Organic geochemical evidence links changes in marine microbes and phytoplankton to ammonite turnover in the Tethyan Middle Jurassic

Abstract

Carbon isotope-based chemostratigraphy is a well-established method for regional or even global correlations, especially at major stratigraphic boundaries which are often marked by global change events. However, background intervals remain less well studied and our knowledge is limited about the drivers of small-scale faunal turnovers which may be reflected in the zonal boundaries of biostratigraphic schemes. Moreover, organic geochemistry is much less used than carbon isotopes in chemostratigraphy. Here, we address these issues by a comprehensive stable isotope and organic geochemical study of three Tethyan Aalenian (Middle Jurassic) sections with good ammonite biostratigraphic control. We assess the linkages between zone-level faunal transitions and minor geochemical perturbations recorded by isotopic and biomarker indicators. A wide range of geochemical analyses (biomarkers, δ¹³C_SAT, δ¹³C_carb, δ¹⁸O_carb, and compound-specific isotope analyses (CSIA)) were performed on samples from three continuous Tethyan Aalenian–lowermost Bajocian sections in the Gerecse Mts. (Hungary) where ammonite zonal boundaries are well constrained. The organic matter is predominantly aquatic in origin with negligible terrigenous admixture. Most of the biostratigraphic boundaries coincide with minor negative Carbon Isotope Excursions in δ¹³C_SAT and phytoplankton-related CSIA. Curves of phytoplankton-related biomarker indicators, including diploptene, dehydroabietic acid methyl ester, and distribution of methylheptadecanes reveal marked changes within the phytoplankton communities at the transitions between ammonite zones and/or subzones. These biomarker results suggest that turnovers of the ammonite faunas were preceded by phytoplankton crises. Thus, biostratigraphic boundaries likely reflect minor environmental changes that first affected the primary producers, then trophic changes propagated through the entire marine food web.