Late Quaternary sapropel sediments in the eastern Mediterranean Sea: Faunal variations and chronology

QUATERNARY

RESEARCH

21, 385-403 (1984)

Late Quaternary Sapropel Sediments in the Eastern Mediterranean Sea: Fauna1 Variations and Chronology DAVID R. MUERDTER,*-1 JAMES P. KENNETT,* *Graduate School of Oceanography, and TDepartment of Geology,

University University

AND ROBERT

of Rhode Island, of South Carolina,

Narragansett, Columbia,

C. THuNELLI’

Rhode Island 02882-1197 South Carolina 29208

Received August 29, 1983 Distinctive planktonic foraminiferal assemblages which characterize particular late Quatemary sapropel layers in deep basin sediments from the eastern Mediterranean Sea have been identified using cluster analysis. Three distinct clusters allow for identification and intercore correlation of the nine sapropels deposited during the last 250,000 yr. Cluster 1, representing sapropel layers S 1 and S9, exhibits low abundances of Neogloboquadrina dutertrei and high abundances of Globigerinoides ruber; Cluster 2, which groups S3, S5, and S7, contains high abundances of G. ruber. N. dutertrei, and Globigerina bulloides, and Cluster 3, which includes samples from S4, S6, and S8, is marked by extremely abundant N. dutertrei and G. bulloides, and rare G. ruber. Analysis of sedimentation rates in 14 cores reveals the following approximate ages for the sapropel layers: S2 = 52,000 yr B.P.; S3 = 81,000-78,000 yr B.P.; S4 = lOO,OOO-98,000 yr B.P.; and S5 = 125,000116,000 yr B.P. As previously suggested, sedimentation rates on the Mediterranean Ridge were determined to be relatively constant during the last 127,000 yr. In contrast, basin sedimentation rates have fluctuated markedly from lower rates during interglacial stage 5 to higher rates during the last glacial episode. These glacial/interglacial differences are most pronounced in the northern Ionian Basin, because of increased terrigenous sediment deposition during glacial episodes. Unusually high biogenic sedimentation rates occurred in an arc south of Crete during the deposition of sapropel S5, probably due to higher productivity in this region.

INTRODUCTION

Distinct black, organic-rich layers of a few centimeters to a few tens of centimeters thick occur in deep-sea Quaternary sediment sequences from the eastern Mediterranean basin (Kullenberg, 1951; Olausson, 1960). These sapropel layers were deposited at water depths greater than 600- 1000 m (Figs. 1 and 2) during synchronous basinwide episodes of anoxia (Olausson, 1961; Ryan, 1972; McCoy, 1974; von Straaten, 1972; Stanley, 1978). The increased stratification of the water column that caused these anoxic episodes has been linked to increased surface water temperatures (von Straaten, 1972; Nesteroff, 1973) or to decreased surface water salinities caused by (1) increased rainfall in the Mediterrat Current address: AMOCO Production Company, 1340 Poydras St., P.O. Box 50879, New Orleans, La. 70150.

nean region (Bradley, 1938; Kullenberg, 1951; Fairbridge, 1972); (2) glacial meltwater input (Olausson, 1961; Ryan, 1972; Thunell et al., 1977; Vergnaud-Grazzini et al., 1977; Thunell and Williams, 1982); (3) rising sea level reconnecting the Black Sea and causing fresh water to flush into the eastern Mediterranean (Olausson, 1961; Ryan, 1972; Thunell et al., 1983); (4) increased fresh water inflow from the Nile River due to African monsoons (Rossignol-Strick et al., 1982; Rossignol-Strick, 1983); or (5) increased inflow of less saline Atlantic water associated with a sea-level rise (Muller, 1973). Kullenberg (1951), Muller (1973), and Jenkins and Williams (1981) have also suggested that an increase in salinity within the entire basin may have immediately preceded sapropel formation and thus as-

385 0033-5894/84 $3.00 Copyright 0 1984 by the University of Washington. All rights of reproduction in any form reserved.

MUERDTER,

386

KENNETT,

AND THUNELL

FIG. 1. Bathymetry of the eastern Mediterranean

&ted in the development of stratification when fresher water was later introduced by any of the above mechanisms. This paper investigates planktonic fGY miniferal assemblages associated .___-___._._I -..-with -. ----- late Quaternary.ss-rosellayers and their chro-_-_ .~

.l-,e.Y

Sea (modified from Carter et al., 1972).

nology. A cluster analysis is used to investigate the differences in planktonic foraminiferal faunas contained within individual sapropels. The ages of the anoxic events that produced the sapropels were determined by averaging the ages of particular

-,I-

19,

. -

-0

ni9- O &&AIION (UNOETERMINEO

RATE

mALYb 1*2

TR171-27

OETERMlN

FIG.

+ALrl187

ALBtlJ8

RCS-181

2. Location of cores used in this study

. .

nc9-

P6510-4

MEDITERRANEAN

sapropel layers from all the cores by integration with a consistent fauna1 or isotope stratigraphy and tephrochronology. General

Sapropel

Characteristics

Previous sedimentological, geochemical, and micropaleontological studies have demonstrated that Quaternary sapropel layers of the eastern Mediterranean Basin have the following general characteristics: (1) They contain a high abundance of organic carbon. Sigl et al. (1978) have defined a sapropel as having an organic carbon content greater than 2%. Sapropels are known to have organic carbon values as high as 16% in a Pliocene sapropel (Sigl et al., 1978). (2) They are usually

SAPROPELS

387

to surrounding sediments (Sigl et al., 1978; Cita et al., 1982). (6) Depletion of #*O occurs in tests of planktonic foraminifera during sapropel formation, an indication of either a salinity decrease or temperature increase in surface waters (Cita et al., 1977; Vergnaud-Grazzini et al., 1977; Williams and Thunell, 1979; Thunell and Williams, 1982; Rossignol-Strick er al., 1982; Thunell et al., 1983). (7) There is increased mineral alteration resulting from the greater organic content. Clays tend to be degraded toward mixedlayer types or chlorite or completely destroyed (Sigl et al., 1976). This degradation lowers the abundance of certain major compounds such as SiO,, Al,O,, TiO,, and K,O compared with the nonsapropel marls and oozes in the same cores (Charnley, 1971, Cita et al., 1977, 1982). (8) Abundant pyrite occurs mainly within the sapropel layers while marcasite occurs below and sometimes near the top of the layers, where reducing conditions were less intense (Robert, 1974; Cita et al., 1977). Crystals of gypsum, often found in or near sapropel layers, form in situ in the deep eastern Mediterranean, possibly produced by bacterial activity in partially reduced environments (Cita et al., 1977). (9) Nine sapropels were deposited during the last 250,000 years (Ryan, 1972) with most occurring during interglacial episodes. Sapropels have been recognized in sediments as old as the middle Miocene in deep-drilled sections (Kidd et al., 1978). The thickness of the layers indicates that each stagnation event lasted a few thousand years.

laminated and underlain by a gray protosapropel zone and overlain by an oxidized layer (Maldonado and Stanley, 1976). The lack of this sequence, associated with several sapropels, is probably the result of complexities such as erosion, nondeposition, or a coring disturbance . (3) They lack benthic microfossils and evidence of bioturbation so that individual lamina and sharp contacts of sapropel layers are preserved. Some sapropels do, however, occasionally contain benthic foraminifera (Parker, 1958; Cita and Podenzani, 1980; Mullineaux and Lohmann, 1981). (4) They contain an unusual planktonic foraminiferal assemblage usually dominated by Neogloboquadrina dutertrei (Kullenberg, 1951; Parker, 1958; Olausson, 1961; Cita et al., 1977, 1982; Thunell et al., 1977), a low salinity species (Ruddiman, 1971; Thunell, 1978a; Loubere, 1981). Thubetween Individual nell and Lohmann (1979) delineated this Differences Sapropel Layers fauna using factor analysis, and Muerdter (1984) used discriminant analysis to identify The sapropel notation used in this paper isochronous nonsapropel sediments with a (Fig. 3) was first employed by McCoy similar assemblage deposited in the (1974) and subsequently modified by Cita shallow, aerated Strait of Sicily. et al. (1977). In this scheme the most recent (5) There is no appreciable change in sapropel is denoted as Sl and older ones total calcium carbonate content compared are consecutively numbered back through

MUERDTER,

388 CHNBl-25 -owl3

LYII-3 FACTOR LOADING -0.8- on 0.8 *.

KENNETT, TR171-24 -o.aoa

08

AND THUNELL TR172-22

TR171-27 -080.00.8 L

-0.8 0.0 0.8

im

im

200

2m

300

4m

5m

~

-.

Ash

7m

FIG. 3. Cores analyzed by Thunell (1978b) and Muerdter (1982) (referred to as Trident data in this report) showing position of sapropel and volcanic ash layers in relation to oscillations of planktonic foraminiferal principal component analysis factor 2-a climatic indicator. Positive values of factor 2 indicate a warmer-water assemblage. Sapropel samples from these cores were analyzed using cluster analysis and marked with pattern indicating cluster allocation as shown in Figure 4.

time. A number of parameters do differ between individual sapropel layers and include the following: (1) Thicknesses vary substantially over the geographic range of an individual layer, but the relative thickness between the various layers remains relatively consis tent. For example, S5 and S6 are usually 3 to 10 times thicker than the other layers. (2) Layers occur within different times of the climatic cycle: during deglaciations (Sl, S5, and S9), during interglaciations (S3, S4, S7, and SB), and during a glacial episode (S6 in isotope Stage 6) (Cita ef al., 1977, 1982; Thunell et al., 1977; RossignolStrick, 1983). (3) The signature of changing organic content is distinct for individual sapropel layers, which is useful for their identification and intercore correlations (Sigl and Mullet-, 1975). For example, sapropel S5

has a distinctive bimodal profile with fluctuations between 2 and 6% organic carbon. (4) The area1 distribution of sapropel S2 is restricted compared to the basin-wide occurrence of all other sapropels. S2 rarely occurs in the Ionian Basin and is thin and often bioturbated in the Levantine Basin (Ryan, 1972). A general thinning trend in sapropel layers from east to west has been noted by ThunelI et al. (1977) in a suite of Trident cores. They suggested that some sapropels are absent from the westernmost Ionian Basin based on the examination of two cores, although Stanley (1978) has reported many cores with sapropel layers from the entire Ionian Basin. Mangini and Dominik (1982) present 23sTh evidence for a large hiatus that may account for the absence of sapropel layers from one of the sapropel-free cores studied by Thunell et al. (1977).

MEDITERRANEAN

(5) Diatom abundances vary greatly between individual sapropel layers. In general, diatoms are rare in Mediterranean sediments, but Olausson (1961), Schrader and Matherne (1981), and Thunell and Williams (1982) observed that a sapropel layer (now identified as S5) contains abundant diatoms in some cores from the Levantine Basin. A few diatoms were also observed by Schrader and Matherne (1981) in sapropel s4. (6) Planktonic foraminiferal faunas differ between the various layers as noted by Olausson (1961) using the data of Parker (1958). Some of the fauna1 differences are directly attributable to the prevailing climatic conditions during sapropel deposition (Thunell and Williams, 1982; Cita et al., 1982; Thunell et al., 1983). The abundance of epipelagic and mesopelagic foraminiferal species also varies between saprope1 layers (Cita et al., 1977, 1982; Williams and Thunell, 1979). Thunell et al. (1977) also observed a lack of N. dutertrei in sapropel Sl. (7) Cita et al. (1977) recorded differences in simple diversity of the planktonic foraminifera between different sapropel layers, with fluctuations from a high (23 species) in S7 to a low (12 species) in S6. Controversy over Chronology of Sapropels

In the past, some controversy has existed over the ages assigned to several sapropel layers. The youngest layer (Sl) has been radiocarbon dated at between 7500 and 9000 yr B.P. (von Straaten, 1972; Maldonado and Stanley, 1976). However, Rossignol-Strick et al. (1982) have reported a core with two distinct sapropel layers of Holocene age separated by a layer of gray nanno ooze. The layers are dated at about 11,800- 10,400 yr B.P. and from 9000-8000 yr B.P., respectively. The base of an underlying protosapropel layer was dated at 16,000 yr B.P. The recent study of Buckley et al. (1982) in which a radiocarbon age of

SAPROPELS

389

10,400 yr B.P. was reported for an interval immediately below the most recent saprope1 would tend to support an age of approximately 9000 yr B .P. for the base of S 1. Controversy also exists over the age of older sapropel layers with two main views. A “younger” stratigraphy arrives at much younger ages for the sapropels based either upon radiocarbon dates and extrapolated sedimentation rates (Blanc-Vernet and Charnley, 1971; Hieke, 1976; Maldonado and Stanley, 1976),or upon an interpretation of a distinct paleoclimatic shift immediately before S5, as the isotope Stage 413 boundary which is dated at approximately 60,000 yr B.P. (Parker, 1958; Hieke, 1976). Maldonado and Stanley (1976) assigned ages of from 25,000 to 23,000 yr B.P. for S2 and from 41,000 to 38,000 yr B.P. for S3. An alternate “older” stratigraphy is based upon the recognition of the sharp paleoclimatic and oxygen isotopic change, immediately below S5, as Termination II which is dated at 127,000 yr B.P. (Broecker and Van Donk, 1970; Hays et al., 1976; Kominz et al., 1979). This older stratigraphy, first proposed by Ryan (1972), is now the more commonly used chronology and is supported by the following internally consistent evidence: (1) additional fauna1 and oxygen isotope studies (Cita et al., 1977, 1982; Thunell et al., 1977, 1983; Vergnaud-Grazzini et al., 1977); (2) tephrochronology, correlations with U/Th-dated raised beaches, and by first and last appearances of certain species of calcareous nannoplankton (Cita and Ryan, 1978; Keller et al., 1978; Thunell et al., 1979; Cita et al., 1982; Blechschmidt et al., 1982); and (3) independent dating, using the 230Th excess method, of Termination II providing an age of 130,000 to 124,000 yr B.P. (Dominik and Mangini, 1979). However, even within the context of the older stratigraphy, different ages have been assigned to a number of sapropel layers,

390

MUERDTER,

KENNETT,

especially the younger ones. S5 is the least controversial, with the sapropel interval beginning at 125,000 yr B.P. and ending at approximately 113,000 yr BP (Ryan, 1972; Keller et al., 1978). The midpoints of sapropels S2, S3, and S4 were dated at approximately 48,000, 78,000, and 100,000 yr B.P., respectively, by Ryan (1972), Cita et al. (1977), and Thunell et al. (1979)) based upon sedimentation rate calculations assuming an age of 126,000 yr B.P. for Termination II. Dominik and Mangini (1979), using the 230Th excess method, estimated the age of sapropel S3 as 76,000-74,000 yr B.P. Keller et al. (1978) and McCoy (1980) assigned much older ages of 54,000 (S2), 90,000 (S3), and 108,000 (S4) yr B.P. based upon the position of sapropels within Substages .5b and 5d in the planktonic foraminiferal paleoclimatic curve. However, this curve is strongly affected by an unusual planktonic foraminiferal assemblage associated with sapropel events. In particular, large increases in frequencies of N. dutertrei in sapropel layers due to salinity changes reduced the relative abundance of warmer-water species, and hence biased the paleotemperature estimates. METHODS AND MATERIALS The planktonic foraminiferal data used in this study are from the reports of the Swedish Deep Sea Expedition, R/V Albatross cores (Parker, 1958), the work of Thunell (1978b), and new work on additional cores. A total of 17 cores from the eastern Mediterranean were used in a cluster analysis and an additional 14 cores for dating the sapropel layers (Fig. 2). The data cover the time interval since 250,000 yr B.P. Parker’s (1958) data have been treated separately because of taxonomic differences. Like Thunell (1978b) we counted the planktonic foraminifera in the > 150-p,m fraction. We also used the same taxonomy. A total of 34 samples was counted over 19 individual sapropel layers in five cores and was used in a cluster analysis (Fig. 3). This is referred to as Trident data after the vessel

AND

THUNELL

from which most of these cores were collected. Core locations, water depths, and core lengths are given in Table 1. The raw data is included in Muerdter (1982). Parker (1958) also counted the planktonic foraminifera in the 150~km fraction of samples from 12 Albatross cores from the eastern Mediterranean basin that contain sapropel layers. Sapropel samples were identified by matching the core depths of the samples counted by Parker (1958) with the position of sapropel layers described by Olausson (1960). This is referred to as the Albatross data. Of 62 sapropel samples identified, 7 were rejected because of apparent fauna1 mixing with nonsapropel assemblages. These sample rejections were based upon one or more of the following criteria: (1) >40 benthic foraminiferal specimens in the sample; (2) > 10% Globorotalia injlata; or (3) >lO% Globorotalia truncatulinoides. Sapropel layers usually have very low frequencies of G. injlata and G. truncatulinoides and lack benthic foraminifera (Cita et al., 1977; Williams and Thunell, 1979). The 12 Albatross cores contain a total of 63 individual sapropel layers, of which 48 were sampled and counted by Parker (1958). Five layers were sampled twice and one three times yielding a total of 55 samples used in our analyses. Since Parker’s report in 1958, the following additional species are generally recognized in quantitative fauna1 studies: Globigerina rubescens, Globigerina calida, and Globigerina falconensis. Forms identified as Globigerina radians by Parker (1958) have been grouped within Globigerina quinqueloba in our study. We have divided the Neogloboquadrina plexus dif-

ferently than Parker (1958). Specimens with five or more chambers in the final whorl were assigned to N. dutertrei (G. eggeri) by Parker (1958). In contrast, we assigned to N. dutertrei those specimens which exhibit an open umbilicus and 4.5 chambers in the final whorl. The cluster and discriminant function analyses used are part of the Biomedical Data Package (BMDP) and Statistical

MEDITERRANEAN

TABLE

Core

1. CORE LOCATIONS,

ID

WATER

391

SAPROPELS

DEPTHS, AND LENGTHS

OF CORES USED IN THIS STUDY

Latitude

Longitude

Water depth Cm)

Core length (cm)

35”12.3’N 35”02’N 34’03’N 33’50’N 35”19’N

16”31.1’E 16”42’E 22”43’E 25’59’E 29”Ol’E

1460 2432 2380 2680 3150

732 575 800 645 520

36’13’N 35’47’N 35’41’N 35”52’N 34”48’N 34’12’N 34”36’N 33’54’N 33”54’N 33’59’N

17”28’E 20”42’E 21”50’E 21’53’E 23’29’E 24”22’E 25”59’E 26”lO’E 28”29’E 31’02’E

3555 2940 4270 3665 3000 2130 2680 2900 2664 2500

1151 735 655 935 721 624 861 938 843 952

and S5 investigation 17”17’E 18”02’E 19”13.7’E 19”41’E 20”09’E 20’=‘06’E 20”43’E 23’25’E 23”25.4’E 23’35.8’E 25”OO.S’E 27’10.9’E 32”23.1’E 32”OO’E

3035 2345 1712 3378 2800 2858 2890 2695 2684 1794 1028 2604 1397 1634

1000 917 658 820 1630 902 830 823 883 1082 1028 764 737 >840

Trigger core length (cm)

Trident data CHN 61-25 LYII-3 TR171-24 TR171-27 TR172-22

55

Albatross data ALB ALB ALB ALB ALB ALB ALB ALB ALB ALB

198 197 196 195 194 193 192 190 189 187

Other cores V lo-69 RC9-191 RC9-190 RC9-189 KS09 RC9-185 V lo-67 V lo-65 RC9-183 RC9- 182 RC9-181 RC9- 179 RC9- 174 P 6510-4

used in chronology 37”135’N 38”11.6’N 38”39.3’N 36”58.7’N 35”09’N 3427.1 ‘N 35”42’N 34’37’N 34”29.8’N 33’48.2’N 33’24.9’N 34”16.3’N 32”57.7’N 32”50’N

Package for the Social Sciences (SPSS), respectively. In the cluster analysis, the input data were not standardized before using the x2 calculating procedure. The discriminant function was calculated by a stepwise selection method using Rao’s V, a generalized distance measure, as the inclusion criterion. Probabilities of group membership were adjusted to the average percentage of the input data. RESULTS AND DISCUSSION Fauna1 Characteristics of Sapropels Cluster analysis-Trident data. Fauna1 differences between individual sapropel layers were determined using cluster anal-

15 38 49 19 33 26 47 40 38 39 36 >150

ysis. Sapropels (Sl to S9) were identified (Fig. 3) based on their stratigraphic position within the faunal paleoclimatic curves produced by a factor analysis of the fauna1 census data (Thunell et al., 1977; Muerdter and Kennett, 1984). Sapropel Sl occurs near the top of four of the cores and S.5 occurs immediately above Termination II. The solitary sapropel in core CHN61-25 occurs in a glacial episode and thus was assigned to S6. Cluster analysis divided these data into three major groups with one outlying sample (Fig. 4 and Table 2). The first group (Cluster 1) is composed exclusively of samples from sapropel S 1. The third cluster

MUERDTER,

392

KENNETT,

AND THUNELL

AMALGAMATION

DISTANCE

FIG. 4. Cluster analysis dendrogram of planktonic foraminiferal data of sapropel layers from Thunell (1978b) and Muerdter (1982). The different clusters are labeled and marked with the patterns which are used in Figure 3. The sole outlier sample was dismissed from further study because it contained >lO% G. in&m, a species rarely found in unbioturbated sapropel.

contains all of the S6 samples and the single S8 sample. Cluster 2 contains all of the other samples from S3, S4, S5, and S7. The only sample not grouped, TR172-22,26 cm, exhibits a fauna uncharacteristic of saprope1 layers (22% G. influta) and thus was eliminated from further analysis. Samples TABLE Trident

from the single sapropel layer in CHN61-25 are grouped with other S6 layers, confirming the original assignment of this saprope1 to S6. The average percentage frequencies of the predominant species in each of the clusters (Table 3) show the main differences be-

2. SAPROPELLAYERSINCLUDEDINEACHCLUSTER Albatross

data

Cluster 1

Sl

Cluster 1

Cluster 2

S3, S4, SS, and s7 S6 and S8

Cluster 2

Cluster 3

Cluster 3 Outliers

data Sl and S9 (and 3 samples from basal part of SS layer) S3 and S5 S4, S6, and S8 S7 (and 2 samples from S3 layer from western Ionian Basin cores)

percentage

bulloides pachyderma quinqueloba glutinata aequilateralis dutertrei universa

G. N. G. G. G. N. 0.

’ Average

injlata truncatulinoides scitula ruber sacculifer tenellus

G. G. G. G. G. G.

(2.4) (1.8) (0.9) (2.3)

6.9 7.0 1.2 5.0

of each species

(0) (0)

in each

cluster

1.1 1.5 16.2 5.5 0.7 3.5 3.0 23.2 8.0

3.9 (5.7) 1.1 (0.9) 9.0 (3.5)

0 0

31.2 (13.8)

63.4 (7.6)

(1.9)

(0) (2.1)

3 (0.8)

deviation

1.0 (2.3) 6.8 (4.7) 24.4 (11.7) 0 (0) 1.9 (1.8) 6.9 (4.1) 3.0 (2.2) 1.3 (1.1) 2.9 (2.5)

47.1 (12.6) 0.6 7.0 20.3 0.7 1.8 3.1 1.5 20.2 4.4

(1.5) (5.1) (9.6) (0.7) (2.8) (4.2) (1.2) (5.0) (3.1)

34.6 (8.8)

5.0 (4.5)

0.8

(1.4)

2 0.1 (0.1) 0 (0)

Cluster 3 (0.6) (0) (5.0)

1.1 (1.7) (0) 0 0.4 (0.8) 16.3 (11.8) 1.6 (1.7) 18.5 (9.5) 0.7 (1.3) 0.6 (2.1) 55.0 (13.6) 1.9 (1.3)

0 3.1

0.2

Albatross data

1 0.9 (1.1) 2.3 (3.4)

OF CLUSTERS

of the average).

COMPOSITION

0 (0) 0.1 (0) 20.1 (10.5) 17.8 (10.7) 13.7 (6.8) 4.0 (3.2) 0.4 (0.2) 36.5 (19.9) 1.1 (1.0)

1.0

0 2.9

0.4

3. FAUNAL

(standard

(2.8) (0) (8.0) (6.1) (1.0) (3.1) (1.2) (8.7) (5.1)

(0) (1.7)

0 2.1

0.1 (0.2) 0 (9)

2 (1.8)

1.2

1 0.2 (0.5)

Cluster

Trident data

TABLE

1.3 3.2 17.4 3.4 3.3 3.3 1.5 30.6 7.8

3.3 4.1 2.0 18.5

(3.6) (3.8) (3.1) (7.4) (2.8) (3.5) (15.2) (5.8) (2.0) (3.8) (1.7) (13.9) (6.6)

Outliers

$

g 3 %

$

3

E P

8 q

394

MUERDTER,

KENNETT,

AND THUNELL

tween the groups. Cluster 1 has very low batross data, but unlike the Trident data, frequencies of N. dutertrei (average 1%) the identities of individual sapropel layers and exceptionally high frequencies of G. represented by each sample were not posruber (63%). Cluster 3 is unusually low in itively known. This analysis grouped the 55 G. ruber and high in N. dutertrei, N. pach- samples into three major clusters (Fig. 5) yderma, G. bulloides, and G. quinqueloba. labeled Cl, C2, and C3. Eight samples that In the western Ionian Sea N. pachyderma did not cluster with these groups have been is prevalent (14-34%) during S6, but this designated as outliers (OL) in Figures 4 and cool-water form does not show any corre- 5. The most important differences between sponding increase in the more eastern Lev- the clusters are shown in Table 3. The clusantine Basin cores. This reflects a temperters show a general pattern down the cores ature-related fauna1 gradient similar to that (Fig. 6), most typically observed in ALB existing in the modem eastern Mediterra189 in which sapropel-related faunas from nean Basin (Thunell, 1978a). The other sap- top to bottom are C2, C3, C2, C3, OL, C3, rope1 layers exhibit intermediate fauna1 and C 1. This pattern, with some variations, values between the extremes of Sl and S6. is generally repeated in ALB 187, 188, 190, Cluster analysis-Albatross data. Clus194, 195, and 198. ter analysis was also carried out on the AlTo link the clusters to specific sapropel layers, the well-studied core ALB 189 was used. This core has been indirectly dated AMALGAMATION DISTANCE using oscillations in planktonic foraminiferal assemblages (Parker, 1958; Ryan, 1972) and oxygen isotope stratigraphy (Emiliani, 1955; Vergnaud-Grazzini et al., 1977). Ryan (1972) and Vergnaud-Grazzini et al. (1977) reinterpreted the climatic stages of Parker (1958) in this core, with Stage 3 becoming Stage 5, Stage 4 becoming Stage 6, and Stage 5 becoming Stage 7 (Fig. 7). Vergnaud-Grazzini and others (1977) have identified sapropel layers S2 through Sl 1 in core ALB 189 (Fig. 7) and Ryan (1972) has identified saprope1 Sl in core ALB 194. This enables the clusters to be related to the individual saprope1 layers as indicated in Table 2. Using these relationships, the sapropel layers in the other Albatross cores were identified based on general spacing between sapropels and on correlations with fauna1 paleoclimatic records (Fig. 6). Nine of the twelve cores contain a very consistent pattern, while three (ALB 191, 193, and 197) are difficult to interpret. Two of these cores (ALB 191 and 193) each contain only one J major fauna1 change and one sapropel, thus FIG. 5. Cluster analysis dendrogram of planktonic providing insufficient evidence for accurate foraminiferal data of sapropel layers from Parker stratigraphic placement. Olausson (1961) (1958).

MEDITERRANEAN 196

197

196

195

194

395

SAPROPELS

193

192

191

I90

169

I66

I87

1 .

2 ?. 2 ? -2

2

s3 -.) .““‘3 54 ..:...

-1, s3 ---

’ ..1111 55

5

4 2. .I.

s4 -3 s5

-v7. I S8 ::a& 2” -

;;;;;R 56 -;

57 -__

5 b 7 5

c d

FIG. 6. Albatross cores (Parker, 1958) showing isotopic stage assignments of Olausson (1960) and this study, position of ash and sapropel layers, and identification of the fauna1 cluster (from Fig. 5) determined for each sapropel layer.

described core ALB 197 as being disturbed, with the sapropel at 142-150 cm appearing to have been redeposited. Therefore saprope1 identifications for ALB 197 (Fig. 6) are tentative, although their sequence can CLIMATIC FAUNAL CURVE

G.RUBER ISOTOPIC CURVE

c Ot’T

2 I

8’801*/..l 0 -2 I

I<

FIG. 7. Paleoclimatic curve aminifera and oxygen isotopic ysis of G. ruber for core ALB et al., 1977). Sapropel layers

based on planktonic forcurve based upon anal189 (Vergnaud-Grazzini are identified.

be generally correlated with the fauna1 data. Several sapropel layers cluster anomalously: Cluster 1 contains several S5 layers; and two S3 layers are not included in any of the three clusters. The S3 samples that are ungrouped are from Ionian Basin cores, whereas S3 layers from the Levantine Basin cores are grouped with S2. The three S5 layers that are grouped in Cluster 1 are from within the bottom part of their respective layers compared with the upper samples which are grouped in Cluster 2. The significance of these differences is discussed below. Comparison of cluster analyses. The clusters found in the two sets of data are fairly similar (Table 2), despite the taxonomic differences. The major difference in sapropel assignments is the inclusion of S4 in Cluster 3 in the Albatross data instead of in Cluster 2 as in the Trident data. The two S7 samples in the Trident data are assigned to Cluster 2 whereas S7 samples are outliers in the Albatross data.

396

MUERDTER,

KENNETT,

As expected, the fauna1 composition of the clusters is similar for both sets of data (Table 3). Cluster 1 is dominated by G. ruber with N. dutertrei conspicuously absent. Cluster 3 exhibits opposite relations with N. dutertrei dominant, but also with high frequencies of G. bulloides and G. quinqueloba. Cluster 2 is intermediate between these extremes, and N. dutertrei. The outliers in the Albatross data contain G. injlatu (average of 3%) and G. truncatulinoides (average of 4%), forms that are absent from most other layers. Several of the outliers also exhibit high frequencies of Orbulina universa or G. glutinata. Several of the differences between clusters can be explained by fauna1 gradients that occurred across the eastern Mediterranean Basin during times of sapropel formation. In the present day, N. pachyderma is sparse in the Levantine Basin, but increases in frequency up to 10% in the western Ionian Basin (Thunell, 1978a). Likewise, during deposition of S6, N. pachyderma was more abundant in cores from the extreme western side of the basin (CHN61-25 and LY3) compared with the more easterly core locations (TR171-24 and 171-27 and ALB 187-194). The difference in average frequencies of N. pachyderma in Cluster 3 between the two data sets (Table 3) results from the samples contained within each group; the Trident data contain six of its eight S6 samples from CHN61-25 and Lynch H-3, whereas five of the eight Albatross samples are from east of the Ionian Basin. The east to west gradient of N. pachyderma is also reflected in the two S3 samples of the Albatross data that are outliers. Both samples are from Ionian Basin cores and contain high frequencies of N. pachyderma that preclude their placement within any of the three clusters. Differences between the two data sets in the frequencies of G. bulloides, G. aequilateralis, and G. tenellus are due in part to taxonomic differences, since G. fulconensis and G. calida were not recognized at the time of Parker’s (1958) study.

AND

THUNELL

Chronology

of Sapropels

To help assist with the age assignment of individual sapropel layers, we undertook an examination of the stratigraphy of many of the available cores containing sapropels. The ages of sapropel layers in cores which had climatic curves based on oxygen isotopic and/or fauna1 data were first analyzed. Then, using a correlation technique, other cores not having an isotopic or fauna1 stratigraphy were examined. Four of the cores from the eastern Mediterranean for which an oxygen isotope stratigraphy exists include: ALB 189 by Emiliani (1955); KS09 by Cita et al. (1977); RC9-181 by Vergnaud-Grazzini et al. (1977) and TR171-22 by Thunell et al. (1977). These cores were plotted on a sedimentation rate diagram (Fig. 8) using datums with the following ages: 4s (xldyr B.P.)

Datum (1) Sapropel S 1 (2) Y-5 Ash Layer (formerly called Ischia Tephra) (3) Termination

II

Reference

8.2 von Straaten (1972) 40 Cita et al. (1977) and Thunell et al. (1979) 127 Broecker and Van Donk (1970)

Also plotted are Termination I at 11,000 yr B.P. and the last glacial maximum at 18,000 yr B.P. Cores TR172-22 and RC9-181 contain all three horizons which fall on the same sedimentation-rate curve when the Y5 Ash Layer is assigned an age of 40,000 yr B.P. KS09 contains a hiatus at the top as noted by Cita et al. (1977). Our work (Figs. 3 and 8) shows that the base of sapropel S5 was deposited close to 125,000 yr ago. This is in agreement with previous age estimates for this sapropel (Ryan, 1972; Cita et al., 1977, 1982; Thunell et al., 1977, 1979; Vergnaud-Grazzini et al.,

MEDITERRANEAN

700 1 800 FIG. 8. Sedimentation rate diagram for four eastern Mediterranean cores that have oxygen isotope stratigraphy. The horizons used to construct the diagram are sapropel Sl at 8200 yr BP., Y-5 Ash Layer at 40,000 yr BP, and Termination II at 127,000 yr B.P. Termination I at 11,000 yr B .P. and the maximum of the last glaciation at 18,000 yr B.P. are also plotted. The bold portions of the lines represent position of sapropel layers.

1977; Keller et al., 1978; Thunell and Williams, 1982). Sedimentation rates for Albatross, R/V Robert Conrad, R/V Vema, and RN Trident cores were analyzed using the three datums: Sl, Y-5 Ash Layer, and the base of S5 (Figs. 8 and 9). The average age for each sapropel was calculated from the dates obtained in the 14 cores (Table 4)

FIG. 9. Sedimentation rate diagrams for eastern Mediterranean cores exhibiting near constant sedimentation rates. Same datums used as those in Figure 8.

SAPROPELS

397

and agree with those previously reported by Ryan (1972), Cita et al. (1977, 1982), and Thunell et al. (1979). Dominik and Mangini (1979) concluded, using 23@Thdata from four Ionian Sea cores, that sedimentation rates have changed markedly with time in the eastern Mediterranean, with higher rates during glacial episodes and lower rates during interglacial episodes. Ryan (1972) noted a similar change but only in several cores from the northern Ionian Sea. In contrast, the four eastern Mediterranean cores for which oxygen isotopic data are available (Fig. 8) all show rather constant sedimentation rates during glacial through interglacial episodes, as do several other cores (Fig. 9). Relatively constant sedimentation rates in this region have previouisly been observed by Thunell et al. (1979) and more recently by Cita et al. (1982). Two possible explanations for these differences are an incorrect age assignment for the Y-5 Ash Layer or regional differences in sedimentation patterns. A number of different ages have been assigned to the Y-5 Ash Layer including 24,000 yr B.P. (Ninkovitch and Heezen, 1967 ; Vergnaud-Grazzini and Herman-Rosenberg, 1969; Keller, 1971; Hieke, 1976; Stanley, 1978); ~27,000 yr B.P. (Fan-and, 1977); 38,000 yr B.P. (Thunell et al., 1978); 40,000 yr B.P. (Cita et al., 1977, 1982); and 41,000 yr B.P. (Delitala et al., 1972). Much of this discrepancy over the age of the Y-5 Ash Layer can be attributed to improper identification of the source of this tephra layer. This unit is often referred to as the “Ischia Tephra” because it was believed to be correlative with the Citara-Serrara tuff on Ischia Island (Keller, 1971; Richardson and Ninkovich, 1976) which has been dated at around 24,000 yr BP However, more recent geochemical work (Thunell et al., 1979) indicates that the Y-5 Ash Layer is actually correlative with the Campanian ignimbrite, which has been dated at 35,000 to 40,000 yr B.P. (Alessio et al., 1971-1974; DiGirolamo and Keller, 1972). As men-

398

MUERDTER, TABLE Sapropel Si s2 s3 s4 SS

KENNETT,

4. AVERAGE

AND THUNELL

AGE OF SAPROPEL LAYERS

Average age

Data source

9000 to 7500 yr B.P. 52,000 yr B.P. 80,600-77,900 yr B.P. 100,500-98,200 yr B.P. 125,000-115,000 yr B.P.

von Straaten (1972) average of 4 samples average of 14 cores average of 13 cores average of 14 cores

tioned above, an age of 40,000 yr B.P. is consistent with a constant sedimentation rate in many eastern Mediterranean cores (Figs. 8 and 9). A hypothetical sedimentation-rate diagram illustrates the implications of using the 24,000 yr B.P. age compared with the 40,000 yr B.P. age for the Y5 Ash Layer (Fig. IO). The diagram was constructed using an age of 8000 yr B .P. for Sl as discussed above, 127,000 yr B.P. for the base of SS, and alternate ages of 24,000 or 40,000 yr B.P. for the Y-5 Ash Layer. An age of 24,000 yr BP. for the Y-5 Ash Layer provides highly variable sedimentation rates (Fig. lo), with highest rates between 35,000 and 8000 yr B.P. Since this interval includes the last glacial maximum with low

FIG. 10. Hypotheticalsedimentationratediagram showing differences of sapropel ages derived by selection of either 25,000 or 40,000 yr B.P. for the age of the Y-5 Ash Layer. Scenario 1 indicates the shift in sapropel ages if the high sedimentation rate began about 35,000 yr B.P. (approximately 2/3 boundary). Scenario 2 shows a lesser shift if the higher rate began at 24,000 yr B.P.

stands of sea level, such a scenario would support the concept of Mangini and Dominik (1982) of higher rates of sedimentation during low stands of sea level. On the other hand, such age assignments would place S3 in isotope Stage 3 or 4, in conflict with the warm assemblages that occur before and after this layer (Ryan, 1972; Cita et at., 1977; Thunell et al., 1977; Muerdter and Kennett, 1984). S2 and S4 would also be displaced away from positions consistent with paleoclimatic data. Even if higher sedimentation rates extended only back to 24,000 yr (Fig. 10; scenario 2), S3 would have an age of about 70,000 yr B.P. and be incorrectly placed within glacial Stage 4. Therefore, constant sedimentation rates and an age of 40,000 yr for the Y-5 Ash Layer appear to be more consistent with our data. The distribution of late Quarternary sediments that seem to exhibit constant sedimentation rates is, however, not basin wide. Constant sedimentation mostly occurs in the western Levantine Basin and south of 35”N in the eastern Levantine Basin, especially associated with the Mediterranean Ridge which is protected from turbidite deposition. In the northern Ionian Basin, clear changes occurred in sedimentation rates between glacial and interglacial episodes (Fig. 11). Cores exhibiting variable sedimentation rates have been collected from this basin (Dominik and Mangini, 1979). Increased sedimentation rates during glacial episodes resulted from increased sediment input into this basin during low stands of sea level (Cita and Ryan, 1978).

MEDITERRANEAN AGE

(IO3

years1

FIG. 11. Sedimentation rate diagram for cores exhibiting different rates between glacial and interglacial episodes. Same datums used as those in Figure 8.

Increased Biogenic ProductivitySapropel S5

Sapropel S5 is unusual because of the distinctly variable thickness over its geographic range and faunal variations through the layer (Thunell and Williams, 1982). Samples from S 1, S9, and the lower part of SS exhibit low frequencies of N. dutertrei 15-z

2OQ

399

SAPROPELS

and very high frequencies of G. ruber and are grouped together in Cluster 1. Conditions affecting the distribution of this planktonic foraminifera must have been similar during all three of these sapropel intervals. All occur near the end of rapid deglaciations. However, anoxic conditions which produced sapropel SS lasted longer than during Sl or S9, as indicated by the much greater thickness of S5. The planktonic foraminiferal assemblage in the upper part of S5 is distinctly different than that in the basal portion, by containing abundant N. dutertrei, G. ruber, and 0. universa. This assemblage is grouped with S3 in Cluster 2. As pointed out by Thunell and Williams (1982), surface water conditions must have changed significantly during the deposition of S5 to create such large-scale fauna1 changes between its lowermost and upper parts. Large variations in the thickness of S5 are apparent with the greatest thicknesses occurring south of Crete and in the northern and eastern Levantine Sea (Fig. 12). To eliminate differences in thickness due to variations in average sedimentation 25*

300

35’E

FIG. 12. Thickness (in cm) of the S5 sapropel layer in sediment cores from the eastern Mediterranean Basin as well as the thickness of the layer divided by the average sedimentation rate for the core. These data define an arc of greatest relative thickness south of Crete.

400

MUERDTER,

KENNETT,

rate of each core, the thickness of S5 was divided by the average sedimentation rate for the core (Figs. 8 and 9). Resulting high values are distributed in an arc south of Crete (Fig. 12). These anomalously higher sapropel thicknesses could have been caused by either (1) differences in the duration of the sapropel intervals or (2) localized increases in sedimentation rates. Differences in the duration for sapropel formation is unlikely because of insufftcient potential isolation of the deep basins. Therefore, it is likely that there existed varying rates of sediment deposition during the formation of S5. Increased sedimentation rates could have been caused by increased terrigenous input or increased biogenic productivity. The former is unlikely because high stands of sea level at the time would have trapped terrigenous sediments on the continental shelf. Also, the localized nature of the anomaly is not consistent with any model invoking differences in eolian sediment deposition. Some evidence exists which indicates that the variable sedimentation rates over the geographic range of S5 resulted from variations in surface-water productivity. Very abundant diatom assemblages have been reported only in S5 from the anomalous areas (Schrader and Matherine, 1981; Thunell and Williams, 1982). In some cases these form thin layers of diatom ooze (Olausson, 1960, 1961). Schrader and Matherne (1981) have postulated that diatom blooms were caused by upwelling in this area. However, Thunell and Williams (1982) have delineated the geographic extent of this area using additional cores and concluded that high diatom abundances were caused by nutrient input from fresh water runoff and not from upwelling.

AND THUNELL

clusters are detected: (a) Cluster 1 includes samples from sapropels Sl, S9, and the base of S5. All these samples exhibit low frequencies of N. dutertrei. (b) Cluster 3 includes samples from sapropels S4, S6, and S8 and is distinguished by extremely abundant N. dutertrei and rare G. ruber. (c) Cluster 2 includes samples from S3, S5, and S7. It is marked by high abundances of G. ruber, N. dutertrei, and 0. universa. (2) Sapropel layers in Albatross cores (Parker, 1958) have been identified using these fauna1 clusters and associated stratigraphy. (3) During times of sapropel deposition, N. pachyderma exhibited a frequency gradient from west to east across the eastern Mediterranean like that of the Recent. Higher frequencies occur in the west in association with cooler surface waters. (4) Analysis of sedimentation rates of 14 cores produced the following approximate ages for sapropel layers. S2 S3 S4 S5

= = = =

52,000 yr 81,000-78,000 yr lOO,OOO-98,000 yr 125,000- 116,000 yr

CONCLUSIONS

Our findings also support an age of approximately 40,000 yr for the Y-5 Ash Layer. (5) Sedimentation rates between glacial and interglacial episodes are relatively constant in the area of the Mediterranean Ridge, but not in other regions. Cores from the northern Ionian Sea exhibit noticeably higher sedimentation rates during glacial episodes reflecting greater terrigenous sediment input during low stands of sea level. (6) Sedimentation rates for sapropel S5 are distinctively higher in an arc south of Crete relative to other areas in the eastern Mediterranean. This increase is in part due to higher biogenic productivity in this area, as reflected in deposition of abundant diatoms.

(1) Cluster analysis has been applied to the planktonic foraminiferal faunas associated with sapropel layers in eastern Mediterranean cores. Three climatically related

We appreciate comments from R. Heath, J. Boothroyd, and T. Glancy, and assistance from Nancy Pen-

ACKNOWLEDGMENTS

MEDITERRANEAN rose. This research was supported by U.S. National Science Foundation Grants OCE75-21262 A01 and OCE79-14594 (Marine Geology and Geophysics).

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