Exceptional Storm Research Articles

High Production of Gigantic Jets by a Thunderstorm Over Indian Ocean

AbstractThirteen gigantic jets (GJ) were observed on 12 February 2020, with a sensitive video camera located at Maïdo Observatory, La Réunion Island in Indian Ocean. They were produced within 68 min by nearby cells embedded in a convective system almost 500 km from Maïdo. The video imagery combined with the lightning activity from GLD360, the cloud top temperature (CTT), the ELF radiations, and the reanalysis of several meteorological parameters allow us to analyze their characteristics and their conditions of production. The altitudes for 12 GJ events are estimated between 85 and 89 km. All jets are of negative polarity and most of them preceded by a positive stroke/pulse in the discharge event. They are produced in sequences of a few minutes, during short pulses of convection within cells in phase of development and associated with dominant positive cloud‐to‐ground lightning flashes. The most luminous GJ produced the strongest Current Moment (CM) maximum, close to 280 kA km, and the largest Charge Moment Change (CMC). The CMCs associated with the GJ events range from about 1,000 C km to close to 5,500 C km, especially thanks to the CM during the trailing phase. Several GJ events exhibit a double structure with two jets slightly shifted in space, most of them occurring within the same field of the video imagery. The environment of this exceptional storm in terms of GJ production, exhibits extreme values of various parameters known to be favorable for GJ production, especially related to the warm cloud depth.

Extreme snowstorms lead to large-scale seabird breeding failures in Antarctica

Climate change increases the frequency and intensity of extreme weather events that negatively impact wildlife, from individuals to whole ecosystems1. In polar environments, such events include heat waves2, anomalous sea ice concentrations3 and storms4. Polar seabirds are adapted to withstand harsh conditions, and although extreme weather events affect their breeding success and other demographic rates, they are thought to affect only a part of the population. Complete breeding failure of an entire population due to extreme environmental conditions is rarely observed5. Here we report how exceptional storm activity in Dronning Maud Land (DML), Antarctica, in the austral summer of 2021/2022 caused almost complete and large-scale breeding failures of the area's three most common seabird species - Antarctic petrel (Thalassoica antarctica), Snow petrel (Pagodroma nivea) and South polar skua (Stercorarius maccormicki).

Evaluation of the Ultimate Collapse Load of a High-Voltage Transmission Tower under Excessive Wind Loads

Several high-voltage transmission towers failed under excessive wind loads in a mountainous and exposed area. This study discusses the efficient and reliable modeling of lattice towers dominantly loaded by wind, a scenario which led to a collapse cascade in a high-voltage transmission line. The ultimate load-bearing capacity had to be estimated and the failure positions identified. Finite Element Analysis was employed through static analyses, Linear Buckling Analyses (LBA) and RIKS analyses (Arc-Length method) in Abaqus 2021. With the purpose of improving the accuracy in the simulation of structural instabilities of complex lattice structures, the model sensitivity to superimposed geometrical imperfections and the joint stiffness of the truss connections were investigated in brace and lattice structure sub-assemblies. Afterwards, linear analyses and non-linear analyses with imperfections were performed on the single tower model. The analysis proved that solely excessive wind can cause such failure on the lattice structures, and the critical structural elements have been correctly identified. The investigation proved that the towers had not been under-designed with respect to the standards valid at the time of erection. However, they were not designed for this exceptional storm event, and evidence was provided that wind alone could bring about the collapse. It is nevertheless not recommended to increase the safety factors in general for the design of such structures, but to base the assumed loading on actual and local wind and service load measurements.

New wind-wave climate records in the Western Mediterranean Sea

This study presents a detailed analysis of changes in wind and wave climate in the Western Mediterranean Sea (WMed), based on 41 years of accurate wind and wave hindcasts. The purpose of this research is to assess the magnitude of recent changes in wave climate and to locate the coastal areas most affected by these changes. Starting from the Theil-Sen slope estimator and the Mann Kendall test, trends in mean and Max significant wave heights (SWH) and wind speed (WS) are analysed simultaneously on seasonal and annual scales. Thus, the new wave records observed since 2010 have been located spatially and temporally using a simple spatial analysis method, while the increases in maximum wave heights over the last decade have been estimated and mapped. This work was motivated by evidence pointed out by several authors concerning the influence of global climate change on the local climate in the Mediterranean Sea and by the increase in the number and intensity of wave storm events over recent years. Several exceptional storms have recently been observed along the Mediterranean coasts, including storm Adrian in 2018 and storm Gloria in 2020, which resulted in enormous damage along the French and Spanish coasts. The results of the present study reflect a worrying situation in large part of the WMed coasts. Most of the WMed basin experiences a significant increasing trend in the annual Max of SWH and WS with evident inter-seasonal variability that underlines the importance of multi-scale analysis to assess wind and wave trends. Since 2013, about half of the WMed coastline has experienced records in wave climate, not recorded at least since 1979, and several areas have experienced three successive records. Several WMed coasts are experiencing a worrying evolution of the wave climate, which requires a serious mobilisation to prevent probable catastrophic wave storms and ensure sustainable and economic development.

Decadal patterns and trends in benthic-pelagic exchange processes

In marine environments, the exchange of particles and solutes between the seafloor and overlying water column, known as benthic-pelagic (B/P) coupling is an important component in many biological and biogeochemical cycles. Key processes and drivers involved in this exchange display strongly seasonal variability, especially in temperate coastal environments. The magnitude and timings of these seasonal patterns however are not identical year-on-year, and the influence of this inter-annual variability on the rate and direction of B/P exchange, as well as the influence of longer term, multi-year trends, are less well understood. In this current study, multi-year temporal patterns of benthic-pelagic solute and particle exchange were investigated on the examples of particulate organic carbon and dissolved inorganic nitrogen time series data, to assess connections between inter- and multi-annual processes and characterize their nature and what drives them. To this end, a decadal (2009–2018) time-series dataset that combines biological, physical, meteorological and chemical measurements from the Western Channel Observatory, Plymouth, UK was analysed in combination with supplementary data from several environmental monitoring agencies. Time-series decomposition using seasonal decomposition with locally estimated scatterplot smoothing revealed that the main causes of inter-annual variability were extreme outlier events, some of which were influential enough to cause multi-annual trends. Stochastic meteorological and biological extremes, such as exceptional storms and phytoplankton blooms explained a large proportion of outlier events in the time series. Global-scale climatic fluctuations, such as North Atlantic Oscillation (NAO) and Southern Oscillation Index were reflected in benthic-pelagic exchange trends when they co-occurred in an additive manner (e.g. positive NAO and El Niño). The importance of multi-parameter long-term observatories, such as the Western Channel Observatory, is highlighted, and the use of transdisciplinary time-series datasets to identify individual events which have large ecosystem-level impacts is demonstrated. In order to identify and monitor long-term effects, such as climate trends or decadal global ocean cycles, multi-decadal sustained observations are of vital importance.

Spatial Distribution of Energy of Subinertial Baroclinic Motions in the Baltic Sea

We explore the basic properties of the “climate” of the field of subinertial motions with periods of 2–12 days in the Baltic Sea. The calculations are performed using the output of the Rossby Center Ocean Model RCO with a spatial resolution of 2 nautical miles for 1961–2005. The field of such motions in the Baltic Sea is strongly asymmetric, with much more energy present in the eastern regions of the sea. Spatial distributions of 5-yr average amplitudes of fluctuations of the main and seasonal pycnocline, near-bottom velocity and kinetic and potential energy reflect this asymmetry and also exhibit extensive variability on scales of a few tens of kilometers. The majority of potential energy of such motions is concentrated in a narrow nearshore strip of the sea with a typical width of <20 km. The largest values of kinetic energy occur along the gently sloping seabed at intermediate depths. The areas of maximum of absolute nearbed velocities only partially match similar areas hosting very large kinetic energy or strong fluctuations of the location of the pycnocline. Both the major properties and spatial details of the discussed distributions exhibit almost no difference for the years 1961–1965 and 2000–2005, except for the maximum seabed velocities that are somewhat larger in 2000–2005 apparently because of exceptional storms in 2001 and 2005.

Integrated sea storm management strategy: the 29 October 2018 event in the Adriatic Sea

Abstract. Addressing coastal risks related to sea storms requires an integrative approach which combines monitoring stations, forecasting models, early warning systems, and coastal management and planning. Such great effort is sometimes possible only through transnational cooperation, which becomes thus vital to face, effectively and promptly, the marine events which are responsible for damage impacting the environment and citizens' life. Here we present a shared and interoperable system to allow a better exchange of and elaboration on information related to sea storms among countries. The proposed integrated web system (IWS) is a combination of a common data system for sharing ocean observations and forecasts, a multi-model ensemble system, a geoportal, and interactive geo-visualisation tools to make results available to the general public. The multi-model ensemble mean and spread for sea level height and wave characteristics are used to describe three different sea condition scenarios. The IWS is designed to provide sea state information required for issuing coastal risk alerts over the analysed region as well as for being easily integrated into existing local early warning systems. This study describes the application of the developed system to the exceptional storm event of 29 October 2018 that caused severe flooding and damage to coastal infrastructure in the Adriatic Sea. The forecasted ensemble products were successfully compared with in situ observations. The hazards estimated by integrating IWS results in existing early warning systems were confirmed by documented storm impacts along the coast of Slovenia, Emilia-Romagna and the city of Venice. For the investigated event, the most severe simulated scenario results provide a realistic and conservative estimation of the peak storm conditions to be used in coastal risk management.

The December 1984 exceptional storm: a reference event for watercourses management and flood control plans in the Saf-Saf wadi watershed, NE of Algeria

The December 1984 exceptional storm: a reference event for watercourses management and flood control plans in the Saf-Saf wadi watershed, NE of Algeria

Fingerprinting changes of source apportionments from mixed land uses in stream sediments before and after an exceptional rainstorm event

The loss of fertile topsoil is one of the principal soil degradation problems in mountain agroecosystems worldwide. Soil erosion rates reach their maximum during exceptional storm events that remove soil particles, especially from unprotected topsoil. In Mediterranean mountainous environments, several centuries of non-irrigated agriculture and the subsequent removal of natural vegetation for developing agriculture has increased the surface area prone to erosion. In addition, the irregularity in exceptional precipitation events results in a great loss of fertile soil, the subsequent siltation of reservoirs and a decrease in water quality. To analyse the soil response to exceptional events, 161 source samples were collected in a 23 km2 catchment that was mostly cultivated at the beginning of the last century. Source samples were distributed over the five main land use/land covers such as agricultural land, pine afforestation, open forest, bare soil and channel bank areas. Furthermore, 20 channel bed sediment samples were collected along the main streams before and after the exceptional storm event to document changes in the sediments. In addition, floodplain sediments were collected to provide a close replication of sediments deposited during regular storm events. Source apportionments were calculated using the FingerPro unmixing model in the pre-event, regular events and post-event scenarios.The unmixing outputs displayed a large variation of source apportionments from the upper part to the lower part of the catchment and from pre- to post-event sediments. After the event, a decrease of >70% of the clay fraction and its associated elements such as Fe, Al, K, Ba, Sr, Rb, Pb, Zn, V, 137Cs, 40K, 232Th and SOC along with a rise in contents of elements associated with the coarse fraction (Si, Nb, Zr, Ti, P and 226Ra) was recorded in the channel bed sediments. At the catchment outlet, the pre-event sediment showed substantial contributions from bare soil (29%) and from agriculture and channel banks, which both reached 35%, while the channel bank was the main source along the catchment ranging between 44 and 71%. The low contribution from soil under natural covers with a mean value <4% underlines the benefits of vegetation to prevent soil loss. In the post-event sediment, the channel bank contribution increased up to 63% at the catchment outlet. Our findings highlight the hazards of exceptional storm events on modifying sediment source contribution and exporting fine sediment.

The ups and downs of a canopy-forming seaweed over a span of more than one century

Canopy-forming seaweeds constitute marine forests that deliver ecosystem services. The worldwide range shift, sharp decline or loss of many of these forests, caused by the cumulative impact of increasing human pressure and climate change, have been widely documented. Contrasting examples, reflecting higher than expected resilience, have been more rarely reported. Here, we took the opportunity of having at our disposal a two-century suite of documents (herbarium vouchers, articles) and a ~120-year observation period, dealing with a long-lived brown seaweed, Cystoseira mediterranea, along a well-explored Mediterranean coastline in the Gulf of Lions, to depict the fate of its populations. In addition, we provided baselines for future surveys, with a high degree of accuracy. The northernmost population, scattered on rare suitable substrates, gradually declined and has been extinct since the 1980s. The length of shore occupied by the southern population showed a long-term decline trend, with two sharp minima followed by partial recovery. The causes of the decline differ between sites and periods: coastal development, pollution, competition with mussels, heatwaves and exceptional storms. Overall, the Gulf of Lions populations reflects long-lasting resilience, higher than expected, and a health status that is better than that reported for many other canopy-forming seaweeds.

Coastal boulder deposit as evidence of an ocean‐wide prehistoric tsunami originated on the Atacama Desert coast (northern Chile)

AbstractA Late Holocene cliff‐top deposit of large boulders well above the limits of modern storm waves is described from the southern coast of the Atacama Desert (northern Chile). The largest moved boulder weighs &gt;40 t and field data point to a flood height &gt;18·5 m above high tide level and an inland penetration greater than 284 m from the cliff edge. The minimum flow velocity needed for particle entrainment was estimated as 10·1 ms−1 and the most likely processes of sediment deposition for different boulders were deduced. The boulder distribution, sorting and orientation of imbricated debris, together with the significant wave height of extreme storms reported and the occurrence of interplate earthquakes in the study area indicate that the deposit records a single event, interpreted here as a tsunami wave train rather than exceptional storm waves. The boulder field was dated to between the 13th and the 16th Centuries ce and possibly correlates with the 1420 Oei orphan tsunami, that affected the eastern coast of Japan. A magnitude of 8·8 to 9·4 has been estimated for the earthquake, which may be one of the larger events of a super‐cycle of earthquakes in the southern Atacama Desert. These cycle‐ending earthquakes involve large rupture areas (lengths in excess of 600 km) and highly destructive ocean‐wide tsunamigenic events.

Storm-wave development of shore-normal grooves (gutters) on a steep sandstone beach face

Shore-normal grooves (gutters) cut into the seabed have been reported widely from the marine geological record. Grooves commonly are spaced regularly across plane, consolidated surfaces in the littoral and sub-littoral zones and may be deeply incised. Despite their common occurrence in the rock record, there are few detailed descriptions of examples from modern environments. Previously reported examples have been ascribed to erosion by wave-induced currents, especially storm-driven near-shore flows. In particular, examples from beach faces have been related to either wave swash or backwash. However, no conceptual model exists to explain the presence of grooves, their morphology or their spacing alongshore.Herein, quasi-regularly spaced grooves on a soft sandstone beach face are described and interpreted to have formed due to wave breaking and swash zone processes consequent upon exceptional storms at sea. The groove morphologies are quantified using terrestrial laser scanning. Numerical modelling of the translation from offshore waves to nearshore breaking waves provides estimates of the swash zone parameters. A consideration of swash zone processes provides an explanation for formation of the grooves. In particular, the swash zone shear stress distribution and consequent bed erosion is a dome-shaped function of distance across the beach face, and this controls the cross-shore variability in groove depths. High-speed sheet flows, such as swash and backwash, develop periodic, shore normal, high and low speed streaks alongshore. Consequent streaky erosion produces the quasi-regular alongshore groove spacings. However, on any given beach face the specific spacing of grooves is likely a property, not only of the local sheet flow attributes, but also of larger-scale morphological forcing. This outcome suggests that spacing is an emergent property of the coupled sheet flow and larger-scale forcing, and thus specific spacings on any beach face remain unpredictable.

Development of a target-site-based regional frequency model using historical information

Nuclear power plants in France are designed to withstand natural hazards. Nevertheless, some exceptional storm surges, considered as outliers, are not properly addressed by classical statistical models. Local frequency estimations can be obtained with acceptable uncertainties if the data set containing the extreme storm surge levels is sufficiently complete and not characterized by the presence of an outlier. Otherwise, additional information such as regional and historical storm surges may be used to mitigate the lack of data and the influence of the outlier by increasing its representativeness in the sample. The objective of the present work is to develop a regional frequency model (RFM) using historical storm surges. Here we propose a RFM using historical information (HI). The empirical spatial extremogram is used herein to form a homogenous region of interest centered on a target site. A related issue regards the reconstitution, at the target site and from its neighbors, of missed storm surges with a multiple linear regression (MLR). MLR analysis can be considered conclusive if available observations at neighboring sites are informative enough and the reconstitution results meet some criteria during the cross-validation process. A total of 35 harbors located on the French (Atlantic and English Channel) and British coasts are used as a whole region with the La Rochelle site as a target site. The Peaks-Over-Threshold frequency model is used. The results are compared to those of an existing model based on the index flood method. The use of HI in the developed RFM increases the representativeness of the outlier in the sample. Fitting results at the right tail of the distribution appear more adequate and the 100-year return level is about 30 cm higher. The 100-year return level in the initial fitting has a return period of about 30 years in the updated fitting.

Extreme Weather Event Triggers Cascade Towards Extreme Turbidity in a Clear-water Lake

Climate forecasts project a global increase in extreme weather events, but information on the consequences for ecosystems is scarce. Of particular significance for lakes are severe storms that can influence biogeochemical processes and biological communities by disrupting the vertical thermal structure during periods of stratification. An exceptional storm passing over northern Germany in July 2011 provided an opportunity to assess the consequences and underlying mechanisms of such extreme events on the interplay between the physics and ecological characteristics of a deep, nutrient-poor lake. Wind speeds were among the most extreme on record. A suite of variables measured throughout the event consistently indicates that a cascade of processes pushed the clear-water lake into an exceptionally turbid state. Specifically, thermocline deepening by the storm-entrained cyanobacteria of a deep chlorophyll maximum located at about 8 m depth into the surface mixed layer. Released from light limitation, intense photosynthesis of the cyanobacteria boosted primary production, increased algal biomass, raised the pH and thus induced massive calcite precipitation to a level never observed within three decades of lake monitoring. As a consequence, water transparency dropped from 6.5 to 2.1 m, the minimum on record for 40 years, and the euphotic zone shrank by about 8 m for several weeks. These results show that cyanobacterial blooms not only are promoted by climate warming, but can also be triggered by extreme storms. Clear-water lakes developing a deep chlorophyll maximum appear to be particularly at risk in the future, if such events become more intense or frequent.

Boulder accumulations related to extreme wave events on the eastern coast of Malta

Abstract. The accumulation of large boulders related to waves generated by either tsunamis or extreme storm events have been observed in different areas of the Mediterranean Sea. Along the eastern low-lying rocky coasts of Malta, five sites with large boulder deposits have been investigated, measured and mapped. These boulders have been detached and moved from the nearshore and the lowest parts of the coast by sea wave action. In the Sicily–Malta channel, heavy storms are common and originate from the NE and NW winds. Conversely, few tsunamis have been recorded in historical documents to have reached the Maltese archipelago. We present a multi-disciplinary study, which aims to define the characteristics of these boulder accumulations, in order to assess the coastal geo-hazard implications triggered by the sheer ability of extreme waves to detach and move large rocky blocks inland. The wave heights required to transport 77 coastal boulders were calculated using various hydrodynamic equations. Particular attention was given to the quantification of the input parameters required in the workings of these equations, such as size, density and distance from the coast. In addition, accelerator mass spectrometry (AMS) 14C ages were determined from selected samples of marine organisms encrusted on some of the coastal boulders. The combination of the results obtained both by the hydrodynamic equations, which provided values comparable with those observed and measured during the storms, and radiocarbon dating suggests that the majority of the boulders have been detached and moved by intense storm waves. These boulders testify to the existence of a real hazard for the coasts of Malta, i.e. that of very high storm waves, which, during exceptional storms, are able to detach large blocks of volumes exceeding 10 m3 from the coastal edge and the nearshore bottom, and also to transport them inland. Nevertheless, the occurrence of one or more tsunami events cannot be ruled out, since radiocarbon dating of some marine organisms did reveal ages which may be related to historically known tsunamis in the Mediterranean region, such as the ones in AD 963, 1329, 1693 and 1743.

Disentangling the history of complex multi-phased shell beds based on the analysis of 3D point cloud data

Abstract We present the largest GIS-based data set of a single shell bed, comprising more than 10,280 manually outlined objects. The data are derived from a digital surface model based on high-resolution terrestrial laser scanning (TLS) and orthophotos obtained by photogrammetric survey, with a sampling distance of 1 mm and 0.5 mm, respectively. The shell bed is an event deposit, formed by a tsunami or an exceptional storm in an Early Miocene estuary. Disarticulated shells of the giant oyster Crassostrea gryphoides are the most frequent objects along with venerid, mytilid and solenid bivalves and potamidid gastropods. The contradicting ecological requirements and different grades of preservation of the various taxa mixed in the shell bed, along with a statistical analysis of the correlations of occurrences of the species, reveal an amalgamation of at least two pre- and two post-event phases of settlement under different environmental conditions. Certain areas of the shell bed display seemingly significant but opposing shell orientations. These patterns in coquinas formed by densely spaced elongate shells may result from local alignment of neighboring valves due to occasional events and bioturbation during the years of exposure. Similarly, the patchy occurrence of high ratios of shells in stable convex-up positions may simply be a result of such “maturity” effects. Finally, we document the difficulties in detecting potential tsunami signatures in shallow marine settings even in exceptionally preserved shell beds due to taphonomic bias by post-event processes.

The contribution of short-waves in storm surges: Two case studies in the Bay of Biscay

This study investigates the contributions of short waves in storm surges through the hindcast of two storms that hit the central part of the Bay of Biscay recently. Despite displaying comparable wind speed and directions in the study area, these two storms induced different storm surges and sea states. Xynthia (27–28th of February 2010) was characterized by large (up to 7m significant wave height Hs) and short-period waves and induced an exceptional storm surge, locally larger than 1.6m. The second storm, Joachim (15–16th of December 2011), was characterized by very large (up to Hs>10m) and long-period waves but only induced a storm surge almost two times lower. To investigate these differences, a new unstructured grid and fully coupled modeling system is applied, with a spatial resolution fine-enough to adequately represent the surf zones over most of the study area (25m). The analysis of the modeling results and the available field observations reveals firstly that the exceptional surge during Xynthia originated from young and steep waves, enhancing surface stress. This particular sea-state is explained by the abnormal track of Xynthia, which restricted the fetch to a few hundred km. The wave radiation stress gradient locally induced setup larger than 0.4m along the coastlines fully exposed to ocean waves, while wave setup in the range 0.1–0.2m was also shown to develop regionally and to propagate in sheltered harbors. Comparatively, wave-enhanced bottom stress appears to be a second-order process and has a more limited impact on storm surges.

The Carrington solar flares of 1859: consequences on life.

The beginning of September 1859 was the occasion of the first and unique observation of a giant solar white light flare, auroral displays were observed at low latitudes and geomagnetic observatories recorded exceptional storms. This paper reviews the impact of the event on the earth system with a special emphasis on living processes using the historical record and current scientific analysis. The data used includes reports from the telegraph operators, mortality and morbidity records, proxies as agricultural production. Comparisons with later solar flare events will be attempted on the basis of the record and the consequences of an event of comparable magnitude to the 1859 set of flares will be discussed.

The role of anomalous SST and surface fluxes over the southeastern North Atlantic in the explosive development of windstorm Xynthia

AbstractIn late February 2010 the extraordinary windstorm Xynthia crossed over southwestern and central Europe and caused severe damage, affecting particularly the Spanish and French Atlantic coasts. The storm was embedded in uncommon large‐scale atmospheric and boundary conditions prior to and during its development, namely enhanced sea‐surface temperatures (SST) within the low‐level entrainment zone of air masses, an unusual southerly position of the polar jet stream, and a remarkable split jet structure in the upper troposphere. To analyse the processes that led to the rapid intensification of this exceptional storm originating close to the subtropics (30°N), the sensitivity of the cyclone intensification to latent heat release is determined using the regional climate model COSMO‐CLM forced with European Centre for Medium‐range Weather Forecasts Reanalysis (ERA)‐Interim data. A control simulation with observed SST shows that moist and warm air masses originating from the subtropical North Atlantic were involved in the cyclogenesis process and led to the formation of a vertical tower with high values of potential vorticity (PV). Sensitivity studies with reduced SST or increased laminar boundary roughness for heat led to reduced surface latent heat fluxes. This induced both a weaker and partly retarded development of the cyclone and a weakening of the PV tower, together with reduced diabatic heating rates, particularly at lower and mid‐levels. We infer that diabatic processes played a crucial role during the phase of rapid deepening of Xynthia and thus to its intensity over the southeastern North Atlantic. We suggest that windstorms such as Xynthia may occur more frequently under future climate conditions due to the warming SSTs and potentially enhanced latent‐heat release, thus increasing the windstorm risk for southwestern Europe.

Lower Permian stems as fluvial paleocurrent indicators of the Parnaíba Basin, northern Brazil

A comprehensive biostratinomic study was carried out with abundant stems from the Lower Permian Motuca Formation of the intracratonic Parnaíba Basin, central-north Brazil. The fossils represent a rare tropical to subtropical paleofloristic record in north Gondwana. Tree ferns dominate the assemblages (mainly Tietea, secondarily Psaronius), followed by gymnosperms, sphenophytes, other ferns and rare lycophytes. They are silica-permineralized, commonly reach 4 m length (exceptionally more than 10 m), lie loosely on the ground or are embedded in the original sandstone or siltstone matrix, and attract particular attention because of their frequent parallel attitudes. Many tree fern stems present the original straight cylindrical to slightly conical forms, other are somewhat flattened, and the gymnosperm stems are usually more irregular. Measurements of stem orientations and dimensions were made in three sites approximately aligned in a W–E direction in a distance of 27.3 km at the conservation unit “Tocantins Fossil Trees Natural Monument”. In the eastern site, rose diagrams for 54 stems indicate a relatively narrow azimuthal range to SE. These stems commonly present attached basal bulbous root mantles and thin cylindrical sandstone envelopes, which sometimes hold, almost adjacent to the lateral stem surface, permineralized fern pinnae and other small plant fragments. In the more central site, 82 measured stems are preferentially oriented in the SW–NE direction, the proportion of gymnosperms is higher and cross-stratification sets of sandstones indicate paleocurrents mainly to NE and secondarily to SE. In the western site, most of the 42 measured stems lie in E–W positions. The predominantly sandy succession, where the fossil stems are best represented, evidences a braided fluvial system under semiarid conditions. The low plant diversity, some xeromorphic features and the supposedly almost syndepositional silica impregnation of the plants are coherent with marked dry seasons. Thick mudstones and some coquinites below and above the sandy interval may represent lacustrine facies formed in probably more humid conditions. The taphonomic history of the preserved plants began with exceptional storms that caused fast-flowing high water in channels and far into the floodplains. In the eastern site region, many tree ferns only fell, thus sometimes covering and protecting plant litter and leaves from further fragmentation. Assemblages of the central and western sites suggest that the trees were uprooted and transported in suspension (floating) parallel to the flow. Heavier ends of stems (according to their form or because of attached basal bulbous root mantle or large apical fronds) were oriented to upstream because of inertial forces. During falling water stage, the stems were stranded on riverbanks, usually maintaining the previous transport orientation, and were slightly buried. The perpendicular or oblique positions of some stems may have been caused by interference with other stems or shallow bars. Rare observed stems were apparently waterlogged before the final depositional process and transported as bedload. The differences of interpreted channel orientations between the three sites are expected in a braided fluvial system, considering the very low gradients of the basin and the work scale in the order of tens of kilometers. The mean direction of the drainage probably was to east and the flows apparently became weaker downstream. This study seems to provide reliable data for paleocurrent interpretations, especially considering areas with scarce preserved sedimentary structures.