Vignette d'image

Biogeochemical changes during the Cenomanian-Turonian Oceanic Anoxic Event (OAE 2)

Auteur(s)
Mort, Haydon
Editeur(s)
Date de parution
2006
Mots-clés
Résumé
Understanding how nutrients behave during sub-oxic environments is crucial in predicting future changes in the ocean-climate system. This study focuses on the role of phosphorus, in its different sedimentary reservoirs, during the Cenomanian-Turonian anoxic event (OAE 2), which occurred during the late Cretaceous (~93.5 Ma). In the geological record this period in the Earth’s history is represented by the widespread occurrence of sediments containing elevated organic matter content and a large positive 13C excursion. Within these lithologies are biological and geochemical signatures that indicate a significant depletion in the amount of oxygen present in the water column. Phosphorus is a macronutrient, used during primary productivity. After the life cycle of the organism, the subsequent destruction of its organic matter on the sea floor releases phosphate at the sediment-water interface. The phosphate can then follow a number of possible biogeochemical pathways, including its mineralization and sorption into authigenic minerals and oxyhydroxides. The path it follows is largely dependant on the concentration of oxygen in the surrounding water. During periods of oxygen deficiency the oxygen required for the degradation of organic matter is derived by reducing electron donors (e.g. MnO2, Fe(OH)3 and SO43-), which are also binding sites for the phosphorus in the water. The reduction in the number of binding sites causes the sediments retention ability for phosphorus to decrease resulting a net increase in the concentration of phosphate in the water column. This phosphate is then available again for primary producers, should it be recycled into the photic zone. By studying the behaviour of phosphorus we attempt infer switches in the mode of sediment accumulation and to consequently reconstruct the oxidation history of OAE 2. To do this a sequential extraction technique (SEDEX) is used to identify the various types (or phases) of phosphorus found in five exposures distributed across the Tethyan Realm (Europe and North Africa) and the Western Interior Seaway (North America). Other proxies are used to consider of regional influences in our data (e.g. 13C, mineralogy, hydrogen and oxygen indices, planktonic foraminiferal biostratigraphy). Phosphorus mass accumulation rates (P MARs) show a distinct trend, similar in each of the five sections. A generalised observation is as follows; (1) an peak in P MARs at the start of the increase in 13C values; (2) a sharp decrease in inorganic-P phases during the isotope excursion itself and a smaller reduction in organic-P. Two sections actually see an enrichment of organic-P during this time. The Corg/Preactive molar ratios (Redfield ratio) begin to increase at this point as does the quantity of organic matter in the sections; (3) A return to ‘background’ values at or just after the start of the isotope plateau. This is interpreted in the following way; (1) an environmental perturbation caused a rapid increase in the amount of phosphate in the ocean, which resulted in the initial increase in P MARs. At present, the most plausible it that increase was caused by a maximum flooding event that caused the rapid reworking of nutrients stored on previously dry land allowing them to settle on the bottom of the ocean but also to become bioavailable. (2) The decrease in inorganic-P was caused by the desorption of phosphate from oxyhydroxides and the reduction in phosphorus authigenesis. We suggest that this phosphate became recycled into the upper water column to further stimulate productivity, which, in-turn, consumed more O2 on the sea floor. Organic-P either increased later (as is seen in sections with a deep paleodepth) or did not decrease very much because of decelerated bacterial degradation of organic matter due to oxygen deficient conditions. The increase in organic matter content after P MARs start to decrease is seen as evidence to support this hypothesis (3) Pre-excursion MAR values are low considering how much phosphate was in the water column suggesting that phosphorus recycling was sustaining the 13C plateau. Intense and sustained primary productivity consumed bottom water oxygen but simultaneously increased atmospheric O2 concentrations. It is possible that atmospheric O2 eventually acted as a negative feedback when the partial pressure of free O2 in the atmosphere became greater than in the mixing layer of the ocean, starting the oxidation of nutrients in the ocean. We present tentative evidence for this with a P curve from an Egyptian section. The preservation of organic matter in black shales would have dramatically reduced the concentration of CO2 in the atmosphere, cooling the climate leading to a reduction in humidity, precipitation, and continent-ocean nutrient fluxes.
Notes
Thèse de doctorat : Université de Neuchâtel, 2006 ; 1933
Identifiants
https://libra.unine.ch/handle/123456789/17742
Type de publication
doctoral thesis
Dossier(s) à télécharger