Starting with the work of Ben Houlton, we have collaborated with the group of Lars Hedin in the Department of Ecology and Evolutionary Biology in isotope studies of terrestrial N cycling. We have also 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.
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16 Publications
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Triple oxygen isotope analysis of nitrate using the denitrifier method and thermal decomposition of N2O
Interaction with ozone transfers its anomalous (non-mass-dependent) 17O enrichment to atmospheric nitrogen oxides and nitrate. The 17O anomaly (Δ17O) in nitrate can be used to identify atmospheric nitrate inputs into terrestrial and aquatic environments as well as to study the role of ozone in the atmosphere s reactive nitrogen cycle. We report…
Dinitrogen fixation in the world s oceans
The surface water of the marine environment has traditionally been viewed as a nitrogen (N) limited habitat, and this has guided the development of conceptual biogeochemical models focusing largely on the reservoir of nitrate as the critical source of N to sustain primary productivity. However, selected groups of Bacteria, including cyanobacteria,…
The contributions of nitrate uptake and efflux to isotope fractionation during algal nitrate assimilation
In order to strengthen environmental application of nitrate N and O isotopes, we measured the N and O isotopic fractionation associated with cellular nitrate uptake and efflux in the nitrate-assimilating marine diatom Thalassiosira weissflogii. We isolated nitrate uptake and efflux from nitrate reduction by growing the cells in the presence of…
Eukaryotic assimilatory nitrate reductase fractionates N and O isotopes with a ratio near unity
In order to (i) establish the biological systematics necessary to interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate (15N/ 14N and 18O/16O) in the environment and (ii) investigate the potential for isotopes to elucidate the mechanism of a key N cycle enzyme, we measured the nitrate N and O isotope effects ( 15ε and 18ε) for nitrate…
Relationship of nitrogen isotope fractionation to phytoplankton size and iron availability during the Southern Ocean Iron RElease Experiment (SOIREE)
The 15N composition of sediments has been used as a proxy for nitrate utilization in surface waters to assess the role of Southern Ocean export production in glacial/interglacial changes in atmospheric CO2 concentration. Interpretation has relied on a temporally constant isotope effect (ε) associated with uptake and assimilation of nitrate by…
Nitrogen isotope evidence for expanded ocean suboxia in the early Cenozoic
The million-year variability of the marine nitrogen cycle is poorly understood. Before 57 million years (Ma) ago, the 15N/14N ratio (d15N) of foraminifera shell-bound organic matter from three sediment cores was high, indicating expanded water column suboxia and denitrification. Between 57 and 50 Ma ago, d15N declined by 13 to 16 per mil in the…
Cenozoic megatooth sharks occupied extremely high trophic positions
A Seasonal Model of Nitrogen Isotopes in the Ice Age Antarctic Zone: Support for Weakening of the Southern Ocean Upper Overturning Cell
In the Antarctic Zone of the Southern Ocean, the coupled observations of elevated diatom-bound 15 N/ 14 N (δ 15 N db ) and reduced export production during the ice ages indicates more complete nitrate (NO 3 − ) consumption. This evidence points to an ice age decline in gross NO 3 − supply from the deep ocean to the surface wind-mixed layer, which…
Enzyme-level interconversion of nitrate and nitrite in the fall mixed layer of the Antarctic Ocean
In the Southern Ocean, the nitrogen (N) isotopes of organic matter and the N and oxygen (O) isotopes of nitrate (NO3 −) have been used to investigate NO3 − assimilation and N cycling in the summertime period of phytoplankton growth, both today and in the past. However, recent studies indicate the significance of processes in other seasons for…
The distinct nitrogen isotopic compositions of low and high molecular weight marine DON
To constrain the sources and cycling of bulk and size-fractionated marine dissolved organic nitrogen (DON), samples were collected for concentration and isotopic (δ 15N) analysis from the western tropical North Atlantic Ocean as well as over the North Australian shelf. Bulk DON concentrations are typically between 4 and 6μM, with low molecular…
Interbasin isotopic correspondence between upper-ocean bulk DON and subsurface nitrate and its implications for marine nitrogen cycling
Measurements to date have shown that both bulk and high molecular weight marine dissolved organic nitrogen (DON) have a 15N/ 14N that is substantially higher than the 15N/ 14N of suspended particulate organic nitrogen (PN susp) found in the same surface waters (with δ 15N of ∼4 to 5‰ and ∼-1 to 1‰, respectively). Moreover, the concentration and…
The flux and isotopic composition of reduced and total nitrogen in Bermuda rain
The concentration and 15N/14N ratio of total nitrogen (TN) were measured in precipitation samples collected at Bermuda between January and December 2000. By correcting for nitrate, analyzed previously, the concentration and δ15N of "reduced" N (RN, i.e., ammonium + organic N) were also determined. The TN precipitation flux (∼ 10-19 mmol N m- 2 yr-…
Nitrate isotopic composition between Bermuda and Puerto Rico: Implications for N2 fixation in the Atlantic Ocean
N and O isotope analyses of water column nitrate between Bermuda and Puerto Rico document a bolus of low-δ15N nitrate throughout the Sargasso Sea thermocline, which we attribute primarily to the input of recently fixed N. Although previous work suggests southward increases in N2 fixation and ventilation age, no meridional trend in nitrate δ15N is…
N isotopic composition of dissolved organic nitrogen and nitrate at the Bermuda Atlantic Time-series study site
To better constrain the dynamics of the dissolved organic nitrogen (DON) pool and the role of N2 fixation in the nitrogen cycle at the Bermuda Atlantic Time-series Study (BATS) site, we measured the 15N/14N ratio of total nongaseous nitrogen (TN) in the upper 250 m and of nitrate in the upper 1000 m of monthly water column profiles from June 2000…
Reduced isotope fractionation by denitrification under conditions relevant to the ocean
Experiments with two well-studied denitrifiers and one recently isolated marine suboxic zone denitrifier show that the cellular-level denitrification N isotope effect ( 15ε) is typically lower than the canonical value of ∼25‰ under many conditions prevalent in the ocean. Across all three strains, 15ε is 10-15‰ at cellular nitrate reduction rates…
North Atlantic ventilation of southern-sourced deep water in the glacial ocean
One potential mechanism for lowering atmospheric CO2 during glacial times is an increase in the fraction of the global ocean ventilated by the North Atlantic, which produces deep water with a low concentration of unused nutrients and thus drives the ocean s biological pump to a high efficiency. However, the data indicate that during glacial times,…