Volcanic carbon cycling in East Lake, Newberry Volcano, Oregon, USA

The carbon cycle in East Lake (Newberry Volcano, Oregon, USA) is fueled by volcanic CO 2 inputs with traces of Hg and H 2 S. The CO 2 dissolves in deep lake waters and is removed in shallow waters through largely diffusive surface degassing and photosynthesis. Escaping gas and photosynthate have low δ 13 C values, leading to δ 13 C(DIC) (DIC—dissolved inorganic carbon) as high as + 5.7‰ in surface waters, well above the common global lake range. A steep δ 13 C depth gradient is further established by respiration and absorption of light volcanic CO 2 in bottom waters. The seasonal CO 2 degassing starts at > 100 t CO 2 /day after ice melting in the spring and declines to ∼ 40 t/day in late summer, degassing ∼ 11,700 t CO 2 /yr. Thus, volcano monitoring through gas fluxes from crater lakes should consider lacustrine processes that modulate the volcanic gas output over time. The flux contribution of a bubbling CO 2 “hotspot” increased from 20% to > 90% of the lake-wide CO 2 flux from 2015 to 2019 CE, by a “toxic gas alert” in July 2020. East Lake is an active volcanic lake with a “geogenic” ecosystem driven primarily by hydrothermal inputs.

Newberry Volcano (Oregon, USA; 43.728°N, 121.210°W) is a Cascade Range back-arc volcano (Carlson et al., 2018), most recently active 1300 yr ago, with two small crater lakes aged ca. 7.5 ka (Jensen and Donnelly-Nolan, 2017). East Lake is a drowned crater with visual evidence of CO 2 inputs from the lake bottom (Lefkowitz et al., 2017). The CO 2 bubbles carry traces of H 2 S and Hg gas but almost no fluids. A bubbly CO 2 -Hg-H 2 S "hotspot" is found along the southeastern beach (Fig. 1). East Lake has no inlets or outlets, a maximum water depth of 55 m, and a 4.2 km 2 surface area. It is frozen in winter and thermally stratified in summer (Lefkowitz et al., 2017). This study aims to determine the magnitude of the volcanic CO 2 input in this carbonate-rich volcanic lake based on 5 years of flux measurements and to map the local carbon cycle based on carbon isotope evidence.

METHODS
We measured lake surface CO 2 fluxes using an accumulation float chamber (e.g., Mazot and Bernard, 2015) and a LI-COR CO 2 detector (model LI-6252). Field samples and measurements were collected annually between June and August from 2015 to 2019, with additional data collected in May 2018 shortly after ice melting. The field data points were treated with se-quential Gaussian simulations (SGSs;Cardellini et al., 2003) to estimate lake-wide CO 2 release rates. Gas samples from the accumulation chamber and ambient CO 2 from air on land and from above the lake were injected into pre-evacuated Exetainer vials for stable isotope analyses. Gas bubbles in the hot-spring pools were collected through water displacement in inverted plastic bottles. Lake depth profiles for pH and temperature were obtained with a YSI Professional Plus five-probe analyzer. Lake water samples were taken at 10 m depth intervals with a Teflon Wildco 2 L water sampler and stored in Exetainers for δ 13 C(DIC) (DIC-dissolved inorganic carbon) analyses after filtration (0.2 μm). Additional details on methods are provided in the Supplemental Material 1 .

RESULTS
The CO 2 escape rates measured between late June and August were similar in 2015-2019, with average CO 2 fluxes of ∼0.2 mol/m 2 /day. The SGS data treatment provided a mean value of 46 ± 10 t CO 2 /day as the typical summer CO 2 evasion rate ( Fig. S1 and Table S1 in the Supplemental Material). The bubbly CO 2 "hotspot" increased in size and intensity over 2015-2019 without impacting the overall flux (see the Supplemental Material). The hotspot flux contribution increased from 20% to >90% of the lake-wide CO 2 flux, which culminated in a "gas alert'" in July 2020 (USGS, 2020). The mid-May 2018 CO 2 flux was ∼95 t CO 2 / day, well outside the summer range and with a different spatial flux pattern. More than 50% of the lake area was emitting >0.5 mol/m 2 /day, whereas the other surveys reached such values over ∼15% of the lake area only, largely in the hotspot area.
Gas samples, with 420-800 ppm CO 2 , taken from the float chamber during CO 2 accumulation runs, are mixtures of ambient and lake CO 2 , with δ 13 C(CO 2 ) from −8‰ to −17‰ (Fig. 2). The δ 13 C(CO 2 ) in ambient gas samples (air) ranged from the fully mixed atmospheric value of ∼−8.7 ‰ at ∼410 ppm CO 2 (NOAA, 2020) to values of −17‰ at 700 ppm CO 2 . In some experiments, we pumped the air component out of the chamber and after some accumulation time, the chamber contained only lake gas, with δ 13 C(CO 2 ) values of −9‰ to −15‰. The δ 13 C(CO 2 ) of hot-spring bubbles ranged from −3‰ to −8‰ (Table S2).
Surface-water pH values varied over the years of record from ∼7.4 in May to 8 in September, with pH values down to ∼6.4 in the hypolimnion. Lake-water profiles show increasing DIC and conductivity with depth (Lefkowitz et al., 2017), mainly due to CO 2 gain from volcanic CO 2 absorption and respiration at depth and CO 2 loss from diffusive CO 2 escape and photosynthesis in the epilimnion.
The 14 C contents of DIC in surface water and at 42 m depth collected in summer 2015 (Table S4)  East Lake sediment carries 2%-12% C org (org-organic) from diatoms, cyanobacteria, and subaquatic vegetation debris (SAV). Cores taken near the CO 2 -rich bubbling zone have the highest C org (8%-12%; see the Supplemental Material). The δ 13 C(C org ) ranges from −17‰ (rich in SAV) to −24‰, with a mean value of −23‰ in surface sediments (Lefkowitz et al., 2017). Mercury concentrations in cores and grab samples throughout the lake range from 0.5 to 4 ppm Hg (Lefkowitz et al., 2017), whereas cores in and around the hotspot have 3-13 ppm Hg (see the Supplemental Material).

DISCUSSION
The data provide a 5 yr record of carbon cycling in East Lake, with implications for its ecosystem functioning and volcano monitoring. Spring melting of the winter ice cover leads to lake overturn, intense CO 2 degassing Sequential Gaussian simulation color scale is located to the right. Hotspot region is outlined in red. In May 2018, the flux had a different distribution pattern and larger overall flux. There were many zones of high CO 2 flux, and the hotspot no longer stood out. Figure 2. Keeling mixing diagram with concentrations and δ 13 C values of CO 2 in ambient air (filled circles) and in accumulation chamber gas (triangles) from our sampling campaign at East Lake (EL; Oregon, USA; 43.7306°N, 121.2098°W). δ 13 C values of "pure lake gas" plot at zero on the x-axis. Air plots on the right-hand side of diagram (∼400 ppm), with blue bar indicating the range of δ 13 C values from air samples taken 5-15 km upwind from the lake. Air data array represents mixing between common air and plant respiration, boat exhaust gas, lake gas above the lake surface, and possibly forest-fire CO 2 . Large blue open circles represent predicted equilibrium isotope compositions of lake gas. Light-green "drawdown" symbols (flat bars) stem from experiments where ambient CO 2 had been pumped out of the chamber. Chamber data represent mixtures of ambient CO 2 and possible lake gas compositions (vertical teal bar). The 2016WY data were analyzed at the University of Wyoming (USA), all other data are from the University of California-Davis (USA); HS samples refer to CO 2 bubbles collected from hot springs. Likely mixing array that covers ∼75% of the data is shown with a pale green band, indicating mixing of slightly contaminated air with "pure lake gas" that has δ 13 C values of −10‰ to −12.5‰. Thick black line represents mixing between standard fully mixed atmospheric CO 2 with an equilibrium lake gas (HS sample); dashed light-blue mixing lines represent the possible extremes of air-lake gas mixing; dashed green line is the air CO 2 -biogenic CO 2 mixing line.
(>100 t CO 2 /day), and the onset of algal productivity. In the fall, the CO 2 flux diminishes to <40 t/day, then the lake again turns over, followed by freezing of surface waters with the cessation of photosynthesis and CO 2 surface degassing. During the winter, the lake accumulates dissolved volcanic CO 2 .
We fitted a power-law function to the CO 2 flux data (Fig. 4) to model seasonal CO 2 escape rates (Supplemental Material), showing that excess winter CO 2 is "blown off" from April through late August. In late summer, the lake evolves toward a steady state with volcanic CO 2 output roughly equaling the volcanic input, estimated at ∼32 t CO 2 /day year-round (Supplemental Material). The lake CO 2 loss of ∼11,700 t/yr is comparable to those from other similarly sized volcanic lakes (Pérez et al., 2011). Over these 5 yr of study, the hotspot growth was most likely caused by a drop in lake level, driven by higher ambient temperatures and lower precipitation levels (NIDIS, 2020). A lower lake level created a larger area across which bubbles could reach the surface and discharge directly into the atmosphere. The 2020 "gas alert" thus was not driven by deep-seated volcanic degassing processes (USGS, 2020).
East Lake has high epilimnial δ 13 C(DIC) values compared to typical global δ 13 C(DIC) lake values (−20‰ to 0‰; Bade et al., 2004) but similar to some other CO 2 -degassing carbonate-rich volcanic lakes (e.g., Mazot et al., 2014). High δ 13 C(DIC) occurs in nonvolcanic lakes with extensive methane generation (e.g., Gu et al., 2004) or strong seasonal algal blooms (e.g., Oren et al., 1995), which are both absent in East Lake. Δδ 13 C(DIC) broadly increases over the season as a result of increasing δ 13 C(DIC) in surface waters and decreasing δ 13 C(DIC) in deeper waters. δ 13 C(DIC) in the epilimnion can increase by ∼2‰ (e.g., between May and August 2018), while deep-water δ 13 C(DIC) can decrease by several per mil after spring homogenization. Δδ 13 C(DIC) is created by photosynthesis and CO 2 degassing in the epilimnion and by respiration and addition of low-δ 13 C volcanic CO 2 in the hypolimnion. The δ 13 C(DIC) depth profiles show the fully developed gradient in August 2017, a lesser gradient in September (storm-related lake mixing), and then near fully mixed conditions in May 2018 after ice melting, indicative of lake turnover (Fig. 3, thick black and orange lines).
The 14 C(DIC) data provide apparent water ages of >10,000 yr (Table S4), but East Lake is only 6500 yr old. Water-budget modeling suggests a water residence time of ∼20 yr (Lefkowitz et al., 2017). The CO 2 -degassing surface waters do not equilibrate with atmospheric CO 2 , and the large flux of "dead" volcanic CO 2 strongly dilutes the atmospheric 14 C input that presumably enters the lake largely through precipitation.
The measured δ 13 C(CO 2 ) values from the accumulation chamber range from −8‰ to −17‰ and cannot be explained as binary mixtures of lake CO 2 gas in equilibrium with lake DIC and fully mixed atmospheric ambient CO 2 (−8.7‰). Equilibrium δ 13 C(CO 2 ) values for lake gas were calculated from isotope mass-balance statements, temperature-dependent intra-species fractionation factors (DeVries et al., 2001), and speciation calculations using the program CO 2 Sys (Pierrot and Wallace, 2006). The epilimnial δ 13 C(DIC) values (+2.5‰ to +5.7‰), temperature (0-18 °C), and pH (7-8.2) provided δ 13 C(CO 2 ) equilibrium values of −2.5‰ to −8.5‰.  Binary mixing of standard ambient air with an equilibrium lake gas composition of −7‰ δ 13 C(CO 2 ) would create a mixing line at the upper section of the data array (Fig. 2, thick black line). Our analyses of local ambient air show a range between fully standard air and a lowδ 13 C component, such as forest and/or ground respiration CO 2 at ∼- 27‰ (Bowling et al., 2002;Chiodini et al., 2008Chiodini et al., , 2011. Newberry summer nights can be below freezing, leading to the formation of a nocturnal atmospheric boundary layer. Soil and forest CO 2 emissions trapped in the atmospheric ground layer may create the observed low δ 13 C values in CO 2 -enriched air (Fig. 2). In addition, low-δ 13 C CO 2 may have been contributed by the common regional forest fires and powerboat exhaust gases, and air collected above the lake may have a lake gas component. The range in δ 13 C(CO 2 ) of potential pure lake gas samples (Fig. 2, teal green bar) spans the calculated equilibrium gas values and the "pure lake gas" chamber samples (light green bar symbols). The latter may be explained with a kinetic isotope effect relative to the calculated equilibrium values. Mixing between the "pure lake gas" samples and slightly contaminated air covers the majority of data points (pale green band), but this solution is not unique.
Thus, the escaping CO 2 has low δ 13 C values, with δ 13 C(CO 2 ) a function of lake composition and a degree of kinetic fractionation that usually depends on the wind speed. The loss of this CO 2 gas is one driver for the heavy δ 13 C(DIC) in shallow waters and the depth gradients that build up over the season.
The second carbon sink from the epilimnion is the photosynthetic flux, which we constrain by the C org burial rate. Only a fraction of the photosynthetic flux is buried; the rest is recycled in the hypolimnion through respiration and oxidation (Cole et al., 1994). Primary organic productivity thus also depletes the DIC in 12 C in epilimnial waters. We calculated the C org burial rate from C org data, the mean sediment mass accretion rate (∼0.05 g/cm 2 / yr) based on core-top 210 Pb ages (Lefkowitz et al., 2017) and a volcanic ash age deeper in one core (ca. 1300 yr B.P. Paulina Lake ash flow layer; Jensen and Donnelly-Nolan, 2017), and bulk dry sediment density data. The mean lake C org burial rate is ∼3.3 mg C/cm 2 /yr, translating into a lake-wide C org burial rate of 140 ± 30 t C/yr.
A preliminary two-box model using the measured carbon fluxes and calculated equilibrium isotope fractionation factors shows that the calculated δ 13 C(DIC) values and isotope gradients broadly match the whole observed δ 13 C(DIC) spectrum (Supplemental Material). The data and modeling indicate that diffusive CO 2 degassing strongly contributes to Δδ 13 C(DIC). A carbon-isotope depth gradient is thus not a precise indicator of primary lake productivity (e.g., McKenzie, 1985) if surface CO 2 degassing occurs as well.
The CO 2 -rich hot-spring bubbles possibly contain primary volcanic CO 2 as suggested by He mantle isotope values (ratio relative to the atmospheric value, R a , = 7.6-8.3; Graham et al., 2009). The δ 13 C(CO 2 ) in discrete bubbles was determined at −3‰ to −8‰, and the box modeling (see the Supplemental Material) demands an input value of ∼-6‰ to −7‰, close to general mantle CO 2 values (Deines and Gold, 1973).
The phosphorus for photosynthesis and silicon for diatom frustule construction are supplied by the geothermal fluids, and fixed nitrogen is supplied by diazotroph cyanobacteria (Lefkowitz et al., 2017). The high levels of CO 2 may stimulate the local primary productivity (Hamdan et al., 2018), and the CO 2 -rich hotspot area has the highest C org contents (as much as 12%; see the Supplemental Material).
Consequently, the lake ecosystem benefits from hydrothermal inputs and is highly productive (6%-12% C org in ash-poor sediment). The high sedimentary Hg levels (Lefkowitz et al., 2017) have no major deleterious impact on the ecosystem, although plankton tows yielded 5 ppm Hg and fish with low parts-per-million Hg values (see the Supplemental Material).

CONCLUSIONS
East Lake is not a North American version of Lake Nyos in Cameroon, where CO 2 accumulated over decades until a catastrophic release took place (e.g., Kusakabe, 2017). At East Lake, accumulated winter CO 2 is released every year during spring and summer. Seasonal ice cover blocking diffusional CO 2 escape occurs in many high-latitude and high-altitude lakes (e.g., Cole et al., 1994). This delay in the release of winter CO 2 may mute the seasonal oscillation in atmospheric CO 2 (Tranvik et al., 2009). Volcano monitoring through gas flux measurements in volcanic lakes must account for lacustrine processes that modulate the gas flux, especially when ice cover occurs. During low-wind periods, the lake is most likely degassing CO 2 with low δ 13 C(CO 2 ) as a result of kinetic fractionation during degassing, whereas during windy periods, the CO 2 flux increases (see the Supplemental Material) and δ 13 C(CO 2 ) becomes close to the equilibrium isotopic composition. The accumulation-chamber data are not an exact replica of natural CO 2 degassing because of the protected environment inside the chamber.
East Lake has a "geogenic" ecosystem that is almost entirely fed by volcanic nutrients, including CO 2 . Its waters have high δ 13 C(DIC) values due to CO 2 degassing and photosynthesis. East Lake's system is dualistic in nature with a hydrothermal supply of good nutrients and harmful toxins, where the good far outpaces the bad for the ecosystem.