Introduction to Research

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n order to develop a predictive understanding of the Earth’s environment, new disciplines have arisen that seek to be quantitative in both measurement and theory. Biogeochemistry represents one such discipline, in which environmental processes are generalized and abstracted in terms of underlying chemistry and elemental mass balance. Two fundamental challenges confront biogeochemistry and its related disciplines. First, the environment is spatially and temporally complex, obscuring the integrated rates of processes that transform and transport biologically important chemicals. Second, the sensitivities of these “biogeochemical fluxes” are difficult to isolate and quantify to the point of developing a predictive understanding of how the fluxes interact. Much of our research has involved the development of subject-specific approaches to these broad challenges. With regard to the first, we have advanced the use of the isotopic composition of dissolved nitrogen species (nitrate and dissolved organic nitrogen in particular) to provide integrative constraints on modern nitrogen cycle processes. With regard to the second, we treat the marine sediment record as an archive of natural experiments from which the underlying controls on the physical and biogeochemical fluxes of the ocean can be determined; in particular, we seek to derive information from the nitrogen isotopic composition of organic matter bound within sedimentary microfossils.

Figure 1
The modern ocean (a, b) and a Southern Ocean-based hypothesis for reduced levels of atmospheric CO2 during glacial times (c, d). The figure shows a generalized depth section running from north to south (a, c) with an expanded view of the Southern Ocean (b,d). From Sigman and Boyle [2000].

A core scientific interest is the role of plant nutrients in the interaction between life and the environment. The following sets of questions, focused on the ocean, have most centrally motivated our work:

The polar oceans are special domains in the ocean where the “major nutrients” nitrogen and phosphorus are not completely consumed by algal growth. What factors control the physical conditions and nutrient status of the polar surface ocean? Over the ice age/interglacial cycles of the last 3 million years, how have the characteristics of the polar ocean affected other regions of the ocean, atmospheric carbon dioxide, and climate?

Focusing on the nitrogen cycle, what terms and rates compose the budget of “fixed” (biologically available) N in the modern ocean? What are the sensitivities of the different inputs and outputs? How have the components of the N budget changed over climate cycles, and how have these changes affected the size of ocean’s fixed N reservoir, the biological fertility of the ocean, and atmospheric CO2?

In the modern low latitude ocean (such as the Sargasso Sea of the North Atlantic), what are the sources of N to phytoplankton, which phytoplankton use which forms of N, and how do physical conditions affect the supply rates of these different N forms? Given these connections, how should low latitude ocean biological fertility respond to climate change, and with what consequences?

In addition, collaborations have broadened our research activities beyond the marine environment, including studies of the terrestrial and atmospheric N cycles.

Our research activities are summarized here under the following categories: (1) isotope method development, (2) laboratory studies of isotope discrimination, (3) studies in the modern ocean, (4) paleoceanographic studies, and (5) model studies of past changes in the geochemistry and physics of the ocean, (6) studies in the atmosphere and terrestrial biosphere. This body of work represents our effort to progress from the introduction of new measurements to the development of the background information needed to make those measurements useful, their application to important questions, and finally a quantitative understanding of the findings in a broader environmental context.