Turbidite‐induced re‐oxygenation episodes of the sediment‐water interface in a diverticulum of the Tethys Ocean during the Oceanic Anoxic Event 1a: The French Vocontian Basin

Abstract

Widespread anoxic events affected the Tethys Ocean during the Mesozoic. The Ocean Anoxic Event 1a (Early Aptian), expressed as the Selli Level or Goguel Level (GL) in European basins. The GL was deposited in the French Vocontian Basin, a semi-enclosed basin connected to the Tethys Ocean. This study presents an integrated approach (Rock Eval, clay minerals, grain-size, inorganic geochemistry and molecular biomarkers), applied to four sections in the basin distributed along a proximal-distal transect. This study shows that the GL was perturbed by turbidites.In particular, the results demonstrate: (a) a homogeneous redox status of the basin that experienced oxic to suboxic conditions, according to trace element distributions and (b) low organic matter contents (total organic carbon ca 1 wt%) in the three sections where turbiditic deposits are observed. The distal, condensed section exhibits the highest organic matter contents (total organic carbon >3 wt%). In addition, the presence of gammacerane and isorenieratene derivatives in the distal sections suggests that the water-column was intermittently stratified, with hypoxia developing in the photic zone. This stratification did not result from strong surface productivity but more likely from: (a) limited renewal of deep water in the basin; (b) reputedly high surface-water palaeotemperatures during the Early Aptian; and (c) the influx of waters, possibly depleted in free oxygen and in some dissolved trace elements, into the basin. The turbiditic inputs, in addition to organic matter dilution in the sediments and a brief rupture of the water-column stratification in the proximal areas of the basin, ventilated the sea floor and more specifically re-oxidized the sedimentwater interface as well as underlying sediments. Such episodes of benthic re-oxygenation could have altered the long-term palaeoredox record, even in the distal sections where reducing conditions prevailed during deposition. In the area deprived of turbiditic input, sedimentary condensation, coupled with low oxygen conditions, furthered organic matter preservation and concentration.

Assessing controls on organic matter enrichments in hemipelagic marls of the Aptian-Lower Albian Blue Marls of the Vocontian Basin (France): an unexpected variability observed from multiple “organic-rich” levels

“…The Goguel, Kilian, Niveau Noir and Fallot levels and hemipelagites show high average contents of I-S, ranging from 32 to 42%. In these groups of samples, average smectite contents are fluctuating from 10% in the Niveau Noir Interval to 30% in the hemipelagites, while the Goguel Level is devoid of smectite in the Sauzeries section (Table S6; this is expected, the section being in the deeply buried part of the basin; see Caillaud et al, 2020). The clay assemblages of the Jacob and Paquier levels are dominated by smectite (44 and 42% on average, respectively), with significant I-S contents (24 and 28%).…”

“…This OL records a firstly negative and then positive d 13 C excursion in the Vocontian Basin (Herrle et al, 2004;Westermann et al, 2013), as is the case for the OAE1a in the Tethyan basins (Menegatti et al, 1998). The Goguel Level contains a total organic carbon (TOC) of 1 to 5 wt% with hydrogen index (HI) values up to 500 mgHC/gTOC, which corresponds to Type-II kerogens, dominated by marine algal-bacterial OM (Bréhéret, 1994(Bréhéret, , 1997Heimhofer et al, 2004Heimhofer et al, , 2006Westermann et al, 2013;Ando et al, 2017;Caillaud et al, 2020). This level was the time-equivalent of the OAE1a (Bréhéret, 1994(Bréhéret, , 1997 and, therefore, it has many equivalent levels in other basins: the Livello Selli in Italian basins, the Fischschiefer in the Lower Saxony Basin, and some other black shales in the Tethyan basins and Atlantic and Pacific Oceans (Föllmi, 2012).…”

“…The Goguel Level The Goguel Level is made up with six horizons of dark, marly, laminated shales (GO 1 to GO 6) amounting to a thickness of about 3.4 m in the Sauzeries section, with each horizon being multi-cm to multi-dm thick (Bréhéret, 1997). However, the laminated horizons are often interbedded with thin turbidites in numerous locations (Friès, 1987;Bréhéret, 1997;Friès and Parize, 2003;Caillaud et al, 2020). For example, the Goguel Level reaches a thickness of 17 m in the Saint-Jaume section, where it is interbedded with thin turbidites and one slump.…”

“…In the Tethyan realm, the deposition of the black shales was accounted for by intense surface-water productivity at the onset of the OAE1a (Weissert, 1990;Föllmi, 1995;Sanfourche and Baudin, 2001;Westermann et al, 2013). In the Vocontian Basin, the model was more specific: the Goguel Level was deposited during a period of sedimentary condensation (Bréhéret, 1997;Heimhofer et al, 2006) in a stratified basin (Caillaud et al, 2020), but, the efficient preservation of OM was depending on the possible occurrence of thin turbidites. Therefore, where thin turbidites were interbedded within the Goguel Level, they favored dilution and remineralization of OM, whereas where the Goguel Level was deposited without turbidites, the concentration and preservation of OM was furthered (see Caillaud et al, 2020 for details).…”

“…Some levels correspond to oceanic anoxic events (OAE1a or OAE1b; Schlanger and Jenkyns, 1976;Arthur et al, 1990;Jenkyns, 2010). Numerous studies have been devoted to these levels (Friès, 1987;Rubino, 1989;Tribovillard, 1989;Tribovillard and Gorin, 1991;Bréhéret, 1994Bréhéret, , 1997Friedrich et al, 2003;Friès and Parize, 2003;Herrle et al, 2003Herrle et al, , 2004Herrle et al, , 2010Heimhofer et al, 2004Heimhofer et al, , 2006Stein et al, 2011;Westermann et al, 2013;Ando et al, 2017;Ferry, 2017;Caillaud et al, 2020). In this paper, we wish to draw up an inventory of the factors involved in the deposition of these dark levels and their enrichment in OM.…”