Neoproterozoic Baltic paleosol: geology and paleoenvironmental interpretation
Date
2015-05-05
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Abstract
Käesoleva doktoritöö eesmärgiks on hinnata Balti murenemiskooriku säilivust, murenemisastet, võimalikke hilisemaid mõjutusi, ning rekonstrueerida paleoklimaatilised tingimused murenemiskooriku moodustumise ajal Eelkambriumis.
Eelkambriumi murenemiskoorikud on haruldased, tavaliselt hilisemates geoloogilistes protsessides moondunud, kuid siiski olulised infoallikad varajase Maa keskkondade uurimiseks. Eelkambriumi noorimat ajajärku, Neoproterosoikumit, iseloomustavad lühiajalised, kuid suureamplituudilised muutused süsinikuringes, hapniku sisalduse tõus atmosfääris tänapäevasele tasemele, globaalsed jäätumised, millele järgnesid lühiajalised nn kasvuhoonekliimaga perioodid ja esimeste hulkraksete loomade ilmumine, mis tähistab bioloogilise mitmekesisuse plahvatusliku arengu algust. Kuigi valdavavalt on Neoproterosoikumi murenemissündmused säilinud pigem kulutuspindade kui terviklike murenemiskoorikutena, siis Ida-Euroopa platvormi kristalsel aluskorral Eestis, Lätis, Leedus ja Loode-Venemaal on säilinud mitmekümne meetri paksune hinnanguliselt 600–560 miljoni aasta vanune murenemiskoorik.
Võrreldes teiste omavanuste murenemiskoorikutega on Balti murenemiskoorik väga hästi säilinud. Murenemiskooriku läbilõiked on ligikaudu 40 meetri paksused ja nende ülemise osa moodustavad kuni 7 meetri paksused savi- ja rauamineraaliderikkad tsoonid, milliste kujunemist seostatakse tavaliselt murenemisega kuumas ja niiskes troopilises kliimas. Selline murenemiskooriku tekkekeskkonna interpretatsioon on aga vastuolus Baltika paleokontinendi tollase asukohaga kõrgetel laiuskraadidel ja seal tüüpiliselt valitsevate parasvöötme kliimatingimustega. Tõenäoliselt kujunes Balti murenemiskoorik lühiajaliselt intensiivistunud murenemise tulemusena, mida võis põhjustada (globaalsetele?) jäätumistele järgnenud nn kasvuhooneperioodid või süsinikuringe suureskaalalise anomaalia põhjustanud Shuram-Wonoka sündmus, mille tagajärjel järsult ja geoloogilises mõttes lühiajaliselt (<10 miljoniks aastaks) tõusnud atmosfääri CO2 sisaldus põhjustas sademete hapestumise ning võimaldas troopikale iseloomuliku murenemiskooriku kujunemist ka kõrgetel laiuskraadidel väljaspool troopilist kliimavöödet.
This thesis addresses the geology and paleoenvironmental interpretation of the Baltic paleosol weathering sequence. Paleosols are remains of ancient soils that are formed at Earth’s surface in direct contact with climatic and environmental conditions that prevailed at the time of soil formation and they hold potentially important information of past environments. Well preserved 600–560 Ma Baltic paleosol forms a major disconformity in the eroded Proterozoic crystalline basement, overlain by the Ediacaran–Paleozoic sedimentary rocks of the former Baltica continent. Because Precambrian paleosols are rather preserved as denudation surfaces than weathering crusts, the Baltic paleosol is possibly one of the few well-preserved examples of paleosol morphogenesis of late Precambrian, a time when modern world emerged. Weathering profiles in the Baltic paleosol extend ca. 40 m below the paleoweathered surface and top with reddish ca. 7 m thick clay (kaolinite) and Fe-mineral (hematite/goethite) rich paleosol horizon. Although the Baltic paleosol stands out from other Precambrian paleosols with its mature and very well-preserved weathering profiles, it has been partially eroded and mineralogical composition of the paleosol has been diagenetically altered at some point after its formation. Several meters thick clay rich paleosol profiles along with Fe-minerals are usually interpreted to represent intense weathering in warm and humid, tropical climate. This interpretation is in conflict with Baltica paleopositions at high latitudes during the paleosol formation which would imply temperate (and seasonal) climate. The most likely scenario is that the Baltic paleosol was indeed formed at extratropical latitudes but during abruptly intensified weathering at the termination of snowball glaciations or possibly Shuram-Wonoka isotope event, when increased temperatures and specifically caused elevated CO2 levels that enhanced soil formation by expanding the tropical weathering zone, for a short time, from intermediate to high latitudes.
This thesis addresses the geology and paleoenvironmental interpretation of the Baltic paleosol weathering sequence. Paleosols are remains of ancient soils that are formed at Earth’s surface in direct contact with climatic and environmental conditions that prevailed at the time of soil formation and they hold potentially important information of past environments. Well preserved 600–560 Ma Baltic paleosol forms a major disconformity in the eroded Proterozoic crystalline basement, overlain by the Ediacaran–Paleozoic sedimentary rocks of the former Baltica continent. Because Precambrian paleosols are rather preserved as denudation surfaces than weathering crusts, the Baltic paleosol is possibly one of the few well-preserved examples of paleosol morphogenesis of late Precambrian, a time when modern world emerged. Weathering profiles in the Baltic paleosol extend ca. 40 m below the paleoweathered surface and top with reddish ca. 7 m thick clay (kaolinite) and Fe-mineral (hematite/goethite) rich paleosol horizon. Although the Baltic paleosol stands out from other Precambrian paleosols with its mature and very well-preserved weathering profiles, it has been partially eroded and mineralogical composition of the paleosol has been diagenetically altered at some point after its formation. Several meters thick clay rich paleosol profiles along with Fe-minerals are usually interpreted to represent intense weathering in warm and humid, tropical climate. This interpretation is in conflict with Baltica paleopositions at high latitudes during the paleosol formation which would imply temperate (and seasonal) climate. The most likely scenario is that the Baltic paleosol was indeed formed at extratropical latitudes but during abruptly intensified weathering at the termination of snowball glaciations or possibly Shuram-Wonoka isotope event, when increased temperatures and specifically caused elevated CO2 levels that enhanced soil formation by expanding the tropical weathering zone, for a short time, from intermediate to high latitudes.
Description
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Keywords
paleontoloogia, neoproterosoikum, maakoor, murenemine, paleoklimatoloogia, paleokeskkond, Balti sünekliis, paleontology, neoproterozoic, crust of the earth, weathering, paleoclimatology, paleoenvironment, Baltic Syneclise