Petrography and the rare earth element composition of apatite in 2 Ga Onega and Pechenga basins, Russia: the environmental settings for phosphogenesis
Date
2015-08-04
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Abstract
Fosfor on bioevolutsiooniliselt võtmetähtsusega element, millel on oluline roll nii elu geneetilises koodis, kui ka organismide ainevahetuses. Tänapäeval toimub fosfogenees (fosfori eemaldamine veekogudest) ja kaasaegsete fosforiitide moodustumine peamiselt mandrilavade äärealadel kus ookeanisüvikutest tõusvad toiteaineterikkad hoovused toidavad pinnakihtide kõrget primaarproduktsiooni. Tüüpiliselt kujunevad fosfaatsete mineraalide (apatiidi) kristalliseerumiseks piisavad fosfaadi kontsentratsioonid settesisestel anoksilise (sulfiidse) – aeroobse keskkonna redokspiiridel, kus fosfori kuhjumist kontrollivad settes elavad ja primaarset orgaanilist ainet lagundavad mikroorganismid. Esimesed fosforirikkad settekivimid moodustusid Paleoproterosoikumis, ligikaudu kaks miljardit aastat tagasi. Seejuures on tähelepanuväärne, et sarnase vanusega fosforiite on leitud paljudest kohtadest üle maakera ning seetõttu võib arvata, et nende väljasettimise põhjustas mingi globaalne sündmus. Doktoritöös uuriti ühtesid maailma vanimaid fosforiite, mis pärinevad kahest erinevast settebasseinist Loode Venemaal. Töö eesmärgiks oli selgitada keskkonnatingimused nende fosforiitide moodustuse ajal ning võrrelda neid tänapäevaste fosfogeneesi keskkondadega. Põhitähelepanu keskendus settelises apatiidis leiduvate haruldaste muldmetallide (lantanoidide) jaotumise ning sisalduste selgitamisele, mis võimaldavad interpreteerida apatiidi tekkekeskkonna redokstingimusi. Uuringute tulemusena selgus, et mõlemas settebasseinis toimus fosfogenees merepõhja settekolonni ülemises osas, kus valitsesid redokstingimused, mis on sarnased kaasaegse fosfogeneesi keskkondadele. See näitab, et tõenäoliselt kontrollisid apatiidi väljasettimist Paleoproterosoikumis ja kontrollivad ka kaasaegsetes setendites sarnased bioloogilised-füüsikalis/keemilised protsessid. Arvatavasti märgib apatiidi väljasettimise algust geoloogilises ajas spetsiifilise anoksilise (sulfiidse) – aeroobse redokspiiri moodustumine sette ülemises mõne cm paksuses osas, mis sai võimalikuks alles peale vaba hapniku ilmumist Maa atmosfääris ligikaudu 2,3 miljardit aastat tagasi.
Phosphorus is an essential and non-substitutable bioelement that limits the ecosystem primary productivity. In the modern P-cycle the major sink of phosphorous and formation of P-rich deposits - phosphorites - occurs at continental margins influenced by ocean upwelling currents. The main sites of modern phosphogenesis are the shallow levels of the sediment column close to the sediment water interface within the anoxic to (sub)oxic diagenetic zones. Recent studies of organic-rich sediments on modern continental margins have revealed the utmost significance of sulphur-oxidizing bacteria mediating a redox-dependent phosphorous-cycling by creating sinks for marine phosphorous, and eventually formation of phosphorites. Phosphorite formation is mainly a Phanerozoic (last 0.5 Ga) phenomenon, but the first significant P-rich sedimentary rocks appear seemingly synchronously worldwide in the Paleoproterozoic rock record at around 2 Ga. Appearance of these deposits have been linked to the oxidation of the atmosphere and establishment of the aerobic, modern-type Earth though little is known about the underlining causes and mechanisms of their formation. The aim of this PhD thesis was to study the environmental conditions of phosphogenesis in two ca 2 Ga old sedimentary basins using the Rare Earth Element analysis of sedimentary phosphates. The environmental conditions of the phosphogenesis recorded in apatite in the Paleoproterozoic Pilgujärvi Sedimentary Formation of the Pechenga basin and in Zaonega Formation, Onega basin are similar, suggesting that the phosphogenic events in these sedimentary basins occurred in a similar way. Initiation of the phosphogenesis in these basins possibly marks the development of specific anoxic(sulfidic)-suboxic redoxclines at shallow sediment depth during the Paleoproterozoic that are very similar to the environmental settings found in modern phosphogenic areas. It is important that phosphogenic events archived in 1.97 Ga Zaonega Formation and in 1.92 Ga Pilgujärvi Sedimentary Formation occurred ca 300-400 Ma after the Great Oxygenation Event at 2.3 Ga, suggesting that time was needed to establish diagenetic conditions and redoxcline conditions suitable for concentration of interstitial phosphate in shallow sediments at sea bottom.
Phosphorus is an essential and non-substitutable bioelement that limits the ecosystem primary productivity. In the modern P-cycle the major sink of phosphorous and formation of P-rich deposits - phosphorites - occurs at continental margins influenced by ocean upwelling currents. The main sites of modern phosphogenesis are the shallow levels of the sediment column close to the sediment water interface within the anoxic to (sub)oxic diagenetic zones. Recent studies of organic-rich sediments on modern continental margins have revealed the utmost significance of sulphur-oxidizing bacteria mediating a redox-dependent phosphorous-cycling by creating sinks for marine phosphorous, and eventually formation of phosphorites. Phosphorite formation is mainly a Phanerozoic (last 0.5 Ga) phenomenon, but the first significant P-rich sedimentary rocks appear seemingly synchronously worldwide in the Paleoproterozoic rock record at around 2 Ga. Appearance of these deposits have been linked to the oxidation of the atmosphere and establishment of the aerobic, modern-type Earth though little is known about the underlining causes and mechanisms of their formation. The aim of this PhD thesis was to study the environmental conditions of phosphogenesis in two ca 2 Ga old sedimentary basins using the Rare Earth Element analysis of sedimentary phosphates. The environmental conditions of the phosphogenesis recorded in apatite in the Paleoproterozoic Pilgujärvi Sedimentary Formation of the Pechenga basin and in Zaonega Formation, Onega basin are similar, suggesting that the phosphogenic events in these sedimentary basins occurred in a similar way. Initiation of the phosphogenesis in these basins possibly marks the development of specific anoxic(sulfidic)-suboxic redoxclines at shallow sediment depth during the Paleoproterozoic that are very similar to the environmental settings found in modern phosphogenic areas. It is important that phosphogenic events archived in 1.97 Ga Zaonega Formation and in 1.92 Ga Pilgujärvi Sedimentary Formation occurred ca 300-400 Ma after the Great Oxygenation Event at 2.3 Ga, suggesting that time was needed to establish diagenetic conditions and redoxcline conditions suitable for concentration of interstitial phosphate in shallow sediments at sea bottom.
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Keywords
apatiit, haruldased muldmetallid, petrograafia, petrogenees, paleoproterosoikum, Petšenga, Loode-Venemaa, apatite, rare earth metals, petrography, petrogenesis, Paleoproterozoic, Pechenga, North-West Russia