Late Quaternary nannofossil indicators of climate change in two deep-sea cores associated with the Leeuwin Current off Western Australia

P IE0 ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 131 (1997)413 432

Late Quaternary nannofossil indicators of climate change in two deep-sea cores associated with the Leeuwin Current off Western Australia H. Okada

a

p. Wells b

a Department of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo 060, Japan. Fax." +81-11-746-0394 b Antarctic Cooperative Research Centre, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001. Fax: +61-02-202973 Received 6 January 1995; accepted 1 June 1995

Abstract

The stratigraphic variations in nannoflora observed in the two deep-sea cores recovered from the offshore Western Australia revealed a sequence of paleoceanographic changes for the last 250 kyr. The dominance of small placolith taxa in the upper-photic flora indicates continuous mild upwelling for the isotope stages 7-5 in this region. The most important controlling factor for the abundance variation of Florisphaera profunda in this region is considered to be the lower-photic temperature rather than the stability of water column or the water turbidity. The significant reduction in the abundance of F. pr~?funda combined with the stable (in the northern core RS96GC21) and reduced (in the southern core RS9-150) abundance of the small taxa suggest an intensified upwelling for the Penultimate Glaciation. The sharp increase in F. profunda abundance indicates a weakening of upwelling at the Last Interglacial Climax (LIC) offshore Western Australia. After isotope stage 5, the existence of upwelling is not clear except in stage 2, when it was not strong enough to produce blooms of Emiliania huxleyi. Among the subordinate taxa, Cah'idiseus leptoporus, Cah'iosolenia murrayi, Neosphaera eoccolithomorpha and Oolithotus fragilis provide meaningful paleoceanographic signals supporting the conclusions of previous studies: a weakening or cessation of Leeuwin Current for the glacial periods. The shift of dominance from small Gephyrocapsa (mostly G. ericsonii) to small Reticulofenestra (mostly R. parvula) can be an useful datum event offshore Western Australia. The reduced diversity of nannoflora during the ir~terglacial periods can be explained by a lessened competition within nannoplankton community caused by particularly favorable conditions for the opportunists, such as E. huxleyi, F. profunda and small placoliths. The variations in nannofossil abundance suggests an increased aeolian flux at the stages 6 and 2 offshore North West Cape, as well as at stage 6 offshore southwestern Australia. © 1997 Elsevier Science B.V.

K:eywords. palaeoceanography; calcareous nannofossil; Indian Ocean; Leeuwin Current; Quaternary

0031-0182/'97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0031-0182(97)00014-X

414

ILL Okada, P. Wells / Pahwogeogrophv, Palaeoclimalolo~y. Pu/aeoecology 131 (1997) 413 432

I. Introduction

1.2. Paleoceanography and previous work

1.1. Present oceanographic condition at (~ffX'hore western Australia

During the Last Glacial Maximum ( L G M ) , reconstructions of late Quaternary sea-surface temperatures off Western Australia have indicated the establishment of large areas of anomalously cold surface water (Prell and Hutson, 1979: Prell et al., 1979, 1980: CLIMAP, 1984; Prell, 1985). The northward advance of the Subtropical Convergence during glacial times (Prell et al., 1979; CLIMAP, 1981: Howard and Prell, 1992) and associated strong offshore winds across the North West Shelf (Webster and Streten, 1978; Prell et al., 1980) would have had the potential to cause upwelling of cold bottom waters. Recent work indicates that towards the end of the Penultimate Glaciation (ePG), sea-surface temperature (SST) was up to 6 C cooler than today off North West Cape, and the situation was again similar at the LGM, with anomalous coolings of up to 7 C off North West Cape (Wells and Wells, 1994: Wells et al., 1994). Major changes in ocean circulations during glacial times, involving the increased importance of the West Australian Current (Prell and Hutson, 1979; Prell et al., 1979, 1980; CLIMAP, 1984; Prell, 1985) and the cessation of flow of the Leeuwin Current (Wells and Wells, 1994) are likely to have been important factors contributing to the establishment of these cold anomalies. Only Wells et al. (1994) have specifically examined evidence in the planktonic and benthic foraminiferal records of this area. for evidence of upwellings. They found that several lines of evidence pointed to good evidence of upwelling during glacial stage 6, but lent only weak support for upwelling during stage 2.

The surface-water circulation of the eastern Indian Ocean is largely controlled by the West Wind Drift, the South Equatorial Current and the Subtropical Convergence (Fig. 1). The strong westerly winds centered between the Subtropical Convergence and Antarctic Convergence in the Southern Ocean drive the West Wind Drift current, and southeast trade winds drive the South Equatorial Currents of the tropics westwards (Tchernia, 1980). Between these wind systems, major anticlockwise gyres are established in the Indian and Pacific Oceans (Fig. 1 ), with poleward flowing currents at the western boundaries, and equatorward-flowing currents at the eastern boundaries of these oceans. An exception to this is the poleward-flowing eastern-boundary Leeuwin Current in the eastern Indian Ocean (Fig. 1). The Leeuwin Current is a narrow southward flow of warm, low-salinity tropical water along the west coast of Western Australia (Cresswell and Golding, 1980), and its depth is limited to mere 50 m and to 200 m in the northern and southern parts, respectively (Pearce, 1991). The current flows all year-round (Church et al., 1989) but is strongest in winter, and during times when the El Nifio Southern Oscillation is not operating (Pearce and Phillips, 1988; Phillips et al., 1991 ). The current is associated with southeast trade winds which cause westward-flowing South Equatorial Currents to drive tropical waters of the Pacific Ocean through the Indonesian Archipelago onto the northwestern shelf of Western Australia. From there, differences in pressure gradient and sea-level cause the near-coastal Leeuwin Current to flow southward along the Western Australian coast, then eastwards across the Great Australian Bight (Cresswell and Golding, 1980; Godfrey and Ridgway, 1985: Pearce and Cresswell, 1985; Pearce and Phillips, 1988; Church et al., 1989; Pearce, 1991: Figs. 1 and 2). Beneath, and adjacent to the Leeuwin Current, the West Australian Current flows northward, as part of a major anticlockwise gyre in the Indian Ocean (Tchernia, 1980).

1.3. Previous Quaternary paleoceanographic studies using nanm~fossils FIorLs'phaera pr~/imda inhibits exclusively in the Iower-photic zone (Okada and Honjo, 1973a) and has been recognized as a powerful tool for Quaternary paleoceanography. The taxon's relative abundance can be utilized as indicators of paleo-waterdepth (Okada, 1983, 1992a), oxygen isotope trends (Li and Okada, 1985), mean water transparency (Ahagon et al., 1993) and changes

H. Okada, P. Wells/Palaeogeography, Palaeoclimatolog)', Palaeoecology 131 (1997) 413 432

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18 ka BP. Fig. I. Aspects of major oceanographic features of the Southern Ocean associated with the history of ocean circulation in the eastern Indian Ocean (adapted from base maps in Tchernia, 1980). Top: during modern times easterly Trade Winds move warm tropical waters of the South Equatorial Current through the Indonesian Archipelago into the northwestern shelf area of Australia, from where differences in pressure gradient and sea-level cause the near-coastal Leeuwin Current (black arrows) to flow southward along the Western Australian coast, then eastwards across the Great Australian Bight. The West Australian Current flows northward beneath and adjacent to the Leeuwin Current, as part of a major anticlockwise gyre in the Indian Ocean. The West Wind Drift is centered between the Subtropical Convergence and Antarctic Convergence, and does not impinge upon the mainland of the Australian Continent. Bottom: During the peak of the Last Ice Age ( 18 ka B.P.) the growth of the polar ice cap (vertical line shading) extended to about 50S near Australia (CLIMAP, 1976). The Subtropical Convergence (STC) and Antarctic Convergence (AnC) moved several degrees to the north, but major oceanographic fronts in the tropics and subtropics showed little movement. This intensified the West Wind Drift belt (Webster and Streten, 1978; Prell et al., 1980). The intensified air stream impinged upon the Australian continent, and established an intensified anticyclonic wind gyre over the Australian Continent, as is evidenced by the orientation of LGM sand dunes (heavy black lines over West Australia (after Kolla and Biscaye, 1977) and increased accumulation of wind-blows quartz (dotted shading; darker areas representing greater concentration) off northwestern Australia (after Kolla and Biscaye, 1977), and off southeastern Australia (Thiede, 1979 ).

in t h e v e r t i c a l s t r u c t u r e o f t h e o c e a n s ( M o l f i n o and McIntyre, 1990a,b; Castradori, 1993). T h e r e h a v e b e e n o n l y a few p r e v i o u s s t u d i e s o f

Q u a t e r n a r y p a l e o c e a n o g r a p h y in t h e I n d i a n O c e a n using calcareous nannoplankton. Biekart (1989) reported late Quaternary nan-

416

H. Okada. P. Wells/Palaeogeography, Palaeoclirnatolog?', Palaeoe('ologv 131 (1997)413 432

Fig. 2. Location of core RS96GC21 and RC9-150 showing their proximity to the modern Leeuwin Current (shaded) with its various temporal jets, eddies and meanders (after Pearce and Cresswell, 1985). The West Australian Current flows anticlockwise northward beneath the Leeuwin Current.

noflora observed in three piston cores recovered from offshore of the Indonesian Archipelago. He found certain correlation between the frequency patterns of carbon isotope and three species groups identified by a principal components analysis. Although no direct relationship was found between the frequency patterns and glacial-interglacial stages, he had suggested a fertility change as the cause of the correlation. Recently, Okada and Matsuoka (1996) studied the nannofossil record of ODP core 716B ( Maldives Ridge, southwest of India) and interpreted major changes in the monsoonal strengths

associated with glacial interglacial cycles, from the abundance trends of F. pr(~[unda and two other lower-photic species. Algirosphaera robusta and Gladiolithus .~labellatus. Spectral analyses of the abundance trends of these three taxa showed a strong 100 kyr cyclicity, indicating recurrent weakening or cessation of monsoons during the glacial intervals which is in accordance with the postulated climatic model of the region (e.g. Prell, 1984). The synchronized abundance fluctuations of these three species also proved the reliability of F. projunda, which is usually the only lower-photic species preserved in deep-sea sediment, as the representative of the entire lower-photic nannoplankton community. Further investigations along these lines (Okada and Matsuoka, in prep.) have indicated other taxa of nannofossils, such as a small placolith group and Emiliania huxleyi, can be used to deduce changes in the vertical structure of the watercolumn, complementing the trends of lower-photic species. Together, these taxa have the potential of providing evidence for paleo-upwelling phenomena in the tropical subtropical regions of the Indian and Pacific Oceans. The purpose of this investigation is to examine the paleoclimate signal of the late Quaternary calcareous nannoplankton that accumulated in two deep-sea cores offshore Western Australia (RS96GC21 and RC9-150: Fig. 2). In particular, this report will examine the floral responses against the documented upwellings and cold surface anomalies.

2. Materials and method

2.1. Samples Samples were collected from cores RS96GC21 (23c46.33'S, 108' 30.04'E; 2100 m water depth) and RC9-150 (31 17'S, 11433.1'E: 2703m water depth) at 5 cm and 10cm intervals, respectively. Core RS96GC21 is a homogeneous, highly bioturbated pale orange to greyish pink foram-rich mud. This core was renamed recently, and it is the same core reported as core BMR96GC21 in Wells et al. (1994).

ILL Okada, P. Wells / Palaeogeography, Palaeoclimatology, Palaeoecology 131 (1997) 413-432

A number of horizons of core RC9-150 (10 40cm, 60cm, 90cm, 150-200cm, 280290 cm and 330-350 cm) contain displaced faunas such as shallow-water dwelling ostracods, pteropods, mollusc fragments and shallow-water benthic foraminifera. Aragonitic needles of ascidian spicules, which were documented as a good indicator of displaced fine particles from the sublittoral zone (Okada, 1992a), occur commonly in many of the samples studied. Although there are some exceptions, ascidian spicules were more abundant within the interglacial sequences than in the glacials. Pteropod (aragonitic marine molluscs) remains are also present at 20-40 cm and 160 cm. In the southeast Indian Ocean, the Aragonite Compensation Depth (ACD) is at present about 500 m (Berger, 1978). The ACD has fluctuated considerably during the last 20,000 yr, dropping to near 3.5 km during the transition period between glacial to Holocene times, and rising to near 1000 m during full glacial conditions (Berger, 1977). Thus the presence of ascidian spicules and pteropods in core material accumulated well below those depths at those times suggests rapid down-slope transport and burial. On the whole, however, the reworking in RC9-150 does not appear to have significantly influenced the oxygen-isotope or foraminiferal paleotemperature data sets, so it is assumed that, although there may be some noise introduced from the down-slope materials, it is worth while to analyze the stratigraphic trend in nannoflora for this core. Since no ascidian spicules and other shallowwater materials were found, core RS96GC21 is free from such a contamination, but the unusual signal of the isotopic trends suggests the presence of hiatuses during isotope stage 1, the most of stages 3 4 and of early stage 5, and stage 7 for this core.

2.2. Core chronology The basic stratigraphy and correlation of the two cores was already discussed in Wells et al. (1994), but finer details of the foraminifera- and isotope-stratigraphy of core RS96GC21 has not

417

been published. Moreover, the observation of nannoflora in this investigation provides a new age datum, necessitating a slight modification for the chronology of this core. The stratigraphic framework for this core is based on biostratigraphic trends of planktonic foraminifera and nannofossils; sea-surface temperature trends in planktonic foraminifera; one AMS date, and from oxygenisotope trends of Globigerinoides sacculi/er. The statistical methods employed to deduce sea-surface temperature and the ~180 data are described in Wells et al. (1994) and Wells and Wells (1994). The revised chronology of core RS96GC21 ( Fig. 3) has been interpreted in the following ways: (1) The Holocene/stage 2 boundary lies near 25 cm, judged by the rapid 61sO enrichment in G. sacculifer, coupled with the rapid fall in sea-surface temperature near there. (2) The L G M lies below 30 cm, and between 40-50 cm, judged by the AMS radiocarbon date of 12,385+100yr at 30cm, and the maximum ~180 enrichment in G. sacculifer over this interval. The slight warming in sea-surface temperature below 30cm is within the error range of the transfer-function method, so would still be consistent with the placement of stage 2 between 25 cm to just below 50 cm. (3) Isotope stages 3 4 and upper portion of stage 5 are either very short or are absent from this core. (4) The beginning of the acme of Emiliania huxleyi occurs at around 60 cm. This datum event is time-progressive (73 85 ka; Thierstein et al., 1977) occurring later in high latitudes. However, its actual occurrence in this core at stage 5, when the tropical Leeuwin Current is postulated to be flowing, restricts this age at close to 85 ka. (5) Isotope stage 5.5 is placed at 99.5 cm, (Wells and Wells, 1994) based on the position of the top of pink G. ruber acme and the corresponding relatively depleted oxygen-isotope values: the leftcoiling event (TL1) is a younger event (Wells and Chivas, 1994). As is discussed later, observation of nannoflora also indicates a possibility that the lowest part of core RC9-150 may be older than reported in Wells and Wells (1994) and Wells et al. (1994).

1t. Oka~kt, P. Wells ,; Palaeogeo~fraphy, Palueoclimatolo~,~v, Palaeoecology 131 ¢ 1997j 413 432

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Identification of the graphic correlation points of Prell et al. (1986), based on the tuned datum's of Imbrie et al. (1984), is possible only for core RC9-150, where sampling resolution is sufficiently high to enable events to be differentiated. The sampling resolution for core RS96GC21 is too low to justify identification of individual events

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within isotopic stages, so this has not been attempted. The average sedimentation rate in core RS96GC21 (0.83 cm/ka) is low, possibly due to the hiatuses• Despite the redeposition of downsloped materials, the rate is also not high in core RC9-150 (2.17 cm/ka). With sampling at 5 10 cm intervals, only an average minimum resolution of

H. Okada, P. Wells / Palaeogeography, Palaeoclimatology, Palaeoecology 131 (1997) 413 432

about l0 ka (RC9-150) to 24 ka (RS96GC21) is possible.

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3. Regional character observed in nannoflora

3.1. Geographical difference in the abundance o1" lower-photic taxa 2.3. Observation and counting of nanno/bssils To study nannofloral composition, smear slides were prepared from crushed, dried core sediment to which 10-30% by weight of glass microbeads had been added, following the method described by O k a d a (1992b). The counting procedure involved three steps: for each smear slide: (1) the absolute abundance of nannofossils was obtained for approximately 100 glass beads, and the ratio of nannofossils/1000 glass beads was calculated for the hypothetical 1:1 mixture of sediment and glass beads; (2) approximately 350 randomly selected specimens were then observed (the " L e v e l - l " count). In this counting, the genera Florisphaera, Emiliania, Gephyrocapsa and Retieulqfenestra were identified to species/ m o r p h o t y p e levels, and all other taxa were lumped together as subordinates. Because we have used a light microscope for the floral investigation, it was impossible to identify small placoliths (<2.5 gin) at the species level. Therefore, small placoliths with a bridge were identified as small Gephyrocapsa while small specimens without bridge were lumped together as small Reticulofenestra. Actually, an observation of a few selected samples under a SEM revealed that the majority of small Gephyrocapsa is Gephyrocapsa ericsonii, and the majority of the small placoliths is Reticulqlbnestra parvula; (3) more than 350 subordinate taxa (the "Level-2" count) were then observed, and their percentage abundance within the subordinate flora, calculated. We also tried to analyze percentage abundances of the subordinate taxa within the total upper-photic flora. However, the stratigraphic trends of the subordinates are completely overshadowed by the dominating major taxa, and no significant information has emerged from this practice. Preservation of nannofossils is good in all samples of core RC9-150 and, although species identification was not difficult, nannofossils are slightly etched in core RS96GC21.

The recent investigation of nannoflora in a tropical Indian Ocean core recovered from a shallow site ( O D P Hole 716B, water depth: 544 m) revealed c o m m o n to abundant occurrences of three lowerphotic taxa, Florisphaera proJunda, Algirosphaera robusta and Gladiolithus flabellatus (Okada and Matsuoka, 1996). Despite the careful examination, however, the later two species were not observed in the two cores studied in this report. Since these three lower-photic species usually coexist in the modern Pacific plankton communities (Okada and Honjo, 1973a,b), their absence in the nannoflora of our two cores confirms that the fragile construction of their coccoliths cannot be preserved in normal deep-sea sediments. Plotting of the " L e v e l - l " count revealed an abundant to dominant occurrence of F. pr~/unda in the northern core RS96GC21, whereas the taxon is much less abundant in the southern core RC9-150 (Fig. 4). The abundance of F. prq/unda during the interglacial periods is approximately 50% in the northern core, and is within a normal range for a temperate to subtropical pelagic region (Ahagon et al., 1993). Since F. prqfunda is known to reduce its abundance significantly in coastal waters ~ ~kada, 1983, 1992a), the lower abundance of this t xon in the southern core compared to that of the northern core can be partly explained by the proximity of the core site to shore. However, the average abundance of less than 10% seems a bit too low for a hemipelagic core collected at offshore distance of a 50 km and water depth of 2700m. Although some regional difference is expected, the F. prqfunda abundance at this site will exceeds the 30% level, if the data of core RC9-150 are compared to the abundance trend observed at similar water depths and/or offshore distance in Central Japan (Okada, 1983, 1992a). Therefore, another cause is suspected for the exceedingly low abundance of the taxon in the southern core. According to the population study in the modern Pacific and Atlantic Oceans, the lowest temper-

ILL Okada, P. Wells/Palaeogeography, Palaeoclimatology, Pa/aeoecology 131 (1997)413 432

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ature F. pro/'unda can tolerate is ca. 1 0 C (Okada and Mclntyre, 1979). The summer SST calculated for core RC9-150 ranges between 16 to 21 ' C except for the upper part of the Holocene where the value reached to 25°C (Wells et al., 1994). This temperature range is approximately 4c'C lower than that of the northern core (Fig. 3). Since F. pr(~/hnda dwells exclusively within the lower-photic zone where temperature is usually several to ten degrees lower than the surface (Okada and Honjo, 1973b), the temperature in the lower-photic zone could be close to or below 10°C for most of the Late Quaternary at the southern core site. Such a temperature would be sufficiently low to suppress the

growth of F. prq/'unda. By comparison, the water temperature in the lower-photic zone was higher than this critical temperature range at the northern core site. Although the reworking of down-sloped materials can dilute F. prq/hnda abundance, the values are extremely low even in the samples in which no ascidian spicules were observed. This excludes reworking as the major cause of the low abundance. Thus, lower temperature is likely to be the most important factor for the diminished occurrence of F. prq/hnda in the southern core. The very low abundance of this taxon observed in a pelagic core V18-222 taken from offshore southeastern

H. Okada, P. Wells/Palaeogeography, Palaeoclimatology, Palaeoecology 131 (1997) 413-432

Australia further supports this reasoning (Wells and Okada, 1996). Presently no data is available for the abundance of F. proJimda within the Leeuwin Current. Because of the extreme shallowness (ca. 50 m) in the northern regime (Phillips et al., 1991), the presence and disappearance of the Leeuwin current will have only limited influence upon the percentage abundance of F. profunda in core RS96GC21. On the other hand, the time-progressive change in strength of the current will have a profound influence on the abundance of F. profunda in the southern core RC9-150, where the depth of the current may extend to 200 m level (Pearce, 1991). Although reworking has disturbed the trend for the stage 1 flora, a significant increase of F. profunda during LIC and a slight increase in Holocene seem to indicate a higher population of this taxon in the lower-photic depth of the Leeuwin Current than in the coastal water that occupied this site during the glacial periods (Fig. 4).

3.2. Phylogenetic succession of the major taxa Stratigraphic variation of the upper-photic flora, derived by subtracting Florisphaera profunda from the "Level-1" count, revealed a succession of dominance by three phylogenetically related small taxonomic groups: Emiliania huxleyi (isotope stages 1 through 4), small Reticulofenestra (upper part of stage 5) and small Gephyrocapsa (lower part of stage 5-7) (Fig. 5). The dominance of E. huxleyi in the latest Quaternary upper-photic nannoflora is a widespread oceanic phenomenon, but the dominance of small Reticulofenestra (mostly Reticulofenestra parvula) during the upper part of stage 5 is unique to the study area. Actually, the only previous record of its dominant occurrence in the latest Quaternary sequence is from the northeastern corner of the Indian Ocean, offshore from the Indonesian Archipelago (Biekart, 1989), where R. parvula was abundant to dominant in the all three cores studied. In core G6-4, recovered from ca. 1600 km northeast of our northern core RS96GC21 (and reported as the least affected by local signals among the Biekart's three cores), R. parvula was especially abundant

421

during the upper part of stage l and stage 3 through the upper part of stages 6: a much longer time interval compare to our result (Fig. 5). The highest abundance in core G6-4 (approximately 50%), however, was reached during stage 5, and it is comparable to our observation in timing and magnitude. Previous records for the dominant occurrence of small Gephyrocapsa (mostly Gephyrocapsa ericsonii) in the latest Quaternary sequence are also limited geographically: from the Gulf of Mexico (Gartner et al., 1983), the Caribbean Sea (Gartner, 1988), and the northeastern corner of the Indian Ocean (Biekart, 1989). In the last example, G. ericsonii was abundant to dominant in a short interval within stage 2 and in the major parts of stages 4-9 in core G6-4 (Biekart, 1989): again much longer than observed in the present study (Fig. 5). Thus, the dominant occurrences of the small placolith taxa have lasted much longer offshore the Indonesian Archipelago, and they extend into stages 1-4 when E. huxleyi universally dominates the flora. Actually, E. huxleyi is an inferior contender for survival in the marginal seas of Eastern Indian Ocean (Okada and Honjo, 1975), and as discussed later, the small placolith taxa are stenothermals. It is, therefore, understandable that the small placoliths could successfully compete with E. huxleyi offshore of the Indonesian Archipelago, where water temperatures have remained higher than those the offshore Western Australia. Recently another succession of dominance including that of small Gephyrocapsa was reported from the Madeira Basin (northeastern Atlantic) and its potential for high resolution biostratigraphy was suggested (Weaver and Thomson, 1993). The succession in the Madeira Basin is: dominant E. huxleyi (stages 1-4), coexistence of abundant E. huxleyi, Gephyrocapsa mulleri and Gephyrocapsa aperta (stage 5), coexistence of abundant G. mulleri (= G. muellerae) and G. aperta (stage 6), dominant G. aperta (stage 7) and dominant Gephyrocapsa caribbeanica (stages 8 12). Because the taxonomy and classification of genus Gephyrocapsa is presently in disarray among specialists, their G. aperta and G. caribbeanica probably correspond to our

tI. Okada, P. Wells / Palaeogeography, Palaeoc/imatology, Palaeoecologv 131 11997) 413 432

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small Gephyrocapsa and medium Gephyrocapsas excluding G. muellerae/margerelii, respectively. These small placoliths are usually minor components of the late Quaternary flora in pelagic realms. For example, in a nearby Pacific core KH84-1, St. 21 (taken from the West Mariana Ridge), no small Reticulo/~,nestra were observed, and small Gephyrocapsa occupied less than several percent of the upper-photic flora through the intervals equivalent to stages 4 - 7 ( M a t s u o k a and Okada, 1989). In that core, no single taxon dominated the flora immediately prior to the acme of E. huxleyi, and the major taxa observed between 85 210 ka are medium forms of Geph)'rocapsa, Umbilicosphaera sibogae, Calcidiscus leptoporus and Rhabdosphaera clavigera (Matsuoka and Okada, 1989). Thus, the successive occurrences of dominant small Gephyrocapsa to dominant small Reticulo-

Ji,nestra is a uniquely local event for offshore Western Australia. The shift of dominance between these two taxa, however, can be useful biostratigraphic datum event applicable to this region. This datum, observed at 225 cm in core RC9-150 (Fig. 5), occurred at the beginning of substage 5.3 (Wells and Wells, 1994), and is likely correlatable to the substage 5.33 (ca. 103 ka) of Martison et al. (1987). In the newly proposed biostratigraphic scheme of Weaver and Thomson (1993) which, as a whole, is not applicable for a global correlation, the sharp shift of dominance from the G. aperta (small Gephyrocapsa) to G. caribbeanica (medium Gephyrocapsa) at stage 7/8 boundary is particularly notable. A similar succession of floral dominance was observed at 4 5 0 c m in core RC9-150, where Wells et al. (1994) identified the base of substage

H. Okada, P. Wells / Palaeogeography, Palaeoclimatology, Palaeoecology 131 (1997) 413-432

7.2 which is convertible to ca. 210 ka using the time table of Martison et al. (1987). Matsuoka and Okada (1989) also found the top of the dominance of medium Gephyrocapsa oceanica at ca. 210 ka in the northwestern Pacific core KH84-1, St. 21, but this age was extrapolated under the assumption of uniform sedimentation rate. Therefore, if Weaver and Thomson's timing for the top of dominance of medium Gephyrocapsa (their G. caribbeanica) is globally synchronous, the lowest section of core RC9-150 (below 450 cm) is correlatable to stage 8. And if a hiatus exists at 450 cm level, the section may be even older (stages 9 12). Since we have no data to further examine this point, however, our age identification for the lowest section of core RC9-150 remains to be ambiguous, at somewhere between stages 7 and 12.

4. Floral response to the climatic changes

4.1. Re,sponse of Florisphaera profunda The stratigraphic variation in percentage abundance of Florisphaera profunda observed in the northern core RS96GC21 (Fig. 4) is generally concordant with that of SSTs (Fig. 3), with the abundance peaks at 5, 60 and 95 cm as well as the significant drops in stages 2 and 6 closely duplicated. The only significant difference occurred at 160 cm where F. profunda abundance dropped while the levels of SSTs rose. The 81sO value, however, registered a sharp decline at this level. Since F. proJimda abundance corresponds directly to the lower-photic temperature rather than the SST, the simultaneous decrease in both the F. profunda abundance and the alsO record may indicates a short-lived thermocline at this time which is possibly correlatable to substage 7.2. The significant reduction in F. profunda abundance during stages 2 and 6 seem to agree with the nutricline model of Molfino and McIntyre (1990a): (1) intensified winds during the glacial periods disturb the stratification in water column, (2) increasing the productivity in the upper-photic layer and (3) resulting in a decrease in F. profunda abundance. This scenario is applicable in tropical waters where the lower-photic temperature within

423

an upwelling is still within a range to growth of

F. profunda (Molfino and Mclntyre, 1990a; Okada and Matsuoka, 1996). However, a cold lowerphotic zone caused by upwelling can be a critical factor to control F. profunda abundance in the subtropical to temperate regions. If an upwelling occurs in the offshore of Western Australia, the resulting drop of lower-photic temperature will result in a reduction of F. proJunda abundance, just as the disturbance of water stratification will do. According to the foraminifera data, the productivity increase at the L G M was much weaker than at the ePG, whereas the reduction of F. profunda abundance was almost at the same levels during these two glacial periods (Fig. 4). Therefore, the lower temperature rather than the increased upperphotic productivity seems to be the more important factor in controlling F. profunda abundance in the study area. Although foraminiferal data indicated a remnant of cold surface water for the LIC (Wells and Wells, 1994; Wells et al., 1994), the generally high abundance of F. profunda during the LIC failed to support this scenario. As will be discussed later, however, a weak upwelling is postulated even for stage 5, and the trend of F. profimda is not to deny the documented data for the cold surface water at LIC. The sharp increase in F. profunda abundance at the LIC is also observable in the southern core RC9-150, but the abundance variation associated with the other glacial/interglacial cycles is not clear (Fig. 4). The blurred signal is chiefly due to the frequent contamination by redeposited downslope sediments in which F. profunda abundance is significantly low. This is particularly the case for the Holocene sequence in which many Ascidian spicules occur, and F. profunda abundance showed no increase despite the significant rise of water temperature.

4.2. Response of the major upper-photic taxa As discussed in the previous section, the prominent feature of nannoflora observed in this investigation is the dominant occurrence of small Gephyrocapsa and small Retieulofenestra through isotope stages 5-7 (Fig. 5). It has been well

424

H. Okada. P. Wells/Palaeogeography, Palaeoclinzatology, Palaeoecology 131 (1997) 413 432

documented that small Gephyrocapsa dominated ( > 8 0 % of the upper-photic flora) the tropical and subtropical upper-photic floras of the middle Pleistocene (0.93 1.25 Ma, according to Gartner, 1988), and this interval, known as the small Gephyrocapsa Zone (Gartner, 1977), is used widely in Quaternary biostratigraphy. Small Gephyrocapsa never reached that level of dominance again in most pelagic realms, but as discussed previously, their dominant recurrence in the latest Pleistocene sequences has been reported from some hemipelagic and marginal seas. The dominance of small Gephyrocapsa has been attributed to the lower temperature and high nutrient level associated with intensified upwellings (Gartner, 1988). A close relationships between the abundant occurrence of Reticulofi~nestra parvula and upwelling was also documented in the Pacific living community (Okada and Honjo, 1973a). A close link between their high abundances and high fertility was also suggested by Biekart (1989). Therefore, it is reasonable to assume that upwelling persisted during stages 5 7 in offshore Western Australia. The abundant occurrence of Florisphaera proJhnda even within stage 6, however, suggests that the strength of upwelling was not too strong. In the tropical Indian Ocean site ODP-716B, F. proJunda and small placoliths (small Gephyrocapsa, small ReticuloJi)nestra and Emiliania huxl~Lvi) showed an inverse trend in stratigraphic abundance, where F. proJunda dominated the flora during the glacial periods when the water stratification is strong, and small placoliths flourished during the interglacial when a stronger monsoon triggered upwellings (Okada and Matsuoka, in prep.). The strengthening of the wind system during the interglacials, however, is unique regional phenomenon of the Asian monsoon. At the Western Australian margins, the climatic change is expected to tbllow a normal pattern of intensified winds during the glacials. The significant drop in the F. pr~/hnda abundance during stages 2 and 6 in core RS96GC21 agrees with this common scenario. Unlike the patterns observed in the tropical Indian Ocean, however, the abundance of the small taxa did not increase (RS96GC21) and was

even reduced (RC9-150) in stage 6, when an intensified upwelling is inferred ( Wells et al., 1994 ). In today's ocean, the lowest temperature for the survival of Gephyrocapsa ericsonii is 1 2 C (Okada and McIntyre, 1979), and the species' maximum concentration occurs between the summer isotherms of 1 8 and 2 5 C (McIntyre et al., 1970). In a study of the modern Pacific nannoplankton community, a large bloom of three small placolith species responding to an upwelling was observed in a subsurface water sample ( 5 0 m , 22.5 C) at 1 0 0 0 ' N , 155005'W (Okada and Honjo, 1973a,b). The population of these species, Gephyrocapsa crassipons, G. ericsonii and R. parvula, was sharply reduced in the lower sample (75 m, 14.8 C) and almost none existed in the higher water sample (30 m, 27.7"C). After studying the nannoflora in the surface sediments of the North Pacific, Roth and Coulbourne (1982) concluded that G. ericsonii is most successful where water temperature is near 1 7 C and nutrients are generally high. All these data indicate that G. ericsonii is an opportunist which blooms only under special circumstances of high nutrient level and an optimal temperature range. The very high abundance of the small taxa observed in the upper part of stage 5 in core RC9-150 may indicate a particularly fertile upper-photic zone with temperatures of > 15 C (Fig. 5). If the upper-photic temperature drops to below 15"C during upwelling, the growth of G. ericsonii will diminish greatly. The reduction in abundance of the small taxa and the drop of summer SST to ca. 1 5 C during stage 6 in the southern core (Wells et al., 1994) confirms this scenario. On the other hand, the summer SST in stage 6 is ca. 22°C in the northern core (Fig. 3), and the consistent high abundance between the lower part of stage 5 through the upper part of stage 7 indicates that the upper-photic temperature did not drop to the critical 1 5 C level even at the ePG in this northern site. Since the percentage abundance of the opportunistic small placoliths did not increase, the increased productivity inferred from the foraminiferal data (Wells et al., 1994) is likely to be caused by organisms other than calcareous nannoplankton. Another opportunist, E. huxh,yi, is a much tougher competitor than medium Gephyrocapsa

H. Okada, P. Wells / Palaeogeography, Palaeoclimatology, Palaeoecology 131 (1997) 413 432

and other medium to large taxa in pelagic realms, and it is no surprise to observe the dominance of E. huxleyi in the upper-photic flora of stages I - 4 in both cores. Moreover, this taxon can tolerate a much colder temperature than G. ericsonii or R. parvula (Okada and McIntyre, 1979), therefore, E. huxleyi is likely to bloom in the colder upwellings that occur in cooler seas such as offshore southwestern Australia. However, the abundance of E. huxleyi did not increase significantly during stage 2 in both cores, despite signs of upwelling such as cold SST anomaly and reduction of F. profunda abundance (Fig. 4). This suggests that an upwelling had occurred during the Last Glacial at offshore Western Australia, but it was not strong enough to trigger a bloom of E. huxleyi. This conclusion is in good agreement with Wells et al. (1994), who had identified an insignificant increase of productivity at the LGM. It is widely regarded that there are two distinctive species of medium to large Gephyrocapsa in today's ocean: Gephyrocapsa oceanica (s. str.) and Gephyrocapsa muellerae. By studying the thanatocoenosis flora in Atlantic and Pacific surface sediments, Geitzenauer et al. (1977) demonstrated that the later taxon (which they referred to as Gephyrocapsa caribbeanica) prefers colder temperatures than the former species. Weaver and Pujol (1988) used the ratio of these two species to identify the time-progressive temperature change in the western Mediterranean. Since G. muellerae evolved ca. 200 ka and became an important component of nannoflora only after ca. 170 ka (Br6h6ret, 1978), their method is applicable only to the upper sections of our cores (stage 1 to the upper part of stage 5). We have tried this method in our cores, and although some fluctuations were observed in core RC9-150, no clear relationship was recognized between this ratio and the SSTs. Possibly the time-resolution of our sample is too large to utilize this method.

4.3. Response of the subordinate taxa Although weak relationships are recognizable for some taxa, in general, paleoenvironmental signals are not distinct in abundance variations of most of the subordinate taxa. Taxa exhibiting weak

425

responses to the glacial/interglacial cycles include Cah'idiscus leptoporus, which increased its abundance during the glacial periods (stages 2 and 6) (Fig. 6). This taxon prefers pelagic environments (Okada, 1992a), and is more common in the fossil flora of central gyres than that of tropical regions (Geitzenauer et al., 1977). The taxon's increased abundance, therefore, is likely to indicate the lessened influence of the near-coastal Leeuwin Current in the glacial periods. Moreover, the abundance is higher in the southern core RC9-150 than in the northern counterpart (Fig. 6). Since C. leptoporus is eurythermal species (Okada and McIntyre, 1979), the temperature difference between the two cores is not a major factor for the observed abundance difference. Therefore, the generally higher abundance of this taxon in the southern core indicates the stronger influence of the West Australian Current at offshore southwestern Australia than off North West Cape throughout the time period studied. Though the abundance is generally low, Neosphaera coccolithomorpha also increased in abundance during the glacial periods in the northern core. This taxon lives mainly in the pelagic sector of subtropical to tropical regions, and its decrease during interglacials is again likely to reflect the increased influence of the non-pelagic Leeuwin Current. Its much reduced occurrence in the southern core is possibly due to the lower temperature. Oolithotus fragilis seems to become less abundant at the L G M and at the ePG in both cores (Fig. 6). Since this species is more common in the lower-photic waters (Okada and Honjo, 1973b), the observed trend is reasonable and concordant with Florisphaera profunda representing the lowerphotic community. Calciosolenia murrayi reduced its abundance during the glacial intervals, particularly in stage 2 in the northern core (Fig. 6). Although the trend is not so clear in the southern core, an increase during stage 5 is notable. In the offshore Indonesian Archipelago, this taxon was more abundant in Core G5-149P2, than in the other two cores studied by Biekart (1989). Core G5-149P2 was recovered from the edge of the northwestern shelf of Western Australia, where the flora is

tt. Okadu, P. WelLs/Palueo~eograp/y. Palaeoclimatoh)~y Pulae¢)ecolok.v 131 (1997) 413 432

426

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expected to reflect the strongest influence o f coastal water a m o n g the three cores studied. The weak trend observed in our study, therefore, seems to indicate an increased influence o f the Leeuwin Current during the interglacial periods. Since the genus Helicosphaera is generally k n o w n to prefer coastal waters, (e.g. Okada, 1992a), the increase o f Helico~?haerct spp. during stages 2 and 6 in the northern core seems unusual ( Fig. 6), especially as the trend is possibly reversed in the southern core. Umbilicosphaera sihogae var. j~)liosa seems to became less a b u n d a n t during stage 6 in both cores (Fig. 6). Since this taxon is more a b u n d a n t in pelagic settings (Okada, 1992a), the seemingly opposite trend of this taxon to that of C. leptoporus is not readily explainable. The trend, however, was not repeated at stage 2, and many irregular fluctuations are observed. Thus, this taxon, which is the second most d o m i n a n t species within the subordinate taxa, does not appear to

be a useful indicator of p a l e o c e a n o g r a p h y in the studied area. The other subordinate taxa not illustrated in Fig. 6 showed even more erratic changes than the six subordinate taxa discussed above. The taxa occurred as c o m m o n elements o f the subordinate flora ("Level-2" counting) are that: Discosphaera tub(/~,ra, Oolithotus [i'agilis vat. cavum, Rhabdosphaera clavi~era, Svracosphaera spp.,

Unlhello,~?haera irregularis, Umbellosphaera tenuis, Unlbilico,v~haera anlillarum and Umbilicosphctera hulburtiana. 4.4. Variations in the sl~e('ies diversiO' qf tota] nannol'lora

The diversity of total nannoflora expressed in the S h a n n o n Wiener Function (Dn) exhibits an obvious response to climatic change. The value was higher in the glacial periods and declined

427

H. Okada, P. Wells/Palaeogeography, Palaeoclimatology, Palaeoecology 131 (1997) 413 432

during the interglacial intervals in both core (Fig. 7). This response is similar to the DR trend of the upper-photic flora, which was observed in a tropical Pacific core L2011 (Li and Okada, 1985). The increased D , value during the glacial periods can be readily explained by the significant reduction of F. profunda and small placoliths. The DR value in Core RC9-150, declined significantly at the Holocene, at the upper parts of stage 5 (150-190 cm) and at stage 7 (410 420 cm) coinciding with the strong dominances of Emiliania huxleyi, small Reticuh~fenestra and small Gephyrocapsa, respectively. (Fig. 5 and Fig. 7). These data indicate a greatly reduced competition in nannoflora triggered by a special oceanographic condition which was particularly suitable for the growth of opportunistic small placoliths. Thus, the stratigraphic trend in species diversity of total nannoflora is concordant between the two cores studied here, and the significant increase of e-

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Since hiatuses are suspected and sedimentation rates are generally low, the determination of an accurate sedimentation rate for each isotope stage is not obtainable in the northern core RS96GC21. Similarly, due to the frequent reworking, the sedimentation rate is probably unreliable for the southern core RC9-150 as well. Nevertheless, the absolute abundance of nannofossils measured by the glass-bead method provides valuable data (Fig. 8) to reconstruct the history of aeolian flux.

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H. Okada, P. I42,11s/ Pulaeogeography. Palaeocl#natoh~gy, Palaeoecoh)gy 131 (1997) 413 432

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The a b u n d a n c e o f nannofossils fluctuates between samples, but significant reductions are observable for the Last and Penultimate Glacials in the northern core. Although an increased productivity associated with an intensified upwelling is reported for the Penultimate Glaciation by foraminiferal data (Wells et al., 1994), small placoliths, most likely to benefit from the intensified upwelling, did not change their a b u n d a n c e during this period (Fig. 4). Since it is likely that coccolith productivity did not change significantly, the d r o p in the absolute a b u n d a n c e o f nannofossils observed in the P G is attributable to an increased accumulation o f aeolian flux reported by Kolla and Biscaye

(1977). D u r i n g the L G M , the foraminiferal data (Wells et al., 1994) and our observation did not detected a significant increase of productivity. The strong decline o f nannofossil abundance at the L G M , therefore, can also be ascribed to the increased flux o f wind-blown materials. A strong relationship was reported between the percentage a b u n d a n c e o f Florisphaera proJimda and sea-water transparency ( A h a g o n et al., 1993), but the reduction of F. pro/unda during stage 6 is similar to that observed for the upper-photic taxa. This data denies a significant influence of increased aeolian flux and support our conclusion for the colder lower-photic temperature for the

ILL Okada, P. Wells/Palaeogeography,

Palaeoclimatology, Palaeoecology 131 (1997) 413 432

decline of F. profunda abundance during the glacials. The patterns of nannofossil abundance in the southern core shows a similar trend to that of the northern core for the PG (Fig. 8). Although the nannofossil record is distorted by the repeated reworkings in the southern core, the interval (300-320cm) where the sharp drop occurred is relatively free of reworking. Thus, it is likely that aeolian dust was falling offshore from southwestern Australia during the Penultimate Glaciation. Because a moderately strong reworking is evident, no attempt was made to analyze the abundance record at the Last Glacial in this core.

5. Conclusions

As the result of our study of the upper Quaternary (last 250 kyr) nannoflora in two deepsea cores off the coast of western Australia, the following statements can be made: (1) The percentage abundance of Florisphaera profunda is significantly lower in the southern core RC9-150 than in the northern core RS96GC21. A lower temperature in the lower photic zone is the main reason for the suppressed occurrence in the southern core. (2) The percentage abundance of F. profunda dropped sharply during stages 2 and 6 in the northern core. A lower temperature in the lower photic zone, rather than the increased productivity in the upper photic zone, is likely to be the main cause for this trend, (3) While F. profunda reduced its abundance significantly, Emiliania huxleyi did not increase its share during stage 2 in the northern core, and the later trend was also true in the southern core. These observations support the finding of Wells et al. (1994), who postulated a weak upwelling and no significant increase of productivity at the L G M offshore Western Australia. (4) The dominance of small taxa of genus Gephyrocapsa and Reticulofenestra in the upperphotic flora indicates continuous, though mostly weak, upwelling during isotope stages 5 7 offshore Western Australia. (5) The peak occurrence of F. profunda at the

429

LIC in the northern core indicates warming of the lower photic zone caused by the weakening of upwelling. (6) The sharp decline in the abundance of F. profunda and lack of increase in the abundance of small placoliths in the northern core indicates intensified upwelling in stage 6. The significant reduction in the abundance of the later taxa in the southern core suggests an intensified upwelling for offshore SW Australia during the Penultimate Glaciation. These results agree with the finding of Wells et al. (1994). (7) The increased abundances of Calcidiscus leptoporus and Neosphaera coccolithomorpha as well as the reductions in Calciosolenia murrayi and Oolithotus fragilis during stages 2 and 6 support the postulated weakening of the Leeuwin Current for the glacial periods. (8) Because of the extreme shallowness of the Leeuwin Current at off northwestern Australia, F. proJimda can not provide significant information for the history of the current in the northern core. However, the taxon's significant increase in LIC and a weak increase in the Holocene of the southern core seem to support the postulated strengthning or rejuvenation of the current for the interglacials. (9) A succession of dominance by three related taxonomic groups occurred commonly in both cores: E. huxleyi (stages 1 4), small ReticuloJenestra (upper part of stage 5) and small Gephyrocapsa (lower part of stage 5 to stage 7). The shift of dominance from small Gephyrocapsa to small Reticulofenestra (mostly Reticulofenestra parvula) can be a useful biostratigraphic event for offshore Western Australia. (10) The species diversity of total nannoflora expressed in the Shannon Wiener Function declined significantly during interglacial periods, and the cause of this trend is reduced competition within the nannoflora caused by the increased dominances of opportunists, such as E. huxleyi, F. profunda and small placoliths. ( 11 ) Variations in coccolith abundance indicates increased fluxes of aeolian dust during the Penultimate and Last Glaciations for off North West Cape. The dust fall is also evident off

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H. Oku&¢, P. Wells ' Pahwogeography, PalaeoclimaloloKv, Palaeoecology 131 (1997) 413 432

southwestern Australia during the Penultimate Glaciation.

Acknowledgements All nannofossil counts were undertaken by HO. PW was funded to visit Yamagata University in May June 1994 by the Japan Society t\)r the Promotion of Science and the Australian Academy of Science, to collaborate in this research. Material from RC9-150 was sampled by P. De Deckker (Geology Department, A N U ) at the LamontDoherty Deep-Sea Sample Repository, supported by the National Science Foundation through Grant OCE-91-01689 and the Office of Naval Research through Grant N00014-90-J-1060. Material from RS96GC21 was collected by PW aboard the AGSO vessel Rig Seismic. We acknowledge the generosity of J. Head (Quaternary Dating Facility, Research School of Pacific Studies, A N U ) , who provided the AMS radiocarbon date for RS96GC21. We also thank anonymous reviewers whose critical comments were useful for the significant improvement of the manuscript. This study was supported by Grant-in-Aid for Scientific Research (No. 05454001) from the Ministry of Educatiom Science and Culture, Japan.

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