Isotope Method Development

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

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

220 Publications

We have previously argued that the Antarctic and subarctic North Pacific are stratified during ice ages, causing to a large degree the observed low CO2 levels of ice age atmospheres by sequestering respired CO2 in the ocean abyss. Here, we suggest a mechanism for the major deglaciations of the late Pleistocene. The mechanism begins with…

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…
This article outlines the isotope systematics of the major processes in the ocean nitrogen (N) cycle, their impact on the isotopic compositions of the major N reservoirs in the ocean, and how the N isotopes in the modern ocean and sedimentary record may record current N fluxes and past changes. The dominant dynamic is of kinetic isotope…
The ocean s biological pump refers to the coupled biological, chemical, and physical processes that work to concentrate carbon and other biologically active elements in the voluminous ocean interior, sequestering them from the surface ocean and the atmosphere. Current research seeks to understand the relationship of the ocean s biological pump…
This article outlines the isotope systematics of processes in the ocean nitrogen (N) cycle and their impact on the isotopic compositions of the major N forms, with an eye toward how the N isotopes may record current N fluxes and past changes. The dominant dynamic is of kinetic isotope discrimination, in which the light isotope of N (14N) is…
A dual-modulation Faraday rotation spectrometer is employed for isotopic ratiometry of nitric oxide (NO) converted from nitrate/nitrite. Excellent linearity of measured NO to dissolved nitrate is demonstrated. Ratiometry of IAEA-NO-3 standards indicates 3 % accuracy. © 2014 OSA.
A dual-modulation Faraday rotation spectrometer is employed for isotopic ratiometry of nitric oxide (NO) converted from nitrate/nitrite. Excellent linearity of measured NO to dissolved nitrate is demonstrated. Ratiometry of IAEA-NO-3 standards indicates ∼3 % accuracy. © 2014 Optical Society of America.
We present real-time optical polarization subtraction for calibration of Faraday rotation spectra. Noise analysis yields minor isotope sensitivity of 3.0 ppbv·Hz-1/2 and 1.7×10-8 rad·Hz-1/2 noise-equivalent angle. Sub-permil ratiometric precision is achieved at integration times >100 s. © 2016 OSA.
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 …
We have adapted the ammonia diffusion method of nitrate extraction for natural-abundance level nitrogen isotopic measurement of oceanic nitrate. The method involves: (1) sample concentration (by boiling or evaporation); (2) conversion of nitrate to ammonia using Devarda s alloy; and (3) the gas-phase diffusion of ammonia onto an acidified glass…
A aeries of climate simulations using on atmospheric general circulation modal shows that maintaining ocean heat transport at close to present-day values, but with otherwise glacial boundary conditions, leads to an enhanced cooling, particularly in the tropics. This is in agreement with recent geochemical evidence from fossil corals, ground waters…
We investigate the response of the calcite lysocline to changes in the export production of the low-latitude surface ocean (the combined equatorial, tropical, and subtroical regions). We employ different CaCO3 throughput schemes in a time-dependent ocean carbon cycle model to separate the CaCO3 production/iysocline balance from the other…
The effect of sea level change on nutrient supply to the anoxic Cariaco demonstrates the fundamental importance of nitrogen (N2) fixation and phosphate to oceanic production. As N2 fixation produces biomass of low δ15N and has been reported to be an important component of the nitrogen cycle in the modern Cariaco Basin, we propose that it…
We present a method for measuring 15N-NH4+ in marine, estuarine and fresh waters. The advantage of this method is that it is broadly applicable to all types of water and it allows measurements in samples with lower ammonium concentrations than has previously been possible. The procedure is a modification of the ammonia diffusion method and uses…
Temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon-based age-estimates of biogenic material that do not agree with true calendar age. This problem is particularly severe beyond the limit of the high-resolution radiocarbon calibration based on tree-ring data, which stretches back only to about 11.8…
Treatment of diatom microfossils from Southern Ocean sediments with hot perchloric acid leaves a diatom-bound N fraction which is 0-4‰ lower in δ15N than the bulk sediment, typically 3‰ lower in recent Antarctic diatom ooze. Results from Southern Ocean surface sediments indicate that early diagenetic changes in bulk sediment N content and δ15N…
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…
High-resolution records of the nitrogen isotopic composition of organic matter (δ15Norg), opal content, and opal accumulation rates from the central Gulf of California reveal large and abrupt variations during deglaciation and gradual Holocene changes coincident with climatic changes recorded in the North Atlantic. Homogenous sediments with…
The surface waters of the modern subarctic Pacific Ocean are isolated from the nutrient-rich waters below by a steep vertical gradient in salinity (halocline), a feature which is a dominant control on upper-ocean stratification in polar environments. The physical processes which maintain the halocline and, in turn, its physical, biological, and…
Twenty years ago, measurements on ice cores showed that the concentration of carbon dioxide in the atmosphere was lower during the ice ages than it is today. As yet, there is no broadly accepted explanation for this difference. Current investigations focus on the ocean s biological pump , the sequestration of carbon in the ocean interior by the…
We report analyses of the nitrogen isotopic composition of nitrate in the eastern Indian and Pacific sectors of the Southern Ocean. In this paper, we focus on the subsurface data as well as data from the deep waters of other ocean basins. Nitrate δ15N is relatively invariant in much of the abyssal ocean (i.e., below 2.5 km), with a value of 4.8±0…
We report a new method for measurement of the isotopic composition of nitrate (NO3-) at the natural-abundance level in both seawater and freshwater. The method is based on the isotopic analysis of nitrous oxide (N2O) generated from nitrate by denitrifying bacteria that lack N2O-reductase activity. The isotopic composition of both nitrogen and…
Titanium and iron concentration data from the anoxic Cariaco Basin, off the Venezuelan coast, can be used to infer variations in the hydrological cycle over northern South America during the past 14,000 years with subdecadal resolution. Following a dry Younger Dryas, a period of increased precipitation and riverine discharge occurred during the…
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 …
We report the results of a field incubation experiment demonstrating a substantial shift in the taxonomic composition of Equatorial Pacific phytoplankton assemblages exposed to CO2 levels of 150 and 750 ppm (dissolved CO2 ∼3 to 25 μM). By the end of the experiment, the phytoplankton community in all samples was dominated by diatoms and Phaeocystis…
The surface water of the marine environment has traditionally been viewed as a nitrogen (N) limited habitat, and this has guided the development of conceptual biogeochemical models focusing largely on the reservoir of nitrate as the critical source of N to sustain primary productivity. However, selected groups of Bacteria, including cyanobacteria,…
We used the nitrogen isotope ratio of algae, suspended particles and nitrate in the water column to track spatial variations in the marine nitrogen cycle in the Mediterranean Sea. Surface PON (5-74 m) was more depleted in 15N in the eastern basin (-0.3±0.5‰) than in the western basin (+2.4±1.4‰), suggesting that nitrogen supplied by biological N2…
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…
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…
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…
In the anoxic Cariaco Basin of the southern Caribbean, the bulk titanium content of undisturbed sediment reflects variations in riverine input and the hydrological cycle over northern tropical South America. A seasonally resolved record of titanium shows that the collapse of Maya civilization in the Terminal Classic Period occurred during an…
Recent evidence from the sulfur isotopic record indicates a transition ∼2.5 billion years ago from an ocean chemistry first dominated by iron and then by sulfide. It has been hypothesized that the selection of metal centers in metalloenzymes has been influenced by the availability of metals through geological time, in particular as a result of…
Rainwater collected on the island of Bermuda between January 2000 and January 2001 shows pronounced seasonal variation in the nitrogen and oxygen isotopic composition of nitrate. Higher 15N/14N and lower 18O/16O ratios are observed in the warm season (April-September) in comparison to the cool season (October-March): The mean δ15N of nitrate for…
Using an ocean box model, we have studied the effect of altered circulation on the oceanic distributions of phosphate (PO4-3) and the 13C/12C and 14C/12C of dissolved inorganic carbon to evaluate competing hypotheses for the cause of observed nutrient depletion and 13C enrichment at intermediate depths of the Atlantic during the last ice age…
The 15N 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 CO2 concentration. Interpretation has relied on a temporally constant isotope effect (ε) associated with uptake and assimilation of nitrate by…
We report measurements of the δ15N of nitrate, suspended particulate nitrogen (PN), and sinking PN from cruises and moored sediment traps in the Subantarctic Zone (SAZ) and Polar Frontal Zone (PFZ) south of Australia. As expected, surface water nitrate δ15N increased as nitrate was consumed during the spring/summer bloom. In contrast, the seasonal…
[1] Below its sill depth, the Santa Barbara Basin (SBB) is commonly suboxic ([O2] ̃ 3 μM), with only brief periods of ventilation. Associated with development of suboxia, the concentration of nitrate decreases with depth into the basin without an associated decrease in phosphate, indicating that a substantial fraction of the nitrate supplied to…
We report the first measurements of coupled nitrogen (N) and oxygen (O) isotopic variations of nitrate (NO3-) during its assimilation by laboratory cultures of marine phytoplankton and derive the N and O kinetic isotope effects for nitrate assimilation by three species of diatoms (Thalassiosira weissflogii, Thalassiosira oceanica, and…
We report 15N/14N measurements of water column nitrate and ammonium, sinking particles, and sediments from the Cariaco Basin, an anoxic marine basin off the coast of Venezuela. Water column denitrification occurring in the basin has only a very small isotopic imprint on nitrate in the basin because nitrate consumption is nearly complete in the…
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…
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…
We report 15N/14N and 18O/16O ratios of nitrate in benthic chamber incubations in the continental shelf sediments of the Santa Monica Bay (SMB) to deconvolve the effects of nitrification and denitrification. Estimates of denitrification rate from benthic flux stoichiometry range from 0.9 to 2.5 mmol N m-2 d -1. Between 46% and 100% of the total…
The low-latitude ocean is strongly stratified by the warmth of its surface water. As a result, the great volume of the deep ocean has easiest access to the atmosphere through the polar surface ocean. In the modern polar ocean during the winter, the vertical distribution of temperature promotes overturning, with colder water over warmer, while the…
Nitrogen and oxygen isotopes of NO3- have been measured in snow and firn from Summit, Greenland. The 15N/ 14N and 18O/16O ratios of NO 3- in recently fallen snow are similar to those of surface snow. Diurnal variation is observed in 15N/ 14N of NO3-, and possibly 18O/16O, suggesting fractionating loss of NO3- from snow during the day, which is…
The 15N/14N of nitrate in the external medium and intracellular pool of the cultured marine diatom Thalassiosira weissflogii (Grun.) Fryxell et Hasle was measured during nitrate assimilation under low light, a 12:12-h light:dark cycle, low temperature, or low iron conditions. The 15N/14N of the nitrate in the medium and the particulate matter both…
To better constrain the dynamics of the dissolved organic nitrogen (DON) pool and the role of N2 fixation in the nitrogen cycle at the Bermuda Atlantic Time-series Study (BATS) site, we measured the 15N/14N ratio of total nongaseous nitrogen (TN) in the upper 250 m and of nitrate in the upper 1000 m of monthly water column profiles from June 2000…
On the basis of the normalization to phosphate, a significant amount of nitrate is missing from the deep Bering Sea (BS). Benthic denitrification has been suggested previously to be the dominant cause for the BS nitrate deficit. We measured water column nitrate 15N/14N and 18O/16O as integrative tracers of microbial denitrification, together with…
Diatom-bound 15N/14N was used to reconstruct the glacial nutrient status of the Subantarctic Zone in the Southern Ocean. Down-core records from both the Pacific and Indian sectors show δ 15N of 5 to 6%‰ during the Last Glacial Maximum and a decrease, coincident with the glacial termination, to values as low as 2‰. The effect of either diatom…