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Chapter 52. Mediterranean deep-sea ostracods, the species poorness of the Eastern Basin, as a legacy of an early Holocene anoxic event

Chapter 52. Mediterranean deep-sea ostracods, the species poorness of the Eastern Basin, as a legacy of an early Holocene anoxic event

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722 D. HARTEN

AND H. J. DROGTB



MATERIALS

AND METHODS

The materials for this study were collected during cruises of N/OCathtrine Laurence (Ligurian

Sea; 1981). B/O Cornide de Saavedra (Alboran and Balearic Seas; 1982), and R/V Tyro (eastern

Mediterranean; 1983) using grabs and boxcoring devices.

For analysis of fauna considered Recent, only the soft surface sediment (1-2 cm thick) was used.

Minimum sample size was ca. 50 cc of raw sediment. Most samples were processed by simple

washing, additional cleaning by boiling with soda being hardly ever necessary. The ostracods

were picked from the fraction coarser than 150 pm.

In collecting, no attempt was made to discriminate between dead and living Recent fauna.

Our data thus relate to taphocoenoses: communities of remains buried, or to be buried, together.

Owing to post-mortem transport, much of which would be downslope, taphocoenotic depth ranges

are expected to be broader than those of the corresponding biotopes.



TEXT-PIG. 1-Schematic map of the Mediterranean showing sampling sites. A: Alboran Sea traqsect (105-1,720 m).

B: Balearic Sea transect (100-2,400 m). C : Ligurian Sea transect (130-2,575 m); two stations lying to the

west of the main transect are not separately shown. D: Libyan Sea transect (165-2,291 m). E: transect off

Alexandria (100-2,880m). F: site of core 353 in the Adriatic Sea used by Van Straaten(1966), Breman (1975),

and Van der Zwaan (1980). Shaded area southwest of Sicily: submerged part of Sicilian Ridge, connecting

Europe with Africa and separating the western from the eastern Mediterranean basin.



RESULTS



Five bottom sampling transects in the Mediterranean are reported, each covering both shallow

and deep waters. Three transects are located in the western basin, the other two lie in the eastern

basin (Text-fig. 1). Identified ostracod taxa are listed in Tables 1-5; to reduce the noise of downslope contamination (the transportation of specimens to greater depth by currents and mass

transporting agents), finds of juveniles have been left out.

All transects show contrasting “shallow” and “deep” assemblages, the transition mostly OCcurring at about 250-300 m depth. The Ligurian Sea seems an exception with the first deep-sea

taxa appearing at more than 400 m. The transition is marked by a faunal turnover and a decrease

in diversity and density with greater depth. The faunal divide may be masked by the effects of



Mediterranean Deep-sea Ostracodr 123



TABLE~-ALJJORAN

SEA. OSTRACOD

DISTRIBUTTON

IN TRANSECT

(SURFACELAYER;ADULTSPECIMENS

ONLY).

depth in m

Aurila spp.

"Bairdia" spp.

Buntonia sublatissima

Celtia quadridentata

Costa edwardsi

Cytherella spp.

Eucythere sp.

Paracytheridea sp.

Pontocypris spp.

Tetracytherura angulosa

Urocythereis sp.

Xestoleberis spp.

Bythocythere sp.

Carinocythereis antiquata

Echinocythereis spp.

Krithe aff. praetexta

Loxoconcha spp.

Monoceratina oblita

Phlyctocythere pellucida

Saptocythere spp.

Bosquetina dentata

Pseudocythere caudata

Pterygocythereis spp.

Henryhowella sarsi

Cytheropteron spp.

Paracypris sp.

Rectobuntonia inflata

Macrocypris ligustica

Argilloecia acuminata

SPP

Krithe monosteracensis

Krithe sp.

Cytherella sp. Breman

Macrocypris adriatica

Microxestoleberis profunda

Parakrithe dimorpha

Pseudocythere armata

Bythocypris tenera

Bathycythere vanstraateni

Krithe keiji

Bythocypris obtusata



.



Polycope inflata



100



500



loo0



I



1



I



20003000

1



1



0

0

0



0

0

0

0

0

0

0



0

0



0

0



0

0



0

0



0



0



0



0

0



0

0

0



0



0



0



0

0



0



0

0

0

0

0

0

0



.

0

.

0

.

.

.

.

0

0



0

0

0

0

0

0

0



0

0



.

0



0

0

0



.

.



0



.

0

.

.

0

0

.

.

0

0



0



0

0

0



0

0



.

0

0

.

0



0



downslope contamination (extremely heavy contamination can be seen in the Libyan Sea transect,

Table 4). Unless their striking depth ranges are, in fact, due to this process, several Polycope species

seem unaffected by the bathymetric divide. Within transects, an increase in shell size with increasing depth can be seen in Argilloecia acuminata (G. W. Miiller) and Bathycythere vanstraateni

Sissingh (Tables 6 and 7).

Between the western and eastern transects, there is reasonable similarity so far as the shallowwater ostracods are concerned. The deep-sea faunas are different, however, because several species

that are quite common in the western basin are lacking entirely in the eastern transects (viz. Bathy-



724 D. HARTEN

AND H. J. DROCPTB



TABLE

~-BALEARICSEA.&TRAC€ID



DISTRIBUTION IN T R A N S E ~(SURFACE LAYER; ADULT SPECIMENS ONLY).



depth in m



100

r



500



lo00

1



2M)o 3000

1



1



"Bairdia" spp.

0

Occultocythereis dohrni

0

Paracypris s p .

0

Paracytheridea s p .

0

Semicytherura spp.

0

Aurila spp.

0

Callistocythere spp.

0

Celtia quadridentata

0

Cytherella spp.

0

Echinocythereis spp.

0

Loxoconcha ex gr. rhomboideao

Hilterrnannicythere sp.

0

0

Tetracytherura angulosa

Urocythereis sp.

0

Xestoleberis spp.

0

Pontocypris spp.

0

Microxestoleberis sp.

0

Argilloecia spp.

0

Pseudocythere caudata

0

Bythocypris lucida

Bosquetina dentata

Parakrithe dimorpha

Pterygocythereis jonesi

Monoceratina mediterranea

Macrocypris succinea

Pseudocythere armata

Cytheropteron spp.

Bathycythere vanstraateni

Rectobuntonia inflata

Argilloecia acuminata

Henryhowella sarsi

Bythocypris tenera

Krithe keiji

Cytherella sp. Breman

Macrocypris adriatica

Krithe monosteracensis

Bythocypris obtusata

Pseudocythere hastata

Polycope tholiformis

vasfiensis

reticulata

inflata

ovalis

demulderi

parareticulata

quadridentata



cythere vanstraateni, Cytherella sp. Breman, Krithe keiji Breman, Krithe monosteracensis (Seguen-



za), and Macrocypris adriatica Breman). The eastern deep-sea faunas are poor, by comparison, and

hardly marked by distinctive genera. They are typically dominated by Polycope spp., not infrequently the only ostracods present, and may further contain Argilloecia acuminata, Bairdia conformis (Terquem), Bythocypris obtusata Sars, and Pseudocythere spp. Pedicythere spp., and Tuberculocythere tetrapteron (Bonaduce, Ciampo and Masoli) are sometimes found but tend to occur

in only small numbers. Bythocypris Zucida Seguenza inhabits shallow waters in the Balearic and Ligurian Seas whilst it apparently lives in the upper part of the deep zone in the Libyan Sea (Bytho-



Mediterranean Deep-sea OstracoCiS 125

TABLE 3-hGURIAN



SEA. OSTRACOD DISTRIBUTION IN TRANSECT (SURFACE LAYER; ADULT SPECIMENS ONLY).



depth in m

"Bairdia" spp.

Bythocypris lucida

Carinocythereis bairdi

Cy theridea neapol itana

Echinocythereis sp.

Parakrithe sp.

Tetracytherura angulosa

Urocythereis sp.

Aurila spp.

Bosquetina dentata

Buntonia spp.

Callistocythere spp.

Cytherella spp.

Krithe aff. praetexta

Loxoconcha spp.

Monoceratina oblita

Occultocythereis dohrni

Pterygocythereis jonesi

Rectobuntonia miranda

Sagmatocythere spp.

Tenedocythere prava

Phlyctocythere pellucida

Pseudocythere caudata

Semicytherura spp.

Macrocypris ligustica

Paradoxostominae

Pontocypris spp.

Xestoleberis spp.

Henryhowella sarsi

Argilloecia spp.

Paracytheridea sp.

Paracypris sp.

Cytheropteron spp.

Pseudocythere armata

hastata

Argilloecia acuminata

Krithe keiji

Bythocypris tenera

Macrocypris adriatica

Bathycythere vanstraateni

Cytherella sp. Breman

Bythocypris obtusata



100



500



1000



20003000



.

.....

..... .

...

.

0..



0..



0.



00.0



..om



mo...



0 .

0.



0 0



Polycope reticulata

tholiformis

inf lata

vasfiensis

n. sp.

ovalis

quadridentata



cypris nomenclature seems confused in the literature; see Plate 1 for usage of B. Iucida and B. obtusuta in this paper).

Several deep-sea cores in the Libyan Sea and off Alexandria penetrated the dark-coloured organic-rich layer (sapropel) generally referred to as S-1 (see Cita et al., 1977) and reached pre-saprope1 sediment underneath (Text-fig. 2). It has become customary to divide Mediterranean organicrich layers into sapropels proper and mere sapropelic layers according to whether their organic

carbon content is greater or smaller than 2 % by weight (Kidd et al., 1978). This distinction is not



726 D. HARTEN



AND



H. J. DROSTE



TABU &LIBYAN



SEA. OSTRACoD DISTRIBUTION IN TRANSECT (SURFACE LAYER; ADULT SPECIMENS ONLY).



depth in m



100

I



Macrocypris succinea

"Bairdia" spp.

Occultocythereis dohrni

Cytherella vulgata

Pontocypris spp.

Argilloecia spp.

Microxestoleberis sp.

Paradoxostominae

Semicytherura spp.

Pseudocythere caudata

Phlyctocythere pellucida

Echinocythereis sp.

Monocera tina ob li ta

Bosquetina dentata

Bairdia conformis

Tetracytherura angulosa

Bythocypris lucida

Pseudocythere aff. caudata

hastata

Pedicythere spp.

Pseudocythere armata

Tuberculocythere tetrapteron

Bythocypris obtusata

Polycope reticulata

rostrata

n. spp.

quadridentata

inflata

vasfiensis

demulderi

ovalis

orbulinaeformis

parareticulata

tholiformis



.



~~



500



loo0



1



I



20003000

I



.

....

.....

.

. .

.......

. ..... .

.......

..

.....

....

. ......

........

.

........

.



I



0.

0.

0.

0.



0 . 0

0.

0 . .

0

0

0



.

.

.



0



.



0



.



0



.



0



.



0 . .



0 . .



0 .



0



0



0.



0 . .

0

.



0.



.

0.



0 . .



0 .

..O



.

a



0



.



0



.



0 . .

.



0 .



O



0 . .

0

.



made in the present paper. Here we are using the term sapropel in its original, loose sense to denote

any markedly organic-rich interbed in an otherwise normal sediment sequence.

Sapropel S-1 is considered to be deposited in the Early Holocene, viz. between about 9,000 and

7,000 YBP (Ryan, 1972; Van Straaten, 1972; Stanley and Maldonado, 1977). Radio-carbon dating

of our Libyan Sea sapropel gave 7,530-1-160 YBP, confirming its identity as S-1.

Barren of benthic fauna itself, S-1 marks a distinct faunal break in our cores: Bathycythere vanstraateni, Cytherella sp. Breman, Krithe monosteracensis, and Macrocypris adriatica are present

below the sapropel but, apart from some rare cases of obvious reworking, absent above it. Some

cores (e.g. core 63, Libyan Sea, Table 8) are suggestive of sustained upcore faunal impoverishment

above the sapropel ; this point needs future elucidation but should not detract from the significant

turnover across S-1. In the pre-sapropel assemblage of cores 66, 67, and 68 (Libyan Sea),

Bythocypris rej7exa Breman is replaced upcore by Bythocypris obtusata.

Argilloecia acuminata and Bathycythere vanstraateni grew considerably larger in the pre-sapropel populations than they do nowadays at comparable, or even greater depths (Tables 6 and 7,

Text-fig. 3). Maximum observed variation of mean shell length and height is on the order of 10%

in these species. Assuming a similar figure for the variation in shell width, the difference in shell

volume amounts to about 30%.



Mediterranean DeepSea Ostracods 121



TABLE5-OFF ALEXANDRIA.

OSTRACOD

DISTRIBUTION IN TRANSECT (SURFACE LAYER;

depth in m



..



100

I



500

I



ADULT SPECIMENSONLY).



loo0

20003000

P



Carinocythereis antiquata

Cytheridea neapolitana

Hiltermannicythere sp.

Krithe aff. praetexta

Loxoconcha ex gr. rhomboidea.

Pseudopsammocythere similis

Sagmatocythere sp.

Aurila convexa

Callistocythere adriatica

Occultocythereis dohrni

Xestoleberis c o m u n i s

Cytherella spp.

Macrocypris succinea

Semicytherura spp.

Acanthocythereis hystrix

0

Bosquetina dentata

Buntonia spp.

Phlyctocythere pellucida

Pterygocythereis jonesi

Argilloecia spp.

Cytheropteron spp.

Pontocypris spp.

Henryhowella sarsi

Monoceratina spp.

Paracytheridea spp.

Bairdia conformis

Tetracytherura angulosa

Parakrithe dimorpha

Argilloecia acuminata

Pedicythere phryne

Pseudocythere hastata

Bythocypris obtusata

Pseudocythere aff. caudata

? Microxestoleberis sp.



.

..

..

.

..

.



Polytope reticulata

inflata

vasfiensis

demulderi

ovalis

quadridentata

truncatula

parareticulata



.



No organic-rich layer is found in cores 66, 67,and 68 (Libyan Sea). However, approximately 15

cm below the top of these cores, there is an abrupt faunal break which is equivalent to the turnover

across S-1 in the other cores (Text-fig. 2, Tables 8 and 9). This suggests that a sapropel was present

initially but has since been removed by some process of submarine erosion leaving a hiatus in the

sedimentary record.



728



core nr:

depth in



m:



63



64



65



66



67



68



1043



1185



1603



1849



2090



2291



O r



10



-



20



-



30



-



40



L cm



below core-top



Sea. Boxcore profiles in sapropelarea. Horizontal lines separate sampled intervals. Dark shading: samples yielding species of the group Bufhycyfhere vunsfrmteni Sissingh-Cytherelfu sp. Breman-Krithe

monosterucensis (Seguenza)-Mucrocypris udriuficu Breman; light shading: samples yielding assemblageswith

these marker species missing; white: barren (sapropel S-1).



TEXT.-PIG. 2-Libym



'



0.6 mrn



A



-



0



0

0



0



0



0



' L

1.2 mrn



I



1.0



1.1



H



Is

0.5



0

0



.



0



0.6 mm



TBCFFIG.3-Length-height plots of sample means listed in Tables 6 and 7. A: Bufhycyfhere vunsfruufeni Sissingh;

B: Argilfoeciu ucumimfu (G.W. Miiller). Open circles: samples taken from surface layer; solid circles: samples taken from subsurface pre-sapropel levels.



729



TABLE6-Bathycythere vanstraateni SISSINGH.SIZE OF



ASTERISKSINDICATE



ADULT LEFT VALVES.



SAMPLES



TAKBN FROM PRESAPROPEL LEVELS.

transect and depth in m



810

1720

900

I500

I235

I849 *

1849'

2090

2291+



Alboran Sea

Balearic Sea

Ligurian Sea

Libyan Sea



T m 7-Argilloecia acuminata (G.W.



length in mm



height in mm



mean



s



mean



1.064

1.085

1.023

1.043

1.022

1.158

1.125

1.109

1.127



0.039



0.609 0.006

0.601

0.581 0.031

0.601 0.016

0.546

0.638 0.018

0.645 0.018

0.613 0.010

0.633 0.027



MULLER).



0.018

0.013



0.018

0.011

0.037

0.018



SIZEOF



n



s



ADULT LEFT VALVES. ASTERISKS



INDICATE SAMPLES TAKEN



FROM PRESAPROPEL LEVELS.



transect and depth in m



Alboran Sea

Balearic Sea

Ligurian Sea

off Alexandria



Libyan Sea



810

I720

900

1500

I235

1580

985

2034

1043,

I849

I849

2090



length in mm



height in onn



mean



s



mean



s



0.510

0.529

0.513

0.545

0.515

0.527

0.520

0.534

0.553

0.565

0.551

0.575



0.010



0.211

0.224

0.208

0.221

0.213

0.214

0.212

0.215

0.222

0,226

0.224

0.228



0.005



0.020

0.007

0.007

0.006

0.009

0.003

0.008

0.017

0.010

0.010

0.019



n



7

4

3

3

9

8

6



0.008



0.002

0.002

0.006

0.007

0.004

0.004

0.008

0.007

0.002

0.008



3

5



7

2

5



TABLE 8 L I B Y A N SEA. OSTRACQD DISTRIBUTION (ADULT SPECIMENS ONLY) IN CORE 63

CORE PROFILE).



HATCHING

INDICATES



(SEE



T~XT-FIG.

2 FOR



BARREN SAPROPEL INTERVAL.



________~ _ _ _ _ ~



cm below core-top

Argilloecia acuminata

Bairdia conformis

Bythocypris lucida

obtusata

Cytheropteron sp.

Paradoxostominae

Pedicythere phryne

Polycope spp.

Pseudocythere spp.

Bathycythere vanstraateni

Cytherella sp. Breman

Henryhowella sarsi

Krithe monosteracensis

Macrocypris adriatica

Xestoleberis sp.



0



2

0

0



5



10



20



16



14



26



. ...

.

.

.

.



.

.



0



0



0

.



0



0



0



.



.



0



0



0

0



.



0



0



.



0



.



0



0



.



0



0



.



3031



730 D. HARTEN

AND H. J. DROSTE



TABLEL LIBYAN SEA. OSTRACOD

DISTRIBUTION



(ADULT SPECIMENS ONLY) IN CORE



66



(SEE



TEXT-FIG.

2 FOR



CORE PROFILE).



cm below core-top



0



2



8



15



21



30



Argilloecia acuminata

Bythocypr is ob tusata

Paradoxostominae

Polytope spp.

Pseudocythere spp.

Bathycythere vanstraateni

Bythocypris reflexa

Cytherella sp. Breman

Krithe monosteracensis

Macrocypris adriatica

Xestoleberis sp.



DISCUSSION

Depth-related Faunal Turnover

Benthonic ostracods, like other benthonic organisms, tend to be bathymetrically distributed in

the sea. Actual depth, in terms of hydrostatic pressure,may play a role but it is abundantly clear from

the pertinent literature that several other factors must be involved as well. It is beyond the scope of

this paper to discuss this issue in any detail. We are only concerned with the existence of typically

shallow and deep faunas which, in most of our transects, merge into one another at about 250-300

m depth. This major divide in the Mediterranean was recognized earlier by Ascoli (1964), Puri

et al. (1969), and Breman (1976a), all of which authors placed it at approximately 250 m.

Division of marine ostracods into distinct shallow and deep assemblages seems world-wide (Peypouquet, 1980) but the boundary lies somewhat deeper in the Mediterranean than it does elsewhere

(outside the Mediterranean it lies between 150 and 200 m; e.g. Van Morkhoven, 1972; Peypouquet,

1980). The world-wide occurrence of the divide taken together with the deeper position in the

Mediterranean with its relatively clear waters supports the idea of a relationship with the photic

zone, in accordance with a suggestion originally put forward by Pokorny (1971). This relationship

may depend on food or oxygen requirements. We tentatively propose that ostracods restricted to

the shallow zone need resources available only in the photic zone and immediately beneath it.

Depth-indifferent forms and the true deep-sea taxa may be less exacting in their requirements.

Sapropel-related Faunal Turnover

Our results are much in line with those of Breman (1975) from core 353 (1,207 m depth) in the

Adriatic Sea. There too, Bathycythere vanstraateni, Cytherella sp. Breman, Krithe monosteracensis,

and Macrocypris adriatica disappear from the record at the base of S-1 ; at some depth below the

sapropel, Bythocypris reJexa is replaced by Bythocypris obtusata, a replacement we also observed

in some of our Libyan Sea cores. Above the sapropel, Breman too found poor faunas mainly composed of Argilloecia acuminata, Bairdoppilata supradentata (= Bairdia conformis of this paper; see

PLATE1-Fig. 1. Bathycythere vunstruuteni Sissingh. ~ 4 5 Balearic

.

Sea, 1,720 m. Fig. 2. Krithe monosterucensis

(Seguenza). x75. Balearic Sea, 1,180m. Figs. 3, 4. Cytherellu sp. Breman. ~ 7 0 Ligurian

.

Sea, 1,810m.

Figs. 5, 6. Krithe keui Breman. X95. Ligurian Sea, 1,580 m. Figs. 7, 8. Bythocypris obfusata SWS. X45.

Balearic Sea, 2,400 m. Figs. 9, 10. Bythocypris fucidu Seguenza. x45. Libyan Sea, 642 m. Fig. 11.

Macrocypris udriuticu Breman. x 30. Ligurian Sea, 2,100 m.



732 D. HARTEN

AND H. J. DROSTE



Bonaduce et ul., 1975), Bythocypris obtusutu. and Polycope spp. Poor Recent faunas of the same

kind were reported by Brambati et ul. (1983) from the deep Cretan Sea.

On account of these similarities, we propose that Buthycythere vunstruuteni, Cytherellu sp. Bre

man, Krithe monosterucensis. and Mucrocypris udriuticu disappeared from the entire eastern

Mediterranean at the onset of sapropel deposition, about 9,000 years ago. This possibly also applies in the case of Krithe keiji, a species described by Breman (1976b) from the “Pleistocene

(?)-Holocene”

of the Adriatic Sea (Breman initially used the preoccupied name K. cuuduta; in

Breman (1978), the specific epitheton was changed into keyi which was an unintentional misspelling

for keiji). We found this form in all our western transects but it is apparently missing in the

eastern ones, both above and below the sapropel.

Still extant in the western Mediterranean, these species became extinct in the eastern basin as a

result of water anoxy associated with sapropel formation. The post-sapropel deep-sea ostracod faunas in the eastern Mediterranean are strongly different from those preceding this layer and much

poorer. This can be considered a direct legacy of the S-1 anoxic event.

Some published reports may seem to rebut our claim. Ascoli (1964, P1. 4 and 1966, P1. 5)

depicted Krithe monosteracensis, Mucrocypris sp. 1 (= M . udriuticu), and Pterygocythereis? sp. 1

(=Buthycythere vunstruuteni) from a core taken at 1,192 m in the Adriatic Sea, believing them to

represent Recent fauna. We subscribe to Breman’s (1976a) suspicions, however, that Ascoli’s pictures in reality refer to fossil specimens from the Pleistocene or Early Holocene which would crop

out at the site of the core. Since Ascoli, as he mentioned in his 1964 paper, used unspecified samples

from core-tops 30 or even 40 cm in length, there is the added possibility that his material did not

come from the actual sediment surface.

The faunal poorness of the present-day deep eastern Mediterranean is very obvious indeed from

the distribution chart in Bonaduce et ul. (1983). Yielding a considerable number of Buthycythere

vunstruuteni, their station Ch 31 (Ionian Sea; 1,087 m) is notably out of tune, however. It is, in fact,

the only eastern Mediterranean station in their chart to yield this species. The same sample is reported to contain Cytherella buthyulis (this is apparently a nomen nudum; the species is probably

identical to C. sp. Breman of this paper) and suspiciously numerous Bythocypris obtusutu. The deviant compositon of this sample and the isolated occurrence strongly suggest that fossil, pre-sapropel material rather than Recent fauna is involved. The composition does not point to reworking, in

our opinion. But pre-sapropel sediment may have cropped out on the sea floor at the site of Ch 31

or be so shallowly buried that it entered the core-tops (of unspecified length) used by Bonaduce

and his co-workers. Note, in this connection, that we found pre-sapropel sediment lying only 14 cm

below the surface in core 68 in the Libyan Sea (Text-fig. 2). The single specimen of Krithe monosterucensis reported by Bonaduce et ul. (1983) from their station Li 3 (Ionian Sea; 2,392 m), again

a sole find in all of the eastern Mediterranean, seems inconclusive.

Sapropels are being generally associated with anoxic bottom-water conditions and the abrupt

disappearance of benthonic ostracods at the base of S-1 is consistent with this. Animals do need free

oxygen for respiration and will die of asphyxia in an anoxic environment. The fact that several preexisting species did not return after the anoxic event was over suggests that they had become

effectively annihilated all over their regional biotope. As sapropel S-1 is geographically restricted to

the eastern Mediterranean (Stanley et ul., 1975; Stanley, 1978; Cita and Grignani, 1982), anoxyinduced extinction would also have been limited to that region. After the annihilation in the east,

recolonisation could only occur from the unaffected western basin, across the Sicilian Ridge. But

the shallowness of this area (present-day sill depth approximately 400 m) and the concurrent

elevated water temperatures to be expected would have tended to counteract re-invasion.

As opposed to the deep-sea fauna, the eastern Mediterranean shallow-water ostracods do not

seem to have suffered much from the S-1 anoxic event. The similarity between our western and



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Chapter 52. Mediterranean deep-sea ostracods, the species poorness of the Eastern Basin, as a legacy of an early Holocene anoxic event

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