The early Toarcian anoxic event (T-OAE) and the associated biotic crisis have received much attention in the last decade. However, the events forewarning the crisis as well as its aftermath are still poorly known. The T-OAE coincides with a prominent carbon isotope negative excursion (T-CIE) that is preceded by an excursion of similar intensity at the Pliensbachian-Toarcian boundary (Hesselbo et al., 2007). The onset of T-CIE occurred some 700 kyr later than the end of the Boundary-CIE (Suan et al., 2008a). This succession of events demonstrates that the T-OAE was a complex suite of environmental perturbations. In this work, we focused on calcareous nannofossil assemblages occurring in the Peniche section (Portugal) during the Boundary-CIE with the aim to understand if calcifying plankton reacted in a similar/different way to the two CIEs. Also, two sections and one borehole located along a W-E transect, along the NW-Tethyan shelf (in the Yorkshire coast, in the E Paris Basin, and in Mecsek Basin, respectively), were investigated to assess which way calcareous nannoplankton recovered after the crisis, and if the recovery was a synchronous event. The production by nannoplankton collapsed during the T-CIE, as demonstrated by the lowest absolute abundance of nannofossils measured in Peniche and other studied sites (Mattioli et al., 2008). Besides this nannofossil abundance decrease, also the size of the incertae sedis Schizosphaerella test was drastically reduced (Suan et al., 2008b). If a similar size decrease is also recorded during the Boundary-CIE, calcareous nannofossil abundances are very high, and assemblages seem not to record an environmental stress. The study of the calcareous nannofossil assemblages along a W-E transect in the NW-Tethyan shelf shows a progressive, but significant decrease in abundance fluxes from W to E, and the lowest fluxes are recorded in the Mecsek Basin that was closer to the oceanic Tethys. A progressive re-colonization of the lower photic zone by deep-dweller nannofossil taxa, mainly Crepidolithus crassus, is observed in the aftermath of the anoxic event, but this re-colonization occurred earlier in the Mecsek Basin, probably because of more effective marine connections with the open-ocean. This set of data indicates that: (1) environmental deterioration was recurrent until it reached its acme during the T-OAE; (2) post-crisis recovery of surface water environments was not synchronous, depending on palaeoceanographic conditions occurring within the western Tethys. Our scenario implies an intrinsically long-lasting suite of events and argues in favour of long-lasting CO2 degassing, most likely related to the emplacement of the large igneous province of Karoo-Ferrar as the main cause of the Toarcian environmental perturbations.
Numerous hyperthermal events have been documented through the Paleocene-Eocene transition. The best known hyperthermal event is the Paleocene-Eocene Thermal Maximum (PETM; around 56Ma), a period that led to surface and bottom water warming of about 5°C within a few millennia at tropical latitudes. It is therefore considered as one of the best analogues of current global warming. The PETM is also characterized by an abrupt 3-4 per mil negative δ13C excursion in deep marine core sediments and by a thin clay-rich layer associated with the PETM onset, most often interpreted as carbonate dissolution due to the shoaling of the CCD. The duration represented by these clays and carbonates is of peculiar interest to constrain the exported carbonate production dynamics of surface ocean and its dissolution throughout the water column. This is key to produce realistic carbon budgets across hyperthermal events.To this end, we generated a new 4 Ma (57.5-53.5) record of extraterrestrial 3He-derived sedimentation rates from pelagic sediments recording at least 10 hyperthermal events at ODP Site 1209 (North Pacific). Our main results indicate that carbonate sedimentation dropped drastically during the PETM onset (minimum of 0.02 cm/ka) and recovered rapidly during the recovery phase of the event (around 0.7 cm/ka). Surprisingly, the sedimentation rate is low (0.3 cm/ka) after the recovery until the Eocene Thermal Maximum 2 (ETM2; around 54Ma). After this major event, the sedimentation rate increased abruptly (0.7 cm/ka) over the last 500 ka of the studied interval due to the overabundance of Zygrhablithus bijugatus a large rod-shaped nannofossil whose ecology is poorly understood yet.Comparisons between the new record of extraterrestrial 3He-derived sedimentation rate and dissolution proxies from this and previous studies lead us to challenge the widely accepted model previously proposed for hyperthermal events, which assumes that the CaCO3 accumulation is mainly controlled by dissolution.
The magnetic stratigraphy of the Maiolica and Calcari ad Aptici pelagic limestones is documented from four Umbro‐Marchean land sections (Italy). The overall biostratigraphic age of the sampled strata encompasses the whole Kimmeridgian–Lower Aptian. The data were gathered from two new sections (Arcevia, Contessa) and by resampling more densely than in the past two additional classical sections (Bosso, Gorgo a Cerbara). A detailed record of the polarity chrons M21n‐M14 and M9‐M0 was derived, and no difference was observed with respect to the sequence of M polarity chrons inferred from oceanic magnetic anomalies. Barely measurable remanent magnetization from chron M14 to M10n at Contessa concurred with previous evidence from Bosso, indicating that the Maiolica from Umbria‐Marche is characterized by a “recording gap” during chrons M14n to M11, possibly due to diagenetic magnetite dissolution related to the Late Valanginian carbon isotope event. At Arcevia and Bosso, the boundary between Maiolica and Calcari ad Aptici occurs at the top of chron M19n and in the upper part of chron M20n, respectively, showing that the onset of Maiolica sedimentation may be diachronous by 1.5–2 Myr along the Umbria‐Marche domain. Arcevia is the most expanded Mid‐Upper Tithonian land section documented so far. Here, the subchrons M20n‐1 and M19n‐1 are found to represent 3.2% and 10.1% in time of chrons M20n and M19n, respectively. Evidence is also provided for a reversal excursion occurring within the M20n‐1 subchron. At Bosso, an excursion and a reversal excursion were found within chrons M16n and M16, respectively.
ABSTRACT Oxfordian deep-shelf deposits of southern Germany and southern Spain are characterized by marl-limestone alternations that are stacked into small-scale, medium-scale, and large-scale depositional sequences. The German sections contain autochthonous sponge reefs and associated fragments of microbialites, whereas the sections studied in Spain display tempestites composed of autochthonous deeper-water and allochthonous shallow-water particles. The facies of the German limestones and marl layers has been analyzed in detail: condensed intervals (implied by glauconitization of particles, abundant cephalopods, intense bioturbation, and generally more marls) are also enriched in sponge reefs and associated particles, and in nannofossils. Limestone-rich intervals, however, contain fewer sponges and fewer nannofossils. Neither bioerosion of sponge reefs nor nannofossil blooms can thus explain the abundance of carbonate mud that forms the limestones. Consequently, it is suggested that most of the carbonate mud is exported from shallow platform areas where carbonate productivity is high. The clay fraction was derived from weathering of massifs in the hinterland. The observed depositional sequences can be correlated between the studied sections in Germany and Spain (situated in different paleotectonic and paleoclimatic domains), and also between deeper-water sections and platform sections. This suggests that they formed through allocyclical processes. Comparision with published time scales implies that the small-scale and medium-scale sequences formed in tune with the 100 kyr and 400 kyr orbital eccentricity cycles, respectively. However, the number of marl-limestone alternations is not always consistent with the expected number of 20 kyr precessional cycles (5 per 100 kyr cycle). The large-scale sequences reflect long-term (third order) sea-level changes. The observed marl-limestone alternations are interpreted to have formed through cyclically varying export of carbonate mud from the platform towards the deep shelf, the variations being controlled by climatically induced high-frequency sea-level fluctuations. Enhanced marl deposition on the deep shelf can be related to sea-level fall causing exposure of the shallow platform and reducing the area of carbonate production, or by rapid sea-level rise (maximum flooding) leading to partial or total drowning of the platform, and/or to retrogradation of facies belts. Enhanced carbonate deposition occurs during transgression, when large production areas are created on the platform, or during late highstands, when progradation is forced and carbonate-mud export enhanced. Depending on the long-term trend of sea-level change on which the high-frequency fluctuations are superimposed, one high-frequency (20 kyr) sea-level cycle can thus create one or two marl-limestone alternations, or only marly deposits. Consequently, one 100 kyr eccentricity cycle may be formed of a variable number of marl-limestone alternations (commonly 2 to 8). The studied deeper-water depositional sequences thus are strongly linked to the history of the adjacent shallow carbonate platforms.
Abstract The flooding of the Lower Jurassic shelf in the North Gondwana Palaeomargin during the early Toarcian occurred on a fragmented and irregular topography affected by differential subsidence – owing to the activity of listric faults along the North–South Axis of Tunisia – that favoured lateral changes in facies and thickness at a kilometric scale. The onset of Toarcian sedimentation (Polymorphum ammonite Zone, NJT5c nannofossil Subzone) in two adjacent sections was characterized by the deposition of limestones under high-energy conditions. The Châabet El Attaris section was located in a depressed sub-basin, and recorded restricted environmental conditions owing to water stagnation and an oxygen-depleted sea bottom. Therefore, dark mudstones developed, with increased total organic carbon contents and enhanced accumulation of redox-sensitive elements. The sedimentation of limestones bearing gutter cast structures is related to gravity flows probably linked to storm activities. These processes favoured the remobilization of sediments at the seafloor, as well as oxygen input to bottom waters, as shown by the record of trace fossils including Zoophycos , Ophiomorpha , and secondarily, Chondrites and Diplocraterion . The thinly interbedded dark mudstones are locally rich in thin-shelled bivalves that re-colonized the sea bottom after the sedimentation of these high-energy deposits, and subsequently underwent mass mortality related to the return of oxygen-depleted conditions. The Kef El Hassine section is located in the upper part of a tilted, less subsident block, as indicated by its reduced thickness compared with the Châabet El Attaris section; the absence of dark mudstones implies oxic conditions. The Polymorphum Zone consists of limestones showing evidence of sedimentation under high-energy conditions, along with hardgrounds. The occurrence of Zoophycos (deep-tiers) in the upper part of some limestone beds of the Polymorphum Zone is linked to minor erosive processes. The top of the high-energy sequence – below the deposits of a marly interval corresponding to the Levisoni Zone – is interpreted as a hardground given the high content of belemnites and Arenicolites , some of them boring on the eroded Zoophycos and Thalassinoides . This study shows that the sedimentary expression of the Jenkyns Event is not uniform across Tunisia, supporting the importance of local conditions in determining the development of anoxic conditions.
'/%3e%3cuse xlink:href='%23a' transform='rotate(72 0 0)'/%3e%3cuse xlink:href='%23a' transform='rotate(-72 0 0)'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1) rotate(72)'/%3e%3c/g%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h1200v600H0z'/%3e%3cpath stroke='%23FFF' stroke-width='60' d='m0 0 600 300M0 300 600 0' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='40' d='m0 0 600 300M0 300 600 0' clip-path='url(%23c)'/%3e%3cpath stroke='%23FFF' stroke-width='100' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='60' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(45.4 0 0 45.4 900 120)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(30 0 0 30 900 120)'/%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 519.022 -457.666) scale(40.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 519.022 -457.666) scale(25)'/%3e%3c/g%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 668.57 -327.666) scale(45.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 668.57 -327.666) scale(30)'/%3e%3c/g%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(50.4 0 0 50.4 900 480)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(35 0 0 35 900 480)'/%3e%3c/svg%3e)
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)