ISSN 1028334X, Doklady Earth Sciences, 2015, Vol. 465, Part 2, pp. 1283–1288. © Pleiades Publishing, Ltd., 2015. Original Russian Text © A.V. Eliseev, V.V. Malakhova, M.M. Arzhanov, E.N. Golubeva, S.N. Denisov, I.I. Mokhov, 2015, published in Doklady Akademii Nauk, 2015, Vol. 465, No. 5, pp. 598–603.
GEOPHYSICS
Changes in the Boundaries of the Permafrost Layer and the Methane Hydrate Stability Zone on the Eurasian Arctic Shelf, 1950–2100 A. V. Eliseeva, V. V. Malakhovab, M. M. Arzhanova, E. N. Golubevab, S. N. Denisova, and Corresponding Member of the RAS I. I. Mokhova Received May 20, 2015
Abstract—By using the model for subsea sediments (SSs) (Institute of Atmospheric Physics, Russian Acad emy of Sciences, IAP RAS) and the general circulation model in the Arctic Ocean–North Atlantic (GCM AONA) (Institute of Computational Mathematics and Mathematical Geophysics, Siberian Branch, Rus sian Academy of Sciences, ICMMG SB RAS), the response of the parameters of the permafrost layer and the methane hydrate stability zone (MHSZ) to external impacts in dependence on the parameters of the problem is considered: the degree of the geothermal heat flux intensity G at the lower (bottom) boundary of the com putation domain of the permafrost layer of subsea sediments and the depth Z of this boundary. DOI: 10.1134/S1028334X15120107
Methane hydrates can exist under highpressure conditions (as high as 120 atm, which is possible, as a rule, in oceanic regions due to water column pressure) or at very cold temperatures (for example, in perma frost regions). The total reserve of methane hydrates is estimated to be 0.5–2.5 thnd GtC [1]. Methane hydrates can be decomposed with the release of CH4 under climate warming or a decrease in the sea level. The release of CH4 from methane hydrates could be a source of a significant greenhouse effect that leads to global warming [2–5]. This could be one of the possi ble reasons for the very warm climatic conditions 55 Ma ago [6, 7]. The current bottom temperature on the Arctic shelf is close to 0°C. At up to 200 m depth of the shelf zone, methane hydrates are thermodynamically unstable. Anyway, they were found during field research works on the Arctic shelf [2, 8]. The occurrence of the MHSZ on the shallow Arctic shelf is constrained by the presence of the shelf permafrost zone. Under the modern climatic conditions, the occurrence of this zone is associated with freezing of the depth of shelf subsea sediments as a result of the ocean regression
a
Institute of Atmospheric Physics, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russia b Institute of Computational Mathematics and Mathematical Geophysics, Siberian Branch, Russian Academy of Sciences, pr. Akad. Lavrentieva 6, Novosibirsk, 630090 Russia email:
[email protected]
during the last glacial cycle. The history of the freezing and degradation of the permafrost layer in the Holocene throughout the Arctic shelf is still poorly known. In particular, there are no data on the perma frost zone thickness in the eastern sector of the Arctic shelf, where the permafrost zone of subsea sediments was found. In addition there is a lack of knowledge about the significance of the geothermal heat flux intensity on the Eurasian shelf. In particular, the G value in this region is about 60 mW m–2 with an uncer tainty of up to 10 mW m–2 [9]. The numerical models, which are used to study the shelf permafrost, differ in the choice of climate scenarios (including the scenario describing the formation period of the shelf perma frost) and the thermophysical parameters of subsea sediments. As a result, different models suggest differ ent depths of the upper and lower boundaries of the permafrost shelf zone [10–12]. Based on numerical simulation, we have studied the dynamics of the deepwater permafrost layer and the methane hydrate stability zone on the Eurasian Arctic shelf under recent climatic conditions and probable climate warming in the future in dependence on the parameters of the model for subsea deposits. In this work we do not discuss the uncertainty of the results obtained due to a lack of knowledge about the parameters of the permafrost layer during the regres sion and the dynamics of the oceanic transgression after the glaciation period. In order to calculate the temperature in the perma frost layer of subsea sediments, the sediment heat transport model (SHTM), developed at IAP RAS
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[10], was used. This model is based on solution of the Stefan problem with a mixed boundary condition and supplemented by the module for calculating the of thermobaric conditions of existence of methane hydrates. In our case, we used the results of calcula tions following the general circulation model in the Arctic Ocean–North Atlantic (GCM AONA), devel oped at the ICM and MG SB RAS, when the bound ary conditions are prescribed [13]. In order to model the climatic conditions of the Eemian interglacial, the SHTM was equilibrated dur ing 3 kyr on the Arctic Ocean shelf at the given tem perature of Tst = –1.8°C at the upper boundary of sub sea sediments. Then, approximately 117 kyr ago, the water was instantaneously removed from the Arctic shelf in the regions with the contemporary depths up to 100 m and the simulation was proceeded further for subaerial conditions at the given surface temperature of Tst = Tst0 + ΔTst0 at Tst0 = –12°C and anomalous ΔTst0, given in accordance with the data obtained dur ing ice drilling at the Vostok station. For the period of 13–5 kyr, the postglacial ocean transgression was taken into account. At the transgres sion the shelf flooding Tst changed instantaneously by –1.5°C. After that the calculation was continued up to 2100. For the period of 1948–2100, we used the field of the sea bottom temperature calculated during a numerical experiment with the GCM AONA, instead of the specified Tst value [14]. In this experiment the atmospheric influence was given according to the NCEP/NCAR reanalysis data (http://www.esrl.noaa. gov/psd/data/gridded/data.ncep.reanalysis.html) over the period of 1948–2005 and the calculation results with the GFDL CM3 climate model (http:// nomads.gfdl.noaa.gov: 8080/DataPortal/cmip5.isp) over the period of 2006–2100. The results were inte grated following the RCP8.5 scenario, which assumes a strong increase in the anthropogenic impact on the climate. The GFDL CM3 climate model is characterized by an extremely large response in the northern polar region for the given scenario of external impact on the system [15]. In the RCP 8.5 scenario, the increase in the bottom temperature for all seasons in most of the Eurasian Arctic in the twentyfirst century ranges from 2 to 4°C. The temperature increase in the Laptev Sea was higher, from 4 to 6°C. The use of the atmospheric model with a high climate sensitivity in the Arctic to the atmospheric impact and of the model with a strong anthropogenic impact on climate suggests that our results serve as an upper boundary for changes in the parameters of the permafrost layer of subsea sediments and MHSZ in the twentyfirst century. In the main simulation, the value of 60 mW m–2 for the geothermal heat flux G at the lower boundary of the computational domain was used. The lower boundary Z of this domain was set at a depth of 1000 m under the ocean floor.
In this calculation scenario, slow degradation of the permafrost layer occurred during the Holocene. In the middle of the twentieth century, the thickness of this layer was about 170–320 m. The lower boundary of the permafrost layer sub sided with increasing sea depth, which was determined by shelf flooding during several transgression periods. Until the end of the twentieth century, the change in the thickness of this layer was mainly determined by the thawing of its boundary caused by the geothermal heat flux (Fig. 1). In 1950–2005 it moved upward by several meters. In the same period, the upper bound ary of the permafrost layer subsided 5–10 m only in some areas of the shelf zone (mostly in the southern part of the Laptev Sea and the western part of the Bar ents Sea) due to an increase in the temperature of bot tom waters. The summer temperature became positive in this period. For most of the Arctic shelf, the varia tions in the depth of the upper boundary of the perma frost layer ht did not exceed several tens of centimeters. The MHSZ occurs in the permafrost zone throughout the shallow shelf with a thickness of 450– 660 m. Its thickness is determined by the thickness of the permafrost layer. The upper boundary of the (Ht) is located inside the permafrost layer at a depth of 140– 220 m above the bottom surface. In general, the change in the thickness of the MHSZ over time fol lows the dynamics of the development of the perma frost zone (Fig. 2). Over the period from 1950 to 2005, the MHSZ lower boundary rises by up to 1.5–2 m. Here, similar to the changes in the ht value, the change in the depth of the upper boundary of the MHSZ (Ht) did not exceed a few tens of centimeters due to the slow temperature increase in the permafrost layer throughout its thickness. The situation changed dramatically in the twenty first century. The displacement rate of the lower boundary of the permafrost layer is insignificant, whereas that of the upper boundary increases sharply. Over the period of 2005–2100, the latter retreated to a depth of 10–20 m on the Arctic shelf (Fig. 1). The response of the boundaries of the methane hydrate sta bility zone to changes in the bottom water temperature is less evident than that of the boundaries of the perma frost layer (Fig. 2). A significant enhancement of the sub sidence for Ht (~1 m) was established only in a narrow zone along the shelf zone boundary. However, the deg radation of the permafrost layer due to the thawing and the rise of its boundary up to 7 m lead to the MHSZ reduction. The maximum displacement of Hb due to the thawing of the bottom boundary is simulated in the eastern sector of the Arctic. Due to the lack of knowledge mentioned about the governing parameters of the problem, additional numerical experiments were conducted, during which the G value was changed from 50 to 60 mW m–2 DOKLADY EARTH SCIENCES
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Downward displacement ht, m 25 6 5 4 3 2 1
1950− 2005
20 15 10 5
6 5 4 3 2 1
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Fig. 1. Change in the depths of the upper and lower boundaries (ht and hb, respectively) of the permafrost layer of subsea sediments in the main calculation model.
Upward displacement Hb, m
1950− 2005
Downward displacement Ht, m 3
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1
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3 2005− 2100
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Fig. 2. Change in the depths of the upper and lower boundaries (Ht and Hb, respectively) of the methane hydrate stability zone (MHSZ).
depending on the calculation method, and the lower boundary of the computational domain varies from 900 to 1200 m.
upper boundary of the permafrost layer played a fun damental role in reducing its thickness in the twenty first century.
Until the beginning of the twentyfirst century, the thawing of the permafrost layer bottom boundary occurred at all G and Z values. In turn, all numerical experiments conducted show that the thawing of the
As can be expected, an increase (a decrease) in G leads to a greater (shallower) depth of the permafrost layer (hb), which varies approximately 2fold in the calculations with G = 65 and 50 mW m–2. Here, a signif
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ELISEEV et al. G = 50 mW m−2, Z = 900 m 1 0 −1 −2 −3 −4 −5 G = 65 mW m−2, Z = 1200 m 8 6 4 2 0
Fig. 3. Deviation of changes in the depth of the lower boundary of the permafrost layer of subsea sediments from the calculated values during the period of 2005–2100 in the case of variations in the intensity of the geothermal heat flux (G) at the lower bound ary of the computational domain and the depth of this boundary (Z).
icant dependence of hb on the depth of the lower boundary of the computational domain Z was revealed. With the increase in depth from 900 to 1200 m, that of the permafrost layer increases one and a half times. However, the degradation rate of the per mafrost layer at the lower boundary was approximately 2–3 cm year–1 for all simulations. In general, with increasing G, the uplift rate of hb increased in the twentieth and early twentyfirst cen turies. However, this dependence is not evident. The dependence of hb on Z is even less clear. A more complex dependence was revealed for the variations in the depth hb depending on values G and Z in the period of 2005–2100 (Fig. 3). The dependence of the displacement of hb in this period on G was non linear. In the case of increase in the degree of the geo thermal heat flux intensity at the lower boundary of the computational domain from 60 (standard value) to 65 mW m–2, throughout the Eurasian shelf the rate of thawing increased. The unexpected result, however, is
that there was acceleration of the thawing in the relatively shallow part of the Eurasian shelf during this period with a decrease in the value G of 60 to 50 mW m–2 noted. As was already noted, the latter is due to the fact that the bottom boundary of the permafrost layer in this shelf zone is located deeper, that is, closer to a geothermal heat source set on the lower boundary of the computa tional domain. The dependence of the displacement of the lower permafrost boundary on Z is noted, but it is less evident in comparison with its dependence on G. In turn, the subsidence rate of the upper boundary of the permafrost layer in 1950–2005 is very weakly dependent on the values G and Z. In the twentyfirst century, the rate of thawing as a whole increases with increasing the value G. Furthermore, at G = 60 mW m–2, an increase in Z leads to a decrease in the subsidence rate, which is insignificant in absolute terms. The lat ter fact is quite important, since the displacement of the upper boundary of the permafrost layer leads to reduction of its thickness in the case of a strong DOKLADY EARTH SCIENCES
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anthropogenic impact (see text above). As a conse quence, the obtained data on changes in the perma frost layer thickness in the twentyfirst century are sta ble in relation to the choice of values G and Z within the specified intervals. The complexity of the influence of G and Z on the variations in hb in 2005–2100 is not evident for the lower boundary of the MHSZ (Hb) in the same period. In particular, a decrease (an increase) in G leads to smaller (larger), significant upward displacement of the MHSZ lower boundary. However, in the period of 2005–2100 this displacement is manifested only in the shallowest (as high as 20 m) eastern part of the Eur asian continental shelf under the following conditions: G = 65 mW m–2, Z = 1200 m and G = 60 mW m–2, Z = 900 m. In this shelf zone, the increase in the degree of geothermal heat flux intensity, where it is given, or the uplift of the lower boundary of the computational domain, leads to deceleration of the upward displace ment of the MHSZ lower boundary. This is connected with the influence of pressure [1] and, consequently, the water column thickness, on the depth of the MHSZ boundary. This influence varies in different bathymetric shelf regions, which may be a reason for the corresponding differences in the relationship between Hb and hb in these regions. This dependence is not manifested at the upper MHSZ boundary (Ht). During the calculation, as a whole, an increase (a decrease) in G and/or a decrease (an increase) in Z contributes to the acceleration (deceleration) of the MHSZ subsidence into the sediment layer. Thus, the complexity of the influence of G and Z on the displacement rate of the MHSZ boundaries, noted above for the boundaries of the permafrost layer, is fur ther complicated by the influence of pressure on the thermodynamic stability of methane hydrates. It should be noted, however, that such an influence is characteristic only for the lower boundaries of the permafrost layer and the zone of thermodynamic sta bility of methane hydrates. Under an intensive anthro pogenic effect on the climatic conditions (for exam ple, for the RCP 8.5 scenario, which is used in this work), this influence plays an insignificant role, but it can be manifested as the relatively weak influence of values Z and G on variations in ht and Ht in the twenty first century. Moreover, during the thawing of the per mafrost layer from below and the corresponding rise in the MHSZ lower boundary, the permafrost layer is still thick enough (hundreds of meters, as a rule). This thickness is enough to “lock” methane hydrates released during dissociation. This fact supplements our conclusion that there is a relatively weak influence of the geothermal heat source intensity at the lower boundary of the computational domain and its depth on the changes that occur at a sufficiently strong anthropogenic impact on the climatic conditions. Thus, for estimation of the displacement of the lower boundaries of the permafrost layer and MHSZ, the available data on the geothermal heat flow are DOKLADY EARTH SCIENCES
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insufficient. At the same time, the choice of the values G and Z does not play a fundamental role for estima tion of the displacement of their upper boundaries. The latter is especially true of the case of intense warming of the bottom water, for example, the imple mentation of the scenario of extreme anthropogenic impact in the twentyfirst century. In particular, in order to reduce the computational burden, the calcu lations can be performed at Z = 1000 m. It should also be noted that the revealed combined influence of the intensity of the geothermal heat source at the lower boundary of the computational domain and its depth can also result in the dependence of the calculated data of current and future degrada tion of the shelf permafrost zone on the parameters of postglacial transgression of the ocean. In particular, the deeper part of the shelf was filled with sediments earlier than shallow zones, which, other things being constant, can be a reason for the smaller depth of the permafrost layer on the deeper shelf, compared with shallow zones. ACKNOWLEDGMENTS This work was supported by Russian Foundation for Basic Research (project nos. 130500652, 1305 12082, 130541432, 140500639, 140500730, 14 0531163, 150502457, and 153521061), programs of the Russian Academy of Sciences, and grants of the President of the Russian Federation and the NSh3894.2014.5 and Ministry of Education and Science of the Russian Federation PNIER RFMEFI61014X0006. REFERENCES 1. F. M. O' Connor, O. Boucher, N. Gedney, et al., Rev. Geophys. 48 (4), RG4005 (2010). 2. V. I. Sergienko, L. I. Lobkovskii, I. P. Semiletov, et al., Dokl. Akad. Nauk 446 (3), 330–335 (2012). 3. I. I. Mokhov, V. A. Bezverkhny, and A. A. Karpenko, Izv. Akad. Nauk, Fiz. Atmos. Okeana 41 (5), 579–592 (2005). 4. I. I. Mokhov, V. A. Bezverkhnii, and A. A. Karpenko, in Extreme Natural Phenomena and Catastrophes, Vol. 1: Assessment and Methods to Decrease the Negative Conse quences of Extreme Natural Phenomena (IFZ RAN, Moscow, 2010), pp. 312–319 [in Russian]. 5. Climate Change 2013: The Physical Science Basis, Ed. by T. Stocker, D. Qin, and G.K. Plattner (Cambridge Univ. Press, New York, 2007). 6. G. R. Dickens, J. R. O’Neil, D. K. Rea, and R. M. Owen, Paleoceanogr. 10, 965–971 (1995). 7. G. S. Golitsyn and A. S. Ginzburg, Doklady Earth Sci., 413 (2), 487490 (2007).
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Translated by Dm. Voroschuk
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