar System, which can provide new insights into the mixing and transport processes that eventually led to planet formation. Such isotopic differences in the components are likely to be found in heavy rare earth elements, such as dysprosium (Dy), erbium (Er), and ytterbium (Yb), because their isotopes have different nucleosynthetic production pathways and the elements have significant differences in volatility; however, these specific elements have yet to be thoroughly investigated in the field of cosmochemistry. As such, we present the first combined Dy, Er, and Yb isotope compositions of sequential acid leachates from the Murchison meteorite, along with multiple bulk meteorites from different taxonomic classes. This work also presents a new method to separate, purify, and accurately measure Dy isotopes. Here we show that resolved Dy, Er, and Yb isotope variations in most bulk meteorites are due to neutron capture processes. However, Dy and Er isotopic compositions of bulk Murchison and Murchison leachates stem from the additions or depletions of a nucleosynthetic component formed by the s-process, most likely mainstream silicon carbide (SiC) grains. In contrast, the Yb isotope compositions of bulk Murchison and Murchison leachates display either unresolved or relatively small isotope anomalies. The disparate isotopic behavior between Dy-Er and Yb likely reflects their differing volatilities, with Dy and Er condensing/incorporating into the mainstream SiC grains, whereas the less refractory Yb remains in the gas phase during SiC formation. This work suggests that Yb is hosted in a non-SiC presolar carrier phase and, furthermore, that mainstream SiC grains may be the primary source of isotopic variation in bulk meteorites.

Kinetic carbon isotope fractionation links graphite and diamond precipitation to reduced fluid sources

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Nico Kueter, Max W. Schmidt, Marvin D. Lilley, Stefano M. Bernasconi

Abstract

At high temperatures, isotope partitioning is often assumed to proceed under equilibrium and trends in the carbon isotope composition within graphite and diamond are used to deduce the redox state of their fluid source. However, kinetic isotope fractionation modifies fluid- or melt-precipitated mineral compositions when growth rates exceed rates of diffusive mixing. As carbon self-diffusion in graphite and diamond is exceptionally slow, this fractionation should be preserved. We have hence performed time series experiments that precipitate graphitic carbon through progressive oxidization of an initially CH4-dominated fluid. Stearic acid was thermally decomposed at 800 °C and 2 kbar, yielding a reduced COH-fluid together with elemental carbon. Progressive hydrogen loss from the capsule caused CH4 to dissociate with time and elemental carbon to continuously precipitate. The newly formed C0, aggregating in globules, is constantly depleted by −6.2±0.3‰ in 13C relative to the methane, which defines a temperature dependent kinetic graphite-methane 13C/12C fractionation factor. Equilibrium fractionation would instead yield graphite heavier than the methane. In dynamic environments, kinetic isotope fractionation may control the carbon isotope composition of graphite or diamond, and, extended to nitrogen, could explain the positive correlation of δ13C and δ15N sometimes observed in coherent diamond growth zones. 13C enrichment trends in diamonds are then consistent with reduced deep fluids oxidizing upon their rise into the subcontinental lithosphere, methane constituting the main source of carbon.

Water controls the seasonal rhythm of rock glacier flow

Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): A. Cicoira, J. Beutel, J. Faillettaz, A. Vieli

Abstract

Rock glaciers are creeping periglacial landforms experiencing strong acceleration during recent atmospheric warming and raising concerns with regard to their future behaviour and stability. High resolution kinematic observations show strong seasonal and multi-annual variations in rock glacier creep, but the linking mechanisms to environmental forcing remain poorly understood and lack quantitative models. Here we investigate the interaction between rock glacier creep and climatic forcing - temperature and precipitation - by developing a novel conceptual and numerical modelling approach. The model is constrained and the results are compared with data from the Dirru Rock Glacier (Vallis - CH). We are able to reproduce the observed velocity variations both in magnitude and phase on seasonal and inter-annual time scales. We find that water from liquid precipitation and snow melt, rather than air temperature, is the main driver of variations in rock glacier creep. Our results imply that the influence of water on rock glacier creep is fundamental and must be considered when investigating the historic and future evolution of rock glaciers.

Triple oxygen isotope investigation of fine-grained sediments from major world's rivers: Insights into weathering processes and global fluxes into the hydrosphere

Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Ilya N. Bindeman, Germain Bayon, James Palandri

Abstract

Continental weathering is accompanied by formation of clays and other secondary minerals and their δ18O and Δ17O values should hence reflect to some extent signatures of meteoric water (δ18OMW) and mean annual temperatures (MAT). Our ability to extract climate information from weathered products across the geologic history relies on analytical methods tested and calibrated against modern climate conditions. We here present triple-oxygen isotope analyses of clay-size sediments from 45 rivers worldwide, as well as δ18O analyses of corresponding silt- and sand-rich detrital fractions, which altogether cover about 25% of the continental area that drained into the oceans, extending from the tropics to polar regions. The majority of studied clays closely approximate weathering products, always having high-δ18O signatures regardless of the bedrock type, and in equilibrium with local meteoric waters. Silts are only ∼1.9‰ lighter on average due to greater detrital dilution. Overall, bulk clays from across different climatic regions do not vary much isotopically; an observation which we attribute to opposing effects of temperature on clay-water fractionation and hydrologic relationship between temperature and δ18OMW. Mathematical inversion of measured clay δ18O and Δ17O values (corrected for detrital contribution) into MAT and δ18OMW, compiled for each studied watershed, returns satisfactory estimates. Globally, triple O isotopes in clays appear to be water-dominated, being controlled almost exclusively by δ18OMW at respective temperature of weathering, with minor effects related to evaporation. Using sand from rivers, correlation of δ18O silts with detrital proportions, and estimated surface outcrop of different rock types, we additionally arrive at a +11.5‰ estimate for the exposed silicate crust undergoing weathering.

Globally-averaged, sediment-flux weighted clay δ18O and Δ17O values are +14.80‰ and −0.164‰, respectively. These values are significantly skewed toward O isotope signatures for the southeast Asia and western Pacific regions, characterized by very high sediment fluxes to the ocean. Using both clay- and silt-size fractions, the total weighted silicate weathering δ18O signature exported to the world's ocean is −2.59‰, almost 50% higher the previous estimate, yielding an ice-free world hydrosphere estimate of −0.78‰.

Overall, the modern river clays represent a snapshot of modern weathering conditions on continents, and associated first-order climatic signatures related to MAT and δ18O of the hydrosphere. This implies that measured increase in δ18O and stepwise decrease in Δ17O in shales in the geologic record capture: evolving global hydrologic cycle upon continental emergence, decrease in global MAT or diagenetic conditions, and decreasing ocean mass via rehydrating of the mantle by subduction of hydrated low-δ18O, high-Δ17O slabs.

Taphonomic bias in exceptionally preserved biotas

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Farid Saleh, Jonathan B. Antcliffe, Bertrand Lefebvre, Bernard Pittet, Lukáš Laibl, Francesc Perez Peris, Lorenzo Lustri, Pierre Gueriau, Allison C. Daley

Abstract

Exceptionally preserved fossil biotas provide crucial data on early animal evolution. Fossil anatomy allows for reconstruction of the animal stem lineages, informing the stepwise process of crown group character acquisition. However, a confounding factor to these evolutionary analyses is information loss during fossil formation. Here we identify that the Ordovician Fezouata Shale has a clear taphonomic difference when compared to the Cambrian Burgess Shale and Chengjiang Biota. In the Fezouata Shale, soft cellular structures are most commonly associated with partially mineralized and sclerotized tissues, which may be protecting the soft tissue. Also, entirely soft non-cuticularized organisms are absent from the Fezouata Shale. Conversely, the Cambrian sites commonly preserve entirely soft cellular bodies and a higher diversity of tissue types per genus. The Burgess and Chengjiang biotas are remarkably similar, preserving near identical proportions of average tissue types per genus. However, the Burgess shale has almost double the proportion of genera that are entirely soft as compared to the Chengjiang Biota, indicating that the classic Burgess Shale was the acme for soft tissue preservation. Constraining these biases aids the differentiation of evolutionary and taphonomic absences, which is vital to incorporating anatomical data into a coherent framework of character acquisition during the earliest evolution of animals.

How to make porphyry copper deposits

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Cin-Ty A. Lee, Ming Tang

Abstract

Much of the world's economic copper resources are hosted in porphyry copper deposits (PCDs), shallow level magmatic intrusions associated mostly with thick (>45km) magmatic arcs, such as mature island arcs and continental arcs. However, a well-known, but unresolved paradox, is that arc magmas traversing thick crust, particularly in continental arcs, are generally depleted in Cu whereas in island arcs, where PCDs are less common, magmas become enriched in Cu. Here, we show that the formation of PCDs requires a complex sequence of intra-crustal magmatic processes, from the lower crust to the upper crust. PCDs form when the crust becomes thick (>45km) enough to crystallize garnet. Garnet fractionation depletes Fe from the magma, which drives sulfide segregation and removal of most of the magma's Cu into the lower crust, leaving only small amounts of Cu in the residual magma to make PCDs. However, because garnet is depleted in ferric iron, the remaining Fe in the magma becomes progressively oxidized, which eventually oxidizes sulfide to sulfate, thereby releasing sulfide bound Cu from the magma into solution. This auto-oxidation of the magma, made possible by deep-seated garnet fractionation, increases the ability of endogenic magmatic fluids to self-scavenge Cu from large volumes of otherwise Cu-poor magmas and then transport and concentrate Cu to the tops of magmatic bodies. Examination of the occurrence of PCDs in the central Andes shows that ore formation occurs when continental arcs reach their maximum thickness (>60km), just before the termination of magmatism.

Graphical abstract

In continental arcs, where oceanic lithosphere subducts beneath continental lithosphere, intracrustal magmatic differentiation likely plays a dominant role in controlling the composition of magmas. Of particular interest are the occurrence of Cu-porphyry systems, which are typically found late in the “life-cycle” of an arc when arc crustal thickness is the greatest. During this time, deep-seated fractionation of garnet-rich cumulates, known as arclogites, results in oxidation of the residual magma, increasing the scavenging power of late stage magmatic fluids and the possibility of making Cu-porphyries.

Control of fluid pressures on the formation of listric normal faults

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): X.P. Yuan, Y.M. Leroy, B. Maillot

Abstract

Listric normal faults are widespread in the extension of the upper crust. Despite major advances in understanding the formation of listric faults through various experiments, the mechanical conditions that allow their formation are highly debated. In particular, Anderson's faulting theory predicts that newly formed normal faults are planar and are dipping at least at 45∘, and in practice, at 60–65∘ for most rock types. Here, we develop Limit Analysis to investigate the formation of a listric fault at the onset of slip linking a deep detachment to the topographic surface. We find that listric normal faults can occur in the brittle upper crust without appealing to viscous or ductile behaviours, nor to flexural stresses. The disequilibrium-compaction fluid pressures, typically observed in many sedimentary basins, are essential for the formation of listric faults. The fluid pressure is hydrostatic down to the fluid-retention depth ZFRD and sustains a higher gradient below this depth. Parametric studies show that the surface slope is also essential for the formation of listric faults even with a gently dipping surface slope (≤4∘), whereas a flat topographic surface leads to a simple Andersonian geometry at the onset of slip. The method is applied to two field examples in order to determine fluid overpressures that best match the fault shapes interpreted from seismic data. For the offshore Niger Delta, the simulated normal faults match the observed listric faults with a very shallow ZFRD = 0.5–0.75 km, and below ZFRD the fluid pressure has a lithostatic gradient, consistent with the observed fluid-pressure profiles. To reproduce a series of listric faults joining on one common low-angle detachment in the NW Gulf of Mexico, we demonstrate that a shallow ZFRD = 0.7–1.1 km is required, below which the fluid pressure increases to the lithostatic pressure on the detachment, in agreement with the fluid-pressure observations.

Late Cretaceous (99-69 Ma) basaltic intraplate volcanism on and around Zealandia: Tracing upper mantle geodynamics from Hikurangi Plateau collision to Gondwana breakup and beyond

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): K. Hoernle, C. Timm, F. Hauff, V. Tappenden, R. Werner, E.M. Jolis, N. Mortimer, S. Weaver, F. Riefstahl, K. Gohl

Abstract

Margins resulting from continental breakup are generally classified as volcanic (related to flood basalt volcanism from a starting plume head) or non-volcanic (caused by tectonic processes), but many margins (breakups) may actually be hybrids caused by a combination of volcanic and tectonic processes. It has been postulated that the collision of the Hikurangi Plateau with the Gondwana margin ∼110 Ma ago caused subduction to cease, followed by large-scale extension and ultimately breakoff of the Zealandia micro-continent from West Antarctica through seafloor spreading which started at ∼85 Ma. Here we report new geochemical (major and trace element and Sr-Nd-Pb-Hf isotope) data for Late Cretaceous (99-69 Ma) volcanism from Zealandia, which include the calc-alkalic, subduction-related Mount Somers (99-96 Ma) and four intraplate igneous provinces: 1) Hikurangi Seamount Province (99-88 Ma), 2) Marlborough Igneous Province (98-94 Ma), 3) Westland Igneous Province (92-69 Ma), and 4) Eastern Chatham Igneous Province (86-79 Ma). Each of the intraplate provinces forms mixing arrays on incompatible-element and isotope ratio plots between HIMU (requiring long-term high μ=238U/204Pb) and either a depleted (MORB-source) upper mantle (DM) component or enriched continental (EM) type component (located in the crust and/or upper mantle) or a mixture of both. St. Helena end member HIMU could be the common component in all four provinces. Considering the uniformity in composition of the HIMU end member despite the type of lithosphere (continental, oceanic, oceanic plateau) beneath the igneous provinces, we attribute this component to a sublithospheric source, located beneath all volcanic provinces, and thus most likely a mantle plume. We propose that the plume material rose beneath the active Gondwana margin and flowed along the subducting lithosphere beneath the Hikurangi Plateau and neighboring seafloor and through slab tears/windows beneath the Gondwana (later to become Zealandia) continental lithosphere. We conclude that both plateau collision, resulting in subduction cessation, and the opening of slab tears/windows, allowing hot asthenosphere and/or plume material to upwell to shallow depths, were important in causing the breakup of Zealandia from West Antarctica. Combined tectonic-volcanic processes are also likely to be responsible for causing breakup and the formation of other hybrid type margins.

Indian monsoon precipitation isotopes linked with high level cloud cover at local and regional scales

Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Di Wang, Lide Tian, Zhongyin Cai, Lili Shao, Xiaoyu Guo, Ran Tian, Yike Li, Yiliang Chen, Chuan Yuan

Abstract

Precipitation stable isotopes preserve historic changes of evaporation in the source regions and precipitation processes, therefore, they can be used to reveal regional hydrological cycle dynamics and paleoclimate reconstructions. In monsoon regions, strong inverse impacts of convection on precipitation isotope ratios, have created a debate regarding the interpretation of isotope records as local climate proxies. The proportions of stratiform to convective precipitation on water isotopes, together with the influence mechanisms on seasonal and interannual scales remain highly uncertain. To further address the influence of precipitation patterns on water isotopes, we used 10 yrs of precipitation isotope data from the southern Tibetan Plateau (TP) to explore the effects of large-scale cloud cover and local climate on precipitation isotopes. Correlation analysis performed between local precipitation δ18O values and different level cloud data, indicated significant negative correlations between precipitation isotopes and high level cloud cover on both seasonal and interannual time scales. This result suggests that high-level convection in the upper moisture transport stream is a main control on precipitation isotopes in the southern TP. The clear and coherent variations of precipitation isotopes with the Southern Oscillation Index and outgoing longwave radiation confirmed that strong convection activity in the moisture source region and during transport significantly depleted heavy isotopes in vapor, producing substantially decreased precipitation δ18O in the study region. These results agree with earlier findings of tree ring cellulose isotope records that correlate with cloud cover, but we emphasized the important role of larger-scale regional cloud cover. We also delineated different maximum correlation zones for seasonal and interannual time scales, likely due to different mechanisms. These findings further improve the interpretation of paleoisotope records from the Indian summer monsoon region.

Astronomically forced climate evolution in a saline lake record of the middle Eocene to Oligocene, Jianghan Basin, China

Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Chunju Huang, Linda Hinnov

Abstract

Following the early Eocene climatic optimum some 50 million years ago, the marine paleoclimate record indicates that a long interval of global cooling took place, punctuated by a series of short-term warming reversals and culminating with the glaciation of Antarctica at the Eocene-Oligocene transition. We investigate a saline lake record from the Qianjiang and Jinghezhen formations, Jianghan Basin, China for the corresponding continental climate response. A 20.25 Myr long astronomical time scale is constructed based on 405-kyr orbital eccentricity cycle tuning of gamma ray (GR) series measured from the Qianjiang and Jinghezhen formations. The halite-rich interval between 41-40.4 Ma correlates well with the Middle Eocene Climate Optimum recorded in the deep-sea δ18O record, confirming predominantly halite deposition during the warming. Silt-mudstone-rich intervals were deposited during ∼2.4 Myr orbital eccentricity minima. The evidence indicates a history of alternating fresh water (humid/cool) and saline water (dry/hot) lake cycles paced by ∼100 kyr orbital eccentricity cycles. Analysis of GR series from the deepest part of the lake indicates strong astronomical forcing of halite deposition throughout the Middle-Late Eocene, and a shift to siliciclastic deposition and cooler climates at ∼33.9 Ma. Early Oligocene wet/cool climate in the Jianghan Basin signals East Asian summer monsoon intensification, and a reduction from northwest winter monsoon in response to Tibetan Plateau uplift. Northern Hemisphere summer insolation minima in the Middle-Late Eocene with temperatures that are 4.6 °C higher than present may provide an analogy for near-future climate change in Jianghan Basin.

Graphical abstract

Importance of permeability and deep channel network on the distribution of melt, fractionation of REE in abyssal peridotites, and U-series disequilibria in basalts beneath mid-ocean ridges: A numerical study using a 2D double-porosity model

Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Boda Liu, Yan Liang

Abstract

A plethora of observations have been made at mid-ocean ridges and mantle sections of ophiolites: presence of tabular replacive dunites, highly depleted LREE in residual abyssal peridotites, U-series disequilibria in fresh basalts, crustal thickness, and highly attenuated seismic and magnetotelluric structures beneath spreading centers. These independent observations can become more powerful if they are jointly analyzed in a self-consistent model. The difficulty for such a model is to resolve and evaluate the effect of fine scale petrologic feature such as dunite channels in a tectonic scale geodynamic model. Here we present a two-dimensional double-porosity ridge model that consists of low-porosity residual lherzolite and harzburgite matrix and high-porosity interconnected channel network. The geodynamic simulation features a spatially distributed channel network and anisotropic permeability that gradually develops as the upwelling mantle is deformed. We compute the porosity, melt and solid flow fields for several choices of channel distribution and permeability model. We use the calculated porosity distribution and velocity fields to model the variations of REE in residual mantle and U-series disequilibria in melts.

The present study underscores the importance of deep channel networks and permeability model to the interpretation of first order geophysical and geochemical observations at mid-ocean ridge spreading centers. The anisotropic permeability of channels can enhance melt focusing by 60%, resulting in thicker crust. The attenuated seismic and magnetotelluric structures require a channel network starting from 60 km depth beneath the ridge axis. The depleted REE patterns in clinopyroxenes in residual abyssal peridotites are also consistent with melt extraction into channels starting from 60 km depth. Although deep channels are conducive to producing U-series disequilibria in eruptible melts, the present model still cannot explain the full ranges of the observed U series disequilibria data in MORB samples. Additional factors, processes, and models are discussed. And finally, we found that, within the uncertainty of the permeability, the porosity varies by three folds and the excess of 230Th varies by up to two folds.

Graphical abstract

Synchrotron X-ray imaging in 4D: Multiscale failure and compaction localization in triaxially compressed porous limestone

Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Lingcao Huang, Patrick Baud, Benoit Cordonnier, François Renard, Lin Liu, Teng-fong Wong

Abstract

Understanding failure and strain localization in porous rock is of fundamental importance in rock physics. Confined compaction experiments on porous rocks have revealed a broad spectrum of failure modes. Techniques such as acoustic emission location and velocity tomography provide kinematic information on the partitioning of damage and localization of strain. Complementary observations on deformed samples using microscopy and microcomputed tomography (μCT) can also be used to image microscale damage and its distribution. Only by synthesizing such measurements on multiple scales could one infer the multiscale dynamics of compaction localization and similar rock failure phenomena. Located at the European Synchrotron Radiation Facility, the HADES rig allows direct in situ 3D imaging of the whole rock sample as it is triaxially compressed. The μCT data provide an integrated perspective of the spatiotemporal evolution of damage and strain localization on scales ranging from grain to continuum. We conducted an experiment on Leitha limestone (initial porosity of ∼22%) at a confining pressure of 20 MPa. With increasing differential stress, the sample strain hardened and two distinct yield points were identified in the stress-strain curve. The spatiotemporal evolution of local porosity and damage were analyzed at multiple scales. At a mesoscopic scale of 10 voxels (65 μm), the time-lapse μCT images reveal the strain partitioning associated with the first yield point and development of strain localization with the second. The latter development of five discrete compaction bands is the first unambiguous observation of such a bifurcation phenomenon in a porous carbonate rock, with geometric attributes comparable to compactions bands observed in porous sandstones. The μCT data on the voxel-scale elucidate in refined details the nucleation and propagation of discrete compaction bands under quasi-static loading, as well as the micromechanical processes, which in the past could only be inferred from a synthesis of kinematic observations of acoustic emissions activity and post-mortem observations of microstructure and damage.

An essential role for sulfur in sulfide-silicate melt partitioning of gold and magmatic gold transport at subduction settings

Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Yuan Li, Lu Feng, Ekaterina S. Kiseeva, Zenghao Gao, Haihao Guo, Zhixue Du, Fangyue Wang, Lanlan Shi

Abstract

Sulfide-silicate melt partitioning controls the behavior of gold in magmas, which is critical for understanding the Earth's deep gold cycle and formation of gold deposits. However, the mechanisms that control the sulfide-silicate melt partitioning of gold remain largely unknown. Here we present constraints from laboratory experiments on the partition coefficient of gold between monosulfide-solid-solution (MSS) and silicate melt (DAuMSS/SM) under conditions relevant for magmatism at subduction settings. Thirty-five experiments were performed in Au capsules to determine DAuMSS/SM at 950-1050 °C, 0.5-3 GPa, oxygen fugacity (fO2) of ∼FMQ-1.7 to FMQ+2.7 (FMQ refers to the fayalite-magnetite-quartz buffer), and sulfur fugacity (fS2) of −2.2 to 2.1, using a piston cylinder apparatus. The silicate melt composition changes from dry to hydrous andesite to rhyolite. The results obtained from electron microprobe and laser-ablation ICP-MS analyses show that the gold solubility in silicate melts ranges from 0.01 to 55.3 ppm and is strongly correlated with the melt sulfur content [S]melt at fO2 of ∼FMQ-1.7 to FMQ+1.6, which can be explained by the formation of complex Au-S species in the silicate melts. The gold solubility in MSS ranges from 130 to 2800 ppm, which is mainly controlled by fS2. DAuMSS/SM ranges from 10 to 14000 at fO2 of ∼FMQ-1.7 to FMQ+1.6, the large variation of which can be fully explained by combined [S]melt and fS2. Therefore, all of the parameters that can directly affect [S]melt and fS2, such as alkali metals, water, FeO, and fO2, can indirectly affect DAuMSS/SM. The mechanisms that control the sulfide-silicate melt partitioning of gold and the other chalcophile elements, such as Ni, Re, and Mo, differ significantly. This is because gold is dissolved mainly as Au-S species in the silicate melts, while the other chalcophile elements are dissolved mainly as metal oxides in the silicate melts. Applying the correlation between DAuMSS/SM and [S]melt to slab melting and arc magmatic differentiation under different redox conditions, we find that ancient to modern slab melts carry negligible to less than 25% of the slab gold to the subarc mantle; however, gold-enrichment can occur in MSS-saturated arc magmas that have differentiated under moderately oxidized conditions with fO2 between FMQ and FMQ+1.6, in particular if the magmatic crystallization follows a fractional crystallization model. We conclude that moderately oxidized magmas with high contents of alkali metals, sulfur, and water, owing to their low DAuMSS/SM and efficient magma-to-fluid transfer of gold and sulfur, have a high potential to form gold deposits.

Theme by Danetsoft and Danang Probo Sayekti inspired by Maksimer