Posts Tagged 'sediment'

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.

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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.

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CaCO3 dissolution in carbonate-poor shelf sands increases with ocean acidification and porewater residence time

Carbonate-poor sandy sediments comprise much of the shelf area, and—despite their low CaCO3 content—contain a significant pool of CaCO3 base available to neutralize ocean acid. Here, we conducted flow-through column experiments on permeable, carbonate-poor sand obtained from Catalina Island, CA, to quantify CaCO3 dissolution across a range of current and future seawater conditions. Using 13C isotope mass balance, we show that dissolution depends both on the CaCO3 saturation state (Ω) of the inflowing seawater, as well as porewater residence time. At current ocean conditions (Ωaragonite =2.4 and Ωcalcite =3.7 at our field site), dissolution was negligible for porewater residence times <1.8 h, but increased thereafter, following sufficient production of CO2 from aerobic respiration. As Ω of inlet water was lowered, simulating future ocean conditions, dissolution began earlier and rates increased. The response to acidification was similar to previously reported observations in carbonate-rich shelf environments, suggesting that carbonate-poor sediments have the potential to support enhanced dissolution in an acidifying ocean, given sufficient CaCO3 substrate. With continued acidification projected to occur this century, these sediments could transition from a net source of acid to the overlying seawater (production of alkalinity to dissolved inorganic carbon, ΔAlk/ΔDIC<1) to net source of buffering capacity (ΔAlk/ΔDIC>1) when overlying seawater Ωaragonite reaches 0.96 to 0.69 (Ωcalcite = 1.50 and 1.07), depending on porewater residence time. In some areas with naturally acidic water, this threshold has already been reached.

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The effects of ocean acidification on microbial nutrient cycling and productivity in coastal marine sediments

Ocean Acidification (OA), commonly referred to as the “other CO₂ problem,” illustrates the current rise in atmospheric carbon dioxide (CO₂) levels, precipitated in large by human-related activity (e.g., fossil fuel combustion and mass deforestation). The dissolution of atmospheric CO₂ into the surface of the ocean over time has reduced oceanic pH levels by 0.1 units since the start of the pre-industrial era and has resulted in wholesale shifts in seawater carbonate chemistry on a planetary scale. The chemical processes of ocean acidification are increasingly well documented, demonstrating clear rates of increase for global CO₂ emissions predicted by the IPCC (Intergovernmental Panel on Climate Change) under the business-as-usual CO₂ emissions scenario. The ecological impact of ocean acidification alters seawater chemical speciation and disrupts vital biogeochemical cycling processes for various chemicals and compounds. Whereby the unidentified potential fallout of this is the cascading effects on the microbial communities within the benthic sediments. These microorganisms drive the marine ecosystem through a network of vast biogeochemical cycling processes aiding in the moderation of ecosystem-wide primary productivity and fundamentally regulating the global climate. The benthic sediments are determinably one of the largest and most diverse ecosystems on the planet. Marine sediments are also conceivably one of the most productive in terms of microbial activity and nutrient flux between the water-sediment interface (i.e., boundary layer). The absorption and sequestering of CO₂ from the atmosphere have demonstrated significant impacts on various marine taxa and their associated ecological processes. This is commonly observed in the reduction in calcium carbonate saturation states in most shell-forming organisms (i.e., plankton, benthic mollusks, echinoderms, and Scleractinia corals). However, the response of benthic sediment microbial communities to a reduction in global ocean pH remains considerably less well characterized. As these microorganisms operate as the lifeblood of the marine ecosystem, understanding their response and physiological plasticity to increased levels of CO₂ is of critical importance when it comes to investigating regional and global implications for the effects of ocean acidification.

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Potential resilience to ocean acidification of benthic foraminifers living in Posidonia oceanica meadows: the case of the shallow venting site of Panarea

This research shows the results regarding the response to acidic condition of the sediment and Posidonia foraminiferal assemblages collected around the Panarea Island. The Aeolian Archipelago represents a natural laboratory and a much-promising study site for multidisciplinary marine research (carbon capture and storage, geochemistry of hydrothermal fluids and ocean acidification vs. benthic and pelagic organisms). The variability and the complexity of the interaction of the ecological factors characterizing extreme environments such as shallow hydrothermal vents did not allow us to carry out a real pattern of biota responses in situ, differently from those observed under controlled laboratory conditions. However, the study provides new insights into foraminiferal response to increasing ocean acidification (OA) in terms of biodiversity, faunal density, specific composition of the assemblages and morphological variations of the shells. The study highlights how the foraminiferal response to different pH conditions can change depending on different environmental conditions and microhabitats (sediments, Posidonia leaves and rhizomes). Indeed, mineral sediments were more impacted by acidification, whereas Posidonia microhabitats, thanks to their buffer effect, can offer “refugia” and more mitigated acidic environment. At species level, rosalinids and agglutinated group represent the most abundant taxa showing the most specific resilience and capability to face acidic conditions.

Continue reading ‘Potential resilience to ocean acidification of benthic foraminifers living in Posidonia oceanica meadows: the case of the shallow venting site of Panarea’

Acidification impacts and acclimation potential of foraminifera

Ocean acidification is expected to negatively affect many ecologically important organisms. Here we explored the response of Caribbean benthic foraminiferal communities to naturally discharging low-pH waters similar to expected future projections for the end of the 21st century. At low-pH (~ 7.7 pH units), low calcite saturation, agglutinated and symbiont-bearing species were relatively more abundant, indicating higher resistance to potential carbonate chemistry changes. Diversity and other taxonomical metrics declined steeply with decreasing pH despite exposure of this ecosystem for millennia to low pH conditions, suggesting that tropical foraminifera communities will be negatively impacted under acidification scenarios SSP3-7.0 and SSP5-8.5. The species Archaias angulatus, a major contributor to sediment production in the Caribbean was able to calcify at conditions more extreme than those projected for the late 21st century (7.1 pH units), but the calcified tests were of lower density than those exposed to high-pH ambient conditions (7.96 pH units), indicating that reef foraminiferal carbonate budget might decrease. Smaller foraminifera were highly sensitive to decreasing pH and our results demonstrate their potential as indicators to monitor increasing OA conditions.

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An evaluation of the efficacy of shell hash for the mitigation of intertidal sediment acidification

Our objectives were twofold: (1) to determine whether the addition of shell hash to intertidal sediments would mitigate porewater acidification and (2) whether its effectiveness was dependent on the type of sediment as described by organic matter (OM) and particle grain size (PGS). Field experiments were conducted at two sites within Burrard Inlet, British Columbia; Maplewood Mudflats (MM), high in OM and silt and Whey-ah-Wichen/Cates Park (WAW), low in OM and an equal PGS among very coarse, coarse, fine sand, and silt. Shell hash was added to triplicate treatment plots matched with triplicate controls at each site and porewater pH measured at flood and ebb tide over eight tidal cycles. Sampling occurred during June and July when tidal cycles were at their maximum inundation and exposure. Porewater pH was significantly greater for ebb versus flood tide and also between sites with MM significantly lower (7.59) as compared to WAW (8.03). Although pH was not mitigated by the shell hash, for WAW, variation in pH was reduced as compared to MM, as indicated by coefficients of variation over the 6-week sampling period. We suggest that the application of shell hash to reduce the impact of ocean acidification (OA) on intertidal sediments will be site dependent. The combined processes of eutrophication in sediments with high OM and respiration of infauna, especially at high densities, could act in concert with OA to create an intertidal region unsuitable for bivalve larvae settlement and development.

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Revisiting the carbonate chemistry of the Sea of Japan (East Sea): from water column to sediment

In this study, we investigated the carbonate system in sediments and water columns from five stations in the Sea of Japan (East Sea) (JES) during the R/V Hakuho Maru KH-10-2 research cruise in the summer of 2010. The total alkalinity (TA) and pH were measured. Adopting a saturation degree of 91% and 80% for the lysocline depth and calcite compensation depth (CCD), respectively, we found that those depths corresponded to 1360 and 1980 m. A comparison of the calcite saturation depths, lysocline depths, and CCD depths obtained for 1999 and 2010 suggests that acidification of the interior of the JES occurred. Sediment cores were retrieved using a multi-corer. In the sediment cores, a sharp decrease in the pH by 0.3–0.4 pH units was observed in the subsurface horizons (0–10 cm) compared with the pH of the seawater from the bottom horizons. The TA in the porewaters was significantly higher than that in the overlying seawater. The anaerobic degradation of organic matter is probably the main cause for the increasing TA in the sediments. The porewaters were significantly undersaturated with calcite and aragonite, except in that from the shallowest station, where the sediments below 7.5 cm were saturated, and even supersaturated, with calcite and aragonite. A linear correlation between the dissolved inorganic carbon and the TA for sediments with a slope of 0.9993 was found, despite there being potentially different ways for the diagenesis of the organic matter to occur. The diagenesis of organic matter in the top sediments of the JES forms gradients of TA and CO2* concentrations on the interface of “bottom water–sediments”. Averaged fluxes of TA and dissolved inorganic carbon (DIC) from the sediments to the bottom waters estimated by means of Fickian diffusion were calculated as 44 and 89 mmol/(m2 year) for TA and DIC, respectively.

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The distribution of intact polar lipid-derived branched tetraethers along a freshwater-seawater pH gradient in coastal East China Sea

In view of the application of microbial branched tetraether lipids (i.e., brGDGTs) in terrestrial pH reconstructions, their potential as seawater pH proxy is investigated from the Yangtze River Estuary (YRE) to the shelf region in the East China Sea (ECS). BrGDGTs occurring as ‘fossil’ core and in ‘living’ intact polar lipids (CL-brGDGTs and IPL-brGDGTs, respectively) in surface sediments are separately analyzed. The results show that riverine IPL-brGDGTs are rapidly lost at the transition from the YRE to the marine settings and sedimentary IPL-brGDGTs are predominantly produced in situ in brackish water environments. Among environmental parameters, surface water pH is a determinant factor controlling the cyclization of IPL-brGDGTs, with higher cyclization at higher pH. Hereby, two IPL-brGDGTs-based seawater pH calibrations are established in the brackish seawater environment in this study: surface pH = 7.28 + 1.08 × #Ringstetra (n = 18, r2 = 0.87, RMSE = 0.05, p < 0.01) and surface pH = 7.13 + 3.82 × f(Ic) + 4.29 × f(IIb’) + 4.21 × f(IIIa) (n = 18, r2 = 0.90, RMSE = 0.04, p < 0.01), which are different from those established in terrestrial environments. This is the first work demonstrating that in situ produced microbial brGDGTs can trace seawater pH change in marine environment, which makes it promising to apply brGDGTs for seawater pH reconstructions in marine settings where the terrestrial inputs are negligible.

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Unraveling ecological signals from a global warming event of the past

This article has related content: Isotopic filtering reveals high sensitivity of planktic calcifiers to Paleocene–Eocene thermal maximum warming and acidification Brittany N. Hupp, D. Clay Kelly, John W. Williams

As we face the increasing threat of global warming and its associated effects, paleontologists and paleoclimatologists alike look to the geological record to investigate how rapid, natural global warming events of the past have impacted the Earth system. One of the most important archives for investigating climate change in the geological past is the marine sediment record (1). In the open oceans, sediment particles, organic matter, and the shells of marine microorganisms, are constantly raining down on the seafloor and accumulating as marine sediments (1). In the relative quiescence of the deep sea, these sediments can build up relatively undisturbed for millions of years (1). Analysis of the chemical signals in these sediments that are influenced by temperature has allowed for the reconstruction of changing global climates throughout the last 70 million years (2).

The first half of the Cenozoic (66 million years to 34 million years ago) was characterized by “hothouse” and “warmhouse” climates, when global temperatures were between 5 °C and 10 °C warmer than the present day (2), and atmospheric CO2 was estimated to be between 500 and 3,000 parts per million (3). Against this backdrop of an already warm world, between 56 million and 46 million years ago, there were a series of rapid global warming events called “hyperthermals” (2). These hyperthermal events are geologically brief, typically <200,000 y in duration, and associated with sharp negative carbon isotope excursions (2). The Paleocene–Eocene thermal maximum (PETM), which occurred ∼56 million years ago, was the largest of these events (2). It was first discovered in the early 1990s as a pronounced shift in the climate records of a deep-sea sediment core from the Southern Ocean (4). Since that time, the PETM has become the most studied Cenozoic hyperthermal, and, due to its potential analogy to anthropogenic climate change, it remains a key interval of Earth history for climatological research.

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Abrupt upwelling and CO2 outgassing episodes in the north-eastern Arabian Sea since mid-Holocene

Identifying the causes and consequences of natural variations in ocean acidification and atmospheric CO2 due to complex earth processes has been a major challenge for climate scientists in the past few decades. Recent developments in the boron isotope (δ11B) based seawater pH and pCO2 (or pCO2sw) proxy have been pivotal in understanding the various oceanic processes involved in air-sea CO2 exchange. Here we present the first foraminifera-based δ11B record from the north-eastern Arabian Sea (NEAS) covering the mid-late Holocene (~ 8–1 ka). Our record suggests that the region was overall a moderate to strong CO2 sink during the last 7.7 kyr. The region behaved as a significant CO2 source during two short intervals around 5.5–4 ka and 2.8–2.5 ka. The decreased pH and increased CO2 outgassing during those abrupt episodes are associated with the increased upwelling in the area. The upwelled waters may have increased the nutrient content of the surface water through either increased supply or weaker export production. This new dataset from the coastal NEAS suggests that, as a potential result of changes in the strength of the El-Nino Southern Oscillation, the region experienced short episodes of high CO2 outgassing and pre-industrial ocean acidification comparable to or even greater than that experienced during the last ~ 200 years.

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Parallel between the isotopic composition of coccolith calcite and carbon levels across Termination II: developing a new paleo-CO2 probe

Beyond the pCO2 records provided by ice core measurements, the quantification of atmospheric CO2 concentrations and changes thereof relies on proxy data, the development of which represents a foremost challenge in paleoceanography. In the paleoceanographic toolbox, the coccolithophores occupy a notable place, as the magnitude of the carbon isotopic fractionation between ambient CO2 and a type of organic compounds that these photosynthetic microalgae synthesize (the alkenones) represents a relatively robust proxy to reconstruct past atmospheric CO2 concentrations during the Cenozoic. The isotopic composition of coeval calcite biominerals found in the sediments and also produced by the coccolithophores (the coccoliths) have been found to record an ambient CO2 signal through culture and sediment analyses. These studies have, however, not yet formalized a transfer function that quantitatively ties the isotopic composition of coccolith calcite to the concentrations of aqueous CO2 and, ultimately, to atmospheric CO2 levels. Here, we make use of a microseparation protocol to compare the isotopic response of two size-restricted coccolith assemblages from the North Atlantic to changes in surface ocean CO2 during Termination II (ca. 130–140 ka). Performing paired measurements of the isotopic composition (δ13C and δ18O) of relatively large and small coccoliths provides an isotopic offset that can be designated as a “differential vital effect”. We find that the evolution of this offset follows that of aqueous CO2 concentrations computed from the ice core CO2 curve and an independent temperature signal. We interpret this biogeochemical feature to be the result of converging carbon fixation strategies between large and small cells as the degree of carbon limitation for cellular growth decreases across the deglaciation. We are therefore able to outline a first-order trend between the coccolith differential vital effects and aqueous CO2 in the range of Quaternary CO2 concentrations. Although this study would benefit from further constraints on the other controls at play on coccolith geochemistry (growth rate, air–sea gas exchange, etc.), this test of the drivers of coccolith Δδ13C and Δδ18O in natural conditions is a new step in the development of a coccolith paleo-CO2 probe.

Continue reading ‘Parallel between the isotopic composition of coccolith calcite and carbon levels across Termination II: developing a new paleo-CO2 probe’

Surface ocean warming and acidification driven by rapid carbon release precedes Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM) is recognized by a major negative carbon isotope (δ13C) excursion (CIE) signifying an injection of isotopically light carbon into exogenic reservoirs, the mass, source, and tempo of which continue to be debated. Evidence of a transient precursor carbon release(s) has been identified in a few localities, although it remains equivocal whether there is a global signal. Here, we present foraminiferal δ13C records from a marine continental margin section, which reveal a 1.0 to 1.5‰ negative pre-onset excursion (POE), and concomitant rise in sea surface temperature of at least 2°C and a decline in ocean pH. The recovery of both δ13C and pH before the CIE onset and apparent absence of a POE in deep-sea records suggests a rapid (< ocean mixing time scales) carbon release, followed by recovery driven by deep-sea mixing. Carbon released during the POE is therefore likely more similar to ongoing anthropogenic emissions in mass and rate than the main CIE.

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Effects of local acidification on benthic communities at shallow hydrothermal vents of the Aeolian Islands (Southern Tyrrhenian, Mediterranean Sea)

Simple Summary

Ocean acidification is causing major changes in marine ecosystems, with varying levels of impact depending on the region and habitat investigated. Here, we report noticeable changes in both meio- and macrobenthic assemblages at shallow hydrothermal vents located in the Mediterranean Sea. In general, the areas impacted by the vent fluids showed decrease in the abundance of several taxa and a shift in community composition, but with a clear biomass reduction evident only for macrofauna. CO2 emissions at shallow hydrothermal vents cause a progressive simplification of community structure and a general biodiversity decline due to the loss of the most sensitive meio- and macrofaunal taxa, which were replaced by the more tolerant groups, such as oligochaetes, or highly mobile species, able to escape from extreme conditions. Our results provide new insight on the tolerance of marine meio- and macrofaunal taxa to the extreme conditions generated by hydrothermal vent emissions in shallow-water ecosystems.

Abstract

The Aeolian Islands (Mediterranean Sea) host a unique hydrothermal system called the “Smoking Land” due to the presence of over 200 volcanic CO2-vents, resulting in water acidification phenomena and the creation of an acidified benthic environment. Here, we report the results of a study conducted at three sites located at ca. 16, 40, and 80 m of depth, and characterized by CO2 emissions to assess the effects of acidification on meio- and macrobenthic assemblages. Acidification caused significant changes in both meio- and macrofaunal assemblages, with a clear decrease in terms of abundance and a shift in community composition. A noticeable reduction in biomass was observed only for macrofauna. The most sensitive meiofaunal taxa were kinorhynchs and turbellarians that disappeared at the CO2 sites, while the abundance of halacarids and ostracods increased, possibly as a result of the larger food availability and the lower predatory pressures by the sensitive meiofaunal and macrofaunal taxa. Sediment acidification also causes the disappearance of more sensitive macrofaunal taxa, such as gastropods, and the increase in tolerant taxa such as oligochaetes. We conclude that the effects of shallow CO2-vents result in the progressive simplification of community structure and biodiversity loss due to the disappearance of the most sensitive meio- and macrofaunal taxa.

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