Response of the pelagic environment to palaeoclimatic changes in the central Mediterranean Sea during the Late Quaternary

Marine Geology 178 (2001) 39±62

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Response of the pelagic environment to palaeoclimatic changes in the central Mediterranean Sea during the Late Quaternary Laura Sbaf® a,d,*, Forese Carlo Wezel a, Nejib Kallel b,c, Martine Paterne b, Isabel Cacho d,e, Patrizia Ziveri f, Nicholas Shackleton d a

UniversitaÁ degli Studi di Urbino, Istituto di Dinamica Ambientale, SOGESTA Campus Scienti®co, LocalitaÁ Crocicchia, 61029 Urbino, Italy Laboratoire des Sciences du Climat et de l'Environment, Laboratoire Mixte CNRS-CEA, Parc du CNRS, 1198 Gif-sur-Yvette Cedex, France c Faculte des Sciences de Sfax, Laboratoire E08/C10, Route de Soukra, B.P. 763, 3038 Sfax, Tunisia d University of Cambridge, Godwin Laboratory for Quaternary Research, New Museums Site, Pembroke Street, Cambridge CB2 3SA, England, UK e Department of Environmental Chemistry, Institute of Chemical and Environmental Research (CSIC), Jordi Girona 18, 08034-Barcelona, Catalonia, Spain f Vrije Universiteit, Faculteit der Aardwetenschappen, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

b

Received 18 September 2000; accepted 21 February 2001

Abstract Three central Mediterranean deep-sea cores have been studied to reconstruct the palaeoclimatic history of the basin over the time interval 34±0 kyr bp. The intensity and duration of the climatic events that occurred in the Mediterranean Sea during the last glacial-postglacial transition were estimated by observing compositional changes in the planktonic foraminifera and calcareous nannoplankton (coccolithophores) assemblages, together with a reconstruction of sea surface temperatures (using 0 the Modern Analogue Technique and Uk37 index), the d 18O signal and pteropod ¯uxes. The application of two independent and well established techniques for the determination of the past SST led to a number of considerations about the accuracy and ef®cacy of the use of such methodologies in the Mediterranean Sea, notoriously dominated by local factors and characterised by a number of independent environments. The recognition of millennial to centennial climatic instabilities, in both the SST and microfossil records, was possible because of the high resolution of the study. A succession of nine main biozones and six subzones, based on the major changes in the planktonic foraminifera records, has been recognised and compared with GRIP and GISP2 ice cores d 18O values and with records from four other studies from the central Mediterranean Sea. During the early phase of the Holocene, a period characterised by relatively higher temperatures and lighter d 18O values has been recognised as being coeval with the `Climatic Optimum' (between 10.5 and 6.1 kyr bp, calendar age). This interval was characterised by an abrupt drop in the pteropods relative abundances and ¯uxes. The pteropodal fossil assemblage may have been affected by a possible event of selective dissolution of the aragonite driven by the shoaling of the ACD. A coeval change in the position of the pycnocline may have been responsible for a strong relative increase in abundance of Neogloboquadrina pachyderma r.c., and a decrease in the abundance of Globorotalia in¯ata in the western sector of the Mediterranean Sea. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Biozonation; SST; Tyrrhenian Sea; Calendar age; Planktonic environment; Late Quaternary

* Corresponding author. Present address: School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK. Tel.: 1441603-592970; fax: 144-1603-507719. E-mail address: l.sbaf®@uea.ac.uk (L. Sbaf®). 0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(01)00185-2

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L. Sbaf® et al. / Marine Geology 178 (2001) 39±62

1. Introduction The Mediterranean Sea has been the focus of intensive studies during the last half-century. The crucial importance of this small and marginal basin is in its capability to register and amplify the smallest climatic variations occurring at the mid-latitude regions. The signals registered by changes in abundance and distribution of fossil micro-organisms represent one of the most powerful tools in many environmental reconstruction, and in the Mediterranean region they provide a reliable and well-documented record at both global and local scales. In this paper, we focus our attention on the Tyrrhenian Sea, the youngest (Pliocene) of the sub-basins that compose the Mediterranean Sea. In the last few decades, several authors have studied the deep-sea sediment successions recovered from this area and have contributed to the reconstruction of the last glacial cycle through the analysis of foraminiferal abundances (Borsetti et al., 1992; Capotondi et al., 1989, 1999; Jorissen et al., 1993; Asioli et al., 1999; Tamburini et al., 1998; Sbaf® et al., 1998; Kallel et al., 1997, 2000; Ariztegui et al., 2000), of coccolithophores (Borsetti and Cati, 1972, 1976; Morlotti and Raf®, 1981; Raf® and Rio, 1981), of pollen (Rossignol-Strick and Planchais, 1989) and of pteropods (Biekart, 1989). The high-resolution proxies obtained in this study have permitted the reconstruction of the palaeoclimatic history of the south Tyrrhenian Sea from 34 kyr bp to the sub-recent. Numerous works about the late Quaternary palaeoceanography of the western Mediterranean Sea revealed dramatic glacial±interglacial variations in both foraminifera associations and their stable isotope records. In particular, the last deglaciation has been extensively studied, although very few records reached a very high detail of study. Past and subpresent sedimentation rates calculated for the CefaluÁ Basin are among the highest found in the Mediterranean Sea and the highest ever reported for the Tyrrhenian Sea. As a result, the resolution achieved is the highest obtained so far for this Mediterranean region. The planktonic associations (foraminifera and coccoliths) show a strong correlation with sea surface temperature variations and with the changes in the physical and chemical properties of the upper water column caused by the climatic instability. Moreover,

the direct in¯uence of North Atlantic Surface Water (Allain, 1960) on the faunal and ¯oral assemblages gives a measure of the close hydrological relationship between the North Atlantic and Mediterranean Sea water masses, and underlines the prominent role of the Mediterranean in the understanding of global climatic evolution. Recently acquired palaeoclimatic records deriving from ice cores (Dansgaard et al., 1993), marine cores (Heinrich, 1988; Bond et al., 1997; Hendy and Kennett, 1999) and terrestrial sites (Follieri et al., 1993; Watts et al., 1996a,b) have demonstrated that major climatic ¯uctuations occurred during the last glacial±interglacial cycle, and Mediterranean records are particularly suitable for studying their intensity and evolution (Rohling et al., 1998; Cacho et al., 1999a,b; Paterne et al., 1999). Detailed studies of past SST performed in the Alboran Sea and in the Tyrrhenian Sea have demonstrated the occurrence of a number of relatively brief cooling events during the last 12 kyr (Cacho et al., 2000), perhaps coeval to short events observed in the North Atlantic (BjoÈrck et al., 1996; Alley et al., 1997). The data collected suggest the direct in¯uence of the ocean in¯ow on at least part of the coolings, largely obliterated by the strong imprint of the climatic regime of whole Mediterranean region (Rohling et al., 1997; De Rijk et al., 1999). Here we propose revised bio-zonation criteria for planktonic foraminifera and coccolith assemblages that integrate the observations made by Jorissen et al. (1993), Tamburini et al. (1998), Asioli et al. (1999) and Capotondi et al. (1999) in the same region with the event stratigraphy introduced by the INTIMATE Group (BjoÈrck et al., 1998) for the GRIP Greenland ice core. We also examine the main climatic changes registered by our cores (BS79-38, BS79-33 and BS7922) during the last glacial cycle, to assess the relationship between our records and those obtained from other cores recovered in the western Mediterranean Sea (MD95-2043), and from the Greenland Summit (GISP2 and GRIP).

2. Methods 2.1. Description of the cores This paper is based on a study of three deep-sea

L. Sbaf® et al. / Marine Geology 178 (2001) 39±62

41

Fig. 1. Location of the cores discussed in this study. The grey arrows indicate the present-day surface circulation of the North Atlantic Ocean.

gravity cores collected from the CefaluÁ Basin (southern Tyrrhenian Sea intraslope basin) during the summer of 1979 within the framework of the Italian C.N.R. project `Oceanography and Sea Bottoms'. BS79-38, BS79-33 and BS79-22 (Fig. 1 and Table 1) are undisturbed and very well preserved cores that consist mostly of grey hemipelagic mud and silty mud, testifying a scarce terrigenous supply. The coarse fraction ….63 mm† consists of foraminiferapteropodal ooze and the terrigenous particles generally represent less than 15% of the total dry weight. The apparent absence of hiatuses and turbidites con®rms the continuity of the sedimentary record, making it suitable for detailed analysis. The protected position of this basin, lying between the upper and the

lower continental shelf, produces a direct control on the ¯ow of terrigenous sediments to the abyssal plain and provides a continuous Late Quaternary geological record. 2.2. Micropalaeontological analyses Samples of about 20 g were taken at 3±5 cm intervals through the sediment, washed over a 63 mm mesh sieve using distilled water, and dry sieved through a 150 mm mesh sieve. Qualitative and quantitative analyses have been performed on the planktonic foraminiferal assemblage for the fraction . 150 mm; split into aliquots each containing at least 400 specimens of planktonic foraminifera. All the shells present

Table 1 Speci®cations of the sediment cores described in this paper Core

Latitude

Longitude

Depth (m)

Length (cm)

Sedimentation rate (cm/kyr)

BS79-38 BS79-33 BS79-22

38824.7 0 N 38815.7 0 N 38823.1 0 N

13834.6 0 E 14801.8 0 E 14823.0 0 E

1489 1282 1449

516 485 523

22.5 17.5 16.7

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L. Sbaf® et al. / Marine Geology 178 (2001) 39±62

Table 2 AMS dating obtained for core BS79-33. The 14C ages are corrected for the reservoir age (,400 years) and converted into calendar age Core

Depth (cm)

Age 14C (yr bp)

Standard deviation (2s)

Sample type (plank. foram.)

Calendar age (yr bp)

BS79-33 BS79-33 BS79-33 BS79-33 BS79-33 BS79-33 BS79-33

115 136 185 225 295 340 450

6310 8160 10830 12910 15480 16990 24120

^ 70 ^ 90 ^ 110 ^ 110 ^ 130 ^ 140 ^ 220

G. ruber alba G. ruber alba G. bulloides G. bulloides G. bulloides G. bulloides G. bulloides

6747 a 8615 a 12226 a 14343 a 17917 a 19655 a 27608 b

a b

Converted into calendar age with the program `calib 4.1' (Stuiver and Reimer, 1993). Converted into calendar age as described in Section 3.

in these sub-samples were then identi®ed and counted and the data expressed as percentages of the total number of planktonic foraminifera. Neogloboquadrina pachyderma right and left coiling were initially counted separately, but because of the low …,1%† and apparently random frequency of N. pachyderma l.c., we rejoined the two taxa. Globigerinoides ruber var. alba and rosea were considered separately. However, factor analyses performed on the planktonic associations have shown a close relationship between several less abundant species belonging to the Globigerinoides genus (G. sacculifer, G. tenellus, G. conglobatus, G. ruber var. rosea) and the species Beella digitata and Globigerinella aequilateralis. Thus, we grouped these taxa under the term `minor species'. Similar grouping criteria were also applied by Pujol and VergnaudGrazzini (1989) in several Alboran Sea cores (Globigerinoides group) and by Rohling et al. (1993) in eastern Mediterranean records (SPRUDTS group). Analyses of the calcareous nannoplankton assemblage of core BS79-33 were performed on smear slides using a light microscope with 1000 times magni®cation. In forty-®ve samples, taken at 10 cm intervals, at least 300 specimens were identi®ed and the percentages (relative abundances) of selected species were calculated. Emiliania huxleyi was by far the most abundant taxon in the sediments (up to 60% during the Holocene) and to minimise statistical errors, a minimum of 100 specimens of species other than E. huxleyi were counted in each sample. Selected samples were also studied using an SEM microscope. The group of the small Gephyrocapsa (G. muellerae, G. ericssoni and G. protohuxleyi) was considered separately from G. oceanica and consists of the

species smaller than 3 mm; Helicosphaera spp. were counted together, except H. carteri, which was the most abundant species belonging to the Helicosphaera genus recognised in our cores. 2.3. Stable isotope measurements Isotopic analyses were performed at the Godwin Laboratory for Quaternary Research, University of Cambridge, UK. Oxygen and carbon isotope stratigraphy was based on three different planktonic species (G. ruber var. alba, G. bulloides and N. pachyderma r.c.). Samples of 35±40 specimens of planktonic fauna picked in the fraction . 180 mm were analysed in a SIRA mass spectrometer with a VG isocarb common acid bath system. The analytical reproducibility of laboratory standard was better than ^0:08½ for d 18O. Isotopic results were calibrated to the Vienna PeeDee Belemnite (VPDB) standard via the NBS19 standard. 3. Time stratigraphic framework Accelerator mass spectrometer (AMS) 14C dates were carried out at the Centre des Faibles RadioactiviteÂs, Gif-sur-Yvette, using the procedure suggested by Arnold et al. (1989). Seven samples of about 10±12 mg of monospeci®c planktonic foraminifera shells (Globigerina bulloides and G. ruber var. alba) were hand-picked in the fraction . 150 mm in core BS79-33 (Table 2). When possible, the samples were selected from high abundance peaks so as to reduce biases deriving from bioturbation. The radiocarbon ages were converted into calendar

L. Sbaf® et al. / Marine Geology 178 (2001) 39±62

43

Fig. 2. Age control points used in the two age models applied to core BS79-33. The solid circles refer to the age model based on: (1) direct 0 comparison between0 BS79-33 d 18O G. bulloides record and the GISP2 d 18O pro®le; (2) direct comparison between BS79-33 Uk37 SST record and MD95-2043 Uk37 SST record (Cacho et al., 1999b); (3) AMS 14C dates calibrated with `calib 4.1' program. The open squares refer to the age model based on: (1) direct comparison between BS79 biozonation and Capotondi et al. (1999) biochronology; (2) AMS 14C dates. The grey arrows show the position of the seven AMS dates.

ages (after Bard et al., 1990) using the `calib 4.1' computer program (Stuiver and Reimer, 1993), which uses a revised marine calibration dataset (Stuiver et al., 1998) and incorporates a time-dependent global ocean reservoir correction of about 400 years for mid-latitude oceans. This program allows the conversion of 14C ages younger than 20,760 years. For older ages, the conversion becomes more dif®cult, because of the poor knowledge of the natural variation in the quantitative production of cosmogenic 14C that occurred over longer time scales. A calendar age for the sample at 450 cm depth was assigned by extrapolating the data from Bard et al. (1990) (Table 2). We successively integrated the ages obtained from the AMS dating with ages obtained by aligning our cores with other well-dated records from the Greenland ice sheet (core GISP2; Grootes et al., 1993; Meese et al., 1997), the Alboran Sea (core MD95-2043, Cacho et al., 1999b) and from the central Mediterranean Sea

(Capotondi et al., 1999). We compared the d 18O record of the GISP2 ice core with the d 18O pro®le obtained for the planktonic species G. 0 bulloides in our cores. Furthermore, we used the Uk37 SST record of core MD95-2043 to provide more detailed series of time control points corresponding to the Holocene and part of Termination I. We also made a direct correlation between our revised biozonation and that proposed by Capotondi et al. (1999) for adjacent areas of the Mediterranean Sea, thus strengthening the existing age model based on 14C analyses by dating levels of important change in the microfaunal record. A continuous time-scale was obtained by linear interpolation assuming invariant sedimentation rates between control points. In this section, we describe two different chronostratigraphic models obtained for our cores as a result of the application of the comparisons summarised above (Fig. 2).

44

L. Sbaf® et al. / Marine Geology 178 (2001) 39±62

The ®rst model (Age Model 1) is based on the AMS dates and on the biostratigraphic study of Capotondi et al. (1999). These authors recognised ten different biozones in a large number of cores collected from the Tyrrhenian and the Adriatic Seas. Most of these zones were readily identi®ed in our cores, providing a temporal cross check between the two studies. The control points for core BS79-33 are summarised in Table 3. The age models for cores BS79-38 and BS79-22 have been obtained by direct comparison with core BS79-33. The second age model (Age Model 2) is based on a careful correlation (tuning) of the oxygen isotope records of BS79 cores to the GISP2 ice core. For this age model, the Holocene interval was tuned to0 core MD95-2043 (Cacho et al., 1999b). The Uk37 index SST record calculated for the Alboran Sea and that calculated for the Tyrrhenian Sea show remarkable similarities (Cacho et al., 2000). A series of ®ve cooling events, named C1±C5 from the youngest to the oldest (Fig. 3) has been recognised in the alkenone record during the last 12 kyr in core BS7938, which presents the more detailed SST study. Similar events have been also observed in core MD952043 and the close resemblance between the two records allowed us to attempt a peak-to-peak correlation. The cooling events will be described in more detail in Section 4.2. This model represents an improvement on the previous one, since it provides a larger number of control points for the interval of time spanning the last glacial period (between 34±15 kyr bp approximately) increasing the accuracy of the chronostratigraphic cover during this interval. The new control points obtained for cores BS79-38, BS79-33 and BS79-22 are summarised in Table 4. The two models show strong similarities during the glacial interval, but highlight a relatively pronounced divergence during the Holocene (Fig. 2) when the differences between Age Model 1 and 2 range between approximately 0 and 1350 years (Fig. 3). In the near future, we intend to collect more data and perform more AMS dating to determine which model is the closest to the real age. However, in this paper we will refer mostly to the second age model, since the ®rst one was already suf®ciently described and applied in Cacho et al. (2000). As a ®rst result, the application of Age Model 2 to

the CefaluÁ Basin cores allowed us to calculate the sedimentation rates, which differ between the three cores and range from 25.6 to 17.5 cm/kyr, respectively for glacial and interglacial intervals with an average sedimentation rate of 19.8 cm/kyr. The mentioned rates are the highest documented for the Tyrrhenian Sea and among the highest registered for the central Mediterranean Sea.

4. Discussion 4.1. Comparison of palaeotemperature estimates The SST record was determined using two distinct approaches, which are by now the most widespread methods for determining palaeotemperatures in deepsea sediments. The ®rst method considers the foraminiferal assemblage and the relative abundance of each species as indicators of particular environmental conditions, in which the temperature of the uppermost water column is the main variable. This method, known as Modern Analogue Technique (MAT) (Hutson, 1980), has recently been used in the Tyrrhenian Sea (Kallel et al., 1997; Paterne et al., 1999) and yields SST records depending on the different growth seasons of the planktonic taxa. MAT is based on a comparison between the relative foraminiferal composition of a fossil sample (in percentages) and a modern database, using its 10 best modern analogues. According to Prell (1985) and Kallel et al. (1997), fossil samples for which good analogues are available have a dissimilarity coef®cient ,0.25. On the other hand, dissimilarity coef®cients .0.3 generally indicate that there are no close modern analogues in the database and that in this case the SST values should be considered with care. In both cores BS79-38 and BS79-33 relatively good modern analogues have been found for a substantial part of the late glacial interval (as far back as 18.5 kyr bp) and for Termination I and the Holocene (average dissimilarity coef®cient respectively 0.19 and 0.17). However, during the LGM interval the average dissimilarity coef®cient is about 0.30±0.33 (Fig. 4b and d). 0 The second method, generally known as Uk37 index, is based on the relative occurrence of the diand tri-unsaturated C37 alkenones (Brassell et al.,

L. Sbaf® et al. / Marine Geology 178 (2001) 39±62 Table 3 Dated control points used in the age model of the core BS79-33 based on 14C dating and on direct comparison with Capotondi et al. (1999) Depth (cm)

Age source

Calculated age References (yr bp)

57 98 115 120 136 150 175 185 204 225 249 295 340 360 450

Boundary 1/2 Boundary 2/3 14 C dating Boundary 3/4 14 C dating Boundary 4/5 Boundary 5/6 14 C dating Boundary 6/7 14 C dating Boundary 7/8 14 C dating 14 C dating Boundary 9/10 14 C dating

2,530 3,985 6,747 7,656 8,615 9,615 10,995 12,225 12,945 14,343 15,343 17,917 19,655 21,968 27,608

Capotondi et Capotondi et This paper Capotondi et This paper Capotondi et Capotondi et This paper Capotondi et This paper Capotondi et This paper This paper Capotondi et This paper

al. (1999) al. (1999) al. (1999) al. (1999) al. (1999) al. (1999) al. (1999) al. (1999)

0

45

1986). These compounds are synthesised only by few Haptophyceae algae, among which E. huxleyi is one of the most abundant. Several studies have demonstrated a 0linear relationship between the unsaturation index Uk37 and the water temperature in which these È ller et al., 1998 and references therein). algae grow (Mu 0 Thus, the Uk37 index has become a very popular SST proxy in palaeoceanographic reconstructions. This relatively new technique has recently been successfully used in the western Mediterranean Sea (Cacho et al., 1999a,b; Doose et al., 1999). The ®rst studies using this method in Tyrrhenian Sea deep-sea cores are discussed by Cacho et al. (2000) and Sbaf® (2000). In Fig. 4a and c we show the results of the simultaneous application of the two SST estimation techniques to cores BS79-33 and BS79-38 and their correlation with planktonic foraminifera d 18O records from the same cores. In both cores we considered the average MAT SST values, calculated from the four

Fig. 3. Application of the Age Models 1 and 2 to the Uk37 SST record of core BS79-38. The differences between the two models are compared 0 with Uk37 SST record from core MD95-2043 (Cacho et al., 1999b) and they go from 0 to 1350 years. The abbreviations C1±C5 represent the positions of cooling events recognised in our SST records and discussed in Section 4.2.

46

L. Sbaf® et al. / Marine Geology 178 (2001) 39±62

Table 4 Dated control points used in the age models of the CefaluÁ Basin cores and based on AMS dating and direct comparison with GISP2 ice core isotopic record (Meese et al., 1997) and MD95-2043 core SST record (Cacho et al., 1999b) Depth (cm)

Age source

Calculated age (yr bp)

Sedimentation rates (cm/kyr)

References

CORE BS79-33 5 75 96 110 140 175 185 195 225 230 295 340 390 430 450 465 480

core BS79-38 core BS79-38 core BS79-28 core BS79-38 core BS79-38 core BS79-38 14 C dating GISP2 ice core 14 C dating BS79-38 14 C dating 14 C dating GISP2 ice core GISP2 ice core 14 C dating GISP2 ice core GISP2 ice core

1,415 5,165 6,783 7,666 10,019 11,785 12,226 12,590 14,343 14,606 17,917 19,655 24,000 25,525 27,608 28,843 30,102

± 18.9 13.0 16.1 12.7 19.8 22.7 27.5 17.1 19.0 19.6 25.9 11.5 26.2 11.3 9.7 11.9

This paper This paper This paper This paper This paper This paper This paper Meese et al. This paper This paper This paper This paper Meese et al. Meese et al. This paper Meese et al. Meese et al.

CORE BS79-38 5 10 35 55 95 125 145 205 230 265 305 320 395 420 495 506

MD95-2043 MD95-2043 MD95-2043 MD95-2043 MD95-2043 MD95-2043 MD95-2043 MD95-2043 MD95-2043 MD95-2043 GISP2 ice core BS79-33 MD95-2043 BS79-33 GISP2 ice core BS79-33

950 1,107 1,877 3,626 4,777 6,071 7,020 10,249 11,303 12,352 14,013 14,606 19,014 20,959 24,000 24,850

± 31.8 32.5 11.4 34.8 23.2 21.1 18.6 23.7 33.4 24.1 25.3 17.0 12.9 24.7 12.9

Cacho et al. (1999b) Cacho et al. (1999b) Cacho et al. (1999b) Cacho et al. (1999b) Cacho et al. (1999b) Cacho et al. (1999b) Cacho et al. (1999b) Cacho et al. (1999b) Cacho et al. (1999b) Cacho et al. (1999b) Meese et al. (1997) This paper Cacho et al. (1999b) This paper Meese et al. (1997) This paper

CORE BS79-22 5 20 32 63 72 81 120 147 156 198 219 267 288

BS79-33 BS79-33 BS79-33 MD95-2043 BS79-33 BS79-33 BS79-33 BS79-33 MD95-2043 BS79-33 BS79-33 BS79-33 BS79-33

1,261 3,626 4,993 6,808 8,058 8,450 10,881 11,753 12,674 14,013 16,443 18,110 19,269

± 6.3 8.8 17.1 7.2 22.9 16.0 30.9 9.8 31.4 8.6 28.8 18.1

This paper This paper This paper Cacho et al. (1999b) This paper This paper This paper This paper Cacho et al. (1999b) This paper This paper This paper This paper

(1997)

(1997) (1997) (1997) (1997)

L. Sbaf® et al. / Marine Geology 178 (2001) 39±62

47

Table 4 (continued) Depth (cm)

Age source

Calculated age (yr bp)

Sedimentation rates (cm/kyr)

References

324 366 425 449 461 508

BS79-33 BS79-33 BS79-33 BS79-33 BS79-33 MD95-2043

20,959 24,534 26,473 28,843 29,514 33,405

21.3 11.7 30.4 10.1 17.9 12.1

This paper This paper This paper This paper This paper Cacho et al. (1999b)

seasonal values provided by this technique, which have been then modi®ed according to a three points running average. A similar comparison has been made between the 0 MAT and the Uk37 index temperature values in recent studies of northern and equatorial Atlantic deep-sea cores (Chapman et al., 1996; Weaver et al., 1999) and of tropical western Paci®c cores (Huang et al., 1997). This comparison allowed the authors to detect the bloom season of the coccolithophore assemblage by 0 observing where the Uk37 temperature estimate lies within the seasonal temperature record provided by the MAT. Although the two methods are not completely analogous and their application is still under discussion for the Mediterranean Sea, the information provided should, in principle contribute towards the understanding of the sea surface water masses. Here, we discuss the reliability of these criteria when applied to our cores. As previously stated, one of the most signi®cant limitations in the application of the MAT technique to Mediterranean Sea sediments is the absence of good analogues during glacial intervals. In particular, some of the down-core faunas observed in our cores (for example, combinations involving substantial numbers of G. scitula and T. quinqueloba described in Section 4.2) have no modern analogues in the Mediterranean Sea. In this case, the use of extraMediterranean analogues would likely compromise the reliability of the results obtained from this method, since faunas in such a basin will bear a strong overprint of biological controls and area-speci®c adaptation. Another problem regarding the accuracy of the absolute values obtained from the MAT measurements in our records is related to the great difference between DSST and Dd 18O in some intervals. Both in core BS79-38 (Fig. 4a) and core BS79-33 (Fig. 4c),

the transition YD-Preboreal, centred at 12.1 kyr bp, is characterised by a MAT DSST of 88C (about 12.58C in summer and 68C in winter, Fig. 5a and b). These values do not agree with the Dd 18O obtained from planktonic foraminifera for the same interval. D of 12 and 68C would correspond to a Dd 18O of about 3±1½, while the real values are respectively 0.75½ for G. bulloides and 0.45½ for G. ruber var. alba. Moreover, the MAT summer SST records show a signi®cant cooling of about 1.5±38C between 11.5 and 6 kyr bp (this interval corresponds to the deposition of sapropel S1 in the eastern Mediterranean and to the so called `Climatic Optimum'). This cooling was indicated by a strong increase in the relative abundance of N. pachyderma (dextral coiling) and by a contemporary decrease of G. in¯ata. This phenomenon has been largely attributed, in the literature (Pujol and Vergnaud-Grazzini, 1989; VergnaudGrazzini and Pierre, 1991; Rohling et al., 1995; Targarona, 1997), to a change in the position of the pycnocline in the water column, rather than a temperature change in the surface waters. On the other hand, the SST values provided by the 0 Uk37 index highlight a much better agreement with the d 18O values down-core. Both records show a similar trend toward cooler conditions during the late Holocene (lower SST and higher d 18O) and the values characterising the main phases of the last deglaciation are easily comparable. For example, the YD-Preboreal transition presents a ,38C DSST, corresponding to a reasonable 18O enrichment of about 0.8½. The alkenone records described in this study have been calibrated on the equation proposed by MuÈller et al. (1998) for the Atlantic Ocean. The calibration proposed by Ternois et al. (1997) and based on sediment trap samples from the north western Mediterranean Sea shows different values when compared to the

48

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0

Fig. 4. (a) BS79-38 correlation between Uk37 index SST (thick line), MAT SST (thin line) and planktonic foraminifera d 18O records (G. ruber X and G. bulloides W). (b) Dissimilarity coef®cient calculated for core BS79-38. This parameter measures the degree of similarity between each fossil sample and ten best modern analogues. For this core the best modern analogues have been found during the last 18.2 kyr bp of the record. 0 (c) BS79-33 correlation between Uk37 index SST (thick line), MAT SST (thin line) and planktonic foraminifera d 18O records (G. ruber X and G. bulloides W) (d) Dissimilarity coef®cient calculated for core BS79-33. For this core the best modern analogues have been found during the last 18.7 kyr bp of the record.

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49

0

k Fig. 5. (a) Four-season SST records of core BS79-38 calculated on the planktonic assemblage (solid circles) in comparison with the U37 index k0 (straight line). (b) Four-season SST records of core BS79-33 calculated on the planktonic assemblage (solid circles) in comparison with the U37 index (straight line).

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open ocean one. A recent study, performed in the Gulf of Lion both on suspended particulate matter and recent sediments (Cacho et al., 1999b), con®rmed the good agreement with Ternois et al. in relation to particulate samples. However, when top sediments were analysed, the general calibration used for the Atlantic Ocean showed more realistic annual values for this area. Such mismatch between sediment-trap based calibration and top-sediments based calibration has already been observed in other oceanographic areas, such as north Atlantic Ocean (Conte et al., 1992) and north Paci®c Ocean (Sikes and Volkman, 1993), and so it is not only a feature of the Mediterranean Sea. This discrepancy remains an open problem, but is not a major consequence in palaeoclimatic reconstructions (Prahl et al., 2000). Moreover, the fact that the sea surface water masses of the Tyrrhenian Sea have also an Atlantic origin, the use of the calibration proposed by MuÈller et al. appears to be of more reasonable coherence. In view of the above considerations regarding the two techniques, their direct comparison would be very dif®cult to interpret. However, we could hypothesize the identi®cation of common features. In this sense, in Fig. 5a and b, it is easy to observe a relatively close match between the alkenone records and the spring/ autumn MAT records during the Holocene (when MAT selects the best analogues of the whole record, the average SST values would be closer to the real values). This could con®rm the existence of two different periods of maximum coccolith production in the Tyrrhenian Sea Ð one during the spring, the main season of the year in terms of enhanced productivity which coincides with the Atlantic Ocean growth season (Conte and Eglinton, 1993), and a second one during the autumn (Vergnaud-Grazzini, 1974). More precisely, it appears that the early Holocene was characterised by a more powerful autumn bloom (seen by observing the closer match between 0 MAT and Uk37 index records) than the late Holocene during which the spring bloom dominated. Past sea surface temperature reconstruction techniques based on changes in the composition of the planktonic foraminifera assemblages have been recently revised and successfully used in the adjacent Atlantic Ocean (Sikes and Keigwin, 1994; Cortijo et al., 1999; Chapman et al., 1996, 2000). Still, the application of these methods in the Mediterranean Sea is

not straightforward, due to the particular behaviour that planktonic foraminifera assume in this small and semi-enclosed basin. The parameters in¯uencing these organisms are considerably more complex than simple temperature effects. Species living at different depths in the water column provide different sorts of information, not necessarily directly connected to the water temperature. Considerably more work is needed on reliable calibration of both MAT and alkenone-based techniques in the Mediterranean Sea. Their application is, however, important to establish a raw palaeoclimatic indicator on which the comparison of other proxies can be made. 4.2. Biozonation of the planktonic association A revised criterion of biozonation (sensu Whittaker et al., 1991) based on the observation of two different planktonic associations (foraminifera and calcareous nannoplankton) and on pteropod relative abundances and ¯uxes to the sea ¯oor, was adopted following previous biostratigraphic schemes proposed for the central Mediterranean Sea (Jorissen et al., 1993; Tamburini et al., 1998; Asioli et al., 1999; Capotondi et al., 1999). The main variations observed in the fossil planktonic record indicate nine different zones and six sub-zones in the last 34 kyr in the south Tyrrhenian Sea. The three cores described in this work present obvious similarities in terms of faunal composition and the zonation was easily applied to all of them. The most prominent features are clearly present in all the sedimentary records. Because of the high sedimentation rates that characterise this area of the Mediterranean Sea, we were also able to detect the small and brief climatic events (e.g. Older and Oldest Dryas) which are often missing from the stratigraphic record, but which are important for a closer and less ambiguous correlation with other pro®les. Biozones are de®ned on the basis of the appearance or disappearance of speci®c taxa, and on their peaks of abundance. Late Quaternary biostratigraphy uses the concept of `assemblage biozones' to refer to the ecological response of organisms to environmental changes, rather than evolutionary changes. In this sense, planktonic foraminifera are important climatic indicators, and we believe that such subdivision of their record, together with the study of coccolith

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51

Fig. 6. Relative abundances of planktonic foraminifera plotted against calendar age in core BS79-22 and scheme of the biozonation applied (see text for explanation).

association and pteropod abundances provides significant insights into the complex dynamics of the Tyrrhenian Sea. The ages at the boundaries of two adjacent zones are expressed in calendar age bp in accordance with Age Model 2 described in Section 3. The chronological cover is therefore relatively robust, even though one of our future targets will be to focus on the elimination of the time uncertainty related to the Holocene period by obtaining more radiocarbon dates. The nine biozones are described as follows, and summarised in Figs. 6 and 7. Biozone 9 (33.7±32.1 kyr bp). This is the oldest interval recognised in our cores, and its dominant

feature is the relatively high abundance of G. ruber var. alba, which represents up to 15% of the total planktonic foraminifera assemblage. This is a shallow dwelling species that prefers well-strati®ed waters (Pujol and Vergnaud-Grazzini, 1995) and proliferates in tropical-subtropical regions (Be and Tolderlund, 1971). Evidence of relatively warm surface waters 0 also comes from the Uk37 index SST record from the Alboran Sea (Cacho et al., 1999b), according to which this period could correspond to the Dansgaard-Oeschger interstadial 5 (Dansgaard et al., 1993). A study of dino¯agellate cysts in core 11-P, also from the Alboran Sea (Targarona, 1997), highlighted an increase in abundance of L. machaerophorum. This

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Fig. 7. Relative abundances of coccoliths (calcareous nannoplankton) plotted against calendar age in core BS79-33 and scheme of the biozonation applied (see text for explanation).

is a warm-transitional species (Turon and Londeix, 1988) that con®rms the presence of relatively high SST values registered in the western Mediterranean Sea. The planktonic microfauna during this period is however dominated by cold taxa among which G. glutinata, N. pachyderma and T. quinqueloba are the most abundant. This interval has only been recognised in core BS79-22, which has the longest sedimentary record. There is therefore a lack of supportive data to con®rm the slightly warm trend indicated by the foraminiferal community. The boundary with biozone 8 is ®xed at the time of the abrupt disappearance of G. ruber var. alba. Biozone 8 (32.1±24.2 kyr bp). This interval is

completely dominated by cold fauna and ¯ora assemblages. At about 26.5 kyr bp. We observed an interruption in the presence of some foraminiferal species and a rapid increase in the pteropod ¯ux (Fig. 8). The bottom part of this interval (subzone 8b, from 32.1 to 26.5 kyr bp) is dominated by G. glutinata (30%), N. pachyderma (20±22%) and secondarily by T. quinqueloba (18%), G. bulloides (15±17%) and G. scitula (15%). The upper part of the interval (subzone 8a, from 26.5 to 24.2 kyr bp) shows a strong increase in the abundance of G. scitula (up to 33% of the assemblage) and a contemporary decrease in N. pachyderma (8±10%) and T. quinqueloba (2±5%). A similar break is also visible in the coccolith association, where zone

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8b shows positive peaks in the distribution of F. profunda and G. oceanica. These two species display the same pattern along the core, increasing in abundance from glacial to interglacial time. The strong similarity between their records, also observed by Flores et al. (1999) in the Agulhas Current domain, South Cape Town, indicates that they may have been controlled by the same factors. According to a study performed by Ziveri et al. (1995) in the California Bight, F. profunda prefers low nutrient concentration waters with low light intensity. In general, strati®cation of the upper surface waters, together with a deep nutricline, favours the development of F. profunda (Mol®no and McIntyre, 1990). This species inhabits the lower photic zone (60±200 m) that occurs above the 128C isotherm (Okada and McIntyre, 1979) and is considered to be a tropical±subtropical taxon (Kleijne, 1993; Knappertsbusch, 1993; Flores et al., 1997). G. oceanica is widely recognised to be a warm water species with a preference for marginal seas (McIntyre and BeÂ, 1967; Okada and Honjo, 1975) and high nutrient content surface waters (Girardeau, 1992). Zone 8a is characterised by a relatively strong increase in H. carteri and S. pulchra. These species show positive peaks coincident with minimum values of F. profunda and G. oceanica (Flores et al., 1999). Biozone 7 (24.2±18.1 kyr bp). This zone spans a long time interval and includes the Last Glacial Maximum, located between 24 and 20.7 kyr bp. At the base of this sector we observed the almost instantaneous appearance of G. ruber var. alba with values of up to 20% of the total foraminiferal count. This rapid event marks the boundary between biozones 8 and 7. G. ruber var. alba is indicative of summer surface temperatures (Pujol and Vergnaud-Grazzini, 1995) and together with positive peaks of E. huxleyi and relatively high values of the sub-tropical taxa G. oceanica and R. clavigera (Kleijne, 1993; Knappertsbusch, 1993; Flores et al., 1997), it con®rms the presence of a milder climate over the western Mediterranean region (Pujol and Vergnaud-Grazzini, 1989; Targarona, 0 1997). The Uk37 index SST record shows a more or less constant trend in both cores BS79-38 (Fig. 4b) and BS79-33 (Fig. 5b) with a weak tendency towards higher temperatures, even though within this interval we observed the lowest SST values recorded (,98C), centred at 24 kyr bp and perhaps corresponding to Heinrich event H2. The effects of this cooling on

53

the Tyrrhenian Sea surface waters are the result of the entrance of waters of polar origin through the Straits of Gibraltar (Paterne et al., 1999). G. bulloides appears to be of low frequency (10±12%) with respect to the average abundance calculated for the whole sedimentary record (20%). Relatively low abundances are also registered for T. quinqueloba and G. scitula. The abrupt decrease in the distribution of G. ruber var. alba represents the boundary between biozones 7 and 6. Biozone 6 (18.1±14.7 kyr bp). This interval is characterised by the near absence of G. ruber var. alba, and by the strongest contribution of G. bulloides to the faunal composition, reaching 30% of the total assemblage. Signals of reorganisation in the ¯oral association suggest the sub-division of this zone into two sub-zones: subzone 6b (from 18.1 to 16.2 kyr bp) is dominated by a few species (small Gephyrocapsa, E. huxleyi, S. pulchra and H. carteri), which together represent more than 90% of the total assemblage. Subzone 6a (from 16.2 to 14.7 kyr bp) is in contrast characterised by a strong peak of F. profunda (rising from near absence to reach 13%), Helicosphaera spp. (up to 5%) and C. leptoporus (up to 4.5%), testifying a tendency towards warmer climatic conditions (Pujos, 1992; Girardeau, 1992; Flores et al., 1997). This warming trend is also clearly visible from the alkenone-based SST and d 18O records (Figs. 4b and 5b) and from the foraminiferal associations, in which during this interval we observe a progressive increase of G. in¯ata and G. ruber var. alba. Some of the species (e.g. S. pulchra and Helicosphaera spp.) show a two-step record, interrupted by a negative peak coinciding with the sub-zone 6b/6a boundary. The boundary with biozone 6 is based on the re-occurrence of G. ruber var. alba, which coincides with the ®rst appearance of G. in¯ataand of the group of `minor species'. Biozone 5 (14.7±13.4 kyr bp). The time span covered by this interval corresponds to the Bùlling± Allerùd transition. The foraminiferal association is dramatically different to that of the previous zone. G. in¯ata and G. ruber var. alba dominate the microfauna representing over 60% of the total assemblage. The base of this zone marks the ®rst appearance of G. truncatulinoides in the fossil record, with low percentages of about 2±3%, whereas the near-disappearance of this species together with an abrupt drop in the

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Fig. 8. Biozonation described in the text (a) compared with pteropods ¯uxes to the sea ¯oor (b) and pteropods relative abundances (c). The acronyms indicated in Fig. 8c are explained as follows: h (high, .30%); m-h (medium-high, 20±30%); m (medium, 15±20%); m-l (mediumlow, 10±15%); l (low, ,10%); na (near absence, ,2%) and refer to the relative abundances of the pteropod group in relation to the total foraminifera (planktonic 1 benthonic) abundances downcore.

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abundance of G. in¯ata and G. ruber var. alba represents the boundary with biozone 4. Some species (G. in¯ata, N. pachyderma and G. ruber) show two frequency peaks, interrupted by a short negative trend at 14 kyr bp corresponding to the Older Dryas cold event, also registered by a slight cooling apparent 0 from both foraminiferal and Uk37 SST records. With the onset of the deglaciation, the SSTs became suf®ciently warm to permit the development of warm forms, such as the `minor species', but during this phase surface waters were also insuf®ciently productive to allow the survival of T. quinqueloba, which recovered only partially during the Younger Dryas (biozone 4) with much lower percentages 0 (about 10%). In the Tyrrhenian Sea, the average Uk37 SSTs during this interval (around 13±148C) are not so high as those obtained by the MAT, which are close to the present day values (178C). This discrepancy could be due to the different seasonal and geographical variability in the distribution of foraminiferal and coccolith assemblage relative to the present day. The coccolith assemblage is characterised by percentages of E. huxleyi similar to those of the present day (over 60%), but the relative abundance of G. oceanica (2%, warm indicator) and of the group of small Gephyrocapsa (30%, cold indicator) are respectively lower and higher than present day values, demonstrating a clear amelioration of climatic conditions relative to the last glacial interval, but with surface temperatures lower than the Holocene average. Biozone 4 (13.4±12.1 kyr bp). The microfaunal composition and the chronostratigraphic reconstruction permit us to recognise this biozone as corresponding to the Younger Dryas cold event. The foraminiferal and coccolith assemblages are similar to those observed for the last glacial interval, with similarly low SST values (10±128C) and high d 18O values (1.8±2.1½). During the coldest interval of this zone, centred at 12.5±13 kyr bp, the microfauna is characterised by the almost total absence of G. ruber var. alba (less than 1%) and G. in¯ata (less than 5%), which were replaced by G. glutinata (35%) and N. pachyderma (30%). G. truncatulinoides and the `minor species' are also absent and G. bulloides is at a minimum (10%). The ¯oral composition does not show a strong difference with respect to the previous zone, with the exception of a decrease in the frequency of F. profunda (3%) and G. oceanica (1%).

55

The boundary with biozone 3 is characterised by many clearly identi®able changes in the planktonic foraminifera composition, in particular an abrupt increase in G. in¯ata, G. truncatulinoides and G. ruber var. alba together with the almost complete disappearance of N. pachyderma. Biozone 3 (12.1±10.5 kyr bp). This interval represents the onset of the Holocene and the de®nitive conclusion of severe glacial conditions. The top end of the zone sees the ®nal disappearance of cold species such as G. scitula and T. quinqueloba, while G. glutinata, although a persistent taxon through most of the cores, never recovers from the rapid warming which follows the YD. The warming trend can also be inferred from the coccolith assemblage, with the frequency peaks of G. oceanica (8%), F. profunda (10%), and the rapid increase in the abundance of U. sigobae (from 0 to 2.5%). During this interval the ¯ux of pteropod shells ….150 mm† to the sea bottom reaches its highest values of 500 pteropods/ yr/cm 2 in core BS79-22 and 400 pteropods/yr/cm 2 in core BS79-38, testifying the good preservation of aragonite during this interval. A strong increase in abundance of N. pachyderma represents the boundary with biozone 2. Biozone 2 (10.5±6.1 kyr bp). This interval, coeval with the deposition of sapropel S1 in the eastern Mediterranean basin, is characterised by a high frequency of N. pachyderma, together with a decrease in abundance of G. in¯ata and a minor decrease in G. ruber var. alba. This change in the composition of the faunal assemblages is re¯ected in an unrealistic decrease in the SST values estimated by the MAT record (a drop of 3.5±48C with respect to the Holocene average). This is due to the fact that N. pachyderma survives in the deep waters within or below the thermocline (Fairbanks and Wiebe, 1980). In the Mediterranean Sea, this non-spinose species has its main food source within the pycnocline, between the North Atlantic Water (NAW) and the Mediterranean Intermediate Water (MIW) masses (Pujol and Vergnaud-Grazzini, 1989). The consequence of this is that N. pachyderma, considered to be a cold-subpolar indicator (Be and Tolderlund, 1971), could provide only an apparent T-driven signal and would bias in such way the total SST determination in this basin. Additionally, other independent proxies such as the 0 Uk37 record and the d 18O pro®les show a warming

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trend, perhaps corresponding to the `Climatic Optimum', and hence do not support the theory of the presumed surface or sub-surface cooling. Two brief cooling events (500±800 years) have been identi®ed 0 in the Uk37 SST record (C5 and C4, Fig. 3), respectively at 10.3 and 8.2 kyr bp. Similar cooling events have also been recognised in the Gulf of CadizAlboran Sea record (Cacho et al. 2000). This time interval is however characterised by strong modi®cations of the water column, since all the fossil organisms considered show changes in abundance. The pteropod association undergoes a period of crisis, with a dramatic decrease of shell deposition to the sea ¯oor and the almost total disappearance of this group from the fossil record. These holoplanktic gastropods survive in a large range of temperatures (10±338C) and salinities (28±45½) (Almogi-Labin, 1982; Almogi-Labin et al., 1998). In view of this, the idea of a strong and abrupt climatic change being the cause of the pteropods decrease appears to be implausible and their near disappearance could re¯ect a shoaling of the ACD (Aragonite Compensation Depth) and the consequent dissolution of the pteropods aragonitic shells. The boundary with biozone 1 corresponds to the abrupt decrease in frequency of N. pachyderma, together with the reoccurrence of G. truncatulinoides. Biozone 1 (6.1-subrecent). This interval presents the typical present-day foraminiferal association, dominated by temperate and sub-tropical taxa. G. in¯ata and G. ruber var. alba represent the most abundant species with more than 60% of relative abundance, followed by G. bulloides (20%) and by the `minor species' (12% average). G. truncatulinoides reaches the highest percentages (up to 15%), testifying the onset of deep winter convection and vertical mixing which characterise the modern Tyrrhenian Sea (Pujol and Vergnaud-Grazzini, 1995). The ¯oral assemblage shows a progressive decrease in the small Gephyrocapsa group towards the top of the core, and a synchronous increase in E. huxleyi, which represents up to 70% of the total community over the last 2.5 kyr bp G. oceanica is present with ¯uctuating values along the whole interval (average 4%) with two main negative peaks, at 5.4 and 2.9 kyr bp. A break in the distribution of the `minor species' group has been used as boundary between subzones 1a and 1b. Subzone 1b is characterised by

a rapid increase in the relative abundance of warm taxa (in particular G. trilobus), from 5 to 15±17%. SST values, calculated using both the MAT and the 0 Uk37 index, are similar to those of the present day (15± 178C), even though there is an overall recent tendency towards colder temperatures, also apparent from the planktonic foraminifera d 18O records (Figs. 4 and 10). 0 During this interval the Uk37 index shows three main cold peaks (C3, C2 and C1, Fig. 3), respectively centred at 5.4, 2.7 and 1.4 kyr bp, as does the G. oceanica record. C3 and C1 are also visible in the Alboran Sea SST record (Cacho et al., 1999b, 2000), whereas C2 appears only in the south Tyrrhenian Sea. 4.3. Comparison with previous studies Reliable biochronology for the last glacial±interglacial cycle in the Mediterranean Sea has been provided by several authors (Jorissen et al., 1993; Tamburini et al., 1998; Asioli et al., 1999; Capotondi et al., 1999). Our high-resolution study (millennial to centennial scale) and the high sedimentation rates that characterise the southern Tyrrhenian basin have allowed us to provide a very detailed biozonation of this region that can include and extend the previous ones. Moreover, the long sedimentary record recovered in two of the three cores studied provides a much longer time-coverage (ca. 34 kyr) of the Late Quaternary. Jorissen et al. (1993) proposed a three-zone subdivision of the last 25 kyr based on the study of planktonic and benthonic assemblages in seven deep-sea cores, mostly located in the south-central Adriatic Sea. These authors placed the boundaries between the three zones where major faunal breaks occurred (at 10.9 and 15.3 kyr bp on their chronology). Thus, it was possible to identify a glacial phase (Zone III), a transitional phase (Zone II) and ®nally a postglacial phase (Zone I). Through direct comparison of the zonation suggested by Jorissen et al. with our record we were able to recognise a close similarity between the two studies, since the Zone III/II boundary corresponds to our biozone 6a/5 boundary, and the Zone II/ I boundary corresponds to our biozone 3/2 boundary. The calendar ages for the Zone III/II and Zone II/I boundaries were calculated using the `calib 4.1' program. Discrepancies may be due to the different

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technique used for the calibration of the radiocarbon ages. Tamburini et al. (1998) applied an informal twostep subdivision of the last glacial post-glacial transition to two gravity cores (MC82-12 and ML83-21) recovered from Latial-Campanian intraslope (eastern Tyrrhenian Sea). In this study, the main boundary was placed at the onset of the Bùlling±Allerùd transition, which corresponds to the biozone 6a/5 boundary in the south Tyrrhenian cores (centred at 14.7 kyr bp). This point corresponds to the ®rst appearance of Mediterranean Sea post-glacial planktonic foraminifera species such as G. truncatulinoides (14.5 kyr bp) and G. in¯ata (15.6 kyr bp). No 14C AMS dates have been provided for these cores, so our comparison is based on direct correlation of the faunal records. A detailed study of the Last Glacial-Holocene transition (18±8 kyr bp) has been carried out by Asioli et al. (1999) for the central-southern Adriatic Sea (cores CM92-43 and IN68-5) and the northern Tyrrhenian Sea (core ET91-18). Within this interval of time, the authors recognised ®ve different ecozones (from VII to III) and seven events, which they correlated with the INTIMATE event stratigraphy (BjoÈrck et al., 1998), also applied in this paper (Fig. 10). The comparison extends to sub-event level (GI-1a to GI-1e), testifying the high resolution of sequences obtained from the Adriatic cores. Their ecozones, based on the Adriatic foraminifera assemblages, correspond, in terms of distributional patterns, to our biozones 6a (end part), 5, 4, 3 and 2 (beginning). However, the timing of these events in the published chronologies is slightly different for the two basins. This may be due to the different chronological approaches used, or to differences in the responses of the planktonic foraminifera in the two seas. Capotondi et al. (1999) based their palaeoclimatic reconstruction of the Late Quaternary of the central Mediterranean Sea on a large number of cores (about 60) collected from both the Adriatic and Tyrrhenian seas. These authors recognised a succession of 10 different ecozones based on the planktonic foraminifera associations. The biochronological reconstruction extends back to 23 kyr bp (radiocarbon age). Many of the ecozones recognised by Capotondi et al. were also detected in this study, even though biozones 9 and 8b have only been identi®ed in our

57

record. Ecozones 3 and 4 of Capotondi et al. correspond to biozone 2, while ecozones 2 and 1 correspond respectively to subzones 1b and 1a. All the other intervals are similar in both studies. Minor differences could be due to the different size fraction used. The comparison between the different studies proposed is summarised in Fig. 9. Finally, correlations with the GRIP ice-core were made to estimate the geographical extent of some smaller climatic events, and to permit a direct comparison of the Tyrrhenian Sea pro®les with the North Atlantic records. The good agreement between the INTIMATE event stratigraphy (BjoÈrck et al., 1998) and our study was a further con®rmation of the high quality of our sedimentary records (Fig. 10). The succession of stadial and interstadial intervals over the last glacial-postglacial transition in the Greenland ice-core record correlates very well with several of our biozones. The boundary between the GS-1 stadial and the Holocene appears to correspond to the biozone 4/3 boundary. Moreover, there is a good coincidence between GS-1 and biozone 4, and GI-1 and biozone 5. The boundary between GS2a and GS-2b is almost coincident with the biozone 7/6b boundary and small differences could be due to either biological factors or different time scales. GS-2c and GI-2 correspond to the lower part of biozone 7. 5. Conclusions A detailed study of the planktonic environment of the Tyrrhenian Sea has permitted the reconstruction of the climatic history of this basin over the past 34,000 years. The western Mediterranean Sea is con®rmed as having a strong sensitivity to the climatic changes that occurred during the last glacial±postglacial cycle in the northern hemisphere. Quantitative and qualitative modi®cations of the planktonic foraminifera and calcareous nannoplankton communities observed in three deep-sea cores from the CefaluÁ Basin exhibit a sequence of biological events summarised in nine successive main time intervals, and six sub-intervals, which are identi®ed as biozones and sub-biozones. The boundaries between adjacent zones are de®ned on the basis of distributional patterns of the most abundant species, and

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Fig. 9. Comparison between the biozonation adopted in this study and previous works on the same area (Jorissen et al., 1993; Tamburini et al., 1998; Asioli et al., 1999; Capotondi et al., 1999).

their ages are determined by calibration with AMS dating and peak-to-peak comparison with previously published records. The analysis of the pteropod ¯ux to the sea ¯oor has also shown a selective dissolution of the aragonite event during the early Holocene (biozone 2), due to a substantial shoaling of the ACD during that time. The simultaneous application of two different 0 methodologies (MAT and the Uk37 index) to determine past SST values in two of the three cores selected permitted the identi®cation of the main climatic changes of the Late Quaternary on the basis of the response of the ¯oral (nannoplankton) and 0 faunal (planktonic foraminifera) communities. The Uk37 index SST record showed ®ve well de®ned cooling events of centennial scale during the Holocene, two within biozone 2 and three within biozone one, also partially recognised in other areas of the western Mediterranean Sea. However, a series of considerations related to the suitability of SST reconstruction techniques'

application to the Mediterranean Sea environment, emphasized an early stage of development, when compared to the results obtained for open oceans. The too simplistic point of view of the MAT and the absence of good modern analogues in large intervals of the late Quaternary make this technique of faint utility in this sector of the Tyrrhenian Sea, at least in relation to highly detailed studies. Nevertheless, the consideration of the MAT results as a guideline in the study of the general evolutionary trend of an oceanographic area, is of common knowledge and of unquestioned utility. On the other side the alkenone-based SST reconstructions, much closer to the real values and in fair agreement with the stable isotope interpretation, would be may be subject to a further improvement with a Mediterranean-speci®c calibration. A conclusive correlation of this work with others well known from close areas of the central Mediterranean Sea con®rmed the good quality of the results, whereas the time span covered, together with the

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Fig. 10. Biochronological scheme proposed in this study in comparison with the INTIMATE event stratigraphy (BjoÈrck et al., 1998) based on GRIP ice core. GISP2 d 18O record has been used to calibrate the planktonic foraminifera d 18O pro®les obtained for core BS79-33 and to provide a ®rst chronological framework.

very high resolution of the study, are the better ever obtained for the Tyrrhenian Sea. In other words, this work has con®rmed that multidisciplinary strategies and multiproxy study performed on late Quaternary cores with very high sedimentation rates can provide a powerful tool for monitoring the palaeoclimatic

evolution of the Mediterranean Sea in relation to the global climatic system over the last glacial±interglacial transition. Such multidisciplinary strategy helps to increase con®dence in results and provides oceanographic background for more effective interpretation of water masses processes.

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