Archive for April, 2008

Des phytoplanctons profitent du réchauffement climatique (in French)

NE JAMAIS désespérer de la nature. Ce pourrait être la leçon de l’étude publiée dans la revue Science, vendredi 18 avril, par une équipe d’océanographes et biologistes : elle met en évidence l’étonnante faculté de certains planctons à prospérer, non seulement en dépit mais grâce à l’augmentation du dioxyde de carbone (CO2) atmosphérique. Une propriété qui déjoue toutes les prévisions.
Continue reading ‘Des phytoplanctons profitent du réchauffement climatique (in French)’

Chalk one up for coccolithophores

Scientists have feared that gradual acidification of the world’s oceans would wreak havoc with organisms that build protective outer shells. But a new finding shows at least three species of coccolithophores — single-celled algae that are major players in the ocean’s cycling of carbon — are responding to ocean acidification by building thicker cell walls and plates of chalk, contrary to what some recent lab experiments have shown.
The difference resulted from conducting experiments that better mimic actual ocean conditions when carbon dioxide from the atmosphere dissolves in seawater, scientists report in the April 18 issue of the journal Science.
Continue reading ‘Chalk one up for coccolithophores’

Seawater pH, pCO2 and [CO32-] variations in the Caribbean Sea over the last 130 kyr; a boron isotope and B/Ca study of planktic foraminifera

Here a new analytical methodology is described for measuring the isotopic composition of boron in foraminifera using multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). This new approach is fast (~10 samples analysed in duplicate per analytical session) and accurate (to better than 0.25 ‰ at 95% confidence) with acceptable sample size requirements (1-3 mg of carbonate). A core top calibration of several common planktic and two benthic species from geographically widespread localities shows a very close agreement between the isotopic composition measured by MC-ICPMS and the isotopic composition of B(OH)4- in seawater (as predicted using the recently measured isotopic equilibrium factor of 1.0272) at the depth of habitat. A down core and core top investigation of boron concentration (B/Ca ratio) shows that the partition coefficient is influenced by [CO32-] complicating the application of this proxy. Nevertheless, it is demonstrated that these two proxies can be used to fully constrain the carbonate system of surface water in the Caribbean Sea (ODP Site 999A) over the last 130 kyr. This reconstruction shows that during much of the Holocene and the last interglacial period surface water at Site 999A was in equilibrium with the atmosphere with respect to CO2. During the intervening colder periods although the surface water pCO2 was lower than the Holocene, it was a minor to significant source of CO2 to the atmosphere possibly due to either an expansion of the eastern equatorial Atlantic upwelling zone, or a more local expansion of coastal upwelling in the southern Caribbean. Such reorganisation of the oceanic carbonate system in favour of a larger source of CO2 to the atmosphere from the equatorial ocean may require mechanisms responsible for lowering atmospheric CO2 during glacial periods to be more efficient than previously supposed.
Continue reading ‘Seawater pH, pCO2 and [CO32-] variations in the Caribbean Sea over the last 130 kyr; a boron isotope and B/Ca study of planktic foraminifera’

A marine nutcracker

It is not farfetched to say that in the marine environment, coral reefs will be the first major ecosystem to be functionally extinguished because of climate change. Of course, many entire small areas of global systems have disappeared already for a number of reasons, from industrial pollution or coastal construction, and many areas of soft substrate have been totally obliterated (trawled for example). But a whole ecosystem with a pan-tropical span? Probably not.

Warming which causes, firstly, widespread bleaching of corals and which is then sufficiently severe and persistent to cause subsequent widespread mortality was not really noticed until the 1970s. It began to be increasingly noticed from the 1980s, and now occurs with frequent if erratic occurrence. The years 1998, 2001/2 and 2005 were seminal. Several predictions (calculations are a better word) have been made that severity and frequency of such events will increase so that the sea temperatures which cause widespread mortality will become a near annual occurrence well within the lifetime of most people alive today. Some suggestions are that, on average – there are many local variations – this will be most severe across a tropical belt and will expand outwards. Some extrapolations (Sheppard 2003) showed that the timing of critical dates are nearest close to the equator, becoming later as one moves polewards, in some oceans at least.

Many suggestions have been made, in both learned and unlearned articles, that the most poleward of reefs might even benefit from this. Many ‘marginal’ reef areas, such as Bermuda, South Africa, or southern Florida have generally cooler temperatures and lower coral and fish diversity. Coral diversity and growth tail off markedly in cooler waters, therefore (so that argument goes) a bit of warming will perhaps encourage more corals and greater variety of corals at greater latitudes. Indeed expansion and contraction of the northern boundary of coral in Florida as climate warmed and cooled over geological time has been well studied (Precht and Aronson, 2004 W.F. Precht and R.B. Aronson, Climate flickers and range shifts of reef corals, Front. Ecol. Environ. 2 (2004), pp. 307–314. Full Text via CrossRefPrecht and Aronson, 2004).

But while such effects clearly occur, it is not the solution to the survival of reefs or coral assemblages because it ignores several things. It ignores the point that reef growth has little connection with coral diversity: for example some highly diverse coral assemblages in the ‘coral triangle’ do not develop into reefs at all for reasons that remain unclear, while some very substantial reefs in some areas support a low coral diversity for biogeographic reasons. It overlooks the point that coral growth and reef growth are not at all the same thing either, and indeed the two become ‘de-coupled’ as temperatures cool, something described with some detail for Arabian regions especially where upwelling is strong (Sheppard et al 1992).

But most importantly perhaps, the argument that corals can perhaps move polewards a bit overlooks ocean acidification. The oceans are having to absorb more CO2 than ever before and, to date, half to two thirds of all CO2 generated since the start of the industrial revolution has been absorbed by the surface layers of the sea. It is, in fact, only the smaller portion which has not been absorbed by the ocean which causes our greenhouse effect and which is giving rise to all those conferences about global climate change and warming. That portion which has been absorbed, however, has changed the pH of the surface ocean by 0.1, which is a 30% increase in H+ ions (Royal Society, 2005). As a result, the complexities of the carbonic acid – bicarbonate – carbonate buffering system mean that calcification by marine life is increasingly curtailed. The aragonite saturation state is falling, and this increasingly restricts calcification by several critical forms of marine life (not only corals of course). This is affecting polar regions first, spreading towards the tropics. Within this century calcification rates could decline by 60% (Kleypas et al 2006) and previously existing, optimal levels of aragonite saturation will exist nowhere at all by 2040. On coral reefs, calcification will decline by about 30% by 30–50 years (ISRS, 2008). Further, there is evidence that coralline algae, which are essential to reef construction, will be affected as much or more than the corals themselves, even though some of them are very tolerant of warm temperatures. This problem of course affects other major calcifying organisms too, especially important of which are planktonic forms – coccolithophores, foraminifera, pteropods, etc., – although those that deposit calcite rather than aragonite will be slightly less vulnerable (Royal Society, 2005). Some pteropods (aragonite, like corals) are already being found with pitted tests in areas in the Southern Ocean where the carbonate saturation layer now extends to near the surface.
Continue reading ‘A marine nutcracker’

Impacts of ocean acidification on marine fauna and ecosystem processes

Oceanic uptake of anthropogenic carbon dioxide (CO2) is altering the seawater chemistry of the world’s oceans with consequences for marine biota. Elevated partial pressure of CO2 (pCO2) is causing the calcium carbonate saturation horizon to shoal in many regions, particularly in high latitudes and regions that intersect with pronounced hypoxic zones. The ability of marine animals, most importantly pteropod molluscs, foraminifera, and some benthic invertebrates, to produce calcareous skeletal structures is directly affected by seawater CO2 chemistry. CO2 influences the physiology of marine organisms as well through acid-base imbalance and reduced oxygen transport capacity. The few studies at relevant pCO2 levels impede our ability to predict future impacts on foodweb dynamics and other ecosystem processes. Here we present new observations, review available data, and identify priorities for future research, based on regions, ecosystems, taxa, and physiological processes believed to be most vulnerable to ocean acidification. We conclude that ocean acidification and the synergistic impacts of other anthropogenic stressors provide great potential for widespread changes to marine ecosystems.
Continue reading ‘Impacts of ocean acidification on marine fauna and ecosystem processes’

Inorganic carbon acquisition in potentially toxic and non-toxic diatoms: the effect of pH-induced changes in seawater carbonate chemistry

The effects of pH-induced changes in seawater carbonate chemistry on inorganic carbon (Ci) acquisition and domoic acid (DA) production were studied in two potentially toxic diatom species, Pseudo-nitzschia multiseries and Nitzschia navis-varingica, and the non-toxic Stellarima stellaris. In vivo activities of carbonic anhydrase (CA), photosynthetic O2 evolution and CO2 and HCO3 uptake rates were measured by membrane inlet MS in cells acclimated to low (7.9) and high pH (8.4 or 8.9). Species-specific differences in the mode of carbon acquisition were found. While extracellular carbonic anhydrase (eCA) activities increased with pH in P. multiseries and S. stellaris, N. navis-varingica exhibited low eCA activities independent of pH. Half-saturation concentrations (K1/2) for photosynthetic O2 evolution, which were highest in S. stellaris and lowest in P. multiseries, generally decreased with increasing pH. In terms of carbon source, all species took up both CO2 and HCO3. K1/2 values for inorganic carbon uptake decreased with increasing pH in two species, while in N. navis-varingica apparent affinities did not change. While the contribution of HCO3 to net fixation was more than 85% in S. stellaris, it was about 55% in P. multiseries and only approximately 30% in N. navis-varingica. The intracellular content of DA increased in P. multiseries and N. navis-varingica with increasing pH. Based on our data, we propose a novel role for eCA acting as Ci-recycling mechanism. With regard to pH-dependence of growth, the ‘HCO3 user’ S. stellaris was as sensitive as the ‘CO2 user’ N. navis-varingica. The suggested relationship between DA and carbon acquisition/Ci limitation could not be confirmed.
Continue reading ‘Inorganic carbon acquisition in potentially toxic and non-toxic diatoms: the effect of pH-induced changes in seawater carbonate chemistry’

Mitigating the atmospheric CO2 increase and ocean acidification by adding limestone powder to upwelling regions

The feasibility of enhancing the absorption of CO2 from the atmosphere by adding calcium carbonate (CaCO3) powder to the ocean and of partially reversing the acidification of the ocean and the decrease in calcite supersaturation resulting from the absorption of anthropogenic CO2 is investigated. CaCO3 could be added to the surface layer in regions where the depth of the boundary between supersaturated and unsaturated water is relatively shallow (250–500 m) and where the upwelling velocity is large (30–300 m a1). The CaCO3 would dissolve within a few 100 m depth below the saturation horizon, and the dissolution products would enter the mixed layer within a few years to decades, facilitating further absorption of CO2 from the atmosphere. This absorption of CO2 would largely offset the increase in mixed layer pH and carbonate supersaturation resulting from the upwelling of dissolved limestone powder. However, if done on a large scale, the reduction in atmospheric CO2 due to absorption of CO2 by the ocean would reduce the amount of CO2 that needs to be absorbed by the mixed layer, thereby allowing a larger net increase in pH and in supersaturation in the regions receiving CaCO3. At the same time, the reduction in atmospheric pCO2 would cause outgassing of CO2 from ocean regions not subject to addition of CaCO3, thereby increasing the pH and supersaturation in these regions as well. Geographically optimal application of 4 billion t of CaCO3 a1 (0.48 Gt C a1) could induce absorption of atmospheric CO2 at a rate of 600 Mt CO2 a1 after 50 years, 900 Mt CO2 a1 after 100 years, and 1050 Mt CO2 a1 after 200 years.
Continue reading ‘Mitigating the atmospheric CO2 increase and ocean acidification by adding limestone powder to upwelling regions’

De l’eau dans le gaz chez le plancton (in French)

«C’est la confusion la plus totale.» avoue Jean-Pierre Gattuso, spécialiste du plancton marin, concernant un sujet doublement «chaud». D’abord parce qu’il s’agit du changement climatique. Mais surtout d’une de ses conséquences qui n’est vraiment étudiée que depuis l’an 2000, explique ce directeur de recherche au Laboratoire d’océanographie de Villefranche-sur-Mer (CNRS, université Pierre et Marie Curie) : l’acidification des océans, provoquée par l’augmentation de la teneur en gaz carbonique de l’air, et ipso facto, des eaux superficielles où vit le plancton océanique, base de la chaîne alimentaire marine. Cette acidification pourrait entraver la fabrication de leur squelette de calcaire par les coraux et certaines algues planctoniques, car, précise Gattuso : «qui dit une eau plus acide dit moins de carbonate, le matériau à partir duquel ces organismes fabriquent leur squelette.» De la chimie simple et vérifiée depuis quelques années par des études sur les coraux et des algues planctoniques comme les foraminifères et les coccolithophores.
Continue reading ‘De l’eau dans le gaz chez le plancton (in French)’

Eocene/Oligocene ocean de-acidification linked to Antarctic glaciation by sea-level fall

One of the most dramatic perturbations to the Earth system during the past 100 million years was the rapid onset of Antarctic glaciation near the Eocene/Oligocene epoch boundary1, 2, 3 (approx34 million years ago). This climate transition was accompanied3 by a deepening of the calcite compensation depth—the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution. Changes in the global carbon cycle4, rather than changes in continental configuration5, have recently been proposed as the most likely root cause of Antarctic glaciation, but the mechanism linking glaciation to the deepening of calcite compensation depth remains unclear. Here we use a global biogeochemical box model to test competing hypotheses put forward to explain the Eocene/Oligocene transition. We find that, of the candidate hypotheses, only shelf to deep sea carbonate partitioning is capable of explaining the observed changes in both carbon isotope composition and calcium carbonate accumulation at the sea floor. In our simulations, glacioeustatic sea-level fall associated with the growth of Antarctic ice sheets permanently reduces global calcium carbonate accumulation on the continental shelves, leading to an increase in pelagic burial via permanent deepening of the calcite compensation depth. At the same time, fresh limestones are exposed to erosion, thus temporarily increasing global river inputs of dissolved carbonate and increasing seawater delta13C. Our work sheds new light on the mechanisms linking glaciation and ocean acidity change across arguably the most important climate transition of the Cenozoic era. Continue reading ‘Eocene/Oligocene ocean de-acidification linked to Antarctic glaciation by sea-level fall’

Can seashells save the world?

Coccolithophores are microscopic marine plants that convert carbon dioxide into chalk. It was thought that rising CO2 and more acid oceans would curb their activity. Instead they are booming – and fighting global warming. Steve Connor reports
Continue reading ‘Can seashells save the world?’

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Ocean acidification in the IPCC AR5 WG II

OUP book