Giant sector-collapse structures (scalloped margins) of the Yangtze Platform and Great Bank of Guizhou,China: Implications for genesis of collapsed carbonate platform margin systems

Sector-collapse structures ranging up to 27 km wide with up to 7.7 km bankward erosion (scalloped margins) and linear escarpments occur along the east-north-east-trending, south-facing margins of the Yangtze Platform and Great Bank of Guizhou. Exposure of one of the structures on the rotated limb of a syncline displays the geometry in profile view. Declivities range from 65° to 90° in the upper wall and decrease asymptotically to the toe. Catastrophic collapses of the margins in both platforms occurred during the late Ladinian as constrained by the ages of strata truncated along the margins and the siliciclastic turbidites that onlap collapse structures. Middle Triassic Anisian and Ladinian platform-edge reef facies and platform-interior facies were truncated along both the Yangtze and Great Bank of Guizhou margins. Lower Triassic facies were also truncated along the Great Bank of Guizhou margin. Gravity transport during the main episodes of collapse occurred as mud-rich debris-flows and as mud-free hyper-concentrated flows. Clasts, several to tens of metres and, exceptionally, hundreds of metres across, were transported to the basin. Following collapse, talus, carbonate turbidites and periplatform-mud accumulated at the toe of slope. Shedding of skeletal grains and carbonate mud indicates active carbonate factories at the margin. Preserved sections of the margins demonstrate that the platforms evolved high-relief, accretionary escarpments prior to collapse. High-relief, without buttressing by basin-filling sediments, predisposed the margins to collapse by development of tensile strain and fracturing within the margin due to the lack of confining stress. The linear geometry of margins and active tectonics in the region supports tectonic activity triggering the collapse. Collapse is thus interpreted to have been triggered by fault movement and seismic shock. Comparison with other systems indicates that evolution from high-relief accretion to tectonic collapse of largely lithified margins resulted in large sector-collapse structures and deposition of a coarse, generally mud-poor breccia apron.     

Controls on carbonate platform architecture and reef recovery across the Palaeozoic to Mesozoic transition: A high-resolution analysis of the Great Bank of Guizhou

Carbonate platforms spanning intervals of global change provide an opportunity to identify causal links between the evolution of marine environment and depositional architecture. This study investigates the controls on platform geometry across the Palaeozoic to Mesozoic transition and yields new stratigraphic and palaeoenvironmental constraints on the Great Bank of Guizhou, a latest Permian to earliest Late Triassic isolated carbonate platform in the Nanpanjiang Basin of south China. Reconstruction of platform architecture was achieved by integrating field mapping, petrography, biostratigraphy, satellite imagery analysis and δ¹³C chemostratigraphy. In contrast to previous interpretations, this study indicates that: (i) the Great Bank of Guizhou transitioned during Early Triassic time from a low-relief bank to a platform with high relief above the basin floor (up to 600 m) and steep slope angles (preserved up to 50°); and (ii) the oldest-known platform-margin reef of the Mesozoic Era grew along steep, prograding clinoforms in an outer-margin to lower-slope environment. Increasing platform relief during Early Triassic time was caused by limited sediment delivery to the basin margin and a high rate of accommodation creation driven by Indosinian convergence. The steep upper Olenekian (upper Lower Triassic) slope is dominated by well-cemented grainstone, suggesting that high carbonate saturation states led to syndepositional or rapid post-depositional sediment stabilization. Latest Spathian reef initiation coincided with global cooling following Early Triassic global warmth. The first Triassic framework-building metazoans on the Great Bank of Guizhou were small calcareous sponges restricted to deeper water settings, but early Mesozoic reef builders were volumetrically dominated by Tubiphytes, a fossil genus of uncertain taxonomic affinity. In aggregate, the stratigraphic architecture of the Great Bank of Guizhou records sedimentary response to long-term environmental and biological recovery from the end-Permian mass extinction, highlighting the close connections among marine chemistry, marine ecosystems and carbonate depositional systems.     

Sedimentary biomarkers reaffirm human impacts on northern Beringian ecosystems during the Last Glacial period

Our understanding of the timing of human arrival to the Americas remains fragmented, despite decades of active research and debate. Genetic research has recently led to the ‘Beringian standstill hypothesis’ (BSH), which suggests an isolated group of humans lived somewhere in Beringia for millennia during the Last Glacial, before a subgroup migrated southward into the American continents about 14 ka. Recently published organic geochemical data suggest human presence around Lake E5 on the Alaskan North Slope during the Last Glacial; however, these biomarker proxies, namely faecal sterols and polycyclic aromatic hydrocarbons (PAHs), are relatively novel and require replication to bolster their support of the BSH. We present new analyses of these biomarkers in the sediment archive of Burial Lake (latitude 68°26′N, longitude 159°10′W m a.s.l.) in northwestern Alaska. Our analyses corroborate that humans were present in Beringia during the Last Glacial and that they likely promoted fire activity. Our data also suggest that humans coexisted with Ice Age megafauna for millennia prior to their eventual extinction at the end of the Last Glacial. Lastly, we identify fire as an overlooked ecological component of the mammoth steppe ecosystem.     

Biomechanical effects of trees in an old‐growth temperate forest

The role of biomechanical effects of trees (BETs) in ecosystem and landscape dynamics is poorly understood. In this study, we aim to (i) describe a widely applicable methodology for quantifying the main BETs in soil, and (ii) analyze the actual frequencies, areas and soil volumes associated with these effects in a mountain temperate old‐growth forest. The research took place in the Boubínský Primeval Forest in the Czech Republic; this forest reserve, predominated by Fagus sylvatica L. and Picea abies (L.) Karst., is among the oldest protected areas in Europe. We evaluated the effects of 4000 standing and lying trees in an area of 10.2 ha from the viewpoint of the following features: tree uprooting, root mounding, bioprotection, trunk baumsteins (rock fragments displaced by trunk growth), root baumsteins, stump hole infilling, trunk and root systems displacements, depressions formed after trunk fall, stemwash, and trunkwash. BETs were recorded in 59% of standing and 51% of lying dead trees (excluding the pervasive soil displacement by thickening trunks and roots and the infilling of decayed stumps). Approximately one tenth of the trees showed simultaneous bioprotective and bioerosion effects. Different tree species and size categories exhibited significantly different biomechanical effects. A bioprotective function was the most frequent phenomenon observed, while treethrows prevailed from the viewpoint of areas and soil volumes affected. The total area influenced by the BETs was 342 m² ha^?1. An additional 774 m² ha^?1 were occupied by older treethrow pit‐mounds with already decayed uprooted trunks. The total volume of soil associated with the studied phenomena was 322 m³ ha^?1, and apart from treethrows, volumes of the living and decaying root systems and bioprotective functions predominated. Other processes were not so frequent but still significant for biogeomorphology. Copyright © 2017 John Wiley & Sons, Ltd.     

Bioindicator role of tintinnid (Protozoa: Ciliophora) for water quality monitoring in Kalpakkam,Tamil Nadu,south east coast of India

The feasibility of a potential bioindicator based on functional groups of microzooplankton tintinnids for bioassessments of water quality status was studied during southwest monsoon (June to September) along the coastal waters of Kalpakkam, India during 2012–2015. The work highlights the following features (1) tintinnid community composed of 28 species belonging to 11 genera and 9 families, revealed significant differences among the four study sites (2) maximum numerical abundance (2224 ± 90 ind. l^? 1) and species diversity (H′ = 2.66) of tintinnid were recorded towards Bay of Bengal whereas minimum abundance (720 ± 35 ind. l^? 1) and diversity (H′ = 1.74) were encountered in the backwater sites, (3) multivariate analyses [RELATE, Biota-environment (BIOENV) and canonical analysis of principal coordinates (CAP)] reveal that chl a, nitrate and phosphate were the potential causative factors for tintinnid distribution. Based on the results, we suggest that tintinnids may be used as a potential bioindicator of water quality status in marine ecosystem.     

Laws,place, history and the interpretation of landforms

The state of an Earth surface system (ESS) is determined by three sets of factors: laws, place, and history. Laws ( L = L₁, L₂, . . . , L_n) are the n general principles applicable to any such system at any time. Place factors ( P = P_1, P_2, . . . , P_m) are the m relevant characteristics of the local or regional environment. History factors ( H = H₁ , H₂, . . . , H_q) include the previous evolutionary pathway of the ESS, its stage of development, past disturbance, and initial conditions. Geoscience investigation may focus on laws, place, or history, but ultimately all three are necessary to understand and explain ESS. The LPH triad is useful as a pedagogical device, illustrated here via application to explaining the world's longest cave (Mammoth Cave, KY). Beyond providing a useful checklist, the LPH framework provides analytical traction to some difficult research problems. For example, studies of the avulsions of three southeast Texas rivers showed substantial differences in avulsion regimes and resulting alluvial morphology, despite the proximity and superficial similarity of the systems. Avulsions are governed by the same laws in all cases [ L (A) = L (B) = L (C)], and the three rivers have undergone the same sea‐level, climate, and tectonic histories, as well as the same general anthropic impacts [ H (A) ≈ H (B) ≈ H (C)]. Though regional environmental controls are similar, local details such as the location of the modern main channel relative to Pleistocene meander channels differ, and thus these place factors explain the differences between the rivers. The LPH framework, or similar types of reasoning, is implicit in many types of geoscience analysis. Explicit attention to the triad can help solve or address many specific problems and remind us of the importance of all three sets of factors. Copyright © 2016 John Wiley & Sons, Ltd.     

Biogeomorphic keystones and equivalents: Examples from a bedrock stream

Biogeomorphic keystone species profoundly impact landscapes, such that their introduction or removal would cause fundamental changes in geomorphic systems. This paper explores the concept of biogeomorphic keystone species by examining the general vs. species-specific biogeomorphic impacts (BGIs) of trees on a limestone bedrock-controlled stream, Shawnee Run, in central Kentucky. Field investigation identified three strong BGIs: (i) biogeomorphic pool formation via bioweathering; (ii) root bank-associated bioprotection; and (iii) avulsion-originated island development linked to bioprotection. This research evaluates these impacts in the context of keystone or other biogeomorphic roles. A field survey was conducted on nine stream reaches, each consisting of 10–12 hydraulic units of riffle, pool, and run. Results suggest that American sycamore (Platanus occidentalis) plays a keystone role by promoting the development of ~42% of pools in the study area. While geomorphic pools are formed by fluvial process–form linkages, these biogeomorphic pools are developed by sycamore root-induced channel bed bioweathering. Only American sycamore and chinquapin oak (Quercus muehlenbergii) exhibited root-bank development amongst 15 different species identified – and thus play a vital role in bank bioprotection. Lastly, trees can promote avulsion-originated island formation by creating erosion-resistant bioprotective patches. Mature trees (in terms of size), particularly large American sycamore and chinquapin oak, dominate Shawnee Run islands with a mean diameter at breast height (DBH) > 40 cm. However, other trees can provide comparable bioprotection, particularly at mature stages. Because its absence would result in fundamentally different stream morphology, sycamore can be considered a biogeomorphic keystone species in Shawnee Run. © 2020 John Wiley & Sons, Ltd.     

System identification for evaluating soil–structure interaction effects in buildings from strong motion recordings

Parametric system identification is used to evaluate seismic soil–structure interaction effects in buildings. The input–output strong motion data pairs needed for evaluations of flexible- and fixed-base fundamental mode parameters are derived. Recordings of lateral free-field, foundation, and roof motions, as well as foundation rocking, are found to be necessary for direct evaluations of modal parameters for both cases of base fixity. For the common situation of missing free-field or base rocking motions, procedures are developed for estimating the modal parameters that cannot be directly evaluated. The accuracy of these estimation procedures for fundamental mode vibration period and damping is verified for eleven sites with complete instrumentation of the structure, foundation, and free-field. © 1998 John Wiley & Sons, Ltd.