Nannoplankton successions in the northern Red Sea during the last glaciation (60 to 14.5 ka BP): Reactions to climate change

Earth and Planetary Science Letters 270 (2008) 271–279

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / e p s l

Nannoplankton successions in the northern Red Sea during the last glaciation (60 to 14.5 ka BP): Reactions to climate change Heiko-Lars Legge a,⁎, Jörg Mutterlose a, Helge W. Arz b, Jürgen Pätzold c a b c

Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Bochum, Germany GeoForschungsZentrum Potsdam, Potsdam, Germany Research Center of Ocean Margins, Fachbereich Geowissenschaften, Universität Bremen, Bremen, Germany

A R T I C L E

I N F O

Article history: Received 8 October 2007 Received in revised form 7 March 2008 Accepted 17 March 2008 Available online 27 March 2008 Editor: H. Elderfield Keywords: Red sea calcareous nannoplankton paleoclimate glacial environment Heinrich events

A B S T R A C T Due to its restricted connection with the Indian Ocean, the desert-enclosed Red Sea is extremely sensitive to global sea level changes and thus ideally suited for paleoceanographic studies of what occurred during the last glaciation. The understanding of its glacial history is, however, still limited. A serious obstacle to obtain satisfactory paleoecological information has been the rarity of microfossil proxy species caused by high salinities. Here, we present a continuous and well-dated calcareous nannoplankton record from the northern Red Sea, covering the interval from 60–14.5 ka BP. Our investigation shows that the composition of the calcareous nannoplankton community varied between ∼ 32 ka BP and 14.5 ka BP in response to rapid environmental changes which are closely correlated to climatic fluctuations described from the North Atlantic region. Heinrich events H3, H2 and H1 are dominated by Emiliania huxleyi. Gephyrocapsa oceanica and especially Gephyrocapsa ericsonii are abundant between H3–H2 and H2–H1. A less pronounced response of the calcareous nannoplankton to the high latitudinal climatic oscillations is documented prior to ∼ 32 ka BP, suggesting that a strong atmospheric coupling between the northern Red Sea and the North Atlantic realm was established in the late Marine Isotope Stage 3. In contrast to the previously held view of a sea level related salinity increase as the major cause for changes of the plankton communities within the glacial Red Sea, we interpret the documented variations as being caused by local hydrographic changes under the atmospheric control from the extratropics. Temperature changes and especially variations of the water stratification appear to be critical selective factors for the calcareous nannoplankton composition. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The Red Sea is a deep ocean basin with a restricted seaway to the Gulf of Aden-Indian Ocean via the shallow (∼ 137 m) Strait of Bab-elMandeb at its southern end (Fig. 1). The climate in the Red Sea area is arid, with low precipitation (10–200 mm/year) and high rates (2.000 mm/year) of evaporation (Edwards, 1987; Sofianos et al., 2002). No permanent river system supplies the Red Sea and the influence of temporary run-off (e.g., from wadis) on the hydrological budget is negligible (Morcos, 1970; Hoelzmann et al., 1998; Siddall et al., 2003). Prevailing winds are basically from NNW. Only south of ∼ 19°N the main wind direction reverses in winter under the influence of the Asian monsoon system (Edwards, 1987). The large-scale water circulation within the Red Sea basin is anti-estuarine (e.g. (Eshel and Naik, 1997)). Warm and normal saline (36.5 psu) surface waters penetrate the basin through the Strait of Bab-el-Mandeb. As this waters move northwards they becomes progressively cooler and saltier (40–41 psu). Due to their buoyancy loss, the journey of the ⁎ Corresponding author. Universitätsstrasse 150, 44780 Bochum, Germany. Tel.: +49 234 32 23673; fax: +49 234 32 14571. E-mail address: [email protected] (H.-L. Legge). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.03.030

surface waters eventually ends in the northernmost Red Sea where southwards flowing intermediate and deep waters are formed (e.g. (Eshel and Naik, 1997; Cember, 1988; Woelk and Quadfasel, 1996)). Nutrient gradients are less pronounced developed than those for salinity and temperature. The southern Red Sea is characterized by a relatively high annual primary production due to the inflow of nutrient rich waters from the Gulf of Aden (for details see (Weikert, 1987)). The open waters of the northern Red Sea area (northern Red Sea and Gulf of Aqaba) resemble seasonal hydrographic patterns and associated phytoplankton dynamics typical of pelagic subtropical settings. The northern Red Sea environment is primarily controlled by the stable stratification of the waters resulting in oligotrophic conditions. Although the close proximity to land and high dust deposition prevent iron-limitation (Lindell and Post, 1995; Acker et al., 2008), input of other nutrients from terrestrial sources is unimportant (no run-off, sparse rainfall). Nutrient content of the surface waters is low. Higher surface water productivity is limited to the cold season; winter mixing held responsible for the upward transport of nutrients from the deeper waters (Edwards, 1987; Weikert, 1987). The resulting fertilization of the surface waters lead to an increase in phytoplankton growth in late winter and spring (Weikert, 1987; Lindell and Post, 1995; Acker et al., 2008). Previous investigations have accentuated a

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Fig. 1. (a) Location of core GeoB 5844-2 in the northern Red Sea and a schematic view of the precipitation regimes at both margins of the subtropical desert belt (north: Mediterranean winter rain, south: southwest summer monsoon). Dashed line shows position of the Intertropical Convergence Zone (ITCZ); thick black arrow indicates wintertime mean position of the Subtropical Jet (STJ) which roughly mark the southern limit of the extratropical Mediterranean winter climate features (Air Ministry Meteorological Office, 1962; Martyn, 1992; Hoskins and Hodges, 2002). The desert belt is shaded in light gray (vegetation-based (Schmithüsen, 1976)). A detail view of the northern Red Sea with the Gulf of Aqaba (GA) and the Gulf of Suez (GS) is shown in the upper right panel (b).

close link between variations of the northern Red Sea hydrography and the winter climate of the Northern Hemisphere mid-high latitudes. Changes in water stratification are thought to be lasting influenced by variations of the extratropical atmospheric circulation (e.g. (Eshel et al., 2000; Arz et al., 2003; Lamy et al., 2006)). Continuous microfossil records from the northern Red Sea, which cover Marine Isotope Stage (MIS) 3 and 2 (Imbrie et al., 1984), are sparse. As a result of the globally low sea level during the last glacial (up to ∼125 m lower than today), water exchange between the Red Sea and the Gulf of Aden via the Strait of Bab-el-Mandeb was drastically reduced (Siddall et al., 2003; Arz et al., 2007). Oxygen isotopic values are exceptional higher in the Red Sea than in the open ocean (Arz et al., 2003; Hemleben et al., 1996). The strongly limited water exchange with the Gulf of Aden, regulated by the lowered sea level, extended the retention period of waters in the Red Sea basin. The resulting high 16O-loss caused by evaporation shifts the oxygen isotope ratio to exceptional high values. The pronounced 18Oenrichment relative to open ocean settings mirrors a salinity increase, characteristic of the glacial Red Sea. Hypersaline conditions, amounting to 50 psu or even above, prevailed during MIS 3 and especially MIS 2 (Arz et al., 2003; Hemleben et al., 1996; Thunell et al., 1988). The disappearance of various microfossil species prelude the “aplanktic zone”, a period characterized by the absence of planktic foraminifera and by a pteropod community mainly consisting of the euryhaline Creseis acicula (Almogi-Labin, 1982; Winter et al., 1983; Fenton et al., 2000). Recent results indicate the presence of calcareous nannoplankton (simply named nannoplankton in the ongoing text) during this hypersaline “aplanktic zone” in the northern Red Sea (Legge et al., 2006).

Fig. 2. Age depth relation of the sediment core GeoB 5844-2 (Arz et al., 2007). The core section/time interval studied here is gray shaded.

We present a nannoplankton record from the desert-enclosed (Schmithüsen, 1976) northern Red Sea (Fig. 1) covering the period between ∼ 60 ka BP and the onset of the Bølling–Allerød warm period at ∼14.5 ka BP (Figs. 2 and 3). Short-termed climatic variations, including Dansgaard–Oeschger (D–O) cycles and Heinrich events, are well documented in the northern North Atlantic region during the time period under consideration (Bond et al., 1993; Dansgaard et al., 1993). D–O cycles, rapid and large amplitude changes observed in the δ18O ice core records from Greenland, reflect variations in air temperature over the northern ice sheets. Several D–O cycles are grouped into multi-millennial intervals characterized by an overall progressive cooling (longer-term cooling cycle). Massive iceberg discharges into the North Atlantic are indicated by extensive layers of coarse-grained debris (Broecker et al., 1992). These Heinrich events, or more precisely their climatic consequences are documented as pronounced cold periods that occurred at the end of the longer-term cooling cycles (Bond et al., 1993; Broecker et al., 1992). D–O cycles, longer-term cooling cycles, and the climatic consequences of Heinrich events (Fig. 3) are not restricted to the North Atlantic region. Various records in the mid-latitudes document similar variations and indicate a strong relation to the climate of the North Atlantic realm (e.g. (Voelker et al., 2002; Rohling et al., 2003)). Given the sensitivity of nannoplankton to autoecological factors (e.g. (Brand, 1994; Ziveri et al., 2004)), our record offers a unique possibility for the examination of millennial-scale environmental changes during the hypersaline period of the Red Sea in response to the longer-term cooling cycles and Heinrich events of the North Atlantic realm. 2. Material and methods The studied sediment core GeoB 5844-2 (Fig. 1) was recovered in 1999 during Meteor cruise 44/3 (Pätzold, 2000) in the vicinity of the Sinai Peninsula (27°42.81′ N, 34°40.90′ E, 963 meter below sea level). For this study, we investigated fossil nannoplankton from the timeinterval ∼60–22 ka BP. To allow a more comprehensive view of the results, we present a complete record of the ∼ 60–14.5 ka BP time period (Figs. 3–6) by including recently published data (Legge et al.,

Fig. 3. Comparison of (a) the SPECMAP δ18O record (stacked (Imbrie et al., 1984)) and (b) the δ18O GRIP ice-core record (on the SFCP timescale (Shackleton et al., 2004)) with results from core GeoB 5844-2 (northern Red Sea): (c) the planktic δ18O record (Arz et al., 2007), (d) the reconstructed sea level curve (for details (Arz et al., 2007)), (e) the alkenone-sea surface temperatures, (f) the relative abundance of E. huxleyi, (g) the relative abundance of G. ericsonii, (h) the relative abundance G. oceanica and (i) the absolute abundance (AA) and accumulation rates (AR) of the total nannoplankton community. The qualitative distribution of diatoms (large Cosinodiscus) is sketched in Fig. 3i above the absolute nannoplankton values. Age control points of core GeoB 5844-2 are indicated by triangles at the bottom (for details see Fig. 2). The trend of the longer-termed cooling cycles and the position of the Heinrich events (H) are sketched at the top. Gray shaded vertical bars show intervals of major SST-drops and E. huxleyi-peaks, which correspond to H3, H2 and H1. Interstadials in δ18O GRIP ice-core record are numbered (1–17). The extent of the “aplanktic zone” (salinity ≥49–50 psu) is sketched at the bottom of the diagram. The dashed vertical line marked the border between the older (60–32 ka BP) and younger (32–14.5 ka BP) core interval.

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shortly after H3 and especially after H2 (Fig. 3h). Fluctuations in the abundance of these three species are also documented in the older part of the record (∼ 60–32 ka BP), there are, however, no analogue abundance patterns as in the younger interval. A comparable distinct species succession of G. oceanica, G. ericsonii and E. huxleyi (order in view of the North Atlantic climate/temperature trend) is virtually absent. In the following discussion we will focus on two aspects: first, the recurrent sequence of G. oceanica, G. ericsonii, and E. huxleyi (=Noelaerhabdaceae-succession) in the younger interval (Section 3.1.), and second, the timing of the onset of the Noelaerhabdaceaesuccession at 32 ka BP (Section 3.2.). 3.1. Fluctuations in the abundance of G. oceanica, G. ericsonii, and E. huxleyi

Fig. 4. Detailed view of the younger core interval (32–14.5 ka BP). Comparison of (a) the alkenone-sea surface temperatures, (b) the absolute abundances (AA) of E. huxleyi (E. hux.) and Gephyrocapsa spp. (=G. ericsonii +G. oceanica; G. spp) and (c) the relative abundances of E. huxleyi (E. hux.) and Gephyrocapsa spp. (G. spp). The dashed graph in Fig. 4b shows an exaggerated version (4x) of the Gephyrocapsa spp. graph to illustrate minor fluctuations of the absolute abundance. The extent of the “aplanktic zone” (salinity ≥49–50 psu) and the qualitative distribution of diatoms (large Cosinodiscus) are sketched.

2006). Sample preparation follows the filtration technique as described by Andruleit (Andruleit, 1996). At least 300 specimens of each sample were examined under scanning electron microscope. The repeatability of the absolute data have been discussed by Herrle and Bollmann (Herrle and Bollmann, 2004). We focus on the abundance patterns of the three Noelaerhabdaceae Emiliania huxleyi (Fig. 3f), Gephyrocapsa ericsonii (Fig. 3g) and Gephyrocapsa oceanica (Fig. 3h), which dominate the upper-photic community of the Red Sea. In addition, we figure the relative abundance of Florisphaera profunda (Fig. 6d). We integrate F. profunda in our discussion because this lower-photic species contributes significantly to the fossil record. Based on AMS (Accelerated Mass Spectrometer) 14C dating and additional paleomagnetic dating points, the stratigraphy framework of core GeoB 5844-2 (Fig. 2) has been established by Arz et al. (Arz et al., 2007). Ages (on the SFCP timescale, (Shackleton et al., 2004)) are announced in calibrated calendar years BP. 3. Results and discussion The varying composition of nannoplankton assemblages defines two intervals (∼ 60–32 ka BP and ∼32–14.5 ka BP). The most striking features in the younger interval (∼ 32–14.5 ka BP) are the recurrent abundance peaks of E. huxleyi, G. ericsonii and G. oceanica. Those intervals, which correspond stratigraphically to Heinrich events H3, H2 and H1 (Fig. 3f), are dominated by E. huxleyi (large sized N4 μm “cold-water form” (Colmenero-Hidalgo et al., 2004)). The longer-term cooling cycles, or more precisely the intervals between H3–H2 and H2–H1, are characterized by increasing values of G. ericsonii (Fig. 3g). In addition, brief increases in abundance of G. oceanica are displayed

3.1.1. Water temperature Biogeographical distribution patterns emphasize the affinity of G. oceanica to the warm waters of the tropical and subtropical latitudes (e.g. (Brand, 1994; Ziveri et al., 2004)). Gephyrocapsa ericsonii flourishes under cooler conditions, while cold waters also inhibit its growth (e.g. (Gartner, 1988; Okada and Wells, 1997)). Emiliania huxleyi tolerates a broad temperature range as documented by its abundant occurrence from the tropics to the subarctic (e.g. (Brand, 1994; Gartner, 1988)). The alkenone-sea surface temperatures (SSTs) available from core GeoB 5844-2 (Arz et al., 2007) show that the Heinrich events H3, H2 and H1 are correlated with pronounced cooling events in the northern Red Sea (Fig. 3e). A weaker response to the high-latitudinal cooling is evident in the older part of the alkenone-record, which also corresponds well to the nannoplankton record. Prior to ∼ 32 ka BP, fluctuations were of lower intensity and the SST always remained above 22.1 °C. Due to the good correlation of our nannoplankton findings with the SSTs, it seems reasonable to envisage a temperaturerelated explanation for the Noelaerhabdaceae-succession. The specific temperature preferences may explain the dominance of E. huxleyi (Figs. 3 and 4) during the maximum cooling intervals (H3, H2, H1) but leaves us with one problem: It remains open why this eurythermal species was displaced by Gephyrocapsa spp. (Figs. 3 and 4) in the transitional intervals (H3–H2, H2–H1). Corroborated by plankton studies, we speculate on temperature-driven variations of the water stratification as the key mechanism. 3.1.2. Winter convective mixing Subtropical marine environments like the northern Red Sea are primarily controlled by the stable stratification of the waters resulting in oligotrophic conditions (regenerated production). Only during the winter period does the breakdown of stratification allow an upwardsmixing of nutrient-rich subsurface waters. The result of this winter mixing is an elevated nutrient level in the photic zone, which favors new production. Since the vast majority of nannoplankton species are adapted to demanding (in particular nutrient limitation) but stable environmental conditions (“K-selection”), the breakdown of stratification initiates an intra-community shift towards only a few, or often only one “opportunistic” nannoplankton species (more r-selected), which dominate in winter and spring (Brand, 1994; Tyrrell and Merico, 2004). Emiliania huxleyi and Gephyrocapsa spp. are the dominating component of nannoplankton assemblages in nutrient-rich environments. Their mass occurrence is characteristic of coastal and centralocean upwelling regions as well as shelf areas (Brand, 1994; Ziveri et al., 2004; Sprengel et al., 2002). In addition, in open subtropical waters they proliferate in highest numbers in response to winter convective mixing (Winter et al., 1979; Haidar and Thierstein, 2001). By comparing the ecological preferences of E. huxleyi with those of Gephyrocapsa spp., maximum E. huxleyi abundances generally occur within better-stratified/weaker mixed waters, while G. oceanica and especially G. ericsonii seem to be adapted to environments of higher

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buoyancy loss, favored especially Gephyrocapsa spp. This approach does not necessarily require a direct competition between Gephyrocapsa spp. and E. huxleyi. Alternatively the physical conditions and the increase of nutrients (nitrate, phosphate and perhaps silica) may have favored more r-selected, non-calcareous/non-preserved phytoplankton (spring-bloom diatoms?) over E. huxleyi (Tyrrell and Merico, 2004; Falkowski et al., 2004). Gephyrocapsa spp. remained in the ecosystem due to its ability to tolerate environments of higher turbulence. We infer that the increase of G. ericsonii in core GeoB 5844-2 reflects significant hydrographic changes driven by the increased strength and influence of cold northern winds in the northern Red Sea region. Still relative high SSTs during the early parts of the cooling cycles may account for the brief increases of G. oceanica. Evidence for the above-described scenario may also comes from planktic diatom. Opal preservation is very rare in the Red Sea. However, the deposition of opal-rich sediments, characterized by abundant large centric diatoms of the genus Cosinodiscus (nearly solely Cosinodiscus oculus-iridis) and low absolute values of the nannoplankton (Fig. 3i), coincide roughly with the Gephyrocapsa spp. dominated intervals. The mass occurrence of C. oculus-iridis in marine sediments is thought to be the result of the “fall-dump” (Kemp et al., 2000), a sedimentation-event initiated by the onset of water mixing in late fall-early winter. It was probably this breakdown of stratification that led later in the cold season to the increase of the Gephyrocapsa spp. Fig. 5. Comparison of (a) the North Atlantic planktic δ18O record (Shackleton et al., 2000) and (b) the Western Mediterranean alkenone-sea surface temperatures (Cacho et al., 1999) with (c) the alkenone-sea surface temperatures from the northern Red Sea (Arz et al., 2007). The onset/range of the Noelaerhabdaceae-succession is sketched at the bottom of the diagram.

turbulence. For example, the nannoplankton communities in the overall weakly mixed pelagic marine ecosystems of the subtropical North Atlantic are dominated by E. huxleyi in winter and spring (Haidar and Thierstein, 2001; McIntyre and Bé, 1967; Okada and McIntyre, 1979). A useful modern analogue for the G. ericsonii-dominated intervals in core GeoB 5844-2 can observed in the northern parts of the western Mediterranean Sea, the Gulf of Lion, and adjoining areas up to the Balearic Islands. The comparison of plankton data (Knappertsbusch, 1993) and hydrological observations (e.g. (Send et al., 1999)) reveals a possible relationship between abundance maxima of G. ericsonii and the overturning of the waters caused by the strong katabatic mistralwind in winter. Moreover, data from the Gulf of Aqaba, the northernmost extension of the Red Sea (Fig. 1), shows high abundances of G. oceanica and especially G. ericsonii during the cold season (Winter et al., 1979; Reiss and Hottinger, 1984). Independent evidence from plankton studies of other algal groups indicates that the structure of the phytoplankton community in the Gulf of Aqaba is strongly affected by the deep (∼ 600 m water depth or more; often no thermocline is present) convective mixing in winter (e.g. (Lindell and Post, 1995; Levanon-Spanier et al., 1979; Post, 2005)), whereas generally a better stratification characterizes the open northern Red Sea (mixing limited to the surface waters; thermocline at ∼150–200 m or slightly deeper). Despite these differences, the primary seasonal patterns described from the Gulf of Aqaba are also evident for the open northern Red Sea (see Introduction). Higher surface water productivity in late winter and spring caused by convective mixing characterize both settings (Acker et al., 2008). Emiliania huxleyi dominates (within the Noelaerhabdaceae) the modern nannoplankton community of the northern Red Sea since the Bølling–Allerød warm period (Legge et al., 2006; Winter, 1982). With regard to the winter mixing intensity, similar conditions like today may have led to high values of E. huxleyi during the mid MIS 3 (before 32 ka BP). We argue that a deep convective mixing due to the surface

Fig. 6. Comparison of the paleoproductivity in the northern Red Sea and the changes in regional precipitation/water balance: (a) variations of the southern desert margin (Reichelt et al., 1992), (b) Chad basin precipitation/evaporation variability (Servant and Servant-Vildary, 1980), (c) Lake level record (in meter below mean sea level) of Lake Lisan (Bartov et al., 2003) and (d) the relative abundance of F. profunda (note reversed axis). Shaded vertical bar indicate the subtropical pluvial as documented by terrestrial proxy records in the subtropics of Africa (e.g. (Nicholson and Flohn, 1980; Rognon, 1996; Hoelzmann et al., 2004)). The arrows in Fig. 6c mark lake level drops of Lake Lisan associated with Heinrich events H5–H1 (Bartov et al., 2003). The paleoflood-record (interval of extreme floods) from the Negev desert (Greenbaum et al., 2006) is sketched in Fig. 6c together with the Lake Lisan curve.

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3.1.3. The glacial salinity increase The major problem in environmental reconstructions is the complexity of the ocean system where various factors (e.g., temperature, salinity, circulation) interact. Because the global sea level decrease co-occur with the long-term trends documented for different parameters during the glacial, it is difficult to exclude a possible impact of salinity on the observed nannoplankton abundance patterns. However, even micropalaeontological studies undertaken in the northern Red Sea region question a simple salinity-related explanation to account for all variations within the planktic foraminifera and pteropod communities. Temperature variations, changes of the water stratification and the availability of food (i.e. productivity of the surface waters) seem to be highly important even in glacial times (Almogi-Labin, 1982; Fenton et al., 2000). The mechanisms proposed above provide a plausible explanation for the Noelaerhabdaceae-succession where the glacial salinity increase is only of secondary importance. The nannoplankton abundance patterns observed in core GeoB 5844-2 show no obvious link to the extent of the hypersaline “aplanktic zone”. Noteworthy is the abundant occurrence of nannoplankton already before the end of this zone. The decrease of nannoplankton abundance may roughly coincide with the onset of the “aplanktic zone “ (at ∼ 30 ka BP in core GeoB 5844-2 (Arz et al., 2003)). The subsequent increase of nannoplankton abundance around the cold-to-warm transition leading into the Bølling–Allerød, however, predates the end of the “aplanktic zone “ (at ∼11 ka BP) by several thousand years. The basic trend of the absolute abundance (low numbers during the glacial/high numbers during the Bølling–Allerød and Holocene (Legge et al., 2006)) found in core GeoB 5844-2 is probably not a Red Sea specific feature. Furthermore, it seems unlikely that a salinity-related reduction of the absolute abundance occurred without a marked imprint on the composition of the dominating nannoplankton species. If nannoplankton was affected, one could anticipate a gradual disappearance of species (i.e. a non-repeating species succession similar to those documented for planktic foraminifera species). Such patterns are not recognizable in our data. We detect instead the reiterated Noelaerhabdaceae-succession, closely correlated with the longer-term cooling cycles and Heinrich events of the North Atlantic realm. A compelling statement based on a detailed comparison with reconstructed sea level changes (the same is true for salinity reconstructions) is hampered by the uncertainties underlying such records (e.g. (Siddall et al., 2003; Arz et al., 2007)). The problem (i.e. the lack of truly independent data) is illustrated in Fig. 3d by the two sea level curves (Arz et al., 2007). Heinrich events H3, H2 and H1 were (temperature corrected curve), or were not (temperature constant curve), accompanied by a temporary reduction of the surface salinity caused by a minor sea level rise. Corroborating evidence against salinity-driven fluctuations of the Noelaerhabdaceae in the northern Red Sea come from the Mediterranean. Nannoplankton data from the western Mediterranean Sea (Colmenero-Hidalgo et al., 2004), although not identical in detail, show comparable patterns to those from the northern Red Sea. Abundance-peaks of E. huxleyi (N4 μm) during Heinrich events and high values of small Gephyrocapsa between the Heinrich events have been observed in this study. 3.2. Onset of the Noelaerhabdaceae-succession As mentioned in the Introduction section, basic phytoplankton patterns in the modern northern Red Sea are not directed by the Indian Ocean. The trend towards lower sea levels and hypersaline conditions mirrors a diminishing communication with the open ocean (e.g. (Siddall et al., 2003; Arz et al., 2007)). Hence, the imprints of climatic signals transmitted from the Gulf of Aden to the Red Sea should be weaker during the peak of glacial conditions. Due to the

great distance from the Strait of Bab-el-Mandeb (nearly 2000 km), this should be especially true for our study area. The nannoplankton abundance patterns and alkenone data documented in core GeoB 5844-2, by contrast, point to a sensitive and fast acting mechanism linking high latitudinal climate oscillations and environmental changes in the northern Red Sea region especially after 32 ka BP. A plausible mechanism fulfilling these requirements seems to be the atmospheric teleconnection with the extratropics and the associated hydrographic changes of the northern Red Sea driven by a reinforced influence of cold air. The observation of a weak response to the high latitudinal cooling in the earlier part of our record (∼60 to 32 ka BP) contrasts with paleoclimatic records from the extratropics. For example, data from the North Atlantic (Fig. 5a, planktic δ18O record from the Iberian Margin (Shackleton et al., 2000)) and the Mediterranean Sea (Fig. 5b, alkenone record from the Alboran Sea (Cacho et al., 1999)) reveal large and rapid environmental fluctuations in response to the high latitudinal cooling already before 32 ka BP. Although environmental changes in the Mediterranean Sea may be partly influenced by Atlantic waters through the Strait of Gibraltar, the primary mechanism for the transmission of the North Atlantic signals to the Mediterranean region is atmospheric forcing (e.g. (Rohling et al., 2003; Cacho et al., 1999; Bartov et al., 2003)). Considering the onset of the Noelaerhabdaceae-succession at ∼ 32 ka BP and the alkenone-SST variations (Fig. 3), a stronger atmospheric connection between the northern Red Sea area and the extratropics established in the late MIS 3. The present-day Mediterranean winter climate (Fig. 1) is affected, in addition to the westerlies responsible for the characteristic winter cyclonic rain, by cold continental and altered maritime polar and arctic air reaching the region via southern and eastern Europe (e.g. (Air Ministry Meteorological Office, 1962; Martyn, 1992; Hoskins and Hodges, 2002)). One of the most distinctive feature of the northern Red Sea area is its close proximity to the tropical-extratropical atmospheric “boundary” (roughly marked by the position of the Subtropical Jet; Fig. 1). The onset of the Noelaerhabdaceae-succession at ∼32 ka BP in the northern Red Sea seem to reflect changes of the tropical-extratropical atmospheric connection, which may have been caused by a southwards shift/modification of the polewards branch of the tropical Hadley cell and an expansion of the extratropical winter climate features into the present-day subtropical desert belt. This shift (probably latitudinal-parallel) and accordingly the more direct and more prolonged influence of the extratropical circulation systems on the northern Red Sea may account for the now undamped atmospheric transfer of the cooling signals (longer-term cooling cycles and Heinrich events) to the northern Red Sea as indicated by the nannoplankton abundance patterns and the alkenone data. Terrestrial records from the northern parts of the Saharan desert belt provide a good link between a climatic change towards cooler and wetter (respectively a positive precipitation-evaporation balance) conditions and the changes of the nannoplankton community structure. The onset of a distinctive “pluvial period” (in absolute terms probably semi-arid or at most semi-humid in large part of the present desert belt) is documented at ∼32(−30) ka BP (e.g. (Nicholson and Flohn, 1980; Servant and Servant-Vildary, 1980; Rognon, 1996; Hoelzmann et al., 2004)). The increased atmospheric pressure gradient, associated with the expansion of the northern ice sheets and the enhanced latitudinal temperature contrast, modulated the atmospheric circulation. A southward shift of the westerlies that allowed Atlantic depression to supply winter-rain towards the desert belt has been proposed (Nicholson and Flohn, 1980; Hoelzmann et al., 2004). The late Pleistocene pluvial period ended at ∼ 22(−20) ka with the onset of arid or even hyper-arid climatic conditions, which characterize the Last Glacial Maximum (LGM; ∼ 22–19 ka BP) and the subsequent interval up to the onset of the Bølling–Allerød (∼ 14.5 ka BP) in wide areas of Africa and Arabia (e.g. (Nicholson and Flohn, 1980; Hoelzmann et al., 2004; Hamilton and Taylor, 1991; Gasse, 2000)). The

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climatic re-orientation at ∼22 ka BP did not disturb the continuation of the Noelaerhabdaceae-succession; G. ericsonii (Fig. 3g) dominated up to the E. huxleyi (Fig. 3f) abundance maximum during Heinrich event H1. On closer examination, however, the climatic re-orientation seems to be manifested in the nannoplankton record by the secular trend of F. profunda (Fig. 6d). Plankton studies in subtropical environments have found maximum abundances of this lower-photic species during summer and fall (Haidar and Thierstein, 2001). These observations are consistent with the idea that F. profunda flourishes in overall stable stratified waters in or below the thermocline/nutricline when surface productivity (and abundance of upper-photic nannoplankton species) is low. Florisphaera profunda has been used (Molfino and McIntyre, 1990) for the reconstruction of nutricline dynamics (high%-abundance of F. profunda = deep nutricline and vice versa). Furthermore, a strong correlation between surface water productivity and the percentage occurrence of F. profunda in marine sediments has been documented (Beaufort et al., 1997). This makes F. profunda particularly interesting as an indicator for changes in paleoproductivity (high %-abundance of F. profunda = low paleoproductivity and vice versa). Following the latter approach, overall high values of F. profunda since ∼22 ka BP indicate low paleoproductivities. The highest productivity/lowest F. profundaabundance is documented between ∼32 ka BP and 22 ka BP. The coincidence (in time) between the interval of higher productivity in the northern Red Sea and the subtropical pluvial period on land (Fig. 6) suggests a linkage between the moisture climate and the trophic state of the northern Red Sea waters. Fenton et al. (Fenton et al., 2000) propose that the glacial conditions were milder in the Gulf of Aqaba than in the Red Sea. Unlike modern conditions, a freshwater inflow to the Gulf of Aqaba supplied by Mediterranean depressions is assumed (Fenton et al., 2000). Higher water levels of the Lake Lisan (Fig. 6c), the progenitor of the Dead Sea, may give evidence of higher regional rainfall of extratropical origin (Bartov et al., 2003; Bartov et al., 2002). There are similarities between fluctuations of the reconstructed lake level and the F. profunda abundance patterns on longer time scale, which may indicate an influence of extratropical precipitation on northern Red Sea fertility. Another possibility is that the African monsoon (Fig. 1), which affected the subtropical desert belt (Fig. 6a) during the late Pleistocene from the south (Rognon, 1996; Rossignol-Strick, 1985; Reichelt et al., 1992), influenced marine fertility. High values of the astronomical monsoon index are documented during the late MIS 3 (peak at ∼32 ka BP, (Rossignol-Strick, 1983)). The typical accompanying eastern Mediterranean sapropel deposition (organic-rich black layer) caused by intensive freshwater discharge and resulting stagnation of bottom water formation is missing in open-ocean records but may be evident near the Nile delta (Rossignol-Strick, 1985). According to Almogi-Labin et al. (Almogi-Labin et al., 1998), this late Pleistocene humid period is documented in the central Red Sea by abundance maxima of epipelagic pteropod species, which should indicate a reduced ventilation of the surface waters and an increase in productivity caused by a strong southwest monsoon. Any explanation linking Red Sea fertility and extratropical or tropical rainfall remains debatable. The high productivity period in the northern Red Sea cannot be linked to tropical precipitation without problems; a significant direct influence of the southwest monsoon should be limited to the southern-central Red Sea region. In addition, because of the conspicuous spatial rainfall differences in the Near East region (e.g. (Horowitz, 1979; Amit et al., 2006; Dayan and Morin, 2006)) care is needed when comparing wetter periods in the northern subhumid/semiarid parts with environmental changes further south (Negev-Gulf of Aqaba-northern Red Sea region). Records from the northern parts reflect primarily the conditions of the mid-latitudinal Mediterranean climate zone. For example, lake-level drops of the Lake Lisan seem to be almost synchronous to the last five Heinrich events (Bartov et al., 2003). Furthermore, pollen spectra and speleothem

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records (δ18O, δ13C) from different localities in Israel illustrate a large degree of climate heterogeneity caused by orography, land-sea contrast and the diversity of climate regimes affecting the region (Horowitz, 1979; Vaks et al., 2006). Oxygen isotopic data from core GeoB 5844-2 (Fig. 3c), overall characterized by a substantial salinity overprint, reveal no distinctive period of surface freshening between 32 ka BP and 22 ka BP. The comparison of the δ18O data (Fig. 3c) with the alkenone-record (Fig. 3e) illustrates the evaporation effect typical of the glacial Red Sea (see Introduction). Pronounced freshening should be recognizable in the δ18O record by an excursion to lower values/a interruption of the secular trend to exceptional high values. Of course, the lack of such excursion does not necessarily exclude a local restricted fresh water supply (e.g., from wadis), but wind-driven variations of the water column properties were maybe more important. Intensified surface water mixing other than in winter season would have interfered the development of environmental conditions especially favorable for F. profunda (stable stratified water column with a deep nutricline). Evidence for increased storminess under overall still dry regional conditions is documented by a recently published paleoflood-record from the hyperarid southern Negev desert (Greenbaum et al., 2006). A conspicuously higher frequency of storms/paleofloods is indicated during the late MIS 3–early MIS 2 (Fig. 4). Interestingly, the authors (Greenbaum et al., 2006) argued for variations of the Red Sea Trough systems (for details (Kahana et al., 2002)) as a possible cause. This implies a profound role of tropical convection intensity (Dayan and Morin, 2006; Kahana et al., 2002). After all, the information available so far only allow speculative statements. Although we note a low F. profunda interval in the northern Red Sea contemporaneous with the pluvial period on land, further research is needed to understand the causal relationship between F. profunda abundance patterns, inferred marine environmental changes/surface productivity and wetness documented by terrestrial proxy records. 4. Conclusion Our record demonstrates that the nannoplankton responded differently to high latitudinal cooling before and after ∼32 ka BP. A strong atmospheric influence from the extratropics is indicated since the late MIS 3. The climatic evolution of the northern subtropics lends support to this finding. A reiterated Noelaerhabdaceaesuccession, closely correlated with the longer-term cooling cycles and Heinrich events of the North Atlantic realm, is detected after 32 ka BP. The three Heinrich events H3, H2, and H1 are dominated by E. huxleyi. Gephyrocapsa oceanica and especially G. ericsonii are abundant between H3 and H2 and between H2 and H1. The present data lead to the assumption that the glacial salinity increase did not cause the Noelaerhabdaceae-successions. Although the glacial Red Sea represents a unique marine environment with regard to the impressive sea level driven salinity increase, the documented nannoplankton changes are probably better explained on the basis of a more unspectacular approach. A plausible mechanism for the observed changes seems to be the atmospheric teleconnection with the extratropics and the associated hydrographic changes driven by an, in relation to present-day and early–mid MIS 3 conditions, reinforced influence of cold northerly air into the northern Red Sea region. Furthermore, we documented a low F. profunda interval contemporaneous with a pluvial period on land between 32 and 22 ka BP. This finding raises further questions with respect to the specific mechanism linking F. profunda abundance patterns, marine productivity and terrestrial environmental changes. Although we cannot present a final explanation for this relationship here, it may provide an interesting approach for further studies to investigate past climatic changes in the northern Red Sea region.

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