American Journal of Science最新文献

{"title":"Late Neoarchean magmatic – metamorphic event and crustal stabilization in the North China Craton","authors":"M. Zhai, Lei Zhao, Xiyan Zhu, Yanyan Zhou, P. Peng, Jing-hui Guo, Qiu-li Li, T. Zhao, Junsheng Lu, Xianhua Li","doi":"10.2475/01.2021.06","DOIUrl":"https://doi.org/10.2475/01.2021.06","url":null,"abstract":"The ca. 2.5 Ga as the time boundary between the Archean and the Proterozoic eons is a landmark, indicating the most important continental crust evolving stage of the Earth, that is, the global cratonization or the formation of supercraton(s) that was unseen before and is unrepeated in the following history of the Earth's formation and evolution. The North China Craton (NCC) is one of the best recorders of the ca. 2.5 Ga event, and therefore studies in the thorough understanding of early Precambrian continental evolution are continuous. The period from 2.8 to 2.6 Ga is the major crustal growth period of the NCC and formed seven micro-blocks. All the micro-blocks in the NCC were surrounded by 2.6 to 2.54 Ga greenstone belts. The clear geological presentations are as follows: (1) Archaic basement rocks in North China (various micro-blocks) experienced strong partial melting and migmatization. The granitoid rocks derived from crustal partial melting include potassium, TTG and monzonitic granitoids, which come, respectively, from continental crust (sedimentary rocks with TTG gneisses), juvenile crust (mafic rocks with TTG gneisses) or mixed crust; (2) the BIF-bearing supracrustal rocks are mainly distribute in greenstone belts. The lithologic associations in the greenstone belts within the NCC are broadly similar, belonging to volcano-sedimentary sequences, with common bimodal volcanic rocks (basalt and dacite) interlayered with minor amounts of komatiites in the lower part, and calc-alkalic volcanic rocks (basalt, andesite and felsic rocks) in the upper part; (3) nearly all old rocks of >2.5 Ga underwent ∼2.52 to 2.5 Ga metamorphism of amphibolite–granulite facies. Most metamorphosed rocks show high-temperature-ultra-high-temperature (HT–UHT) characteristics and record anticlockwise P–T paths, albeit a small number of granulites seemingly underwent high-pressure granulite facies metamorphism and record clockwise P–T paths; (4) ∼2.5 Ga mafic dikes (amphibolites), granitic dikes (veins) and syenitic–ultramafic dikes developed across these archaic basements and were strongly deformed or un-deformed; (5) the extensive 2.52 to 2.48 Ga low-grade metamorphic supracrustal covers has been recognized in eastern, northern and central parts of the NCC, which are commonly composed of bi-modal volcanic rocks and sedimentary rocks. The above mentioned ∼2.5 Ga geological rocks and their characters imply that the seven micro-blocks have been united through amalgamation to form the NCC. The metamorphosed late Neoarchean greenstone belts, as syn-formed mobile belts, welded the micro-blocks at the end of the Neoarchean. However, the metamorphic thermal grades of the greenstone belts are lower than those of the high-grade terranes within the micro-blocks, suggesting that the latter might have developed under a higher geothermal gradient than the former. Besides, the greenstone belts surround the various micro-blocks in the late Neoarchean when both the old continental cr","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"206 - 234"},"PeriodicalIF":2.9,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"69322170","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"India in the Nuna to Gondwana supercontinent cycles: Clues from the north Indian and Marwar Blocks","authors":"W. Wang, Peter A. Cawood, M. Pandit","doi":"10.2475/01.2021.02","DOIUrl":"https://doi.org/10.2475/01.2021.02","url":null,"abstract":"Evolution of the Indian Block can be traced through Earth's Phanerozoic and Precambrian supercontinent cycles. The Paleoproterozoic tectonostratigraphic record of the North Indian Block and the Aravalli Delhi Fold Belt in the Nuna supercontinent assembly shows a close link with the events in the Cathaysia Block of South China. Accretion of the two terranes is documented by 1.97 to 1.92 Ga continental arc igneous rocks and 1.91 to 1.81 Ga syn- and post-collisional magmatism in the Lesser Himalaya, along with 1.88 to 1.86 Ga granulite metamorphism in both continental blocks. The connection between the North Indian Block and the Cathaysia Block continued through Nuna dispersal and was followed by the accretion of a series of terranes/microcontinents along the western margin of this united North India-Cathaysia Block during Rodinia assembly (ca. 1.0 Ga). This is recorded by accretion of the Marwar Block to the North Indian Block and Yangtze Block to the Cathaysia Block. Long-lived active continental margins continued along Marwar (NW India), Yangtze, Madagascar and the Seychelles until ca. 720 Ma that jointly occupied a peripheral or even independent paleoposition in the Rodinia reconstructions. The eastern margin of India sutured with the Western Australia-Mawson blocks along the Kunnga Orogen during the final assembly of Gondwana in the early Paleozoic, whereas microcontinental blocks including south Qiangtang and north Lhasa, were accreted to the northern margin of Gondwana in the vicinity of India. The collision of this ensemble of blocks with Africa (western Gondwana) is marked by the East African Orogen/Mozambique Belt, extending through central east Africa, Madagascar, South India and Antarctica. However, further north, India was separated from the Arabian-Nubian Shield by an embayment of the proto-Tethys that remained integral until the breakup of Gondwana. The accretion of Laurussia to Gondwana in the mid-Paleozoic during the assembly of Pangea corresponds with lithospheric extension along the northern margin of India (Gondwana) and separation of several continental blocks including South China, south Qiangtang, and north Lhasa, which then drifted northward across the Paleo-Tethys to collide with the Asian segment of Pangea in the Permo-Triassic.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"83 - 117"},"PeriodicalIF":2.9,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"69322081","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Reconstruction of the original extent of the Tertiary pre-volcanic gravels in the northern Sierra Nevada (CA): Implications for the range's Paleotopography","authors":"C. Tipp, E. Gabet","doi":"10.2475/12.2020.01","DOIUrl":"https://doi.org/10.2475/12.2020.01","url":null,"abstract":"The ancient auriferous gravels that helped spawn the California Gold Rush have figured prominently in investigations of the Cenozoic history of the Sierra Nevada. These fluvial sediments, scattered throughout the northern half of the range, are the remnants of much larger deposits that accumulated throughout the Eocene and Early Oligocene. In this study, we present a reconstruction of the original extent of the gravels developed according to a few simple rules. The ancient auriferous gravels that helped spawn the California Gold Rush have figured prominently in investigations of the Cenozoic history of the Sierra Nevada. These fluvial sediments, scattered throughout the northern half of the range, are the remnants of much larger deposits that accumulated throughout the Eocene and Early Oligocene. In this study, we present a reconstruction of the original extent of the gravels developed according to a few simple rules. This reconstruction suggests that large swaths of the northern Sierra Nevada were once buried under broad alluvial plains, a result consistent with previous work. The reconstruction also supports the hypothesis that the gravels accumulated behind high ridges along the Sierra Nevada foothills, with the Yuba River providing an important outlet. Moreover, gravel deposits on two high peaks indicate that the gravels may have buried the Feather River watershed up to the modern crest of the range. Finally, on the basis of our reconstruction, we estimate that the total volume of the gravels was, at a minimum, ∼200 km3.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"320 1","pages":"815 - 850"},"PeriodicalIF":2.9,"publicationDate":"2020-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42864099","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Zn2+-Pb2+-doped calcite shrub fabrics: Abiotic morphogenesis of travertine-like dripstone encrustation at the Jersey Zinc Mine, southeastern British Columbia","authors":"P. Broughton","doi":"10.2475/12.2020.02","DOIUrl":"https://doi.org/10.2475/12.2020.02","url":null,"abstract":"A cm-thick calcite dripstone on the floor of the abandoned Jersey Zinc Mine in southeastern British Columbia, western Canada, consists of a mixture of shrub and radial fibrous elongated columnar crystal fabrics that resulted from Zn2+ and Pb2+ doping of the calcite crystallographic lattice structure. These heavy metal elements were sourced from sulfide mineralized veins in the overlying limestone beds. Zn2+ and Pb2+ substitution for Ca2+ reduced the size of the calcite crystal structure, impacting configuration of the calcite crystal fabrics. Lower concentration levels of dopants resulted in a radial fibrous elongated columnar crystal fabric with high inter-crystalline porosity. Increased levels of Zn2+ and Pb2+ dopants resulted in a crystal splitting process forming the shrub fabric. Growth of normally flat rhombohedral face on the external surface of the dripstone encrustation was disrupted by mixtures of isotropic and anisotropic growth rates. Inhibited growth rates resulted in early stage crystal splitting processes, forming parallel arrays of precursor crystallites aligned as step-down micro-terraces separated by dam-like micro-barrage partitions with normal growth rates. Shrub fabrics developed below the encrustation surface as ion-laden dripwater films infiltrated the 20% to 25% inter-crystalline porosity. This early stage of crystal splitting transitioned into a shrub fabric substrate, each consisting of fan-shaped crystal arrays distributed along a main stem, all in optical continuity. Extreme levels of Zn2+ dopant resulted in split crystal formation of micro-spherulites on the external surface of the dripstone. Spherulite neomorphism occurred, resulting in replacement of the crystallites by an individual calcite spar during envelopment by the columnar crystal domain fabric. The spheroid forms were preserved within the columnar fabric as spheroid zones of Zn-calcite and outlined by microcrystalline ferroan calcite. The shrub fabrics have morphological similarity to dendritic branching shrubs commonly associated with biotic and abiotic travertine and some biotic tufa deposits, which form under very different depositional conditions. Abiotic Zn2+-Pb2+ dopants absorbed into the calcite crystal structure of this mine floor encrustation precluded the necessity for carbonate precipitation from supersaturated carbonate water or precursor microbial induced nucleation sites, unlike similar travertine and tufa shrub fabrics elsewhere.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"320 1","pages":"851 - 891"},"PeriodicalIF":2.9,"publicationDate":"2020-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44150446","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Constraints on surface temperature 3.4 billion years ago based on triple oxygen isotopes of cherts from the Barberton Greenstone Belt, South Africa, and the problem of sample selection","authors":"D. Lowe, D. Ibarra, N. Drabon, C. Chamberlain","doi":"10.2475/11.2020.02","DOIUrl":"https://doi.org/10.2475/11.2020.02","url":null,"abstract":"ABSTRACT Studies of Earth's surface temperature before 3.0 Ga have focused heavily on the oxygen isotopic composition of silica-rich sedimentary rocks called cherts. Interpretation of the results have suggested early surface temperatures ranging from as high as 70 ± 15 °C down to those that differ little from modern values. A major controversy centers on whether differences in the oxygen isotopic compositions of cherts over time reflect changing surface temperatures, changing ocean isotopic composition, or post-depositional diagenetic and metamorphic effects. We here present results of triple oxygen measurements of 3.472 Ga to 3.239 Ga cherts from the Barberton Greenstone Belt, South Africa. The best preserved samples based on geological evidence have Δ'17O and δ'18O values that plot generally on or near the equilibrium fractionation line for silica precipitated out of modern, ice-free sea water. Geologic considerations allow many potentially useful samples to be eliminated for paleotemperature analysis because of proximity to younger mafic intrusions or interactions with meteoric waters during deposition, both of which tend to lower preserved isotopic values. Our results of triple-O isotopic analyses of a suite of samples representing deposition under open marine, shallow shelf conditions suggest that Archean surface temperatures were well above those of the present day, perhaps as high as 66 to 76 °C. They demonstrate that geologic context, including depositional setting and post-depositional history, requires careful assessment before the significance of oxygen isotopic results can be evaluated.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"320 1","pages":"790 - 814"},"PeriodicalIF":2.9,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45647645","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Determining the origin of inclusions in garnet: Challenges and new diagnostic criteria","authors":"T. Griffiths, G. Habler, R. Abart","doi":"10.2475/11.2020.01","DOIUrl":"https://doi.org/10.2475/11.2020.01","url":null,"abstract":"ABSTRACT Permian metapegmatite garnets from the Koralpe region (Eastern Alps, Austria) contain abundant submicrometer- to micrometer-sized inclusions of rutile, corundum, Fe-Mn phosphate, ilmenite, xenotime, zircon, and apatite. Variations in inclusion abundance, phase assemblage, habit, and size define sector and concentric zones in the garnets, tracing low-indexed garnet facets. Zoning resulted from a process occurring at the garnet-melt interface, homogeneous along each facet, but sensitive to its crystallographic plane. Furthermore, inclusion and host lattices interacted, generating host-inclusion crystallographic orientation relationships (CORs). These phenomena exclude inclusion formation via overgrowth of pre-existing phases, infiltration of fluids/melts, or dissolution-reprecipitation. Magmatic garnet rims contain rutile needles up to 100 μm long, showing an interface-dependent shape-preferred orientation (SPO) that cannot be explained by exsolution models. Furthermore, the COR distribution for needles is unique, and implies large 3D lattice mismatches. These phenomena suggest that needles originated via oriented heterogeneous nucleation at the garnet interface and subsequent simultaneous growth of both phases. The origin of equant inclusions in core domains is less clear. With some assumptions, integrated compositions remain compatible with closed system exsolution or open system precipitation (OSP) involving divalent cation loss. Still, the oriented interface nucleation hypothesis seems to better explain the fact that the frequency of rutile-garnet CORs varies strongly not only between cores and rims but also between garnet core domains. Inclusion formation by oriented interface nucleation and simultaneous growth can explain many observations commonly attributed to exsolution, making distinguishing between these two mechanisms a challenge. We suggest interface-dependence of SPOs and COR frequencies as criteria for identifying inclusion formation via oriented nucleation at an interface and subsequent simultaneous growth.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"320 1","pages":"753 - 789"},"PeriodicalIF":2.9,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49140778","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Zircon U-Pb geochronology and Nd-Pb isotope geochemistry of Blue Ridge basement in the eastern Great Smoky Mountains, U.S.A.: Implications for the Proterozoic tectonic evolution of the southeastern Laurentian margin","authors":"D. Moecher, F. C. Harris, E. A. Larkin, R. J. Quinn, K. B. Walsh, D. F. Loughry, Eric D. Anderson, S. Samson, A. Satkoski, E. Tohver","doi":"10.2475/10.2020.02","DOIUrl":"https://doi.org/10.2475/10.2020.02","url":null,"abstract":"The Mesoproterozoic to Paleozoic history of the southeastern Laurentian margin involved repeated collisional and accretionary tectonomagmatic events that reworked and recycled older continental crust of preceding events. The Great Smoky Mountains Basement Complex (GSMBC) within the southern Appalachian Blue Ridge exposes complexly deformed orthogneiss and paragneiss that preserve a record of Laurentian margin evolution from ca. 1.9 Ga to 450 Ma. The GSMBC consists primarily of: (1) 1.34 to 1.31 Ga (pre-Elzevirian) granodioritic orthogneiss and entrained mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (correlated with pre-Elzevirian crustal components in the Adirondack, Green Mountains, New Jersey Highlands, and French Broad massifs), (2) ca. l.15 to 1.05 Ga augen and granitic orthogneiss produced during Shawinigan and Ottawan phases of Grenville-age magmatism and metamorphism, respectively, and (3) paragneiss derived from protoliths with either Grenville-age (1.1–1.0 Ga) or post-Grenville (Neoproterozoic) depositional ages based on presence/absence of ca. 1.0 Ga metamorphic zircon and 1.9 to 1.1 Ga detrital zircon. All lithologies experienced Taconian metamorphism and variable migmatization. Pre-Ottawan paragneiss exhibits major detrital zircon ages modes at 2.0 to 1.6 and 1.4 to 1.3 Ga that require a component of older Proterozoic crust in the sediment source region. Detrital zircon grains in post-Ottawan paragneiss exhibits the full spectrum of Grenville-age modes that correlate with the five phases of Grenville magmatic/metamorphic events in eastern Laurentia. These paragneiss samples also contain scattered 750 to 600 Ma detrital zircon grains that constrain their maximum depositional age to late Neoproterozoic. The sedimentary protoliths of the latter paragneiss consist largely of detritus from exhumation of all Grenville crustal age components during post-orogenic exhumation and crustal extension leading up to Late Neoproterozoic breakup of Rodinia. Most zircon U-Pb age systematics exhibit variable discordance that can be attributed to disturbance of the U-Pb system and/or new zircon growth during either high-grade Ottawan (ca. 1.04 Ga) or Taconian (ca. 0.46 Ga) regional metamorphism and migmatization. Neodymium TDM model ages for granodioritic orthogneiss and paragneiss range from 1.8 to 1.6 Ga, indicating that the rocks were derived from recycling of Proterozoic crust (that is, they are not juvenile), consistent with the 1.9 to 1.6 Ga detrital zircon grains in pre-Ottawan paragneiss and with 1.8 to 1.7 Ga inherited zircon in some 1.33 Ga orthogneisses and a 1.35 Ga xenolith. Whole rock Pb isotope compositions of GSMBC rocks overlap the field of compositions characteristic of Amazonian crust and of other basement rocks from the south-central Appalachians. The Pb isotopes and geochronology in orthogneiss, mafic xenoliths, and pre-Ottawan paragneiss are consistent with a correlation of the GSMBC with t","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"320 1","pages":"677 - 729"},"PeriodicalIF":2.9,"publicationDate":"2020-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2475/10.2020.02","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45128692","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Redox constraints on a Cenozoic imbalance in the organic carbon cycle","authors":"M. Galvez","doi":"10.2475/10.2020.03","DOIUrl":"https://doi.org/10.2475/10.2020.03","url":null,"abstract":"Over geological timescales, variations in atmospheric O2 are typically attributed to the imbalance between the weathering of organic carbon (OC) and reduced sulfur on land, the major sink terms for atmospheric O2, and the burial of OC and reduced sulfur in marine sediments, the major source terms of O2 to the atmosphere. But the Fe cycle matters too. Using a compilation of C, Fe, S and H fluxes between the Earth's exosphere, continents, and mantle reservoirs, I demonstrate that hydrothermal weathering of the oceanic lithosphere and volcanic degassing of SO2 have acted as net sinks of O2, amounting to ca. 2.7 ± 1.1 Tmol O2/y, over the Cenozoic. Near constancy of atmospheric oxygen concentrations over the same interval of time suggests that this igneous sink is compensated by the sedimentary cycles of C, S and Fe. The net subduction and accretion of OC likely made the dominant contribution and, therefore, operated as a net source of atmospheric O2 over the last 50-60 Myr. This result implies that redox steady-state in the Cenozoic is dynamically maintained by a net input of solar (photosynthetic) energy, ∼ 0.8 to 2 EJ/y, into the lithospheric cycles of C, S and Fe.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"320 1","pages":"730 - 751"},"PeriodicalIF":2.9,"publicationDate":"2020-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2475/10.2020.03","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48887369","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Miocene to Pleistocene glacial history of West Antarctica inferred from Nunatak geomorphology and cosmogenic-nuclide measurements on bedrock surfaces","authors":"P. Spector, J. Stone, G. Balco, Trevor R. Hillebrand, M. Thompson, T. Black","doi":"10.2475/10.2020.01","DOIUrl":"https://doi.org/10.2475/10.2020.01","url":null,"abstract":"We report geomorphic observations and cosmogenic-nuclide measurements on bedrock surfaces from three isolated nunatak groups in West Antarctica: the Pirrit Hills and Nash Hills, located in the Weddell Sea sector, and the Whitmore Mountains, located on the Ross-Weddell divide. The objectives of this paper are to (i) establish a chronology for landscape development at these sites and (ii) quantify the long-term history of ice-thickness variations in West Antarctica. These nunataks display relic alpine landscapes on which weathered bedrock surfaces are superimposed. In the Pirrit Hills, an erosional trimline is etched into alpine ridges and separates smooth-crested ridges below from serrated ridges above. Below the trimline, geomorphic evidence indicates repeated frozen-based ice cover, while above the trimline evidence for ice cover is entirely absent. There is also no geomorphic evidence for thicker-than-present ice at the Whitmore Mountains. Cosmogenic nuclide concentrations in the oldest bedrock surfaces from the Whitmore Mountains and from above the Pirrit Hills trimline indicate uninterrupted exposure for ∼12 Myr at extraordinarily low erosion rates. This places a lower limit on the timing of the formation of alpine landscapes in West Antarctica, and we hypothesize that this occurred during the relatively warm climates prior to the mid-Miocene cooling. The absence of evidence for thicker ice at the Whitmore Mountains is consistent with the hypothesis that the divide was typically thinner than present during Pleistocene glacial periods due to reduced accumulation. Bedrock surfaces below the trimline have much lower nuclide concentrations and are most easily explained by a scenario of repeated frozen-based ice cover and occasional subglacial plucking, which is consistent with geomorphic observations. Bedrock surfaces near the modern ice level appear to have been covered more than half of the time, while higher elevation surfaces indicate progressively less cover. The trimline and associated geomorphic features are very similar to a prominent trimline in the Ellsworth Mountains, and we conclude that these are, in fact, part of the same feature. The Ellsworth trimline is hypothesized to have formed during the mid-Miocene cooling and the transition from alpine to continental glaciation, and our results are consistent with this hypothesis.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"320 1","pages":"637 - 676"},"PeriodicalIF":2.9,"publicationDate":"2020-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2475/10.2020.01","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45324159","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A process-based ecosystem model (Paleo-BGC) to simulate the dynamic response of Late Carboniferous plants to elevated O2 and aridification","authors":"J. White, I. Montañez, Jonathan P. Wilson, C. Poulsen, J. McElwain, W. DiMichele, M. Hren, S. Macarewich, J. Richey, W. Matthaeus","doi":"10.2475/09.2020.01","DOIUrl":"https://doi.org/10.2475/09.2020.01","url":null,"abstract":"Ecosystem process models provide unique insight into terrestrial ecosystems by employing a modern understanding of ecophysiological processes within a dynamic environmental framework. We apply this framework to deep-time ecosystems made up of extinct plants by constructing plant functional types using fossil remains and simulating—as close as possible—the in vivo response of extinct taxa to their paleoclimatic environment. Ecosystem process models provide unique insight into terrestrial ecosystems by employing a modern understanding of ecophysiological processes within a dynamic environmental framework. We apply this framework to deep-time ecosystems made up of extinct plants by constructing plant functional types using fossil remains and simulating—as close as possible—the in vivo response of extinct taxa to their paleoclimatic environment. To accomplish this, foliar characteristics including maximum stomatal conductance, distance from leaf vein to stomata, and cuticular carbon and nitrogen were input as model parameters derived from measurements of well-preserved Pennsylvanian-age fossil leaves. With these inputs, we modeled a terrestrial tropical forest ecosystem dominated by “iconic” plant types of the Pennsylvanian (∼323–299 Ma) including arborescent lycopsids, medullosans, cordaitaleans, and tree ferns using a modified version of the process model BIOME-BGC, which we refer to as Paleo-BGC. Modeled carbon and water—and, for the first time, nitrogen—budgets of a tropical ecosystem from Euramerica driven by daily meteorology are simulated using the Global Circulation Model GENESIS 3.0. Key findings are: lycopsids have the lowest daily leaf water potential, soil water content, surface runoff, and degree of nitrogen leaching indicating an intensive water use strategy compared to medullosans, cordaitaleans, and tree ferns that have increasingly lower simulated water use, greater surface, and nitrogen loss in this order; modeled vegetation response to aridification, which was caused by reduced precipitation and intensified through the close of the Carboniferous and into the Permian shows that lycopsids and medullosans have the lowest tolerance for precipitation decrease compared to cordaitaleans and tree ferns, consistent with the paleobotanical record of occurrence of floral turnovers through the Middle Pennsylvanian through earliest Permian; elevated atmospheric pO2, hypothesized as characteristic for the latter half of the Pennsylvanian and early Permian (∼299–272 Ma), caused higher atmospheric pressure reducing plant transpiration, higher surface water runoff rates, and increased nitrogen export for all plant types simulated, manifested most strongly in the lycopsid dominated ecosystems—with overall only a small reduction in net daily assimilation (≈1 μmol CO2 m−2 s−1). Both aridification and elevated atmospheric oxygen reduced transpiration, increased water retention in soils, with higher surface runoff. With more discharge, enhanced and high","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"320 1","pages":"547 - 598"},"PeriodicalIF":2.9,"publicationDate":"2020-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2475/09.2020.01","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45635877","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}