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.
We present real-time optical polarization subtraction for calibration of Faraday rotation spectra... more We present real-time optical polarization subtraction for calibration of Faraday rotation spectra. Noise analysis yields minor isotope sensitivity of 3.0 ppbv·Hz<sup>-1/2</sup> and 1.7×10<sup>-8</sup> rad·Hz<sup>-1/2</sup> noise-equivalent angle. Sub-permil ratiometric precision is achieved at integration times >100 s.
Canadian Journal of Fisheries and Aquatic Sciences, Feb 1, 2020
The15N/14N ratio of the fish-native organic matter preserved in fish otoliths (or δ15Noto) may al... more The15N/14N ratio of the fish-native organic matter preserved in fish otoliths (or δ15Noto) may allow for reconstruction of fish trophic history and changes in food webs. To support this application, ground-truthing data are needed on the relationships among the δ15N of diet, of fish tissue (e.g., white muscle tissue, δ15Nwmt), and δ15Noto. Using a highly sensitive method for N isotope analysis, δ15Notowas compared with δ15Nwmtin 24 teleost species. Within a species, the difference between δ15Notoand δ15Nwmt(Δδ15No-w) varied little across individuals, confirming the utility of δ15Nototo reconstruct δ15Nwmtchanges for a given species. Across species, δ15Notoand δ15Nwmtwere highly correlated. However, Δδ15No-wvaried systematically across species. Phylogeny, the concentrations of total N and amino acids, and life history were ruled out as the main cause for the observed variation in Δδ15No-w. δ15Notowas lowest relative to δ15Nwmtin species producing larger otoliths. We propose that δ15Notois elevated by isotopically fractionating metabolism of the organic matrix, which is less important when otolith growth is fast and thus when the otolith is large.
Abstract The nitrogen isotopes (δ15N) in the organic fraction of accretionary hard part structure... more Abstract The nitrogen isotopes (δ15N) in the organic fraction of accretionary hard part structures, such as fish otoliths, may provide life histories of dietary change. We performed controlled experiments to validate the dynamics of the isotopic signal incorporation into biominerals following dietary shifts and also compared whole-otolith and serial sampling approaches for diet reconstruction. Laboratory-reared Atlantic croaker (Micropogonias undulatus) were switched from a high quality, high δ15N diet (δ15N = 10.7‰) to one of two lower quality, lower δ15N diets (δ15N = 5.9 or 3.7‰). Using the oxidation-denitrifier method, including cleaning protocols required for fossil otoliths, we measured both otolith-bound δ15N (δ15Noto) of whole-otolith subsamples (δ15Nwhole) and sequentially micromilled otolith powders (δ15Nmicro) and compared these results to white muscle tissue and liver δ15N obtained through traditional techniques. Both δ15Nwhole and δ15Nmicro recorded the diet switch, although sampling limitations muted the signal in δ15Nmicro, especially in the slowly growing otoliths of the fish on the lowest quality diet, in which otolith accretion was particularly slow. The timescales at which otoliths and tissues approached the new δ15N after the diet switch varied, but the slowest was for muscle and the fastest was for liver. For δ15Nwhole, there were two factors: (1) the turnover time of the N provided to the otolith and (2) that the otolith is accreting and thus integrating over the entire life history of the fish. Using a model to account for the accretionary growth, turnover time for N supplying otolith growth ranged from 12 to 16 days. In both soft tissues and otoliths, the trophic discrimination factor (the δ15N elevation relative to diet) appears to have been lower in the larval fish, prior to the onset of the diet switches. This raises questions about the interpretation of the δ15Noto of otolith cores; however, the core is typically a minute portion of the total otolith. We conclude that (1) δ15Nwhole is useful for tracking diet despite the whole-life integration of the δ15N signal and (2) δ15Nmicro also records diet, although higher resolution tracking will require further optimization of δ15Nmicro sampling.
Deep-sea Research Part Ii-topical Studies in Oceanography, Nov 1, 2014
ABSTRACT We conducted a survey of the natural abundance 15N/14N ratio (δ15N) of particulate organ... more ABSTRACT 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 in relation to the presence of seasonal sea ice. The data reveal a pattern of increasing δ15N northward and eastward (POM: 2.1 to 14.7‰; frustule-bound N: 4.9 to 20.7‰; zooplankton: 6.4 to 18.0‰), with POM δ15N reaching~9‰ higher than that of water-column nitrate. Higher δ15N in each of these plankton fractions was largely associated with stations covered by sea ice. POM δ15N collected concurrently from within sea ice was not sufficiently 15N-enriched to explain the elevated water-column values. Rather, the δ15N of water-column POM under sea ice appears to derive from the assimilation of ammonium released from shelf sediments. Water-column ammonium δ15N was between 28 and 63‰, most likely due to partial nitrification in the sediment and overlying water column. The high δ15N of this ammonium is effectively transmitted to phytoplankton under sea ice because light limitation from the ice cover delays the springtime nitrate assimilation that yields algal biomass with a lower δ15N. Despite this seasonal explanation, published δ15N data from sediment traps, summertime zooplankton, and surface sediment indicate that a shoreward and northward δ15N increase – albeit of a weaker magnitude – is perennial, suggesting that the δ15N of the total annual fixed N supply (including both ammonium and nitrate) also increases shoreward and northward. This requires that the partial nitrification of ammonium underlying the spatial pattern in δ15N is at least partly coupled to denitrification in the sediments that preferentially removes 14N, causing the total fixed N reservoir on the shelf to evolve toward higher δ15N. Shelf geometry and the consequent benthic-pelagic coupling of N cycling thus seem to underlie the spatial pattern in the mean annual δ15N of plankton, while sea-ice cover causes the high δ15N of ammonium on the shelf to be most strongly reflected by the production occurring in the winter and early spring. Our results provide a basis for tracing the geographic and seasonal origins and trophic transfer of N in the Bering shelf ecosystem.
The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Ea... more The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Earth. The major role of the Agulhas Current in driving significant heat and salt fluxes is well known, yet its biogeochemical fluxes remain largely uncharacterised. Here, we use nitrate isotopes (δ15N, δ18O, and Δ(15-18) = δ15N-δ18O) to evaluate nutrient supply mechanisms that ultimately support new production in the southwest Indian Ocean. Across the greater Agulhas region, thermocline nitrate-δ15N is lower (4.9-5.8‰) than the underlying Subantarctic Mode Water source (δ15N of 6.9‰) and the upstream source regions (where nitrate-δ15N ranges from 6.4-7.0‰), which we attribute to local N2 fixation. Using a one-box model to simulate the newly-fixed nitrate flux, we estimate a local N2 fixation rate of 7-25 Tg N.a-1, amounting to ~30-95% of the whole Indian Ocean nitrogen gain estimated by models. Thermocline and mixed-layer nitrate Δ(15-18) is also low, due to both N2 fixation and coupled p...
The greater Agulhas Current region is an important component of the climate system, yet its influ... more 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 waters. The deep and intermediate waters record processes occurring remotely, including partial nitrate assimilation in the Southern Ocean and denitrification in the Arabian Sea. In the thermocline and surface, tropically sourced waters are biogeochemically distinct from adjacent subtropically sourced waters, confirming inhibited lateral mixing across the current core. (Sub)tropical thermocline nitrate δ15N is lower (4.9–5.8‰) than the sub‐thermocline source, Subantarctic Mode Water (6.9‰); we attribute this difference to local N2 fixation. Using a one‐box model to simulate the newly fixed nitrate flux, we estimate a local N2 fixation rate of 7–...
We present real-time optical polarization subtraction for calibration of Faraday rotation spectra... more We present real-time optical polarization subtraction for calibration of Faraday rotation spectra. Noise analysis yields minor isotope sensitivity of 3.0 ppbv·Hz<sup>-1/2</sup> and 1.7×10<sup>-8</sup> rad·Hz<sup>-1/2</sup> noise-equivalent angle. Sub-permil ratiometric precision is achieved at integration times >100 s.
Canadian Journal of Fisheries and Aquatic Sciences, Feb 1, 2020
The15N/14N ratio of the fish-native organic matter preserved in fish otoliths (or δ15Noto) may al... more The15N/14N ratio of the fish-native organic matter preserved in fish otoliths (or δ15Noto) may allow for reconstruction of fish trophic history and changes in food webs. To support this application, ground-truthing data are needed on the relationships among the δ15N of diet, of fish tissue (e.g., white muscle tissue, δ15Nwmt), and δ15Noto. Using a highly sensitive method for N isotope analysis, δ15Notowas compared with δ15Nwmtin 24 teleost species. Within a species, the difference between δ15Notoand δ15Nwmt(Δδ15No-w) varied little across individuals, confirming the utility of δ15Nototo reconstruct δ15Nwmtchanges for a given species. Across species, δ15Notoand δ15Nwmtwere highly correlated. However, Δδ15No-wvaried systematically across species. Phylogeny, the concentrations of total N and amino acids, and life history were ruled out as the main cause for the observed variation in Δδ15No-w. δ15Notowas lowest relative to δ15Nwmtin species producing larger otoliths. We propose that δ15Notois elevated by isotopically fractionating metabolism of the organic matrix, which is less important when otolith growth is fast and thus when the otolith is large.
Abstract The nitrogen isotopes (δ15N) in the organic fraction of accretionary hard part structure... more Abstract The nitrogen isotopes (δ15N) in the organic fraction of accretionary hard part structures, such as fish otoliths, may provide life histories of dietary change. We performed controlled experiments to validate the dynamics of the isotopic signal incorporation into biominerals following dietary shifts and also compared whole-otolith and serial sampling approaches for diet reconstruction. Laboratory-reared Atlantic croaker (Micropogonias undulatus) were switched from a high quality, high δ15N diet (δ15N = 10.7‰) to one of two lower quality, lower δ15N diets (δ15N = 5.9 or 3.7‰). Using the oxidation-denitrifier method, including cleaning protocols required for fossil otoliths, we measured both otolith-bound δ15N (δ15Noto) of whole-otolith subsamples (δ15Nwhole) and sequentially micromilled otolith powders (δ15Nmicro) and compared these results to white muscle tissue and liver δ15N obtained through traditional techniques. Both δ15Nwhole and δ15Nmicro recorded the diet switch, although sampling limitations muted the signal in δ15Nmicro, especially in the slowly growing otoliths of the fish on the lowest quality diet, in which otolith accretion was particularly slow. The timescales at which otoliths and tissues approached the new δ15N after the diet switch varied, but the slowest was for muscle and the fastest was for liver. For δ15Nwhole, there were two factors: (1) the turnover time of the N provided to the otolith and (2) that the otolith is accreting and thus integrating over the entire life history of the fish. Using a model to account for the accretionary growth, turnover time for N supplying otolith growth ranged from 12 to 16 days. In both soft tissues and otoliths, the trophic discrimination factor (the δ15N elevation relative to diet) appears to have been lower in the larval fish, prior to the onset of the diet switches. This raises questions about the interpretation of the δ15Noto of otolith cores; however, the core is typically a minute portion of the total otolith. We conclude that (1) δ15Nwhole is useful for tracking diet despite the whole-life integration of the δ15N signal and (2) δ15Nmicro also records diet, although higher resolution tracking will require further optimization of δ15Nmicro sampling.
Deep-sea Research Part Ii-topical Studies in Oceanography, Nov 1, 2014
ABSTRACT We conducted a survey of the natural abundance 15N/14N ratio (δ15N) of particulate organ... more ABSTRACT 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 in relation to the presence of seasonal sea ice. The data reveal a pattern of increasing δ15N northward and eastward (POM: 2.1 to 14.7‰; frustule-bound N: 4.9 to 20.7‰; zooplankton: 6.4 to 18.0‰), with POM δ15N reaching~9‰ higher than that of water-column nitrate. Higher δ15N in each of these plankton fractions was largely associated with stations covered by sea ice. POM δ15N collected concurrently from within sea ice was not sufficiently 15N-enriched to explain the elevated water-column values. Rather, the δ15N of water-column POM under sea ice appears to derive from the assimilation of ammonium released from shelf sediments. Water-column ammonium δ15N was between 28 and 63‰, most likely due to partial nitrification in the sediment and overlying water column. The high δ15N of this ammonium is effectively transmitted to phytoplankton under sea ice because light limitation from the ice cover delays the springtime nitrate assimilation that yields algal biomass with a lower δ15N. Despite this seasonal explanation, published δ15N data from sediment traps, summertime zooplankton, and surface sediment indicate that a shoreward and northward δ15N increase – albeit of a weaker magnitude – is perennial, suggesting that the δ15N of the total annual fixed N supply (including both ammonium and nitrate) also increases shoreward and northward. This requires that the partial nitrification of ammonium underlying the spatial pattern in δ15N is at least partly coupled to denitrification in the sediments that preferentially removes 14N, causing the total fixed N reservoir on the shelf to evolve toward higher δ15N. Shelf geometry and the consequent benthic-pelagic coupling of N cycling thus seem to underlie the spatial pattern in the mean annual δ15N of plankton, while sea-ice cover causes the high δ15N of ammonium on the shelf to be most strongly reflected by the production occurring in the winter and early spring. Our results provide a basis for tracing the geographic and seasonal origins and trophic transfer of N in the Bering shelf ecosystem.
The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Ea... more The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Earth. The major role of the Agulhas Current in driving significant heat and salt fluxes is well known, yet its biogeochemical fluxes remain largely uncharacterised. Here, we use nitrate isotopes (δ15N, δ18O, and Δ(15-18) = δ15N-δ18O) to evaluate nutrient supply mechanisms that ultimately support new production in the southwest Indian Ocean. Across the greater Agulhas region, thermocline nitrate-δ15N is lower (4.9-5.8‰) than the underlying Subantarctic Mode Water source (δ15N of 6.9‰) and the upstream source regions (where nitrate-δ15N ranges from 6.4-7.0‰), which we attribute to local N2 fixation. Using a one-box model to simulate the newly-fixed nitrate flux, we estimate a local N2 fixation rate of 7-25 Tg N.a-1, amounting to ~30-95% of the whole Indian Ocean nitrogen gain estimated by models. Thermocline and mixed-layer nitrate Δ(15-18) is also low, due to both N2 fixation and coupled p...
The greater Agulhas Current region is an important component of the climate system, yet its influ... more 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 waters. The deep and intermediate waters record processes occurring remotely, including partial nitrate assimilation in the Southern Ocean and denitrification in the Arabian Sea. In the thermocline and surface, tropically sourced waters are biogeochemically distinct from adjacent subtropically sourced waters, confirming inhibited lateral mixing across the current core. (Sub)tropical thermocline nitrate δ15N is lower (4.9–5.8‰) than the sub‐thermocline source, Subantarctic Mode Water (6.9‰); we attribute this difference to local N2 fixation. Using a one‐box model to simulate the newly fixed nitrate flux, we estimate a local N2 fixation rate of 7–...
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