WHAT WE CAN AND WHAT WE CANNOT LEARN ABOUT THE EDIACARAN CLIMATE FROM THE OXYGEN ISOTOPIC SIGNATURE OF WEATHERING MATERIAL: A CASE OF THE EAST EUROPEAN CRATON

The recent studies on the Ediacaran of the western margin of the East European Craton (EEC) have identified several profiles of well-developed lateritic weathering crusts (paleosols) (Livamägi et al., 2018). The aim of the present study was to use the oxygen isotopic signature of weathering products to constrain weathering conditions during the Ediacaran.

Samples from two weathering crusts and five horizons of siltstones, representing sediments formed by redepositon of the underlying weathering crusts, were used. Clay fractions were obtained from the bulk rocks and subjected to two different selective dissolution procedures: 1) dissolution of Fe (hydr)oxides with DCB treatment and 2) dissolution of kaolinite with NaOH solution. Original clay fractions and reaction residues were characterized using XRD, ICP-OES, and δ¹⁸O measurement.

Mineral compositions of all samples were dominated by kaolinite, hematite, and mixed-layer illite-smectite (I-S). Selective dissolution of kaolinite was found to alter I-S and interfere with the isotopic investigation, so kaolinite-hematite isotopic geothermometer could not be used. Based on theoretical δ¹⁸O of water in equilibrium with hematite, two groups of samples can be distinguished: those which crystallized in equilibrium with water of meteoric origin under hot climate and those which crystallized in equilibrium with isotopically light water (δ¹⁸O < -10 ‰). The latter may be interpreted as a sign of post weathering overprint. No systematic differences between isotopic signatures of weathering crusts and siltstones were noticed.

The results of present study are in general agreement with the Ediacaran weathering under hot climate (Liivamägi et al., 2018). The obtained results lead also to the conclusion that not all samples represent equilibrium conditions with the Ediacaran environment. The presence of I-S suggests post-depositional alterations of the original smectite, confirmed by K-Ar dating (Liivamägi et al., 2018).

Acknowledgments:

This study was financed by the Polish National Science Centre MAESTRO grant 2013/10/A/ST10/00050.

References:

LIIVAMÄGI et al. (2018): Paleosols on the Ediacaran basalts of the East European Craton: a unique record of paleoweathering with minimum diagenetic overprint. – Precambrian Research, 316, 66-82.