A high resolution mineralogical study (bulk-rock and clay-fraction) was
carried out upon the hemipelagic strata of the Angles section (Vocontian
Basin, SE France) in which the Valanginian positive C-isotope excursion
occurs. To investigate sea-level fluctuations and climate change
respectively, a Detrital Index (DI: (phyllosilicates and
quartz)/calcite) and a Weathering Index (WI: kaolinite/(illite +
chlorite)) were established and compared to second-order sea-level
fluctuations. In addition, the mineralogical data were compared with the
High Nutrient Index (HNI, based on calcareous nannofossil taxa) data
obtained by Duchamp-Alphonse et al. (2007), in order to assess the link
between the hydrolysis conditions recorded on the surrounding continents
and the trophic conditions inferred for the Vocontian Basin. It appears
that the mineralogical distribution along the northwestern Tethyan
margin is mainly influenced by sea-level changes during the Early
Valanginian (Pertransiens to Stephanophorus ammonite Zones) and by
climate variations from the late Early Valanginian to the base of the
Hauterivian (top of the Stephanophorus to the Radiatus ammonite Zones).
The sea-level fall observed in the Pertransiens ammonite Zone (Early
Valanginian) is well expressed by an increase in detrital inputs (an
increase in the DI) associated with a more proximal source and a
shallower marine environment, whereas the sea-level rise recorded in the
Stephanophorus ammonite Zone corresponds to a decrease in detrital
influx (a decrease in the DI) as the source becomes more distal and the
environment deeper. Interpretation of both DI and WI, indicates that the
positive C-isotope excursion (top of the Stephanophorus to the
Verrucosum ammonite Zones) is associated with an increase of detrital
inputs under a stable, warm and humid climate, probably related to
greenhouse conditions, the strongest hydrolysis conditions being reached
at the maximum of the positive C-isotope excursion. From the Verrucosum
ammonite Zone to the base of the Hauterivian (Radiatus ammonite Zone)
climatic conditions evolved from weak hydrolysis conditions and, most
likely, a cooler climate (resulting in a decrease in detrital inputs) to
a seasonal climate in which more humid seasons alternated with more arid
ones. The comparison of the WI to the HNI shows that the nutrification
recorded al: the Angles section from the top of the Stephanophorus to
the Radiatus ammonite Zones (including the positive C-isotope shift), is
associated with climatic changes in the source areas. At that time,
increased nutrient inputs were generally triggered by increased
weathering processes in the source areas due to acceleration in the
hydrological cycle under greenhouse conditions This scenario accords
with the widely questioned palaeoenvironmental model proposed by Lini et
al., (1992) and suggests that increasing greenhouse conditions are the
main factor that drove the palaeoenvironmental changes observed in the
hemipelagic realm of the Vocontian Basin, during the Valanginian
positive C-isotope shift. This high-resolution mineralogical study
highlights short-term climatic changes during the Valanginian, probably
associated to rapid changes in the C-cycle. Coeval Massive
Parana-Etendeka flood basalt eruptions may explain such rapid
perturbations. (C) 2011 Elsevier B.V. All rights reserved.