Linking invertebrate burrow distributions (neoichnology) to physicochemical stresses on a sandy tidal flat: implications for the rock record

Invertebrate burrow distributions (neoichnology) across a modern tidal flat are presented as an analogue for interpreting the ichnology of palaeo‐tidal‐flat successions. Burrow distributions are linked to physical and chemical (physicochemical) stresses to establish the main controls on the distribution of biosedimentary structures. Across the tidal flat, there is clear heterogeneity in both the diversity of traces and the intensity of burrowing. This heterogeneity reflects a myriad of physicochemical stresses, with the sedimentation rate dominating burrow distributions. Across all substrates, the total area occupied by organisms rarely exceeds 3% of the tidal‐flat surface, and is commonly <1%; this equates to a bioturbation index value of one. To reach bioturbation index values of two to six, sediments must be available to biogenic reworking and/or recolonization. With an increasing sedimentation rate, substrates are rapidly buried and re‐exposed, which limits the time when a substrate is available to colonization. For palaeo‐ichnological studies, this research presents several key results. (i) Burrow cross‐cutting relationships in tidal‐flat successions commonly reflect natural heterogeneities in the areal distribution of infaunal communities, rather than infaunal tiering. (ii) Ichnofabric analysis of palaeo‐tidal flats with a high sedimentation rate would yield fabrics that reflect heterogeneities in the areal distribution of infaunal communities rather than variability in the physicochemical stresses of the environment. (iii) The composite trace‐fossil assemblage of tidal flats cannot be attributed to a single ichnofacies, but instead comprises elements typical of multiple ichnofacies. (iv) The main controls on trace assemblages across tidal flats in fully marine settings are sedimentological and include the sedimentation rate and, to a lesser extent, grain size.     

Paleoproterozoic crustal evolution of the Tarim Craton: Constrained by zircon U–Pb and Hf isotopes of meta-igneous rocks from Korla and Dunhuang

The Tarim Craton is one of three large cratons in China. Presently, there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis, especially an in situ Lu–Hf isotope analysis of zircon. In this study, Precambrian basement rocks from the Kuluketage and Dunhuang Blocks in the northeastern portion of the Tarim Craton have been analyzed for combined in situ laser ablation ICP-(MC)-MS zircon U–Pb and Lu–Hf isotopic analyses, as well as whole rock elements, to constrain their protoliths, forming ages and magma sources. Two magmatic events from the Kuluketage Block at ∼2.4 Ga and ∼1.85 Ga are revealed, and three stages of magmatic events are detected in the Dunhuang Block, i.e., ∼2.0 Ga, ∼1.85 Ga and ∼1.75 Ga. The ∼1.85 Ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings, suggesting that the Kuluketage and Dunhuang Blocks are part of the uniform Precambrian basement of the Tarim Craton. Zircon Hf model ages of the ∼2.4 Ga magmatism indicate that the crust of the Tarim Craton may have been formed as early as the Paleoarchean period. The ∼2.0 Ga mafic rock from the Dunhuang Block was formed in an active continental margin setting, representing an important crustal growth event of the Tarim Craton in the mid-Paleoproterozoic that coincides with the global episode of crust formation during the assembly of the Columbia supercontinent. The ∼1.85 Ga event in the Kuluketage and Dunhuang Blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the Columbia supercontinent, while the ∼1.75 Ga magmatism in the Dunhuang Block resulted from a mixture of the reworked Archean crust with juvenile magmas and was most likely related to a post-collisional episode.     

http://www.sciencedirect.com/science/article/pii/S1674987112001570

The supercontinent cycle,by which Earth history is seen as having been punctuated by the episodic assembly and breakup of supercontinents,has influenced the rock record more than any other geologic phenomena,and its recognition is arguably the most important advance in Earth Science since plate tectonics.It documents fundamental aspects of the planet’s interior dynamics and has charted the course of Earth’s tectonic,climatic and biogeochemical evolution for billions of years.But while the widespread realization of the importance of supercontinents in Earth history is a relatively recent development,the supercontinent cycle was first proposed thirty years ago and episodicity in tectonic processes was recognized long before plate tectonics provided a potential explanation for its occurrence.With interest in the supercontinent cycle gaining momentum and the literature expanding rapidly,it is instructive to recall the historical context from which the concept developed.Here we examine the supercontinent cycle from this perspective by tracing its development from the early recognition of long-term episodicity in tectonic processes,through the identification of tectonic cycles following the advent of plate tectonics,to the first realization that these phenomena were the manifestation of episodic supercontinent assembly and breakup.     

华北克拉通南缘潘河~1.5Ga正长岩的厘定及其构造意义

华北克拉通是否曾是Columbia超大陆的组成部分并具有一致的裂解过程一直存在争议,而已知的中元古代非造山岩浆组合集中分布于华北克拉通北缘是引起有关裂解过程争论的重要原因。作者近来在华北克拉通南缘,河南省卢氏县潘河地区,厘定了一组中元古代走向近东西的正长岩脉群。正长岩LA-ICP-MS锆石U-Pb年龄为1469±8Ma(MSWD=1.4)。岩石类型主要为黑云母正长岩,局部有少量霞石正长岩。岩体的SiO2变化于51.3%～65.7%之间,碱含量较高,K2O+Na2O总量8.32%～13.0%,铝含量较高(Al2O3=18.7%～20.0%),为碱性过铝质岩类。岩体轻重稀土分馏较为显著(La/Yb)N=14.5～34.5,具有显著的Eu负异常(δEu=0.2～0.3),强烈富集Rb、Ba、Th等大离子亲石元素和Nb、Ta、Nd、Tb等高场强元素,而Sr、Ti、P、Eu等元素显著亏损。岩石εHf(t)=-14.79～+8.16,Hf模式年龄为2.93～1.74Ga,指示成岩物质主要来源于富集岩石圈地幔的部分熔融并有地壳物质的加入。岩体形成于板内伸展构造环境,这表明华北克拉通的南缘也同时记录了华北克拉通曾经在中元古代发生过大规模的伸展作用,其伸展过程可能与Columbia超大陆的裂解有关。     

Optimizing fixed observational assets in a coastal observatory

Proliferation of coastal observatories necessitates an objective approach to managing of observational assets. In this article, we used our experience in the coastal observatory for the Columbia River estuary and plume to identify and address common problems in managing of fixed observational assets, such as salinity, temperature, and water level sensors attached to pilings and moorings. Specifically, we addressed the following problems: assessing the quality of an existing array, adding stations to an existing array, removing stations from an existing array, validating an array design, and targeting of an array toward data assimilation or monitoring.     

Dynamic characteristics and horizontal transports of internal solitons generated at the Columbia River plume front

River plume front-generated internal solitons play an important role in the interaction between the plume and coastal waters. The internal solitons drive a non-harmonic velocity field, resulting in a horizontal transport that carries plume water seaward and redistributes nutrients and sediments. In this study, we present observations of internal solitons generated at the Columbia River plume front that separates the new, tidal plume, older plume and coastal waters. Scale analyses suggest that the plume front-generated internal solitons are highly non-linear waves, and their dynamic properties do not conform to any weakly non-linear theory. Thus, a high-order Korteweg–de Vries (KdV) theory is used to analyze the internal solitons. The comparison between theoretical values and cruise data shows that the high-order KdV model is much better than the weakly non-linear theories for prediction of the soliton dynamic parameters. Based on the model, we develop theoretical and numerical solutions of the soliton-induced upper layer horizontal transport and Lagrangian water parcel transport distance, which shows that the water particle drift, during the internal soliton passage, is as far as 1 km, and demonstrates the role of the internal solitons on the exchange between the plume and ambient coastal water. Energy fluxes caused by the internal solitons are estimated using the high-order KdV theory. The leading soliton fluxes 2.0×10³ W m⁻¹ per unit crest length, and carries energy of 4.2×10⁵ J m⁻¹. The total energy carried by the eight internal solitons is 1.6×10⁶ J m⁻¹, about 70% of the total frontal energy.     

扬子陆块西缘古元古代基性侵入岩LA-ICP-MS锆石U-Pb年龄和Hf同位素及其地质意义

扬子陆块西缘康滇南北向构造带内发育大量古元古代基性岩脉（墙）,它们对于认识扬子陆块早期的构造演化具有重要意义。对四川会理地区侵入到元古界通安组内的辉长岩进行LA-ICP-MS锆石U-Th-Pb同位素测定,获得的207Pb/206Pb加权平均年龄为1694Ma±16Ma（MSWD=0.9）,代表了该辉长岩的形成和侵位时代,这一结果指示会理地区通安组应为古元古代地层。辉长岩全岩地球化学具有类似于N-MORB的特征。辉长岩锆石初始（176Hf/177Hf）i值为0.281881~0.281982,对应的εHf（t）为6.2~9.8,单阶段亏损地幔Hf模式年龄tDM1为1738~1883Ma,平均值为1803Ma。这些数据表明古元古代辉长岩来源于亏损地幔,指示古元古代晚期扬子西缘之下存在亏损地幔。结合扬子西缘大量古老的碎屑锆石,推测扬子陆块西缘存在太古宙—古元古代的结晶基底。与扬子陆块西缘古元古代晚期基性侵入岩形成有关的幔源岩浆事件明显晚于扬子陆块古元古代碰撞造山及造山后的伸展作用,记录了扬子陆块古元古代晚期一次伸展过程,可能是Columbia超大陆裂解作用在扬子陆块的响应。     

海南三亚中元古代变质砾岩的发现及其对Columbia超大陆裂解的指示

海南岛中元古代沉积记录对于探讨华南陆块早期大陆演化过程起关键作用。对海南三亚三郎岭地区奥陶系干沟村组砂岩-板岩组合中的变砾岩形成时代进行了重新厘定。碎屑锆石U-Pb同位素年龄分析结果表明,65颗锆石207Pb/206U年龄范围为2691~1350 Ma,且分成2691~2444 Ma、1838~1632 Ma、1540~1350 Ma三组,形成约1699 Ma、约1440 Ma 2个主要峰值和约2461 Ma的1个次要峰值。基于最年轻锆石年龄为1350 Ma,且没有出现海南岛常见的1250~1100 Ma和1000~900 Ma年龄记录,将该套变质砾岩沉积时代限定在1350~1250 Ma,而并非前人划属的奥陶纪,这是海南岛首次发现中元古代的砾岩建造。该套变砾岩砾石成分成熟度高、结构成熟度中等,表现为裂谷盆地高密度的碎屑流沉积特征,是研究区Columbia超大陆裂解晚期的沉积响应。变质砾岩与劳伦大陆西部的Belt-Purcell超群下部沉积地层具有相似的碎屑锆石年龄组成和年龄谱特征,暗示中元古代哥伦比亚超大陆中,海南岛与劳伦西部靠近。     

Tree‐ring records unveil long‐term influence of the Pacific Decadal Oscillation on snowpack dynamics in the Stikine River basin,northern British Columbia

Long hydroclimate records are essential elements for the assessment and management of changing freshwater resources. These records are especially important in transboundary watersheds where international cooperation is required in the joint planning and management process of shared basins. Dendrochronological techniques were used to develop a multicentury record of April 1 snow water equivalent (SWE) for the Stikine River basin in northern British Columbia, Canada, from moisture‐sensitive white spruce (Picea glauca) tree rings. Explaining 43% of the instrumental SWE variability, to our knowledge, this research represents the first attempt to develop long‐term snowpack reconstructions in northern British Columbia. The results indicated that 15 extreme low April 1 SWE events occurred from 1789 to the beginning of the instrumental record in 1974. The reconstruction record also shows that the occurrence of hydrological extremes in the Stikine River basin is characterized by persistent below‐average periods in SWE consistent with phase shifts of the Pacific Decadal Oscillation (PDO). Spectral analyses indicate a very distinct in‐phase (positive) relationship between the multidecadal frequencies of variability (~40 years) extracted from the SWE tree‐ring reconstruction and other reconstructed winter and spring PDO indices. Comparison of the reconstructed SWE record with other tree‐ring‐derived PDO proxy records shows coherence at multidecadal frequencies of variability. The research has significant implications for regional watershed management by highlighting the hydrological response of the Stikine River basin to prior climate changes.     

High-pressure and ultrahigh-temperature metamorphism at Komateri, northern Madurai Block, southern India

We report for the first time the evidence for prograde high-pressure (HP) metamorphism preceding a peak ultrahigh-temperature (UHT) event in the northernmost part of the Madurai Block in southern India. Mg–Al-rich Grt–Ged rocks from Komateri in Karur district contain poikiloblastic garnet with numerous multi-phase inclusions. Although most of the inclusion assemblages are composed of gedrite, quartz, and secondary biotite, rare staurolite + sapphirine and spinel + quartz are also present. The X_Mg (=Mg/[Fe+Mg]) of staurolite (0.45–0.49) is almost consistent with that reported previously from Namakkal district in the Palghat–Cauvery Shear Zone system (X_Mg = 0.51–0.52), north of the Madurai Block. The HP event was followed by peak UHT metamorphism at T = 880–1040 °C and P = 9.8–12.5 kbar as indicated by thermobarometric computations in the Grt–Ged rock and associated mafic granulite. Symplectic intergrowth of spinel (X_Mg = 0.50–0.59, ZnO < 1.7 wt.%) and quartz, a diagnostic indicator of UHT metamorphism, probably formed by decompression at UHT conditions. The rocks subsequently underwent retrograde metamorphism at T = 720–760 °C and P = 4.2–5.1 kbar. The P–T conditions and clockwise exhumation trajectory of the Komateri rocks, comparable to similar features recorded from the Palghat–Cauvery Shear Zone system, suggest that the Madurai Block and the Palghat–Cauvery Shear Zone system underwent similar HP and UHT metamorphic history probably related to the continent–continent collision during the final stage of amalgamation of Gondwana supercontinent.