Decimetre-Thick Encrustations of Calcite and Aragonite on the Sea-Floor and Implications for Neoarchaean and Neoproterozoic Ocean Chemistry

  1. Wladyslaw Altermann2 and
  2. Patricia L. Corcoran3
  1. D. Y. Sumner

Published Online: 12 MAR 2009

DOI: 10.1002/9781444304312.ch5

Precambrian Sedimentary Environments: A Modern Approach to Ancient Depositional Systems

Precambrian Sedimentary Environments: A Modern Approach to Ancient Depositional Systems

How to Cite

Sumner, D. Y. (2002) Decimetre-Thick Encrustations of Calcite and Aragonite on the Sea-Floor and Implications for Neoarchaean and Neoproterozoic Ocean Chemistry, in Precambrian Sedimentary Environments: A Modern Approach to Ancient Depositional Systems (eds W. Altermann and P. L. Corcoran), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304312.ch5

Editor Information

  1. 2

    Institut für Allgemeine und Angewandte Geologie, Ludwig-Maximilians-Universität München, Luisenstrasse 37, D-80333 Munich, Germany

  2. 3

    Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, B3H 3J5, Canada

Author Information

  1. Geology Department, University of California, Davis, CA 95616, USA

Publication History

  1. Published Online: 12 MAR 2009
  2. Published Print: 18 FEB 2002

ISBN Information

Print ISBN: 9780632064151

Online ISBN: 9781444304312

SEARCH

Keywords:

  • decimetre-thick encrustations of calcite and aragonite on sea-floor - neoarchaean and neoproterozoic ocean chemistry;
  • sea-floor encrusting calcite and aragonite crystals;
  • geochemical constraints;
  • neoproterozoic cap carbonates;
  • neoarchaean–neoproterozoic comparisons

Summary

Centimetre- to metre-tall aragonite pseudomorphs are abundant in Neoarchaean marine carbonates and less common, but present, in some Proterozoic carbonates. Neoarchaean carbonates also contain substantial proportions of decimetre-thick calcite encrustations on the sea-floor, whereas their abundance declines through time. In contrast, the relative proportion of micritic sediment increases during Proterozoic time. The precipitation of large aragonite crystals and thick layers of calcite on the sea-floor implies that crystal growth rates were very high relative to sedimentation rates. For metre-tall Neoarchaean fans in shallow subtidal depositional environments, crystal growth rates must have substantially exceeded estimates of modern aragonite cement precipitation rates. High aragonite saturation states in sea water could produce rapid precipitation rates. The paucity of micrite suggests that nucleation of carbonate in the water column was limited. These conditions need to be maintained for millions of years and may be due to the presence of chemical inhibitors that slow crystal nucleation and precipitation rates. Rapid local crystal growth may reflect the local absence of inhibitors and globally low carbonate accumulation rates relative to calcium influx to the oceans. Some Neoproterozoic ‘cap carbonates’, which immediately overly glacial deposits, contain large aragonite pseudomorphs, similar to those common in Neoarchaean carbonates. Unlike Neoarchaean carbonates, however, sea-floor calcite encrustations are rare, and micrite precipitation was abundant. These differences suggest that the circumstances leading to the growth of Neoproterozoic large aragonite fans were different. The limited stratigraphic distribution of aragonite pseudomorphs also suggests that changes in ocean dynamics may have produced a temporary increase in carbonate saturation states, in contrast to the long-term maintenance of high supersaturation required by the distribution of Neoarchaean aragonite pseudomorphs.