A lively controversy still exists between different authors dealing with the timing of northern Tethyan platform drowning and the Early Aptian oceanic anoxic event (OAE 1a). To the present day, there is no consensus if the OAE 1a black shales must be attributed to the Deshayesites weissi or the Deshayesites deshayesi zone (see discussion in Moreno-Bedmar et al., 2009). OAE 1a black shale deposition has been traditionally attributed to the Deshayesites weissi zone (Gradstein et al., 2004). Despite this disagreement about the biostratigraphic timing, several authors postulate a relation between biotic perturbations and environmental changes linked to OAE 1a, e. g. the disappearance of coral-rudist reefs related with the demise of the northern Tethyan Urgonian platforms in the Helvetic Alps (Weissert et al., 1998; Follmi et al., 2008). In the central and southern Tethyan realm (Istria, Oman), OAE 1a is likely expressed as the transient mass occurrence of microencrusters (Lithocodium-Bacinella) and the coeval demise of the characteristic mid-Cretaceous framework-builders (rudists, corals). Chemostratigraphic data indicate that these microbial blooms coincide with the Deshayesites weissi zone (Huck et al., 2010, Rameil et al, 2010). These observations raise the question whether northern Tethyan platform drowning is coeval to microbial bloom periods in the central and southern Tethys? The analysis of all available literature and unpublished evidence demonstrates that well constrained age data are surprisingly scarce and controversial. The goal of the present research project is to compile a chemostratigraphic framework for the northern Tethyan platform drowning (Haute-Savoie, SE France) in order to shed light on the temporal constraints of platform drowning versus pelagic black shale deposition versus microbial blooms.
Abstract In the aftermath of major Phanerozoic biocrises, diverse metazoan-dominated reef ecosystems were commonly replaced by microbial carbonate-producing communities. Apart from the loss of metazoan competitors, the factors causing pervasive microbial carbonate production in shallow-water platform settings are not completely understood. Amongst others, outstanding warm temperatures coupled with low-oxygen waters were proposed as possible triggers. This study focuses on late Aptian shallow marine carbonates deposited on the Apennine carbonate platform (ACP) in the central Tethys. By establishing an integrated high-resolution chemostratigraphic framework for two sections of the ACP, the coeval onset of pervasive bacinelloid growth is discovered, indicating a platform-wide shift from a metazoan-dominated ecosystem to microbial carbonate production. The initial phase of microbial proliferation coincides with the final stage of the so-called late Aptian “cold snap” and the subsequent temperature increase, which was paralleled by a significant sea-level rise. Our results contrast with observations from the early Aptian Oceanic Anoxic Event 1a, where a similar shift toward microbial “bacinelloid” carbonate production has been linked to exceptionally warm conditions and hypoxia.
Terrestrial and marine sedimentary archives covering the Valanginian interval (136.8-133.9 Ma, Ogg et al., 2004) display a distinct positive delta13C-isotope excursion (CIE) of ∼2.5 permil (Lini et al., 1992; Grocke et al., 2005). The carbon isotope shift spans ∼2.0 Ma and has been interpreted to reflect severe perturbations of the Early Cretaceous carbon cycle and paleoenvironmental conditions. According to different authors, the Valanginian CIE was accompanied by enhanced volcanic activity of the Parana-Etendeka large igneous flood basalts, enhanced pCO2 (Lini et al., 1992; Weissert et al., 1998), widespread biocalcification crisis (Erba et al., 2004) and a distinct climatic cooling as evidenced by ice-rafted debris and glendonites from high-latitude sites. In addition, the positive CIE was assigned to be the result of an anoxic event, named the Weissert OAE (Erba et al., 2004). In this study, we investigate the composition and distribution of sedimentary organic matter (OM) deposited in a hemipelagic setting before, during, and after the Valanginian CIE. The aim of this study is to provide a detailed view on possible changes in OM deposition during a time of major paleoenvironmental and climatic stress. The chosen approach combines sedimentological and chemostratigraphical information (delta13Ccarb) with geochemical analysis of the bulk OM (incl. TOC, C/N, delta13Corg, Rock-Eval) and biomarker data. For this study, hemipelagic deposits located in the basinal part of the Vocontian Trough (SE France) covering the late Valanginian to early Hauterivian (Campylotoxus Zone to Radiatus Zone) (Greselle 2007) have been sampled on a high resolution (sampling spacing of ∼2/m). A total of three sections has been logged (La Charce, Vergol, Morenas), which consist of hemipelagic marl-limestone alternations and which allow for the construction of a composite succession. The delta13Ccarb values range between ∼0.1 and 2.7 permil and show a characteristic stratigraphic trend typical for this time interval, including a prominent positive CIE. The high-resolution delta13Ccarb record allows for detailed correlation and comparison with existing chemostratigraphic records across this event. TOC values fluctuate between 0.20 and 4.05%, Rock-Eval pyrolysis results depict HI values of 134 to 383 mgHC/g TOC and OI values of 19 to 160 mg CO2/g TOC indicating the predominance of marine OM with only minor terrestrial inputs in all investigated samples. The aliphatic fraction of the OM extractable by organic solvents is dominated by n-alkanes, isoprenoids, and a variety of hopanes and steranes. No distinct changes during the CIE in the abundances of biomarkers specific for Dinoflagellates and methanotrophic bacteria are observed, pointing to no significant response of the marine biota in this basin to the carbon cycle perturbation. There is no indication for an anoxic water column during the CIE. Steranes show slightly enhanced values for the plateau phase of the excursion, and increasing values during the decline of the delta13C shift (e.g. dinosterane) what may just as well be due to the cooling episode or a change in sea-level (Melinte and Mutterlose, 2001). At this stage, the detailed analysis of the sedimentary OM does not provide evidence for the existence of an OAE or enhanced accumulation/preservation of OM associated with the Valanginian CIE. These findings point to paleoenvironmental changes on continents rather than in marine settings as causes for the isotope shift.
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