Application of carbon isotope and rare earth elements as recorders of environmental conditions in the aftermath of the Paleoproterozoic Lomagundi-Jatuli Event
Kuupäev
2019-11-25
Autorid
Ajakirja pealkiri
Ajakirja ISSN
Köite pealkiri
Kirjastaja
Abstrakt
Vaba hapniku ilmumine atmosfääri ~2,4 miljardit aastat tagasi tõi endaga kaasa põhimõttelised muudatused atmosfäär-ookean süsteemis ning (bio)-geokeemilistes aineringetes. Hapniku kontsentratsiooni kasvuga käis kaasas teadaolevalt suurim häiring Maa süsinikuringes (Lomagundi-Jatuli Isotoopsündmus – LJE), mille jooksul moodustusid ebaharilikult raske süsiniku isotoopkoostisega merelised karbonaadid. Täna enimlevinud arusaama järgi langes O2 sisaldus pärast LJE lõppu, 2,06 miljardit aastat tagasi 0,1% tänapäevasest tasemest, mis kasvas uuesti alles Neoproterosoikumi lõpus. Antud doktoritöös uuriti kahte LJE-järgset läbilõiget Loode Venemaal – 1,98 Ga vanuselist Zaonega kihistut ning 1,92 Ga vanuselist Pilgujärvi settekivimite kihistut. Neid läbilõikeid on varasemalt kasutatud globaalsete keskkonnatingimuste kirjeldamiseks pärast LJE-d, kuid saadud tulemused on jõudnud vasturääkivate järeldusteni. Doktoritöös uuriti nendes kihistustes esineva karbonaatide ja apatiitide petrograafiat, mineraloogiat, süsiniku isotoopkoostist ning haruldaste muldmetallide käitumist. Doktoritööst järeldub, et Zaonega kihistus esinev negatiivne süsinikuisotoopide ekskursioon on ennekõike seotud settebasseinile iseäralike protsessidega (nt metaani oksüdeerimisel moodustunud kerge süsiniku isotoopkoostisega karbonaadid), mida ei saa kasutada avaookeani kirjeldamiseks. Lisaks näitas haruldaste muldmetallide käitumine, ennekõike redoksensitiivse Ce, et Zaonega ja Pilgujärvi settekivimite kihistu moodustumisel pidi atmosfäär-ookean süsteemis olema piisavalt vaba hapnikku, et Ce(III) oksüdeeritaks Ce(IV), põhjustades nii teiste haruldaste muldmetallidega võrreldes Ce vaesustunud geokeemilise signaali (negatiivse Ce-anomaalia). See seab kahtluse alla globaalse hapnikutaseme languse pärast Lomagundi-Jatuli Isotoopsündmust ning viitab kõrgenenud hapnikutaseme püsimisele.
The first appearance of free atmospheric oxygen ~2.4 billion years ago brought about fundamental changes in the atmospheric-ocean system and biogeochemical elemental cycles. The increase in oxygen concentration was accompanied by the largest perturbation in global carbon cycle called the Lomagundi-Jatuli Event (LJE), during which marine carbonates of anomalously enriched in heavy carbon isotope were precipitated. According to the most widely held notion, the O2 levels dropped to no more than 0.1% from present atmospheric levels after the termination of LJE 2.06 billion years ago and only increased again at the end of the Neoproterozoic. This thesis focuses on two post-LJE successions in north-western Russia – the 1.98 Ga Zaonega Formation and the 1.92 Ga Pilgujärvi Sedimentary Formation. These successions have previously been used to describe the global post-LJE environmental conditions, however, with contradicting results. This thesis characterises the petrography, mineralogy, carbon isotope composition and behaviour of rare earth elements of carbonates and apatites in these Formations. The thesis concludes that the negative carbon isotope excursion in the Zaonega Formation is primarily due to basin-specific processes (e.g., carbonates enriched in light carbon isotope were influenced by methane oxidation) that cannot be directly used to describe the global ocean. In addition, the behaviour of rare earth elements, particularly redox-sensitive Ce, showed that during the formation of the post-LJE Zaonega and Pilgujärvi sedimentary rocks, sufficient oxygen concentration was required in the atmosphere-ocean system in order to decouple Ce from other rare earth elements (negative Ce-anomaly). This calls into question the global decline in oxygen levels following the Lomagundi-Jatul Isotope Event and suggests prolonged elevated oxygen levels at the time.
The first appearance of free atmospheric oxygen ~2.4 billion years ago brought about fundamental changes in the atmospheric-ocean system and biogeochemical elemental cycles. The increase in oxygen concentration was accompanied by the largest perturbation in global carbon cycle called the Lomagundi-Jatuli Event (LJE), during which marine carbonates of anomalously enriched in heavy carbon isotope were precipitated. According to the most widely held notion, the O2 levels dropped to no more than 0.1% from present atmospheric levels after the termination of LJE 2.06 billion years ago and only increased again at the end of the Neoproterozoic. This thesis focuses on two post-LJE successions in north-western Russia – the 1.98 Ga Zaonega Formation and the 1.92 Ga Pilgujärvi Sedimentary Formation. These successions have previously been used to describe the global post-LJE environmental conditions, however, with contradicting results. This thesis characterises the petrography, mineralogy, carbon isotope composition and behaviour of rare earth elements of carbonates and apatites in these Formations. The thesis concludes that the negative carbon isotope excursion in the Zaonega Formation is primarily due to basin-specific processes (e.g., carbonates enriched in light carbon isotope were influenced by methane oxidation) that cannot be directly used to describe the global ocean. In addition, the behaviour of rare earth elements, particularly redox-sensitive Ce, showed that during the formation of the post-LJE Zaonega and Pilgujärvi sedimentary rocks, sufficient oxygen concentration was required in the atmosphere-ocean system in order to decouple Ce from other rare earth elements (negative Ce-anomaly). This calls into question the global decline in oxygen levels following the Lomagundi-Jatul Isotope Event and suggests prolonged elevated oxygen levels at the time.
Kirjeldus
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Märksõnad
carbon, isotopes, rare earth metals, Paleoproterozoic, carbon cycle, carbonate rocks, Russian Northwest