Conservative mixing of alkaline groundwater plume with saline waters at
Hamelin Pool recorded in Ba isotopic compositions of stromatolitic
carbonates
Abstract
Hamelin Pool, W.-Australia, is a natural laboratory to study
stromatolite formation in a hypersaline lagoon. Stromatolitic aragonites
sampled at supratidal, intertidal and subtidal environments show gradual
increase in Ba isotopic compositions (d138Bacarb = -0.12-0.57‰) with
decreasing Ba/Ca following conservative mixing between groundwater and
seawater. We observe fingerprints of the groundwater end-member in a
supratidal sample showing particular light d138Bacarb corresponding with
elevated Mn/Sr and low d18Ocarb as indicator for discharge from the
local Tamala Limestone aquifer. In contrast, stromatolitic carbonates
formed in equilibrium with a saline end-member show heavy d138Bacarb.
Lagoonal d138Bafluid ranges from 0.44-0.6‰ at increasing PSU (3.6-65.6).
The calculated partition coefficient for Ba into stromatolites is lowest
at the subtidal (logDBa=0.45) and highest at the groundwater discharge
site (logDBa=0.86). Injection of groundwaters into Hamelin Pool likely
contributes to enhanced aragonite precipitation at up to 0.32mm/yr,
possibly catalysed by nucleation within or onto extra-polymeric
substances of microbially diverse mats.