Elisabetta Erba

Volcanism, CO2 and palaeoclimate: a Late Jurassic–Early Cretaceous carbon and oxygen isotope record

A composite Tethyan Late Jurassic–Early Cretaceous carbon and oxygen isotope curve is presented. C-isotope data provide information on the evolution and perturbation of the global carbon cycle. O-isotope data are used as a palaeotemperature proxy in combination with palaeontological information. The resulting trends in climate and in palaeoceanography are compared with biocalcification trends and oceanographic conditions favouring or inhibiting biocalcification. Positive C-isotope anomalies in the Valanginian and Aptian correlate with episodes of increased volcanic activity regarded as a source of excess atmospheric carbon dioxide. A major warming pulse accompanies the Aptian but not the Valanginian C-isotope event. The observed change in Early Aptian temperatures could have triggered the destabilization of sedimentary gas hydrates and the sudden release of methane to the biosphere as recorded as a distinct negative carbon isotope pulse preceding the positive excursion. Both C-isotope anomalies are accompanied by biocalcification crises that may have been triggered by pCO2-induced changes in climate and in surface water chemistry. Elevated nutrient levels in river-influenced coastal waters and in upwelling regions further weakened marine calcification. These conditions contrast with ‘normal’ trophic conditions prevailing in the latest Jurassic and favouring biocalcification. The C- and O-isotope curves record a stable mode of carbon cycling and stable temperatures. We conclude that biocalcification is mostly triggered (and inhibited) by CO2 conditions in the atmosphere–ocean system.

Onset of the Mid‐Cretaceous greenhouse in the Barremian‐Aptian: Igneous events and the biological, sedimentary, and geochemical responses

Basalts and biostratigraphy dated at 125–120 Ma from the Ontong Java and Manihiki Plateaus in the western Pacific evidence the largest volcanic event in Earth history in at least the past 160 m.y. The intervening Nova-Canton Trough rifted at about 121–118 Ma, and a number of guyots and seamounts formed concurrently or slightly later. Geological events that probably were responses to these volcanic/tectonic events occurred in the following chronostratigraphic order. Biotic fluctuations began at about 122.5 Ma. At about 122.0 Ma, 87Sr/86Sr began to decline slowly. Metal concentrations of Co, Mn, Pb, Yb, and Cu in sediments peaked at about 121.5–121.2 Ma. Changes in planktonic communities and sedimentation culminated in a nannoconid “crisis” just prior to 120.5 Ma and in the Selli black shale (OAE la) at about 120.5–119.5 Ma. A sharp drop in δ13C occurred at the beginning of the Selli event and rebounded into a longer positive excursion that reached a peak after the Selli event at about 119.5–118.5 Ma. At 120.5 Ma, 87Sr/86Sr declined rapidly and reached a minimum at about 116–113 Ma. We speculate that the intensity of these latter responses suggests a corresponding peak in volcanic/tectonic activity at about 121–119 Ma.

Carbon-isotope stratigraphy recorded by the Cenomanian–Turonian Oceanic Anoxic Event: correlation and implications based on three key localities

We present new, detailed carbon-isotope records for bulk carbonate, total organic carbon (TOC) and phytane from three key sections spanning the Cenomanian–Turonian boundary interval (Eastbourne, England; Gubbio, Italy; Tarfaya, Morocco), with the purpose of establishing a common chemostratigraphic framework for Oceanic Anoxic Event (OAE) 2. Isotope curves from all localities are characterized by a positive carbon-isotope excursion of c. 4‰ for TOC and phytane and c. 2.5‰ for carbonate, although diagenetic overprinting appears to have obliterated the primary carbonate carbon-isotope signal in at least part of the Tarfaya section. Stratigraphically, peak δ13C values for all components are followed by intervals of high, near-constant δ13C in the form of an isotopic plateau. Recognition of an unambiguous return to background δ13C values above the plateau is, however, contentious in all sections, hence no firm chemostratigraphic marker for the end-point of the positive isotopic excursion can be established. The stratigraphically consistent first appearance of the calcareous nannofossil Quadrum gartneri at or near the Cenomanian–Turonian boundary as established by ammonite stratigraphy, in conjunction with the end of the δ13C maximum characteristic of the isotopic plateau, provides a potentially powerful tool for delimiting the stratigraphic extent and duration of OAE 2. This Oceanic Anoxic Event is demonstrated to be largely, if not wholly, confined to the latest part of the Cenomanian stage.

Nannofossils and superplumes: The Early Aptian “nannoconid crisis”

A group of calcareous nannoplankton named nannoconids experienced a crisis in the early Aptian and recovered only later in the late Aptian after a period of virtual absence. Although no extinctions occurred, the widespread nature of the “nannoconid crisis” suggests a global causal factor. This crisis is recorded within the Chiastozygus litterarius nannofossil and Globigerinelloides blowi planktonic foraminiferal zones, postdates magnetic chronozone M0 by approximately 300 kyr, and precedes the oceanic anoxic subevent 1a and associated δ13C anomaly by some 40–100 kyr. Selective dissolution and anoxia cannot explain the crisis, because nannoconids are dissolution-resistant forms and their crisis clearly precedes the deposition of anoxic sediments. At least 1 m.y. prior to the “nannoconid crisis,” the onset of a nannoplankton speciation event may be the response of nannofloras to a major rise in relative sea level. The “nannoconid crisis” seems to be synchronous with the early Aptian volcanic eruptions in the Pacific Ocean. Hence calcareous nannoplankton were severely affected by the “superplume” volcanic episode. The coccolithophorid bloom/nannoconid crisis was possibly induced by the excessive CO2 levels in the atmosphere and/or caused by changes in nutrient content of oceanic surface waters. Fertility was enhanced by rapid turnover of nutrients due to the abnormal volcanic activity and accelerated transfer of nutrients from the continents into the oceans under warm and humid conditions of the mid-Cretaceous greenhouse climate. The “nannoconid crisis” may represent a competition between phytoplankton groups for nutrients or, more likely, competition between different calcareous nannoplankton. The biologic affinity and mode of life of Nannoconus are unknown, because there is no modern analog of this genus. However, comparison of Lower Cretaceous nannofossil assemblages with modern nannoplankton cummunities suggests that nannoconids, like extant Florisphaera profunda, possibly inhabited the lower photic zone. Concentrations of nutrients in the upper euphotic zone may have triggered blooms of coccolithophorids and nannoconid depletion. This model implies that the “nannoconid crisis” is the result of an abrupt, major change in the structure of surface waters caused directly or indirectly by the “superplume.” The adjustments of the biosphere to the new paleoceanographic and climatic conditions required some 40–100 kyr before changing into abnormally high primary productivity and deposition of organic carbon-rich sediments with dinoflagellates outcompeting nannoplankton.

N2-fixing cyanobacteria supplied nutrient N for Cretaceous oceanic anoxic events

The abundance of specific membrane lipids, 2-methylhopanoids, indicates that cyanobacteria played a key role in the seemingly global deposition of black shales during the early Aptian (ca. 120.5 Ma) and late Cenomanian (ca. 93.5 Ma) oceanic anoxic events. Organic matter–rich sediments deposited during these events are characterized by a 15N content typical of newly fixed N2, indicating that cyanobacterial N2 fixation was the main source for nutrient N. We propose that denitrification and anaerobic ammonium oxidation effectively cut off the return of nutrient N from the anoxic deep waters to the photic zone in the oceans, giving N2-fixing cyanobacteria a competitive advantage over algae during these oceanic anoxic events.

Black shale deposition on the northwest African Shelf during the Cenomanian/Turonian oceanic anoxic event: Climate coupling and global organic carbon burial

High-resolution geochemical records from a depth transect through the Cenomanian/Turonian (C/T) Tarfaya Basin (northwest African Shelf) reveal high-amplitude fluctuations in accumulation rates of organic carbon (OC), redox-sensitive and sulphide-forming trace metals, and biomarkers indicative of photic zone euxinia. These fluctuations are in general coeval and thus imply a strong relationship of OC burial and water column redox conditions. The pacing and regularity of the records and the absence of a prominent continental signature suggest a dynamic depositional setting linked to orbital and higher-frequency forcing. Determining the dominant frequency depends on the definition of the most pronounced oceanic anoxic event (OAE2) and its duration. We propose that eccentricity is the main forcing factor at Tarfaya and controlled fluctuations in wind-driven upwelling of nutrient-rich, oxygen-depleted intermediate waters from the adjacent Atlantic and the periodic development of photic zone and bottom water euxinia on the mid-Cretaceous northwest African shelf. Accumulation records clearly identify the basin center as the primary site of sediment deposition with highest temporal variability and an up to six-fold increase in OC burial from similar to2 g/m(2) . yr prior to the OAE2 to similar to12 g/m(2) . yr during the OAE2. Photic zone and bottom water euxinia alternated with periods of greater oxygenation of the water column in response to climate forcing. Mass balance calculations imply that similar to2% of the overall global excess OC burial associated with the OAE2 was deposited in the Tarfaya Basin, an area that represented only similar to0.05% of the total global C/T ocean floor. In fact, the lateral extent of similar black shales along the African continental margin indicates that this part of the ocean contributed significantly to the global increase in organic carbon burial during the OAE2.

Calcareous nannofossils and Milankovitch cycles: the example of the Albian Gault Clay Formation (southern England)

Abstract A quantitative study of calcareous nannofossil assemblages was carried out on the Albian Gault Clay Formation (England). This unit consists of grey marly claystones and marlstones with phosphatic nodules and glauconitic layers as minor lithologies. Two cores were analyzed in detail to focus on changes in nannofloral assemblages and consider their significance. Preservation of calcareous nannofossils is excellent and therefore diagenesis cannot be invoked to explain, not even partially, the changes registered in assemblages. Fast Fourier spectral analysis was performed on percentages of selected nannofossil species. Fluctuations in abundance of both Biscutum constans and Watznaueria barnesae, regarded as fertility and non-fertility indices, respectively, appear to be orbitally driven because spectral analysis showed periodicities of approximately 41 kyr (obliquity cycle), and 100 kyr (short eccentricity cycle). In all the spectra, the axial obliquityc cycle is a strong signal, as expected at these high latitude sites from the Boreal realm. The same pattern was observed in another fertility controlled taxon, Zygodiscus cf. erectus, but cross-correlation shows that its abundance plot is displaced with respect to that of B. constans. Orbital perturbations induce variations in climate and ocean circulation because they influence the Earths insolation and seasonality. During times of high seasonality, strong gradients cause vigorous oceanic circulation, a better cycling of nutrients and higher fertility, whereas during times of low seasonality, low gradients induce more sluggish circulation and lower fertility. The carbonate content of the Gault Clay cores is not correlatable to the higher fertility indices, as the abundance curves of B. constans, W. barnesae, and Z. cf. erectus are displaced with respect to CaCO3 content variations. This also means that cyclicity in nannofossil assemblages is not a result of diagenesis but an original signal. Abundance fluctuations of Repagulum parvidentatum and combined Parhabdolithus asper/P. splendens, colder and warmer water indicators, respectively, provide evidence for orbital periodicities suggesting that also cyclic variations of surface water temperature might have influenced the nannofossil assemblage patterns within the Gault Clay.

Valanginian Weissert oceanic anoxic event

Biotic changes in nannofossils and radiolarians associated with the Valanginian δ 1 3 C anomaly are documented at Ocean Drilling Program Hole 1149B in the Pacific Ocean: they are coeval and similar to those previously documented in the Tethys, suggesting a global perturbation of marine ecosystems. A marked increase in abundance of Diazomatolithus, absence of nannoconids, and a Pantanellium peak characterize the Valanginian δ 1 3 C excursion. Such changes are interpreted as being due to global enhanced fertility and a biocalcification crisis under conditions of excess CO 2 . The occurrence of organic C-rich black shales in the Southern Alps and in the Pacific in the interval corresponding to the δ 1 3 C excursion suggests a Valanginian oceanic anoxic event (OAE). Volcanism of the Parana-Etendeka large igneous province (ca. 132 Ma) was presumably responsible for an increase of CO 2 , triggering a climate change and accelerated hydrological cycling, possibly causing an indirect fertilization of the oceans. Widespread nutrification via introduction of biolimiting metals at spreading ridges could have significantly increased during the Gondwana breakup and simultaneous tectonic events in three separate oceans. There is no paleontological or δ 1 8 O evidence of warming during the Valanginian OAE. On the contrary, both nannofossils and oxygen isotopes record a cooling event at the climax of the δ 1 3 C excursion. Weathering of basalts and burial of organic C-rich black shales were presumably responsible for CO 2 drawdown and establishment of reversed greenhouse conditions.

Barremian-Aptian calcareous plankton biostratigraphy from the Gorgo Cerbara section (Marche, central Italy) and implications for plankton evolution

Abstract The Barremian-Aptian boundary interval of the Gorgo Cerbara section (Marche, central Italy) was revisited in order to improve stratigraphic correlations and investigate plankton evolution. A very close sampling at cm scale was carried out in the upper Maiolica and basal Scisti a Fucoidi formations. The latter formation includes the Livello Selli, the landward sedimentary expression of oceanic anoxic subevent OAE la. The study of closely spaced samples revealed that (1) the first diversification among planktonic foraminifera, marked by the appearance of the genus Globigerinelloides , occurred during the Barremian (this event was dated by means of ammonites); thus the first occurrence (FO) of both Globigerinelloides duboisi and Globigerinelloides blowi cannot be used to identify the Aptian; (2) the appearance of Rucinolithus irregularis is the biostratigraphic event which best approximates the Barremian-Aptian boundary; and (3) chron M0, being slightly younger than the FO of R. irregularis , is very close to this boundary. Semiquantitative and quantitative analyses of planktonic foraminifera, calcareous nannofossils and radiolarians show that the three groups fluctuate in abundance and assemblage composition throughout the studied interval. An important change in plankton distribution patterns occurs within the G. blowi foraminiferal Zone and C. litterarius nannofossil Zone, where planktonic foraminifera, radiolarians and calcareous nannofossils start to show large-scale, higher frequency fluctuations in abundance; fluctuations in the three groups are out of phase. These new plankton distribution patterns are interrupted by the Livello Selli, which is preceded and followed by ‘critical intervals’. The distribution and composition of all three planktonic groups lead to the interpretation of the ‘critical intervals’ as representing increased nutrient contents in the surface water and the Livello Selli as a very high fertility event. Among planktonic foraminifers the hedbergellids seem to indicate a more eutrophic habitat than the globigerinelloids. In calcareous nannofossil assemblages, the abrupt crisis affecting the nannoconids prior to deposition of the Livello Selli suggests that these nannofossils are characteristic of more oligotrophic conditions, whereas Zygodiscus erectus is indicative of a more eutrophic environment. Based on our data, the early Aptian Livello Selli has the same oceanographic significance as the latest Cenomanian Livello Bonarelli, i.e., it represents a high fertility event on a global scale.

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.