Daniel Sigman

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Daniel Sigman is Dusenbury Professor of Geological and Geophysical Sciences at Princeton University. He studies the global cycles of biologically active elements, in particular, nitrogen and carbon, and he is active in the development of analytical techniques for studying nitrogen in the environment.

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New Insights Into the Ocean as a Source of Atmospheric N in the North Atlantic Region

Nitrate isotopic gradients in the North Atlantic Ocean and the nitrogen isotopic composition of sinking organic matter

Deep-sea Research Part I-oceanographic Research Papers, Mar 1, 2019

Abstract Biogeochemical processes within the Atlantic basin alter the nitrogen (N) and oxygen (O)... more Abstract Biogeochemical processes within the Atlantic basin alter the nitrogen (N) and oxygen (O) isotopic compositions (δ15N and δ18O) 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 δ15N and δ18O measurements along the 2013 CLIVAR/GO-SHIP Atlantic section A16N from 6°S to 61°N are used to calculate the δ15N and δ18O of regenerated nitrate added to the interior at different latitudes and depths along the section. The δ15N of nitrate being regenerated is similar among depths and covaries with δ15N of the nitrate supply to overlying surface waters. These observations are consistent with regenerated nitrate deriving dominantly from sinking N, rather than from dissolved organic or suspended particulate organic N that is circulated through the ocean interior. The δ15N of regenerated nitrate never declines below the δ15N of the nitrate supply, consistent with the dominance of subsurface nitrate over N2 fixation and atmospheric N deposition in fueling net community production. In the low latitudes (

Revisiting nutrient utilization in the glacial Antarctic: Evidence from a new method for diatom-bound N isotopic analysis

Paleoceanography, Jul 1, 2004

Isotopic measurements of diatom-bound nitrogen, using a wet chemical oxidation combined with the ... more Isotopic measurements of diatom-bound nitrogen, using a wet chemical oxidation combined with the ''denitrifier'' method for nitrate analysis, show significant offsets from previously published combustion-based measurements. This offset is attributed to a gaseous nitrogen blank associated with the diatom's opal frustule. Moreover, experimentation with multiple chemical cleaning protocols demonstrates that diatom microfossils from the clay-rich sediments of the glacial Antarctic are more difficult to clean than Holocene materials. New downcore profiles from the Antarctic show no change in the diatom-bound N 15 N/ 14 N between the last glacial and the Holocene in the Atlantic sector, and the elevation of glacial diatom-bound 15 N/ 14 N relative to the Holocene in the Indian sector is smaller than in previous measurements. These data suggest no change in the degree of nitrate utilization in the Atlantic sector and at most a 20% increase (from $25 to 45%) in the Indian sector. The new measurements suggest that, during the last ice age in the Atlantic sector of the Antarctic, the atmospheric source of biologically available iron was not so great as to become significant relative to the iron supply from below. Given the apparent spatial variability in the degree of nitrate drawdown, more work is required to develop an adequate picture of the glacial Antarctic nutrient field.

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Global Nitrogen Cycle: Critical Enzymes, Organisms, and Processes for Nitrogen Budgets and Dynamics

Chemical Reviews, Jun 12, 2020

Nitrogen (N) is used in many of life's fundamental biomolecules, and it is also a participant... more Nitrogen (N) is used in many of life's fundamental biomolecules, and it is also a participant in environmental redox chemistry. Biogeochemical processes control the amount and form of N available to organisms ("fixed" N). These interacting processes result in N acting as the proximate limiting nutrient in most surface environments. Here, we review the global biogeochemical cycle of N and its anthropogenic perturbation. We introduce important reservoirs and processes affecting N in the environment, focusing on the ocean, in which N cycling is more generalizable than in terrestrial systems, which are more heterogeneous. Particular attention is given to processes that create and destroy fixed N because these comprise the fixed N input/output budget, the most universal control on environmental N availability. We discuss preindustrial N budgets for terrestrial and marine systems and their modern-day alteration by N inputs from human activities. We summarize evidence indicating that the simultaneous roles of N as a required biomass constituent and an environmental redox intermediate lead to stabilizing feedbacks that tend to blunt the impact of N cycle perturbations at larger spatiotemporal scales, particularly in marine systems. As a result of these feedbacks, the anthropogenic "N problem" is distinct from the "carbon dioxide problem" in being more local and less global, more immediate and less persistent.

Nitrogen Isotopes in the Ocean

Elsevier eBooks, 2009

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The dual isotopes of deep nitrate as a constraint on the cycle and budget of oceanic fixed nitrogen

Deep-sea Research Part I-oceanographic Research Papers, Sep 1, 2009

We compare the output of an 18-box geochemical model of the ocean with measurements to investigat... more We compare the output of an 18-box geochemical model of the ocean with measurements to investigate the controls on both the mean values and variation of nitrate d 15 N and d 18 O in the ocean interior. The d 18 O of nitrate is our focus because it has been explored less in previous work. Denitrification raises the d 15 N and d 18 O of mean ocean nitrate by equal amounts above their input values for N 2 fixation (for d 15 N) and nitrification (for d 18 O), generating parallel gradients in the d 15 N and d 18 O of deep ocean nitrate. Partial nitrate assimilation in the photic zone also causes equivalent increases in the d 15 N and d 18 O of the residual nitrate that can be transported into the interior. However, the regeneration and nitrification of sinking N can be said to decouple the N and O isotopes of deep ocean nitrate, especially when the sinking N is produced in a low latitude region, where nitrate consumption is effectively complete. The d 15 N of the regenerated nitrate is equivalent to that originally consumed, whereas the regeneration replaces nitrate previously elevated in d 18 O due to denitrification or nitrate assimilation with nitrate having the d 18 O of nitrification. This lowers the d 18 O of mean ocean nitrate and weakens nitrate d 18 O gradients in the interior relative to those in d 15 N. This decoupling is characterized and quantified in the box model, and agreement with data shows its clear importance in the real ocean. At the same time, the model appears to generate overly strong gradients in both d 18 O and d 15 N within the ocean interior and a mean ocean nitrate d 18 O that is higher than measured. This may be due to, in the model, too strong an impact of partial nitrate assimilation in the Southern Ocean on the d 15 N and d 18 O of preformed nitrate and/or too little cycling of intermediate-depth nitrate through the low latitude photic zone.

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N and O Isotopes of Nitrate in the GISP2 Ice Core: Implications for the Interpretation of Ice-Core Nitrate Profiles

AGU Spring Meeting Abstracts, May 1, 2001

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The Nitrogen and Oxygen Isotope Composition of Porewater Nitrate from Bering Sea Sediments

AGUFM, Dec 1, 2002

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Changes in Ice Age Nitrate Consumption, Productivity, and Biological Pump Efficiency can be Explained by the Internal Cycling of Iron

AGUFM, Dec 1, 2016

Secular decline of seawater calcium increases seawater buffering and pH

AGU Fall Meeting Abstracts, Dec 1, 2015

The isotopic composition of ammonium in marine rainwater

AGUFM, Dec 1, 2012

Surviving a High Nutrient-Low Chlorophyll (HNLC) region: insights to the internal cycling of nitrogen and iron in the eastern equatorial Pacific

American Geophysical Union eBooks, Feb 1, 2016

The effect of benthic nitrogen cycling on the δ 15 N and δ 18 O of water-column nitrate

GeCAS, Sep 1, 2003

$Research paper thumbnail of Nitrate isotope fractionations during biological nitrate reduction: Insights from first principles theoretical modeling$

Nitrate isotope fractionations during biological nitrate reduction: Insights from first principles theoretical modeling

AGUFM, Dec 1, 2010

The chemical composition of organic nitrogen in marine rainwater and aerosols

AGU Fall Meeting Abstracts, Dec 1, 2010

ABSTRACT The current state of knowledge on organic nitrogen in the atmosphere is very limited. At... more ABSTRACT The current state of knowledge on organic nitrogen in the atmosphere is very limited. Atmospheric water soluble organic nitrogen (WSON) is a subset of the complex water soluble organic matter measured in atmospheric aerosols and rainwater; as such, it impacts cloud condensation processes and aerosol chemical and optical properties. In marine and continental atmospheric deposition, the organic N fraction can be 20-80% of total N potentially influencing receiving ecosystems. Therefore, atmospheric WSON plays an important role in both atmospheric chemistry and the global biogeochemical N cycle. However, the sources (i.e., anthropogenic vs. terrestrial vs. marine), composition (e.g., reduced or oxidized N), potential connections to inorganic N (NO3- and NH4+), and spatio-temporal variability of atmospheric WSON are largely unknown. Samples were collected on or near the island of Bermuda (32.27°N, 64.87°W), which is located in the western North Atlantic and experiences seasonal changes in transport that allow for study of both anthropogenically and primarily marine influenced air masses. Rainwater samples (n=7) and aqueous extracted aerosol samples (n=4) were analyzed by positive ion ultra-high resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) to characterize the chemical composition of the water soluble organic N on a per compound level. We found ~ 800 N containing compounds in 8 compound classes. The CHON+ compound class contained the largest number of N compounds (~ 460). Compared to continental rainwater [Altieri et al., ES&amp;T, 2009], the CHON+ compounds in the marine samples are as dominant in number, yet have less regular patterns and lower O:C ratios for comparable N:C ratios. In fact, average O:C ratios of all N containing compound classes were lower in the marine samples than in continental rainwater samples. No organosulfates or nitrooxy-organosulfates were detected in the marine samples, both of which are known secondary anthropogenic compounds detected in continental rainwater. This suggests that the lifetime of these compounds is insufficient for transport to the remote marine atmosphere. Possible explanations for the differences between continental and marine samples include 1) anthropogenic emissions might lead to more secondary formation closer to the source regions, 2) there are more oxidants in air masses dominated by anthropogenic emissions leading to higher O:C ratios, or 3) higher NOx concentrations produce more water soluble carbonyls and thus more secondary formation via aqueous reactions in the anthropogenic environment. The marine rainwater and aerosols also had a large number of mixed functional compounds (i.e., sulfur and phosphorous present with nitrogen) suggesting a potentially increased importance of organic S and organic P in remote marine environments. In addition to the detailed compositional information above, we will also discuss the influence of air mass origin on the sources of WSON using air mass back trajectory data, and the potential inter-relationships of inorganic N and WSON will be investigated using the nitrogen and oxygen isotopic ratios of nitrate.

Controls on the Nitrogen and Oxygen Isotopic Composition (δ 15 N, δ 18 O, δ 17 O) of Atmospheric Nitrate in Princeton, NJ

AGUFM, Dec 1, 2004

The oxygen isotopic composition of atmospheric nitrate reflects the oxidative mechanisms that con... more The oxygen isotopic composition of atmospheric nitrate reflects the oxidative mechanisms that convert NOx to HNO3, while the nitrogen isotopic composition of atmospheric nitrate may reflect different NOx source signatures and/or fractionations related to NOx chemistry [Michalski et al., 2003; Hastings et al., 2003; Freyer et al., 1993]. New analysis techniques are capable of determining the 15N/14N, 18O/16O and 17O/16O

Correction to “Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin”

Global Biogeochemical Cycles, Feb 1, 2006

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Glacial/interglacial changes in the isotopes of nitrate from the Greenland Ice Sheet Project 2 (GISP2) ice core

Global Biogeochemical Cycles, Dec 1, 2005

The 15 N/ 14 N and 18 O/ 16 O ratios of nitrate in the Greenland Ice Sheet Project 2 (GISP2) (Sum... more The 15 N/ 14 N and 18 O/ 16 O ratios of nitrate in the Greenland Ice Sheet Project 2 (GISP2) (Summit, Greenland) ice core are much higher in ice from the last glacial period than in the pre-industrial Holocene, despite the lack of a significant glacial/interglacial change in nitrate concentration. While both the 15 N/ 14 N and 18 O/ 16 O records are anticorrelated with snow accumulation rate, neither is satisfactorily explained by accumulation changes or post-depositional processes. The similarity in the glacial/ interglacial change in 15 N/ 14 N from several different Greenland ice cores and the large amplitude of this change relative to observed seasonal variation raise the possibility that the isotopes of nitrate in ice cores indicate a large-scale glacial/interglacial change in the isotopic composition of atmospheric NO x . The glacial/interglacial change in 18 O/ 16 O is best explained by a greater contribution of HNO 3 production from hydrolysis of N 2 O 5 , which has implications for reconstruction of past atmospheric oxidant levels. Although isotope effects associated with NO x photochemistry and nitrate scavenging have not been fully characterized, the 15 N/ 14 N data may indicate glacial/interglacial changes in the relative contributions from different natural sources of NO x on a hemispheric or global scale.

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$Research paper thumbnail of Relationship of nitrogen isotope fractionation to phytoplankton size and iron availability during the Southern Ocean Iron RElease Experiment (SOIREE)$

Relationship of nitrogen isotope fractionation to phytoplankton size and iron availability during the Southern Ocean Iron RElease Experiment (SOIREE)

Limnology and Oceanography, May 1, 2003

The 15 N composition of sediments has been used as a proxy for nitrate utilization in surface wat... more The 15 N 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 CO 2 concentration. Interpretation has relied on a temporally constant isotope effect () associated with uptake and assimilation of nitrate by phytoplankton. To investigate the reliability of this approach, we examined the relationships between the 15 N compositions of dissolved nitrate, bulk and size-fractionated (200, 70, 20, 5, 1 m) suspended particulate organic nitrogen (PON), and sinking particles obtained from sediment traps during the Southern Ocean iron release experiment (SOIREE). We found variations in phytoplankton nitrogen isotopic compositions with both cell size and iron availability. ␦ 15 N PON increased by Ͼ2‰ with increasing size, both within and outside the ironenriched patch. In comparison to unfertilized waters, ␦ 15 N PON within the iron-fertilized patch was a further 3-4‰ higher in those size fractions dominated by large diatoms (20-70, 70-200 m). We speculate that this iron response might result from (1) variation in of nitrate utilization or (2) an iron-stimulated shift from ammoniumbased to nitrate-based production. Comparing the ␦ 15 N of the large diatom-dominated size fractions to the ␦ 15 N of nitrate suggests relatively low values of 4-5‰, in contrast to estimated values of 7-10‰ from seasonal nitrate depletion and export production. This suggests that higher glacial ␦ 15 N in Southern Ocean sediments could, in part, reflect increases in iron availability, dominant cell size, and possibly growth rates, and these effects must be considered in any quantitative scaling of ␦ 15 N variations, including those of diatom-bound ␦ 15 N, to the extent of nitrate utilization.

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Sinking organic matter spreads the nitrogen isotope signal of pelagic denitrification in the North Pacific

Geophysical Research Letters, Apr 25, 2009

Culture studies of denitrifying bacteria predict that denitrification will generate equivalent gr... more Culture studies of denitrifying bacteria predict that denitrification will generate equivalent gradients in the d 15 N and d 18 O of deep ocean nitrate. A depth profile of nitrate isotopes from the Hawaii Ocean Time-series Station ALOHA shows less of an increase in d 18 O than in d 15 N as one ascends from abyssal waters into the denitrificationimpacted mid-depth waters. A box model of the ocean nitrate N and O isotopes indicates that this is the effect of the low latitude nitrate assimilation/regeneration cycle: organic N sinking out of the surface spreads the highd 15 N signal of pelagic denitrification into waters well below and beyond the suboxic zone, whereas the nitrate d 18 O signal of denitrification can only be transmitted by circulation in the interior.

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