The late Hauterivian Faraoni oceanic anoxic event in the western Tethys: Evidence from phosphorus burial rates

Abstract

In the uppermost Hauterivian sediments of the western Tethys, a short-lived anoxic event (Faraoni event) is documented both in the form of an interval enriched in organic matter (pelagic realm) and in a condensed interval enriched in glauconite and phosphate (shelf realm). This latter interval represents the onset of a drowning episode on the Helvetic carbonate platform along the northern tethyan margin that lasted throughout the early Barremian. This drowning episode marks a turning point in the way the platform carbonate factory functioned: during the Hauterivian carbonate production was dominated by heterozoans, whereas during the late Barremian a photozoan assemblage developed that is preserved in the so-called Urgonian limestone. The late Hauterivian Faraoni oceanic anoxic event is of particular interest because it is not accompanied by a major positive shift in y 13 C unlike other oceanic anoxic events during the Cretaceous (Valanginian, early Aptian, Cenomanian-Turonian boundary).We have analyzed four (hemi-)pelagic sections with regards to their phosphorus content to better understand the palaeoceanographic conditions related to this anoxic event and the associated changes in the shallow-water carbonate factory. The sections are located in Angles (SE France), Fiume-Bosso and Gorgo a Cerbara (central Italy), and Veveyse de Châtel-St. Denis (west Switzerland). We calculated phosphorus mass accumulation rates by using a cyclostratigraphic approach in order to obtain an adequate age model. We observe a comparable and correlatable long-term trend for the four sections, which suggests that the phosphorus mass accumulation rates and temporal changes therein are representative for the western tethyan pelagic realm. The Faraoni event is marked by a minimum in phosphorus accumulation and a positive shift in the C org /P tot ratios, which is interpreted as a reflection of the decreased capacity of storing and preserving phosphorus in oxygen-depleted sediments. Moreover, the onset in the decrease in phosphorus accumulation coincides with a sea level rise, while the Faraoni level itself corresponds to a maximum flooding interval. This phase of sea-level rise may have been important in the establishment of marine connections between the boreal and tethyan realms and, as such, in the exchange of nutrient-enriched waters. The model for the origin of the Faraoni oceanic anoxic event proposed here incorporates these aspects together with a positive feedback loop generated by phosphorus regeneration and a negative feedback loop related to changes in the ocean oxygen cycle.The subsequent long-term changes in phosphorus burial rates during the Barremian suggest that the Faraoni event marks the onset of a long period of environmental instability with regards to platform growth, leading to periodic phases of eutrophication * Corresponding author. Tel.: +41 32 718 26 05; fax: +41 32 718 26 01.E-mail address: stephane.bodin@unine.ch (S. Bodin).and drowning of the northern tethyan carbonate platform. This environmental cr...

Biostratigraphy, sedimentology and sequence stratigraphy of the latest Hauterivian – Early Barremian drowning episode of the Northern Tethyan margin (Altmann Member, Helvetic nappes, Switzerland)

“…Consequently, the sequences Ha6 to Ba1 are lacking in these sections and/or condensed within the basal phosphatized hard-170 S. Bodin et al ground. A notable point is the fact that the Ba2 sequence lacks evidence for a LST deposit in all studied sections, as well as in the basinal section of Angles (e.g., Arnaud, 2005;Bodin et al 2006). This is compatible with the here-proposed hypothesis that the highly-condensed basal phosphatized hardground is representative of a type 3 SB.…”

Platform-induced clay-mineral fractionation along a northern Tethyan basin-platform transect: implications for the interpretation of Early Cretaceous climate change (Late Hauterivian-Early Aptian)

“…8). The absence of kaolinite and the dominance of smectite in most of the Hauterivian sediments may reflect a true climatic signal, which is in agreement with the type of carbonate platform that developed at that time, under high nutrient conditions (e.g., van de Schootbrugge et al, 2003;Bodin et al, 2006a). During the latest Hauterivian, the appearance of kaolinite indicates a change from a seasonally contrasted towards a more humid and warm (intertropical) climate that can be linked to an acceleration of the hydrologic cycle, and with that an increase in continental run-off.…”

“…During the latest Hauterivian, the appearance of kaolinite indicates a change from a seasonally contrasted towards a more humid and warm (intertropical) climate that can be linked to an acceleration of the hydrologic cycle, and with that an increase in continental run-off. The related enhanced supply of phosphorus and of Dissolved Inorganic Carbon (DIC) to the ocean may have been triggered by intensified biogeochemical weathering, as already suggested by Bodin et al (2006a) and Godet et al (2006). The latter linked the very stable evolution of the d 13 C record observed from the base of the P. mortilleti to the top of the C. darsi ammonite zone to an increase of the oceanic DIC volume in the ocean, which may have buffered the d 13 C signal against paleoceanographic change (see Bartley and Kah (2004) for a detailed explanation of the DIC buffering effect on the d 13 C record).…”

Evidence for oxic conditions during oceanic anoxic event 2 in the northern Tethyan pelagic realm

“…Total P analysis was performed on bulk-rock samples, following the procedure described in Bodin et al (2006). The concentration of PO 4 , expressed in ppm, is obtained by calibration with known standard solutions, using an UV/Vis photospectrometer (Perking Elmer UV/Vis Photospectrometer Lambda 10, l ¼ 865 nm) with a mean precision of 5%.…”