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.
References
19 Publications
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Nitrate isotopic gradients in the North Atlantic Ocean and the nitrogen isotopic composition of sinking organic matter
Biogeochemical processes within the Atlantic basin alter the nitrogen (N) and oxygen (O) isotopic compositions (δ 15 N and δ 18 O) of nitrate before this nutrient is carried from the upper water column into the deep ocean by the formation of North Atlantic Deep Water (NADW). Here, nitrate δ 15 N and δ 18 O measurements along the 2013 CLIVAR/GO…
Tropical Dominance of N2 Fixation in the North Atlantic Ocean
To investigate the controls on N2 fixation and the role of the Atlantic in the global ocean s fixed nitrogen (N) budget, Atlantic N2 fixation is calculated by combining meridional nitrate fluxes across World Ocean Circulation Experiment sections with observed nitrate 15N/14N differences between northward and southward transported nitrate. N2…
Aerobic respiration along isopycnals leads to overestimation of the isotope effect of denitrification in the ocean water column
The nitrogen (N) isotopes provide an integrative geochemical tool for constraining the fixed N budget of the ocean. However, N isotope budgeting requires a robust estimate for the organism-scale nitrogen isotope effect of denitrification, in particular as it occurs in water column denitrification zones (εwcd). Ocean field data interpreted with the…
Nitrate isotope distributions on the US GEOTRACES North Atlantic cross-basin section: Signals of polar nitrate sources and low latitude nitrogen cycling
We report and interpret the nitrate δ15N and δ18O distributions in the cross-basin transect of the US GEOTRACES North Atlantic expedition. Lownitrate δ15N observed in the shallowthermocline of the centralwestern portion of the basin (reaching as lowas 2.5%vs. air; 2.3%lower than deep Atlantic nitrate) reflects the remineralization of nitrogen…
The Angola Gyre is a hotspot of dinitrogen fixation in the South Atlantic Ocean
Biological dinitrogen fixation is the major source of new nitrogen to marine systems and thus essential to the ocean’s biological pump. Constraining the distribution and global rate of dinitrogen fixation has proven challenging owing largely to uncertainty surrounding the controls thereon. Existing South Atlantic dinitrogen fixation rate…
The Agulhas Current Transports Signals of Local and Remote Indian Ocean Nitrogen Cycling
The greater Agulhas Current region is an important component of the climate system, yet its influence on carbon and nutrient cycling is poorly understood. Here, we use nitrate isotopes (δ15N, δ18O, Δ(15–18) = δ15N–δ18O) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating…
Instabilities Across the Agulhas Current Enhance Upward Nitrate Supply in the Southwest Subtropical Indian Ocean
The Agulhas Current, like other western boundary currents (WBCs), transports nutrients laterally from the tropics to the subtropics in a subsurface “nutrient stream.” These nutrients are predominantly supplied to surface waters by seasonal convective mixing, to fuel a brief period of productivity before phytoplankton become nutrient-limited…
Iron fertilization of the subantarctic ocean during the last ice age
John H. Martin, who discovered widespread iron limitation of ocean productivity, proposed that dust-borne iron fertilization of Southern Ocean phytoplankton caused the ice age reduction in atmospheric carbon dioxide (CO2). In a sediment core from the Subantarctic Atlantic, we measured foraminifera-bound nitrogen isotopes to reconstruct ice age…
Southern Ocean dust-climate coupling over the past four million years
Dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean. Indeed, dust supply to the Southern Ocean increases during ice ages, and iron fertilizationg of the subantarctic zone may have contributed up to 40-parts per million…
Laboratory Assessment of the Impact of Chemical Oxidation, Mineral Dissolution, and Heating on the Nitrogen Isotopic Composition of Fossil-Bound Organic Matter
Fossil-bound organic material holds great potential for the reconstruction of past changes in nitrogen (N) cycling. Here, with a series of laboratory experiments, we assess the potential effect of oxidative degradation, fossil dissolution, and thermal alteration on the fossil-bound N isotopic composition of different fossil types, including…
The GEOTRACES Intermediate Data Product 2014
The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as…
Effect of iron limitation on the isotopic composition of cellular and released fixed nitrogen in Azotobacter vinelandii
Most biological nitrogen transformations have characteristic kinetic isotope effects used to track these processes in modern and past environments. The isotopic fractionation associated with nitrogen fixation, the only biological source of fixed nitrogen (N), provides a particularly important constraint for studies of nitrogen cycling. Nitrogen…
Erratum: Deglacial pulses of deep-ocean silicate into the subtropical North Atlantic Ocean (Nature (2013) 495 (495-498) DOI: 10.1038/nature12006)
Deglacial pulses of deep-ocean silicate into the subtropical North Atlantic Ocean
Growing evidence suggests that the low atmospheric CO 2 concentration of the ice ages resulted from enhanced storage of CO 2 in the ocean interior, largely as a result of changes in the Southern Ocean. Early in the most recent deglaciation, a reduction in North Atlantic overturning circulation seems to have driven CO 2 release from the Southern…
Deglacial nitrogen isotope changes in the Gulf of Mexico: Evidence from bulk sedimentary and foraminifera-bound nitrogen in Orca Basin sediments
Constraining variations in marine N2-fixation over glacial-interglacial timescales is crucial for determining the role of the marine nitrogen cycle in modifying ocean productivity and climate, yet paleo-records from N2-fixation regions are sparse. Here we present new nitrogen isotope (δ15N) records of bulk sediment and foraminifera test-bound (FB)…
Detailed sedimentary N isotope records from Cariaco Basin for terminations I and V: Local and global implications
For the last deglaciation and Termination V (the initiation of MIS 11 at around 430 ka) we report high-resolution sedimentary nitrogen isotope (δ15N) records from Cariaco Basin in the Caribbean Sea. During both terminations the previously reported interglacial decrease in δ15N clearly lags local changes such as water column anoxia as well as…
Elevated 15N/14N in particulate organic matter, zooplankton, and diatom frustule-bound nitrogen in the ice-covered water column of the Bering Sea eastern shelf
We conducted a survey of the natural abundance 15N/14N ratio (δ15N) of particulate organic matter (POM), diatom frustule-bound nitrogen (δ15NDB), and zooplankton from water column material collected with net tows across the eastern Bering Sea shelf in late winter of 2007 and 2008, to investigate the N dynamics of primary and secondary production…
Cleaning methods for the isotopic determination of diatombound nitrogen in non-fossil diatom frustules
Ground-truthing the stable isotope ratio of diatom frustule-bound organic nitrogen (N) as a paleoceanographic proxy of phytoplankton nutrient consumption calls for studies of modern diatoms from cultures and the field. This work has been hindered by the lack of a method to prepare fresh diatom material, which has significant geochemical…
Analytical improvements and assessment of long-term performance of the oxidation–denitrifier method
The analysis of the nitrogen (N) isotopic composition of organic matter bound to fossil biomineral structures (BB-δ15N) using the oxidation–denitrifier (O–D) method provides a novel tool to study past changes in N cycling processes. Methods: We report a set of methodological improvements to the O–D method, including (a) a method for sealing the…