The Contribution of Shore Thermoabrasion to the Laptev Sea Sediment Balance

THE CONTRIBUTION OF SHORE THERMOABRASION TO THE LAPTEV SEA SEDIMENT BALANCE

F.E. Are

Petersburg State University of Means of Communications, Moskovsky av. 9, St.-Petersburg, 190031, Russia e-mail: [email protected]

Abstract A schematic map of Laptev Sea shore dynamics is compiled for the first time, using available published data. It shows the distribution of thermoabrasion shores, mean long-term shore retreat rates, and areas of seabed ero- sion and accretion. The amount of sediment released to the sea from the 85 km Anabar-Olenyok section of the coast is calculated, as an example, at 3.4 Mt/year. These results are compared with published data on sediment transport of rivers running into the Laptev Sea. Estimates of the Lena River discharge range from 12 to 21 Mt/year, of which only 2.1 to 3.5 Mt may reach the sea. The analysis shows that the input of thermoabrasion at mean shoreline retreat rates of 0.7 to 0.9 m/year is at least of the same order as the river input and may great- ly exceed it.

Introduction ble to determine the input from shore thermoabrasion offshore more accurately. Thousands of kilometres of Arctic sea coast retreat at rates 2-6 m/year under the action of thermoabrasion1 Maps of shore dynamics are needed to solve this (Are, 1985; Barnes et al., 1991). Tens of square kilome- problem and others related to climate change impacts. tres of Arctic land therefore are consumed by the sea Such maps have been compiled for about 650 km of every year. This is a special kind of marine transgres- Alaska northern shores (Reimnitz et al., 1988; Barnes et sion occurring with a constant sea level. Obviously this al., 1991). Mean annual amount of sediments supplied process has to be taken into account by various eco- into the sea due to shore thermoabrasion have been cal- nomic activities on the coast. It also plays an important culated and presented. Similar maps may be drawn up role in the evolution of the Arctic environment. Sea for the Arctic shores of Russia. A schematic map of the level has risen during the past century and is expected Laptev Sea shore dynamics is compiled here as a first to continue rising, reaching most probably 50 cm above approach (Figure 1). the present level by the year 2100 (Climate change... , 1996). Therefore the rate of shore thermoabrasion may Compiling the Laptev Sea shore accelerate considerably. dynamics map

The thermoabrasion of shores is considered by per- Very little reliable data are published on Laptev Sea mafrost scientists as a separate cryogenic process, but it shore retreat rates. Most measurements are from short is an important part of a complicated land-ocean inter- sections of the coast. Schematic maps of shore dynamics action. In particular thermoabrasion is a source of sedi- are compiled for Khatanga Bay, for a part of Anabar- ment for the sea derived from the land. Therefore it Olenyok coast and for Vankina Guba Bay (Zigarev and affects the sediment balance of Arctic seas. But the Sovershaev, 1984; Are, 1985; Novikov, 1984; Grigoryev, importance of this agent has not been determined reli- 1996; Medkova, 1994). Satisfactory information on the ably until now. The sediment discharge of rivers has lithology of coastal sediments is derived from geologi- long been considered in Russia as the main terrestrial cal maps at 1:1,000,000 and larger scales. The input to the marine sediment balance. More recently, Permafrost-landscape map of Yakutia ASSR of very rough comparisons showed that river and shore 1:2,500,000 scale (1991) gives only a rough idea of the input may be of the same order of magnitude ice content of coastal sediment. The height of the cliffs (Suzdalsky, 1974; Shuysky 1983). At present it is possi- and nearshore bathymetry were obtained from the

1 Thermoabrasion is the process of erosion of coast and shoreface, composed of icebonded permafrost or ice, under the com- bined action of mechanical and thermal energy of the sea. Shores subjected to thermoabrasion are called thermoabrasional (Are, 1988).

F.E. Are 25 topographic maps of 1: 25,000 and other scales. Other provides evidence of the equality of the shore retreat information was gathered from numerous scientific rate and of the ice complex2 thermodenudation rate. publications. All these sources were used to compile the The latter is calculated using the seasonal sums of mean map presented in Figure 1. daily positive air temperatures (Are, 1985).

The resultant directions of wave energy, directions of The areas of sea floor erosion and sediment accretion longshore sediment movement and areas of sediment in the vicinity of Anabar-Olenyok coast and around accretion in the bays were drawn on the map according Bolshoy Lyakhovsky island are drawn on the map to Sovershaev (1980). The directions of sediment trans- according to Klyuyev (1967, 1970, Klyuyev, Kotyukh, port in the straits of Novosibirsky archipelago were 1985). derived from Klyuyev (1967) and maps of surface cur- rents (Atlas of the Arctic, 1985; World Ocean Atlas, The coastal plain between the mouths of the Anabar 1980). and Olenyok rivers is composed of an ice complex con- taining 60-80 % of particles less then 0.05 mm in diame- The stable shores supplying no sediment to the sea, ter. According to Klyuyev (1970), the floor of the Laptev shown on the map with a bold line, represent either sta- Sea in this area is composed of sand in water depths ble shores not exposed to the destructive influence of between 4-6 m and 20-25 m. Near the shore and in the sea, or those composed of hard rock - liparite, deeper areas the bottom sediment is silty. Klyuyev andesite and basalt. Shores where nothing is known are (1970) believes that sandy bottoms represent erosional represented by a thin line. areas of the sea floor. Lindemann (1994) and Benthien (1994) support this viewpoint and suggest that a sandy The shores composed of soft rock are classed as com- bottom in the Laptev Sea marks areas of sea floor ero- paratively stable. These are characterised by a very low sion, whereas silty and clayey bottom - indicates sedi- rate of retreat, but the cliffs of such shores, if rather ment accretion. This hypothesis is used in compilation high, may supply a considerable amount of sediment to of Figure 1 to distinguish the areas of erosion and accre- the sea. tion offshore.

Prograding shores are shown on the map along the Much of the information contained in Figure 1 is front of the Lena and Yana river deltas because these based on several assumptions and its reliability cannot deltas are growing according to many hydrological be proven at present, but the reasons for choosing of publications. But there is published data for eroding any designation used may be explained. For example, delta shores also. For example Grigoryev (1993) reports why was the south-eastern coast section of Taymyr that the northern shores of the Lena delta in its eastern peninsula designated as comparatively stable shore? part undergo erosion in places. Intensive erosion of This part of the coast is composed of loose marine, allu- shores was observed along the Yana river delta vial and marine-alluvial sediments along half its length (Grigoryev, 1966). The front of the Yana river delta is according to the State Geological Map at 1:1,000,000 framed by two young marine terraces up to 10 km wide scale. The other half is composed of soft rock in 5 according to State Geological Map of 1:1,000,000 scale. separate sections. The mean shoreface slope out to the So the dynamics of delta shores are rather complicated. 10 m isobath is steeper than typical conditions in the Further investigations are needed to understand these Laptev Sea, suggesting that this part of the shore under- processes. goes wave erosion. The form of the shoreline demon- strates that the blocks of soft rock offer shore protection. Most shore retreat rates shown on the map are These blocks form capes with bays between them. The derived from direct field measurements or a compari- bays extend as much as 5 km inland. Probably the bays son of aerial photographs (Sovershaev, 1980; Novikov, are created by thermoabrasion of shores composed of 1984; Are, 1985; Grigoryev, 1996). The values shown on loose deposits during last 5,000 years when sea level the northern and southern coasts of Dm. Laptev Strait was stable. So the present position of the shoreline in are determined in an indirect way. Various published the bays should be near an equilibrium state. If so, the descriptions of these cliffed coasts suggest that their rate of shore retreat should be limited now by the ero- visual appearance has not changed during the last 80 sion rate of the sections composed by soft rocks. So it years (Vollosovich, 1915; Romanovsky, 1963; Grigoryev, seems most reasonable to designate the shore section as 1996). These cliffs, composed of an ice complex, have a a comparatively stable one. narrow thermoterrace at the base and a vertical upper part. Preservation of this morphology for many years

2 Perennially frozen ice-rich unconsolidated Quaternary deposits with a large content of ice wedges are called ice complex in Russian permafrost literature. The total ice content of these sediments is as large as 95 % by volume. Ice complex is widespread on Laptev Sea coast.

26 The 7th International Permafrost Conference Figure 1. Schematic map of Laptev Sea shore dynamics. 1 - stable shores supplying no sediments into the sea, 2 - comparatively stable shores built by soft rock, 3 - slowly retreating thermoabrasion shores (<2m/year, 4 - rapidly retreating thermoabrasion shores (>2m/year), 5 - prograding shores of delta, 6 - areas of sea floor erosion, 7 - areas of sediment accretion, 8 - resultant direction of wave energy, 9 - direction of sediment longshore movement, VGB - Vankina Guba Bay, DLS - Dmitry Laptev Strait, BLI - Bolshoy Lyakhovsky Island.

F.E. Are 27 Sediment supplied to the sea by shore. The erosion rate is as high as 4 cm/year thermoabrasion of Anabar-Olenyok coast (Klyuyev, 1967, 1970; Klyuyev, Kotyukh, 1985). Therefore the value h max = 5 m was taken for the cal- The following data are needed to calculate the amount culations of sediment amount. According to Klyuyev of sediment supplied to the sea by thermoabrasion: the (1970) the shoreface retreats parallel to itself implying shore retreat rate (V , m/year) or shore advance rate, that retreat of the shore is continuous. the height of the cliffs (H, m), position of the lower boundary of shoreface (average maximum wave base A part of this coast, 69.5 km long, is composed of ice complex or thermokarst depression deposits. The latter hmax , m), lithology of sediments composing the coast consist mainly of thawed and subsided ice complex. and shoreface, volumetric ice content of these sedi- 3 Therefore the height of the cliffs crossing thermokarst ments and their dry bulk density (B, t/m ). depressions can be considered as a product of thaw set- tlement of ice complex. This implies that the quantity of Among the parameters needed, the least studied and sediment supplied to the sea by thermoabrasion of cliffs understood is lower boundary of the shoreface, which composed of ice complex and thermokarst depression corresponds to the maximum depth of wave influence deposits (per metre of shoreline) is equal. Therefore all on the bottom (h max , m). This boundary is usually shores composed of ice complex or thermokarst depres- well-defined geomorphologically for deep seas (depths sion deposits are considered in calculations as having > h max ) along the abrasion coasts composed of sand, the height of cliffs in thermokarst depressions (10 m). coarse-grained sediments or rock. In such situations the Other parts of the coast 15.5 km long are composed of wave base is marked by the edge of an underwater sediments having zero thaw settlement and cliffs about accretion terrace (Zenkovich, 1962). In shallow seas, the 1 m high. lower boundary of the shoreface may be associated with the isobath where the comparatively steep slope of Little is known about the thickness of ice complex in shoreface changes into a rather gentle slope of transi- this area. In some places the lower ends of ice-wedges tion zone. However, in some beach profiles the change are below sea level. As a first approximation it is of inclination is indistinguishable. In some areas the assumed that thaw settlement of all deposits which boundary between the shoreface and the transition occur below sea level is zero. The dry bulk density of zone may be indicated by a change in bottom sediment silty sands is approximately 1.5 t/m3. from sandy on the shoreface and to silty in the transi- tion zone (Reineck and Singh, 1980). Using the data listed and methodology of calculations developed and described in detail by the author (Are, About a half of the Laptev Sea is shallow. Waves in press) the volume of sediment supplied to the sea by rework the sea floor everywhere up to several hundred thermoabrasion along 85 km of the Anabar-Olenyok kilometres from the shore. This impact is so strong that coast is as follows. during severe fall storms the waves throw sand and silt onto the deck of ships (Klyuyev, 1970). Obviously it is For the section composed of ice complex and unreasonable to consider erosion of the sea floor at such thermokarst depression deposits distances from the coast as shore erosion. In this situa- tion the notion of shoreface becomes meaningless. However it is necessary to locate the boundary between M = B ¥V¥ H ¥ D + B ¥ V ¥ h max shore erosion and sea-floor erosion in order to calculate D = 1.5 ¥ 2 ¥ 10 ¥ 69500 +1.5 ¥ 2 ¥ 5 ¥ 69500 = the amount of sediment volume supplied to the sea by destruction of these shores. At present it is possible to 2 085 000 + 1 042 500 = 3 127 500 t/year. use only assumed values of h max , which cannot be substantiated reliably. where D - is the length of the coast section. The first term in the equation represents input of shore area The Anabar-Olenyok coast is one of the longest and above sea level and the second one - that below sea best studied sections of the Laptev Sea shores retreating level (shoreface). For the sections with low cliffs because of thermoabrasion. The mean retreat rate of a section of this 85 km long coast (Figure 1, dotted line M = 1.5 ¥ 2 ¥ 0.5 ¥ 15 500 + 1.5 ¥ 2¥ 5 ¥ 15 500 = box) is 2 m/year, as determined by comparing aerial photographs taken in 1949 and 1971 (Are, 1985). 23 250 + 232 500 = 255 750 t/year

Erosion of the shallows occurs everywhere in this area and the total supplied from all 85 km of coast about out to the 5 m isobath at a distance of 6-8 km from the 2 108 250 + 1 275 000 = 3.4 Mt/year.

28 The 7th International Permafrost Conference Sediment discharge from rivers draining into and h max = 10 m seem to be reasonable for this the Laptev Sea calculation. V = M : [B ¥ D ¥ (H + h )] = 21,000,000 : The results of calculations of sediment input from the max Anabar-Olenyok coast can be compared with the data [1.5 ¥ 1,000,000 ¥ (5 + 10)] = 0.9 m/year. on sediment discharge from the rivers. Monitoring of water and sediment discharge from the main rivers Taking the shoreline length as 1300 km gives (Anabar, Khatanga, Lena, Olenyok, Yana ) has been car- V = 0.7 m/year. From the authorÕs experience, 0.7- ried out during the last 50 years. It is found that about 0.9 m/year would not be an exaggerated estimate for 75 % of the total riverine water input into the Laptev the mean retreat rate of Laptev Sea thermoabrasion Sea is supplied by the Lena River. The sediment dis- shores. charge from this river represents about 80 % of the total discharge from all the rivers (Alabyan et al., 1995). The Conclusions quantitative evaluations of Lena sediment discharge by different investigators range from 11.8 to 21 Mt/year A schematic map of Laptev Sea shore dynamics is (Doronina, 1962; Alabyan et al., 1995; Ivanov and drawn up for the first time using available published Piskun, 1995; Ivanov and Piskun, in press; Waterways ... data. The map indicates the current state of understan- , 1995). No explanation for the diversity of results is ding and demonstrates that our knowledge of Laptev given. All data published characterise the sediment Sea shore evolution is quite poor. The map may be used yield upstream from the delta. V.V. Ivanov and A.A. for planning future investigations. Piskun (1995) believe that special full-scale investiga- tions are needed to determine the amount of sediments Calculation of the sediment amount supplied to the coming into the sea. Alabyan et al. (1995) state that only Laptev Sea from shore thermoabrasion of Anabar- 2.1-3.5 Mt/year of the total Lena River suspended sedi- Olenyok coast and comparison of this amount with the ment discharge enters the sea. But unfortunately in the sediment yield of the Lena River shows that the inputs publication cited no reason is given for this extremely of shore thermoabrasion and river yield to the sediment important statement. No data on bed load discharge are balance of the Laptev Sea are at least of the same order . available. So it seems that the riverine input into the It is very likely that the contribution of thermoabrasion sediment balance of the Laptev Sea is no better studied is greater than the contribution of rivers. than the input of sediment from shore thermoabrasion. Therefore only a rough and conservative comparison of thermoabrasion and river input is appropriate at this Acknowledgements stage. The author wishes to thank the German Ministry of Nevertheless the calculations and data on Lena River Science and Technology (BM BF Grant No. 525 4003 sediment discharge cited above demonstrate that the OG0517A) and Institute of Earth Cryosphere SB RAS, input of thermoabrasion into the sediment balance of Tyumen, Russia who supported this research. It was the Laptev Sea cannot be neglected; furthermore the carried out on the base of German-Russian scientific co- two inputs should be considered of the same magni- operation and owing to support of AWI, Bremerhaven tude at least. and GEOMAR, Kiel.

According to the map in Figure 1, the total length of The author would like also to thank personally Dr. H. Laptev Sea shore undergoing thermoabrasion is esti- Kassens and E. Reimnitz for numerous discussions. The mated to be in the range 1000-1300 km (Khatanga bay reviews by Donald Forbes and Robert Taylor greatly shores excluded). The 85 km length of Anabar-Olenyok helped to improve the manuscript especially the poor coast represents only 6-8 % of it. If only 2.1-3.5 Mt/year English of the author. of river sediment discharge enter the sea as stated by Alabyan et al. (1995), the total input of shore erosion prevails. Another way of supporting this statement is as follows.

Let us calculate the shore retreat rate needed to sup- ply into the sea an amount of sediment equivalent to the maximum estimate of Lena River sediment dis- charge (21 Mt/year, 80 % of total riverine input) using the foregoing methodology. The mean values H = 5 m

F.E. Are 29 References

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