Posts Tagged 'paleo'

Atmospheric carbon dioxide reconstruction and ocean acidification deduced from carbon isotope variations across the Triassic–Jurassic boundary in the Qiangtang Area, Tibetan Plateau

The end-Triassic mass extinction was one of the five most profound Phanerozoic extinction events. This event was accompanied by a series of significant environmental changes, of which the most notable is the emergence of warm climate and the world-wide disappearance of carbonate platform. C isotope is one of the main means of reconstructing palaeoenvironment, however, there are very
limited studies on Asia and Oceania in the East Tethys region. In China, continuous marine strata through the J/T boundary are widespread in the Qiangtang area of Tibet (Chen Lan et al., 2017), which provide us abundant
research materials to study the environmental geological evolution during the T–J transition in Asian and even eastern Tethys.

Continue reading ‘Atmospheric carbon dioxide reconstruction and ocean acidification deduced from carbon isotope variations across the Triassic–Jurassic boundary in the Qiangtang Area, Tibetan Plateau’

The role of calcification in carbonate compensation

The long-term recovery of the oceans from present and past acidification is possible due to neutralization by the dissolution of biogenic CaCO3 in bottom sediments, that is, carbonate compensation. However, such chemical compensation is unable to account for all features of past acidification events, such as the enhanced accumulation of CaCO3 at deeper depths after acidification. This overdeepening of CaCO3 accumulation led to the idea that an increased supply of alkalinity to the oceans, via amplified weathering of continental rocks, must accompany chemical compensation. Here we discuss an alternative: that changes to calcification, a biological process dependent on environmental conditions, can enhance and modify chemical compensation and account for overdeepening. Using a simplified ocean box model with both constant and variable calcification, we show that even modest drops in calcification can lead to appreciable long-term alkalinity build-up in the oceans and, thus, create overdeepening; we term this latter effect biological compensation. The chemical and biological manifestations of compensation differ in terms of controls, timing and effects, which we illustrate with model results. To better predict oceanic evolution during the Anthropocene and improve the interpretation of the palaeoceanographic record, it is necessary to better understand biological compensation.

Continue reading ‘The role of calcification in carbonate compensation’

Carbon burial in deep-sea sediment and implications for oceanic inventories of carbon and alkalinity over the last glacial cycle

Although it has long been assumed that the glacial–interglacial cycles of atmospheric CO2 occurred due to increased storage of CO2 in the ocean, with no change in the size of the “active” carbon inventory, there are signs that the geological CO2 supply rate to the active pool varied significantly. The resulting changes of the carbon inventory cannot be assessed without constraining the rate of carbon removal from the system, which largely occurs in marine sediments. The oceanic supply of alkalinity is also removed by the burial of calcium carbonate in marine sediments, which plays a major role in air–sea partitioning of the active carbon inventory. Here, we present the first global reconstruction of carbon and alkalinity burial in deep-sea sediments over the last glacial cycle. Although subject to large uncertainties, the reconstruction provides a first-order constraint on the effects of changes in deep-sea burial fluxes on global carbon and alkalinity inventories over the last glacial cycle. The results suggest that reduced burial of carbonate in the Atlantic Ocean was not entirely compensated by the increased burial in the Pacific basin during the last glacial period, which would have caused a gradual build up of alkalinity in the ocean. We also consider the magnitude of possible changes in the larger but poorly constrained rates of burial on continental shelves, and show that these could have been significantly larger than the deep-sea burial changes. The burial-driven inventory variations are sufficiently large to have significantly altered the δ13C of the ocean–atmosphere carbon and changed the average dissolved inorganic carbon (DIC) and alkalinity oncentrations of the ocean by more than 100µM, confirming that carbon burial uxes were a dynamic, interactive component of the glacial cycles that significantly modified the size of the active carbon pool. Our results also suggest that geological sources and sinks were significantly unbalanced during the late Holocene, leading to a slow net removal flux on the order of 0.1PgCyr−1 prior to the rapid input of carbon during the industrial period.

Continue reading ‘Carbon burial in deep-sea sediment and implications for oceanic inventories of carbon and alkalinity over the last glacial cycle’

Tracking the Paleocene‐Eocene Thermal Maximum in the North Atlantic: A shelf‐to‐basin analysis with a regional ocean model

The Paleocene‐Eocene Thermal Maximum (PETM), a transient greenhouse climate interval spurred by a large release of carbon to the ocean‐atmosphere ca. 56 million years ago, provides a geological point of comparison for potential effects of anthropogenic carbon emission. Geochemical proxies and fossil assemblages offer insight into the continental shelf response to the PETM, but global ocean‐atmosphere models cannot resolve shelf processes at sufficient resolution for model‐data comparisons. We present high‐resolution simulations of the pre‐PETM and PETM North Atlantic basin using the Regional Ocean Modeling System (ROMS), including a resolved continental shelf along the eastern margin of North America in the Salisbury Embayment. ROMS’ high‐resolution, terrain‐following coordinate system permits greater vertical resolution and eddy resolution along continental margins while also capturing open‐ocean processes. We find that during the PETM, benthic oxygen concentration ([O2]) in the Salisbury Embayment decreases 18% to an average state of year‐round mild hypoxia, while average benthic calcite saturation (Ω) declines from 4.4 to 2.3. These benthic decreases are driven largely by enhanced benthic oxic respiration, which occurs despite no increase in shelf productivity. Instead, increased respiration stems from less vigorous off‐shelf transport of organic matter due to (a) weakened along‐shelf water currents and (b) weakened coastal upwelling that forces productivity closer to the shelf seafloor. Model results do not include riverine inputs, which would have further lowered benthic [O2] and Ω. Our data suggest lowered benthic calcite saturation and mild hypoxia as an upper bound on the oxygenation state of the Salisbury Embayment seafloor during the PETM.

Continue reading ‘Tracking the Paleocene‐Eocene Thermal Maximum in the North Atlantic: A shelf‐to‐basin analysis with a regional ocean model’

Hyperthermal-driven mass extinctions: killing models during the Permian–Triassic mass extinction

Many mass extinctions of life in the sea and on land have been attributed to geologically rapid heating, and in the case of the Permian–Triassic and others, driven by large igneous province volcanism. The Siberian Traps eruptions raised ambient temperatures to 35–40°C. A key question is how massive eruptions during these events, and others, could have killed life in the sea and on land; proposed killers are reviewed here. In the oceans, benthos and plankton were killed by anoxia–euxinia and lethal heating, respectively, and the habitable depth zone was massively reduced. On land, the combination of extreme heating and drought reduced the habitable land area, and acid rain stripped forests and soils. Physiological experiments show that some animals can adapt to temperature rises of a few degrees, and that some can survive short episodes of increases of 10°C. However, most plants and animals suffer major physiological damage at temperatures of 35–40°C. Studies of the effects of extreme physical conditions on modern organisms, as well as assumptions about rates of environmental change, give direct evidence of likely killing effects deriving from hyperthermals of the past.

Continue reading ‘Hyperthermal-driven mass extinctions: killing models during the Permian–Triassic mass extinction’

CO2 storage and release in the deep Southern Ocean on millennial to centennial timescales

The cause of changes in atmospheric carbon dioxide (CO2) during the recent ice ages is yet to be fully explained. Most mechanisms for glacial–interglacial CO2 change have centred on carbon exchange with the deep ocean, owing to its large size and relatively rapid exchange with the atmosphere1. The Southern Ocean is thought to have a key role in this exchange, as much of the deep ocean is ventilated to the atmosphere in this region2. However, it is difficult to reconstruct changes in deep Southern Ocean carbon storage, so few direct tests of this hypothesis have been carried out. Here we present deep-sea coral boron isotope data that track the pH—and thus the CO2 chemistry—of the deep Southern Ocean over the past forty thousand years. At sites closest to the Antarctic continental margin, and most influenced by the deep southern waters that form the ocean’s lower overturning cell, we find a close relationship between ocean pH and atmospheric CO2: during intervals of low CO2, ocean pH is low, reflecting enhanced ocean carbon storage; and during intervals of rising CO2, ocean pH rises, reflecting loss of carbon from the ocean to the atmosphere. Correspondingly, at shallower sites we find rapid (millennial- to centennial-scale) decreases in pH during abrupt increases in CO2, reflecting the rapid transfer of carbon from the deep ocean to the upper ocean and atmosphere. Our findings confirm the importance of the deep Southern Ocean in ice-age CO2 change, and show that deep-ocean CO2 release can occur as a dynamic feedback to rapid climate change on centennial timescales.

Continue reading ‘CO2 storage and release in the deep Southern Ocean on millennial to centennial timescales’

Modelling determinants of extinction across two Mesozoic hyperthermal events

The Late Triassic and Early Toarcian extinction events are both associated with greenhouse warming events triggered by massive volcanism. These Mesozoic hyperthermals were responsible for the mass extinction of marine organisms and resulted in significant ecological upheaval. It has, however, been suggested that these events merely involved intensification of background extinction rates rather than significant shifts in the macroevolutionary regime and extinction selectivity. Here, we apply a multivariate modelling approach to a vast global database of marine organisms to test whether extinction selectivity varied through the Late Triassic and Early Jurassic. We show that these hyperthermals do represent shifts in the macroevolutionary regime and record different extinction selectivity compared to background intervals of the Late Triassic and Early Jurassic. The Late Triassic mass extinction represents a more profound change in selectivity than the Early Toarcian extinction but both events show a common pattern of selecting against pelagic predators and benthic photosymbiotic and suspension-feeding organisms, suggesting that these groups of organisms may be particularly vulnerable during episodes of global warming. In particular, the Late Triassic extinction represents a macroevolutionary regime change that is characterized by (i) the change in extinction selectivity between Triassic background intervals and the extinction event itself; and (ii) the differences in extinction selectivity between the Late Triassic and Early Jurassic as a whole.

Continue reading ‘Modelling determinants of extinction across two Mesozoic hyperthermal events’

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Ocean acidification in the IPCC AR5 WG II

OUP book