Qianyu Li

Biogeographic impact of the Leeuwin Current in southern Australia since the late middle Eocene

Abstract The Leeuwin Current can be tracked from the western margin of the Australian continent to the southern margin by the record of fossilized organisms typical of warm-water marine environments. This transport smudges the normal latitudinal asymmetry in the biotas of opposing oceanic and continental margins, in which the eastern margins of oceans are cooler than the western margins and warmer biotas are restricted to lower latitudes in the east. The most comprehensive record is in the large benthic foraminifera, although fossils of benthic invertebrates, nektonic nautiloids and planktonic protists are also informative. In addition, organic biomarker hydrocarbons in stranded bitumens and resins demonstrate that they have travelled the same route from their ultimate sources in Cenozoic sedimentary basins and modern tropical rainforests of the Indonesian Archipelago. The earliest spoor of the current is in the later middle Eocene, at which time the part-deflection of counter-gyral circulation in the Indian Ocean to the southeast was stimulated by the accelerated opening of the oceanic gap between Australia and Antarctica. Thus the origin of the Leeuwin Current is twice the age of the previously suggested origin in the Miocene. The current turns on and turns off in the Great Australian Bight during the late Quaternary in concert with the interglacials and the glacials at scales of 10 5 yr. The switch can be seen in the faunal succession of planktonic foraminifera which are consistent with the neritic benthic faunas of the central gulfs: both communities show that, at the warm peak of the last interglacial, the current transported biota across the Bight more strongly than it has during the Holocene. The Cenozoic record of the past 40 Ma is in the same mode at 10 6 yr scales: the relevant fossils are found concurrently at major marine transgressions and warming reversals of the overall fall in global temperature. However, the fossil pattern is due to transport on the activated Leeuwin Current, not merely to general warming and the spread of friendly environments to higher latitudes.

Quaternary bryozoan reef mounds in cool-water, upper slope environments: Great Australian Bight

Bryozoan reef mounds are common features in the geological record, occurring within mid-ramp, slope paleoenvironments, especially in Paleozoic carbonate successions, but until now have not been recorded from the modern ocean. Recent scientific drilling in the Great Australian Bight (Ocean Drilling Program Leg 182) has confirmed the existence of shallow subsurface bryozoan reef mounds in modern water depths of 200–350 m. These structures have as much as 65 m of synoptic relief, and occur both as single mounds and as mound complexes. They are unlithified, have a floatstone texture, and are rich in delicate branching, encrusting and/or nodular-arborescent, flat-robust branching, fenestrate, and articulated zooidal bryozoan growth forms. The muddy matrix is composed of foraminifers, serpulids, fecal pellets, irregular bioclasts, sponge spicules, and calcareous nannofossils. The 14C accelerator mass spectrometry dates of 26.6–35.1 ka indicate that the most recent mounds, the tops of which are 7–10 m below the modern seafloor, flourished during the last glacial lowstand but perished during transgressive sea-level rise. This history reflects changing oceanographic current patterns; strong upwelling during lowstands, and reduced upwelling and lowered trophic resources during highstands. Large specimens of benthic foraminifers restricted to the mounds confirm overall mesotrophic growth conditions. The mounds are similar in geometry, scale, general composition, and paleoenvironments to older structures, but lack obvious microbial influence and extensive synsedimentary cementation. Such differences reflect either short-term local conditions or long-term temporal changes in ocean chemistry and biology.

Palaeoceanographic significance of recent foraminiferal biofacies on the southern shelf of Western Australia: a preliminary study

The southern shelf of Western Australia lies close to the Subtropical Convergence, in a region strongly influenced by the warm Leeuwin Current from the north and the cold, more massive Western Australian Current from the south. Fresh Holocene and relict Pleistocene foraminiferal specimens are mixed in dredged sediment samples, similar in composition to those from other parts of the southern Australian margin. The Holocene planktonic assemblages are dominated in the west by subtropical forms (Globigerinoides trilobus s.l., Globorotalia menardii and Neogloboquadrina dutertrei ) and in the east by the temperate species Globorotalia inflata. Three Holocene benthic assemblages are distributed from the inner shelf to the upper slope, also showing a strong longitudinal gradation from west to the east. The change from warm-water assemblages to temperate assemblages is progressive and continuous, but the southwest corner is marked by the southerly limit of some larger benthic taxa including Heterostegina, and the area off Esperance is the furthermost extent of an abundant, though patchy, living Amphisorus‐Marginopora association. This W! E gradation indicates that the Leeuwin Current has played a key role in influencing the distribution of foraminifera as from at least the last interglacial, because this pattern exists not only in the Holocene but also in the relict foraminiferal biofacies.

FORAMINIFERAL BIOFACIES ON THE MID-LATITUDE LINCOLN SHELF, SOUTH AUSTRALIA: OCEANOGRAPHIC AND SEDIMENTOLOGICAL IMPLICATIONS

Foraminiferal assemblages on the southern mid-latitude Lincoln Shelf comprise mixtures, more strongly in shallow waters, of Holocene and relict Pleistocene specimens. Over 200 benthic and 15 planktonic species were recorded, and a higher diversity was found in the central and deeper parts of the shelf and slope. Cluster analysis identified five assemblages or biofacies. Nearshore assemblage A (<50 m) and inner mid-shelf assemblage B (50–90 m) are dominated by relict and Recent shallow-water species. The outer mid-shelf assemblage C (90–120 m) contains more cibicidids and is transitional between the shallow- and deep-water assemblages. Outer shelf assemblage D (120–170 m) is characterized by cibicidids and anomalinids and by a higher planktonic and benthic diversity, and it can be subdivided into three sub-assemblages D1-D3. Unlike others, assemblage D was not recognized from the most western transect samples. The outer shelf to slope assemblage E (170–400 m) is typically deep-water, having forms like Hoeglundina elegans, Pullenia bulloides and Melonis affinis. n nThe western, Great Australian Bight sector has the larger epifaunal benthic Sorites-Marginopora group, while the infauna is higher in the eastern, Neptune sector of the shelf. We attribute these contrasts to the influence of the warm Leeuwin Current and to the mixing between gulf, shelf and oceanic waters, respectively. A former lagoonal environment is largely responsible for the accumulation of relict tests during lower sea-level periods of the late Pleistocene when climate was more arid. The lack of Recent sediments preserved on the inner shelf is considered to be due to strong wave abrasion causing sediment starvation. Richer forarniniferal assemblages from the outer shelf to upper slope parallel an increasingly calcareous sediment. Faunal evidence indicates a warmer, nearly oligotrophic condition on Lincoln Shelf, compared to the mesotrophy on the adjacent Lacepede Shelf. This difference may be due to the separation of these two shelves by Kangaroo Island acting as a local oceanographic (environmental) and biogeographic barrier.

Mixed foraminiferal biofacies on the mesotrophic, mid-latitude Lacepede Shelf, South Australia

The Lacepede Shelf on the southern Australian passive continental margin narrows from 200 km off the River Murray to 50 km to the southeast (the Bonney Shelf) and 70 km to the west. It is mostly deeper than 40 m. Sediments are Holocene and late Pleistocene quartzose clastics and bryozoan/ mollusc carbonates. Dredged samples from four transects yield over 200 foraminiferal species. Test preservation in any single taxon ranges from fresh to brown (or relict). Plankton is rare on the shelf, but increases to 10% or more in 100 m and deeper waters. Relict benthics, representing products of previous low sealevels, are concentrated off the River Murray mouth. Mixed deepand shallow-water assemblages are common in midto inner-shelf sites and also on the eastern Bonney Shelf, where there is seasonal wind-driven upwelling. A cluster analysis determined five major assemblages: I, slope and deeper water (>130 m); II, mid to outer shelf (60-130 m); III, upwelling center, mixing A (<82 m on Bonney Shelf); IV, mixing B (150338 m on Bonney Shelf; 35-70 m on other transects); and V shallow relict (<55 m off River Murray mouth). These mixed thanatocoenoses have been generated in the past 300 Ka during sealevel fluctuations over tens of meters. The rarity of fresh tests on the inner to middle shelves indicates a low deposition rate of recent foraminifera due to high energy, particularly wave abrasion. It is anticipated that some fossil assemblages may similarly be strongly mixed over 103104 years.

Origin of Late Pleistocene Bryozoan Reef Mounds; Great Australian Bight

Bryozoan-rich biogenic mounds grew periodically on the prograding carbonate slope of the central Great Australian Bight throughout Pliocene-Pleistocene time. Cores from three ODP Leg 182 drill sites provide a record of mound growth during the last 300,000 years over a stratigraphic thickness of similar to 150 m. These mounds, the first such structures described from the modern ocean, grew between paleodepths of 100 and 240 m; we infer that the upper limit of growth was established by swell wave base, and the lower boundary was fixed by an oligotrophic water mass. Detailed chronostratigraphy, based on radiometric and U-series dating, benthic foraminifer stable-isotope stratigraphy, and planktonic foraminifer abundance ratios, confirms that buildups flourished during glacial lowstands (even-numbered marine isotope stages) but were largely moribund during interglacial highstands and are not extant today. Mound floatstones are compositionally a mixture of in situ bryozoans comprising 96 genera and characterized by fenestrate, flat robust branching, encrusting, nodular-arborescent, and delicate branching growth forms. The packstone matrix comprises autochthonous and allochthonous sand-size bryozoans, benthic and planktonic foraminifers, serpulids, coralline algae, sponge spicules, peloids, and variable glauconite and quartz grains, together with mud-size ostracods, tunicate spicules, bioeroded sponge chips, and coccoliths. Intermound, allochthonous packstone and local grainstone contain similar particles, but they are conspicuously worn, abraded, blackened, and bioeroded. An integrated model of mound accretion during sea-level lowstands begins with delicate branching bryozoan floatstone that increases in bryozoan abundance and diversity upward over a thickness of 5-10 m, culminating in thin intervals of grainstone characterized by reduced diversity and locally abraded fossils. Mound accumulation was relatively rapid (30-67 cm/ky) and locally punctuated by rudstones and firmgrounds. Intermound highstand deposition was comparatively slow (17-25 cm/ky) and typified by meter-scale, fining-upward packages of packstone and grainstone or burrowed packstone, with local firmgrounds overlain by characteristically abraded particles. Mound growth during glacial periods is interpreted to have resulted from increased nutrient supply and enhanced primary productivity. Such elevated trophic resources were both regional and local, and thought to be focused in this area by cessation of Leeuwin Current flow, together with northward movement of the subtropical convergence and related dynamic mixing.

Foraminiferal biostratigraphy and depositional environments of the mid‐Cenozoic Abrakurrie Limestone, Eucla Basin, southern Australia

The Abrakurrie Limestone from the central part of the southern Australian margin contains abundant bryozoan, molluscan and other calcareous skeletons including many poorly preserved foraminifers. Limited planktonic and benthic foraminiferal data suggest that the limestone was deposited during the Late Oligocene and Early Miocene. It is believed to have been deposited at about 28–21 Ma, as the local manifestation of eustatic second‐order supercycle TB1. The limestone accumulated mainly in water depths around 100 m, for its benthic fauna was largely composed of mid to outer shelf taxa, particularly the cibicidids and eponidids. There was a major faunal change between the infauna‐dominated assemblage in the middle, and probably also lower, Abrakurrie Limestone and the epifauna‐dominated assemblage in the upper member. Amphistegina lessonii made its first but rare and episodic appearance in the lower member, and the species became abundant and widespread only in the upper member. Likewise, such species as Elp...

Southern Australian endemic and semi-endemic foraminifera: a preliminary report

The Cenozoic in southern Australia contains many foraminifera endemic to the region in neritic (intermediate- to shallow-water) facies. They were mostly epifaunal and inhabited waters to some 300 m deep. This endemism is first obvious in the later Eocene when Maslinella, Crespinina and Wadella, among others, evolved. More than half of the Eocene endemic species disappeared in the Eocene or Oligocene. There followed in the Oligocene the evolution of such species as Parrellina imperatrix and Astrononion centroplax. The Miocene was a time of slightly reduced endemism and is characterized by migration into the region of many larger (sub)tropical taxa such as Lepidocyclina and Cycloclypeus. The long-ranging genus Notorotalia emerged about 50 Ma ago and is still common in modern southern mid-latitude waters. The youngest common extant species which made their first appearance in the Pliocene–Quaternary include Discorbis dimidiatus and Parredicta porifera, both with a test up to 1.5 mm in diameter. A similar pattern has been recorded in New Zealand. Four phases of endemism can be recognized: later Eocene, Oligocene, Miocene and Pliocene–Quaternary. It appears that the four phases were all stimulated in response to major marine transgressions, respectively the Wilson Bluff ( = Khirthar), Aldingan, Clifton–Longfordian and Hallet Cove–Glanville transgressions. Probably they signal four important stages in the transformation of water masses along the southern continental margin.

Ecostratigraphy and sequence biostratigraphy, with a neritic foraminiferal example from the Miocene in Southern Australia

The Miocene oscillation is a second order interruption of Cainozoic cooling and falling sea level with a time of warming and relatively high sea level at its zenith, the Miocene climatic optimum. The second order trajectories in putative global sea level, in oceanic δ 18O (a proxy for climate change) and δ 13C (a proxy for major change in nutrient regime), are punctuated by third order changes, including the Mi glacials. Southern‐temperate foraminiferal assemblages provide a profile of neritic biofacies through the oscillation. Plankton and benthos fit the second order and third order scenarios well: there are numerous resonances from the global ocean in this regional neritic realm. The cycle TB2 is a biofacies entity as well as a physical entity, and so too are most of its third order components. The influence of the Monterey carbon excursion and the climatic optimum are visible in this biotic succession. We corroborate the prediction that a trophic resource continuum expands in a warming and transgressi...

Miocene climatic oscillation recorded in the Lakes Entrance oil shaft, southern Australia; reappraisal of the planktonic foraminiferal record

The stratigraphic section at Lakes Entrance accumulated on a narrow platform in a neritic environment, close to the interaction of the East Australian Current and the West Wind Drift. The biostratigraphic succession of planktonic foraminiferal events first presented by D.G. Jenkins in 1960 has been slightly revised and correlated with the integrated Miocene geochronology. To extend biostratigraphy to ecostratigraphy, we have revised the systematics and nomenclature of the planktonic taxa and profiled the faunal succession in sixteen assemblages falling into three groups. 1, From the later middle Miocene to late Miocene occurred assemblages XI to XVI, typified by the collapse of woodi/bulloides ratio and resurgence of spinose species, as well as a comeback by the cancellate and globorotaliid forms. 2, Assemblages IX to X range from the latest early Miocene to early middle Miocene (upper N7 to N10O equivalents), with greatest amplitudes in fluctuation in species diversity and other metrics. 3, The early Miocene contained assemblages I to VIII, in which a rising woodi/bulloides ratio was accompanied by abundant microperforates but decline in spinose and cancellate species and in the planktonic/benthic ratio. At the second order or 107 years scale, the Exxon sealevel curve rises sporadically through the early Miocene and falls sporadically to its lowest level in the late Miocene, broadly congruent with the pelagic oxygen isotopes, which indicate an early Miocene rise and a major decline into the late middle Miocene. At the third order and 10 years scale, there may be promise of synchrony between the Mi glacial cycles and the marginal sequences. It is noteworthy that, at Lakes Entrance, there are about ten putatively global sequence boundaries and ten Mi glaciations spanning the time in which fifteen neritic faunal assemblages were recognized. Spikes in the woodi/bulloides ratio fit between the Mi glaciations and fall in the vicinity of maximum flooding surfaces. However, they fit lows, not highs, in planktonic diversity and planktonic/benthic ratios, suggesting intensified estuarine-type runoff as the control in this neritic setting. The perturbation in all measures increased in the 1.5 m.y. spanning assemblages IX and X straddling the early/middle Miocene boundary. At this time of peak warming and transgression, of stratification of pelagic water and planktonic communities, and bunching of third-order sequences, the biosphere was at its most sensitive and volatile and most responsive to perturbations (which led the big drops in climate and sealevel). INTRODUCTION In a paper published in Micropaleontology, D.G. Jenkins (1960) described 11 planktonic foraminiferal biozones from the Lakes Entrance oil shaft in east Gippsland, Victoria (Crespin, 1947). His pioneer work laid a foundation for mid-Tertiary foraminiferal biostratigraphy in the extratropical southern hemisphere, for several of the zones established at Lake Entrance were confirmed with some modification in New Zealand (Jenkins 1966, 1971), confirmed (again with some modification) in South Australia (Ludbrook and Lindsay 1969), and still are in use today (Jenkins 1985, 1993; Horibrook et al. 1989). In southern Australia, biostratigraphic correlations to this zonal scheme are discussed also in Wade (1964), McGowran et al. (1971), Abele et al. (1976), Heath and McGowran (1984), and Carter (1990). Why do we return to this neritic section 35 years later, when the focus of microplanktonic biostratigraphy has shifted long since into the deep ocean? Our major motive is the central position of the neritic record in the geohistory and the biohistory of southern Australia and the key position of the Lakes Entrance section in that record. Within the four allostratigraphic supersequences comprising the Cenozoic stratigraphic record in Australia (McGowran 1979a), the late Oligocene to late Miocene is represented by sequence three, a complete second-order cycle of transgression-regression in terms of sequence stratigraphy. A warming and subsequent cooling in global climate can be detected as an overall trend at 107 years scale, and the same seems to be true of global sealevel. At the next order down, we expect similar trends at 106 years scale, as shown by the match between warming episodes and the extratropical migrations of neritic larger foraminifera (McGowran 1979a, 1986). The planktonic foraminiferal record in the Lakes Entrance section is expected to reveal changes at these scales. The Gippsland Basin including the Lakes Entrance section faces the region of interaction between the Eastern Australian Current and the West Wind Drift (text-fig. 1). Planktonic foraminiferal communities would have advanced into and retreated from the neritic zone as the watermasses sloshed back and forth across the continental margin during times of climatic changes, and the local neritic record should reveal these changes. At the same time, however, immigrating faunas may have been filtered as they moved into less-than-optimal environments away from the open ocean (compare, e.g. Kennett 1985). Such matters characterize the ecostratigraphic dimension of biostratigraphy and they require detailed quantitative profiling of the entire faunal succession. This paper summarizes our findings and extends the discussion beyond a preliminary report on planktonic foraminifera (McGowran and Li 1993). A consideration of several interesting implications is deferred to a companion paper on the benthic assemblages at the same Lakes Entrance section (Li and McGowran 1997). micropaleontology, vol. 43, no. 2, pp. 129-148, text-figures 1-9, plates 1-3, tables 1-2, 1997 129 This content downloaded from 207.46.13.172 on Sat, 15 Oct 2016 04:43:12 UTC All use subject to http://about.jstor.org/terms Brian McGowran and Qianyu Li:Miocene climatic oscillation, Lakes Entrance oil shaft: reappraisal of the planktonic foraminiferal record TEXT-FIGURE 1 Locality map off eastern Australia showing present circulation (A) and the tectonic zones of the Gippsland Basin with Miocene channelling (B) (after Hocking et al. 1976 and Hegarty et al. 1986). THE SECTION AND DATA BASE The Lakes Entrance Oil Shaft was sunk in 1941-1945 about 1.6km northeast of Lakes Entrance. Crespin (1947) determined the upper part of the section as: Recent to Pleistocene (post-Kalimnan stage), 0-lOft, lower Pliocene (Kalimnan), 10-150ft, upper Miocene (Mitchellian), 150-208ft, middle Miocene (Bairnsdalian), 208-524ft. Thus the Miocene/Pliocene boundary was identified at about 150ft. Unfortunately, the top 200ft of the section was sampled at only two points. Jenkins (1960) study spanned the sampled section from 212ft to 1204ft, upper Miocene to upper Oligocene, a total thickness of 992ft (ca.302m), and so does ours. The section was deposited near the edge of the Lakes Entrance Platform in a shelf environment, and might have been influenced by Miocene channelling (text-fig. 1). The platform is a narrow tectonic unit between the highlands of Palaeozoic rocks that were uplifted in the Neogene and the Central Deep of the Gippsland Basin (Abele et al. 1976; Hegarty et al. 1986). The lower part of the sampled section consists of micaceous and glauconitic marls and silts assigned to the Metung Marl Member of the Lakes Entrance Formation. The overlying marls and limestones are part of a calcareous succession subject to somewhat complex lithostratigraphic nomenclature (Abele et al. 1976), but they can be assigned collectively to the Gippsland Limestone. In a broader context the strata record the major cycle of transgression-regression from late Oligocene to early Pliocene in the Gippsland Basin, and are classified as the Seaspray Group (Abele et al. 1976; Thompson 1986). All 228 samples were washed, dried and examined and 128 of them, or more than every second sample, were selected for a quantificative analysis of planktonic and benthic foraminifera. On average, over 500 specimens 63mm were picked and identified from each sample. As reported by Jenkins (1960), planktonic foraminifera are rich and diverse although the benthics usually dominate the assemblages. Planktonic profiles were based on about 15,000 individuals belonging to 65 species. A principal component factor analysis assembled several factors, but only the factor 1 for species groups seems relevant and is discussed here. All planktonic foraminiferal species are listed in the Appendix and most of them are illustrated in Plates 1-3, including some topotypes of the species described by Jenkins (1960). Data files recording the detailed distribution of individual species can be obtained from the Micropaleontology Press or the authors. BIOSTRATIGRAPHY AND JUSTIFICATION OF FAUNAL PROFILES We confirm most of Jenkins (1960) first and last appearance datums (text-fig. 2) which are correlated to Berggrens updated (in Berggren et al. 1995), integrated Miocene geochronology including the planktonic foraminiferal Mor N-zones (text-fig. 3). This does not mean that we recognized the N-zones; instead, we emphasize first appearance (FA) and last appearance (LA) datums, as advocated by Heath and McGowran (1984) and Lindsay (1985). Our correlations lack precision for two reasons: 1) there are few direct ties from this midlatitude site to the standard N-zones, and 2) we lack a geomagnetic record. The first appearance of Globoquadrina dehiscens is used to mark the boundary between subzones N4a and N4b, and thus our Oligocene/Miocene boundary (see also Heath and 130 This content downloaded from 207.46.13.172 on Sat, 15 Oct 2016 04:43:12 UTC All use subject to http://about.jstor.org/terms Micropaleontology, vol. 43, no. 2, 1997