Improving shallow‐water carbonate chemostratigraphy by means of rudist bivalve sclerochemistry

Abstract

Deep-time shallow-marine carbonate platforms record distinct biotic responses to climatic and environmental stressors. Unfortunately, precise temporal assignment of these biotic responses is often problematical due to poor biostratigraphic control and/or a significant diagenetic overprint of the neritic bulk carbonate chemostratigraphic inventory. An accurate stratigraphic framework is essential to better understand the causal relation between biotic events recorded by carbonate platforms and environmental changes that, for instance, culminated in mass extinction events or prolonged episodes of oceanic anoxia. Here we provide an integrated carbon and strontium-isotope stratigraphy of the Early Cretaceous subtropical Provence carbonate platform in SE France that is based solely on pristine low-Mg calcite from rudist bivalves. Carbon-isotope data of geochemically screened rudist fragments enabled reconstruction of a characteristic Barremian pattern including the Mid-Barremian Event (MBE) that allowed for a precise correlation with stratigraphically well-constrained Tethyan shallow-water and hemipelagic reference sections. In order to evaluate ontogenetic carbon-isotope changes and the overall variability of the shell-derived carbon-isotope data, numerous sclerochronological carbon-isotope profiles of individual large rudist shells are presented. Strontium-isotope stratigraphy supports the carbon-isotopebased age of the studied sections, but also provides unequivocal evidence for a major hiatus in the depositional record covering large parts of the Late Barremian. In contrast to biostratigraphic and bulk carbonate chemostratigraphic archives, the here established chronostratigraphy of carbonate platform evolution in the southern Provence region demonstrates a twofold resurgence of rudist-rich carbonate platform production during the Early Aptian and arguably the latest Early Aptian.

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

“…A cross plot in Figure a shows that this correlation is moderate in strength ( R 2 = 0.47) and highly statistically significant ( p ≪ 0.05). Note that positive correlations between δ 13 C and δ 18 O records have been put forward by previous authors as evidence for disequilibrium fractionation and signs of diagenetic alteration (e.g., Huck & Heimhofer, ; McConnaughey, ; Steuber, ). However, the evidence cited above (see also Figure ) shows that stable isotope samples originated from the most pristine parts of the T. sanchezi outer shell layer, rendering diagenetic alteration unlikely.…”

“…Since the rudists became extinct at the Cretaceous‐Paleogene boundary (Steuber et al, ), no close living relatives are available for comparison. As a result, even though rudist taxa have featured in several sclerochronology and geochemistry studies (e.g., Al‐Aasm & Veizer, , ; Gili et al, ; Gili & Götz, ; Huck & Heimhofer, ; Ross & Skelton, ; Skelton, ; Steuber, , ; Steuber et al, ), several aspects of their paleobiology remain uncertain. This uncertainty limits the potential of rudist shells as geochemical archives for climate and environmental change during the Cretaceous.…”

Disentangling shallow‐water bulk carbonate carbon isotope archives with evidence for multi‐stage diagenesis: An in‐depth component‐specific petrographic and geochemical study from Oman (mid‐Cretaceous)

“…). Although the elemental and isotopic composition of biogenic hard parts (Immenhauser et al ., ; Schöne, ) are known to severely depend on metabolic processes, previous studies have shown that bivalve shells might act as a suitable archive for mid‐Cretaceous secular carbon isotope changes (Huck et al ., ; Huck & Heimhofer, ). Unfortunately, the petrographic inspection of bivalve shell fragments derived from the Jabal Madar section provides clear evidence for a moderate recrystallization and partial silicification of the fibrous low‐Mg calcite ultrastructure.…”

“…10. Carbon isotope based correlation of the 'filtered' Jabal Madar record with age-calibrated neritic (Cluses) and basinal (Angles, Gorgo a Cerbara) reference records Sprovieri et al, 2006;Huck et al, 2011;Stein et al, 2011;Huck & Heimhofer, 2015). Note extremely low Barremian carbonate preservation rates as calculated from the Jabal Madar section.…”

Late Jurassic to Cretaceous evolution of the eastern Tethyan Hawasina Basin (Oman Mountains)

“…). The strontium concentration between 9·8 m and 60·5 m varies between 1000 and 1200 ppm (Table ) which is typical for biogenic low‐Mg calcite from rudist shells of the same age (Steuber & Veizer, ; Huck et al ., ; Huck & Heimhofer, ). The uppermost 20 m of the lower Huwar 2 Member (75 to 95 m) consist of micritic limestone with low strontium concentrations of 500 to 700 ppm.…”