Record of plate boundary metamorphism during Gondwana breakup from Lu–Hf garnet geochronology of the Alpine Schist,New Zealand

The Zealandia portion of the Pacific–Gondwana margin underwent widespread extension, fragmentation, separation and subsidence during the final stages in the breakup of Gondwana. Although these processes shaped the geology of New Zealand, their timing and the timing of subduction cessation in the region remain unclear. To investigate the timing of these processes, we used Lu–Hf garnet geochronology to date six samples of the Alpine Schist, which represents the metamorphic section of the former Zealandia margin. The garnet dates range from 97.3 ± 0.3 to 75.4 ± 1.3 Ma. Compositional zoning in garnet indicates that the spread in ages results from diachronous metamorphism in the upper plate at the Pacific–Gondwana margin, occurring concurrently with rifting of Zealandia from East Gondwana via opening of the Tasman Sea. Clear spatial trends in the timing of garnet growth throughout the Alpine Schist are absent, indicating that either regional age trends were offset by post‐metamorphic deformation, or that metamorphism did not result from a single regional heat source, and was instead driven by short‐duration, spatially dispersed processes such as episodic fluid‐fluxing or mechanical heating. Diachronous metamorphism of the Alpine Schist can be attributed to heat conduction from the rising upper mantle during widespread extension, progressive burial and heating of accretionary wedge sediments during ongoing horizontal shortening, or fluid‐fluxing sourced from a subducting and dehydrating Hikurangi Plateau. These results indicate that during separation of Zealandia from East Gondwana in the late Cretaceous, the crust at the Pacific–Gondwana margin remained hot, potentially facilitating the extensive thinning of the Zealandia lithosphere during this time.     

The distinct metamorphic stages and structural styles of the 1.94–1.86 Ga Snowbird Orogen,Northwest Territories,Canada

Palaeoproterozoic orogenesis within the Archean southeastern Rae craton is related to the initial amalgamation of Laurentia. Characterizing the accompanying tectonic processes during this time has been complicated due to polymetamorphism, which results in the obscuring of the age record of the terranes involved. To improve the knowledge of the tectonic evolution of the South Rae Craton, petrologic and structural analyses are applied in conjunction with in situ trace element chemistry, inclusion barometry, U–Pb monazite and titanite, and Lu–Hf garnet chronology. The data robustly constrain Palaeoproterozoic pressure–temperature–time paths of major deformational events along the southeastern Rae craton margin. D₁ occurred between 1.94 and 1.93 Ga in the Dodge-Snowbird domain, which included prograde burial of metasedimentary rocks, deposited at 2.2–2.0 Ga, and the development of migmatitic layering and east-southeast trending folds (S₁, F₁). Peak metamorphism is recorded in metasedimentary units at c. 1.93 Ga when rocks reached conditions of 9.0–10.5 kbar and 810–830°C. Within the Dodge-Snowbird domain, D₂ imparted north-northeast trending open folds and associated axial planar cleavage (S₂, F₂) between 1.93 and 1.90 Ga during east-west compression that appears to have been synchronous with cooling and exhumation. Later D₂ deformation, localized within the Wholdaia Lake shear zone (WLsz; S_T1), developed in the footwall of this thrust-sense structure at 1,873 ± 5 Ma at conditions of 9.5–11.0 kbar and 820–850°C. The hangingwall Dodge-Snowbird domain had already cooled to below 300°C by then, indicating a significant structural and metamorphic break across the domain's western boundary. A new phase of unroofing (D₃) involved pervasive amphibolite- to greenschist facies extensional shearing (S_T2) within the WLsz, which overprinted S_T1 foliations between 1.87 and 1.86 Ga. Continued greenschist facies shearing younger than 1.86 Ga likely ended by c. 1.83 Ga when lamprophyre dykes cut the structure, which was followed by cooling until c. 1.80 Ga. This work highlights the utility and application of multiple chronometers (zircon, monazite, titanite, garnet) along with structural and petrologic analysis that together can resolve precise orogenic cycles in polymetamorphic terranes that may otherwise be undetected. The time-resolved P–T–D histories derived here enable more robust interpretations regarding the nature and evolution of 1.9 Ga tectonism along the southeast Rae craton margin, which may be used to refine models for Laurentian terrane amalgamation.     

Seasonality of tropospheric ozone and water vapor over Delhi,India: a study based on MOZAIC measurement data

Tropospheric distributions of ozone (O₃) and water vapor (H₂O) have been presented based on the Measurements of OZone and water vapor by Airbus In-Service AirCraft (MOZAIC) data over the metro and capital city of Delhi, India during 1996–2001. The vertical mixing ratios of both O₃ and H₂O show strong seasonal variations. The mixing ratios of O₃ were often below 40 ppbv near the surface and higher values were observed in the free troposphere during the seasons of winter and spring. In the free troposphere, the high mixing ratio of O₃ during the seasons of winter and spring are mainly due to the long-range transport of O₃ and its precursors associated with the westerly-northwesterly circulation. In the lower and middle troposphere, the low mixing ratios of ∼20–30 ppbv observed during the months of July–September are mainly due to prevailing summer monsoon circulation over Indian subcontinent. The summer monsoon circulation, southwest (SW) wind flow, transports the O₃-poor marine air from the Arabian Sea and Indian Ocean. The monthly averages of rainfall and mixing ratio of H₂O show opposite seasonal cycles to that of O₃ mixing ratio in the lower and middle troposphere. The change in the transport pattern also causes substantial seasonal variation in the mixing ratio of H₂O of 3–27 g/kg in the lower troposphere over Delhi. Except for some small-scale anomalies, the similar annual patterns in the mixing ratios of O₃ and H₂O are repeated during the different years of 1996–2001. The case studies based on the profiles of O₃, relative humidity (RH) and temperature show distinct features of vertical distribution over Delhi. The impacts of long range transport of air mass from Africa, the Middle East, Indian Ocean and intrusions of stratospheric O₃ have also been demonstrated using the back trajectory model and remote sensing data for biomass burning and forest fire activities.     

Environmental Governance Meets Reality: A Micro-Scale Perspective on Sustainability Certification Schemes for Oil Palm Smallholders in Jambi,Sumatra

Abstract

Multi-stakeholder sustainability certification schemes have become a favorite instrument for applying good governance, though studies indicate their inefficiency at the producer level. In this study, we used a mixed-method approach to first, map the institutional context of independent oil-palm smallholders in rural Sumatra while, second, reflecting upon the impact of the Smallholder Standard proposed by the Roundtable on Sustainable Palm Oil on smallholder management practices. We hold that non-recognition of micro-scale perspectives within governance processes may partially explain noncompliance with certification principles among smallholders. The Smallholder Standard appears unable to mitigate challenges important for smallholders, who in turn cannot properly comply with it, due to problems including weather instability and high management costs. We suggest that certification schemes need to work on some overlooked but essential preconditions of good governance, namely gaining micro-level visibility and acceptance.     

The geological setting of tourmalinite at Rum Jungle,N.T., Australia — genetic and economic implications

Tourmalinite is a common rock type associated with Proterozoic strata-bound mineral deposits. Although common, it is often difficult to recognise in the field, leading to misidentification. It occurs as a conformable banded quartz-tourmaline lithological unit comprising at least 15% and as much as 50% of the rock. At Rum Jungle, tourmalinite occurs within the oldest sediments (arenites and magnesites) as distinct lenses, as facies equivalents of quartz-magnetite units and mafic schists (tuffs?) and distal equivalents of polymetallic sulfides. Distinct layering, slump folding, rip-up clasts and the association with diagenetic pyrite suggest a sedimentary environment. Enechelon fracturing of the fine-grained, light green tourmaline crystals spectacularly supports pre-deformation formation. The crystals are optically and chemically zoned parallel to the c axis, with irregular growth lamellae width — which supports a pre-regional metamorphic origin. Analyses show the tourmaline to be the Mg-rich variety “dravite”. Most tourmalinites are interpreted as subaqueous marine deposits. It is more likely that they form in lacustrine, shallow water, evaporitic environments, particularly continental rifts. Suitable B-bearing fluids can be generated by hotspring activity and mobilized by CO₂-rich fluids. Association with chemical sediments suggests tourmalinites also have a chemical sediment precursor. Ample evidence at Rum Jungle supports the notion of a continental rift environment, which was the site of deposition of fluvial arenites and alkaline, evaporitic lake sediments. Localised hot-spring activity contributed B-bearing fluids which precipitated chemical sediments according to the pertaining pH, temperature etc. Diagenetic alteration produced the tourmalinite now present. These tourmalinites are comparable to those of similar age elsewhere e.g. Sullivan, Broken Hill. They can be genetically modelled upon Recent borate concentrations, all of which occur in continental rift environments.     

Inventory and evolution of glacial lakes since the Little Ice Age: Lessons from the case of Switzerland

Retreating glaciers give way to new landscapes with lakes as an important element. In this study, we combined available data on lake outlines with historical orthoimagery and glacier outlines for six time periods since the end of the Little Ice Age (LIA; ~1850). We generated a glacial lake inventory for modern times (2016) and traced the evolution of glacial lakes that formed in the deglaciated area since the LIA. In this deglaciated area, a total of 1192 lakes formed over the period of almost 170 years, 987 of them still in existence in 2016. Their total water surface in 2016 was 6.22 ± 0.25 km². The largest lakes are > 0.4 km² (40 ha) in size, while the majority (> 90%) are smaller than 0.01 km². Annual increase rates in area and number peaked in 1946–1973, decreased towards the end of the 20th century, and reached a new high in the latest period 2006–2016. For a period of 43 years (1973–2016), we compared modelled overdeepenings from previous studies to actual lake genesis. For a better prioritization of formation probability, we included glacier-morphological criteria such as glacier width and visible crevassing. About 40% of the modelled overdeepened area actually got covered by lakes. The inclusion of morphological aspects clearly aided in defining a lake formation probability to be linked to each modelled overdeepening. Additional morphological variables, namely dam material and type, surface runoff, and freeboard, were compiled for a subset of larger and ice-contact lakes in 2016, constituting a basis for future hazard assessment.     

Extended search for correlation between solar mean magnetic field Bison data and coronal mass ejections

The Birmingham Solar-Oscillations Network (BiSON) has acquired high-precision solar mean magnetic field (SMMF) data on a 40-s cadence for a decade. We present attempts to compare such data from recent years with the occurrence of coronal mass ejections (CMEs) as recorded by LASCO, using correlation techniques applied to measurements from different BiSON instruments to maximise the sensitivity to CME-related SMMF responses. SMMF measurements were recorded at the time of occurrence of several hundred CMEs. No CME event shows a convincing response in our SMMF data at short periods setting a threshold amplitude of 12 mG. By averaging data sets we are able to set lower thresholds, which depend somewhat on the distribution of response strengths. A brief summary of the very first results of this study is also given in Chaplin et al.     

Ice-avalanche scenario elaboration and uncertainty propagation in numerical simulation of rock-/ice-avalanche-induced impact waves at Mount Hualcán and Lake 513, Peru

The interest in numerical simulation of cascading processes involving mass movements and lakes has recently risen strongly, especially as the formation of new lakes in high-mountain areas as a consequence of glacier recession can be observed all over the world. These lakes are often located close to potentially unstable slopes and therewith prone to impacts from mass movements, which may cause the lake to burst out and endanger settlements further downvalley. The need for hazard assessment of such cascading processes is continuously rising, which demands methodological development of coupled numerical simulations. Our study takes up on the need for systematic analysis of the effect of assumptions taken in the simulation of the process chain and the propagation of the corresponding uncertainties on the simulation results. We complemented the research of Adv Geosci 35:145-155, 2014 carried out at Lake 513 in the Cordillera Blanca, Peru, by focusing on the aspects of (a) ice-avalanche scenario development and of (b) analysis of uncertainty propagation in the coupled numerical simulation of the process chain of an impact wave triggered by a rock/ice avalanche. The analysis of variance of the dimension of the overtopping wave was based on 54 coupled simulation runs, applying RAMMS and IBER for simulation of the ice avalanche and the impact wave, respectively. The results indicate (a) location and magnitude of potential ice-avalanche events, and further showed (b) that the momentum transfer between an avalanche and the impact wave seems to be reliably representable in coupled numerical simulations. The assessed parameters—initial avalanche volume, friction calibration, mass entrainment and transformation of the data between the models—was decisive of whether the wave overtopped or not. The overtopping time and height directly characterize the overtopping wave, while the overtopping volume and the discharge describe the overtopping hydrograph as a consequence of the run-up rather than the wave. The largest uncertainties inherent in the simulation of the impact wave emerge from avalanche-scenario definition rather than from coupling of the models. These findings are of relevance also to subsequent outburst flow simulation and contribute to advance numerical simulation of the entire process chain, which might also be applied to mass movements other than rock/ice avalanches.     

Oceanographic controls on shallow‐water temperate carbonate sedimentation: Spencer Gulf,South Australia

Spencer Gulf is a large (ca 22 000 km²), shallow (<60 m water depth) embayment with active heterozoan carbonate sedimentation. Gulf waters are metahaline (salinities 39 to 47‰) and warm‐temperate (ca 12 to ?28°C) with inverse estuarine circulation. The integrated approach of facies analysis paired with high‐resolution, monthly oceanographic data sets is used to pinpoint controls on sedimentation patterns with more confidence than heretofore possible for temperate systems. Biofragments – mainly bivalves, benthic foraminifera, bryozoans, coralline algae and echinoids – accumulate in five benthic environments: luxuriant seagrass meadows, patchy seagrass sand flats, rhodolith pavements, open gravel/sand plains and muddy seafloors. The biotic diversity of Spencer Gulf is remarkably high, considering the elevated seawater salinities. Echinoids and coralline algae (traditionally considered stenohaline organisms) are ubiquitous. Euphotic zone depth is interpreted as the primary control on environmental distribution, whereas seawater salinity, temperature, hydrodynamics and nutrient availability are viewed as secondary controls. Luxuriant seagrass meadows with carbonate muddy sands dominate brightly lit seafloors where waters have relatively low nutrient concentrations (ca 0 to 1 mg Chl‐a m^?3). Low‐diversity bivalve‐dominated deposits occur in meadows with highest seawater salinities and temperatures (43 to 47‰, up to 28°C). Patchy seagrass sand flats cover less‐illuminated seafloors. Open gravel/sand plains contain coarse bivalve–bryozoan sediments, interpreted as subphotic deposits, in waters with near normal marine salinities and moderate trophic resources (0·5 to 1·6 mg Chl‐a m^?3) to support diverse suspension feeders. Rhodolith pavements (coralline algal gravels) form where seagrass growth is arrested, either because of decreased water clarity due to elevated nutrients and associated phytoplankton growth (0·6 to 2 mg Chl‐a m^?3), or bottom waters that are too energetic for seagrasses (currents up to 2 m sec^?1). Muddy seafloors occur in low‐energy areas below the euphotic zone. The relationships between oceanographic influences and depositional patterns outlined in Spencer Gulf are valuable for environmental interpretations of other recent and ancient (particularly Neogene) high‐salinity and temperate carbonate systems worldwide.