A Zn isotope perspective on the rise of continents

作者：Pons M L^*; Fujii T; Rosing M; Quitte G; Telouk P; Albarede F

来源：Geobiology, 2013, 11(3): 201-214.

DOI：10.1111/gbi.12030

Zinc isotope abundances are fairly constant in igneous rocks and shales and are left unfractionated by hydrothermal processes at pH%26lt;5.5. For that reason, Zn isotopes in sediments can be used to trace the changing chemistry of the hydrosphere. Here, we report Zn isotope compositions in Fe oxides from banded iron formations (BIFs) and iron formations of different ages. Zinc from early Archean samples is isotopically indistinguishable from the igneous average (66Zn similar to 0.3 parts per thousand). At 2.92.7Ga, 66Zn becomes isotopically light (66Zn%26lt;0 parts per thousand) and then bounces back to values %26gt;1 parts per thousand during the similar to 2.35Ga Great Oxygenation Event. By 1.8Ga, BIF 66Zn has settled to the modern value of FeMn nodules and encrustations (similar to 0.9 parts per thousand). The Zn cycle is largely controlled by two different mechanisms: Zn makes strong complexes with phosphates, and phosphates in turn are strongly adsorbed by Fe hydroxides. We therefore review the evidence that the surface geochemical cycles of Zn and P are closely related. The Zn isotope record echoes Sr isotope evidence, suggesting that erosion starts with the very large continental masses appearing at similar to 2.7Ga. The lack of Zn fractionation in pre-2.9Ga BIFs is argued to reflect the paucity of permanent subaerial continental exposure and consequently the insignificant phosphate input to the oceans and the small output of biochemical sediments. We link the early decline of 66Zn between 3.0 and 2.7Ga with the low solubility of phosphate in alkaline groundwater. The development of photosynthetic activity at the surface of the newly exposed continents increased the oxygen level in the atmosphere, which in turn triggered acid drainage and stepped up P dissolution and liberation of heavy Zn into the runoff. Zinc isotopes provide a new perspective on the rise of continents, the volume of carbonates on continents, changing weathering conditions, and compositions of the ocean through time.

• 出版日期2013-5