Life's influence on the sedimentary record: The interplay of ocean chemistry, circulation, and the biological pump

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.” 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. 1.9 Ga carbonate cements reveal a substantially reduced carbon isotope gradient with depth relative to the modern gradient, 0.5 versus 2‰. Simulations using a two-box model of the Paleoproterozoic ocean suggest that the small gradient may be due to high levels of atmospheric carbon dioxide in the Paleoproterozoic, which would have damped the effects of biological pumping.; A second box modeling study investigates the role of biological pumping in creating deepwater anoxia. 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.; 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. We find that climatic warming consistent with Permian paleoclimatological evidence reduces ocean circulation rates and lowers deepwater oxygen concentrations to anoxic levels.; 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.