Zigzag microstructure in rugose corals: A possible indicator of relative seawater Mg/Ca ratios

Abstract

Low-Mg calcite (LMC) and high-Mg calcite (HMC) have been identified confidently as original biominerals in different rugose corals. Devonian Tabulophyllum had a LMC skeleton, whereas Pennsylvanian Lophophyllidium had a HMC skeleton. Zigzag microstructure, a diagenetic microstructure in rugose corals, may be related to early migration of Mg2+ ions during transformation of HMC into LMC. Therefore, zigzag microstructure may indicate that the original mineral was HMC even in specimens that subsequently converted to LMC. Stratigraphically, corals with zigzag microstructure are confined to the late Paleozoic, which is coincident with the “aragonite sea” of P.A. Sandberg. Hence, fluctuating Mg/Ca ratios may have directly controlled oscillations in marine carbonate chemistry and the mineral composition of rugose corals so that HMC skeletons (as indicated by zigzag microstructure) formed during intervals with “aragonite seas” and LMC skeletons (e.g., Tabulophyllum ) formed during intervals with “calcite seas.”