A water sample plots below the Global Meteoric Water Line (GMWL) on a delta-D vs delta-18O diagram, with higher delta-18O than expected for its delta-D. What process most likely caused this?
ARadioactive decay of deuterium
BEvaporation from an open water body, which kinetically fractionates 18O more than D relative to equilibrium, producing an evaporation line with slope 3-5 (below the GMWL slope of 8)
CMixing with seawater
DGeothermal heating of groundwater
During evaporation under non-equilibrium conditions, the kinetic fractionation factor for 18O/16O is larger relative to its equilibrium value than for D/H, because the diffusivity difference is proportionately larger for the lighter oxygen isotopes. This causes evaporated water to plot along a line with slope 3-5 rather than the GMWL slope of 8. Samples below the GMWL have experienced evaporative enrichment -- common in arid-region lakes, irrigation return flow, and soil water.
Question 2 True / False
Delta-15N values increase by approximately 3-4 per mil with each trophic level in food webs.
TTrue
FFalse
Answer: True
Metabolic processes preferentially excrete the lighter 14N isotope (in urine, feces, ammonia), leaving the organism's tissues enriched in 15N relative to its diet. This ~3.4 per mil per trophic level enrichment is remarkably consistent across ecosystems and is used to determine trophic position in ecological studies. A top predator (trophic level 4) would be enriched by ~10 per mil over the baseline primary producers.
Question 3 Short Answer
Explain how nitrogen isotopes distinguish between agricultural fertilizer runoff and sewage as sources of nitrate contamination in a river.
Think about your answer, then reveal below.
Model answer: Synthetic fertilizer nitrogen is produced by the Haber-Bosch process from atmospheric N2, so it has delta-15N near 0 per mil (similar to air). Animal waste and sewage have delta-15N of +10 to +20 per mil because biological processing (volatilization of 14N-enriched ammonia) leaves the residual nitrogen enriched in 15N. Nitrate in the river with delta-15N near 0 indicates fertilizer origin; values above +10 per mil indicate sewage or manure. Intermediate values suggest mixing. Paired delta-15N and delta-18O of nitrate can further discriminate sources, since synthetic fertilizer nitrate has distinctive delta-18O values.
The source materials have fundamentally different isotopic compositions due to their different formation pathways, making delta-15N a direct tracer of contamination origin.