Posts Tagged 'sediment'

Calcareous nannoplankton response to a high CO2 world: evidence from sediment traps (Aegean and Ionian Seas) and Pliocene paleofluxes

One of the most enigmatic features of long-term Cenozoic climatic evolution, with some analogue potential for present/ future global climate change, is the last sustained warm and high-atmospheric CO2 interval in Earth’s history. The Pliocene is the most recent period in Earth’s history when average global temperature, atmospheric CO2 concentrations, and sea level were higher than today. This time period offers an appropriate interval to understand the climatic processes of a warm, high CO2 world, similar to the ongoing climatic conditions. Also, due to the high absorption capacity of the Eastern Mediterranean to anthropogenic CO2, the study area (Aegean and Ionian seas) is an ideal location to assess the impact of anthropogenic ocean acidification on calcifying organisms. The main objective of the present study is to investigate calcareous nannoplankton fluxes in the NE Mediterranean Sea as recorded by sediment traps and paleoceanographic records. The study material is collected from sediment traps in the Aegean and Ionian Seas and from the sedimentary record of the Eastern Mediterranean Deep Sea Drilling Project (DSDP Leg42A, Site 378). In the present study, coccolith fluxes from sediment traps were examined and compared in different sites of the Aegean and Ionian Seas. Data were compared in order to define the spatial and seasonal variability in assemblage composition and coccolithophore fluxes. The present study reflects in the coccolithophore export productivity the context of biogenic sedimentation in the water column. Furthermore, a water and sediment trap samples (N.Aegean Sea) analysis was carried out and through the comparison with data derived from surface sediment of the same site, valuable information were provided on the alterations observed in coccolithophore assemblage composition during their export from the euphotic zone to the seafloor. In addition, the morphometric analysis of coccoliths contributed to the investigation of water masses in the water column of the North Aegean Sea. In the DSDP core data we focus on the “warm Pliocene” interval, after the Zanclean “flooding” phenomenon in the Aegean after the Messinian Mediterranean Salinity Crisis (Zanclean reflooding). According to the detailed biostratigraphy and the derived age model, this study presents a composite dataset of the two boreholes of DSDP-Leg42A-Site 378 for the interval 3.8-5.08Ma. Subsequently, we studied how coccolithophores adapted to the Pliocene environment by quantifying their abundance through paleofluxes, species composition and correlation with geochemical paleo-indices analyses performed on the core material. In addition, the DSDP sedimentary record provided evidence on the Zanclean reflooding mechanism in the Cretan Basin. This study aims to improve our understanding of long-term adaptation strategies of calcareous nannoplankton in warm, high-CO2 climates by combining present-day evidence with Lower Pliocene fossil time series.

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Acidification impacts and acclimation potential of Caribbean benthic foraminifera assemblages in naturally discharging low-pH water (update)

Ocean acidification (OA) is expected to negatively affect many ecologically important organisms. Here we report the response of Caribbean benthic foraminiferal assemblages to naturally discharging low-pH waters with a composition similar to that expected for the end of the 21st century. At low pH ∼ 7.8 and low saturation state with respect to calcite (Ωcalcite< 4), the relative abundance of hyaline, agglutinated, and symbiont-bearing species increased, indicating higher resistance to potential carbonate chemistry changes. Diversity and other taxonomical metrics (i.e., richness, abundance, and evenness) declined steeply with decreasing pH despite exposure of this ecosystem to low-pH conditions for millennia, suggesting that tropical foraminiferal communities will be negatively impacted under acidification scenarios SSP3-7.0 (Shared Socioeconomic Pathways) and SSP5-8.5. The species Archaias angulatus, a major contributor to sediment production in the Caribbean, was able to calcify at more extreme conditions (7.1 pH) than those projected for the late 21st century, but the calcified tests had a lower average density than those exposed to higher-pH conditions (7.96), indicating that reef foraminiferal carbonate production might decrease this century. Smaller foraminifera were particularly sensitive to low pH, and our results demonstrate their potential use to monitor OA conditions.

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An experimental study on post-mortem dissolution and overgrowth processes affecting coccolith assemblages: a rapid and complex process

Coccolith dissolution together with post-mortem morphological features are immensely important phenomena that can affect assemblage compositions, complicate taxonomic identification as well as provide valuable palaeoenvironmental insights. This study summarizes the effects of pH oscillations on post-mortem coccolith morphologies and the abundances and compositions of calcareous nannoplankton assemblages in three distinct types of material—(i) Cretaceous chalk, (ii) Miocene marls, and (iii) late Holocene calcareous ooze. Two independent experimental runs within a semi-enclosed system setting were realized to observe assemblage alterations. One experiment was realized with the presence of bacteria and, in contrast, the second one inhibited their potential effect on the studied system. The pH was gradually decreased within the range of 8.3–6.4 using a reaction of CO2 with H2O forming weak carbonic acid (H2CO3), thereby affecting CO32-. Further, a subsequent overgrowth study was carried out during spontaneous degassing accompanied by a gradual pH rise. The experiment revealed that the process and intensity of coccolith corrosion and subsequent overgrowth build-ups are influenced by a plethora of different factors such as (i) pH and associated seawater chemistry, (ii) mineral composition of the sediment, (iii) the presence of coccoliths within a protective substrate (faecal pellets, pores, pits), and (iv) the presence/absence of bacteria. Nannoplankton assemblages with corroded coccoliths or with coccoliths with overgrowth build-ups showed that the observed relative abundances of taxa experienced alteration from the original compositions. Additionally, extreme pH oscillations may result in enhanced morphological changes that make coccoliths unidentifiable structures, and might even evoke the absence of coccoliths in the fossil record.

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Investigating the effect of ocean acidification (natural and anthropogenic) on the size of Emiliania huxleyi from late Holocene sediments of the north Aegean sea (NE Mediterranean)

The impact of ocean acidification on calcareous nannoplankton has been debated among researchers. This study focused to enrich the available data on coccolith size and calcification for the cosmopolitan species Emiliania huxleyi and assess their connection to natural and anthropogenic environmental changes. The analysis was based on the M2 core from Athos basin (North Aegean Sea, Greece). In total, 80 samples were selected and processed in laboratory to prepare for Scanning Electron Microscope (SEM) imaging. About 4000 E. huxleyi coccoliths were inspected under the SEM and their morphometric values were calculated. Morphometric values displayed fluctuations across the core depths, which were compared to the age model and multiproxy analyses of previous studies in the same area (Gogou et al., 2016; Skampa et al., 2019; Dimiza et al., 2020). Evident changes were based mainly to the Relative Tube Width (RTW), with a tendency towards slightly increased calcified coccoliths within the Little Ice Age (c. 1200-1850 AD). Afterwards, during the Instrumental Period (c. 1850-present) values show a decreasing pattern. It is possible that human activities, especially in the last century, have affected the marine equilibrium with higher atmospheric CO2 absorption, environmental parameters changes and depletion of bioavailable carbonate ions. Although naturally induced environmental changes in the Northern Aegean could mask the clear effect of ocean acidification on E. huxleyi, these data may contribute to a potential tool for environmental monitoring in the context of tackling future climate change.

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Denitrification and N2O emission in estuarine sediments in response to ocean acidification: from process to mechanism

Graphical abstract

Global estuarine ecosystems are experiencing severe nitrogen pollution and ocean acidification (OA) simultaneously. Sedimentary denitrification is an important way of reactive nitrogen removal but at the same time leads to the emission of large amounts of nitrous oxide (N2O), a potent greenhouse gas. It is known that OA in estuarine regions could impact denitrification and N2O production; however, the underlying mechanism is still underexplored. Here, sediment incubation and pure culture experiments were conducted to explore the OA impacts on microbial denitrification and the associated N2O emissions in estuarine sediments. Under neutral (in situ) conditions, fungal N2O emission dominated in the sediment, while the bacterial and fungal sources had a similar role under acidification. This indicated that acidification decreased the sedimentary fungal denitrification and likely inhibited the activity of fungal denitrifiers. To explore molecular mechanisms, a denitrifying fungal strain of Penicillium janthinellum was isolated from the sediments. By using deuterium-labeled single-cell Raman spectroscopy and isobaric tags for relative and absolute quantitation proteomics, we found that acidification inhibited electron transfers in P. janthinellum and downregulated expressions of the proteins related to energy production and conservation. Two collaborative pathways of energy generation in the P. janthinellum were further revealed, that is, aerobic oxidative phosphorylation and TCA cycle and anoxic pyruvate fermentation. This indicated a distinct energy supply strategy from bacterial denitrification. Our study provides insights into fungi-mediated nitrogen cycle in acidifying aquatic ecosystems.

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Dynamics of carbonate characteristics of the Kara Sea waters in the late autumn season of 2021

The field data characterizing the dynamics of the carbonate system, the level of surface water corrosivity with respect to aragonite, and CO2 fluxes in the ocean–atmosphere system in the Kara Sea in the late autumn season are investigated. The study of carbonate characteristics of waters was carried out on board the R/V Akademik Mstislav Keldysh in October 2021. At that time, the waters of the main part of the studied area of the Kara Sea absorbed carbon dioxide from the atmosphere intensely; in the invasion zone, the average CO2 flux was 30.2 ± 35.5 mmol m–2 day–1. It is shown that, among the Arctic seas in the season under study, the open water area of the Kara Sea was one of the most significant sinks for atmospheric CO2. The change in the flux direction occurred in a narrow coastal zone; supersaturation of waters with CO2 was recorded in the estuarine regions, where the contribution of inland waters exceeded 50%. It is found that, with a small contribution of river waters (<10%), the change in the surface water temperature determined more than 90% of the spatial variability of рСО2. In the late autumn season, the surface waters of the Kara Sea were mainly supersaturated with calcium carbonate; an extremely low aragonite saturation level was found only in the estuarine regions, the areas of maximum influence of river waters.

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Role of oceanic abiotic carbonate precipitation in future atmospheric CO2 regulation

The oceans play a major role in the earth’s climate by regulating atmospheric CO2. While oceanic primary productivity and organic carbon burial sequesters CO2 from the atmosphere, precipitation of CaCO3 in the sea returns CO2 to the atmosphere. Abiotic CaCO3 precipitation in the form of aragonite is potentially an important feedback mechanism for the global carbon cycle, but this process has not been fully quantified. In a sediment-trap study conducted in the southeastern Mediterranean Sea, one of the fastest warming and most oligotrophic regions in the ocean, we quantify for the first time the flux of inorganic aragonite in the water column. We show that this process is kinetically induced by the warming of surface water and prolonged stratification resulting in a high aragonite saturation state (ΩAr ≥ 4). Based on these relations, we estimate that abiotic aragonite calcification may account for 15 ± 3% of the previously reported CO2 efflux from the sea surface to the atmosphere in the southeastern Mediterranean. Modelled predictions of sea surface temperature and ΩAr suggest that this process may weaken in the future ocean, resulting in increased alkalinity and buffering capacity of atmospheric CO2.

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Biotic and paleoceanographic changes across the Late Cretaceous Oceanic Anoxic Event 2 in the southern high latitudes (IODP sites U1513 and U1516, SE Indian Ocean)

Abstract

Oceanic Anoxic Event 2, spanning the Cenomanian/Turonian boundary (93.9 Ma), was an episode of major perturbations in the global carbon cycle. To investigate the response of biota and the paleoceanographic conditions across this event, we present data from International Ocean Discovery Program sites U1513 and U1516 in the Mentelle Basin (offshore SW Australia; paleolatitude 59°–60°S in the mid-Cretaceous) that register the first complete records of OAE 2 at southern high latitudes. Calcareous nannofossils provide a reliable bio-chronostratigraphic framework. The distribution and abundance patterns of planktonic and benthic foraminifera, radiolaria, and calcispheres permit interpretation of the dynamics of the water mass stratification and provide support for the paleobathymetric reconstruction of the two sites, with Site U1513 located northwest of the Mentelle Basin depocenter and at a deeper depth than Site U1516. The lower OAE 2 interval is characterized by reduced water mass stratification with alternating episodes of enhanced surface water productivity and variations of the thickness of the mixed layer as indicated by the fluctuations in abundance of the intermediate dwelling planktonic foraminifera. The middle OAE 2 interval contains lithologies composed almost entirely of radiolaria reflecting extremely high marine productivity; the low CaCO3 content is consistent with marked shoaling of the Carbonate Compensation Depth and ocean acidification because of CaCO3 undersaturation. Conditions moderated after deposition of the silica-rich, CaCO3-poor rocks as reflected by the microfossil changes indicating a relatively stable water column although episodes of enhanced eutrophy did continue into the lower Turonian at Site U1516.

Key Points

  • Documentation of first complete record of the Late Cretaceous Oceanic Anoxic Event 2 (OAE 2) at southern high latitudes (60°S) in the Indian Ocean
  • Dynamics of the water mass stratification inferred from distribution patterns of foraminifera, radiolaria, calcispheres
  • OAE 2 is characterized by alternating episodes of enhanced surface water productivity and variations of the thickness of the mixed layer
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Calcification of planktonic foraminifer Neogloboquadrina pachyderma (sinistral) controlled by seawater temperature rather than ocean acidification in the Antarctic Zone of modern Southern Ocean

Neogloboquadrina pachyderma (sinistral), the dominant planktonic foraminiferal species in the mid-to-high latitude oceans, represents a major component of local calcium carbonate (CaCO3) production. However, the predominant factors, governing the calcification of this species and its potential response to the future marine environmental changes, are poorly understood. The present study utilized an improved cleaning method for the size-normalized weight (SNW) measurement to estimate the SNW of N. pachyderma (sin.) in surface sediments from the Amundsen Sea, the Ross Sea, and the Prydz Bay in the Antarctic Zone of the Southern Ocean. It was found that SNW of N. pachyderma (sin.) is not controlled by deep-water carbonate dissolution post-mortem, and can be therefore, used to reflect the degree of calcification. The comparison between N. pachyderma (sin.) SNW and environmental parameters (temperature, salinity, nutrient concentration, and carbonate system) in the calcification depth revealed that N. pachyderma (sin.) SNWs in the size ranges of 200–250, 250–300, and 300–355 µm are significantly and positively correlated with seawater temperature. Moreover, SNW would increase by ∼30% per degree increase in temperature, thereby suggesting that the calcification of N. pachyderma (sin.) in the modern Antarctic Zone of the Southern Ocean is mainly controlled by temperature, rather than by other environmental parameters such as ocean acidification. Importantly, a potential increase in calcification of N. pachyderma (sin.) in the Antarctic Zone to produce CaCO3 will release CO2 into the atmosphere. In turn, the future ocean warming will weaken the ocean carbon sink, thereby generating positive feedback for global warming.

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Calcification response of planktic foraminifera to environmental change in the western Mediterranean Sea during the industrial era

The aim of this work is to investigate the variability of planktic foraminifera calcification in the northwestern Mediterranean Sea on seasonal, interannual and pre-industrial Holocene time scales. This study is based on data from a 12-year-long sediment trap record retrieved in the in the Gulf of Lions and seabed sediment samples from the Gulf of Lions and the promontory of Menorca. Three different planktic foraminifera species were selected based on their different ecology and abundance: Globigerina bulloides, Neogloboquadrina incompta, and Globorotalia truncatulinoides. A total of 273 samples were weighted in both sediment trap and seabed samples. As the traditionally used sieve fractions method is considered unreliable because of the effect of morphometric parameters on the foraminifera weight, we measured area and diameter to constrain the effect of these parameters. The results of our study show substantial different seasonal calcification patterns across species: G. bulloides showed a slight calcification increase during the high productivity period, while both N. incompta and G. truncatulinoides display a higher calcification during the low productivity period. The comparison of these patterns with environmental parameters revealed that Optimum Growth Conditions temperature and carbonate system parameters are the most likely to influence seasonal calcification in the Gulf of Lions. Interannual analysis suggest that both G. bulloides and N. incompta slightly reduced their calcification between 1994 and 2005, while G. truncatulinoides exhibited a constant and pronounced increase in its calcification that translated in an increase of 20 % of its shell weight for the 400–500 µm narrow size class. While our data suggest that carbonate system parameters are the most likely environmental parameter driving foraminifera calcification changes over the years.

Finally, comparison between sediment trap data and seabed sediments allowed us to assess the changes of planktic foraminifera calcification during the late Holocene, including the preindustrial era. Several lines of evidence strongly indicate that selective dissolution did not bias the results in any of our data sets. Our results showed a clear calcification reduction between pre-industrial Holocene and recent data with G. truncatulinoides experiencing the largest calcification decrease (32–40 %) followed by N. incompta (20–27 %) and G. bulloides (18–24 %). Overall, our results provide evidence of clear reduction in planktic foraminifera calcification in the Mediterranean most likely associated with ongoing ocean acidification and consistent with previous observations in other settings of the world’s oceans.

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Planktonic foraminifera organic carbon isotopes as archives of upper ocean carbon cycling

The carbon cycle is a key regulator of Earth’s climate. On geological time-scales, our understanding of particulate organic matter (POM), an important upper ocean carbon pool that fuels ecosystems and an integrated part of the carbon cycle, is limited. Here we investigate the relationship of planktonic foraminifera-bound organic carbon isotopes (δ13Corg-pforam) with δ13Corg of POM (δ13Corg-POM). We compare δ13Corg-pforam of several planktonic foraminifera species from plankton nets and recent sediment cores with δ13Corg-POM on a N-S Atlantic Ocean transect. Our results indicate that δ13Corg-pforam of planktonic foraminifera are remarkably similar to δ13Corg-POM. Application of our method on a glacial sample furthermore provided a δ13Corg-pforam value similar to glacial δ13Corg-POM predictions. We thus show that δ13Corg-pforam is a promising proxy to reconstruct environmental conditions in the upper ocean, providing a route to isolate past variations in δ13Corg-POM and better understanding of the evolution of the carbon cycle over geological time-scales.

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Offshore extinctions: ocean acidification impacting interstitial fauna

As problematic as global warming, ocean acidification is a widespread problem, but the consequences of the interstitial fauna are still underrated. The biodiversity within sandy beaches is out of measurement, and its loss will be significantly felt. Estimations of the number of species are still vague. Acting as a key role in the trophic net, the interstitial organisms are threatened by pH value changes. Changing the pH values is already linked with less species richness and weakness of the sea community. The sediments may not be a sufficient buffer. Beyond this, there is another environmental problem aggravating the scenario. The decreasing complexity in the sand structure generated by the destruction of biological-generated sediments will impact the local biodiversity. Other environmental situations such as lack of sufficient O2 levels may be an aggravating combination. Here, I propose a protocol to observe if occur offshore extinctions, the veiled extinctions of interstitial fauna.

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Environmental change and carbon-cycle dynamics during the onset of Cretaceous oceanic anoxic event 1a from a carbonate-ramp depositional system, Abu Dhabi, U.A.E.

Highlights

  • Negative δ13C excursion at onset of OAE1a recorded in carbonate-ramp deposits.
  • Time-series analysis shows relative complete record of C3 segment of OAE1a.
  • Evidence for short-lived carbonate dissolution event at the negative δ13C peak of C3.
  • Discussion of effects of seawater temperature, pH, and diagenesis on δ18O record.

Abstract

We report the first high-resolution sedimentological and geochemical record of the negative carbon-isotope excursion (CIE) at the onset of the early Aptian oceanic anoxic event (OAE) 1a from a carbonate-ramp depositional environment, analysed from a well core from c. 2500 m depth, 100 km offshore Abu Dhabi, United Arab Emirates. Time-series analysis of stable oxygen isotope values and concentrations of Si, Al, and Ti resulted in durations of the C3 and C4 segments of the CIE that support relative completeness of the C3 segment and high sediment preservation rates of c. 13 cm/kyr of the studied sedimentary sequence. Stable oxygen-isotope ratios of bulk carbonates are interpreted to indicate two episodes of cooling, separated by rapid warming during the peak of the negative CIE. The contributions of diagenesis and seawater pH on the bulk oxygen-isotope record will have affected the palaeoclimatic signal and are critically discussed. A major shift in oxygen isotope values at the peak of the negative CIE in the C3 segment coincides with relatively carbonate-poor, marly deposits, time-equivalent with other, global evidence for a reduction of carbonate saturation of sea-surface water. According to our chemo- and cyclostratigraphic calibration, this episode of low carbonate saturation of seawater reflects a pulse of major volcanic CO2 release from the Ontong-Java large igneous province that was sufficiently short to have escaped internal buffering by the dynamics of the ocean lysocline.

Continue reading ‘Environmental change and carbon-cycle dynamics during the onset of Cretaceous oceanic anoxic event 1a from a carbonate-ramp depositional system, Abu Dhabi, U.A.E.’

Environmental change and carbon-cycle dynamics during the onset of Cretaceous oceanic anoxic event 1a from a carbonate-ramp depositional system, Abu Dhabi, U.A.E.

Highlights

  • Negative δ13C excursion at onset of OAE1a recorded in carbonate-ramp deposits.
  • Time-series analysis shows relative complete record of C3 segment of OAE1a.
  • Evidence for short-lived carbonate dissolution event at the negative δ13C peak of C3.
  • Discussion of effects of seawater temperature, pH, and diagenesis on δ18O record.

Abstract

We report the first high-resolution sedimentological and geochemical record of the negative carbon-isotope excursion (CIE) at the onset of the early Aptian oceanic anoxic event (OAE) 1a from a carbonate-ramp depositional environment, analysed from a well core from c. 2500 m depth, 100 km offshore Abu Dhabi, United Arab Emirates. Time-series analysis of stable oxygen isotope values and concentrations of Si, Al, and Ti resulted in durations of the C3 and C4 segments of the CIE that support relative completeness of the C3 segment and high sediment preservation rates of c. 13 cm/kyr of the studied sedimentary sequence. Stable oxygen-isotope ratios of bulk carbonates are interpreted to indicate two episodes of cooling, separated by rapid warming during the peak of the negative CIE. The contributions of diagenesis and seawater pH on the bulk oxygen-isotope record will have affected the palaeoclimatic signal and are critically discussed. A major shift in oxygen isotope values at the peak of the negative CIE in the C3 segment coincides with relatively carbonate-poor, marly deposits, time-equivalent with other, global evidence for a reduction of carbonate saturation of sea-surface water. According to our chemo- and cyclostratigraphic calibration, this episode of low carbonate saturation of seawater reflects a pulse of major volcanic CO2 release from the Ontong-Java large igneous province that was sufficiently short to have escaped internal buffering by the dynamics of the ocean lysocline.

Continue reading ‘Environmental change and carbon-cycle dynamics during the onset of Cretaceous oceanic anoxic event 1a from a carbonate-ramp depositional system, Abu Dhabi, U.A.E.’

Calcification, dissolution and test properties of modern planktonic foraminifera from the central Atlantic Ocean

The mass of well-preserved calcite in planktonic foraminifera shells provides an indication of the calcification potential of the surface ocean. Here we report the shell weight of 8 different abundant planktonic foraminifera species from a set of core-to sediments along the Mid-Atlantic Ridge. The analyses showed that near the equator, foraminifera shells of equivalent size weigh on average 1/3 less than those from the middle latitudes. The carbonate preservation state of the samples was assessed by high resolution X-ray microcomputed tomographic analyses of Globigerinoides ruber and Globorotalia truncatulinoides specimens. The specimen preservation was deemed good and does not overall explain the observed shell mass variations. However, G. ruber shell weights might be to some extent compromised by residual fine debris internal contamination. Deep dwelling species possess heavier tests than their surface-dwelling counterparts, suggesting that the weight of the foraminifera shells changes as a function of the depth habitat. Ambient seawater carbonate chemistry of declining carbonate ion concentration with depth cannot account for this interspecies difference. The results suggest a depth regulating function for plankton calcification, which is not dictated by water column acidity.

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Increased ocean acidification by upwelling intensification in southern Tethyan margin during the PETM: implication for foraminiferal record

The upper Thanetian–lowermost Ypresian succession in Tunisia is part of an extensive high-productivity upwelling regime in the southern Tethyan margin. As in several modern coastal upwelling systems, the upwelling strengthening regionally accentuated sustained acidification conditions, which prevailed in the Roman Bridge area (Central Tunisia). The poor-carbonate sedimentation, associated with the bad preservation state of calcifiers, points to the expansion of carbonate undersaturation in the water column and deep-sea sediments. The upwelling of deep CO32− and dissolved oxygen-depleted water significantly put calcifiers under chemically stressed habitats. Foraminiferal dwarfism, decrease in abundance and diversity, and especially occurrence of abundant dissolved and fragmented shells could account for the severe carbonate-corrosive waters. The spoiled primary morphological characteristics of benthic foraminifera emphasize the alkalinity increase in the deep marine waters. The well-preserved organic matter in the Roman Bridge sediments suggested a relatively minor role of remineralization in CaCO3-unsaturated waters. The expansion of carbonate-depleted water in the Roman Bridge area was principally driven by upwelled deep depleted-carbonate waters. These findings highlight the challenge to predict the response of the marine ecosystem to rising ocean acidification in upwelling strengthening regions in the future.

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Assessing the effects of ocean warming and acidification on the seagrass Thalassia hemprichii

Seagrass beds serve as important carbon sinks, and it is thought that increasing the quantity and quality of such sinks could help to slow the rate of global climate change. Therefore, it will be important to (1) gain a better understanding of seagrass bed metabolism and (2) document how these high-productivity ecosystems are impacted by climate change-associated factors, such as ocean acidification (OA) and ocean warming (OW). A mesocosm-based approach was taken herein in which a tropical, Western Pacific seagrass species Thalassia hemprichii was cultured under either control or OA-simulating conditions; the temperature was gradually increased from 25 to 31 °C for both CO2 enrichment treatments, and it was hypothesized that this species would respond positively to OA and elevated temperature. After 12 weeks of exposure, OA (~1200 ppm) led to (1) increases in underground biomass and root C:N ratios and (2) decreases in root nitrogen content. Rising temperatures (25 to 31 °C) increased the maximum quantum yield of photosystem II (Fv:Fm), productivity, leaf growth rate, decomposition rate, and carbon sequestration, but decreased the rate of shoot density increase and the carbon content of the leaves; this indicates that warming alone does not increase the short-term carbon sink capacity of this seagrass species. Under high CO2 and the highest temperature employed (31 °C), this seagrass demonstrated its highest productivity, Fv:Fm, leaf growth rate, and carbon sequestration. Collectively, then, it appears that high CO2 levels offset the negative effects of high temperature on this seagrass species. Whether this pattern is maintained at temperatures that actually induce marked seagrass stress (likely beginning at 33–34 °C in Southern Taiwan) should be the focus of future research.

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Ocean acidification alters the predator – prey relationship between hydrozoa and fish larvae

Anthropogenic CO2 emissions cause a drop in seawater pH and shift the inorganic carbon speciation. Collectively, the term ocean acidification (OA) summarizes these changes. Few studies have examined OA effects on predatory plankton, e.g. Hydrozoa and fish larvae as well as their interaction in complex natural communities. Because Hydrozoa can seriously compete with and prey on other higher-level predators such as fish, changes in their abundances may have significant consequences for marine food webs and ecosystem services. To investigate the interaction between Hydrozoa and fish larvae influenced by OA, we enclosed a natural plankton community in Raunefjord, Norway, for 53 days in eight ≈ 58 m³ pelagic mesocosms. CO2 levels in four mesocosms were increased to ≈ 2000 µatm pCO2, whereas the other four served as untreated controls. We studied OA-induced changes at the top of the food web by following ≈2000 larvae of Atlantic herring (Clupea harengus) hatched inside each mesocosm during the first week of the experiment, and a Hydrozoa population that had already established inside the mesocosms. Under OA, we detected 20% higher abundance of hydromedusae staged jellyfish, but 25% lower biomass. At the same time, survival rates of Atlantic herring larvae were higher under OA (control pCO2: 0.1%, high pCO2: 1.7%) in the final phase of the study. These results indicate that a decrease in predation pressure shortly after hatch likely shaped higher herring larvae survival, when hydromedusae abundance was lower in the OA treatment compared to control conditions. We conclude that indirect food-web mediated OA effects drove the observed changes in the Hydrozoa – fish relationship, based on significant changes in the phyto-, micro-, and mesoplankton community under high pCO2. Ultimately, the observed immediate consequences of these changes for fish larvae survival and the balance of the Hydrozoa – fish larvae predator – prey relationship has important implications for the functioning of oceanic food webs.

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Global record of “ghost” nannofossils reveals plankton resilience to high CO2 and warming

RELATED PERSPECTIVE

Fossil imprints from oceans of the past

Ghosts of the past

The marine geological records of some past global warming events contain relatively few nannoplankton fossils, the lack which some interpret as being evidence of the impact of ocean acidification and/or related environmental factors on biocalcification. Slater et al. present a global record of imprint, or “ghost,” nannofossils throughout several of those intervals during the Jurassic and Cretaceous periods (see the Perspective by Henderiks). This finding implies that a literal interpretation of the fossil record can be misleading, and demonstrates that nannoplankton were more resilient to past warming events than traditional fossil evidence would suggest. —HJS

Abstract

Predictions of how marine calcifying organisms will respond to climate change rely heavily on the fossil record of nannoplankton. Declines in calcium carbonate (CaCO3) and nannofossil abundance through several past global warming events have been interpreted as biocalcification crises caused by ocean acidification and related factors. We present a global record of imprint—or “ghost”—nannofossils that contradicts this view, revealing exquisitely preserved nannoplankton throughout an inferred Jurassic biocalcification crisis. Imprints from two further Cretaceous warming events confirm that the fossil records of these intervals have been strongly distorted by CaCO3 dissolution. Although the rapidity of present-day climate change exceeds the temporal resolution of most fossil records, complicating direct comparison with past warming events, our findings demonstrate that nannoplankton were more resilient to past events than traditional fossil evidence suggests.

Continue reading ‘Global record of “ghost” nannofossils reveals plankton resilience to high CO2 and warming’

Effect of different pCO2 concentrations in seawater on meiofauna: abundance of communities in sediment and survival rate of harpacticoid copepods

The amount of CO2 dissolved in the ocean has been increasing continuously, and the results using climate change models show that the CO2 concentration of the ocean will increase by over 1000 ppm by 2100. Ocean acidification is expected to have a considerable impact on marine ecosystems. To find out about the impacts of ocean acidification on meiofaunal communities and copepod groups, we analyzed the differences in the abundance of meiofauna communities in sediment and the survival rate of harpacticoid copepod assemblages separated from the sediment, between 400 and 1000 ppm pCO2 for a short period of 5 days. In experiments with communities in sediments exposed to different pCO2 concentrations, there was no significant difference in the abundance of total meiofauna and nematodes. However, the abundance of the harpacticoid copepod community was significantly lower at 1000 ppm than that at 400 ppm pCO2. On the other hand, in experiments with assemblages of harpacticoid copepods directly exposed to seawater, there was no significant difference in their survival rates between the two concentrations. Our findings suggest that a CO2 concentration of 1000 ppm in seawater can cause changes in the abundance of specific taxa such as harpacticoid copepods among the meiofauna communities in sediments.

Continue reading ‘Effect of different pCO2 concentrations in seawater on meiofauna: abundance of communities in sediment and survival rate of harpacticoid copepods’

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