Rare earth elements in the terminal Ediacaran Bambui Group carbonate rocks (Brazil): evidence for high seawater alkalinity during rise of early animals

Author(s)

Paula-Santos, Gustavo Macedo ^[1] ; Caetano-Filho, Sergio ^[2] ; Enzweiler, Jacinta ^[1] ; Navarro, Margareth S. ^[1] ; Babinski, Marly ^[2] ; Guacaneme, Cristian ^[2] ; Kuchenbecker, Matheus ^{[3, 4]} ; Reis, Humberto ^[5] ; Trindade, Ricardo I. F. ^[6]

Affiliation

^[1] Univ Estadual Campinas, Inst Geociencias, Rua Carlos Gomes 250, BR-13083855 Campinas, SP - Brazil

^[2] Univ Sao Paulo, Inst Geociencias, Rua Lago 562, BR-05508080 Sao Paulo, SP - Brazil

^[3] Univ Fed Vales Jequitinhonha & Mucuri, Inst Ciencia & Tecnol, Lab Estudos Tecton, Rodovia MGT 367, Km 583, BR-39100000 Diamantina, MG - Brazil

^[4] Univ Fed Minas Gerais, Ctr Pesquisas Prof Manoel Teixeira da Costa, Av Antonio Carlos 6627, BR-66273127 Belo Horizonte, MG - Brazil

^[5] Univ Fed Ouro Preto, Dept Geol, Escola Minas, Lab Modelagem Tecton, Campus Morro Cruzeiro, BR-35400000 Ouro Preto, MG - Brazil

^[6] Univ Sao Paulo, Inst Astron Geofis & Ciencias Atmosfer, Rua Matao 1226, BR-05508090 Sao Paulo, SP - Brazil

• Document type: Journal article
• Source: Precambrian Research; v. 336, JAN 2020.
• Web of Science Citations: 0

Abstract

Rare earth elements plus yttrium (REY) mass fractions of ancient carbonate rocks are used to track changes in chemistry of past seawater. Here we investigate REY patterns in two carbonate sections from the Ediacaran Bambui Group, Sao Francisco Basin (Brazil), which comprise its two lowermost transgressive-regressive second order sedimentary sequences. Shale normalised distributions vary with the sequence stratigraphy framework. In the basal 2nd-order sequence, carbonate samples from the basal sequence transgressive systems tract display light REY (LREY) distributions slightly depleted to enriched that reflect input of freshwater, possibly in a post glacial episode. Upwards, carbonate rocks from the early highstand systems tract (EHST) yielded LREY enriched distributions, which progressively turns into LREY shale normalized depleted distributions on samples from the late highstand systems tract (LHST). This portion of the sequence also displays Y positive anomaly in some cases. Carbonate samples from the upper second-order sequence do not display coherent patterns. Ce/Ce{*} values > 1 in most samples throughout the two sections suggest permanent anoxia of seawater. The REY change from the EHST to LHST in the basal sequence marks an important paleoenvironmental overturn in the basin, with increasing alkalinity in seawater driving REY fractionation and LREY depletion. Confinement of the basin in the inner areas of West Gondwana due the uplift of marginal neoproterozoic orogens probably changed the weathering style of source areas to more congruent, thus delivering a higher ionic influx to a restricted setting, increasing alkalinity during LHST. Cloudina sp. fragments were reported in this stage of the Bambui Group and in carbonate rocks with high Sr mass fractions in other West Gondwana basins, supporting the hypothesis that the high alkalinity of seawater during late Ediacaran may have driven the appearance of the first biomineralizing organisms. (AU)

• FAPESP's process: 17/00399-1 - Chemical weathering and nutrient input into late Ediacaran ocean and their relationship to life diversification
• Grantee: Gustavo Macedo de Paula Santos
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 16/06114-6 - The Neoproterozoic Earth System and the rise of biological complexity
• Grantee: Ricardo Ivan Ferreira da Trindade
• Support Opportunities: Research Projects - Thematic Grants