Phosphorus and Carbon burial during Cretaceous oceanic anoxic events, links with climatic changes

Titre du projet

Description

The main goal of this project is get a better understanding of mechanisms, which triggered Cretaceous oceanic anoxic events (OAE), more particularly, the paleoclimatic conditions which characterized the onset of major anoxic events and their evolution. Our recent work demonstrated that the onset of the Cenomanian-Turonian OAE was triggered by a short-lived but significant increase in phosphorus burial. Then bottom waters became anoxic and switched from being a P sink to a P source, sustaining the productivity in a positive feedback loop. Increased aridity, removal of atmospheric CO2 by burial as organic carbon in black shales and reduction in nutrient availability may have been factors bringing about the return to more oxic oceans. The key point is therefore the climatic fluctuations that triggered the onset and the termination of OAEs and these are still poorly understood. These climatic changes appear to be linked to huge volcanic activity (LIPS) and may paradoxically be responsible for both onset and termination of OAEs, by first increasing greenhouse conditions by large CO2 release and secondly by decreasing temperature due to increased weathering, which leads to a drop in CO2. Such a scenario first needs to be tested on several OAE’s in different geographic regions and at different geologic times. We propose to investigate 2 Cretaceous OAE events, at the Cenomanian-Turonian and Coniacian-Santonian transitions, which present different levels of anoxia, from moderate and gradual (Coniacian-Santonian), to intense and abrupt (Cenomanian-Turonian). Specifically, our study will focus on bulk and clay mineralogy, phosphorus and its speciation coupled to organic carbon accumulation (TOC), stable oxygen and carbon isotopes, Sr/Ca, Mg/Ca trace-elements and high-resolution quantitative micropaleontology and palynology to evaluate biotic effects of anoxia, changes in climate, primary productivity and volcanic activity. The sections to be analyzed will be selected in different paleogeographic areas at different paleodepths. Obtained P and C burial data will be compared with our data on the Cenomanian-Turonian OAE. To resume, the present proposal will try to answer two crucial questions concerning the origin of the OAEs: What triggered the P Accumulation at the onset of the ?13C shift of OAE2 and: is P MARs showing the same distribution pattern in other OAEs (e.g. during the Coniacian-Santonien OAE events). In recent years, a renewed effort to understand greenhouse conditions, global biogeochemical cycles, and feedback mechanisms, has focused on the Cretaceous anoxic events as a natural laboratory of the past and analog for the future greenhouse warming. It is hoped that understanding the greenhouse world climatic evolution which led to such events, and implies increase in atmospheric CO2, changes in productivity, and biotic survival and extinction rates, will help predict negative consequences of future greenhouse warming due to anthropogenic emissions of CO2.

Chercheur principal

Adatte, Thierry

Foellmi, Karl

Statut

Completed

Date de début

1 Juillet 2008

Date de fin

30 Juin 2011

Chercheurs

Matera, Virginie

Bomou, Brahimsamba