A high-resolution belemnite geochemical analysis of Early Cretaceous (Valanginian-Hauterivian) environmental and climatic perturbations

International audienceThe Early Cretaceous Weissert event, characterized by a positive carbon isotope excursion and coincident with the Paraná-Etendeka volcanism, saw a biogeochemical chain of events that ultimately led to an increase in carbon burial. A conclusive link between the Paraná-Etendeka volcanism and its impact upon the environment remains, however, elusive. Here we reconstruct temperature through the Weissert event from Mg/Ca ratios of belemnites from the Vocontian Trough (France) and SE Spain and use carbon isotopes to link our temperature reconstruction to marine records of carbon cycling. We provide evidence that the Paraná-Etendeka volcanism, unlike some large igneous provinces, did not cause a climate warming. The case can be made for cooling in the last stages of the Weissert event, which possibly reflects substantial CO 2 drawdown. In the absence of warming and consequent accelerated hydrological cycling and the relatively long duration of the eruptive phase of the Paraná-Etendeka, an alternate trigger for increased fertilization of the oceans is implicated

Carbon cycle history through the Jurassic–Cretaceous boundary: A new global δ13C stack

publisher: Elsevier articletitle: Carbon cycle history through the Jurassic–Cretaceous boundary: A new global δ13C stack journaltitle: Palaeogeography, Palaeoclimatology, Palaeoecology articlelink: http://dx.doi.org/10.1016/j.palaeo.2016.03.016 content_type: article copyright: Copyright © 2016 Published by Elsevier B.V

Changements paléoenvironnementaux et production carbonatée hémipélagique de la marge nord-ouest téthysienne durant le Valanginien (approches minéralogique, micropaléontologique et géochimique)

Le Valanginien (Crétacé inférieur, ~135 Ma) se caractérise à l échelle du globe, par une diminution drastique de la production carbonatée et une eutrophisation accrue de l écosystème marin. Le paroxysme des perturbations coïncide avec l enregistrement d une anomalie positive du 13C dans les sédiments carbonatés. L objectif de ce travail est de comprendre, à haute résolution (<100 ka), les interactions majeures existant entre le climat, l altération-érosion sur le continent et la productivité primaire du Valanginien, dans la bande intertropicale du domaine téthysien. Il repose sur une approche minéralogique (roche totale et fraction argileuse), micropaléontologique (nannofossiles calcaires et calpionelles) et géochimique (éléments chimiques et isotopes stables) des archives des marnes de la coupe d Angles (Bassin vocontien, SE de la France).Après avoir contrôlé le potentiel d altération diagénétique de chacun des traceurs, l étude simultanée à haute résolution de la production carbonatée primaire, des conditions trophiques vocontiennes ainsi que de l intensité de l érosion/altération sur les continents a permis d établir les changements paléocéanographiques, paléoenvironnementaux et paléoclimatiques du Bassin vocontien au Valanginien. Ils se matérialisent par une crise des principaux organismes producteurs de carbonates (nannofossiles calcaires et calpionelles), engendrée par une fertilisation accrue de la zone photique suite à une intensification de l altération/érosion du continent, dans un scénario global de climat à effet de serre.The Valanginian time interval (lower Cretaceous, ~135 Myr) is documented by a widespread eutrophication of marine ecosystems associated with a crisis of carbonate producing biota. The paroxysm of the perturbations coincides with a global positive carbon isotope excursion.The objective of this study is to explore the relationship between climate, continental weathering, and primary productivity in the northwestern Tethys. This work focuses on the Angles section, located in the hemipelagic realm of the vocontian Basin. This site offers the possibility for a high-resolution micropalaeontological (calcareous nannofossils and calpionellids) mineralogical (bulk rock and clay minerals) and geochemical (elemental geochemistry and stable isotopes) investigation. The high resolution study of the vocontian primary production and the trophic conditions associated with the continental weathering pattern allowed to provide new insight on the palaeoceanographic, palaeoenvironmental and palaeoclimatic changes of the vocontian Basin during the Valanginian. They are characterized by a crisis of the carbonate producing biota triggered by higher fertility conditions in the photic zone, through an increase continental runoff and erosion, in a global greenhouse climate scenario.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Climate and sea-level variations along the northwestern Tethyan margin during the Valanginian C-isotope excursion: Mineralogical evidence from the Vocontian Basin (SE France)

A high resolution mineralogical study (bulk-rock and clay-fraction) was carried out upon the hemipelagic strata of the Angles section (Vocontian Basin, SE France) in which the Valanginian positive C-isotope excursion occurs. To investigate sea-level fluctuations and climate change respectively, a Detrital Index (DI: (phyllosilicates and quartz)/calcite) and a Weathering Index (WI: kaolinite/(illite + chlorite)) were established and compared to second-order sea-level fluctuations. In addition, the mineralogical data were compared with the High Nutrient Index (HNI, based on calcareous nannofossil taxa) data obtained by Duchamp-Alphonse et al. (2007), in order to assess the link between the hydrolysis conditions recorded on the surrounding continents and the trophic conditions inferred for the Vocontian Basin. It appears that the mineralogical distribution along the northwestern Tethyan margin is mainly influenced by sea-level changes during the Early Valanginian (Pertransiens to Stephanophorus ammonite Zones) and by climate variations from the late Early Valanginian to the base of the Hauterivian (top of the Stephanophorus to the Radiatus ammonite Zones). The sea-level fall observed in the Pertransiens ammonite Zone (Early Valanginian) is well expressed by an increase in detrital inputs (an increase in the DI) associated with a more proximal source and a shallower marine environment, whereas the sea-level rise recorded in the Stephanophorus ammonite Zone corresponds to a decrease in detrital influx (a decrease in the DI) as the source becomes more distal and the environment deeper. Interpretation of both DI and WI, indicates that the positive C-isotope excursion (top of the Stephanophorus to the Verrucosum ammonite Zones) is associated with an increase of detrital inputs under a stable, warm and humid climate, probably related to greenhouse conditions, the strongest hydrolysis conditions being reached at the maximum of the positive C-isotope excursion. From the Verrucosum ammonite Zone to the base of the Hauterivian (Radiatus ammonite Zone) climatic conditions evolved from weak hydrolysis conditions and, most likely, a cooler climate (resulting in a decrease in detrital inputs) to a seasonal climate in which more humid seasons alternated with more arid ones. The comparison of the WI to the HNI shows that the nutrification recorded al: the Angles section from the top of the Stephanophorus to the Radiatus ammonite Zones (including the positive C-isotope shift), is associated with climatic changes in the source areas. At that time, increased nutrient inputs were generally triggered by increased weathering processes in the source areas due to acceleration in the hydrological cycle under greenhouse conditions This scenario accords with the widely questioned palaeoenvironmental model proposed by Lini et al., (1992) and suggests that increasing greenhouse conditions are the main factor that drove the palaeoenvironmental changes observed in the hemipelagic realm of the Vocontian Basin, during the Valanginian positive C-isotope shift. This high-resolution mineralogical study highlights short-term climatic changes during the Valanginian, probably associated to rapid changes in the C-cycle. Coeval Massive Parana-Etendeka flood basalt eruptions may explain such rapid perturbations. (C) 2011 Elsevier B.V. All rights reserved

Summer and Autumn Insolation as the Pacemaker of Surface Wind and Precipitation Dynamics Over Tropical Indian Ocean During the Holocene: Insights From Paleoproductivity Records and Paleoclimate Simulations

International audienceAbstract Insolation is the engine of monsoon and Walker circulations over the tropical Indian Ocean. Here, we present Holocene coccolith‐related net primary productivity (NPP) signals from two sediment cores retrieved in the wind‐driven coastal upwelling systems off southern India and southern Sumatra. Upwelling‐induced NPP is enhanced during summer and autumn and is a powerful tool to reconstruct atmospheric features at a seasonal scale. Our records indicate that during summer and autumn, westerly winds off southern India strengthened from the early‐Holocene (EH) to late‐Holocene (LH), while southeasterly winds off southern Sumatra strengthened from the EH to mid‐Holocene (MH) and weakened from the MH to LH. Comparisons with previous paleoclimate records and simulations, allow us to confirm such wind patterns at a regional scale and identify distinct atmospheric features associated to insolation before and after the MH. From the EH to MH, as the insolation in the Northern Hemisphere weakens during summer and strengthens during autumn, the equatorial Indian Ocean is characterized by more vigorous Walker and monsoon circulations in summer and autumn, respectively. From the MH to LH, as the insolation weakens in the Northern Hemisphere during summer and over the equator during autumn, the equatorial Indian Ocean is influenced by a general reinforcement of the Walker circulation during both seasons, a feature that we relate to a modern negative IOD‐like mode. The changes in wind result in increasing precipitation over Indonesia and India from EH to MH and over Indonesia from MH to LH as India is getting dryer

Coccolith counting and reconstructed primary productivity results of core MD00-2354

Coccolith assemblages of core MD00-2354 are used for reconstructing primary productivity in the northwestern Arabian Sea. The results cover the last 23,000 years. The attached data are original coccolith counting by using a polarized light microscope, and primary productivity. Counting result of each coccolith species is shown by the sum of counting number (n) in all fields of view. Primary productivity (gC m^-2 yr^-2) is calculated by an empirical equation that convert the percentage of lower euphotic zone species Florisphaera profunda to primary productivity. Details can be found in the related article

Variations of Primary Productivity in the Northwestern Arabian Sea during the last 23,000 Years and Their Paleoclimatological Implications

The Arabian Sea (AS) is one of the most productive oceanic regions in the world due to several monsoon-related processes that can increase nutrients contents in the euphotic zone. Previous studies of the imprint of oceanic Primary Productivity (PP) in AS sediments yielded diverse results depending on the studied area and the chosen proxies, with unprecise paleoclimatic interpretations. Here, we provide multi-decennial PP and coastal upwelling dynamic records off northern Oman over the last 23 kyr, based on the analysis of coccoliths from sediment core MD00-2354. Our results have been compared with previous paleoenvironmental records as well as new modelling data to get precise paleoclimatic interpretations. We document higher PP and weaker coastal upwelling during the Last Glacial Maximum relative to the Holocene, and significant millennial-scale variations over the last deglaciation corresponding to the fluctuations of the Atlantic Meridional Overturning Circulation strength. Higher PP and weaker upwelling are found during cold stadials, while lower PP and stronger upwelling during the warm interstadial. We propose that the increases of PP were driven by increased bioavailable nutrient content in surface waters under both stronger winter monsoon conditions that strengthened the convective mixing, and higher aeolian inputs. Over the Holocene, stronger upwelling and slightly lower PP are found during the Early-Mid Holocene, when higher summer insolation triggered stronger summer monsoon. At that time, the lower PP was probably the result of restricted advection of eutrophic summer upwelling seawater under negative wind stress curls and less aeolian inputs. Key Points Past primary productivity and upwelling in the NW Arabian Sea are reconstructed by coccolith assemblages from a marine sediment core Primary productivity was relatively high during the cold periods compared to the warm periods of the last 23 kyr Primary productivity changes were mainly controlled by winter monsoon through changes in convective mixing and by aeolian inputs Plain Language Summary Ocean Primary Productivity is the production rate of organic carbon from inorganic carbon by ocean phytoplankton through photosynthesis, in which nutrients and sunlight are necessary. In the tropical oceans, this rate is usually limited by the availability of nutrients in the sunlit ocean, which is controlled by physical processes such as seawater upward motion, ocean mixing, and dust storms. All these processes play a significant role in the northwestern Arabian Sea today. It is surrounded by arid lands that provide the highest amount of airborne material in the world and it is influenced by the Indian monsoon system that drives seawater upward motion and mixing in summer and winter, respectively. However, not much is known about their impact on Primary Productivity in the past. This work aims to reconstruct changes of Primary Productivity in the northwestern Arabian Sea over the past twenty thousand years and understand by which process(es) they are impacted, based on a model-data comparison approach. We’ve found that Primary Productivity was higher during cold periods than during warm periods. The changes were mainly controlled by both, winter monsoon through changes in convective mixing, and dust storms through changes in nutrient-enrich aeolian inputs