Carbonate preservation and climatic changes in the central Red Sea during the last 380 kyr as recorded by pteropods

ELSEVIEiR

Marine Micropaleontology 33 (1998) 87-107

Carbonate preservation and climatic changes in the central Red Sea during the last 380 kyr as recorded by pteropods Ahuva Almogi-Labin

a**,Christoph Hemleben b, Dieter Meischner ’

’ Geological Survey of Israel, 30 Malkhe Yisrael St., Jerusalem 95501, Israel b Institut und Museum Geologie und Paliiontologie, Universiry of Tiibingen, Sigwartst. IO, D-72076 Tiibingen, Germany (’Institut,fiir Geologie und Paliiontologie, Abteilung Sediment-Geologie, Universiry of GGttingen, Goldschmidstr: 3, D-37077 Giittingen, Germany

fir

Received 30 April 1997; revised version received 8 July 1997; accepted 19 July 1997

Abstract Numerical abundances of pteropods and planktic foraminifera, and the mode of pteropod preservation, were determined in core KL 11 taken from the central part of the Red Sea. The abundance of pteropods (shells/g dry sediment) was compared to that of planktic foraminifera (% Pt/Pf + Pt) - a technique that permits detection of changes in carbonate preservation for the last 380 kyr. The numerical abundance of pteropods is influenced by the properties of the water column, and preservation is influenced by the bottom water. During the last -200 kyr (except during isotope stage 5.5) carbonates are in general well preserved. During this period the abundance pattern of the pteropods and planktic foraminifera is very similar and follows the climatic signal of the Red Sea with high numbers during the interglacial stages, changing to very low numbers during glacial maximum conditions. The similar abundance trends of the two groups, and between them and the S180 curve, indicates abundance is strongly linked to salinity. From isotope stage 6 to the bottom of the core the pteropods occur in low numbers, unlike the planktic foraminifera, which continue to display the high-amplitude glacial-interglacial cyclicity. The deviations, mainly during interglacial stages, between the abundance pattern of the two groups and the low % Pt/Pf + Pt values, indicate a significant change in carbonate preservation. Distinctive carbonate dissolution intervals are recognized in the Red Sea, correlating to large scale deep water dissolution events of the ‘mid-Brunhes dissolution cycle’ in the Indian Ocean. The anti-estuarine circulation pattern of the Red Sea prevents a direct connection between the deep water masses of the two oceans and rules out the likelihood that changes in the deep water circulation caused these carbonate dissolution events. The numerical variations between nonmigratory epipelagic and migratory mesopelagic pteropods were used to evaluate changes in the structure of the water column. Abundance maxima of mesopelagic pteropods, as in the recent Red Sea, indicate an aerated water column with ~0.5 ml 02/l oxygen concentrations at the minimum zone. Mesopelagic abundance maxima coincide commonly with negative monsoonal index values indicating a more aerated water column connected to increasing aridity in the Red Sea region. Abundance maxima of epipelagic pteropods indicate a strongly stratified water column, at times causing severe depletion in oxygen at intermediate water depths. Epipelagic peak events coincide often with positive monsoon index values implying an overall milder and more humid climate in the Red Sea, probably associated with enhanced precessional-controlled southwest monsoon activity. 0 1998 Elsevier Science B.V. All rights reserved.

Keywords: Red Sea; Pteropoda; Quatemary paleoclimate; carbonate preservation

* Corresponding author. E-mail: [email protected] 0377-8398/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII s0377-8398(97)00034-0

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1. Introduction Pteropods, holoplanktic gastropods with aragonitic shells, are common members of the calcareous zooplankton community in open ocean environments (Be and Gilmer, 1977). The flux of pteropod shells (> 150 pm) to the sea floor is high, and in certain regions it is similar to that of the planktic foraminifera. The abundances of pteropod shells in deep sea sediments are different from those of the live pteropod populations. The decreasing CaCOs saturation in the deep ocean causes selective dissolution of the aragonitic shells (Berner, 1977) to such a degree that a pteropod ooze is quite rare in deep ocean sediments. Well-preserved pteropod shells which can be used for paleoceanographic reconstruction are limited to shallow tropical and subtropical ocean environments (Arabian Sea, e.g. Herman and Rosenberg, 1969), to environments which border carbonate platforms (Droxler et al., 1988, 1990), or to marginal seas with anti-estuarine circulation like the Red Sea and the Mediterranean Sea (Reiss et al., 1980). These water bodies are regarded as ‘carbonate traps’ able to preserve pteropod-rich sediments in their deep waters (Berner, 1977; Berger, 1978). The sporadic and often irregular records of pteropods in Quaternary sediments prevents effective widespread use of the group. Nevertheless, in the Red Sea, the well-preserved continuous late Quatemary pteropod record, is extremely useful for paleoceanographic reconstruction. A fairly precise stratigraphic correlation for the latest Pleistocene-Holocene was established throughout the northern Red Sea-Gulf of Aqaba area, based on distinct Pteropoda datum layers (AlmogiLabin, 1982). Estimation of past variations in temperature, salinity and productivity were inferred from the pteropod record (Herman and Rosenberg, 1969; Chen, 1969; Risch, 1976; Yusuf, 1978; Reiss et al., 1980; Almogi-Labin, 1982; Ivanova, 1985; Locke and Thunell, 1988; Almogi-Labin et al., 1986, 1991). The Red Sea pteropod species have two lifestyles non-migratory (epipelagic) and die1 migratory (mesopelagic) (Weikert, 1982, 1987, AlmogiLabin, 1984; Auras-Schudnagies et al., 1989). The epipelagic group consist of 10 taxa comprising less than 25% of the core top and Holocene assemblage. The mesopelagic group (the die1 migratory species) consists of four species and comprises more than

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33 (1998) 87-107

75% of the assemblage. Variations in the abundance of epipelagic and mesopelagic pteropods may be controlled by fluctuations of the oxycline (AlmogiLabin et al., 1991). Numerous studies of the late Quaternary paleoceanography of the Red Sea reveal pronounced glacial-interglacial variations in foraminifera and pteropoda assemblages and their stable isotopes. Especially the last glacial-interglacial cycle is quite well documented. Only a few older strata - as old as the top of the penultimate glacial [top stable isotope stage (IS) 61 - were analyzed. Documenting the pteropod record beyond the last glacial-interglacial cycle may reveal more about the long- and shortrange climatic fluctuations in this marginal, partly isolated, low-latitude sea. A continuous sedimentary record with well preserved pteropods and foraminifera was found in core KL 11, one of a series of long cores obtained from the central part of the Red Sea. It contains the last four glacial-interglacial cycles (isotope stages l11.2) based on the detailed and correlatable stable isotope record representing the last 380 kyr (Hemleben et al., 1996). The long time span covered by this core, and its location in intermediate water, below the oxygen minimum zone (OMZ) within the central Red Sea (Fig. l), make this core suitable for investigating changes in water column properties. The present work was carried out in order to study the response of the epi- and mesopelagic pteropods to mixed layer and intermediate water column fluctuations in the Red Sea over a considerable time span. Estimation of bottom water properties was derived from the fluctuations in the mode of pteropod preservation. 1.1. Hydrographic and paleoceanographic

setting

The Red Sea is an elongate, deep, semi-enclosed marginal sea connected with the open ocean through the shallow and narrow Bab-el-Mandeb straits. The climate of the Red Sea and its neighboring East African and Arabian landmasses is arid, with very low annual precipitation and runoff, and high rates of evaporation (Morcos, 1970). The circulation pattern is anti-estuarine with warm and normal-marine Gulf of Aden surface water (summer, 30°C 36.5%0; winter, 25.5”C, 36.5%0) entering through the Bab-elMandeb straits, becoming cooler and saltier - due

A. Almogi-hbin

et al. /Marine

Fig. 1. Core location in the Red Sea (R/V ‘Meteor’ cruise 5 leg, 2): Station 174 (18”46,3’N, 39”19.9’E), core KL I 1, water depth 825 m.

to evaporation - as it moves northward. The warm and shallow water mass lies above a cooler (21.7”C) and saltier (40.5%0) water body that extends from 100 m depth to the sea bottom (Morcos, 1970). Below 100 m the oxygen content decreases rapidly and reaches a minimum between 200 and 650 m depth. The oxygen concentration at the minimum level is about 1.75 ml 02/l in the north, decreasing southward to about 0.5 ml 02/l at 20”N. Below 700 m the bottom water is well aerated (2 ml 02/l) (Neumann and McGill, 1962; Woelk and Quadfasel, 1996). The deep water is seasonally renewed in winter by oxygen-rich dense surface water formed in the Gulf of Suez. Two additional sources contributing to the replenishment of the Red Sea intermediate water mass are the cooler dense surface water of the northemmost Red Sea and the outflowing water from the Gulf of Aqaba (Cember, 1988; Eshel et al., 1994). During glacial low sea level stands, when the shallow Gulf of Suez (55-75 m maximal water depth) often

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33 (1998) 87-107

89

dried up, these two sources seem to have contributed significantly to the Red Sea deep water. The pronounced oxygen depletion in the OMZ of the southem Red Sea results from two independent factors: the increasing distance from the areas of deep water formation and the increased take up of oxygen due to higher primary productivity levels in the southern Red Sea (Morcos, 1970; McGill, 1973; Sakthivel, 1973a; Weikert, 1987). During the southwest monsoon, nutrient-rich Gulf of Aden water enters the Red Sea as a subsurface current flowing as far as 18-20”N (Maillard and Soliman, 1986). These waters cause a pronounced seasonal increase in primary productivity supporting high standing stocks of zooplankton and increased oxygen demand at intermediate water depths, where this biomass is decomposed (Sakthivel, 1973a; McGill, 1973; Weikert, 1987). Properties of the water column are controlled mainly by the Bab-el-Mandeb straits dynamics and by the regional climate. Changes in sea level accompanying glacial-interglacial cycles determine the volume of water moving from the Gulf of Aden into the Red Sea, and hence its degree of isolation (Thunell et al., 1988; Hemleben et al., 1996). Fluctuations in the intensity and geographic extent of the monsoon rainfall influence the surface waters and the climate in this region (Pachur et al., 1990; Gasse et al., 1990; Hemleben et al., 1996). Stable isotope records, as well as other micropaleontological and sedimentological data, point to large hydrographic fluctuation during the Late Pleistocene and early Holocene (Reiss et al., 1980; Almogi-Labin et al., 1986, 1991; Locke and Thunell, 1988; Thunell et al., 1988; Hemleben et al., 1996). The salinity of the surface water fluctuated between 38.5%0 - the present-day value - and approximately 53%0 (Hemleben et al., 1996) during the last glacial maximum (LGM). Deep water salinity, today 40.5%0, was approximately 55%0 during the LGM. The decrease in surface water salinity to the present-day value occurred in stages during the Deglaciation. The present-day surface water salinity at the core site KL-11 was established during the mid-Holocene (Almogi-Labin et al., 1991). Oxygen concentrations varied considerably. Anoxic conditions predominated at the end of the deglaciation period, and low oxygen levels prevailed during IS 3. Well-aerated bottom water existed in the northern

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90

Red Sea during the middle to late Holocene, as well as during long intervals of the last interglacial (Luz and Reiss, 1983; Almogi-Labin et al., 1986, 1991). 2. Material and methods Core KL 11 was obtained from the central part of the Red Sea (Fig. 1) by the German R/V ‘Meteor’, cruise 5, leg 2, at station 174 (18”46.3’N, 39”19.9’E, 825 m water depth). A continuous and undisturbed sequence is revealed by X-ray radiographs (Clasen

I.S.

6’“O(%oPDB) (In) 3

2

I

0 -I -2 -3

0

No. Pterouods /e_ .

% Coarse Fraction 0

(‘63 pm) 20 40 60

and Gehrke, 1990). A stable oxygen isotope stratigraphy (Fig. 2a) was established for the core by analysis of the planktic foraminifer GZobigerinoides ruber (Hemleben et al., 1996) and by correlating this record with the global record (Imbrie et al., 1984). Hundred eighty three samples were taken along the 20.93 m long core every 5-25 cm, selected according to lithological variations and X-ray character. Between 53 and 93 cm samples were taken each centimeter (cf. Almogi-Labin et al., 1991). Micropaleontological preparation included drying

No. Planktic Foraminifera /g

No. Pteropod Pteronods/Planktic Forams&Pteropods (%) Specie&nple

0

1

no planktic foraminifera

; 3 I 5

6

I

8

9

18 0

1

I

I

I

I

I

I

_

Fig. 2. (a) Planktic foraminiferal (Globigerinoides ruber) oxygen isotope record of core KL 11 showing a wide range glacial-interglacial amplitude (Hemleben et al., 1996). Numbers on the right side of the record indicate oxygen isotope stages (IS). (b) The coarse fraction content (% >63 pm) of the sediments. (c) The numerical abundance of pteropodslg dry sediment. Note that below 12 m the group occurs in very low numbers. (d) Numerical abundance of planktic foraminiferdg dry sediment. The distribution pattern of pteropods (in the upper 12 m) and that of the planktic foraminifera (throughout) follows the climatic signal of the Red Sea. (e) The relative abundance of pteropods relative to the total of pteropods and planktic foraminifera. During glacial maximum conditions high % Pt/Pf + Pt values result from the higher salinity tolerance of pteropods. Intervals with low % Pt/Pf + Pt values (<50%) represent intensive pteropod loss associated with enhanced dissolution of aragonite. (f) The diversity (number of species/sample) of pteropods is high during interglacial stages; intermediate during the glacial stages and very low during extreme glacial maximum conditions.

A. Almogi-Labin et al. /Marine Micropaleontology 33 (1998) 87-107

at 50°C and weighing the dried material. About 2 g of dried material was washed through a 63 pm sieve, dried at 50°C and the residue larger than 63 pm was weighed again. As in previous studies of pteropods in the Red Sea (Almogi-Labin, 1982; Almogi-Labin et al., 1986, 1991; Locke and Thunell, 1988), the ~149 pm size fraction was used in the present study. The sieved samples were split into an aliquot containing more than 300 pteropods. All specimens were identified and counted. Aragonitic pteropods are more susceptible to dissolution than the more resistant calcitic planktic foraminifers, thus the numerical relation % Pt/Pf + Pt (the percentage of pteropods relative to total planktic foraminifera and pteropods in the > 149 pm size fraction) was used as an indicator of the state of pteropod preservation. The pteropod shells were grouped into five classes of preservation, following Almogi-Labin et al. (1986): transparent, opaque, internal molds with original aragonite, internal molds with no original aragonite, and shells filled and/or encrusted with authigenic aragonite. All the counted specimens were separated for further study and are deposited at the Geological Survey of Israel. 3. Results The sediments of core KL 11 consist of a nanno-foraminiferal to nanno-foraminifera-pteropoda1 ooze. The sediments are mostly light gray, yellowish brown to brown and often bioturbated. The sequence is interrupted by the so-called ‘hard layers’ (Degens and Ross, 1969) between 0.93 and 1.83 m depth, and a distinct dark grey and laminated layer between 0.82 and 0.93 m depth (Almogi-Labin et al., 1991). The ‘hard layers’ are composed of lithified aragonite and contain aragonite-encrusted pteropod shells. This interval was deposited during IS 2. A very dark grey horizon above the ‘hard layers’ has a high total organic carbon (TOC) content of up to 1.65 wt% and a low carbonate content of lo-20% CaC03. This interval was deposited during the late Deglaciation-Early Holocene. 3.1. Pteropoda abundance and diversity The coarse grain fraction >63 pm usually comprises lo-20% of the sample weight (Fig. 2b). This

91

fraction includes a calcareous skeletal component (consisting of planktic foraminifers and pteropods), and authigenic carbonate. The authigenic component is restricted mainly to glacial periods and consist of high-magnesium carbonate (HMC) concentrated in nodules and pteropod fillings, as well as secondary aragonite encrusting pteropod shells. Coarse-grained silicates comprise an additional component, which occurs predominantly during the glacial stages. Coarse-material spikes, occurring in glacial stages 6, 8, 10 and especially during the LGM, are associated with aragonite encrusted pteropod shells. Apart from these spikes the uniformity in the % weight of the coarse fraction is striking, especially as compared to the frequent variations in the abundance of pteropods and planktic foraminifers along the core (Fig. 2c, d) as well as variations in the abundance of pteropod shells filled with HMC or secondary aragonite. Interglacial-glacial fluctuations in pteropod abundance are found mainly in the upper 10 m of the core (Fig. 2c) with higher numbers during the interglacial stages. This pattern of distribution is similar to that of the planktic foraminifera (Fig. 2d) and parallels the stable oxygen isotope curve (Fig. 2a). High pteropod abundances are associated with the interglacial stages 1 and 5 with peak abundance of 50007000 specimens/g dry sediment. During the glacial stages the pteropods number less than 2000 specimens/g dry sediment, and they drop to extremely low values between 100 and 400 specimens/g during the glacial maxima of stages 2 and 6. In the lower part of the core pteropods occur in low numbers, less than 1000 specimens/g dry sediment, with a slight increase during glacial stage 10. Pteropods were absent from the sedimentary record during interglacial event 9.3. Unlike the general downcore decline in the numerical abundance of pteropods, the planktic foraminifera occur in a distinct cyclic pattern, with high numbers during the interglacial stages and low numbers during the glacial stages (Fig. 2d). The numerical relation of pteropods and planktic foraminifera, is used as an indicator of the state of pteropod preservation (Bemer, 1977). Aragonitic pteropods are more susceptible to dissolution than the calcitic planktic foraminifers. The pteropoda/planktic foraminifera numerical relation (% Pt/Pf + Pt) in core top (Almogi-Labin et al.,

92

A. Ahqi-L&n

et al. /Marine

1986, 1991) and in Holocene sediments is around 50’6 (Fig. 2e). During the glacial stages 3 and 4, high (% Pt/Pf + Pt values, >SO%, occur often, probably due to impoverishment of the planktic foraminifera as compared to the pteropods. A % Pt/Pf + Pt value of 1OOYois found during IS 2 when the pteropods were the only surviving calcareous planktic group. Downcore, below 10 m depth, the percentage of % Pt/PP + Pt decreases gradually to values below 50%, tollowing the general decrease in abundance of the pteropods in general. Nevertheless the ratio is clearly higher during the glacial compared to the interglacial stages, reflecting on one hand the increasing impoverishment of planktic i’oraminifera as compared to the pteropods during the glacial stages, and the larger loss of pteropods by dissolution during the interglacial stages. The number of pteropod species (simple diversity) varies along the core (Fig. 2f) parallel to the stable oxygen isotope curve. During the interglacial stages species diversity was high, with Y-12 species per sample. During the glacial stages diversity decreased to S-7 species per sample. A monospecific pteropod population occurs during the LGM, when all the planktic foraminifera disappeared from the central and northern Red Sea. The similarity between the diversity curve and the stable oxygen isotope curve was interrupted during interglacial stage 9.3 when no pteropods are preserved. This interval probably represents a period of intense aragonite dissolution and is different from previous interglacial stages with high pteropod diversity. 3.2. Pteropod assemblage

composition

Abundance per g dry sediment and relative abundance of the seven most common pteropod species out of the total sixteen species known to occur in the Red Sea are presented in Figs. 3 and 4. The seven dominant species comprise 9.5-100% of the total assemblage. The other species are rare, with relative abundances usually below 1%. The Red Sea pteropods belong ecologically either to the epipelagic non-migratory group (Fig. 3b-d, Fig. 4bd) or to the mesopelagic migratory species (Fig. 3eh). The distribution of the common species varies frequently, with peaks lasting over short periods (Table 1).

Micropaleontology

33 (19%)

87-107

The mesopelagic Limacina infatu is the dominant species in Recent and middle-late Holocene sediments, comprising 65-75% of the assemblage (Fig. 3f, Fig. 4f). Downcore the species’ abundance varies considerably, with peaks (>50%) during both interglacial and glacial stages (Table 1). L. in&a is absent during the middle part of glacial stage 10 and during most of the last glacial cycle (IS 2-4), except for a short interval during the middle part of IS 3. The epipelagic L. trochiformis, ranking second in relative abundance along the core, is a minor component in Recent sediments, but it is an important component of the assemblage both during glacial and interglacial stages (Fig. 3d), and becomes the dominant species during IS 3 and 10. The highest abundance was recorded during the early Holocene and the deglaciation period. High abundances on the order of 600-I 000 specimens/g dry sediment were recorded during most of IS 3, parts of IS 5, and the top of IS 7. Creseis virgula s.1. includes variants like Creseis virgula virgula, Creseis virgula conica and Creseis chierchiae. All these taxa were grouped under C. virgula s.1. because downcore the majority of the shells consist of early embryonic parts which look alike in the three taxa. The three species belong to the epipelagic group living in the mixed zone (Be and Gilmer, 1977). The identification of C. chierchiae was possible only in the youngest record of the last 13 kyr thanks to the preservation of adult specimens (Almogi-Labin et al., 1991). Creseis virgula s.1. ranks third in abundance along the core. In Recent and Late Holocene sediments it is a minor component comprising 4-8% of the assemblage (Fig. 3~). In general its relative abundance is higher during the glacial stages, though it is a common species also in interglacial stages. High concentrations of C. virgula (above 400 specimens/g dry sediment) were recorded in the early Holocene and during parts of IS 3 and IS 6 (Fig. 4~). Otherwise the species occurs in low numbers, between 100 and 300 specimens per g dry sediment. At the beginning of 1s 2, C. virgula comprises up to 85% of the assemblage. Creseis acicula, ranking fourth in pteropod species abundance, is an epipelagic species comprising 4-10% of the assemblage during the Holocene (Fig. 3b). Downcore it decreases in abundance below 5%. During maximal glacial conditions of IS 6 and

93

A. Almogi-Labin et al. /Marine Micropaleontology 33 (1998) 87-107

Epipelagic Pteropods

St80(%o PDB) m-0

3

2

1

0

-1

Cacicula t%) -2

Cvirgula (%)

-.1

Mesopelagic Pteropods

I

L.trochiformis (%)

L.buIimoides I (%)

L.Mata (%f

Ssubule (%)

C convexa 1%)

50

6

9

12

15

18

21

Fig. 3. (a) Planktic foraminiferal (Globigerinoides ruber) oxygen isotope record of core KL I1 (Hemleben right side of the record indicate oxygen isotope stages (IS). The relative abundance of different epipelagic pteropod species.

mainly of IS 2, this species is the only calcareous planktic organism found in the sediment forming a monospecific population that survived the extremely high surface water salinity (Hemleben et al., 1996). A prominent C. acicula abundance peak was found during the last deglaciation period (at 0.91-0.92 m depth). During the LGM when the species formed a monospecific population, its total abundance was extremely low and ranged between 100 and 300 specimens/g dry sediment. Below IS 2 it becomes a minor component of the assemblage, usually with less than 100 specimens/g dry sediment (Fig. 4b). a mesopelagic species, Limacina bulimoides, ranks fifth among the pteropods in total and relative abundance (Fig. 3e, Fig. 4e). This species is rare

et al.. 1996). Numbers on the (b-d) and mesopelagic (e-h)

during the last 2-3 kyr (Almogi-Labin et al., 1991) but was more common during the middle Holocene, and in general more abundant during the interglacial stages. It is absent during the last glacial stages 2 and 3, during most of the penultimate glacial IS 6 and during most of IS 10. The highest relative abundance of L. bulimoides is found during IS 4 and 11.2 with high relative abundance occurring also during parts of IS 5, 7 and 8. An abundance of more than 500 specimens/g dry sediment was found in the middle part of IS 1, and during IS 5.1, 5.5, 7.2, 7.3 and base of IS 10. Two other mesopelagic species which occur in the core are Clio convexa and Styliola subula (Fig. 3gh, Fig. 4g-h). Clio convexa lives at present in the

A. Almogi-Labin et al. /Marine Micropaleontology 33 (1998) 87-107

94

Epipelagic Pteropods 6 I80 (!%oPDB) (rn) 3

2

1 0 -1 -2 -3

Cacicula &I

I.S. 0

Mesopelagic Pteropocls

C.virgula L.trochiformi L.bulimoides i (g) (9) 031

500 1000 0

No

0

so0 loo0

0

so0 lc00

0

L.infIata C&3) 2000

S.subula Cconvexa @I (9) 4000

0

500 0

500

18

Fig. 4. (a) Planktic foraminiferal (G2obigerinoides ruber) oxygen isotope record of core KL 11 (Hemleben et al., 1996). Numbers on the right side of the planktic record indicate oxygen isotope stages (IS). The numerical abundance (no. specimens/g dry sediment) of epipelagic (b-d) and mesopelagic (e-h) pteropod species. Distinct interglacial (higher) and glacial (lower) numerical abundance cycles are recognized in the upper 12 m of the core. Below this depth the signal is reduced due to the very low numerical abundance.

Red Sea and comprises 4-5% of the late Holocene assemblage. It is a rare species, confined to the interglacial stages and often to their most developed conditions (i.e. IS 1, 5.5, 7.1, 9.1, 11.2). The highest abundance of C. convexa of up to 360 specimens/g dry sediment was found in the late Holocene sequence. Styliola subula is absent in the Red Sea sediments after the base of IS 4. Downcore it occurs in both interglacial and glacial stages, though in the latter only in transitional conditions. Maximal abundance of S. sub&a was recorded during IS 6.5, with 570 specimens/g dry sediment.

3.3. Mode of pteropod preservation The state of aragonite shell preservation varies in the core. The five different classes of preservation used by Almogi-Labin et al. (1986, 1991) in Red Sea cores were applied here (Fig. 5). The least altered transparent shells are common during most of the Holocene and during glacial IS 3 (Fig. Sb). Downcore, below 4 m depth, this class becomes rare. Opaque white shells are less frequent during the Holocene but are the most common class during parts of IS 3 and earlier during IS 5.1-5.3, 6-8 and

A. Almogi-Labin Table I Frequency

maxima of pteropod

Depth in core KL 11

et al. /Marine

Micropaleontology

33 (1998) 87-107

species in core KL 11, central Red Sea Isotope stage

Pteropod

species

Relative abundance

Reference

(%I

(ml 0.0-0.25 0.25-0.46 0.46-0.73 0.83-0.85 0.85-0.87 0.87-0.89 0.89-0.91 0.92-l .5 1.55-l .80

1 2/l 2/l

2 2 2-3

1.80-2.65 2.65-2.85 2.90-3.65 3.9-4.25 4.3-5.6 5.6-6.25 6.25-6.75 6.75-6.95 6.95-7.19 7.19-7.35 7.35-7.5 7.6-8.25 9.0-9.3 9.55-9.75 9.9-10.3 10.6-l 1.35 11.35-12.25 12.4-12.9 13.1-13.2 13.3-13.9 13.9-15.0 15.7-16.9 17.8-18.15 18.15-19.1 19.4-19.65 19.65-19.9 19.9-20.3 20.3-20.9 References: study.

95

3 4-5.1 5.1-5.3 5.3-5.4 5.4-5.5 6.1 6.2 6.3 6.3 6.4 6.4-6.5 6.7 6.7-7.1 7.1-7.3 7.4 7.4-7.5 8.1 8.2-8.4 8.4-8.5 9-9.3 10.1-10.2 10.2-11.1 11.1 11.1 11.2 11.2-11.3 1 = Herman,

L. inflata L. bulimoides

and L. inflata

L. inflata L. tmchtfonnis and C. chierchiae L. trochtformis L. trochtformis and C. chierchiae C. chierchiae C. acicula C. virgula L. trochiformis L. inflata L. trochiformis and C. virgula L. bulimoides L. infata L. injata and L. trochiformis L. bulimoides and L. trochifotntis L. inflata C. acicula L. trochifotmis L. bulimoides L. inflata S. subula L. trochiformis L. inflata and S. subula L. bulimoides L. inflata and L. bulimoides L. trochiformis L. bulimoides C. virgula and L. trochtformis L. bulimoides and L. inflata L. inflata and S. subula L. inflata L. trochiformis S. subula and L. bulimoides L. trochtfiomis L. inflrrta L. bulimoides

1968; 2 = Chen, 1969; 3 = Almogi-Labin,

parts of IS 10 (Fig. 5~). In most of the core, the majority of pteropods are preserved with their original aragonite shells (Fig. 6b). Quantitatively they are very abundant during the Holocene and IS 5 (Fig. 6~). Below IS 5 this class of shells falls below 2000 specimens/g dry sediment. Shells which are preserved as molds, with remnants of the original aragonite, are abundant in distinct intervals in the middle and lower parts of the core (Fig. 5d).

1982; 4 = Ivanova,

69-75 75-79 62-72 74-90 42-59 57-72 52 91-100 75-84 61-89 77-90 75-97 32-67 43-67 70-86 49-6 1 81 44-89 52-57 21-41 47-89 45-46 51-53 47-62 49-62 49-88 43-57 40-58 73-80 63-84 59-82 39-50 46-95 66-91 60 40-78 40-89 1985; 5 = Almogi-Labin

1, 2, 2, 5, 2, 5, 5,

2, 3, 4, 5, 6 3,5, 6 3, 5, 6 6 3,5, 6 6 6 1, 2, 3,4,5, 6 2, 3, 6 2, 3, 6 1, 1, 2, 2, 2, 2, 6

2, 2, 3, 3, 3, 3,

3, 4, 6 3, 4, 6 6 4, 6 6 6

3, 2, 2, 3, 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

6 3, 6 6 6

et al., 1991; 6 = this

Molds with no remnants of the original aragonite are increasingly common further downcore, starting at the transition of IS 6/5 and during parts of IS 8-11 (Fig. 5e). The latter two classes were grouped under ‘Molds’ (Fig. 6d). The abundance of these classes, which includes both molds with and without remnants of original aragonite is extremely low, usually less than 300 specimens/g dry sediment (Fig. 6e). Pteropod shells with secondary aragonite

96

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Transparent

et al. /Marine

shells

2

1

0

-1

-2

-3

0

50

W) 100 0

50

33 (1998) 87-107

Molds + original shells (“h)

Opaque shells

(%) (m) 3

Micropaleontology

100

0

50

Aragonite overgrowth (%)

Molds /no original shells (%) 100

0

50

100

0

50

100

no planktic foraminifera

L

b

Fig. 5. (a) Planktic foraminiferal (Globigerinoides tuber) oxygen isotope record of core KL I I (Hemleben et al., 1996). Numbers on the right side of the record indicate oxygen isotope stages (IS). Relative abundance of the various modes of pteropod preservation (following Almogi-Labin et al., 1986): (b) transparent shells; (c) opaque shells; (d) internal pteropod molds with original aragonite shells; (e) internal pteropod molds with no original aragonite shells: (f) authigenic aragonite encrusting or infilling pteropod shells.

overgrowths are restricted to glacial maximum intervals of IS 10, 8 and 6. This class of preservation is dominant during IS 2 (Fig. 5f) though at low abundances of 100-400 specimens/g dry sediment (Fig. 6g). 4. Discussion

4.1. Preservation the carbonate

of the pteropodal system

aragonite and

4.1.1. Pteropoda abundance cycles - the 6”O climatic signal Pteropods and planktic foraminifera are the principal biogenic components in the coarse fraction. The abundance patterns of the two groups resemble

each other in the upper part of the core (Fig. 2c-d) and closely follow the S’sO climate signal of the Red Sea (Fig. 2a) with greatest abundances during the interglacials. During glacials, and especially during their maxima, both groups drop to extremely low numbers. During the last 380 kyr large glacialinterglacial (G/I), c?‘~O shifts on the order of 3.5 5.5%0 are recorded in the Red Sea (Hemleben et al., 1996) as compared to the 1.2-1.3%0 shift of the global oceanic record (Fairbanks, 1989). The large shifts are attributed to a significant salt increase during periods of lowered sea level. The surface water salinity at the core site was at least 53%0 during the LGM, a period which is characterized by anomalously high A”0 values (Hemlehen et al., 1996). The similar G/I abundance pattern of the two

A. Almogi-Labin

et al. /Marine

Mlcropaleonrology

_?3 (IYY8j

87-107

No. Ongmal

li18 0 (?&IPDB)

(ml 3

2

I S. Original Aragonite shells (%)

Aragonite sh& ig

Molds (%I

No Molds/g

97

% shells with Aragonite overgrowth

No. sheils with Aragonite overgrowth ig

1 0 -I -2 -3

1 1 ,

-r---?--i

&

Fig. 6. (a) Planktic-foraminiferal (Globigerinoides ntber) oxygen Isotope record of core KL 11 (Hemleben et al.. 1996). Numbers on the right side of the record inchcate oxygen isotope stages (IS). The leiatire (b) and total a,,undance \c) 01 wcil preserved ttra~qarem and opaque ptcrupod shells grouped as ol-rginal &li~,! Kelativi: id) and total abun,lancc :, : If ~te~q& r.i.Adb ;inciudiug tnolti, n.,:, 0: without the origin,J aragonite
groups which persisted between IS 6 and the Recent (except during IS 5.5) is best expressed by the high linear correlation (R* = 0.9 1) among the two groups (Fig. 7). The similar abundance trends of the pteropods and foraminifera, and between them and the S’*O curve, suggest that salinity exerted a strong influence on the abundance pattern of both groups. 4.1.2. Pteropoda abundance cycles - upper salini9 limits qf pteropods and planktic,foramintfera In core top and Holocene sediments the numericai abundance of pteropods and planktlc forammifers are similar (Fig. 2e). During the Increased cahnity of glacial intervals (Hemleben et ai.. 1996). the planktic foraminifera appear to be more impoverished as compared to the pteropods. I‘hrs IS agatn observeu during glacial and glacial maximal cc?ndltlons ot’ 1’:

2. 3 and 6. The differences between the numerical abundance of the two groups suggest a somewhat greater tolerance of the pteropods to the increased salinity. 4.1.3. Pteropod abundance cycles - loss oj pteropods In the lower part of the core the pteropod curve shows a large deviation from the characteristic glaclai-mterglaclal pattern observed in the upper part of the core. Dissim&rity is observed also between the nlanktic toraminifera and the oteropod abundance patterns (linear correlation R’ = 0.04) dunnp IS ‘/-I 1.2 ( t;ig. 7 ). The decreasmg numencal abundance ot the pteropods downcore seems to reflect a contmuous pteropod loss which is more enhanced ~IIIIW the mrercTlaclal stages compared to

98

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Micropaleontology

33 (1998) 87-107

6000

8000

No. Pteropods / g Fig. 7. The linear correlation (no. samples = 183) between planktic foraminifera and pteropoda numerical abundance (no. specimens/g dry sediment) shows a high correlation of R* = 0.91 during IS I-5.4, 6 (based on 109 samples). A deterioration in pteropod preservation causes a poor linear correlation of R2 = 0.31 among these groups (based on 7 samples) during IS 5.5 and no correlation R2 = 0.04 during IS 7-l 1 (based on 67 samples).

the glacial stages (cf. Fig. 2c-e). During earlier interglacial intervals a flourishing planktic foraminifera fauna, similar to that of IS 1 and IS 5.1-5.4, occurred contemporaneously with low numbers of pteropods. The numerical % Pt/Pf + Pt values, which measure the state of pteropod preservation (aragonite vs. calcite), shows a strong pteropod loss at IS 5.5 as well as during IS 7, 9 and 11 compared to interglacial periods without pteropod loss where the % Pt/Pf f Pt values are around 50% (Fig. 2e; Berner, 1977; Almogi-Labin et al., 1986, 1991). The loss of pteropods involves all the species in the assemblage with no specific preferential dissolution.

The G/I pteropod abundance pattern, controlled by ecological constraints such as salinity, is maintained also by the change in number of pteropod species/sample (= simple diversity). The diversity (Fig. 2f) follows the 6t80 climate signal (Fig. 2a) with a high pteropod diversity during the most developed interglacial intervals. An intermediate diversity characterizes the glacial stages while during the LGM only a monospecific pteropod fauna survived the highest salinity ever recorded during the last 380 kyr. During interglacial stages 5.5, 7, 9 and 11 when the numerical abundance of the group and the % Pt/Pf + Pt values are low, the pteropod diversity

A. Almogi-Labin

et al. /Marine

(Fig. 2f) continues to be high and of an interglacial type. The continuous high diversity, even during periods with intensive pteropod loss, indicates that the loss includes all the existing species with no preferential dissolution of specific taxa. From pteropod dissolution experiments (J. Erez and A. AlmogiLabin, unpublished results) certain pteropod species like L. in$ata. and C. acicula are similarly prone to dissolution. Other species like Cuvierina columnella and S. sub&a are more resistant. Of these two species C. columnella does not occur in the Red Sea while S. subula is rare. Comparison of the two curves of pteropod abundance and % Pt/Pf + Pt values provides a good estimate of the pteropod loss, or in other words, of the aragonite dissolution pattern. The combination of the pteropod abundance and the % Pt/Pf + Pt allows to differentiate between three independent proxies which control the pteropod abundance pattern (Fig. 8): (1) the Red Sea climate signal (i.e. the normal glacial-interglacial pteropod abundance variation); (2) different ecological upper salinity limits of the pteropods and the planktic foraminifera groups; (3) aragonite dissolution intervals (ADI). 4.1.4. Pteropod loss - a global cause? The AD1 are recognized as periods of low % Pt/Pf + Pt values (((50%). The low % Pt/Pf + Pt values result from the enhanced pteropod loss combined with the increase in the total abundance of the remaining, more resistant, planktic foraminifera/g dry sediment (e.g. IS 9). The major AD1 occur during IS 5.5, 7.1-7.3, 7.5-8.3, 8.5-10.0, 11.0-11.2 (Fig. 2a, c, e). These intervals coincide with carbonate dissolution events in deep water sediments of the Indian and Pacific Oceans, corresponding to the upper part of the well known and widely distributed ‘mid-Brunhes dissolution cycle’ of deep sea carbonate sediments (Peterson and Prell, 1985; Farrell and Prell, 1989; Droxler et al., 1990; Bassinot et al., 1994). The intensity of the dissolution pulses increases downcore and corresponds to the long-term dissolution oscillation pattern shown by Bassinot et al. (1994). The strongest dissolution pulse during the last 380 kyr happened in the Red Sea during IS 9.3 where pteropods are not preserved in the sedimentary record. The synchrony of carbonate dissolution pulses in the Indian Ocean and AD1 in the Red

Micropaleontology

33 (1998) 87-107

99

Sea suggests a common cause for these events. At present, well-preserved aragonite-bearing carbonates are found in deep water Red Sea sediments (with in situ water column values of Qaagoniteof 2.7 at 1400 m water depth in the northern Red Sea) and a general rough estimate of the aragonite compensation depth at -2 km water depth (Berger, 1978; Krumgalz and Erez, 1984). In the adjacent northwestern part of the Indian Ocean the aragonite compensation depth is estimated to lie at -0.5 km water depth. Surface Gulf of Aden water enters the Red Sea and flows northward as a surface water current up to the northernmost region where intermediate and bottom water are produced (Cember, 1988). Thus the deep and intermediate water masses of the Red Sea and the Indian Ocean are separated by the Red Sea’s antiestuarine circulation pattern (Morcos, 1970). The lack of direct Indian Ocean-Red Sea deep water connection may rule out the possibility considered by Bassinot et al. (1994) that the changes in NADW production may have an effect on the global deep sea carbonate budget. On the other hand the suggestion by Bassinot et al. (1994) that the long-term dissolution cycles affect the entire water column gains support from Red Sea results. Along with the idea that the Red Sea AD1 follow the Indian Ocean deep sea carbonate dissolution pattern the pteropod loss in the Red Sea may be associated with an oxygenated bottom water coupled with enhanced supply of organic matter (Luz et al., 1984; Almogi-Labin et al., 1986). Pteropod loss is expressed by low numbers of pteropods and low % Pt/Pf + Pt values (Fig. 2c, e, Fig. 8). Another indicator of pteropod loss are the increasing numbers of pteropod shells preserved as molds (Fig. 6d). Milliman et al. (1969) suggested that molding processes accompanied by magnesian calcite precipitation represent inversion products of aragonite dissolution. Almogi-Labin et al. (1986) found that in cases where only 30-70% of the specimens are molds, the loss of original pteropods exceeds 80%. In extreme cases where only molds were left the residual assemblage represents less than 10% of the original assemblage. The comparison between the % Pt/Pf + Pt percentage curve and the percentage of molds (Fig. 2a, d, e) shows that only certain AD1 [during IS 5.5 (compare to Almogi-Labin et al., 1986), IS 8.4, 9.1 and 11.2.1 coincide with enhanced pteropod molding. During

A. Almogi-Labin et al. /Marine Micropaleontology 33 (1998) 87-107

100

0 (!%aPDB)

6 l8

(m) 3

2

1

0

-1

I.S. No. Pteropods/ g

-2 -3

Pteropods/Planktic Foram+Pterop. (?%I)

Controls of Pteropod Abundance Pattern in the Red Sea Normal G/I Pt signal Pteropoda salinity tolerance

no plauktic foraminifera

Normal G/I Pt signal

6 5_

/

Pteropod loss Pteropoda salinity tolerance Normal G/I Pt signal

Pteropod loss Normal G/l Pt signal Pteropod loss I

c

Normal Gil Pt signal

Pteropod loss

18 1t - 11.0 11.2

21

I

I

I

a I

I

I

1 I

Fig. 8. (a) Planktic foraminiferal (Globigerinoides ruber) oxygen isotope record of core KL 11 (Hemleben et al., 1996). Numbers on the right side of the record indicate oxygen isotope stages (IS). The comparison between the numerical abundance of pteropods/g dry sediment (b) and the relative abundance of pteropods vs. pteropods and planktic foraminifera (c) shows that the pteropod abundance curve is influenced by three independent proxies (d), i.e. the normal glacial-interglacial pteropod signal (= Normal G/I Pt signal); the pteropoda higher salinity tolerance, and the aragonite dissolution intervals presented here as intervals of pteropod loss.

other ADI, like those of IS 7.1-7.2, 7.5, 8.3, 8.5 and part of IS 11.2, the enhanced pteropod dissolution is not accompanied by molding. In the most extreme cases like that of IS 9.3, the all pteropods dissolved leaving no record. Considering the stability of the carbonate system intervals with aragonite loss ac-

companied by high magnesian calcite precipitation may be closer to carbonate saturation than periods when the dissolved aragonite left no traces. Regional carbonate dissolution pulses in deep sea sediments are recognized by fluctuations in the coarse fraction content (Peterson and Prell, 1985;

A. AlmogiLabin et al. /Marine Micropaleontology 33 (1998) 87-107

Williams et al., 1985; Bassinot et al., 1994). This indicator seems to be sensitive in tracing carbonate dissolution cycles above and within the Indian Ocean lysocline. In the Red Sea the use of the coarse fraction content as an indicator of carbonate dissolution is confined to the early interglacial stages. During IS 5.5, 7.5, 9.1 and 9.3 the low coarse fraction content pulses are associated with ‘lows’ in planktic foraminifera concentration (compare Fig. 2b and d). These dissolution pulses of calcitic shells occur within the AD1 and probably mark a certain culmination of carbonate dissolution in the Red Sea. The low coarse fraction content during glacial stages accompanied by high % Pt/Pf + Pt values (Fig. 2b, e) seems to reflect the general fauna1 impoverishment following the climatic change of the Red Sea rather than a dissolution pulse. Single coarse fraction spikes during IS 8 and 10 are caused by increased authigenic high magnesian calcite and/or aragonite precipitation. A long period occurred during IS 2, with precipitation inside and outside pteropod shells, or as nodules of several hundreds microns (Milliman et al., 1969; Almogi-Labin et al., 1986). The increase in the coarse fraction due to authigenic precipitation, occurred only when biogenic accumulation is very low (Fig. 2b-d). The amount of the authigenic component of the coarse fraction is difficult to estimate because it cannot be separated from the biogenic one. Thus, the coarse fraction content may be used as an indicator of the carbonate system of the Red Sea only for periods of no molding (e.g. during IS 3, and intervals within IS 5, 6, 7, 8 and 10 (Fig. 2b and Fig. 6b, d). 4.2. Pteropoda events and paleoclimate The numerical and relative abundances of epipelagic and mesopelagic pteropods varied frequently during the last 380 kyr (Figs. 3, 4 and 9). Variations in the abundances of epipelagic (nonmigratory) and mesopelagic (migratory) pteropods indicate fluctuation in the nature and position of the oxycline (Almogi-Labin et al., 1991). At present the oxygen content decreases rapidly below 100 m depth and reaches a minimum in the intermediate water mass between 200 and 650 m depth (Neumann and McGill, 1962; Woelk and Quadfasel, 1996). At the core site the minimum oxygen concentration is about

101

0.5 ml 02/l at -400 m depth. A high relative abundance of mesopelagic pteropods (-75%) is associated with the present-day OMZ (Fig. 9; cf. Weikert, 1982) but further oxygen depletion would eliminate the mesopelagic component from the pteropod assemblage. A more intense and vertically more extended OMZ would favor the increase of epipelagic, mixed-layer inhabitant pteropods in the fossil assemblage. Shifts in the OMZ and its oxygen content are climatically controlled. At present, renewal of the intermediate and deep water masses is regulated by seasonal cooling (mainly during winter) in the northernmost Red Sea and in the Gulf of Suez (Cember, 1988; Eshel et al., 1994; Woelk and Quadfasel, 1996). Periods with a more aerated OMZ, like the present, may reflect arid intervals or periods with even colder and dryer winters than at present. During these arid/hyperarid periods dense northernmost Red Sea surface water is produced because of the strong S-N evaporation gradient and flows southward isopycnally presumably towards intermediate water depths. A strong and well developed OMZ indicates a slower rate of intermediate water replenishment resulting from a milder and more humid climate in central and northern Red Sea and a smaller S-N evaporation gradient. An additional, climatically controlled factor strengthens the OMZ in the central and southern Red Sea during milder and more humid climatic phases. This factor is connected to an increase in oxygen demand in the intermediate water body and may also be related to periods of increased primary productivity associated with enhanced, precessional controlled, SW monsoon activity (Weikert, 1987). During the last glacial-interglacial cycle, mesopelagic pteropods were dominant in the interglacial stages (i.e. IS 1, 5) while the epipelagic forms were associated mainly with the glacial stages (Herman and Rosenberg, 1969; Almogi-Labin, 1982; Ivanova, 1985; Almogi-Labin et al., 1986, 1991). This distinct cyclic pattern is less clear in earlier G/I cycles, and even in the last G/I cycle, there are exceptions to this simple pattern (Fig. 9). During certain interglacial intervals epipelagic pteropods show a high relative abundance; in contrast, during glacial intervals mesopelagic dominance occurs quite commonly. Fluctuation between epi- and mesopelagic

A. Almogi-Labin et al. /Marine Micropaleontology 33 (19981 87-107

102

Mesopelagic Pteropods (%)

6 ‘*O (%o PDB) (ka) 3

2

1

0

-1

-2

-3

I

I

I

I

I

I

J

0

100

Epipelagic Pteropods (%)

1 no

50

Monsoon Index arid humid

lanktic foraminifera

Fit5 5

b -

150 6 200 7 250 8 300

- no pteropods 1_ I 350

400

1

a I

I

I

I

I

t-f--n-60 -30

Fig. 9. (a) Planktic foraminiferal (Globigerinoides ruber) oxygen isotope record Numbers on the right side of the planktic record indicate oxygen isotope stages mesopelagic (b) and the epipelagic (d) pteropods with the monsoon index (c). Note with negative monsoon indices pointing to a general arid period. The association of indicate generally more humid conditions.

dominance intervals alternate between and within G/I cycles following the precession index pattern of Berger and Loutre (1991). Hemleben et al. (1996)

0

30

60

of core KL 11 following Hemleben et al. (1996); (IS). Correlation of the relative abundance of the that most of the mesopelagic maxima are associated epipelagic pteropods with positive monsoon indices

suggested that the Indian Ocean monsoon influences Red Sea climates because of the enhanced variance in the precession band compared to the global cli-

A. Almogi-Lubin et al. /Marine Micropaleontology 33 (1998) 87-107

matic record. Orbitally driven increase of monsoonal rainfall may add to the humidity in the Red Sea region. At the same time an increase in the primary productivity is expected to occur in the central and southern Red Sea. High relative abundances of epipelagic pteropods during IS 3.1, 3.3, 5.3, 5.5, 6.4 (partial), 6.5, 7.1, 7.3, 7.5, 8.3 and 8.5 coincide with positive monsoon index values that correspond in general to SW monsoon-derived humid climates (Fig. 9). Negative monsoon indices, implying a more arid climate, are associated with high relative abundances of mesopelagic pteropods during IS 5.1, 6.6, 7.2, 7.4, 8.2, 8.4, 8.6 and 9.2. Exceptions to this general monsoon-controlled pattern occur during glacial maximum intervals of IS 2, 6.2 and 10 when epipelagic pteropods dominate at times of negative monsoon indices. The dominance of the epipelagic species during these intervals results mainly from a wide ecological tolerance of certain epipelagic species like C. acicula and C. virgula (Fig. 3b, c; Chen, 1969; Almogi-Labin, 1982) to the elevated salinity of glacial maximum conditions (Hemleben et al., 1996). During these glacial extremes most of the calcareous planktic organisms, including most of the pteropods, disappear or nearly disappear from the Red Sea sediments (Berggren, 1969; Reiss et al., 1980), and the remaining forms are the ecologically resistant epipelagic pteropod species. During certain humid phases (characterized by positive monsoon indices) mesopelagic abundances reach maxima during IS 1, 3.2, 5.2, 6.3, 9.1, and 11.2. During these intervals the climate derived by the southwest monsoon probably did not extend further north within the Red Sea system. As a result, well aerated intermediate water, related to arid climate, supported a mesopelagic fauna. The mesopelagic group consists mainly of four die1 migratory species (Figs. 3 and 4). Limacina inflata and C. convexa presently live in the central Red Sea. Limacina in.ata, the most common living species, migrates down to the OMZ core (down to -400 m depth) and C. convexa migrates down to -700 m depth (Weikert, 1982, 1987). Limacina bulimoides, at present a rare species (Fig. 3e), was a common member of the pteropod community between 5 and 3 kyr ago, comprising up to 50% of the population (Edelman, 1996). Increased abundance of L. bulimoides is related to a more aerated OMZ at

103

times of increased aridity in the northernmost Red Sea (Almogi-Labin et al., 1991). Periods with high relative abundance of L. btdimoides (Fig. 3e) occur during IS 4-5.1, 5.5, 6.2, 7.2-7.4, 8.2, 8.4-8.5 and 11.2. Hemleben et al. (1996) calculated the difference between mean ocean S180 (6,) records and the planktic S’8O record from core KL 11, central Red Sea, and estimated salinity anomalies during the last 380 kyr. High abundances of L. bulimoides are characterized by salinity anomalies elevated by up to 4.8%0 during interglacial stages and by up to 9.7%0 during glacial stages (Fig. 3e; Hemleben et al., 1996, fig. 2b). The positive salinity anomalies coinciding with negative monsoon index values point to a general aridification during times of L. bulimoides abundance peaks. Holocene abundance maxima of L. injata, the most common mesopelagic die1 migratory pteropod, are associated with well aerated OMZ though to a somewhat lesser degree compared to periods with L. buEimoides abundance (Almogi-Labin et al., 1991). Limacina inflata abundance maxima contribute considerably to the general mesopelagic group signal (Fig. 3f, Fig. 9) and they seem to indicate in general hydrographic and consequently climatic conditions similar to the present. Clio convexa, the recent deepest intermediate water dweller species, is a minor component of the pteropod community, occurring always in low numbers (Fig. 3g, Fig. 4g). It occurs either together with L. injata or L. bulimoides abundance maxima and is restricted to interglacial intervals with S’s0 values below 0.0%0 and with negative to slightly positive salinity anomalies (Hemleben et al., 1996, fig. 2b). This species seems to be a sensitive indicator of initial salinity changes in the frequently fluctuating system. C. convexa is absent during certain well aerated interglacial and interstadial intervals because it prefers only lower salinities. Styliola subula is another minor component of the mesopelagic group that rarely constitutes up to 65% of the pteropod assemblage (Fig. 3g, Fig. 4g). It occurs during both interglacial and transitional glacial stages in association with either the epipelagic C. virgula and L. trochiformis or the mesopelagic L. bulimoides and L. inJata. The species disappeared from the Red Sea at the base of IS 4 and is not present in the Arabian Sea today (Sakthivel, 1973b). The ecological require-

104

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ments of Styliola subula are known mainly from the northern Indian, northern Atlantic and the northern Pacific oceans. Styliola subula is a deep mesopelagic, central gyre species with a distinct early to late winter abundance peak (McGowan, 1971; Be and Gilmer, 1977; Wormuth, 1981; Almogi-Labin et al., 1988). Sakthivel (1973b) suggested that the absence of this species from the northern Arabian Sea is related to the very low oxygen content of the waters below the thermocline. The infrequent distribution of this species in the central Red Sea might represent periods mainly in the winter of relatively well aerated, deep intermediate water. In addition these maxima may be connected to a better fauna1 exchange between the Red Sea and the Gulf of Aden at times of weakening of the OMZ in the northern Indian Ocean. Creseis acicula, C. virgula s.1. and L. trochiformis are epipelagic species, living in the mixed layer with small-scale vertical migrations within this zone (Be and Gilmer, 1977; Wormuth, 1981). Creseis acicda, a common component of the living Red Sea pteropod population (Weikert, 1982; Almogi-Labin, 1984), occurs in general in low relative and total abundance (Fig. 3b, Fig. 4b). During glacial maximum conditions of IS 6 (e.g. IS 6.2) and mainly of IS 2 it constitutes a monospecific population. The presence of this species during the anomalous high saline conditions of the LGM was recorded previously by Chen (1969) and was attributed to the wide salinity tolerance of this species and its ability to survive in extremely high salinity. This view was also supported by the salinity estimates of Hemleben et al. (1996). The absence of similar C. acicula spikes in older maximum glacial stages indicates that conditions were less extreme as compared to those of the LGM. Creseis virgula and L. trochiformis together constitute the majority of the epipelagic group (Fig. 3c-d, Fig. 4c-d, Fig. 9). From living observations and stable isotope studies (Wormuth, 1981; Almogi-Labin, 1984; Auras-Schudnagies et al., 1989; Almogi-Labin et al., 1991; Jasper and Deuser, 1993) it turns out that these species are restricted to the upper 50 to 100 m at most with C. virgula being a somewhat shallower mixed-layer dweller. Abundance fluctuations between the two species indicates changes in the thickness of the mixed layer. Thus the common epipelagic signal (Fig. 9) might be fur-

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ther separated into times with C. virgula dominant intervals reflecting a shallower mixed layer zone and periods with L. trochiformis dominance reflecting a thicker mixed layer interval. Fluctuations in the position of the mixed layer may reflect change within an overall humid climatic phase. 5. Conclusions High-amplitude fluctuations in the numerical abundance of pteropods were found during the last -200 kyr in central Red Sea. These fluctuations seem to be controlled to a large extent (except during isotope stage (IS) 5.5) by the climatic changes in the Red Sea region. The climate induces a prolific pteropod population with high numbers during the less saline interglacial periods, and low numbers during the glacial stages that are characterized by a significant salt increase. Numerical abundance records of pteropods associated with variations in the relative abundance of pteropods to planktic foraminifers are used to trace early aragonite dissolution in otherwise well preserved calcareous sediments. The coupling of these two records seems to trace early stages of carbonate dissolution in deep sea sediments accumulating above the ACD, as in the case of the deep water sediments in the Red Sea. Carbonate preservation deteriorates in the central Red Sea below the base of IS 6. Intervals with moderate to advanced stages of aragonite dissolution, reflected by enhanced pteropod loss, were recognized at IS 5.5, 7.1-7.3, 7.5-8.3, 8.5-10.0, and 11.O- 11.2. These intervals coincide with carbonate dissolution events in Indian Ocean deep water of the upper part of the ‘mid-Brunhes dissolution cycle’, The co-occurrence of carbonate dissolution events in the Red Sea and in the Indian Ocean, with no direct deep water connection between the two basins, makes changes in deep water circulation as the primary cause of these wide-spread deep water dissolution events unlikely. Variations in the abundance of epipelagic and mesopelagic pteropods reflect fluctuations in the nature and position of the oxygen minimum zone (OMZ) in the central Red Sea. These fluctuations are climatically controlled. High abundances of mesopelagic pteropods are often associated with

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times of negative monsoon index implying a generally arid period in the Red Sea region similar to the present-day situation. High relative abundance of epipelagic pteropods frequently coincide with positive monsoon indices that correspond in general to an enhanced SW monsoon activity. During these periods the strengthening of the OMZ is related to milder and more humid climates, coupled with enhanced primary productivity in the central and southern Red Sea. Specific abundance maxima of mesopelagic and epipelagic pteropods are hydrographically controlled. Abundance maxima of the die1 migratory mesopelagic Limacina bulimoides, a present-day rare species, occur during periods of positive salinity anomalies as compared to mean ocean ~5~~0(8,) (Hemleben et al., 1996). These anomalies coincide with negative monsoon index values, implying the conditional relation between abundance maxima of L. bulimoides and the most aerated OMZ at times of increased aridity in the Red Sea region. Limacina injlata, the present-day dominant pteropod, is a deepwater mesopelagic species that proliferates at times of water column conditions similar to the presentday configuration. Clio convexa, probably the deepest mesopelagic dwellers in the Red Sea, is restricted to the lowest salinity intervals during interglacial stages, and thus seems to be a sensitive salinity indicator. Creseis acicula, an epipelagic species, is the most euryhaline pteropod surviving during high hypersaline conditions (-53%0, Hemleben et al., 1996) of IS 2. Otherwise it is a minor component of the assemblage. Creseis virgula s.1. and Limacina trochiformis are two common epipelagic, mixedlayer dwellers, Their abundance peak coincides with positive monsoon indices that correspond in general to southwest-monsoon-derived humid climates. Acknowledgements

We thank the master, crew and scientists aboard R/V ‘Meteor’ (cruise 5) for their help and assistance. We thank J. Lipps and two anonymous reviewers for their constructive and helpful comments. We gratefully acknowledge H. Erlenkeuser (University of Kiel, Germany) for carrying out the stable isotope study. We also thank R. Ott for helping with programs and figures and I. Breitinger for helping

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with the manuscript (both University of Tubingen, Germany). We acknowledge the assistance received from. H. Hemo, S. Ashkenazi (both of the Geological Survey of Israel, Jerusalem). The research was supported by a grant from the German-Israeli Foundation for Scientific Research and Development and the Deutsche Forschungsgemeinschaft (He 697/7 and Me 267/28). References Almogi-Labin, A., 1982. Stratigraphic and paleoceanographic significance of late Quaternary pteropods from deep-sea cores in the Gulf of Aqaba (Elat) and northernmost Red Sea. Mar. Micropaleontol. 7, 53-72. Almogi-Labin, A., 1984. Population dynamics of planktic foraminifera and pteropoda - Gulf of Aqaba, Red Sea. Proc. R. Ned. Akad. Wet. Ser. B Palaeontol. Geol. Phys. Chem. Antropol. 87, 48 l-5 11, Almogi-Labin, A., Luz, B., Duplessy, J.-C., 1986. Quaternary paleo-oceanography, pteropod preservation and stableisotope record of the Red Sea. Palaeogeogr., Palaeochmatol., Palaeoecol. 57, 195-211. Almogi-Labin, A., Hemleben, Ch., Deuser, W.G., 1988. Seasonal variation in the flux of euthecosomatous pteropods collected in a deep sediment trap in the Sargasso Sea. Deep Sea Res. 35. 441-464. Almogi-Labin, A., Hemieben, Ch., Meischner, D., Erlenkeuser, H., 1991. Paleoenvironmental events during the last 13,000 years in the Central Red Sea as recorded by pteropoda. Paieoceanography 6, 83-98. Auras-Schudnagies, A., Kroon, D., Ganssen, G., Hemleben, Ch., Van Hinte, I.E., 1989. Distributional pattern of planktonic foraminifers and pteropods in surface waters and top core sediments of the Red Sea, and adjacent areas controlled by the monsoonal regime and other ecological factors. Deep-Sea Res. 36, 1515-1533. Bassinot, EC., Beautort, L., Vincet, E., Labeyrie, L.D., Rostek, F., Muller, PJ., Quidelleur, X., Lancelot, Y., 1994. Coarse fraction fluctuations in pelagic carbonate sediments from the tropical Indian Ocean: a 1500-kyr record of carbonate dissolution. Paleoceanography 9, 579-600. Be, A.W.H., Gilmer, R.W., 1977. A zoogeographic and taxonomic review of euthecosomatous pteropoda. In: Ramsay, A.T.S. (Ed.), Oceanic Micropaleontology, Vol. 1. Academic, San Francisco, CA, pp. 733-808. Berger, A., Loutre, M.F., 1991. Insolation values for the climate of the last 10 million years. Quat. Sci. Rev. 10, 297-317. Berger, W.H., 1978. Deep-sea carbonate: pteropod distribution and the aragonite compensation depth. Deep Sea Res. 2, 447452. Berggren, W.A., 1969. Micropaleontologic investigations of Red Sea cores - summation and synthesis of results. In: Degens, E.T., ROSS, D.A. (Eds.), Hot Brines and Recent Heavy Metal Deposits in the Red Sea. Springer, Berlin, pp. 329-335.

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