Kenneth A. McGee

Global distribution of atmospheric phosphorus sources, concentrations and deposition rates, and anthropogenic impacts

A worldwide compilation of atmospheric total phosphorus (TP) and phosphate (PO4) concentration and deposition flux observations are combined with transport model simulations to derive the global distribution of concentrations and deposition fluxes of TP and PO4. Our results suggest that mineral aerosols are the dominant source of TP on a global scale (82%), with primary biogenic particles (12%) and combustion sources (5%) important in nondusty regions. Globally averaged anthropogenic inputs are estimated to be similar to 5 and 15% for TP and PO4, respectively, and may contribute as much as 50% to the deposition over the oligotrophic ocean where productivity may be phosphorus-limited. There is a net loss of TP from many (but not all) land ecosystems and a net gain of TP by the oceans (560 Gg P a(-1)). More measurements of atmospheric TP and PO4 will assist in reducing uncertainties in our understanding of the role that atmospheric phosphorus may play in global biogeochemistry.

Soil efflux and total emission rates of magmatic CO2 at the Horseshoe Lake tree kill, Mammoth Mountain, California, 1995–1999

Abstract We report the results of eight soil CO 2 efflux surveys by the closed circulation chamber method at the Horseshoe Lake tree kill (HLTK) — the largest tree kill on Mammoth Mountain. The surveys were undertaken from 1995 to 1999 to constrain total HLTK CO 2 emissions and to evaluate occasional efflux surveys as a surveillance tool for the tree kills. HLTK effluxes range from 1 to >10,000 g m −2 day −1 (grams CO 2 per square meter per day); they are not normally distributed. Station efflux rates can vary by 7–35% during the course of the 8- to 16-h surveys. Disturbance of the upper 2 cm of ground surface causes effluxes to almost double. Semivariograms of efflux spatial covariance fit exponential or spherical models; they lack nugget effects. Efflux contour maps and total CO 2 emission rates based on exponential, spherical, and linear kriging models of survey data are nearly identical; similar results are also obtained with triangulation models, suggesting that the kriging models are not seriously distorted by the lack of normal efflux distributions. In addition, model estimates of total CO 2 emission rates are relatively insensitive to the measurement precision of the efflux rates and to the efflux value used to separate magmatic from forest soil sources of CO 2 . Surveys since 1997 indicate that, contrary to earlier speculations, a termination of elevated CO 2 emissions at the HLTK is unlikely anytime soon. The HLTK CO 2 efflux anomaly fluctuated greatly in size and intensity throughout the 1995–1999 surveys but maintained a N–S elongation, presumably reflecting fault control of CO 2 transport from depth. Total CO 2 emission rates also fluctuated greatly, ranging from 46 to 136 t day −1 (metric tons CO 2 per day) and averaging 93 t day −1 . The large inter-survey variations are caused primarily by external (meteorological) processes operating on time scales of hours to days. The externally caused variations can mask significant changes occurring at depth; a striking example is the masking of a degassing event generated at depth and detected by a soil gas sensor network in September 1997 while an efflux survey was in progress. Thus, occasional efflux surveys are not an altogether effective surveillance tool for the HLTK, and making them effective by greatly increasing their frequency may not be practical.

Three‐year decline of magmatic CO2 emissions from soils of a Mammoth Mountain Tree Kill: Horseshoe Lake, CA, 1995–1997

We used the closed chamber method to measure soil CO2 efflux over a three-year period at the Horseshoe Lake tree kill (HLTK)—the largest tree kill on Mammoth Mountain in central eastern California. Efflux contour maps show a significant decline in the areas and rates of CO2 emission from 1995 to 1997. The emission rate fell from 350 t d−1 (metric tons per day) in 1995 to 130 t d−1 in 1997. The trend suggests a return to background soil CO2 efflux levels by early to mid 1999 and may reflect exhaustion of CO2 in a deep reservoir of accumulated gas and/or mechanical closure or sealing of fault conduits transmitting gas to the surface. However, emissions rose to 220 t d−1 on 23 September 1997 at the onset of a degassing event that lasted until 5 December 1997. Recent reservoir recharge and/or extension-enhanced gas flow may have caused the degassing event.

Annual cycle of magmatic CO2 in a tree-kill soil at Mammoth Mountain, California: Implications for soil acidification

Time-series sensor data reveal significant short-term and seasonal variations of magmatic CO 2 in soil over a 12 month period in 1995–1996 at the largest tree-kill site on Mammoth Mountain, central-eastern California. Short-term variations leading to ground-level soil CO 2 concentrations hazardous and lethal to humans were triggered by shallow faulting in the absence of increased seismicity or intrusion, consistent with tapping a reservoir of accumulated CO 2 , rather than direct magma degassing. Hydrologic processes closely modulated seasonal variations in CO 2 concentrations, which rose to 65%–100% in soil gas under winter snowpack and plunged more than 25% in just days as the CO 2 dissolved in spring snowmelt. The high efflux of CO 2 through the tree-kill soils acts as an open-system CO 2 buffer causing infiltration of waters with pH values commonly of H+ ṁha −1 ṁyr −1 , mobilization of toxic Al 3+ , and long-term decline of soil fertility.

Total sulfur dioxide emissions and pre‐eruption vapor‐saturated magma at Mount St. Helens, 1980–88

SO2 from explosive volcanism can cause significant climatic and atmospheric impacts, but the source of the sulfur is controversial. TOMS, COSPEC, and ash leachate data for Mount St. Helens from the time of the climactic eruption on 18 May 1980 to the final stages of non-explosive degassing in 1988 give a total SO2 emission of 2 Mt. COSPEC data show a sharp drop in emission rate that was apparently controlled by a decreasing rate of magma supply. A total SO2 emission of only 0.08 Mt is estimated from melt inclusion data and the conventional assumption that the main sulfur source was pre-eruption melt; commonly invoked sources of “excess sulfur” (anhydrite decomposition, basaltic magma, and degassing of non-erupted magma) are unlikely in this case. Thus melt inclusions may significantly underestimate SO2 emissions and impacts of explosive volcanism on climate and the atmosphere. Measured CO2 emissions, together with the H2O content of melt inclusions and experimental solubility data, indicate the Mount St. Helens dacite was vapor-saturated at depth prior to ascent and suggest that a vapor phase was the main source of sulfur for the 2-Mt of SO2. A vapor source is consistent with experimental studies on the Mount St. Helens dacite and removes the need for a much debated shallow magma body.

Rates of volcanic CO2 degassing from airborne determinations of SO2 Emission rates and plume CO2/SO2: test study at Pu′u ′O′o Cone, Kilauea Volcano, Hawaii

We present an airborne method that eliminates or minimizes several disadvantages of the customary plume cross-section sampling method for determining volcanic CO2 emission rates. A LI-COR CO2 analyzer system (LICOR), a Fourier transform infrared spectrometer system (FTIR), and a correlation spectrometer (COSPEC) were used to constrain the plume CO2/SO2 and the SO2 emission rate. The method yielded a CO2 emission rate of 300 td−1 (metric tons per day) for Pu′u ′O′o cone, Kilauea volcano, on 19 September 1995. The CO2/SO2 of 0.20 determined from airborne LICOR and FTIR plume measurements agreed with the CO2/SO2 of 204 ground-based samples collected from vents over a 14-year period since the Pu′u ′O′o eruption began in January 1983.

Geochemical evidence for a magmatic CO2 degassing event at Mammoth Mountain, California, September–December 1997

Recent time series soil CO2 concentration data from monitoring stations in the vicinity of Mammoth Mountain, California, reveal strong evidence for a magmatic degassing event during the fall of 1997 lasting more than 2 months. Two sensors at Horseshoe Lake first recorded the episode on September 23, 1997, followed 10 days later by a sensor on the north flank of Mammoth Mountain. Direct degassing from shallow intruding magma seems an implausible cause of the degassing event, since the gas released at Horseshoe Lake continued to be cold and barren of other magmatic gases, except for He. We suggest that an increase in compressional strain on the area south of Mammoth Mountain driven by movement of major fault blocks in Long Valley caldera may have triggered an episode of increased degassing by squeezing additional accumulated CO2 from a shallow gas reservoir to the surface along faults and other structures where it could be detected by the CO2 monitoring network. Recharge of the gas reservoir by CO2 emanating from the deep intrusions that probably triggered deep long-period earthquakes may also have contributed to the degassing event. The nature of CO2 discharge at the soil-air interface is influenced by the porous character of High Sierra soils and by meteorological processes. Solar insolation is the primary source of energy for the Earth atmosphere and plays a significant role in most diurnal processes at the Earth surface. Data from this study suggest that external forcing due largely to local orographic winds influences the fine structure of the recorded CO2 signals.

Airborne detection of diffuse carbon dioxide emissions at Mammoth Mountain, California

We report the first airborne detection of CO2 degassing from diffuse volcanic sources. Airborne measurement of diffuse CO2 degassing offers a rapid alternative for monitoring CO2 emission rates at Mammoth Mountain. CO2 concentrations, temperatures, and barometric pressures were measured at ∼2,500 GPS-referenced locations during a one-hour, eleven-orbit survey of air around Mammoth Mountain at ∼3 km from the summit and altitudes of 2,895–3,657 m. A volcanic CO2 anomaly 4–5 km across with CO2 levels ∼1 ppm above background was revealed downwind of tree-kill areas. It contained a 1-km core with concentrations exceeding background by >3 ppm. Emission rates of ∼250 t d−1 are indicated. Orographic winds may play a key role in transporting the diffusely degassed CO2 upslope to elevations where it is lofted into the regional wind system.

Quiescent hydrogen sulfide and carbon dioxide degassing from Mount Baker, Washington

Volcanic H2S emission rate data are scant despite their importance in understanding magma degassing. We present results from direct airborne plume measurements of H2S and CO2 on a 21-orbit survey at eleven different altitudes around Mount Baker volcano in September 2000 utilizing instrumentation mounted in a light aircraft. Measured emission rates of H2S and CO2 were 5.5 td−1 and 187 td−1 respectively. Maximum concentrations of H2S and CO2 encountered within the 4-km-wide plume were 75 ppb and 2 ppm respectively. Utilizing the H2S signal as a marker for the plume allows the corresponding CO2 signal to be more easily and accurately distinguished from ambient CO2 background. This technique is sensitive enough for monitoring weakly degassing volcanoes in a pre-eruptive condition when scrubbing by hydrothermal fluid or aquifers might mask the presence of more acid magmatic gases such as SO2.

Airborne volcanic plume measurements using a FTIR spectrometer, Kilauea Volcano, Hawaii

A prototype closed-path Fourier transform infrared spectrometer system (FTIR), operating from battery power and with a Stirling engine microcooler for detector cooling, was successfully used for airborne measurements of sulfur dioxide at Kilauea volcano. Airborne profiles of the volcanic plume emanating from the erupting Pu′u′O′o vent on the East Rift of Kilauea revealed levels of nearly 3 ppm SO2 in the core of the plume. An emission rate of 2,160 metric tons per day of sulfur dioxide was calculated from the FTIR data, which agrees closely with simultaneous measurements by a correlation spectrometer (COSPEC). The rapid spatial sampling possible from an airborne platform distinguishes the methodology described here from previous FTIR measurements.