Variability of the western Mediterranean Sea surface temperature during the last 25,000 years and its connection with the Northern Hemisphere climatic changes

. Sea surface temperature (SST) profiles over the last 25 kyr derived from alkenone measurements are studied in four cores from a W-E latitudinal transect encompassing the Gulf of Cadiz (Atlantic Ocean), the Alboran Sea, and the southern Tyrrhenian Sea (western Mediterranean)..The results document the sensitivity of the Mediterranean region to the short climatf(cid:127) changes of the North Atlantic Ocean, particularly those involving the latitudinal position of the polar front. The amplitude of the SST oscillations increases toward the Tyrrhenian Sea, indicating an amplification effect of the Atlantic signal by the climatic regime of the Mediterranean region. All studied cores show a shorter cooling phase (700 years) for the Younger Dryas (YD) than that observed in the North Atlantic region (1200 years). This time diachroneity is related to an intra-YD climatic change documented in the European continent. Minor oscillations in the southward displacement of the North Atlantic polar front may also have driven this early warming in the studied area. During the Holocene a regional diachroneity propagating west to east is observed for the SST maxima, 11.5-10.2 kyr B.P. in the Gulf of Cadiz, 10-9 kyr B.P. in the Alboran Sea, and 8.9-8.4 kyr B.P. in the Thyrrenian Sea. A general cooling trend from these SST maxima to present day is observed during this stage, which is marked by short cooling oscillations with a periodicity of 730+40 years and its harmonics. of an internal standard mixture containing n-nonadecan-l-ol, n-hexatriacontane, and n- tetracontane, dry sediments were in an ultrasonic bath with dichloromethane. The extracts were hydrolyzed with 6% potassium hydroxide in methanol, and the alkenones were recovered with hexane. The solvent was evaporated to dryness with a N 2 stream, and the extracts were finally redissolved with toluene and derivatized with bis(trimethylsilyl)trifluoroacetamide


Introduction
The Mediterranean Sea is a semienclosed water body surrounded by large continental masses with only one narrow connection to the Atlantic Ocean. The hydrodynamics of the basin is controlled by the North Atlantic water inflow, wind regime, and climate of the surrounding lands. Recent  ]. In addition, noticeably vertical and lateral fluxes are related to frontal regions developed at the boundaries between the Atlantic jet and surrounding waters [Peinert and Miquel, 1994]. Therefore, although the cores selected for study are located at almost the same latitude (36ø-38øN), they monitor different stages of alteration of the Atlantic Water entering into the Mediterranean Sea.

Sea Surface Temperature (SST) Reconstruction
SST was obtained from the relative composition of C37 unsaturated alkenones through the uK'3, index [Brassell et al., 1986]. Sediment samples (---2 g) were freeze-dried and manually grounded. After addition of an internal standard mixture containing n-nonadecan-l-ol, n-hexatriacontane, and ntetracontane, dry sediments were extracted in an ultrasonic bath with dichloromethane. The extracts were hydrolyzed with 6% potassium hydroxide in methanol, and the alkenones were recovered with hexane. The solvent was evaporated to dryness with a N 2 stream, and the extracts were finally redissolved with toluene and derivatized with bis(trimethylsilyl)trifluoroacetamide before instrumental analysis.
Alkenones were analyzed with a Varian gas chromatograph Model 3400 equipped with a septurn programmable injector and a flame ionization detector. The instrument was equipped with a CPSIL-5 CB column coated with 100% dimethylsiloxane (film thickness of 0.12 m). Hydrogen was the carrier gas (50 cm/s). The oven temperature was programmed from 90 ø to 140øC at 20øC/rain, then to 280øC at 6øC/min (holding time 25 rain), and finally, to 320øC at 10øC/rain (holding time of 6 min). The injector was programmed from 90øC (holding time of 0.3 rain) to 320øC at 200øC/min (final holding time was 55 rain). For further details on analytical conditions used in this study, see Villanueva et al. [1997]. Replication of a sediment sample with similar lipid content and uK'3? index (n = 5) showed a standard deviation of +0.15øC in temperature estimation.

Discussion
The  Table 1).  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  The most surprising feature of SST evolution during the YD in all our profiles concerns to the brief duration of the cold phase (--,700'years), which contrasts with extent of the isotopic plateau recorded in the same cores for this period (---1100 years). This means that the onset of the final YD warming started ,--600 years earlier in the Mediterranean SST (12,250 years B.P.) than over Greenland (11,650 years B.P.). This warming phase was extremely abrupt in the Alboran Sea (3.3øC per 55 years) and was suddenly interrupted by a short (1 øC) reversal which ended in parallel with the GS-1 on GISP2. Cores M39-008 (Gulf of Cadiz) and BS79-38 (Tyrrhenian Sea) also show a short reversal just after the YD warming although the lower resolution and/or age model uncertainties prevent a precise correlation of this extremely short event (260 years). The causes of these timing anomaly in our YD records could be regarded as a chronological problem in the framework of the Greenland Ice Sheet Project 2 (GISP2) ice core and/or the studied profiles.

4), the so-called
The YD in Greenland ice records lasted 1200 years with slight differences in the absolute timing, 12,800-11,600 years B.P. in  GISP2 and 12,700-11,500

Further work on high-resolution YD records from low North Atlantic latitudes (---35øN) would be required for a better assessment of the polar front movement during this period.
Several records from the northwestern European continent document the occurrence of climatic changes during the YD, pointing to a warmer episode during the latter phase [Walker, 1995]. Sediments from Lake Gosciaz (Poland) show two main phases: the earlier, which was longer, colder, and drier, and the latter, which w•.s shorter and milder [Goslar et al., 1993]. The transition between these two phases is assigned to--600 years ]. This warming phase was shortly interrupted by a cooling period similar to that recorded by SST in Alboran Sea. All this evidence strongly supports that this climatic variation over Europe was related to the YD signature in our cores and therefore that wind regime changes were involved in this early warming phase. Comparison of the diverse YD records mentioned above seem to indicate that this climatic disturbance increased its amplitude toward the south.

Holocene
All studied areas show the warmest SST values at the early Holocene followed by a slow cooling trend. Nevertheless, the timing of the Holocene maximum SST change between the areas: 11.5-10.2 kyr B.P. in the Gulf of Cadiz, 10-9 kyr B.P. in the Alboran Sea, and 8.9-8.4 kyr B.P. in the Tyrrhenian Sea. This indicates a regional diachroneity in the occurrence of the Holocene.optimum, which propagates from west to east.
The general Holocene cooling trend is shortly interrupted by the occurrence of some short cooling events (1 ø-2øC), which are more intense in the Tyrrhenian Sea (2.5ø-3øC). This short-term .variability can be better analyzed in core MD 95-2043, the one studied at higher resolution and with more abundant •4C AMS ages. Six short cold events (A,C1-AC6; 1ø-1.5øC) have been identified (Figure 4 and Table 2). The younger events (AC1-AC3) lasted 900-1500 years, while th• older events (AC4-AC6) were considerably shorter (250-400 years) but with the same thermal intensity (1.5øC).
A perio•licity of,-,730+40 years is observed between the oldest cold spells (AC6-4; see Table 2). Then, these periods lengthen after the Holocene optimum, but they always occur in harmonic tones of this 7304-40 years periodicity (AC3:---730 x 3; AC2:  (Table 2), although the lower resolution and less accurate age model do not allow a strict correlation between them. Only few of them, AC3 and AC4 in the Alboran Sea and CC1 and CC2 in the Gulf of Cadiz, can be well correlated ( Figure  2 and Table 2 Holocene SST show a cooling trend from start to present in all the studied areas and reflect a regional diachroneity, occurring earlier (10-11.5 kyr B.P.) in the areas under higher oceanic influence and later (8.4-8.9 kyr B.P.) in the enclosed basin sites.
A sequence of short SST cooling events (1.5ø-3øC) observed in the cores studied at high resolution is consistent with a climatic periodicity of---730ñ40 years. These coolings are in agreement with the Holocene variability recorded in other areas. They were transmitted to the Mediterranean Sea by the Atlantic inflowing water, but strong winter winds may have amplified these oscillations in the Tyrrhenian Sea.