Cinzia Bottini

A volcanic CO2 pulse triggered the Cretaceous Oceanic Anoxic Event 1a and a biocalcification crisis

The Aptian Oceanic Anoxic Event 1a (OAE1a, ca.120 Ma ago) is one of the most prominent of a series of geologically brief intervals in the Cretaceous characterized by the deposition of organic carbon–rich sediments. OAEs reflect major perturbations in the global carbon cycle evidenced by sedimentary carbon isotope records. However, the triggering mechanisms for OAEs remain controversial. Here we present a bulk-rock and molecular (marine and terrestrial bio-markers) C isotope record at unprecedented time resolution, from the Cismon section of northern Italy, that shows that OAE1a conditions were reached over a period of several thousands of years through a stepwise perturbation of the carbon cycle. The documented sequence of events is most compatible with a trigger associated with increased CO 2 emissions, possibly leading to a doubling of p CO 2 , which in turn caused larger C isotope fractionation in marine and terrestrial organisms and a major biotic crisis in the calcareous nannoplankton. Our data also show that a release of isotopically light carbon from partial methane hydrate dissociation probably played a minor role in the OAE1a carbon cycle perturbation.

Calcareous Nannoplankton Response to Surface-Water Acidification Around Oceanic Anoxic Event 1a

Acidification of the Ancient Oceans Ocean acidification fueled by rising levels of atmospheric CO2 is likely to become a major challenge for ocean ecosystems. Understanding how marine biota responded to similar events in Earths history may provide clues as to what to expect—and what to prevent—in the future. To this end, Erba et al. (p. 428) present a detailed stratigraphic and geochemical characterization of 120-million-year-old marine sediments from a time when the oceans acidified because of a massive outgassing of volcanic CO2. Microscopic fossils in the sediments, such as calcareous nannoplankton, show evidence of having responded to this major disruption through species-specific adaptations like deforming and shrinking their cells. These changes allowed these abundant and diverse organisms to avoid extinction, even through a subsequent global depletion of ocean oxygen levels. Rather than going extinct, calcareous plankton adapted to ocean acidification ~120 million years ago. Ocean acidification induced by atmospheric CO2 may be a major threat to marine ecosystems, particularly to calcareous nannoplankton. We show that, during the Aptian (~120 million years ago) Oceanic Anoxic Event 1a, which resulted from a massive addition of volcanic CO2, the morphological features of calcareous nannofossils traced the biological response to acidified surface waters. We observe the demise of heavily calcified nannoconids and reduced calcite paleofluxes at the beginning of a pre-anoxia calcification crisis. Ephemeral coccolith dwarfism and malformation represent species-specific adjustments to survive lower pH, whereas later, abundance peaks indicate intermittent alkalinity recovery. Deepwater acidification occurred with a delay of 25,000 to 30,000 years. After the dissolution climax, nannoplankton and carbonate recovery developed over ~160,000 years under persisting global dysoxia-anoxia.

Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a

The early Aptian Oceanic Anoxic Event (OAE 1a) resulted from an exceptional set of interactions between the geosphere, the biosphere, and the ocean-atmosphere system. We present new Re-Os data from two sites spanning OAE 1a in the Tethys and Pacific Oceans. The patterns of variation in the seawater Os-isotope composition from both sites are very similar, and together they constrain the timing and duration of continental weathering in relation to the large-scale volcanic activity of the Ontong Java Plateau. The dominant feature through the OAE is an interval of ∼880 k.y. when the Os-isotope composition of the global ocean was exceptionally unradiogenic, implicating unambiguously the Ontong Java Plateau as the trigger and sustaining mechanism for OAE 1a. A relatively short-lived (∼100 k.y.) Os-isotope excursion to radiogenic compositions in the Tethyan record is clearly linked to an abrupt perturbation to the global carbon cycle, and is fully consistent with the Pacific record. These highly distinctive features of seawater Os in contemporaneous samples from three high-resolution sections, two of which were very remote from the Ontong Java Plateau, indicate that ocean mixing at that time was very efficient. The results suggest that OAE 1a was also related to rapid global warming and elevated rates of silicate weathering both on the continents and in the oceans.

High sea-surface temperatures during the early Aptian Oceanic Anoxic Event 1a in the Boreal Realm

The early Aptian was characterized by the widespread occurrence of anoxia in the oceans, known as Oceanic Anoxic Event (OAE) 1a. Intense degassing from submarine volcanic plateaus presumably resulted in high atmospheric CO2 concentrations, culminating in greenhouse conditions. OAE 1a can, therefore, be considered as a past “natural experiment” important to understand the future evolution of our climate. Paleotemperature estimates for OAE 1a are, however, predominantly based on bulk oxygen isotopes, which are susceptible to diagenetic overprinting, while TEX86 paleotemperature estimates are limited in number or derived from stratigraphically poorly constrained sections. Here we reconstructed for the first time sea-surface water temperatures (SSTs) based on the TEX86 paleothermometer from an OAE 1a section from the middle northern latitudes (39°N paleolatitude). We find a SST rise starting prior to OAE 1a and reaching a maximum during the event, with SSTs of ~31–34 °C, 4–9 °C higher than those of older Hauterivian–lower Aptian sediments from the same sedimentary basin. The end of OAE 1a is marked by relatively lower SSTs of ~30 °C. These observations are supported by belemnite-based oxygen-isotope (d18OBel) data and calcareous nannofossils. Our integrated data set clearly indicates that “super greenhouse” conditions prevailed during OAE 1a at northern latitudes. SSTs are similar to those estimated for coeval low-latitudinal sites, suggesting that an equable warm climate, with reduced latitudinal gradients, characterized the early Aptian.

Early Cretaceous chalks from the North Sea giving evidence for global change

Among calcareous nannofossils, important primary producers in Jurassic and Cretaceous oceans, nannoconids were carbonate rock-forming organisms. During the late Barremian and early Aptian (~126 to 122 million years ago), nannoconids went through a crisis culminating during the Oceanic Anoxic Event 1a. Here we present nannofossil and geochemical data from a section of early Barremian-early Aptian age from the North Sea, recording the earliest chalks ever known in the Boreal Realm. These middle-late Barremian chalks were generated by blooming of endemic nannoconids under relative warm and arid conditions. A subsequent decrease of nannoconids in the latest Barremian coincides with increased nutrient and clay input. This nannoconid decline, also detected at low latitudes, was associated with the Ontong Java Plateau emplacement. We conclude that nannoconids were rock forming also at high latitudes, under clear and oligotrophic waters. Their decline was related to increased continental runoff under reinforced greenhouse conditions.

CALCAREOUS NANNOPLANKTON RESPONSE TO THE LATEST CENOMANIAN OCEANIC ANOXIC EVENT 2 PERTURBATION

Morphometric analyses were performed on Biscutum constans , Zeugrhabdotus erectus , Discorhabdus rotatorius and Watznaueria barnesiae specimens from five sections spanning the Cenomanian-Turonian boundary interval including Oceanic Anoxic Event (OAE) 2 (~ 94 Ma). The study provides evidence for size fluctuations and dwarfism of B. constans during OAE 2, followed by a partial recovery at the end of the event: this taxon appears to be the most sensitive species, with similar and coeval size trends in all the analyzed sections. Conversely, morphometry shows negligible or unsystematic coccolith variations in Z. erectus , D. rotatorius and W. barnesiae . The comparison of OAE 2 data with those available for the early Aptian OAE 1a and latest Albian OAE 1d, indicates that B. constans repeatedly underwent size reduction and temporary dwarfism possibly implying that the same paleoenvironmental factors controlled calcification of B. constans during subsequent OAEs although the amplitude of B. constans coccolith reduction is significantly larger for OAE 1a than OAE 2. Paleoceanographic reconstructions suggest that ocean chemistry related to the amount of CO 2 and toxic metal concentrations played a central role in B. constans coccolith secretion, while temperature and nutrient availability do not seem to have been crucial. Contrary to OAE 1a, Z. erectus , D. rotatorius and W. barnesiae appear to be substantially unrelated to OAE 2 paleoenvironmental stress, possibly because of different degrees of perturbation.

Mid-Cretaceous paleoenvironmental changes in the western Tethys

We present a continuous record of surface water temperature and fertility variations through the latest Barremian–Cenomanian interval (ca. 27 Myr) based on calcareous nannofossil abundances from the western Tethys. The nannofossil temperature index, calibrated with TEX86 sea surface temperatures, suggests that warmest (34–36 C) conditions were reached during oceanic anoxic event (OAE) 1a onset, the Aptian–Albian boundary interval hyperthermals (113, Kilian level and Urbino level OAE 1b) and during a ca. 4 Myr long phase in the middle Albian. Coolest temperatures (29 C) correspond instead to the late Aptian. Generally warm conditions characterized the Albian followed by a progressive cooling trend that started in the latest Albian (at the Marne a Fucoidi–Scaglia Bianca Formation transition). Temperate conditions occurred in the Cenomanian with frequent short-term variations highlighted by abundance peaks of the cold-water nannofossil species E. floralis and R. parvidentatum. Mid-Cretaceous surface water fertility was rather fluctuating and mostly independent from climatic conditions as well as from black shales intervals. Intense warming and fertility spikes were systematically associated only with black shales of OAE 1a and of the Aptian–Albian boundary hyperthermals. The Albian–Cenomanian rhythmic black shales are, in fact, associated with varying long-term climatic and fertility conditions. The similarity of western Tethys climatic and fertility fluctuations during OAE 1a, OAE 1b, the middle Albian and OAE 1d with nannofossil-based records from other basins indicated that these paleoenvironmental conditions were affecting the oceans at supra-regional to global scale.

CALCAREOUS NANNOFOSSILS AT THE TRIASSIC/JURASSIC BOUNDARY: STRATIGRAPHIC AND PALEOCEANOGRAPHIC CHARACTERIZATION

In this work, calcareous nannofossils are identified for the first time in the uppermost Triassic sequence of the Lombardy Basin (Southern Calcareous Alps, Italy). Two zones are recognized, namely the NT2b (latest Triassic) and the NJT1 (earliest Jurassic). Two species resulted to be good markers to constrain the TJB interval: Prinsiosphaera triassica and Schizosphaerella punctulata . Nannofossil data are calibrated with C isotopic chemostratigraphy obtained for carbonate and organic matter. Size reduction of P. triassica and a decline in the abundance of Triassic nannofossils are detected soon after the “precursor Carbon Isotope Excursion (CIE) and culminated during the “initial negative CIE” characterized by lowest nannofossil abundances and small-sized P. triassica . The extinction of Triassic nannofossils occurred in distinctive steps within the “initial negative CIE”, while the Jurassic S. punctulata is first observed at the base of the “main negative CIE”. The latest Triassic nannofossil decline in abundance, size reduction and extinctions, represent a progressive deterioration associated to the Central Atlantic Magmatic Province (CAMP) volcanism. Our findings are consistent with nannofossil changes at supraregional scale and indicate that the massive CAMP flood basalts were preceded by initial volcanic pulses. We speculate that a combination of climate change, fertilization and ocean acidification started to influence the calcification process prior to the “initial negative CIE”. Nannoplankton extinctions were not simultaneous and might imply limited capacity for adaptation in the early stages of evolutionary history. However, originations of new taxa soon after the disappearance of Triassic forms suggest the ability to rapidly overcame extreme stressing conditions.

Past excess CO2 worlds: biota responses to extreme warmth and ocean acidification

More than half a gigaton of CO 2 per year is recycled in the Earth interior at convergent margins. At least 40% of this CO2 returns to the atmosphere via igneous activity at subduction zones. Experimental and thermodynamic modelling of phase relationships at high pressure indicate that decarbonation or carbonate dissolution in fluids account for only a portion of CO 2 released, and that dry carbonate melts are feasible only if thermal relaxation occurs in a subduction environment. The subducted oceanic crust includes a range of gabbroic rocks, altered on rifts and transforms, with large amounts of An-rich plagioclase. It has been shown that epidote disappearance with pressure depend on the normative anorthite content of the bulk composition considered; we therefore expect that altered gabbros might display a much wider pressure range where epidote persists, potentially affecting the location and the nature of volatiles release. New experimental data from 3.7 to 4.6 GPa, 750 C to 1000C are intended to unravel the effect of variable bulk and volatile compositions in model eclogites, enriched in the normative anorthite component (An 37 and An45). Experiments are performed in piston cylinder apparatus and multianvil machine, using both single and, buffered, double capsule techniques. Garnet, clinopyroxene and coesite form in all syntheses. Lawsonite was found to persist at 3.7 GPa, 750 C, with both dolomite and magnesite; at 3.8 GPa, 775-800 C, fluid saturated conditions, epidote coexists with kyanite, dolomite and magnesite. The anhydrous assemblage garnet, omphacite, aragonite, kyanite is found at 4.2 GPa, 850 C. At 900 C, fluid-rich conditions, a silicate fluid/melt of granitoid composition, a carbonatitic melt and Na-carbonate are observed upon quenching. Close to fluidsaturation, 3.8-4.2 GPa, 870 900 C, garnet and Na-rich clinopyroxene coexist with a carbonatitic liquid, Mg-calcite/dolomite and aragonite. The carbonatitic melt is enriched in Ca compared to liquids previously obtained in dry carbonated experiments. Sandwich experiments, at variable volatile content and variable CO 2:H2 ratio, have been designed to demonstrate attainment of equilibrium and to challenge the suggested supercritical nature of the carbonate liquid produced. H 2O strongly depresses liquidus surface for Ca-rich carbonates allowing the generation of liquids efficiently scavenging volatiles, Ca and Si from the slab. Hydrous silicate carbonate liquids are therefore feasible if a H 2O source is provided along a relatively warm subducting path. Such source may correspond to fluids flushed from the dehydrating serpentinized lithosphere or to an internal source, i.e. epidote. In warm subduction zones, the possibility of extracting hydrous carbonatitic liquids from a variety of gabbroic rocks offers new scenarios on the metasomatic processes in the lithospheric wedge of subduction zones and a new mechanism for recycling carbon. Such liquids are expected to be extremely reactive in a percolated mantle wedge, where they generate carbonate pyroxenites, a fertile CO2 source for magmatism at subduction zones.