Turbulence de type Burgers dans la zone de surf
Dans cette article nous montrons que la turbulence d'onde en zone de surf interne a de nombreuses similarités avec la turbulence de Burgers. En nous appuyant sur des résultats théoriques concernant la turbulence de Burgers nous proposons un modèle décrivant les spectres d'énergie en zone de surf interne. Notre modèle est ensuite validé par rapport à des données expérimentales. A partir de mesures de l'élévation des vagues, notre modèle permet d'estimer le coefficient de diffusion turbulente et de prédire le spectre de dissipation de l'énergie.
(29/08/2022)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Charcoal in deep-sea sediments: fire history and climate change
Fire results today from interactions between climate, vegetation and human activities and occurs in most terrestrial ecosystems. It has potential feedbacks to climate, through the emission of CO2, trace gases and aerosols that influence the global carbon cycle, the atmospheric chemistry and the radiative balance. How fire regimes (including frequency and intensity) may respond to climatic change can be explored by modelling. Empirical-based models however lack the impact of climatic change on vegetation and it is necessary to evaluate process-based models with paleofire data. Marine sedimentary charcoal records provide the opportunity to explore drivers of fire beyond the range of recent climates at different time-scales (from centennial, millennial to orbital scales). I will focus on two subtropical regions, the southwestern Iberian Peninsula and the southern Africa where millennial- and orbital-scale fire variability was observed. Results suggest a strong orbital and millennial climate control on fires and highlight the main role of vegetation type and fuel amount in driving fires in these subtropical regions. Contrary to the conventional expectation that fire increases with higher temperatures and increased drought, fires may increase also under cooler and/or wetter climates. Data-model comparison is still challenging as paleofire records provide charcoal measurements (abundance and morphology) not directly comparable to fire regime metrics. I will develop on calibrating charcoal in marine sediments showing recent results from the Mediterranean region and off the African continent. Results suggest the abundance and morphology of charcoal detect specific fire regimes, in particular fires of high intensity in mixed vegetation in Iberia and in graminoid-mixed ecosystems in Africa. These results might be a springboard for converting marine charcoal records into past fire regime history.
(29/08/2022)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
When Eastern India Oscillated Between Desert Versus Savannah‐Dominated Vegetation
During the last glacial period, the tropical hydrological cycle exhibited large variability across orbital and millennial timescales. However, the response of the Indian summer monsoon (ISM), its related impact on terrestrial ecosystems, and associated forcing mechanisms remain controversial. Here we present a marine record of pollen-inferred vegetation changes suggesting that eastern India shifted from woody-savanna mosaics during Marine Isotopic Stage 3 to grasslands during the Last Glacial Maximum resulting from large-scale drying. Our data shows that ISM maximum is in phase with obliquity and precession maxima suggesting a dominant role of the Indian Ocean interhemispheric temperature gradient on glacial ISM variability. Persistent and abrupt dryland expansions of varying magnitude suggest rapid-scale onset of aridity during Heinrich Stadial events and during the Toba eruption. We propose that the amplitude of ISM drought events are initiated by high latitude and volcanic forcings, although modulated by precession.
(Geophysical Research Letters. vol. 49, n° 0094-8276, 28/08/2022)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, UAlg, IPMA, IFP, MEAE, CNRS
Probability-based preservational variations within the early Cambrian Chengjiang biota (China)
The Chengjiang biota (Yunnan Province, China) is a treasure trove of soft-bodied animal fossils from the earliest stages of the Cambrian explosion. The mechanisms contributing to its unique preservation, known as the Burgess Shale-type preservation, are well understood. However, little is known about the preservation differences between various animal groups within this biota. This study compares tissue-occurrence data of 11 major animal groups in the Chengjiang biota using a probabilistic methodology. The fossil-based data from this study is compared to previous decay experiments. This shows that all groups are not equally preserved with some higher taxa more likely to preserve soft tissues than others. These differences in fossil preservation between taxa can be explained by the interaction of biological and environmental characteristics. A bias also results from differential taxonomic recognition, as some taxa are easily recognized from even poorly preserved fragments while other specimens are difficult to assign to higher taxa even with exquisite preservation.
(PeerJ. vol. 10, n° 2167-8359, 23/08/2022)
MEC, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, U of S, UNIL
Preservation of exopolymeric substances in estuarine sediments
The surface of intertidal estuarine sediments is covered with diatom biofilms excreting exopolymeric substances (EPSs) through photosynthesis. These EPSs are highly reactive and increase sediment cohesiveness notably through organo-mineral interactions. In most sedimentary environments, EPSs are partly to fully degraded by heterotrophic bacteria in the uppermost millimeters of the sediment and so they are thought to be virtually absent deeper in the sedimentary column. Here, we present the first evidence of the preservation of EPSs and EPS-mineral aggregates in a 6-m-long sedimentary core obtained from an estuarine point bar in the Gironde Estuary. EPSs were extracted from 18 depth intervals along the core, and their physicochemical properties were characterized by (i) wet chemical assays to measure the concentrations of polysaccharides and proteins, and EPS deprotonation of functional groups, (ii) acid–base titrations, and (iii) Fourier transform infrared spectroscopy. EPS-sediment complexes were also imaged using cryo-scanning electron microscopy. EPS results were analyzed in the context of sediment properties including facies, grain size, and total organic carbon, and of metabolic and enzymatic activities. Our results showed a predictable decrease in EPS concentrations (proteins and polysaccharides) and reactivity from the surface biofilm to a depth of 0.5 m, possibly linked to heterotrophic degradation. Concentrations remained relatively low down to ca. 4.3 m deep. Surprisingly, at that depth EPSs abundance was comparable to the surface and showed a downward decrease to 6.08 m. cryo-scanning electron microscopy (Cryo-SEM) showed that the EPS complexes with sediment were abundant at all studied depth and potentially protected EPSs from degradation. EPS composition did not change substantially from the surface to the bottom of the core. EPS concentrations and acidity were anti-correlated with metabolic activity, but showed no statistical correlation with grain size, TOC, depth or enzymatic activity. Maximum EPS concentrations were found at the top of tide-dominated sedimentary sequences, and very low concentrations were found in river flood-dominated sedimentary sequences. Based on this observation, we propose a scenario where biofilm development and EPS production are maximal when (i) the point bar and the intertidal areas were the most extensive, i.e., tide-dominated sequences and (ii) the tide-dominated deposit were succeeded by rapid burial beneath sediments, potentially decreasing the probability of encounter between bacterial cells and EPSs.
(Frontiers in Microbiology. vol. 13, n° 1664-302X, 18/08/2022)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, Unibas, IC2MP [Poitiers], UP, INC-CNRS, CNRS, GEOPS, INSU - CNRS, CNRS, BIC, UB, INSERM, CNRS, AORI, UTokyo, BGS, UB, CNRS
Contrasting effects of siderophores pyoverdine and desferrioxamine B on the mobility of iron, aluminum, and copper in Cu-contaminated soils
Siderophores are biogenic metallophores that can play significant roles in the dynamics of a range of metals, including Cu, in the soil. Understanding the impact of siderophores on the mobility and the availability of metals in soil is required to optimize the efficiency of soil remediation processes such as phytoextraction. This study compared the ability of siderophores desferrioxamine B (DFOB) and pyoverdine (Pvd) to mobilize metals in a series of Cu-contaminated soils, and investigated the extent their metal mobilization efficiency changed over time and with the level of Cu contamination of the soil. Siderophores were supplied (or not) to Cu-contaminated soils and metal mobility was assessed through their total concentration in 0.005 M CaCl2 extract. DFOB selectively mobilized Fe and Al while Pvd also mobilized Cu and Ni, Co, V and As but to a lesser degree. The 1:1 relationship between DFOB in the CaCl2 extract and Fe + Al mobilized from the solid phase suggests that DFOB mobilized metals by ligand-controlled dissolution. The accumulation of Cu in soil enhanced the adsorption of DFOB and Pvd at the surface of soil constituents and the mobilization of Fe to the detriment of Al by the two siderophores. The metal mobilization efficiency of DFOB and to a lesser extent of Pvd decreased over 22 days. According to N-15-Pvd analyses, Pvd degradation at least partly contributed to the progressive reduction in the metal mobilization efficiency of Pvd. The processes behind these results and the relevance of these results for manipulating the availability of Cu (and Fe) in soil are discussed.
(Geoderma. vol. 420, n° 0016-7061, pp. 115897, 15/08/2022)
UMR ISPA, Bordeaux Sciences Agro, INRAE, LPG, UM, UA, INSU - CNRS, CNRS, Nantes univ - UFR ST, Nantes Univ, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Within-species variation of seed traits of dune engineering species across a European climatic gradient
Within-species variation is a key component of biodiversity and linking it to climatic gradients may significantly improve our understanding of ecological processes. High variability can be expected in plant traits, but it is unclear to which extent it varies across populations under different climatic conditions. Here, we investigated seed trait variability and its environmental dependency across a latitudinal gradient of two widely distributed dune-engineering species (Thinopyrum junceum and Calamagrostis arenaria). Seed germination responses against temperature and seed mass were compared within and among six populations exposed to a gradient of temperature and precipitation regimes (Spiekeroog, DE; Bordeaux, FR; Valencia, ES; Cagliari, IT, Rome, IT; Venice, IT). Seed germination showed opposite trends in response to temperature experienced during emergence in both species: with some expectation, in populations exposed to severe winters, seed germination was warm-cued, whereas in populations from warm sites with dry summer, seed germination was cold-cued. In C. arenaria, variability in seed germination responses disappeared once the seed coat was incised. Seed mass from sites with low precipitation was smaller than that from sites with higher precipitation and was better explained by rainfall continentality than by aridity in summer. Within-population variability in seed germination accounted for 5 to 54%, while for seed mass it was lower than 40%. Seed trait variability can be considerable both within- and among-populations even at broad spatial scale. The variability may be hardly predictable since it only partially correlated with the analyzed climatic variables, and with expectation based on the climatic features of the seed site of origin. Considering seed traits variability in the analysis of ecological processes at both within- and among-population levels may help elucidate unclear patterns of species dynamics, thereby contributing to plan adequate measures to counteract biodiversity loss.
(Frontiers in Plant Science. vol. 13, n° 1664-462X, pp. 1-11, 11/08/2022)
JNU, ROMA TRE, ROMA TRE, UniCa, CIDE, CSIC, DZMB, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Compilation of Southern Ocean sea-ice records covering the last glacial-interglacial cycle (12–130 ka)
Antarctic sea ice forms a critical part of the Southern Ocean and global climate system. The behaviour of Antarctic sea ice throughout the last glacial-interglacial (G-IG) cycle (12 000-130 000 years) allows us to investigate the interactions between sea ice and climate under a large range of mean climate states. Understanding both temporal and spatial variations in Antarctic sea ice across a G-IG cycle is crucial to a better understanding of the G-IG regulation of atmospheric CO 2 , ocean circulation, nutrient cycling and productivity. This study presents 28 published qualitative and quantitative estimates of G-IG sea ice from 24 marine sediment cores and an Antarctic ice core. Sea ice is reconstructed from the sediment core records using diatom assemblages and from the ice core record using sea-salt sodium flux. Whilst all regions of the Southern Ocean display the same overall pattern in G-IG sea-ice variations, the magnitudes and timings vary between regions. Sea-ice cover is most sensitive to changing climate in the regions of high seaice outflow from the Weddell Sea and Ross Sea gyres, as indicated by the greatest magnitude changes in sea ice in these areas. In contrast the Scotia Sea sea-ice cover is much more resilient to moderate climatic warming, likely due to the meltwater stratification from high iceberg flux through "iceberg alley" helping to sustain high sea-ice cover outside of full glacial intervals. The differing sensitivities of sea ice to climatic shifts between different regions of the Southern Ocean has important implications for the spatial pattern of nutrient supply and primary productivity, which subsequently impact carbon uptake and atmospheric CO 2 concentrations changes across a G-IG cycle.
(Climate of the Past. vol. 18, n° 1814-9324, pp. 1815 - 1829, 10/08/2022)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Wave-induced mean currents and setup over barred and steep sandy beaches
Wind-generated surface waves breaking in the nearshore cause an increase in mean water levels, the wave setup, which can represent a significant fraction of storm surges developing both along open coasts and over sheltered areas such as coastal lagoons and estuaries. A common way to simulate the wave setup is to assume a balance between the barotropic gradient and the divergence of the depth-integrated wave-averaged momentum flux (radiation stress) associated with breaking waves in the surf zone. Field observations collected at several sandy beaches revealed that this depth-integrated approach could largely underestimate the wave setup close to the shoreline (by up to a factor of 2). The present study builds on Guérin et al. (2018) and further investigates how representing the depth-varying wave-induced forcing in modelling systems can improve the prediction of wave setup across the surf zone. We use data collected during two major field campaigns at Duck, N.C., combined with simulations with SCHISM, a threedimensional (3D) phase-averaged modelling system employing the vortex-force formalism to represent the effects of waves on currents. The ability of SCHISM to reproduce the surf zone circulation is first assessed with data collected during October 1994 (Duck94), which serve as a classical benchmark for 3D hydrostatic oceanic circulation models. The wave setup dynamics are then analysed during a storm event that occurred during SandyDuck. Consistent with the results of Guérin et al. (2018), we find that resolving the depth-varying nearshore circulation results in increased and improved wave setup predictions across the surf zone. At the shoreline, depth-integrated approaches based on the vortex-force formalism or the radiation stress concept underestimate the maximal wave setup by 10-15% and 30% on the 1:14 foreshore slope, respectively. An analysis of the 3D cross-shore momentum balance reveals that the vertical mixing is the second most important contributor (10-15% across the surf zone) to the simulated wave setup after the wave forces (80-90%), followed by the vertical advection whose contribution increases with the beach slope (up to 10% at the shoreline). Simulations performed with a phase-resolving numerical model suggest that the largest discrepancies observed at the shoreline in past studies likely originate from swash-related processes, highlighting the difficulties to disentangle wave and swash processes on steep foreshores in the field.
(Ocean Modelling. vol. 179, n° 1463-5003, pp. 102110, 04/08/2022)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LIENSs, INSU - CNRS, ULR, CNRS, VIMS
Antarctic sea ice over the past 130 000 years – Part 1: a review of what proxy records tell us
(Climate of the Past. vol. 18, n° 1814-9324, pp. 1729 - 1756, 02/08/2022)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS