Testing icehouse cyclicity and seawater dynamics on an ancient carbonate platform with Nd isotopes (Carboniferous, southern Kazakhstan)

Over the past decades, neodymium isotopes have received considerable attention in palaeoceanography as a tool for reconstructing past seawater circulation, local weathering inputs, and sea-level changes. In this study, we have investigated the Nd isotope composition of Serpukhovian (Carboniferous) carbonates of a shallow-water succession to test icehouse cyclicity and seawater dynamics on the Karatau carbonate platform in southern Kazakhstan. The cyclic succession formed in response to glacio-eustasy and composed of subtidal and intertidal limestones displays a large variation in the ε_Nd(326 Ma) values from –1.6 to +4.3, corresponding to differences in the isotopic composition of two seawater masses present in the adjacent Uralian–Turkestan Ocean during the Serpukhovian, highly radiogenic deep water and less radiogenic surface water. The Nd isotope excursions within the icehouse cycles are more complex than simple transgressive-regressive cycles. They probably reflect a temporal pattern of the sub-Milankovitch climatic perturbations during the interglacial periods in the Carboniferous. The episodic appearance of rich brachiopod communities was forced by the inflow of highly radiogenic, nutrient-rich waters, presumably driven by upwelling. Nd isotope analyses of cyclic intertidal and subtidal carbonates have great potential to produce high-resolution records of seawater dynamics on shallow-water carbonate platforms.

This study was supported by the Polish National Science Centre, grant No. 2013/11/B/ST10/04751.