mid-Pleistocene Climate Transition Research Articles

Record of Deep-Sea, Benthic Elongate-Cylindrical Foraminifera Across the Eocene-Oligocene Transition in the North Atlantic Ocean (ODP Hole 647A)

The Eocene-Oligocene transition (EOT) at ∼34 Ma constitutes one of the major episodes of climate change that occurred in the last 50 myr, associated with the establishment of permanent Antarctic ice sheets and the initiation of meridional overturning circulation. We present a high-resolution quantitative study (>125-μm fraction) of a group of calcareous, deep-sea, benthic foraminifera with elongate-cylindrical morphologies (Elongate Gp.) across the EOT at high latitudes in the North Atlantic Ocean (ODP Hole 647A). This group experienced significant declines in abundance and diversity during periods of global cooling, disappearing almost completely during the mid-Pleistocene Climate Transition. The Elongate Gp. was a common component of deep-sea benthic foraminiferal assemblages at Hole 647A (average 20%). The family Stilostomellidae dominated during the late Eocene and at the EOT; the Nodosariidae slightly dominated during the early Oligocene. During the EOT at Hole 647A, an abrupt decline in the abundance and diversity of agglutinated foraminifera occurred, but not a distinct taxonomic turnover of the Elongate Gp. Siphonodosaria jacksonensis was the most abundant species and showed the most significant changes in relative abundance across the EOT, from almost no record to a peak of >50% of total benthic foraminifera coincident with the lowest diversity and highest absolute abundance of the Elongate Gp. We attribute these changes to the influence of more than one mechanism, including increased productivity, deepening of the calcite compensation depth, and more vigorous deep-ocean circulation.

Evolution of Ocean Temperature and Ice Volume Through the Mid-Pleistocene Climate Transition

Earth's climate underwent a fundamental change between 1250 and 700 thousand years ago, the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum.

241 Cyclic vegetation changes during the Mid-Pleistocene climate transition around Osaka Bay, southwest Japan

241 Cyclic vegetation changes during the Mid-Pleistocene climate transition around Osaka Bay, southwest Japan

Planktic foraminifera-based sea-surface temperature record in the Tasman Sea and history of the Subtropical Front around New Zealand, over the last one million years

Abstract Planktic foraminiferal assemblages in a composite section from two cores (MD06-2989/2986) off the west coast of New Zealand's South Island (42–43.5°S) provide a 1 myr (MIS 31–1) sea-surface temperature (SST) record (~ 3–4 kyr resolution) in the Tasman Sea. A significant overall faunal change occurred near the end of the mid-Pleistocene Climate Transition (MPT) at ~ 600 ka (MIS 15). Mean annual SSTs were estimated using the Artificial Neural Network (ANN) method. Glacial–interglacial (G–I) cycles in the MPT had ~ 9 °C SST range, whereas in the post-MPT SST range was ~ 6–7 °C. The SST and faunal changes imply that the Subtropical Front (STF) migrated ~ 6° in latitude to lie just north of the sites in MPT glacials (MIS 28–16), but only migrated 3–5° north in post-MPT glacials. These G–I latitudinal migrations of the STF west of New Zealand contrast with the situation east of New Zealand, where migrations of both the STF and Subantarctic Front were prevented by the Chatham Rise and Campbell Plateau. Subtropical Water is inferred to have only flowed around the south of New Zealand (as it does today) during MIS 11 and the Late Pleistocene–Holocene (MIS 5–1).

Significant increase in relief of the European Alps during mid-Pleistocene glaciations

The effect of expanded glaciation on the relief of mountain ranges is debated. Chronometric and model data from the European Alps suggest that relief in the Rhone Valley was enhanced as a result of the increased incision of the valley following the mid-Pleistocene climate transition.

Southern Ocean dust–climate coupling over the past four million years

Dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean. Indeed, dust supply to the Southern Ocean increases during ice ages, and 'iron fertilization' of the subantarctic zone may have contributed up to 40 parts per million by volume (p.p.m.v.) of the decrease (80-100 p.p.m.v.) in atmospheric carbon dioxide observed during late Pleistocene glacial cycles. So far, however, the magnitude of Southern Ocean dust deposition in earlier times and its role in the development and evolution of Pleistocene glacial cycles have remained unclear. Here we report a high-resolution record of dust and iron supply to the Southern Ocean over the past four million years, derived from the analysis of marine sediments from ODP Site 1090, located in the Atlantic sector of the subantarctic zone. The close correspondence of our dust and iron deposition records with Antarctic ice core reconstructions of dust flux covering the past 800,000 years (refs 8, 9) indicates that both of these archives record large-scale deposition changes that should apply to most of the Southern Ocean, validating previous interpretations of the ice core data. The extension of the record beyond the interval covered by the Antarctic ice cores reveals that, in contrast to the relatively gradual intensification of glacial cycles over the past three million years, Southern Ocean dust and iron flux rose sharply at the Mid-Pleistocene climatic transition around 1.25 million years ago. This finding complements previous observations over late Pleistocene glacial cycles, providing new evidence of a tight connection between high dust input to the Southern Ocean and the emergence of the deep glaciations that characterize the past one million years of Earth history.

Ice-rafting from the British–Irish ice sheet since the earliest Pleistocene (2.6 million years ago): implications for long-term mid-latitudinal ice-sheet growth in the North Atlantic region

The Plio-Pleistocene intensification of Northern Hemisphere continental ice-sheet development is known to have profoundly affected the global climate system. Evidence for early continental glaciation is preserved in sediments throughout the North Atlantic Ocean, where ice-rafted detritus (IRD) layers attest to the calving of sediment-loaded icebergs from circum-Atlantic ice sheets. So far, Early-Pleistocene IRD deposition has been attributed to the presence of high-latitudinal ice sheets, whereas the existence and extent of ice accumulation in more temperate, mid-latitudinal regions remains enigmatic.Here we present results from the multiproxy provenance analysis of a unique, Pleistocene-Holocene IRD sequence from the Irish NE Atlantic continental margin. There, the Challenger coral carbonate mound (IODP Expedition 307 site U1317) preserved an Early-Pleistocene record of 16 distinctive IRD events, deposited between ca 2.6 and 1.7 Ma. Strong and complex IRD signals are also identified during the mid-Pleistocene climate transition (ca 1.2 to 0.65 Ma) and throughout the Middle-Late Pleistocene interval. Radiogenic isotope source-fingerprinting, in combination with coarse lithic component analysis, indicates a dominant sediment source in the nearby British–Irish Isles, even for the oldest, Early-Pleistocene IRD deposits. Hence, our findings demonstrate, for the first time, repeated and substantial (i.e. marine-terminating) ice accumulation on the British–Irish Isles since the beginning of the Pleistocene. Contemporaneous expansion of both high- and mid-latitudinal ice sheets in the North Atlantic region is therefore implied at the onset of the Pleistocene. Moreover, it suggests the recurrent establishment of (climatically) favourable conditions for ice sheet inception, growth and instability in mid-latitudinal regions, even in the earliest stages of Northern Hemisphere glacial expansion and in an obliquity-driven climate system.

Revisiting the Quaternary development history of the western New Caledonian shelf system: From ramp to barrier reef

The present paper is based on the study of a new core (Ténia 3,151.20m long) and revisitation of previous core data from the western New Caledonian barrier reef. Based on variations in lithology and stable isotope signatures, and the occurrence of unconformity surfaces, the Ténia 3 sequence is subdivided into ten units, regarded as deposited during interglacial, high sea levels. Five detrital and two framework facies are delineated using their biological and textural attributes, including coral growth forms and assemblages. These facies are interpreted as representative of outer to inner carbonate shelf (ramp to barrier reef) environments. Variations in the facies within each stratigraphic unit occur mainly as vertical transitions from lower energy (and probably deeper) to higher energy (probably shallower) settings. The succession of depositional events is reconstructed using all available age criteria including lithostratigraphy, magnetostratigraphy, uranium-series dating, and nannofossil biostratigraphy. Shallow-water, coral communities appear to have been developed as soon as carbonate deposition started at around 1.2million years (Ma), but the initiation of typical coral reef tracts is likely to have occurred not prior to 0.40million years during Marine Isotope Stage (MIS) 11. Carbonate deposition previous to MIS 11 is considered to have operated along an open shelf margin, i.e. a ramp or non-rimmed platform. Based on the re-examination of the core dataset previously acquired from the New Caledonian reefs, the coral communities, dominated by massive forms (mainly poritids) are assumed to have remained stable in composition over the past 1.4Ma. Similarly, as indicated by a preliminary study of pollen assemblages in the core, the composition of terrestrial flora, dominated by Podocarpaceae and Araucariaceae, has not apparently suffered significant change over the past 1Ma, and reflects climate conditions cooler and drier than today. Mean subsidence rates are derived tentatively over the past 1Ma from the stratigraphic position at present reached by the top of depositional units; mean rates of ≤0.08mmyr−1 for the western New Caledonian margin are consistent with past sea-level estimates. The Quaternary depositional history of the New Caledonian shelf margin appears to have been controlled by both the amplitude of sea-level changes and by differing vertical tectonic movements. Comparison between the depositional shelf histories from western New Caledonia, north-eastern Australia, southern Belize and the Ryukyu Islands supports the assumption that coral populations over the early and mid Pleistocene developed as non-reefal communities along ramp systems, not forming continuous outer-shelf reef rims. At a global scale, Quaternary reef-rimmed platforms are likely to have initiated after the Mid-Pleistocene Climate Transition, from marine isotope stage 11, at about 0.4Ma.

MIS 21 and the Mid-Pleistocene climate transition: Climate and sea-level variation from a sediment core in Osaka Bay, Japan

We report climate and sea-level variation for the marine oxygen isotope stage (MIS) 21, encompassing the end of the Mid-Pleistocene climate transition (MPT), based on pollen, diatom, and sulfur records from a 50-m thick sequence in a core from Osaka Bay. An extremely warm climate coincided with the sea level highstand of substage 21.5, when the warm-temperate element Quercus (Cyclobalanopsis) exceeds 40% of total arboreal pollen. This was followed by a warm-temperate to temperate and humid climate that continued until the end of MIS 21. A linear age model shows that climate was dominated by precessional cyclicity, with an inverse correlation between temperature and precipitation. The postglacial sea-level rise reached its highest peak in substage 21.5, when paleo-Osaka Bay reached its maximum extent including Kyoto and Nara Basins. At this time pelagic diatoms were dominant in the central part of the bay. Sea level dropped below the Osaka Bay sill (about −60m at present) during substage 21.4, followed by a rise above the sill in substage 21.3, and a drop at 21.2. Sea level remained below the sill during substage 21.1. The thermal maximum and sea level peak occurred just after the rapid postglacial sea level rise, after which there was a gradual decline in temperature and sea level accompanied by precession-related oscillations; these features are typical of the post-MPT interglacials dominated by 100-ka cyclicity. These features may be a sign of termination of the MPT.

Impact of the Middle Miocene climate transition on elongate, cylindrical foraminifera in the subtropical Pacific

Fifty-eight species of elongate, cylindrical benthic foraminifera (here referred to as the Extinction Group) belonging to genera that became extinct during the mid-Pleistocene Climate Transition (MPT), were documented (similar to 50 kyr resolution) through the early middle Miocene (15-13 Ma) in two sites on opposite sides of the subtropical Pacific Ocean (ODP Sites 1146, South China Sea; ODP Site 1237, southeast Pacific). The study was undertaken to investigate the response of the Extinction Group (Ext. Gp) to the major cooling during the middle Miocene Climate Transition (MCT) to look for clues that might explain the causes of the extinction during the glacials of the mid-Pleistocene Climate Transition. Ext. Gp faunal differences between the two sites (attributed to regional and bathymetric differences in food supply to the seafloor) are greater than those that occurred through the 2 myr time span at either site. The middle Miocene Climate Transition was not an interval of enhanced species turnover or a decline in Ext. Gp abundance, in contrast to the major extinctions that occurred during the mid-Pleistocene Climate Transition. Distinct changes in the composition of the Ext. Gp faunas did occur through this time (more pronounced in Site 1237). At both sites the pre-middle Miocene Climate Transition faunas were transformed into their post-middle Miocene Climate Transition composition during the period of major cooling (14.0-13.7 Ma). During this transition interval the faunal composition swung back and forth between the two end member faunas. These faunal changes are attributed to changes in productivity (decrease in South China Sea, increase in southeast Pacific), brought about by major changes in global climate and continental aridity. (C) 2010 Elsevier B.V. All rights reserved.

Pliocene to Pleistocene palynoflora and vegetation in Italy: State of the art

Fifty-four Italian pollen sites, spanning the last 5.3 Ma, have been the object of an updated synthesis. The chronostratigraphic framework of floral and vegetational events illustrates the development of cooler climates since the Pliocene. Floral and vegetational response to glacial/interglacial cycles, as well as major taxa replacements have been analysed with special attention to latitudinal and altitudinal gradients and to physiographic reorganizations. The pollen flora shows marked changes both at the beginning of the Pleistocene as well as at the time of the Mid-Pleistocene climate transition, when a major decrease in temperature, during both glacial and interglacial phases, occurred. Alternations of Artemisia steppe and thermophilous forest mark the overall glacial–interglacial vegetation changes. However, in northern Italy, the latter are rather expressed by an alternating spread of altitudinal coniferous forest (mainly Picea), without significant expansion of steppe vegetation, and thermophilous forest. More complex vegetational cycles than those pointed out by earlier syntheses are also testified by the detection of an alpine vegetation spreading during glacial phases and a wooded steppe in the earlier phases of interglacials, in some southern marine sections. Such diversified patterns could be associated with obliquity related warm/humid–cold/dry “interglacial”–“glacial” cycles superimposed by precession related warm/dry–cold/humid cycles. However, the role of depositional processes as well as taphonomic biases should not be ignored in the reconstruction of vegetation dynamics.

CENOZOIC RECORD OF ELONGATE, CYLINDRICAL, DEEP-SEA BENTHIC FORAMINIFERA IN THE INDIAN OCEAN (ODP SITES 722, 738, 744, 758, AND 763)

A group of ,100 species of elongate, cylindrical deep-sea benthic foraminifera (families Stilostomellidae, Pleurostomellidae, Nodosariidae) with complex, often constricted apertural structures, became extinct during increasingly cold glacial periods in the late Pliocene to mid-Pleistocene Climate Transition (MPT, ,2.6–0.6 Ma). We document the evolutionary history and architecture of this Extinction Group (Ext. Gp) through the Cenozoic at four lower bathyalupper abyssal Indian Ocean sites (ODP 722, 744/738, 758, 763), seeking clues to the cause of this morphologicallytargeted extinction episode late in the Cenozoic. Eighty percent of the 116 Ext Gp. species present in the Cenozoic of the Indian Ocean originated globally in the Eocene or earlier, compared with 23–37% of other Quaternary deep-sea foraminifera. The Ext. Gp had its peak species richness and relative and absolute abundances in the late Eocene. The rapid warming of the Paleocene-Eocene Thermal Maximum, that resulted in a loss of 30–50% of deep-sea foraminiferal species, had no impact on the Ext. Gp in the one Indian Ocean section (ODP 744/738) studied. Major Cenozoic changes in the Ext. Gp, including increased species turnover, changes in dominant species, a decline in abundance, loss in diversity, and finally extinction, mostly occurred during the middle Eocene to early Oligocene, middle to late Miocene, and late Pliocene to middle Pleistocene. These were times of stepped increase in the volume of polar ice, global oceanic cooling, surface-water eutrophication, seasonality of phytoplankton production, deep-water ventilation, and southern deep-water carbonate corrosiveness. The final decline and disappearance of the Ext. Gp began in the late Miocene at high latitudes (744/738), but not until the late Pliocene (758, 763) or MPT (722) at lower latitudes. We hypothesize that the loss of the Ext. Gp of deep-sea foraminifera may have been caused by the decline or demise of their specific food source (detrital phytoplankton or bottom-dwelling microbes) that was abundant in the Greenhouse World and was decimated by the stepwise cooling, ventilation, or eutrophication of the oceans that began in the middle and late Eocene.

Evidence for the earliest Pleistocene hominid activity in the Nihewan Basin of northern China

The Nihewan Basin in North China contains the densest concentration of early Pleistocene Paleolithic sites outside Africa. This paper introduces a new archaeological site complex at Madigou (MDG) that was systematically excavated from 2011 to 2014 in the northeastern part of the Nihewan Basin. The site contains fossils and well-preserved stone artefacts in fluvio-lacustrine sediments. Our magnetostratigraphic results situate the MDG sedimentary sequence in the early Brunhes normal chron and the late Matuyama reverse chron, including the Jaramillo normal subchron. The MDG artifact layers are positioned within the pre-Jaramillo Matuyama chron, with an estimated age of ca. 1.2 Ma, close to the onset of Mid-Pleistocene climate transition. The MDG core and flake technology includes bipolar flaking of siliceous dolomite cobbles, and freehand flaking of chert and brecciated chert block fragments. Mammalian fauna and pollen compositions indicate that the MDG hominins lived in an open habitat varying from lightly-wooded grassland to an ecosystem dominated by sparse steppe near the shore of the Nihewan paleolake. Our combined results in the fields of archaeology, paleontology, palynology and magnetochronology suggest that innovations in technological behavior may correlate with adaptations to high environmental variability during the start of Mid-Pleistocene climate transition.

Cenozoic record of elongate, cylindrical, deep-sea benthic foraminifera in the North Atlantic and equatorial Pacific Oceans

In the late Pliocene–middle Pleistocene a group of 95 species of elongate, cylindrical, deep-sea (lower bathyal–abyssal) benthic foraminifera became extinct. This Extinction Group (Ext. Gp), belonging to three families (all the Stilostomellidae and Pleurostomellidae, some of the Nodosariidae), was a major component (20–70%) of deep-sea foraminiferal assemblages in the middle Cenozoic and subsequently declined in abundance and species richness before finally disappearing almost completely during the mid-Pleistocene Climatic Transition (MPT). So what caused these declines and extinction? In this study 127 Ext. Gp species are identified from eight Cenozoic bathyal and abyssal sequences in the North Atlantic and equatorial Pacific Oceans. Most species are long-ranging with 80% originating in the Eocene or earlier. The greatest abundance and diversity of the Ext. Gp was in the warm oceanic conditions of the middle Eocene–early Oligocene. The group was subjected to significant changes in the composition of the faunal dominants and slightly enhanced species turnover during and soon after the rapid Eocene–Oligocene cooling event. Declines in the relative abundance and flux of the Ext. Gp, together with enhanced species loss, occurred during middle–late Miocene cooling, particularly at abyssal sites. The overall number of Ext. Gp species present began declining earlier at mid abyssal depths (in middle Miocene) than at upper abyssal (in late Pliocene–early Pleistocene) and then lower bathyal depths (in MPT). By far the most significant Ext. Gp declines in abundance and species loss occurred during the more severe glacial stages of the late Pliocene–middle Pleistocene. Clearly, the decline and extinction of this group of deep-sea foraminifera was related to the function of their specialized apertures and the stepwise cooling of global climate and deep water. We infer that the apertural modifications may be related to the method of food collection or processing, and that the extinctions may have resulted from the decline or loss of their specific phytoplankton or prokaryote food source, that was more directly impacted than the foraminifera by the cooling temperatures.

Rock magnetic properties of the Gardar Drift sedimentary sequence, Site IODP U1314, North Atlantic: Implications for bottom current change through the mid-Pleistocene

The sediment obtained at Integrated Ocean Drilling Program (IODP) Site U1314 on the Gardar Drift in the eastern flank of the Reykjanes Ridge was analyzed using rock magnetic techniques to study the depositional environment through the mid-Pleistocene climate transition. The study area is under the influence of Iceland–Scotland Overflow Water (ISOW). Consequently, the sedimentary record represents directly ISOW behavior over time. A change in the depositional environment was documented as magnetic property changes that are the result of changes in concentration and grain size of magnetic minerals, both of which decrease upward from around 0.9 Ma. Because these shifts are accompanied by a change in magnetic fabrics, which is interpreted as a change in bottom current directions from E–W to NNE–SSW, it is considered that the sedimentation pattern inferred from magnetic records at the site was modified by a change in the bottom current through the middle Pleistocene.

Productivity and continental denudation history from the South China Sea since the late Miocene

Late Miocene–Recent micropaleontological and geochemical records from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea (SCS) indicate that increase and decrease in abundance of siliceous plankton may be controlled mainly by the input of nutrients derived from land and provided by upwelling. A high export production event — a “biogenic bloom” event — occurred in the southern SCS between 12 and 6 Ma. During this period, high ratios of smectite/(illite + chlorite), smectite/quartz and Al/K indicate a high weathering intensity of the Asian continent, possibly due to the intensification of the East Asian Summer Monsoon (EASM), which may have increased the net flux of nutrients to the ocean, both directly through terrestrial input and indirectly through upwelling activity. A drop in Ba/Ti, Al/Ti and Ca/Ti values around 6 Ma may indicate a lowering of productivity, possibly due to the large consumption of sea surface nutrients by the “biogenic bloom”. Alternatively, it may indicate a shift in terrigenous input source area. At about 5.4 Ma, a decrease in weathering intensity, as indicated by a sudden decrease in the values of smectite/(illite + chlorite), smectite/quartz and Al/K, might have led to a sudden decrease of terrestrial nutrient input to the SCS. We suggest that the biogenic bloom ended when nutrients in surface waters were exhausted, because of a decrease in supply as well as a decrease in upwelling intensity due to weakening of the EASM. As a result, radiolarians were absent in the studied area between ~ 6 and 3.2 Ma. At ~ 3.2 Ma, radiolarians began to recover, possibly because the start of Northern Hemispheric glaciation and the rapid uplift of the Tibet Plateau led to intensification of the East Asian monsoon. After the Mid-Pleistocene Climate Transition at 0.9 Ma, the abundance and mass accumulation rates of radiolarians increased, probably as a result of increased upwelling activity driven by the increasing intensity of the summer monsoon.

The effect of submerged plateaux on Pleistocene gyral circulation and sea-surface temperatures in the Southwest Pacific

Uniquely in the Southern Hemisphere the New Zealand micro-continent spans the interface between a subtropical gyre and the Subantarctic Circumpolar Current. Its 20° latitudinal extent includes a complex of submerged plateaux, ridges, saddles and basins which, in the present interglacial, are partial barriers to circulation and steer the Subtropical (STF) and Subantarctic (SAF) fronts. This configuration offers a singular opportunity to assess the influence of bottom topography on oceanic circulation through Pleistocene glacial – interglacial (G/I) cycles, its effect on the location and strength of the fronts, and its ability to generate significant differences in mixed layer thermal history over short distances. For this study we use new planktic foraminiferal based sea-surface temperature (SST) estimates spanning the past 1 million years from a latitudinal transect of four deep ocean drilling sites. We conclude that: 1. the effect of the New Zealand landmass was to deflect the water masses south around the bathymetric impediments; 2. the effect of a shallow submerged ridge on the down-current side (Chatham Rise), was to dynamically trap the STF along its crest, in stark contrast to the usual glacial–interglacial (G–I) meridional migration that occurs in the open ocean; 3. the effect of more deeply submerged, downstream plateaux (Campbell, Bounty) was to dynamically trap the SAF along its steep southeastern margin; 4. the effects of saddles across the submarine plateaux was to facilitate the development of jets of subtropical and subantarctic surface water through the fronts, forming localized downstream gyres or eddies during different phases in the G–I climate cycles; 5. the deep Pukaki Saddle across the Campbell-Bounty Plateaux guided a branch of the SAF to flow northwards during each glacial, to form a strong gyre of circumpolar surface water in the Bounty Trough, especially during the mid-Pleistocene Climate Transition (MIS 22-16) when exceptionally high SST gradients existed across the STF; 6. the shallower Mernoo Saddle, at the western end of the Chatham Rise, provided a conduit for subtropical water to jet southwards across the STF in the warmest interglacial peaks (MIS 11, 5.5) and for subantarctic water to flow northwards during glacials; 7. although subtropical or subantarctic drivers can prevail at a particular phase of a G–I cycles, it appears that the Antarctic Circumpolar Current is the main influence on the regional hydrography. Thus complex submarine topography can affect distinct differences in the climate records over short distances with implications for using such records in interpreting global or regional trends. Conversely, the local topography can amplify the paleoclimate record in different ways in different places, thus enhancing its value for the study of more minor paleoceanographic influences that elsewhere are more difficult to detect. Such sites include DSDP 594, which like some other Southern Ocean sites, has the typical late Pleistocene asymmetrical saw-tooth G–I climate pattern transformed to a gap-tooth pattern of quasi-symmetrical interglacial spikes that interrupt extended periods of minimum glacial temperatures.

Origin of orbital periods in the sedimentary relative paleointensity records

Orbital cycles with 100 kyr and/or 41 kyr periods, detected in some sedimentary normalized remanence (relative paleointensity) records by power spectral analysis or wavelet analysis, have been attributed either to orbital forcing of the geodynamo, or to lithologic contamination. In this study, local wavelet power spectra (LWPS) with significance tests have been calculated for seven relative paleointensity (RPI) records from different regions of the world. The results indicate that orbital periods (100 kyr and/or 41 kyr) are significant in some RPI records during certain time intervals, and are not significant in others. Time intervals where orbital periods are significant are not consistent among the RPI records, implying that orbital periods in these RPI records may not have a common origin such as orbital forcing on the geodynamo. Cross-wavelet power spectra (|XWT|) and squared wavelet coherence (WTC) between RPI records and orbital parameters further indicate that common power exists at orbital periods but is not significantly coherent, and exhibits variable phase relationships, implying that orbital periods in RPI records are not caused directly by orbital forcing. Similar analyses for RPI records and benthic oxygen isotope records from the same sites show significant coherence and constant in-phase relationships during time intervals where orbital periods were significant in the RPI records, indicating that orbital periods in the RPI records are most likely due to climatic ‘contamination’. Although common power exists at orbital periods for RPI records and their normalizers with significant coherence during certain time intervals, phase relationships imply that ‘contamination’ (at orbital periods) is not directly due to the normalizers. Orbital periods are also significant in the NRM intensity records, and ‘contamination’ in RPI records can be attributed to incomplete normalization of the NRM records. Further tests indicate that ‘contamination’ is apparently not directly related to physical properties such as density or carbonate content, or to the grain size proxy κ ARM/ κ. However, WTC between RPI records and the grain size proxy ARM/IRM implies that ARM/IRM does reflect the ‘contamination’ in some RPI records. It appears that orbital periods were introduced into the NRM records (and have not been normalized when calculating RPI records) through magnetite grain size variations reflected in the ARM/IRM grain size proxy. The orbital power in ARM/IRM for some North Atlantic sites is probably derived from bottom-current velocity variations that are orbitally modulated and are related to the vigor of thermohaline circulation and the production of North Atlantic Deep Water (NADW). In the case of ODP Site 983, the orbital power in RPI appears to exhibit a shift from 41-kyr to 100-kyr period at the mid-Pleistocene climate transition (∼750 ka), reinforcing the climatic origin of these orbital periods. RPI records from the Atlantic and Pacific oceans, and RPI records with orbital periods eliminated by band-pass filters, are highly comparable with each other in the time domain, and are coherent and in-phase in time-frequency space, especially at non-orbital periods, indicating that ‘contamination’, although present (at orbital periods) is not debilitating to these RPI records as a global signal that is primarily of geomagnetic origin.

Paleoceanography of the mid-Pleistocene South China Sea

High-frequency fluctuations in paleoenvironmental proxies from the South China Sea, including stable isotopes and abundance of planktonic foraminifers, nannofossils, radiolarians, and palynomorphs, reveal a dynamic local response to the stepwise development of the mid-Pleistocene climate transition (MPT). These proxies indicate a dramatic drop in sea surface temperature (SST) at about 900 ka, the first largest SST decrease in the region during the Quaternary. Estimated winter SST declined from 24–25 °C to 17–18 °C in the northern and from 26–27 °C to 23–24 °C in the southern South China Sea. Subsequent changes in the thermocline depth and faunal–floral turnovers imply a period of about 300 ka in the final stage of the MPT. Winter monsoons increased at ∼900 ka and reached a maximum strength toward the end of the MPT when summer monsoons also strengthened in interglacials. As a result, thermal gradient between the northern and southern South China Sea increased substantially, with stronger winter monsoon influence in the north and warm and saline conditions in the south especially during glacial periods. These N–S paleoceanographic contrasts indicate an initial establishment of the modern-styled semi-enclosed South China Sea about 900 ka ago when passages in the south started to become completely exposed during glacial lowstands. Coupled with deep water cooling and ventilation, uplift of the sill depth in the Bashi Strait to near the present-2400 m during this period caused sudden decline and extinction of Pacific Deep Water benthic foraminifers in the isolated deep sea basin. Together with data from the oceanic western Pacific, these results further imply a considerable weakening of the western Pacific warm pool during MIS 23-22 and in subsequent glacial periods. While the MPT may have invoked high latitude processes especially an increased ice volume, tropical processes more likely have facilitated the restoration of heat and energy to the western Pacific in each interglacial rebound. Planktonic δ 13C maxima on eccentricity periocities leading major cooling events during the Quaternary indicate the important role of global carbon reservoir changes due to low as well as high latitude processes in past climate change.

Glacial–interglacial ocean climate variability from planktonic foraminifera during the Mid-Pleistocene transition in the temperate Southwest Pacific, ODP Site 1123

Planktonic foraminiferal assemblages and artificial neural network estimates of sea-surface temperature (SST) at ODP Site 1123 (41°47.2'S, 171°29.9'W; 3290 m deep), east of New Zealand, reveal a high-resolution history of glacial–interglacial (G–I) variability at the Subtropical Front (STF) for the last 1.2 million years, including the Mid-Pleistocene climate transition (MPT). Most G–I cycles of ~100 kyr duration have short periods of cold glacial and warm deglacial climate centred on glacial terminations, followed by long temperate interglacial periods. During glacial–deglacial transitions, maximum abundances of subantarctic and subtropical taxa coincide with SST minima and maxima, and lead ice volume by up to 8 kyrs. Such relationships reflect the competing influence of subantarctic and subtropical surface inflows during glacial and deglacial periods, respectively, suggesting alternate polar and tropical forcing of southern mid-latitude ocean climate. The lead of SSTs and subtropical inflow over ice volume points to tropical forcing of southern mid-latitude ocean-climate during deglacial warming. This contrasts with the established hypothesis that southern hemisphere ocean climate is driven by the influence of continental glaciations.Based on wholesale changes in subantarctic and subtropical faunas, the last 1.2 million years are subdivided into 4-distinct periods of ocean climate. 1) The pre-MPT (1185–870 ka) has high amplitude 41-kyr fluctuations in SST, superimposed on a general cooling trend and heightened productivity, reflecting long-term strengthening of subantarctic inflow under an invigorated Antarctic Circumpolar Current. 2) The early MPT (870–620 ka) is marked by abrupt warming during MIS 21, followed by a period of unstable periodicities within the 40–100 kyr orbital bands, decreasing SST amplitudes, and long intervals of temperate interglacial climate punctuated by short glacial and deglacial phases, reflecting lower meridional temperature gradients. 3) The late MPT (620–435 ka) encompasses an abrupt decrease in the subantarctic inflow during MIS 15, followed by a period of warm equable climate. Poorly defined, low amplitude G–I variations in SSTs during this interval are consistent with a relatively stable STF and evenly balanced subantarctic and subtropical inflows, possibly in response to smaller, less dynamic polar icesheets. 4) The post-MPT (435–0 ka) is marked by a major climatic deterioration during MIS 12, and a return to higher amplitude 100 kyr-frequency SST variations, superimposed on a long term trend towards cooler SSTs and increased mixed-layer productivity as the subantarctic inflow strengthened and polar icesheets expanded.