BASED ON BENTHIC FORAMINIFERS FROM DSDP LEG . NEOGENE PALEOBATHYMETRY OF THE MEDITERRANEAN BASED ON BENTHIC FORAMINIFERS FROM DSDP LEG 42A Ramil Wright, Beloit College, Beloit Wisconsin1

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  • 41. NEOGENE PALEOBATHYMETRY OF THE MEDITERRANEANBASED ON BENTHIC FORAMINIFERS FROM DSDP LEG 42A

    Ramil Wright, Beloit College, Beloit Wisconsin1

    ABSTRACTQuantitative and qualitative bathymetric analysis of Neogene

    benthic foraminiferal faunas from eight drill sites in the Mediterra-nean Sea confirms the presence of deep basins immediately afterthe deposition of the evaporites of the Messinian. Analysis of thefaunas of two sites which penetrated the evaporitic sequenceestablishes the existence of deep basins in both the eastern andwestern Mediterranean prior to the Messinian. Although therecovery of samples near the upper and lower boundaries of theevaporites was not continuous there is no evidence of the presenceof faunas which are transitional between the deep water assem-blages and the shallow water Messinian faunas. These observationsall lend considerable support to a deep basin desiccation model forthe formation of the evaporites.

    INTRODUCTION

    Reconstruction of the Neogene paleobathymetry ofthe Mediterranean area has been based primarily onoutcrop studies of structural basins which have beenuplifted around the margins of the Mediterranean, oncores taken along the margin, and on geophysicalstudies of the central deep basins. The first deep basincores were taken during DSDP Leg 13 and yieldedpreliminary conclusions based on a qualitative exami-nation of the benthic foraminifers by W. Maync (Ryan,Hs, et al., 1973) and occasional ostracode carapaces(Benson, 1973).

    This study is based on the benthic foraminiferscollected from the Leg 42A sites (Figure 1) andexamined qualitatively and quantitatively to provideestimates of water depth during the Neogene.

    BATHYMETRIC ZONATION

    The utilization of benthic foraminifers as bathymet-ric indicators is dependent on comparisons between theNeogene fauna and occurrences of the same andrelated species in the modern Mediterranean andadjacent waters. The modern Mediterranean is not atypical ocean basin. The restriction to oceanic circula-tion caused by the 320-meter deep sill at Gibralter hascreated a water mass structure whose deep bottomtemperatures (13C) are warmer than those of waterof similar depth in the rest of the ocean basins of theworld. Consequently, the resulting vertical zonation ofbenthic foraminifers in the Mediterranean differs fromthat of the adjacent Atlantic at depths greater than1300 meters. Qualitative studies of the Recent bathyalbenthic foraminiferal fauna in the Mediterranean

    'Current address: Florida State University, Tallahassee, Florida.

    (Blanc-Vernet, 1969; Colom, 1974; Parker, 1958;Todd, 1958) indicate that many elements of the mod-ern Atlantic deep-sea benthic foraminiferal fauna(Phleger et al., 1953) are missing from the Mediterra-nean today. Living benthic foraminiferal faunas in theMediterranean today display changes in species com-position to depths of 1000-1300 meters. Below thesedepths there is little change in the species compositionof the faunal assemblages. However, a quantitativestudy of the Recent Mediterranean fauna (Cita andZocchi, in press) revealed that deeper zones could beidentified on the basis of whole fauna abundance, thenumber of individuals per species and the number ofspecies.

    As a consequence of these differences between theMediterranean fauna and those of "normal" oceanbasins, the bathymetric zonation scheme of the openocean (Hedgepth, 1957) used in previous DSDPstudies (Ingle, 1973) is unsuitable for work in theMediterranean. A bathymetric zonation (Table 1) hasbeen specifically designed for the Mediterranean and isbased on the distribution of the benthic foraminifersand on the hyposgraphic curve of the Mediterranean.

    BENTHIC FORAMINIFERS AS BATHYMETRICINDICATORS

    Benthic foraminifers are sensitive to a variety ofenvironmental factors. It is not clear whether or notthey respond to depth per se or only to depth-relatedfactors such as temperature, salinity, light intensity,nutrient distribution, and oxygen concentration. How-ever, it is clear that some species have coincident andwell-defined upper depth limits in various ocean ba-sins. Bandy and Chierici (1966) recognized severalsuch "isobathyal" species in the Mediterranean. Moreweight was given to these species when paleobathymet-ric interpretations were made from the Mediterraneancores.

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    20 30 40

    B A L E A R I C

    0TYRRHENIAN' ,32

    40E30-

    Figure 1. Location of drill sites, DSDP Leg 42A (in relation to those of Leg 13).

    TABLE 1Bathymetric Zonation, Mediterranean Sea

    Zone

    Inner neriticOuter neriticUpper epibathyalLower epibathyalUpper mesobathyalMid mesobathyalLower mesobathyalInfrabathyal

    Upper Limit (m)

    050

    150-200500-700

    1000-1300180025004000

    Lower Limit (m)

    50150-200500-700

    1000-1300180025004000

    Whenever- possible, the paleobathymetric interpreta-tions of this study were made by analogy with thefauna of the Mediterranean today. However, a numberof species were encountered in the cores which are notliving in the modern Mediterranean, but are living inother seas and oceans of the world. Where theseparticular species occur in areas whose benthic fora-minifers are well studied such as the Gulf of Gascogne(Caralp et al., 1970; Pujos-Lamy, 1973), the Gulf ofMexico (Parker, 1954; Phleger and Parker, 1951), theGulf of California (Bandy, 1961), the Pacific Ocean(Bandy and Arnal, 1957; Cushman, 1933, 1942; Todd,1965; Uchio, 1960), and the Andaman Sea (Frerichs,1970), their recorded depth distribution was utilized.Those species whose depth distribution in several ofthese areas was consistent were given more weight inthe interpretations than rare species or those whosedepth ranges display great variation.

    Because foraminiferal tests may be transporteddownslope, it was necessary to eliminate from consid-eration any obviously reworked specimens. Such speci-mens were recognized in various ways. The presence ofunlikely assemblages such as the occurrence together of

    Ammonia spp. and Elphidium spp. with bathyal speciesof Cibicidoides and Bulimina indicate downslope trans-port of the first two genera. Another indication ofdisplacement is the presence of worn and abradedspecies mixed with well-preserved specimens of otherspecies. Not all displaced specimens are abraded norare they always obviously out of place. Consequently,the enclosing sediment was examined for signs ofgrading, size sorting, and abundant detrital materialwhich might indicate downslope transport. Becausemodern faunas may also be displaced downslope,heavy reliance was placed on the upper depth limits ofRecent species when drawing analogies for paleodepthdeterminations. The modern downslope transport oftests may be unrecognized today and consequently therecorded lower depth limits of some species may beconsiderably greater than is actually the case; henceupper depth limits are of special significance.

    Those species representing the deepest habitat in afossil assemblage were assumed to be in place andthose species indicative of shallower depths wereconsidered as possible downslope contiminants. Conse-quently, the paleobathymetric interpretations based onthe deepest dwelling assemblages represent a minimumestimate of the paleodepth.

    However, some modification of this minimum esti-mate can be made if one considers the abundances ofselected species and also if one evaluates the morphol-ogy of selected species.

    Many Recent benthic foraminifers exhibit a verticalquantitative distribution which approximates a Gauss-ian curve (Theyer, 1971). Consequently, the abun-dance of a species in an assemblage may give clues todepth and should be considered. Rare occurrences of acommon species probably indicate that it lies near its

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  • NEOGENE PALEOBATHYMETRY BASED ON BENTHIC FORAMINIFERS

    upper or lower depth limit whereas abundant speci-mens of a common species may be indicative of apaleodepth well below the upper and well above thelower depth limits. As an example, consider threecommon Mediterranean species (Bandy and Cherici,1966): Hoeglundina elegans has its upper depth limitat about 100-150 meters and increases in abundance to700 m; Gyroidina altiformis has its upper depth limit at200 meters, is abundant below 600 meters, and veryabundant below 800 meters; Cibicides wuellerstorfi isfound rarely as shallow as 500 meters and is mostcommon below 1000-1200 meters.

    A sample containing abundant specimens of H.elegans, rare specimens of G. altiformis, and no speci-mens of C. wuellerstorfi, would be evaluated as lyingbetween 200-500 meters and probably closer to 200meters in depth. A sample containing abundant H.elegans and G. altiformis but no C. wuellerstorfi wouldbe evaluated as representing a depth of about 500meters. Although the deepest dwelling species in theassemblage, G. altiformis, has an upper limit of 200meters, the deeper estimate of 500 meters would beassigned on the basis of the abundances. This utiliza-tion of abundance is used with care as downslopetransport can mask actual abundances and give adepth estimate that is too shallow. Only the commonspecies can be utilized in the manner.

    Some genera and species exhibit morphologicalvariation with depth and recognition of these ecophe-notypic specimens may give some clues to the waterdepth. A survey of the literature on depth-relatedmorphological changes in selected genera (Boltovskoyand Wright, 1976, p. 242) shows that species ofPullenia become more spherical with depth, Cibicidesbecomes more oblate and exhibits more prominentsutures, the species of Eponides tend to becomesmaller, Gyroidina becomes larger, Bolivina developssinuate sutures, ornamentation on specimens of Trifa-rina becomes less pronounced, and hispid Uvigerinabecome dominant over costate forms. These phenom-ena were all used in the evaluat