The Strontium Isotope Record of Zavkhan Terrane Carbonates: Strontium Isotope Stability Through the Ediacaran-Cambrian Transition

Abstract

First order trends in the strontium isotopic (87Sr/86Sr) composition of seawater are controlled by radiogenic inputs from the continent and non-radiogenic inputs from exchange at mid-ocean ridges. Carbonates precipitated in seawater preserve trace amounts of strontium that record this isotope ratio and therefore record the relative importance of mid-ocean ridge and weathering chemical inputs to sea water composition. It has been proposed that environmental changes during the Ediacaran-Cambrian transition may have enabled the rapid diversification of life commonly named the “Cambrian explosion.” Proposed environmental changes include 2.5x increase in mid-ocean ridge spreading at the Ediacaran-Cambrian boundary and large continental fluxes sediment into oceans. These hypotheses rely on a poorly resolved strontium isotope curve to interpret Ediacran-Cambrian seawater chemistry. A refined strontium isotope curve through this time period may offer insight into the environmental conditions of the early Cambrian.

New age models and detailed mapping in the Zavkhan terrane in west-central Mongolia provide the context necessary for robust geochemical analysis. This study aims to better resolve the coarse strontium isotope curve for the early Cambrian period by analyzing carbonate sequences in the Zavkhan basin. These carbonate sections are rapidly deposited, have undergone little diagenesis, and are likely to preserve a primary seawater signal. Strontium isotope analysis of these sequences was carried out to determine changes in hydrothermal activity and weathering fluxes during this time period. Recompiling these data with a global dataset of strontium isotopes through this time period indicates a stable strontium isotope signal through much of the early Cambrian. These data do not support previous hypotheses attributing the driving mechanism for the early Cambrian transition from Mg-dominated to Ca-dominated seas to increased sea floor spreading rates.