A marine reservoir correction for the Houtman-Abrolhos Archipelago, East Indian Ocean, Western Australia

High-precision analysis using accelerator mass spectrometry (AMS) was performed upon known-age Holocene and modern, pre-bomb coral samples to generate a marine reservoir age correction value (ΔR) for the Houtman-Abrolhos Archipelago (28.7°S, 113.8°E) off the Western Australian coast. The mean ΔR value calculated for the Abrolhos Islands, 54 ± 30 yr (1σ) agrees well with regional ΔR values for Leeuwin Current source waters (N-NW Australia-Java) of 60 ± 38. The Abrolhos Islands show little variation with ΔR values of the northwestern and north Australian coast, underlining the dominance of the more equilibrated western Pacific-derived waters of the Leeuwin Current over local upwelling. The Abrolhos Islands ΔR values have remained stable over the last 2896 yr cal BP, being also attributed to the Leeuwin Current and the El Niño Southern Oscillation (ENSO) signal during this period. Expected future trends will be a strengthening of the teleconnection of the Abrolhos Islands to the climatic patterns of the equatorial Pacific via enhanced ENSO and global warming activity strengthening the Leeuwin Current. The possible effect upon the trend of future ΔR values may be to maintain similar values and an increase in stability. However, warming trends of global climate change may cause increasing dissimilarity of ΔR values due to the effects of increasing heat stress upon lower-latitude coral communities.

Stable oxygen isotope record from a high-latitude reef off Western Australia

A core from a coral colony of Porites lutea was analysed for stable oxygen isotopic composition*. A 200-year proxy record of sea surface temperatures from the Houtman Abrolhos Islands off west Australia was obtained from coral delta18O. At 29°S, the Houtman Abrolhos are the southernmost major reef complex of the Indian Ocean. They are located on the path of the Leeuwin Current, a southward flow of warm, tropical water, which is coupled to Indonesian throughflow. Coral delta18O primarily reflects local oceanographic and climatic variability, which is largely determined by spatial variability of the Leeuwin Current. However, coherence between coral delta18O and the current strength itself is relatively weak. Evolutionary spectral and singular spectrum analyses of coral delta18O demonstrate a high variability in spectral composition through time. Oscillations in the 5-7-y, 14-15-y, and quasi-biennial bands reflect teleconnections of local sea surface temperature (SST) to tropical Pacific climate variability. Deviations between local (coral-based) and regional (instrument) SST contain a cyclic component with a period of 15 y. Coral delta18O suggests a rise in SST by 0.6°C since AD 1944, consistent with available instrumental SST records. A long-term warming by 1.4°C since AD 1795 is inferred from the coral record.

Age, annual coral growth rates and stable isotopes of coral sample Ningaloo

116-year record of coral skeletal delta18O is presented from a colony of Porites lutea from Ningaloo Reef, western Australia. Interannual variability of sea-surface temperatures (SST) inferred from skeletal delta18O is dominated by a 9.5-year period, and may constitute a characteristic signal of the Leeuwin Current. On long-terms coral skeletal delta18O indicates a near-continuous increase of SST at Ningaloo Reef over one century. The skeletal delta18O time series was checked for the presence of seasonal cooling events resulting from major volcanic eruptions. An ~1 °C cooling is evident following the eruption of Pinatubo in 1991, which reproduces the results of previous investigations. However, only weak or no signals can be related to the eruptions of Krakatau (1883) and Agung (1963).

Response of deep-sea benthic foraminifera to Late Quaternary climate changes, southeast Indian Ocean, offshore Western Australia

The Late Quaternary benthic foraminifera of four deep-sea cores off Western Australia (ODP 122-760A, ODP 122-762B, BMR96GC21 and RC9-150) have been examined for evidence of increased surface productivity to explain the anomalously low sea-surface paleotemperatures inferred by planktic foraminifera for the last and penultimate glaciations. The delta13C trends of Cibicidoides wuellerstorfi, and differences between the delta13C trends of planktics (Globigerinoides sacculifer) and benthics (C. wuellerstorfi) in the four cores indicate that during stage 6 bottom waters were significantly depleted in delta13C, and strong delta13C gradients were established in the water column, while during stage 2 and the Last Glacial Maximum, delta13C trends did not differ greatly from that of the Holocene. Two main assemblages of benthic foraminifera were identified by principal component analyses: one dominated by Uvigerina peregrina, another dominated by U. proboscidea. Abundance of these Uvigerinids, and of taxa preferring an infaunal microhabitat, and of Epistominella exigua and Bulimina aculeata indicate that episodes of high influx of particulate organic matter were established in most sites during glacial episodes, and particularly so during stage 6, while evidence for upwelling during the Last Glacial Maximum is less strong. The Penultimate Glaciation upwellings were established within the areas of low sea-surface paleotemperature indicated by planktic foraminifera. During the Last Interglacial Climax, upwelling appears to have been established in an isolated region offshore from a strengthened Leeuwin Current off North West Cape. Last Glacial Maximum delta13C values of C. wuellerstorfi at waterdepths of less than 2000 m show smaller than global mean glacial-interglacial changes suggesting the development of a deep hydrological front. A similar vertical stratification/bathyal front was also established during the Penultimate Glaciation.

Organic-walled dinoflagellate cysts analysis and statistical analysis of 116 surface sediment samples from the eastern Indian Ocean

Assemblages of organic-walled dinoflagellate cysts (dinocysts) from 116 marine surface samples have been analysed to assess the relationship between the spatial distribution of dinocysts and modern local environmental conditions [e.g. sea surface temperature (SST), sea surface salinity (SSS), productivity] in the eastern Indian Ocean. Results from the percentage analysis and statistical methods such as multivariate ordination analysis and end-member modelling, indicate the existence of three distinct environmental and oceanographic regions in the study area. Region 1 is located in western and eastern Indonesia and controlled by high SSTs and a low nutrient content of the surface waters. The Indonesian Throughflow (ITF) region (Region 2) is dominated by heterotrophic dinocyst species reflecting the region's high productivity. Region 3 is encompassing the area offshore north-west and west Australia which is characterised by the water masses of the Leeuwin Current, a saline and nutrient depleted southward current featuring energetic eddies.

Grain-size distributions and bulk chemistry of sediment core MD00-2361

The terrigenous fraction of sediments from a deep-sea sediment core recovered from the northwestern Western Australian continental slope offshore North West Cape, SE Indian Ocean, reveals a history of Western Australian climate throughout the last 550 ka. End-member modelling of a data set of grain-size distributions (n = 438) of the terrigenous sediment fraction allows to interpret the record in terms of aeolian and fluvial sediment deposition, both related to palaeo-environmental conditions in the North West Cape area. The data set can be best described by two aeolian end members and one fluvial one. Changes in the ratio of the two aeolian end members over the fluvial one are interpreted as aridity variations in northwestern Western Australia. These grain-size data are compared with bulk geochemical data obtained by XRF scans of the core. Log-ratios of the elements Zr/Fe and Ti/Ca, which indicate a terrigenous origin, corroborate the grain-size data. We postulate that the mid- to late Quaternary near North West Cape climate was relatively arid during the glacial and relatively humid during the interglacial stages, owing to meridional shifts in the atmospheric circulation system. Opposite to published palaeo-environmental records from the same latitude (20°S) offshore Chile and offshore Namibia, the Australian aridity record does not show the typical southern hemisphere climate variability of humid glacials and dry interglacials, which we interpret to be the result of the relatively large land mass of the Australian continent, which emphasises a strong monsoonal climatic system.

Phytoplankton composition and biomass during leg 7 of Galathea 3 cruise across the southern Indian Ocean

Phytoplankton composition and biomass was investigated across the southern Indian Ocean. Phytoplankton composition was determined from pigment analysis with subsequent calculations of group contributions to total chlorophyll a (Chl a) using CHEMTAX and, in addition, by examination in the microscope. The different plankton communities detected reflected the different water masses along a transect from Cape Town, South Africa, to Broome, Australia. The first station was influenced by the Agulhas Current with a very deep mixed surface layer. Based on pigment analysis this station was dominated by haptophytes, pelagophytes, cyanobacteria, and prasinophytes. Sub-Antarctic waters of the Southern Ocean were encountered at the next station, where new nutrients were intruded to the surface layer and the total Chl a concentration reached high concentrations of 1.7 µg Chl a/L with increased proportions of diatoms and dinoflagellates. The third station was also influenced by Southern Ocean waters, but located in a transition area on the boundary to subtropical water. Prochlorophytes appeared in the samples and Chl a was low, i.e., 0.3 µg/L in the surface with prevalence of haptophytes, pelagophytes, and cyanobacteria. The next two stations were located in the subtropical gyre with little mixing and general oligotrophic conditions where prochlorophytes, haptophytes and pelagophytes dominated. The last two stations were located in tropical waters influenced by down-welling of the Leeuwin Current and particularly prochlorophytes dominated at these two stations, but also pelagophytes, haptophytes and cyanobacteria were abundant. Haptophytes Type 6 (sensu Zapata et al., 2004), most likely Emiliania huxleyi, and pelagophytes were the dominating eucaryotes in the southern Indian Ocean. Prochlorophytes dominated in the subtrophic and oligotrophic eastern Indian Ocean where Chl a was low, i.e., 0.043-0.086 µg total Chl a/L in the surface, and up to 0.4 µg Chl a/L at deep Chl a maximum. From the pigment analyses it was found that the dinoflagellates of unknown trophy enumerated in the microscope at the oligotrophic stations were possibly heterotrophic or mixotrophic. Presence of zeaxanthin containing heterotrophic bacteria may have increased the abundance of cyanobacteria determined by CHEMTAX.

Benthic foraminifera and stable isotope record of ODP Leg 182 sites

Cores from Sites 1129, 1131, and 1132 (Ocean Drilling Program (ODP) Leg 182) on the uppermost slope at the edge of the continental shelf in the Great Australian Bight reveal the existence of upper Pleistocene bryozoan reef mounds, previously only detected on seismic lines. Benthic foraminiferal oxygen isotope data for the last 450,000 years indicate that bryozoan reef mounds predominantly accumulated during periods of lower sea level and colder climate since stage 8 at Sites 1129 and 1132 and since stage 4 at the deeper Site 1131. During glacials and interstadials (stages 2-8) the combination of lowered sea level, increased upwelling, and absence of the Leeuwin Current probably led to an enhanced carbon flux at the seafloor that favored prolific bryozoan growth and mound formation at Site 1132. At Site 1129, higher temperatures and downwelling appear to have inhibited the full development of bryozoan mounds during stages 2-4. During that time, favorable hydrographic conditions for the growth of bryozoan mounds shifted downslope from Site 1129 to Site 1131. Superimposed on these glacial-interglacial fluctuations is a distinct long-term paleoceanographic change. Prior to stage 8, benthic foraminiferal assemblages indicate low carbon flux to the seafloor, and bryozoan mounds, although present closer inshore, did not accumulate significantly at Sites 1129 and 1132, even during glacials. Our results show that the interplay of sea level change (eustatic and local, linked to platform progradation), glacial-interglacial carbon flux fluctuations (linked to local hydrographic variations), and possibly long-term climatic change strongly influenced the evolution of the Great Australian Bight carbonate margin during the late Pleistocene.

Benthic foraminiferal abundances from eight sites of the Northwest Australian shelf

Foraminiferal analysis of Miocene to recent strata of the Northwest Shelf of Australia is used to chart West Pacific Warm Pool (WPWP) influence. The assemblage is typified by "larger" foraminifera with ingressions of the Indo-Pacific "smaller" taxa Asterorotalia and Pseudorotalia at around 4 Ma and from 1.6 to 0.8 Ma. A review of recent and fossil biogeography of these taxa suggests their stratigraphic distribution can be used to document WPWP evolution. From 10 to 4.4 Ma a lack of biogeographic connectivity between the Pacific and Indian Ocean suggests Indonesian Throughflow (ITF) restriction. During this period, the collision of Australia and Asia trapped warmer waters in the Pacific, creating a central WPWP biogeographic province from the equator to 26°N. By 3 Ma Indo-Pacific species migrated to Japan with the initiation of the "modern" Kuroshio Current coinciding with the intensification of the North Pacific Gyre and Northern Hemisphere ice sheet expansion. Indo-Pacific taxa migrated to the northwest Australia from 4.4 to 4 Ma possibly because of limited ITF. The absence of Indo-Pacific taxa in northwest Australia indicates possible ITF restriction from 4 to 1.6 Ma. Full northwest Australian biogeographic connectivity with the WPWP from 1.6 to 0.8 Ma suggests an unrestricted stronger ITF (compared to today) and the initiation of the modern Leeuwin Current. The extinction of some Indo-Pacific species in northwest Australia after 0.8 Ma may be related to the effects of large glacial/interglacial oscillations and uplift of the Indonesian Archipelago causing Indonesian seaway restriction.

Geochemical measurements of the Tasmanian Gateway from ODP Hole 189-1172A

Tectonic changes that produced a deep Tasmanian Gateway between Australia and Antarctica are widely invoked as the major mechanism for Antarctic cryosphere growth and Antarctic Circumpolar Current (ACC) development during the Eocene/Oligocene (E/O) transition (34-33 Ma). Ocean Drilling Program (ODP) Leg 189 recovered near-continuous marine sedimentary records across the E/O transition interval at four sites around Tasmania. These records are largely barren of calcareous microfossils but contain a rich record of siliceous- and organic-walled marine microfossils. In this study we integrate micropaleontological, sedimentological, geochemical, and paleomagnetic data from Site 1172 (East Tasman Plateau) to identify four distinct phases (A-D) in the E/O Tasmanian Gateway deepening that are correlative among ODP Leg 189 sites. Phase A, prior to 35.5 Ma: minor initial deepening characterized by a shallow marine prodeltaic setting with initial condensation episodes. Phase B, 35.5-33.5 Ma: increased deepening marked by the onset of major glauconitic deposition and inception of energetic bottom-water currents. Phase C, 33.5-30.2 Ma: further deepening to bathyal depths, with episodic erosion by increasingly energetic bottom-water currents. Phase D, <30.2 Ma: establishment of stable, open-ocean, warm-temperate, oligotrophic settings characterized by siliceous-carbonate ooze deposition. Our combined evidence indicates that this early Oligocene Tasmanian Gateway deepening initially produced an eastward flow of relatively warm surface waters from the Australo-Antarctic Gulf into the southwestern Pacific Ocean. This "proto-Leeuwin" current fundamentally differs from previous regional reconstructions of eastward flowing cool water (e.g., a "proto-ACC") during the early Oligocene and thereby represents an important new constraint for reconstructing regional- to global-scale dynamics for this major global change event.

Neohornibrookella sorrentae (Chapman and Crespin, 1928) and allied ostracod taxa from the Neogene of southeastern Australia: systematic and palaeoceanographical relationships, palaeoecology and palaeobiogeography

Three closely allied shallow marine taxa, Neohornibrookella sorrentae (Chapman and Crespin), Neohornibrookella glyphica (Neil), and Neohornibrookella nepeani sp. nov. are recorded from latest early Miocene to late Pliocene strata in southeastern Australia. These taxa, together with Neohornibrookella quadranodosa (Holden) from the Miocene of Midway Island (Northwestern Hawaiian Islands), form a morphologically distinct group of relatively large species (the sorrentae-group) within the genus Neohornibrookella Jellinek. Latitudinal expansion of the subtropical and warm-temperate climatic belts together with the influence of warm western boundary surface currents associated with the North and South Pacific gyres, are likely to have played key roles in the Miocene dispersal of this species group. Species of the sorrentae-group first migrated south from equatorial west Pacific regions into southeastern Australia during the early Miocene, under the influence of the East Australian Current. During three time intervals (i) latest early Miocene, (ii) latest late Miocene and (iii) earliest late Pliocene, forceful pulses of the East Australian Current played a significant role in propelling the widespread distribution of thermophilic Neohornibrookella species across southeast Australian shallow marine realms. During intervening middle and late Miocene times, Neohornibrookella species are only sporadically present across the Bass Strait region of southeast Australia, indicating a weaker East Australian Current influence and the cooling influence of coastal upwelling. During the mid early Pliocene Neohornibrookella species disappeared from the western Bass Strait region, suggesting the complete exclusion of East Australian Current waters from this region. This was probably due to the counteracting influence of the eastward flowing Zeehan Current (extension of the Leeuwin Current) impinging on the western Bass Strait region. This mid early Pliocene palaeobiogeographical partition in Bass Strait, defined by the distribution of sorrentae-group species, is here termed the Bassian Gateway. The two species, N. sorrentae and N. glyphica, occur concurrently during the mid Miocene in southeast Australia, but are associated with different lithofacies. It is hypothesised that there is a heterochronic evolutionary relationship expressed in the ornament of these two species. The thaerocytherid genera Neohornibrookella Jellinek, Tenedocythere Sissingh and Bosasella Bonaduce are here included in the new ostracod subfamily Tenedocytherinae.

Composition of Cape Leeuwin Manganese nodule deposit off Western Australia

In 1970 a large deposit of ferromanganese nodules was discovered on the floor of the Indian Ocean southwest of Cape Leeuwin by the research vessel USNS Eltanin. This discovery, which was based largely on bottom photographs from about 20 stations, was discussed by Frakes (1975) and Kennett and Watkins (1975, 1976). The photographs suggest that the deposit spreads, nearly continuously, over 900 000km^2, and cores showed that the nodules are essentially confined to the sediment surface. Kennett and Watkins (op. cit.) pointed to the abundance of ripple and scour marks and current-formed lineations on the present surface, and of extensive disconformities in the cores, as evidence of strong present and past bottom currents in the region. They suggested that the current action had resulted in very low sedimentation rates, which had allowed the nodule field, named by them (1976) the 'Southeast Indian Ocean Manganese Pavement', to develop. In early 1976 the authors used the research vessel HMAS Diamantina for a 10-day cruise in the region to sample the nodules in order to study their chemistry and mineralogy. During the cruise 9 stations were occupied, 8 of them successfully (Figure 1), and about 2000 nodules were recovered from the sea bed. The apparatus used was a light box dredge on the ships hydrowire, which had a breaking strain of about one tonne. Although an attempt was made to reoccupy Eltanin photographic stations, it should be noted that positioning was by celestial navigation, so errors of up to 10 km are possible.