Anne-Christine Da Silva

Sedimentology, magnetic susceptibility and isotopes of a Middle Frasnian carbonate platform: Tailfer section, Belgium

SummaryThe Tailfer section (Belgium, northern border of the Dinant Synclinorium) exposes Middle Frasnian shallow-water limestones. This paper proposes a sedimentological analysis based on classical petrology, which leads to the identification of 19 fourth order sequences, forming three third order system tracts. This sedimentological analyses is complemented by carbon isotopic and magnetic susceptibility (MS) data (based on the relation between MS and lithogenic input). Each system tracts is characterised by a particular evolution of the isotope and MS curve:--Biostromal Unit: the biostromes are built by lamellar and branching stromatoporoids and massive metazoans. MS values are weak, in relation with the important distance from landmasses and lesser amounts of lithogenic contribution. The carbon isotope values are close to Frasnian seawater values.--Lagoonal Unit: it corresponds to a lagoonal facies succession, from inter- to supratidal zones, rich inAmphipora, paleosiphonocladalesUmbella, and pellets, alternating with paleosoils. MS values are high, related to landmass proximity and high lithogenic input. The carbon isotopic curve shows strongly negative values, close to −7%., resulting probably from a continental influence.--Lagoonal and biostromal Unit: it consists of a lagoonal succession with some biostromal interruptions. MS values are lower than in the lagoonal unit and the isotopic values are higher (close to 0%.), related to an increase in distality.

Magnetic susceptibility and its relationship with paleoenvironments, diagenesis and remagnetization: examples from the Devonian carbonates of Belgium

To better understand the origin of the initial magnetic susceptibility (χin) signal in carbonate sequences, a rock magnetic investigation that includes analysis of acquisition curves of the isothermal remanent magnetization (IRM) and hysteresis parameters, was undertaken on Devonian carbonates from the Villers and Tailfer sections, Belgium. Both sections are divided into a lower unit, dominated by biostromal and external ramp facies (biostromal unit) and an upper unit, only consisting of lagoonal facies (lagoonal unit). The variations in χin signal are mainly driven by magnetite variation, mostly pseudo-single-domain (PSD) magnetite. Clay minerals, pyrite, hematite and obviously calcite and dolomite are also present but their contribution to the χin pattern is not significant. There is a correlation between detrital proxies (Zr, Rb, Al2O3, TiO2) and χin for the Tailfer biostromal unit and the entire Villers section. The pervasive presence of fine-grained magnetite is interpreted as related to remagnetization. In absence of external fluids, the iron released during the smectite to illite transition remains in situ. In those situations χin may reflect an inherited primary synsedimentary signal. In the lagoonal unit of the Tailfer section, remagnetization appears to have obscured the original detrital information prompting the need for an evaluation of the composition of the susceptibility signal for individual case studies.

Chemostratigraphy and magnetic susceptibility of the Late Devonian Frasnian–Famennian transition in western Canada and southern China: implications for carbon and nutrient cycling and mass extinction

Abstract We investigate the Late Devonian Frasnian–Famennian extinction interval in western Alberta and south China to shed light on the palaeoecological and palaeoceanographic conditions that characterize this biotic crisis. Both the Lower and Upper Kellwasser events are documented in western Canada. Only the Upper Kellwasser event has been evaluated in south China. Our multiproxy geochemical approach reveals that these events are characterized by positive δ13C and δ15N excursions and increasing magnetic susceptibility (Canada/China) and increases in detrital (Al, Si, Ti, Zr), productivity (Cu, Ni, Zn) and redox (Mo, U, V) elemental proxies (Canada). We interpret these trends as part of a systemic palaeoecological shift associated with the development of widespread terrestrial forests and their alteration of chemical–mechanical weathering patterns. Increase in detrital proxies is thus interpreted as resulting from pedogenically driven weathering on the continents that nutrified epeiric and continental margin seas. High biological productivity led to eutrophication and development of suboxic to anoxic conditions during both events and probably euxinic conditions during the Upper Kellwasser event in western Canada. Positive δ13C excursions are the telltale signature of excessive carbon burial, while redox proxies and δ15N records indicate suboxic–anoxic denitrifying conditions.

The astronomical calibration of the Givetian (Middle Devonian) timescale (Dinant Synclinorium, Belgium)

Abstract Recent advances in radiometric dating result in significant improvements in the geological timescale and provide better insight into the timing of various processes and evolutions within the Earths system. However, no radiometric ages are contained within the Givetian. Consequently, the absolute ages of the Givetian Stage boundaries, as well as the stages duration, remain poorly constrained. As an alternative, the analysis of sedimentary cycles allows for the estimation of the duration of this stage. We examined the high-resolution magnetic susceptibility signals of four Givetian outcrops in the Givet area for a possible astronomical imprint, to fully understand the rates of evolutionary and environmental change. All four sections are firmly correlated and wavelet analyses of the magnetic susceptibility signals reveal the imprint of astronomical eccentricity forcing. The highly stable 405 kyr cycles constrain the duration of the Givetian Stage at 4.35±0.45 Myr, which is in good agreement with the International Chronostratigraphic Chart (5.0 Myr). The studied sections also exhibit an imprint of obliquity, suggesting a climatic teleconnection between low and high latitudes. The corresponding microfacies curves demonstrate similar astronomical imprint, and thereby indicate that the observed 105 year-scale cyclicity is the result of climatic and environmental change.

Astronomical age constraints and extinction mechanisms of the Late Triassic Carnian crisis

The geological record contains evidence for numerous pronounced perturbations in the global carbon cycle, some of which are associated with mass extinction. In the Carnian (Late Triassic), evidence from sedimentology and fossil pollen points to a significant change in climate, resulting in biotic turnover, during a time termed the ‘Carnian Pluvial Episode’ (CPE). Evidence from the marine realm suggests a causal relationship between the CPE, a global ‘wet’ period, and the injection of light carbon into the atmosphere. Here we provide the first evidence from a terrestrial stratigraphic succession of at least five significant negative C-isotope excursions (CIE)’s through the CPE recorded in both bulk organic carbon and compound specific plant leaf waxes. Furthermore, construction of a floating astronomical timescale for 1.09 Ma of the Late Triassic, based on the recognition of 405 ka eccentricity cycles in elemental abundance and gamma ray (GR) data, allows for the estimation of a duration for the isotope excursion(s). Source mixing calculations reveal that the observed substantial shift(s) in δ13C was most likely caused by a combination of volcanic emissions, subsequent warming and the dissociation of methane clathrates.

Insight into the development of a carbonate platform through a multi-disciplinary approach: a case study from the Upper Devonian slope deposits of Mount Freikofel (Carnic Alps, Austria/Italy)

The development and behavior of million year-scaled depositional sequences recorded within Palaeozoic carbonate platform has remained poorly examined. Therefore, the understanding of palaeoenvironmental changes that occur in geological past is still limited. We herein undertake a multi-disciplinary approach (sedimentology, conodont biostratigraphy, magnetic susceptibility (MS), and geochemistry) of a long-term succession in the Carnic Alps, which offers new insights into the peculiar evolution of one of the best example of Palaeozoic carbonate platform in Europe. The Freikofel section, located in the central part of the Carnic Alps, represents an outstanding succession in a fore-reef setting, extending from the Latest Givetian (indet. falsiovalis conodont zones) to the Early Famennian (Lower crepida conodont zone). Sedimentological analysis allowed to propose a sedimentary model dominated by distal slope and fore-reef-slope deposits. The most distal setting is characterized by an autochthonous pelagic sedimentation showing local occurrence of thin-bedded turbiditic deposits. In the fore-reef slope, in a more proximal setting, there is an accumulation of various autochthonous and allochthonous fine- to coarse-grained sediments originated from the interplay of gravity-flow currents derived from the shallow-water and deepwater area. The temporal evolution of microfacies in the Freikofel section evolves in two main steps corresponding to the Freikofel (Unit 1) and the Pal (Unit 2) limestones. Distal slope to fore-reef lithologies and associate changes are from base to top of the section: (U1) thick bedded litho- and bioclastic breccia beds with local fining upward sequence and fine-grained mudstone intercalations corresponding, in the fore-reef setting, to the dismantlement of the Eifelian–Frasnian carbonate platform during the Early to Late Frasnian time (falsiovalis to rhenana superzones) with one of the causes being the Late Givetian major rift pulse; (U2) occurrence of thin-bedded red nodular and cephalopod-bearing limestones with local lithoclastic grainstone intercalations corresponding to a significant deepening of the area and the progressive withdrawal of sedimentary influxes toward the basin, in relation with Late Frasnian sea-level rise. MS and geochemical analyses were also performed along the Freikofel section and demonstrate the inherent parallel link existing between variation in MS values and proxy for terrestrial input. Interpretation of MS in terms of palaeoenvironmental processes reflects that even though distality remains the major parameter influencing MS values, carbonate production and water agitation also play an important role.

Lower Carboniferous ramp sedimentation of the Central Alborz Basin, northern Iran: integrated sedimentological and rock-magnetic studies

Abstract The Lower Carboniferous Mobarak Formation in the Alborz Basin (northern Iran) was deposited along the northeastern margin of Gondwana in a carbonate ramp setting. This paper focuses on the Tournaisian stratigraphic interval of this formation that crops out at the Jaban section in the southwestern Central Alborz Basin. The following facies associations, representing different ramp palaeoenvironments, have been identified: (1) mudstone–wackestone outer-ramp facies; (2) crinoidal to skeletal grainstone–packstone mid-ramp facies; (3) peloidal to crinoidal grainstone–packstone inner-ramp facies; and (4) coastal facies, which include a variety of microbial laminated to oncoidal grainstones and mudstones with evaporitic pseudomorphs. This ramp profile was affected by frequent storms that were responsible for the formation of several skeletal to non-skeletal shoals in the distal mid-ramp to the most proximal inner-ramp areas. The development of the skeletal to non-skeletal shoals along the sea side of the ramp formed a semi-enclosed lagoon sensitive to the influence of both high tides and storm surges. The magnetic susceptibility (χin) of all the samples was measured and compared with that of the facies from which the sample was taken. There is a clear link between χin and the facies; the average χin values were higher for the distal facies than for the proximal facies. The χin profile of this Lower Carboniferous carbonate sequence reflects stratigraphic variations in response to relative changes in sea level and the input of detrital materials. In the context of the sequence stratigraphic framework, the average χin values for lowstand and transgressive systems tract deposits are higher than for the highstand systems tract deposits. The clear link between χin and facies indicates at least a partly preserved primary χin signal related to the detrital inputs. However, to obtain a better understanding of the nature and origin of the minerals carrying the χin, we performed hysteresis measurements on selected samples. It appears that the χin signal is mainly carried by low-coercivity ferromagnetic minerals such as magnetite, with a mixture of relatively coarse grains (detrital fraction) and ultra-fine grains (probably formed during diagenesis).

Timing and pacing of the Late Devonian mass extinction event regulated by eccentricity and obliquity

The Late Devonian envelops one of Earth’s big five mass extinction events at the Frasnian–Famennian boundary (374 Ma). Environmental change across the extinction severely affected Devonian reef-builders, besides many other forms of marine life. Yet, cause-and-effect chains leading to the extinction remain poorly constrained as Late Devonian stratigraphy is poorly resolved, compared to younger cataclysmic intervals. In this study we present a global orbitally calibrated chronology across this momentous interval, applying cyclostratigraphic techniques. Our timescale stipulates that 600 kyr separate the lower and upper Kellwasser positive δ13C excursions. The latter excursion is paced by obliquity and is therein similar to Mesozoic intervals of environmental upheaval, like the Cretaceous Ocean-Anoxic-Event-2 (OAE-2). This obliquity signature implies coincidence with a minimum of the 2.4 Myr eccentricity cycle, during which obliquity prevails over precession, and highlights the decisive role of astronomically forced “Milankovitch” climate change in timing and pacing the Late Devonian mass extinction.Understanding of Late Devonian mass extinction mechanisms is poor due to imprecise stratigraphies. Here, using cyclostratigraphic techniques, the authors present a global orbitally-calibrated chronology and reveal the key role of astronomically-forced Milankovitch climate change.

Early Cretaceous climate changes recorded in Magnetic Susceptibility and color index variations of the Lower Liupanshan Group, Central China

This study carried out comprehensive analysis on sedimentology, magnetic susceptibility (chi(lf)) and color data of the continental sediments of the Liupanshan Group in Central China so as to obtain climatic change information during the 129.14-122.98 Ma interval. Based on the results of the chi(lf) and of the redness (a*), the section can be divided into two segments: (1) 129.14-126.3 Ma, with the lowest chi(lf) values and strongly variable relatively high values of redness and (2) 126.3-122.98 Ma, with high chi(lf) values and relatively low redness. Analysis of the lithology and facies as well as the magnetic minerals and their contents points to a detrital origin of the magnetic minerals and this allow us to interpret the relationship between magnetic susceptibility variations and climate changes. Our study shows that the climate was significantly dry and hot during the whole studied interval although the interval between 126.3 Ma and 122.98 was a little bit cooler with increased humidity.

Grand Challenges (and Great Opportunities) in Sedimentology, Stratigraphy, and Diagenesis Research

NERC Yorkshire Integrated Catchment Solutions Programme [NE/P011160/1]; NERC National Capability project Climate Linked Atlantic Sector Science Programme (CLASS)