References of "Lefèbvre, Vincent"
     in
Bookmark and Share    
Full Text
Peer Reviewed
See detailPaleoproductivity during the middle Miocene carbon isotope events: A data-model approach
Diester-Haass, Liselotte; Billups, Katharina; Jacquemin, Ingrid ULg et al

in Paleoceanography (2013), 28

To what extent are individual middle Miocene eccentricity-scale benthic foraminiferal carbon isotope maxima (the so-called CM events) related to changes in marine export productivity? Here we use benthic ... [more ▼]

To what extent are individual middle Miocene eccentricity-scale benthic foraminiferal carbon isotope maxima (the so-called CM events) related to changes in marine export productivity? Here we use benthic foraminiferal accumulation rates from three sites in the Pacific and Southern Oceans and a geochemical box model to assess relationships between benthic foraminiferal δ13C records, export productivity, and the global carbon cycle. Results from Deep Sea Drilling Project Hole 588 and Ocean Drilling Program Site 747 show a distinct productivity maximum during CM 6 at 13.8 Ma, the time of major expansion of ice on Antarctica. Productivity maxima during other CM events are only recorded at high-latitude Site 747. A set of numerical experiments tests whether changes in foraminiferal δ13C records (CM events) and export productivity can be simulated solely by sea level fluctuations and the associated changes in global weathering-deposition cycles, by sea level fluctuations plus global climatic cooling, and by sea level fluctuations plus invigorated ocean circulation. Consistent with data, the periodic forcing of sea level and albedo (and associated weathering cycles) produces δ13C variations of the correct temporal spacing, albeit with a reduced amplitude. A productivity response of the correct magnitude is achieved by enhancing ocean circulation during cold periods. We suggest that the pacing of middle Miocene δ13C fluctuations is associated with cyclical sea level variations. The amplitude, however, is muted perhaps due to the competing effects of a time-lagged response to sea level lowstands but an immediate response to invigorated ocean circulation during cold phases. [less ▲]

Detailed reference viewed: 28 (8 ULg)
Full Text
Peer Reviewed
See detailDid a Katian Large Igneous Province trigger the Late Ordovician glaciation ? A hypothesis tested with a carbon cycle model
Lefebvre, Vincent; Servais, Thomas; François, Louis ULg et al

in Palaeogeography, Palaeoclimatology, Palaeoecology (2010), 296

The Ordovician is known as a period with high temperatures and high sea levels and a cooling event at the end of the period, leading to the Hirnantian glaciation and the !rst of the ‘big !ve’ mass ... [more ▼]

The Ordovician is known as a period with high temperatures and high sea levels and a cooling event at the end of the period, leading to the Hirnantian glaciation and the !rst of the ‘big !ve’ mass extinctions of marine life. The cause of this cooling event remains unclear. Several authors correlate it with a drop in atmospheric pCO2 to a threshold permitting the ice accumulation on the Gondwana supercontinent. However, what are the reasons of the atmospheric pCO2 decrease? Here, we follow the hypothesis initiated by Barnes (2004) that an Ordovician superplume event occurred during the Middle to Late Ordovician. Such an event would not only have a large impact on the Ordovician biodiversi!cation (Barnes, 2004) but it would also be responsible for the climatic upheaval during the Late Ordovician by the emplacement of a low latitudinal continental basaltic province that had an impact on the atmospheric pCO2. There is no direct evidence of a superplume event or of basaltic traps and the present study is therefore a hypothetical modelling approach where we demonstrate with a numerical box model, including carbon, alkalinity and phosphorus cycles coupled with a 1D climate model (energy balance model-EBM) (François and Walker,1992), that such a scenario allows to explain both the mid-Ashgill (Katian) global warming event, known as the Boda Event (Fortey and Cocks, 2005), and the subsequent Late Ordovician (Hirnantian) glaciation. Because silicate weathering is enhanced upon warm and wet climate, we try to constrain the size and the latitudinal emplacement of the basaltic province leading to a suf!cient consumption of atmospheric pCO2 to the threshold proposed by Herrmann et al. (2004 a, b) to initiate a glaciation on Gondwana. [less ▲]

Detailed reference viewed: 32 (0 ULg)
Full Text
Peer Reviewed
See detailMid-Miocene paleoproductivity in the Atlantic Ocean and implications for the global carbon cycle
Diester-Haass, Liselotte; Billups, Katharina; Groecke, Darren R et al

in Paleoceanography (2009), 24

A prominent, middle Miocene (17.5-13.5 Ma) carbon isotope excursion ubiquitously recorded in carbonate sediments has been attributed to enhanced marine productivity and sequestration of C-13 depleted ... [more ▼]

A prominent, middle Miocene (17.5-13.5 Ma) carbon isotope excursion ubiquitously recorded in carbonate sediments has been attributed to enhanced marine productivity and sequestration of C-13 depleted organic carbon in marine sediments or enhanced carbon burial in peat/lignite deposits on land. Here we test the hypothesis that the marine delta C-13 record reflects a change in productivity with proxy records from three Atlantic Ocean sites (Deep Sea Drilling Program Site 608 and Ocean Drilling Program Sites 925 and 1265). Our multiproxy approach is based on benthic foraminiferal accumulation rates, elemental ratios (Ba/Al and P/Al), the delta C-13 of bulk sedimentary organic matter, and dissolution indices. We compare these proxies to benthic foraminiferal delta C-13 values measured on the same samples. Our results indicate that marine paleoproductivity in the Atlantic Ocean is not related to the benthic foraminiferal delta C-13 excursion. A numerical box model confirms that marine productivity cannot account for the delta C-13 maximum. The model shows that sequestration of 1.5 x 10(18) mol C in the terrestrial realm over a period of 3 Ma leads to a 0.9 parts per thousand delta C-13 increase in the deep ocean, which is near the observed records. Therefore, an increase in continental organic carbon sequestration is the most plausible way to enrich the ocean's carbon pool with C-13, which is consistent with coeval lignite deposits worldwide. The delta C-13 values of bulk sedimentary organic matter parallel the delta C-13 of dissolved inorganic carbon as reflected by benthic foraminiferal delta C-13 values suggesting no significant change in atmospheric pCO(2) levels over the investigated period. [less ▲]

Detailed reference viewed: 26 (1 ULg)
See detailAtmospheric Carbon Dioxide and Climate Over Phanerozoic Times
François, Louis ULg; Lefèbvre, Vincent; Goddéris, Yves et al

Conference (2006, December)

The atmospheric CO2 mixing ratio has fluctuated widely over the Phanerozoic, according to the estimates from available proxy records. Because atmospheric CO2 is a major greenhouse gas, these fluctuations ... [more ▼]

The atmospheric CO2 mixing ratio has fluctuated widely over the Phanerozoic, according to the estimates from available proxy records. Because atmospheric CO2 is a major greenhouse gas, these fluctuations should have led to significant climatic variations. The "classical" view is indeed that atmospheric CO2 has been the main driver of the Earth's climate history. On long-term time scales, the atmospheric CO2 level is the result of the balance between CO2 inputs from volcanoes or oxidation of old organic carbon (kerogen) in exposed rocks and outputs through silicate weathering or organic carbon deposition. Existing model reconstructions of the Phanerozoic history of atmospheric CO2 are based on such budgets. Recent data and model experiments currently challenge these models. First, the carbon cycle may be more complex than represented in the earliest models. In particular, silicate weathering depends on numerous factors, which are not obvious to model or are poorly known over the Phanerozoic. Mountain uplift is one such factor, which has been much debated in the last decade. Lithology is another example: basalts weather much more rapidly than other silicate rocks and the emplacement of large basaltic areas on the continents may trigger glaciations. Continental configuration is also more important than previously thought, as indicated by recent model experiments on super-continent fragmentation coupling geochemical and climate models. Problems of "classical" Phanerozoic CO2 models are also well illustrated by the fact that the most recent estimates of CO2 degassing show very little variation between the Cretaceous and the present, a period when large changes in CO2 have occurred, whereas degassing is the most important forcing of CO2 evolution in long-term carbon cycle models. Second, CO2 is not the only driver of climate evolution. This obvious fact has largely been forgotten in Phanerozoic studies. What the proxies tell us on paleo-atmospheric CO2 is not always in line with what we know about paleoclimatic records. For instance, the proxies suggest relatively high CO2 levels during the Late Ordovician glaciations. Similarly, the Late Jurassic now appears to be colder than earlier thought, while again proxies suggest high atmospheric CO2 at that time. The mid-Miocene climate warming, which occurs simultaneously with a drop in CO2, provides another example. This latter change in CO2 is unanimously reflected in all proxies and, so, this decoupling between CO2 and climate cannot arise from uncertainties on the reconstructed CO2 levels or from dating problems, as might be the case of the former two examples. Other climatic drivers than CO2 clearly need to be considered. In this respect, vegetation- climate feedbacks have been completely disregarded in long-term climatic studies. Cenozoic cooling is, however, accompanied by a progressive transition from closed forests to more widespread grasslands and deserts on the continental areas, a change which must have had major impacts on the surface albedo and the water cycle. [less ▲]

Detailed reference viewed: 32 (2 ULg)