Calibration Methodology for the Scripps 13C/12C and 18O/16O stable Isotope program 1992-2018

This report details calibration method for measurements of 13C/12C and 18O/16O ratios of atmospheric CO2 by the Scripps CO2 program from 1992-2018.  The method depends principally on repeat analysis of CO2 derived from a suite of high-pressure gas cylinders filled with compressed natural air pumped at La Jolla.  The first set of three cylinders were given isotopic assignments in 1994 based on comparisons with material artifacts NBS16, NBS17, and NBS19.  Six cylinders subsequently brought into service were assigned values by comparing directly or indirectly with this first set.  A tenth cylinder with natural CO2 in air was obtained from MPI Jena.  Aliquots of CO2 from these cylinders, which serve as secondary standards, were extracted into heat-sealed glass ampoules (“flame-off tubes”) before introduction into the mass spectrometer.  Some of these ampoules have been stored for many years before analysis, allowing long-term isotopic drift of the cylinders to be quantified.  All secondary standards contain natural levels of N2O.  The method corrects for any detected drift, while also applying corrections for N2O interference, for isobaric interferences (“Craig correction”) and for an inter-lab offset identified in early comparisons with the isotope lab at the University of Groningen.  The Jena cylinder was found to be drifting upwards in δ18O at a rate of +0.10 ‰ per decade.  Five of the other nine cylinders were found to be drifting downwards in δ18O, δ13C, or both, at rates of up to -0.11‰ per decade.  The secondary standards were applied uniformly across a transition to a new mass spectrometer in 2000, thereby establishing continuity across this transition.  Results are presented also for instrumental precision based on replicate analyses of standards.  Drift-corrected analyses of the Jena cylinder establishes offsets of +0.037 ‰ in δ13C and +0.041 ‰ in δ18O between the Scripps and JRAS isotopic scales (Scripps more positive)

Precise atmospheric oxygen measurements with a paramagnetic oxygen analyzer

A methodology has been developed for making continuous, high-precision measurements of atmospheric oxygen concentrations by modifying a commercially available paramagnetic oxygen analyzer. Incorporating several design improvements, an effective precision of 0.2 ppm O-2 from repeated measurements over a 1-hour interval was achieved. This is sufficient to detect background changes in atmospheric O-2 to a level that constrains various aspects of the global carbon cycle. The analyzer was used to measure atmospheric O-2 in a semicontinuous fashion from air sampled from the end of Scripps Pier, La Jolla, California, and data from a 1-week period in August 1996 are shown. The data exhibit strongly anticorrelated changes in O-2 and CO2 caused by local or regional combustion of fossil fuels. During periods of steady background CO2 concentrations, however, we see additional variability in O-2 concentrations, clearly not due to local combustion and presumably due to oceanic sources or sinks of O-2. This variability suggests that in contrast to CO2, higher O-2 sampling rates, such as those provided by continuous measurement programs, may be necessary to define an atmospheric O-2 background and thus aid in validating and interpreting other O-2 data from flask sampling programs. Our results have also demonstrated that this paramagnetic analyzer and gas handling design is well suited for making continuous measurements of atmospheric O-2 and is suitable for placement at remote background air monitoring sites

Atmospheric potential oxygen: New observations and their implications for some atmospheric and oceanic models

Measurements of atmospheric O2/N2 ratios and CO2 concentrations can be combined into a tracer known as atmospheric potential oxygen (APO ≈ O2/N2 + CO2) that is conservative with respect to terrestrial biological activity. Consequently, APO reflects primarily ocean biogeochemistry and atmospheric circulation. Building on the work of Stephens et al. (1998), we present a set of APO observations for the years 1996-2003 with unprecedented spatial coverage. Combining data from the Princeton and Scripps air sampling programs, the data set includes new observations collected from ships in the low-latitude Pacific. The data show a smaller interhemispheric APO gradient than was observed in past studies, and different structure within the hemispheres. These differences appear to be due primarily to real changes in the APO field over time. The data also show a significant maximum in APO near the equator. Following the approach of Gruber et al. (2001), we compare these observations with predictions of APO generated from ocean O2 and CO2 flux fields and forward models of atmospheric transport. Our model predictions differ from those of earlier modeling studies, reflecting primarily the choice of atmospheric transport model (TM3 in this study). The model predictions show generally good agreement with the observations, matching the size of the interhemispheric gradient, the approximate amplitude and extent of the equatorial maximum, and the amplitude and phasing of the seasonal APO cycle at most stations. Room for improvement remains. The agreement in the interhemispheric gradient appears to be coincidental; over the last decade, the true APO gradient has evolved to a value that is consistent with our time-independent model. In addition, the equatorial maximum is somewhat more pronounced in the data than the model. This may be due to overly vigorous model transport, or insufficient spatial resolution in the air-sea fluxes used in our modeling effort. Finally, the seasonal cycles predicted by the model of atmospheric transport show evidence of an excessive seasonal rectifier in the Aleutian Islands and smaller problems elsewhere. Copyright 2006 by the American Geophysical Union

Interpreting the seasonal cycles of atmospheric oxygen and carbon dioxide concentrations at American Samoa Observatory

We present seven years of atmospheric O2/N2 ratio and CO2 concentration data measured from flask samples collected at American Samoa. These data are unusual, exhibiting higher short-term variability, and seasonal cycles not in phase with other sampling stations. The unique nature of atmospheric data from Samoa has been noted previously from measurements of CO2, methyl chloroform, and ozone. With our O2 data, we observe greater magnitude in the short-term variability, but, in contrast, no clear seasonal pattern to this variability. This we attribute to significant regional sources and sinks existing for O2 in both hemispheres, and a dependence on both the latitudinal and altitudinal origins of air masses. We also hypothesize that some samples exhibit a component of "older" air, demonstrating recirculation of air within the tropics. Our findings could be used to help constrain atmospheric transport models which are not well characterized in tropical regions

On the role of large bubbles in air-sea gas exchange and supersaturation in the ocean

A parameterization of bubble-induced gas exchange is presented in which the bubble contribution to gas exchange is expressed in terms of separate transfer velocities for ingassing (Kbin) and outgassing (Kbout). The difference between the ingassing and outgassing velocities (Kbin − Kbout) is further separated into two components, the first caused by the injection of small bubbles into the water, the second caused by gas exchange across the surface of hydrostatically compressed larger bubbles. It is argued that both Kbout and the exchange contribution to the difference Kbin − Kbout should be largely independent of the dissolved concentrations of the major gases N2 and O2. A simple model is presented which allows Kbout and the exchange contribution to the difference Kbin − Kbout to be estimated. The model incorporates data from laboratory simulation experiments on the bubble production spectrum. The results indicate that bubbles larger than 0.05 cm in radius, which have often been assumed to play a negligible role, contribute significantly to bubble-induced gas exchange and supersaturation in the ocean. The model is used to explore the sensitivity of bubble-induced gas exchange to the overall air entrainment rate, size and depth distributions of the bubbles, and to the gas exchange rates across the surface of individual bubbles. The model suggests that bubbles may make an important contribution to overall gas exchange at windspeeds above 10 m sec−1. In this regime gas transfer velocities should depend, not just on diffusivity, but also on the solubility of the gases. It is suggested that Kb(out) should scale roughly as α−0.3D0.35 where α is the solubility and D is the diffusivity. The model results, in combination with measurements on inert gas supersaturations, suggest that the global-mean supersaturation of CO2 induced by bubbles is not larger than 0.3% and most probably is around 0.08%. A major uncertainty results from a lack of information on production rates and distributions of large bubbles. Several possible experiments are proposed for improving estimates of bubble-induced gas exchange and supersaturation

Special considerations for updating the primary in situ Mauna Loa record to the X12 scale

We report here on several details that apply to updating the primary in situ Mauna Loa CO2 record from the Scripps CO2 program to the X12 calibration scale from the previous X08A scale.  The update incorporates revised determinations of the primary reference gases on the constant-volume mercury-column manometer (CMM) based on improved assessments of manometer volumes and other parameters (Keeling et al., 2016).  The changes, which impact data only after 1974, are typically of order 0.1 to 0.2 ppm or less at ambient concentrations. In this update, we also incorporate two changes that are unrelated to the revised manometric determinations and are relevant only for the in situ Mauna Loa record but not relevant for flask data from Mauna Loa or other stations. The first involves a reassignment of a secondary calibration cylinder used for the in situ Mauna Loa measurements starting April 2015.  The second involves eliminating a +0.12 ppm adjustment that had been applied only to the in situ Mauna Loa record to correct for offset between flasks and the continuous analyzer.&nbsp

Carbon dioxide measurements by the Scripps O2 program. 2020 Update

This report documents changes to the CO2 calibration scale of the Scripps O2 program to bring it in line with the SIO X12 scale. The update involves new assignments on six primary CO2 reference gases used by the O2 program, which were used to assign cubic coefficients to the instrument response function for the Siemens CO2 analyzer used by the O2 program.  The new assignments correct for drift in the CO2 concentration of these cylinders which is now well resolved. The update also entails changing the functional form for the instrument response function for the Licor CO2 analyzer, also used by the Scripps O2 program, from a cubic to a double inverse hyperbole. This update impacts all data from the Scripps O2 program from program inception onwards.  We designate the updated CO2 scale as the VH344-2020 scale. The previous scale, which had no designation, is now called the VH344-2015 scale.  The changes from the VH344-2015 to VH433-2020 scales involve changes at the level of a few tenths of a ppm, typically towards higher CO2 .  This report also compares the VH344-2020 scale to the NOAA X2007 scale based on tanks provided by Britt Stephens of the National Center of Atmospheric Research

Analysis of the mean annual cycle of the dissolved oxygen anomaly in the World Ocean

A global climatology of the dissolved oxygen anomaly (the excess over saturation) is created with monthly resolution in the upper 500 m of the ocean. The climatology is based on dissolved oxygen, temperature and salinity data archived at the National Oceanographic Data Center. Examination of this climatology reveals statistically significant annual cycles throughout the upper 500 m of the World Ocean, though seasonal variations are most coherent in the North Atlantic, where data density is greatest. Vertical trends in the phase and amplitude of the annual cycle are noted. The cycle in surface waters is characterized by a summer maximum and a winter minimum, consistent with warming and high rates of photosynthesis during the summer, and cooling and entrainment of oxygen-depleted water during the winter. In low and middle latitudes, the amplitude increases with depth and the maximum occurs later in the year, a trend consistent with the seasonal accumulation of oxygen associated with the shallow oxygen maximum. At a depth that varies between about 30 and 130 m, the phase of the annual cycle undergoes an abrupt shift. We call this depth the oxygen nodal depth. Below the nodal depth, the annual cycle is characterized by an early-spring maximum and a late-fall minimum, consistent with a cycle dominated by respiration during the spring and summer and replenishment of oxygen from the atmosphere by ventilation during the fall and winter. Below the nodal depth, the amplitude of the annual cycle generally decreases with depth, indicative of decreasing respiration and ventilation rates, or less seasonality in both processes. We postulate that the nodal depth in middle and high latitudes corresponds closely to the summertime compensation depth, where photosynthesis and net community respiration are equal. With this interpretation of the nodal depth and a simple model of the penetration of light in the water column, a compensation light intensity of 1 W m−2 (4μE m−2 s−1) is deduced, at the low end of independent estimates. Horizontal trends in the phase and amplitude of the annual cycle are also noted. We find that the nodal depth decreases toward the poles in both hemispheres and is generally greater in the Southern Hemisphere, patterns found to be consistent with light-based estimates of the compensation depth. The amplitude of the annual cycle in the oxygen anomaly increases monotonically with latitude, and higher latitudes lag lower latitudes. In the North Atlantic and North Pacific, the amplitude of the annual cycle tends to increase from east to west at all depths and latitudes, as expected considering that physical forcing has greater seasonal variability in the west. The tropics and the North Indian Ocean have features that distinguish them from other regions. Below about 75 m, these waters have pronounced annual cycles of the oxygen anomaly that are shown to be caused mainly by wind-driven adiabatic displacements of the thermocline. A semiannual cycle of the oxygen anomaly is found in the surface waters of the North Indian Ocean, consistent with the known semiannual cycle of surface heat flux in this region

On the Linkage between Antarctic Surface Water Stratification and Global Deep-Water Temperature

The suggestion is advanced that the remarkably low static stability of Antarctic surface waters may arise from a feedback loop involving global deep-water temperatures. If deep-water temperatures are too warm, this promotes Antarctic convection, thereby strengthening the inflow of Antarctic Bottom Water into the ocean interior and cooling the deep ocean. If deep waters are too cold, this promotes Antarctic stratification allowing the deep ocean to warm because of the input of North Atlantic Deep Water. A steady-state deep-water temperature is achieved such that the Antarctic surface can barely undergo convection. A two-box model is used to illustrate this feedback loop in its simplest expression and to develop basic concepts, such as the bounds on the operation of this loop. The model illustrates the possible dominating influence of Antarctic upwelling rate and Antarctic freshwater balance on global deep-water temperatures