We have developed new methods for natural abundance isotope ratio measurement of several biologically available and typically dissolved forms of N that are common in the environment: the 15N/14N, 18O/16O, and 18O/17O/16O of nitrate, the 15N/14N of total dissolved N (i.e. dissolved organic N in waters lacking nitrate and ammonium), and the 15N/14N of ammonium. For paleoceanographic work, we developed new techniques for the 15N/14N of the microfossil-bound N of diatoms (made of opal), planktonic foraminifera (calcite), and scleractinian corals (aragonite). All of these approaches have as their cornerstone the “denitrifier” method for nitrate isotopic analysis, in which nitrate (NO3-) is converted to nitrous oxide (N2O) gas by a strain of denitrifying bacteria that lacks an active N2O reductase, followed by analysis of the product N2O with a stable isotope ratio mass spectrometer.
Coupling of the denitrifier method, which yields the 15N/14N and 18O/16O of nitrate (NO3-)(1,2), with other steps to yield the 15N/14N of dissolved organic N (DON, 3), microfossil-bound organic N (4), and dissolved ammonium (NH4+, 5). “MS” indicate mass spectrometric analysis.
References
225 Publications
Previous studies have suggested that during the late Pleistocene ice ages, surface-deep exchange was somehow weakened in the Southern Ocean s Antarctic Zone, which reduced the leakage of deeply sequestered carbon dioxide and thus contributed to the lower atmospheric carbon dioxide levels of the ice ages. Here, high-resolution diatom-bound nitrogen…
Previous studies suggest that meridional migrations of the Antarctic Circumpolar Current may have altered wind-driven upwelling and carbon dioxide degassing in the Southern Ocean during past climate transitions. Here, we report a quantitative and continuous record of the Antarctic Circumpolar Current latitude over the last glacial-interglacial…
The nitrogen isotopic composition of time-series sediment trap samples, dissolved NO3/-, and surficial sediments was determined in three regions along the margin of the eastern North Pacific: Monterey Bay, San Pedro Basin, and the Gulf of California (Carmen and Guaymas Basins). Complex physical regimes are present in all three areas, and each is…
Atmospheric water soluble organic nitrogen (WSON) is a subset of the complex organic matter in aerosols and rainwater, which impacts cloud condensation processes and aerosol chemical and optical properties and may play a significant role in the biogeochemical cycle of N. However, its sources, composition, connections to inorganic N, and…
Emissions of anthropogenic nitrogen (N) to the atmosphere have increased tenfold since preindustrial times, resulting in increased N deposition to terrestrial and coastal ecosystems. The sources of N deposition to the ocean, however, are poorly understood. Two years of event-based rainwater samples were collected on the island of Bermuda in the…
Emissions of anthropogenic nitrogen (N) to the atmosphere have increased tenfold since preindustrial times, resulting in increased N deposition to terrestrial and coastal ecosystems. The current sources of N deposition to the ocean, however, are poorly understood. To investigate the sources of nitrate in rainwater deposited to the ocean, two years…
Global models estimate that the anthropogenic component of atmospheric nitrogen (N) deposition to the ocean accounts for up to a third of the ocean s external N supply and 10% of anthropogenic CO2 uptake. However, there are few observational constraints from the marine atmospheric environment to validate these findings. Due to the paucity of…
N2 fixation in low-latitude surface waters dominates the input of fixed nitrogen (N) to the global ocean, sustaining ocean fertility. In the Caribbean Sea, higher foraminifera-bound (FB-)δ15N indicates a decline in N2 fixation during ice ages, but its cause and broader implications are unclear. Here, we report three additional Atlantic FB-δ15N…
Dissolved oxygen (O2) is essential for most ocean ecosystems, fuelling organisms’ respiration and facilitating the cycling of carbon and nutrients. Oxygen measurements have been interpreted to indicate that the ocean’s oxygen-deficient zones (ODZs) are expanding under global warming1,2. However, models provide an unclear picture of future ODZ…
The shoaling of the Central American Seaway (CAS) around 4.6 Ma (million years ago) is thought to have enhanced the Gulf Stream, strengthening the Atlantic Meridional Overturning Circulation and potentially influencing the evolution of Pliocene climate. Paleoclimate records indicate a buildup of heat and salt in the Caribbean and changes in the…
The Younger Dryas cooling 12,700 years ago is one of the most abrupt climate changes observed in Northern Hemisphere palaeoclimate records. Annually laminated lake sediments are ideally suited to record the dynamics of such abrupt changes, as the seasonal deposition responds immediately to climate, and the varve counts provide an accurate estimate…
Tropical forests account for one third of terrestrial primary production and contribute significantly to the land carbon sink 1,2. The future of this sink relies critically on forest interactions with nutrient cycles 3-5. Humid montane tropical forests are often thought to be rich in phosphorus, but to contain low levels of bioavailable nitrogen 6…
In a piston core from the central Bering Sea, diatom microfossil-bound N isotopes and the concentrations of Opal, biogenic barium, calcium carbonate, and organic N are measured over the last glacial/interglacial cycle. Compared to the interglacial sections of the core, the sediments of the last ice age are characterized by 3‰ higher diatom-bound…
In piston cores from the open subarctic Pacific and the Okhotsk Sea, diatom-bound δ15N (δ15Ndb), biogenic opal, calcium carbonate, and barium were measured from coretop to the previous glacial maximum (MIS 6). Glacial intervals are generally characterized by high δ15Ndb (∼8‰) and low productivity, whereas interglacial intervals have a lower δ15Ndb…
Phytoplankton in the Antarctic deplete silicic acid (Si(OH)4) to a far greater extent than they do nitrate (NO3-). This pattern can be reversed by the addition of iron which dramatically lowers diatom Si(OH)4:NO3- uptake ratios. Higher iron supply during glacial times would thus drive the Antarctic towards NO3- depletion with excess Si(OH)4…
An essential element of modern ocean circulation and climate is the Atlantic meridional overturning circulation (AMOC), which includes deep-water formation in the subarctic North Atlantic. However, a comparable overturning circulation is absent in the Pacific, the world’s largest ocean, where relatively fresh surface waters inhibit North Pacific…
Atmospheric deposition of nitrogen (N) and base cations was measured for 5-7 years on the island of Hawaii and for 1.5 years on Kauai. On Hawaii, mean annual fluxes of K+, Mg2+, and Ca2+ were 15, 17, and 13 kg ha-1 yr-1, respectively. Fog interception was the largest deposition pathway. Sea salt contributed the majority of cations, although…
The link between similarity in amino acid sequence,for ammonia monooxygenase (AMO) and isotopic discrimination for ammonia oxidation (εAMO) was investigated in β-subdivision ammonia-oxidizing bacteria. The isotope effects for ammonia oxidation in pure cultures of the nitrifying strains. Nitrosomonas marina, Nitrosomonas C-113a, Nitrosospira tenuis…
We report a novel method for measurement of the oxygen isotopic composition (18O/16O) of nitrate (NO3-) from both seawater and freshwater. The denitrifier method, based on the isotope ratio analysis of nitrous oxide generated from sample nitrate by cultured denitrifying bacteria, has been described elsewhere for its use in nitrogen isotope ratio …
Seabirds transfer nutrients from the ocean to their nesting island, potentially altering nitrogen (N) cycling within adjacent terrestrial and marine ecosystems. Yet, the processes involved in seabird-N transfer along the land–sea continuum remain elusive. Using δ15N and δ18O measurements of groundwater nitrate, we demonstrate the role of brackish…
The nitrogen isotopic composition (15N/14N ratio, or δ15N) of enameloid-bound organic matter (δ15NEB) in shark teeth was recently developed to investigate the biogeochemistry and trophic structures (i.e., food webs) of the ancient ocean. Using δ15NEB, we present the first nitrogen isotopic evidence for trophic differences between shark taxa from a…
The equatorial Pacific Ocean is one of the major high-nutrient, low-chlorophyll regions in the global ocean. In such regions, the consumption of the available macro-nutrients such as nitrate and phosphate is thought to be limited in part by the low abundance of the critical micro-nutrient iron1. Greater atmospheric dust deposition2 could have…
North Atlantic (NA) deep-water formation and the resulting Atlantic meridional overturning cell is generally regarded as the primary feature of the global overturning circulation and is believed to be a result of the geometry of the continents. Here, instead, the overturning is viewed as a global energy-driven system and the robustness of NA…
A growing number of paleoceanographic observations suggest that the ocean s deep ventilation is stronger in warm climates than in cold climates. Here we use a general ocean circulation model to test the hypothesis that this relation is due to the reduced sensitivity of seawater density to temperature at low mean temperature; that is, at lower…
Chilean margin sedimentary N isotope records have been the focus of paleoceanographic studies examining the extent of water-column denitrification in the eastern South Pacific in the past. Here we use 15N/14N of nitrate and surface sedimentary N along the Chilean coast to investigate the relative contributions of water-column denitrification and…
During the last glacial period, the North Atlantic regionexperienced pronounced, millennial-scale alternations between cold, stadial conditions and milder interstadial conditions - commonly referred to as Dansgaard-Oeschger oscillations - as well as periods of massive iceberg discharge known as Heinrich events1. Changes in Northern Hemisphere…
We investigate the response of the 15N/14N of oceanic nitrate to glacial/interglacial changes in the N budget, using a geochemical box model of the oceanic N cycle that includes N2 fixation and denitrification in the sediments and suboxic water column. This model allows us to quantify the isotopic response of different oceanic nitrate pools to…
Nitrogen fixation is crucial for maintaining biological productivity in the oceans, because it replaces the biologically available nitrogen that is lost through denitrification. But, owing to its temporal and spatial variability, the global distribution of marine nitrogen fixation is difficult to determine from direct shipboard measurements. This…
We report nitrate nitrogen (N) and oxygen (O) isotope measurements from the seasonally sea ice covered Polar Antarctic Zone (PAZ) south of the Southern Antarctic Circumpolar Front. The 15N/14N and 18O/16O ratios of nitrate both increase into the summertime surface mixed layer, in strong correlation with the upward decrease in nitrate concentration…
Recent studies provide seasonally and spatially resolved information on the isotopic characteristics of nitrate supply and N cycling in Southern Ocean surface waters. The new data improve our understanding of the nitrate supply to the Antarctic surface and its isotopic characteristics, especially with regard to the summertime subsurface minimum…
We report nitrate (NO3-) nitrogen isotope ratios for seawater samples collected in the Subantarctic Zone of the Southern Ocean during both winter and summer as part of the Australian Antarctic CRC Subantarctic Zone (SAZ) Project. The concentration and 15N/14N of the wintertime surface nitrate are very close to those of the subantarctic thermocline…
Zinc (Zn) is vital to marine organisms. Its active uptake by phytoplankton results in a substantial depletion of dissolved Zn, and Zn bound to particulate organic matter replenishes dissolved Zn in the ocean through remineralization. However, we found that particulate Zn changes from Zn bound to phosphoryls in cells to recalcitrant inorganic pools…
We investigated the potential of coral skeleton δ15N (CS-δ15N) records for tracking anthropogenic-N sources in coral reef ecosystems. We produced a 56 yr-long CS-δ15N record (1958–2014) from a reef flat in Guam that has been exposed to varying 1) levels of sewage treatment 2) population density, and 3) land use. Increasing population density (from…
Historical coral skeleton (CS) δ18O and δ15N records were produced from samples recovered from sedimentary deposits, held in natural history museum collections, and cored into modern coral heads. These records were used to assess the influence of global warming and regional eutrophication, respectively, on the decline of coastal coral communities…
We tested the hypothesis that the nitrogen (N) isotopic signature (δ15N) of coral skeletal organic material (CS-δ15N) matches that of the coral tissue and also reflects the δ15N of water column fixed N, such that CS-δ15N can be used as a proxy for spatio-temporal oceanic N isotope distributions. Strong correlations between the δ15N of skeletal…
Ongoing human activities are known to affect nitrogen cycling on coral reefs, but the full history of anthropogenic impact is unclear due to a lack of continuous records. We have used the nitrogen isotopic composition of skeleton-bound organic matter (CS-δ15N) in a coastal Porites coral from Magnetic Island in the Great Barrier Reef as a proxy for…
Salinity-driven density stratification of the upper Arctic Ocean isolates sea-ice cover and cold, nutrient-poor surface waters from underlying warmer, nutrient-rich waters. Recently, stratification has strengthened in the western Arctic but has weakened in the eastern Arctic; it is unknown if these trends will continue. Here we present…
The cyclic growth and decay of continental ice sheets can be reconstructed from the history of global sea level. Sea level is relatively well constrained for the Last Glacial Maximum (LGM, 26,500 to 19,000 y ago, 26.5 to 19 ka) and the ensuing deglaciation. However, sea-level estimates for the period of ice-sheet growth before the LGM vary by …
The fraction of phytoplankton growth that leads to the rain of organic carbon out of the sunlit surface ocean ("export production") is central to the ocean s sequestration of atmospheric carbon dioxide. Nitrate assimilation has long been taken as a measure of export production; however, this has recently been questioned by suggestions that much of…
Despite slow nutrient supply to the subtropical surface ocean, its rates of annual inorganic carbon drawdown and net oxygen production are similar to those of nutrient-rich high latitude waters. This surprisingly rapid carbon drawdown, if due to the production and export of marine biomass, cannot be explained in terms of known nutrient supply…
Phytoplankton growth is potentially limited by the scarcity of biologically available forms of nitrogen such as nitrate and ammonium. In the subtropical ocean gyres, water column stratification impedes the upward flux of nitrate to surface waters. Phytoplankton in these waters are assumed to rely largely on ammonium and other forms of nitrogen…
The Sargasso Sea is characterized by strong summertime stratification that is thought to drive oligotrophy, but export production is surprisingly similar to that of high-latitude regions with ample major nutrient supply. Here we use the summer-to-fall progression in the northwestern Sargasso Sea to investigate the relationship between upper ocean…
The nitrogen-isotope record preserved in Southern Ocean sediments, along with several geochemical tracers for the settling fluxes of biogenic matter, reveals patterns of past nutrient supply to phytoplankton and surface-water stratification in this oceanic region. Areal averaging of these spatial patterns indicates that reduction of the CO2 leak …
Roughly 240 million years ago (Ma), scleractinian corals rapidly expanded and diversified across shallow marine environments. The main driver behind this evolution is uncertain, but the ecological success of modern reefbuilding corals is attributed to their nutritional symbiosis with photosynthesizing dinoflagellate algae. We show that a suite of…
We report concentration and nitrogen and oxygen isotopic measurements of nitrate, total dissolved nitrogen, and particulate nitrogen from Antarctic landfast sea ice, covering almost the complete seasonal cycle of sea ice growth and decay (from April to November). When sea ice forms in autumn, ice algae growth depletes nitrate and accumulates…
Ocean circulation supplies the surface ocean with the nutrients that fuel global ocean productivity. However, the mechanisms and rates of water and nutrient transport from the deep ocean to the upper ocean are poorly known. Here, we use the nitrogen isotopic composition of nitrate to place observational constraints on nutrient transport from the…
We report nitrogen (N) isotopic measurements of nitrate, total dissolved nitrogen, and particulate nitrogen from Antarctic pack ice in early and late spring. Salinity-normalized concentrations of total fixed N are approximately twofold higher than in seawater, indicating that sea ice exchanges fixed N with seawater after its formation. The…
It is understood that the global mean ocean nitrate δ15N is set by the δ15N of the input of fixed nitrogen (N) to the ocean (mostly N2 fixation) and the net isotopic discrimination of fixed N loss (mostly denitrification). Here, we demonstrate that, in addition to the fixed nitrogen input/output budget, the isotopic discrimination of nitrate…
Both the nitrogen (N) isotopic composition (δ 15 N) of the nitrate source and the magnitude of isotope discrimination associated with nitrate assimilation are required to estimate the degree of past nitrate consumption from the δ 15 N of organic matter in Southern Ocean sediments (e.g., preserved within diatom microfossils). It has been suggested…
This chapter reviews the ongoing efforts to use sediment and ice core records to understand the dynamics of oceanic fixed nitrogen (N), focusing on recent glacial-interglacial cycles. Research has, up to this point, followed the reductionist approach of trying to reconstruct either changes in the ocean N budget (largely through low latitude…