Deep-water stratigraphic cyclicity and carbonate mud mound development in the Middle Cambrian Marjum Formation, House Range, Utah, USA

Abstract In mid‐Middle Cambrian time, shallow‐water sedimentation along the Cordilleran passive margin was abruptly interrupted by the development of the deep‐water House Range embayment across Nevada and Utah. The Marjum Formation (330 m) in the central House Range represents deposition in the deepest part of the embayment and is composed of five deep‐water facies: limestone–argillaceous limestone rhythmites; shale; thin carbonate mud mounds; bioturbated limestone; and cross‐bedded limestone. These facies are cyclically arranged into 1·5 to 30 m thick parasequences that include rhythmite–mound, rhythmite–shale, rhythmite–bioturbated limestone and rhythmite–cross‐bedded limestone parasequences. Using biostratigraphically constrained sediment accumulation rates, the parasequences range in duration from ≈14 to 270 kyr. The mud mounds are thin (<2 m), closely spaced, laterally linked, symmetrical domes composed of massive, fenestral, peloidal to clotted microspar with sparse unoriented, poorly sorted skeletal material, calcitized bacterial(?) filaments/tubes and abundant fenestrae and stroma‐ tactoid structures. These petrographic and sedimentological features suggest that the microspar, peloids/clots and syndepositional micritic cement were precipitated in situ from the activity of benthic microbial communities. Concentrated growth of the microbial communities occurred during periods of decreased input of fine detrital carbonate transported offshore from the adjacent shallow‐water carbonate platform. In the neighbouring Wah Wah Range and throughout the southern Great Basin, coeval mid‐Middle Cambrian shallow‐water carbonates are composed of abundant metre‐scale, upward‐shallowing parasequences that record high‐frequency (10⁴?10⁵ years) eustatic sea‐level changes. Given this regional stratigraphic relationship, the Marjum Formation parasequences probably formed in response to high‐frequency sea‐level fluctuations that controlled the amount of detrital carbonate input into the deeper water embayment. During high‐frequency sea‐level rise and early highstand, detrital carbonate input into the embayment decreased as a result of carbonate factory retrogradation, resulting in the deposition of shale (base of rhythmite–shale parasequences) or thin nodular rhythmites, followed by in situ precipitated mud mounds (lower portion of rhythmite–mound parasequences). During the ensuing high‐frequency sea‐level fall/lowstand, detrital carbonate influx into the embayment increased on account of carbonate factory pro‐ gradation towards the embayment, resulting in deposition of rhythmites (upper part of rhythmite–mound parasequences), reworking of rhythmites by a lowered storm wave base (cross‐bedded limestone deposition) or bioturbation of rhythmites by a weakened/lowered O₂‐minimum zone (bioturbated lime‐ stone deposition). This interpreted sea‐level control on offshore carbonate sedimentation patterns is unique to Palaeozoic and earliest Mesozoic deep‐water sediments. After the evolution of calcareous plankton in the Jurassic, the presence or absence of deeper water carbonates was influenced by a variety of chemical and physical oceanographic factors, rather than just physical transport of carbonate muds.     

Petrogenesis of Dunite Xenoliths from Koolau Volcano, Oahu, Hawaii: Implications for Hawaiian Volcanism

Ultramafic xenoliths from Koolau Volcano on the island of Oahu,Hawaii, are divided into spinel lherzolite, pyroxenite, anddunite suites. On the basis of a study of the petrography andmineral compositions of 43 spinel lherzolites, 12 pyroxenites,and 20 dunites, the following characteristics of the dunitesin relation to the other nodule types and to Hawaiian lavasemerge. (1) The forstente content of olivines in the Koolaudunites (Fo_82.6-Fo_{89 7} ) overlap those of Hawaiian tholeiiticand alkalic lavas and are generally lower than those in abyssallherzolites and dunites and in Koolau spinel lherzolites. (2)Most of the dunites contain no orthopyroxene, all except twocontain chrome spinel, and a few contain interstitial plagioclaseand clinopyroxene. (3) Chrome spinels from the Koolau dunitesare distinctly higher in Cr/(Cr+Al), lower in Mg/(Mg+ Fe²⁺)and higher in TiO₂ than those from abyssal basalts and peridotites.Chrome spinels in the dunites correspond closely in compositionto chrome spinels in Hawaiian tholeiitic and alkalic lavas.(4) The abundance of dunite relative to other nodule types decreasesoutward from the central part of the volcano. The dunites areinterpreted not as residues of partial fusion of the mantlebut as crystal accumulations stored at shallow depths beneaththe central part of Koolau Volcano and derived from picriticmagmas parental to the shield-building tholeiitic lavas.     

Granulite Metamorphism, Fluid Buffering, and Dehydration Melting in the Madras Charnockites and Metapelites

Interlayered and cofolded charnockites and metapelites of thetype charnockite area near Madras were metamorphosed under granulitefades conditions. Fe-Mg partitioning between orthopyroxene,garnet, and biotite indicates that chemical equilibrium wasapproached under similar P-T conditions in the two rock suites.Several geothennometers and geobarometers give P-T values whichconverge at 750–800?C and 6.5–7.5 kb. Computations utilizing data from high pressure phase equilibriumexperiments of Bohlen et al. (1983a) and Wones & Dodge (1977)point to several significant relations regarding the behaviourof H₂O during the granulite metamorphism. a_H2O values, computedfrom Bohlen et al.'s (1983a) reversal data and the a-X modelfor phlogopite after Bohlen et al. (1980), show distinctly lowermagnitudes in metapelites (0.10–0.16) than in charnockites(0.23–0.34). No systematic spatial gradients exist withinthe charnockites or metapelites, and a_H2O has similar valuesin metapelite exposures widely separated in the field. Theseimply an internal, rather than an external (e.g., by CO2 influx),control of the fluids. Applying the algebraic method developed by Rumble (1976), Gibbsanalysis in the system K₂O-MgO-FeO-Fe₂O₃-Al₂O₃-SiO₂-TiO₂-H₂Oshows that the chemical potentials of H₂O and to O₂, as monitoredagainst biotite composition and , exhibit gradients with respect to X_Mg in the two rock suites under isothermal-isobaricconditions. µ_H2O was found to decrease with X_Mg^bt in both,while µ_O2 increases with decreasing X_Mg^bt in metapelitesbut increases sympathetically with X_Mg^bt in charnockites. Thesefindings point out again that µ_H2O and µ_O2 wereinternally buffered. The absence of graphite in the metapelites,at an estimated f_O2 = 10^–14.7 b, also argues against anexternal influx of CO₂ and, inter alia, supports internal buffering.A complementary enquiry into variations of a_TIO2 reveals aninverse relation between a_TIO2 and a_H2O, suggesting a similarcontrol for a_TIO2. The inferences from biotite dehydration equilibria, when combinedwith the P-T data and with several field and chemical featuresof these rocks noted earlier (Sen, 1974), make dehydration meltinga distinct possibility for the Madras rocks. It is argued thatthe low a_H2O and high a_TIO2 ({small tilde} 0.9) observed inthe metapelites have been caused by a greater extent of meltingin the precursors of metapelites, which were more hydrous thanthose of charnockites, coupled with preferential partitioningof Ti into the residual rocks—thus strengthening the casefor dehydration melting.     

Investigating the multiscale teleconnections of Madden–Julian oscillation and monthly rainfall using time-dependent intrinsic cross-correlation

Natural Hazards - This study proposes a novel ensemble empirical mode decomposition (EEMD) time-dependent intrinsic cross-correlation (TDICC)-coupled framework to investigate the correlation...     

C-isotope stratigraphy and paleoenvironmental changes across OAE2 (mid-Cretaceous) from shallow-water platform carbonates of southern Mexico

The stratigraphic and geochemical record of the mid-Cretaceous (Cenomanian–Turonian) Oceanic Anoxic Event 2 (OAE2) has been studied in numerous Tethyan and proto-Atlantic hemi-pelagic/pelagic successions, but little data comes from nearshore carbonate successions from the proto-Pacific region. Here we present the results of a combined stratigraphic and δ¹³C study of C–T platform carbonates from southern Mexico, which were deposited within the proto-Pacific. Two scales of sedimentary cyclicity are recognized. High-frequency peritidal and subtidal cycles (0.4–8 m) display little evidence of cycle-capping subaerial exposure and are not correlative between sections; these relationships suggest that the amplitudes of high-frequency sea-level changes were minimal during the peak mid-Cretaceous greenhouse. Longer-term transgressive-regressive sequences (18–40⁺ m) are correlated between sections, and using δ¹³C trends, can be correlated with sequences developed in northern Europe and India.The Mexican successions were sampled at a high resolution (~ 10 ky) for stable isotopes (inorganic, organic carbon and oxygen), total organic carbon, insoluble residues, and trace metals. The δ¹³C_carb curve matches global trends (including 6 distinct isotopic stages) permitting identification of OAE2 despite the lack of characteristic anoxic facies. Using the δ¹³C_carb trends, we tie the previously identified ammonite, planktonic foram, and nannofossil biostratigraphy from England and the Western Interior seaway of Colorado into the Mexican sections. The initiation of OAE2, defined by an abrupt positive 3–4‰ δ¹³C shift, coincides with a long-term sea-level rise, though the sedimentary expression of the deepening is no greater than that observed for any of the other sea-level events across the studied interval. OAE2 termination (transition from gradually decreasing to background δ¹³C values) is not associated with a particular sea-level trend. Stratigraphic changes in insoluble residues (proxy for continental sediment discharge) across OAE2 are not correlative between sections and do not show consistent systematic relationships with δ¹³C or sea-level variations, therefore do not support the hypothesis that OAE2 was associated with increased continental-derived nutrient influx. Two peaks in trace metal concentrations coincide with the abrupt increase in δ¹³C ratios (onset of OAE2) and during the transition from elevated-to-decreasing δ¹³C values (near the C–T stage boundary). These trends are similar to those recorded in coeval deposits of the Western Interior seaway, and are consistent with the hypothesis that OAE2 development was related to the release of reduced metals during the short-lived (< 1 My) Caribbean oceanic plateau basalt eruption. In this scenario, oxidation of the metals depleted the existing low dissolved-O₂ concentrations and thermally-buoyant plumes of seawater enriched in biolimiting elements mixed with surface waters, stimulated primary productivity, and further reduced O₂ concentrations leading to widespread anoxia and a large positive δ¹³C shift.     

土耳其西部新生代斑岩成矿岩浆氧逸度-挥发份元素组成演化特征——来自锆石和磷灰石组成约束

特提斯成矿域中段的土耳其西部集中产出了与俯冲、碰撞-后碰撞、伸展动力学背景有关的斑岩型铜、钼、金矿床。前人已完成了针对这些斑岩型矿床构造背景的大量研究工作,但对于区内不同构造背景下斑岩型矿床的成矿岩浆源区性质、成矿元素-挥发分含量和成矿物质演化关系尚未有系统研究。而这些研究将对认识中特提斯构造域晚白垩世以来在新特提斯洋俯冲、欧亚板块-Tauride-Anatolide板块碰撞和伸展过程中斑岩矿床形成时地壳厚度、壳幔相互作用方式及成矿物质演化过程具有重要意义。本研究选择土耳其西段三个斑岩成矿带(Tavsanli带、Biga半岛成矿带和Afyon-Konya带)内五个斑岩型矿床的成矿岩体与同期侵入岩-火山岩,开展锆石微量元素和磷灰石主量元素研究,限定碰撞与伸展环境下斑岩型矿床成矿岩浆的形成温度、氧逸度条件及其与岩浆形成时地壳厚度的关系,利用磷灰石F-Cl-S含量组成揭示熔体挥发分和硫元素的物质组成,进而约束新特提斯洋俯冲-碰撞-伸展过程中斑岩系统的深部物质演化规律。研究结果表明,土耳其西部新生代斑岩型矿床成矿岩浆锆石大部分落于高水含量-高氧逸度区间,具有相似的稀土元素标准化配分曲线。从始新世到中新世,锆石氧逸度Ce~(4+)/Ce~(3+)比值呈现出先降低(斑岩型Cu-Mo矿到斑岩型Mo矿)后升高(斑岩型Mo-Cu矿到斑岩Au矿)的趋势,且相对于斑岩型Mo矿和Mo-Cu矿,斑岩型Au矿和Cu-Mo矿成矿岩体的锆石形成时具有相对较高的氧逸度。绝大部分斑岩型矿床成矿岩体的锆石Eu_N/EU_N~*位于0.4～0.7之间,但斑岩型Mo矿和斑岩型Mo-Cu矿床的锆石具有相对较低的Eu_N/Eu_N~*比值,可能是由于在结晶时熔体受斜长石结晶影响较大。锆石微量元素显示(Yb/Gd和Hf/Y-Th/U)锆石组成大部分受岩浆房中角闪石±榍石±磷灰石分离结晶控制。根据锆石Ti温度计估算土耳其西部斑岩型矿床成矿岩体及其相关岩体的形成温度在650～900℃之间,结晶温度从斑岩型Au矿、斑岩型Cu-Mo/Mo-Cu矿至斑岩型Mo矿呈现递减趋势。对于熔体的挥发分与硫含量组成始新世-渐新世斑岩型Cu-Mo与Mo矿床成矿岩浆相对具有高F和低Cl组成,中新世伸展环境形成的斑岩型Mo-Cu矿和斑岩型Au矿床成矿岩浆Cl含量普遍较高。与成矿岩体同期的暗色包体或基性岩脉中磷灰石计算获得的熔体硫含量均大于侵入体对应熔体的硫含量,且具有不均一的含量组成,表明基性岩浆注入可能为岩浆房提供硫。结合区域动力学和地壳厚度估算,本文认为触发土耳其西部新生代斑岩矿床形成的动力学机制是:在新特提斯洋向北单向汇聚的背景下,北部始新世-渐新世斑岩矿床受控于碰撞后俯冲的新特提斯洋板片(Vardar洋)后撤-回转-断离过程;南部中新世斑岩矿床的形成则受控于爱琴海板片俯冲控制的地壳伸展-减薄过程。北侧Izmir-Ankara-Erzincan缝合带附近的Tavsanli与Biga半岛斑岩成矿带始新世-渐新世斑岩型矿床的形成与熔融-同化-储存-均一过程(MASH)有关,深部地壳热区过程(DCHZ)与中新世Afyon-Konya带斑岩型矿床的形成有关。     

Integrating value-chain approach with participatory multicriteria analysis for sustainable planning of a niche crop in Indian Himalayas

The value-chain approach(VCA)was used for exploring how a niche crop,namely large cardamom(Amomum subulatum Roxb.),can be developed sustainably to increase incomes and enhance rural livelihoods in the mountains.Large cardamom is a high-value,agro-climatically suitable,and non-perishable spice crop grown in the Himalayan region.Originating in Sikkim,the crop is important to the local economy and is sold in both domestic and international markets;however,its production in India has been declining significantly in recent years.Using VCA and participatory methods of data collection the study helped elicit value chain actors'perspectives on various reasons for the decline as well as potential strategies to improve the cardamom value chain.The results present the multiple environmental and climatic(e.g.water stress),social(e.g.women participation)and institutional(e.g.lack of trust between actors)challenges and corresponding upgrading strategies.Through participatory multi-criteria analysis(MCA),the strategies were prioritized based on the overall preferences of multiple stakeholders.The analysis considered eight criteria and 25 options,or strategies,and found that that marketability and profitability were the most important criteria,and increasing cardamom production,the most important optiontrait.Stakeholder discussions along with sensitivity analysis also showed how these options could be implemented and revealed how criteria preferences can change with expert inputs.This combination of VCA and MCA,because it supports participatory decision-making and balances multiple dimensions of sustainable development,can be effectively deployed in planning for agricultural development in the mountains.     

Numerical Simulations of the Microscale Heterogeneities of Turbulence Observed on a Complex Site

We simulate the microscale heterogeneities of turbulent variables observed at a complex site for different wind directions. The atmospheric computational fluid dynamics (CFD) results are compared with an ensemble of 36 months of data collected at the experimental site SIRTA “Site Instrumental de Recherche par Télédétection Atmosphérique”, located near Paris (France) in a semi-urban environment. The experimental data show that the normalized turbulent kinetic energy (TKE) k/U ² (where k is TKE and U is the wind speed) at 10-m height, for two different locations, is highly dependent on wind direction and strongly influenced by trees. These measurements show a strong increase of the normalized TKE downstream of the forest canopies with a large variability within the 36-month period in part due to the variation of the tree foliage. The numerical simulations are carried out using the CFD code Code_Saturne with the standard k?ε closure, in neutral stratification. The buildings are taken into account explicitly in the mesh and the forested areas are modelled with two approaches: the classical roughness wall law and a drag porosity. A comparison has been performed between the calculated values and the median of measured values of the normalized TKE and the normalized friction velocity, for each wind sector of 10°. A very good agreement is obtained with the drag porosity model, whereas the classical roughness law leads to a strong underestimation downstream of the forested areas. However, this large improvement of the results using the drag porosity model can only be obtained with a refinement of the grid, especially in forested areas, and an accurate land-use map.