Isotopic evidence that recent agriculture overprints climate variability in nitrogen deposition to the Tibetan Plateau

The stable isotopes of nitrogen in nitrate archived in polar ice have been interpreted as reflecting a shift in reactive nitrogen sources or changes in atmospheric chemical reactivity. Here, we present a novel concentration and isotopic record of nitrate (delta N-15-NO3-) from a central Tibetan Plateau ice core over the last similar to 200 years. We find that nitrate concentration increased from 6.0 +/- 2.3 mu eq/L (mean +/- 1 sigma) in the preindustrial period (prior to 1900s) to 7.3 +/- 2.7 mu eq/L in post-1950. Over the same time period, the delta N-15-NO3- decreased from 8.7 +/- 3.7 parts per thousand to 4.2 +/- 3.1 parts per thousand, with much larger interannual variation in delta N-15-NO3- during the preindustrial period. We present a useful framework for quantifying the sensitivity of the isotopic composition of atmospheric nitrate to changes in both sources and chemistry (gas and aerosol phase). After 1950, nitrogen deposition is primarily driven by fertilizer use, leading to significant increases in concentration and decreases in delta N-15-NO3-. The large interannual variability of ice core delta N-15-NO3- in the preindustrial reflects natural processes, namely the El Nino Southern Oscillation (ENSO) and dust events. Our results highlight a new connection between the nitrogen cycle and ENSO, and the overprinting of natural climate signals by recent anthropogenic increases in reactive nitrogen release.