ALBERT

Description: Slab breakoff is one of the primary processes in the evolution of many collisional orogens. In the Tibet-Himalaya orogen, the timing of breakoff of the Neo-Tethyan slab remains controversial because of a scarcity of solid evidence. This study reports the discovery of Eocene gabbros, dated at 45.0 ± 1.4 Ma (in situ U-Pb age of titanite) using secondary ion mass spectrometry, from the eastern segment of Tethyan Himalaya in southern Tibet. These rocks show geochemical characteristics similar to those of HIMU (high μ)–type oceanic island basalt and have depleted Sr-Nd isotopes [ 87 Sr/ 86 Sr(t) = 0.70312–0.70317; Nd (t) = +4.9 to +5.0]. It is suggested that the gabbros stand as the first direct evidence for partial melting of the asthenosphere followed by rapid magma ascent with negligible crustal contamination. This event, combined with results from relevant studies along the Indus-Yarlung suture zone, is best explained by a sudden and full-scale detachment of subducted Neo-Tethyan slab at great depth. The breakoff model may account for coeval tectonomagmatic activities (development of small-scale, short-lived magmatism and subsequent termination of the Gangdese arc magmatism) in southern Tibet and for the abrupt slowdown (ca. 45 Ma) of Indo-Asia convergence.

Description: 〈span〉The Naqing section in South China is a representative carbonate slope succession in the eastern Paleo-Tethyan realm. It encompasses four global stratotype section and point (GSSP) candidates for the Carboniferous Period. High-resolution magnetic susceptibility measurements through the section have variations that correlate with lithological cycles of lime mudstone, wackestone, and packstone. Astronomical calibration of ~3 m sedimentary cycles to a 405 k.y. orbital eccentricity cycle period aligns other significant, shorter sedimentary cycles to periods recognizable as short orbital eccentricity (136 k.y., 122 k.y., and 96 k.y.), obliquity (31 k.y.), and precession (22.9 k.y. and 19.7 k.y.). The orbital eccentricity has long-period modulations with 2.4 m.y., 1.6 m.y., and 1.2 m.y. periods, and the obliquity has a 1.2 m.y. modulation cycle. The astronomical calibration indicates durations of 7.6 m.y., 8.1 m.y., 8.5 m.y., 2.87 m.y., and 4.83 m.y. for the Serpukhovian, Bashkirian, Moscovian, Kasimovian, and Gzhelian Stages, respectively. The calibrated durations of the 25 conodont zones collectively indicate a 33.9 m.y. time scale. Biochronological correlation of the Paleo-Tethyan and pan-Euramerican records significantly refines the global chronostratigraphy for the Serpukhovian Stage and the Pennsylvanian subsystem. This new Paleo-Tethyan astronomical time scale opens a new window for understanding the late Paleozoic icehouse world.〈/span〉

Description: 〈span〉Recycled crustal components have been widely identified in the source of continental basalts with geochemical features similar to oceanic island basalts (OIBs). However, the mechanism of how these recycled materials are involved remains highly debatable. Here we conduct comprehensive geochemical analyses (including whole-rock, olivine, and melt inclusion) and numerical modeling on Late Cretaceous Ji’an basalts from South China interior, aiming to investigate the possible role of recycled crustal components in basalt petrogenesis driven by the subducted paleo-Pacific oceanic plate. The Ji’an basalts show geochemical characteristics akin to OIBs and have depleted asthenospheric mantle-like Sr-Nd-Pb-Hf isotopic compositions with moderately radiogenic Os. Their olivine-hosted melt inclusions have low H〈sub〉2〈/sub〉O and highly negative δD values and olivine phenocrysts are mainly characterized by depletion of 〈sup〉18〈/sup〉O with δ〈sup〉18〈/sup〉O values lowering to 3.9‰. These features are consistent with positive Sr and Eu anomalies in some whole-rock samples. The combined geochemical data suggest that the primary magmas were derived from an asthenospheric mantle enriched by melts from an altered gabbroic oceanic crust, which had experienced intensive dehydration. Further numerical modeling shows that melting of the dehydrated oceanic crust can occur along the torn flank of the subducting lithosphere, in the case that the slab is strongly thinned and fractured. The low δ〈sup〉18〈/sup〉O preserved in olivine and the estimated slab age (〈300 Ma) from the radiogenic whole-rock Os and Pb compositions also require the involvement of a recently recycled slab, probably represented by the subducted paleo-Pacific oceanic plate. Rollback of the subducting paleo-Pacific slab might create a slab window, in which melt from the torn/fractured slab reacted with the upwelling asthenosphere to form an enriched mantle source for the Ji’an basalts and similar counterparts.〈/span〉

Description: The emergence and expansion of animal life on Earth represents a dramatic shift in the structure and complexity of the biosphere. A lack of firm constraints on surface oxygen levels during the mid-Proterozoic has resulted in heated debate as to whether the rise and earliest diversification of animals was directly linked to a change in environmental oxygen levels or, instead, simply reflects the timing of innovations in gene expression and developmental regulation and was independent of a direct environmental trigger. Here, we present chromium (Cr) isotope data from marine black shales that provide evidence for minimal Cr oxidation throughout the mid-Proterozoic leading up to the diversification of eukaryotes and the rise of animals during the late Neoproterozoic. This observation requires very low background oxygen levels (〈1% of present atmospheric levels). Accepting previously proposed estimates of p O 2 levels needed to induce Cr isotope fractionation, our data provide support for the persistence of an Earth system in which baseline atmospheric p O 2 would have been low enough to inhibit the diversification of animals until ca. 800 Ma. More generally, evidence for a delayed rise of atmospheric oxygen strongly suggests that environmental factors have played a fundamental role in controlling the emergence and expansion of complex life on Earth.

Description: Large earthquakes alter physical and chemical processes at Earth’s surface, triggering landslides, fracturing rock, changing large-scale permeability, and influencing hydrologic pathways. The resulting effects on global chemical cycles are not fully known. Here we show changes in the dissolved chemistry of the Min Jiang, a river in the Yangtze River (China) headwaters, following the A.D. 2008 M w 7.9 Wenchuan earthquake. Total solute fluxes transported by the Min Jiang increased after the earthquake, accompanied by an ~4 x increase in Na*/Ca ratios (where Na* is Na + corrected for atmospheric and evaporite contributions) and a 0.000644 ± 0.000146 increase in 87 Sr/ 86 Sr isotopic ratios. These changes are consistent with enhanced contribution from silicate sources. We infer that the CO 2 consumption rate via silicate-derived alkalinity increased 4.3 ± 0.4 times. If similar changes are associated with other large earthquakes, enhanced solute export could directly link tectonic activity with weathering and alkalinity fluxes that supply nutrients to ecosystems, influence seawater chemistry evolution, and steer Earth’s long-term carbon cycle and climate.

Description: Movement of CO 2 from the atmosphere into land via photosynthesis and root respiration, the subsequent formation of bicarbonate in soil, and its storage in groundwater or precipitation as CaCO 3 in dryland soils are major processes in the global carbon cycle. Together, inorganic carbon as soil carbonate (~940 PgC) and as bicarbonate in groundwater (~1404 PgC) surpass soil organic carbon (~1530 PgC) as the largest terrestrial pool of carbon. Yet, despite general agreement about its huge size as a carbon pool, controversy about the potential of inorganic carbon to sequester atmospheric CO 2 remains unresolved. We suggest that the controversy stems from the absence of a lexicon and propose a classification scheme that uses (1) calcium source illustrated by two widely recognized chemical reactions, and (2) the concept of carbonate "generations." When calcium is derived from preexisting carbonate, an equilibrium reaction occurs that does not sequester carbon in soil carbonate but does sequester carbon in groundwater until bicarbonate precipitates as CaCO 3 . When calcium is derived from silicate minerals, a unidirectional reaction occurs that sequesters carbon in both soil carbonate and groundwater. The generations concept shows that carbon sequestration occurs only in the first generation when calcium is released directly from silicates. This classification not only enhances communication and provides a framework for quantifying amounts of fossil fuel carbon that can be sequestered within a geoengineering context, it provides more precise language for discussing the terrestrial carbon cycle through geologic time.

Description: A curved belt consisting of terranes or blocks and associated late Paleozoic to early Mesozoic sutures are features of the southeastern part of the Tibetan Plateau. The southeastern Tibetan Plateau Paleotethyan belt is divided into a northwestern segment (central Tibetan Plateau) and a southern segment (the Sanjiang orogenic belt). The Paleotethyan tectonic history of this belt remains a subject of considerable debate. A major controversy concerns whether or not it evolved as a single genetic element. This paper provides new geochronological, geochemical, and structural data to constrain the Paleotethyan tectonics of this belt. A detailed field study across three sections of the Sanjiang orogenic belt has identified two volcaniclastic successions separated by a regional angular unconformity in the northwestern segment, and one continuous volcaniclastic succession in the southern segment. New zircon U-Pb ages reveal that: (1) the lower succession of the northwestern segment was produced by three volcanic phases at ca. 272 Ma, 253–247 Ma, and 240–235 Ma; (2) the upper succession of the northwestern segment formed during a shorter period along the Yushu–Yidun arc from ca. 227 to 211 Ma; and (3) the single volcaniclastic succession of the southern segment was produced continuously from ca. 260 to 210 Ma. Whole-rock geochemical data including trace elements show that the late Paleozoic to early Mesozoic magmatic activity has a continental arc–like affinity. The spatial distribution and field relationships of the magmatic rocks define two continuous volcanic arcs in the southeastern Tibetan Plateau, which correspond to two phases of arc-like magmatism. The Late Triassic Yushu–Yidun arc belt is exposed along the northern half of the northwestern segment to the south of the western Jinsha–Garzê–Litang suture. Southward subduction along the western Jinsha–Garzê–Litang suture is likely to have generated the Yushu–Yidun arc. The second arc (Jomda–Weixi–Yunxian) was diachronous and is continuous through the southeastern Tibetan Plateau. The northwestern part of the Jomda–Weixi–Yunxian arc belt is overlain by the Yushu–Yidun arc from the north. From northwest to southeast, the cessation of magmatism in the Jomda–Weixi–Yunxian arc varies from the Middle Triassic (ca. 235 Ma) to the Late Triassic (ca. 210 Ma), whereas the onset of magmatism appears to have been in the Early Permian throughout the arc. Zircon Lu-Hf isotopic data suggest that large degrees of melting of a depleted mantle source produced the earliest magmas in each segment of the Jomda–Weixi–Yunxian arc. The similarity in zircon ages of the ophiolitic units and their distribution imply that the Longmu Co–Shuanghu suture is a continuation of the Changning–Menglian suture. The spatial relationship between the Jomda–Weixi–Yunxian arc and the Longmu Co–Shuanghu–Changning–Menglian suture suggests that diachronous eastward subduction of part of the Paleotethys along the suture produced the Jomda–Weixi–Yunxian arc belt. Widespread folds of the continental arc–volcaniclastics with orogen-parallel hinges reflect compression perpendicular to the orogen, subsequent to arc magmatism. As such, the Paleotethyan tectonics of the southeastern Tibetan Plateau are characterized by two-phase subduction and subsequent two-phase continent-continent collision.

Description: 〈span〉〈div〉Abstract〈/div〉The Naqing section in South China is a representative carbonate slope succession in the eastern Paleo-Tethyan realm. It encompasses four global stratotype section and point (GSSP) candidates for the Carboniferous Period. High-resolution magnetic susceptibility measurements through the section have variations that correlate with lithological cycles of lime mudstone, wackestone, and packstone. Astronomical calibration of ∼3 m sedimentary cycles to a 405 k.y. orbital eccentricity cycle period aligns other significant, shorter sedimentary cycles to periods recognizable as short orbital eccentricity (136 k.y., 122 k.y., and 96 k.y.), obliquity (31 k.y.), and precession (22.9 k.y. and 19.7 k.y.). The orbital eccentricity has long-period modulations with 2.4 m.y., 1.6 m.y., and 1.2 m.y. periods, and the obliquity has a 1.2 m.y. modulation cycle. The astronomical calibration indicates durations of 7.6 m.y., 8.1 m.y., 8.5 m.y., 2.87 m.y., and 4.83 m.y. for the Serpukhovian, Bashkirian, Moscovian, Kasimovian, and Gzhelian Stages, respectively. The calibrated durations of the 25 conodont zones collectively indicate a 33.9 m.y. time scale. Biochronological correlation of the Paleo-Tethyan and pan-Euramerican records significantly refines the global chronostratigraphy for the Serpukhovian Stage and the Pennsylvanian subsystem. This new Paleo-Tethyan astronomical time scale opens a new window for understanding the late Paleozoic icehouse world.〈/span〉

Description: Slab breakoff is one of the primary processes in the evolution of many collisional orogens. In the Tibet-Himalaya orogen, the timing of breakoff of the Neo-Tethyan slab remains controversial because of a scarcity of solid evidence. This study reports the discovery of Eocene gabbros, dated at 45.0 ± 1.4 Ma (in situ U-Pb age of titanite) using secondary ion mass spectrometry, from the eastern segment of Tethyan Himalaya in southern Tibet. These rocks show geochemical characteristics similar to those of HIMU (high μ)–type oceanic island basalt and have depleted Sr-Nd isotopes [ 87 Sr/ 86 Sr(t) = 0.70312–0.70317; Nd (t) = +4.9 to +5.0]. It is suggested that the gabbros stand as the first direct evidence for partial melting of the asthenosphere followed by rapid magma ascent with negligible crustal contamination. This event, combined with results from relevant studies along the Indus-Yarlung suture zone, is best explained by a sudden and full-scale detachment of subducted Neo-Tethyan slab at great depth. The breakoff model may account for coeval tectonomagmatic activities (development of small-scale, short-lived magmatism and subsequent termination of the Gangdese arc magmatism) in southern Tibet and for the abrupt slowdown (ca. 45 Ma) of Indo-Asia convergence.

Description: Several tectonic models have been proposed for the Neoproterozoic amalgamation of the South China block during the assembly of Rodinia. However, the timing of the end of arc magmatism between the two subblocks in the South China block (i.e., the Yangtze and Cathaysia blocks) remains controversial because it is unclear whether the 860–820 Ma magmatic rocks and coeval sedimentary basins in this area are related to subduction or plume activity. Here, we present new detrital zircon U-Pb and Hf isotopic data for the sediments directly overlying early Neoproterozoic arc volcanic rocks in this region. These data reveal a rhythmic change in source coincident with a progressive increase in the amount of juvenile and old crustal detritus within these sediments. This result, combined with the presence of a fining-upward grain-size trend and horizontal bedding within these sediments, provides evidence of bidirectional sources that are consistent with a backarc setting. The juvenile crustal material within these sediments was sourced from adjacent arc terranes to the east, whereas the old crustal detritus was derived from the Yangtze block to the west. In addition, sensitive high-resolution ion microprobe zircon U-Pb dating of mafic rocks within equivalent sedimentary sequences yielded ages of ca. 860–840 Ma, and these mafic rocks have arc- or mid-ocean-ridge basalt–like geochemical features that indicate the initiation of backarc spreading associated with Neoproterozoic NW-directed subduction. The data from the sediments and mafic rocks suggest the presence of a backarc basin system at ca. 860–820 Ma within the southeastern margin of the Yangtze block. This in turn indicates that Rodinia assembly was not completed until ca. 820 Ma, with the South China block possibly acting as a connection between a Neoproterozoic Andean-type active continental margin and Grenvillian belts on the paleo–western margin of the Rodinia supercontinent.