Subdaily‐Scale Chemical Variability in a Torreites Sanchezi Rudist Shell: Implications for Rudist Paleobiology and the Cretaceous Day‐Night Cycle

Abstract

This study presents subdaily resolved chemical records through fossil mollusk shell calcite. Trace element profiles resolve periodic variability across ~40‐μm‐thin daily growth laminae in a Campanian Torreites sanchezi rudist bivalve. These high‐resolution records are combined with seasonally resolved stable isotope and trace element records that allow shell‐chemical variability to be discussed on both seasonal and daily scale. A combination of layer counting, spectral analysis of chemical cyclicity and chemical layer counting shows that the rudist precipitated 372 daily laminae per year, demonstrating that length of day has increased since the Late Cretaceous, as predicted by astronomical models. This new approach to determine the length of a solar day in geologic history through multiproxy chemical records at subdaily resolution yields considerably more control on the uncertainty of this estimate. Daily chemical variability exceeds seasonal variability in our records, and cannot be explained by diurnal temperature changes. Instead, we postulate that rudist shell chemistry is driven on a daily scale by changes in light intensity. These results together with those of stable isotope analyses provide strong evidence that Torreites rudists had photosymbionts. Bivalve shell calcite generally preserves well. Therefore, this study paves the way for daily‐scale reconstructions of paleoenvironment and sunlight intensity on geologic time scales from bivalve shells, potentially allowing researchers to bridge the gap between climate and weather reconstructions. Such reconstructions improve shell chronologies, document environmental change in warm ecosystems, and widen our understanding of the magnitude of short‐term changes during greenhouse climates.

The Giant Marine Gastropod Campanile Giganteum (Lamarck, 1804) as a High‐Resolution Archive of Seasonality in the Eocene Greenhouse World

“…In the latter case, care must be taken to focus high‐resolution transects on (parts of) cross sections that are perpendicular to the growth increments (e.g., along the columella; see S16) and to use methods that limit depth averaging, especially if the three‐dimensional growth of the gastropod is not well understood. Techniques that would potentially enable researchers to reach these daily to subdaily resolutions include Laser Ablation ICP‐MS analyses on small spot sizes (<20 μm diameter, or rectangular spots oriented parallel to the increments, e.g., Warter & Müller, 2017; de Winter et al, 2020), synchrotron‐radiation XRF measurements (<10 μm spot sizes; Yoshimura et al, 2013) or Electron Microprobe Analyses (spot size ~1 μm; Freitas et al, 2009) for trace element analyses, and (nano)SIMS for in situ stable isotope analyses (spot sizes <10 μm; Sano et al, 2012).…”

Paleoceanography of the Late Cretaceous northwestern Tethys Ocean: Seasonal upwelling or steady thermocline?

“…resectus bivalve grew with a weekly periodicity and formed its shell almost year-round. These two features are also usually observed in modern algae-bearing tridacnids [ 45 ] and in other supposedly photoautotrophic rudists [ 33 ].…”

“…It is highly unlikely that this stable isotope pattern resulted from kinetic fractionation effects, because rudists precipitated their shells in isotopic equilibrium with the ambient water [ 32 ]. Correlation between δ 13 C and δ 18 O has been observed in many rudist species [ 33 , 65 ] and is a typical feature observed in modern photosymbiont-bearing bivalves, e.g., tridacnids [ 45 , 46 ]. In photosymbiotic bivalves, such a trend results from the enhanced uptake of 12 C from the ambient water by the symbiotic algae during periods of high insolation and warm water conditions (i.e., more negative shell δ 18 O values) [ 45 ].…”

“…Late Cretaceous shallow water habitats were dominated by rudist bivalves, which majorly contributed to the formation of peri-Tethyan deposits [ 28 ]. Rudist shells, especially those of the elevator morphotypes [ 29 ], have been extensively used in sclerochronological studies [ 22 , 30 – 33 ]. To estimate deeper sub-annual water temperatures, inoceramid shells represent a promising proxy archive.…”

“…In a previous study, this correlation was interpreted as a sign for kinetic isotope effects, which prompted the authors to assume disequilibrium fractionation for the rudist species Torreites sanchezii [ 32 ]. More recent structure morphological and sclerochronological evidences, however, strongly indicate that such stable isotopic patterns derived from a photosynthetic lifestyle [ 33 , 44 ]. It is known that symbiotic algal activity can strongly affect the δ 13 C signal of shell [ 45 ].…”