Destabilization of Carbon Dioxide Hydrates in the Ocean. Resherle Verna Department of Earth Science, University of Southern California

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1 Verna 1 Destabilization of Carbon Dioxide Hydrates in the Ocean Resherle Verna Department of Earth Science, University of Southern California I. ABSTRACT This paper features the investigation of the change in solubility of carbon dioxide (CO 2 ) hydrates in the ocean. The area of interest, the JADE Hydrothermal Field, is in the Western Pacific Ocean near the Okinawa Trough Back-arc Basin. Located at approximately 1,443 meters (m) below sea level, the hydrothermal field is home to a hydrothermal vent that ejects CO 2 -rich fluids. Upon encountering the surrounding seawater, such fluids turn into hydrates that accumulates near the hydrothermal vent. Underneath the layer of CO 2 hydrates, exists an unknown quantity of liquid CO 2. Depending on temperature and pressure, CO 2 may exist in a solid, liquid, or gas phase. In attempt to create a situation analogous to the high-end Intergovernmental Panel on Climate Change (IPCC) scenario for the 21 st century, a forcing of about 800 parts-per million (ppm) of CO 2 is imposed on the atmosphere using cgenie, an earth systems model. Such forcing is imposed to determine if the CO 2 hydrates in the JADE Hydrothermal Field will encounter a temperature maximum that will cause them to dissipate, which would effectively release the liquid CO 2 that it serves as a cap for. As a key component of the carbon cycle, the ocean serves as a major carbon sink due to its ability to capture atmospheric CO 2. Although the global carbon budget accounts for the CO 2 that is being ejected from hydrothermal vents, the existence of liquid CO 2 underneath layers of CO 2 hydrates have not been taken into consideration. The destabilization of these CO 2 hydrates may lead to several amplifications and feedbacks. Though the forcing imposed on the simulation produced a global warming on the order of 4 C, such warming was not sufficient to trigger the destabilization of the CO 2 hydrates in the JADE Hydrothermal Field. II. INTRODUCTION As global warming becomes an evolving pressing issue, it is important to understand the science that serves as its foundation. CO 2 serves as the main greenhouse gas that amplifies the problem of global warming. Therefore, it is important to understand how of carbon interacts with the various components of the earth s systems. Naturally, carbon cycles in and out of the atmosphere, ocean, and the terrestrial biosphere. The Global Carbon Project calculates the global carbon budget from emissions resulting from fossil fuel combustion and oxidation and cement production, emissions from land-use change, the growth rate of atmospheric CO 2 concentration,

2 Verna 2 the sequestering of CO 2 in the ocean and on land, and takes into account a budget imbalance (Le Quere et al., 2017). Before the industrial revolution, the atmospheric concentration of CO 2 was estimated to be approximately 277 parts-per million (ppm). Since then, the concentration of CO 2 in the atmosphere has increased largely due to anthropogenic activity and now sits slightly above 400 ppm (Le Quéré et al., 2017). The continuous increase of atmospheric CO 2 calls for a constant re-evaluation of the global carbon budget. Figure 1. This figure depicts the flux of carbon that is being exchanged between the different reservoirs (Global Carbon Budget, 2016). In 1977, scientists discovered the first series of hydrothermal vents while exploring an oceanic spreading ridge near the Galapagos Islands (NOAA, 2017). Since this initial discovery, a plethora of hydrothermal vents have been found mostly near divergent and convergent plate boundaries at various depths in the ocean. Near these vents, a thick layer of sediment typically forms due to the fluids pumping out of the vents turning into solids upon contact with the surrounding seawater (Lupton et al., 2006). At approximately 1,443 m below sea level, in the Mid-

3 Verna 3 Okinawa Trough Back-arc Basin, scientists have found CO 2 -rich fluid bubbles emerging from the seafloor in the JADE Hydrothermal Field, which covers an area of 1000 m by 200 m (Sakai et al., 1990). Such fluids contains approximately 86% CO 2, 3% hydrogen sulfide (H 2 S), and 11% of methane plus hydrogen (CH 4 + H 2 ) (Sakai et al., 1990). The observation of these CO 2 fluid bubbles indicates the existence of liquid CO 2 capped under a layer of CO 2 hydrates (Sakai et al., 1990). In addition, it is believed that such fluids originate from the same magmatic source of a nearby hydrothermal vent that pumps out a CO 2 -rich solution (Sakai et al., 1990). Figure 2. This schematic depicts the process taking place both outside and inside of hydrothermal vents (Lupton et al., 2006). Research question/hypothesis: Although the global carbon budget considers the amount CO 2 that is ejected from hydrothermal vents, it does not consider the liquid CO 2 that is capped off by CO 2 hydrates. Depending on temperature and pressure, CO 2 can exist in a solid, liquid, or gas phase. Thus, we hypothesize that if a forcing of 800 ppm of CO 2 is imposed on the atmosphere, then it will simulate

4 Verna 4 a situation analogous to the high-end IPCC scenario, producing a global warming on the order of 4 C. We believe that such warming will cause the seawater surrounding the JADE Hydrothermal Field to reach a temperature maximum in which the nearby CO 2 hydrates will destabilize. Figure 3. This graph shows the stability of CO2 hydrates for a given temperature and pressure (Chow, 2014). III. METHODS Temperature Maximum The current ocean temperature near the JADE hydrothermal field is approximately 2.5 C, which was established with the use of Ocean Data View. Upon determining this temperature, we calculated the temperature maximum that the CO 2 hydrates in this hydrothermal field would have to encounter to dissipate. Looking at a CO 2 phase diagram (figure 3), we determined that the

5 Verna 5 surrounding seawater would have to increase from approximately 2.5 C to 9.5 C. At 9.5 C, the ocean would be too warm for CO 2 to exist as hydrates at a depth of approximately 1,443 m. Porosity We then proceeded to estimate the amount of liquid CO 2 that is stored underneath the layer of CO 2 hydrates. Since the thickness of the layer of CO 2 hydrates is unknown, we estimate it to be approximately 0.15 m for the purposes of this paper. From this information, we estimate the volume of the layer of hydrate to be the following: Volume of CO 2 hydrate layer = 1000 m 200 m 0.15 m = 30,000 m, (Eq. 1) By obtaining this volume, an estimation of porosity can be established. We calculate porosity to obtain a measurement of the possible amount of liquid CO 2 that is saturated in the layer of CO 2 hydrates. Due to seawater and liquid CO 2 being immiscible, we calculate the measure of liquid CO 2 under the assumption that the layer of hydrates is saturated only with liquid CO 2. In this region of the ocean, porosity is estimated to be approximately 40% (Wang et al., 2011). In addition, the density of CO 2 at 2.5 C is approximately 1, (Figure 4). Using these values yields the 0 following quantity of liquid CO 2 : / Volume of liquid CO 2 = 30,000 m, 0.60 = 18,000 m, Mass of liquid CO 2 = / 0 18,000 m, = kg (Eq. 2 & 3)

6 Verna 6 Figure 4. This graph shows the density of CO 2 as a function of temperature (Prof. Stott). cgenie Simulations To run this experiment, we made use of cgenie, which is a grid-enabled earth system model. We ran three simulations which simulated a total of 11,335 years of data. Our first simulation was a SPIN-Up run used to simulate 10,000 years before the industrial revolution. In this run, the concentration of atmospheric CO 2 was kept at a constant value of 278 ppm. Our second run spanned from the years 1765 to 2100 to simulate the start of the industrial revolution and beyond. The objective of this run was to create conditions analogous to the high-end IPCC scenario for the 21 st century to produce a global warming on the order of 4 C. In this run, a forcing of 800 ppm of CO 2 was imposed on the atmosphere. Our third simulation spanned from the years 2100 to The purpose of this run was to enable the global mean surface temperature to reach a new equilibrium temperature. In this run, the concentration of atmospheric CO 2 was kept constant at 800 ppm. The data obtained from these simulations were analyzed in UV-CDAT, which is a climate data analysis open-source software.

7 Verna 7 III. RESULTS & DISCUSSION Our first run produced a global mean surface air temperature equilibrium of approximately 12.5 C at the end of the 10,000-year period, with the atmospheric concentration of CO 2 remaining at 278 ppm. In our second run, the global mean surface air temperature increased as the concentration of atmospheric CO 2 approached 800 ppm. By the year 2100, the global mean surface air temperature increased to approximately 16.2 C. In our third run, the global mean surface air temperature reached a new equilibrium temperature of approximately 16.5 C by the year However, the actual global mean surface air temperature equilibrium was approximately 14.5 C before the start of the industrial revolution. An average air surface temperature of approximately 12.5 C is too cold for that period. It was determined that cgenie underestimated the warming associated with the doubling of atmospheric CO 2 by 2 C. To fulfill the purposes of this paper, we added 2 C to both the surface air and ocean temperatures that were generated by cgenie. Year cgenie Temp. ( C) Temp. After Correction ( C) Figure 5a. This graph shows the correction made to the global mean surface air temperature projected by cgenie. The corrections were made by adding 2 C to the cgenie temperatures.

8 Verna 8 History of Global Atmospheric Temperature Temperature ( C) Time (years) Figure 5b. This graph depicts the history of the global mean surface air temperature across all three simulations ran in cgenie. We found that at a depth of approximately 1,443 m, the ocean temperature increased from 1.5 C (3.5 C after correction) to 2.7 C (4.7 C after correction), which is a difference of 1.7 C. This increase in temperature did not provide an ample amount of warming to trigger the destabilization of the CO 2 hydrates located in the JADE Hydrothermal Field. For these CO 2 hydrates to destabilize, the temperature in that region of the ocean would need to increase to approximately 9.5 C.

9 Verna 9 Temperatures at ~1,443 m depth in the Year 1765 Figure 6a. This graph shows the temperature of the ocean at a depth of approximately 1,443 m to be about 1.5 C (3.5 C after correction) in the year 1765 (UV-CDAT). Temperatures at ~1,443 m depth in the Year 2800 Figure 6b. This graph shows the temperature of the ocean at a depth of approximately 1,443 m to be about 2.7 C (4.7 C after correction) in the year 2800 (UV-CDAT).

10 Verna 10 Difference Between 2800 & 1765 Temperatures at ~1,443 m Depth Figure 6c. This graph represents the difference between figures 6a & 6b, which is about 1.2 C (UV- CDAT). However, for CO 2 hydrates near hydrothermal vents located at depths less than 700 m, this increase in temperature provides a sufficient amount of warming to cause them to destabilize. For example, at a depth of approximately 641 m, the current ocean temperature is about 6.2 C. In the 1765, cgenie estimates the ocean temperature at that depth to be 5.6 C (7.6 C after correction). By the 2800, the ocean temperature increases to 7.2 C (9.2 C after correction), which surpasses the temperature maximum of 9 C and would effectively trigger the destabilization of CO 2 hydrates at the depth. Temperatures at 641 m depth in the Year 1765

Verna 11 Figure 7a. This graph shows the temperature of the ocean at a depth of approximately 641 m to be about 5.6 C (7.7 C after correction) in the year 1765 (UV-CDAT).

11 Verna 11 Figure 7a. This graph shows the temperature of the ocean at a depth of approximately 641 m to be about 5.6 C (7.7 C after correction) in the year 1765 (UV-CDAT). Temperatures at 641 m depth in the Year 2800 Figure 7b. This graph shows the temperature of the ocean at a depth of approximately 641 m to be about 7.2 C (9.2 C after correction) in the year 2800 (UV-CDAT). Difference Between 2800 & 1765 Temperatures at 641 m Depth Figure 7c. This graph represents the difference between figures 7a & 7b, which is about 1.7 C (UV-CDAT). Depending on temperature and pressure, CO 2 can exist in a solid, liquid, or gas phase. Therefore, CO 2 hydrates that exists at depths lower than 1,443 m will require less warming to provoke their destabilization. Hydrothermal vents are located at depths between m throughout the world s

to destabilization in the event of such oceanic warming. Figure 8a.

12 Verna 12 oceans. Therefore, there exists a myriad of CO2 hydrate layers that will be susceptible to destabilization in the event of such oceanic warming. Figure 8a. This map highlights the global distribution of deep-sea and shallow-sea hydrothermal vents (Tarasov et al). Global Temperatures at 641 m Depth in the Year 2800

13 Verna 13 Figure 8a. This map depicts the global variation of oceanic temperatures at a depth of about 641 m. The CO 2 hydrates located in the regions in yellow, ranging from 6 C (8 C after correction) and 8 C (10 C after correction), would be susceptible to destabilization (UV-CDAT). Uncertainty in Results There are a few factors that contribute to the overall uncertainty in the results of this research. The first factor is associated with the settings of cgenie. The atmospheric component of cgenie does not simulate winds and therefore does not consider wind pattern variability. The second factor contributing to the uncertainty of the results are the thickness of the CO 2 hydrate layer. There is simply a lack of knowledge regarding such measurement. In addition, the amount liquid CO 2 underneath the layer of CO 2 hydrates is also unknown and thus serves as another contributing factor in the uncertainty of the results. IV. CONCLUSION The discovery of hydrothermal vents has advanced our understanding in various areas of research. In addition, it has contributed to the development of new investigations. For this project, we aimed to simulate a situation analogous to the high-end IPCC scenario for the 21 st century, which projects a global warming on the order of 5 C. We hypothesized that if a forcing of 800 ppm of CO 2 is imposed on the atmosphere, then the temperature of the seawater surrounding the JADE Hydrothermal Field will reach a temperature maximum that will provoke the destabilization of CO 2 hydrates at that depth, which would effectively release the liquid CO 2 that is capped underneath. We analyzed the change in temperature in the region produced by cgenie. At a depth of approximately 1,443 meters below sea level, we found that such forcing on the atmosphere does not produce a sufficient amount of warming to trigger the destabilization of

14 Verna 14 the CO 2 hydrates found in the JADE Hydrothermal Field. However, hydrothermal vents exist at depths from 0 to 4000 m. We found that for shallow hydrothermal vents, ranging from m, the forcing on the atmosphere did produce an ample amount of warming that would result in nearby CO 2 hydrates destabilizing. Although the global carbon budget accounts for some imbalance, the existence of liquid CO 2 underneath layers of CO 2 hydrates has not been taken into consideration. Though, we do not know the exact amount of liquid CO 2 in the ocean, the possibility of this unknown amount interacting with the rest of the ocean and the atmosphere may disrupt the global carbon cycle. Such disruption may lead to several amplifications and feedbacks that our society would have to address. V. ACKNOWLEDGEMENTS I would like to thank Professor Stott for the guidance and for the materials that allowed for the creation of this research. I would also like to thank Jun Shao for helping me run the simulations on cgenie. Lastly, I would like to thank John Yu for providing me with a laptop so that I could access UV-CDAT.

15 Verna 15 VI. REFERENCES Chow, Aaron (2014). Ocean Carbon Sequestration by Direct Injection, CO2 Sequestration and Valorization, Mr. Victor Esteves (Ed.), InTech, DOI: / Available from: Le Quere, C., et al (2017). Global Carbon Budget Earth System Science Data Discussions, Vol. 2017, pg. 1-79, doi: /essd Lupton, J., et al. (2006). Submarine venting of liquid carbon dioxide on a Mariana Arc volcano. Geochem. Geophys. Geosyst., 7, Q08007, doi: /2005GC NOAA. What is a hydrothermal Vent? NOAA, 10 Oct. 2017, Accessed on 5 Dec Sakai, H., et al. (1990). Unique chemistry of the hydrothermal solution in the mid-okinawa Trough Backarc Basin. Geophysical Research Letters, 17:12, doi: /gl017i012p Wang, X., Hutchinson, D., Wu, S., Yang, S., Guo, Y. (2011). Elevated gas hydrate saturation within silt and silty clay sediments in the Shenhu area, South China Sea. Journal of Geophysical Research, Vol. 116, B05102, doi: /2010jb

: 1.9 ppm y -1

: 1.9 ppm y -1 Atmospheric CO 2 Concentration Year 2006 Atmospheric CO 2 concentration: 381 ppm 35% above pre-industrial Atmoapheric [CO2] (ppmv) 4001850 1870 1890 1910 1930 1950 1970 1990 2010 380 360 340 320 300 280