Isotopic Analysis of Jurassic (Callovian) Mollusks From the Christian Malford Lagerstätte (Uk): Implications for Ocean Water Temperature Estimates Based on Belemnoids

Abstract

Isotopic data (C and O) derived from Callovian (Middle Jurassic) mollusks (bivalves, ammonites and belemnoids, including true belemnites and Belemnotheutis) are presented from a narrow stratigraphic interval in the Christian Malford Lagerstätte, UK. The exceptionally well-preserved mollusks include aragonite-calcite pairs precipitated by individual belemnite animals that enable an assessment of possible ''vital'' effects and the reliability of using belemnite calcite to determine ocean water compositions. The oxygen isotope data derived from the calcitic rostra of the belemnites (Cylindroteuthis) show modest variability, ranging from 21.2 to 0.9%o (V-PDB), while their accompanying aragonitic phragmocones range from 21.4 to 0.0%o. Data derived from the ammonite Kosmoceras show some scatter, with oxygen isotope values varying from 23.6 to 20.2 %o. The aragonite data from Cylindroteuthis, Kosmoceras and Belemnotheutis all overlap, suggesting they inhabited similar (surface) water depths. However, the corresponding data from the calcitic rostra of the Cylindroteuthis specimens suggest temperatures , 5uC cooler. As we have analyzed aragonite-calcite pairs, the discrepancy cannot be explained by environmental effects. Though clearly a vital effect, it is difficult to resolve whether the temperatures derived from the aragonite (phragmocone) are too warm or from the calcite (rostrum) are too cool. Consequently, the applicability of standard paleotemperature equations to Cylindroteuthid belemnite rostra remains unproven. Sequentially sampled ontogenetic isotope data derived from Belemnotheutis phragmocones reveal only modest d 18 O variation, consistent with limited movement between warmer (shallower) and cooler (deeper) waters. A coincidental systematic pattern of d 13 C enrichment may signal changes in metabolic activity associated with a shift in ecology or feeding with age.

Ontogenetic trajectories of septal spacing in Early Jurassic belemnites from Germany and France, and their palaeobiological implications

“…On the other hand, the phragmocone was originally composed of aragonite (Hall & Kennedy ; Price et al . ) resulting in a preservation potential lower than that of the rostrum, because calcite is in general a more stable mineral than aragonite (Tucker ). Phragmocones of belemnites are divided by septa into chambers, which are homologous with internal shells of cuttlefish and external shells of nautiloids and ammonoids, suggesting that the phragmocones of belemnites served as a buoyancy device (Monks et al .…”

First steps in reconstructing Early Jurassic sea water temperatures in the Andean Basin of northern Chile based on stable isotope analyses of oyster and brachiopod shells

“…At the same time, it has to be noted that the majority of data from Europe is based on belemnites, which are known to commonly record lower temperatures than co-occurring brachiopods and bivalves (e.g., Dera et al 2011;Alberti et al 2012aAlberti et al , 2019. The cause of this difference might include different life habits between the faunal groups, but is still strongly debated in literature (Fürsich et al 2005;Mutterlose et al 2010;Alberti et al 2012aAlberti et al , 2012bPrice et al 2015;Dera et al 2016;Hoffmann et al 2016).…”

Marine temperatures underestimated for past greenhouse climate

“…Here we present new clumped isotope data derived from co-occurring calcite and aragonite in individual exceptionally well preserved belemnites ( Cylindroteuthis ) 42 from the Callovian-aged Christian Malford Lagerstätte (Fig. 1 ).…”

“…Our warm temperatures are more in line with the geographical distributions of temperature-sensitive fossil flora and fauna, as well as the results of climate models with increased CO 2 levels, and demonstrate that at least several common belemnite genera lived neither deep in the water column nor in hypersaline conditions.

Figure 1 ( A ) Sampling location (Christian Malford) in the U.K., denoted by star, with the outcrop of Callovian sediments indicated by grey band, after Price et al 42 . ( B ) Palaeogeographic reconstruction of the Tethyan Realm during the Middle Jurassic, after Dera et al 63 .

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