Bibliography - T. R. Knutson

1. Sigman, Daniel, P. J. DiFiore, M. P. Hain, C. Deutsch, Y. Wang, D. Karl, T. R. Knutson, K. K. Lehman, and S. Pantoja, 2009: The dual isotopes of deep nitrate as a constraint on the cycle and budget of oceanic fixed nitrogen. Deep Sea Research I, 56(9), doi:10.1016/j.dsr.2009.04.007 1419-1439
   [ Abstract ]

   We compare the output of an 18-box geochemical model of the ocean with measurements to investigate the controls on both the mean values and variation of nitrate δ¹⁵N and &delta:¹⁸O in the ocean interior. The &delta:¹⁸O of nitrate is our focus because it has been explored less in previous work. Denitrification raises the δ¹⁵N and &delta:¹⁸O of mean ocean nitrate by equal amounts above their input values for N₂ fixation (for δ¹⁵N) and nitrification (for &delta:¹⁸O), generating parallel gradients in the δ¹⁵N and &delta:¹⁸O of deep ocean nitrate. Partial nitrate assimilation in the photic zone also causes equivalent increases in the δ¹⁵N and &delta:¹⁸O of the residual nitrate that can be transported into the interior. However, the regeneration and nitrification of sinking N can be said to decouple the N and O isotopes of deep ocean nitrate, especially when the sinking N is produced in a low latitude region, where nitrate consumption is effectively complete. The δ¹⁵N of the regenerated nitrate is equivalent to that originally consumed, whereas the regeneration replaces nitrate previously elevated in &delta:¹⁸O due to denitrification or nitrate assimilation with nitrate having the &delta:¹⁸O of nitrification. This lowers the &delta:¹⁸O of mean ocean nitrate and weakens nitrate &delta:¹⁸O gradients in the interior relative to those in δ¹⁵N. This decoupling is characterized and quantified in the box model, and agreement with data shows its clear importance in the real ocean. At the same time, the model appears to generate overly strong gradients in both &delta:¹⁸O and δ¹⁵N within the ocean interior and a mean ocean nitrate &delta:¹⁸O that is higher than measured. This may be due to, in the model, too strong an impact of partial nitrate assimilation in the Southern Ocean on the δ¹⁵N and &delta:¹⁸O of preformed nitrate and/or too little cycling of intermediate-depth nitrate through the low latitude photic zone.

2. Donner, S. D., T. R. Knutson, and Michael Oppenheimer, 2007: Model-based assessment of the role of human-induced climate change in the 2005 Caribbean coral bleaching event. Proceedings of the National Academy of Sciences of the United States of America, 104(13), doi:10.1073/pnas.0610122104 5483-5488
   [ Abstract ]

   Episodes of mass coral bleaching around the world in recent decades have been attributed to periods of anomalously warm ocean temperatures. In 2005, the sea surface temperature (SST) anomaly in the tropical North Atlantic that may have contributed to the strong hurricane season caused widespread coral bleaching in the Eastern Caribbean. Here, we use two global climate models to evaluate the contribution of natural climate variability and anthropogenic forcing to the thermal stress that caused the 2005 coral bleaching event. Historical temperature data and simulations for the 1870–2000 period show that the observed warming in the region is unlikely to be due to unforced climate variability alone. Simulation of background climate variability suggests that anthropogenic warming may have increased the probability of occurrence of significant thermal stress events for corals in this region by an order of magnitude. Under scenarios of future greenhouse gas emissions, mass coral bleaching in the Eastern Caribbean may become a biannual event in 20–30 years. However, if corals and their symbionts can adapt by 1– 1.5°C, such mass bleaching events may not begin to recur at potentially harmful intervals until the latter half of the century. The delay could enable more time to alter the path of greenhouse gas emissions, although long-term ‘‘committed warming’’ even after stabilization of atmospheric CO₂ levels may still represent an additional long-term threat to corals.

Direct link to page: http://cmi.princeton.edu/bibliography/results.php?author=3974