Mid-Cretaceous Paleoenvironmental Changes in the Western Tethys
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Clim. Past, 14, 1147–1163, 2018 https://doi.org/10.5194/cp-14-1147-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Mid-Cretaceous paleoenvironmental changes in the western Tethys Cinzia Bottini and Elisabetta Erba Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, 20133 Milan, Italy Correspondence: Cinzia Bottini ([email protected]) Received: 22 March 2018 – Discussion started: 27 March 2018 Revised: 24 July 2018 – Accepted: 27 July 2018 – Published: 10 August 2018 Abstract. We present a continuous record of surface wa- 1 Introduction ter temperature and fertility variations through the latest Barremian–Cenomanian interval (ca. 27 Myr) based on cal- careous nannofossil abundances from the western Tethys. The mid-Cretaceous was generally characterized by a The nannofossil temperature index, calibrated with TEX86 warm climate with prevailing super-greenhouse conditions, sea surface temperatures, suggests that warmest (34–36 ◦C) weak latitudinal temperature gradients and the absence of conditions were reached during oceanic anoxic event (OAE) ice sheets (e.g., Clarke and Jenkyns 1999; Föllmi, 2012; 1a onset, the Aptian–Albian boundary interval hyperthermals Friedrich et al., 2012; Bodin et al., 2015; O’Brien et al., (113, Kilian level and Urbino level OAE 1b) and during a 2017 and reference therein). Compilation of available sea 18 ca. 4 Myr long phase in the middle Albian. Coolest tempera- surface temperatures (SSTs) based on δ O and TEX86 re- tures (29 ◦C) correspond instead to the late Aptian. Generally calibrated by O’Brien et al. (2017) suggests SSTs between 25 ◦ warm conditions characterized the Albian followed by a pro- and 40 C through the Aptian–Turonian interval at low lati- gressive cooling trend that started in the latest Albian (at the tudinal sites with maximum temperatures reached during the Marne a Fucoidi–Scaglia Bianca Formation transition). Tem- latest Cenomanian–Turonian “thermal maximum”. However, perate conditions occurred in the Cenomanian with frequent the studies performed over the last decades have demon- short-term variations highlighted by abundance peaks of the strated that the mid-Cretaceous was not continuously char- cold-water nannofossil species E. floralis and R. parvidenta- acterized by greenhouse conditions since cooling interludes tum. Mid-Cretaceous surface water fertility was rather fluc- were identified (e.g., Kemper, 1987; Hochuli et al., 1999; tuating and mostly independent from climatic conditions as Price, 1999; Herrle and Mutterlose, 2003; Mutterlose et al., well as from black shales intervals. Intense warming and fer- 2009; McAnena et al., 2013; Millán et al., 2014; Bottini et al., tility spikes were systematically associated only with black 2015). Moreover, the ocean–atmosphere system experienced shales of OAE 1a and of the Aptian–Albian boundary hy- phases of transient but sometimes prolonged paleoenviron- perthermals. The Albian–Cenomanian rhythmic black shales mental perturbation in relation to widespread ocean anoxia, are, in fact, associated with varying long-term climatic and known as oceanic anoxic events (OAEs), which included the fertility conditions. The similarity of western Tethys climatic early Aptian OAE 1a, the early Albian OAE 1b, the latest Al- and fertility fluctuations during OAE 1a, OAE 1b, the mid- bian OAE 1d and the latest Cenomanian OAE 2 (see Weissert dle Albian and OAE 1d with nannofossil-based records from and Erba 2004 and Jenkyns, 2010, for a review). The major- other basins indicated that these paleoenvironmental condi- ity of paleoenvironmental studies focused on OAEs and little tions were affecting the oceans at supra-regional to global is known about the intervals between these extreme events. scale. 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 in- terval is not available. Published by Copernicus Publications on behalf of the European Geosciences Union. 1148 C. Bottini and E. Erba: Mid-Cretaceous paleoenvironmental changes Important information regarding paleoenvironmental con- gree? (3) How does the nannofossil temperature index cor- ditions can be gathered from nannofossils since calcare- relate with other independent proxies of paleotemperature? ous nannoplankton is extremely sensitive to changes in sur- (4) Are there any limitations in using calcareous nannofossils face water chemical and physical conditions. Specifically, as paleoenvironmental tracers? (5) Is there a systematic cor- decades of studies performed on calcareous nannofossils im- relation between black shales and high or low fertility and/or proved the knowledge about the paleoecology affinity of temperature? some species (e.g., Roth and Krumbach, 1986; Bralower, 1988; Premoli Silva et al., 1989a, b; Watkins, 1989; Coccioni 2 Material and methods et al., 1992; Erba et al., 1992; Erba, 1994; Herrle and Mut- terlose, 2003; Herrle et al., 2003a; Tiraboschi et al., 2009), 2.1 Studied sites which were also used to calculate temperature and nutri- ent indices (e.g., Herrle et al., 2003a, b; Bornemann et al., In this work we investigated three nearby sections of the 2005; Watkins et al., 2005; Browning and Watkins 2008; Umbria–Marche basin (Fig. 1): (1) the Monte Petrano sec- Tiraboschi et al., 2009; Pauly et al., 2012; Mutterlose and tion, spanning the pre-OAE 1d to the early Turonian interval, Bottini, 2013; Bottini et al., 2015; Kanungo et al., 2018) over (2) Le Brecce, covering the OAE 1d, and (3) the Furlo sec- specific intervals of the Cretaceous. tions through the late Cenomanian–early Turonian interval. The sedimentary successions of the Umbria–Marche basin The sections were selected to cover coeval intervals in order are ideal for long-term studies of paleoenvironmental condi- to discriminate any effect of selective diagenesis from pri- tions since they represent continuous pelagic records through mary paleoecological signals. the mid-Cretaceous in the Tethys Ocean. The Aptian–Albian The studied sequence comprises the Scaglia Bianca For- interval corresponds to the Marne a Fucoidi Formation char- mation that lies above the Marne a Fucoidi Formation (lower acterized by varicolored marlstones and marly limestones Aptian–upper Albian) and is followed by the Scaglia Rossa with black shales deposited in the early Aptian (Selli level Formation (lower Turonian–middle Eocene). Four members OAE 1a), late Aptian “113 level”, “Kilian level” at the were distinguished within the Scaglia Bianca and described Aptian–Albian boundary and early Albian (Urbino level by Coccioni and Galeotti (2003). OAE 1b) (e.g., Coccioni et al., 1987, 1989; Erba, 1988, The Furlo section, 30 m thick, is located in an abandoned 1992a; Erba et al., 1989). Over the middle–late Albian, the quarry (Beaudoin et al., 1996, Mitchell et al., 2008, Lanci et Umbria–Marche basin was characterized by the deposition al., 2010; Gambacorta et al., 2015) in the Furlo Gorge area, of rhythmic black shale (Herbert and Fischer, 1986; Pre- 25 km southeast of Urbino (Fig. 1). The section includes the moli Silva et al., 1989b; Erba, 1992a; Galeotti et al., 2003; MCE I and OAE 2. The Monte Petrano section covers 80.5 m Tiraboschi et al., 2009). The sedimentation changed in the and is located along a dirt road not far from the village of late Albian as represented by the Scaglia Bianca Formation Moria (Schwarzacher, 1994; Giorgioni et al., 2012; Gamba- consisting of whitish pelagic limestones with chert nodules, corta et al., 2014; Fig. 1); it includes the OAE 1d, MCE I bands and radiolarian layers. The Scaglia Bianca Formation and OAE 2. The Le Brecce section, 20 m thick, is located includes (i) the Pialli level (Coccioni, 2001; Gambacorta et 3 km west of the village of Piobbico at the km 34 point on al., 2015), which is the sedimentary expression of OAE 1d, the state road 257-Apecchiese, close to the Piobbico drill site (ii) rhythmic black shale and chert layers correlating with the (Tiraboschi et al., 2009; Gambacorta et al., 2015; Fig. 1). The mid-Cenomanian event I (MCE I) (Coccioni and Galeotti, section covers the interval encompassing OAE 1d, and a fault 2003; Gambacorta et al., 2015), and (iii) a ∼ 1 m thick in- is present above the OAE 1d interval at ca. 16.5 m (Gabriele terval of black shales and radiolarian-rich sands representing Gambacorta, personal communication, 2018). the Bonarelli level (Bonarelli, 1891), which is the regional The stratigraphic framework for the investigated sections sedimentary expression of the latest Cenomanian OAE 2. is based on integrated lithostratigraphy and carbon-isotope Here, we present new quantitative nannofossil data for the stratigraphy calibrated with calcareous nannofossil bios- western Tethys Ocean (Umbria–Marche basin, Italy) used to tratigraphy presented in Gambacorta et al. (2015). Three pos- derive surface water temperature and fertility during the late itive excursions in the δ13C record identify the OAE 1d, Albian–early Turonian. The new dataset is integrated with MCE I and OAE 2. Black shale layers of the Pialli level the nannofossil data previously collected from the same area (at Le Brecce and Monte Petrano) are restricted to the lower for the latest Barremian–late Albian time interval (Tiraboschi part of the OAE 1d carbon-isotope excursion. The MCE I (at et al., 2009; Bottini et al., 2015) to gain a long-term