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Benthic foraminifera from the Carnarvon Ramp reveal variability in Leeuwin Current activity (Western Australia) since the Pliocene
Abstract
Benthic foraminiferal assemblages from a ~300 m deep core from an outer carbonate-ramp site off Western Australia (International Ocean Discovery Program Core U1460A) were examined to reconstruct the paleoceanographic evolution of the Carnarvon Ramp and the warm surficial Leeuwin Current (LC) for the last 3.54 Ma. Of the identified 179 benthic foraminiferal species, occurrences of the 15 most abundant taxa were assessed using Q-mode Cluster Analysis and Non-Metric Dimensional Scaling. Diversity, equitability, planktonic/benthic index, microhabitat preference, and sedimentary parameters such as lithology and sponge spicule content were analyzed to gather information about past intermediate- and surface-water circulation. Relative abundances of infaunal and epifaunal species were applied to indicate changes in organic-matter supply and oxygenation at the sea floor. Influence of upwelling was recognized by a high infaunal species ratio, with dominance by Uvigerina peregrina, Lagena annellatrachia and Trifarina bradyi. Epifaunal species such as Hanzawaia nipponica and Hyalinea florenceae gradually became more abundant around 1.14 Ma, indicating increased ventilation and establishment of the paleo-LC. A more substantial change was initiated by 0.91 Ma as marked by key species Spirorutilus carinatus and Rotorbinella sp., together with increased faunal diversity, benthic foraminiferal accumulation rates, and evidence for suspension feeding sponges. With the LC flow suppressing upwelling, and better ventilated waters entering the shelf, the environment favored epifaunal agglutinates, rotalids, and miliolids, while buliminids decreased. Under high-flow conditions of the LC, sponge spicules and skeletal carbonate production reached an optimum at ~0.6 Ma before returning to modern conditions. Supported by these observations, we propose the following paleoceanographic evolution of the Carnarvon Ramp: During the late Pliocene to mid Pleistocene (3.54–0.91 Ma) conditions of deep-water upwelling from the Western Australian Current and Indian Ocean Gyre indicate the absence of the capping LC on the outer carbonate ramp. A transitional phase started in the mid Pleistocene (1.14–0.61 Ma). The paleo-LC triggered gradual oxygenation at the sediment-water interface, which coincided with an increase in carbonate sedimentation rates, and waning sea-surface productivity. During a third phase, mid Pleistocene to present (0.91–0 Ma), the LC's intensity and flow rates peaked at ~0.6 Ma. Benthic foraminiferal accumulation rates reached a high, then decreased to present-day rates. For short periods, sea-surface productivity was moderately enhanced, likely due to fluctuating LC persistence or landward shift during glacial maxima.
... Note the close correlation between data from Hole A and B; b) abundance of detrivore Uvigerina spp. and facultative anaerobe Trifarina bradyi from U1460 benthic foraminifera abundance data of Haller et al. (2018), with a clear cross-over in the two abundance during the 900-ka event (Mis 24e22); c) inverted Uvigerina peregrina Mg/Ca ratio derived intermediate water temperatures (IWT, in C) from DSDP Site 593 (see Fig. 1). Note the IWT drop and concomitant spicule increase in MIS 20e19; d) global sea level from ODP Site 1123 (Elderfield et al., 2012), note the inception of large (>45 m below present marked by the red shaded areas) sea level drops during MIS-24; e) Lower central deep-water U. peregrina derived d 13 C records from ODP Site 1123 (Elderfield et al., 2012). ...
... The bottom water community changes at Site U1460 include changes in siliceous sponge spicule accumulation (Gallagher et al., 2017, Fig. 7a) and shifts in benthic foraminifer assemblages (Haller et al., 2018). Both serve as proxies for LUC conditions and indicate that significant changes in bottom water conditions occurred over the MPT at Site U1460 (Fig. 7). ...
... Both serve as proxies for LUC conditions and indicate that significant changes in bottom water conditions occurred over the MPT at Site U1460 (Fig. 7). In particular, we note a shift in benthic foraminifer assemblages during the MPT that Haller et al. (2018) had already observed (Fig. 7b). The authors interpreted this change in the foraminifer community as a decrease in overall nutrient flux after the MPT at Site U1460. ...
The Middle Pleistocene Transition (MPT) represents a major change in Earth's climate state, exemplified by the switch from obliquity-dominated to ∼100-kyr glacial/interglacial cycles. To date, the causes of this significant change in Earth's climatic response to orbital forcing are not fully understood. Nonetheless, this transition represents an intrinsic shift in Earth's response to orbital forcing, without fundamental changes in the astronomical rhythms. This study presents new high-resolution records of International Ocean Discovery Program (IODP) Site U1460 (eastern Indian Ocean, 27°S), including shallow marine productivity and organic matter flux reconstructions. The proxy series covers the interval between 1.1 and 0.6 Ma and provides insights into Pleistocene Leeuwin Current dynamics along the West Australian shelf. The large >45 m global sea level drop during the marine isotope stage (MIS) 22–24 is marked in our data, suggesting that the MPT led to large-scale changes in Indian Ocean circulation patterns and surface water conditions. We consider shelf exposure (and thus the “Sahul-Indian Ocean Bjerknes mechanism”) as a possible key process to increase the upwelling of nutrient-rich sub-Antarctic Mode waters through the Leeuwin Undercurrent along the Australian shelf. We conclude that the shoaling of nutrient-rich lower-thermocline waters enhanced mid-latitude productivity patterns in the eastern Indian Ocean across the 900-ka event.
... The LDX, is conventionally used to assess the depths of the aragonite lysocline as a function of oceanic water masses (Gerhardt & Henrich, 2001). However, aragonite dissolution by bottom-waters is highly unlikely at IODP Site U1460, which is situated far above the regional aragonite lysocline (Sabine et al., 2002) and likely was never in significantly deeper water throughout the studied time interval (Haller et al., 2018). The LDX at IODP Site U1460 can therefore be used to assess post-depositional dissolution within the sediment. ...
... The low sulphate reduction potential could result from the low availability of total organic matter at this site, with overall low alkenone concentrations and TOC ranging only between 0.1% and 0.4% (Fig. 3). These low alkenone and TOC values likely result from the generally oligotrophic conditions at IODP Site U1460 during the last ca 0.6 Myr, as indicated by benthic foraminifera assemblages (Haller et al., 2018). ...
Aragonite and high‐Mg calcite are abundant in modern, neritic carbonate systems but almost absent in their fossil counterparts. Dissolution of these metastable mineral phases commonly leaves no visible trace in the sedimentary record, compromising the derivation of palaeoenvironmental information from the rock record. The upper 25 m of Integrated Ocean Drilling Program (IODP) Site U1460 on the outer ramp of the western Australian Shelf were investigated to study shallow burial (tens of metres) marine diagenesis in organic‐carbon poor sediments using microscopic, total organic carbon, biomarkers and mineralogical analysis in combination with porewater geochemistry. Aragonite dissolution is negligible at the seafloor but intensifies ca 5 m below, even though bulk porewaters are supersaturated for aragonite. This apparent contradiction likely results from dissolution in undersaturated microenvironments. Aragonite dissolution below 5 to 6 m is on average more intense in interglacial compared to glacial intervals. The presence of disseminated framboidal pyrite and porewater results indicate that minor sulphate reduction is active at IODP Site U1460. Sulphate reduction is probably limited by the low organic matter content (ca 0.2%). It is well‐known from the literature that incipient sulphate reduction can lead to a drop in pH and consequently to carbonate dissolution. It is therefore assumed that the slightly higher concentration of organic matter in the interglacial intervals allowed increased aragonite dissolution during sulphate reduction compared to glacial beds. Low amounts of dolomite cement (<15%) start to form at the same depth (5 to 6 m) as aragonite dissolution intensifies. Dolomite formation and aragonite dissolution also show covariance on a metre‐scale below 5 to 6 m, indicating that a low carbonate saturation state might enhance dolomite formation. This mechanism provides an indirect link between dolomite formation, aragonite dissolution and orbital cycles. The outcome of this study, therefore, contributes to a better understanding of differential diagenesis in marine carbonates.
... Bolivina robusta (Haller et al., 2018;Kaiho, 1994;Zhao et al., 2018) Deep infaunal Chilostomella oolina (Kaiho, 1994;McGann & Conrad, 2018;Wang et al., 2018) Infaunal Fursenkoina rotundata (Das et al., 2017;Kaiho, 1994;Patarroyo & Martínez, 2015) Deep faunal species (Bubenshchikova et al., 2010;Jorissen et al., 1998). Gyroidinoides cibaoensis is found in a wider range of oxygen conditions with variable food availability (De & Gupta, 2010). ...
... Cassidulina laevigata, is a shallow infaunal taxon (Fontanier et al., 2002), typifying relatively cold waters with high organic matter content (Pascual et al., 2020), and is tolerant of moderate oxygen depletion in the bottom and pore water under high organic flux rates Nardelli et al., 2014;Sen Gupta & Machain-Castillo, 1993). Bolivina robusta is an open ocean infaunal species found in temperate to the subtropical middle to upper bathyal depths (Haller et al., 2018;B. Zhao et al., 2018). ...
The East China Sea (ECS) seasonally receives a high organic input due to the terrestrial organic matter influx, which is controlled by the East Asian Summer Monsoon (EASM), and the increased productivity driven by upwelling of the subsurface Kuroshio Current (KC). Changes in benthic foraminiferal assemblage composition in combination with paleoceanographic proxy data (CaCO3 (%), TOC (%), δ¹³Cpf, and δ¹⁸Obf) are used to reconstruct bottom water oxygenation and organic export flux variability over the last 400 kyr in the ECS. Multivariate analyses of benthic foraminiferal census data identified six biofacies characteristic of varying environmental conditions. These results suggest that enhanced EASM precipitation and KC upwelling directly influenced organic export flux and bottom water oxygen content in the ECS. The ECS bottom water was suboxic during Marine Isotope Stage (MIS) 11 to 8; suboxic to dysoxic between MIS 7 and 6, strongly dysoxic between mid‐MIS 5 and 4, and exhibited high variability between MIS 3 and 1. Spectral analysis of relative abundances of representative genera Quinqueloculina (oxic), Bulimina (suboxic), and Globobulimina (dysoxic) reveals a robust 23 kyr signal, which we attribute to precessionally‐paced changes in surface productivity and bottom water oxygenation related to KC variability over the past 400 kyr.
... Bolivina robusta (Haller et al., 2018;Kaiho, 1994;Zhao et al., 2018) Deep infaunal Chilostomella oolina (Kaiho, 1994;McGann & Conrad, 2018;Wang et al., 2018) Infaunal Fursenkoina rotundata (Das et al., 2017;Kaiho, 1994;Patarroyo & Martínez, 2015) Deep faunal species (Bubenshchikova et al., 2010;Jorissen et al., 1998). Gyroidinoides cibaoensis is found in a wider range of oxygen conditions with variable food availability (De & Gupta, 2010). ...
... Cassidulina laevigata, is a shallow infaunal taxon (Fontanier et al., 2002), typifying relatively cold waters with high organic matter content (Pascual et al., 2020), and is tolerant of moderate oxygen depletion in the bottom and pore water under high organic flux rates Nardelli et al., 2014;Sen Gupta & Machain-Castillo, 1993). Bolivina robusta is an open ocean infaunal species found in temperate to the subtropical middle to upper bathyal depths (Haller et al., 2018;B. Zhao et al., 2018). ...
... Water circulation along the Rowley Shelf is influenced by shallow currents that originate in the West Pacific Warm Pool, including the shallow (i.e., <300 m deep) and warm, low salinity Leeuwin Current, which carries warm waters southward along the coast of Western Australia and suppresses coastal upwellings (Gallagher et al., 2018;Haller et al., 2018;Hatcher et al., 1991;Holloway, 1995;James et al., 2004;Stephen et al., 2009). Water circulation is also influenced by tide, with the tidal range reaching up to 6 m along the coast and generating strong tide-induced currents in the cross-shelf direction, which in turn generate internal tides (Condie and Andrewartha, 2008;Holloway, 2001;Katsumata, 2006). ...
Palaeoshorelines and associated palaeo-coastal features are studied to reconstruct past sea level, climate, and depositional environments. Their identification typically depends on direct field observations and is therefore challenging in marine environment, where the interpretation mostly relies on sparse geophysical data. This review presents, based on 118 published case studies, a summary of morphological evidences that can be used to identify submerged relict coastal features worldwide, using only geophysical data. Four coastal feature categories that can be used as palaeoshoreline indicators were identified: (1) beach ridges of wind and wave origin; (2) shoreface strata; (3) marine terraces; and (4) coral-reef terraces.
In light of this proposed classification, an area of ~200,000 km² was investigated along the Rowley Shelf (North West Shelf, Australia), a carbonate-dominated platform, based on the integration of high-resolution bathymetry (i.e., seismic-derived bathymetry, satellite-derived bathymetry, multibeam echosounder bathymetry, spot depth soundings) and 2D reflection seismic lines. Relict features were discriminated from modern bedforms using five criteria: (1) stratigraphic position; (2) emersion features; (3) similarity with modern and published analogues; (4) integration of modern ocean conditions; and (5) evidence of early cementation. In total, over 500 submerged relict coastal features were identified, making this review the most comprehensive catalogue published to date.
Relict features are concentrated over specific depths, referred to as modal sea-level depths (MSLDs), which correspond to depths where the relative sea level remained stable over long periods of time. Nine MSLDs are observed at 20, 35, 50, 60, 70, 80, 90, 105 and 125 m below sea level. Each MSLD is the result of the accumulation of coastal features through multiple glacial/ interglacial cycles. Most of the features may nevertheless be related to the last glacial sea-level fall and were likely formed between Marine Isotopes Stages (MIS) 5 and 2.
The analysis of the submerged coastal features indicates that the overall shelf morphology is controlled by the repartition of these features, and that, while in a carbonate province, their formation is related to wind, tide, fluvial and wave processes. The higher concentration of fluvial relict features at shallower depths and of tide-influenced relict features at greater depths suggest that fluvial runoffs were limited during glacial periods. This, in turn, supports the hypothesis of a prevalent dry climate during glacial periods and in contrast, of a humid climate during interglacial periods. Finally, the study reveals that most of modern coral reefs of the Rowley Shelf are growing on top of relict coastal features and that seabed ridges previously interpreted as drowned coral reefs are, in fact, likely to be relict coastal features formed through clastic processes.
Results from this study will support the identification of submerged palaeoshorelines on continental shelves around the globe and highlight the influence of associated relict coastal features on shelf morphologies. Additionally, this study provides new insights on processes shaping carbonate provinces.
... The other current, the WAC (Figure 1a), is a cold, high-salinity, nutrient-rich current (Spooner, Deckker, Barrows, & Fifield, 2011), which influences water masses as deep as 2,000 m (Tchernia, 1980). The Leeuwin Undercurrent (LU; Haller, Hallock, Hine, & Smith, 2018), which forms part of the WAC, is a high velocity current (0.32-0.4 m/s) with its core at a depth of 400 m (Figure 1a; Woo & Pattiaratchi, 2008). The LU is interpreted as a prolongation of Flinders Current that flows along southern margin of Australia (Woo & Pattiaratchi, 2008). ...
Deep‐marine deposits provide a valuable archive of process interactions between sediment gravity flows, pelagic sedimentation, and thermo‐haline bottom‐currents. Stratigraphic successions can also record plate‐scale tectonic processes (e.g. continental breakup and shortening) that impact long‐term ocean circulation patterns, including changes in climate and biodiversity. One such setting is the Exmouth Plateau, offshore NW Australia, which has been a relatively stable, fine‐grained carbonate‐dominated continental margin from the Late Cretaceous to Present. We combine extensive 2D (~40,000 km) and 3D (3,627 km²) seismic reflection data with lithologic and biostratigraphic information from wells to reconstruct the tectonic and oceanographic evolution of this margin. We identified three large‐scale seismic units (SUs): (1) SU‐1 (Late Cretaceous) – 500 m‐thick, and characterised by NE‐SW‐trending, slope‐normal elongate depocentres (c. 200 km long and 70 km wide), with erosional surfaces at their bases and tops, which are interpreted as the result of contour‐parallel bottom‐currents, coeval with the onset of opening of the Southern Ocean; (2) SU‐2 (Palaeocene – Late Miocene) – 800 m‐thick and characterised by: (i) very large (amplitude, c. 40 m and wavelength, c. 3 km), SW‐migrating, NW‐SE‐trending sediment waves, (ii) large (4 km‐wide, 100 m‐deep), NE‐trending scours that flank the sediment waves, and (iii) NW‐trending, 4 km wide and 80 m deep turbidite channel, infilled by NE‐dipping reflectors, which together may reflect an intensification of NE‐flowing bottom currents during a relative sea‐level fall following the establishment of circumpolar‐ocean current around Antarctica; and (3) SU‐3 (Late Miocene – Present) – 1000 m‐thick and is dominated by large (up to 100 km³) mass‐transport complexes (MTCs) derived from the continental margin (to the east) and the Exmouth Plateau Arch (to the west), and accumulated mainly in the adjacent Kangaroo Syncline. This change in depositional style may be linked to tectonically‐induced seabed tilting and folding caused by collision and subduction along the northern margin of the Australian plate. Hence, the stratigraphic record of the Exmouth Plateau provides a rich archive of plate‐scale regional geological events occurring along the distant southern (2000 km away) and northern (1500 km away) margins of the Australian plate.
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Late Miocene to mid-Pleistocene sedimentary proxy records reveal that northwest Australia underwent an abrupt transition from dry to humid climate conditions at 5.5 million years (Ma), likely receiving year-round rainfall, but after ~3.3 Ma, climate shifted towards an increasingly seasonal precipitation regime. The progressive constriction of the Indonesian Throughflow likely decreased continental humidity and transferred control of northwest Australian climate from the Pacific to the Indian Ocean, leading to drier conditions punctuated by monsoonal precipitation. The northwest dust pathway and fully established seasonal and orbitally controlled precipitation were in place by ~2.4 Ma, well after the intensification of northern hemisphere glaciation. The transition from humid to arid conditions was driven by changes in Pacific and Indian Ocean circulation and regional atmospheric moisture transport, influenced by the emerging Maritime Continent. We conclude that the Maritime Continent is the switchboard modulating teleconnections between tropical and high-latitude climate systems.
Tectonically induced changes in oceanic seaways had profound effects on global and regional climate during the Late Neogene. The constriction of the Central American Seaway reached a critical threshold during the early Pliocene ~4.8–4 million years (Ma) ago. Model simulations indicate the strengthening of the Atlantic Meridional Overturning Circulation (AMOC) with a signature warming response in the Northern Hemisphere and cooling in the Southern Hemisphere. Subsequently, between ~4–3 Ma, the constriction of the Indonesian Seaway impacted regional climate and might have accelerated the Northern Hemisphere Glaciation. We here present Pliocene Atlantic interhemispheric sea surface temperature and salinity gradients (deduced from foraminiferal Mg/Ca and stable oxygen isotopes, δ¹⁸O) in combination with a recently published benthic stable carbon isotope (δ¹³C) record from the southernmost extent of North Atlantic Deep Water to reconstruct gateway-related changes in the AMOC mode. After an early reduction of the AMOC at ~5.3 Ma, we show in agreement with model simulations of the impacts of Central American Seaway closure a strengthened AMOC with a global climate signature. During ~3.8–3 Ma, we suggest a weakening of the AMOC in line with the global cooling trend, with possible contributions from the constriction of the Indonesian Seaway.
Sediment dynamics exert large control over coral reef geomorphological evolution and are vital to understanding past and present geomorphic responses. Large benthic foraminifera (LBF) live in the algal reef flats, and their tests (shells) are transported post-mortem by waves and currents onto back-reef environments, including sand aprons. This study investigated the patterns of transport linking surficial and downcore sediments in samples from three sand aprons with different wave exposures at One Tree Reef on the southern Great Barrier Reef (Australia). Six LBF genera represented up to 32% of the sediments analysed. Lagoonward transport increased LBF test abrasion and sediment bulk density. Sediment grain size and LBF abundance in sediments also decreased with lagoonward transport. Sediment transport patterns indicated by LBF species used as tracer were consistent with the prominent E-SE wave environment. A novel taphofacies approach was used to describe stratigraphic layers in downcore sediments based on LBF test abrasion and abundance. Varied sediment deposition rates did not affect the LBF test abrasion signature downcore. It appears that Baculogypsina sphaerulata has been the dominant species for at least 3 ka. Tests that were deposited slowly exhibited less or the same levels of abrasion than those that were rapidly deposited. It appears that test abrasion is primarily determined by the distance travelled rather than the influence of increased age or chemical dissolution.
Invasion of amphisteginid foraminifera in the Adriatic Sea
Martin R. Langer . Gloria H. Mouanga
Biological Invasions
DOI 10.1007/s10530-016-1070-0
ABSTRACT: Amphisteginid foraminifera are larger
symbiont-bearing foraminifera that are abundant in
tropical and subtropical reef and shelf regions of the
world’s oceans. There is now unequivocal evidence
that climate change has led to an expansion of tropical
belts. As temperatures rise, surface isotherms are
shifting poleward and habitat ranges are moving
towards higher latitudes. Fueled by substantial warming,
the Mediterranean has been identified as one of
the most severely affected areas. This study documents
for the first time that amphisteginid foraminifera
have now crossed the Strait of Otranto and
invaded the eastern Adriatic coast along southern
Albania. Sampling on the opposite side along the
southern part of the Italian boot shows, however, that
amphisteginid foraminifera have not yet colonized the
western coast of the Adriatic. The asymmetric invasion
of the Adriatic displays spatial heterogeneity,
where the progressive colonization follows the major
surface currents with a northerly flow along the eastern
coast and a southerly return along the western
coastline. Previous sampling along Ionian Sea sample
sites provides a baseline chronology allowing computations
of range expansion rates. The observed recent
rate of range expansion in Amphistegina is computed
between 4.0 and 10 km/year-1 and provides strong
support for current species model projections. Based
on the new data, the range boundary shift is projected
to lead to a total northward range expansion of 5.2�
latitude in the year 2100. The latitudinal range
extension computed for the Mediterranean is thus
almost twice as large as those computed for the
southern hemisphere. The ongoing range extension
into new areas of the Mediterranean Sea is shown to
trigger changes in community structures with potential
consequences for ecosystem functioning. Numerical
abundances and impacts of invasive amphisteginids
are greatest at shallow depths\20 m. Evidence is now
accumulating that the invasion of amphisteginid
foraminifera results in (1) a loss of benthic foraminiferal
biodiversity, (2) alterations of foraminiferal
community structures and (3) shifts in abundances of
functional groups of foraminifera.
3D exploration seismic data were interpreted to investigate the locations and characteristics of submarine slope failures along the continental slope in the offshore Carnarvon Basin on Australia's North West Shelf. Seisnetics™, a patented genetic algorithm was used to process the 3D seismic data to extract virtually all peak and trough surfaces in an unbiased and automated manner. The extracted surfaces were combined in a 3D visual database to develop a seafloor digital terrain model that extends from the continental slope to the Exmouth Plateau. The 3D data were used to map the subsurface extent and geometry of landslide failure planes, as well as to estimate the thickness and volumes of slide deposits. This paper describes the geomorphic characteristics of six of the survey areas. Geomorphic mapping shows the presence of slope failures ranging from small (<3 km across) to moderate (<10 km across) scale debris flows, rotational block failures, translational slides and topple failures, as well as large scale (>20 km across) mass transport complexes (MTC). The features are associated with debris flow chutes, turbidity flow channels, and debris fields. Analysis of failure planes show prominent grooves or striations related to the mobilization of slide material down both the continental slope and Exmouth Plateau and into the Kangaroo Syncline. Submarine slope failures can occur at the continental shelf break in approximately 200 m to 300 m of water and run out to the Exmouth Plateau surface in approximately 1,100 m to 1,400 m water depths. The largest individual slides in the survey areas have widths of >30 km and minimum run-out lengths of 75 km, though associated turbidity flow deposits likely extend much further. The subsurface expression of the large MTCs illustrates a history of sediment accumulation along the mid-slope followed by repeated slope failure and debris run-out. Sediment accumulation and slope failure processes are actively occurring along the continental slope and submarine landslides thus are a major driver of hazard to subsea infrastructure development. Smaller slides seem to occur more frequently than large slides and thus may pose a greater hazard to subsea infrastructure than large infrequent MTCs.
In this study, the annual and interannual variations of the Leeuwin Current at 32°S off the Western Australian coast are investigated. The mean annual cycle and the El Niño and La Niña composites of the Leeuwin Current temperature structures are obtained by linearly fitting a Taylor expansion to historical upper ocean temperature data. A temperature-salinity relationship is used to derive the salinity field and geostrophy is assumed to calculate the current velocity. The downward tilting of the isotherms toward the coast and the strength of a near-surface core of the low-salinity water indicate the seasonal variation of the Leeuwin Current. Seasonally, the Leeuwin Current has the maximum poleward geostrophic transport of 5 Sv (106 M3 s-1) during June-July. Interannually, the Leeuwin Current is distinctly stronger during a La Niña year and weaker during an El Niño year. The annual average poleward geostrophic transports in the mean, the El Niño and La Niña years are 3.4, 3.0, and 4.2 Sv respectively. Variations of the Leeuwin Current structure on annual and interannual timescales are coastally trapped. A linear relationship between the coastal sea level deviation at Fremantle and the Leeuwin Current transport is derived, which justifies and calibrates the usage of the Fremantle sea level as an index for the strength of the Leeuwin Current.
The paper summarises a structural analysis of the NNEtrending
Gorgon Platform, which hosts the Gorgon Gas Field,
using the Triton 3D seismic survey. Located approximately
130 km offshore in water depths of 100-200 m the Gorgon
Platform forms the southern termination of the Rankin Trend
in the Northern Carnarvon Basin, North West Shelf Australia.
Structural analysis identified four populations of
extensional faults that constrain the evolution of the Gorgon
Platform; (1) uppermost Triassic–Cretaceous NNE-trending
extensional syn-rift faults, (2) Upper Jurassic–lowermost-
Cretaceous WNW-trending extensional syn-rift faults, (3)
Cretaceous polygonal post-rift fault tiers, and (4) Neogene
faults attributed to gravity driven scarp collapse and masstransport
complexes. Seismic attribute and fault displacement/
orientation analyses showed that initially segmented fault
systems formed and were subsequently connected laterally and
vertically by both soft- and hard-linkage relay structures.
Changes in the orientations of principle stresses and/or
reactivation of an antecedent structural fabric may account
for the uppermost Triassic to Cretaceous NNE-trending faults
and perpendicular Upper Jurassic to lowermost Cretaceous
WNW-trending faults. This Triassic to Cretaceous synrift
fault system influences the location and orientation of
overlying Upper Cretaceous to Neogene post-rift polygonal
faults and mass-transport-related deformation.
This study illustrates the progressive evolution of the
Gorgon Platform with implications for understanding the
distribution, segmentation, linkages and ages of extensional
faults within the Northern Carnarvon Basin. Fault analyses
demonstrate that extension oblique to pre-existing uppermost
Triassic to Cretaceous NNE-trending faults was partitioned
into optimally oriented extensional fault populations and that
a soft vertical linkage exists between syn-rift extensional fault
systems and the overlying post-rift polygonal faults as well as
to faults attributed to mass-transport processes.
Fossils of marine microorganisms such as planktic foraminifera are among the cornerstones
of palaeoclimatological studies. It is often assumed that the proxies derived from their shells
represent ocean conditions above the location where they were deposited. Planktic foraminifera,
however, are carried by ocean currents and, depending on the life traits of the
species, potentially incorporate distant ocean conditions. Here we use high-resolution ocean
models to assess the footprint of planktic foraminifera and validate our method with proxy
analyses from two locations. Results show that foraminifera, and thus recorded palaeoclimatic
conditions, may originate from areas up to several thousands of kilometres away,
reflecting an ocean state significantly different from the core site. In the eastern equatorial
regions and the western boundary current extensions, the offset may reach 1.5 �C for species
living for a month and 3.0 �C for longer-living species. Oceanic transport hence appears to be
a crucial aspect in the interpretation of proxy signals.
The areal extent of the Maritime Continent (the islands of Indonesia and surrounding region) has grown larger by ~60% since 5 Ma. We argue that this growth might have altered global climate in two ways that would have contributed to making recurring ice ages possible. First, because rainfall over the islands of the Maritime Continent not only is heavier than that over the adjacent ocean, but also correlates with the strength of the Walker Circulation, the growth of the Maritime Continent since 5 Ma may have contributed to the cooling of the eastern tropical Pacific since that time. Scaling relationships between the strength of the Walker Circulation and rainfall over the islands of the Maritime Continent and between sea-surface temperature (SST) of the eastern tropical Pacific and the strength of easterly wind stress suggest that the increase in areal extent of islands would lead to a drop in that SST of 0.75°C. Although only a fraction of the 3-4°C decrease in SSTs between the eastern and western tropical Pacific, the growth of the Maritime Continent may have strengthened the Walker Circulation, increased the east-west temperature gradient across the Pacific, and thereby enabled ice sheets to wax and wane over Canada since 3 Ma. Second, because the weathering of basaltic rock under warm, moist conditions extracts CO2 from the atmosphere more rapidly than weathering of other rock or of basalt under cooler or drier conditions, the increase in weathering due to increasing area of basalt in the Maritime Continent may have drawn down enough CO2 from the atmosphere to affect global temperatures. Simple calculations suggest that increased weathering of basalt might have lowered global temperatures by 0.25°C, possibly important for the overall cooling.
Piston cores collected from IODP drilling platforms (and its predecessors) provide the best long-term geological and climatic record of marine sediments worldwide. Coring disturbances affecting the original sediment texture have been recognized since the early days of coring, and include deformation resulting from shear of sediment against the core barrel, basal flow-in due to partial stroke, loss of stratigraphy, fall-in, sediment loss through core catchers, and structures formed during core recovery and on-deck transport. The most severe disturbances occur in non-cohesive (sandy) facies, which are particularly common in volcanogenic environments and submarine fans. Although all of these types of coring disturbances have been recognized previously, our contribution is novel because it provides an easily accessible summary of methods for their identification. This contribution gives two specific examples on the importance of these coring disturbances. We show how suck-in of sediments during coring artificially created very thick volcaniclastic sand layers in cores offshore Montserrat and Martinique (Lesser Antilles). We then analyze very thick, structureless sand layers from the Escanaba Trough inferred to be a record of the Missoula mega-floods. These sand layers tend to coincide with the base of core sections, and their facies suggest coring disturbance by basal flow-in, destroying the original structure and texture of the beds. We conclude by outlining and supporting IODP-led initiatives to further reduce and identify coring disturbances, and acknowledge their recent successes in drilling challenging sand-rich settings, such as during IODP Expedition 340.
Old Gradients
The surface ocean temperature gradient between the warmer Western Equatorial Pacific and the cooler Eastern Equatorial Pacific is smaller during El Niño episodes than during neutral periods or during La Niñas. Some reconstructions of Pacific Ocean sea surface temperatures (SST) covering periods before ∼3 million years ago have suggested a permanent El Niño–like state. Zhang et al. (p. 84 ; see the Perspective by Lea ) present data from a biomarker-derived proxy for SST that indicate a sizable east-west gradient has existed for the past 12 million years, contradicting the concept of a permanent El Niño–like state existed.
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A comprehensive, but simple-to-use software package for executing a range of standard numerical analysis and operations used in quantitative paleontology has been developed. The program, called PAST (PAleontological STatistics), runs on standard Windows computers and is available free of charge. PAST integrates spreadsheettype data entry with univariate and multivariate statistics, curve fitting, time-series analysis, data plotting, and simple phylogenetic analysis. Many of the functions are specific to paleontology and ecology, and these functions are not found in standard, more extensive, statistical packages. PAST also includes fourteen case studies (data files and exercises) illustrating use of the program for paleontological problems, making it a complete educational package for courses in quantitative methods.
Seafloor organic matter flux from marine primary productivity is quantified, and the range of annual flux rates is calculated and compared to the counts of benthic foraminifera at 382 surface sediment stations from the equatorial Guinea Basin to the Arctic Ocean. Benthic foraminifera show high variability in flux range dependent distributional patterns, with maximum deviations at lowest percentages. The occurrence of a single species covers flux ranges within one to three orders of magnitude. Only a small number of species shows a correlation of this broad range of organic fluxes versus percentages in a count. For C. wuellerstorfi a functional relationship for the recalculation of flux rates from percentages in a count can be given within a standard deviation below 2 g organic carbon [m 2 yr 1]. However, such functions have to be restricted to a specific size range counted. The patterns of dominance more closely scale the environmental optimum of the species in general. For interspecific combinations, these patterns identify the ranges of overlap, where it is impossible to distinguish between higher or lower fluxes on the basis of faunal composition. This is quantified for the co-occurrence of C. wuellerstorfi and U. peregrina near 20% for one species. On an ocean wide scale, a number of taxa can be used to define threshold values for the nutritive needs of the assemblages, most pronounced within annual flux ranges at 2-3 g org. C [m-2]. Different trophic needs of species can be attributed to their infaunal, epibenthic, or opportunistic behavior respectively, and examples for the flux dependent takeover in dominance are given. These quantifications may offer approximations for flux rate dependent faunal patterns in surface sediments and for the detection of flux rate dependent faunal fluctuations in the Quaternary record.
To understand the gradual global cooling during the mid-Pliocene (3.5-2.5 Myr ago) one needs to consider the tectonical constriction of tropical seaways, which affected ocean circulation and the evolution of climate. Here we use paired measurements of σ8O and Mg/Ca ratios of planktonic foraminifera to reconstruct the Pliocene hydrography of the western tropical Indian Ocean (Site 709C) and changes in the Leeuwin Current in the eastern subtropical Indian Ocean (Site 763A) in response to Indonesian Gateway dynamics. Today, the Indonesian Throughflow (ITF) and, subsequently, the warm southward flowing Leeuwin Current off Western Australia are essential for the polar heat transport in the Indian Ocean. During 3.5-3 Ma, sea surface temperatures significantly dropped in the Leeuwin Current area, becoming since ∼3.3 Ma 2°C-3°C cooler than the rather unchanged sea surface temperatures from the eastern and western tropical Indian Ocean. We refer this drop in sea surface temperatures to a weakened Leeuwin Current with severe climatic effects on Western Australia induced by a tectonically reduced surface ITF. We suggest that this reduced surface ITF led to a diminished poleward heat transport in the Indian Ocean resulting in a weakened Leeuwin Current and possibly to cooling of the Benguela upwelling system.
Sediment samples have been examined from 51 locations between the Narrows Bridge and Fremantle Harbour in the Swan River estuary. All have yielded foraminifera and all but one provided data in which distinctive trends can be recognized. Foraminiferal number (number of specimens in 10 g dry weight of sample) decreases upstream until near Narrows Bridge, where it increases again. Values are > 2500 for most samples through Fremantle Harbour to Blackwall Reach and just past Point Walter. The higher numbers correlate with the outside of the bends in the river, reflecting a higher energy, more marine influence. The highest value is similar to 10,000 at Station 2. Values of 500-2500 occur in the inside of the river bends between Fremantle and Blackwall Reach, probably correlating with a slightly lower energy environment. There is a separate region of 500-2500 values in a narrow strip from the Narrows Bridge into Melville Water, probably due to the influx of river-borne nutrients; the composition of faunas contributing to this high value is quite different from that between Fremantle and Blackwall Reach. Two regions of lower values (100-500 and < 100) are recognizable, both confined to Melville Water, the lower values apparently coinciding with the mud-filled basin of the river. In the Fremantle Harbour - Blackwall Reach section of the river, Ammonia cf. aoteana (Finlay) makes up < 5% of the foraminiferal fauna, but > 50% of faunas near the Narrows Bridge and in most of Melville Water, excluding the shallower parts particularly to the northwest. In contrast to A. cf. aoteana, miliolids make up > 50% of faunas in the southern side of Blackwall Reach and extend just into Melville Water. They make up < 5% of the fauna where A. cf. aoteana is > 50%. The inverse relationship between A. cf. aoteana and miliolids is very marked. Elphidium constitutes up to 90% of the foraminiferal fauna, but this polyspecific estuarine genus is concentrated towards the marine end of the estuary. Diversity in foraminifera decreases and dominance increases upstream.
The carbonate sediments of the Western Australian shelf in the Indian Ocean host diverse assemblages of benthic foraminifera. These shelf environments are dominated by the southward-flowing Leeuwin Current, which impacts near-surface circulation and influences biogeographic ranges of Indo-Pacific warm-water foraminifera. Analyses of outer-ramp to upper-slope sediments (127-264 m water depth) at four different sites (some with replicates) revealed 185 benthic species. A shift from benthic to planktonic foraminifera was accompanied by a decrease in ''larger'' benthic foraminifera below the lowermost euphotic zone. Fisher a and proportions of buliminid and textulariid taxa increased with water depth, as miliolids and rotaliids decreased in proportion. Cluster analyses on the 125 to 250 lm and 250 to 850 lm size fractions revealed distinct assemblages, with the former distinguishing between deeper and shallower sites, and the latter distinguishing between the Carnarvon Ramp site and the three sites on the northwestern shelf (NWS). The assemblage shift with depth was likely caused by rapidly changing physical conditions in the upper thermocline. The assemblage differences between the NWS and the Carnarvon Ramp site indicate limited horizontal transport and migration rates on the outer shelf below the influence of the Leeuwin Current. Similarity in bottom-water temperature at the studied sites indicates that water mass characteristics, biogeographic history, and/or possibly diversity in benthic shelf habitats, rather than temperature and depth, are responsible for differences between the two regions.
Three textural features seem especially useful in classifying those carbonate rocks that retain their depositional texture (1) Presence or absence of carbonate mud, which differentiates muddy carbonate from grainstone; (2) abundance of grains, which allows muddy carbonates to be subdivided into mudstone, wackestone, and packstone; and (3) presence of signs of binding during deposition, which characterizes boundstone. The distinction between grain-support and mud-support differentiates packstone from wackestone—packstone is full of its particular mixture of grains, wackestone is not. Rocks retaining too little of their depositional texture to be classified are set aside as crystalline carbonates.
We compare paleoproductivity proxy records from a set of gravity cores from the Exmouth Plateau (≈ 19°S, 113°E, 950 to 2250 m) and the Perth Basin (≈ 27°S, 111°E, 2750 m) in the southeastern Indian Ocean. In general, these proxies indicate higher surfaceocean productivity in this region at the Last Glacial Maximum (LGM, isotope Stage 2). LGM sediment accumulation rates and the accumulation rates of biogenic sediment components (CaCO3 and organic carbon) are a factor of 1.5 to 2 higher than Holocene values. Benthic foraminiferal abundances and accumulation rates are both higher in glacial sediments, as are the concentrations and accumulation rates of authigenic uranium in the sediments. These benthic foraminiferal abundance and authigenic uranium data suggest higher surface ocean productivity during the glacial, but we cannot yet relate them to carbon flux quantitatively. In contrast to these three approaches, a productivity proxy based on paired-species benthic foraminiferal δ13C differences shows little glacial-Holocene change. Possible explanations for this disagreement are discussed.
Together, the data suggest that the glacial productivity off Western Australia was elevated relative to Holocene values, and support the hypothesis that a north-flowing west Australian current led to coastal upwelling and enhanced primary productivity off western Australia during the Last Glacial Maximum. However, glacial productivity was high only relative to the low productivity characteristic of this region in the modern ocean. We see no evidence of strong upwelling similar to that observed in the modern ocean off the southwestern coasts of Africa and South America.
Two hundred and thirty-six benthonic species and six planktonic species are identified among Formainifera present in Holocene sediment of Exmouth Gulf, in a water depth of 5-30 m. The benthonic microfauna comprises 20 agglutinated species (including 9 Lituolida, 1 Trochamminida, and 10 Textulariida), 74 porcellaneous species (Miliolida), and 142 hyaline species (including 4 Spirillinida, 27 Lagenida, 37 Buliminida, and 75 Rotaliida). Abundant species, at least at one site, include Ammotium australiensis, Textularia foliacea, Textularia lateralis, and Textularia oceanica among the agglutinated types; Parahauerinoides fragilissimus, Peneroplis pertusus, Planispirinella exigua, Pseudomassilina australis, Quinqueloculina arenta, Q. philippinensis, Q. sp 8, Sigmoihauerina involuta, Sorites marginalis, Triloculina tricarinata among the porcellaneous species; and Ammonia parkinsoniana, Amphistegina lessonii, A. sp cf. A. Papillosa, A. radiata, Asterorotalia gaimardi, Cibicides sp cf C. refulgens, Discorbinoides patelliformis, Elphidium sp cf E. advenum, E. crispum, E. sp 1, Heterostegina depressa, Operculina ammonoides, Pararotalia nipponica, and Rosalina cosymbosella among the hyaline (Rotaliida) species.
Australia's western margin is adjacent to a low–moderate-relief, semi-arid hinterland extending from northern tropical to southern temperate latitudes. Swell waves occur throughout, and cyclonic storms and tidal influences decline from north to south. The margin is influenced by the poleward-flowing, warm, nutrient-poor Leeuwin Current. There is limited upwelling and localized downwelling of saline water on to the shelf. The North West Shelf (NWS) is an ocean-facing ramp with palimpsest sediments – formed during Marine Isoptope Stage (MIS) 3 and 4; stranded ooids and peloids formed early during the post-Last Glacial Maximum (LGM) sea-level rise – and Holocene particles. Changing oceanography during sea-level rise profoundly affected sediment character.
The SW Shelf (SWS) comprises the subtropical sediment-starved Carnarvon Ramp in the north and the incipiently rimmed, flat-topped, steep-fronted Rottnest Shelf in the south. The inner Carnarvon Ramp includes the Ningaloo Reef and hypersaline Shark Bay. The mid ramp is relict or stranded foraminifer-dominated sand, and represents attenuated carbonate production due to downwelling incursions of Shark Bay water on to the ramp; the outer ramp is planktic foraminiferal sand or spiculitic mud. Rottnest Shelf has coralline algal-encrusted hardgrounds, larger symbiont-bearing foraminifers with abundant cool-water elements including bryzoans, molluscs and smaller foraminifers. The SWS is transitional between warm- and cool-water carbonate realms.
Although extreme-wave events are frequent along the northwestern coast of Western Australia and tsunamis in 1994 and 2006 induced considerable coastal flooding locally, robust stratigraphical evidence of prehistoric tropical cyclones and tsunamis from this area is lacking. Based on the analyses of X-ray computed microtomography (μCT) of oriented sediment cores, multi-proxy sediment and microfaunal analyses, optically stimulated luminescence (OSL) and 14C-AMS dating, this study presents detailed investigations on an allochthonous sand layer of marine origin found in a back-barrier depression on the NW Cape Range peninsula. The event layer consists of material from the adjacent beach and dune, fines and thins inland, and was traced up to ~400 m onshore. Although a cyclone-induced origin cannot entirely be ruled out, the particular architecture and fabric of the sediment, rip-up clasts and three subunits point to
deposition by a tsunami. As such, it represents the first stratigraphical evidence of a prehistoric, mid-Holocene tsunami in NW Western Australia. It was OSL-dated to 5400-4300 years ago, thus postdating the regional mid-Holocene sea-level highstand.
Smear slides provide a rapid, simple and cheap technique for examining silt-size marine sediments. The very small sample size required (only 1–2 mm3) means that even archive cores can be sampled without damage. The technique needs little preparation and simple equipment. Their usefulness for rapidly describing cored sediment sequences has been demonstrated by their continual use during the 15 years of the Deep Sea Drilling Project. Today, smear slide analysis constitutes the primary investigative tool, albeit qualitative, in the initial analysis of cored sediments and is likely to remain so. Smear slides have wide application: besides sediment cores, they can be used to examine sediment attached to dredged rocks, lithified borehole samples and unconsolidated terrestrial sediments and soils. Smear slides are especially useful in determining small scale variability in a sediment.
During the preparation of the Atlas on the physical oceanography of the Indian Ocean (WYRTKI, 1971) a comprehensive analysis of its oceanographic conditions had to be undertaken to construct meaningful maps, sections and diagrams, and to interpret the structure and circulation of this ocean, which is in so many ways different from the other oceans. On a large scale 3 distinct circulation systems can be delineated. These are:
I.
the seasonally changing monsoon gyre
II.
the south hemispheric subtropical anticyclonic gyre
III.
the Antarctic waters with the Circumpolar Current
The objective of this work was to review the distribution of benthic foraminiferal species at the western European continental margin from 43-58°N, determine their diversity, and generate a standardized taxonomy based on 44 publications (1913-2010) and unpublished information. Qualitative and quantitative data based upon foraminiferal occurrences and species abundances were included together with supplementary sedimentological and hydrographical data. From the species inventory, as well as from differences in morphological, physical, and hydrographic conditions in the study area, we defined six regions. The investigation of 2902 stations revealed 1486 species, of which 26% are synonymous. Most of the species have a hyaline test and live free, on or in the sediment. We recorded 608 species whose distributions were confined only to one of the six regions. Quantitative faunal data showed a general diversity increase from shelf to slope and two mid-slope diversity maxima, one located on the Basque continental margin at 550-850-m water depth and the other west of Ireland at 700-1100 m. In addition, the number of living species on the shelf generally increased from N-S. The latitudinal vs. depth distribution of six dominant species showed an irregular, lobate distribution pattern for the shelf regions. These species displayed similar distribution patterns on the continental slope, despite different modes of life, and different food and substrate preferences. This suggests that they have the same ecohabitat throughout their depth range. The faunal distribution pattern revealed close relationships between the different regions despite their varying hydrologic regimes. A delineation of one or two regions based on faunal criteria has not been attempted to date. There were no major distinctions in diversity among the six regions of the NE Atlantic, but the whole area exhibits an interregional diversity (γ-diversity of 16), similar in magnitude to that of the Gulf of Mexico. In some parts of the study area, gaps in data coverage and differences in foraminiferal taxonomy and hydrographic conditions prevented the calculation of diversity indices.
Recent benthonic foraminifera from middle bathyal-abyssal water depths (2500-4600 m) in the southeast Indian Ocean consist primarily of calcareous species, but with agglutinated species present in most samples. The presence of calcareous benthonic foraminiferal assemblages to depths of 4600 m suggests that the CCD is deeper than 4600 m in the southeast Indian Ocean. The majority of the 33 taxa found in this study are rotalid species (26), with miliolid species (4) and agglutinated forms (3) of secondary importance. The number of taxa found in any one sample ranges from 12 to 25. The dominant rotalids are Epistominella umbonifera, Uvigerina spp., Globocassidulina subglobosa, Epistominella exigua, Planulina wuellerstorfi, and Oridorsalis tener. The four miliolid species are Pyrgo murrhina, Quinqueloculina cf. weaveri, Quinqueloculina venusta, and Ophthalmidium pusillum. The most common agglutinated species are Siphotextularia catenata and Eggerella bradyi, while Karreriella bradyi is uncommon. Faunal/water-mass associations are similar to those found in the Atlantic by other workers.
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. profunda 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, Calcidiscus leptoporus, Calciosolenia murrayi, Neosphaera coccolithomorpha 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 interglacial 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.
The Indonesian Throughflow acts as a major switchboard in the global
thermohaline circulation, and its variability is strongly related to
tropical climate dynamics on shorter and longer timescales. During the
Holocene and Pleistocene, fluctuating sea surface temperature and
salinity patterns in the Western Pacific Warm Water Pool and Indonesian
Seas and variations in East Asian monsoon strength mainly controlled the
intensity and hydrological characteristics of the throughflow.
Additionally, glacial/deglacial sea-level change strongly influenced
throughflow volume in shallow sections of many passages (i.e. the
southern part of the Timor passage on the NW Australian shallow shelf)
thus altering the related heat transfer between oceans.
The tectonic history of the Indonesian Gateway ultimately controlled the
long-term evolution of the throughflow. During the Pliocene, changes in
the position and geometry of the inflow passages (Mindanao Passage to
the North and Halmahera Passage to the south) in relation to the
tropical Pacific front significantly modified the climatic role of the
tropical Indian and Pacific Oceans, resulting in reduced atmospheric
heat transport from the tropics to high latitudes. However, the precise
timing of major restriction in the surface and thermocline water flow is
difficult to ascertain. The early evolution of the Indonesian Gateway
was characterized by tectonic restriction of the deep water pathway
between the Pacific and Indian Oceans at approximately 25 Ma. By the
early Miocene, the Indonesian Gateway was already closed as a deep water
pathway between the Pacific and Indian Oceans.
The flow of water from the western Pacific to the eastern Indian Ocean
through the Indonesian archipelago is governed by a strong pressure
gradient. Dynamic height computations determine the average sea level
difference as 16 cm and show that most of the pressure gradient is
contained in the upper 200 m. Sea level data from Davao in the
Philippines and from Darwin in Australia are used to determine the
annual signal and the interannual variations of the pressure gradient
for the years 1966 to 1985. The annual signal has a maximum during the
southeast monsoon in July and August and a minimum in January and
February. Interannual variations are not related to the Southern
Oscillation because sea level is low at both stations during El
Niño events, and thus there is little influence on the sea level
difference. The mechanism of the through flow is discussed, but a
determination of its numerical value will have to await direct
measurements. A comparison of the sea level difference with results from
a numerical model by Kindle shows satisfactory agreement. It is
concluded that the variability of the through flow can be monitored by
sea level measurements.
We investigate the combination of environmental factors that influence the distribution patterns of benthic foraminiferal tests (> 63 μm) in a topographically varied region crossed by both the Subtropical and Subantarctic Fronts, south-east of New Zealand. Seafloor sample sites, extending from outer shelf (50 m) to abyssal (5000 m) depths, are bathed by five different water masses, and receive phytodetritus from Subtropical, Subantarctic and Circumpolar surface water masses. Eight mappable associations are recognised by Q-mode cluster analysis of the benthic foraminiferal census data. Similar associations are identified using cluster analysis based solely on the presence or absence of species. Canonical correspondence analysis and a correlation coefficient matrix were used to relate the faunal data to a set of environmental proxies. These show that factors related to water depth (especially decreasing food supply with increasing depth) are the most significant in determining the overall foraminiferal distribution. Other contributing factors include surface water productivity and its seasonality; bottom water ventilation; energetic state of the benthic boundary layer and resulting substrate texture; and bottom water carbonate corrosiveness. Three shallow-water associations (50–700 m), dominated by Cassidulina carinata, Trifarina angulosa, Globocassidulina canalisuturata, Gavelinopsis praegeri, and Bolivina robusta, occur in coarse substrates on the continental shelf, and on the crests and upper slopes of four seamounts under well-oxygenated, high energy regimes, and high food input. Three mid bathyal to upper abyssal associations (500–3300 m), dominated by Alabaminella weddellensis, C. carinata, and Epistominella exigua, occur in biopelagic sandy mud, beneath a region of strongly seasonal food supply, with their composition influenced by total food flux, ventilation (Oxygen Minimum Zone), and bottom current strength. An unusual lower bathyal association (1200–2100 m), dominated by T. angulosa and Ehrenbergina glabra, occurs in a belt of coarser sandy substrate that runs along the crest of the submarine plateaux slopes beneath the strongly-flowing Subantarctic Front-related currents. A deep abyssal association (3500–5000 m), dominated by Nuttallides umbonifer and Globocassidulina subglobosa, occurs on the abyssal plain beneath oligotrophic lower Circumpolar Water south-east of the Subantarctic Front and is strongly influenced by the cold, carbonate-corrosive conditions.The faunal composition of this subantarctic region is most similar to adjacent southern New Zealand, but with the addition of a few significant Antarctic elements (e.g., E. glabra, Globocassidulina crassa).
The history of coastal surface currents in the eastern Indian Ocean over the last ∼ 130 kyr has been studied through the application of the FI-2 transfer function to planktic foraminifera in deep-sea cores in a N-S transect offshore Western Australia.The concept of sea-surface temperature (SST) gradient residuals is the amount by which the sea-surface temperature gradient of a given time differs from that in the modern sea. The SST gradient residuals for several episodes of climate extremes in the Late Quaternary [the last glacial maximum (∼ 20 kyr B.P.), the last interglacial maximum ( ∼ 122-120 kyr B.P.), and at the end of the penultimate glaciation ( ∼ 130 kyr B.P.)] indicate that areas north of ∼ 18°S off Western Australia have changed very little during the extremes in climate over the last ∼ 130 kyr, while south of ∼ 18°S there have been significant changes in surface-water temperatures.Large areas of anomalously cool surface-water were established off North West Cape (22°S–24°S) during both glacial and interglacial extremes. These reflect the increased influence of the West Australian current on the surface-water circulation. During the last glacial maximum and at the end of the penultimate glaciation, cold sea-surface temperature anomalies were established near to the coast, indicating that the Leeuwin Current did not warm the coastal zone as it does today. During the last interglacial maximum the Leeuwin Current flowed as an intensified current in a narrow zone close to the coast, but did not significantly warm surface-waters offshore as far as 20°–25°S, where a zone of cold water was maintained.
We identified 164 taxa of benthic foraminifera in 35 selected box-core top samples collected on the Australian-Irian Jaya continental margin in waterdepths between 60 and 2119 m, along three systematically sampled transects across the Banda Arc. The bathymetric distribution pattern of the benthic foraminiferal faunas is related to the oceanographic situation of this area, where the watermasses of the Indian Ocean collide with the watermasses of the Pacific Ocean. With the results of cluster analyses and empirical depth-ranges of “isobathyal” taxa, four faunal depth-zones and four subzones can be distinguished:
Trends in the abundance, diversity and taxonomic composition of ‘live’ (rose Bengal stained) foraminiferal assemblages (0-1 cm layer, >63-μm fraction) were analysed in replicate multiple corer samples collected at the Porcupine Abyssal Plain (48° 50’ N, 16° 30’ W, 4850 m water depth) over a 13-yr period (1989-2002). Total densities were significantly higher in 1996-2002 compared to 1989-1994, a change coincident with a spectacular rise in the density of the holothurian Amperima. However, total densities exhibited no significant relation to seasons or any significant correlation with modelled organic matter flux, the North Atlantic Oscillation (NAO) index, Amperima densities, or megafaunal assemblage composition. Over the same period, species richness and diversity measures decreased and dominance increased, although not significantly. Multivariate analyses revealed three assemblages represented by samples collected in 1989-1994, 1996-July 1997 and October 1997-October 2002. These reflected temporal changes in the densities of higher taxa and species. Trochamminaceans, notably a small undescribed species, increased from 5-9% (1989-1994) to 29-40% (1996-2002) of the assemblage with a corresponding rise in absolute abundance. Species of Hormosinacea and Lagenammina also tended to increase in density from 1996/1997 onwards. Rotaliids, dominated by Alabaminella weddellensis and Epistominella exigua, showed a bimodal distribution over time with peak densities in May 1991 (32%) and September 1998 (28%) and lowest densities in 1996-1997. Responses by these species to seasonal phytodetritus inputs probably explain the relative abundance of E. exigua, and to a lesser extent A. weddellensis, in 1989 and 1991 when phytodetritus was present. A qualitative change in the phytodetrital food, repackaging of food by megafauna, increased megafaunal disturbance of the surficial sediment, or a combination of these factors, are possible explanations for the dominance of trochamminaceans from 1996 onwards. The miliolid Quinqueloculina sp. was virtually absent in multicore samples (0-1 cm, >63-μm fraction) from 1989-1994, peaked in September 1996 (22%) when degraded phytodetritus was present on core surfaces, was less common in March 1997, and thereafter was relatively uncommon. However, horizontally sliced box-core samples (0-5 cm, >250-μm fraction) revealed that large specimens were more abundant in March 1997, and also were concentrated in deeper sediment layers, than in September 1996. We suggest that Quinqueloculina sp. migrated to the sediment surface in response to a 1996 flux event, grew and reproduced, before migrating back into deeper layers as the phytodetrital food became exhausted. Overall, the abyssal time-series revealed decadal-scale changes among shallow-infaunal foraminifera, more or less coincident with changes in the megafauna, as well as indications of shorter-term events related to seasonally-pulsed phytodetrital inputs.
A data set of benthic foraminiferal faunas counted in 138 surface samples from the Mediterranean Sea has been used to investigate whether the bathymetrical distribution of the dominant taxa is controlled by the amount of labile organic matter transported to the sea floor. We find that most of the major taxa show a clear W to E shallowing of their upper or lower depth limit, coinciding with a W to E decrease in the surface water primary production, and in the estimated flux of the labile organic matter to the sea floor. This observation implies that the bathymetrical succession of these taxa is indeed determined by the organic flux. In the western Mediterranean, we find successions from more oligotrophic taxa at greater water depths to more eutrophic taxa in more shallow water. Towards the eastern Mediterranean, most eutrophic taxa tend to become increasingly rare, or even to disappear, whereas the more oligotrophic taxa show a clear shoaling of their depth range. Deep infaunal taxa are mainly limited to the western part of the Mediterranean. This is explained by their dependency on a relatively elevated organic flux, and by the fact that the bacterial stocks on which they feed may become unattainable when the redox front is positioned too deep in the sediment. The close similarity between the flux level controlling our main faunal boundary, and the flux levels coinciding with important faunal changes in other parts of the world ocean, suggests that a flux level of about 2–3g labile Cm−2y−1 level corresponds to a benthic ecosystem threshold value of global importance.
Around bathyal hydrocarbon seeps of Green Canyon, Gulf of Mexico, a community of foraminifera has colonized mats of Beggiatoa, a chemolithotrophic, sulfide-oxidizing bacterium. The O2-H2S boundary zone in these mats is likely to be within 1-2 mm of the sediment-water interface. Judging by Rose-Bengal staining, small populations of some foraminiferal species survive under the Beggiatoa mats, down to a substrate depth of at least 2 or 3 cm, where the sediment is black and the pore fluid is sulfidic. These species, including Bolivina albatrossi, Bolivina ordinaria, Cassidulina neocarinata, Gavelinopsis translucens, Osangularia rugosa, and Trifarina bradyi, are microaerophiles or facultative anaerobes. The subsurface distributions and densities of Bolivina albatrossi suggest that it is the most tolerant of anoxia and sulfide toxicity. Transmission electron microscopy of 12 individuals of diverse species failed to confirm the presence of live foraminifera under the bacterial mats. The cytoplasm, when present, was degraded, although the structure of some organelles suggests that some of the individuals had died only recently. Initial stable-isotope analyses of empty tests of three species (Uvigerina peregrina, Bolivina subaenariensis, and Lenticulina sp.) from previously studied seep sites show anomalously negative and wide-ranging δ13C values (-1.3‰ to -3.6‰ PDB) that are typical of seep CO2 plumes, but not of associated sediment-pore fluids. However, the carbon-isotopic signature in the dominant species (especially in Bolivina albatrossi) living under Beggiatoa mats in deep-bathyal Green Canyon is presently unknown. Thus, whether species of foraminifera invading the anoxic layers under Beggiatoa mats can construct tests in these microhabitats remains unresolved.