Holocene sea-level rise evidenced in Hells Bells 234U/238U ratio and geochemical composition

Abstract

Hells Bells are underwater secondary carbonates discovered in sinkholes (cenotes) southeast of Cancun on the north-eastern Yucatán Peninsula, Mexico. These authigenic calcite precipitates, reaching a length of up to 4 m, most likely grow in the pelagic redoxcline. Here, we report on detailed 230Th/U-dating and in-depth geochemical and stable isotope analyses of specimens from cenotes El Zapote, Maravilla and Tortugas. Hells Bells developed during the end of MIS5b/c (~96–90 thousand years ago) and again since the early Holocene, with active growth until present day. The temporal evolution of the geochemistry and isotope composition of Hells Bells calcites appears closely linked to the mid to late Holocene sea-level rise, which reflects changing hydrological conditions of the aquifer. A stabilization of sea level combined with aquifer occlusion during the past ~8 thousand years probably led to a reduction in hydraulic conductivity and a desalinization of the freshwater layer as indicated by decreasing Sr/Ca values. In addition, initial (234U/238U) activity ratios (δ234U0) in the Bells calcite decrease from 55 to 15‰ as sea level converges toward its present state. We propose that the Holocene sea-level rise drives desalinization and subsequent deceleration of leaching of excess 234U from the previously unsaturated bedrock.