The first dramatic rise in atmospheric oxygen to concentrations above 10^-5 present atmospheric level (PAL), known as the Great Oxidation Event (GOE), was initiated during the early Proterozoic Eon c. 2.43-2.32 billion years (Gyrs) ago [1,2].  Although atmospheric O₂ concentrations are generally accepted to have remained below 1% PAL for at least 1.5 Gyrs following the GOE [3], high atmospheric O₂ build up occurred during the Lomagundi carbon isotope excursion (LE) at the latest stage of the GOE [4]. The LE is the most pronounced and longest-lived carbon isotope excursion in Earth’s history that took place c. 2.22-2.06 Gyrs ago [4,5]. It reflects increased organic carbon (C_org) burial resulting from high primary productivity at the time of high phosphorous flux to the ocean associated with intense acidic chemical weathering of landmasses. However, mechanisms responsible for such high C_org sequestration are not yet fully resolved, nor has it been possible to precisely quantify the magnitude and expansion of oxygenation within the coeval atmosphere-ocean system.

Here, we studied diagenetic concretions of pyrite and carbonate and their host black shales of the Francevillian Group, southeast Gabon, deposited during the LE. Light sulfur (δ³⁴S ‰, VCDT) and carbon (δ¹³C_carb ‰, VPDB) isotope ratios indicate that both pyrite and carbonate formed in sediments through microbial sulfate reduction and C_org remineralization, respectively. Selenium isotopic ratios (δ^82/76Se ‰, NIST3149) of the pyrite concretions and their host shales are dominated by highly negative values as low as –3‰, which is strong evidence for partial reduction of selenium oxyanions (SeO_x^2-) in the sediment below an oxygenated seawater column. Collectively, the data suggests an oxygenated water column in the Francevillian Basin with a large SeO_x^2- reservoir that continuously resupplied these electron acceptors to the sediment and prevented their quantitative reduction. The studied black shales host putative, fossilized large colonial multicellular organisms that had the ability to laterally and vertically migrate within the sediments [6]. We propose that bioturbation by these organisms allowed an increased flux of electron acceptors (e.g., O₂, NO₃^–, SeO_x^2-, SO₄^-) into the sediments and pushed the microbial sulfate reduction and methanogenesis zones downward. As a consequence, CH₄ and H₂S generated in these zones were re-oxidized in more oxic upper levels of the sediments, which prevented them from escaping to the water column. An increase in ecosystem complexity thus likely aided C_org sequestration to the sediments and O₂ accumulation in the atmosphere-ocean system during the LE.

[1] Bekker et al. (2004), Nature, 427, 117–120. [2] Holland (2006), Philos. Trans. R. Soc. B 361, 903–91. [3] Colwyn et al. (2014), Geobiology, DOI: 10.1111/gbi.12360. [4] Karhu and Holland (1996), Geology, 24, 867–870. [5] Bekker (2014), Encyclopedia of Astrobiology, Springer-Verlag, 1–6. [6] El Albani et al. (2019), Proc. Natl. Acad. Sci. USA, 116, 3431–3436.

How to cite: Ossa Ossa, F., König, S., Hofmann, A., Bekker, A., Spangenberg, J. E., and Schoenberg, R.: Increase in ecosystem complexity aided organic carbon sequestration during the c. 2.22-2.06 Ga Lomagundi carbon isotope excursion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10828, https://doi.org/10.5194/egusphere-egu21-10828, 2021.

During the late Pliensbachian–Toarcian interval, several global events were recognized as cooling and sea-level fall, warming, and sea-level rise which were driven by the changes in the concentration of the atmospheric CO₂. The latter one caused a second-order mass extinction during the Jenkyns Event in the early Toarcian, which was sedimentologically represented by black shales on the epicontinental carbonate-siliciclastic ramps of the Neotethyan Boreal-Atlantic Foraminiferal Biome (BAFB). While in the distal, hemipelagic-pelagic parts of the Western Neotethys, in the Mediterranean Foraminiferal Biome (MFB, Zsiborás & Görög, 2020, Görög & Zsiborás, 2020) with condensed Ammonitico Rosso carbonates, the black shales are usually missing.

The Bakonycsernye (Hungary) sections gave firstly the opportunity for the investigation of foraminifers from the upper Pliensbachian–Toarcian Ammonitico Rosso successions belong to the MFB. Thus, our goals were to show out the effect of the Jenkyns Event on the foraminiferal assemblages in this biome and based on the literature, to compare them with the ones of the MFB and BAFB.

Foraminiferal fauna was extracted by glacial acetic acid from 22 samples of three sections. Microfacies studies, taxonomic and palaeoecological analyses (abundance, diversity, morphogroups, and ecozones) were used for the evaluation of the palaeoenvironmental changes. For the comparison, a review of the literature was carried out to collect similar quantitative data about the assemblages of other regions.

The upper Pliensbachian was affected by a cooling event and a global sea-level fall (JPlie8) which was characterized by a diverse ammodiscinid-lenticulinid-ichthyolariid-nodosariid mixed epi- and infaunal assemblage in Bakonycsernye. During the rapid global warming and sea-level rise event (JToa1) in the lower Toarcian, ammodiscinids and ornamented ichthyolariids disappeared thus the assemblage changed into a less diverse lenticulinid-nodosariid-polymorphinid infauna-dominated assemblage. In the middle–upper Toarcian, epifaunal grazer spirillinids reigned the assemblages followed by lenticulinids and nodosariids.

The microfossils indicated low to moderate oxygen levels in the upper Pliensbachian–lower Toarcian, high from the middle Toarcian. From then, the opportunist epifaunal spirillinids appeared in mass. They could graze on a microbial and/or fungal biofilm or could be fed on marine snow. Thus, the existence of the main foraminiferal groups depended on the microbial activity which increased during the warmer intervals.

Based on the ratio of the foraminiferal morphogroups, the foraminiferal fauna of Bakonycsernye was similar to others from the Mediterranean Biome and it was remarkably different from Boreal Atlantic ones. The calculated provinciality indices showed no provinciality between the foraminiferal fauna of Bakonycsernye and other Western and Southern European localities in the upper Pliensbachian–upper Toarcian interval. These results also proved the global warming event linked to the increased atmospheric CO₂ level caused by the Karoo-Ferrar LIP.

Görög, Á., & Zsiborás, G. (2020). Foraminiferal Faunal Changes in the Upper Pliensbachian–Toarcian Ammonitico Rosso Sections (Bakonycsernye, Hungary). Journal of Foraminiferal Research, 50(4), 342–372.

Zsiborás, G., & Görög, Á (2020). Aalenian–lower Bajocian foraminiferal fauna from the Ammonitico Rosso series of Bakonycsernye (Hungary). Part 2: Palaeoecological and palaeobiogeographical aspects.  Journal of Foraminiferal Research, 50(1): 73–88.

How to cite: Görög, Á. and Zsiborás, G.: The biotic response of the Late Pliensbachian–Toarcian foraminiferal assemblages to the environmental perturbations in the pelagic and neritic environments of the Neotethys ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13108, https://doi.org/10.5194/egusphere-egu21-13108, 2021.

The Cretaceous was punctuated by interludes of widespread deposition of organic-rich sediments (black shales) in the oceans and epicontinental seas, named Oceanic Anoxic Events (OAE)s, representing major alterations in the global carbon budget. The early Aptian OAE 1a (ca. 120 Ma) coincided with a global paleoclimatic and paleoenvironmental perturbation which lasted for ca. 1.1 Myrs probably triggered by volcanogenic CO₂ emissions associated with the emplacement of the Ontong Java Plateau. To date, there is a comprehensive characterization of OAE 1a paleoceanographic conditions and paleoecology of surface-waters while less information is available for bottom-water evolution. In this regard, benthic foraminifera are ideal to characterize deep-water oxygen levels and the organic carbon flux. We present a high-resolution study of benthic foraminiferal assemblages across OAE 1a in the Cismon Core (western Tethys, Lombardy Basin, Northern Italy). Contrarily to many sites, the Cismon Core yields benthic foraminifera also in the Selli Level thus providing information about deep-water conditions during OAE 1a. Our data are indicative of fluctuations in bottom-water oxygenation and organic-matter flux to the sea-floor prior to, during and after OAE 1a. The integration of the new benthic foraminiferal data with calcareous nannofossil and planktonic foraminiferal datasets is here used to produce a model of surface- to bottom-water paleowater evolution through the latest Barremian-early late Aptian. In particular, the new data show coeval changes in bottom- and surface-waters conditions prior to and at the onset of OAE 1a. Anoxia was reached during the core of the negative carbon isotope anomaly, under maximum warming and higher surface-water fertility. Conversely, the repopulation of benthic foraminifera postponed the plankton recovery. Benthic foraminifera data at Cismon show, for the first time, evidence of a repopulation event during the OAE 1a suggestive for a slight increase in the supply of oxygen to the seafloor during the Selli Level deposition.

How to cite: Bottini, C., Giraldo-Gómez, V. M., Petrizzo, M. R., and Erba, E.: Surface- and deep-water response to the Early Aptian OAE 1a in the western Tethys, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8045, https://doi.org/10.5194/egusphere-egu21-8045, 2021.

It is largely accepted that climate plays a pivotal role in the diversification of shallow-water communities, with special regards to larger foraminifera (LF), also because increase of surface water temperatures is often accompanied by change in trophic conditions. The shift from widespread eutrophic to oligotrophic conditions in shallow seas probably contributed to the LF differentiation during Paleocene-Eocene times. However, there are few recent attempts to quantify the changes in biodiversity and to correlate them with the global climatic events of the Paleogene. We concentrated our attention on the group of rotaliids, resilient taxa that partially survived after the mass extinction occurred at the end of the Cretaceous.

Our data show that their differentiation at genus level was very rapid, reaching its maximum already in the late Danian SB2 Zone. Specific diversification, instead, culminated in late Thanetian SB4 Zone. A second peak in specific diversity is recorded during the Cuisian (upper part of the Ypresian), then rotaliid diversity steadily declined, as long as other groups of larger foraminifers, especially Alveolina and Nummulites, became more competitive and proliferated with a large number of species up to the Bartonian SB17 Zone, when a significant drop in rotaliid biodiversity is recorded.

Differently to other taxonomic groups, i.e., alveolinids and nummulitids, for which a single genus during the whole Eocene generated numerous species, rotaliid genera are usually characterized by a low number of species, possibly due to the re-opening of ecological niches after the abrupt decrease of diversity that followed the PETM event. The competition with other K-strategist LF probably contributed to the decline of rotaliids in the middle Eocene up to the MECO event, where a last dramatic drop is recorded.

The major changes appear strictly linked to warming events such as the Late Danian Event (LDE, starting of the generic diversification of rotaliids), Paleocene Eocene Termal Maximum (PETM, faunal turnover followed by abrupt decrease in both generic and specific diversity), Early Eocene Climatic Optimum (EECO, increase in number of K-strategists under widespread oligotrophic conditions) and Middle Eocene Climatic Optimum (MECO, ultimate drop in diversity and competition with other larger foraminifers).

This study was funded by the Italian Ministry of Education and Research (MIUR), funds PRIN 2017: project “Biota resilience to global change: biomineralization of planktic and benthic calcifiers in the past, present and future” (prot. 2017RX9XXY).

How to cite: Benedetti, A., Papazzoni, C. A., and Bosellini, F. R.: Rise and fall of rotaliids in the Paleocene-Eocene: the influence of climate and trophic competition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7245, https://doi.org/10.5194/egusphere-egu21-7245, 2021.

The Oligo-Miocene Transition (OMT) is one of the most important climatic transitions of younger earth history. This short period of climate warming coincides with a few biotic turnovers. OMT follows the Late Oligocene Warming Event which marks the last warming pulse of a generally cool interval and represents a time frame that could potentially fit well with modern climate change predictions.

The Case Cné section located within the Tertiary Piedmont Basin (TPB) represents a gradual transgressive event, which shows the drowning of a locally developed reef complex and a development of a deeper marine sedimentary setting influenced by gravity flow mechanics. Larger foraminifera association indicate a late Oligocene (SBZ23) time and this seems confirmed by Sr isotopes data.

By usage of sedimentological, semi-quantitative microfacies and geochemical analysis the sedimentary history of the section could be reconstructed and divided into four major phases. 1) The growth and establishment of the reef directly on the metamorphic substrate, 2) its development over the basement and the construction of a modest reefal body, 3) the slow drowning of the reef complex due to enhanced prograding fluvial activity and finally 4) the onset of gravity flows passing to turbiditic influence which cap the transgression and that continue regionally throughout into the Miocene. 

The benthic fauna seems to register the warming period by change in biodiversity and abundance. Below the warming event, larger foraminfera are rather sparse over the section and the benthic community seems dominated by suspension feeders. Toward the top of the section, where the LOWE seems to occur, the gravity flows transport a very large amount of operculinid foraminifera that are well adapted to warm and eutrophic conditions as the ones that possibly characterized the LOWE time span in this tectonically active region.

How to cite: Briguglio, A., Vannucci, M. G., Bruzzone, C., Crobu, S., Lutaj, E., and Piazza, M.: Late Oligocene Warming Event (LOWE) possibly preserved on top of a reef drowning sequence in NW Italy: insights from an integrated stratigraphic approach., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3360, https://doi.org/10.5194/egusphere-egu21-3360, 2021.

The biogeochemical and physical processes occurring in the oceans are pivotal in controlling the atmospheric carbon dioxide (CO_2atm) concentration. In particular, the biological pump uptakes CO_2atm through photosynthesis, whereas the carbonate pump produces CO₂ during CaCO₃ production. On the other hand, the accumulation of microbiota-derived carbonates into the deep-sea sediments favors the CO₂ surplus buffering during high carbonate dissolution episodes. To unravel the role of calcifying marine microbiota on CO_2atm regulation, we studied the microfossil contents from two sectors of the Western Pacific Ocean with potential different behaviors towards CO₂ buffering: the Ocean Drilling Program Site 1209 (NW Pacific) and the IMAGES Core MD 97-2114 (SW Pacific). The selected time interval covers the last 300 kyr – i.e. the most recent glacial-interglacial cycles, with particular attention to the Last Interglacial (LIG; 129-116 kyr) which is a good modern-analogue within the geological records. Analyses of calcareous nannofossil assemblages provided data on coccolith carbonate production and fluxes, primary productivity, and carbonate dissolution, which are essential to investigate the carbonate and biological pumps’ efficiency on CO_2atm uptake.

Our data confirm that the calcareous nannofossils contributed significantly to the ocean carbon cycle through both the biological and carbonate pumps, constituting up to 50-70% of the carbonate stocked into the sediments of the studied sites. In both the oceanic sectors, we recorded  higher coccolith carbonate fluxes mostly during deglaciations, and thus coccolithophores’ production acted as a negative feedback on CO_2atm uptake contributing to the CO_2atm abrupt increase which characterizes the deglacial-interglacial transitions of the last 800 kyr. Instead, higher carbonate dissolution episodes generating positive feedback on CO_2atm occurred during glacial inceptions and interglacials at Site 1209, and during glacial phases at Core MD 97-2114 due to the stronger intensity of the highly corrosive Deep Western Boundary Current (DWBC). Therefore, changes in coccolithophore production influenced the saturation state of deep waters in a time ranging from 10 to 100 kyr at both sites. The comparison between the two oceanic areas allows identifying significant differences in the processes involved in the CO₂ buffering. In the NW Pacific site the biological pump is probably more efficient owing to the high productivity of coccolithophorids together with the presence of diatom blooms during glacial phases. At the southern site the biological pump consists only of coccolithophorids, but the coccolith carbonate fluxes are almost doubled compared to the northern site testifying a major efficiency of the carbonate pump on CO₂ production. On the other hand, the higher amount of CaCO₃ stocked into the southern sediments and the higher efficiency of the physical pump related to the DWBC strongly contributed to the CO_2atm uptake during glacials. Thus, these two oceanic sectors seem to be decoupled and, therefore, it could be possible that their combined effect in a long-term trend can result in a balance.

How to cite: Bordiga, M., Lupi, C., Cabrini, M., and Cobianchi, M.: Coccolithophores’ efficiency towards CO2 buffering in the Western Pacific Ocean during the last 300 kyr, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7735, https://doi.org/10.5194/egusphere-egu21-7735, 2021.

The global ocean acts as a climate regulator through the uptake of Earth’s excess heat and the absorption of about 30% of anthropogenic CO₂ emissions since 1750.  Southern Ocean waters are warming faster than the global ocean average and their low temperatures and moderate alkalinity make this region especially vulnerable to ocean acidification. Coccolithophores are a major group of calcifying phytoplankton and an important component of the Southern Ocean carbon cycle. Controlled laboratory experiments on Emiliania huxleyi (the most abundant coccolithophore) over a broad range of carbonate chemistry scenarios suggest that this taxon may be susceptible to ongoing environmental change. However, it remains uncertain whether Southern Ocean coccolithophore populations have been modified by environmental change during the industrial era. The main reason for this knowledge gap is the lack of observational data since the onset of the Industrial Revolution. In particular, continuous monitoring of key Southern Ocean ecosystems only started a few decades ago, a period insufficiently long to permit assessments of whether anthropogenic impacts on the environment have affected coccolithophore populations beyond their natural state. In order to overcome this limitation, here we take advantage of the preservation capacity of coccolithophores in the sedimentary record to provide a benchmark of their pre-industrial state. We compare the morphotype assemblage composition and morphometric parameters in coccoliths of E. huxleyi from a suite of Holocene-aged sediments south of Tasmania with annual sediment trap records retrieved at the Southern Ocean Time Series observatory in the Australian sector of the Subantarctic Zone. Our results suggest that carbonate dissolution in the sediments reduced the coccolith mass and length of the coccoliths but, coccolith thickness appeared to be decoupled from dissolution. The biogeographical distribution of coccolith thickness in subtropical and subantarctic sediments mirrored the distribution of E. huxleyi morphotypes, highlighting the important role of E. huxleyi assemblage composition on the control of coccolith morphometrics. Moreover, comparison of coccolith assemblages from the sedimentary record with those collected from subantarctic sediment traps indicates that modern E. huxleyi coccoliths are about 2% thinner than those from the pre-industrial Holocene. The subtle change in coccolith thickness is in stark contrast with previous work that documented a dramatic reduction in shell calcification in the planktonic foraminifera Globigerina bulloides that resulted in a shell-weight decrease of 30-35%, most likely induced by ocean acidification. Overall, our results underscore the variable sensitivity of different marine calcifying plankton groups to ongoing environmental change in the Southern Ocean.

How to cite: Rigual-Hernandez, A. S., Sierro, F. J., Flores, J. A., Sánchez-Santos, J. M., Eriksen, R. S., Moy, A. D., Abrantes, F., Bostock, H., Nodder, S. D., González-Lanchas, A., and Trull, T. W.: Has (anthropogenic) climate change driven subantarctic Emiliania huxleyi populations beyond their natural state?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9748, https://doi.org/10.5194/egusphere-egu21-9748, 2021.

Hypoxia is widely distributed in coastal benthic habitats and is driven by warming, nutrient pollution and the diurnal cycles of photosynthesis and respiration. Benthic sessile species, such as the soft shell clam Mya arenaria, are commonly exposed to oxygen fluctuations in their habitats which might negatively impact the performance and metabolism of clams. To determine the effects of different oxygen regime on metabolism and behavior of M. arenaria, we exposed the clams for 21 days to chronic (constant) hypoxia at 20% of air saturation, fluctuating (cyclic) hypoxia (~10-50% of air saturation) and normoxia (100% of air saturation). To mimic conditions occurring in coastal hypoxic zones, CO2 and pH levels varied with the oxygen. We assessed the digging performance, bioirrigation capacity and bioenergetics of the clams. Acclimation to constant or cyclic hypoxia did not affect the oxygen consumption of the clams, but the oxygen consumption rates declined at low ambient oxygen concentrations regardless of the acclimation to different oxygen regimes. Clams acclimated to constant hypoxia mainly used lipids, whereas clams acclimated to cyclic hypoxia used carbohydrates as energy fuel. Clams acclimated to constant or cyclic hypoxia dug slower compared to the clams acclimated to normoxia. Furthermore, bioirrigation capacity decreased in clams acclimated to constant hypoxia. Our results indicate that constant and cyclic hypoxia impair bioturbation and bioirrigation capacity of clams which has implications for their ecological function as ecosystem engineers in benthic soft bottom habitats. 

How to cite: Ouillon, N., Forster, S., Jarret, A., Sokolov, E., and Sokolova, I.: The influence of different oxygen regimes on metabolism and behavior of a soft shell clam Mya arenaria, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12620, https://doi.org/10.5194/egusphere-egu21-12620, 2021.