Malcolm B. Hart

Microfossil Assemblages and the Cenomanian-Turonian (late Cretaceous) Oceanic Anoxic Event

The effects of the Cenomanian-Turonian Oceanic Anoxic Event (OAE) in the Chalk Sea of NW Europe have been investigated using published macrofossil records combined with new detailed sedimentological, foraminiferal, ostracod, calcareous nannofossil, dinoflagellate cyst and stable-isotope data from Dover, England. The ranges of individual fossil species are displayed against lithostratigraphic logs, and their relation to the Cenomanian-Turonian boundary (defined using macrofaunal data) is discussed. A positive carbon stable-isotope excursion, indicating the stratigraphic extent of the OAE, spans the stage boundary. Correlation with successions elsewhere in NW Europe suggests that the OAE was isochronous, and that major biostratigraphic marker horizons are characterised by distinctive δ13C values. All microfossil groups display uppermost Cenomanian abundance and diversity minima which correspond closely to the peak of the carbon stable-isotope excursion. We propose that the OAE was a phase of increased upwelling which led to a widespread expansion and intensification of the oxygen-minimum zone in the oceans. As a result, increasingly dysaerobic bottom waters developed within the Chalk Sea, and were responsible for progressive disappearances in the benthonic microfauna, including the extinctions of many typical Cenomanian taxa. At the same time the oxygen-minimum zone rose in the water column, causing the extinction of deeper water planktonic foraminifera and then the gradual loss of intermediate-water groups. A temporary disappearance of dinoflagellate cysts and a proliferation of calcispheres were associated with these events. As the OAE waned, new species gradually evolved to fill niches left vacant following the extinctions of Cenomanian taxa. The appearance of these new species defines the base of the Turonian, the stage division being a direct consequence of the OAE. It is concluded the OAEs provide a major mechanism for controlling rates of evolution and extinction throughout the Phanerozoic.

Latest Cenomanian—earliest Turonian low-oxygen tolerant benthonic foraminifera: a case-study from the Sergipe basin (N.E. Brazil) and the western Anglo-Paris basin (southern England)

Abstract Distinctive sedimentological, geochemical, isotopic and micropalaeontological response characterise the nature of sediments deposited under hypoxic/anoxic bottom conditions during the latest Cenomanian to earliest Turonian of most of the worlds basins. The composition of the biota is a direct response to overall environmental conditions. The recognition of distribution patterns of low-oxygen tolerant microfauna is, therefore, of primary importance in evaluating and reconstructing the regional palaeoceanographic settings where such events took place. A case-study is presented from two coeval stratigraphic sequences, the Sergipe basin (northeastern Brazil) and the western Anglo-Paris basin (southern England). The associated benthonic foraminiferal assemblages from oxygen depleted environments consist mostly of “opportunistic”, r-selected species. Basic patterns in community trophic structures appear to have primarily dictated the composition of the biota. The assemblages from outer shelf to upper slope oxygen depleted environments are commonly characterised by: (1) low-diversity; (2) limited intraspecific variation; (3) predominance (c. 40–60% of the total foraminiferal assemblage) of epifaunal, deposit feeding, plano/concavo-convex, trochospiral, calcareous-hyaline morphotypes (gavelinellids, conorboidids, rosalinids and bagginids); (4) variable numbers (c. 10–30%) of infaunal, bacterial/detrital scavengers, tapered and elongate morphotypes (turrilinids, bolivinids, buliminellids, fursenkoinids, polymorphinids and caucasinids); (5) epifaunal/shallow infaunal, deposit feeding, lenticular morphotypes (coiled vaginulinids) (c. 5–19%); (6) variable numbers (c. 10–30%) of agglutinated specimens (lituolids, discamminids, eggerellids, textulariids, trochamminids, ammodiscids, tritaxiids and valvulinids); and (7) dominance (about 60–80%) of minute (c. 100–200 μm in maximum diameter, width or length) calcareous-hyaline taxa during widespread dysaerobic/quasi-anaerobic episodes. Highly oxygen depleted bottom conditions are marked by the absence of benthonic microfauna. Minor taxonomic differences between the two studied areas are mostly due to palaeobiogeographic provincialism.

Cretaceous foraminiferal morphogroup distribution patterns, palaeocommunities and trophic structures: a case study from the Sergipe Basin, Brazil

Foraminiferal studies have been used in palaeo-environmental reconstructions of the marine Cretaceous succession (upper Aptian to Maastrichtian) of the Sergipe Basin, in northeastern Brazil. The foraminiferal assemblages show broadly three types of response to changes in environment: (1) variations in morphotypes of the taxa present; (2) changes in specific and generic diversity; and (3) changes in relative abundance. Twelve palaeocommunities, characterised by the relative dominance of the major foraminiferal groups, can be recognised in the succession. Their palaeoenvironmental distribution is proposed as a model with reference to the Sergipe Cretaceous sequence. An intimate relationship is inferred among foraminiferal association distribution patterns, trophic structures (community feeding strategy, dwelling habits, substrate niche patterns) and water-mass conditions (depth-related in part). It is suggested that the distribution patterns may be a direct response of the functional adaptive morphology of the foraminiferal tests to individual characteristics of behavioural structure (preferential dwelling microhabitat and trophic strategy versus environment). The approach is a simple, yet very powerful tool, for the interpretation of foraminiferal palaeocommunities and palaeoceanographic research. It may also permit interpretation of palaeocommunity strategies in terms of adaptation rate and selection response (i.e. “r-selection” versus “k-selection”) to variable environmental conditions.

The evolution and biodiversity of Cretaceous planktonic Foraminiferida

Abstract Planktonic Foraminiferida originated in the Triassic[?] or Early Jurassic but only expanded markedly in the Early to mid-Cretaceous. It is during the mid-Late Cretaceous that they evolved into a wide range of morphotypes and were able to colonise a much greater range of environments and regions. During the mid-Cretaceous, in particular, they developed a range of strategies which allowed them to move from eutrophic environments into oligotrophic environments, especially those much deeper in the water column. The general increase and/or turnover of taxa was punctuated by a number of major “events”, e.g., the Bonarelli event in the latest Cenomanian. Between these major biotic crises there are a number of smaller events, some of which can be attributed to limited anoxic events, although others appear to be more related to major eustatic changes. The major changes in the fauna are described and problems identified.

Submarine pyroclastic deposits formed at the Soufrière Hills volcano, Montserrat (1995–2003): What happens when pyroclastic flows enter the ocean?

The Soufriere Hills volcano, Montserrat, West Indies, has undergone a series of dome growth and collapse events since the eruption began in 1995. Over 90% of the pyroclastic material produced has been deposited into the ocean. Sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea. The coarse components (pebbles to boulders) are deposited proximally from dense basal slurries to form steep-sided, near-linear ridges that intercalate to form a submarine fan. The finer ash-grade components are mixed into the overlying water column to form turbidity currents that flow over distances >30 km from the source. The total volume of pyroclastic material off the east coast of Montserrat exceeds 280 × 106 m3, with 65% deposited in proximal lobes and 35% deposited as distal turbidites.

Late Pleistocene tephrochronology of marine sediments adjacent to Montserrat, Lesser Antilles volcanic arc

The recent history of the Soufrière Hills volcano, Montserrat, Lesser Antilles volcanic arc, is deduced using data obtained from a submarine core collected in 2002. The core contains concentrations of ash and several tephra layers, which are identified by the abundance of glass shards, dense and poorly vesiculated particles, and scoria. The tephra layers have been dated using micropalaeontology and stable isotope stratigraphy. Tephra layers in a marine sediment core off the coast of Montserrat record the volcanic history of South Soufrière Hills–Soufrière Hills volcano back to 250 ka. Eight layers are composed of dense juvenile ash related to dome eruptions, five of which can be directly correlated to dated domes or related pyroclastic flow sequences on land. Six layers are composed of pumiceous glassy ash and relate to significant explosive eruptions. A marker sequence of basalt tephra layers is dated at 124–147 ka and is correlated with construction of the South Soufrière Hills basaltic stratocone. Pelagic sediments between the main tephra layers have low abundances of volcanogenic components (<15%) and suggest long periods (c. 104 years) of dormancy or low activity.

Late Cretaceous anoxic events in the Brazilian continental margin

Abstract A combined geochemical and micropalaeontological study of Cenomanian to Maastrichtian pelitic sediments from the Brazilian continental margin has been undertaken. The organic carbon contents, Rock-Eval pyrolysis results and biological marker distributions, together with the taxonomic selection and distribution patterns of microfossil assemblages indicate that intermittent anoxic events occurred in the Brazilian marginal basins during the Cenomanian-Santonian with deposition taking place in progressively deeper waters from neritic to bathyal. Such events are characterised by the following: 1. (i) a predominance among the benthonic foraminifera of low-diversity, small-sized specimens (mainly composed of gavelinellids, buliminids and nodosariids) indicating “dysaerobic” bottom conditions; 2. (ii) levels with virtually no benthonic foraminifera and a well-developed planktonic biota (foraminifera, radiolarians and diatoms) indicating “anaerobic” bottom conditions with oxygenated surface waters; 3. (iii) the occurrence of organic rich calcareous mudstone sediments; 4. (iv) features in the biological marker distributions such as high concentrations of 28,30-bisnorhopane and 25,28,30-trisnorhopane (up to 130 ppm), a predominance of phytane over pristane, and medium to high concentrations of nickel and vanadyl porphyrins (up to ca 5700 ppm). The widely recognized Cenomanian-Turonian and Santonian “oceanic anoxic events” (Schlanger and Jenkyns, 1976; Ryan and Cita, 1977 and Jenkyns, 1980) are therefore manifest in the Cassipore, Ceara, Sergipe/Alagoas and Campos basins within the Brazilian continental margin, as is the presence of such events in the Coniacian. In contrast, the Campanian-Maastrichtian appears to be a time interval when deposition under oxygenated conditions produced sediments with low organic carbon contents and poor hydrocarbon source potential, containing highly diversified and abundant calcareous and agglutinated benthonic foraminifera.

Modern seawater acidification: the response of foraminifera to high-CO2 conditions in the Mediterranean Sea

Abstract: The seas around the island of Ischia (Italy) have a lowered pH as a result of volcanic gas vents that emit carbon dioxide from the sea floor at ambient seawater temperatures. These areas of acidified seawater provide natural laboratories in which to study the long-term biological response to rising CO2 levels. Benthic foraminifera (single-celled protists) are particularly interesting as they have short life histories, are environmentally sensitive and have an excellent fossil record. Here, we examine changes in foraminiferal assemblages along pH gradients at CO2 vents on the coast of Ischia and show that the foraminiferal distribution, diversity and nature of the fauna change markedly in the living assemblages as pH decreases.

Isotopic evidence for Early to mid-Cretaceous ocean temperature variability

Abstract Stable isotopic measurements have been made on planktonic and benthic foraminifera of Early to mid-Cretaceous (Early Albian–Cenomanian) age from DSDP site 463 located in the Pacific Ocean. The degree of alteration of the foraminifera has been assessed through the application of scanning electron microscopy. The oxygen isotope data support the view that during the Cenomanian ‘globigerine’ foraminifera, such as the genus Hedbergella , inhabited near-surface waters, whilst the flattened, keeled, morphotypes lived deeper within the water column. The carbon isotope data reveal a more complex pattern as the δ 13 C values from planktonic species essentially show a large overlap, which is interpreted in terms of a weak δ 13 C gradient existing in the upper part of the ocean. In contrast, the oxygen isotope data from planktonic foraminifera from the Late Albian display no discernible depth stratification pattern and the δ 13 C data also show a large overlap of values indicative of a complete disruption of the upper part of the thermocline. A relatively low surface to bottom water temperature gradient within the Early–Middle Albian part of the succession may reflect a well-mixed ocean. Inferred cool ocean temperatures (and/or high salinities) for the Early Albian may reflect increased oceanic poleward heat transport. An increase in temperature (and/or decrease in salinity) in the Cenomanian suggests a reduction of the poleward heat flux, promoting the build-up of limited polar ice during this period of tropical warmth.

Late Cretaceous anoxic events, sea-level changes and the evolution of the planktonic foraminifera

Summary This paper investigates the relationship, if any exists, between the evolution of the planktonic foraminifera and the presence of anoxic water masses in the water column. The relationship with changes in sea-level over the expanded Cretaceous shelves is also discussed. New data from DSDP Legs 80 and 95, coupled with that from southern England, north-east England and the North Sea Basin will be used to assess the evolution of the microfauna.