T160 Critical Earth-Life Transitions: The Marine Perspective

Monday, 20 October 2014
1:00 PM - 5:00 PM
114/115

CALCIUM ISOTOPES IN EVAPORITES CONSTRAIN RELATIVE SULFATE, CALCIUM, AND DIC LEVELS IN PHANEROZOIC AND PROTEROZOIC SEAWATER

Paper number: T160-5
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Abstract

The Ca-isotope behavior of marine evaporite formations represents a new, independent method for tracing the composition of ancient seawater, in particular the relative concentrations of sulfate, calcium, and DIC (dissolved inorganic carbon). The concentrations of these species influence global calcification and organic matter respiration rates, and are thus metrics for the evolution of carbonate sedimentation and oceanic redox. Ca-isotope analyses of carbonate and sulfate facies of evaporites provide new constraints on the behavior of these components during broad periods of geological time, including intervals where few other data on seawater chemistry exist.

Because Ca isotopes are fractionated during calcite and gypsum/anhydrite formation, the Ca-isotope composition of evaporite sequences which progress to halite saturation is sensitive to the relative concentrations of sulfate and DIC to that of calcium. In modern seawater, [SO4] + ½ [DIC] is in excess of [Ca], leading to the removal of most Ca and a distinct Ca-isotope enrichment with progressive evaporation. If [Ca] is instead greater than [SO4] + ½ [DIC], significant Ca remains in the brine and a much smaller isotopic enrichment occurs. Phanerozoic evaporite sequences from the Mediterranean (Messinian), Sergipe-Alagoas (Cretaceous), Delaware (Permian), and Michigan Basins (Silurian) demonstrate Ca-isotope behavior that is consistent with estimates of [SO4] and [Ca] from previous work on halite fluid inclusions.

Analyses of the Society Cliffs Formation (1.2 Ga) and Pethei Group (1.8 Ga), which both lack preserved halite but preserve evidence for its former presence, show an absence of Ca-isotope enrichment. These results suggest that during these intervals of Paleo- and Mesoproterozoic time, neither SO4 nor DIC was abundant enough to consume the available Ca during evaporite formation. These new limits on marine SO4 and DIC have important implications for global geochemical cycles during the Proterozoic.