Marine Regression Research Articles

Detrital Zircon U‐Pb Ages and Geochemistry of the Silurian to Permian Sedimentary Rocks in Central Inner Mongolia, China: Implications for Closure of the Paleo‐Asian Ocean

The Solonker suture zone has long been considered to mark the location of the final disappearance of the Paleo‐Asian Ocean in the eastern Central Asian Orogenic Belt (CAOB). However, the time of final suturing is still controversial with two main different proposals of late Permian to early Triassic, and late Devonian. This study reports integrated whole‐rock geochemistry and LA‐ICP‐MS zircon U‐Pb ages of sedimentary rocks from the Silurian Xuniwusu Formation, the Devonian Xilingol Complex and the Permian Zhesi Formation in the Hegenshan‐Xilinhot‐Linxi area in central Inner Mongolia, China. The depositional environment, provenance and tectonic setting of the Silurian‐Devonian and the Permian sediments are compared to constrain the tectonic evolution of the Solonker suture zone and its neighboring zones. The protoliths of the silty slates from the Xuniwusu Formation in the Baolidao zone belong to wacke and were derived from felsic igneous rocks with steady‐state weathering, poor sorting and compositional immaturity. The protoliths of metasedimentary rocks from the Xilingol Complex were wackes and litharenites and were sourced from predominantly felsic igneous rocks with variable weathering conditions and moderate sorting. The Xuniwusu Formation and Xilingol Complex samples both have two groups of detrital zircon that peak at ca. 0.9–1.0 Ga and ca. 420–440 Ma, with maximum deposition ages of late Silurian and middle Devonian age, respectively. Considering the ca. 484–383 Ma volcanic arc in the Baolidao zone, the Xuxiniwu Formation represents an oceanic trench sediment and is covered by the sedimentary rocks in the Xilingol Complex that represents a continental slope sediment in front of the arc. The middle Permian Zhesi Formation metasandstones were derived from predominantly felsic igneous rocks and are texturally immature with very low degrees of rounding and sorting, indicating short transport and rapid burial. The Zhesi Formation in the Hegenshan zone has a main zircon age peak of 302 Ma and a subordinate peak of 423 Ma and was deposited in a back‐arc basin with an early marine transgression during extension and a late marine regression during contraction. The formation also crops out locally in the Baolidao zone with a main zircon age peak of 467 Ma and a minor peak of 359 Ma, and suggests it formed as a marine transgression sedimentary sequence in a restricted extensional basin and followed by a marine regressive event. Two obvious zircon age peaks of 444 Ma and 280 Ma in the Solonker zone and 435 Ma and 274 Ma in Ondor Sum are retrieved from the Zhesi Formation. This suggests as a result of the gradual closure of the Paleo‐Asian Ocean a narrow ocean sedimentary environment with marine regressive sedimentary sequences occupied the Solonker and Ondor Sum zones during the middle Permian. A restricted ocean is suggested by the Permian strata in the Bainaimiao zone. Early Paleozoic subduction until ca. 381 Ma and renewed subduction during ca. 310–254 Ma accompanied by the opening and closure of a back‐arc basin during ca. 298–269 Ma occurred in the northern accretionary zone. In contrast, the southern accretionary zone documented early Paleozoic subduction until ca. 400 Ma and a renewed subduction during ca. 298–246 Ma. The final closure of the Paleo‐Asian ocean therefore lasted at least until the early Triassic and ended with the formation of the Solonker suture zone.

Sedimentation in a synclinal shallow‐marine embayment: Coniacian of the North Sudetic Synclinorium, SW Poland

AbstractThis study is a reconstruction of the Coniacian palaeogeographic and palaeoenvironmental development in the North Sudetic Basin, a synclinal trough within the Late Cretaceous Central European seaway linking the Boreal and Tethyan marine provinces. The basin formed as an early side effect of the Alpine orogeny combined with the mid‐Cretaceous eustasy, and crucial stages of its evolution occurred during the Coniacian. The basin in the early Coniacian was a long and narrow shallow‐marine embayment with a hypothetical (non‐preserved) bayhead strait funnelling tidal currents. Coalescing tidal sand ridges formed a littoral platform that prograded from the bayhead zone along the basin axis, impinged on laterally by the basin‐margin shoreface and local river deltas. A mid‐Coniacian forced marine regression and closure of the bayhead strait, attributed to the Alpine tectonism combined with eustasy, brought about a dramatic change in the basin, whereby the basin‐wide littoral sand platform emerged and turned briefly into a denudated coastal plain. The late Coniacian eustatic marine transgression formed an in‐place growing coastal sand barrier at the outer edge of the former littoral platform, sheltering a paralic limno‐lagoonal plain with peat‐forming mires. The coastal barrier was eventually drowned by the sea and maximum marine flooding occurred, followed by a normal regression recorded as a rapidly upwards‐shallowing succession of offshore‐transition to fluvio‐deltaic deposits. This case study of the sedimentation pattern in an evolving, tectonically controlled marine embayment contributes to the existing facies models for estuarine embayments formed by a passive marine drowning of large fluvial or glacial valleys.

Paaliup Qarmangit 1 site geoarchaeology: Taphonomy of a Thule‐Inuit semi‐subterranean dwelling in a periglacial context in northeastern Hudson Bay

AbstractA multidisciplinary geoarchaeological investigation was performed at Paaliup Qarmangit 1 site (JjGj‐14), located in a valley affected by periglacial processes in Nunavik, Canada. A taphonomic approach was carried out to document site formation processes on the site of a single‐family semi‐subterranean winter Thule‐Inuit house. Off‐site geomorphological and stratigraphical analyses enabled us to reconstruct the sedimentary environment in the valley since the last glaciation. Valley shoreline sediments deposited during the marine regression were reworked by eolian, runoff, and periglacial processes. The site was first used by Dorset people after 143‐327 cal A.D. and then by the Thule‐Inuit between 1317–1413 cal A.D. and 1466–1642 cal A.D. Strategic features of the site, such as the availability of building material, food, and water resources may explain its sucessive use by two different cultures. On‐site, stratigraphical, micromorphological, and macrofossil analyses showed the predominance of niveo‐eolian and nivation processes in landscape and site formations. The archaeological record reveals the action of niveo‐eolian processes throughout the Medieval Climatic Optimum and the Little Ice Age, likely due to local factors, site location, and the bowl‐shape of the semi‐subterranean house. Macrofossil data and radiocarbon dating indicate postdepositional natural and/or anthropogenic reworking of Dorset artefacts.

High frequency (4th order) sequence stratigraphy of Early Miocene deltaic shorelines, offshore Texas and Louisiana

Abstract Well logs and 3D seismic reflection data are integrated to image Lower Miocene depocenters of the upper Texas and westernmost Louisiana coastal and offshore areas. Although previously interpreted at a large scale (3rd order cycles), the detailed stratigraphy and depositional history of the early Miocene succession has not been fully developed. The upper lower Miocene interval from Robulus L (MFS10) to Amphistegina B (MFS9) has been further subdivided into five 4th order cycles (each 200–300 m thick) to provide finer scale stratigraphic detail and interpretation of depositional environments. Results of the finer scale correlations are presented in a series of sandstone percent maps. Overall, the maps display a strike-elongate (sub-parallel to the present day coastline) orientation for the sandstone bodies which thin to the southeast (basinward). Net sandstone thickness is relatively low in older cycles and increases in younger cycles suggesting an overall progradation of the deltaic system into the mapped area through time. Continued marine regression and southward movement of the shoreline are also confirmed by seismic amplitude maps. After a major transgression associated with Robulus L deltaic progradation occurred under rising sea level conditions. Deltaic sediments prograded southward and sandstone brought to the upper Texas coast was distributed laterally by longshore currents to form strandplain coast lines. Maximum regression occurred during the interval MFS9_2 to MFS9_3 when a deltaic depocenter formed offshore Texas in east High Island area. However, sandstone thickness progressively decreased laterally to the east (offshore Louisiana) where more shale was present and marine processes reworked deltaic derived sandstones into shore parallel bars. There was a clear retreat of the shoreline during the youngest cycle marking the beginning of transgression associated with Amphistegina B. Detailed stratigraphic interpretation (at a 4th order scale) shows more variability in the dominant shoreline processes during early Miocene than previously thought. Recognition of sedimentary systems variability at a 4th order scale is critical for improved hydrocarbon exploration and in understanding potential future CO2 storage in the area.

Cold adaptation in the Asian tiger mosquito's native range precedes its invasion success in temperate regions.

Adaptation to environmental conditions within the native range of exotic species can condition the invasion success of these species outside their range. The striking success of the Asian tiger mosquito, Aedes albopictus, to invade temperate regions has been attributed to the winter survival of diapause eggs in cold environments. In this study, we evaluate genetic polymorphisms (SNPs) and wing morphometric variation among three biogeographical regions of the native range of A. albopictus. Reconstructed demographic histories of populations show an initial expansion in Southeast Asia and suggest that marine regression during late Pleistocene and climate warming after the last glacial period favored expansion of populations in southern and northern regions, respectively. Searching for genomic signatures of selection, we identified significantly differentiated SNPs among which several are located in or within 20kb distance from candidate genes for cold adaptation. These genes involve cellular and metabolic processes and several of them have been shown to be differentially expressed under diapausing conditions. The three biogeographical regions also differ for wing size and shape, and wing size increases with latitude supporting Bergmann's rule. Adaptive genetic and morphometric variation observed along the climatic gradient of A. albopictus native range suggests that colonization of northern latitudes promoted adaptation to cold environments prior to its worldwide invasion.

Stratigraphy and Palaeontology of mid-Carboniferous Sequences at Phu Bo Bit in Loei Province, Thailand

The rocks from the Phu Bo Bit section consist of shale and limestone along with an abundant and diverse invertebrate fauna consisting of brachiopods, corals, bryozoans and crinoids, interpreted as having been deposited in shallow marine or shelf environments. At Phu Bo Bit, thick limestone lenses in shale contain microfossils. Based on the foraminiferan and algae assemblage, the stratigraphic sequence of Phu Bo Bit extends from the late Bashkirian to the early Moscovian. According to lithofacies and palaeontology, the Phu Bo Bit section represents the change from carbonate to siliciclastic sedimentation, which provides new evidence on the marine regression during the mid-Carboniferous.

Vegetation dynamics during the last 7500 years on the extreme southern Brazilian coastal plain

For the first time in the extreme southern Brazilian coastal plain, we analyzed palynomorphs in a continuous sequence, using an outcrop profile of relict muds in Hermenegildo Beach (33°42′12″S, 53°18′57″W), Santa Vitória do Palmar, Rio Grande do Sul state. Five radiocarbon dates were obtained. The results revealed the plant succession in an ancient water reservoir near the coastal line. 7500–4000 BP: Marine organisms and freshwater taxa, with herbaceous marsh at the margins. Marine taxa were markedly present from 6000 to 5000 BP, the time of the maximum transgression. Regionally, the vegetation was dominated by grasslands and forest groupings. 4000–2000 BP: Regressive marine phase, with an increased record of freshwater organisms due to increased rainfall. A large increase in herbaceous marsh components resulted in a gradual filling-in of the water reservoir and the development of regional vegetation. 2000–1760 BP: With the end of the marine regression, progressive desalinization promoted the local herbaceous marsh development. Regionally, grassland and forest also developed, indicative of climate amelioration. Local plant succession ceased during this phase of herbaceous marsh ca. 1760 BP, when beach sands covered the sedimentary sequence. This study reveals the phases of a hydrosere in a coastal water reservoir, influenced by marine oscillations, with gradual desalinization and filling-in that culminated in an herbaceous marsh deposit. These results provide a basis for understanding many other similar relict muds spread throughout the coastal plain in the extreme south of Brazil.

A sedimentary depositional and diagenetic model of a Pleistocene/Holocene coastal formation in Alexandria, Mediterranean Sea, Egypt.

Late Quaternary deposition along the northwest coastal plain of Egypt was influenced by sea level fluctuations as documented by a succession of Quaternary carbonate ridges (up to eight or nine) running parallel to the present shoreline.This study focuses upon depositional and diagenetic aspects related to the last glacial cycle and aims to the reconstruction of the coastal sedimentary paleo-depositional environments of the “Gabal El-Kor Island”, located along the Mediterranean coast of Egypt near Alexandria.The sedimentary succession cropping out at Gabal El-Kor Island is interpreted as a part of the Pleistocene second coastal ridge, dated at about 100 Ka.This study revealed that during the last interglacial period (Marine Isotopic Stage 5, MIS 5), marine deposits mostly made of bioclastic grains and ooids were formed and their early diagenetic alteration was achieved under marine subaqueous conditions. After marine regression, during glacial periods, a phase of subaerial exposure followed. This was associated with transport and accumulation of the exposed carbonate sands from the shore inland by wind action. In the postglacial, the sea level rose up to its present level and a lagoon formed behind Gabel El-Kor Island. The most relevant diagenetic events took place in three distinct stages: MIS 5, between MIS 4 and MIS 2, and during the early and middle Holocene MIS 1. Salient diagenetic events refer to vadose, meteoric, and marine phreatic environments respectively.

The role of island physiography in maintaining genetic diversity in the endemic Tyrrhenian wall lizard (Podarcis tiliguerta)

AbstractSea level oscillations occurred during the Pleistocene have strongly affected islands’ physiography by changing area, elevation, and even the number of islands rising above the sea level. Such changes had direct consequences on island genetic diversity by promoting genetic admixture during glacial marine regressions while fragmentation or even extinctions occurred because of marine transgressions. Here we investigated the effect of islands’ physiographic changes on the mitochondrial diversity of 84 individuals of the Tyrrhenian wall lizard (Podarcis tiliguerta) from different islands and island groups surrounding Corsica and Sardinia. The La Maddalena Archipelago that is the more complex of the studied island groups, showed the highest genetic diversity, while we detected decreasing genetic diversity in island groups with lower number of islands and elevation. The genetic imprint we found seems to have been shaped by eustatic changes occurred in the Mediterranean basin during Mid‐Pleistocene times. Indeed, a relevant component of ancient lineages was found on those island groups among which elevation of at least one island allowed populations to survive during the Mid‐Pleistocene marine transgressions. In contrast, we found evidence of recent gene flow between populations inhabiting islands or island groups characterized by low elevation. Our results suggest that many features such as number of islands and their elevation, may provide predictive information to identify those islands or “archipelagos” that deserve special attention in terms of conservation priority.

Legacy of a Pleistocene bacterial community: Patterns in community dynamics through changing ecosystems

Bacterial communities are resilient to the environmental changes, yet the effect of long term ecological changes on bacterial communities remain poorly explored. To study the effect of prolonged environmental changes, a 25 m long sediment core was excavated from a paleo beach ridge located on the Cauvery delta, south east coast of India. Geological evidences suggested that the site has experienced multiple marine transgressions and regressions. The three paleosols from Vettaikaraniruppu (VKI) beach ridge, VKI-2 (2.8 m bgl; 3 kybp), VKI-5 (7.2 m bgl; 6 kybp) and VKI-14 (24.5 m bgl; 146 kybp) was chosen for bacterial community analysis based on their formation period. Bacterial community structure of paleosols was reconstructed using V3 hypervariable region of bacterial 16S rDNA targeted Illumina sequencing. The VKI-5 sediment layer which formed under marine environment contained highest bacterial diversity, and the community was a mix up of terrestrial and marine bacterial population. The final community VKI-2 exhibited an approximate structural pattern witnessed in the native bacterial community VKI-14 which formed during marine regression. Furthermore, marine transgression and regression experienced in VKI resulted in the formation of distinct biogeographic patterns.

Reconstruction holocene environmental changes in North-Western Pacific in relation to paleorecord from Shikotan Island

Results of a paleolimnological investigated of a well-dated lake sediment section from Shikotan Island (Southern Kurils) showed that from ca 8.0 to 5.8 cal ka BP warm and humid period corresponding to middle Holocene optimum took place. Cooling thereafter corresponds to Neoglacial. A reconstructed from ca 0.9 to ca 0.58 cal ka BP warm period can be correlated to a Medieval Warm Period. Cooling after 0.58 cal ka BP can be correlated with the LIA. Marine regression stages were identified at ca 6.2-5.9, 5.5-5.1 and 1.07-0.36 cal ka BP. The general chronology of major climatic events of Holocene in the island is in accordance with the climate records from the North Pacific region. Revealed spatial differences in timing and magnitude of the Late Holocene climatic episodes (LIA, MWP) within region needs further investigations.

Different speciation processes in a cryptobenthic reef fish from the Western Tropical Atlantic

The saddled blenny Malacoctenus triangulatus is a widely distributed species of cryptobenthic reef fish that occurs from the Caribbean to southeastern Brazil, including the oceanic islands. Subtle morphological differences have been observed between populations, suggesting some degree of structuring along its distribution, especially between insular and coastal environments. In this study, we conducted phylogeographic analyses of M. triangulatus based on mitochondrial (cytochrome oxidase I and cytochrome b) and nuclear (rhodopsin) genes, including sequences of M. brunoi, a closely related species endemic to the oceanic islands of southeastern Brazil. Three highly structured lineages were identified within the M. triangulatus complex: one restricted to the Caribbean province probably isolated by the Amazon barrier, and two in the Brazilian province, one in the northeastern oceanic islands (NOI) and another along the coast (including M. brunoi). This result indicates that divergent evolutionary processes have driven the evolution of the saddled benny in the Tropical Southwestern Atlantic: an ancient isolation of the NOI lineage during the Neogene and a recent ecological speciation event in the southeastern oceanic islands, which were connected to the coast during Pleistocene marine regressions. Together, these results provide insights on the evolutionary patterns and oceanographic barriers in the Western Tropical Atlantic.

Reconstruction of Holocene Environmental Changes in North-Western Pacific in Relation to Paleorecord from Shikotan Island

Results of a paleolimnological investigation of a well-dated lake sediment section from Shikotan Island (Southern Kurils) showed that from ca 8.0 to 5.8 cal ka BP a warm and humid period corresponding to middle Holocene optimum took place. Cooling thereafter corresponds to Neoglacial. A reconstructed from ca 0.9 to ca 0.58 cal ka BP warm period can correspond to a Medieval Warm Period. Cooling after 0.58 cal ka BP can be correlated with the LIA. Marine regression stages were identified at ca 6.2–5.9, 5.5–5.1 and 1.07–0.36 cal ka BP. The general chronology of major climatic events of Holocene in the island is in accordance with the climate records from the North Pacific region. Revealed spatial differences in timing and magnitude of the Late Holocene climatic episodes (LIA, MWP) in the region needs further investigations.

Can Tenax Extraction Be Used as a Surrogate Exposure Metric for Laboratory-Based Bioaccumulation Tests Using Marine Sediments?

The Tenax technique was used as an alternative exposure metric to assess the bioavailability of polychlorinated biphenyls (PCBs) from contaminated marine sediments. The sediments used were collected from 2 Superfund sites, New Bedford Harbor (MA, USA) and Gould Island (RI, USA). No sieving was conducted for either sediment after arrival, and sediments were stored in stainless steel drums at 2.8 to 4.0 °C in the dark until use. Exhaustive chemical extractions, single-point 24-h Tenax extractions, and 14-d bioaccumulation tests using the amphipod Leptocheirus plumulosus were conducted for both sediments. The sum of 119 PCB congeners from total exhaustive chemical extraction in the New Bedford Harbor and Gould Island sediments were 1084 and 188.2 µg/g organic carbon, respectively. The PCB concentrations from the bioaccumulation tests and Tenax extractions showed that both exposure metrics followed a similar trend in amount and distribution of PCB congeners. The results from both exposure metrics were fit into a log-log linear regression, and then compared with a previously developed log-log linear model for freshwater organisms. The results showed that although the marine data fell within the prediction intervals of the freshwater linear model, the marine regression followed a lower trajectory due to the differences in both the slopes and intercepts between the marine and freshwater regressions. The present study showed a strong relationship between Tenax and marine invertebrate PCB concentrations. Environ Toxicol Chem 2019;38:1188-1197. © 2019 SETAC.

Late Holocene coastal dynamics, climate pulses and low terraces in the coast of the state of São Paulo, southeast, Brazil

The lower Ribeira de Iguape stream – located in São Paulo, Brazilian southern coast, exhibits complex geomorphologic characteristics that resulted from multiple processes developed during the late Holocene. The present study proposes discussions about the possible relations between short-term climate events from the late Holocene (Climate Pulses) promoting the coastal dynamics and genesis of different levels of low fluvial-marine and marine terraces located in the study area, a subject rarely considered in other studies in humid tropical regions. The procedure of work included grain-size analyses and optically stimulated luminescence (OSL) dating of sediment samples collected from different depositional packages. Four depositional events resulted from transgressive and regressive events geochronologically related with climate pulses from the late Holocene were identified. The events can be summarized as follows: I) Marine Transgression (∼3.5–2.7 ka); II) Marine Regression (2.7–1.8 ka); III) Marine Transgression (1.8–0.65 ka); IV) Marine Regression (0.65–0.10 ka).

Marine Coastline Polygonal Ridges and Surface Roughness Development on a Salt-Crusted Playa: Recognition by Structure-from-Motion Photogrammetry

Salt crust is a normal landform in drying-out salt lake basins or marine regression coastlines, but the surface evolution processes over a decadal or even centenary period are not well understood due to poor data records. Microrelief characteristics control erodibility and erosivity, which will significantly influence wind erosion and dust emission. It is essential to classify the microrelief pattern of salt crust for mapping its spatial distribution and evaluating the environmental process. A desiccated inland tail-end lake would be an example of the coastline surface evolution after regression and represent a good case study of salt crust because of the fewer exogenic process interruptions. For this paper, field work was performed in the Lop Nur playa in China, about 90° E, 40° N, which used to be a salt lake half a century ago. Ground-based photos of the salt crust were acquired and imported into structure-from-motion (SfM) software to produce a fine centimeter-scale digital elevation model (DEM). Two indexes were introduced and extracted from the digital elevation model to classify various types of salt crust: roughness was calculated to evaluate the magnitude and the gray-level co-occurrence matrix (GLCM) score was derived to describe the structure pattern of the salt crust. Moreover, in this paper, sedimentary features during different parts of a playa evaporation cycle are reviewed and peculiar kinds of salt crust found on Lop Nur are further discussed.

Global evolution and paleogeographic distribution of mid-Cretaceous orbitolinids.

Members of the Larger Benthic Foraminiferal (LBF) family Orbitolinidae occurred from the Cretaceous to the Paleogene, however, they were most diverse during the mid-Cretaceous, and dominated the agglutinated LBF assemblages described from limestones of that period. Various orbitolinid species have been used to zone and date lithologies formed in the shallow, warm waters of the Aptian to the early Cenomanian, and many, sometimes inaccurate, generic and sub-generic nomenclatures have been proposed to differentiate the often-subtle morphological changes that orbitolinids exhibit over time. Also, until now, it has not been possible to develop an effective global overview of their evolution and environmental development because descriptions of specimens from Asia have been relatively rare. Following our recent study of over 1800 orbitolinid-rich thin sections of material from 13 outcrops of Langshan limestone, from the Southern Tibetan Plateau, and from the Barito Basin, South Kalimantan, Indonesia, it has been possible to compare the stratigraphic ranges of these orbitolinids with previously described Tethyan and American forms, based on the use of a planktonic zonal (PZ) scheme, itself tied to the most recent chronostratigraphic scale. This has allowed the reconstruction of the phylogenetic and paleogeographic evolution of the orbitolinids from their Valanginian origin in the Tethys. Although the Tethys remained the paleogeographic centre for the orbitolinids, it is inferred here for the first time that a bi-directional paleogeographic migration of some orbitolinid genera occurred from the Tethys to the Americas and also to the Western Pacific region. Our observations and dating suggest that global marine regressions in the Aptian were coincident with, and may well have facilitated, these orbitolinid transoceanic migrations. Migration stopped however after rising sea level in the early Albian appears to have again isolated these provinces from each other. Tectonic forces associated with the subduction of the Farallon Plate and further sea level raises led to the opening of the Western Interior Seaway in North America, which correlates with, and may have been the cause of, the middle Albian (top of PZ Albian 2) extinction of the American orbitolinids. The extinction of the orbitolinids revealed that the Western Pacific province was split into two sub-provinces, with extinction occurring at the end of the early Albian (top of PZ Albian 1) in the Northwest Pacific sub-province, and at the end of the Albian (top of PZ Albian 4) in the subprovince that is today South East Asia (on the margins and west of the Wallace Line). The final near extinction of the orbitolinids occurred at the end of the Cenomanian in the Tethyan province, which coincides with, and may have been caused by, global anoxic oceanic events that correlate with a near-peak Mesozoic eustatic sea level high-stand that led to the overall global collapse of the paleotropical reef ecosystem at that time.

Provenance and diagenesis of Guamá Sandstone, northeastern Pará, Brazil: A Silurian link between the Amazonas and Parnaíba basins

The Guamá Sandstone, exposed in small isolated outcrops in the northeastern region of the State of Pará (Brazil), which are surrounded by Cenozoic sediments, represents a Silurian relic of the Amazonas and Parnaiba basins connected during the Paleozoic and separated only since the Triassic period through the development of the Marajó Basin. Previous outcrop-based studies showed that the Guamá Sandstone is a fine to medium-grained quartzarenite, with high compositional and textural maturity, which was deposited in a coastal marine setting (foreshore/shoreface), correlatable with the environment of the Silurian Nhamundá Formation of the Amazonas Basin. The principal primary sources of the sandstone are associated with rocks formed during the Transamazonico and Brasiliano Cycles according to isotopic dating of zircon. In order to better understand provenance and diagenesis of the Guamá Sandstone core samples of a 50 m deep bore hole, located 6 km north of the town of São Miguel do Guamá, have been studied. The sandstone is usually bioturbated and massive, but planar stratification (foreshore) and cross bedding (shoreface), the latter in the lower part of the profile, are also present indicating a transgressive to regressive coastline. In addition to quartz, the Guamá Sandstone is composed of the following detrital accessory minerals: zircon, tourmaline, rutile, epidote, mica, ilmenite and chromite while kamacite occurred only sporadically indicating, at least for the latter, origin from Fe-Ni-meteorites. Provenance data obtained from the composition of zircon and tourmaline grains, as well as from internal textures of zircon, point to primary igneous and metamorphic source rocks of the Guamá Sandstone. While zircon is principally of magmatic origin, tourmaline grains are derived dominantly from metapelitic, metapsammitic and granitoid rocks. However, pre-existing sediments probably represent, to a lesser extent, the ultimate direct source as suggested by the well-rounded grains of the quartz arenite and worn tourmaline overgrowths. The diagenetic phases of the sandstone include syntaxial quartz overgrowths, microquartz, kaolinite booklets, illite and anatase. The quartz overgrowths are pseudo-zoned and engulf parts of the kaolinite booklets. These may be surrounded by microquartz which usually replaced quartz overgrowths. Illite, which is rare, is probably the product of replacement of kaolinite, and anatase may be related to partial dissolution of ilmenite. The main diagenetic processes including precipitation of quartz and kaolinite began during eodiagenesis, in shallow burial conditions, and proceeded through mesodiagenesis as suggested by multi-episodic quartz zoning and formation of illite. The early precipitation of kaolinite is related to alteration of feldspar and mica caused through flushing of meteoric water during forced marine regression and uplift, probably still in Silurian time. The sources for quartz cementation may include kaolinitization of feldspar and pressure dissolution, the latter associated to concavo-convex and sutured grain contacts, although these types of contact are relatively rare. Therefore, external silica-rich solutions likely occurred.

Past hybridisation and introgression erased traces of mitochondrial lineages evolution in the Neotropical silver catfish Rhamdia quelen (Siluriformes: Heptapteridae)

The Neotropical species complex Rhamdia quelen is composed of at least six mitochondrial lineages. Three of these occur in sympatry in several regional basins, which encompass La Plata basin, Patos-Merin basin system and the coastal lagoons draining to SW Atlantic Ocean. Based on both mitochondrial cytochrome b gene and 10 nuclear microsatellite loci markers, this study aims to investigate the genetic diversity pattern and the reproductive isolation among R. quelen mitochondrial lineages. Past hybridisation and introgression were evidenced among at least two mitochondrial lineages. The ancestral structure recovered in this study was divided into two groups, which could have diverged in the last marine regression. The recent population structure of R. quelen species complex is mostly recovered following the geographic distribution pattern. We delimited seven management units, three inhabiting riverine environments and four associated to different coastal lagoons. Lagoon populations, unlike the riverine ones, would have diverged in a scenario with null or restricted gene flow and small size population, possibly related to bottleneck events. Population genetic structure should be considered for conservation legislation, fishery management and aquaculture regulation. Additionally, the present genetic structure could aggravate the impact of specimen translocation and escapees from aquaculture farms.

Evolutionary and demographic correlates of Pleistocene coastline changes in the Sicilian wall lizardPodarcis wagleriana

AbstractAimEmergence of coastal lowlands during Pleistocene ice ages might have provided conditions for glacial expansions (demographic and spatial), rather than contraction, of coastal populations of temperate species. Here, we tested these predictions in the insular endemic Sicilian wall lizardPodarcis wagleriana.LocationSicily and neighbouring islands.MethodsWe sampled 179 individuals from 45 localities across the whole range ofP. wagleriana. We investigated demographic and spatial variations through time using Bayesian coalescent models (Bayesian phylogeographic reconstruction, Extended Bayesian Skyline plots, Isolation‐with‐migration models) based on multilocusDNAsequence data. We used species distribution modelling to reconstruct present and past habitat suitability.ResultsWe found two main lineages distributed in the east and west portions of the current species range and a third lineage restricted to a small area in the north of Sicily. Multiple lines of evidence from palaeogeographic (shorelines), palaeoclimatic (species distribution models), and multilocus genetic data (demographic and spatial Bayesian reconstructions) indicate that these lineages originated in distinct refugia, located in the north‐western and south‐eastern coastal lowlands, during Middle Pleistocene interglacial phases, and came into secondary contact following demographic and spatial expansions during the last glacial phase.Main conclusionsThis scenario of interglacial contraction and glacial expansion is in sharp contrast with patterns commonly observed in temperate species on the continent but parallels recent findings on other Mediterranean island endemics. Such a reverse expansion–contraction (EC) dynamic has been likely associated with glacial increases of climatically suitable coastal lowlands, suggesting this might be a general pattern in Mediterranean island species and also in other coastal regions strongly affected by glacial marine regressions during glacial episodes. This study provides explicit predictions and some methodological recommendations for testing the reverseECmodel in other region and taxa.