Life's Influence on the Sedimentary Record: The Interplay of Ocean Chemistry, Circulation, and the Biological Pump

Open Access
Author:
Hotinski, Roberta Michelle
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 28, 2000
Committee Members:
  • Hiroshi Ohmoto, Committee Member
  • Lee Kump, Committee Chair
  • Michael Allan Arthur, Committee Member
  • Raymond Gabriel Najjar Jr., Committee Member
Keywords:
  • ocean stagnation
  • Pethei
  • Permian
  • Precambrian
  • carbon isotopes
  • anoxia
  • biogeochemical cycles
Abstract:
In the modern ocean, the distributions of a variety of chemical elements are controlled by a combination of physical circulation and the metabolic activities of marine organisms. While mixing acts to homogenize ocean chemistry, chemical gradients in oxygen, phosphate, and carbon isotopes are created through a process known as "biological pumping," the transfer of elements from surface to deep waters via the production, sinking, and decomposition of organic matter. Similar processes must have been at work in ancient oceans, but chemical patterns of certain time periods preserved in the geologic record are very different than today's, prompting many authors to suggest different operation of either biological pumping or circulation during those time intervals. Accurate interpretation of ancient chemical patterns, however, requires disentangling the competing effects of biology, physics and changes in bulk ocean composition, which is difficult through intuition alone. This study utilizes observation and numerical models to explore what combination of factors best explains chemical signals preserved in the geologic record and how the biological pump has operated through earth history. For the Paleoproterozoic, the efficacy of biological pumping is evaluated through a combination of field data and numerical modeling. Samples of carbonate cements collected from the Pethei Platform, a 1.9 Ga stromatolitic reef, reveal a substantially reduced carbon isotope gradient with depth relative to the modern gradient, 0.5 versus 2‰. Such a small gradient would conventionally be interpreted as indicating a weak biological pump, but steady-state simulations using a two-box model of the Paleoproterozoic ocean suggest that the lack of a carbon isotope gradient may instead be due to high partial pressures of carbon dioxide in the Paleoproterozoic, which would increase the ocean's dissolved inorganic carbon content and damp the effects of biological pumping. A second box modeling study investigates the role of biological pumping in creating deepwater anoxia in the post-Carboniferous Phanerozoic. Previous work had suggested that anoxia might be the product of either increased or decreased thermohaline circulation. Using open-system 3- and 4-box models of ocean circulation and nutrient cycling, we show that different mechanisms may operate to create anoxia on long and short timescales, and that estimates of ocean oxygenation are sensitive to the circulation scheme and vertical resolution of box models. Overall, we find that changes in low-latitude productivity and thermohaline circulation interactively cause dysoxic to anoxic conditions on long timescales, while reduction of convective mixing induces rapid transitions to persistent dysoxia or anoxia in all simulations. Finally, the relative contributions of biological pumping and ocean circulation to Permian deepwater anoxia were evaluated using a general circulation model of the ocean coupled with a biogeochemical model of phosphate and oxygen cycling. Previous authors had postulated that stagnation of ocean circulation led to deepwater anoxia in this time period. We find that climatic warming consistent with Permian paleoclimatological evidence reduces ocean circulation rates and lowers deepwater oxygen concentrations to anoxic levels. Although the competing factors controlling ocean chemistry make extracting a causal mechanism for ancient chemical patterns complicated, these numerical experiments demonstrate the utility of numerical models in deconvolving the competing influences of marine biological activity, circulation, and ocean composition. Our results suggest that large differences in chemical patterns among geologic time periods do not require cessation of either productivity or circulation, and that the sedimentary record of ocean chemistry is consistent with the presence of an active biological pump throughout earth history.