The utility of isotopic distributions in the environment is premised on knowledge of the magnitudes of isotope discrimination by individual biogeochemical reactions, which is most often gained through lab studies of cultured organisms. Our current focus is on the isotope effects of nitrate-consuming processes, in particular, nitrate assimilation by photosynthetic organisms (central to studies of nutrient supply and uptake in the surface ocean) and denitrification by heterotrophic bacteria (central to studies of the global ocean’s input/output budget of fixed N). While the core goal of this work is to help with environmental and paleoceanographic application of the N isotopes, the sensitivities of isotope effects also offer insight into cell physiology and enzyme mechanism.
Increase in nitrate δ15N and δ18O during nitrate assimilation by three species of diatoms and E. huxleyi grown under various environmental conditions. a) Change in nitrate δ15N relative to initial nitrate δ15N plotted as a function of 1n(f) (f being the fraction of initial nitrate that remains in the medium). The slopes of individual experiments approximate the N isotope effect on NO3-(15ε). Dashed lines show slopes for 15ε of 5 and 25‰. b) The δ15N vs. δ18O change in NO3-. The slope of the linear regression and its standard error (1.00 ± 0.01) are inclusive of all the experiments combined. Dashed lines show slopes of 1.1 and 0.9, for comparison. From Granger et al. (2004)
References
The oxygen content of the oceans is susceptible to climate change and has declined in recent decades1, with the largest effect in oxygen-deficient zones (ODZs)2 , that is,mid-depth ocean regions with oxygen concentrations <5 μmol kg −1 (ref. 3). Earth-system-model simulations of climate warming predict that ODZs will expand until at least…
Nitrogen isotopes are widely used to study the trophic position of animals in modern food webs; however, their application in the fossil record is severely limited by degradation of organic material during fossilization. In this study, we show that the nitrogen isotope composition of organic matter preserved in mammalian tooth enamel …
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…
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…
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 …
Biological dinitrogen fixation is the major source of new nitrogen to marine systems and thus essential to the ocean’s biological pump. Constraining the distribution and global rate of dinitrogen fixation has proven challenging owing largely to uncertainty surrounding the controls thereon. Existing South Atlantic dinitrogen fixation rate…
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…
Fossil-bound organic material holds great potential for the reconstruction of past changes in nitrogen (N) cycling. Here, with a series of laboratory experiments, we assess the potential effect of oxidative degradation, fossil dissolution, and thermal alteration on the fossil-bound N isotopic composition of different fossil types, including…
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…
We present a dual-modulation Faraday rotation spectrometer with in-line optical subtraction for differential measurement of nitric oxide (NO) isotopologues. In-situ sample referencing is accomplished via differential dual-cell measurements, with 3.1 ppbv/rt(Hz) (15NO) sensitivity through 15 cm optical path length. Our system operates at 1.9x…
For times prior to those represented by the air trapped in Antarctic ice core records, the concentration of CO2 in the atmosphere must be reconstructed using geochemical proxies. The δ13C of particulate organic carbon (POC) produced in ocean surface waters has previously been observed to covary with the concentration of CO2 in the water…
The GEOTRACES Intermediate Data Product 2017 (IDP2017) is the second publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2016. The IDP2017 includes data from the Atlantic, Pacific, Arctic, Southern and Indian oceans, with about twice the data volume of the…