mid-Pleistocene Transition Research Articles

Diverse manifestations of the mid-Pleistocene climate transition

The mid-Pleistocene transition (MPT) is widely recognized as a shift in paleoclimatic periodicity from 41- to 100-kyr cycles, which largely reflects integrated changes in global ice volume, sea level, and ocean temperature from the marine realm. However, much less is known about monsoon-induced terrestrial vegetation change across the MPT. Here, on the basis of a 1.7-million-year δ13C record of loess carbonates from the Chinese Loess Plateau, we document a unique MPT reflecting terrestrial vegetation changes from a dominant 23-kyr periodicity before 1.2 Ma to combined 100, 41, and 23-kyr cycles after 0.7 Ma, very different from the conventional MPT characteristics. Model simulations further reveal that the MPT transition likely reflects decreased sensitivity of monsoonal hydroclimate to insolation forcing as the Northern Hemisphere became increasingly glaciated through the MPT. Our proxy-model comparison suggests varied responses of temperature and precipitation to astronomical forcing under different ice/CO2 boundary conditions, which greatly improves our understanding of monsoon variability and dynamics from the natural past to the anthropogenic future.

Cenozoic tectonic and paleoenvironmental evolution of northwestern China: Evidence from two deep boreholes in the Jartai Basin

To study the Cenozoic tectonic and paleoenvironmental evolution of northwestern China, we drilled two deep boreholes, JLT1 and JLT2, in the Jartai Basin. Magnetostratigraphy, stratigraphy studies were conducted to establish a chronological framework. Sedimentary facies were reconstructed using lithology, geophysical logs (gamma ray (GR) and resistivity (R)) and measurements of grain size, sediment color (L*, a* and b*) and magnetic susceptibility and its frequency dependence. The stratigraphic sequence of borehole JLT2 is similar to that of Paleogene sediments in basins along the northeastern Tibetan Plateau, which may be the result of the far-field effect of the uplift of the Tibetan Plateau during the Paleocene, via faulting activity. The results of borehole JLT1 indicate that the basin was occupied by alternating desert, lacustrine and alluvial fan environments since ca. 6.7 Ma. During ca. 6.7–3.6 Ma, alternations of aeolian sand and lacustrine facies may reflect climatic cooling and drying, and at that time the area may have been an important sediment source for the aeolian Red Clay of the Loess Plateau. During ca. 3.6–2.1 Ma, alluvial fan development also reflected the uplift of the northeastern Tibetan Plateau, and the area acted as a source region for dust supplied to the Loess Plateau and the Shanxi Graben. The alternations of aeolian sand and lacustrine deposits since ca. 2.1 Ma may have resulted from glacial–interglacial variations of the East Asian Monsoon. Since ca. 1.7 Ma, lacustrine deposition developed, because of the enormous discharge of the Yellow River into the Jartai Basin. A tectonic movement at ca. 0.8 Ma is reflected by lacustrine red clay at the top of borehole JLT1. After the mid-Pleistocene transition, the modern landscape of the Jartai Basin and its adjacent regions were formed.

Effects of Periodic Forcing on a Paleoclimate Delay Model

We present a study of a delay differential equation (DDE) model for the Mid-Pleistocene Transition (MPT). We investigate the behavior of the model when subjected to periodic forcing. The unforced model has a bistable region consisting of a stable equilibrium along with a large-amplitude stable periodic orbit. We study how forcing affects solutions in this region. Forcing based on astronomical data causes a sudden transition in time and under increase of the forcing amplitude, moving the model response from a non-MPT regime to an MPT regime. Similar transition behavior is found for periodic forcing. A bifurcation analysis shows that the transition is due not to a bifurcation but instead to a shifting basin of attraction. While determining the basin boundary we demonstrate how one can accurately compute the intersection of a stable manifold of a saddle with a slow manifold in a DDE by embedding the algorithm for planar maps proposed by England, Krauskopf, and Osinga [SIAM J. Appl. Dyn. Syst., 3 (2004), pp. 161–190] into the equation-free framework by Kevrekidis and Samaey [Rev. Phys. Chem., 60 (2009), pp. 321–344].

Increased dust deposition in the Parece Vela Basin since the mid- Pleistocene inferred from radiogenic Sr and Nd isotopes

Abstract Eolian dust deposition is crucial to paleoclimate reconstructions of dust source areas and may have played an important role in the climate during the mid-Pleistocene transition (MPT). Here, we present the radiogenic Sr and Nd isotopic composition and excess barium (Baxs) of a sediment core that was recovered from the Parece Vela Basin (PVB) in the northwestern Pacific. The new record indicates that these detrital sediments are primarily composed of dust from western and central Chinese deserts and volcanic materials from the islands in and around the PVB. A two end-member Sr isotope model was applied to distinguish Asian dust from volcanic materials and to reconstruct the mass accumulation rates (MARs) of dust over the last 2.1 Myr. Starting at ~1.2 Myr, the dust MARs doubled relative to those in the Early Pleistocene, which was coupled with the frequent congruence of minimum eccentricity and low obliquity. We argue that the congruent minimum eccentricity and low obliquity may have triggered the climate change before and during the MPT and resulted in the aridity of East Asia and increased dust input. The coupling of dust MAR, Baxs MAR and atmospheric CO2 concentration and their rapid increase since the MPT strongly suggest that the internal positive feedback of dust input might be another mechanism behind dust input to the PVB since the MPT.

Single foraminifera Mg/Ca analyses of past glacial-interglacial temperatures derived from G. ruber sensu stricto and sensu lato morphotypes

The ratio of magnesium to calcium (Mg/Ca) in foraminiferal shells is commonly used as a proxy for past ocean temperature. Recent advances in elemental analyses now enable single-specimen measurements of planktic foraminifera and thus, can provide information on past seasonal and interannual variability, owing to the near-monthly lifespan of foraminifera. In this study, we explore the temperature variance recorded by Mg/Ca in tests of foraminifera Globigerinoides ruber, a planktic species that occurs throughout the year in tropical waters. Using LA-ICP-MS, we characterize Mg/Ca variability in single specimens of two morphotypes of G. ruber picked from a sediment core retrieved offshore New Caledonia. We provide an estimate of the range of calcification temperatures for these morphotypes during five interglacial-glacial cycles over the last 1.55 Ma. First, we find significant and systematic differences between the morphotypes and second, the temperature difference between morphotypes does not remain constant through time. Our results highlight a progressive increase in surface-water temperatures during interglacials and a progressive decrease in glacial subsurface water temperatures. These changes in surface and subsurface temperatures potentially highlight a change in the stratification of the water column over the Mid-Pleistocene Transition. We conclude that single-specimen Mg/Ca on foraminiferal morphotypes can offer unique perspectives on paleoenvironmental reconstructions.

Sea-Air CO2 Exchange in the SW Iberian Upwelling System during Two Contrasting Climate Cycles: 860–780 ka and 630–520 ka

Analysis of planktonic and benthic foraminifers’ accumulation rates from the Iberian margin reveal a substantial change in the biogenic ocean-atmosphere CO2 exchange during the Mid-Pleistocene Transition (MPT; ~800–650 ka from present). Such changes resulted from the major reorganisations in both surface and deep-water circulation that occurred in the North Atlantic at the time, and affected the behaviour of this upwelling region as a CO2 uptake/release area during climate cycles before and after the MPT. During Marine Isotope Stages (MIS) 21-MIS 20 (860–780 ka), this margin acted mostly as an uptake area during interglacials and early glacials. During glacial maxima and terminations it would be neutral because, although surface production and export were very low, carbon storage occurred at the seafloor. During MIS 15-MIS 14 (630–520 ka), the pattern was the opposite, and the Iberian margin worked as a neutral, or as a source area during most interglacials, while during glacials it acted as an important uptake area. Present findings support the idea that glacial/interglacial atmospheric pCO2 oscillations are partly driven by alterations in the meridional overturning circulation that results in substantial variations of the biological pump, and carbon sequestration rate, in some high-productivity regions.

Closure of the Bering Strait caused Mid-Pleistocene Transition cooling

The Mid-Pleistocene Transition (MPT) is characterised by cooling and lengthening glacial cycles from 600–1200 ka, thought to be driven by reductions in glacial CO2 in particular from ~900 ka onwards. Reduced high latitude upwelling, a process that retains CO2 within the deep ocean over glacials, could have aided drawdown but has so far not been constrained in either hemisphere over the MPT. Here, we find that reduced nutrient upwelling in the Bering Sea, and North Pacific Intermediate Water expansion, coincided with the MPT and became more persistent at ~900 ka. We propose reduced upwelling was controlled by expanding sea ice and North Pacific Intermediate Water formation, which may have been enhanced by closure of the Bering Strait. The regional extent of North Pacific Intermediate Water across the subarctic northwest Pacific would have contributed to lower atmospheric CO2 and global cooling during the MPT.

A two million year record of low-latitude aridity linked to continental weathering from the Maldives

Indian-Asian monsoon has oscillated between warm/wet interglacial periods and cool/dry glacial periods with periodicities closely linked to variations in Earth’s orbital parameters. However, processes that control wet versus dry, i.e. aridity cyclical periods on the orbital time-scale in the low latitudes of the Indian-Asian continent remain poorly understood because records over millions of years are scarce. The sedimentary record from International Ocean Discovery Program (IODP) Expedition 359 provides a well-preserved, high-resolution, continuous archive of lithogenic input from the Maldives reflecting on low-latitude aridity cycles. Variability within the lithogenic component of sedimentary deposits of the Maldives results from changes in monsoon-controlled sedimentary sources. Here, we present X-ray fluorescence (XRF) core-scanning results from IODP Site U1467 for the past two million years, allowing full investigation of orbital periodicities. We specifically use the Fe/K as a terrestrial climate proxy reflecting on wet versus dry conditions in the source areas of the Indian-Asian landmass, or from further afield. The Fe/K record shows orbitally forced cycles reflecting on changes in the relative importance of aeolian (stronger winter monsoon) during glacial periods versus fluvial supply (stronger summer monsoon) during interglacial periods. For our chronology, we tuned the Fe/K cycles to precessional insolation changes, linking Fe/K maxima/minima to insolation minima/maxima with zero phase lag. Wavelet and spectral analyses of the Fe/K record show increased dominance of the 100 kyr cycles after the Mid Pleistocene Transition (MPT) at 1.25 Ma in tandem with the global ice volume benthic δ18O data (LR04 record). In contrast to the LR04 record, the Fe/K profile resolves 100-kyr-like cycles around the 130 kyr frequency band in the interval from 1.25 to 2 million years. These 100-kyr-like cycles likely form by bundling of two or three obliquity cycles, indicating that low-latitude Indian-Asian climate variability reflects on increased tilt sensitivity to regional eccentricity insolation changes (pacing tilt cycles) prior to the MPT. The implication of appearance of the 100 kyr cycles in the LR04 and the Fe/K records since the MPT suggests strengthening of a climate link between the low and high latitudes during this period of climate transition.

Deep-sea ostracod faunal dynamics in a marginal sea: biotic response to oxygen variability and mid-Pleistocene global changes

AbstractDeep-sea benthic ostracod assemblages covering the last 2 Myr were investigated in Integrated Ocean Drilling Program Site U1426 (at 903 m water depth) in the southern Sea of Japan. Results show that (1) orbital-scale faunal variability has been influenced by eustatic sea-level fluctuations and oxygen variability and (2) secular-scale faunal transitions are likely associated with the mid-Brunhes event (MBE, ~0.43 Ma) and the onset of the Tsushima Warm Current (TWC, ~1.7 Ma).Krithe,Robertsonites, andAcanthocythereisare the three most abundant genera throughout the core, accounting for 78.5% of total specimens. Multiple-regression tree analysis indicated that the TWC, the MBE, and oxygen content are the significant controlling factors of ostracod dominance. Changes in assemblages exhibit decline and recovery patterns corresponding to orbital-scale cyclicity of sea-level changes. In the Sea of Japan marginal ocean setting, this cyclicity shows a close relationship with bottom-water oxygen variability since the onset of the TWC influx. The MBE amplified the influence of the TWC and oxygen variability to the deep-sea ecosystem through larger sea-level fluctuations.Acanthocythereis dunelmensis, a circumpolar species, dominates before the TWC onset. After the TWC onset and during the mid-Pleistocene transition (MPT, ~1.2–0.7 Ma)Krithespp., known for their low-oxygen tolerance, substantially increase under moderate oxygen depletion. At the end of the MPT,Krithedominance diminishes and is replaced byRobertsonites hanaiiandPropontocyprisspp. after the MBE. The post-MBE assemblage, characterized byR. hanaii, suggests a slightly warmer environment under the development of the TWC. In addition, the post-MBE high-amplitude climate system may have caused the increased abundance of active-swimmingPropontocyprisspp. due to their superior migration ability. Benthic ecosystems in marginal seas are sensitive and vulnerable to both short- and long-term climatic changes, and the MBE is suggested to be a global biotic event affecting benthic ecosystems substantially.

Change in the North Atlantic circulation associated with the mid-Pleistocene transition

Abstract. The southwestern Iberian margin is highly sensitive to changes in the distribution of North Atlantic currents and to the position of oceanic fronts. In this work, the evolution of oceanographic parameters from 812 to 530 ka (MIS20–MIS14) is studied based on the analysis of planktonic foraminifer assemblages from site IODP-U1385 (37∘34.285′ N, 10∘7.562′ W; 2585 m b.s.l.). By comparing the obtained results with published records from other North Atlantic sites between 41 and 55∘ N, basin-wide paleoceanographic conditions are reconstructed. Variations of assemblages dwelling in different water masses indicate a major change in the general North Atlantic circulation during MIS16, coinciding with the definite establishment of the 100 ky cyclicity associated with the mid-Pleistocene transition. At the surface, this change consisted in the redistribution of water masses, with the subsequent thermal variation, and occurred linked to the northwestward migration of the Arctic Front (AF), and the increase in the North Atlantic Deep Water (NADW) formation with respect to previous glacials. During glacials prior to MIS16, the NADW formation was very weak, which drastically slowed down the surface circulation; the AF was at a southerly position and the North Atlantic Current (NAC) diverted southeastwards, developing steep south–north, and east–west, thermal gradients and blocking the arrival of warm water, with associated moisture, to high latitudes. During MIS16, the increase in the meridional overturning circulation, in combination with the northwestward AF shift, allowed the arrival of the NAC to subpolar latitudes, multiplying the moisture availability for ice-sheet growth, which could have worked as a positive feedback to prolong the glacials towards 100 ky cycles.

Orbital-scale evolution of the Indian summer monsoon since 1.2 Ma: Evidence from clay mineral records at IODP Expedition 355 Site U1456 in the eastern Arabian Sea

Clay mineral assemblages at International Ocean Discovery Program Expedition 355 Site U1456 drilled in the eastern Arabian Sea have been investigated to reveal the sediment provenances and reconstruct the erosion/weathering patterns in the western Himalaya and Indian subcontinent, thus constraining the evolution of the Indian summer monsoon and its forcing mechanism since 1.2 Ma. The clay mineral assemblages at Site U1456 mainly comprise smectite (with an average value of 59%) and illite (with an average value of 33%), with chlorite (with an average value of 5%) and kaolinite (with an average value of 3%) as minor constituents. In terms of sediment provenance, our results indicate that illite and chlorite are predominantly derived from the Indus River, which originates from the western Himalaya and Karakoram, while smectite is primarily sourced from the Narmada River and Tapti River in the Deccan Traps, with a non-negligible contribution from the Indus River in some cases. Variations in smectite/(illite + chlorite) ratio are ultimately controlled by the Indian summer monsoon, which is characterized by approximate glacial/interglacial cyclicity, showing higher values (i.e., enhanced chemical weathering) during interglacial periods. In addition, a major shift in smectite/(illite + chlorite) ratio at 0.9 Ma is correlated to the Mid-Pleistocene Transition (∼1.2–0.9 Ma). Both the Marine Isotope Stage 13 event (533–478 ka) and the Mid-Brunhes Event (∼430 ka) are also recorded in the clay mineral proxies. Based on the spectral analysis, smectite/(illite + chlorite) ratio displays a transition from nonprimary periodicity (29-kyr) to strong eccentricity (100-kyr) and precession (22-kyr) periodicities at approximately 0.9 Ma, which corresponds to the Mid-Pleistocene Transition. Our study indicates that the variability of the Indian summer monsoon at orbital timescales is controlled by high-latitude (i.e., ice volume) and low-latitude (i.e., summer insolation) processes.

The impact of eustatic sea-level fluctuations, temperature variations and nutrient-level changes since the Pliocene on tropical carbonate platform (Xisha Islands, South China Sea)

Sedimentological, paleontological, and geochemical data from core XK-1 in the Xisha Islands, South China Sea, allow a detailed documentation of the evolution of the reef-bank system from the early Pliocene to the late Pleistocene and of the aeolianite deposits from the late Pleistocene to the Holocene. A bank characterized by heterozoan carbonates occurred until 1.7 Ma. Between 1.7 and 0.2 Ma, the system turned into a reef dominated by photozoan carbonates. After 0.2 Ma, the aeolianite deposits, composed of heterozoan skeletal grains, occupied this area. A Pliocene to Pleistocene decrease in the abundance of planktonic foraminifera and some benthic foraminifer species generally associated with deep water settings indicates a long-term shallowing-upwards trend, resulting from the large-scale eustatic sea-level falls. Seven exposure horizons are recorded in core XK-1, due to high-frequency sea-level changes. The thickness of the reef body in the individual cycles increased after the Middle Pleistocene transition. This is interpreted as a consequence of the increase in duration of the short-term sea-level highstands. The formation of heterozoan aeolianite deposits after 0.2 Ma is probably caused by climate cooling, related with the intensification of East Asian winter monsoon. The rapid increase in coral abundance at ~1.7 Ma correlates well with a sharp decline in nutrient level, recorded by decreases in geochemical nutrient indexes values, declines in bryozoan and red algae (rhodolith) abundance, and decreases in the abundance of foraminifer species indicative of elevated nutrient-level. This correlation indicates that the carbonate factory turnovers were also affected by nutrient-level fluctuations, which could be linked to a paleoceanographic reorganization. These results demonstrate that together with sea-level fluctuations, also paleoceanographic changes had a significant effect on the Pliocene-Holocene tropical carbonate system.

Paleoceanographic history of the Japan Sea over the last 9.5 million years inferred from radiolarian assemblages (IODP Expedition 346 Sites U1425 and U1430)

Previous studies showed that the evolution of the Japan Sea paleoceanography since the Miocene has been influenced by the regional tectonism (e.g., opening/closing of the connecting seaways) and regional/global climate. In the Japan Sea, Expedition 346 of the Integrated Ocean Drilling Program (IODP) retrieved core sediments dating back to the Miocene at two sites (U1425 and U1430). In this study, we reconstruct shallow-to-deep-water hydrography of the Japan Sea during the Mio-Pliocene based on radiolarian assemblages at Sites U1425 and U1430 considering the local tectonism and changes in global/regional climate. Our data suggest that glacioeustatic sea-level changes have probably had an influence on the local paleoceanography between 9.5 and 7.0 Ma. Indeed, warm water probably flowed from the North Pacific into the Japan Sea when sea level was high via shallow central and eastern seaways. In addition, the sill depth of the northern seaway was probably close to 1000 m between 9.5 and 7.8 Ma and had probably allowed inflow of oxygen minimum zone water from the North Pacific to the Japan Sea when sea level was high. In contrast, our data imply that Cycladophora nakasekoi, an endemic species to the Japan Sea, dominated between 9.5 and 7.3 Ma when sea level was low. Our data also suggest a progressive shoaling of the sill for the period since 7.8 Ma and that global climatic events such as such the late Miocene cooling (7.5–5.5 Ma) and the early Pliocene warmth have had a sustained influence on the Japan Sea. During the mid-Pliocene, a deep cooling of the subsurface to intermediate water of the Japan Sea likely occurred because species related to subarctic subsurface to intermediate waters were dominant between 5 and 3.8 Ma. The Northern Hemisphere Glaciation (ca. 3.0–2.7 Ma) and Mid-Pleistocene Transition (1.2–0.8 Ma) have both likely intensified the cooling of the Japan Sea.

The Mid-Pleistocene Transition induced by delayed feedback and bistability

The Mid-Pleistocene Transition, the shift from 41 kyr to 100 kyr glacial-interglacial cycles that occurred roughly 1 Myr ago, is often considered as a change in internal climate dynamics. Here we revisit the model of Quaternary climate dynamics that was proposed by Saltzman and Maasch (1988). We show that it is quantitatively similar to a scalar equation for the ice dynamics only when combining the remaining components into a single delayed feedback term. The delay is the sum of the internal times scales of ocean transport and ice sheet dynamics, which is on the order of 10 kyr. We find that, in the absence of astronomical forcing, the delayed feedback leads to bistable behaviour, where stable large-amplitude oscillations of ice volume and an equilibrium coexist over a large range of values for the delay. We then apply astronomical forcing. We perform a systematic study to show how the system response depends on the forcing amplitude. We find that over a wide range of forcing amplitudes the forcing leads to a switch from small-scale oscillations of 41 kyr to large-amplitude oscillations of roughly 100 kyr without any change of other parameters. The transition in the forced model consistently occurs near the time of the Mid-Pleistocene Transition as observed in data records. This provides evidence that the MPT could have been primarily a forcing-induced switch between attractors of the internal dynamics. Small additional random disturbances make the forcing-induced transition near 800 kyr BP even more robust. We also find that the forced system forgets its initial history during the small-scale oscillations, in particular, nearby initial conditions converge prior to transitioning. In contrast to this, in the regime of large-amplitude oscillations, the oscillation phase is very sensitive to random perturbations, which has a strong effect on the timing of the deglaciation events.

Plio-Pleistocene sedimentary record from the Northwind Ridge: new insights into paleoclimatic evolution of the western Arctic Ocean for the last 5 Ma

Sediment core HLY0503-03JPC from the top of the Northwind Ridge provides the first confirmed Plio-Pleistocene record from the western Arctic Ocean, with calcareous microfossils uniquely preserved to ca. 5 Ma. Results are compared to nearby core P1-93AR-P23 from the ridge slope, which was previously used to reconstruct early Quaternary sea-ice conditions in the region (Polyak et al. in Quat Sci Rev 79:145–156, 2013), and is now re-dated to at least the late Pliocene. Ages were estimated primarily from strontium isotope stratigraphy on benthic foraminifers. Based on multiple physical, paleomagnetic, elemental geochemical, and paleobiological (foraminifers) proxies, we identify three major stratigraphic divisions (Units 1, 2a, and 2b) roughly representing upper to middle (“glacial”) Quaternary, lower Quaternary to Pliocene, and lower Pliocene to possibly upper Miocene (undated). Benthic foraminiferal assemblages were utilized to evaluate paleo-sea-ice conditions, while other proxies were used to interpret paleocirculation and sediment transport processes. Early Quaternary and older sediments indicate diminutive effect from glaciations, reduced sea-ice conditions, and a periodic strong current impact on the ridge top, possibly due to an enhanced Atlantic water flow. Ages derived from the first foraminiferal tests appearing at ca. 5 Ma likely indicate a redeposition pulse that we attribute to the onset of Pacific water throughflow via the Bering Strait. A large hiatus above this level in JPC3 spans most of the Pliocene. The Unit 2a/1 boundary, estimated to ca. 0.8 Ma, is marked by an abrupt faunal and sedimentary change, which is consistent with the major climatic shift that occurred during this time (Mid-Pleistocene Transition). Unit 1 exhibits a strong control from glacial cyclicity, with a progressive expansion of the Laurentide Ice Sheet primarily affecting the study region, and mostly perennial sea-ice conditions. Overall results suggest that the Pliocene and early Pleistocene may provide relevant paleoclimatic analogs for the rapidly changing Arctic environments of today.

A theory of Pleistocene glacial rhythmicity

Abstract. Variations in Northern Hemisphere ice volume over the past 3 million years have been described in numerous studies and well documented. These studies depict the mid-Pleistocene transition from 40 kyr oscillations of global ice to predominantly 100 kyr oscillations around 1 million years ago. It is generally accepted to attribute the 40 kyr period to astronomical forcing and to attribute the transition to the 100 kyr mode to a phenomenon caused by a slow trend, which around the mid-Pleistocene enabled the manifestation of nonlinear processes. However, both the physical nature of this nonlinearity and its interpretation in terms of dynamical systems theory are debated. Here, we show that ice-sheet physics coupled with a linear climate temperature feedback conceal enough dynamics to satisfactorily explain the system response over the full Pleistocene. There is no need, a priori, to call for a nonlinear response of the carbon cycle. Without astronomical forcing, the obtained dynamical system evolves to equilibrium. When it is astronomically forced, depending on the values of the parameters involved, the system is capable of producing different modes of nonlinearity and consequently different periods of rhythmicity. The crucial factor that defines a specific mode of system response is the relative intensity of glaciation (negative) and climate temperature (positive) feedbacks. To measure this factor, we introduce a dimensionless variability number, V. When positive feedback is weak (V∼0), the system exhibits fluctuations with dominating periods of about 40 kyr which is in fact a combination of a doubled precession period and (to smaller extent) obliquity period. When positive feedback increases (V∼0.75), the system evolves with a roughly 100 kyr period due to a doubled obliquity period. If positive feedback increases further (V∼0.95), the system produces fluctuations of about 400 kyr. When the V number is gradually increased from its low early Pleistocene values to its late Pleistocene value of V∼0.75, the system reproduces the mid-Pleistocene transition from mostly 40 kyr fluctuations to a 100 kyr period rhythmicity. Since the V number is a combination of multiple parameters, it implies that multiple scenarios are possible to account for the mid-Pleistocene transition. Thus, our theory is capable of explaining all major features of the Pleistocene climate, such as the mostly 40 kyr fluctuations of the early Pleistocene, a transition from an early Pleistocene type of nonlinear regime to a late Pleistocene type of nonlinear regime, and the 100 kyr fluctuations of the late Pleistocene. When the dynamical climate system is expanded to include Antarctic glaciation, it becomes apparent that climate temperature positive feedback (or its absence) plays a crucial role in the Southern Hemisphere as well. While the Northern Hemisphere insolation impact is amplified by the outside-of-glacier climate and eventually affects Antarctic surface and basal temperatures, the Antarctic ice-sheet area of glaciation is limited by the area of the Antarctic continent, and therefore it cannot engage in strong positive climate feedback. This may serve as a plausible explanation for the synchronous response of the Northern and Southern Hemisphere to Northern Hemisphere insolation variations. Given that the V number is dimensionless, we consider that this model could be used as a framework to investigate other physics that may possibly be involved in producing ice ages. In such a case, the equation currently representing climate temperature would describe some other climate component of interest, and as long as this component is capable of producing an appropriate V number, it may perhaps be considered a feasible candidate.

A 1 Ma sea surface temperature record from the North Atlantic and its implications for the early human occupation of Britain

AbstractThe British Quaternary sequence has an exceptionally rich record of Palaeolithic archaeology up to 1 Ma. In this study, we reinvestigate foraminifera-based sea surface temperature (SST) reconstructions from the two marine core records (Ocean Drilling Program Site 980 and M23414) that are most relevant to the climatic history of the British Isles, consequently allowing the evolution of SST over the past 1 Ma to be studied. This is then compared with long-term changes with the British archaeological record in order to understand in greater detail the changing patterns of climatic forcing and the major climatic transitions that were the background environmental drivers against which patterns of early human occupation occurred. These include the mid-Pleistocene revolution, the mid-Brunhes event, and changing patterns of isotopic substage complexity. Significantly, however, the SST record indicates that the Marine Oxygen Isotope Stage 15–13 interval was characterised by the most prolonged period of consistently warm conditions of the entire 1 Ma interval in the northeast Atlantic. This unique climatic period correlates with the first major proliferation of archaeological sites in northwest Europe. The article concludes by discussing the significance of these climatic shifts for our understanding of early human occupation in this region.

Changes in the distribution of Uvigerinidae species over the past 775 kyr: Implications for the paleoceanographic evolution of the Japan Sea

The distribution of benthic foraminifera of the Uvigerinidae family, which are abundant in middle to upper Pleistocene samples from the Yamoto Basin in the Japan Sea, have been analyzed quantitatively and used for paleoceanographic interpretations. Twenty-three Uvigerinidae species are reported from IODP Site U1426, 6 of which are present in at least three or more samples with 10% or more abundance and considered as dominant species. The abundance and ecological preferences of dominant Uvigerinidae species, comprised of Uvigerina mediterranea, Uvigerina yabei, Uvigerina sp.1, Uvigerina peregrina suggest dysoxic conditions, with high influx of organic matter between 775 and 475 ka in response to the enhanced flow of the Tsushima Warm Current. The end of MIS 13 marked the onset of more extreme glacial interglacial climate cycles in the Japan Sea. Glacial interglacial alternations of Uvigerina peregrina and Trifarina angulosa from 475 ka to the Holocene indicate major fluctuations in circulation and productivity within this semi-enclosed basin. Both U. peregrina and T. angulosa respond to 100 kyr variability, reflecting the paleoclimatic shift and global ice volume changes associated with the Middle Pleistocene Transition.

Complex Climate Response to Astronomical Forcing: The Middle-Pleistocene Transition in Glacial Cycles and Changes in Frequency Locking

Through the past few million years large ice sheets have repeatedly grown and disappeared on the Northern hemisphere. These are the Pleistocene glaciations. They are related to the changing solar heating of the Earth due to changes in Earth's orbit and axis of rotation. The climate response to these changes is highly non-trivial and non-linear, expressing the complex nature of the climate system. Many aspects of glacial cycles still need a convincing explanation, one particular mystery being the change from approximately 40 kyr (kilo year) glacial cycles to approximately 100 kyr cycles around 1 million years ago. This transition is called the middle Pleistocene transition (MPT). Here we review some conceptual models to explain the dynamics of glacial cycles and possible dynamical causes of the MPT. We especially focus on the well studied van del Pol oscillator as a conceptual model for the glacial cycles and propose that the MPT is a result of changes in frequency locking of the climate system to the astronomical forcing. This is compared to a recently presented model that relates the MPT to a transcritical bifurcation in the structure of a generic critical/slow manifold for a fast-slow dynamical system.

Green sea turtle (Chelonia mydas) population history indicates important demographic changes near the mid-Pleistocene transition

The increased availability of genome sequences has provided remarkable advances in our understanding of the evolutionary history of non-model species. One important consideration in evolutionary studies is the role of demographic history in shaping contemporary levels and distribution of genetic variation. In green sea turtles (Chelonia mydas), a draft genome sequence has recently been made available, yet little is known regarding how past demographic events have shaped genomic variation in populations of this species. In this study, single nucleotide polymorphisms were identified in the green sea turtle’s genome and used to reconstruct past demographic events. It was found that this green sea turtle population, from the South China Sea, experienced a marked expansion ca. 0.8 million years ago near the mid-Pleistocene transition (MPT). Simulations revealed that the past demographic history can at least partially be explained by changes in population structure and gene flow; possibly associated with the climatic and geomagnetic events occurring since the MPT. The results demonstrate the importance in considering the effects of gene flow when reconstructing historical changes in population size and provide an extensive set of genomic resources for future evolutionary studies of green sea turtles.