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Introduction

In 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 exceedingly difficult to isolate and quantify to the point of developing a predictive understanding of how the fluxes interact. Much of my research has involved the development of subject-specific approaches to these broad challenges. With regard to the first, I have advanced the use of the isotopic composition of dissolved nitrogen species (nitrate and dissolved organic nitrogen in particular) to provide integrative constraints on nitrogen cycle processes. With regard to the second, I am among those who 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, I seek to derive information from the nitrogen isotopic composition of organic matter bound within sedimentary microfossils. 


My core scientific interest is the role of plant nutrients in the interaction between life and the environment. Two sets of questions, focused on the ocean, have most centrally motivated my 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 nitrogen cycling, 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 fertility of the ocean, and atmospheric CO2?

In addition, recent collaborations with colleagues and students have broadened my research activities beyond the marine environment, involving me in studies of the terrestrial and atmospheric N cycles as well.


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

 

Isotope method development

We have developed new methods for natural abundance isotope ratio measurement of several biologically available and typically dissolved forms of N that are common in the environment: the 15N/14N, 18O/16O, and 18O/17O/16O of nitrate, the 15N/14N of total dissolved N (i.e. dissolved organic N in waters lacking nitrate and ammonium), and the 15N/14N of ammonium. For paleoceanographic work, we developed a new technique for the 15N/14N of diatom microfossil-bound N. All of these techniques have as their cornerstone the “denitrifier” method for nitrate isotopic analysis, in which nitrate (NO3-) is converted to nitrous oxide (N2O) gas by a strain of denitrifying bacteria that lacks an active N2O reductase, followed by analysis of the product N2O with a stable isotope ratio mass spectrometer.


Kaiser, J., et al. (in press), Triple oxygen isotope analysis of nitrate using the denitrifier method and thermal decomposition of N2O, Anal Chem.

Granger, J., et al. (2006), A method for nitrite removal in nitrate N and O isotope analyses, Limnol Oceanogr-Meth, 4, 205-212.

Knapp, A. N., et al. (2005), N isotopic composition of dissolved organic nitrogen and nitrate at the Bermuda Atlantic time-series study site, Global Biogeochem Cy, 19, 10.1029/2004GB002320.

Robinson, R. S., et al. (2004), Revisiting nutrient utilization in the glacial Antarctic: Evidence from a new method for diatom-bound N isotopic analysis, Paleoceanography, 19, 10.1029/2003PA000996.

Casciotti, K. L., et al. (2002), Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method, Anal. Chem., 74, 4905-4912.

Sigman, D. M., et al. (2001), A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater, Anal Chem, 73, 4145-4153.

Holmes, R. M., et al. (1998), Measuring 15N-NH4+ in marine, estuarine and fresh waters: An adaptation of the ammonia diffusion method for samples with low ammonium concentrations, Mar Chem, 60, 235-243.

Sigman, D. M., et al. (1997), Natural abundance-level measurement of the nitrogen isotopic composition of oceanic nitrate: an adaptation of the ammonia diffusion method, Mar Chem, 57, 227-242.

 

Laboratory studies of isotope discrimination

The utility of isotopic distributions in the environment is premised on knowledge of the magnitudes of isotope discrimination by individual biogeochemical reactions, which is most often gained through lab studies of cultured organisms. Our current focus is on the isotope effects of nitrate-consuming processes, in particular, nitrate assimilation by photosynthetic organisms (central to studies of nutrient supply and uptake in the surface ocean) and denitrification by heterotrophic bacteria (central to studies of the global ocean’s input/output budget of fixed N). Most recently, graduate student Kristen Karsh has taken up this work.

Granger, J., et al. (2004), Coupled nitrogen and oxygen isotope fractionation of nitrate during assimilation by cultures of marine phytoplankton, Limnol Oceanogr, 49, 1763-1773.

Needoba, J. A., et al. (2004), The mechanism of isotope fractionation during algal nitrate assimilation as illuminated by the 15N/14N of intracellular nitrate, J Phycol, 40, 517-522.

Casciotti, K. L., et al. (2003), Linking diversity and stable isotope fractionation in ammonia-oxidizing bacteria, Geomicrobiol J, 20, 335-353.

 

Studies in the modern ocean

In projects carried out in various regions of the open ocean and in previously well-studied isolated basins, we are developing and applying the N and O isotope ratios of nitrate and dissolved organic N as integrative signals of spatially and temporally variable processes. The goal is to gain insight into both the input/output budget of biologically available N in the ocean and the internal cycling of this nutrient.

Difiore, P. J., et al. (2006), Nitrogen isotope constraints on subantarctic biogeochemistry, J Geophys Res-Oceans, 111, -.

Knapp, A. N., et al. (2005), N isotopic composition of dissolved organic nitrogen and nitrate at the Bermuda Atlantic time-series study site, Global Biogeochem Cy, 19, 10.1029/2004GB002320.

Lehmann, M. F., et al. (2005), Origin of the deep Bering Sea nitrate deficit: Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate fluxes, Global Biogeochem Cy, 19, -.

Sigman, D. M., et al. (2005), Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin, Global Biogeochem Cy, 19, -.

Lehmann, M. F., et al. (2004), Coupling the 15N/14N and 18O/16O of nitrate as a constraint on benthic nitrogen cycling, Mar Chem, 88, 1-20.

Thunell, R. C., et al. (2004), Nitrogen isotope dynamics of the Cariaco Basin, Venezuela, Global Biogeochem Cy, 18, 10.1029/2003GB002185.

Karsh, K. L., et al. (2003), Relationship of nitrogen isotope fractionation to phytoplankton size and iron availability during the Southern Ocean Iron RElease Experiment (SOIREE), Limnol Oceanogr, 48, 1058-1068.

Lourey, M. J., et al. (2003), Sensitivity of d15N of nitrate, surface suspended and deep sinking particulate nitrogen to seasonal nitrate depletion in the Southern Ocean, Global Biogeochem Cy, 17, 10.1029/2002GB001973.

Karl, D., et al. (2002), Dinitrogen fixation in the world’s oceans, Biogeochemistry, 57/58, 47-98.

Pantoja, S., et al. (2002), Stable isotope constraints on the nitrogen cycle of the Mediterranean Sea water column, Deep-Sea Res Pt I, 49, 1609-1621.

Sigman, D. M., and K. L. Casciotti (2001), Nitrogen isotopes in the ocean, in Encyclopedia of Ocean Sciences, edited by J. H. Steele, et al., pp. 1884-1894, Academic Press, London.

Sigman, D. M., et al. (2001), A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater, Anal Chem, 73, 4145-4153.

Sigman, D. M., et al. (2000), The d15N of nitrate in the Southern Ocean: Nitrogen cycling and circulation in the ocean interior, J Geophys Res-Oceans, 105, 19599-19614.

Altabet, M. A., et al. (1999), The nitrogen isotope biogeochemistry of sinking particles from the margin of the Eastern North Pacific, Deep-Sea Res Pt I, 46, 655-679.

Sigman, D. M., et al. (1999b), The d15N of nitrate in the Southern Ocean: Consumption of nitrate in surface waters, Global Biogeochem Cy, 13, 1149-1166.

 

Studies in the terrestrial biosphere, the atmosphere, and ice cores

Starting with the work of Ben Houlton, I have collaborated with Lars Hedin of Ecology and Evolutionary Biology at Princeton in isotope studies of terrestrial N cycling. My involvement in this topic is encouraged by the excellence of my Princeton colleagues in terrestrial ecology and the opportunities for cross-fertilization in studies of ocean and land. With recent Princeton Ph.D. Meredith Hastings, I worked on the nitrate N and O isotopes in rain, snow, and ice as tracers of reactive nitrogen sources and processing in the modern and ancient atmosphere. Jan Kaiser, a recent postdoc with Michael Bender, also did work in this area while at Princeton.

Hastings, M. G., et al. (2005), Glacial/interglacial changes in the isotopes of nitrate from the Greenland Ice Sheet Project 2 (GISP2) ice core, Global Biogeochem Cy, 19, -.

^Houlton, B. Z. (2005), Isotopic Evidence for the Climate Dependence of Nitrogen Cycles Across Old Tropical Rainforests, Mt. Haleakala, Hawaii, Ph.D. thesis, 152 pp, Princeton University, Princeton, New Jersey.

Hastings, M. G., et al. (2004), Seasonal variations in N and O isotopes of nitrate in snow at Summit, Greenland: Implications for the study of nitrate in snow and ice cores, J Geophys Res-Atmos, 109, 10.1029/2004JD004991.

Hastings, M. G., et al. (2003), Isotopic evidence for source changes of nitrate in rain at Bermuda, J Geophys Res-Atmos, 108, 10.1029/2003JD003789.

Carrillo, J. H., et al. (2002), Atmospheric deposition of inorganic and organic nitrogen and base cations in Hawaii, Global Biogeochem Cy., 16, 10.1029/2002GB001892.

 

Paleoceanographic studies

In the area of paleoceanographic and paleoclimate measurements, my group’s activities are varied, much of it in collaboration with my close colleague Gerald Haug and his group at GFZ Potsdam. A particular analytical focus of mine is the isotopic composition of organic N bound within microfossils as a tool to reconstruct past ocean changes. Work at Princeton in this area began with recent postdoc Rebecca Robinson Graham and continues with graduate students Brigitte Brunelle and Abby Ren.

Brunelle, B. G., et al. (in press), Evidence from diatom-bound nitrogen isotopes for Subarctic Pacific stratification during the last ice age and a link to North Pacific denitrification changes, Paleoceanography.

Yancheva, G., et al. (in press), Influence of the Intertropical Convergence Zone on the East Asian Monsoon, Nature.

Haug, G. H., et al. (2005), North Pacific seasonality and the glaciation of North America 2.7 million years ago, Nature, 433, 821-825.

Jaccard, S. L., et al. (2005), Glacial/interglacial changes in subarctic North Pacific stratification, Science, 308, 1003-1006.

Robinson, R. S., et al. (2005), Diatom-bound 15N/14N: New support for enhanced nutrient consumption in the ice age Subantarctic, Paleoceanography, 20, 10.1029/2004PA001114.

Robinson, R. S., et al. (2004), Revisiting nutrient utilization in the glacial Antarctic: Evidence from a new method for diatom-bound N isotopic analysis, Paleoceanography, 19, 10.1029/2003PA000996.

Sigman, D. M., et al. (2004), Polar ocean stratification in a cold climate, Nature, 428, 59-63.

Haug, G. H., et al. (2003), Climate and the collapse of Maya civilization, Science, 299, 1731-1735.

Sigman, D. M., and G. H. Haug (2003), Biological Pump in the Past, in Treatise On Geochemistry V.6: The Oceans and Marine Geochemistry, edited by H. Elderfield, et al., pp. 491-528, Elsevier Pergamon, Oxford.

Brzezinski, M. A., et al. (2002), A switch from Si(OH)4 to NO3- depletion in the glacial Southern Ocean, Geophys Res Lett, 29, 10.1029/2001GL014349.

Haug, G. H., et al. (2001), Southward migration of the intertropical convergence zone through the Holocene, Science, 293, 1304-1308.

Sigman, D. M., and E. A. Boyle (2000), Glacial/interglacial variations in atmospheric carbon dioxide, Nature, 407, 859-869.

Haug, G. H., et al. (1999), Onset of permanent stratification in the subarctic Pacific Ocean, Nature, 401, 779-782.

Sigman, D. M., et al. (1999a), The isotopic composition of diatom-bound nitrogen in Southern Ocean sediments, Paleoceanography, 14, 118-134.

Haug, G. H., et al. (1998), Glacial/interglacial variations in production and nitrogen fixation in the Cariaco Basin during the last 580 kyr, Paleoceanography, 13, 427-432.

Francois, R., et al. (1997), Contribution of Southern Ocean surface-water stratification to low atmospheric CO2 concentrations during the last glacial period, Nature, 389, 929-935.

 

Model studies of past changes in the geochemistry and physics of the ocean

As a rule, I work to interpret the data generated in my lab as quantitatively as possible. In our paleoceanographic work, my collaborators and I have used geochemical box models. By collaborating with GFDL and AOS researchers (in particular, recent AOS graduate student Curtis Deutsch and recent postdoc Agatha de Boer), I have begun to be involved in the use of more complex numerical ocean/atmosphere models. Graduate student Peter DiFiore is also undertaking such work.

de Boer, A. M., et al. (in press), The effect of global ocean temperature change on deep ocean ventilation, Paleoceanography.

Deutsch, C., et al. (2004), Isotopic constraints on glacial/interglacial changes in the oceanic nitrogen budget, Global Biogeochem Cy, 18, 10.1029/2003GB002189.

Sigman, D. M., et al. (2004), Polar ocean stratification in a cold climate, Nature, 428, 59-63.

Sigman, D. M., and G. H. Haug (2003), Biological Pump in the Past, in Treatise On Geochemistry V.6: The Oceans and Marine Geochemistry, edited by H. Elderfield, et al., pp. 491-528, Elsevier Pergamon, Oxford.

Sigman, D. M., et al. (2003), Evaluating mechanisms of nutrient depletion and 13C enrichment in the intermediate-depth Atlantic during the last ice age, Paleoceanography, 18, 10.1029/2002PA000818.

Sigman, D. M., and E. A. Boyle (2001), Palaeoceanography - Antarctic stratification and glacial CO2 - Sigman and Boyle reply, Nature, 412, 606-606.

Sigman, D. M., and E. A. Boyle (2000), Glacial/interglacial variations in atmospheric carbon dioxide, Nature, 407, 859-869.

Hughen, K. A., et al. (1998), Deglacial changes in ocean circulation from an extended radiocarbon calibration, Nature, 391, 65-68.

Sigman, D. M., et al. (1998), The calcite lysocline as a constraint on glacial/interglacial low-latitude production changes, Global Biogeochem Cy, 12, 409-427.