Lacustrine Systems Research Articles

Tectonically conditioned record of continental interior paleoclimate during the Carnian Pluvial Episode: The Upper Triassic Los Rastros Formation, Argentina

Abstract Discerning paleoclimate parameters in depositional systems of the continental interior is challenging because the system response and stratigraphic record of climate are controlled by tectonic processes and are mediated through landscape and hydrological evolution of fluvial lacustrine systems. Climate and tectonic signals cannot be deconvolved from stratigraphic patterns alone but require additional information or data sets that directly record climate or tectonic influence. The Carnian Los Rastros Formation in northwest Argentina provides an excellent case study that integrates an appropriate range of information in a system with strong climate and tectonic signals, being deposited in part during the Carnian Pluvial Episode and spanning the active rift phase of the Ischigualasto–Villa Unión Basin. We examined the stratigraphic and spatial patterns of carbon (C) and oxygen (O) stable isotopes in lacustrine carbonates from the Los Rastros Formation in multiple parts of the basin to constrain paleohydrological conditions and paleotemperatures. Practically all C and O isotope values are characterized by negative values: δ18Ocarb –11.6‰ and –15.7‰ (χ average –13.1‰; 1σ = 1.6) and δ13Ccarb –2.6‰ to –8.0‰ (χ average –5.1‰; 1σ = 2.1), reflecting the latitude, altitude, and continentality of the lake system and its vegetated and humid catchment area. Stratigraphic patterns of stable isotope data from two different localities (Cerro Bola North and Cerro Bola South) show a change from short water-residence time to long residence time and back to short residence time. This contrasts with sedimentologic, organic geochemical, and small-scale stratigraphic patterns that indicate an overfilled lake basin, which is expected to contain a completely open-hydrology isotopic signature. Paleotemperatures calculated from marginal lacustrine carbonates show a warm and quite variable paleothermal range consonant with their continental interior position and with Global Climate Models estimates for high paleolatitudes. Warmer paleotemperatures (linked to aridity, probably smaller lake size, and less thermal mass) precede the Carnian Pluvial Episode, whereas relatively cooler paleotemperatures coincide with the Carnian Pluvial Episode (linked to humidity, probably larger lake size, and more thermal mass). Carbon and oxygen stable isotope signatures integrated with sedimentologic and physiographic information allow us to propose that tectonics, specifically, half-graben tilting during the active synrift phase, dominated over climate effects as the cause of hydrological fluctuations of this system, even during the Carnian Pluvial Episode. Without appropriate stratigraphic-tectonic context, single-proxy reconstructions of continental-interior paleoclimate can be misleading. A robust interpretation of climate effects requires characterization of tectonic effects, geomorphology, paleohydrology, and sedimentary system responses.

Triple oxygen isotope systematics of evaporation and mixing processes in a dynamic desert lake system

Abstract. This study investigates the combined hydrogen deuterium and triple oxygen isotope hydrology of the Salar del Huasco, an endorheic salt flat with shallow lakes at its centre that is located on the Altiplano Plateau, N Chile. This lacustrine system is hydrologically dynamic and complex because it receives inflow from multiple surface and groundwater sources. It undergoes seasonal flooding, followed by rapid shrinking of the water body at the prevailing arid climate with very high evaporation rates. At any given point in time, ponds, lakes, and recharge sources capture a large range of evaporation degrees. Samples taken between 2017 and 2019 show a range of δ18O between −13.3 ‰ and 14.5 ‰, d-excess between 7 ‰ and −100 ‰, and 17O-excess between 19 and −108 per meg. A pan evaporation experiment conducted on-site was used to derive the turbulence coefficient of the Craig–Gordon isotope evaporation model for the local wind regime. This, along with sampling of atmospheric vapour at the salar (-21.0±3.3 ‰ for δ18O, 34±6 ‰ for d-excess and 23±9 per meg for 17O-excess), enabled the accurate reproduction of measured ponds and lake isotope data by the Craig–Gordon model. In contrast to classic δ2H–δ18O studies, the 17O-excess data not only allow one to distinguish two different types of evaporation – evaporation with and without recharge – but also to identify mixing processes between evaporated lake water and fresh flood water. Multiple generations of infiltration events can also be inferred from the triple oxygen isotope composition of inflow water, indicating mixing of sources with different evaporation histories. These processes cannot be resolved using classic δ2H–δ18O data alone. Adding triple oxygen isotope measurements to isotope hydrology studies may therefore significantly improve the accuracy of a lake's hydrological balance – i.e. the evaporation-to-inflow ratio (E / I) – estimated by water isotope data and application of the Craig–Gordon isotope evaporation model.

Mercury methylation in oxic aquatic macro-environments: a review

Mercury methylation in aquatic environments is a key process that incorporates this neurotoxin into the food chain and ultimately the human diet. Mercury methylation is considered to be essentially biotic and mainly driven by sulfate-reducing bacteria present in the bottom sediments in aquatic systems. However, in recent decades, many researchers have shown that this methylation also occurs in oxic layers in conjunction with a high content of particulate organic matter and localized depletion of dissolved oxygen. The goals of this review are to summarize our current understanding of Hg methylation in water columns of both marine and freshwater environments, as well as to highlight knowledge gaps and future research needs. Most of the literature showed that suspended particles (known as marine and lake snow) could be the microenvironment in which Hg methylation could occur across oxic water columns, because they have been recognized as a site of organic matter mineralization and as presenting oxygen gradients around and inside them. To date, the majority of these studies concern marine environments, highlighting the need for more studies in freshwater environments, particularly lacustrine systems. Investigating this new methylmercury production environment is essential for a better understanding of methylmercury incorporation into the trophic chain. In this review, we also propose a model which attempts to highlight the relative importance of a MeHg epilimnetic path over a MeHg benthic-hypolimnetic path, especially in deep lakes. We believe that this model could help to better focus future scientific efforts in limnic environments regarding the MeHg cycle.

Distinct responses of late Miocene eolian and lacustrine systems to astronomical forcing in NE Tibet

AbstractEast Asian summer monsoon (EASM) and winter monsoon (EAWM) variability on orbital time scales during the late Miocene remains poorly constrained. Climate models reveal variable responses of the EASM and EAWM to astronomical forcing, but there is a lack of empirical evidence from the geological record to validate these results. In this study, we used time series analysis to reconstruct climatic changes and orbital forcing from eolian and lacustrine sediment archives from NE Tibet during the late Miocene. Analysis of magnetic susceptibility data demonstrates that lacustrine sediments in the Tianshui Basin (Yaodian section) show dominant ∼100 k.y. eccentricity forcing in the late Miocene (ca. 10.25–8 Ma). In contrast, eolian deposits in the Jianzha Basin (Jiarang section) show significant 405 k.y. eccentricity and 41 k.y. obliquity forcing over the same interval with weak evidence for ∼100 k.y. eccentricity cycles. Grain size data from the Yaodian section support a lacustrine origin of these sediments and also support previous work demonstrating aridification in NE Tibet after ca. 8.6 Ma. Taken together, our analyses highlight markedly different orbital forcing responses of lacustrine and eolian sedimentary systems during the late Miocene. We suggest that the dominant ∼100 k.y. lacustrine cycles in the Yaodian section, which were mainly controlled by EASM variability, may have been linked to Antarctic ice sheet and/or low-latitude insolation modulations related to precession amplitude modulation by eccentricity. In contrast, the orbital signature of eolian sediments in the Jiarang section was significantly influenced by the EAWM and can be linked to variability in meridional temperature/pressure gradients.

Temporal changes in the relative abundance of anadromous Arctic charr, brown trout, and Atlantic salmon in northern Europe: Do they reflect changing climates?

AbstractWarming temperatures resulting from climate change may alter the distribution and abundance of many freshwater fish species, especially those in northern latitudes. Owing to interspecific differences in temperature adaptations and tolerances, warming may lead to changes in the fish community as a result of shifts in the abundance of co‐existing species.We investigated how increased temperatures have potentially affected the abundance and catch composition of anadromous salmonids in northern Europe. We used national angling catch statistics over a period of several decades (1993–2018) as a proxy of relative annual abundance of adult Atlantic salmon (Salmo salar), brown trout (Salmo trutta), and Arctic charr (Salvelinus alpinus) in water courses in Iceland and in northern Norway. These are the only locations in the world where the three species coexist naturally as anadromous forms, and where they are distributed across latitudinal gradients characterised by varying temperatures.Interpolated data on average annual air temperatures proximate to the study rivers increased 1–1.7°C in northern Norway and 1.0–1.5°C in Iceland during the 26‐year study period.While the overall total number of fish caught has remained relatively stable during the period of study, gradual climate warming has been accompanied by a marked change in the catch composition of the three species in both countries. Increased temperatures were related to a decreased proportion of Arctic charr and an increased proportion of brown trout, while no temperature effect was found for Atlantic salmon. Controlling for the difference between fluvial and lacustrine systems, brown trout has begun to replace Arctic charr in all regions, whereas the relative catch of Atlantic salmon has been stable.Interpretation of angling data can facilitate understanding of longer‐term trends in catch data and provide insights into possible underlying changes in fish species and alert managers to shifts in abundance that might be mitigated by management actions. However, care must be taken to avoid the confounding effects that changes in fisheries management measures, angler preferences, and effort might have on the metrics of relative change used to make inferences about long‐term trends.

Assessment of Two Techniques for Remediation of Lacustrine Rocky Reef Spawning Habitat

AbstractRocky reef habitats in lacustrine systems constitute important areas for lithophilic‐spawning fishes. Interstitial spaces created by the structure of rocky reefs form microenvironments where incubating embryos and juvenile fish are potentially protected from predators and physical displacement. However, if interstitial spaces are filled or blocked by sediment or biofouling, the reef structure may lose these benefits. Common practices to restore reef habitat include augmentation of existing reef structures or construction of new reefs, though these practices can be costly. We explored an alternative approach for reef remediation. In 2018, we developed two benthic sled cleaning devices that used either propulsion or pressurized water jets and were towed behind a small vessel to clean reefs. We used the devices to clean two impaired natural rocky reefs in Saginaw Bay, Lake Huron. We indexed effectiveness of cleaning by measured changes in substrate relative hardness before and after cleaning. A biological response to reef cleaning was also measured by egg deposition of fall (Lake WhitefishCoregonus clupeaformis) and spring (WalleyeSander vitreus) lithophilic spawners. We found that our propulsion cleaning device was more effective in increasing substrate relative hardness than was the water jet device, although this was not consistent among all study locations. We also found that egg deposition on study plots was variable, but in general, egg deposition was highest on study plots that had the greatest increases in relative hardness post‐cleaning. The practicality of cleaning devices is likely related to the magnitude of site‐specific degradation. Our results indicate that the use of these or similar devices can potentially increase the quality of spawning habitat by displacing sediments that have deposited on reef structures.

The tectono-stratigraphic evolution of the Fucino Basin (central Apennines, Italy): new insights from the geological mapping of its north-eastern margin.

ABSTRACT We present the geological map of the north-eastern margin of the Fucino Basin, which is mainly characterized by Plio-Quaternary continental deposits that show transition from deeper-water lacustrine environment, marginal lacustrine system, and fluvial facies. These deposits unconformably overlie upper Messinian Lago-Mare sediments and pre-orogenic carbonate succession. The occurrence of Caspiocypris tiberina in the Plio-Quaternary lacustrine sediments, coupled with the normal magnetic polarity of Casa Colombaia section, led to suggest the onset of the sedimentation of these continental deposits to the late Piacenzian (2.58–3.04 Ma). The upper Messinian deposits are characterized by ostracod assemblages related to the Loxocorniculina djafarovi zone, pointing to the last Lago-Mare event of the Messinian Salinity Crisis (5.40–5.33 Ma). This new stratigraphic framework may be useful to refine the long-term slip rates of the still active normal faults that affect the north-eastern margin of the Fucino Basin.

Marine-lacustrine systems during the Eocene in northern South America – Palynological evidence from Colombia

The Neotropics have experienced a series of marine incursions and the development of lacustrine systems at different intervals during the Cenozoic. One of these events occurred during the middle-late Eocene, an interval of time related to one of the most prolific oil reservoirs in Colombia, the Mirador Formation. Previous studies have identified marginal and marine environments during the late Eocene, especially in the Putumayo basin and the central Llanos foothills of Colombia. However, it is still not clear if the marine incursion is restricted to the late Eocene, or if the incursion started early. To investigate this, we analyzed a stratigraphic section located in the Llanos Foothills of Colombia. We studied the palynological content of 57 samples to stablish a temporal framework for the interval and to estimate the degree of marine, terrestrial and/or lacustrine influence during the deposition. The marine influence is especially marked during the middle Eocene and show a decreasing trend that finally reach values close to zero in the early Oligocene. Differences in the contribution of microforaminifera, dinoflagellate cysts, and freshwater algae are evident throughout the section. The corridor proposed to explain the marine incursion during the late Eocene, was active at least since the middle Eocene.

Hazardous heavy metals in the pristine lacustrine systems of Antarctica: Insights from PMF model and ERA techniques

A comprehensive study was presented on the ecological risk, distribution, and quantitative source apportionment of heavy metals in the selected lacustrine systems of Schirmacher Hills using various environmental indices and methods. A total of 25 sediment samples from 16 lakes were collected around scientific research stations and analyzed for metals. Geochemical approaches and ecological risk assessment methods were implemented to characterize and evaluate the contamination level and associated risk in the lacustrine systems. Moreover, statistical techniques and a positive matrix factorization (PMF) model were indorsed to understand metals' association and apportion their probable sources. Results revealed that most of the heavy metals (mean concentration in ppm) such as Al (77,504.09), Cd (1.36), Co (29.52), Cr (102.75), Cu (65.19), Fe (57,632.87), Mn (679.05), Ni (49.13), Pb (10.11), and Zn (253.78) are originated from natural weathering of source rocks (78.53%) followed by human-induced actions/ station activities coupled with atmospheric deposition (21.47%). Environmental risk assessment (ERA) techniques suggest that the lakes in the study area are under minimal to moderate enrichment/ contamination category and experienced minimal to adverse biological effects where metal toxicity risk is minimal.

Tectono-stratigraphic evolution of a cretaceous intracontinental rift: Example from Rio do Peixe Basin, north-eastern Brazil

The Rio do Peixe Basin is part of a set of aborted rifts with NE-SW trend along the NE region of Brazil. This axis was originated in response to the stress caused by the fragmentation of Pangea during the Mesozoic, and effectively installed during the Cretaceous event that defined the Brazilian equatorial margin. The detailed structural framework allowed the recognition of the Brejo das Freiras fault zone which presents NE-SW trend and, locally, ENE-WSW. This fault zone, strongly influenced by the basement fabric, deformed the layers and originated drag folds, fault-bending folds, and fault-propagation gentle folds. The half-graben fault margin developed alluvial fans, triggered by tectonic activity and geologically instantaneous creation of accommodation space. At the central portion it predominates a lacustrine sedimentation, while fluvial systems are responsible for carrying sedimentary supply from the flexural margin towards the depocenter. Since tectonics controls sedimentation on rifts, the tectonic systems tracts correspond to an arrangement of contemporaneous depositional systems associated with basin evolutional stages. In the syn-rift sequence, corresponding to Rio do Peixe Group nine stratigraphic units were recognized, bounded by flooding surfaces and drowning unconformities which record cycles of expansion and contraction of the lacustrine system. The tectonic systems tracts related to the initial stages of the rift installation were identified, followed by moments of high and low tectonic activity. The seismic and the well data support both the tectono-sedimentary evolution of the Brejo das Freiras Sub-Basin and the seismic-stratigraphic model proposed in this work. Different correlations between accommodation and sedimentation along a half-graben basin are expected in moments of tectonic activity and quiescence. The use of seismic attributes and filters improved the visualization and extraction of geological features, in both, structural (using pseudo-relief and curvature attributes) and stratigraphic interpretations (RMS amplitude and cosine phase). The understanding of spatial and temporal distribution of depositional systems responsible for a basin filling allows predicting potential petroleum systems.

Age and Paleoenvironmental Significance of the Frazer Beach Member—A New Lithostratigraphic Unit Overlying the End-Permian Extinction Horizon in the Sydney Basin, Australia

The newly defined Frazer Beach Member of the Moon Island Beach Formation is identified widely across the Sydney Basin in both outcrop and exploration wells. This thin unit was deposited immediately after extinction of the Glossopteris flora (defining the terrestrial end-Permian extinction event). The unit rests conformably on the uppermost Permian coal seam in most places. A distinctive granule-microbreccia bed is locally represented at the base of the member. The unit otherwise consists of dark gray to black siltstone, shale, mudstone and, locally, thin lenses of fine-grained sandstone and tuff. The member represents the topmost unit of the Newcastle Coal Measures and is overlain gradationally by the Dooralong Shale or with a scoured (disconformable) contact by coarse-grained sandstones to conglomerates of the Coal Cliff Sandstone, Munmorah Conglomerate and laterally equivalent units. The member is characterized by a palynological “dead zone” represented by a high proportion of degraded wood fragments, charcoal, amorphous organic matter and fungal spores. Abundant freshwater algal remains and the initial stages of a terrestrial vascular plant recovery flora are represented by low-diversity spore-pollen suites in the upper part of the unit in some areas. These assemblages are referable to the Playfordiaspora crenulata Palynozone interpreted as latest Permian in age on the basis of high precision Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry (CA-IDTIMS) dating of thin volcanic ash beds within and stratigraphically bracketing the unit. Plant macrofossils recovered from the upper Frazer Beach Member and immediately succeeding strata are dominated by Lepidopteris (Peltaspermaceae) and Voltziopsis (Voltziales) with subsidiary pleuromeian lycopsids, sphenophytes, and ferns. Sparse vertebrate and invertebrate ichnofossils are also represented in the Frazer Beach Member or in beds immediately overlying this unit. The Frazer Beach Member is correlative, in part, with a thin interval of organic-rich mudrocks, commonly known as the “marker mudstone” capping the Permian succession further to the north in the Bowen, Galilee and Cooper basins. The broad geographic distribution of this generally <5-m-thick mudrock unit highlights the development in eastern Gondwana of extensive, short-lived, shallow lacustrine systems with impoverished biotas in alluvial plain settings in the immediate aftermath of the end-Permian biotic crisis.

Community assembly of adult odonates in lacustrine systems of an understudied world heritage site of south-eastern Zimbabwe

Odonata (dragonflies and damselflies) are efficacious for management and conservation efforts in freshwaters. In recent times, increased effort has gone into enhancing awareness, data and information on dragonflies among scientists and policymakers. Here, we examined the Odonata community of dams within the Matobo National Park, a world heritage site in southern Zimbabwe. Specifically, we determined diversity of larval and adult odonate in five dams over one year. Several physico-chemical parameters were measured, including pH, total dissolved salts (TDS), electrical conductivity (EC). Further, we assessed how habitat types (i.e., detritus, sand, gravel, plant type) affect Odonata communities. In addition, surveys were conducted to identify critical threats to Odonata in the Matobo National Park and surrounding areas. Broadly, results showed that Odonata nymphs and volant adults did not change predictably across all sampling occasions. Further, pH was positively correlated with Libellulidae and Platycnemididae, whereas TDS and EC were positively correlated with Libellulidae, Platycnemididae and Macromiidae. Contrariwise, Gomphidae were found to be negatively correlated to TDS, EC and pH. The threat analyses identified natural system modifications, agricultural expansion and intensification, as well as human intrusions and disturbance as the major threats to Odonata and freshwater resources in Matobo National Park. Taken together, these data provide baseline data that will be useful for future monitoring of threats and subsequently conservation strategies in the Matobo National Park and other protected areas in Southern Africa.

Concentrations of Poorly Studied Technology-Critical Elements Ge, Zr, Mo, Sn, Sb, Hf, Ta, W and Ti in the Sediments of Three Northern Egyptian Lakes

Thirty five Sediment samples were collected from three Northern Egyptian Lakes namely El Burullus, El Manzala and El Bardawil to evaluate for the first time the concentrations of poorly studied technology-critical elements, namely Ge, Zr, Mo, Sn, Sb, Hf, Ta, W and Ti, and how that may be linked to local water quality parameters and other geochemical factors. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to determine element’s concentration. The results were compared with Upper Continental Crust (UCC) and Nile sediment (ANS). Lake sediments generally have relatively low contents of most elements. The average concentration of Zr, Hf, Ta and W are lower than UCC and ANS in three Lakes. El Bardawil Lake attained the lowest average concentration for all elements except Mo. Correlation analysis revealed that water quality and geochemical properties did not play any important role affecting the metal distribution and the studied metals are generally not affected by the complex lacustrine system of these lakes and their spatial distribution still not affected by the different anthropogenic activates.

Response of the lacustrine flora in East Asia to global climate changes across the K/Pg boundary

The global climate during the latest Cretaceous became variable, with several global warming and cooling trends in a context of a greenhouse Earth. The responses of marine ecosystems to these climate events are relatively well known worldwide; however, lacustrine responses are poorly known due to the less frequent and discontinuous terrestrial fossil records. The relative changes in charophyte diversity in continuous lacustrine sedimentary sequences from two basins in East Asia, i.e., Songliao and Jiaolai, are considered here for the first time to establish their correlation with global climate changes during the Late Cretaceous and K/Pg boundary, compared with the well-studied south European Ibero-Armorican Island. Lacustrine deposits correlated with the Campanian–Maastrichtian Boundary Event (CMBE), related to a long cooling period provided a relatively low diversity in East Asia. In contrast, we detected a diverse charophyte flora in lacustrine deposits correlated with the Mid-Maastrichtian Event (MME) in East Asia in a global warming trend. A higher charophyte diversity was further found in the end-Cretaceous global warming event due to speciation under the background of latest Maastrichtian warming event (LMWE). During the LMWE, Characeae species such as Microchara cristata reduced its size significantly due to environmental stress probably related to the Deccan volcanism. On the other hand, a general tendency of increasing the gyrogonite size in new Characeae taxa has been detected in populations extracted from lower Danian deposits, probably related to stable palaeoenvironmental conditions in a global cooling context. This study represents the first attempt to correlate the response of the charophyte flora to global climate changes in permanent lacustrine systems during the three main Late Cretaceous–early Danian climatic turnovers. • Changes of charophyte flora is a sensitive proxy to record climate changes. • Characeae species reduced their size due to environmental stress. • Increasing size in new Danian Characeae taxa is related to global cooling.

Depositional controls on lacustrine coquinas from an early cretaceous rift lake: Morro do Chaves Formation, Northeast Brazil

The Morro do Chaves Formation is a hybrid sedimentary succession composed of shell beds interbedded with mudstones and sandstones deposited during the rifting process of the Gondwana in a complex lake rift system, forming one of the thickest coquina deposits around the world. Despite being deeply investigated over the last decades in outcrops, the entire stratigraphic organization is not well understood yet, mainly because of the absence of continuous well core data, in addition to challenging geometries and facies distribution of the shell beds. This study focus on the characterization of facies and facies associations, and the development of a conceptual depositional model for the Morro do Chaves Formation, based on the analysis of four continuous well cores and the stratigraphic correlation among them. The described carbonate and carbonate-hybrid rocks are composed mainly of bivalves that form flat shell banks developed during dry periods with low tectonic activity. The siliciclastic rocks (mudstones, sandstones and hybrid rocks) occur interbedded with the carbonate rocks and indicates periods of active tectonic and/or climatic wet periods. The deposits are arranged in a hierarchical organization of 6 m-scale T-R cycles showing aggradation and progradation patterns. The system is controlled mainly by allocyclic (e.g., tectonics and climate) and the autocyclic (e.g., waves and currents) processes. The maximum regressive surface record an important change in the environment with a gradual increase of terrigenous input into the basin until the fast creation of accommodation space for the deposit of extensive shales that drowned and mark the end of the shell bank system. This study on facies, geometries and stratigraphic surfaces adds new and useful knowledge for helping in the understanding of complex coquina reservoirs and is applicable in the exploration of new areas with correlate lacustrine systems.

Oil shale potential of the lower cretaceous Jiufotang Formation, Beipiao Basin, Northeast China

Lacustrine oil shale is an important unconventional energy source in China. Previous exploration has mainly focused on stable deep lake deposits, whereas lacustrine oil shale layers in unstable sedimentary settings, which are widely distributed in Cretaceous strata, having received little research attention. The present paper is focused on the Aptian Jiufotang Formation in the Beipiao Basin. In order to study the impact of gravity flows and volcanism on oil shale formation, petrology, organic geochemistry, molecular geochemistry and elemental geochemistry were used to characterize the oil shales, which are very rich in well-preserved fossils (“Jehol Biota”). The predominant macerals in the high quality oil shales are lamalginite and telalginite, suggesting organic matter was derived mainly from phytoplankton and aquatic algal-bacterial organisms. A ternary diagram of C27-C28-C29 steranes showed increasing contribution of land plants in organic-lean units. Element indices such as Sr/Ba, Ca/(Ca + Fe), V/(V + Ni), δCe, Cu/Zn, Th/U, Sr/Cu, Fe/Mn, Ti/Al, TOC/S, as well as biomarker ratios (pristane/phytane, gammacerane index) and the index of compositional variability (ICV), show that high-quality oils shales were deposited under anoxic freshwater conditions. Terrigenous detrital input was low, and a warm and humid palaeoclimate, as well as a stable tectonic setting, likely prevailed. At times, low-density turbidites were accompanied by the input of terrestrial detritus, promoting a greater degree of lake oxygenation, thereby reducing oil shale potential. Volcanic eruptions triggered the occurrence of low-density turbidites due to emission of greenhouse gases which facilitated a drier climate. Periods of high-quality oil shale formation can therefore be linked to times of weak volcanism and tectonic activity, in which the organic matter was deposited under conditions of warm and humid paleoclimate, with enhanced preservation due to anoxic bottom waters in the lacustrine system. The organic-rich unit B within the Jiufotang Formation seems suitable for the application of in-situ oil shale conversion.

Water depth variation and its impact on carbonate content and oxygen isotopes: A study from a satellite lake near Lake Qinghai over the past 7.6 kyr

The oxygen isotopic composition of sedimentary carbonate (δ18Ocarb) provides information on the regional water balance between precipitation and evaporation in a lacustrine system. A volume effect was proposed to explain the abnormally low δ18Ocarb values in Lake Qinghai on the northern Tibetan Plateau, under the conditions of high evaporation and low water levels, as observed during the early Holocene. Volume effect theory states that the small volume of the lake and its sensitivity to isotopic signals from freshwater discharge control the δ18Ocarb values. In this study, we present 7.6-kyr records of carbonate content and oxygen isotopes from Lake Gahai, a small and saline satellite lake near Lake Qinghai. The records demonstrate that the water level and lake size of Lake Gahai considerably changed over the past 7.6 kyr, and the volume effect was responsible for the δ18Ocarb variations. The records also suggest contrasting responses of carbonate precipitation to water depth across different timescales. For example, over the past 7.6 kyr, the carbonate content decreased with increasing lake water levels. This can be explained by gradual water freshening under low evaporation conditions and enhanced connection between Lake Qinghai and Lake Gahai. On the centennial scale, however, the carbonate content varied significantly and was positively correlated with Lake Gahai's water level. Such a variation is probably due to the periodically lower lake water levels and a stronger influence of terrestrial clastic material, which contributed to the dilution of the carbonate content under centennial cool and dry periods. Therefore, in order to advance our understanding of past hydroclimatic variations in the climatically sensitive region, information across different timescales should be separated.

Reservoir quality of tight oil plays in lacustrine rift basins: Insights from early Cretaceous fine-grained hydrothermal dolomites of the Erlian Basin, NE China

There has been increasing recognition of fine-grained dolomitic sediments in continental rift basins, but research has tended to focus on their petrography, geochemistry and genesis, rather than the pore characteristics that determine tight oil and gas reservoir quality. This paper presents a classification of pore types for dolomitic sediments influenced by hydrothermal processes from the early Cretaceous Erlian Basin, and examines relations between pore types to infer underlying genetic processes and controls as a precursor to predicting their distribution. We recognize three pore types: (1) primary porosity, which includes inter-crystalline pores between hydrothermal minerals within granules and nano-scale pores within fine matrix; (2) secondary dissolutional porosity, including inter-crystalline dissolved pores in granules, mouldic pores in laminae and bandings and selectively dissolved pores in argillaceous matrix, as well as large vugs in breccias; and (3) fracture porosity, including low-angle dissolved fractures, bedding-parallel microfractures and high-angle tectonic fractures. Both hydrothermal and hydrocarbon-related fluids caused significant dissolution, generating secondary pores that form the largest volume of reservoir storage. Channeling of aggressive fluids via low-angle fractures localized dissolution in the matrix and significantly enhanced permeability. Vertical fractures have a lesser impact on porosity but contribute to improved pore network connectivity. These relationships reflect dolomite accumulation within a tectonically-active extensional setting, with associated fault and fracture networks providing conduits for hydrothermal fluids. These fluids are responsible for formation of the dolomitic reservoir rocks and their diagenesis and may result in a characteristic suite of pore types. The high heat flux ensured rapid maturation of source rocks and may have played an important role in generating additional dissolutional porosity. Given the tight nature of reservoirs in such lacustrine systems, a systematic understanding of the nature and evolution of the pore network is key to identifying productive “sweet spots” in the vicinity of faults for de-risking future hydrocarbon exploration and also in developing such settings for geothermal energy.

GEOLOGIA DA ÁREA NORDESTE DA FOLHA POÇO DA CRUZ, BACIA DE JATOBÁ, NORDESTE DO BRASIL

A Bacia de Jatobá está localizada na porção centro-sul do estado de Pernambuco e no norte da Bahia e Alagoas, possui uma área com cerca de 5000 km², direção preferencial NE-SW e registro sedimentar com aproximadamente 3 km. Esta bacia do tipo rifte foi desenvolvida a partir dos eventos distensivos da ruptura do supercontinente Gondwana e marca o limite norte do sistema rifte abortado Recôncavo-Tucano-Jatobá. A sucessão estratigráfica da área de estudo, localizada no distrito Campos, Ibimirim (PE), compõe as sequências Sinéclise, Início de Rifte e Clímax de Rifte da bacia. A partir da utilização de seções colunares e identificação de fácies sedimentares foram reconhecidas seis unidades estratigráficas, além de uma cobertura quaternária. As duas unidades flúvio-marinhas inferiores estão relacionadas aos registros sedimentares siluro-devonianos da sequência Sinéclise, enquanto os sedimentitos lacustres e fluvio-eólicos das unidades superiores marcam a sequência de Início de Rifte e Clímax de Rifte. A Formação Aliança, discutida de forma particular neste trabalho, representa os primeiros registros sedimentares da sequência de Início de Rifte de idade Neojurássica (andar Dom João). Essa unidade foi depositada sob um sistema lacustre, com níveis carbonáticos decimétricos altamente fossilíferos dispostos em espessos pacotes pelíticos, sugerindo deposição em momentos de variações energéticas de um lago raso. O detalhamento estratigráfico desses depósitos, associado a estudos petrográficos e paleontológicos, traz uma nova compreensão acerca do sistema deposicional que atuou nesse ambiente e os primeiros processos que deram início à abertura do rifte durante o Jurássico Superior.

Avian remains from the Toro Negro Formation (Neogene), Central Andes of Argentina

We describe new avian remains from the lower levels of the Toro Negro Formation (~7-6 Ma, U–Pb), exposed in Quebrada de la Troya between Vinchina and Jagüé towns in La Rioja Province, Argentina. The Toro Negro Formation is composed of a thick continental sequence (~2.4 km) of sandstones, conglomerates and mudstones deposited in both fluvial and lacustrine systems at the base of the unit. The avifauna of Toro Negro is low in diversity, and the majority of the remains are fragmentary and isolated. However, it is interesting in that it includes carnivorous birds such as a new species of a large eagle (Vinchinavis paka gen. et sp. nov.), scavengers (condors) and large herbivores (rheas). On the whole, the occurrences documented within the Toro Negro Formation confirm the presence of taxa with strong temperate warm preferences that typically inhabits open areas with arboreal patches within an arid-semiarid zone. The palaeoenvironmental characteristics of the fauna are confirmed as fully compatible with the evidence previously obtained through sedimentology and facies analysis.