Stable isotopes (δ ¹³ C, δ ¹⁸ O) and element ratios (Mg/Ca, Sr/Ca) of Jurassic belemnites, bivalves and brachiopods from the Neuquén Basin (Argentina): challenges and opportunities for palaeoenvironmental reconstructions

Abstract

Fossils from the Jurassic succession of the Neuquén Basin (Argentina) were analysed for their stable isotope (δ13C, δ18O) and elemental (Mg/Ca, Sr/Ca) composition. Mg/Ca ratios point to comparatively stable temperature conditions from the Bajocian to Early Oxfordian and during the Tithonian, but do not allow a reliable reconstruction of absolute water temperatures. Sr/Ca ratios follow the general global pattern indicating water exchange between the basin and the open ocean. The δ18O values can be translated into water temperatures between 20 to 25 °C for most of the studied intervals with possible shorter cold spells in the Late Pliensbachian, Bajocian, and Late Tithonian. However, precise temperature reconstructions are complicated by bivalve shells from the northern/central part of the basin pointing to local fluctuations in the δ18O values of seawater. Potential reasons for these variations are discussed, but it seems most likely that they are caused by phases of enhanced freshwater input leading to meso- to brachyhaline conditions in the northern study areas. The present article therefore exemplifies the particular challenges for temperature reconstructions in marginal seas and highlights the opportunities of combining different geochemical proxies to disentangle the influence of different environmental parameters.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5173466

Dynamic environmental conditions recorded by the trace fossil Teichichnus and event beds during deposition of the basal Vaca Muerta Formation in the central Neuquén Basin (Argentina)

“…It was separated from the Palaeo-Pacific by a volcanic arc forming an island chain (e.g., Howell et al, 2005), but alternatively a continuous peninsula has been suggested (Vicente, 2005). Water exchange between Palaeo-Pacific and Neuquén Basin was thus more or less restricted but Sr/Ca ratios of marine carbonates nonetheless imply a rather permanent connection between both water bodies since the Sr/Ca values are in the same range (e.g., Alberti et al, 2020).…”

Upper Jurassic (Tithonian) belemnites from the Neuquén Basin, Argentina

“…1), Upper Jurassic-Lower Cretaceous belemnite records are sparser and poorly known. Oxfordian Belemnopsis with European affinities have been mentioned from La Manga Formation in southern Neuquén (Doyle et al, 1996;Riccardi et al, 2011) and from the Callovian-Oxfordian of Vega de la Veranada, in northern Neuquén (Alberti et al, 2020). Younger not-illustrated records from Mendoza Province include an indeterminate rostrum fragment from the Tithonian of Arroyo Serrucho (Krantz, 1928) and some thin incomplete rostra from the upper Tithonian of the Vaca Muerta Formation in Bardas Blancas (Gerth, 1925; here referred as Cañada de Leiva).…”

Organic-rich, fine-grained contourites in an epicontinental basin: The Upper Jurassic-Lower Cretaceous Vaca Muerta Formation, Argentina

“…Moreover, times of carbonate deposition might be associated with cooler climates. For instance, during deposition of the Picún Leufú Formation carbonates in the southern part of the basin (late Tithonian-early Berriasian, Leanza et al, 2020), Alberti et al (2020) recorded a cold snap (18 • C) using bivalve oxygen isotope data. However, Gómez-Dacal et al ( 2018) calculated relatively warm temperatures (~25 • C) in the northern Neuquén Basin at the same time.…”