Paul D. Quay

Carbon isotope fractionation during gas-water exchange and dissolution of CO2

The kinetic and equilibrium fractionation effects for 13C during CO2 gas transfer (ek and eag−g) have been measured in acidified distilled water. The equilibrium fractionation effects between bicarbonate and carbonate and gaseous C02 (eHCO3−g and eCO3−g) have been measured in NaHC03 and NaHC03 + Na2C03 solutions, respectively, from 5° to 25°C. The measured fractionations, except eCO3−g, agreed with earlier work to within 0.2‰. eCO3−g was about 2‰ smaller than most values previously reported. The temperature dependence of the fractionation for 13C between bicarbonate and carbonate and gaseous CO2 was found to be eHCO3−g = −(0.141 ± 0.003)T(°C) + 0.05)‰ and eCO3−g = −(0.052 ± 0.03) T(°C)E+ (7.22 ± 0.46)‰ respectively. The fractionation during gas dissolution was eCO3−g = −0.03)T(°C) + (1.31 ± 0.06%. and the kinetic effect during gas transfer, ek, was −0.81 ± 0.16‰ at 21°C and −0.95 ± 0.20‰ at 5°C. The equilibrium fractionation between total DIC in seawater and CO2 in air (eDIC−g) was measured and compared with that calculated from the concentration of aqueous CO2, HC03− and CO3= and individual fractionations between the three C species and CO2 gas. The measured and calculated results showed a significant difference of up to 0.2‰. We hypothesize that carbonate ion complexes likely complicate the calculation of eDIC−g from individual C species. We obtain the following empirical function of eDIC−g in seawater vs. temperature and the carbonate fraction (fCO3), eDIC−g = (0.014 ± 0.01) TfCO3 − (0.105 ± 0.002) T + (10.53 ± 0.05)%., when 0.05 < fCO3 < 0.20and5°C< T < 25°C

Changes in atmospheric carbon-14 attributed to a variable sun.

The 14C production rate in the upper atmosphere changes with time because the galactic cosmic-ray flux responsible for 14C production is modulated by the changes in solar wind magnetic properties. The resulting changes in the atmospheric 14C level are recorded in tree rings and are used to calculate past 14C production rates from a carbon reservoir model that describes terrestrial carbon exchange between the atmosphere, ocean, and biosphere. These past 14C production rate changes are compared with 14C production rates determined from 20th-century neutron flux measurements, and a theory relating 14C production and solar variability, as given by geomagnetic Aa indices and sunspot numbers, is developed. This theory takes into account long-term solar changes that were previously neglected. The 860-year 14C record indicates three episodes when sunspots apparently were absent: A.D. 1654 to 1714 (Maunder minimum), 1416 to 1534 (Sp�rer minimum), and 1282 to 1342 (Wolf minimum). A less precisely defined minimum occurred near A.D. 1040. The part of this record after A.D. 1645 correlates well with the basic features of the historical record of sunspot numbers. The magnitude of the calculated 14C production rates points to a further increase in cosmic-ray flux when sunspots are absent. This flux was greatest during the Sp�rer minimum. A record of approximate sunspot numbers and Aa indices for the current millennium is also presented.

Oceanic Uptake of Fossil Fuel CO2: Carbon-13 Evidence

The δ13C value of the dissolved inorganic carbon in the surface waters of the Pacific Ocean has decreased by about 0.4 per mil between 1970 and 1990. This decrease has resulted from the uptake of atmospheric CO2 derived from fossil fuel combustion and deforestation. The net amounts of CO2 taken up by the oceans and released from the biosphere between 1970 and 1990 have been determined from the changes in three measured values: the concentration of atmospheric CO2, the δ13C of atmospheric CO2 and the δ13C value of dissolved inorganic carbon in the ocean. The calculated average net oceanic CO2 uptake is 2.1 gigatons of carbon per year. This amount implies that the ocean is the dominant net sink for anthropogenically produced CO2 and that there has been no significant net CO2 released from the biosphere during the last 20 years.

Young organic matter as a source of carbon dioxide outgassing from Amazonian rivers

Rivers are generally supersaturated with respect to carbon dioxide, resulting in large gas evasion fluxes that can be a significant component of regional net carbon budgets. Amazonian rivers were recently shown to outgas more than ten times the amount of carbon exported to the ocean in the form of total organic carbon or dissolved inorganic carbon. High carbon dioxide concentrations in rivers originate largely from in situ respiration of organic carbon, but little agreement exists about the sources or turnover times of this carbon. Here we present results of an extensive survey of the carbon isotope composition (13C and 14C) of dissolved inorganic carbon and three size-fractions of organic carbon across the Amazonian river system. We find that respiration of contemporary organic matter (less than five years old) originating on land and near rivers is the dominant source of excess carbon dioxide that drives outgassing in medium to large rivers, although we find that bulk organic carbon fractions transported by these rivers range from tens to thousands of years in age. We therefore suggest that a small, rapidly cycling pool of organic carbon is responsible for the large carbon fluxes from land to water to atmosphere in the humid tropics.

Loss of organic matter from riverine particles in deltas

Abstract In order to examine the transport and burial of terrigenous organic matter along the coastal zones of large river systems, we assessed organic matter dynamics in coupled river/delta systems using mineral surface area as a conservative tracer for discharged riverine particulate organic matter (POM). Most POM in the rivers studied (n = 6) is tightly associated with suspended mineral material; e.g., it is sorbed to mineral surfaces. Average organic loadings in the Amazon River (0.67 ± 0.14 mg C m−2), the river for which we have the largest dataset, are approximately twice that of sedimentary minerals from the Amazon Delta (∼0.35 mg C m−2). Stable carbon isotope analysis indicate that approximately two-thirds of the total carbon on the deltaic particles is terrestrial. The combined surface-normalized, isotope-distinguished estimate is that >70% of the Amazon fluvial POM is not buried in the delta, consistent with other independent evidence (Aller et al., 1996). Losses of terrestrial POM have also been quantified for the river/delta systems of Columbia in the USA, Fly in New Guinea, and Huange-He in China. If the losses of riverine POM observed in these river/delta systems are representative of rivers worldwide, then the surface-constrained analyses point toward a global loss of fluvial POM in delta regions of ∼0.1 x 1015 g C y−1.

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.

Abyssal Water Carbon-14 Distribution and the Age of the World Oceans

The carbon-14 distribution in the abyssal waters of the world oceans indicates replacement times for Pacific, Indian, and Atlantic ocean deep waters (more than 1500 meters deep) of approximately 510, 250, and 275 years, respectively. The deep waters of the entire world ocean are replaced on average every 500 years.

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.

Atmospheric14C changes resulting from fossil fuel CO2 release and cosmic ray flux variability

A high-precision tree-ring record of the atmospheric14C levels between 1820 and 1954 is presented. Good agreement is obtained between measured and model calculated 19th and 20th century atmospheric Δ14C levels when both fossil fuel CO2 release and predicted natural variations in14C production are taken into account. The best fit is obtained by using a ☐-diffusion model with an oceanic eddy diffusion coefficient of 3 cm2/s, a CO2 atmosphere-ocean gas exchange rate of 21 moles m−2 yr−1 and biospheric residence time of 60 years. For trees in the state of Washington the measured 1949–1951 atmospheric Δ14C level was20.0±1.2%. below the 1855–1864 level. Model calculations indicate that in 1950 industrial CO2 emissions are responsible for at least 85% of the Δ14C decline, whereas natural variability accounts for the remaining 15%.

Mesoscale Eddies Drive Increased Silica Export in the Subtropical Pacific Ocean

Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawai`i. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.