Bibliography - K. K. Lehman
- 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.
- Onstott, T. C., D. McGown, J. Kessler, B. S. Lollar, K. K. Lehman, and S. M. Clifford, 2006: Martian CH4: sources, flux and detection. Astrobiology, 6(2), doi:10.1089/ast.2006.6.377 377-395
[ Abstract ]Recent observations have detected trace amounts of CH4 heterogeneously distributed in the
martian atmosphere, which indicated a subsurface CH4 flux of 2 X 105 to 2 X 109 cm-2 s-1.
Four different origins for this CH4 were considered: (1) volcanogenic; (2) sublimation of hydrate-
rich ice; (3) diffusive transport through hydrate-saturated cryosphere; and (4) microbial
CH4 generation above the cryosphere. A diffusive flux model of the martian crust for He, H2,
and CH4 was developed based upon measurements of deep fracture water samples from South
Africa. This model distinguishes between abiogenic and microbial CH4 sources based upon
their isotopic composition, and couples microbial CH4 production to H2 generation by H2O
radiolysis. For a He flux of 105 cm-2 s-1 this model yields an abiogenic CH4 flux and a microbial
CH4 flux of 106 and 109 cm-2 s-1, respectively. This flux will only reach the martian
surface if CH4 hydrate is saturated in the cryosphere; otherwise it will be captured within
the cryosphere. The sublimation of a hydrate-rich cryosphere could generate the observed
CH4 flux, whereas microbial CH4 production in a hypersaline environment above the hydrate
stability zone only seems capable of supplying 105 cm-2 s-1 of CH4. The model predicts
that He/H2/CH4/C2H6 abundances and the C and H isotopic values of CH4 and the C isotopic
composition of C2H6 could reveal the different sources. Cavity ring-down spectrometers represent
the instrument type that would be most capable of performing the C and H measurements
of CH4 on near future rover missions and pinpointing the cause and source of the CH4
emissions.
- Sigman, Daniel, J. Granger, P. J. DiFiore, K. K. Lehman, R. Ho, G. Cane, and A. van Geen, 2005: Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin. Global Biogeochemical Cycles, 19(GB4022), doi:10.1029/2005GB002458
[ Abstract ]Water column depth profiles along the North Pacific margin from Point Conception
to the tip of Baja California indicate elevation of nitrate (NO3) 15N/14N and 18O/16O
associated with denitrification in the oxygen-deficient thermocline waters of the
eastern tropical North Pacific. The increase in δ18O is up to 3% greater than in δ15N,
whereas our experiments with denitrifier cultures in seawater medium indicate a 1:1
increase in NO3
δ18O and δ15N during NO3
consumption. Moreover, the maximum in
NO3
δ18O is somewhat shallower than the maximum in NO3
δ15N. These two
observations can be summarized as an anomaly from the 1:1 δ18O-to-δ15N relationship
expected from culture results. Comparison among stations and with other data indicates
that this anomaly is generated locally. The anomaly has two plausible interpretations: (1)
the addition of low-δ15N NO3
to the shallow thermocline by the remineralization of
newly fixed nitrogen, or (2) active cycling between NO3
and NO2
(coupled NO3
reduction and NO2
oxidation) in the suboxic zone.
Direct link to page: http://cmi.princeton.edu/bibliography/results.php?author=4122
