S. G. Detlisenko, G. A. Tarasov. Wyprawy Geograficzno na Spitsbergen. of Russian Academy of Sciences

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1 S. G. Detlisenko, G. A. Tarasov Murmansk Marine Biological Institute Wyprawy Geograficzno na Spitsbergen UMCS, Lublin 1992 of Russian Academy of Sciences FOSSIL COMPLEXES OF BIVALVE MOLLUSCS WITH PREVALENCE OF CHLAMYS ISLANDICA FROM COASTAL MORAINES OF TRESKELEN AREA OF THE HORNSUND FIORD (WEST SPITSBERGEN) The carbonate remains of macrofauna from moraines deposits of the Hornsund Fiord were collected in July 1991 during the expedition conducted by Murmansk Marine Biological Institute on board R/V Pomor". The location of moraines and their height above sea level are shown by Tarasov et al. (in present book). A paleofaunistic complex of bivalve molluscs into deposits of moraine No. 1 is presented by the following forms (in order of significance): Chlamys islandica, Hiatella arctica, Mya truncata, Elliptica elliptica, Tridonta borealis. In the deposits of moraine No. 2 there were Chlamys islandica, Mya truncata, Tridonta borealis v. placenta. In the deposits of moraine No. 3 there were found Chlamys islandica, Elliptica elliptica, Mya truncata, Astarte crenata. Hiatella arctica was prevalent in the deposits of moraine No. 4. Besides bivalve molluscs in the deposits of moraines No. 1, 2, 4, the well-preserved conglomerates of the red algae of Lithothamnion genus were observed which apparently points at a relatively small inhabitation depth (no more than m) of organisms of the complex at large. The crusted and lumped colonies of the bryozoans Celleporina ventricosa, Celleporina sp., Porella princeps, Porella minuta, Microporella ciliata, the tubes of sedentary polychaets (Spirorbis genus) and the beds of destructed shells of Balanus sp. were found on the Chlamys islandica valves (mainly in moraine No. 1). The absolute geological age of Chlamys islandica valves found by the method of carbon isotopes analysis, is equal to 8380 ± 80 years for shells from moraine No. 1 and 7090 ±200 years for shells from moraine No. 2. Nevertheless, a taxonomic likeness of the paleofaunistic complexes of bivalve molluscs in all moraines (besides, perhaps, moraine No. 4) shows the identity of their biocenotic origin. The absence in complex No. 4 of the most molluscs valves found in other moraines, is accounted by the fact that moraine location on the island was subjected to especially heavy ploughing effect of glacier, or by incomplete moraine deposits baring. The larger Chlamys islandica, Mya truncata shells and less solid small Tridonta borealis, Elliptica elliptica, Astarte crenata valves could be pounded and grained into detritus. 101

2 An increase of fragmentariness degree of Chlamys islandica valves and their size from moraine No. 1 to 2 and from moraine No. 2 to 3 inversely proportionally to height of deposits above the present sea level can serve as corroboration of that assumption. Comparatively good safety of the Chlamys islandica shells permits to analyze the rate and temperature conditions of growth of these molluscs in lifetime. The yearly accretions of valves height was determined by width of growth rings standed out on the valves as sculptural zones, often with heterogeneous pigmentation (Denisenko, 1979). Based on the value of these increases the curves of individual growth were built (Fig. 1). Themean curves of the group growth for the valves from diverse moraines are rather different, but all of them have an evident bend at the age of 3-4 years (Fig. 2). That shows impossibility of determination of the growth of these molluscs by a generally used asymptotic Bertalanfy's equation. Such difficulties can be overcome by the method for stage-by-stage growth description using two equations: the parabolic one for the age up to 5 years and the asymptotic one for the age ove 5 years (Denisenko, 1989). However, we have already shown that Chlamys islandica growth can be well determined by Gomperts equation (Denisenko, 1985), which is mainly used for a vertebrate growth description: U _U e-c0/a -a' (t-40) where H, is the height of valve of mollusc at the age t, H c o is the theoretically maximal valve height (mm), c 0 is the start specific rate of growth, a is the constant, e is the base of natural logarithm, t is the age (years), t is the age from that the growth is submitted to Gomperts law. The equations calculated for the valves from various moraines are as follows: H =108 5 * e~ * e * c-0-6 ) H = ц ' _ * e * e-o-3i9 * (t-0.809) * g * * ( ) О) (2) (3) The results of cluster analysis have shown that the theoretical curves of group growth described by these equations are most similar to curves for the modern Chlamys islandica of the West Murman, Northern Norway areas and west coast of the West Spitsbergen, where the mean water temperature in summer at present is 1-4 C. Therefore, it is interesting to know in what temperature conditions the molluscs, valves which were found in the moraine deposits, existed. To answer this question we have calculated the parameters of the Bertalanfy's equation for a period of decelerated growth of fossil valves. We showed before (Denisenko, 1989) that a constant к" for that equation was connected by the exponential relation with water temperature. The calculated values of constants for the 102

3 molluscs from different moraines are as follows: kl =0.103, k2 = 0.113, k3 = According to the relation shown above, the average water temperatures in summer of 3.7 C, 4.0 C and 4.5 C are corresponding to these constants. A slight temperature increase of fossil molluscs habit and diminution of geological age of their valves as lowering of moraines bedding height above a modern sea level can testify to a past gradual (during several centuries, at least) rise in temperature during a period of regression on shelf glacier in the Hornsund Fiord. The Chlamys islandica grounds in this area seem to have existed rather long and separately formed deposits from the valves of molluscs leaved in the various temperature conditions. When a glacier for thefirst time moved west the longest distance along thefiord, the bottom sediments with shells of scallops that inhabited at temperature of 3.7 C about 8380 years ago, were cut off and transported the longest distance (moraine No. 1). The middle layer (moraine No. 2) with shells of molluscs, which developed at a temperature of 4.0 C over 7090 years ago after thawing of the first and most extensive glacier tissue, was ploughed up and moved to a shorter distance when a glacier particularly come back again. A next layer (moraine No. 3) with valves of the molluscs that have habited at 4.5 C, was tilled later and moved a smallest distance by a third tissue of glacier. The results of reconstruction for the paleotemperatures of the Chlamys islandica habitat using a valves growth rate show the insignificant differences between a thermal regime in sublittoral zone of the Hornsund Fiord before glacier formation and a modern one. Likeness of species composition investigated paleophaunistic complexes and morden benthic communities from Svalbard offshore confirms this. In spite of repeated warm and cold water periods in North-Eastern Atlantic during last 10 thousands years the dominant species Chlamys islandica was and still is the Barents Sea as one of characteristic faunistic forms for hard bottom deposits. Last objective gives definite confidence for perspective planning of rational fishery of scallops resources in the Barents Sea, which is connected with large expenditures. REFERENCES Denisenko S. G., 1979: (Nekotoryje melody opredelenija vozrasta islandskogo grebeshka.) Some methods for determination of the Iceland scallop age. In: Biologija i individual'noje razvitije nekotorykh vozmozhnykh objektov marikuptury v morjakh Evropejskogo Severa. Apatity, Kola Branch of the USSR Acad. Sci. Press, p (in Russian). Denisenko S. G., 1985: (Nekotoryje osobennosti ekologii i rosta islandskogo grebeshka (Mollusca: Bivalvia) iz chetvertichnikh otlozhenij rajona Kopylova (Barencevo more). Some peculiarities of ecology and growth of the Iceland scallop (Mollusca: Bivalvia) from Quaternary 103

4 deposits of the Kopylov area (Barents Sea). In: Problemy chetvertichnoj paleoekologii i paleogeografii Barentceva i Belogo morej. Tezisy dokladov Vsesojuznoj konferentcii (Murmansk, 1985). Murmansk, CNTI Press, p (in Russian). Denisenko S. G., 1989: (Ekologija i resursy islandskogo grebeshka v Barencevom more.) Ecology and resources of the Iceland scallop in the Barents Sea. Apatity, KolaSci. Centre of USSR Acad. Sci. Press, 138 p. (in Russian). Tarasov G. A., Denisenko S. G., Matishov G. G., Pogodina J. A.: Moraine complexes and certain questions concerning paleoecology of the Treskelen region in the Hornsund Fiord (West Spitsbergen) Wyprawy Geograficzne na Spitsbergen. UMCS Lublin.

5 t; Fig. 1. Curves of individual growth of Chlamys islandica from moraines No. 1, 2, 3, deposits. On absciss axis the age of molluscs (years) is shown; on ordinate axis the height of valve (mm). 105

6 Fig. 2. Empirical curves of the group growth of Chlamys islandica from moraines No. 1, 2, 3. Fig. 3. Theoretical curves of the group growth of Chlamys islandica from moraines No. 1, 2,