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The exaggerated radiocarbon age of deposit-feeding molluscs in calcareous environments JOHN ENGLAND, ARTHUR S. DYKE, ROY D. COULTHARD, ROGER MCNEELY AND ALEC AITKEN
England, J., Dyke, A. S., Coulthard, R. D., McNeely, R. & Aitken, A. 2013 (April): The exaggerated radiocarbon age of deposit-feeding molluscs in calcareous environments. Boreas, Vol. 42, pp. 362–373. 10.1111/j.15023885.2012.00256.x. ISSN 0300-9483. Throughout northern Canada, live-collected, pre-bomb, deposit-feeding marine molluscs from calcareous sediments yield greater apparent radiocarbon ages than do suspension feeders. We explore the size of this effect in a set of 57 paired datings of deposit feeders, mainly Portlandia arctica, and suspension feeders, mainly Hiatella arctica and Mya truncata, collected from both calcareous and non-calcareous Holocene sediments. Deposit feeders from calcareous sediments are older than their suspension-feeding counterparts by as much as 2240⫾130 14 C years. This is attributed to the uptake of ‘old’ bicarbonate derived from calcareous bedrock. The age discrepancy between suspension and deposit feeders in calcareous terrain is non-systematic in space and time, thereby invalidating the application of a correction. In contrast, the age comparisons are concordant at sites located on the Precambrian Shield. In terrestrial environments underlain by carbonate, previous acceptance of dates on deposit feeders led to erroneous interpretations of deglaciation and relative sea-level history, in both the North American and the Eurasian Arctic. This has prompted several researchers to exclude deposit feeders from their late Quaternary reconstructions. The same chronological problem of deposit-feeding molluscs now needs to be more widely acknowledged by the marine community. John England (e-mail:
[email protected]) and Roy D. Coulthard (e-mail:
[email protected]), Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada, T6G 2E3; Arthur S. Dyke (e-mail:
[email protected]) and Roger McNeely (e-mail:
[email protected]), Geological Survey of Canada, Ottawa, ON, Canada, K1A 0E8; Alec Aitken (e-mail:
[email protected]), Department of Geography & Planning, University of Saskatchewan, Saskatoon, SK, Canada, S7N 5C8; received 30th September 2011, accepted 3rd February 2012.
While attempting to measure the 14C marine reservoir effect in molluscs, by radiocarbon dating the shells of animals collected live prior to nuclear bomb testing, we noted that deposit feeders from calcareous substrates gave larger apparent ages than did suspension feeders (McNeely et al. 2006). A similar age difference was noted in the Holocene fossil record from the Queen Elizabeth Islands, Arctic Canada, where deposit feeders yielded unexpectedly greater radiocarbon ages than did suspension feeders from the same sediments (England 1997, 1999; Dyke et al. 2002). Similarly, by far the earliest and most problematic dates from the Champlain Sea, a carbonate-floored basin of southern Canada, are on deposit-feeding molluscs (Portlandia arctica and Macoma balthica; Dyke 2004). Concerns about the validity of dates on deposit feeders from calcareous terrains led to exclusion of these data in recent syntheses of deglaciation in Arctic Canada and North Greenland (Dyke 2004; England et al. 2004, 2006; Funder et al. 2011). Because of the limited availability of live-collected, pre-bomb, deposit feeders, we decided to further explore this problem by assembling and expanding a record of paired datings of Portlandia arctica (a deposit feeder) and common suspension feeders (mainly Hiatella arctica and Mya truncata) from raised marine deposits across the Canadian Arctic Archipelago (CAA, Fig. 1). Here, we report that the DOI 10.1111/j.1502-3885.2012.00256.x
deposit feeders do indeed yield older ages, and, more importantly, unpredictably so. This builds upon previous evidence that P. arctica yields exaggerated ages in comparison with accompanying species (e.g. Forman & Polyak 1997; England et al. 2003, 2004; Mangerud et al. 2006; Funder et al. 2011). This difference led us to hypothesize that suspension-feeding molluscs derive the carbonate for their shells from the bicarbonate in freely circulating ocean water, whereas deposit-feeding molluscs instead derive their shell carbonate from the pore waters in the deposits from which they feed (e.g. Mangerud et al. 2006). If these deposits are calcareous, some of the bicarbonate in the pore water will have been derived from ancient carbonate rock, not in equilibrium with the ocean water (Mangerud 1972). Even in non-calcareous substrates, the pore waters may have a longer residence time than does freely circulating ocean water. An unfortunate consequence of the fact that ancient limestone was, like modern shells, formed in isotopic equilibrium with dissolved sea water bicarbonate (d13C ~0‰; Keith & Weber 1964) is that the carbon isotope fractionation in the shells does not allow us to distinguish between a calcareous pore-water origin and an ocean-water origin for the shell carbonate. Nonetheless, consideration of an enhanced 14C marine reservoir effect present in deposit-feeder age determinations allows us to resolve certain inconsistencies in the © 2012 The Authors Boreas © 2012 The Boreas Collegium
Exaggerated radiocarbon ages of molluscs 31
90°W
38 30
Arctic Ocean
19 1,2
20-22
Axel Heiberg Is.
24
3-5
Ellesmere Is.
NS
6
7,8,32
18,33 ES 34
23
35
nland
120°W
363
9
Gree
150°W
N. Str.
BOREAS
60°W
36 14
15-17 25
37 10-13
Melville Is. 27 Bathurst Is. Barrow
39
Str
Baffin Bay
Victoria Is.
Baffin Is
26
land
28
120°W
Fig. 1. Place names and locations of direct comparisons and associations presented in the text and Table 1. NS = Nansen Sound; N. Str. = Nares Strait; ES = Eureka Sound.
66°N
Main
land
0
deglacial and sea-level history of northernmost North America. What we describe as the ‘Portlandia effect’ has only recently been taken into consideration by researchers establishing chronologies for marine cores collected from the carbonate-floored channels of the CAA (Pien´kowski et al. 2011). This significant chronological effect doubtless applies to deposit feeders in all carbonate terrains, and calls into question data gathered from other sedimentary terrains where carbonate may still be present. Below we present the results of paired dates on suspension- and deposit-feeding molluscs whose true ages are necessarily concordant because they inhabited the same sediment. We comment on geochronological problems that can be better understood if an enhanced Portlandia effect is accepted. The deposit feeders most commonly used in radiocarbon dating in the CAA are members of the genera Macoma and Portlandia, and rarely Yoldia, Yoldiella, Nuculana, and Nucula. Previously, Portlandia has been preferred for dating ice retreat and the postglacial marine limit throughout Arctic Canada because of its reputation as a ‘facies fauna’, that is, as an aggressive colonizer of substrates under sediment-laden, glacier-proximal waters (Ockelmann 1958; Funder 1978; Lubinsky 1980; Syvitski et al. 1989; Blake 1992; Aitken & Gilbert 1996; Dyke 2004; England et al. 2004). The problem that we identify
500
29
Cana
da
1000 km 90°W
Foxe 66°N Basin
unfortunately affects a large body of radiocarbon age determinations ostensibly providing minimum ages for deglaciation across the CAA, as well as control points for postglacial relative sea-level (RSL) curves.
Radiocarbon dating We consider 97 radiocarbon dates on molluscs collected from 39 sites spanning the CAA and NW Greenland (Fig. 1, Table 1). Of these, 63 dates are based on AMS analyses of individual valves. All the site comparisons have in common one or more dates on P. arctica and predominantly include H. arctica, representing suspension feeders, because it is the common denominator throughout most of the study area. We have subdivided our site comparisons into two populations (Table 1): (i) ‘direct comparisons’, which comprise those sites where different species have been collected from the same enclosing sediments and are assumed to be contemporaneous; and (ii) ‘associations’, which comprise sites where different species have been collected from adjacent deposits, all related to the deglacial marine limit, which is locally contemporaneous (Andrews 1970). The shells attributed to ‘associations’ were collected from deep-water marine rhythmites that stratigraphically extend to within 10 m
Location
Direct comparisons Ellesmere Island 1a Esayoo Bay 1b Esayoo Bay 2a Greely Fiord 2b Greely Fiord 2c Greely Fiord 2d Greely Fiord 3a Muskox Bay 3b Muskox Bay 3c Muskox Bay 4a Chandler Fiord6 4b Chandler Fiord6 4c Chandler Fiord6 4d Chandler Fiord6 5a Black Rock Vale 5b Black Rock Vale 6a Radmore Harbour 6b Radmore Harbour 6c Radmore Harbour 7a John Richardson Bay 7b John Richardson Bay 7c John Richardson Bay 8a John Richardson Bay 8b John Richardson Bay 9a Herschel Bay7 9b Herschel Bay7 9c Herschel Bay7 10a Makinson Inlet 10b Makinson Inlet 10c Makinson Inlet 11a Makinson Inlet 11b Makinson Inlet 12a Piliravijuk Bay 12b Piliravijuk Bay 12c Piliravijuk Bay 13a Inner Piliravijuk Bay 13b Inner Piliravijuk Bay 14a Vendom Fiord 14b Vendom Fiord 15a Bjorne Peninsula 15b Bjorne Peninsula 16a Bjorne Peninsula 16b Bjorne Peninsula 17a Bjorne Peninsula 17b Bjorne Peninsula 18a Fosheim Peninsula 18b Fosheim Peninsula
Site
TO-8090 TO-8089 TO-8085 TO-141 TO-8086 S-2645 S-2108 TO-435 TO-8094 GSC-3179 TO-4474 GSC-1812 TO-8093 TO-8092 TO-8091 TO-3464 TO-3778 TO-8098 TO-3774 TO-8081 TO-8082 TO-8083 TO-8084 TO-225 TO-226 GSC-2516 AA-23620 TO-9487 TO-9488 GSC-3180 AA-23628 Beta-111699 TO-224 GSC-2519 AA-23633 Beta-119914 TO-9877 TO-9878 CAMS-61418 GSC-6410 CAMS-61414 GSC-6408 CAMS-61417 GSC-6406 TO-2240 TO-2611
Laboratory dating no.1
P. arctica H. arctica* P. arctica P. arctica H. arctica* M. truncata P. arctica P. arctica M. truncata* P. arctica P. arctica P. arctica H. arctica* P. arctica H. arctica* P. arctica P. arctica H. arctica* P. arctica P. arctica H. arctica* P. arctica H. arctica* P. arctica M. calcarea M. calcarea* P. arctica P. arctica H. arctica* P. arctica H. arctica* P. arctica P. arctica H. arctica* P. arctica H. arctica* P. arctica H. arctica* P. arctica M. truncata* P. arctica M. truncata* P. arctica M. truncata* P. arctica Echinoderm spicules*
Taxon dated (comparator species marked by *)2
4150 3570 9370 9160 8800 8890 9270 7800 7540 8270 7960 7740 7040 7670 6640 8040 8060 7370 7350 7640 5650 6760 4860 9250 9420 9340 10 010 9880 9430 9670 8850 9750 9560 9330 8550 8500 8170 6730 9780 9200 4610 4980 4770 4380 9970 9310
Conventional radiocarbon age (14C a BP)3
90 60 80 180 80 150 260 80 70 135 90 90 70 70 70 60 60 80 60 70 70 70 90 50 150 50 65 70 60 55 75 70 60 50 60 80 60 90 40 45 50 55 50 50 90 90
⫾
90 100 115 70 160 90 90 90 85 80 100 110 60
1700 1990 1900 -90 80 580 450 820 420 230 50 1440 580
125
100 100
670 690
660
100
1030
75
150 115 115
1230 920 700
70
270 105
1730 260
390
115 200
570 360
-370
110
⫾4
580
Age of P. arctica relative to compared species (14C years)
4
4
-4
7
10
1
4 3
7
5 4
-1 1
12
14 15
5 5
8
6 6 5
6 2
4 2
4
Difference ()5
Marine clay Marine clay Marine silt Marine silt Marine silt Foreset beds Marine silt Marine silt Marine silt Marine silt Marine silt Marine silt Marine silt Marine silt Marine silt Marine silt Marine silt Marine silt Deglacial silt Deglacial silt Deglacial silt Marine silt Marine silt Surface Surface Surface Marine silt Marine silt Marine silt Marine silt Marine silt Clayey silt Glaciomarine silt Clayey silt Bedded sand Bedded sand Silt Silt Marine silt Marine silt Glaciomarine mud Glaciomarine mud Foreset beds Foreset beds Clayey silt Silty clay
Enclosing material
3 3 105 105 105 95 70 70 70 65 58 46 >65 ~50 ~50 36 36 30 53 53 53 8 8 11 11 11 ~58 ~58 ~58 65 ~71 ~38 ~40 ~40 ~58 ~58 ~15 ~15 ~113 ~113 ~19 ~19 8–12 8–12 90 87
Sample elevation (m a.s.l.)
Table 1. Radiocarbon dates on suspension-feeding and deposit-feeding molluscs in both ‘direct comparisons’ and ‘associations’.
>3 >3 >105–ⱕ124 >105–ⱕ124 >105–ⱕ124 >95–ⱕ124 >70–ⱕ110 >70–ⱕ110 >70–ⱕ111 83 ⱕ87 >72 ⱕ83 70 70 >36–ⱕ72 >36–ⱕ72 >36–ⱕ72 ⱕ82 ⱕ82 ⱕ82 >8 >8 >11–>130 >11–>130 >11–>130 >58–ⱕ115 >58–ⱕ115 >58–ⱕ115 >65–ⱕ93 >71–ⱕ93 >40 >40 >40 >58–ⱕ72 >58–ⱕ72 ⱕ62 ⱕ62 >113 >113 >19–ⱕ24 >19–ⱕ24 >8–ⱕ19 >8–ⱕ19 >90–ⱕ141 >87–ⱕ141
Related relative sea level (m a.s.l.)
80°51′ 80°51′ 80°43′ 80°43′ 80°43′ 80°43′ 81°46′ 81°46′ 81°46′ 81°41′ 81°40′ 81°43′ 81°41′ 81°47′ 81°47′ 80°30′ 80°30′ 80°30′ 80°13′ 80°13′ 80°13′ 80°08′15″ 80°08′15″ 78°36′ 78°36′ 78°36′ 77°23′ 77°23′ 77°23′ 77°32.2′ 77°32′ 77°19′ 77°19′ 77°19.2′ 77°19′30″ 77°19′30″ 78°03′ 78°03′ 77°49′04″ 77°49′04″ 77°52′36″ 77°52′36″ 77°52′ 77°52′ 79°56′ 79°56′
Lat. N
81°47′ 81°47′ 80°35′ 80°35′ 80°35′ 80°35′ 65°43′ 65°43′ 65°43′ 69°08′ 69°07′ 69°23′ 69°08′ 67°37′ 67°37′ 70°43′ 70°43′ 70°43′ 71°16′ 71°16′ 71°16′ 71°22′30″ 71°22′30″ 74°45′ 74°45′ 74°45′ 81°33′ 81°33′ 81°33′ 81°32′ 81°32′ 82°01.2′ 82°01.2′ 82°01.2′ 82°25′ 82°25′ 82°14′ 82°14′ 86°49′34″ 86°49′34″ 86°53′5.6″ 86°53′5.6″ 86°53′ 86°53′ 84°17′ 84°17′
Long. W
This paper This paper This paper England (1990) This paper England (1990) England (1983) England (1997) This paper England (1983) Smith (1999) England (1983) This paper This paper This paper England (1996) England (1996) This paper England (1996) This paper This paper This paper This paper Blake (1992) Blake (1992) Blake (1992) England et al. (2004) England et al. (2004) this paper Blake (1981) England et al. (2004) England et al. (2004) Blake (1993) Lowdon & Blake (1978) England et al. (2004) England et al. (2004) This paper This paper England et al. (2004) England et al. (2004) This paper This paper This paper England et al. (2004) Bell (1996) Bell (1996)
References
364 John England et al. BOREAS
P. arctica H. arctica*
TO-9479
TO-9478
Aurland Fiord head
Aurland Fiord, E. arm Aurland Fiord, E. arm Aurland Fiord, central Aurland Fiord, central Aurland Fiord, central Surprise Fiord, W. central Surprise Fiord, W. central
20c
10
Ellice Hills
Associations E Ellesmere Island 30a Ida Bay 30b Ida Bay 30c Simmonds Bay 31a Alert 31b Alert 31c Wood River 31d Wood River
29b
GSC-1775 GSC-1668 SI-3300 GSC-1815 TO-8097 S-1985 TO-10945
BGS-2466
TO-10685
Committee Bay 29a Ellice Hills7 9
Beta-208161 Beta-208162
NE Baffin Island 28a Erik River7 28b Erik River7
7
TO-11340 TO-11341
SW Melville Island 27a Liddon Gulf 27b Liddon Gulf
GSC-6279
TO-8080
Victoria Island 26a Wollaston Peninsula
Wollaston Peninsula
TO-5664a TO-5664b
Bathurst Island 25a Young Inlet 25b Young Inlet
P. arctica P. arctica H. arctica* P. arctica P. arctica B. glacialis8 H. arctica*
C. ciliatum*
P. arctica
P. arctica H. arctica*
P. arctica H. arctica*
H. arctica*
P. arctica
P. arctica D. greenlandicus*
P. arctica P. arctica H. arctica*
A. borealis*
GSC-6416
8530 8770 7270 10 500 10 390 9690 9420
9190
9260
8610 8600
13 540 11 680
11 100
10 760
10 590 9500
9980 9590 8810
4240
4370
8440
9120
B. glacialis8 H. arctica*
9860
9310
9480
8580
9220
9890
8460
10 700 9240
P. arctica
P. arctica
GSC-3744 TO-8099 TO-9882
26b
8
H. arctica*
B. glacialis
P. arctica
CAMS-61419
TO-9474
TO-9475
TO-9476
TO-9463
NW Greenland 24a Hall Land 24b Hall Land 24c Hall Land
23b
23a
22c
22b
22a
21b
21a
TO-9462
Aurland Fiord head
20b
TO-9494
Aurland Fiord head
20a
H. arctica*
TO-9458
Nansen Sound
19c
P. arctica P. arctica
GSC-4740 TO-9459
Axel Heiberg Island 19a Nansen Sound 19b Nansen Sound
100 210 70 105 160 1075 60
180
70
40 40
100 90
50
90
160 70
70 80 60
30
40
70
60
80
70
80
70
70
100
120
50 70
120 220 120 170
1080 970
195
55
135
105
175
90 100
50
90
105
105
100
120
130 140
1260 1500
70
10
1860
-340
1090
1170 780
130
680
1420
170
640
1310
2240 780
7 5
8 6
1
1
10
-3
5
9 6
2
6
10
2
5
8
14 5
Silt Silt Silt Silt Silt Bottomset silt Silt
Marine silt and clay Marine silt and clay
Stoney marine silt Stoney marine silt
Glaciomarine silt Glaciomarine silt
Stoney sand and silt Glaciomarine silt
Marine silt Marine silt
Marine silt Marine silt Marine silt
Stoney marine silt
Stoney marine silt
Clay
Clay
Clay
Bottomset muds
Silt Massive stoney silt Massive stoney silt Massive stoney sand Massive stoney sand Massive stoney sand Bottomset muds
81°00′15″ 78°17.7′ 78°17.7′
>7 >5–ⱕ9 >5–ⱕ9
~7
83 83 88 113–116 120 ~45 ~96
152
156
35 35
43 41
106
106
110 110
110 110 110
5–9
5–9
81°32′ 81°31′ 81°17′ 82°27′ 82°27′ 82°30′ 82°30′
67°57′38″
ⱕ250
ⱕ104 >83–ⱕ104 95 120 >120 92–ⱕ125 96–ⱕ125
67°57′38″
70°36′ 70°36′ ⱕ250
68–68–106
112 112
81°42′20″ 81°42′20″ 81°42′20″
81°00′15″
>7
~7
>110–ⱕ140 >110–ⱕ141 >110–ⱕ142
81°00′15″
>7
~7
27
81°00′11″
80°58′
>10–ⱕ38
~10–15
81°00′11″
80°58′
>10–ⱕ38
~10–15
>27–ⱕ45
80°58′
>10–ⱕ38
~10–15
>27–ⱕ45
81°01′
>45–ⱕ68
~45
27
81°01′ 81°01′
>45–ⱕ68 >45–ⱕ68
45 ~45
68°58′ 69°07′ 69°25′ 62°45′ 62°40′ 63°15′ 63°10′
88°52′38″
88°52′38″
72°34′ 72°34′
114°03′ 114°03′
117°06′30″
117°06′30″
98°59.5′ 98°59.5′
59°36′ 59°36′ 59°36′
90°38.4′
90°38.4′
93°52′21″
93°52′21″
93°52′21″
93°44′16″
93°44′16″
94°22′
94°22′
94°22′
91°40′
91°40′ 91°40′
England (1978) England (1978) England (1983) England (1978) this paper England (1983) this paper
Little (2006)
Little (2006)
This paper This paper
England et al. (2009) England et al. (2009)
Dyke & Savelle (2000)
This paper
Bednarski (2003) Bednarski (2003)
England (1985) This paper This paper
This paper
This paper
This paper
This paper
This paper
This paper
This paper
This paper
This paper
This paper
This paper
Bednarski (1998) This paper
BOREAS
Exaggerated radiocarbon ages of molluscs 365
I(GSC)-18 GSC-1707 GSC-48
Victoria Island 39a Peel Point 39b Peel Point 39c Richard Collinson Inlet P. arctica13 P. arctica H. arctica*
P. arctica H. arctica*
P. arctica M. truncata & H. arctica* P. arctica P. arctica H. arctica* P. arctica P. arctica M. truncata* P. arctica H. arctica* P. arctica M. truncata & H. arctica*
P. arctica H. arctica*
Taxon dated (comparator species marked by *)2
12 820 13 000 11 730
10 550 8840
9970 9190 9540 9220 7740 9440 9335 8655 9110 8140 9680 8590
8460 7720
Conventional radiocarbon age (14C a BP)3
340 70 80
90 70
90 40 155 45 85 70 70 90 60 45 80 50
90 70
⫾
115 115 75 95
785 680 970 1090
1090 1270
350 105
115
175 95
1800 1480
1710
100
115
⫾4
780
740
Age of P. arctica relative to compared species (14C years)
3 9
11
9
9
5 5
8 12
6
5
Difference ()5
Clay Clay Surface
Fine sand Marine silt
Clayey silt Fine sand and silt Fine sand Fine sand Fine sand and silt Silt Silt Surface Glaciomarine silt Silt Silty-clay Upper beaches
Marine silt Marine silt
Enclosing material
67–70 67–70 67
57 43
90 ~135 70 70–74 75–78 72 97 99 68 78 -3212 ~101
~15 ~67
Sample elevation (m a.s.l.)
82–85 82–85 85
>55–ⱕ80 >43–ⱕ80
>90–ⱕ141 >136–ⱕ140 >80 >80 >75 >72–ⱕ118 >97–ⱕ110 >99 >72 >78–ⱕ101 – >101
~88 84
Related relative sea level (m a.s.l.)
73°18′ 73°18′ 72°34′
82°03′ 82°03′
79°56′ 79°59′ 79°02′ 79°02′ 79°01′ 78°39′ 78°32′ 78°31′ 78°11′ 78°13′ 77°34′ 77°34′
80°10′ 80°10′
Lat. N
114°30′ 114°30′ 113°53′
88°08′ 87°45′
84°17′ 85°35.3′ 81°31′ 81°31′ 81°28′ 86°43′ 86°41′ 87°14′ 84°08′ 84°34′ 85°06′ 85°05′
71°11′ 71°28′
Long. W
Walton et al. (1961) Lowdon & Blake (1976) Dyck & Fyles (1962)
Evans (1990) Evans (1990)
Bell (1996) this paper Hodgson (1985) Hodgson (1985) Hodgson (1985) O’Cofaigh et al. (2000) O’Cofaigh et al. (2000) O’Cofaigh et al. (2000) Hodgson (1985) O’Cofaigh et al. (2000) Smith (1998) England et al. (2004)
England (1996) England (1996)
References
1 Laboratory designations: AA = University of Arizona.; Beta = Beta Analytic; CAMS = Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory; GSC = Geological Survey of Canada; I = Isotopes Inc.; S = Saskatchewan Research Council; SI = Smithsonian Institution; TO = IsoTrace Laboratory. TO and AA samples were dated by Accelerator Mass Spectrometry (AMS). Beta-116131 is an AMS date; all others are conventional 14C dates. 2 Prior to dating, all samples were abraded by brush or dremel tool, rinsed in de-ionized water and pre-leached as much as possible in dilute HCl, commonly >40%. Details of GSC sample preparation can be found in the references cited in McNeely & Brennan (2005). 3 GSC dates are reported as updated by McNeely & Brennan (2005). We report all GSC dates as conventional radiocarbon dates with 1 standard deviation, rather than 2 as originally reported by the laboratory. All samples were corrected for isotopic fractionation to a base of d13C=-25‰. AA-, Beta-, CAMS-, GSC- (except GSC-48) and TO- dates were reported normalized by the laboratory. S-dates, SI-dates, I(GSC)-18 and GSC-48 were originally non-normalized radiocarbon dates. They are here reported as conventional radiocarbon dates normalized according to Donahue et al. (1990). I(GSC)-18 and GSC-48 were normalized assuming d13C=1.02⫾1.32‰ (Coulthard et al. 2010), S-2108 was normalized for the measured d13C of 1.8‰, and SI-3300 was normalized for the measured d13C of 0.3‰. 4 The uncertainty of these offsets was derived using standard statistical procedures (i.e. the square root of the sum of the squared standard errors of the compared ages). 5 We calculate the number of standard deviations by taking the difference of the sample ages in a site-specific comparison (14C years), and then dividing this by the sum of their standard errors (1), always rounding up. 6 All the radiocarbon dates listed from Chandler Fiord were collected from thick deglacial rhythmites underlying the dissected delta marking the marine limit. The sample sites were within 0.5 km of one another and hence should be the same age. Sample sites 4a and 4d are from the same rhythmites and provide the best comparison for this area. 7 Note that Herschel Bay, Erik River and Ellice Hills are underlain by crystaline rocks of the Precambrian Shield, whereas all other samples are underlain by sedimentary bedrock. 8 B. glacialis is included here as this species also generally dates older than associated taxa. 9 reported by Little (2006) by the sample code BU-0202-A. 10 Associations refer to samples that relate to the same relative sea level (deglacial marine limit) but are not collected from the same bed. 11 Sample 33a is the same as 18a (above). The comparison with sample 33b is made here as it provides an independent date on the same deglacial shoreline at ~140 m a.s.l. 12 P. arctica collected from the basal part of the lake core presently below sea level. The sample is assumed to represent the deglacial environment and hence is comparable to the local marine limit at ~101 m a.s.l. 13 I(GSC)-18 is the same sample as GSC-1707, which was a cross-check of this early date. The species was originally reported as Yoldia arctica.
TO-473 TO-855
NW Ellesmere Island 38a Cape Armstrong 38b Cape Armstrong
TO-3450 TO-4198
TO-2240 Beta-116769 GSC-3388 GSC-1978 GSC-3397 AA-23587 AA-23593 AA-23592 GSC-2719 GSC-6037 TO-4757 Beta-116771
John Richardson Bay John Richardson Bay
32a 32b
Laboratory dating no.1
W Ellesmere Island 33a11 Fosheim Peninsula 33b Fosheim Peninsula 34a Thumb Mountain, Irene Bay 34b Thumb Mountain, Irene Bay 34c Thumb Mountain, Irene Bay 35a Raanes Peninsula 35b Raanes Peninsula 35c Eureka Sound 36a Starfish Bay 36b Starfish Bay 37a Hoved Island 37b Hoved Island
Location
Site
Table 1. Continued
366 John England et al. BOREAS
Exaggerated radiocarbon ages of molluscs 31
90°W
120°W
38
Pearya
1000 km
500
Arctic Ocean
30 1,2 19
20-22 NS
6 7,8,32
18,33
ES
35
23 Fig. 2. Generalized geology of Arctic Canada and adjacent NW Greenland showing the distribution of Palaeozoic and younger sedimentary rocks (containing varying amounts of carbonate; blue) vs. Precambrian Shield (pink; modified after Trettin 1991 and Harrison et al. 2011). Note that northern Ellesmere Island (red) constitutes Pearya, a diverse area of igneous and metamorphic rocks unrelated to the Precambrian Shield (Trettin 1987). Black dots show the distribution of direct comparisons and associations whose site numbers correspond to those in Table 1. Note that only site numbers 9, 28 and 29 are located on the Precambrian Shield. All remaining sites are located on Palaeozoic and younger sedimentary rocks that contain some carbonate. The majority of modern (live-collected) age comparisons reported by McNeely et al. (2006) border the contact of Precambrian Shield with Paleozoic carbonate, northern Foxe Basin (black circle).
24
3-5
34
9
nland
150°W
367
Gree
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60°W
36 14
15-17 25
37 10-13
Baffin Bay
27 Barrow
39
Str
Pre
cam
26
bria 28 nS hie
ld
120°W
66°N
(vertically) of the marine limit. Shells within these sediments should be younger than the marine limit by no more than 330 years (and probably less) based on the slowest rate of postglacial emergence in Arctic Canada during deglaciation (ranging from 3 to 7 m century-1; e.g. Blake 1975). All the radiocarbon age determinations cited are reported as conventional radiocarbon dates unless otherwise noted (normalized to a base of d13C=-25‰; Table 1). To simplify the comparisons, we have not applied a marine reservoir correction (Coulthard et al. 2010).
Results We present 29 sites spanning the CAA (Fig. 1) that include 42 direct comparisons (Table 1 and footnotes therein). The differences between the ages vary from 2240⫾130 years (site 19) to 10⫾55 years (site 28), with P. arctica yielding the older date in all but one case. The uncertainty of these offsets was derived using standard statistical procedures (i.e. the square root of the sum of the squared standard errors of the compared ages). The difference between paired datings is 4 or greater in 31 out of 42 direct comparisons. The greatest statistical difference is 15 (site 7), and the smallest difference is less than 1 (site 28; Table 1). We calculate the number
Prec
29
ambr
ian S
hield
90°W
Foxe 66°N Basin
of standard deviations by taking the difference of the sample ages in a site-specific comparison (14C years), and then dividing this by the sum of their standard errors (1), always rounding up. The mean errorweighted difference between all direct comparisons is 590⫾605 years. We also present 10 additional localities that include 15 associations whose ages should be contemporaneous (within ⱕ330 years). Nonetheless, the differences between paired datings at these sites vary from 1800⫾175 years (site 34) to 680⫾115 years (site 35; Table 1). In all but one of the associations, the paired dates differ by 4 or more. The largest difference is 12 (site 34), whereas the smallest difference is 3 (site 39). The mean weighted difference of all the associations is 1095⫾330 years. In summary, of all the paired datings (direct comparisons and associations), 45 out of the 57 examples differ by 4 or greater, and commonly by much more (Table 1). The mean error-weighted difference of all 57 paired datings is 690⫾595 years, emphasizing the unpredictability of the differences between sample comparisons at specific sites. The overwhelming majority of paired datings (50/57) were collected from carbonate terrain, including clastic rocks that contain carbonate (Fig. 2). In contrast, we note that direct age comparisons of P. arctica and suspension feeders collected from deglacial sites underlain
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John England et al.
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13 000 12 000
Direct Comparisons (carbonate terrain) Associations (carbonate terrain)
Suspension feeder age (14C a BP)
11 000 10 000
Direct Comparisons (Precambrian shield) Direct Comparisons (heterogeneous terrain)
9000 8000 7000 6000 5000 4000 3000 3000
4000
5000
6000
7000
8000
9000 10 000 11 000 12 000 13 000 14 000
P. arctica age ( C a BP) 14
by the Precambrian Shield yield ages that are statistically indistinguishable at 1 (sites 28, 29; Fig. 2; Table 1). Sites where age differences are of intermediate or low value coincide with more heterogeneous bedrock (e.g. a mixture of carbonate and siliciclastic and/or nearby Precambrian Shield, sites 13, 21, 23; Fig. 2; Table 1). Collectively, these differences are plotted for comparison (Fig. 3).
Discussion Forman & Polyak (1997) considered several possible causes for their exaggerated ages of P. arctica in the Barents and Kara seas, including ‘old carbon sources from stream or glacier input’ and isotopically depleted (14C) pore water (cf. McCorkle et al. 1990). These earlier considerations were further advanced by observations from the Norwegian Arctic (Mangerud et al. 2006). They proposed that the Group Protobranchia (including the genera Portlandia, Yoldia, Yoldiella, Nucula, Nuculana and Pseudomalletia) would tend to yield exaggerated ages owing to carbonate-rich pore water and/or the ingestion of carbonate particles and organic matter, including coal. This was supported by two dates on Nuculana pernula from Svalbard, now considered to be ~1600 years ‘too old’ for the earliest shoreline they could be assigned to. As an alternative explanation, we note that although a long residence time of pore water itself would contribute to a greater apparent age, even if it was solely derived from the ocean water above it, this does not explain the notable
Fig. 3. Plot of direct comparisons and associations of suspension feeders with P. arctica derived from Table 1. Comparisons with concordant dates, and therefore no Portlandia effect, should fall on the line. Most comparisons with P. arctica ages are shifted to the right of the line and are older by an average of 690⫾595 years. This large variability demonstrates the non-systematic error inherent in P. arctica dates. Note, however, that P. arctica comparisons from the Precambrian Shield and heterogeneous terrain fall on or very close to the 1:1 line, demonstrating concordance with suspension feeders in non-calcareous environments, and reinforcing the validity of these dates.
concordance of P. arctica and suspension-feeder dates from the Precambrian Shield and other siliciclastic terrain reported here. Therefore, dissolved pore-water residence time cannot be a significant cause of our exaggerated ages. Based on the numerous P. arctica age comparisons presented here, we conclude that the exaggerated age of P. arctica records its access to dissolved Palaeozoic carbonate within the sediment pore water in and adjacent to carbonate-rich terrain. In the sites presented in this study, dates obtained on P. arctica are commonly >500 years older than those obtained on other species, and in one-third of the cases they are >1000 years older (Table 1; Fig. 3). These age differences are therefore non-trivial and fall well beyond the reported statistical error (1 or 2) of individual 14C measurements in this study. Furthermore, these discordant ages do not appear to show any systematic spatial variation across the Queen Elizabeth Islands – indeed they extend over 1000 km into the southwestern CAA, where they are also documented on Melville and Victoria Island (sites 26, 27, 39; Table 1). Although many comparisons with P. arctica reveal large differences (>1000 years) at the heads of fiords (e.g. sites 4, 6, 14, 20, 34, 36), where isolation from the open ocean could be a contributing factor, these results are matched by similar age differences at open coastal locations (e.g. sites 19, 24, 31, 39). Moreover, we note that these discordant ages are not restricted to deglaciation (8 to 11 ka BP), as might be expected because of the abundant carbonate rock flour then released from retreating tidewater glaciers. Rather, large age differences persist well after deglaciation in deposits as young
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as 3 to 5 ka BP (sites 1 and 8, respectively; Fig. 3; Table 1). Pre-bomb, live-collected molluscs from the CAA similarly demonstrate that P. arctica ages typically exceed those on suspension feeders by ⱖ2 (McNeely et al. 2006). The absolute difference in these (modern) comparisons (n=18) is commonly
