The Early Aptian Oceanic Anoxic Event.

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The Early Cretaceous (Aptian Age), 120 Ma.

The geological records show that large and rapid global warming events occurred repeatedly during the course of Earth history. The growing concern about modern climate change has accentuated interest in understanding the causes and consequences of these ancient abrupt warming events. The early Aptian Oceanic Anoxic Event (OAE1a, 120 Ma) represents a geologically brief time interval characterized by rapid global warming, dramatic changes in ocean circulation including widespread oxygen deficiency, and profound changes in marine biotas. During the event, black shales were deposited in all the main ocean basins. It was also associated with the calcification crisis of the nannoconids, the most ubiquitous planktic calcifiers during the Early Cretaceous. Their near disappearance is one of the most significant events in the nannoplankton fossil record.

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Scanning electron microscope photos of different nannofossil assemblages from Early Cretaceous chalks from the North Sea (adapted from Mutterlose & Bottini, 2013)

Calcareous nannoplankton represent a major component of oceanic phytoplankton. Their calcareous skeletons can be found in fine-grained pelagic sediments in high concentrations and the biomineralization of coccoliths is a globally significant rock-forming process. The ‘nannoconid decline’ is related to the emplacement of the Ontong Java Plateau (OJP). The  CO2 released by the flood basalts was the main player in the climatic events. However, records from the Pacific and Tethys realms demonstrate that during OAE 1a the  major shift in global oceanic osmium composition occurs well after the onset of the nannoconid crisis. Previous studies argued that the nannoconid crisis was caused by ocean acidification due to numerous pulses of CO2 and methane. The Ontong Java Plateau is a massive, submerged seafloor.  It covers an area of about 1,900,000 square kilometers. It  was emplaced ca. 120 Ma, with a much smaller magmatic pulse of ca. 90 Ma. The CO2 release was too late, and too gradual, to have caused the calcification crisis in the nannoconids by ocean acidification

References:

Naafs, B. D. A. et al., Gradual and sustained carbon dioxide release during Aptian Oceanic Anoxic Event 1a, Nature Geosci. http://dx.doi.org/10.1038/ngeo2627 (2016)

Jenkyns, H. C. (2010), Geochemistry of oceanic anoxic events, Geochem. Geophys. Geosyst., 11, Q03004, doi:10.1029/2009GC002788.

Response of marine ecosystems to the PETM.

Biotic change among foraminifera during and after the PETM. (From Speijer et al., 2012)

Biotic change during and after the PETM. (From Speijer et al., 2012)

The Paleocene-Eocene Thermal Maximum (PETM; 55.8 million years ago), was a short-lived (~ 200,000 years) global warming event attributed to a rapid rise in the concentration of greenhouse gases in the atmosphere. It was suggested that this warming was initiated by the melting of methane hydrates on the seafloor and permafrost at high latitudes. During the PETM, around 5 billion tons of CO2 was released into the atmosphere per year, and temperatures increased by 5 – 9°C. This event was accompanied by other large-scale changes in the climate system, for example, the patterns of atmospheric circulation, vapor transport, precipitation, intermediate and deep-sea circulation and a rise in global sea level.

Dinoflagellate cysts: Adnatosphaeridium robustum and Apectodinium augustum (From Sluijs and Brinkhuis, 2009)

Dinoflagellate cysts: Adnatosphaeridium robustum and Apectodinium augustum (From Sluijs and Brinkhuis, 2009)

The rapid warming at the beginning of the Eocene has been inferred from the widespread distribution of dinoflagellate cysts. One taxon in particularly, Apectodinium, spans the entire carbon isotope excursion (CIE) of the PETM. The distribution of Apectodinium is linked to high temperatures and increased food availability.

The most disruptive impact during the PETM was likely the exceptional ocean acidification and the rise of the calcite dissolution depth, affecting marine organisms with calcareous shells (Zachos et al., 2005). When CO2 dissolves in seawater, it produce carbonic acid. The carbonic acid dissociates in the water releasing hydrogen ions and bicarbonate. The formation of bicarbonate then removes carbonate ions from the water, making them less available for use by organisms.

Nannofossil abundance changes during the PETM. (From Kump, 2009.)

Nannofossil abundance changes during the PETM. (From Kump, 2009.)

The PETM onset is also marked by the largest deep-sea mass extinction among calcareous benthic foraminifera (including calcareous agglutinated taxa) in the last 93 million years. Similarly, planktonic foraminifera communities at low and high latitudes show reductions in diversity, while larger foraminifera are the most common constituents of late Paleocene–early Eocene carbonate platforms.

The response of most marine invertebrates (mollusks, echinoderms, brachiopods) to paleoclimatic change during the PETM is poorly documented.

Coccolithus bownii and Toweius pertusus

Coccolithus bownii and Toweius pertusus (Adapted from Bown and Pearson, 2009)

The PETM is also associated with dramatic changes among the calcareous plankton,characterized by the appearance of transient nanoplankton taxa of heavily calcified forms of Rhomboaster spp., Discoaster araneus, and D. anartios as well as Coccolithus bownii, a more delicate form.

The combination of global warming and the release of large amounts of carbon to the ocean-atmosphere system during the PETM has encouraged analogies to be drawn with modern anthropogenic climate change. The current rate of the anthropogenic carbon input  is probably greater than during the PETM, causing a more severe decline in ocean pH and saturation state. Also the biotic consequences of the PETM were fairly minor, while the current rate of species extinction is already 100–1000 times higher than would be considered natural. This underlines the urgency for immediate action on global carbon emission reductions.

Comparison of the effects of anthropogenic emissions (total of 5000 Pg C over 500 years) and PETM carbon release (3000 Pg C over 6 kyr) on the surface ocean saturation state of calcite. From Zeebe, 2013

Comparison of the effects of anthropogenic emissions (total of 5000 Pg C over 500 years) and PETM carbon release (3000 Pg C over 6 kyr) on the surface ocean saturation state of calcite. From Zeebe, 2013

 

References:

Maria Rose Petrizzo, The onset of the Paleocene–Eocene Thermal Maximum (PETM) at Sites 1209 and 1210 (Shatsky Rise, Pacific Ocean) as recorded by planktonic foraminifera, Marine Micropaleontology, Volume 63, Issues 3–4, 13 June 2007, Pages 187-200

Zeebe RE and Zachos JC. 2013 Long-term legacy ofmassive carbon input to the Earth system: Anthropocene versus Eocene. Phil Trans R Soc A 371: 20120006. http://dx.doi.org/10.1098/rsta.2012.0006.

Wright JD, Schaller MF (2013) Evidence for a rapid release of carbon at the Paleocene-Eocene thermal maximum. Proc Natl Acad Sci USA 110(40):15908–15913.

Kump, L.R., T.J. Bralower, and A. Ridgwell. 2009. Ocean acidification in deep time. Oceanography 22(4):94–107, http://dx.doi.org/10.5670/oceanog.2009.100

Raffi, I. and de Bernardi, B.: Response of calcareous nannofossils to the Paleocene-Eocene Thermal Maximum: observations on composition, preservation and calcification in sediments from ODP Site 1263 (Walvis Ridge – SW Atlantic), Mar. Micropaleontol., 69, 119–138, 2008.

Robert P. SPEIJER, Christian SCHEIBNER, Peter STASSEN & Abdel-Mohsen M. MORSI; Response of marine ecosystems to deep-time global warming: a synthesis of biotic patterns across the Paleocene-Eocene thermal maximum (PETM), Austrian Journal of Earth Sciences. Vienna. 2012. Volume 105/1.

Brief introduction to the Toarcian oceanic anoxic event.

Early Jurassic reconstruction (From Wikimedia Commons)

Early Jurassic reconstruction (From Wikimedia Commons)

In Earth history there have been relatively brief intervals when a very significant expansion of low-oxygen regions occurred throughout the world’s oceans. In mid-1970s the discovery of black shales at many drill sites from the Atlantic, Indian, and the Pacific Ocean led to the recognition of widespread anoxic conditions in the global ocean spanning limited stratigraphic horizons. In 1976, Schlanger and Jenkyns termed these widespread depositional black shale intervals “Oceanic Anoxic Events” (Takashima et al, 2006). This was one of the greatest achievement of the DSDP (Deep Sea Drilling Project).

The Toarcian OAE, Weissert OAE, OAE 1a, and OAE 2 are global-scale anoxic events associated with prominent positive excursions of δ13C and worldwide distribution of black shales. Two models have been proposed to explain it: the stagnant ocean model (STO model) and the expanded oxygen-minimum layer model (OMZ model). Deep-water warming may have also contributed to a decrease in oxygen solubility in the deep ocean and may have triggered the dissociation of large volumes of methane hydrate buried in sediments of the continental margins.

Time scale [Gradstein et al., 2005] illustrating the stratigraphic position and nomenclature of OAEs (From Jenkyns, 2010).

Time scale [Gradstein et al., 2005] illustrating the stratigraphic position and nomenclature of OAEs (From Jenkyns, 2010).

In the Jurassic and Cretaceous oceans, the calcareous nannoplankton was the most efficient rock-forming group, for that reason the characterization of calcareous nannofloras in OAE intervals are used to improve our understanding of the marine ecosystem and biological processes such as photosynthesis (biological pump) and biomineralisation (carbonate pump) that affect the organic and inorganic carbon cycle, as well as adsorption of atmospheric CO2 in the oceans (Erba, 2013). Calcareous nannoplankton represent a major component of oceanic phytoplankton, ranging in size  from 0.25 to 30 μm. The first records are from the Late Triassic. Their calcareous skeletons can be found in fine-grained pelagic sediments in high concentrations and the biomineralization of coccoliths is a globally significant rock-forming process.

The early Toarcian Oceanic Anoxic Event  (T-OAE; ∼183 mya) in the Jurassic Period is considered as one of the most severe of the Mesozoic era. It’s associated with a major negative carbon isotope excursion, mass extinction, marine transgression and global warming (Huang, 2014, Ullmann et al., 2014). The T-OAE has been extensively studied in the past three decades although there is no general consensus about the causes or triggering mechanisms behind this event. During the peak of the perturbation corresponding to this event, calcareous nannofossils collapsed.

 

Schizosphaerella punctulata (adapted from Clémence, 2014)

Schizosphaerella punctulata (adapted from Clémence, 2014)

Schizosphaerella is a nannofossil of uncertain biological affinities with a large globular test with two interlocking sub-hemispherical valves formed from a geometric arrangement of equidimensional crystallites with an average value of 10.5 μm in the major axis. During the Early Jurassic, suffered a major drop in abundance, and a reduction in size. The average values drastically decrease down to 8.3 μm around the interval corresponding to the T-OAE. This event is know as ‘Schizosphaerellid crisis’, ‘calcareous nannofossil crisis’ or ‘disappearance event’ (Erba 2004, Clémence, 2014). Four main hypotheses have been proposed to account for the nannoplankton biocalcification crisis through the early Toarcian: (1) a strong stratification of the water column and the development of an oxygen-minimum zone; (2) the discharge of low salinity arctic waters through the Laurasian seaway; (3) high values in atmospheric pCO2; and (4) a rapid warming (Clémence, 2014).

Results from the Paris Bassin as in other localities indicates that the increasing greenhouse conditions may have caused acidification in the oceans, hampering carbonate bio-mineralisation, and provoking a dramatical loss in the CO2 storage capacity of the oceans. The CO2 induced changes in seawater chemistry likely affected the calcification potential of both neritic and pelagic systems, as evidenced by drops of platform-derived carbonate accumulation and drastic reductions in size of the main carbonate producer Schizosphaerella.

The better understanding of the Mesozoic ocean-climate system and the formation of OAEs would help us to predict environmental and biotic changes in a future greenhouse world.

References:

Marie-Emilie Clémence: Pattern and timing of the Early Jurassic calcareous nannofossil crisis.  Palaeogeography, Palaeoclimatology, Palaeoecology, 2014/doi: 10.1016/j.palaeo.2014.06.022.

Elisabetta Erba, Calcareous nannofossils and Mesozoic oceanic anoxic events, Marine Micropaleontology 52 (2004) 85 – 106

Bown, P.R., Lees, J.A., Young, J.R., (2004), Calcareous nannoplankton evolution and diversity through time. In: Thierstein, H.R., Young, J.R. (Eds.), Coccolithophores From Molecular Processes to Global Impact. Springer, Amsterdan, pp. 481–508.

Jenkyns, H. C. (2010), Geochemistry of oceanic anoxic events, Geochem. Geophys. Geosyst., 11, Q03004, doi:10.1029/2009GC002788.

 

Brief introduction to Calcareous nannoplankton

Coccolithus pelagicus (Wallich, 1871) Schiller, 1930 Lower Palaeocene-Recent. From UCL

Coccolithus pelagicus (Wallich, 1871) Schiller, 1930
Lower Palaeocene-Recent. From UCL

Calcareous nannoplankton represent a major component of oceanic phytoplankton, ranging in size  from 0.25 to 30 μm. The first records are from the Late Triassic. Their calcareous skeletons can be found in fine-grained pelagic sediments in high concentrations and the biomineralization of coccoliths is a globally significant rock-forming process.  This heterogeneous group includes coccoliths, discoasters and nannoconids.

Coccolithophores are unicellular marine golden-brown algae differing from other Chrysophyta in having two flagella and a third flagella-like appendage called a haptonema. They also posses calcified scales, called coccoliths, at some stage in their life as a protective armour that eventually falls to the ocean floor to build deep-sea ooze and fossil chalks.

Micrantholithus obtusus Stradner, 1963. Berriasian-Upper Aptian. From UCL

Micrantholithus obtusus Stradner, 1963. Berriasian-Upper Aptian. From UCL

The discoasters are an extinct group of stellate calcareous nannofossils and the nannoconids are cone-shaped microfossils very useful in Cretaceous biostratigraphy in the absence of other groups.

Typically coccolithophores are autotrophic but they can be heterotrophs under certain environmental conditions. They are restricted to the photic zone of the water column (0–200 m depth). The algal cell is generally spherical and includes  two golden-brown pigment, a nucleus, two flagella of equal length and a haptonema, mitochondria, vacuoles and the Golgi body which is the site of coccolith secretion in many species.

Diagram of a living coccolithophore cell

Diagram of a living coccolithophore cell

In some living genera there is also an alternation between a motile and a non-motile stage. The first one has a flexible skeleton with coccoliths embedded in a pliable cell membrane and in the non-motile stage, the calcification of the membrane forms a rigid shell called a coccosphere.

Coccoliths are composed of calcium carbonate in the form of calcite with a low amount of  magnesium, although it has been some of vaterite or aragonite. It is thought they are formed for  protection from intense sunlight, to concentrate light, buoyancy control, or for the biochemical efficiency of the cell.

Syracolithus (modern holococcolith) and Prediscophaera (Cretaceous heterococcolith).

Syracolithus (modern holococcolith) and Prediscophaera (Cretaceous heterococcolith).

The coccolith morphology is the basis for classification of both living and fossil members of the group. They can be divided in two basic morphological types: heterococcoliths and holococcoliths. While the holococcoliths are usually formed by rhombohedral calcite and always disintegrate after they are shed, the heterococcoliths provide the bulk of the microfossil record. They are built of different submicroscopic elements such as plates, rods and grains imbricated into a relatively rigid structure.

In some cases, some living coccolithophores, like Scyphosphaera produce two layers of morphologically distinct coccoliths (dithecism).

Scyphosphaera apsteinii. Credits image:  Ian Probert, Markus Geisen.

Scyphosphaera apsteinii. Credits image: Ian Probert, Markus Geisen.

Ehrenberg in 1836, was the first to use the term “coccoliths” while he was studying  the chalk from the island of Rugen in the Baltic Sea, but he thought they had an inorganic origin. G. C. Wallich  in 1860, was the first to suggest  the organic origin of coccoliths. Later, in 1872, the HMS Challenger expedition recovered coccospheres from the upper water layers and correctly concluded that they were the skeletons of calcareous algae.

Coccolithophores has a great radiation in the Early Jurassic, an event that parallels the radiation of the peridinialean dinoflagellate cysts and it’s related to the opening of the Atlantic Ocean. During the Late Cretaceous, there was a second radiation that led to the deposition of chalk in several areas of continental plataform,  but were very affected by the extinction event at the end of the Cretaceous. Since then, Coccolithophores have regained their dominance in tropical and temperate waters but are significantly less diverse than in the Mesozoic.

Coccoliths and discoasters has an extraordinary value as biostratigraphic markers for the Mesozoic and Cenozoic, and are good indicators of surface water chemistry and reflect surface productivity.

References:

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Jörg Mutterlose, André Bornemann, Jens O. Herrle, Mesozoic calcareous nannofossils — state of the art, Paläontologische Zeitschrift, March 2005, Volume 79, Issue 1, pp 113-133.