Abstract
AbstractWidespread 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 sediment‐water 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.
Highlights
The Tethys Ocean was affected by widespread anoxia as demonstrated by the presence of organic carbon enriched horizons found world-wide (Schlanger and Jenkyns, 1976; Jenkyns, 2010)
The Goguel Level (GL) was explored through four sections in the Vocontian Basin, using a multi-proxy approach: the organic geochemistry is used to determine the abundance and nature of organic matter (OM), and, in addition to field observations, the inorganic geochemistry, grain-size of the non-carbonate fraction and clay-mineral assemblage identification allow the depositional conditions prevailing during the Oceanic Anoxic Events (OAEs) 1a in this diverticulum of the Tethys Ocean to be defined
This study demonstrates that OAEs were not uniform but rather heterogeneous
Summary
The Tethys Ocean was affected by widespread anoxia as demonstrated by the presence of organic carbon enriched horizons found world-wide (Schlanger and Jenkyns, 1976; Jenkyns, 2010) These Oceanic Anoxic Events (OAEs) were catastrophic environmental issues because the absence of oxygen profoundly disturbed the whole marine ecosystem biota leading to mass extinctions. The aim of the study is to assess the role of turbiditic activity in the enrichment and preservation of organic matter (OM) For this purpose, the GL was explored through four sections in the Vocontian Basin, using a multi-proxy approach: the organic geochemistry is used to determine the abundance and nature of OM, and, in addition to field observations, the inorganic geochemistry, grain-size of the non-carbonate fraction and clay-mineral assemblage identification allow the depositional conditions prevailing during the OAE 1a in this diverticulum of the Tethys Ocean to be defined
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