Amino acid geochronology of raised beaches in south west Britain

Quaternary Science Reviews, Vol. 4, pp. 279-318, 1985. Printed in Great Britain. All rights reserved.

0277-3791/85 $0.00 + .50 Copyright ~) 1986 Pergamon Journals Ltd.

AMINO ACID GEOCHRONOLOGY OF RAISED BEACHES IN SOUTH WEST BRITAIN

D.Q. Bowen and G.A. Sykes

University of London, Queen's Building, Department of Geography, Royal Holloway and Bedford New College, Egham, Surrey TW20 OEX, U.K. (Formerly at The University College of Wales, Deparlment of Geography, Aberystwyth) Alayne Reeves (nee Henry)

Department of Geography, The University College of Wales, Aberystwyth, Dyfed SY23 3DB, U.K. G.H. Miller and J.T. Andrews

INSTAAR & Geological Sciences, University of Colorado, Boulder, Colorado 80309, U.S.A. J.S. Brew

University of London, Queen's Building, Department of Geography, Royal Holloway and Bedford New College, Egham, Surrey TW20 OEX, U.K. P.E. Hare

Geophysical Laboratory, Carnegie Institution of Washington, Washington D.C., U.S.A. With Contributions from: Department of Geology, The University College of Wales, Aberystwyth, Dyfed SY23 3DB, U.K. Sandra Hughes Department of Geography, The University Collegeof Wales, A berystwyth, U.K. (present address: Godalming College, Godalming, Surrey, U.K.) H.C.L. James Faculty of Environmental Studies, Bulmershe College, Reading RG6 1HY, U.K. D.B. Smith British Geological Survey, Newcastle-Upon-Tyne, U.K. D.N. Mottershead Edge Hill College, Ormskirk, Lancs L39 4QP, U.K. D.G. Jenkins Earth Sciences, The Open University, Milton Keynes, U.K. J.T. Hollin INSTAAR, University of Colorado, Boulder, Colorado 80309, U.S.A. A.M. McCabe Department of Geography, University of Ulster, Newton Abbey, Co. Antrim, Ulster BT37 OQB, U.K. D.D. Harkness RadiocarbonLaboratory, SURRC, East Kilbride, Glasgow G75

A.R. Wyatt

OQU, U.K. 279

280

D.Q. Bowen et al. Based on (1) the epimerization of L:isoleucine to D:aUoisoleucine (D/L ratios) in Patella vulgata, Littorina littorea, L. littoralis, L. saxatilis, Littorina species and Nucella lapillus from raised beaches in south west Britain, (2) statistical analysis of the D/L ratios, and (3) lithostratigraphic and geomorphic evaluation, three (D/L) Stages are proposed. The D/L ratios for all the species measured are converted to a Patella vulgata standard. The three (D/L) Stages are: (1) The Minchin Hole (D/L) Stage, D/L ratios 0.175 + 0.014, defined at a stratotype in Minchin Hole Cave, Gower, Wales. (2) A provisionally defined, but as yet, unamed (D/L) Stage, because of the current unavailability of a suitable stratotype, with D/L ratios of 0.135 + 0.014. (3) The Pennard (D/L) Stage, D/L ratios 0.105 _ 0.016, defined at a stratotype in Minchin Hole Cave, Gower, Wales. Two geochronological models of the three high sea-level events representing the D/L Stages are constrained by uranium-series age determinations on stalagmite interbedded with marine beds in Minchin Hole and Bacon Hole Caves, Gower, Wales. A potential 'fixed point' in model evaluation is an age determination which is equivalent to Oxygen Isotope Sub-stage 5e (122 ka). The two models are:-

Oxygen Isotope Stage and age (ka BP) (D/L) Stage Pennard

D/L Ratio 0.105 ± 0.016

Model la ~ 5e(122)

Unnamed Minchin Hole

0.135 ± 0.014 0.175 ± 0 . 0 1 4

7(186-245)

Model lb

Model 2

~5a(ca. 80) ~5c(ca. 100) 5e(122) 7(186-245)

5e(122) 7(194 ?) 7(216 ?)

The ages of the raised beaches are used to constrain the timing and extent of glaciation in the area. The extent of the Late Devensian glaciation is fixed by reference to localities where raised beaches of the Pennard (D/L) Stage outcrop, as well as by using (unpublished) data on molluscs incorporated into glacial deposits. Two pre-Devensian glaciations antedate the Minchin Hole (D/L) Stage. The Irish Sea Glaciation of the Bristol Channel (Irish Sea 'older drift' of South Wales/Fremington Till of North Devon) is ascribed to Oxygen Isotope 10 or earlier, while the Paviland (Welsh) Glaciation is ascribed to Oxygen Isotope Stage 8 or earlier. An earlier glaciation (Berwyn Glaciation) is inferred from erratics of known Welsh provenance in Essex.

INTRODUCTION Principles of amino acid geochronology are discussed in Miller and H a r e (1980). The m e t h o d relies on the t i m e - d e p e n d e n t degradation of protein in the carbonate matrix of molluscan shells. T h e reaction used most frequently in geochronological studies is the epimerization of the protein amino acid L-isoleucine to its non-protein diastereoisomer Dalloisoleucine. T h e ratio of the two isomers (D/L) in a shell is a measure of the time elapsed since the death of the organism. In a m o d e r n shell the ratio is effectively zero and progressive inversion proceeds to an equilibrium value of ca. 1.30 (Miller, 1982). Because the reaction is also t e m p e r a t u r e d e p e n d e n t (Miller and H a r e , 1980) the D/L ratio can only be directly correlated between shells which have experienced a similar

Amino Acid Geochronology

281

temperature history. Thus a fundamental assumption of the method is that correlation, in the absence of any supporting independent geochronological control, is made throughout a region which has experienced a uniform integrated thermal history. For the study region (Fig. 1) present-day mean annual air temperatures are 11.9°C at Swansea (for Gower) and 10.7°C at Portland. By using a wide variety of proxy evidence, it may be assumed, that at least for the time represented by the data herein, that the temperature history throughout the region has been uniform. A further consideration relates to the need to use monospecific samples for direct comparison. This is not always possible when correlation between beds is required so that 6 basic species are used in this study. Fortunately they frequently overlap in their incidence so that different species which epimerize at essentially the same rate can be identified and correction factors applied to relate a variety of species to a common base - - in this study to Patella vulgata. Data from 34 sites and 6 species are used. Out of 204 possible combinations of these only 78 are present in the data set. The total number of data.values (D/L ratio measurements) is 336. The maximum number of sites at which a single species is found is 27 (for Patella vulgata), and the maximum number of data points for a single species is 135 (also for Patella vulgata).. Given the assumptions and constraints outlined it is possible to correlate directly between stratigraphic units throughout the region and to establish a regional aminostratigraphy (Miller and Hare, 1980). In this way a model of the reaction kinetics is not required nor is a precise model of the nature and degree of past climatic changes. Philosophically the fundamental approach in this study is to evaluate stratigraphically the experiment performed by nature by measuring the D/L ratios. To convert these to a geochronological scale requires independent calibration which is discussed later. The species used in this study are: Patella vulgata, Littorina littorea, Littorina littoralis, Littorina saxatilis, Littorina sp. and Nucella lapillus.

PREVIOUS WORK The first D/L ratios on marine molluscs for the British Isles were reported by Andrews et al. (1979) and Miller et al. (1979). Of relevance to the present study is the former (Andrews et al., 1979) when D/L ratios from Minchin Hole Cave, Gower (outer beach), Portland (East), Dorset, Middle Hope (Swallow Cliff), Avon, Saunton, Devon and the Burtle Beds, Somerset, were presented. They concluded that the data showed that two or possibly three sea-level events were represented by raised beaches, but were unable to determine whether these occurred during the whole or part of marine Isotope Stage 5, or in part from an earlier interglacial. This continued the dilemma apparent when two raised beach units had been exposed at Minchin Hole Cave, Gower (Sutcliffe and Bowen, .1973), which had been evaluated against the marine time scale (Bowen, 1973c). Subsequent determinations of P. vulgata D/L ratios from the older of the two units (the inner beach) of k = 0.14 (Bowen, 1981) as compared with )2 = 0.099 from the younger unit (outer beach) confirmed the separate identity of two high sea-level events. In an attempt to relate the emerging amino acid time scale to a geochronological one a uranium-series determination on travertine encasing shells of Patella vulgata ( A A R 0.123 +

282

D.Q. Bowen et al.

I,.I.I ¢-

E O-

o

O.

on" ~ z ~.

O'

o

.~

to

iiiiiiiiiiiiiiii!iii!i!iiii~!i!~

o

N::!iii!i!!i::iii:~i::i::i~~ %

m

i:!:!:E!!i!ij!: jEi!iiiiiii!i:.'~:

%

%

%

~Jc~ %

I I

°A

Amino Acid Geochronology

283

0.024) from Jersey (Keen et al., 1981) gave an age of ,1,~-12 .,,+~4 ka BP. They suggested therefore, that one of the groups recognised by Andrews et al. (1979) could be ascribed to Isotope Sub-stage 5e of the marine scale (Shackleton and-Opdyke, 1973). Subsequently the available evidence was considered in a review by Wehmiller (1982) who showed that comparisons between fast and slow epimerizing species and correlation between marine and non-marine species presented major conflicts when age determination was attempted. In particular he suggested that it was possible that the Group 2 data of Andrews et al. (1979) could well represent an age equivalent to Stage 7 of the marine isotope scale. His identification of potentially conflicting interpretations of data from the Burtle Beds of Somerset is still unresolved. Further D/L ratios on Littorina littoralis from Portland (East) and Broughton Bay, Gower, were presented to justify correlation with beach fragments broadly regarded as Ipswichian (Stage 5) in south-west Britain by Campbell et al. (1982). Davies (1983, 1984) presented the first systematic study of AAR's from raised beach units in southern Britain. She identified two major groups of interglacial age which were ascribed to Stage 7 and Substage 5e of the marine isotope scale (Davies, 1983). Davies further presented D/L data from eleven species from raised beaches on the Isle of Portland (Davies and Keen, 1985). It was argued that the age of the Portland West raised beach was time equivalent to Isotope Stage 7 and the Portland East raised beach was time equivalent to Isotope Sub-stage 5e. Correlation was made with other raised beaches throughout southern Britain, as had previously been made in her earlier paper (Davies, 1983). Using 20 samples consisting of Patella (1), Littorina littorea (2), Littorina sp. (1), Littorina littoralis (1), M a c o m a (8) and Corbicula (7), Andrews et al. (1984) investigated raised beach units and other marine deposits at the head of the Severn Estuary. Although they were unable to resolve the conflicts noted by Wehmiller (1982) they suggested that the data showed a high sea-level event during Sub-stage 5e, an earlier high sea-level event when a storm beach was deposited at Swallow Cliff but of indeterminate age, and a still earlier event at ca. 400 to 600 ka when fluvial deposits, with Corbicula, accumulated close to a contemporary sea-level which was antedated by a cold stage when local glaciation occurred. With the exception of the data presented by Andrews et al. (1985) which used D/L ratios involving the sample preparation method currently in use at the amino acid geochronology laboratories at the University of Colorado at Boulder, at the University of Bergen, Norway, "at the University of London (Royal Holloway & Bedford New College) and formerly at the University College of Wales, Aberystwyth, all the other data presented in the references cited employed preparation methods which have been supplanted (Miller, 1985). The data presented in this paper, therefore, can only be directly compared with previous work as in Andrews et al. (1985).

METHOD o-alloisoleucine/L-isoleucine ratios (D/L) are determined by ion exchange liquid chromatography on a purpose designed automated amino acid analyser with a fluorescence detection system (Appendix 1). D/L ratios are based on the peak heights as calculated by an Apple IIe microcomputer running the IMI Inc. chromatochart system. The concentration

284

D.Q. Bowen et at.

of other amino acids is calibrated by the addition to the sample of a known amount of the non-protein amino acid Norleucine. Each species has a characteristic amino acid signature (chromatogram) which changes systematically with time. Should this be uncharacteristic, possible causes are reviewed: these could include, incorrect identification of a shell fragment (unlikely in this study because entire shells were used mostly), incomplete removal of surface contaminants, or contamination during preparation. The data reported here consists of the 'Total' amino acid fraction: that is, the 'Free' fraction (released by natural hydrolysis), and those amino acids still peptide bound. Preparation and analytical procedures are given by Miller and Hare (1980) and Miller (1982). Details of the procedures followed in this study are given in Appendix 2. Sample preparation takes place entirely within one vessel. This is necessary because fractionation of the sample can occur because of adsorption of high molecular weight protein residues on vial walls if transfers are made prior to acid hydrolysis of the sample (Miller et al., 1982; Miller, 1985). Of previously reported British D/L ratios only those on raised beaches reported by Andrews et al. (1985), and on non-marine molluscs measured by Sandra Hughes (reported in Holyoak and Preece, 1986) used the now approved method of sample preparation in one vial. Two standards are used in the London (previously Aberystwyth) amino acid geochronology laboratory: B D H standard B containing a known concentration of alloisoleucine which is used for testing peak resolution and retention times, and an internal laboratory standard. During the preparation of the D/L ratios reported here this was run 50 times giving a mean value of 0.140 + 0.009 with a coefficient of variation of 6.2%. Most samples were analysed at Aberystwyth (ABER) but some were also run at Boulder (AAL) (Table 1). Samples are collected from discrete lithostratigraphic units. In general they are taken from below surface levels, although within individual units it has been shown that no inherent variability of D/L ratios can be detected. On geomorphic grounds it can be shown that the majority of raised beach exposures have only recently been exposed from beneath a cover of periglacial deposits thus eliminating any possible variation because of differential heating history due to shallow burial. In some cases, stacked lithostratigraphic units yield shells with different D/L ratios clearly from two different populations (e.g. at Minchin Hole Cave, Gower). In other cases, isolated single raised beach units contain two or more populations of different ages. This, of course, is predictable given that more than one high sea-level event has occupied the coastal zone, an occurrence long appreciated (e.g. Bowen, 1973b, 1973c). The problems of mixed populations and their recognition are discussed later (see Classification). Procedurally, the recommendation that three individual shells of a single species are analysed (e.g. Miller, 1982) is followed whenever possible. If the D/L ratios from a stratigraphic unit differ by more than 10% of the mean value then additional shells are analysed to determine whether an inherent variability of the samples occurs if more than one population is represented (Miller and Hare, 1980). This procedure is followed in the majority of cases but in some instances an insufficient sample is available. In such cases it is usually possible to reach a decision on the age of an outcrop by comparative means, particularly if there are other immediately adjacent ones. These are clearly evident in Table 1.

285

Amino Acid Geochronology

TABLE 1. D/L ratios for individual localities

Locality

LAB ID

Species

Godrevy, St. lves Bay, Cornwall SW579433 basal conglomerate ABER 681A B C

2, cr

Shells n

Runs n

0.161 0.165 0.207

0.178 + 0.021

1

3

Fistral Beach North, Newquay, Cornwall SW799624 basal sandrock A B E R 680A Pv B C

0.189 0.175 0.193

0.186 + 0.008

3

3

Trebetherick Point, Cornwall SW926780 basal conglomerate A B E R 708A

Lsax

0.113

1

1

Pv

0.130 0.113 0.121 0.098 0.103 0.086 0.112 0.105 0.088 0.121 0.109 0.092 0.102

0.113 + 0.012

5

5

0.101 + 0.011

3

5

0.102 + 0.012

5

5

0.173 0.182 0.126 0.119 0.109 0.204 0.189 0.194 0.146 0.137 0.151

0.178 + 0.005

2

2

0.118 + 0.007

1

3

0.196 + 0.006

1

3

0.145 + 0.006

3

3

0.196 0.206 0.202 0.103 0.110 0.107 0.142 0.145

0.201 + 0.004

3

3

0.107 + 0.003

1

3

0.144 + 0.002

1

2

Broadhaven, South Pembrokeshire SS978942 cemented beach gravel ABER 782A B C D E A B E R 783A B A B E R 784* A B E R 785A B C D E Baggy Point - - Pencil Rock, Devon SS425403 basal conglomerate ABER 676A C A B E R 678A B C A B E R 677A B C sandrock ABER 679A

Pv

D/L Ratio Peak Height

Lira

Llrs

Pv Lira

N1

Pv

B C

Saunton - - Chesil Cliff, Devon SS435381 basal conglomerate ABER 674A B C A B E R 675A B C base of sandrock A B E R 709A C

Pv

NI

Pv

286

D.Q. Bowen et al.

TABLE 1 (continued)

Locality

LAB ID

Species

D/L Ratio Peak Height

~, ~r

Shells n

Runs n

0.096 0.106 0.121 0.091 0.112 0.103 0.117 0.104 0.109 0.108

0.105 ± 0.011

5

5

0.108 + 0.005

5

5

0.088 0.094 0.103 0.109 0.131 0.104

0.095 + 0.006

3

3

0.115 ± 0.012

3

3

0.107 0.119 0.100 0.118 0.128 0.097 0.092 0.097

0.109 _+ 0.008

3

3

0.123 + 0.005

2

2

0.095 + 0.002

3

3

0.096 0.086 0.084 0.087 0.121 0.091 0.103 0.124 0.136 0.124

0.089 + 0.005

3

3

0.105 --. 0.015

5

5

0.130 __. 0.006

2

2

0.155 0.183 0.166 0.168 0.189 0.190

0.168 + 0.012

1

3

0.190 + 0.001

1 1

1 2

0.158 0.146 0.135

0.146 + 0.009

3

3

Broughton, Gower SS418930 cemented beach gravel

A B E R 865A B C D E A B E R 866A B C D E

Lira

Llrs

Worms Middle Head, Gower SS389876 cemented beach gravel

A B E R 842A B C A B E R 843A B C

Pv Lira

Worms Inner Head, Gower SS392875 cemented beach gravel

A B E R 839A B C ABER 840A B A B E R 841A B C

Pv

Lira Llrs

Rhosili South, Gower SS402873 cemented beach gravel

ABER 844A B C ABER 845A B C D E A B E R 846A B

Pv

Lira

Llrs

Butterslade, Gower SS422869 cemented beach gravel

ABER 356B C D ABER 444A ABER 357B C

Pv Litt sp. NI

Overton Cliff, Gower SS455848 cemented beaeh gravel

A B E R 853A B C

Pv

Amino Acid Geochronology

287

TABLE 1 (continued)

Locality

LAB ID

Overton West, Gower SS460848 cemented beach gravel A B E R 849A B C D E A B E R 851A B D E C A B E R 852A B ABER 850A B C D E Overton Mere, Gower SS463846 cemented beach gravel ABER 847A B A B E R 848A B C D E Horton, Gower SS484854 Horton Upl~er Beach partly cemented, mostly unconsolidated gravel in a red sand-clay matrix

A B E R 819A B C A B E R 818A B C D E A B E R 820A B C D

Honon Lower Beach, Gower SS484854 cemented beach gravel A A L 2841A B C

Species

Pv

Lira

Lira Llrs Nl

Nl

D/L Ratio Peak Height

x", tr

Shells n

Runs n

0.099 + 0.010

5

5

0.131 _+ 0.009

4

0.097 +_ 0.005

1 2

1 2

0.122 + 0.010

3

3

0.153 + 0.002

2

2

0.139 0.126 0.087 0.095 0.098 0.093 0.094

0.133 + 0.007

2

2

0.093 +_ 0.004

5

5

0.136 0.130 0.124 0.136 0.174 0.156 0.198 0.181 0.162 0.154 0.198 0.171

0.130 + 0.005

1

3

0.177 + 0.015

1 4

1 4

- 0.171 + 0.017

4

4

0.113 _+ 0.014

3

3

0.097 0.107 0.081 0.107 0.105 0.135 0.139 0.116 0.132 0.170 0.101 0.092 0.119 0.111 0.135 0.151 0.155 o

Pv Lira

Lira

Llrs Llrs

NI

Litt sp.

0.097 0.110 0.131

288

D.Q. Bowen et al.

T A B L E 1 (continued)

Locality

L A B ID

~, cr

Shells n

Runs n

0.168 0.194 0.168 0.174 0.174 0.165 0.168 0.178 0.179 0.154 0.177 0.196

0.174 + 0.008

9

9

0.176 + 0.017

3

6

0.086 0.114 0.093 0.104 0.087 0.123 0.090 0.145 0.118 0.108 0.097 0.164 0.128 0.144

0.100 + 0.013

7

9

0.117 ___0.018

4

4

0.136 __. 0.008

1 2

1 2

0.117 0.117 0.118 0.126 0.122 0.109 0.111 0.094 0.108 0.105

0.120 ___0.004

5

5

0.105 + 0.006

5

5

Pv

0.110 0.105 0.105 0.102 0.098

0.104 _+ 0.004

5

5

Pv

0.122 0.101

0.112 + 0.011

2

2

Species

D/L Ratio Peak Height

Minchin Hole Cave, Gower SS562868 Inner Sand Beach

A A L 4603A B C. D E

Pv

A A L 4604A B C D A A L 4601A B A A L 4602**

Lira

Minchin Hole Cave, Gower SS562868 Outer Gravel Beach

A B E R 400* A B E R 854A A B E R 855A B C D E A A L 4600A B C D E A B E R 401A B

Pv

Lira

Lira NI

Minchin Hole Cave, Gower SS562868 Bed 4a

A B E R 398A B C D E A B E R 399A B C D E

Llrs

Lsax

Minchin Hole Cave, Gower SS562868 Bed 4b

A B E R 396B D F A B E R 397B D

Bacon Hole Cave, Gower SS559868 Bed 3

A A L 4630A B

289

Amino Acid Geochronology T A B L E 1 (continued)

Locality

LAB ID

Species

D/L Ratio Peak Height

~, tr

Shells n

Runs n

0.093 0.104 0.137 0.128 0.145 0.131 0.125 0.105 0.088 0.136 0.097 0.114 0.104 0.101 0.115 0.110 0.099 0.097 0.198 0.163 0.156 0.152 0.168 0.124 0.119 0.124 0.100 0.111 0.109 0.102 0.112 0.138 0.127 0.109 0.101 0.094 0.093 0.111 0.098

0.113 + 0.017

18

18

0.167 + 0.016

5

5

0.122 + 0.002

3

3

0.114 + 0.012

8

8

0.099 + 0.006

5

5

0.109 0.119 0.103 0.108 0.114 0.111 0.113 0.109 0.111 0.113 0.084

0.111 + 0.006

5

5

0.107 ___ 0.011

6

6

0.095 0.092 0.106

0.098 + 0.006

3

3

Bacon Hole Cave, Gower SS559868 Bed 4

A B E R 856A B A B E R 858C E A B E R 859C D A B E R 342C A A L 2842A B C D E F A A L 4627A B C A A L 4629A B A B E R 858A B D A B E R 859A B A A L 4626A B C A B E R 860A B C D E A B E R 857A B C A B E R 861A B C D E

Pv

Pv

Lira

Llrs

Lsax

Hunts Bay West, Gower SS562868 cemented beach gravel

A A L 3988A B C D E A B E R 3986A B C D E F

Pv

Lira

Hunts Bay East, Gower SS565867 cemented beach gravel

A B E R 452A B C

Lira

290

D.Q. Bowen et al. TABLE 1 (continued)

Locality

LAB ID

Species

A B E R 451A B A B E R 453A A B E R 499A A B E R 450A B C

Llrs Llrs Litt sp. NI

A B E R 862A B C D E A B E R 863A B C D E A B E R 864A B C

Pv

NI

D/L Ratio Peak Height

,~, ~r

Shells n

0.225 + 0.009

1 1 1 2

1 1 1 2

0.109 + 0.001

2

2

0.099 + 0.010

5

5

0.090 _+ 0.009

1 4

1 4

0.120 + 0.002

1

3

0.118 0.140 0.140 0.149 0.124 0.137 0.126 0.122 0.135 0.115 0.124 0.133 0.145 0,125 0.149 0.143 0.131

0.134 + 0.011

5

5

0.127 + 0.008

5

5

0.129 __- 0.005

2

4

0.135 _+ 0,010

2

2

0.141 _ 0.007

3

5

0.117 0.125 0.118 0.120 0.114 0.114 0.167 0.176 0.153 0.160 0.155 0.129 0.155 0.137

0.118 + 0.004

6

8

0.164 + 0.009

4

6

0.144 + 0.011

4

8

0.225 0.111 0.095 0.233 0.216 0.109 0.108

Runs n

Langland, Gower SS613871 cemented beach gravel

Lira Lira

Llrs

0.090 0,107 0.102 0.084 0.110 0.154 0.088 0.083 0.105 0.083 0.119 0.117 0.123

Shoalstone, Devon SX939568 cemented beach gravel

ABER 637A B A B E R 641A B C ABER 638A B C A B E R 642A B ABER 639* A B E R 644A A B E R 644B C A B E R 640* A B E R 643A B

Lira

Llrs

Lsax Litt sp. NI

Thatcher Rock, Devon SX9462 cemented beach gravel

A B E R 625A B C ABER 628A B A B E R 634* ABER 629A B C ABER 631" ABER 630* ABER 632A B ABER 635*

Pv

Lira

Llrs

291

Amino Acid Geochronology T A B L E 1 (continued)

Locality

LAB ID A B E R 627A B A B E R 633A B

Species

D/L Ratio Peak Height

$, tr

Shells n

Runs n

Lsax

0.137 0.124 0.147 0.136

0.136 + 0.008

4

4

Pv

0.184 0.183 0.175 0.164 0.173 0.181 0.164 0.161 0.186 0.182 0.150 0.138 0.110 0.110 0.101 0.152 0.200

0.181 + 0.004

3

3

0.173 + 0.007

3

3

0.163 + 0.002

2

2

0.173 __+.0.016

3

3

0.115 + 0.014

4

4

0.176 +_ 0.024

2

2

0.095 + 0.014

12

12

0.099 + 0.017

5

5

0.101 + 0.012

5

5

0.098 + 0.005

3

3

0.098 + 0.007

1

3

Hope's Nose, Torquay, Devon SX9463 basal conglomerate

SX947634

sandrock 1 SX947634 basal beach gravel

SX948634 sandrock 2 SX948634

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A B E R = Aberystwyth Amino Acid Geochronology Lab. A A L = I N S T A A R Amino Acid Geochronology Lab., Boulder, Colorado. Species are abbreviated (see text); * = one shell, 3 analyses, ** = one shell, 4 analyses.

292

D.Q. Bowen et al.

When data from an individual lithostratigraphic unit are given, the mean D/L ratio for a particular species, standard deviation, number of shells and number of analyses is shown: for example, Patella vulgata (or P. vulgata) 0.174 + 0.008 (9)(9). All the data is given on Table 1.

CLASSIFICATION The six species used in this study epimerize at approximately similar rates as is evident from D/L ratios measured on all six species or combinations of species from discrete stratigraphic units. Figures 2 to 7 show the data for individual species from different sites, Fig. 8 shows all the data for all sites. Classification rests largely on the proposed stratotype of Minchin Hole Cave, Gower, South Wales (Fig. 12) where two marine beds are separated by a periglacial unit (Sutcliffe and Bowen, 1973; Bowen, 1973c). It is further based on statistical analysis (below) and on an evaluation of site factors of geomorphology (past and present) and lithostratigraphy. The two marine units at Minchin Hole Cave show that sea-level has been at approximately the same height on at least two occasions (Bowen, 1973c). When D/L ratios from other raised beaches are compared with characteristic ones from the two marine units at Minchin Hole Cave it is evident that the re-working of fossil populations by younger sealevel events has occurred. While this criterion is based primarily on lithostratigraphically constrained data it is still difficult to differentiate true mixed populations from data that are merely 'noisy'. Re-working of beach sediment and its included fauna is possible in two principal ways given a background of multiple sea-level occupation of the present coastal zone. (1) By the incorporation of an older fauna, and associated sediment, into a younger marine unit. This is probably the most common mode of re-working and it is noticeable that most re-worked shells consist of the more robust hydrodynamic forms such as the Littorinids and Nucellids rather than the more fragile Patellids, although on occasion, probably si.te-dependent, these too are thought to have been re-worked. The number of instances of re-working in this way, however, is not great (Fig. 8). Outstanding examples of younger beaches adjacent to older ones are provided by unconformable juxtaposition of the two Pleistocene beaches at Minchin Hole Cave (Fig. 12) and numerous instances of contemporary beaches at the same elevation as Pleistocene ones. (2) Because younger beaches occur at the same elevation as older ones and sometimes lie against them (Fig. 12) or show young sediment and fauna overlying older units, it is possible for younger fauna to be emplaced on or within essentially older units. The detection of this must rest heavily on an interpretation of the contextual coastal geomorphology and lithostratigraphy.

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STATISTICAL ANALYSIS The primary purpose of the statistical analysis of the data is not to test the significance of specific prior hypotheses, but rather to determine if any structure is shown by the data. Initial inspection indicated that the standard deviation of AA,R at a site tended to increase with the mean, indicating that a log-normal error distribution is more appropriate than normal. This is compatible with the normal practice of quoting D/L accuracy as a coefficient of variation rather than as an absolute value. It was considered plausible that the different species could produce systematic biases in AAR's that could be partially or wholly eliminated by the use of an appropriate statistical model. Consequently a linear model of the form: log10 (AAR) = SITE + SPECIES + E R R O R was investigated using a linear regression program with indicator variables to represent the different sites and species. The main question of interest is whether the inclusion of the species terms (in addition to the site terms) would improve predictive accuracy. The appropriate criterion for judging this is that of Akaike (McCullagh and Nelder, 1983) which states that if the inclusion of an extra term can reduce the scaled deviance (residual sum of squares divided by variance) by at least two units then the predictive power of the model is improved. In fact the inclusion of the species terms improved deviance by more than four per extra term and achieved the tighter criterion of significance at the one per cent level using Snedecor's F test. The analysis revealed a number of statistical outliers to the model being fitted and these were removed iteratively. After an initial trial fit the data point with largest standardized residual, if greater than three, was removed and the model refitted. Five data points were eliminated in this way and the largest remaining residual was 3.0 (there is about a 0.6 probability of such a large residual in 331 data points), and there were 14 residuals greater than 2.0 (compared to an expected 16). Tests on the residuals indicated no departures from normality. The regression constants for the different species were then used to convert all the data to

a Patella vulgata equivalent. For example, the regression constant for Nucella lapillus is 0.045, i.e. the overall variance is minimised if the predicted value of log (AAR) is increased by 0.045 for all N. lapillus data points. To compensate for these higher values N. lapillus values of log (AAR) should be reduced by 0.045, or equivalently, A A R be factored by antilog (-0.045), 0.90. The conversion factors and their 95% confidence intervals are shown in Table 2. By using the conversion factors, data for all species can be converted into the common currency of a single species (Patella vulgata) rather than having to limit considerations only to data from a single species, so that the full data set of 336 analyses from 34 sites can be used rather than, at most, 135 analyses from 27 sites. This modified, Patella vulgata equivalent, data set was analysed to see what could be said about ranking the data in A A R (and hence assumed chronological) order, and the extent to which the data forms clusters. The site rankings are shown in Fig. 9. The points plotted are

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two sites just meet then their mean values are between 1.0 to 1.4 standard errors apart, and the probability of incorrect ordering is between 0.16 and 0.08. The final question asked of the data concerns the grouping of sites into bands of similar A A R value: in particular how many groups are present, and which sites belong to each group? Two models were considered as possible descriptions of the temporal formation of the sites. The first of these assumes that the beaches in any one group were created simultaneously, and each group was created at a different time. Thus all sites within a group should be of the same age, and the different groups will represent different ages. This pattern can be determined statistically by finding a partitioning of sites which maximises the between-groups variance and minimises the within-groups variance for a given number of groups. The method is essentially that of Edwards and Cavalli-Sforza (1965). By considering goodness-of-fit with different total numbers of groups the plausible alternatives can be reduced to a manageable subset of alternative numbers of groups. The actual analysis was conducted by means of a stepwise regression program. The dependent variable was taken to be the modified AAR's rather than their logged values since A A R is very nearly linear with time for the ranges present in these data. However, since the error distribution is assumed to be log-normal there is a degree of inconsistency in this choice. In order to determine if this is likely to be a critical problem all analyses were duplicated using logged values as dependent variable. The results of the two yielded results identical in all but minor details. The partitioning was determined by the use of indicator variables, one for each site, whose values are unity for all data points at that site or any site ranked below it, and zero otherwise. Thus if the tenth indicator variable, say, were the only one present in the regression equation other than a constant term, then all sites up to and including the tenth in rank order would be assigned a single mean value, and all those above the tenth would be given a different mean. The partition would in effect be between the sites ranked tenth and eleventh. If two indicator variables are included then the fitted parameters interact in such a way as to define three groups separated by the two partition points. The stepwise regression chooses those indicator variables which minimise the variance (or closely approximates the minimum) for each of various total numbers of indicators present. Thus a sequence of optimal, or near optimal, partitions into 2, 3, 4 etc. groups is produced. The goodness-of-fit of the regressions, considered as a function of the number of groups, can be used to indicate what numbers of groups represent plausible alternatives without over-fitting or underfitting. For any given number of groups the regression variables used indicate which sites should be placed in which groups. Considering numbers of groups first the question is whether an improvement in model fit is worth the inclusion of an extra parameter (i.e. group of sites) in the model. A rough guide is to consider an extra parameter if it produces a decrease in scaled deviance of between 2 and 6 (McCullagh and Nelder, 1983). For both A A R and log (AAR) this criterion limits the plausible choice to either six groups or seven. The former is shown in Fig. 10. The sevengroup alternative is formed by sub-partitioning group 3 so that either Shoalstone or Thatcher Rock comprise a single-site group. The boundaries between groups are not clearcut and can move approximately one rank up or down between groups 4 and 5, and one rank up between 1 and 2.

297

Amino Acid Geochronology

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(5) 0.104 (+0.002), (6) 0.096 (+0.003).

This partitioning has been based on the assumption that the ranking of sites is as indicated, but, as described previously, there is a degree of uncertainty in the ranking itself because of the uncertainty in estimating the site means. The groupings should therefore be considered as six distinct A A R levels that appear to be present in the data rather than as six groups of sites, and the decision as to which group a given site belongs will depend on its standard error as well as its mean. The six groups described above are those derived from the first model of site evolution. The second model assumes that sites are created at a uniform rate for certain periods of time interspersed with interregnums when no sites are created (high sea-levels and low sealevels respectively). The analysis of this model differs from that of the first only in the additional inclusion of a 'slope' term, proportional to rank, in the regression equation. Instead of the groups forming the fiat steps of model 1, the steps slope upwards to the right. The groups have A A R ' s lying on parallel lines with step-like displacements at the partition points. The results of the analysis indicate that the data can be plausibly put into three or four groups. The former case is shown in Fig. 10 and the additional group is formed by the

298

D.Q. Bowen et al.

single site of Shoalstone. Comparison of Figs 10 and 11 shows that the main difference in the implications of the two models is that groups 4 to 6 of model 1 are united into a single group in model 2. The sites of group 3 of model 1 remain something of an anomaly in both models. The assumption of model 2 that the slope of the line within each group be equal is rather artificial, but the aim of having the two models is to demonstrate the implications of contrasting assumptions, and further refinements are unlikely to be worthwhile. Further, fitting model 2 to logged data effectively gives different slopes (in terms of unlogged AAR's) to the different groups yet yields identical conclusions, so that the restriction is unlikely to be critical. As with model 1 the grouping is that of A A R values rather than of sites, The certainty with which a site can be assigned to a group will depend on the standard error of the estimate of site mean. The two statistical alternatives can now be evaluated with geological and other information. The twofold separation recognised on lithostratigraphic grounds is present on -+i ¢0

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Amino Acid Geochronology

299

both alternatives (Figs 10 and 11). Here it is relevant to observe that both the Baggy Point (N. Devon) and Saunton (N. Devon) basal conglomerate beds would also have fallen in group 1 of both alternatives had the single Littorina littorea at Baggy Point and single Nucella Iapillus at Saunton been omitted from the data processed. The basal conglomerate at both localities is overlain by a sandrock unit which gives different D/L ratios (Table 1, Fig. 13). Figure 13 shows the stratigraphic relationship at these localities together with the D/L ratios from each lithostratigraphic unit. If the two outlying D/L ratios are ignored two lithologically distinct units at both localities, a basal conglomerate overlain by a sandrock unit, show intrinsically characteristic D/L ratios. If the outlying young D/L ratios are not 'noisy' components of the data then alternative explanations are: (a) that the youngest D/L ratios [0.118 ___0.007 (1)(3) from L. linorea at Baggy Point; 0.107 + 0.003 (1)(3) from N. lapillus at Saunton] indicate the age of both the marine units, both of which contain reworked faunas of different ages; or (b) that the youngest D/L ratios relate to the same sealevel event as the sandrock units (with their slightly higher ratios) and that the underlying basal conglomerate units have been re-worked in the manner described above; or (c) that the youngest D/L ratios represent a sea-level event younger than sea-level events represented respectively by the basal conglomerate and sandrock and that the young shells have been introduced into the basal conglomerate unit. Explanation (c) appears to be the most likely because the local geomorphology of the raised shore-platform and the manner of its dissection shows that both basal conglomerate units and sandrock units could have been cliffed by a younger sea-level event. Here the contemporary analogue of Minchin Hole Cave, where two older marine units, could conceivably be in contact with today's sealevel is relevant. Employing this analogue, and because both the basal conglomerate and sandrock units at both sites show D/L ratios characteristic of established groups, the basal units are ascribed to the oldest group of the statistical analysis (Figs 10 and 11). At Hope's Nose the relationships between lithostratigraphy and D/L stratigraphy are clearer: basal gravel [P. vulgata 0.173 _+_ 0.0016 (3)(3)] is overlain by a sandrock unit with a mixed population [P. vulgata 0.176 + 0.024 (2)(2) and 0.115 + 0.014 (4)(4)]. The first statistical alternative (Fig. 10) which assumes that the bands of D/L ratios formed at the same time may be refined by detailed lithostratigraphic and biostratigraphic information from Minchin Hole and Bacon Hole Caves, Gower (Henry, 1984). Minchin Hole Bed 4b cannot represent a sea-level event separate from Bed 4a because it is part of Sutcliffe's (1981) terrestrial Earthy Breccia Series. Furthermore biostratigraphic correlation between Minchin Hole Beds 4a and 4b with Bacon Hole Beds 3 and 4 (Henry, 1984) shows that groups 4 and 5 of statistical model 1 (Fig. 10) are the same age. The alternative model (Fig. 11), which assumes that within each group of D/L ratios different sites have different ages (i.e. created at different times within a specific sea-level event) actually shows the refinement based on such considerations in that the earlier groups (alternative 1) 4, 5 and 6 are grouped together as group 3. The most likely break point within the data outside group 1 of both versions, which is constrained by the stratigraphic situation at Minchin Hole Cave, is between groups 2 and 3 of the second alternative (Fig. 11). Here the sites of group 2 do show somewhat higher D/L ratios than sites in group 3. Shoalstone falls close to the partition between the two groups but on balance seems marginally higher (Table 1, Fig. 11) than group 4. Outstanding questions remain. Do some of the anomalously low D/L ratios from the Portland (East) raised beach demonstrate a sea-level younger than group 3 of the second

300

D.Q. Bowenet al.

model (Fig. 11)? Here it is noted that the first model (Fig. 10) does suggest such a possibility. But the fundamental assumption of that model, namely that each group formed at the same, virtually instamaneous, time, requires testing by further sampling of 'contemporary' faunas on present day beaches. If groups 2 and 3 of the second model (Fig. 11) formed during and throughout the same high sea-level then epimerization amounting to some 0.056 occurred during that event. Existing data on epimerization throughout the Holocene makes this unlikely.

THE (D/L)STAGES Groups of D/L ratios have been classified and described as 'Groups' with a numbering system backwards in time (e.g. Andrews et al., 1979), or 'Events' using the same system of numbering (e.g. Boulton et al., 1982), or 'Aminozones' (Nelson, 1981; Miller, 1982). The groups of D/L ratios presented here are classified into (D/L)Stages because the concept of the zone seems insufficiently important in the hierarchy of stratigraphic terminology to classify events which appear to be more or less coeval with and equal in rank to Stages defined by other criteria: e.g. pollen biostratigraphy in northwest Europe (Mitchell et al., 1973), mammalian biostratigraphy in Britain (e.g. Sutcliffe, in press), or on a wider front, Oxygen Isotope Stratigraphy (e.g. Shackleton and Opdyke, 1973; Imbrie et al., 1984). Three (D/L)Stages are proposed. The Minchin Hole (D/L)Stage and the Pennard (D/L)Stage are clearly separated not only on D/L criteria but also by lithostratigraphy and mammalian biostratigraphy (see previous discussion). There are grounds, however, for recognising an intermediate (D/L)Stage with different D/L ratios. This seems likely on the basis of the cluster analysis as well as the incidence of individual localities which give D/L ratios exclusive to this proposed Stage: e.g. Shoalstone, Devon, or Overton Cliff, Gower. It is possible, however, that future work or new discoveries will show that the unnamed (D/L)Stage belongs to either the Minchin Hole or Pennard (D/L)Stages.

THE MINCHIN HOLE (D/L)STAGE Stratotype

The stratotype for this (D/L)Stage is at the coastal cave of Minchin Hole, Pennard, Gower (SS562868 a UK national grid reference). The importance of this cave, which is also the stratotype for the Pennard (D/L)Stage, lies in its stratigraphical relationships first clearly exposed by Sutcliffe and Bowen (1973), the availability of D/L ratios from several lithostratigraphic units, some uranium-series age calibration from stalagmite of the D/L ratios (Sutcliffe and Currant, 1984), an availability of a mammalian (Sutcliffe, 1981; Sutcliffe and Currant, 1984) and molluscan biostratigraphy (Evans, unpubl, data), as well as the historical importance of the site (Falconer, 1868; George, '1932; Bowen, 1966). Figure 12 is a longitudinal section normal to the shore line and shows the stratigraphical relationships in the cave. The unit which is stratotype for the Minchin Hole (D/L)Stage is the Inner Sand Beach, which is intertidal in origin (Sutcliffe and Bowen, 1973; Bowen, 1977a; Sutcliffe, 1981; Henry, 1984; Sutcliffe and Currant, 1984). Its base rests on the marine abraded Dinantian Limestone floor of the cave and it is directly overlain by a red

301

Amino Acid Geochronology TABLE 3. D/L Stages and characteristic D/L ratios for the different species and the standardised (to Patella vulgata) D/L ratios D/L Stage

Stratotype

Mean D/L ratios

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Minchin Hole Cave

Pv(Standard) 0.105 + 0.016 (196)(204) 2 Pv 0.105 + 0.015 (79)(81) Lira 0.107 + 0.016 (52)(54) Llrs 0.110 + 0.013 (37)(39) Lsax 0.102 + 0.007 (14)(14) Lift. sp. 0.108 _ 0.014 (4)(4) NI 0.115 + 0.016 (8)(10)

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undesignated

Pv(Standard) 0.135 + 0.014 (42)(53) Pv 0.133 + 0.015 (13)(14) Lira 0.147 + 0.018 (9)(11) Llrs 0.135 + 0.013 (9)(13) Lsax 0.134 _+ 0.008 (6)(8) Litt. sp. 0.135 ___ 0.010 ( 2 ) ( 2 ) N/0.141 _+ 0.007 (3)(5)

Minchin Hole

Minchine Hole Cave

Pv(Standard) 0.175 + 0.014 (42)(52) ] Pv 0.178 ___ 0.014 (27)(31) Llra 0.176 + 0.017 (3)(6) Llrs 0.177 + 0.015 (4)(4) Litt. sp. 0.178 + 0.006 (4)(4) Nl 0.183 + 0.016 (6)(9)

*The Stage name 'Pennard' has been used previously in the sense that it referred to the Outer Gravel Beach at Minchin Hole Cave and all the other raised beaches of Gower which were ascribed to the 'last' (Oxygen Isotope Sub-stage 5e) interglacial (Bowen, 1977b). That usage is now superseded by this new D/L definition. i Two N. lapillus (Horton upper beach) are removed from this calculation (see Fig. 8). 2Two N. lapillus (Overton Mere) are added, (see Fig. 8).

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Inner talus

;~'~~'," ','~,' ' ' '~ 'i 'I'llllllll[ll ' ' ' ' L I I III I I

FIG. 12. Diagrammatic and reconstructed longitudinal section (not to scale) of the beds at Minchin Hole Cave, Gower (after Sutcliffe and Bowen, 1973; Sutcliffe, 1981; Sutcliffe and Currant, 1984; Henry, 1984). Bed 1: Inner Sand Beach; Bed 2: Lower Red Cave Earth; Bed 3: Outer Gravel Beach; Bed 4a: T.N. George's (1932) Neritoides Beach; Bed 4b/4c: Sutcliffe's (1981) Earthy Breccia Series. Black = stalagmite: The outer talus is mostly washed away and the inner talus is unexcavated. 'HSL" is the highest recorded sea-level witnessed (1972-1984). Note that present day marine shingle lies on a "platform" and against a "cliff" composed of the Outer Gravel Beach, just as the Outer Gravel Beach similarly overlies the Inner Sand Beach: that is, three different aged sea-levels are, more or less at the same elavation.

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D.Q. Bowen et al.

cave earth (Fig. 12). D/L ratios measured by the method which involved transferring a solution between vials, which is no longer used, gave a mean value of 0.14 (Bowen, 1981) and 0.22 + 0.02 (4)(5) (Davies, 1983) for Patella vulgata, 0.22 for Littorina littorea (Davies, 1984) and 0.24 +_. 0.01 (2)(2) for Nucella lapillus (Davies, 1983). These are now superseded by the following definitive D/L ratios for the stratotype: Patella vulgata 0.174 __. 0.008 (9)(9); Littorina littorea 0.176 + 0.017 (3)(6). The Inner Sand Beach is overlain by a red cave earth unit which consists of limestone scree, frost riven from the cave roof and walls, with a matrix of inwashed red silty clay which is characteristic of colluvial beds in coastal Gower generally (Bowen, 1970). The red cave earth has yielded a large form of Microtus oeconomus which is indicative of severe climatic conditions (Sutcliffe, 1981; Sutcliffe and Currant, 1984).

Fistral Beach North, Newquay, Cornwall (SW799624) (James unpubl, data) Basal sandrock. Three shells of Patella vulgata give a D/L ratio of 0.186 ___0.008 (3)(3).

Godrevy, St. Ives Bay, Cornwall (SW579433) (James unpubl, data) Basil conglomerate. Only one shell of Patella vulgata was analysed from this site: three measurements of the D/L ratio gave a mean value of 0.178 + 0.021 (1)(3). (Postscript: this D/L ratio has subsequently been replicated on further shells of Patella vulgata by the London A A Laboratory).

Hope's Nose, Torquay, Devon (SX948634 and 947634) (Ussher, 1904; Orme, 1960) Cemented conglomerate and sandrock overlying a raised shore-platform cut across Devonian Limestone at '24 ft (7.3 m) above H.W.M.S.T.' (Orme, 1960). Samples were collected from two outcrops: (a) at the northern end of the site, where the outcrop runs normal to the shore along a coastal indentation in bedrock (SX948634); and (b) at the southern end (SX947634). (a) The northern exposure has been studied recently by Mottershead and Gilbertson (unpubl. data) and independently by James (unpubl. data). Samples were collected by James from a basal beach gravel: P. vulgata D/L ratio 0.173 + 0.016 (3)(3). From an overlying sandrock unit 'Sandrock 2' Mottershead collected shells of Patella vulgata which fall into two D/L populations: P. vulgata 0.176 + 0.024 (2)(2) and 0.115 + 0.014 (4)(4). This is consistent with the investigations of Mottershead and Gilbertson (unpubl. data) of the sequence of slope and other deposits overlying the marine beds at Hope's Nose. In particular there is evidence for more than one sea-level event and more than one episode of slope deposit formation. The integrity of the basal gravel, however, seems intact, and the sequence shows beds of one sea-level event [Minchin Hole (D/L)Stage] overlain by those of a younger one [Pennard (D/L)Stage]. (b) At the southern exposure of the Hope's Nose raised beach unit James collected shells from a basal conglomerate and overlying sandrock (Sandrock 1). Basal Conglomerate (SX947634): P. vulgata 0.181 __. 0.004 (3)(3), Littorina sp. 0.173 + 0.007 (3)(3). Sandrock 1 (SX947634): P. vulgata 0.163 + 0.002 (2)(2).

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303

Butterslade, Gower, West Glarnorgan (SX422869) (George, 1932; Jenkings et al., 1984) Cemented beach gravels with Patella vulgata D/L 0.168 + 0.012 (1)(3), Littorina species D/L 0.168 (1)(1) and Nucella lapillus 0.190 ___0.001 (1)(2) give characteristic ratios for the Minchin Hole (D/L)Stage. These have been previously reported (Jenkins et al., 1984).

Baggy Point-Pencil Rock, Devon (SS425403) This is a basal conglomerate lying on a raised platform. It is overlain by sandrock (James,

unpubl, data) (Fig. 12). The D/L ratios from this site have already been discussed (see Classification) and an interpretation for the anomalies present has been proposed. The D/L ratios are: Patella vulgata 0.178 + 0.005 (2)(2), Nucella lapillus 0.196 + 0.006 (1)(3), and Linorina linorea O.118 + 0.007 (1)(3). As discussed earlier, this is an example of a younger sea-level event [Pennard (D/L)Stage], introducing fauna into the sediments of an earlier one (Minchin Hole (D/L)Stage).

Saunton-Chesil Cliff, Devon (SS435381) This is a basal conglomerate lying on a platform. It is overlain by sandrock (James,

unpubl, data) (Fig. 13). Similarly to Baggy Point-Pencil Rock anomalous D/L ratios are shown to be the product of modification by a younger sea-level event. The D/L ratios are: Patella vulgata 0.201 + 0.004 (3)(3) and NucelIa lapiUus 0.107 + 0.003 (1)(3) (see

Classification). Horton Upper Beach, Gower, West Glamorgan (SS484854) (Wirtz, 1953; Bowen, 1970, 1971, 1977a) This anomalous raised beach, consisting of clasts and shells in a red sand-clay matrix, lies a few metres above and laterally away from the Horton Lower B~ach (above) ascribed to the Pennard (D/L)Stage. Nucella lapillus 0.177 + 0.015 (4)(4) gives characteristic D/L ratios for the Minchin Hole (D/L)Stage. But Littorina littorea 0.130 + 0.005 (1)(3) and Littorina littoralis 0.136 (1)(1) give D/L ratios of a younger sea-level event. The Horton Upper Beach has, therefore, been subject to re-working not only by the sea-level event represented by the D/L ratios of 0.130 + 0.005 but also almost certainly by that of the Pennard (D/L)Stage sea-level event whose deposits lie adjacent to it (Horton Lower Beach). Thus this site shows evidence of multiple sea-level occupation on three occasions.

THE UNNAMED

(D/L)STAGE

Overton Cliff, Gower, West Glamorgan (SS455848) (George, 1932) Cemented beach gravel with Patella vulgata D/L ratios of: 0.146 ___0.009 (3)(3).

Baggy Point-Pencil Rock, Devon (SS425403) (Fig. 13) A sandrock unit overlying basal conglomerate (James, unpubl, data). D/L ratios for Patella vulgata are 0.145 + 0.006 (3)(3). This site has been discussed in the section on classification.

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D.O. Bowen et al.

1

-Sandrock -Pv 0.145 -Lara 0.118 Pv 0.178 NI 0.196

FIG. 13. Diagrammatic sections of the exposures at Baggy Point, Pencil Rock (left) and Saunton, Chesil Cliff (right). Heights in m O.D. The open circles indicate a basal conglomerate lying on a raised platform (James, unpubl, data).

Saunton-Chesil Cliff, Devon (SS435381) (Fig. 13) A sandrock unit overlying the basal conglomerate previously noted (James, unpubl. data). D/L ratios for Patella vulgata are: 0.144 + 0.002 (1)(2). This site has been discussed in the section on classification.

Thatcher Rock, Devon (SX944628) (Hunt, 1888; Shannon, 1927; Orme, 1960) This lies on a raised platform at 33 ft (10 m) OD (Orme, 1960). The D/L ratios show a spread of values which could indicate a certain amount of re-working: Littorina saxatilis 0.136 + 0.008 (4)(4), Littorina littoralis O. 144 + 0.011 (4)(8), Littorina littorea O. 164 ___0.009 (4)(6), but Patella vulgata gives D/L ratios of 0.118 ___0.004 (6)(8).

Shoalstone, Devon (SX939568) (Ussher and LLoyd, 1933; Orme, 1960) This consists of a cemented beach gravel on a raised platform. The beach unit is overlain by head. Other than for Patella vulgata, all the species used in this study have been collected and give characteristic D/L ratios for this unnamed high sea-level event. This site is the best available on stratotypic grounds but it is not used as such. The D/L ratios are: Littorina littorea 0.134 +_ 0.001 (5)(5), Littorina littoralis 0.127 _+ 0.008 (5)(5), Littorina saxatilis 0.129 +_ 0.005 (2)(2), Littorina species 0.135 _+ 0.010 (2)(2) and Nucella lapillus 0.141 _+ 0.007 (3)(5).

Amino Acid Geochronology THE PENNARD

305

(D/L)STAGE

Stratotype The stratotype is at the coastal cave of Minchin Hole, Pennard, Gower, West Glamorgan (SS562868). Reasons for the importance of this stratotype are given under the Minchin Hole (D/L)Stage stratotype description (Fig. 12). The stratotype consists of Beds 3, 4a, 4b and 4c of Henry (1984). The base of Bed 3 lies unconformably across the top of Bed 2 and the bedrock geology of Carboniferous Limestone. Bed 3 corresponds with the outer beach of Sutcliffe and Bowen (1973) and the Patella beach of George (1932) (note, however, that the Patella beach as defined by George in Gower is not. everywhere of the same age, as the data in this paper show). Bed 2, on which Bed 3 lies, is the Red Cave Earth of Sutcliffe and Bowen (1973) which contains a cold fauna (Sutcliffe, 1981). Bed 4a is a bright brown sandy clay with some limestone fragments containing shells and mammalian bones and presumably corresponds to the Neritoides Beach of George (1932) according to Sutcliffe (1981). Minchin Hole Bed 4b corresponds to the Earthy Breccia Series of Sutcliffe (1981). Bed 4b does not occur on the east side of the cave but is replaced by Minchin Hole Bed 4c. The top of the stratotype is placed at the top of Beds 4b and 4c below Bed 5 (Henry, 1984). Beds 3, 4a, 4b and 4c all lie in conformable stratigraphic sequence. On the eastern side of the cave a block of stalagmite, not in situ, rests on the surface of both Minchin Hole Bed 3 (Outer Gravel Beach) and Minchin Hole Bed 4a and is embedded in the base of Bed 4c (Earthy Breccia Series of Sutcliffe, 1981).

Minchin Hole Bed 3 (Outer Gravel Beach) This gives the following ratios: Patella vulgata 0.100 + 0.013 (7)(9), Littorina littorea 0.117 + 0.018 (4)(4), but with a possible re-worked population: Nucella lapillus 0.136 + 0.008 (2)(2) and one Littorina littorea 0.164 (1)(1) which may represent a still older population (Table 1, Fig. 8).

Minchin Hole Bed 4a ('Neritoides Beach' of T.N. George, 1932) This gives the following D/L ratios: Littorina littoralis 0.120 + 0.004 (5)(5), Littorina saxatilis 0.105 _+ 0.006 (5)(5). Minchin Hole Bed 4b gives the following D/L ratios: Patella vulgata 0.104 + 0.004 (5)(5). The mean value of Patella vulgata from both Beds 3 and 4b is 0.101 + 0.011 (12)(14). The mean values for each species from all the stratotype beds at Minchin Hole Cave are:

Patella vulgata Littorina littorea Littorina littoralis Littorina saxatalis Nucella lapillus

0.101 0.117 0.120 0.105 0.136

+ + + + +

0.011 0.018 0.004 0.006 0.008

(12)(14) (4)(4) (5)(5) (5)(5) (2)(2)

Hope's Nose, Torquay, Devon (SX948634) (Mottershead and Gilbertson unpubl, data; James unpubl, data) Sandrock overlying basal beach gravel (see Minchin Hole D/L Stage) D/L ratios are: Patella vulgata O.176 _+ 0.024 (2)(2) (population 1); and 0.115 _+ 0.014 (4)(4) (population 2)

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D.Q. Bowenet al.

Re-working of marine faunas is evident and has previously been discussed [see Minchin Hole (D/L)Stage]. Trebetherick Point, Cornwall (SW926780) Basal beach conglomerate overlying a raised platform (Arkell, 1943). One D/L ratio from a specimen of Littorina saxatilis gives 0.113 (1)(1). Bacon Hole Cave, Pennard, West Glamorgan (SS559868) D/L ratios are presented from units described by Henry (1984) and Stringer et al. (1986) (Table 1). These lie mostly on a raised shore platform at 12 m O.D. and extend into the cave proper. Uranium-series dates from stalagmite within the sequence are considered later (Geochronology). Bacon Hole Bed 3 (Henry, 1984), which is the Sandy Breccio-Conglornerate of Stringer et al. (1986). Patella vulgata D/L ratios of 0.112 + 0.011 (2)(2)were obtained. Bed 4 (Henry, 1984) which corresponds to the Sandy Cave Earth of Stringer et al. (1986) and appears to correspond with the Yellow Shelly Sand of Benson (1852) contains two populations (Table 1, Fig. 8). The younger, of Patella vulgata, gives characteristic D/L ratios of 0.113 +__0.017 (18)(18), eleven of these were analysedat Boulder (AAL): 0.106 + 0.012 (11)(11) and seven at Aberystwyth (ABER): 0.123 + 0.017 (7)(7); the older gives D/L ratios of 0.167 ___ 0.016 (5)(5). A further 8 shells of Littorina littoralis gave D/L ratios of 0.114 ___0.012 (8)(8), while 5 shells of Littorina saxatilis give D/L ratios of 0.099 + 0.006 (5)(5), 3 shells of Littorina linorea give D/L ratios of 0.122 + 0.002 (3)(3). Sandra Hughes (1984, and in press) measured D/L ratios of 0.13 + 0.02 (5)(5) from the land snail Cepaea nemoralis from the Sandy Cave Earth of Stringer et al. (1986) (Bed 4 of Henry, 1984). These correspond to similar ones she measured on the same species from Tattershall, Lincolnshire, reported by Holyoak and Preece (1985) Tattershall Castle 0.10 _.+ 0.01 (3)(3) and Tattershall Thorpe 0.13 + 0.02 (5)(5). Uranium-series dates on Cepaea nemoralis are available at Tattershall as well as a TL date from calcareous silt (see Geochronology). Correlation between Bed 3, 4a, 4b and 4c of Minchin Hole on the one hand with Beds 3 and 4 of Bacon Hole (using the nomenclature of Henry, 1984) is possible (Table 4). This amino acid correlation (aminostratigraphy) is supported by general lithostratigraphy and in particular by correlation of vertebrate faunas between Minchin Hole Beds 4a and 4b with Bacon Hole Beds 3 and 4 (Henry, 1984). The following are common to both Minchin Hole Beds 4a and 4b and also Bacon Hole Beds 3 and 4: Dama dama (Linnaeus), Panthera leo (Linnaeus), Microtus agrestis (Linnaeus), Clethrionomys glareolus (Schreber), Arvicola sp. Apodemus sylvaticus (Linnaeus), Sorex cf. araneus, Sorex cf. minutus, and Rana sp./Bufo sp. Further correlation is possible by 23°Th dates available on broken stalagmite fragments from within Bed 4 of Henry (1984) (Geochronology). Stringer et al. (1986) argue that the stalagmite formed on the Sandy Cave Earth prior to deposition of the Shelly Sand (Stringer et al., 1986, p. 61). Overton West, Gower, West Glamorgan, (SS460848) Cemented beach gravel (George, 1932) with D/L ratios Patella vulgata of 0.099 + 0.001 (5)(5) and Littorina littoralis 0.097 + 0.005 (2)(2) giving characteristic values. But both

Amino Acid Geochronology

307

Linorina littorea and Nucella lapillus give higher D/L ratios showing re-working of faunas (Table 1). Horton Lower Beach, Gower, West Glamorgan (SS484854) Cemented beach gravel lying in a fault-guided gully (Davies, 1984) somewhat below and laterally beyond the 10 m platform which carries the Horton Upper Beach (Bowen, 1971). Three shells only identified as Littorina species give D/L ratios of 0.113 + 0.014 (3)(3). The marine deposit contains head in its upper layers but lies seawards of the drift composed of two lithofacies of head (Wirtz, 1953; Bowen, 1971) overlain by loess (Case, 1977, 1983). The foraminifera content of this beach is quite different from the upper beach (Henry, 1984).

Hunts Bay West, Gower, West Glamorgan (SS562868) Cemented beach gravel overlying raised platform (George, 1932; Bowen, 1970, 1971, 1977a; Henry, 1984; Bowen and Henry, 1984). D/L ratios are: Patella vulgata 0.111 + 0.006 (5)(5), and Littorina littorea 0.107 + 0.011 (6)(6). The marine unit is overlain by colluvium and head which contains some recycled glacial material (Bowen, 1970, 1971).

Worms Head (lnner Head), Gower, West Glamorgan (SS392875) Cemented beach gravel on a shore platform (George, 1932). D/L ratios on Patella vulgata of 0.109 + 0.008 (3)(3), Littorina littorea O.123 + 0.005 (2)(2), and Littorina littoralis 0.095 + 0.002 (3)(3) are characteristic. The marine unit is overlain by limestone head and glacial beds (George, 1932, 1933).

Broadhaven, South Pembrokeshire, Dyfed (SS978942) Cemented beach gravel lying on a raised platform and in gullies below it (Dixon, 1921; John, 1970). Patella vulgata 0.113 + 0.012 (5)(5), Littorina littorea 0.101 _+ 0.011 (3)(5) and Littorina linoralis O.102 + 0.012 (5)(5) show characteristic D/L ratios. The marine beds are overlain by sand rock and undivided limestone head (Bowen, 1973b, 1974).

Broughton Bay, Gower, West Glamorgan (SS418930) Cemented beach gravel lying on a low raised platform (George, 1932). D/L ratios from Littorina littorea of 0.105 + 0.011 (5)(5) and Littorina littoralis 0.108 + 0.005 (5)(5) are characteristic of the Pennard (D/L)Stage. The marine bed is overlain by limestone head and glacial deposits (Campbell et al., 1982).

Worms Head (Middle Head), Gower, West Glamorgan (SS389876) Cemented beach gravel on raised shore platform (George, 1932) contains Patella vulgata with D/L ratios of 0.095 + 0.006 (3)(3) and Littorina littorea with D/L ratios of 0.115 + 0.012 (3)(3) which includes one specimen with a D/L ratio of 0.131. The marine unit is overlain by limestone head.

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D.Q. Bowenet al.

Rhosili (South), Gower, West Glamorgan (SS404873) Cemented beach gravel (George, 1932) with shells giving characteristic ratios for the Pennard (D/L)Stage: Patella vulgata, 0.089 + 0.005 (3)(3), Littorina littorea 0.105 + 0.015 (5)(5), with some indication of re-worked Littorina littoralis (Table 1). The marine bed is overlain by limestone head partly formed locally of frost-rived Dinantian fossils (Bowen, 1970) and (Case, 1983).

Langland Bay, Gower, West Glamorgan (SS613871) Cemented beach gravel on a raised shore platform (George, 1932; Bowen, 1970, 1971, 1977a). Except for one shell of Littorina littorea (D/L ratio 0.154), all the other D/L ratios are characteristic of the Pennard (D/L)Stage: Patella vulgata 0.099 + 0.010 (5)(5), Littorina littorea 0.090 + 0.009 (4)(4) and Littorina littoralis 0.120 _+ 0.002 (1)(3). The raised beach unit is overlain by limestonehead and by glacial deposits (George, 1933; Bowen, 1970, 1971, 1977a).

Overton Mere, Gower, West Glamorgan (SS463846) Cemented beach gravel (George, 1932) contains Littorina littorea with characteristic D/L ratios of 0.093 + 0.004 (5)(5), but, unusually, with a possible mixed population of Patella vulgata (Fig. 8, Table 1).

Hunts Bay East, Gower, West Glamorgan (SS562867) Cemented beach gravel on platform co-extensive with the present inter-tidal one (George, 1932; Bowen, 1970, 1971, 1977a; Henry, 1984; Bowen and Henry, 1984). D/L ratios are: Littorina littorea 0.098 + 0.006 (3)(3), one shell of Littorina littoralis 0.111, one shell identified as Littorina species, 0.095, and 2 shells of Nucella lapillus O.109 + 0.001 (2)(2). But a mixed population is evident from D/L ratios of 0.225 _+ 0.009 (2)(2) on two shells of Nucella lapillus and one shell of Littorina littoralis 0.225. These show derivation from an event older than the Minchin Hole (D/L)Stage and represent the oldest marine fauna identified in the data presented in this study (Fig. 8). The marine bed is overlain by limestone head which consists of two lithofacies which together partially complete a cyclothem ascribed to one cold stage (Bowen, 1970, 1971, 1977a), although Mitchell (1972) would equate lithostratigraphy (i.e. the two separate lithofacies) with a notional chronostratigraphy and thus recognise two cold Stages ('Wolstonian' and 'Devensian') subsequent to the raised beach event.

Portland (EasO, Portland Bill, Isle of Portland, Dorset (SY678684) Cemented beach gravel on a raised platform (Prestwich, 1875; Arkell, 1943; BadenPoweU, 1930; Davies and Kenn, 1985). D/L ratios are: Patella vulgata 0.095 + 0.014 (12)(12), Littorina littorea 0.099 + 0.017 (5)(5), Littorina littoralis 0.101 + 0.012 (5)(5), Littorina saxatilis 0.098 + 0.005 (3)(3) and Nucella lapillus 0.098 + 0.007 (1)(3). Of some interest are the large numbers of shells with anomalously low D/L ratios (Table 1).

Amino Acid Geochronology

309

GEOCHRONOLOGY Using lithostratigraphic succession, notably at Minchin Hole Cave, Gower, .tO interpret a statistical analysis of all the available D/L data, it has been suggested that 3 (D/L)Stages can be proposed (Table 3). , This may be explored further using 234Uranium/23°Thorium dates on stalagmite fragments found interbedded in the lithostratigraphic sequences in Bacon Hole Cave and Minchin Hole Cave, Gower (Stringer et al., 1986; Sutcliffe and Currant, 1984). Correlation between the units at Bacon Hole and Minchin Hole is possible by biostratigraphy and by D/L aminostratigraphy (Table 4). The large uncertainties in the Useries ages are because of low uranium concentrations in all samples (Stringer et al., 1986). Based on such error terms it must be assumed that there is no statistical difference in the age of dates given for'Bacon Hole or Minchin Hole Caves. The mean age of 122 _+ 9 from Bacon Hole could, therefore, show that these units correspond in time with Oxygen Isotope Sub-stage 5e of the marine time-scale (Shackleton and Opdyke, 1973; Imbrie et aL, 1984). Using this as a base-line, alternative age models for the D/L data can be explored (Table 6). Model la: This assumes that both the Pennard and the unnamed (D/L)Stages are time equivalent to Oxygen Isotope Sub-stage 5e (122 ka BP). If the two D/L groupings of 0.105 (late) and 0.135 (early) are part of the same sea-level event, this dispenses with the need to recognise some mixed populations. The Minchin Hole (D/L)Stage then probably corresponds to all, or part, of Oxygen Isotope Stage 7 (186 to 245 ka BP, Imbrie et al., 1984). Model lb: Because the amount of epimerization shown by Littorina (Miller, pers. commun.) and Macoma (unpubl. data standardised to P. vulgata) during the Late-glacial and Holocene corresponds to some 0.030 it seems doubtful that groupings of D/L ratios of 0.105 and 0.135 belong to the same sea-level event. It is also doubtful if sea-level was at and above its present height for 10,000 years or so during Sub-stage 5e. Studies of the Holocene marine transgression have shown that sea-level has been at, or about, its present position for about 5,000 years (Tooley, 1974); while work on the Ipswichian (5e?) marine transgression by West (1972) showed that the high sea-level event occurred during Ipswichian Pollen Zone 'f' (of the old nomenclature) or PZ Ip IIb (of the new classification). In neither case is it likely that sea-level was at or above its present height for long enough to result in shells showing a variation in epimerization rate amounting to some 0.030. It is probable, therefore, that more than one sea-level event is represented by the respective mean D/L ratios of 0.135 and 0.105. One possibility is that two, if not three high sea-levels of Stage 5 are all present: that is, Sub-stage 5e at 122 ka BP, Sub-stage 5c at ca. 100 ka BP and Sub-stage 5a at ca. 80 ka BP. Thus the low D/L ratios from Portland (East), together with some from elsewhere, e.g. Hunts Bay and Rhosili, might represent the youngest sea-level event shown by these data. On this evaluation the Minchin Hole (D/L)Stage remains ascribed to Oxygen Isotope Stage 7, or part of it. At Bacon Hole Cave, Gower, where, potentially, all or most of the time corresponding to Oxygen Isotope Stage 5, is represented, some constraints on this model occur. The sea-level event shown by Bed 4 of Henry (1984) or the Sandy Breccio-conglomerate and Shelly Sand of Stringer et al. (1986), with P. vulgata D/L ratios of 0.113 _+ 0.017 (18)(18) contains an

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D.Q. Bowenet al.

interglacial mammalian fauna consistent with local temperate woodland (Stringer et al., 1986). This is overlain by Bed 5 (Henry, 1984), the Grey Clays, Silts and Sands (Stringer et al., 1986), which indicate a falling sea-level and temperature: mammoth is recorded for the first time in the sequence (Stringer et al., 1986). In turn this is overlain by Beds 6 and 7 (Henry, 1984), the Upper Sands of Stringer et al. (1986), which are aeolian in origin. These are succeeded by Bed 8 (Henry, 1984), the Upper Cave Earth, which contains a 'comparatively restricted mammalian fauna of "interglacial" character' (Stringer et al., 1986). Overlying this is a stalagmite floor (Table 4) with a U-series date 81 _+ 18 ka BP (Stringer et al., 1986). If the Sandy Cave Earth and Shelly Sand (Bed 4) correspond to Oxygen Isotope Sub-stage 5e, as is indicated by the U-series determinations, then the Upper Cave Earth (Bed 8) could be time equivalent to either or both Sub-stages 5c or 5a, and the overlying stalagmite time-equivalent to Sub-stage 5a. This would mean that no sealevel events time equivalent to Sub-stages 5c and 5a are recorded at this site. Note, however, that the cave opens on to a raised platform at an elevation of 12 m O.D. so that it may have been above such sea-levels during Sub-stages 5c and 5a. D/L ratios on Cepaea nemoralis from the Sandy Cav.e Earth (Stringer et al., 1986) (Bed 4 of Henry, 1984) are similar to those measured on the same species from Tattershall, Lincolnshire (Hughes, 1984 and unpubl, data, and in Holyoak and Preece, 1985) (see previously). At Tattershall Uranium-series determinations on shells of Cepaea nemoralis na+18 ka BP, and ~,,~-20 ln1+25 ka BP. A give ages of 76+I ° _ ka BP, 94+_1° ka BP, ~-,-16 Thermoluminescence date on calcareous silt gave an age of 114 +_ 16 ka BP. The U'sefies age of 121+_~4 ka BP on the travertine cement of the raised beach at Belle Houge Cave, Jersey, Channel Islands, is not included in this study for the following re aso as: (1) originally the Patella vulgata D/L ratios were measured as 0.123 + 0.024 (Keen et al., 1981), but were subsequently re-measured using the preparation method now current. The new D/L ratios were 0.135 + 0.016 which, surprisingly, were higher than the original ones (Andrews et al., 1985) (generally the current preparation gives lower D/L ratios). Further collection by James and subsequent analysis (in the London University Laboratory) showed low amino acid concentrations in shells of Patella vulgata; (2) U-series ages on the Portland raised beach by Rowe and Atkinson (1985) have given Holocene ages. (3) Jersey lies to the south of the study region at 49 ° 13' north (compared with 51036 ' at Broadhaven or 50015 ' north at Godrevy; moreover mean annual temperature at Jersey is I°C higher than it is at localities on the southern margin (e.g. Portland) as well as the northern margin of the study area (e.g. Swansea).

Model 2: Because of the lithostratigraphic and biostratigraphic separation by a cold climate bed at Minchin Hole Cave, Gower, of the Minchin Hole and Pennard (D/L)Stages it could be assumed that the possible sea-level event represented by D/L ratios of 0.135 is closer in time to the Pennard (D/L)Stage than it is to the Minchin Hole (D/L)Stage. If, on the other hand, it is closer to the Minchin Hole Stage, then it is possible that D/L ratios of 0.135 can be ascribed to the later part of Oxygen Isotope Sub-Stage 7c and D/L ratios of 0.175 can be ascribed to earlier in Stage 7. The Pennard (D/L)Stage is ascribed, on this model, to Sub-stage 5e (122 ka).

Amino Acid Geochronology

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This model is most consistent with the evidence from Baggy Point and Chesil Cliff, Saunton in North Devon (above) where two lithostratigraphic units ascribed to the D/L subdivisions at 0.175 and 0.135 respectively occur. It seems unlikely that they are separated by a full glacial event (Oxygen Istotope Stage 6) and more probable they could be closer together in time: e.g. time equivalent to subdivisions of Stage 7. This is also consistent with two D/L faunas represented in the Horton Upper Beach indicating two sea-level events. Pending further discoveries, or the application of other dating methods (an ESR dating programme is being undertaken), geochronological calibration of the D/L ratio data is presented in terms of the two basic models above. As will be evident, however, the fundamental importance of each model is that a (D/L)Stage corresponding to Oxygen Isotope 7, or part of it, is represented by raised beaches which can be used as calibration of the glacial sequence (below). SEA-LEVEL, GLACIAL AND PERIGLACIAL EVENTS An implication from model lb, as discussed above, is that the 'Barbados sea-level model' (see e.g. Cronin, 1983) is inapplicable in south-west Britain. As such it derives support from the coastal plain of the eastern USA, where high sea-levels correlated with Sub-stages 5e, 5c and 5a of the Oxygen Isotope Scale have been recognised (Szabo, 1985). As indicated earlier the anomalously low D/L ratios from the Portland (east) raised beach could well correspond to the sea-level event of Sub-stage 5a. The sea-level events recognised in either model show that the coastline, with its raised platforms at different levels (Orme, 1960; Wright, 1967) has been repeatedly re-occupied by the sea (Bowen, 1973c, 1973d). This complexity of evolution extends to the now degraded cliffs landwards of the platforms and potentially for some of the still existing accumulations of periglacial and colluvial slope deposits at their base. Sequences of coastal slope deposits overlying raised beaches of Minchin Hole (D/L)Stage age merit detailed reexamination to see how complete the subsequent record may be. On the b~sis of Nucella lapillus D/L ratios of 0.225 + 0.009 (2)(2) and one sample of Littorina littoralis 0.225 (1)(1), the oldest D/L ratios presented here, from Hunts Bay East, Gower, which can be tentatively ascribed to Oxygen Isotope Stage 9, the raised coastal platforms with their complementary cliffs, are at least as old as Stage 9 (303 to 339 ka BP). The D/L data given here does not demonstrate any neotectonic uplift, but unpublished data from Sussex and Somerset shows that localised uplift has occurred within the geologically recent past. Some of the low D/L ratios (e.g. from Portland) may also point to localised uplift since the sea-level events they represent. Lithostratigraphic and field relationships between the D/L marine (raised beach) record and glacial and periglacial deposits allows constraints to be put on the timing and extent of glaciation. According to either model put forward two important conclusions emerge: (1) glaciation only postdated raised beaches of the Pennard (D/L)Stage at Langland Bay and Broughton Bay, Gower, South Wales (Oxygen Isotope Stage 5e [or 5c or 5a]); (2) glaciation did not occur locally in south Dyfed, most of south Gower, or anywhere on the shores and coastal hinterland of the Bristol Channel and Celtic Sea after the Minchin Hole (and unnamed, in model 2) (D/L)Stages (Oxygen Isotope Stage 7).

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The principle of using raised beaches of an assumed age to delimit the extent of glaciation (Bowen, 1973a, b, 1974) has now been amplified by the D/L stratigraphy calibrated by uranium-series dates. The age of the glaciation postdating the raised beaches in Gower is demonstrably post Oxygen Isotope Stage 5 in age and unpublished data of D/L ratios of marine molluscs included in till show it cannot be earlier than Late Devensian. Two, probably three, earlier glaciations are older than Oxygen Isotope Stage 7 [Minchin Hole (D/L)Stage] because their deposits nowhere overlie raised beaches of that age (cf. Bowen, 1973b). Except at West Angle Bay, at the entrance to Milford Haven, where estuarine interglacial deposits overlie a till (Dixon, 1921), pre-Devensian glacial deposits in situ nowhere occur in stratigraphic sequence with a raised beach or associated sediments. Unfortunately the vegetational history inferred from pollen analysis of the West Angle deposits precludes confident correlation with sites elsewhere in Britain (Stevenson and Moore, 1982). In Gower, however, the field relationships between the raised beaches of the Minchin Hole (D/L)Stage and glacial deposits of two previous glaciations are known (Bowen, Jenkins, Reid and Catt, unpubl, data). Two glaciations are recognised: the earlier crossed Gower from north-north-west to south-south-east, as indicator erratics and heavy minerals show (see Bowen, 1970). This 'Irish Sea' Glaciation probably deposited the shelly calcareous till at Fremington in north Devon where Zeuner (1949), followed by Bowen (1969), showed that its deposition antedated the local raised beaches of Barnstaple Bay. The outer limit of this glaciation, here called the 'Fremington Glaciation' is probably along the north coast and immediate coastal hinterland of south west England and, as Mitchell and Orme (1967) showed bisects the Isles of Scilly (Fig. 1), Where sediments relating to this glaciation occur in coastal exposures they have been redistributed downslope by colluvial and periglacial processes and as such are technically 'head' deposits which postdate formation of the raised beaches on which they lie (Bowen, 1969, 1973b). A second glaciation antedating the raised beaches of the Minchin Hole (D/L)Stage has recently been proved by drilling in Gower and has been named the Paviland Glaciation (Fig. 1). Unlike the previous one, glaciation was by ice from an exclusively Welsh source to the north. Still earlier, the most extensive glaciation of all is recorded by North Wales erratic clasts in the fluviatile/outwash deposits of the Kesgrave Formation of Essex (Rose and Allen, 1974). Such a powerful glaciation from the mountains of north and central Wales would probably have extended well to the south of the Bristol Channel and the Isles of SciUy. It is here called the Berwyn Glaciation after the Berwyn Mountains from where some of its igneous erratics in Essex derive. The age of these glaciations may be constrained to some extent with reference to the raised beach D/L Stages together with the Oxygen Isotope Scale. Two models are presented (Table 5): (1) the first, in which the maximum age for glaciation antedating the Devensian Stage is given by the D/L ratios of the Minchin Hole Stage: that is, Oxygen Isotope Stage 7. In both models minimum ages only, are given for the glacial events; and (2) the second in which the maximum age for glaciation antedating the Devensian Stage is indicated by the oldest D/L ratios from Hunts Bay East, Gower, which could be time-equivalent to Oxygen Isotope Stage 9.

313

Amino Acid Geochronology TABLE 5. Possible models constraining the extent and age of glaciations in south west Britain (N.E. Atlantic margin). The critical minimum ages are for the marine (D/L) stage ascribed to Oxygen Isotope Stage 7, or the derived faunas ascribed to Oxygen Isotope Stage 9. Data for the Early Devensian Glaciation, which hitherto has only been demonstrated north of the study area, are from D/L ratios of molluscs in glacial deposits (unpubl. data) Glaciation Oxygen Isotope Stage 2 4 5 6 7 8 9 10 12 14

Model 1

Model 2

Late Devensian Late Devensian Early Devensian Early Devensian [marine (D/LStage(s)] [marine (D/LStage(s)] ~Paviland glaciation mixed D/L faunas ~>Irish Sea glaciation ~>Paviland glaciation ~Berwyn glaciation ~>Irish Sea glaciation ~Berwyn glaciation

But because no appreciable glaciation occurred during Stage 10 in the northern hemisphere it is likely that both models require modification accordingly.

CONCLUSIONS Based on D/L ratios of six species which all epimerize at much the same rate, some lithostratigraphic control, some geochronological calibration by U-series ages, and partly assisted by a cluster analysis, the following conclusions have been reached. (1) T h r e e high stands of sea-level (in detail there may have been more) are recognised on the basis of the D/L ratios on marine molluscs. Two major ones, the Minchin Hole and Pennard (D/L)Stages, are confirmed by lithostratigraphic and biostratigraphic separation. A third, intermediate (D/L)Stage is proposed on the basis of separate D/L ratios, its occurrence as discrete outcrops and statistical integrity; but it has yet to be lithostratigraphically subdivided from the two other (D/L)Stages. (2) Using calibration based on uranium-series ages from stalagmites in coastal caves where they are interstratified with the raised beaches two geochronological models are presented in Table 6. (3) Using these D/L data from marine stratigraphic units constraints can be placed on the timing and extent of glaciation in south west Britain, a critical area on the north-east margin of the Atlantic Ocean. Because of uncertainty introduced by the mixed molluscan population at Hunts Bay East, which has a bearing on the minimum age for the last local glaciation of the stratotypic area of south Gower, two models are presented for preDevensian glaciations (Table 5). The timing of the Late Devensian Glaciation is closely constrained by radiocarbon dated D/L screened molluscs in shelly drift (Bowen, McCabe,

D.Q. Bowen et al.

314

TABLE 6. Age estimates for the (D/L) Stages. Model la does not conform to the "Barbados model' of sea-level change. Ages are from SPECMAP (Imbrie et al., 1984), with some approximations (5a and 5e), and gross estimates for Sub-stages of Stage 7 Oxygen Isotope Stage and Age (ka BP) (D/L) Stage

D/L Ratio

Pennard

0.105 ± 0.016

Unnamed

0.135 ± 0.014

Minchin Hole

0.175 ± 0 . 0 1 4

Model la

Model lb

Model 2

5a(ca. 80) 5c(ca. 100)

5e(122)

5e(122)

7(194 ?)

7(186-245)

7(216?)

5e(122) 7(186-245)

Sykes, R e e v e s and H a r k n e s s , unpubl, data); while Early Devensian Glaciation occurs, farther north, in parts of Ireland (Bowen, M c C a b e , Sykes, Reeves, unpubl, data). (4) Coastal g e o m o r p h o l o g y , in terms of raised beaches, raised platforms and adjacent cliffs, is the product of a complex history. In terms of this study all that can be said is that the landforms fashioned in the solid geology are at least as old as the oldest sediments (D/L ratios of 0.225 + 0.009 (2)(2) = Oxygen Isotope Stage 9 = 303-339 ka BP?) resting on t h e m (Bowen, 1973b); they m a y , of course, be older, but in view of some neotectonic evidence in southern Britain (unpubl. data) it is unlikely that they are much older than the oldest faunas identified. ACKNOWLEDGEMENTS Except for the D/L ratios measured at the Amino Acid Geochronology Laboratory at INSTAAR, Colorado University, Boulder, Colorado and a small number (e.g. Godrevy, Hope's Nose and Jersey) run at the University of London AAG Lab (Royal HoUoway and Bedford New College), the majority of samples were run while the latter laboratory was based at The University College of Wales, Aberystwyth. There the original HPLC system was installed by P.E. Hare. The present system, on which the data in this paper were measured, was built and operated by G.A. Sykes. Thanks are due to The University College of Wales, Aberystwyth, who supported the establishment of the laboratory and to the following technical staff: R.O. Jones, J.P. Rooke and D. Francis (drillers and workshop) and to J.S. Orzechowski. The London Laboratory has been supported by the Royal Holloway and Bedford New College and the University of London. Thanks are due to R. Watt, the laboratory technician. Many have helped by providing samples and our thanks is due to them: J.M. Hodgson and R. Shephard-Thorn. The radiocarbon dates referred to in the text were provided by D.D. Harkness at the NERC East Kilbride Radiocarbon Laboratory and form part of a joint project between D.Q. Bowen, D.D. Harkness, G.A. Sykes, A. Reeves and A.M. McCabe on shelly glacial deposits in Ireland and the Irish Sea. We acknowledge useful discussion at different times with the following: J. Wehmiller, J. Rose, D.G. Sutherland, W.D. McCoy, K. Lajoie and his team at Menlo Park, Julie Brigham-Grette, J. Mangerud;'H. Peter-Serjup, N.J. Shackleton and W.G. Jardine. All the diagrams were drawn by Giily Corran (RHBNC) and the text was prepared by Kathy Roberts with help from Pam Cardwell, whom we thank. The research has been supported by NERC (GR3/5192).

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APPENDIX 1 Separation of amino acids is achieved by chromatography on 25 x 0.24 internal diameter (I.D.) cm column packed with Durram DC4A ion exchange resin and maintained at approximately 65°C. Sample injection is by an LKB 2153 Autoinjector with 1291 dispenser accessory which provides variable sample volumes. The Amino Acids are eluted by a step gradient of 67 mM Sodium Citrate buffer pH 3.12, 3.8 and 33 mM Sodium Borate buffer pH 10.5 containing 170 mM NaC1 and 2.7 mM EDTA. Buffer flow rate is maintained at 0.10 ml/min by an Eldex A-30-S pump at a back pressure of 400-700 psi depending on column age. Eluant from the column is mixed with O P A reagent, 0.49 M Potassium Borate Buffer pH 10.5 containing 1 ml of Bri] 35/1 and 15 ml O P A solution (0.750 g OPA, 15 ml Methanol, 300 ~.l Mercaptoethanol). Reagent flow is maintained at 0.11 ml/min under nitrogen pressure and the reaction takes place in a reaction coil (80 x 0.005 I.D. cm). The amino acids are detected as O P A derivatives by a Gilson 121 fluorometer. Control of sample injection, buffer switching and data collection are by an Apple IIe computer using IMI Chromatochart software in conjunction with IMI adalab card and chomadapt interface, a second adalab card monitors data with a five fold attenuation. The system can plot and integrate the peaks and save the raw data on disk from up to 16 analyses.

APPENDIX 2 Sample Preparation Procedure for Total Fraction (1) The shell is identified to the genetic or specific level. Distinguishing characteristics and the part of the shell to be used for the analysis are noted on a sample preparation card. (2) The shell is cleaned of adhering sediment either mechanically, or in an ultra sonic bath. If present, the periostracum is also removed. (3) A complete shell or shell fragment (50-100 mg) is weighed to the nearest 0.1 mg and the weight is recorded on the sample preparation card. The shell is placed in a clean test tube labelled with a laboratory identification number ( A B E R # ) and the species. (4) Sufficient 2 M HCl is added to dissolve 1/3 of the shell by weight (1.0 i~l of 2 M HCl dissolves 100 mg of CaCo3). Distilled water is added to completely cover the shell in the solution. (5) When the reaction is completed the solution is poured off and the shell is rinsed 4-5 times in distilled water with a final rinse in ultrapure water. The sample is shaken vigorously during this process with the mouth of the test tube covered by cling film. The shell is then air dried. (6) The shell sample is re-weighed to the nearest 0.1 mg and placed in a clean'sample vial which is labelled with the Lab. ID number and species. (7) Sufficient 7 M HCl plus norleucine is added to completely dissolve the shell (0.02 ml 7 M HCI + norleucine x sample weight in mg = ml 7 M HCI + norleucine to be added). It is important that this reaction proceeds to completion. (8) The vial is flushed with nitrogen gas and sealed. It is then hydrolysed at 110°C for 22 hours. (9) The sample is allowed to cool and is then placed in a centrifuge for 2-3 minutes at 3,000 rpm. The vial is opened and dried down on a hot plate under nitrogen gas. (10) The sample is rehydrated with pH 2 solution ([weight of sample in rag]/25 = ml pH 2 to be added). (pH 2 solution is 330 ml analytical grade water adjusted to pH 2 with hydrochloric acid.)