Holocene sedimentation rates on the hellenic outer ridge: A comparison by 14C and 230Thexcess methods

Sedimentary Geology, 32 (1982) 99--110

99

Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands

HOLOCENE SEDIMENTATION RATES ON THE HELLENIC OUTER RIDGE: A COMPARISON BY 14C AND 23°Thexce~s METHODS

JOHN THOMSON

Institute of Oceanographic Sciences, Wormley, Godalming, Surrey GU8 5UB (Great Britain) (Received May 19, 1981; revised and accepted November 10, 1981)

ABSTRACT Thomson, J., 1982. Holocene sedimentation rates on the Hellenic Outer Ridge: a comparison by 14C and 23°Thexce~ methods. Sediment. Geol., 32: 99--110. An examination of the most recent Eastern Mediterranean sapropel ($1 or A ) i s made by 14C and 23°Whexcess techniques, to test the observation (Mangini and Dominik, 1979) that sapropels in this area often represent periods of slower than average sedimentation. Two cores from the Hellenic Outer Ridge with long clear-cut sapropel sections were analysed. Together the 23°Whexcess and 14C data are equivocal and suggest a complex sedimentation process. The t4C ages call for high sedimentation rates in the sapropel sections, while the estimated 230Thexcess values in the sapropel sections are relatively high, and would normally be taken to indicate a slower sedimentation rate than in the overlying carbonate ooze. The 14C ages are more direct and ought to be valid for the core sites. From consideration of a model based on water-column supply of 23°Whexcess, however, it is suggested that the sapropel in the local area as a whole probably accumulated at a rate similar to, or slightly higher than, the carbonate ooze subsequently deposited.

INTRODUCTION

In a recent paper concerned with late Quaternary sapropels of different ages from the Mediterranean Ridge (Mangini and Dominik, 1979), the systematics of the uranium and thorium isotopes were investigated. Two important conclusions of the work were that, firstly, the 234U isotope is preferentially mobile in these organic and uranium-rich reduced layers, a p h e n o m e n o n observed elsewhere in the marine environment in slowly accumulating, oxic sediments (Ku, 1965) and in phosphorites (Kolodny and Kaplan, 1970). Secondly, arguments based on 23°Th budgets indicated that the sapropel layers often accumulate at rates slower than the long-term average rate in the same cores (Dominik and Mangini, 1979). This latter conclusion may appear at first surprising as high organic contents tend to be associated with high accumulation rates, whereas slower accumulation rates allow a greater time for organic degradation to occur before burial. It is n o t 0037-0738/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

100

clear, however, that such a straightforward concept is useful for the Eastern Mediterranean where the sediments switch periodically from oxic to anoxic and back, probably driven b y periods of anaerobic deep-water conditions, as discussed for example b y McCoy (1974) or Stanley (1978). The idea that sapropel horizons form at slower rates than average, if generally true, could be important in framing new models on sapropel formation conditions, and would require to be a c c o m m o d a t e d in any successful model. This paper reports an investigation of the most recent sapropel layer, designated A (Hieke et al., 1973, Hieke, 1976) or S~ (McCoy, 1974; Stanley and Moldonado, 1977, Stanley et al., 1978) in the different terminologies used, Relatively little information was gathered for the $1 sapropel b y Mangini and Dominik (1979), b u t it offers the best o p p o r t u n i t y for a direct comparison of 14C and 2a°Th methods. This is because the ages involved allow use of 14C dating and mean that potentially disruptive effects such as 234U migration have a shorter period in which to establish themselves. The material available was from a relatively small area on the Hellenic Outer Ridge in the Ionian Sea which showed a variable thickness of the $1 sapropel. PROCEDURES

Core locations

Cores were available from R.R.S. "Discovery" Cruise 104 in 1979. An area of protected high ground on the Hellenic Outer Ridge was sought in an effort to minimise the possibility of coring slumped sediment. The coring site had a fairly complex local topography, with water depths around 2900 m. Cores were taken with an I.O.S. 30 cm square b o x corer (Peters et al., 1980) and a Hydrowerkst~itten Kastenlot corer with a 2 m, 15 cm square section b o x barrel. Four cores in all were taken, three of which showed horizontal bedding, with a buff-coloured calcareous ooze top (thickness 22-28 cm) overlying a black sapropel layer of thickness varying from core to core (thickness 6.5--73 cm) which in turn overlay a grey calcareous lutite which was apparently resistant to corer penetration. The fourth core had a complex folding and pinching structure in the sapropel layer. Samples from one b o x core (10103 No. 1B; 36°09.6'N, 20°28.5~E; 2880 m) and one

Fig. 1. P h o t o g r a p h o f K a s t e n core 1 0 1 0 3 No. 8 K o n c o l l e c t i o n . T h e core t o p is t o t h e left, w i t h s a p r o p e l ( b l a c k s e d i m e n t ) c o n t a c t s at 28 a n d 101 c m . T h e t o t a l core l e n g t h in f r a m e is a b o u t 1 1 0 c m .

101

Kastenlot core (10103 No. 8K; 36°09.4'N, 20°28.6'E; 2895 m) were taken for this work. Both cores had calcareous ooze sections of 28 cm, but the sapropel section was from 28--43 cm in the box core and 28-101 cm in the Kastenlot core. The sapropel top and b o t t o m contacts were rather sharp in both cases (Fig. 1).

Radiochemical analyses Analyses for uranium and thorium isotopes were made by alpha spectrometry. Samples (around 2 g of dried sediment) were spiked with a calibrated 232 U 228Th tracer and totally dissolved by a method based on potassium fluoride and sodium/potassium pyrosulphate fusions adapted from Sill et al. (1974). Uranium and thorium fractions were separated by anion exchange and electroplated for counting. Radiocarbon analyses were made by the N.E.R.C. Radiocarbon Laboratory. Where samples contained sufficient organic matter, two analyses were made on each. These are termed here "inorganic" and "organic" based on an operational distinction. The inorganic (calcium carbonate) analyses are from the carbon dioxide liberated by phosphoric acid, and the organic analyses are from the carbon dioxide liberated by chromic acid oxidation of the residue from the phosphoric acid treatment.

Other analyses Calcium carbonate and "organic carbon" analyses were made by Leco analyser. The calcium carbonate was estimated gravimetrically via carbon dioxide evolved on hydrochloric acid treatment. The carbon content corresponding to this value was subtracted from the total carbon content of a pyrolised sample, again by gravimetric carbon dioxide determination, to yield an organic carbon value. The coefficients of variation for the carbonate and organic carbon methods are 1 and 9.5% respectively. Sediment density was determined by weighing known volumes of sediment, taken at the time of collection, before and after drying at 105 ° C. RESULTS AND DISCUSSION

Radiocarbon analysis The results of the radiocarbon analysis are listed for the two cores in Table I. The total range of ages for the sapropel samples is from 6395 to 9145 yrs, respectively at the top and b o t t o m of the long section in core No. 8K where the data have more resolution. The duration of the $1 sapropel has been estimated at 7000 to 9000 yrs (McCoy, 1974; Stanley and Maldonado, 1977) from various areas of the Eastern Mediterranean, including the Ionian Sea (Stanley, 1978) and the Hellenic Trench to the east of the cores here

102 TABLE I

Radiocarbon ages of the studied samples Depth

Laboratory

(cm)

No.

14 C age (yrs)

Core 1 0 1 0 3 No. 8 K : 0 - - 10

S R R 1706

Inorganic

Organic 1 8 - - 25 2 9 - - 36 *

S R R 1707 S R R 1708

5 3 - - 63 *

8 R R 1709

6 5 - - 75 *

S R R 1710

90--100 *

S R R 1711

Inorganic

105--115

S R R 1712

Organic Inorganic

Core 1 0 1 0 3 No. 1B: 0-- 8

S R R 1713

Inorganic

2 8 - - 33 *

S R R 1714

3 7 - - 43 *

S R R 1715

4 3 - - 62

S R R 1716

Inorganic Inorganic

Organic Inorganic Organic Inorganic Organic

Organic Inorganic Organic Inorganic Organic Inorganic

2915 2590 6530 8095 6395 7840 7460 7765 7280 8675 9145 12320

-+ 50 2 280 _+ 60 + 80 + 90 -+ 50 -~ 70 + 60 + 100 -+ 55 -+ 130 + 60

3450 4120 8730 8510 9015 8830 11350

+ 50 +_ 240 + 60 _+ 90 -+ 55 -+ 100 -+ 70

* Sapropel sample.

(Stanley et al., 1978). This confirms the layer here to be the S, sapropel. Further, the data are good evidence that very little, it any, older material has been included the sapropel. The radiocarbon ages from both core tops are non-zero because of the finite thickness of sediment taken, and possibly because of bioturbation. (From observation of the cores on deck, it is believed that the interfaces were retrieved.) A b o x model has recently been developed to unravel the complexity of '"C ages near the surface resulting from bioturbation (Erlenkeuser, 1980). With literature data from several deep-sea carbonate cores, it was demonstrated that it is possible to obtain a near-zero age for the sediment interface if sufficiently detailed radiocarbon analyses against depth are available. It is notable that the 6--9 cm mixed-layer radiocarbon data listed there ( 2 5 0 0 - - 4 5 0 0 yrs) are similar to the t o p m o s t ages here. Utilisation of equation 9 (Erlenkeuser, 1980) with the top t w o inorganic data points of core No. 8K yields a surface age of 1 8 2 0 yrs. This can be considered to be a high estimate however, as the top datum covers a depth range of 10 cm, deeper than most observed mixed layers. If bioturbation does exist in the pelagic sediment overlying the sapropel here, it must have been established fairly

103 recently, as the underlying sapropel layer tops show no evidence of overlying sediment worked in (Fig. 1). It would be expected that both inorganic and organic radiocarbon ages from the same depth should be similar (but not necessarily identical) if both are formed from the same carbon pool of the water column (Erlenkeuser, 1980), neglecting any possible supply of terrestrial carbon. While this is generally the case for these two cores, a considerable proportion of the calcium carbonate present throughout is in the form of large pteropod tests which should be readily separated from the bulk of the sediment in any flow process. This may be the reason for the off-trend inorganic datum at 29--36 cm in core No. 8K. Stanley et al. (1978) have discussed the high, variable sedimentation rates found in the Hellenic Trench in terms of a seismically affected, complex physiography controlling sediment redistribution. Sediment flows are therefore localised in extent and in time, and the resultant lithologies are difficult to correlate on a wide scale. It would appear that some similar process is recorded in the two sapropel sections here. For example, in core No. 1B the two sapropel radiocarbon ages, biassed as they are towards the beginning of the event, result in sedimentation rates of 34 cm/103 yr and 30 cm/103 yr from the organic and inorganic data respectively. In core No. 8K a regression line through the four organic data points yields a rate of 24 cm/103 yr, while the more scattered inorganic data give 116 cm/103 yr. It will be shown subsequently that rates of this magnitude are at variance with the 23°Th data, leading to the conclusion that while the 14C data can estimate accumulation rates for the two sites, they cannot be representative of the local area as a whole. Uranium and thorium isotope systematics The experimental data are presented in Table II. It is evident that these data demonstrate similar features to those found by Mangini and Dominik (1979): enhanced uranium concentrations in the sapropel layer with 234U/ 238U activity ratios tending towards the seawater value of 1.14, and high 23°Th specific activities in the sapropel samples by comparison with the remainder of the samples. As only a short period has elapsed since formation of this sapropel, however, little 23°Th has been developed from the authigenic 234U and there is little or no apparent 23°Thexcess in the sapropels. This fact, together with the 234U/238U activity ratio, is good evidence, in addition to that provided by Mangini and Dominik, that the enhanced uranium contents are derived from seawater. An eventuality which might affect the isotope spike dilution technique for thorium employed would be disequilibrium between 228Th and 232Th in the sample. This is because 228Th in the alpha spectra is from both the sample and the spike. In practice the sample contribution is subtracted from the peak integral to yield the spike contribution by assuming that the sample

104 TABLE II Uranium and thorium isotope, calcium carbonate and organic data of the studied samples Depth (cm)

U (ppm)

Th (ppm)

Th/U

234U/ 2ss U activity ratio

6.18 + 0.23 5.71+0.16 6.67 + 0.21 5.20 + 0.16 5.79 -+0.44 3.94+0.14 4.13 + 0.16 5.65 -+0.25 6.28 -+0.23 6.92 -+0.20

4.2 +_0.2 4.2 +0.2 0.66 + 0.02 0.34 + 0.01 0.39 -+0.03 0.30_+0.01 0.23 + 0.01 0.37 -+0.02 1.21 -+0.06 1.31 -+0.06

1.00 + 0.03 0.97-+0.05 1.11 -+0.02 1.14 _+0.02 1.11-+0.02 1.12-+0.02 1.12 -+0.03 1.08 -+0.02 1.06 -+0.04 1.07 -+0.03

6.56 5.67 5.10 5.69 6.54

4.3 3.5 0.85 0.48 1.06

0.97 1.02 1.11 1.15 1.12

Core 10103 No. 8 K

0-- 3 17-- 20 29-- 32 * 11-- 44 * 53-- 56" 65-- 68* 78-- 81 * 91-- 94 * 104--107 (duplicated)

1.47 -+0.04 1.37-+0.05 10.1 + 0.2 15.4 + 0.4 15.0 -+0.3 13.1 -+0.3 18.0 -+0.7 15.2 -+0.3 5.2 -+0.2 5.3 _+0.2

Core 10103 No. 1B

0-10-28-33-43--

10 19 33 * 37 * 62

1.53 1.61 6.01 11.9 6.16

-+0.04 -+0.07 +0.16 + 0.3 -+0.16

+ 0.31 + 0.16 +0.23 -+0.26 + 0.17

-+0.2 -+0.2 -+0.04 -+0.02 -+0.04

-+0.03 -+0.05 -+0.03 -+0.02 -+0.03

* Sapropel sample. Quoted uncertainties are based on 1 sigma counting statistics.

228Th/232Th a c t i v i t y r a t i o is u n i t y . T o c h e c k this a s s u m p t i o n , f o u r samples f r o m t h e s a p r o p e l l a y e r o f t h e cores were r u n w i t h o u t spike a d d e d , a n d in each case a secular e q u i l i b r i u m value (1.00 w i t h i n e x p e r i m e n t a l u n c e r t a i n t y ) was f o u n d . I t m a y be c o n c l u d e d t h a t t h e relatively high values o f ~3°Th specific activity f o u n d in t h e s a p r o p e l layers are real. T o evaluate t h e c o m p o s i t i o n s o f t h e t o t a l 23°Th specific activities in t h e sapropel, it is a s s u m e d t h a t t h e r e is a detrital c o n t r i b u t i o n , an i n g r o w t h c o n t r i b u t i o n f r o m a u t h i g e n i c 234U i n c l u d e d f r o m seawater, a n d an a u t h i g e n i c c o m p o n e n t (23°Thexcess) derived f r o m seawater. This is a similar p r o c e d u r e t o t h a t a d o p t e d b y Mangini a n d D o m i n i k ( 1 9 7 9 ) b u t t h e details differ slightly. Table I I I lists these evaluations. T h e detrital c o n t r i b u t i o n is calculated f r o m t h e 232Th activity m e a s u r e d , a s s u m i n g t h a t t h e T h / U ratio in t h e o v e r l y i n g l a y e r is also r e p r e s e n t a t i v e o f t h e detrital material in t h e sapropel, a n d t h a t t h e d e c a y c h a i n 238U ~ 234U -* 23°Th is in radioactive~,equilibrium. I t is c o n s e q u e n t o n this a s s u m p t i o n t h a t t h e detrital 23°Th/232Th activity ratio is 0.76. This m a y be c o n s i d e r e d t o be t h e least f i r m l y based a s p e c t o f t h e calc u l a t i o n , as t h e detrital s o u r c e m a y be d i f f e r e n t f o r t h e t w o s e d i m e n t a r y units. F o r c o m p a r i s o n , a r a t i o close t o u n i t y is usually f o u n d in river-borne

105

230Th / 232 Th activity ratio

234 U (dpm/g)

230Th (dpm/g)

23° Thexce~ (dpm/g)

2.53 2.84 3.71 4.02 3.96 4.09 4.98 3.89 3.10 2.93

-+ 0.09 +- 0.07 +- 0.11 +- 0.11 -+ 0.27 + 0.15 +0.18 +- 0.17 +- 0.10 +- 0.07

1.10 1.00 8.34 13.1 12.4 11.0 15.0 12.3 4.10 4.23

+ 0.03 + 0.04 +_0.16 + 0.4 -+ 0.2 -+ 0.3 +-0.6 + 0.3 +__0.14 +- 0.13

3.81 3.93 6.01 5.07 5.57 3.92 4.99 5.34 4.72 4.92

+ 0.11 + 0.08 + 0.13 +- 0.11 +- 0.31 + 0.08 +-0.13 +- 0.15 +- 0.13 +- 0.11

2.71 2.93 ------0.62 0.69

2.41 3.13 4.09 3.80 2.63

+ 0.10 +- 0.08 -+ 0.16 + 0.16 +- 0.06

1.11 1.23 4.99 10.3 5.14

+- 0.03 --4"0.05 + 0.13 + 0.16 +- 0.13

3.83 4.31 5.07 5.26 4.18

+- 0.15 -+ 0.10 +- 0.16 +- 0.18 + 0.09

CaCO 3 (%)

Organic carbon

(%)

53.8 57.2 41.5 48.2 49.9 59.5 53.9 48.3

0.3 0.2 2.3 2.2 2.3 2.0 2.5 2.9

-4--0.19 +- 0.17

45.9

0.3

2.72 -+ 0.15 3.08 + 0.11 ----

52.5 53.0 44.9 46.7 47.6

0.2 0.3 3.3 2.9 0.3

+ 0.11 + 0.09

T A B L E III E s t i m a t e d c o m p o n e n t s o f t o t a l 230 T h c o m p o s i t i o n Depth (era)

23Su (dpm/g)

232Th (dpm/g)

23°Th (dpm/g) total

23°Th (dpm/g) detrital *

23°Th 230 Thexcess (dpm/g) (dpm/g) ingrowth +

Core 1 0 1 0 3 No. 8 K 29--32 7.55 41--44 11.5 53--56 11.2 65--68 9.79 78--81 13.5 91--94 11.4

1.62 1.26 1.41 0.96 1.00 1.37

6.01 5.07 5.57 3.92 4.99 5.34

1.24 0.96 1.08 0.73 0.76 1.05

0.51 0.85 0.82 0.73 1.03 0.84

4.26 3.26 3.67 2.46 3.20 3.45

Core 1 0 1 0 3 No. 1B 28--33 4.49 33--37 8.89

1.24 1.38

5.07 5.26

0.95 1.05

0.29 0.63

3.83 3.58

* Detrital 230Th = 232Th d p m / g - 1.31. + I n g r o w t h 23°Th = [23sU d p m / g - - (232Th - 1 . 3 1 ) ] X 1.14 X [1 - - e x p ( - - 8 0 0 0 X In 2 75,200)]

106 TABLE IV 23°Th/232 Th activity ratios in shallow water sediments Area

Core (No. of samples)

Mean 23°Th/232 Th

Reference

Long Island Sound, U.S.A.

DEEP-R-1 (12) NWC-R-6 (17) NAR-1 (6) C-2a (8)

0.83 0.83 0.86 0.76

D-1 (7)

0.67 + 0.03

28.5 km (4)

0.96 -+0.16

3.0 km (5)

1.02 _4-0.14

Cochran 1979 Cochran 1979 Cochran 1979 Cochran and Aller 1979 Cochran and Aller 1979 Elsinger and Moore 1980 Elsinger and Moore 1980

Narraganset Bay, U.S.A. New York Bight, U.S.A.

Winyah Bay, U.S.A.

+ 0.04 + 0.03 + 0.04 -+ 0.03

detritus (Scott, 1968) and in shallow water sediments (Table IV). A value of unity would have minor consequences on the conclusions. The authigenic component is calculated by subtracting a detrital contribution from the total uranium as above, increasing by a factor of 1.14 to allow for the higher 234U activity, and estimating the ingrowth of 23°Th over 8000 yrs, the mean sapropel age. The authigenic component, 23°Thexcess, is then obtained by subtraction of the other two components from the total measured. As most 23°Thexcess methods depend on a decrease in the specific activity (75,200 yr half life) with depth to determine sedimentation rates (Ku, 1976), these 23°Thexcess data cannot be used in the conventional manner over the short period involved here. Instead a comparison is made of the data with those anticipated from water column supply, using the more direct 14C data to calibrate ages and rates. Aggregated values are used since the 14C indicates the two sediment types of interest to be unmixed. An expression for supply of a radioactive species like 23°Th supplied to sediments at steady state (i.e. production and supply rate from the water column equals decay rate in the sediment column) is given by Krishnaswami et al. (1971): P = p • s "Ao/~.

(1)

where P is the rate of deposition per unit area and unit time, p is the in situ (dry) sediment density, s is the sedimentation rate in length per unit time, Ao is the activity at the sediment interface in activity per unit dry weight in the sediment, and ~ is the disintegration constant of the nuclide in question. For the water column here of 2890 m, the 23°ThexCe~s PrOduced from 234U, P, may be calculated from the seawater uranium concentration and 234U/23sU activity ratio (3.3 pg/1 and 1.14 respectively; Ku et al., 1977) as 813 atoms/ cm2/min.

107 TABLE V Sediment densities, core 10103 No. 8K Depth (cm)

4--6

16--18

22--24

31-33 *

52-54 *

72-74 *

91-93 *

108-110

In situ

1.47

1.57

1.48

1.33

1.31

1.28

1.36

1.60

0.77

0.94

0.77

0.61

0.57

0.53

0.59

1.02

Wet density In situ

Dry density * Sapropel sample.

In the few thousand years involved, the value of Ao is taken as the mean 23°Thexcess from the calcareous o o z e tops (2.86 dpm/g) corrected for 23°Th decay in the mean age of 3 5 0 0 yr. Along with the mean density for this section from Table V, this yields a sedimentation rate o f 3.1 cm/103 yr from eq. (1). From the 14C mean sedimentation rate of 4 cm/103 yr (28 cm deposited in 7 0 0 0 yrs), a predicted value o f 2.26 dpm/g for Ao is f o u n d from eq. (1). It follows that the material being laid d o w n at this site has a higher specific activity than could be obtained from the overhead water depth. This may be a reflection o f the horizontal m o v e m e n t and passage time o f sediment in

OCore No 8K +Core No. 18 "~Mean of both data sets.

/// //o~,/

/ ~. E o.

o

/Vo

/////

? /o

/////

////

~0.81 /// // /// / 232Th, d p m/g

Fig. 2. Plot of e s t i m a t e d 23°Thexcess activities versus measured 232Th activities in the sapropel sections. The solid line is a regression line of 23°Thexcess on 232Th, while the broken line is drawn through the origin and the mean of the two data sets.

108 seawater from source to the Hellenic Ridge, as this location is surrounded b y deeper water in three directions and b y the ridge itself to the south. Applying the mean sapropel 23°Thexcess specific activity (3.45 dpm/g), corrected for decay over 8000 yr, a sedimentation rate of 3.5 cm/103 yr is obtained. This is an order of magnitude lower than the rates in the sapropel indicated b y the radiocarbon data which imply 23°Thexcess values around 0.4--0.5 dpm/g. Again these estimates use the water column production rate which did n o t balance for the calcareous ooze section. There is of course no guarantee that transport routes remain unaltered during periods of sapropel formation, and indeed it has been suggested on mineralogical evidence that t h e y do n o t (Dominik and Stoffers, 1978). Nevertheless, with the same value of P used for comparative purposes, the sapropel sedimentation rate is calculated as 13% higher than that of the calcareous ooze, despite its 21% higher 23°Thexcess, because of the difference in situ sediment densities. This illustrates clearly the importance of density in such calculations. In cases where sediment density alters markedly d o w n core, Behrens (1980) has p r o m o t e d conversion to a constant porosity, and such a procedure appears worthwhile in cores containing sapropel sections. One n o t e w o r t h y feature of the estimated sapropel 23°Thatches values is that they correlate loosely with the 232Th specific activities (Fig. 2). As 232Th is found in the n o n ~ a r b o n a t e fraction of sediments (Heye, 1969), this may be a function o f 23°Th~ce~ uptake on fine particles during transport through seawater. On this explanation, a regression line through the origin would be expected, consistent with the observations within the error of the data. CONCLUSIONS Detailed analysis results on the $1 sapropel and carbonate ooze sections examined here do n o t determine their relative rates of accumulation unequivocally, despite the simple, horizontally b e d d e d stratigraphy of the cores. Whereas the 14C ages indicate sedimentation rates of a b o u t 4 cm/103 yr for the carbonate ooze, a rapid rate of 24--34 cm/103 yr is found for the sapropel. The 23°Thexce~s data on the other hand are more consistent with a sedimentation rate almost an order o f magnitude lower for the sapropel, based on water column supply considerations. Expressed in a different way, the 14C data suggest accelerated sediment accumulation during the sapropel event at the two sites studied here relative to the long-term mean rates, while the 23°Thexcess data are of the expected magnitude for the long term rate. This, together with the fact that the four cores taken in the same small area exhibited a variable thickness of sapropel section, suggests that some sediment redistribution process was active in the area during the period of sapropel formation and resulted in a gathering of material to these t w o sites. Without more detailed survey information it is n o t possible to speculate further on possible causes for this. An important feature of the generally

109

consistent '4C data trend is that the different sediment types are unmixed with each other or with older sediments, despite the complex sediment transport processes to the coring sites indicated by the 23°Thexcess data. The simplest resolution of this contradictory evidence, then, appears to be that the 14C data reflect anomalously rapid sediment accumulation at the coring sites relative to the general area during the sapropel event, while the estimated 23°Thexcess data reflect the transport of material, particularly noncarbonate material, through seawater to the area as a whole before the inferred sediment redistribution in this period. Given the above, it is possible to conclude from the formal model, based on water column supply of 23°Thexc~ss only, that what indications there are suggest that regional accumulation rates of the carbonate and sapropel were similar, with those during sapropel formation possibly being slightly greater. This work illustrates that 23°The~¢~ data on their own must be treated with some caution in the absence of some other control. Such controls might be consistent stratigraphy over a wide area, or a balance of 23°Thatches water column supply and sediment inventory. The importance of density determinations where lithology changes drastically is also noteworthy. These observations do not affect the validity of the findings of Mangini and Dominik (1979) but suggest aspects to which attention should be paid in future work on this theme. Further, the model to treat 23°Thexce~ data in sapropels, on which their calculations were based, is supported by this characterisation of the most recent sapropel. ACKNOWLEDGEMENTS

I wish to thank Dr. D.D. Harkness (NERC Radiocarbon Laboratory, S.U.R.R.C. East Kilbride) for the radiocarbon analyses, Mrs. H.E. Sutherland for the carbonate and organic carbon analyses and Drs. T.L. Ku and T.R.S. Wilson for comments on a draft manuscript. REFERENCES Behrens, E.W:, 1980. On sedimentation rates and porosity. Mar. Geol., 35: M11--M16. Cochran, J.K., 1979. The Geochemistry of 226Ra and 22SRa in Marine Deposits. Thesis, Yale University, New Haven, Conn., 260 pp. Cochran, J.K. and Aller, R.C., 1979. Particle reworking in sediments from the New York Bight apex: evidence from 234Th/23SU disequilibrium. Estuarine Coastal Mar. Sci., 9: 739--747. Dominik, J. and Mangini, A., 1979. Late Quaternary sedimentation rate variations on the Mediterranean Ridge, as results from the 23°Th excess method. Sediment. Geol., 23: 95--112. Dominik, J. and Stoffers, P., 1978. The influence of Late Quaternary stagnations on clay sedimentation in the Eastern Mediterranean Sea. Geol. Rundsch., 68: 302--317. Elsinger, R.J. and Moore, W.S., 1980. 226Ra behaviour in the Pee Dee River--Winyah Bay estuary. Earth Planet. Sci. Lett., 48: 239--249. Erlenkeuser, H., 1980, ~4C age and vertical mixing of deep-sea sediments. Earth Planet. Sci. Lett., 47: 319--326.

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