Exploring Icehouse Cyclicity Pattern and Seawater Dynamics on an Ancient Carbonate Platform With Nd Isotopes (Carboniferous, Southern Kazakhstan)

Abstract

Over the past decades, neodymium (Nd) isotopes have received considerable attention in paleoceanography as a tool for reconstructing past seawater circulation, local weathering inputs, and sea‐level change. In this study, we have investigated the Nd isotope composition of a shallow‐water Serpukhovian (Carboniferous) carbonate succession to explore icehouse cyclicity pattern 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 εNd(326 Ma) values from −1.6 to +4.3, corresponding to differences in the isotopic composition of two seawater masses present in the Uralian–Turkestan Ocean during the Serpukhovian: (a) highly radiogenic deep waters and (b) less radiogenic surface waters. 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 Carboniferous interglacial intervals. The episodic appearance of rich brachiopod communities was forced by the inflow of highly radiogenic, nutrient‐enriched 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.