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Deep-sea coral evidence for lower Southern Ocean surface nitrate concentrations during the last ice age. Proc Natl Acad Sci U S A 2017; 114:3352-3357. [PMID: 28298529 DOI: 10.1073/pnas.1615718114] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The Southern Ocean regulates the ocean's biological sequestration of CO2 and is widely suspected to underpin much of the ice age decline in atmospheric CO2 concentration, but the specific changes in the region are debated. Although more complete drawdown of surface nutrients by phytoplankton during the ice ages is supported by some sediment core-based measurements, the use of different proxies in different regions has precluded a unified view of Southern Ocean biogeochemical change. Here, we report measurements of the 15N/14N of fossil-bound organic matter in the stony deep-sea coral Desmophyllum dianthus, a tool for reconstructing surface ocean nutrient conditions. The central robust observation is of higher 15N/14N across the Southern Ocean during the Last Glacial Maximum (LGM), 18-25 thousand years ago. These data suggest a reduced summer surface nitrate concentration in both the Antarctic and Subantarctic Zones during the LGM, with little surface nitrate transport between them. After the ice age, the increase in Antarctic surface nitrate occurred through the deglaciation and continued in the Holocene. The rise in Subantarctic surface nitrate appears to have had both early deglacial and late deglacial/Holocene components, preliminarily attributed to the end of Subantarctic iron fertilization and increasing nitrate input from the surface Antarctic Zone, respectively.
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Casciotti KL. Nitrogen and Oxygen Isotopic Studies of the Marine Nitrogen Cycle. ANNUAL REVIEW OF MARINE SCIENCE 2016; 8:379-407. [PMID: 26747521 DOI: 10.1146/annurev-marine-010213-135052] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
The marine nitrogen cycle is a complex web of microbially mediated reactions that control the inventory, distribution, and speciation of nitrogen in the marine environment. Because nitrogen is a major nutrient that is required by all life, its availability can control biological productivity and ecosystem structure in both surface and deep-ocean communities. Stable isotopes of nitrogen and oxygen in nitrate and nitrite have provided new insights into the rates and distributions of marine nitrogen cycle processes, especially when analyzed in combination with numerical simulations of ocean circulation and biogeochemistry. This review highlights the insights gained from dual-isotope studies applied at regional to global scales and their incorporation into oceanic biogeochemical models. These studies represent significant new advances in the use of isotopic measurements to understand the modern nitrogen cycle, with implications for the study of past ocean productivity, oxygenation, and nutrient status.
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Affiliation(s)
- Karen L Casciotti
- Department of Earth System Science, Stanford University, Stanford, California 94305;
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Martínez-García A, Sigman DM, Ren H, Anderson RF, Straub M, Hodell DA, Jaccard SL, Eglinton TI, Haug GH. Iron fertilization of the Subantarctic ocean during the last ice age. Science 2014; 343:1347-50. [PMID: 24653031 DOI: 10.1126/science.1246848] [Citation(s) in RCA: 73] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Abstract
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 nitrate consumption, burial fluxes of iron, and proxies for productivity. Peak glacial times and millennial cold events are characterized by increases in dust flux, productivity, and the degree of nitrate consumption; this combination is uniquely consistent with Subantarctic iron fertilization. The associated strengthening of the Southern Ocean's biological pump can explain the lowering of CO2 at the transition from mid-climate states to full ice age conditions as well as the millennial-scale CO2 oscillations.
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Jaccard SL, Hayes CT, Martínez-García A, Hodell DA, Anderson RF, Sigman DM, Haug GH. Two modes of change in Southern Ocean productivity over the past million years. Science 2013; 339:1419-23. [PMID: 23520109 DOI: 10.1126/science.1227545] [Citation(s) in RCA: 169] [Impact Index Per Article: 15.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Export of organic carbon from surface waters of the Antarctic Zone of the Southern Ocean decreased during the last ice age, coinciding with declining atmospheric carbon dioxide (CO(2)) concentrations, signaling reduced exchange of CO(2) between the ocean interior and the atmosphere. In contrast, in the Subantarctic Zone, export production increased into ice ages coinciding with rising dust fluxes, thus suggesting iron fertilization of subantarctic phytoplankton. Here, a new high-resolution productivity record from the Antarctic Zone is compiled with parallel subantarctic data over the past million years. Together, they fit the view that the combination of these two modes of Southern Ocean change determines the temporal structure of the glacial-interglacial atmospheric CO(2) record, including during the interval of "lukewarm" interglacials between 450 and 800 thousand years ago.
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Affiliation(s)
- S L Jaccard
- Geological Institute, Department of Earth Sciences, ETH Zurich, Zurich, Switzerland.
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Karsh KL, Granger J, Kritee K, Sigman DM. Eukaryotic assimilatory nitrate reductase fractionates N and O isotopes with a ratio near unity. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2012; 46:5727-35. [PMID: 22534036 DOI: 10.1021/es204593q] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/11/2023]
Abstract
In order to (i) establish the biological systematics necessary to interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate ((15)N/(14)N and (18)O/(16)O) 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 ((15)ε and (18)ε) for nitrate reduction by two assimilatory eukaryotic nitrate reductase (eukNR) enzymes. The (15)ε for purified extracts of NADPH eukNR from the fungus Aspergillus niger and the (15)ε for NADH eukNR from cell homogenates of the marine diatom Thalassiosira weissflogii were indistinguishable, yielding a mean (15)ε for the enzyme of 26.6 ± 0.2‰. Both forms of eukNR imparted near equivalent fractionation on N and O isotopes. The increase in (18)O/(16)O versus the increase in (15)N/(14)N (relative to their natural abundances) was 0.96 ± 0.01 for NADPH eukNR and 1.09 ± 0.03 for NADH eukNR. These results are the first reliable measurements of the coupled N and O isotope effects for any form of eukNR. They support the prevailing view that intracellular reduction by eukNR is the dominant step in isotope fractionation during nitrate assimilation and that it drives the (18)ε:(15)ε ≈ 1 observed in phytoplankton cultures, suggesting that this O-to-N isotope signature will apply broadly in the environment. Our measured (15)ε and (18)ε may represent the intrinsic isotope effects for eukNR-mediated N-O bond rupture, a potential constraint on the nature of the enzyme's transition state.
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Affiliation(s)
- Kristen L Karsh
- Department of Geosciences, Princeton University, Guyot Hall, Princeton, New Jersey 08544, United States.
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Barnes RT, Raymond PA. Land-use controls on sources and processing of nitrate in small watersheds: insights from dual isotopic analysis. ECOLOGICAL APPLICATIONS : A PUBLICATION OF THE ECOLOGICAL SOCIETY OF AMERICA 2010; 20:1961-1978. [PMID: 21049883 DOI: 10.1890/08-1328.1] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Studies have repeatedly shown that agricultural and urban areas export considerably more nitrogen to streams than forested counterparts, yet it is difficult to identify and quantify nitrogen sources to streams due to complications associated with terrestrial and in-stream biogeochemical processes. In this study, we used the isotopic composition of nitrate (delta15N-NO3- and delta18O-NO3-) in conjunction with a simple numerical model to examine the spatial and temporal variability of nitrate (NO3-) export across a land-use gradient and how agricultural and urban development affects net removal mechanisms. In an effort to isolate the effects of land use, we chose small headwater systems in close proximity to each other, limiting the variation in geology, surficial materials, and climate between sites. The delta15N and delta18O of stream NO3- varied significantly between urban, agricultural, and forested watersheds, indicating that nitrogen sources are the primary determinant of the delta15N-NO3-, while the delta18O-NO3- was found to reflect biogeochemical processes. The greatest NO3- concentrations corresponded with the highest stream delta15N-NO3- values due to the enriched nature of two dominant anthropogenic sources, septic and manure, within the urban and agricultural watersheds, respectively. On average, net removal of the available NO3- pool within urban and agricultural catchments was estimated at 45%. The variation in the estimated net removal of NO3- from developed watersheds was related to both drainage area and the availability of organic carbon. The determination of differentiated isotopic land-use signatures and dominant seasonal mechanisms illustrates the usefulness of this approach in examining the sources and processing of excess nitrogen within headwater catchments.
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Affiliation(s)
- Rebecca T Barnes
- Yale University, School of Forestry and Environmental Studies, 210 Prospect Street, New Haven, Connecticut 06511, USA.
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Cassar N, Bender ML, Barnett BA, Fan S, Moxim WJ, Levy H, Tilbrook B. Response to Comment on "The Southern Ocean Biological Response to Aeolian Iron Deposition". Science 2008; 319:159; author reply 159. [DOI: 10.1126/science.1150011] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Affiliation(s)
- Nicolas Cassar
- Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
- Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, P.O. Box 308, Princeton, NJ 08542, USA
- Commonwealth Scientific and Industrial Research Organisation, Wealth from Oceans Flagship and Antarctic Climate and Ecosystem Cooperative Research Center, Hobart, Tasmania 7001, Australia
| | - Michael L. Bender
- Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
- Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, P.O. Box 308, Princeton, NJ 08542, USA
- Commonwealth Scientific and Industrial Research Organisation, Wealth from Oceans Flagship and Antarctic Climate and Ecosystem Cooperative Research Center, Hobart, Tasmania 7001, Australia
| | - Bruce A. Barnett
- Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
- Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, P.O. Box 308, Princeton, NJ 08542, USA
- Commonwealth Scientific and Industrial Research Organisation, Wealth from Oceans Flagship and Antarctic Climate and Ecosystem Cooperative Research Center, Hobart, Tasmania 7001, Australia
| | - Songmiao Fan
- Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
- Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, P.O. Box 308, Princeton, NJ 08542, USA
- Commonwealth Scientific and Industrial Research Organisation, Wealth from Oceans Flagship and Antarctic Climate and Ecosystem Cooperative Research Center, Hobart, Tasmania 7001, Australia
| | - Walter J. Moxim
- Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
- Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, P.O. Box 308, Princeton, NJ 08542, USA
- Commonwealth Scientific and Industrial Research Organisation, Wealth from Oceans Flagship and Antarctic Climate and Ecosystem Cooperative Research Center, Hobart, Tasmania 7001, Australia
| | - Hiram Levy
- Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
- Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, P.O. Box 308, Princeton, NJ 08542, USA
- Commonwealth Scientific and Industrial Research Organisation, Wealth from Oceans Flagship and Antarctic Climate and Ecosystem Cooperative Research Center, Hobart, Tasmania 7001, Australia
| | - Bronte Tilbrook
- Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
- Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, P.O. Box 308, Princeton, NJ 08542, USA
- Commonwealth Scientific and Industrial Research Organisation, Wealth from Oceans Flagship and Antarctic Climate and Ecosystem Cooperative Research Center, Hobart, Tasmania 7001, Australia
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