Detrital Sediments Research Articles

The chromitites of the Neoproterozoic Bou Azzer ophiolite (central Anti-Atlas, Morocco) revisited

• Intermediate-Cr and high-Cr chromitites in the Bou Azzer ophiolite, Morocco. • Chromitites crystallized from fore-arc basalts (FAB) and boninites, respectively. • Fore-arc position of the ophiolite in a subduction-initiation geodynamic setting. • Super-reduced phases (moissanite and native Cu) formed during serpentinization. The Neoproterozoic Bou Azzer ophiolite in the Moroccan Anti-Atlas Panafrican belt hosts numerous chromitite orebodies within the peridotite section of the oceanic mantle. The chromitites are strongly affected by serpentinization and metamorphism, although they still preserve igneous relicts amenable for petrogenetic interpretation. The major, minor and trace element composition of unaltered chromite cores reveal two compositional groups: intermediate-Cr (Cr# = 0.60 – 0.74) and high-Cr (Cr# = 0.79 – 0.84) and estimates of parental melt compositions suggest crystallization from pulses of fore-arc basalts (FAB) and boninitic melts, respectively, that infiltrated the oceanic supra-subduction zone (SSZ) mantle. A platinum group elements (PGE) mineralization dominated by Ir-Ru-Os is recognized in the chromitites, which has its mineralogical expression in abundant inclusions of Os-Ir alloys and coexisting magmatic laurite (RuS 2 ) and their products of metamorphic alteration. Unusual mineral phases in chromite, not previously reported in this ophiolite, include super-reduced and/or nominally ultra-high pressure minerals moissanite (SiC), native Cu and silicates (oriented clinopyroxene lamellae), but “exotic” zircon and diaspore have also been identified. We interpret that clinopyroxene lamellae have a magmatic origin, whereas super-reduced phases originated during serpentinization processes and diaspore is linked to late circulation of low-silica fluids related to rodingitization. Zircon grains, on the other hand, with apatite and serpentine inclusions, could either have formed after the interaction of chromitite with mantle-derived melts or could represent subducted detrital sediments later incorporated into the chromitites. We offer a comparison of the Bou Azzer chromitites with other Precambrian ophiolitic chromitites worldwide, which are rather scarce in the geological record. The studied chromitites are very similar to the Neoproterozoic chromitites reported in the Arabian-Nubian shield, which are also related to the Panafrican orogeny. Thus, we conclude that the Bou Azzer chromitites formed in a subduction-initiation geodynamic setting with two-stages of evolution, with formation of FAB-derived intermediate-Cr chromitites in the early stage and formation of boninite-derived high-Cr chromitites in the late stage.

Sedimentary environment and organic enrichment mechanisms of lacustrine shale: A case study of the Paleogene Shahejie Formation, Qikou Sag, Bohai Bay Basin

Recent Chinese shale oil (or gas) exploration efforts have focused on the Shahejie Formation of Bohai Basin. However, this work has been handicapped by a lack of state-of-the-art investigations of the targeted shale. Specifically, there is a need for an improved understanding of the role that paleoenvironmental aspects of the basin played in the organic enrichment of these deposits. The present study provides the insights into the paleoenvironmental and depositional history of various of shales of the Es1 Member of Paleogene Shahejie Formation, Qikou Sag, Bohai Basin, and addresses the controlling factors of organic matter (OM) accumulation and preservation in these deposits based on petrographic characteristics, organic geochemistry, and major and trace elements data. Our results suggest that the organic-enriched Es1 oil shale is a desirable exploration target, especially felsic and mixed shale. The felsic shale, mixed shale, dolostone and carbonate shale accumulated under relatively humid climatic conditions but in saline lacustrine environment. Climate, salinity, and redox conditions appear to have varied at least three times during the depositional history of the studied interval. Paleoenvironmental conditions favored stable productivity levels and only a modest supply of clastic detritus sediment resulting in a diminished rate of deposition. Felsic and mixed shale deposits that have the greatest source potential accumulated under more humid atmospheric and oxygen-depleted environmental conditions than dolostone and carbonate shale. The present study suggests infers that organic carbon content of the Es1 oil shale was controlled principally by salinity and secondarily by primary productivity and redox conditions.

Formation, dispersion and accumulation of terra rossa on the Cayman Islands

AbstractUnconsolidated and lithified terra rossa fills surface depressions, joints, fractures and cavities throughout the karst landscapes on the Cayman Islands. The terra rossa is formed from kaolinite, halloysite, vermiculite, hydrotalcite, böhmite, gibbsite, goethite, hematite, anatase, quartz, calcite, dolomite, halite and X‐ray amorphous material. Locally, phosphate derived from bird guano and biofragments is present. Lithified terra rossa is characterized by numerous generations of fractures that are lined with clay and filled with multiple phases of calcite cement. There are no obvious geographic or stratigraphic differences between the terra rossa found on the three islands. A residual origin for the terra rossa is unlikely given that the Oligocene to Pleistocene limestone/dolostone bedrock contains <1% non‐carbonate material. Detrital sediments cannot be the source because none developed on these islands, given that they are surrounded by deep oceanic waters. The terra rossa probably originated largely from wind‐blown volcanic dust or Saharan dust. Various geochemical ratios and rare earth element profiles indicate that that the Cayman terra rossa has a strong affinity to Saharan dust that is frequently blown into the Caribbean region each year. The Cayman terra rossa is characterized by SiO2/Al2O3 ratios (<1.1) that are significantly lower than those associated with terra rossa on other islands, such as Jamaica and Barbados. The low SiO2/Al2O3 ratios of the Cayman terra rossa is due, at least in part, to the fact that quartz is rare to absent. X‐ray amorphous material is a common component in many of the Cayman samples. Although difficult to date precisely, the formation of terra rossa on the Cayman Islands appears to have been an ongoing process for at least the last 5 Myr. This time frame is consistent with the origin of the Sahara Desert that came into existence with aridification of North Africa about 5 Ma.

Petrographical and mineralogical study of detrital strata near and within the Ballık travertine deposit (SW Turkey): architecture of a mixed clastic–carbonate succession

The Ballik area (SW Turkey) was studied as a mixed clastic–continental carbonate reservoir analogue, in which kilometre wide and up to 70-m-thick tufa and travertine lithologies are found in an envelope of detrital sediments, which locally strongly interfinger with these porous carbonates. Former studies focussed on the carbonate lithologies, since they are considered as pre-salt analogues. This study aims to describe the adjacent non-carbonate lithologies, unravel their depositional setting, and address their influence on the overall sedimentary architecture. This study relies on an extensive field campaign, during which 142 samples of all different detrital lithologies were collected. Optical, fluorescence, cathodoluminescence, and Scanning Electron Microscopy (SEM) yielded important insights in the petrography of these lithologies, based on which 5 main lithologies were differentiated: i.e., (1) laminated marls, (2) polygenetic conglomerates, (3) massive marls, (4) tabular sandstones, and (5) coquina accumulations. These were interpreted to represent three different sedimentary facies corresponding to lacustrine, fluvial, and shoreline facies. The (clay) mineralogy of lacustrine sediments was extensively studied by bulk and clay-specific XRD. In this respect, special emphasis was laid on the depositional setting of the lacustrine facies, in which both authigenic palygorskite and poorly ordered dolomite were identified. Petrophysical properties of 16 plugs were determined by He porosimetry and N2-permeability, indicating that the detrital sediments are characterised by poor reservoir properties. The latter causes them to act, after assumed burial compaction, as potential barriers within a continental carbonate reservoir system.

Elemental geochemistry of surface sediments from Manakudy estuary, south‐west coast of India: Inferences to sources of elements and their accumulation

Manakudy estuary is located in the southernmost tip of the Indian sub‐continent, which is essential to study the sediment geochemistry as it receives limited input of sediments during the rainy season from its catchment zones. The elemental geochemical variation of sediments is to understand the mechanism influencing the estuarine environment, and it becomes baseline data to assess the environmental impact in the near future. Twenty surface sediments samples were collected at different seasons, and the grain size results showed that the estuary sediments were carpeted by the mixture of sand, silt, clay, organic matter (OM), and CaCO3. The seasonal variation of elements is slightly more in the pre‐monsoon than in the post‐monsoon season. Fe showed maximum variation, while Cd showed minimum variation in total heavy metal concentrations of detrital sediments. The order of abundance of metals based on their mean values is as Fe > Mn > Cr > Zn > Cu > Pb > Ni > Co > Cd in pre‐monsoon and Fe > Mn > Zn > Cr > Ni > Co > Cu > Pb > Cd > in post‐monsoon, respectively. Different elemental ratios and enrichment factors were used to assess the abundance of major and trace elements in both seasons. The ecological risk assessment clearly shows the higher concentration of a few metals in the estuarine sediments during post‐ monsoon. Correlation and factor analysis show that from geogenic along with the anthropogenic input of industrial effluents, agriculture, urban, municipal wastes and slum sewage are the major sources for derived metals in the estuarine sediments.

Composição de elementos maiores de rochas fonte, solos e sedimentos derivados de rochas graníticas em clima subtropical úmido: contribuição aos estudos de proveniência sedimentar

O estudo dos processos sedimentares e da modificação da composição detrítica entre rocha, solo e sedimento é fundamental para compreender o fracionamento composicional nos primeiros estágios do ciclo sedimentar. Estudos em ambientes atuais são necessários para desvendar a inter-relação complexa entre os processos que geram todo o espectro composicional dos sedimentos detríticos. Neste trabalho, a influência dos processos sedimentares na composição dos sedimentos foi avaliada utilizando-se amostras de rochas fonte, solos e sedimentos detríticos em uma bacia hidrográfica na qual rochas graníticas estão expostas em clima subtropical úmido. As amostras foram separadas em oito intervalos granulométricos de seixo a lama. A composição geoquímica dessas amostras foi analisada por Inductively Coupled Emission Optical Spectroscopy — ICP-EOS. A relação entre a composição química e o tamanho de grão foi analisada por métodos estatísticos utilizando-se diagramas, gráficos de dispersão, análise dos componentes principais e avaliação da estrutura de covariância via análise biplot. Os resultados demonstram o controle do tamanho de grão na composição dos solos e sedimentos e têm implicação no entendimento da influência do intemperismo e do transporte no fracionamento geoquímico. A análise da relação entre a composição de rocha, solo e sedimento em ambientes atuais contribui para a construção e tem potencial para a construção de modelos quantitativos que permitam determinar o efeito dos processos sedimentares na composição das rochas siliciclásticas.

Formation of Tridymite and Evidence for a Hydrothermal History at Gale Crater, Mars

AbstractIn August 2015, the Curiosity Mars rover discovered tridymite, a high‐temperature silica polymorph, in Gale crater. The existing model for its occurrence suggests erosion and detrital sedimentation from silicic volcanic rocks in the crater rim or central peak. The chemistry and mineralogy of the tridymite‐bearing rocks, however, are not consistent with silicic volcanic material. Using data from Curiosity, including chemical composition from the Alpha Particle X‐ray Spectrometer, mineralogy from the CheMin instrument, and evolved gas and isotopic analyses from the Sample Analysis at Mars instrument, we show that the tridymite‐bearing rocks exhibit similar chemical patterns with silica‐rich alteration halos which crosscut the stratigraphy. We infer that the tridymite formed in‐place through hydrothermal processes and show additional chemical and mineralogical results from Gale crater consistent with hydrothermal activity occurring after sediment deposition and lithification.

Influence of Hydraulic Connectivity on Carbon Burial Efficiency in Mackenzie Delta Lake Sediments

AbstractThe Arctic is undergoing accelerated changes in response to ongoing modifications to the climate system, and there is a need for local to regional scale records of past climate variability in order to put these changes into context. The Mackenzie Delta region in northern Canada is populated by numerous small shallow lakes. They are classified as no‐, low‐, and high‐closure (NC, LC, and HC, respectively) lakes, reflecting varying degrees of connection to the river main stem, and have different sedimentation characteristics. This study examines sedimentological (mineral surface area, grain size), carbon isotopic (bulk and molecular‐level) and inorganic isotopic (neodymium) characteristics of sediment cores from three lakes representing each class. We find that HC lake sediments exhibit strikingly different properties from the other lake sediments. Specifically, they are characterized by higher organic carbon loadings per unit mineral surface area and with relatively minor influence from allochthonous, petrogenic (rock‐derived) organic carbon. In contrast, LC and NC lakes have the potential to record basin‐scale climatic changes at a high resolution by virtue of enhanced detrital sedimentation. Overall the delta lakes have the capacity to bury about 2 MtC year−1, with little changes in the last 200 years. However, in the (near) future, an increased number of high closure lakes might change the carbon burial efficiency of the Mackenzie Delta as they seem to retain less carbon than NC and LC lakes.

Stratigraphic revision and reconstruction of the deep-sea fan of the Voirons Flysch (Voirons Nappe, Chablais Prealps)

The Voirons Flysch (Caron in Eclogae Geologicae Helvetiae 69:297–308, 1976), is a flysch sequence aggregated into the sedimentary accretionary prism of the Chablais and Swiss Prealps. Its palaeogeographic location is still debated (South Piemont or Valais realm). We herein present a stratigraphic revision of the westernmost unit of the former Gurnigel Nappe sensu Caron (Eclogae Geologicae Helvetiae 69:297–308, 1976): the Voirons Flysch. This flysch is subdivided into three lithostratigraphic units at the formation level (the Voirons Sandstone, the Vouan Conglomerate, the Boëge Marl), with an additional unit (Bruant Sandstone) of uncertain attribution, ranging from the early Eocene to probably the late Eocene. We further propose a new model of the depositional setting of the deep-sea of the Voirons Flysch based on palaeocurrent directions, the overall geometry and sedimentary features. This model depicts an eastward deflected deep-sea fan. The stratigraphic record of the proximal part of this fan is fairly complete in the Voirons area, whereas its most distal part is only represented by one small exposure of thinly bedded sandstones in the Fenalet quarry. The stratigraphic evolution of the Voirons Flysch shows two major disruptions of the detrital sedimentation at the transition between Voirons Sandstone—Vouan Conglomerate and Vouan Conglomerate—Boëge Marl. The cause of these disturbances has to be constrained in the framework of the palaeogeographic location of the Voirons Flysch.

Size-dependent chemical depletion of sediments in glacial environments: a case study of Mandakini Valley, central Himalaya, India

ABSTRACT The size-dependent chemical maturity of sediment cycling in a high-altitude glacial/periglacial environment was studied by examining the sediments of the Chorabari Glacier, Mandakini Valley, central Himalaya, India. Sediments evacuated from glacial debris and a periglacial area were studied via particle-size analysis, X-ray fluorescence, and X-ray diffraction of different grain-size fractions. The results suggest a clear control of grain-size fraction over chemical weathering and release of its attendant nutrients from the glacial substrate. Elemental indices suggest the release of nutrients is principally controlled through comminution and sub-glacial chemical weathering of leachable minerals such as feldspar, mica, and apatite. Owing to biogeochemical cycles, the release of nutrients like phosphorous and calcium is predominantly controlled through mineral dissolution process of aluminosilicate rocks containing apatite, carbonate minerals and organic complexes. Overall, the size-dependent chemical maturity of glacial sediments suggests a much more chemically active system than previously thought. This study is of particular importance for downstream weathering tracer studies of detrital sediments.

Paleoenvironmental reconstruction of gold-bearing BIF from the Archean Cuiabá deposit based on petrographic and geochemical studies

In the world-class Cuiabá gold deposit, located in the Archean Rio das Velhas greenstone belt in Brazil, metamorphosed carbonate-dominated banded iron formations represent the main lithology hosting gold mineralization. The deposit's stratigraphy consists of a basal metavolcanic unit with carbonaceous metasedimentary rocks, followed by a layer of gold-bearing BIF and thin layer of carbonaceous metapelite, a upper metavolcanic unit, and turbiditic metapelites associated with metavolcanoclastic rocks. The BIF is essentially composed of fine-grained quartz and siderite, with variable contents of carbonaceous matter. It is assumed that these chemical sedimentary rocks, classified as Algoma-type BIF, have been formed in the deep-sea environment, relatively close to volcanic spreading centers. This hypothesis is supported by the BIF's geochemical signature, which in general presents a positive anomaly of La (La/La*paas = 0.64–3.43), Eu (Eu/Eu*paas = 1.27–4.05) and Y, reflecting characteristics inherited from seawater and high-temperature hydrothermal fluids (>250 °C). In addition, the commonly observed superchondritic Y/Ho ratio values reinforce the influence of seawater on the rock's geochemistry. The absence of Ce anomalies concerns the oxygenation conditions of seawater, indicating low oxygen availability. The low content of Al2O3 (<0,27%) and other immobile elements, typical of detrital sediments, corroborate the interpretation that these rocks have been deposited in a deep marine environment with low siliciclastic influence. Subordinately, ferruginous metachert, BIF with magnetite and BIF with silicates are also present in the Cuiabá gold deposit. This diversity may be related to specific changes in the basin during the deposition of ferruginous chemical sedimentary rocks - changes that may indicate, for example. variations in the detritic support or availability of carbonaceous matter. Furthermore, geochemical studies from the lower and upper mafic metavolcanic units of the Cuiabá gold deposit helped to constrain and provide a broader understanding of the depositional basin's geotectonic context. Consequently, a genetic model that includes the paleoenvironment of this sequence is proposed. Both rocks of the lower and upper units have very similar mineralogical composition, but register significant geochemical differences. The lower unit is a calc-alkaline basalt/andesite from a continental arc, interpreted as having been developed during the Rio das Velhas II event (2800-2760 Ma), while the upper unit is defined as basalt with a transitional affinity to tholeiitic, possibly corresponding to an ocean floor volcanism.

Geochemical, mineralogical and sedimentological analyses of reworked sediments (new) in the syn- to post-rift Middle Cretaceous-Quaternary detrital deposits from western Atlantic margin of Cameroon: evidence from sedimentation-erosion alternation in the context of passive margin evolution

Reworked detrital sediments were discovered in the syn-rift to post-rift Cretaceous to Quaternary (Pleistocene) detrital deposits of the Douala coastal sub-basin in the eastern margin of the South Atlantic Ocean at West Cameroon. This contribution presents the geochemical, mineralogical, and sedimentological data to constrain the origin of these reworked sediments. The investigated samples are characterized by the high values of Plagioclase Index of Alteration (PIA, 97.83–99.90), Chemical Index of Alteration (CIA, 71.9–99.08), and Chemical Index of Weathering (CIW, 98.67–99.90). Data gleaned from these indices suggest that the reworked sediments and their potential source rocks experienced severe weathering. The A-CN-K plot along with the mineralogical indicates a probably post-depositional K-enrichment. The high light rare earth elements (LREE)/heavy rare earth elements (HREE) ratios (15.82–79.13) suggest that the reworked were mainly derived from felsic igneous rocks. This interpretation is further confirmed by the Zr versus TiO2, TiO2/Al2O3, Th/Co versus La/Sc, and La/Th versus Th/Yb plots, which indicate felsic igneous and silicic source rocks. Meanwhile, the high variability of LREE/HREE ratios (15.82–79.13) and the positive Eu anomalies (1.10–1.32) of studied samples could indicate a little contribution of mafic source rocks. Analysis of the distribution of the heavy minerals (e.g. tourmaline, aluminum silicate, and rutile) revealed that these felsic and silicic source rocks are mainly granite and gneiss, probably belonging to the Nyong Group and the Western part of the Neoproterozoic Yaounde Group. The morphoscopic analyses of the host rocks (very angular to angular shapes) have revealed that the reworked sediments have proximal to subproximal onshore origins, probably from former sedimentary deposits, as is indicated by variation of their color. The tectonic discriminant diagram indicates the Oceanic Island Arc and Active Continental Margin which are inconsistent with the geological history of the Douala coastal sub-basin and could be explained by the recycling effect experienced by studied sediments which have been deposited in the coastal plain to the fluvial environment in the Low System Track context.

Lake sedimentary processes and vegetation changes over the last 45k cal abpin the uplands of south‐eastern Amazonia

ABSTRACTThis study addresses paleoclimate influences in a southern Amazonia ecotone based on multiproxy records from lakes of the Carajás region during the last 45k cal abp.Wet and cool environmental conditions marked the initial deposition in shallow depressions with detrital sediments and high weathering rates until 40k cal abp.Concomitantly, forest and C3 canga plants, along with cool‐adapted taxa, developed; however, short drier episodes enabled expansion of C4 plants and diagenetic formation of siderite. A massive event of siderite formation occurred approximately 30k cal abpunder strong drier conditions. Afterwards, wet and cool environmental conditions returned and persisted until the Last Glacial Maximum (LGM). The LGM was marked by lake‐level lowstands and subaerial exposure. The transition from the LGM to the Holocene is marked by the onset of oscillations in temperature and humidity, with an expansion of forest and canga plants. Cool taxa were present for the last time in the Carajás region ~ 9.5–9k cal abp.After 10k cal abp, shallow lakes became upland swamps due to natural infilling processes, but the current vegetation types and structures of the plateaus were acquired only after 3k cal abpunder wetter climatic conditions.

Middle Eocene carbonate platforms of the westernmost Tethys

A study of the paleoenvironmental evolution of the middle Eocene platforms recognized in the westernmost Tethys has been carried out in the well-exposed middle Eocene succession from Sierra Espuña-Mula basin (Betic Cordillera, S Spain). Eight microfacies (Mf1 to Mf8) have been recognized, based mainly on fossil assemblages (principally larger benthic foraminifera), and rock texture and fabric. The fossiliferous assemblage can be assigned to the ‘subtropical’ heterozoan association or to the low-latitude ‘foralgal facies’, which are dominated by non-framework building, light-dependent biota such as perforate larger benthic foraminifera, coralline algae, and sometimes green algae and solitary corals. Larger benthic foraminifer assemblages, corresponding from euphotic to oligophotic conditions and the large surface showed, suggest a progressive marine ramp under essentially oligotrophic conditions. Eventually, supply of detrital sediments from the continent and/or upwelling currents increases the nutrients of marine waters. Comparison with other Tethyan sectors allows stating that coral-reef buildups (z-corals) were widespread on shallow platforms of the central and eastern Tethys Ocean, but these were neither of great dimensions nor dominant because of the much more dominant presence of larger benthic foraminifera. Moreover, these coral constructions were completely absents in the westernmost Tethys. The dominance of larger benthic foraminifera and the absence of z-corals in the westernmost Tethys are explained by particular paleogeographic features due to the occurrence of a narrow and deep oceanic branch (i.e., the Maghrebian Flysch Basin) connecting the Tethys with the Atlantic Ocean. The various issues regarding the morphological characters and evolution of larger benthic foraminifera in the study area, such as sizes of tests, specific diversity and/or intraspecific variability, number of appearances and last occurrences during the middle Eocene are analyzed and compared with those appearing in other Tethyan sectors. In addition, the early to late Bartonian boundary is recognized in the study area as critical for the biological change as in other shallow-marine environments along the Tethys margins.

Sedimentary record of climate change across the Eocene/Oligocene transition from the Qaidam Basin, northeastern Tibetan Plateau

The Eocene/Oligocene transition (EOT) was marked by a major environmental reorganization accompanied by an abrupt shift of the Earth system from “greenhouse” to “icehouse” conditions, which is well documented in marine sedimentary records. Although there are an increasing number of terrestrial records documenting this major climatic reorganization, high-resolution sedimentologic and petrographic evidence from terrestrial contexts is sparse, and the processes driving the EOT which affected the formation of continental detrital sediments are unclear. Here we present evidence from lithofacies, lithology, sediment color, and sediment grain size from a section in the northern part of the NE Tibetan Plateau (Tibetan Plateau), which spans the EOT. The results indicate that major changes in the grain size (coarsening), chromaticity a* (redness increasing), thick gypsum layers (disappearance), structural maturity (decreasing roundness and sorting), sediment compositional maturity (increasing content of lithic, feldspar and mafic minerals) of silt and sandstone occurred at ~33.8 Ma, which were considered to be a response to the EOT. The synchronous changes in sedimentological and petrological characteristics and weathering proxies in the Qaidam Basin (Qaidam Basin) and neighboring basins suggest that the climate-forced weakening of weathering and erosion may have been the direct cause of those proxy changes the direct cause of those proxy change. Global cooling is speculated to have been the first-order driving factor in regulating climatic evolution and weathering responses in the Qaidam Basin and even the Asian interior at the EOT. A conceptual model is presented which summarizes the mechanisms responsible for the observed changes in sedimentary characteristics across the EOT.

Modelling Ediacaran metazoan–microbial reef growth

AbstractThroughout the Phanerozoic, sessile metazoans grew in close association with various microbial carbonates to form reefs. The first metazoans with calcareous hard‐parts appeared in the terminal Ediacaran,ca550 million years ago, and these also commonly grew associated with microbial mats, thrombolites and stromatolites, to form the oldest known metazoan–microbial reefs. These hard‐parts also formed the first skeletal (bioclastic) carbonate sediments, which increased to dominate shallow marine carbonate sedimentary production during the Phanerozoic. Here the growth dynamics and sedimentary interactions between Ediacaran microbial–metazoan reefs and their bioclastic products are described based on reef complexes from the Nama Group, Namibia (ca547 Ma), and the first three‐dimensional numerical models are constructed to parametrize these dynamics. These reefs are observed to form large domal mounds and columns neighbouring locally high rates of bioclastic sediment accumulation, and commonly evolve to flat‐topped surfaces overlain by sediment. This model is parsimonious, embodying a single dynamic rule:metazoans attach to microbialite mounds which grow radially into free space shedding bioclasts in proportion to metazoan production. Models evolve mounds into columns due to spatial competition, and produce an oscillating interplay between mound expansion and smothering that results in an inter‐fingering of microbialite mound and detrital sediment. This dynamic is shown to be non‐linear in the proportion of bioclastic sediment produced. Smothering by bioclastic sediment is also demonstrated to both reduce the rate (by volume) of subsequent mound growth, and to overwhelm the growing mat surfaces of stromatolites: after a threshold is reached in the rate of bioclastic sediment deposition, sediment terminates all stromatolite growth. These models show that the general characteristics of field observations can be explained by a single dynamic rule, and that reef‐sourced bioclasts may make an important local contribution to Ediacaran to Cambrian microbial–metazoan reef dynamics – a contribution that remains important throughout the Phanerozoic.

Mineralogy and geochemistry of deeply-buried marine sediments of the Vaca Muerta-Quintuco system in the Neuquén Basin (Chacay Melehue section), Argentina: Paleoclimatic and paleoenvironmental implications for the global Tithonian-Valanginian reconstructions

The Vaca Muerta-Quintuco (VM-Q) system of Tithonian-early Valanginian age was studied in the Chacay Melehue section of the Neuquén Basin (western Argentina) by means of sedimentological, mineralogical and geochemical analyses in order to determine the main driving factors that triggered the paleoenvironmental change from a carbonate ramp (Vaca Muerta Formation, VMFm) to a mixed siliciclastic/carbonatic marine environment (Quintuco Formation, QFm). The VMFm was divided into two stratigraphic intervals: Lower VMFm (LVMFm) and Upper VMFm (UVMFm), whereas the QFm is subdivided into the Puesto Barros Member (PBMb) and the Cerro La Visera Member (CVMb), which can be correlated to other sections in the basin (e.g., Puerta Curaco). Isolated, turbiditic sandstone beds, correlated to the Huncal Member, are included in the QFm.The LVMFm (Tithonian) and the UVMFm (Berriasian) are constituted by dark, well-laminated marls, mudstones, calcite concretions and tuffs. The PBMb (earlier early Valanginian) is constituted by marls and sandstones, whereas the CVMb (later early Valanginian) is constituted by marls, mudstones, siltstones, sandstones and coquinas. The LVMFm (Total organic carbon, TOC ~ 1–4 wt%) is characterized by the enrichment of redox sensitive trace elements (RSTE), where the enrichment of Ni and Cu suggest high productivity in the water column, and the enrichment of Mo, U, V points to sea bottom anoxia, with periods of increased oxygenation as deduced from higher P concentrations in marls and mudstones. The clay mineral association is constituted by mixed-layer illite/smectite formed by the transformation of smectitic layers. The predominance of smectite in coeval sucessions, less overprinted by burial diagenesis, suggests a temperate and semi-arid climate in the adjacent continent. The UVMFm (TOC ~ >1 wt%) is characterized by a gradual decrease of the RSTE pointing to a decrease in productivity and a slight increase in the oxygenation of the sea bottom. In addition, a change towards more humid conditions in the continent is inferred by both the increase in the Chemical Index of Alteration and the Al2O3/TiO2 ratio. The PBMb (TOC ~1 wt%) has even lower content of RSTE indicating diminished sea water productivity and a gradual rise of the oxygenation of the sea bottom. In this interval, increased illite contents suggest periods of enhanced physical weathering, probably related to the tectonic uplift of the Huincul Ridge. The RSTE in the CVMb (TOC <1 wt%) documents a fully oxygenated sea bottom, where the productivity of the water column was negligible. The presence of kaolinite and the increment of the detrital sedimentation in the CVMb indicate a change towards more humid conditions in the hinterlands. The enhanced runoff caused by this paleoclimatic change towards more humid conditions that started in the early Berriasian and increased during the later early Valanginian triggered the change from carbonate ramp to mixed siliciclastic/carbonatic to siliciclastic marine paleoenvironments. The organic carbon isotope composition (δ13Corg vs. VPDB) of the VMFm ranges between −30.0 and −23.4‰, whereas in the QFm values range between −29.0 and −23.9‰. Two positive carbon isotope excursions (PCIE) are recorded in the system: PCIE-A in the lower part of the VMFm (early Tithonian) with δ13Corg values ~ −25‰, and PCIE-B in the upper part of the QFm (later early Valanginian) with δ13Corg values ~ −24.5‰. The shift in δ13Corg at PCIE-B is up to +4.2‰ and marks the onset of the Weissert Event. This is the first time that the onset of this event is recorded in the Neuquén Basin, within the Lissonia riveroi ammonite zone. The results of our study confirm that clay mineralogy, trace elements and stable isotopes are valuable proxies of past ocean-climate variability even in sediment deposits that underwent near-surface and deep burial diagenesis.

Depositional and environmental controlling factors on the genesis of Quaternary tufa deposits from Bonito region, Central-West Brazil

The Pleistocene and Holocene tufas of the Serra da Bodoquena (Bonito region, Brazilian Midwest) consist of several deposits related to a karstic system developed in the carbonatic rocks of Corumbá Group. Tufas currently forming in the Serra da Bodoquena area (Mimoso and Formoso rivers) were investigated for a better understanding of which environmental changes control the deposition of different types of tufa facies. The Mimoso River displays waterfalls, cascades, barriers and large pools as morphological elements. The topographic breaks on the bedrock of this river control the distribution of these main morphological elements. Five facies were identified: (i) stromatolitic boundstones, characterized by an alternation of dense and porous laminae; (ii) phytohermal boundstones, corresponding to bryophyte cushions; (iii) phytoclastic rudstones, formed by the accumulation of leaves and plant fragments; (iv) oncoidal rudstones, composed by coated grains with concentric laminae (oncoids); and (v) unconsolidated detrital sediment, formed by bioclasts and tufa fragments. SEM images indicated the presence of microorganisms such as Oocardium stratum Nägeli 1849 and Phormidium incrustatum Gomont ex Gomont 1892, as well as diatoms and fungi filaments colonizing tufa samples. On the other hand, the Formoso River is more sinuous, lacking significative topographic breaks and presents flooded sub-environments, such as inactive barriers and other shallow fluvial areas, containing unconsolidated detrital sediment, lime mud and oncoidal rudstones. No microbial remains have been observed. The Bonito region has a humid warm climate with a rainy summer season and a dry winter season, and temperatures barely varying through year. The seasonality of rainy and dry seasons influences both the physicochemical changes of water and, indirectly, the microbial communities of tufa. However, the very different facies displayed by each river, Mimoso and Formoso, are not directly linked to climate. The stepped profile of the Mimoso River allows fast flowing waters producing fast CO2 outgassing that leads to high calcite precipitation rates. The gentle slope of the Formoso River and its slow flowing waters leads to slow passive CO2 outgassing and photosynthesis controlling calcite precipitation at lower rates than in the Mimoso River.

New insights into lake responses to rapid climate change: the Younger Dryas in Lake Gościąż, central Poland

The sediment profile from Lake Gościąż in central Poland comprises a continuous, seasonally resolved and exceptionally well‐preserved archive of the Younger Dryas (YD) climate variation. This provides a unique opportunity for detailed investigation of lake system responses during periods of rapid climate cooling (YD onset) and warming (YD termination). The new varve record of Lake Gościąż presented here spans 1662 years from the late Allerød (AL) to the early Preboreal (PB). Microscopic varve counting provides an independent chronology with a YD duration of 1149+14/–22 years, which confirms previous results of 1140±40 years. We link stable oxygen isotopes and chironomid‐based air temperature reconstructions with the response of various geochemical and varve microfacies proxies especially focusing on the onset and termination of the YD. Cooling at the YD onset lasted ~180 years, which is about a century longer than the terminal warming that was completed in ~70 years. During the AL/YD transition, environmental proxy data lagged the onset of cooling by ~90 years and revealed an increase of lake productivity and internal lake re‐suspension as well as slightly higher detrital sediment input. In contrast, rapid warming and environmental changes during the YD/PB transition occurred simultaneously. However, initial changes such as declining diatom deposition and detrital input occurred already a few centuries before the rapid warming at the YD/PB transition. These environmental changes likely reflect a gradual increase in summer air temperatures already during the YD. Our data indicate complex and differing environmental responses to the major climate changes related to the YD, which involve different proxy sensitivities and threshold processes.

The use of nutrient budget approach for informing eutrophication management in urbanised shallow coastal lakes: A case study from Lake Nokoué in Benin

Lake Nokoué in Benin supports the livelihoods of more than 500.000 people through fish production and sand mining. However, its health is jeopardised by eutrophication and excessive growth of the invasive water hyacinth, which dies off and sink to sediment during dry seasons when the lake's salinity increases due to tidal intrusion. Water-hyacinth constitutes then a potential source of internal nutrient loading through the decomposition of detritus and therefore can exacerbate eutrophication problems. An important management question is whether the removal of water hyacinth before it dies would significantly reduce eutrophication problems through the removal of nutrients. To compare the amount of nutrients removed by water-hyacinth biomass to the amount of nutrients transported into the Lake, mass-balance approach has been taken. Nutrient concentrations, water hyacinth cover, and hydrological inflows were measured over a 12 month period from November 2016 to October 2017. Water-hyacinth biomass estimation was based on a calibrated NDVI and field measurement relationship. In total the hyacinth covered up to 18 km2 (10%) of the lake, corresponding to 80,000 tonnes of dry weight which sink to the bottom each year when salinity rise. The potential internal nutrient loaded from hyacinth to the system was estimated to 1021 tonnes of nitrogen (8% of external input) and 231 tonnes of phosphorus (40% of external input). Considering that the tidal nutrient output compensates almost all the external nutrient inputs (about 70%), part of the nutrient available for eutrophication must come from internal sources, likely through the decomposition of sediment detritus.