J. Stutsman

The isotopic composition of atmospheric methane

Measurements of the 13C/12C, D/H and 14C composition of atmospheric methane (CH4) between 1988 and 1995 are presented. The 13C/12C measurements represent the first global data set with time series records presented for Point Barrow, Alaska (71°N), Olympic Peninsula, Washington (48°N), Mauna Loa, Hawaii (20°N), American Samoa (14°S), Cape Grim, Australia (41°S), and Baring Head, New Zealand (41°S). North-south trends of the 13C/12C and D/H of atmospheric CH4 from air samples collected during oceanographic research cruises in the Pacific Ocean are also presented. The mean annual δ13C increased southward from about −47.7 ‰ at 71°N to −41.2 ‰ at 41°S. The amplitude of the seasonal cycle in δ13C ranged from about 0.4 ‰ at 71°N to 0.1 ‰ at 14°S. The seasonal δ13C cycle at sites in tropical latitudes could be explained by CH4 loss via reaction with OH radical, whereas at temperate and polar latitudes in the northern hemisphere seasonal changes in the δ13C of the CH4 source were needed to explain the seasonal cycle. The higher δ13C value in the southern (−47.2 ‰) versus northern (−47.4 ‰) hemisphere was a result of interhemispheric transport of CH4. A slight interannual δ13C increase of 0.02±0.005 ‰ yr−1 was measured at all sites between 1990 and 1995. The mean δD of atmospheric CH4 was −86±3 ‰ between 1989 and 1995 with a 10 ‰ depletion in the northern versus southern hemisphere. The 14C content of CH4 measured at 48°N increased from 122 to 128 percent modern between 1987 and 1995. The proportion of CH4 released from fossil sources was 18±9% in the early 1990s as derived from the 14C content of CH4.

Carbon isotopic composition of atmospheric CH4: Fossil and biomass burning source strengths

The 13C/12C of atmospheric methane (CH4) was measured at Point Barrow (71°N, 156°W), Olympic Peninsula (48°N, 126°W), Mauna Loa (19°N, 155°W), and Cape Grim (41°S, 144°E) between 1987 and 1989. The global average δ13CPDB from these measurements (n = 208) was −47.20 ± 0.13%o. The lowest mean annual δ13C value of-47.61 ± 0.14‰ was measured at Point Barrow with values increasing to -47.03 ± 0.14‰ at Cape Grim. The seasonal cycle in the δ13C of CH4 was greatest at Point Barrow, with an amplitude of 0.5‰, and varied inversely with concentration. The isotopic fractionation during CH4 oxidation is calculated to be 0.993 ± 0.002 based on the measured CH4 concentration and δ13C values. The 14C content of atmospheric CH4, measured at monthly intervals at the Olympic Peninsula site between 1987 and 1989, is increasing at 1.4 ± 0.5 pM yr−1, primarily owing to 14CH4 release from nuclear reactors. The global average 14C content of 122 pM for CH4 implies a fossil methane source strength that is 16% of the total source. The global mean δ13C of −47.2‰, when coupled with the 14C results, implies that ∼11% of the total CH4 release rate is derived from biomass burning. These results indicate for a total CH4 source of ∼550 Tg yr−1 that natural gas release accounts for ∼90 Tg yr−1 and biomass burning yields ∼60 Tg yr−1. Preliminary analyses of the δ13C data using a three-dimensional chemical tracer model indicate that the observed meridional gradients in the annual average δ13C and concentration of CH4 are most closely matched with a CH4 source scenario in which 11% of the CH4 is derived from biomass burning.

Surface layer carbon budget for the subtropical N. Pacific: δ13C constraints at station ALOHA

Abstract The rate of biological organic carbon export at the time series station ALOHA in the subtropical N. Pacific (23° N 158° W ) has been estimated from monthly measurements of dissolved inorganic carbon (DIC) and the δ 13 C of the DIC in the surface layer between 1994 and 1999. The most consistent feature of the annual DIC and δ 13 C cycles occurs during summer (April–September) when there is a 14 μmol kg −1 decrease in DIC and a 0.1‰ increase in δ 13 C . During this period, when the mixed layer depth is ∼50 m , the DIC decrease corresponds to a loss of 4.9 mmol m −2 d −1 . The mean p CO 2 of the surface ocean during the summer is ∼10 μatm lower than atmospheric p CO 2 yielding a net air–sea CO 2 invasion of 1.6±1.0 mmol m −2 d −1 . Horizontal advection, as a result of Ekman and geostrophic velocities, yields a DIC loss of 1.1±0.7 mmol m −2 d −1 . Insignificant alkalinity changes indicate that CaCO 3 loss is negligible. Surface layer DIC and DI 13 C budgets were used to solve for two carbon fluxes, the net community production (NCP) and the upward supply of DIC resulting from vertical mixing at the base of the mixed layer. During the summer, the calculated rates of NCP and upward DIC supply are 7.2±2.9 and 1.8±3.9 mmol m −2 d −1 , respectively. Thus the measured DIC and δ 13 C changes during the summer at ALOHA indicate that the DIC draw down is primarily the result of NCP exceeding DIC supplied via air–sea CO 2 exchange and vertical diffusion. Over an annual cycle, the DIC and DIC 13 budgets indicate that NCP at 6.8±3.4 mmol m −2 d −1 is approximately balanced by the upward DIC flux of 6.5±4.8 mmol m −2 d −1 resulting from vertical mixing. These NCP rates agree well with previously published estimates at ALOHA of 6– 7.4 mmol m −2 d −1 . Extrapolating a NCP rate of 7 mmol m −2 d −1 at ALOHA to the global subtropical ocean yields a rate of 6.3 Gt C yr −1 that represents more than half of the estimated global ocean organic carbon export rate of ∼11 Gt C yr −1 .

Primary production and carbon export rates across the subpolar N. Atlantic Ocean basin based on triple oxygen isotope and dissolved O2 and Ar gas measurements

Gross photosynthetic O2 production (GOP) rates in the subpolar North Atlantic Ocean were estimated using the measured isotopic composition of dissolved oxygen in the surface layer on samples collected on nine transits of a container ship between Great Britain and Canada during March 2007 to June 2008. The mean basin-wide GOP rate of 226 ± 48 mmol O2 m−2 d−1 during summer was double the winter rate of 107 ± 41 mmol O2 m−2 d−1. Converting these GOP rates to equivalent 14C-based PP (14C-PPeqv) yielded rates of 1005 ± 216 and 476 ± 183 mg C m−2 d−1 in summer and winter, respectively, that generally agreed well with previous 14C-based PP estimates in the region. The 14C-PPeqv estimates were 1–1.6× concurrent satellite-based PP estimates along the cruise track. A net community production rate (NCP) of 87 ± 12 mmol O2 m−2 d−1 (62 ± 9 mmol C m−2 d−1) and NCP/GOP of 0.35 ± 0.06 in the mixed layer was estimated from O2/Ar and 17Δ measurements (61°N 26°W) during spring bloom conditions in May 2008. Contrastingly, a much lower long-term annual mean NCP or organic carbon export rate of 2.8 ± 2.7 mol C m−2 yr−1 (8 ± 7 mmol C m−2 d−1) and NCP/GOP of 0.07 ± 0.06 at the winter mixed layer depth was estimated from 15 years of surface O2 data in the subpolar N. Atlantic collected during the CARINA program.

Atmospheric 14CO: A tracer of OH concentration and mixing rates

Time series measurements of the ground level 14CO concentration were made at Olympic Peninsula, Washington (48°N), and Point Barrow, Alaska (71°N), between 1991 and 1997. Measurements of the meridional gradients of the 14CO concentration at sea level were made during five oceanographic cruises in the Pacific Ocean between 55°N and 65°S during 1991–1995. These measurements were combined with earlier time series measurements of atmospheric 14CO at 41°S and 77°S [Brenninkmeijer, 1993] and at 13°N [Mak and Southon, 1998] and meridional transects of 14CO at 6–8 km [Mak et al., 1994]. These 14CO data sets were analyzed using a two-dimensional atmospheric circulation and chemistry model in order to determine the tropospheric OH concentration that could explain the temporal and spatial trends in 14CO. Additionally, the interannual trend in tropospheric methyl chloroform concentration and the stratospheric time history of bomb 14CO2 were simulated by the model. The results of this analysis indicate that an average tropospheric OH concentration of ∼l0×l05 radicals cm−3 explains both the 14CO and methyl chloroform trends. The model-predicted 14CO concentrations, however, are sensitive to the rate of stratosphere-troposphere exchange and horizontal mixing in the troposphere. Model predictions of tropospheric 14CO at high latitudes improved when the stratosphere-troposphere exchange rate was slowed, based on the results of the stratospheric bomb 14CO2 model simulation. Substantial improvement in the model 14CO simulations occurred with increased horizontal diffusion rates in the troposphere and lower cosmogenic 14CO production rates. Significantly lower 14CO concentrations (∼50%) are observed in the Southern versus Northern Hemisphere. Model simulations indicate that either higher tropospheric horizontal mixing or higher OH concentrations in the Southern Hemisphere can explain the hemispheric asymmetry in 14CO.