RETRACTED: Pattern and timing of the Early Jurassic calcareous nannofossil crisis

Abstract

abstract Article history:Received 27 April 2014Received in revised form 12 June 2014Accepted 23 June 2014Available online 30 June 2014Keywords:Early ToarcianCalcareous nannofossilsBiocalcification crisisCarbonate production TheToarciancalcareousnannofossilcrisisassociatedwiththeearlyToarcianOceanicAnoxicEvent(T-OAE)intheEarlyJurassicperiodisthoughttorepresentoneofthemostimportantbiocalcificationcrisesduringtheMesozo-ic,occurringsimultaneouslywithaprofounddisturbanceofthecarboncycle.However,thecausesarestillunderdebate, particularly with regard to the pattern, timing of the biocalcification crisis, relative roles of intrinsic andextrinsic processes as drivers of the crisis, and also causal mechanisms of the T-OAE.In this study, a new quantification of Toarcian calcareous nannofossil abundance and size is presented for theSancerre borehole (Paris Basin, France). Beyond the recognition of a severe biocalcification crisis defined bythe major drop in abundance, and the reduction in size of the most important pelagic carbonate producerSchizosphaerella punctulata, for the first time, this study proposes an insight into the pace and timing of thenannoplanktoncrisis.AtSancerre,thecarbonateproductionofthelowerToarciansedimentspreviouslyattribut-ed to obliquity forcing of climate allows to estimate a duration of ~210 kyr for the biocalcification crisis andof ~120 kyr for the shift towards lower carbon isotope values. The onset of the biocalcification crisis marked bya fertility event lasted~60kyr,andthe calciumcarbonate valuesremainedlow for ~150 kyr;thesubsequent re-covery of carbonate and nannoplankton lasted ~60 kyr and N550 kyr, respectively. Additionally, a link betweenthe biocalcification crisis, the seawater palaeotemperature, and the carbon isotope steps can be demonstrated.This covariance provides compelling evidence of fundamental change in the response of the climatic warmingand the carbon cycle systems triggering the biocalcification crisis. These observations indicate that thebiocalcification crisis can be regarded as a direct or indirect consequence of a global warming. Moreover, a defi-ciency of the biological pump is proposed here as a complementary casual mechanism for explaining thenegative carbon-isotope excursion.© 2014 Elsevier B.V. All rights reserved.Contents1. Introduction 572. TheSancerreborehole:geologicalsettingandpalaeogeographyoftheParisBasin 573. Materialandmethods 583.1. Quantificationofnannofossilabundanceandvolume 583.2. Analyticalanalysis 584. Results 594.1. Calcareousnannofossilpreservation 594.2. Stratigraphicdistributionofthecalcareousnannoplankton 594.3. Clusterandprincipalcomponentanalyses 594.4. Evolution of Schizosphaerella size 605. Discussion 605.1. ThecalcareousnannoplanktonresponsetotheT-OAEenvironmentalchanges 605.1.1. Patternsinnannofossilassemblageswithinpre-eventstableenvironmentalconditions 605.1.2. TheT-OAEperturbationasrecordedbynannofossilassemblages 605.1.3. Theenvironmentalrecoveryasrecordedbynannofossilassemblages 605.2. Expressionandevolutionofthebiogeniccarbonateproduction 615.3. AtimescaleforthenannoplanktoncrisisacrosstheT-OAE 61