Mid-Cretaceous paleoenvironmental changes in the western Tethys

Cinzia Bottini,Elisabetta Erba

doi:10.5194/cp-14-1147-2018

Abstract

Abstract. We present a continuous record of surface water temperature and fertility variations through the latest Barremian–Cenomanian interval (ca. 27 Myr) based on calcareous nannofossil abundances from the western Tethys. The nannofossil temperature index, calibrated with TEX86 sea surface temperatures, suggests that warmest (34–36 ∘C) conditions were reached during oceanic anoxic event (OAE) 1a onset, the Aptian–Albian boundary interval hyperthermals (113, Kilian level and Urbino level OAE 1b) and during a ca. 4 Myr long phase in the middle Albian. Coolest temperatures (29 ∘C) correspond instead to the late Aptian. Generally warm conditions characterized the Albian followed by a progressive cooling trend that started in the latest Albian (at the Marne a Fucoidi–Scaglia Bianca Formation transition). Temperate conditions occurred in the Cenomanian with frequent short-term variations highlighted by abundance peaks of the cold-water nannofossil species E. floralis and R. parvidentatum. Mid-Cretaceous surface water fertility was rather fluctuating and mostly independent from climatic conditions as well as from black shales intervals. Intense warming and fertility spikes were systematically associated only with black shales of OAE 1a and of the Aptian–Albian boundary hyperthermals. The Albian–Cenomanian rhythmic black shales are, in fact, associated with varying long-term climatic and fertility conditions. The similarity of western Tethys climatic and fertility fluctuations during OAE 1a, OAE 1b, the middle Albian and OAE 1d with nannofossil-based records from other basins indicated that these paleoenvironmental conditions were affecting the oceans at supra-regional to global scale.

Highlights

The mid-Cretaceous was generally characterized by a warm climate with prevailing super-greenhouse conditions, weak latitudinal temperature gradients and the absence of ice sheets (e.g., Clarke and Jenkyns 1999; Föllmi, 2012; Friedrich et al, 2012; Bodin et al, 2015; O’Brien et al, 2017 and reference therein)
In all the three sites nannofossil assemblages are characterized by relatively high abundances (> 40 %) of Watznaueria barnesiae (Figs. 2, 3 and 4) possibly indicative of heavily altered samples (e.g., Thierstein and Roth, 1991), we exclude the possibility that nannofossil abundances were controlled by diagenesis since dissolution-prone species (e.g., D. rotatorius, B. constans, Z. erectus) are relatively abundant and there is not a systematic correlation between lithology and nannofossil abundance and/or preservation
A description of the major trends of the total nannofossil abundance and of the relative abundance of the taxa used for the calculation of the temperature index (TI) and nutrient index (NI) is given for the three investigated sections

Summary

Introduction

The mid-Cretaceous was generally characterized by a warm climate with prevailing super-greenhouse conditions, weak latitudinal temperature gradients and the absence of ice sheets (e.g., Clarke and Jenkyns 1999; Föllmi, 2012; Friedrich et al, 2012; Bodin et al, 2015; O’Brien et al, 2017 and reference therein). The majority of paleoenvironmental studies focused on OAEs and little is known about the intervals between these extreme events. This implies that, despite SST compilations (e.g., Clarke and Jenkyns, 1999; Friedrich et al, 2012; O’Brien et al, 2017), a continuous long-term reconstruction of paleoecological and paleoclimatic conditions over the Aptian–early Turonian interval is not available

Methods

Results

Discussion

Conclusion