143 research outputs found
Dynamique climatique de l'océan Pacifique ouest équatorial au cours du PléistocÚne récent
M. Edouard BARD, Professeur, CollĂšge de France - Co-directeur M. Luc BEAUFORT, ChargĂ© de Recherche CNRS, Aix-en-Provence - Directeur de thĂšse Mme. Elsa CORTIJO, ChargĂ©e de Recherche CNRS, Gif-sur-Yvette - Examinateur M. Patrick DE DECKKER, Professeur, Australian National University - Rapporteur M. Dick KROON, Professeur, Vrije Universteit Amsterdam - Rapporteur M. Nicolas THOUVENY, Professeur, UniversitĂ© de la MĂ©diterranĂ©e -ExaminateurThe western equatorial Pacific warm pool (WPWP), warmest ocean area of the globe, fuels the meridian monsoon and zonal Walker atmospheric circulation cells. Using planktonic foraminifera assemblages, stable isotope geochemistry and morphometry, and coccolithophorids assemblages, we investigated past climate dynamics of this area during the Late Pleistocene. We developped a new planktonic foraminifera transfer function which is not biased by the ecosystem structure like was the CLIMAP study. Using this method, the last 185 kyrs SSTs in the WPWP oscillated between 27 and 29.5°C, in agreement with alkenones estimates. The âdeepâ atmospheric convection over the WPWP was thus a stable feature of the Late Pleistocene climate. These temperatures appear to be modulated by the precession cycles which also force a rocking of the Indo-Pacific thermocline, following an âEl Niño-Southern Oscillation-likeâ mechanism. At millenial time-scale, the high-latitudes Dansgaard-Oeschger cycles were find in past changes in East Asian winter monsoon strength from the northern edge of the WPWP. A 1500 yrs cycle imprints this monsoon record, which is not ice volume modulated, and thus not driven by some high-latitude process. A potential climatic feedback is attributed to thermal dissociation of methane gas-hydrates from low-latitude sedimentary margins. Indeed, a high-resolution of carbon isotopic changes in the WPWP, exhibits the occurrence of large methane gas-hydrates releases during the last glacial stage.L'ocĂ©an Pacifique ouest Ă©quatorial, zone ocĂ©anique la plus chaude du globe, est au cĆur des cellules de circulation atmosphĂ©rique de mousson (mĂ©ridienne) et de Walker (zonale). Les assemblages des foraminifĂšres planctoniques, leur gĂ©ochimie et leur morphomĂ©trie ainsi que les assemblages de coccolithophoridĂ©s nous ont permis de reconstruire la dynamique climatique de cette zone au PlĂ©istocĂšne rĂ©cent Nous avons dĂ©veloppĂ© une nouvelle fonction de transfert non biaisĂ©e par la structure des Ă©cosystĂšmes comme dans l'Ă©tude CLIMAP. Les tempĂ©ratures de surface des derniers 185 ka reconstruites par cette mĂ©thode varient entre 27 et 29.5°C, en accord avec les estimations des alcĂ©nones. La convection atmosphĂ©rique liĂ©e Ă ces tempĂ©ratures Ă©levĂ©es a donc persistĂ© au cours du PlĂ©istocĂšne rĂ©cent. Ces tempĂ©ratures sont modulĂ©es par les cycles orbitaux de prĂ©cession qui forcent Ă©galement Ă cette Ă©chelle de temps le balancement de la thermocline des ocĂ©ans Indo-Pacifique, suivant un mĂ©canisme similaire Ă l'El Niño-Oscillation Australe. A l'Ă©chelle du millĂ©naire, la dynamique climatique rapide des cycles de Dansgaard-Oeschger des hautes latitudes est dĂ©celĂ©e dans les variations d'intensitĂ© de la mousson d'hiver Est Asiatique du Pacifique ouest Ă©quatorial. Cette mousson est marquĂ©e par un cycle de 1500 ans, indĂ©pendant du volume global des glaces, donc d'un forcage des hautes latitudes. Un rĂŽle d'amplificateur des changements climatiques rapides est attribuĂ© Ă la dissociation thermale des clathrates de mĂ©thane des marges sĂ©dimentaires de basses latitudes. En effet, un enregistrement isotopique Ă haute rĂ©solution des isotopes du carbone dĂ©montre l'existence de dĂ©gazages catastrophiques de ces clathrates de mĂ©thane pendant le dernier stade glaciaire
Worldwide Genotyping in the Planktonic Foraminifer Globoconella inflata: Implications for Life History and Paleoceanography
The planktonic foraminiferal morpho-species Globoconella inflata is widely used as a stratigraphic and paleoceanographic index. While G. inflata was until now regarded as a single species, we show that it rather constitutes a complex of two pseudo-cryptic species. Our study is based on SSU and ITS rDNA sequence analyses and genotyping of 497 individuals collected at 49 oceanic stations covering the worldwide range of the morpho-species. Phylogenetic analyses unveil the presence of two divergent genotypes. Type I inhabits transitional and subtropical waters of both hemispheres, while Type II is restricted to the Antarctic subpolar waters. The two genetic species exhibit a strictly allopatric distribution on each side of the Antarctic Subpolar Front. On the other hand, sediment data show that G. inflata was restricted to transitional and subtropical environments since the early Pliocene, and expanded its geographic range to southern subpolar waters âŒ700 kyrs ago, during marine isotopic stage 17. This datum may correspond to a peripatric speciation event that led to the partition of an ancestral genotype into two distinct evolutionary units. Biometric measurements performed on individual G. inflata from plankton tows north and south of the Antarctic Subpolar Front indicate that Types I and II display slight but significant differences in shell morphology. These morphological differences may allow recognition of the G. inflata pseudo-cryptic species back into the fossil record, which in turn may contribute to monitor past movements of the Antarctic Subpolar Front during the middle and late Pleistocene
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Multiproxy assessment of the western equatorial Pacific hydrography during the last 30 kyr
Sea surface temperature (SST) and salinity of the Western Pacific Warm Pool (WPWP) reflect global climate effects such as the El NiñoâSouthern Oscillation phenomenon. However, reconstructions of past changes in the WPWP from the geologic record vary depending on the specific proxy record used. Here we develop a multiproxy record of the last deglaciation from a radiocarbon-dated sediment core (MD97-2138) retrieved in the heart of the WPWP. SST reconstructions for the past 30,000 years based on planktonic foraminiferal Mg/Ca (Globigerinoides ruber and Globigerinoides sacculifer), alkenone unsaturation index, and foraminiferal transfer functions differ notably. Mg/Ca-based SST estimates from the surface dwelling species G. ruber in MD97-2138 indicate a larger surface cooling (3° ± 0.6°C) during the Last Glacial Maximum (LGM) than inferred from Mg/Ca ratios in G. sacculifer (2.3° ± 0.7°C), statistical transfer functions based on planktonic foraminiferal species assemblages, and Ukââ 0 (1°â2°C). These estimates are consistent with estimates from other WPWP cores, thereby suggesting that the discrepancy is due to proxy compatibility rather than differences in cores qualitity. Postdepositional dissolution above the lysocline might have altered the Mg/Ca-based temperature estimates in our site, but this effect is insufficient to resolve discrepancies between Mg/Ca in G. ruber and the other proxies. We suggest that the lower estimates obtained from Mg/Ca in G. sacculifer, faunal transfer functions, and Ukââ 0 might reflect subsurface temperature changes rather than strict surface estimates. Accounting for potential artefacts, including dissolution and bioturbation, we suggest that the glacial WPWP SST was about 2.5° ± 0.7°C cooler than during the Holocene, whereas the subsurface/upper thermocline temperature change was only about 1.8° ± 0.7°C. Interpreting variations in ÎŽÂčâžOSW in terms of salinity changes suggests a possibly slight decrease in surface salinity at the site of MD97-2138 during the LGM. Though LGM freshening in MD97-2138 is not robust to postdeposition dissolution effects, this inferred freshening appears to be a general feature of the western equatorial Pacific
North Atlantic Midlatitude Surface-Circulation Changes Through the Plio-Pleistocene Intensification of Northern Hemisphere Glaciation
The North Atlantic Current (NAC) transports warm salty water to high northern latitudes, with important repercussions for ocean circulation and global climate. A southward displacement of the NAC and Subarctic Front, which separate subpolar and subtropical water masses, is widely suggested for the Last Glacial Maximum (LGM) and may have acted as a positive feedback in glacial expansion at this time. However, the role of the NAC during the intensification of Northern Hemisphere glaciation (iNHG) at ~3.5 to 2.5 Ma is less clear. Here we present new records from Integrated Ocean Drilling Program Site U1313 (41°N) spanning ~2.8â2.4 Ma to trace the influence of Subarctic Front waters above this midâlatitude site. We reconstruct surface and permanent pycnocline temperatures and seawater ÎŽ18O using paired Mg/CaâÎŽ18O measurements on the planktic foraminifers Globigerinoides ruber and Globorotalia crassaformis and determine abundances of the subpolar foraminifer Neogloboquadrina atlantica. We find that the first significant glacial incursions of Subarctic Front surface waters above Site U1313 did not occur until ~2.6 Ma. At no time during our study interval was (sub)surface reorganization in the midlatitude North Atlantic analogous to the LGM. Our findings suggest that LGMâlike processes sensu stricto cannot be invoked to explain interglacialâglacial cycle amplification during iNHG. They also imply that increased glacial productivity at Site U1313 during iNHG was not only driven by southward deflections of the Subarctic Front. We suggest that nutrient injection from coldâcore eddies and enhanced glacial dust delivery may have played additional roles in increasing export productivity in the midlatitude North Atlantic from 2.7 Ma.t. Funding for
this research was provided by IODP
France (C. T. B.) and the German
Research Foundation (DFG) (grant OF
2544/2 to O. F.). I. B. is grateful to the UK
IODP for financial support for shipboard
and post-cruise participation in IODP
Exp. 306. C. T. B., K. T., T. D. G., L. V., C. S.,
and M. E. acknowledge OSU Pythéas.
M. M. R. acknowledges support by the
USGS Land Change Science Program
Machine learning techniques to characterize functional traits of plankton from image data
Plankton imaging systems supported by automated classification and analysis have improved ecologists' ability to observe aquatic ecosystems. Today, we are on the cusp of reliably tracking plankton populations with a suite of lab-based and in situ tools, collecting imaging data at unprecedentedly fine spatial and temporal scales. But these data have potential well beyond examining the abundances of different taxa; the individual images themselves contain a wealth of information on functional traits. Here, we outline traits that could be measured from image data, suggest machine learning and computer vision approaches to extract functional trait information from the images, and discuss promising avenues for novel studies. The approaches we discuss are data agnostic and are broadly applicable to imagery of other aquatic or terrestrial organisms
Stable sea surface temperatures in the western Pacific warm pool over the past 1.75 million years
About 850,000 years ago, the period of the glacial cycles changed from 41,000 to 100,000 years. This mid-Pleistocene climate transition has been attributed to global cooling, possibly caused by a decrease in atmospheric carbon dioxide concentrations(1,2). However, evidence for such cooling is currently restricted to the cool upwelling regions in the eastern equatorial oceans(3,4), although the tropical warm pools on the western side of the ocean basins are particularly sensitive to changes in radiative forcing(5,6). Here we present high-resolution records of sea surface temperatures spanning the past 1.75 million years, obtained from oxygen isotopes and Mg/Ca ratios in planktonic foraminifera from the western Pacific warm pool. In contrast with the eastern equatorial regions, sea surface temperatures in the western Pacific warm pool are relatively stable throughout the Pleistocene epoch, implying little long-term change in the tropical net radiation budget. Our results challenge the hypothesis of a gradual decrease in atmospheric carbon dioxide concentrations as a dominant trigger of the longer glacial cycles since 850,000 years ago. Instead, we infer that the temperature contrast across the equatorial Pacific Ocean increased, which might have had a significant influence on the mid-Pleistocene climate transition
Toward automated picking of foraminifera ? â the FIRST (Foraminifera Image Recognition and Sorting Tool) project
International audienc
An automated system for imageâbased identification of foraminifera using convolutional neural networks
International audienc
Toward automated picking of foraminifera ? â the FIRST (Foraminifera Image Recognition and Sorting Tool) project
International audienc
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