The onset of modern central Asian atmospheric circulation is traditionally linked to the interplay of surface uplift of the Mongolian and Tibetan-Himalayan orogens, retreat of the Paratethys sea from ...central Asia and Cenozoic global cooling. Although the role of these players has not yet been unravelled, the vast dust deposits of central China support the presence of arid conditions and modern atmospheric pathways for the last 25 million years (Myr). Here, we present provenance data from older (42-33 Myr) dust deposits, at a time when the Tibetan Plateau was less developed, the Paratethys sea still present in central Asia and atmospheric pCO2 much higher. Our results show that dust sources and near-surface atmospheric circulation have changed little since at least 42 Myr. Our findings indicate that the locus of central Asian high pressures and concurrent aridity is a resilient feature only modulated by mountain building, global cooling and sea retreat.
Resources for foreign aid come under attack when parties that care little for international affairs come to power. Internationally focused parties of the left and right, however, prefer to use aid as ...a tool to pursue their foreign policy goals. Yet varying goals based on left-right ideology differentiate the way donors use foreign aid. We leverage sector aid to test hypotheses from our Partisan Theory of Aid Allocation and find support for the idea that domestic political preferences affect foreign aid behavior. Left-internationalist governments increase disaster aid, while parochial counterparts cut spending on budget assistance and aid that bolsters recipients' trade viability. Conservative governments favor trade-boosting aid. We find consistent, nuanced, evidence for our perspective from a series of Error Correction Models (ECMs) and extensive robustness checks. By connecting theories of foreign aid to domestic politics, our approach links prominent, but often disconnected, fields of political research and raises important questions for policymakers interested in furthering the efficacy of development aid.
The number of subduction zones that facilitated the northward translation of the Anatolide‐Tauride continental terrane derived from Gondwana to the southern margin of Eurasia at the longitude of ...western Turkey is debated. We hypothesized that if two north dipping subduction zones facilitated incipient collision in western Turkey, a late Cretaceous arc would have formed within the Neotethys and along the southern margin of Eurasia. To determine if an island arc formed within the Neotethys we investigated the sedimentary record of the Central Sakarya basin, which was deposited along the southern margin of Eurasia from 85 to 45 million years ago. Detrital zircon deposited within the lower levels of the Central Sakarya basin (the Değirmenözü Formation) are associated with south to north‐directed paleocurrents and exhibit a unimodal late Cretaceous age peak sourced from isotopically juvenile mantle melts. Zircon maximum depositional ages from the Değirmenözü Formation cluster between 95 and 90 Ma and are 5–10 Myr older than biostratigraphic depositional ages. Between 95 and 80 Ma, a 12‐unit shift from mantle to crustal derived εHf values occurs in the overlying Yenipazar Formation. We explain the absence of Paleozoic, Eurasian‐sourced detrital zircon, the rapid shift from mantle to crustal derived εHf values, and lag time in terms of passive margin subduction within an isolated intra‐oceanic subduction zone, whose island arc was reworked from south to north into the Central Sakarya basin during incipient collision. Thus, widely outcropping late Cretaceous plutonic rocks within Eurasia must have belonged to an additional convergent margin.
Key Points
We leveraged the isotopic signature of subducted passive margin to test incipient collision models in northwest Turkey
Passive margin subduction resulted in the development of an isolated intra‐oceanic basin
Two north dipping subduction zones facilitated incipient collision in the region
Pedogenic carbonate is widespread at mid latitudes where warm and dry conditions favor soil carbonate growth from spring to fall. The mechanisms and timing of pedogenic carbonate formation are more ...ambiguous in the tropical domain, where long periods of soil water saturation and high soil respiration enhance calcite dissolution. This paper provides stable carbon, oxygen and clumped isotope values from Quaternary and Miocene pedogenic carbonates in the tropical domain of Myanmar, in areas characterized by warm (>18°C) winters and annual rainfall up to 1,700 mm. We show that carbonate growth in Myanmar is delayed to the driest and coldest months of the year by sustained monsoonal rainfall from mid spring to late fall. The range of isotopic variability in Quaternary pedogenic carbonates can be solely explained by temporal changes of carbonate growth within the dry season, from winter to early spring. We propose that high soil moisture year‐round in the tropical domain narrows carbonate growth to the driest months and makes it particularly sensitive to the seasonal distribution of rainfall. This sensitivity is also enabled by high winter temperatures, allowing carbonate growth to occur outside the warmest months of the year. This high sensitivity is expected to be more prominent in the geological record during times with higher temperatures and greater expansion of the tropical realm. Clumped isotope temperatures, δ13C and δ18O values of tropical pedogenic carbonates are impacted by changes of both rainfall seasonality and surface temperatures; this sensitivity can potentially be used to track past tropical rainfall distribution.
Plain Language Summary
Soil carbonates are the focus of many continental paleoenvironmental studies because their isotopic composition records many features of the local environment (such as the type and density of vegetation, annual or warm season temperatures, and aridity). Soil carbonates are commonly studied in temperate and arid areas; in those environments, carbonates form during warm months when soils dry. Soil carbonates are rarer but present in the tropical domain, where their isotopic systematics and formation processes have been barely studied. This study provides stable isotopic data from soil carbonates in the tropical monsoonal domain of Myanmar, which is characterized by warm (>18°C), dry winters and abundant summer rainfall. We show that these soil carbonates grow during the coldest months of the year and follow different dynamics and isotope systematics than those of temperate and arid areas. We show that high soil wetness and warm temperatures year‐round make carbonate growth particularly sensitive to the seasonal distribution of rainfall in the tropical domain. This seasonal sensitivity complicates the interpretation of soil isotopic data from past tropical ecosystems. We suggest that isotopic data from tropical paleoenvironments can be used as a proxy to reconstruct past rainfall distribution instead of average (or summer) environmental features.
Key Points
Tropical soil carbonates under monsoonal rainfall grow in winter and early spring
The cold‐season bias in carbonate growth is promoted by warm (>15°C) winter temperatures and by high soil water content in summer and fall
Clumped isotope temperatures in tropical paleosols are likely impacted by changes of rainfall distribution
The strong present-day Asian monsoons are thought to have originated between 25 and 22 million years (Myr) ago, driven by Tibetan-Himalayan uplift. However, the existence of older Asian monsoons and ...their response to enhanced greenhouse conditions such as those in the Eocene period (55-34 Myr ago) are unknown because of the paucity of well-dated records. Here we show late Eocene climate records revealing marked monsoon-like patterns in rainfall and wind south and north of the Tibetan-Himalayan orogen. This is indicated by low oxygen isotope values with strong seasonality in gastropod shells and mammal teeth from Myanmar, and by aeolian dust deposition in northwest China. Our climate simulations support modern-like Eocene monsoonal rainfall and show that a reinforced hydrological cycle responding to enhanced greenhouse conditions counterbalanced the negative effect of lower Tibetan relief on precipitation. These strong monsoons later weakened with the global shift to icehouse conditions 34 Myr ago.
Aims. The Sun shows strong variability in its magnetic activity, from Grand minima to Grand maxima, but the nature of the variability is not fully understood, mostly because of the insufficient ...length of the directly observed solar activity records and of uncertainties related to long-term reconstructions. Here we present a new adjustment-free reconstruction of solar activity over three millennia and study its different modes. Methods. We present a new adjustment-free, physical reconstruction of solar activity over the past three millennia, using the latest verified carbon cycle, 14C production, and archeomagnetic field models. This great improvement allowed us to study different modes of solar activity at an unprecedented level of details. Results. The distribution of solar activity is clearly bi-modal, implying the existence of distinct modes of activity. The main regular activity mode corresponds to moderate activity that varies in a relatively narrow band between sunspot numbers 20 and 67. The existence of a separate Grand minimum mode with reduced solar activity, which cannot be explained by random fluctuations of the regular mode, is confirmed at a high confidence level. The possible existence of a separate Grand maximum mode is also suggested, but the statistics is too low to reach a confident conclusion. Conclusions. The Sun is shown to operate in distinct modes – a main general mode, a Grand minimum mode corresponding to an inactive Sun, and a possible Grand maximum mode corresponding to an unusually active Sun. These results provide important constraints for both dynamo models of Sun-like stars and investigations of possible solar influence on Earth’s climate.
•The simulated mean optimum planting date for maize in Iowa, USA corresponds to USDA-NASS 18% planting progress.•The simulated optimum date has advanced by 0.13 days/year from 1980 to 2015.•Climate ...change scenarios affected crop yields much more than the optimum planting dates.•Future investments in planting technologies can buffer climate variability.
Planting date and cultivar selection are major factors in determining the yield potential of any crop and in any region. However, there is a knowledge gap in how climate scenarios affect these choices. To explore this gap, we performed a regional scale analysis (11 planting dates × 8 cultivars × 281 fields × 36 weather years × 6 climate scenarios) using the APSIM model and pSIMS software for Iowa, the leading US maize (Zea mays L.) producing state. Our objectives were to determine how the optimum planting date (optPD) changes with weather scenarios and cultivars and the potential economic implications of planting outside the optimum windows. Results indicated that the mean optPD corresponds to the US Department of Agriculture, National Agriculture Statistics Service (USDA-NASS) 18.4% planting progress (April 28th) in Iowa. The optPD was found to be advancing by –0.13 d yr−1 from 1980 to 2015. A 1 °C increase in mean temperature increased the length of the growing season by 10 days while the optPD changed by –2 to + 6 days, depending on cultivar. Under a more realistic scenario of increasing the minimum temperature by 0.5 °C, decreasing the maximum temperature by 0.5 °C, increasing spring rainfall by 10% and decreasing summer rainfall by 10%, the optPD only changed by –2 days compared to current trends, however, yield increased by 6.6%. Analysis of historical USDA-NASS planting durations indicated that on average, the planting duration (1–99% statewide reported planting progress) is 44 days, while it can be as low as 21 days in years with favorable weather. A simple economic analysis illustrated a potential revenue loss up to $340 million per year by planting maize outside the optimum window. We conclude that future investments in planting technologies to accelerate planting, especially in challenging weather years, as well as improved optPD × cultivar recommendations to farmers, will provide economic benefits and buffer climate variability.
The Leech River Shear Zone (LRSZ) on southern Vancouver Island separates the metasedimentary schists of the Leech River Complex (LRC) from the accreted oceanic plateau of the Siletz‐Crescent terrane. ...The juxtaposition of these rock units suggests a possible origin as a subduction plate boundary but tectonic context of the LRSZ has yet to be fully established. We present field, microstructural, petrological, and geochronological observations that constrain the structural and metamorphic history of LRSZ. The mylonite zone of the LRSZ straddles the lithologic contact between the schists of the LRC and basalts of the Siletz‐Crescent terrane. Foliation orientations, a steeply plunging stretching lineation, and kinematic indicators all suggest reverse‐sinistral motion. Compositions of garnet in the schist and amphibole in the metabasalt record synkinematic growth at temperature and pressure conditions of 550°C–570°C and 450–490 MPa. These metamorphic conditions require elevated geotherms that are consistent with plate models that position the Kula‐Farallon Ridge and Yellowstone Hotspot in the region in the Eocene (∼50 Ma). Detrital zircon U‐Pb age distributions for the Leech River Schist have Paleocene maximum depositional ages and are similar to the Upper Nanaimo Group that unconformably overlies the Wrangellia terrane. Our ages support early Paleocene deposition of the schist in a subduction trench/slope environment followed by underthrusting and underplating. These results establish the exhumed mylonite zone of the LRSZ as a paleo‐plate interface that was active during Eocene subduction.
Key Points
Leech River Shear Zone confirmed as a hot, Eocene‐age paleo‐subduction interface
High‐temperature metamorphism produced by ridge subduction during accretion of Siletz‐Crescent terrane
New detrital zircon ages from the Leech River Schist support near‐trench deposition during Paleocene
Paleogeographic maps are essential tools for understanding Earth system dynamics. They provide boundary conditions for climate and geodynamic modelling, for analysing surface processes and biotic ...interactions. However, the temporal and spatial distribution of key features such as seaways and mountain belts that govern climate changes and biotic interchange differ between various paleogeographies that require regular updates with new data and models. We developed a reproducible and systematic approach to paleogeographic reconstruction and provide a set of worldwide Cenozoic paleogeographic maps at 60, 40 and 20 Ma. We followed a six-stage methodology that integrates an extensive review of geological data into a coherent plate tectonic model using the open source software GPlates. (1) We generated a global plate kinematic model, and reconstructed intensely-deformed plate boundaries using a review of structural, paleomagnetic and other geologic data in six key regions: the Andes, the North American Cordillera, the Scotia Arc, Africa, the Mediterranean region and the Tibetan-Himalayan collision zone. (2) We modified previously published paleobathymetry in several regions where continental and oceanic crust overlap due to differences in the plate models. (3) We then defined paleoshorelines using updated fossil and geologic databases to locate the terrestrial to marine transition. (4) We applied isostatic compensation in polar regions and global eustatic sea level adjustments. (5) Paleoelevations were estimated using a broad range of data including thermochronology and stable isotopes, combined with paleobotanical (mostly pollen and leaf physiognomy), structural and geomorphological data. We address ongoing controversies on the mechanisms and chronology of India-Asia collision by providing alternate reconstructions for each time slice. We finally discuss the implications of our reconstructions on the Cenozoic evolution of continental weatherability and review methodological limitations and potential improvements. Future addition of new data, tools and reconstructions can be accommodated through a dedicated interactive website tool (https://map.paleoenvironment.eu/) that enables users to interactively upload and download data and compare with other models, and generate their own plots. Our aim is to regularly update the models presented here with new data as they become available.
Strain resulting from the collision of India with Asia has caused fundamental changes to Asian drainage patterns, but the timing and nature of these changes are poorly understood. One frequently ...proposed hypothesis involves the connection of the palaeo Tsangpo drainage to a precursor to the Irrawaddy River of central Myanmar in the Palaeogene. To test this hypothesis, we studied the provenance of Palaeogene fluvio-clastic sedimentary rocks that crop out in central Myanmar, namely the Late Middle Eocene-Early Oligocene Pondaung and Yaw Formations. Isotopic analysis on bulk-rock and petrographic data indicate a primary magmatic arc source, and a secondary source composed of recycled, metamorphosed basement material. Although the exact location of both sources is hardly distinguishable because Burmese and Tibetan provinces share common lithological features, the presence of low-grade metamorphic fragments, the heterogeneity in Sr-Nd isotopic values of bulk sediments and westward-directed palaeoflow orientations indicate a proximal source area located on the eastern Asian margin. Central Myanmar was the locus of westward-prograding deltas opening into the Indian Ocean, supplied by the unroofing of an Andean-type cordillera that extended along the Burmese margin. We found no evidence to support a palaeo Tsangpo-Irrawaddy River, at least during the Late Eocene.
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