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Robertson, A.H.F., Emeis, K.-C., Richter, C., and Camerlenghi, A. (Eds.), 1998 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 160

9. CALCAREOUS NANNOFOSSILS IN THE BASAL OF THE EASTERN : REMARKS ON PALEOCEANOGRAPHY AND SAPROPEL FORMATION1

Davide Castradori2

ABSTRACT

This paper presents the results of a study on nannofossil assemblages in the uppermost and basal Zanclean sedi- ments recovered from Ocean Drilling Program (ODP) Holes 969B (south of Crete) and 967A (north of the Eratosthenes Sea- mount) located in the Eastern Mediterranean Sea. Focus of the research is on (1) the micropaleontological signature of the basically nonmarine facies of the uppermost Messinian interval; (2) the correlation of the Messinian/Zanclean boundary outside the Mediterranean by means of nannofossil biostratigraphy; and (3) the mechanism of sapropel formation in the earliest . The main results are summarized as follows: 1. The Messinian sediments are characterized by assemblages dominated by diagenetic resistant and long-ranging species, absence of typical upper –Messinian species, high abundance of reworked taxa of Cenozoic and , and high abundance of gypsum crystals and dolomite. All evidence listed above point to the absence of a primary marine signature in the nannofossil assemblages of the sediments. A noticeable exception, constituted by anomalous abundance of Sphenolithus spp., which suggests a possible (marginal) marine signature, is documented. 2. The recovery of some specimens of Triquetrorhabdulus rugosus and of acutus overlapping each other in the basal Zanclean suggests that, contrary to what has been reported by most authors, the last appearance datum (LAD) of T. rugo- sus and the first appearance datum (FAD) of C. acutus do not exactly coincide. This was already reported from the Ceara Rise (ODP Leg 154), where the FAD of C. acutus was found immediately below the cyclostratigraphically correlated position of the Messinian/Zanclean boundary. These closely spaced bioevents can be considered as a valuable tool, in addition to magneto- stratigraphy and astrocyclostratigraphy, for the recognition and worldwide traceability of the Messinian/Zanclean boundary. 3. Based on a model by Molfino and McIntyre, fluctuation in abundance of Florisphaera profunda, a peculiar nannoplank- ton species living in the lower part of the photic zone, is interpreted to represent nutrient availability within different levels of the photic zone. Thus, positive peaks of F. profunda were connected by the author to the presence of a deep chlorophyll maxi- mum (DCM) at times of sapropel formation, a concept previously introduced by Rohling and Gieskes for the mid- to time interval. Abundance peaks of Florisphaera profunda are recorded in all five sapropel layers recovered in the basal Zanclean of ODP Hole 969B. Therefore, the DCM model may be tentatively applied also to the earliest Pliocene and probably to the whole sapropel sequence of the Mediterranean area, since the late . The recovery of abundance peaks of pentaradiatus in most sapropels (together with the decreased abundance of Coccolithus pelagicus in some of them), suggests that these sapropels probably correlate with the marls (and not the lime- stones) of the rhythmically bedded sedimentary sequences of the land sections in and Calabria (southern ), thus con- firming previous assumptions by De Visser, Hilgen, and others.

BACKGROUND AND OBJECTIVES boundary was formally proposed in the Capo Rossello section by Cita (1975a) and subsequently proved to correlate with the fifth pre- This paper illustrates part of the results of a multidisciplinary cession-related astronomical cycle below the Thvera magnetic event study on the Messinian/Zanclean boundary based on data from four (Zijderveld et al., 1991; Hilgen, 1991b; Hilgen and Langereis, 1993). Ocean Drilling Program (ODP) sites, two from the Western Mediter- From a biostratigraphic point of view, the Messinian/Zanclean ranean (Leg 161, Sites 975 and 978) and two from the Eastern Med- boundary is usually regarded as poorly constrained in the Mediterra- iterranean (Leg 160, Sites 967 and 969). The sites were selected to nean area, because of the basically nonmarine nature of the upper form a transect that may show different geographic, geologic, and pa- Messinian sediments (e.g., Arenazzolo Formation in Sicily) and of leoenvironmental settings. the lack of globally recognized nannofossil and foraminiferal bio- The idea of a multidisciplinary stratigraphic research originated events in the very basal Zanclean. In particular, nannofossil species, from the need to address, on a previously unavailable marine record, such as Triquetrorhabdulus rugosus, Ceratolithus acutus, and C. the problem of the restoration of open-marine conditions at the end of rugosus, which define the zonal boundary of the oceanic standard zo- the Messinian salinity crisis. This event is of key importance from nation (Martini, 1971; Okada and Bukry, 1980), are rare in the Med- both paleoceanographic and chronostratigraphic points of view. Con- iterranean stratigraphic record. cerning the latter, the problem of the selection of an appropriate Glo- The discussion in the scientific community about the Messinian/ bal Standard Stratotype-section and Point (GSSP) of the Zanclean Zanclean boundary is really animated (see Berggren et al., 1995b, for strongly depends upon the demonstration of the absence of a a review). To summarize, there are three possible options for the for- significant hiatus across the Messinian/Zanclean boundary. This malization of a GSSP of the Zanclean Stage (as base of the Pliocene ): (1) the base of the Trubi in the Capo Rossello area; (2) an ap- propriate level in an extra-Mediterranean section selected to corre- spond to the base of the Trubi (counting five precession-related cy- 1 Robertson, A.H.F., Emeis, K.-C., Richter, C., and Camerlenghi, A. (Eds.), 1998. Proc. ODP, Sci. Results, 160: College Station, TX (Ocean Drilling Program). cles below the Thvera magnetic event); (3) an appropriate level in an 2 AGIP S.p.A., Laboratori Bolgiano, via Maritano 26, 20097 San Donato Milanese, extra-Mediterranean section selected to correspond approximately to Italy. [email protected] the base of the Gilbert (Benson and Hodell, 1994), thus decou-

113 D. CASTRADORI pling the Messinian/Zanclean stage boundary from the Miocene/ and Pestiaux, 1987; Chepstow-Lusty et al., 1989), will provide addi- Pliocene series boundary (see discussions in Berggren et al., 1995a, tional information on the paleoenvironmental conditions at times of 1995b). sapropel formation. Whatever option will be preferred by the International Commis- sion on Stratigraphy (ICS) (and ratified by the International Union of Geological Sciences), the position of the Messinian/Zanclean bound- MATERIAL AND METHODS ary in the Mediterranean with respect to biohorizons recognized in the world oceans is relevant to the solution of the problem. This paper Site 969 is located on the Mediterranean Ridge, some 100 km presents some information on this matter, together with quantitative south of Crete, at a water depth of about 2200 m (Fig. 1). The location and semiquantitative data on nannofossil assemblages that character- was selected to recover a sedimentary record of sapropel formation ize the sapropel layers intercalated in the basal Zanclean of Sections on the ridge that separates the Ionian (to the west) and the Levantin 160-969B-11H-5 and 6. (to the east) basins. A secondary goal was the recovery of a well- The origin of sapropels is another problem that gave rise to many developed Messinian–Zanclean sequence to address the problem of different genetic theories in the last decades (see Rohling, 1994, for the restoration of open-marine conditions following the Mediterra- an overview). In synthesis, sapropels, which are organic matter–rich, nean “salinity crisis” (Emeis, Robertson, Richter, et al., 1996). dark-colored pelagic layers interbedded in the land and marine Med- Site 967 is located on the northern slope of the Eratosthenes Sea- iterranean record since times (and particularly in the mount, at a water depth of 2552 m, in a position critical for testing the Pliocene and Pleistocene; Kidd et al., 1978), may have originated hypothesis of tectonic underthrusting of the Seamount beneath the is- from increased primary production in the photic zone (e.g., Calvert, land of Cyprus (Fig.1). Subsidiary goals of Site 967 were the recon- 1983; Sutherland et al., 1984; Pedersen and Calvert, 1990; Calvert et struction of paleoenvironments at the Messinian/Zanclean boundary al., 1992), from anoxia in the bottom waters (e.g., Olausson, 1961, and the recovery of a Pliocene/Pleistocene succession more expand- Ryan, 1972; Thunell, 1979; Thunell and Williams, 1983; Thunell et ed and better resolved with respect to that of Site 966, which is locat- al., 1977; Cita et al, 1977; Muerdter, 1984), or from a complex inter- ed on top of the Eratosthenes at a much shallower depth (Emeis, Rob- play of the two (e.g., Rohling and Gieskes, 1989; Rohling, 1991; ertson, Richter, et al., 1996). Rohling and Hilgen, 1991; Castradori, 1993; Rohling, 1994). Sampling for this study was limited to the uppermost meters of Even if anoxia of the bottom waters was the primary mechanism, Messinian sediments and to the lowermost meters of the basal Zan- it was probably triggered by decreased salinity of the surficial waters clean. Twenty-four samples were collected from Sections 160-969B- that are, therefore, the environment where the entire process originat- 11H-5 through 11H-7, and 22 samples from Sections 160-967A- ed. That is why studies of planktonic living in the upper 14X-1 through 13H-1. In particular, sampling of the basal Zanclean part of the water column may play a critical role for unraveling the at Hole 969B was focused on the alternation of sapropels and normal whole story. Since the 1960’s, the assemblage composition and iso- pelagic marls to obtain a preliminary picture of their peculiar nanno- topic signature of planktonic foraminifers have been investigated in fossil assemblages. In contrast, the absence of sapropel layers in the detail, initially in mid-Pleistocene to Holocene sapropels (e.g., basal Zanclean of Site 967 suggested a more regular sampling inter- Olausson, 1961; Cita et al., 1977, Rohling and Gieskes, 1989), then val (2−3 samples per section) throughout foraminifer Zones MPl 1 in those of mid-Pliocene to Pleistocene age that outcrop on land (par- and basal MPl 2 (Cita, 1975b). ticularly in southern Italy; e.g., Lourens et al., 1992), and lastly in up- For each sample, a smear slide was prepared and analyzed with an per Miocene sapropels of Crete (Nijenhuis et al., 1996). optical microscope at 1000×. However, Discoaster abundances were Recently, calcareous nannofossils (the fossil calcareous cover of determined at 630× magnification. No special technique was applied planktonic algae) have also been studied to decipher the formation to clean or concentrate the biogenic fraction to retain the original mechanisms of mid-Pleistocene to Holocene sapropels (Castradori, composition of the nannofossil assemblages. 1993). This study highlighted increased abundance of Florisphaera Distribution of calcareous nannofossils in the basal Pliocene of profunda (the only species of calcareous nannoplankton that inhabits the Mediterranean area is rather poorly known, possibly because of the lower part of the photic zone) in most sapropel layers, with re- the local scarcity of oceanic marker species (see above). Therefore, a spect to the underlying and overlying normal pelagic marls. Based on semiquantitative description of the whole nannofloral assemblage a study by Molfino and McIntyre (1990a, 1990b) that demonstrated a clear link between the abundance variations of F. profunda and the nutrient availability at different depths of the photic zone, it was pro- posed that the increased abundances of F. profunda in the sapropel layers witnessed the presence of a Deep Chlorophyll Maximum (DCM). The presence of a distinct DCM at times of sapropel forma- tion was initially hypothesized by Rohling and Gieskes (1989), main- ly on the evidence of increased abundance of neogloboquadrinids. The increased primary production in a DCM was then considered as the triggering mechanism for the formation of most sapropel layers of mid-Pleistocene to Holocene age (Rohling and Gieskes, 1989; Castradori, 1993), although anoxia may have played a key role, par- ticularly for the formation of those sapropels with a relatively lower organic carbon content (Rohling, 1994). A similar scenario has been hypothesized also for the formation of late Pliocene and early Pleis- tocene sapropels (Lourens et al., 1992) and, recently, for the late Mi- ocene sapropelites that outcrop on Crete (Nijenhuis et al., 1996). In this paper, I will try to prove or disprove whether the DCM model may be applied also to the formation of earliest Pliocene Figure 1. Location map of the two sites studied. Site 969 is located on the sapropels. To do so, the abundance variations of F. profunda will be Mediterranean Ridge (at a water depth of 2200 m) on a topographic high that analyzed across five sapropel layers. In addition, the abundance fluc- separates the Ionian (to the west) and the Levantin (to the east) basins. Site tuations of different Discoaster species, a group with peculiar paleo- 967 is located on the northern slope of the Eratosthenes Seamount (at a water ecological preferences extinct since the late Pliocene (e.g., Backman depth of 2552 m), just to the south of the inferred subduction front.

114 CALCAREOUS NANNOFOSSILS IN THE BASAL ZANCLEAN was considered suitable. Results are reported in Tables 1 and 2 (in- cus, small Reticulofenestra, Dictyococcites spp., Coccolithus cluding a visual evaluation of presence and relative abundance of pelagicus, Sphenolithus moriformis; mineral components, such as gypsum and dolomite). The total abun- 3. Absence of typical upper Tortonian–Messinian species (e.g., dance of nannofossils is recorded as abundant (A = more than 30 Discoaster quinqueramus, Discoaster berggrenii, Amauroli- specimens/1 field of view), common (C = 10−30 specimens/1 field of thus spp.); view), or scarce (S = 1−10 specimens/1 field of view) and the overall 4. High abundance of reworked taxa of Cenozoic and Cretaceous preservation is qualitatively classified as good (G), moderate (M), or age; and poor (P). Semiquantitative abundance of single species is reported 5. High abundance of gypsum crystals (often twin crystals, Pl. 1, according to the following classes: Figs. 1−4) and dolomite.

A = abundant (more than 10 specimens per field of view); All the evidence listed above point to the absence of a primary − marine signature in the Messinian sediments investigated. Samples C = common (1 10 specimens per field of view); − − S = scarce (1 specimen every 1−10 fields of view); 160-967A-14X-1, 54 57 cm, and 14X-1, 72 75 cm are character- R = rare (1 specimen in more than 10 fields of view); and ized, however, by an anomalous abundance of Sphenolithus spp. RR = very rare (1 specimen in standard analysis). (mostly Sphenolithus gr abies/moriformis; Pl. 1, Fig. 6). Such a pecu- liar feature, noted also in some samples from the uppermost Messin- In addition, a quantitative analysis was performed on the Dis- ian of the western Mediterranean ODP Sites 975 and 978 (D. coaster and Amaurolithus groups and on Florisphaera profunda. Castradori, unpubl. data), cannot be explained if reworking was re- Discoaster and Amaurolithus were counted on a fixed area of each sponsible for the observed assemblages. This anomalous signature slide (10 mm2; Backman and Shackleton, 1983; Rio et al., 1990). In may witness the existence of temporary (marginal) marine settings Tables 1 and 2, the abundance of single species of Discoaster and predating the Zanclean ingression. This is obviously a preliminary in- Amaurolithus is reported as number of specimens/mm2, whereas in terpretation that needs further investigation. Samples 160-969B-11H-6, 124−126 cm, and 160-967A-13H-4, Figures 2 and 3, percentage values of each species with respect to the − entire group are reported to avoid discrepancies caused by the differ- 25 27 cm, bear the first evidence of primary marine assemblages of ent quantity of sediment smeared onto a slide. Zanclean age. Assemblages are characterized by very abundant cal- The variations in abundance of F. profunda were recorded on careous nannofossils with moderate to good preservation. The abun- what is probably more a semiquantitative than a truly quantitative ba- dance of reworked taxa, gypsum crystals, and dolomite clearly de- sis. Following Castradori (1993) and also partially Matsuoka and creases upward, although they may be rather abundant in some lay- Okada (1989), the abundance of this species in a given sample is ers. computed as number of its specimens compared to 100 specimens of all other species. This counting procedure is justified by the peculiar Biostratigraphy and Chronostratigraphy morphologic features of F. profunda (thin, gray, small calcite plates of the Messinian/Zanclean Boundary of polygonal shape; Pl. 2, Fig. 1) and by its unique habitat, the lower part of the photic zone (compared to the middle and upper parts where all the other species live). The Messinian/Zanclean boundary, as defined at the base of the Trubi Formation in Sicily, can now be correlated outside the Medi- terranean on the basis of magnetostratigraphy and astrocyclostratig- RESULTS AND DISCUSSION raphy (counting five precession-related cycles below the base of the Thvera magnetic event; see above). In most biostratigraphic schemes, the Messinian/Zanclean boundary is bracketed by the last In this section, the analytical data (Tables 1, 2; Figs. 2, 3) will be appearance datum (LAD) of Discoaster quinqueramus (below) and presented and discussed according to their relevance for: (1) paleon- the LAD of Triquetrorhabdulus rugosus and first appearance datum tological and sedimentological characteristics of the uppermost (FAD) of Ceratolithus acutus (above) (Berggren et al., 1995a, 1995b; Messinian; (2) biostratigraphy and chronostratigraphy of the Messin- Raffi and Flores, 1995). Therefore, in terms of the nannofossil zona- ian/Zanclean boundary; and (3) paleoceanography and mechanism of tion of Okada and Bukry (1980), the Messinian/Zanclean boundary sapropel formation in the basal Zanclean. in the ocean domain is usually correlated to Subzone 10a. Very recently, Backman and Raffi (1997) documented, from the Paleontological and Sedimentological Characteristics Ceara Rise, that C. acutus actually appears before the extinction level of the Uppermost Messinian of T. rugosus. The astrocyclostratigraphic calibration of the LAD of D. quinqueramus (5.537 Ma), FAD of C. acutus (5.372 Ma), and The stratigraphic intervals from Samples 160-969B-11H-7, 40− LAD of T. rugosus (5.231 Ma), obtained by Backman and Raffi 42 cm, to 11H-6, 128−129 cm, and from Samples 160-967A-14X-1, (1997), clearly demonstrates that the Messinian/Zanclean boundary 119−122 cm, to 13H-4, 78−80 cm, are assigned to the Messinian on (5.32 Ma) is very well constrained outside the Mediterranean, on the the basis of sedimentological features (Emeis, Robertson, Richter, et basis of nannofossil biostratigraphy. al., 1996), characteristics of the fossil assemblages (discussed in this In the Mediterranean, the LAD of D. quinqueramus is not docu- section), and biostratigraphic signature of the overlying marine Zan- mented because of the essentially nonmarine character of the deposits clean (see below). of that time interval (upper Messinian evaporitic and postevaporitic With the exception of Samples 160-967A-13H-6, 79−81 cm, and interval). The last specimens of T. rugosus and the first specimens of 13H-6, 99−101 cm, which were prepared from Zanclean sediments C. acutus should in principle be detectable in the first meters of the that had been mechanically intruded into the Messinian sequence by Zanclean Stage. Unfortunately, the record of both species is very drilling disturbances, the samples analyzed from the Messinian inter- scattered, and they are seldom reported in the literature. In particular, val are characterized by the following (also see Tables 1, 2): Gartner (in Cita and Gartner, 1973) reported C. acutus in the basal Zanclean of the Capo Rossello section, starting from 6 m above the 1. Relatively high abundance of calcareous nannofossils, with Messinian/Zanclean boundary, whereas he did not mention the pres- moderate to poor preservation; ence of T. rugosus. On the contrary, Rio et al. (1984) found a few T. 2. Assemblages dominated by diagenetic resistant and long-rang- rugosus in the first 6 m of the basal Zanclean (again in the Capo Ros- ing Neogene species, such as Reticulofenestra pseudoumbili- sello section), but they did not find any C. acutus.

115 D. CASTRADORI ). Abun- 2

Remarks

T. rugosus T. rugosus T. rugosus T. C. acutus

ganic matter ganic Or

Dolomite

Gypsum crystals Gypsum

raminifers o F

rked o w re Cenozoic

ed rk o w re Cretaceous

undance ab total Discoaster

orhabdulus rugosus orhabdulus iquetr r T

tolithus acutus tolithus a Cer

(circular outline) (circular pseudoumbilicus R.

tals and dolomite are purely qualitative.

spp. a acosphaer Thor

(see footnote) (see ofunda pr a Florisphaer

spp. a Umbilicosphaer

spp. a cosphaer a Syr

spp. a Scyphosphaer

ouweri br Discoaster

lithus delicatus lithus o Amaur

lithus primus/tricorniculatus lithus o Amaur

variabilis . gr Discoaster

adiatus pentar Discoaster

ulus c sur Discoaster

“Material and Methods” section [this chapter] for abundance keys) and quantitative when in numbers (number of specimens/mm when in numbers and quantitative keys) “Material and Methods” section [this chapter] for abundance

a stalis a Helicosphaer

Scapholithus fossilis Scapholithus

alaris c inter Discoaster

spp. Dictyococcites

spp. a Rhabdosphaer

spp. Discoaster

ei macintyr Calcidiscus

lcidiscus leptoporus lcidiscus a C

spp. a ntosphaer o P

a intermedia a Helicosphaer

a carteri a Helicosphaer CRCCR 5CS/CR70 C80C/ACRCCS 77 CC 133CRCCC2CC/A2 155 S/CC2 920321*12C 165105** R R RC RCR 3115961213S/CRS80 R S

Table 1. Calcareous nannofossil distribution chart of the uppermost Messinian and basal Zanclean in Hole 969B. chart of the uppermost distribution nannofossil 1. Calcareous Table

icus g pela Coccolithus

a Reticulofenestr small

moriformis/abies . gr Sphenolithus

a pseudoumbilicus a Reticulofenestr

tion a preserv Nannofossil

undance

Nannofossil ab Nannofossil crys of gypsum and Methods” section [this chapter]). Abundance is related to 100 specimens of all other species (see “Material cm from top of section of top from cm

F. profunda F.

Section Core dance of 11H5145-147A G SAARCRCAC CC 181151335945S/CRC150 A RR311 RRC Presence of of of A C 100 RR311 RC1R R140 RRC R Presence 823621546SR G SAARCRCAC C 3-5AG/MCAAC/ACRCCS Presence A CC11H5 11H 177 CAC 511H 14S/CRS60 511H S 30-32 CC 511H 50-52 181151335945S/CRC150 A 511H 70-72 S R A 5 G/M11H5145-147A 19-20AG/MCAAAA 91-93 A 120-12211H C 29-30AG/MAAAA M 262611124 A 6 A G/M11H6 39-40AG/MAAAA A G/M11H6 C R 49-50AG/MAAAA A G 9-10 C/A S/C C/A CR AA C/A11H6 R C/A A R C M C11H6 S S AC/AAACRSCRS A A RRRR R CRSRC R11H A A C S/C C 6 M11H A C A S/C 611H 59-60 C A C S 611H 69-70 A S A R A 611H R AA 79-80 C R/S S A 6 RRR/S 11H A C C 90-91 M S 6 A S G/M11H C C 98-99 C C/A R/S 6 A C 108-109 G/M11H A A A A A 6 117-118 R A G/M11H6128-129A S 8-10CM/PCCCCS/CRSS/CRS R A C A A 124-126 C/A A G/M G/M11H S C C C R C/A C C/A 6 A A A G/M11H7 C A A A C/A R C/A 139-140 A11H740-42C/AM/PACACCR G/M A S S A C C/A C R RRRR 1 CA A AA A A A A C/A C A S C 1 8 A A C R M 22 C/A C C C/A A 8 R C C A 1 17 A A R R A S R C * S C/AS/C C C C/A * C S 13 A 1 A R/S * A C 14 S/C C C C/A R S S S C 7 R 1 3 104 C 13 C 9 C C R/S 2 33 C 83 R R R R C S/C S 4 2 1 S/C S S C 53 C 1 C AA 8 11 1 2 1 S 80 1 60 38 AC 1 C S/C 6 28 40 1 S 7 R CA S 57 41 C R S C 5 24 C/A * 14 108 R CC A S S 3 R S 2 S * S 1 40 15 28 R 180 S 1 19 S 15 C 46 R 1 20 3 85 137 C 23 C R 55 C 35 S A 5 C 1 45 S/C 82 R 20 C/A S/C 2 R C 6 50 1 1 6 150 2 RR 66 C/A 12 1 232 40 C/A * 5 47 51 S 6 A C 15 2 C/A S S 53 R S A 4 S RR 2 60 S C 84 14 33 35 80 C R C 141 R 160 A 86 S S 180 C A A C 200 180 R R S cf 191 S A S A R 117 S Presence of 94 R 96 R C 153 S Presence of 68 R S R S S S S R S C R S R C S S S R C S/C C A Base of primary marine assemblages C/A A C A Notes: * = detected only in extra analysis. Relative abundances are semiquantitative when in letters (see are semiquantitative abundances analysis. Relative Notes: * = detected only in extra

116 CALCAREOUS NANNOFOSSILS IN THE BASAL ZANCLEAN is related to F. profunda F. Remarks

bundance of bundance

Dolomite

Gypsum crystals Gypsum

Foraminifers Cenozoic reworked Cenozoic

SAS SAAS S A C of sphenoliths abundance “Anomalous” R A C of sphenoliths abundance “Anomalous”

S/C C A Cretaceous reworked Cretaceous

RARC AR RC AAS RR AC RS AAS AAS

total abundance total Discoaster

spp. Thoracosphaera

spp. Umbilicosphaera

sp. Ceratolithus

Calcidiscus macintyrei Calcidiscus

Scapholithus fossilis Scapholithus

spp. Rhabdosphaera

te are purely qualitative estimates. te are purely qualitative

(circular outline) (circular pseudoumbilicus R.

spp. Syracosphaera

spp. Scyphosphaera

(see footnote) (see profunda Florisphaera

spp. Discoaster

Discoaster intercalaris Discoaster

Discoaster pentaradiatus Discoaster

Discoaster brouweri Discoaster

Amaurolithus delicatus Amaurolithus

Amaurolithus primus/tricorniculatus Amaurolithus

spp. Pontosphaera

intermedia Helicosphaera

variabilis gr. Discoaster

Discoaster surculus Discoaster

“Material and Methods” section [this chapter] for abundance keys) and quantitative when in numbers (number of specimens/mm2). A and quantitative keys) “Material and Methods” section [this chapter] for abundance

Calcidiscus leptoporus Calcidiscus

Reticulofenestra pseudoumbilicus Reticulofenestra

Helicosphaera carteri Helicosphaera

spp. Table 2. Calcareous nannofossil distribution chart of the uppermost Messinian and basal Zanclean in Hole 967A. distribution nannofossil 2. Calcareous Table Dictyococcites

moriformis/abies gr. Sphenolithus

Reticulofenestra small

Coccolithus pelagicus Coccolithus

Nannofossil preservation Nannofossil

Nannofossil abundance Nannofossil

cm from top of section of top from cm

Section Core 100 specimens of all other species (see “Material and Methods” section [this chapter]). Abundance of gypsum crystals and dolomi of gypsum 100 specimens of all other species (see “Material and Methods” section [this chapter]). Abundance 417-5 P RSARRRR RRRR 72-75C 14X1 321-1GMASCA/2.1.R10.1.2.20.10SS RR RRR 86.0 AASCC/AC/A24.015.0RS1.07.018.025.02.02.0130RSRSR 13H 19-21AG/M 13H 113H 1 RRR 27-2913H2 1 77-7913H A 117-119 RR 13H G/M A A 213H G/M G/M 3 RR 94-96 R13H AA AA 3 AA 25-27 C13H C A 3 C 69-7113H G/M S A 113-115 4 S R R 79-81S/CP 13H G/M A C 4 SCC/ASRSRRR C 25-27 S C13H513-15C/APSSSSSS G/M A C/A C/A 4 S/C 2.5 78-80 C AA A13H5 A A A A 109-111 1.5 A AA R/S13H C A C A A A M S S 16.013H 5.0 3.0 5 S C 2.0 P S/C P A13H C/A C R/S R S 103-105 6 8.0 S/C 13.0 C C S/CA13H C C 0.5 S 6 C C C 37-39 0.513H C A R/S C 6 4.0 11.0 2.5 43.0 P C 79-81 C S/C13H 6.0 1.0 A C C R 99-101b 6 S 11.0 1.0 10.0 2.0 R/S14X S S/C 2.0 8.0 R/S 9.0 S A A 99-101a 7 7.0 M 4.0 27.0 S/C 32.0 C 9.0 G/M 3.0 1.0 S R C/A S 5.0 R 1 M S/C 27.0 29-31 C14X1119-122CP 4.0 A S/C C/A S C/A R R 4.0 S/C C/A 54-57 P A A 2.0 S 2.0 0.1 S S 1.0 22.0 40 C/A R R 40 1.0 1.1 1.0 120 A R C/A S S/C 1.0 8.0 R P P S S S 140 S C/A S S C/A S C 70 S S S S R S R/S S/C C C R S C C/A S S S/C R/S S/C 0.8 0.8 130 S R S R A 1.7 4.6 1.0 S R C S/C S R/S R 50 R R R S C/A S R R S/C R R S 0.2 R S C/A 0.1 R R R RR 0.3 C R 0.3 R 0.4 R R 1.1 R R S/C 2.7 S/C 0.1 R R 0.2 S 0.2 86.0 0.1 0.1 R S 0.2 0.2 C/A RR R/S RR RR R R R 40.0 18.0 R RR S/C 35.0 7.5 RR R S R R 64.0 R R 44.0 R S R R R R R R S R R R R S S R R 8.6 1.7 C 3.2 S S C A A R A R S R R S C C C C Zanclean assemblage (drilling disturbances) Base of primary marine assemblages Zanclean assemblage (drilling disturbances) S C/A S C Notes: Relative abundances are semiquantitative when in letters (see are semiquantitative abundances Notes: Relative

117 D. CASTRADORI

Discoaster F. profunda * D. gr. variabilis D. surculus D. pentaradiatus D. brouweri total abundance % % % % 969B (n/mm2) Core 11H 0 100 200 050100 150 050100 150 050100 150 050100 150 0 200 400 0 0 0 0 0 0

50 50 50 50 50 50

Sect. 5 100 100 100 100 100 100

Zanclean

150 150 150 150 150 150

200 200 200 200 200 200

Sect. 6 250 250 250 250 250 250

300 300 300 300 300 300 Messinian

"sapropel" No quantitative data (see range chart) *abundance compared to pelagic marl 100 specimens of other species

Figure 2. Quantitative abundance pattern of selected nannofossil species in the basal Zanclean of Hole 969B. For explanation on analytical methods, see the “Material and Methods” section (this chapter).

In this study, I found a few very well-preserved specimens of T. that its stratigraphic range is restricted to the basal Zanclean, but it rugosus (Pl. 1, Fig. 7) up to the topmost part of Section 160-969B- obviously needs further investigation. 11H-5 (last sample studied). Moreover, two specimens of C. acutus (Pl. 1, Fig. 5) were found in Samples 160-969B-11H-6, 69−70 cm, Paleoceanography and Mechanism and 11H-6, 59−60 cm. Specimens of Ceratolithus sp. (probably C. of Sapropel Formation acutus) were also detected at Sections 160-967A-13H-2 and 13H-3 (see Table 2). I did not observe specimens of D. quinqueramus in any The following focuses on sediments from Hole 969B in which of the analyzed samples. five sapropel layers were recovered in the basal Zanclean. There were My biostratigraphic findings indicate that: no sapropel layers recovered from this stratigraphic interval in Hole 967A. 1. Contrary to what reported by Bukry (1973), Okada and Bukry (1980), and Berggren et al. (1995a, 1995b) the LAD of T. rug- Florisphaera profunda osus and the FAD of C. acutus do not exactly coincide, a de- tectable overlap being present in the basal Zanclean. This is in Table 1 and Figure 2 show the abundance of F. profunda in the good agreement with what reported by Backman and Raffi basal Zanclean of Hole 969B. It is clearly evident that this species has (1997) from the Ceara Rise. an abundance pattern strictly linked to the sapropel/nonsapropel pat- 2. The FAD of C. acutus is probably the best approximation of tern. In fact, abundance peaks of F. profunda are present in all the Messinian/Zanclean boundary, in terms of nannofossil bio- sapropel layers, and particularly in the lower four (Pl. 2, Fig. 1). It stratigraphy. should be emphasized that nannofossil assemblages within sapropels are usually well preserved, often better than the normal underlying Other data obtained in the present study seem promising from bio- stratigraphic and chronostratigraphic points of view. In particular, the and overlying pelagic marls (see also Müller, 1978, 1985; Violanti et presence of rather numerous specimens of Helicosphaera intermedia al., 1991; Castradori, 1993). Therefore, I consider the quantitative (in samples with virtually no reworking) in the basal Zanclean of pattern of F. profunda detected in these sediments as a genuine prod- Holes 969B and 967A is rather surprising (Pl. 2, Fig. 5). As far as I uct of abundance variations in the primary nannoplankton popula- know, this typical Miocene species has never been reported in the tion. F. profunda shows some abundance fluctuations also in sedi- basal Pliocene. I suggest to investigate further the upper part of the ments from Hole 967A (Table 2; Fig. 3) that are not directly related stratigraphic range of H. intermedia, whose LAD may prove useful to any peculiar sedimentological features. as a subsidiary tool for the identification of the basal Zanclean. Another unusual morphotype is a Reticulofenestra sp. with circu- Discoaster and Amaurolithus lar outline and overall dimensions of ~6–8 µm (Pl. 2, Fig. 2). This morphotype is abundant throughout Sections 160-969B-11H-5 and Abundance variations of the Discoaster and Amaurolithus 11H-6 and in Sections 160-967A-13H-2 through 13H-4. I was not groups, as well as of their different species, are reported in Tables 1 able to find reports of this species in the literature. This may mean and 2 and Figures 2 and 3.

118 CALCAREOUS NANNOFOSSILS IN THE BASAL ZANCLEAN

Figure 3. Quantitative abundance pattern of selected nannofossil species in the basal Zanclean of Hole 967A. For explanation on analytical methods, see the “Material and Methods” section (this chapter).

Discoaster spp. are present in very high abundances (up to 300, Coccolithus pelagicus and usually >100 specimens/mm2), especially when compared to cor- responding values obtained in the Western Mediterranean (our data, On a semiquantitative basis, the abundance variations of this spe- ODP Sites 975 and 978) and in the Tyrrhenian Sea (Rio et al., 1990). cies show distinctive features. First of all, this species seems general- Generally speaking, Discoaster variabilis is the most abundant ly less abundant in the Eastern than in the Western Mediterranean species throughout. Discoaster pentaradiatus, D. surculus, and D. (Müller, 1985; our data, ODP Sites 975 and 978). Moreover, intervals brouweri are often well represented, the latter two particularly in sed- of remarkably low abundances are recorded. In sediments from Hole iments from Hole 967A. 969B, two intervals of strongly decreased abundance coincides with In sediments from Hole 969B, there seems to be a connection be- the lower sapropel of Section 6 and with the lower sapropel of Sec- tween increased total Discoaster abundance and the sapropel layers, tion 5. In Hole 967A, an interval of almost complete absence of C. particularly the lower two sapropels of Section 160-969B-11H-5. pelagicus was detected from Samples 160-967A-13H-2, 94−96 cm, Probably more obvious is the presence of abundance peaks of D. pen- up to 13H-1, 27−29 cm (uppermost sample studied). taradiatus within the two sapropels of Section 160-969B-11H-6 and in the lower two of Section 160-969B-11H-5. D. variabilis shows Summary negative excursions in the same layers, both as “absolute” number per mm2 (Table 1) and as a percentage of the Discoaster assemblage The analytical results outlined above provide important insights (Fig. 2). on the mechanism of sapropel formation. The only species that shows With abundance values up to 18 specimens/mm2, Amaurolithus an abundance pattern clearly related to the sapropel pattern is F. pro- spp. are much more abundant than in corresponding intervals of the funda. In addition, D. pentaradiatus and C. pelagicus seem partially Tyrrhenian Sea (Rio et al., 1990) and of the Western Mediterranean sensitive to the mechanism of sapropel formation. (our data, ODP Sites 975 and 978). No straightforward relationship is The abundance peaks of F. profunda suggest a genetic relation- noted between abundance variations of this group and the sapropel ship between the basal Zanclean sapropels and those of mid-Pleis- pattern. tocene to Holocene age (Castradori, 1993). In both cases, the in- creased abundance of F. profunda is interpreted as a result of the Scyphosphaera spp. presence of a DCM, that is of enhanced primary production in the lower part of the photic zone. The DCM forms as a result of de- Tables 1 and 2 record the semiquantitative abundance variations creased salinity of surficial waters in the source area of the Mediter- of this taxonomic group (Pl. 2, Figs. 3−4 and 6−7). It can be noted ranean Intermediate Water (MIW), near (Rohling and that Scyphosphaera spp. are always well represented, and often abun- Gieskes, 1989). This reduced salinity is caused by higher than normal dant, in the Zanclean sediments. Relative abundance of this group runoff of the Nile River (Rossignol-Strick, 1985) and enhanced pre- was also noted in ODP Site 132 (Raffi and Rio, 1979), whereas it has cipitation over the land bordering the Levantine Basin to the east sporadic occurrences in the basal Zanclean of the Capo Rossello sec- (Rohling and Hilgen, 1991) at times of increased seasonality in the tion (Rio et al., 1984). No easily discernible relation is observed be- Northern Hemisphere (i.e., perihelion in the boreal summer; see tween the abundance variations and the sapropel/nonsapropel pat- Rohling, 1994, for a review). This reduced salinity would cause a tern. shoaling (up to the lower part of the photic zone) of the pycnocline

119 D. CASTRADORI separating the MIW from the overlying surficial waters. This rising Discoaster group, and particularly D. pentaradiatus, showing warm- determines the upward mixing of new nutrients into the lower photic water preferences; Backman and Pestiaux, 1987, Thunell et al., zone (where F. profunda lives), with the development and mainte- 1991a) or a low primary production in the upper part of the photic nance of a DCM. In such a setting, primary production is mainly zone (the Discoaster as a group being an indicator of oligotrophic “new” production, which greatly increases the export of organic mat- conditions; Chepstow-Lusty et al., 1989), or even both conditions. In ter from the photic zone (e.g., Legendre and Le Fevre, 1989; Eppley, fact, in the DCM model, the increased primary production is confined 1989). Therefore, the abundance of F. profunda is a proxy for the in- to the lower part of the photic zone, whereas the upper layers may be creased export of organic matter from the photic zone. characterized by relatively low primary production and may be also From studying calcareous nannofossils that live in the upper part by higher temperatures (at least in summer). The latter would be re- of the water column, no conclusion can be reached about the devel- sponsible for the observed decreased abundances of C. pelagicus, this opment of anoxia in the bottom waters. In my opinion, anoxia, con- species being a well-known cold-water indicator (e.g., McIntyre and sidered the primary mechanism of sapropel formation by many au- Bé, 1967; McIntyre et al., 1970; Geitzenauer et al., 1976). thors (see “Background and Objectives”), may have played an impor- The relatively high abundances of Scyphosphaera spp. recorded tant role, in particular for the formation of those sapropels with a not in the present study and in the Western Mediterranean (Raffi and Rio, so high organic carbon content (according to Rohling, 1994). How- 1979; our data, ODP Sites 975 and 978) are of problematic interpre- ever, the abundance variations of F. profunda demonstrate that in- tation. Unfortunately, the paleoecology of this group is largely un- creased primary production is an important factor, and probably the known. However, it seems to show a preference for neritic, near- triggering mechanism, for the formation of (most) sapropel layers. shore environments (Perch-Nielsen, 1985). The abundance pattern of the Discoaster group and of Cocco- Very preliminarily, it may be suggested that the high abundance lithus pelagicus suggests an interesting relationship between the of this group may have originated during the earliest times of the Zan- sapropel/nonsapropel pattern in the basal Zanclean of ODP Hole clean transgression, when the basically nonmarine environments 969B and the limestone-marl couplets that characterize the time characterizing most of the Mediterranean area were rapidly trans- equivalent Trubi Formation widely outcropping in Sicily and Cala- gressed by the incoming waters, which gave rise for a while to widely bria (Southern Italy). distributed neritic environments. Then, scyphosphaerids may have The couplets of the Trubi Formation are unanimously thought to proliferated, at least for a short time interval, in the overlying open- reflect the periodicity of the Earth precession cycle (~21−23 k.y.) marine sediments. (e.g., Hilgen, 1991a, 1991b; Thunell et al., 1991a, 1991b). However, the mechanism through which the astronomical signal is transferred into the sedimentary record is far from clear. CONCLUDING REMARKS According to De Visser et al. (1989), the marl is the product of higher fertility and temperature of the surficial waters (witnessed by The analytical data presented here allowed me to characterize the increased abundance of Neogloboquadrina acostaensis, lower δ18O, uppermost Messinian sediments and provide some constraints on the and higher Barium content) at times of increased runoff. The marls correlation of the Messinian/Zanclean boundary outside the Mediter- were considered by these authors as the “equivalent” of the sapropels ranean by means of calcareous nannofossils. characterizing the Monte Narbone Formation, overlying the Trubi in The more interesting outcome of this paper is probably the possi- Sicily and Calabria (see also Lourens et al., 1996). On this assump- bility to apply the DCM model of sapropel formation to the basal tion, Hilgen (1991a, 1991b) proposed his astronomically calibrated Zanclean. In this scenario, enhanced export of organic matter toward time scale, where marl layers (in the Trubi) and sapropels (in the the seafloor results from an increased primary production in the lower Monte Narbone) are correlated with periods of increased seasonality part of the photic zone (DCM). As for mid-Pleistocene to Holocene in the Northern Hemisphere (perihelion in the boreal summer at times sapropels (Castradori, 1993), the abundance variations of F. profun- of maximum eccentricity). da are of key importance, even in the basal Zanclean, as a proxy of a On the contrary, Thunell et al. (1991a, 1991b) interpreted the similar scenario. Since the DCM model has been applied also to the limestones (and not the marls) as the result of increased primary pro- formation of late Pliocene and early Pleistocene sapropels (Lourens duction connected with localized upwelling phenomena in the Medi- et al., 1992) and, very recently, of sapropelites outcrop- terranean region. They indicated the abundance of Globigerina bul- ping on Crete (Nijenhuis et al., 1996), it appears that the same mech- loides, benthic foraminifers, Coccolithus pelagicus (indicating cold anism may have triggered the formation of most sapropel layers in upwelled waters), and small placoliths as the results of the increased the sedimentary record of the Mediterranean area. primary production in the limestones, whereas abundant Globiger- inoides spp. and Discoaster spp. (particularly D. pentaradiatus) and scarcity of benthic foraminifers would be the result of warm water ACKNOWLEDGMENTS and lower productivity. The analytical results of the present study, that is, increased abun- My sincere thanks are due to Maria Bianca Cita for her always en- dance of D. pentaradiatus (and probably of the Discoaster as a couraging and supportive attitude; to Kay Emeis and Angelo Camer- group) in most sapropels and decreased abundance of C. pelagicus in lenghi for useful advice; to Wuchang Wei, Carl Richter, and an anon- two of them, indicate, when compared with data of Thunell et al. ymous reviewer for helpful comments; to Stefano Torricelli for tech- (1991a, 1991b), that the sapropels recovered in Hole 969B are prob- nical help; and to Renata and Luca, for their patience. ably to be correlated to the marly layers of the Trubi Formation, in agreement with the interpretation by De Visser et al. (1989), Hilgen REFERENCES (1991a, 1991b), and Lourens et al. (1996). This conclusion comes from a simple comparison of analytical data from the different lithol- Backman, J., and Pestiaux, P., 1987. 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Rohling, E.J., 1991. A simple two-layered model for shoaling of the Eastern Thunell, R., Rio, D., Sprovieri, R., and Vergnaud-Grazzini, C., 1991b. An Mediterranean pycnocline due to glacio-eustatic sea level lowering. Pale- overview of the post-Messinian paleoenvironmental history of the West- oceanography, 6:537−541. ern Mediterranean. Paleoceanography, 6:143−164. ————, 1994. Review and new aspects concerning the formation of east- Thunell, R.C., and Williams, D.F., 1983. Paleotemperature and paleosalinity ern Mediterranean sapropels. Mar. Geol., 122:1−28. history of the eastern Mediterranean during the late Quaternary. Palaeo- Rohling, E.J., and Gieskes, W.W.C., 1989. Late Quaternary changes in Med- geogr., Palaeoclimatol., Palaeoecol., 44:23−39. iterranean intermediate water density and formation rate. Paleoceanogra- Thunell, R.C., Williams, D.F., and Kennett, J.P., 1977. Late Quaternary pale- phy, 4:531−545. oclimatology, stratigraphy and sapropel history in Eastern Mediterranean Rohling, E.J., and Hilgen, F.J., 1991. The eastern Mediterranean climate at deep-sea sediments. Mar. Micropaleontol., 2: 371−388. times of sapropel formation: a review. Geol. Mijnbouw, 70:253−264. Violanti, D., Grecchi, G., and Castradori, D., 1991. Paleoenvironmental Rossignol-Strick, M., 1985. Mediterranean Quaternary sapropels, an imme- interpretation of core Ban88-11GC (Eastern Mediterranean, Pleistocene- diate response of the African Monsoon to variation of insolation. Palaeo- Holocene) on the grounds of , Thecosomata and calcareous geogr., Palaeoclimatol., Palaeoecol., 49:237−263. nannofossils. Il Quatern., 4:13−39. Ryan, W.B.F., 1972. Stratigraphy of late Quaternary sediments in the Eastern Zijderveld, J.D.A., Hilgen, F.J., Langereis, C.G., Verhallen, P.J.J.M., and Mediterranean. In Stanley, D.J. (Ed.), The Mediterranean Sea: Strouds- Zachariasse, W.J., 1991. Integrated magnetostratigraphy and biostratigra- burg, PA (Dowden, Hutchison and Ross), 149−169. phy of the upper Pliocene-lower Pleistocene from the Monte Singa and Sutherland, H.E., Calvert, S.E., and Morris, J.R., 1984. Geochemical studies Crotone areas in Calabria (Italy). Earth Planet. Sci. Lett., 107:697−714. of the recent sapropel and associated sediment from the Hellenic Outer Ridge, eastern Mediterranean Sea, I. Mineralogy and chemical composi- tion. Mar. Geol., 56:79−92. Thunell, R.C., 1979. Pliocene-Pleistocene paleotemperature and paleosalin- ity history of the Mediterranean Sea: results from Deep Sea Drilling Project Sites 125 and 132. Mar. Micropaleontol., 4:173−187. Thunell, R., Rio, D., Sprovieri, R., and Raffi, I, 1991a. Limestone-marl cou- Date of initial receipt: 6 January 1997 plets: origin of the Early Pliocene Trubi marls in Calabria, southern Italy. Date of acceptance: 8 May 1997 J. Sediment. Petrol., 61:1109−1122. Ms 160SR-005

Plate 1. 1, 2. Large twin crystals of gypsum; 975×. Sample 160-967A-14X-1, 72–75 cm; (1) parallel light; (2) crossed nicols. 3, 4. Twin crystals of gypsum; 975×. Sample 160-969B-11H-7, 40–42 cm; (3) parallel light; (4) crossed nicols. 5. Ceratolithus acutus Gartner and Bukry; 975×. Sample 160-969B-11H-6, 69– 70 cm; crossed nicols. 6. “Anomalous” abundance of Sphenolithus spp. (mostly S. gr. abies/moriformis); 975×. Sample 160-967A-14X-1, 54–57 cm; crossed nicols. 7. Triquetrorhabdulus rugosus Bramlette and Wilcoxon; 975×. Sample 160-969B-11H-5, 3–5 cm; parallel light.

122 CALCAREOUS NANNOFOSSILS IN THE BASAL ZANCLEAN

Plate 2. 1. Abundance of Florisphaera profunda Okada and Honjo (small, grey, polygonal calcite plates) in a sapropel layer; 975×. Sample 160-969B-11H-5, 91–93 cm; crossed nicols. 2. Reticulofenestra sp., with circular outline; 975×. Sample 160-969B-11H-6, 19–20 cm; crossed nicols. 3–7. Different morphotypes of Scyphosphaera spp.; all 975×. Sample 160-969B-11H-6, 124–126 cm; parallel light. 5. Helicosphaera intermedia Martini; 975×. Sample 160-969B-11H-5, 91–93 cm; crossed nicols. 8. Reticulofenestra pseudoumbilicus Gartner; 975×. Sample 160-969B-11H-5, 3–5 cm; crossed nicols.

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