The Calcium Isotope (δ^44/40Ca) Record Through Environmental Changes: Insights From the Late Triassic

Abstract

The stable calcium (Ca) isotopic composition of marine carbonate archives (δ 44/40 Ca) was proposed to be an important palaeo-environmental and palaeo-oceanographic proxy, mainly because of the linkage between the Ca and C geochemical cycles. The continental weathering of Ca bearing carbonates and silicates is the main source of Ca to the ocean, while authigenic, biogenic and diagenetic carbonate precipitation in the marine realm is the main Ca sink (DePaolo, 2004). The global Ca fluxes play a key control on the CO 2 level in the atmosphere and on the related oceanic-atmospheric system as well as on climate regulation throughout the geological record (Fantle & Tipper, 2014;Gussone et al., 2020). Variations in the Ca isotopic composition of marine carbonate archives through time, that is, as a consequence of Ca mineral weathering, carbonate precipitation and subse-Abstract Calcium isotopes (δ 44/40 Ca) are particularly useful in palaeo-environmental studies due to the key role of carbonate minerals in continental weathering and their formation in seawater. The calcium isotope ratio can provide hints on past changes in the calcium fluxes, environmental shifts, ecological factors and alternatively diagenesis of carbonate rocks. The investigation of the Late Triassic calcium isotope record offers a great opportunity to evaluate such factors in a time interval that witnessed important environmental and ecological turnovers, such as the first appearance of calcareous nannoplankton, ocean acidification and periods of elevated extinction rates. In this study, we present a δ 44/40 Ca data set from the upper Norian (Upper Triassic) through the lower Hettangian (Lower Jurassic) interval. The isotope records reveal two globally significant signals: a ∼ 0.20‰ decrease through the early Rhaetian (Upper Triassic) and a small, negative (∼0.14‰) excursion corresponding to the emplacement of the Central Atlantic Magmatic Province, at the end of the Triassic. The possible explanations for these signals are changes in the isotopic ratio of the continental calcium influx to the ocean due to the high chemical weathering rate of carbonates and possibly ocean acidification, respectively. The considerable (∼0.15-0.30‰) offset in δ 44/40 Ca between study areas is likely the combined result of local differences in lithology and early marine diagenesis. The major evolutionary step represented by the first occurrence of calcareous nannoplankton did not have at this time a determining role on the calcium isotopic signature of the marine carbonates.Plain Language Summary During Earth's history, profound environmental changes happened, which are recorded in geological samples and accessible through proxy methods. We measured Late Triassic carbonates for their Ca isotopic composition. The Late Triassic witnessed important environmental and ecological turnovers, such as the first appearance of calcareous nannoplankton, mass extinction and ocean acidification associated with CO 2 emission from a large volcanic province. These multiple events help...