Carbon and oxygen isotopic composition of a recent megapolygon-spelean limestone, fisheman by, South Australia

Abstract

In the supratidal zone of broad peritidal flats at Fisherman Bay, in the semi-arid climatic zone of South Australia, a diagenetic limestone containing megapolygons, tepee structures and speleothems is forming by cementation of nearsurface marine carbonates. Cementation occurs as aragonite precipitates from upwardly-flowing saline springs formed by groundwater of meteoric origin which has flowed seawards across the coastal plain. The 180 content of the emerging spring waters δ 18O, −2.45%osmow) is higher than that of local rainfall ( δ 18O ∼-3.5%o) but has not reached the positive values found in other highly evaporative environments, such as the sabkhas of the Persian Gulf. The 13C content of the dissolved carbonate ( δ 13C, −9.1‰ PDB) is slightly higher than that of altered Tertiary marine carbonate sediments ( δ 13C, −8.7 to −6.3%o) which form that part of the coastal aquifers in which the dissolved carbonate is generated. Aragonite precipitating in cavities within the limestone has an isotopic composition ( 13C; −4.4%o; δ 18 O +26.7% o) which reflects that of the emergingwaters. Elsewhere, higher rates of evaporation and evolution of C0 2 produce a range of isotopically heavier cements. The isotopic composition of the host limestone is marginally below that of the local uncemented intertidal marine carbonates ( δ 13C, +0.8 to +4.5%o; δ 18O, +33.0 to +34.3%o) but it is within the general range for Holocene and Pleistocene marine carbonates. Despite the highely evaporative climate, the isotopic characteristics of the spring waters remain distinctively nonmarine. Evaporative increases in 18O content are limited by the partly confined nature of the coastal aquifer system and, in the near-surface environment, by the overlying limestone. The 3C content of the dissolved carbonate in the aquifers increases considerably as a result of water-sediment interactions but only towards a limit set by the isotopically light composition of the aquifer carbonates. The limited influence of the groundwaters on the bulk isotopic composition of the limestone is a consequence of their seawater-like chemical composition, which minimizes dissolution and alteration of the original marine carbonates. Ancient examples of cementation of peritidal carbonates by aragonite-precipitating upward-flowing spring waters should be recognizable as thin lenses located at sub-aerial exposure surfaces. Such lenses would contain juxtaposed isotopically-different non-marine cavity cements and unaltered marine sediments.