Nitrogen biogeochemistry and ancient oceanic anoxia

Open Access
Author:
Junium, Christopher Kendall
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 06, 2010
Committee Members:
  • Michael Allan Arthur, Dissertation Advisor
  • Michael Allan Arthur, Committee Chair
  • Katherine Haines Freeman, Committee Member
  • Lee Kump, Committee Member
  • Jason Philip Kaye, Committee Member
Keywords:
  • nitrogen cycle
  • nitrogen isotopes
  • black shale
  • porphyrins
Abstract:
This study is an exploration of the links between nitrogen biogeochemistry and ancient oceanic anoxia. The goal of this dissertation is to answer the question: Is enhanced N2-fixation a necessary response to widespread oceanic anoxia? Understanding the N-cycle is important because N is one of the primary nutrients limiting carbon fixation on Earth. Over geologic time scales, N availability, along with that of P and Fe, impacts the regulation of atmospheric CO2 and climate through the limitation of carbon fixation by photoautotrophs in the oceans and on land. This work focuses on understanding the geologic record of the nitrogen cycle during episodes of ancient oceanic oxygen deprivation during the mid-Cretaceous and Neoproterozoic, and the processes controlling the preservation of N-cycle proxies in Holocene surface sediments of the Peru Margin. Under anoxic conditions, nutrient N is lost from the ocean through microbial metabolic processes but P is more efficiently recycled (Ingall and Janke, 1993). It could be envisioned that intervals of more widespread marine anoxia would significantly impact the balance of the marine nutrient cycles, affecting biological productivity. As geoscientists, we provide a unique perspective that can help answer some of the most important questions regarding the evolution of the N-cycle through time, the biological evolution of the earth, and the potential impacts of natural and anthropogenic climate change. To assess the state of the ancient nitrogen cycle I have focused on the isolation and N-isotopic analysis of chlorophyll derivatives (e.g. porphyrins and chlorins), and bulk organic extracts. Utilization of porphyrins and chlorins for compound-specific nitrogen isotope analysis requires an in-depth analysis of the processes that control their transformation and preservation over geologic time and in modern environments. A significant proportion of this work focuses on the abundances and distribution of porphyrins and chlorins in addition to N-isotopic analysis. In this study, initial investigation focused on the preserved chlorophyll derivatives of the Cretaceous strata recovered from the Demerara Rise. This work yielded unexpected discoveries of high abundances of bicycloalkanoporphyrins (BiCAPs), present as free bases (metal free) and Zn and VO2+ complexes. The occurrence of Zn bicycloalkanoporphyrins represents the first occurrence of primary Zn porphyrins found in the geologic record. Structural confirmation of the chlorin mesochlorophyllone in the Demerara Rise black shales represents the oldest such occurrence in the geologic record by over 70 million years; its presence suggests that the abundant bicycloalkanoporphyrins in the Demerara Rise sediments are derived from chlorophyll a, the only possible precursor for mesochlorophyllone. The stratigraphic distribution of BiCAPS is controlled, foremost, by metal availability in the water column and sediments rather than early diagenesis Eh/pH conditions, or post depositional thermal maturity. Titration of the local water-column metal reservoir by sulfide during Oceanic Anoxic Event II (OAE II) resulted in high concentrations of FB BiCAPs and very low concentrations of metallo-BiCAPs. The highest total concentrations of porphyrins are found where metal concentrations are highest, suggesting that porphyrin preservation is enhanced by the increased stability that results from formation of metal complexes. Paradoxically, the total concentration of porphyrins is lowest during the heart of OAE II, in an interval of higher TOC where enhanced organic matter preservation would be expected; this may be the result of decreased preservation of tetrapyrroles in the absence of the stabilizing effect of metals. The nitrogen isotopic composition of BiCAPs confirms that the δ15N of dissolved inorganic nitrogen becomes 15N-depleted probably in response to expanded nitrogen fixation during Oceanic Anoxic Event II. These data support a strong spatial and temporal link between nitrogen fixation and loss of nutrient nitrogen via suboxic metabolisms. I have also found that the δ15N values of the three porphyrins are systematically different despite a common chlorophyll source; the origin of this difference is related to nitrogen isotopic effects associated with the formation of metal complexes. These results demonstrate that direct reconstruction of primary phototroph biomass from porphyrins can be misleading without a full assessment of the δ15N of the range of structures present in ancient sediments. Analysis of the δ15N record of bulk sediments and co-occurring chlorins from Peru Margin surface sediments demonstrates that downslope transport and degradation of organic matter results in an isotopic depletion of bulk sedimentary nitrogen. Despite an order of magnitude decrease in the sedimentary concentration of chlorins downslope, their δ15N values remain constant, demonstrating that chlorin degradation causes no significant nitrogen isotopic effects. These data suggest that studies that utilize bulk δ15N for paleoceanographic studies in dynamic environments need to account for possible diagenetic effects even in low oxygen settings. The factors controlling carbon burial in the Neoproterozoic are illustrated by the range of processes associated with the deposition of the Kwagunt Formation sediments. Shallow epicratonic rift basins associated with the break-up of Rodinia may have been may have been significant depocenters for burial of organic carbon and aiding in the drawdown of CO2 prior to the Snowball Earth glaciations. Microbial mat communities played an integral role in this process by providing efficient burial of carbon in shallow environments. The δ15N record does not confirm the presence of a euxinic deep ocean during the mid-Neoproterozoic but it suggests that the range of nutrient regimes inferred by δ15N record can be put into the context of modern of modern processes.