Radiocarbon dates, Mya arenavia phase of the Champlain Sea

JOHN A. ELSON Department of Geological Sciences, McGill University, Montreal, Quebec

Received January 3, 1969 Accepted for publication February 4, 1969

The history of the Champlain Sea is divided into an early cold period of subarctic water termed the Hiatella phase that lasted from its inception, about 11 800 years ago (Two Creeks time), until a low water phase about 10 800 years ago (Younger Dryas or Valders time). This was followed by an interval of warmer boreal water, here called the Mya arenaria phase, that lasted from about 10 800 years ago until 10 200 years ago or the end of the Champlain Sea.

General descriptions, previously unpublished, of Groningen dates GrN 2031 (10 870 k 100 B.P.) GrN 2032 (10 450 + 80 B.P.), GrN 2034 (10 590 + 100 B.P.), and GrN 2035 (10 330 + 100B.P.) are presLnted. These dates record the Mya arenaria phase of the Champlain Sea, which may have had an initial rising (transgressive) phase, though the stratigraphic evidence, supported by three sets of radiocarbon dates, is not wholly conclusive.

Introduction Previously unpublished details of four radio-

carbon dates on shell material, measured by Hessel de Vries in 1959 are presented here. It was expected that they would appear in a Groningen date list long ago. The dates are part of a project that was not completed because half of the specimens were lost in the confusion at the time de Vries died and efforts to replace some of them have not succeeded. The project involved dating the Hiatella and Mya arenaria phases (Elson and EIson 1959) and other events in the history of the Champlain Sea. Two radiocarbon samples, Gro 1696, 10 210 _+ 80 years B.P. (GrN 1696, 10 450) from 140 ft (42.7 m) above sea level and Gro 1697, 11 245 + 110 years B.P. (GrN 1697, 11 490) from 565 ft (172.2 m) above sea level (de Vries and Barendsen 1958) gave the range of time of the Champlain Sea near Montreal. Newer dates (Mott 1968) extend the range back to 11 880 years B.P. (GSC 505) and forward to 10 200 B.P. (L604 D).

For the purpose of this paper the Champlain Sea refers to the late Pleistocene marine sub- mergence of the St. Lawrence Lowland west of Quebec City, including the Ottawa and Lake Champlain valleys.

EcologicaI Studies of Champlain Sea Fossils In her classic ecological study of Champlain

Sea fossils Goldring (1922) did not consider the changes that must have occurred in the salinity and temperature as the sea shoaled and as influx of fresh water changed during contem- porary glacier recession and crustal movements.

To ascertain if the fossil fauna reflected such changes, and to see if any particular fossils might be useful for identifying specific water-planes in the former islands and north shore of the Cham- plain Sea where tracing of strandlines is other- wise almost impossible, the writer collected fossils from more than 40 littoral occurrences within a radius of about 60 miles (-96 km) of Montreal. Macrofossils (mainly pelecypods) were identified and Hiatella, Macoma balthica, Mytilus edulis, and Mya arenaria were measured and statistical values of length, weight, and for Mytilus the growth rate, were obtained. Published records of the modern occurrence of these species were also studied, some new collections measured, and an effort made to relate the param- eters available from the fossils to temperature and salinity of modern fauna. A geochemical approach was impractical at the time of this study (1956-1963) and in any case would not have provided a method of field identification of a strandline. Some results of the study were published (Elson and Elson 1959; Elson 1964), but the data are still incomplete and have been presented only orally.

In brief, Hiatella arctica is abundant in all littoral deposits above 250 ft (76.2 m) altitude in the vicinity of Montreal and in some shallow water deposits at lower levels. The modal length of Hiatella decreases from about 22 mm at 500 ft (152.4 m) a.s.1. to about 19 mm at 200 ft (60.9 m). The mean weight of valves 21 mm long decreases from about 0.4 to 0.3 g in the same interval. The trends of this species suggest a decrease of salinity and an increase of temperature. Modern Hiatella live where salinity is greater

Canadian Journal of Earth Sciences, 6, 367 (1969)

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368 CANADIAN JOURNAL OF EARTH SCIENCES. VOL. 6, 1969

than 26%, and summer temperatures are from 0 to 10 "C.

The modal length of Macoma balthica in- creases from about 13 mm at 500 ft (152.4 m) a.s.1. to 17 mm at 200 ft (60.9 m); the mean weight of the 15 mm long valves decreases from about 0.16 to 0.14 g, but the scatter is large for this parameter. The trends of this species, which tolerates salinities as low as 6%,, suggest that the Champlain Sea warmed as it shoaled.

Shells of Mytilus edulis occur at most levels, but were too sparse for statisticalmeasurements of length or weight. Growth rates read from the rest-period lines were similar to specimens collecLed from Hudson Bay (Churchill, Manitoba) by the oceanographic vessel Calanus (Lubinsky 1958). The species tolerates temperatures from about 2 to 20 "C and salinity as low as 5%,. The salinity of the Churchill area is about 23%,. Sparse published and unpublished data suggest that a linear relation between modal class length and salinity may exist; if so, fossils from Uplands (near Ottawa), Ontario, and Mont St. Hilaire, Quebec fall in a range of salinities between 15%, and 20%,. These occurrences are within the last half of the Champlain Sea time.

Mya arenaria occurs mainly below about 250 ft (76.2 m) a.s.1. near Montreal and below 320 ft (97.6 m) in the Oka Mountains 30 miles (48.2 km) west of Montreal. This mollusk requires summer temperatures to be higher than 5 "C and tolerates salinity as low as 5%,. Small specimens live together with a small freshwater snail that has tolerance for salt water in the St. Lawrence estuary at St. Jean Port Joli, Quebec. From the literature and my own collections from Long Island to GaspC, modal length appears to increase with temperature and so does the weight of a given size class (33 mm was used). Fossils from the Champlain Sea are small (modal classes 18-26 mm) and light in weight, suggesting low salinity and perhaps relatively warm water, if weight is signxcant. The suggestion of warmth is surprising, but the gentle coast, and embayments of the south side of the Champlain Sea may have resulted locally in summer water temperatures perhaps as high as 20 "C.

Considerable caution, if not scepticism, must be used in interpreting ecological parameters from single species, but the combined use of the four common and easily recognized species mentioned above permits a generalized interpretation. It

was concluded that the Champlain Sea was at first cold, with summer temperatures of about 2 to 3 "C and a salinity of more than 26%,. This was the Hiatella phase. Subsequently shoaling of the sea resulting from crustal uplift, the result- ing reduction of exchange with the open ocean, and the withdrawal of the ice margin from nearby permitted the water to warm and reduced the salinity. This was the Mya arenaria phase; its salinity probably decreased from about 20%, during its maximum extent to 6 %, at the end, and summer temperatures were warmer than 5 "C.

Radiocarbon Dates

The four radiocarbon dates presented in this paper were selected to give the age range of the Mya arenaria phase of the Champlain Sea and to find out if the highest occurrences of M. arenaria were synchronous. This phase is characterized by a "boreal" marine fauna including abundant littoral Mya arenaria, Macoma balthica, and Mytilus edulis. Some Hiatella do occur in littoral sediments, but they are not ubiquitous as they are in the earlier colder phase. Macoma balthica and Mytilus edulis are larger than in the deposits of the earlier colder Hiatella phase. Mya arenaria does not occur in the cold-water phase. The highest occurrences of Mya arenaria range from about 175 ft (53.3 m) near Mount Johnson, Quebec, to about 270 ft (82.3 m) near Drum- mondville and 320 ft (97.6 m) near St. Joseph du Lac, and seem to delimit a warped water plane, possibly corresponding to the Plattsburg level of Chapman (1937, Fig. 16).

Because of the long delay in publication and the fact that the dates have undergone some metamor- phosis, a comment on their history is in order: the dates Gro 203 1,2,4, and 5 presented here were given to me in preliminary form by H. de Vries in a letter dated August 20, 1959 and confirmed in a letter from H. deWaard dated 22 December 1960. They were listed by me in a field trip guidebook (Elson 1962). Parry and MacPherson (1964) listed dates that must be these, using the Gro designation, but they acknowledged no source of information. De Vries mentioned that a correction of about 240 years would be applicable to Groningen dates during his visit to Montreal in June 1959, but that the correction would be deferred by the laboratory pending further

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ELSON: RADIOCARBON DATES, CHAMPLAIN SEA 3 69

inter-laboratory checks. The corrections were published by Vogel and Waterbolk (1963). Thereafter I used the revised dates designated GrN, notably in an oral presentation entitled "Ecological aspects of the history of the Cham- plain Sea" presented to the travelling seminar on Paleoecology, which was held at McGill Univer- sity on 2 April 1965. When Pierre LaSalle asked permission to use the dates, I gave him the corrected ones (LaSalle 1966), which were subsequently cited by Mott (1968).

GrN2031 10870 f 100 y. B.P. (GRO 2031 10 630 $- 100)

St. Philombne Station, Quebec (45" 16.5' N. Lat., 73" 44.5' W. Long.), altitude 140 $- 3 ft (42.7 f 1 m). Mya arenaria, valves articulated and in growth position on the surface of a gravel bed (ice-contact stratified drift at least 15 ft (4.6 m) thick) overlain by about 4 ft (1.2 m) of laminated clay in beds 1 to 3 cm thick contain- ing sparse Macoma balthica, also with articulated valves in growth position, near the base; the upper part of the clay is barren of macrofossils. The clay is overlain by about 3 ft (1 m) of sand and gravel containing freshwater shells (Lamp- silis, Sphaerium, Pisidium, and a small Lymnaeid snail). Collected 30 August, 1958.

Elson (1962) interpreted this and other sections in the same gravel pit that expose as much as 5 ft (1.5 m) of sand with abundant Mya arenaria and Macoma balthica between the clay and the lower gravel, as representing a relatively shallow brackish sea that gradually deepened and then turned fresh to become the Lampsilis Lake. The lake shoaled and its waves reworked parts of the gravel ridge at St. Philo- mbne. The lake is thought to have formed when crustal uplift in the region west of Quebec City eliminated the strait connecting the Champlain Sea to the ocean. The Lampsilis Lake may have discharged both southward through the Lake Champlain valley (through the Fort Ann outlet of the earlier Glacial Lake Vermont) and through a channel across the emerged lowland southwest of Quebec City. Subsequently, an increase in discharge through the Lampsilis Lake may have increased the rate of erosion of the outlets, and the less-resistant sedimentary rocks at Quebec eroded more rapidly than the crystalline rocks at Fort Ann (Chapman 1937, p. 105), so that the main outlet gorge developed at Quebec. This

could have been accomplished without much increase of discharge. However, it was roughly 9000 years ago that the waters of Glacial Lake Agassiz were diverted eastward from the Missis- sippi system through the post-glacial Great Lakes into the St. Lawrence system. This event increased the area of the St. Lawrence watershed by a factor of two or three and hence also the discharge. The accelerated erosion of the outlet through soft rocks would have been an inevitable effect.

Prest (1962) suggested that the Mya arenaria at St. Philombne may be instead Myapseudoaren- aria, which lives in deep water rather than within and slightly below the tidal range as does Mya arenaria. No specimens of M. pseudo- arenaria have been available to me for com- parison, but after conferring with Mrs. Ida Lubinsky, who had a similar problem in studying collections made in the Arctic by the Fisheries Research Board's oceanographic vessel Calanus, I see no reason to change my identification. Prest (1962) acknowledges that the stratigraphy exposed at St. Philombne is "puzzling", but offers no alternative explanation.

An alternative hypothesis that can be suggested involves rapid currents produced in this part of the Champlain Sea by tidal or wind-induced seiches.' Such violent currents can produce, in fairly deep water, sediments that have many characteristics of shallow-water sediments. A reduction in the violence of seiches because of either the elimination of the tidal exchange or a general reduction of the depth and area of the sea, could have resulted in the relatively quiet sedimentation of the clay overlying the sand without involving a change in water level. The seiche currents hypothesis would also explain why a large area of Potsdam sandstone north of Covey Hill lacks any cover of marine or other

'A seiche is a periodic oscillation of a body of water having a period of resonance controlled by the dimensions of the containing basin. Seiches in large lakes often are caused by differences in atmospheric pressure. Changes in water level in the order of several feet occur at the ends of the basin. In the central area (node) the level remains constant and horizontal currents alternate in direction synchronously with the fluctuations of.levels at the ends. The fundamental periods of seiches ~n the Champlain Sea can be estimated by the use of a formula given by Bascom (1964, p. 97). During the deepest Hiatella phase it was about 6.5 hours, and for the later, shallower Mya arenaria phase it was about 10.3 hours. The nodal line was just north of the Champlain Valley, which thus must have been affected little by seiches of the main St. Lawrence basin.

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370 CANADIAN JOURNAL OF EARTH.SCIENCES. VOL. 6, 1969

TABLE I Sets of radiocarbon dates on littoral materials suggesting a rise of water level in the Champlain Sea.

(Dates are on shell except GSC 454, which is whalebone.)

Age Altitude No. Wears b . ~ . ) Location (ft a d . ) Reference

Group 1 Y216 10 850k 330 Uplands, Ottawa, Ont. 323 Preston et,fl. 1925 Y215 10 630+ 330 Hull, Quebec 392 Group 2 L604 A 10 700 + 200 Upl%ds, Ott!wa, ?pt. 260 O l y an! Brocker 1?,61 L604 B 10 550+200 265 GSC 454 10 420k 150 300 Dyck et al. 1966 L604 D 10 200k200 Ottawa, Ontario 350 Dyck and Fyles 1962; Mott 1968 Group 3 GrN 2031 10 870k 105 St. Philomhne, Q;e. 140 This paper

9 , 9 , GrN 1696 10 450k 180 185 de Vries and Barendsen 1958

surficial sediments. It does not explain the fossil sequence however.

A relative rise of water level still remains the most favored hypothesis. A fluctuation (fall) of sea level might have been associated with an ice advance, for instance to the St. Narcisse moraine, about 10 900 years ago. A climatic fluctuation that may have caused this is represented by the Younger Dryas pollen zone in Denmark during which there was a marine regression in Europe (Hansen 1965). This was contemporary with part of the Valders readvance in North America. The subsequent rise (transgression) of sea level in the order of 50 ft (1 5 m), is represented by the marine day at St. Philomgne.

Alternatively, a still mobile crust could have been isostatically down-warped at the periphery of an expanding ice sheet if the advance occurred as a surge without climatic deterioration. How- ever, a surge would not likely have produced a sharply defined moraine ridge such as the St. Narcisse moraine.

As well as the St. PhilomCne stratigraphy, three groups of radiocarbon dates within the champlain Sea (Table I) support the concept of a relative rise in sea level. UnfortunateIy because of the possible error in interpreting the depth range of fossils, and the statistical counting errors of the radiocarbon dates, they are not conclusive evidence. The date differences range from 220 to 520 years and the altitude ranges are from 45 to 90 ft (13.7-27.4 m). Only in the case of radio- carbon dates Y-216 and Y-215 (Table I) is the counting error greater than the range of dates. Dates within groups 1 and 3 are no more than about 20 miles apart, and not in the direction of,

maximum tilt. Misinterpretation of the depth range of the fossils and their enclosing sediments, though approximate at best, is not likely to account for all the differences in altitude. Though none of the data are conclusive in themselves sea level does appear to have been as much as 40 ft (12.2 m) lower about 10 800 years ago than it was about 10 500 years ago in this region. This is in accord with one of the fluctua- tions suggested by Curray (1965).

GPN 2032 10 450 + 80 y. B. P. (GRO 2032 10 210 + 100)

Mount Johnson, Quebec (45" 20.5' N. Lat., 73" 09' W. Long.), altitude 175 f 7 ft (53.3 4 2.1 m). Mya arenaria in a sand spit extending southeast from Mount Johnson, shells about 7 to 10 ft (2.1-3 m) below the surface in fine gravel with large Macoma balthica and some Mytilus edulis. This is the highest occurrence of M. arenaria found on Mount Johnson. Other fossils occur to an altitude of at least 285 ft (86.9 m). The topography is unfavorable for the deposition or preservation of marine sediments above this altitude. Collected 2 August 1958.

GrN 2034 10 590 + 100 y. B.P. (GRO 2034 10 350 + 100)

Duncan Station, Quebec. Ditch on south side of Highway 9 (45" 48' N. Lat., 72" 38.5'W. Long.) altitude 265 ) 5 ft (80.9 + 1.5 m), 1 mile (1.6 km) northwest of Duncan Station. Mya arenaria at base of 3-5-ft (1-1.5 m) medium-grained sand resting on till. The shells are concentrated in a bed at the base of the sand, which accumulated in a shallow embayment. Collected 9 Sept. 1958. Mytilus and Macoma balthica are also abundant.

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ELSON: RADIOCARBON DATES, CHAMPLAIN SEA 371

This dates the highest occurrence of M. arenaria found in this vicinity. The date can be compared with L 441-C, 9430 f 250 years, from the base of the St. Germain bog which is 4 miles (6.4 km) northeast of the shell locdity at an altitude of 300 ft (91.4 m) (Terasmae 1960). The bog date gives a minimum age for the withdrawal of the sea from this area.

GUN 2035 10 330 100 y. B.P. (GRO 2035 10 090 k 100)

St. Joseph du Lac, Quebec (45" 32.5' N. Lat.: 74" 02.5' W. Long.), altitude 320 f 5 ft (97.6 f 1.5 m). Mya arenaria, valves articulated and in growth position in medium to coarse sand forming foreset beds of a wave-built terrace on a mass of ice-contact stratified drift. Collected 23 August 1958. Incorrectly listed as 10 290 years B.P. in Mott (1968). The water level represented was probably 10-20 ft (3-6 m) above the position of the shells. This is the highest observed occur- rence of M. arenaria in the Oka Mountains, but their distribution here may be limited by the narrow, steep littoral area available. The Cham- plain Sea was deeper and probably colder here than at Mount Johnson and Duncan Station (near Drummondville), and this may not be necessarily synchronous with the highest Mya arenaria water plane on the south side of the St. Lawrence Valley. Hiatella, Mytilus, and Macoma balthica are also present. Mytilus are especially abundant in the topset beds of the terrace. Balanus hameri occurs in the lower parts of some of the foreset beds, which have an amplitude of 40 to 50 ft (12.2-15.2 m).

Interpretation The four radiocarbon dates described above

show that the boreal Mya arenaria phase of the Champlain Sea began roughly 10 900 years ago and lasted until 10 300 years ago, an interval o! 600 years. The marine water lasted until at least 10 200 years ago (Mott 1968, L604 D) and was probably the Mya phase to the end. The dates suggest that sea level stood low in the St. Lawrence Lowlands at the beginning of the episode, at a time that may correlate with the St. Narcisse advance, which extended into the Champlain Sea, and with the Younger Dryas pollen zone in Europe, which falls within the Valders of North America. Sea level in the St. Lawrence Lowlands then rose, but the evidence for this is not con- clusive. The Mya phase stood at its highest level

near Drummondville, Quebec, about 10 600 years ago. The relatively shallow water on the south side of the Champlain Sea was probably warmer than that on the deeper northern side, which was closer to the ice margin and retained some of the characteristics of the sub-Arctic Hiatella phase for a longer time. The ages of the highest Mya arenaria vary locally depending on the presence of suitable shore-line topography, but are within a range of about 300 years, which is reasonably consistent and may be the limit of accuracy of the method. More dates might delineate a water plane even in the absence of good shoreline features.

Acknowledgments I am grateful to my late colleague, Dr. Hessel

de Vries, for his interest, encouragement, friend- ship, and persistent arguments supporting the validity of radiocarbon dates of marine shells at a time when the prevailing opinion was that all mollusk shells were contaminated by ancient carbon. I thank the Groningen Radiocarbon Laboratory for releasing the dates. Paul MacClin- tock and Jaan Terasmae joined us in discussions in the field that gave moral support to this dating program. Mrs. Ida Lubinsky gave advice on the problem of Mya pseudoarenaria, and Dr. Dean Fisher kindly arranged access to the Fisheries Research Board's collections from the Calanus expeditions. My wife Jeanne assisted in collection and study of the specimens and pre- pared the manuscript. I am grateful to all these people.

BASCOM, W. 1964. Waves and beaches. Anchor Books, Doubleday and Co., Garden City, N.Y.

CHAPMAN, D. H. 1937. Late-glacial and postglacial history of the Champlain valley. Amer. J. Sci.. 34. , ,

pp. 89-124. CURRAY, J. R. 1965. Late Quaternary history, continental

shelves of the United States. ImThe Quaternary of the United States (H. E. Wright; Jr., and D. C. Frey, Eds.), pp. 723-735.

DYCK, W. and FYLES, J. G. 1962. Geological Survey of Canada radiocarbon dates I. Radiocarbon, 4, pp. 13-26.

DYCK, W., LOWDON, J. A., FYLES, J. G., and BLAKE, W., JR. 1966. Geological Survey of Canada radiocarbon dates V. Radiocarbon, 8, pp. 96-127.

ELSON, J. A. and ELSON, J. B. 1959. Phases of the Champlain Sea indicated by littoral moHusks (Abstr.). Bull. Geol. Soc. Amer., 70, p. 1956.

Emx, J. A. 1962. Pleistocene geology between Montreal and Covey Hill. In New England Intercollegiate Geol, Conf. Guide Book u. 1-1. Clark, Ed.). 54th Ann. Meet., Montreal, Oct. 5-7, 1962, p. 61-66.

ELSON, J. A. 1964. Late Pleistocene water bodies in the St. Lawrence Lowland (Abstr.). Geol. Soc. Amer., Spec. Paper 76, p. 54.

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372 CANADIAN JOURNAL OF EARTH SCIENCES. VOL. 6, 1969

GOLDRING, WINNIFRED. 1922. The Champlain Sea; evidence of its decreasing salinity southward as shown by the character of the fauna. New York State Mus. Bull., Nos. 232-240, pp. 153-194.

WSEN, SIGURD. 1965. The Quaternary of Denmark. In The Quaternary, Vol. I (Kalervo Rankama, Ed.), New York, Interscience, Wiley and Sons, pp. 1-90.

LASAILE, PIERRE. 1966. Late Quaternary vepta tm and glacial history in the St. Lawrence Lowlands, Canada. Leidse Geol. Mecledel., 38, pp. 91-128.

LUBWSKY, IDA. 1958. Sludies on hf}di/ics edrrlis L. of the "Calanus" expeditions to Hudson Bay and Ungava Bay. Can. J. Zool., 36, gp. 869-881.

Mom, R. J. 1968. A radiocarbon-dated marine algal bed of the Champlain Sea episode near Ottawa, Ontario. Can. J. Earth Sci.. 5. nu. 319-324.

OLSEN, E. A. and BROECKER, WT S. 1981. Lamont natural radiocarbon measurements VII. Radiocarbon, 3.

PARRY, J. T. and MACPIERSON, J. C. 1964.'The St. Faustin - St. Narcisse moraine and the Champlain Sea. Rev. gkograph. Montrkal, 18, pp. 235-248.

PREST, V. K. 1962. Surficial geology and soils of the Montreal area, discussion and closure. Natl. Res. Council Can., Assoc. Comm. on Soil and Snow Mechanics, Tech. Memo. 73 (E. Pemer and J. Butler Eds.), pp. 163-185.

PRESTON, R. S., PERSON, E., and DEEVEY, E. S. 1955. Yale natural radiocarbon measurements 11. Science, 122, pp. 954-960.

TERASMAE, JAAN, 1960. A palynological study of post- glacial deposits in the St. Lawrence Lowlands. .Geol. Surv. Can., Bull. 56, pp. 1-22.

VOGEL, J. C. and WATERBOLK, H. T. 1963. Groningen radiocarbon dates IV. Radiocarbon, 5, pp. 163-202.

DE VRIES, HESSEL, and BARENDSEN, G. W. 1958. Groningen radiocarbon dates 111. Science, 128, pp. 1550-1556.

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