Investigation of phosphorus cycle dynamics associated with organic carbon burial in modern (North Pacific) and ancient (Devonian and Cretaceous) marine systems: Strengths and limitations of sequentially extracted (SEDEX) phosphorus data.

摘要

Periods of time characterized by widespread accumulation of organic carbon (Corg) --rich deposits have been interpreted to form as a result of increased rates of marine primary production and oxygen-deficient conditions in bottom and pore waters. Yet, debates persist regarding the source of nutrients (in particular phosphorus (P)) to marine surface waters during such intervals. In this study I attempt to address the role of P-cycle dynamics in C-burial processes using a sequential extraction procedure (SEDEX). The main focus of the study is two intervals of the geologic past characterized by widespread deposition of Corg --rich facies.;The first section of this dissertation (Chapter 2) explores the hypothesis that SEDEX results may be biased by post-burial diagenesis of reactive P phases. Observations spanning a range of depositional environments and geologic ages do not support this hypothesis and thus it is concluded that SEDEX provides the best available analytical approach for generating interpretable data for sedimentary P in the geologic past. The second section of this dissertation includes two chapters that examine the role of P cycling on Corg burial during the Cenomanian--Turonian (C--T) Oceanic Anoxic Event (OAE) 2. Chapter 3 focuses on SEDEX results and supporting geochemical observations from a transect of sites that sample epicontinental to oceanic depositional environments. Chapter 4 describes modeling experiment designed to evaluate the roles of terrestrial input and nutrient recycling on the marine P budget during the C--T interval which provides confirmation of the hypotheses that increased terrestrial input of P could initiate OAE-like conditions while P recycling could maintain nutrient fluxes during OAE 2. An observational study of the Late Devonian was designed to assess the roles of terrestrial nutrient input versus nutrient cycling is presented in Chapter 5. The results are similar those observed for OAE 2. The final chapter (6) provides evidence of transport of authigenic phosphate minerals to marine sediments from terrestrial sources. Such observations lead to questions regarding current residence time estimates of dissolved P in the modern ocean and interpretations of C/P ratios as a productivity and redox proxy in modern and ancient marine environments.