Beachrocks record Holocene events, including natural disasters

Abstract

Rocks that form by cementation of sediments in intertidal parts of beaches are known as beachrocks. Modern beachrocks are restricted to warm climatic belts between latitudes 35EN and 35ES. Pottery fragments and debris from humans (including World War II relics such as shell casings) affirm that cementation in the rocks is still occurring today. The rate of cementation of sedimentary particles into beachrocks is very rapid. The cement between the particles may include aragonite, high-magnesian calcite, and/or low-magnesian calcite. All beachrocks sampled worldwide, except samples from Java, Indonesia, and Bardawil (Pelusium, Nile Delta), Egypt, have similar δ 13C and δ 18O signatures ranging from +0.1 to +5.2 and from +2.2 to −3.5, respectively. By contrast, beachrock from Java produced δ13C‰PDB −18.0 and δ 18O‰PDB −14.8, and from Bardawil (Pelusium, Nile Delta) δ13C‰PDB −39.3 to −3.3 and δ 18O‰PDB 3.8 to −36.6. The Krakatoa explosion on Java in Indonesia of August 27, 1883 has been designated as “the day the world exploded,” and was involved in its beachrock formation. The carbonate beachrock samples of dolomite from Bardawil yielded anomalous negative δ 13C values that suggest the carbon was derived from methane, which had formed at the expense of original gypsum and dolomite. Bacteria converted methane to CO2, thus making isotopically light bicarbonate available for precipitation. The timing (3,300–3,500 bp) appears to be related to the Exodus event as described in Jewish tradition.