Method development for Li isotopic analysis via multi-collector ICP-mass spectrometry application to Cretaceous-Paleogene bounda
Method development for Li isotopic analysis via multi-collector ICP-mass spectrometry application to Cretaceous-Paleogene bounda
Lithium als bijkomend bewijs voor uitsterven dinosauriërs door meteorietinslag.
Gent – Bijkomend bewijs voor het uitsterven van de dinosauriërs ten gevolge van de apocalyptische klimaatveranderingen werd gevonden via het element lithium. Deze klimaatveranderingen werden gelinkt met een meteorietinslag die 66 miljoen jaar geleden plaatsvond. Onderzoekers van de UGent hebben, in samenwerking met collega’s van de Vrije Universiteit Brussel en de Universiteit van Cambridge (VK), hiervoor bewijs kunnen leveren door onderzoek op kleiafzettingen, die gevormd zijn ten tijde van de meteorietinslag.
Meteorietinslag
De wetenschap is het er al jaren over eens dat er omstreeks 66 miljoen jaar geleden voor de vijfde maal een massaal uitsterven van planten- en diersoorten op onze planeet plaatsvond. Hiertoe behoorden ook de dinosaurussen. Er was gedurende lange tijd onzekerheid over de oorzaak van deze massale uitsterving. Tot de jaren 80 van de vorige eeuw werd vulkanisme als verklaring naar voren geschoven. In 1980 echter werd in de miljoenen jaren oude kleiafzetting in Italië een verhoogde concentratie van bepaalde chemische elementen vastgesteld. De overmaat aan deze elementen kan enkel van buitenaardse oorsprong zijn. Enkele jaren later werd een gigantische krater van zo’n 200 km doorsnede ontdekt op het Yucatán schiereiland te Mexico. De wetenschap vermoedt dat een buitenaards projectiel van 10 km doorsnede richting de aarde is gekatapulteerd door een botsing van twee grote asteroïden in ons zonnestelsel. De kracht van de impact was zo groot dat de buitenaardse elementen over de hele wereld terug te vinden zijn. De effecten van dergelijke impact hebben de geologische geschiedenis van onze planeet mede bepaald. De gevolgen van de impact konden in deze studie in kaart gebracht worden met behulp van lithium. Lithium is een scheikundig element, een metaal.
Lithium werd al eerder gebruikt voor onderzoek naar de impact van de meteorietinslag. Hierbij werd er gefocust op lithium in zeewater. Dit onderzoek echter kijkt voor de eerste keer naar kleimonsters van dezelfde ouderdom als het uitsterven van de dinosauriërs.
Klimaatverandering
Lithium uit kleimonsters van verschillende diepte en dus ouderdom (tussen de 64,7 en 71 miljoen jaar oud) werd op eenzelfde manier geanalyseerd. Het resultaat wordt geplaatst in het kader van de meteorietinslag. Tom Croymans, onderzoeker aan de UGent vertelt het volgende: “De meteorietinslag veroorzaakte enorme veranderingen in het klimaat. De kracht, die bij de kratervorming plaatsvond, creëerde immers een gigantische stofwolk. De planeet was gedurende lange tijd in duisternis gehuld. De afwezigheid van zonlicht veroorzaakte ontbossing en de aanhoudende (zure) regen zorgde wereldwijd voor een sterke toename in de aantasting van het aardoppervlak. Ons resultaat bevestigt dat scenario”. De afname in de vegetatie beïnvloedde vanzelfsprekend ook de dieren die hiervan afhankelijk waren zoals de dinosaurussen.
Lithium
Het element lithium komt voor in twee verschillende vormen (isotopen), een zwaar isotoop met massagetal 7 (7Li), en een licht isotoop, met massagetal 6 (6Li). De twee verschillende lithiumvormen vertonen verschillende eigenschappen bij de aantasting van gesteenteoppervlakten. De verhouding van het zwaardere isotoop over het lichtere isotoop (7Li/6Li verhouding) kan dus worden gebruikt als een indicator voor aantasting van gesteenten zoals kleien.
In deze studie werd informatie gewonnen uit deze 7Li/6Li verhouding in kleihoudende kalksteenafzettingen uit de Umbria-Marche regio in Italië.
Het resultaat van deze studie vertoont exact ter hoogte van de afzetting van 66 miljoen jaar geleden een piek in de 7Li/6Li verhouding ten opzichte van de eerdere en latere afzettingen. Deze piek kan enkel verklaard worden door een zeer sterke aantasting van de kleien vermoedelijk ten gevolge van de meteorietinslag.
Laboratoriumwerk
Het ontwikkelen van de methodiek voor de analyse vormde de grootste uitdaging tijdens het onderzoek.
In een stofvrij laboratorium wordt door middel van een speciaal ontwikkelde procedure lithium in zuivere vorm geïsoleerd. ”Alleen al de voorbereiding van het scheidingsprotocol nam 19 uur non-stop laboratoriumwerk in beslag.” aldus Croymans.
Eenmaal lithium geïsoleerd, wordt overgegaan tot de meting via een massaspectrometer. Dit instrument meet het aantal zwaardere (7Li) en lichte (6Li) lithiumisotopen dat de detectoren per tijdseenheid bereikt. Waarna de 7Li/6Li verhouding kan berekend worden. ”Het gebruikte type instrument in deze studie is uniek in Vlaanderen”, aldus de onderzoeker.
De resultaten werden reeds warm onthaald op de internationale Environmetal Isotopes conferentie in Ascona te Zwitserland.
In de toekomst zullen andere sites op dezelfde wijze bestudeerd worden om het wereldwijde effect van de meteorietinslag in kaart te brengen.
Volledige titel van de scriptie:Method development for Li isotopic analysis via multi-collector ICP-mass spectrometry application to Cretaceous-Paleogene boundary clays from the Umbria-Marche sequence.
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