Can carbonate hardground cements allow us to reconstruct the evolution of pore water chemistry?

The formation of carbonate cements within sediments aids with lithification and impacts sediment permeability and porosity. The formation of these cements is often driven by microbial processes that remove kinetic inhibitors to abiotic carbonate precipitation in the open ocean. As the trace element composition of carbonate cement should reflect a combination of temperature and the formational fluid chemistry, changes in the elemental ratios along the growth axis may record variations in these conditions. We would expect, for example, that the Mg/Ca ratio would evolve to higher values as carbonate is precipitated from pore fluids, removing more calcium than magnesium from the pore fluids. The degree of this progression, along with changing trace element ratios, should provide insight into the various drivers of carbonate cement formation and the degree of closed-versus-open system behavior during cement precipitation. Results from electron microprobe analysis of carbonate cements found in hardgrounds through the Phanerozoic show evolving Mg/Ca ratios in carbonate cements along the growth axis. To further investigate these trends, high resolution laser ablation work will be presented results for major and trace element ratios along the growth axes. Additionally, high-resolution strontium isotope measurements support seawater conditions for much of the earliest cements. In combination, these results demonstrate the ability of early carbonate cements to both capture seawater chemistry and demonstrate the chemical conditions ideal for the precipitation of subsurface, or authigenic, carbonate during times of divergent seawater chemistry.