Millennial-scale Climate Oscillations Research Articles

Deciphering the Variations and Mechanisms of Global Land Monsoons during Marine Isotope Stage 3

Abstract Marine Isotope Stage 3 (MIS 3) is characterized by significant millennial-scale climatic oscillations between cold stadials and mild interstadials, which presents a valuable case for understanding hydrological response to abrupt climate change. Through a set of coupled model simulations, our results broadly show an antiphased interhemispheric change in land monsoonal precipitation during the present-day relative to MIS 3 interstadial and the stadial–interstadial transition, with a general decrease in the Northern Hemisphere but an increase in the Southern Hemisphere. The antiphased pattern is largely caused by the change in orbital insolation during the present-day relative to MIS 3 interstadial, whereas by the weakened Atlantic meridional overturning circulation during the interstadial–stadial transition. However, there are obvious discrepancies in precipitation response and underlying mechanisms among individual monsoon domains and across different periods. Based on the moisture budget analysis, we indicate that the dynamic factor mainly explains the decreased monsoonal rainfall in the Northern Hemisphere during the present-day relative to the MIS 3 interstadial, whereas the thermodynamic term is largely responsible for the increased precipitation in the Southern Hemisphere. In contrast, the dynamic factor plays an important role in the variation of precipitation over all the monsoon zones from the MIS 3 interstadial to stadial states, with the thermodynamic term mainly contributing to the decreased tropical monsoonal precipitation in the colder Northern Hemisphere. Our results help improve the understanding of global monsoon variations under intermediate glacial climate conditions and shed light on their behaviors under potentially rapid climate change in the future.

Legacies of millennial-scale climate oscillations in contemporary biodiversity in eastern North America.

The Atlantic meridional overturning circulation (AMOC) has caused significant climate changes over the past 90 000 years. Prior work has hypothesized that these millennial-scale climate variations effected past and contemporary biodiversity, but the effects are understudied. Moreover, few biogeographic models have accounted for uncertainties in palaeoclimatic simulations of millennial-scale variability. We examine whether refuges from millennial-scale climate oscillations have left detectable legacies in the patterns of contemporary species richness in eastern North America. We analyse 13 palaeoclimate estimates from climate simulations and proxy-based reconstructions as predictors for the contemporary richness of amphibians, passerine birds, mammals, reptiles and trees. Results suggest that past climate changes owing to AMOC variations have left weak but detectable imprints on the contemporary richness of mammals and trees. High temperature stability, precipitation increase, and an apparent climate fulcrum in the southeastern United States across millennial-scale climate oscillations aligns with high biodiversity in the region. These findings support the hypothesis that the southeastern United States may have acted as a biodiversity refuge. However, for some taxa, the strength and direction of palaeoclimate-richness relationships varies among different palaeoclimate estimates, pointing to the importance of palaeoclimatic ensembles and the need for caution when basing biogeographic interpretations on individual palaeoclimate simulations. This article is part of the theme issue 'Ecological novelty and planetary stewardship: biodiversity dynamics in a transforming biosphere'.

Dansgaard–Oeschger events in climate models: review and baseline Marine Isotope Stage 3 (MIS3) protocol

Abstract. Dansgaard–Oeschger (D–O) events, millennial-scale climate oscillations between stadial and interstadial conditions (of up to 10–15 ∘C in amplitude at high northern latitudes), occurred throughout the Marine Isotope Stage 3 (MIS3; 27.8–59.4 ka) period. The climate modelling community up to now has not been able to answer the question of whether our climate models are too stable to simulate D–O events. To address this, this paper lays the ground-work for a MIS3 D–O protocol for general circulation models which are used in the International Panel for Climate Change (IPCC) assessments. We review the following: D–O terminology, community progress on simulating D–O events in these IPCC-class models (processes and published examples), and evidence about the boundary conditions under which D–O events occur. We find that no model exhibits D–O-like behaviour under pre-industrial conditions. Some, but not all, models exhibit D–O-like oscillations under MIS3 and/or full glacial conditions. Greenhouse gases and ice sheet configurations are crucial. However most models have not run simulations of long enough duration to be sure which models show D–O-like behaviour, under either MIS3 or full glacial states. We propose a MIS3 baseline protocol at 34 ka, which features low obliquity values, medium to low MIS3 greenhouse gas values, and the intermediate ice sheet configuration, which our review suggests are most conducive to D–O-like behaviour in models. We also provide a protocol for a second freshwater (Heinrich-event-preconditioned) experiment, since previous work suggests that this variant may be helpful in preconditioning a state in models which is conducive to D–O events. This review provides modelling groups investigating MIS3 D–O oscillations with a common framework, which is aimed at (1) maximising the chance of the occurrence of D–O-like events in the simulations, (2) allowing more precise model–data evaluation, and (3) providing an adequate central point for modellers to explore model stability.

A 50 kyr record of eolian sedimentation in the Eastern Arabian Sea – Dust deposition changes synchronous with the Northern Hemisphere Climatic Oscillations

The Late Pleistocene Northern Hemisphere (NH) climate was extremely variable, owing to the abrupt warm Dansgaard–Oeschger (D/O) events and cold Heinrich events in the Greenland and North Atlantic. Although the close coupling between the Indian monsoon and the Northern hemisphere climatic fluctuations has been previously reported from several western and northern Arabian Sea records, these millennial-scale climatic oscillations are not clearly defined in the Eastern Arabian Sea (EAS) possibly because many of the past records are from the continental margin which are vulnerable to sediment slumps and slides. Here, we present a 50 kyr eolian sedimentation record from a bathymetric high off Goa in the EAS, reconstructed using the environmental magnetic parameter S-ratio (a measure of the relative proportions of ferromagnetic and canted antiferromagnetic minerals) and the mass accumulation rate of iron and magnesium (FeMAR and MgMAR). S-ratio record shows striking similarity with the δ18O record from the Greenland GISP2 ice core, indicating the control of the temperature fluctuations of high northern latitudes on the low-latitude climate variability. The cold Heinrich events are marked by lowered S-ratios indicating less eolian input while the D/O interstadials are marked by increased S-ratios illustrating the intensified summer monsoon wind strength and the increased eolian supply during these periods. The mass accumulation rate of Mg and Fe reveals high values during 29–18 kyr reflecting the increased source area aridity and dust transport during the last glacial period. The spectral analysis of the S-ratio record reveals significant periodicities centered at 24 kyr and 1.3 kyr establishing the orbital and northern hemisphere controls on the EAS climate.

Millennial-scale climate oscillations over the last two climatic cycles revealed by a loess–paleosol sequence from central Asia

• Multiscale climate variability is decomposed by using EEMD method in central Asia. • Northern high-latitude temperature regulated the millennial moisture variability. • Higher insolation during the MIS6.5 promoted the moisture oscillations. Characteristics of millennial-scale climate variability over central Asia during the last two climatic cycles remain unclear. Here, multi-time scale climate variability during the previous two climatic cycles over central Asia is demonstrated using a 50-m thick loess–paleosol sequence and the ensemble empirical mode decomposition method. Decomposed orbital-scale components show that climate oscillation across central Asia is particularly influenced by northern high-latitude ice volume and solar insolation, while the millennial-scale moisture variability shows similar periodicities to the rhythms of North Atlantic cooling. Millennial-scale moisture variability exhibits a pattern of long duration/high amplitude during interglacial periods and short duration/low amplitude during glacial periods, a pattern also reflected in loess sections from the Chinese Loess Plateau (CLP) and CH 4 records from Antarctic ice -cores. These characteristics differ from the high-latitude temperature variability recorded in NGRIP δ 18 O data, which indicates that millennial-scale moisture oscillation in central Asia is possibly regulated by the northern high-latitude temperature, with lower temperatures limiting moisture fluctuations. We also find that only four millennial-scale moisture oscillations can be clearly detected during the MIS6.5 period, during which increased summer insolation provided favorable conditions to trigger abrupt climate events. However, high-resolution climate proxies that are more sensitive to environmental change are required to detect moisture variability during extreme cold and dry periods.

Heinrich summers

Millennial-scale climate oscillations of the last ice age registered in Greenland and Antarctic ice cores did not always vary in unison. A striking example is that the strongest Antarctic warming episodes occurred during Heinrich episodes in the North Atlantic region. Although the bipolar seesaw affords a possible explanation for such anti-phasing, it does not account for the equally striking observation that climate varied in unison between the hemispheres about half the time. Such phasing differences suggest the need for an alternative hypothesis in which the polar regions at times responded in unison to common forcing, and at other times left the impression of a bipolar seesaw. We posit that this impression arose from the effect of warmer-than-usual summers on continental ice sheets adjacent to the North Atlantic Ocean during each Heinrich episode. The relatively warm Heinrich summers produced discharges of meltwater and icebergs of sufficient volume to stimulate very cold winter conditions from widespread sea ice on a freshened ocean surface. The intervals between Heinrich episodes featured relaxation of sea-ice-induced winter severity from reduced summertime influx of meltwater and icebergs, indicating relatively cooler summer conditions. It is postulated that the causative variations in freshwater fluxes were driven by a climate signal most evident in Antarctic ice cores but also recognized in other paleoclimate records in both polar hemispheres. We suggest that this widespread signal arose from changes in the latitude and strength of the austral westerlies and the resulting effect on the western Pacific tropical warm pool, a mechanism dubbed the Zealandia Switch.

Timing and mechanisms of sediment accumulation and pedogenesis: Insights from the Po Plain (northern Italy)

The relationships between pedogenetic processes and fluvial-coastal dynamics in the Po Plain have been reconstructed through sedimentological analysis and correlation of ca. 170 core data chronologically constrained by 376 radiocarbon dates. Vertically stacked, weakly developed paleosols within Upper Pleistocene and Holocene mud-prone strata testify to intermittent pedogenesis, periodically interrupted by overbank sedimentation. Individual paleosols are laterally traceable for tens of km and exhibit A-Bk-Bw, A-Bk or A-Bw profiles. Stratigraphically ordered 14 C calibrated ages from A organo-mineral horizons testify to slow aggradation during 4–6 thousand years-long exposure periods. Burial ages, with an error of few centuries, are provided by plant debris at the top of A horizons. Millennial-scale climate oscillations and glacio-eustasy are the main drivers of the pedo-sedimentary evolution of the area during the last 50 kyr. Upper Pleistocene paleosols (P1-P3) developed in well-drained floodplain environments, during relatively warm periods. Paleosol burial occurred during colder phases. High-sediment-supply during the Last Glacial Maximum hindered pedogenesis and led to the accumulation of 3–10 m-thick overbank strata. Widespread soil development (paleosol PH) occurred at the end of Last Glacial Maximum, following the retreat of Alpine glaciers and the afforestation of Apennine drainage basins. At distal locations, paleosol PH was progressively buried under estuarine sediments during the Holocene phases of post-glacial sea-level rise. Beyond the area of marine influence, burial ages of paleosol PH change from a place to another without specific spatial trends and reflect upstream fluvial sedimentation dominated by avulsions and deposition of spatially restricted alluvial units. Holocene (H1−H2) paleosols show a poor correlation potential and laterally variable degree of maturity that reflect avulsive sedimentation patterns and crevassing. This paper provides insights on the timing and mechanisms of formation and burial of weakly-developed paleosols. The outcomes of this research are applicable to similar Quaternary alluvial systems, and may help interpreting ancient paleosol-bearing successions. • Po Plain pedostratigraphy reveals relations between pedogenetic and alluvial events. • Upper Pleistocene-Holocene stacked paleosols testify to intermittent pedogenesis. • Radiocarbon dates permitted to estimate the duration of the main pedogenetic phases. • Climate change and eustasy controlled Late Pleistocene pedo-sedimentary evolution. • Discontinuous Holocene paleosols reflect avulsive sedimentation and crevassing.

Linking the Mediterranean MIS 5 tephra markers to Campi Flegrei (southern Italy) 109–92 ka explosive activity and refining the chronology of MIS 5c-d millennial-scale climate variability

Explosive activity preceding the ~40 ka Campanian Ignimbrite (CI) eruption in the Neapolitan volcanic area, Southern Italy, has long been speculated based on the occurrences of widespread tephra layers, with a Campanian geochemical signature, such as the C-22, X-5, and X-6, preserved in Mediterranean Marine Isotope Stage (MIS) 5 sedimentary records. However, previous studies of pre-CI pyroclastic units occurring in close proximity of the Neapolitan volcanoes, including Campi Flegrei, Somma-Vesuvius, Ischia and Procida islands, did not allow a conclusive identification of the near-source equivalents of these tephra markers. Here we present a comprehensive characterization of four pyroclastic units from the Campanian Plain, comprising major and trace element glass compositions, Sr Nd isotopes and 40 Ar/ 39 Ar dating. Our data allowed the identification of the medial equivalents of the MIS 5 tephra markers, including the widespread C-22, X-5, and X-6 tephra, and their assignment to previously undocumented Campi Flegrei activity between 109 and 92 ka. In addition to substantially extending Campi Flegrei explosive activity deeper in time, and thus providing the basis for a revaluation of its history, our findings provide new precise radioisotopic dating to better constrain the chronology of the millennial scale climatic oscillations of the MIS 5c-d in the Mediterranean area and possibly on a larger scale. • X-6, X-5 and C-22 act as fundamental markers for the Mediterranean MIS 5 stratigraphy • This widespread markers, and two additional tephra, originated in Campi Flegrei volcanic area • These add to the history of this volcano five eruptions occurred between 109 and 92 ka • Their high-precision dating allows the timing MIS 5 climatic events to be precisely assessed

Abrupt Quaternary Ocean-ice Events in the Arctic: Evidence from the Ostracode Rabilimis

The Arctic Ocean has experienced orbital and millennial-scale climate oscillations over the last 500 kilo-annum (ka) involving massive changes in global sea level and components of the Arctic cryosphere, including sea-ice cover, land-based ice sheets and ice shelves. Although these climate events are only partially understood, micropaleontological studies utilizing ostracodes and benthic foraminifera have demonstrated that major changes in faunas have occurred at different timescales that signify ecosystem regime changes linked to sea-ice cover, surface productivity, bottom temperature and other factors. In addition to faunal changes characterizing glacial-interglacial cycles, Arctic sediments contain several unusual faunal events that cannot be explained by orbital-scale sea level and cryospheric changes. One indicator of such events involves the ostracode Rabilimis mirabilis (Brady 1868), a shallow-water species that inhabits continental shelves in the modern Arctic. We conducted studies of the stratigraphic distribution of R. mirabilis in cores from the Northwind, Mendeleev, Lomonosov, and Alpha Ridges; the Siberian and North American (Beaufort Sea) continental margins; and the Lincoln Sea off North Greenland and in the northern Greenland Sherard Osborn Fjord. Evidence from these records suggests that this species occurs as a fossil in deeper water sediment cores on the upper parts of submarine ridges (mainly 700-900 meters water depth, mwd), in significant numbers (from 1%to 50% of total ostracodes) during Marine Isotope Stages (MIS) 5a (125-109 ka), MIS 4 (71-57 ka), and MIS 3 (57-29 ka). Furthermore, it occurs in cores from various depths on the Siberian margin, the Beaufort and Lincoln Seas during MIS 1 (the Holocene, approx. 11-0 ka). These occurrences involve well-preserved, stratigraphically consistent adult and juvenile populations, which are autochthonous in nature and not caused by downslope transport or ice rafting. Based on their age and associated paleoceanographic conditions in the Arctic, we interpret these R. mirabilis events as signifying basin-ward migration during abrupt changes in growth and decay of massive ice shelves and may be useful as biostratigraphic markers.

Direct astronomical influence on abrupt climate variability

Changes in the magnitude of millennial-scale climate variability (MCV) during the Late Pleistocene occur as a function of changing background climate state over tens of thousands of years, an indirect consequence of slowly varying incoming solar radiation associated with changes in Earth’s orbit. However, whether astronomical forcing can stimulate MCV directly (without a change in the background state) remains to be determined. Here we use a comprehensive fully coupled climate model to demonstrate that orbitally driven insolation changes alone can give rise to spontaneous millennial-scale climate oscillations under intermediate glacial conditions. Our results demonstrate that an abrupt transition from warm interstadial to cold stadial conditions can be triggered directly by a precession-controlled increase in low-latitude boreal summer insolation and/or an obliquity-controlled decrease in high-latitude mean annual insolation, by modulating North Atlantic low-latitude hydroclimate and/or high-latitude sea ice–ocean–atmosphere interactions, respectively. Furthermore, contrasting insolation effects over the tropical versus subpolar North Atlantic, exerted by obliquity or precession, result in an oscillatory climate regime, even within an otherwise stable climate. With additional sensitivity experiments under different glacial–interglacial climate backgrounds, we synthesize a coherent theoretical framework for climate stability, elaborating the direct and indirect (dual) control by Earth’s orbital cycles on millennial-scale climate variability during the Pleistocene. Millennial-scale climate oscillations can arise from orbital forcing alone during relatively stable glacial climate states, according to an analysis of high- and low-latitude climate proxy records as well as climate modelling.

Timing of late quaternary glaciations in the Yankti Kuti valley of the upper Kali Ganga catchment, Northern India

The Yankti Kuti valley which is located in the transitional climatic zone of the upper Kali Ganga catchment of northern India has witnessed multiple glacier advances since Marine Isotopic Stage 3 (MIS 3). The oldest glacial advance in the valley occurred during early MIS-3 and is dated to 52 ± 3 ka (52.8 ± 3 and 50.4 ± 3 ka), the second advance occurred during late MIS 3 and is dated to 35 ± 2 ka (36.4 ± 1.8 and 34.5 ± 2.2 ka). The termination of this stage occurred at ∼30.3 ± 2 ka and is reflected in the vertical shrinkage of the glacier. The third glacial advance which was more restricted is dated to the Last Glacial Maximum (LGM) 22 ± 1 ka (22.2 ± 1.1 and 21.4 ± 1.2 ka). Following this, a glacial advance is dated to 16.1 ± 1 ka corresponding to the Older Dryas cooling of the late glacial period. A subsequent minor re-advance is dated to 4.3 ± 2.0 ka (4.4 ± 0.4 and 4.2 ± 0.1 ka) and can be attributed to the globally identified 4.2 ka cooling (arid) event. The study is in accordance with earlier observations made from the monsoon-dominated central Himalayan region. It provides a robust chronology and climatic evidence indicating significant ice volume depicted by the moraine height during MIS 3, implying that the moisture-deficient valleys of semi-arid Himalayan regions respond sensitively to enhance precipitation. The study suggests a regional synchronicity of glacier response to climate variability since MIS 3 and was in accordance with the synoptic-scale, climatic perturbation triggered by the North Atlantic millennial-scale climate oscillations.

Late Quaternary glacial advances and equilibrium-line altitude changes in a semi-arid region, Suru Basin, western Himalaya

This study examines the late Quaternary glacial history of Suru Basin and its associated tributary valleys (i.e., Achambur, Kangriz, Shafat, Rangdum, and Pensilugnpa), western Himalaya, based on the data collected from field surveys (2015–2019), satellite images, and optically stimulated luminescence (OSL) dating. The glaciers in Suru Basin have fluctuated greatly throughout the Marine Isotope Stages (MIS) 3 and 2 to the Little Ice Age (LIA), and are critical in understanding the linkages between regional and global climate change. Here, we present a detailed record of five major glacial advances of decreasing magnitude, dated 33–23 ka, 16 ka, 13–11 ka, 10–7.3 ka, 2.8–2.3 ka, and a minor advance dated 0.7–0.4 ka. These five major glacial advances are named from oldest to youngest as Suru-I to V, based on their optical chronology, geographical position, moraine stratigraphy, and glacially polished rock surfaces. Whereas, the equilibrium-line altitude (ELA) ascended by ∼538 m from the oldest glacial advance (Suru-I) to the present-day, followed by four other events (Suru-II, III, IV, and V) with ascended ELAs of ∼522, ∼399, ∼243, and 215 m, respectively. Suru Basin has lost ∼502 km2 in its glaciated area and ∼163 km3 in ice volume from Suru-I to the present-day. The magnitude of glacier extent decreased from the MIS 3 and 2, early to late-Holocene, and LIA. The timing of these glacial advances is coarsely in agreement with North Atlantic millennial-scale climate oscillations (Heinrich events), signifying the probable correlation between these abrupt climate changes and glacial fluctuations in this semi-arid region of the Himalaya.

Orbital- and millennial-scale Antarctic Circumpolar Current variability in Drake Passage over the past 140,000 years

The Antarctic Circumpolar Current (ACC) plays a crucial role in global ocean circulation by fostering deep-water upwelling and formation of new water masses. On geological time-scales, ACC variations are poorly constrained beyond the last glacial. Here, we reconstruct changes in ACC strength in the central Drake Passage in vicinity of the modern Polar Front over a complete glacial-interglacial cycle (i.e., the past 140,000 years), based on sediment grain-size and geochemical characteristics. We found significant glacial-interglacial changes of ACC flow speed, with weakened current strength during glacials and a stronger circulation in interglacials. Superimposed on these orbital-scale changes are high-amplitude millennial-scale fluctuations, with ACC strength maxima correlating with diatom-based Antarctic winter sea-ice minima, particularly during full glacial conditions. We infer that the ACC is closely linked to Southern Hemisphere millennial-scale climate oscillations, amplified through Antarctic sea ice extent changes. These strong ACC variations modulated Pacific-Atlantic water exchange via the “cold water route” and potentially affected the Atlantic Meridional Overturning Circulation and marine carbon storage.

The Zealandia Switch: Ice age climate shifts viewed from Southern Hemisphere moraines

Two fundamental questions about the ice-age climate system await satisfactory resolution. First, if summer solar radiation intensity truly controls the orbital signature of the last glacial cycle, then why were major climatic shifts, including the last termination, globally synchronous? Second, what caused the millennial-scale climate oscillations superimposed on this cycle? We address these questions from a Southern Hemisphere perspective focused on mid-latitude mountain ice fields. We put particular emphasis on the last glacial termination, which involved both orbital-scale and millennial-scale climate elements and has generally well-resolved chronological control.Sustained retreat of mountain glaciers, documented by detailed mapping and chronology of glacial landforms in the Southern Alps and southern Andes, marked the termination of the last ice age, beginning ∼18 kyrs ago and involved a change from glacial to near-interglacial atmospheric temperature within a millennium or two. A rapid poleward shift of the Subtropical Front, delineating the northern margin of the Southern Ocean, ∼18 kyrs ago implies a concurrent poleward shift of the austral westerlies and leads us to hypothesize a southern origin for the dominant phase of the last glacial termination. Together with interhemispheric paleoclimate records and with results of coupled ocean-atmosphere climate modeling, these findings suggest a big, fast, and global end to the last ice age in which a southern-sourced warming episode linked the hemispheres. We posit that a shift in the Southern Ocean circulation and austral westerly wind system, tied to southern orbital forcing, caused this global warming episode by affecting the tropical heat engine and hence global climate.Central to this hypothesis, dubbed the ‘Zealandia Switch’, is the location of the Australia and Zealandia continents relative to Southern Hemisphere oceanic and atmospheric circulation. Coupled ocean-atmosphere climate modeling shows that the locus of the austral westerlies, whether in a more equatorward position representing a glacial-mode climate or in a poleward-shifted position marking interglacial-mode climate, has profound effects on oceanic and associated atmospheric linkages between the tropical Pacific and the Southern Ocean. Shifts in the austral westerlies have global climatic consequences, especially through resulting changes in the greenhouse gas content of the atmosphere and altered heat flux from the tropical Pacific into the Northern and Southern Hemispheres. We suggest that the last glacial termination was a global warming episode that led to extreme seasonality in northern latitudes by stimulating a flush of meltwater and icebergs into the North Atlantic from adjoining ice sheets. This fresh-water influx resulted in widespread North Atlantic sea ice that caused very cold boreal winters, thus amplifying the annual southward shift of the Intertropical Convergence Zone and the monsoonal rain belts. We further suggest that muted manifestations of the Zealandia Switch mechanism were responsible for smaller, recurring millennial-scale climate oscillations within the last glacial cycle.

A gradual transition into Greenland interstadial 14 in southeastern China based on a sub-decadally-resolved stalagmite record

Greenland stadials and interstadials (GS and GIS) are prominent features in ice core records of the last glacial period and are characterized by millennial-scale climate oscillations with sub-decadal- to decadal-scale hydroclimatic shifts. Over the past two decades, studies of Chinese stalagmite records have revealed corresponding Chinese Stadials/Interstadials (CS/CIS). However, the CS/CIS shifts and their corresponding forcings across the Asian monsoon region are still not fully understood. In this study, we present a 230Th-based sub-decadally-resolved stalagmite δ18O record from Xianyun Cave in southeastern China covering the period 58.2–50.8 kyr BP (before 1950 AD) and including GIS (and CIS) 16–14. We used GIS (CIS) 15.1 (peak point) and 15.2 (first point) to chronologically link the high and low-latitude records. We found that the onset of GIS 14 in our record occurred earlier than in an ice core record from Greenland and stalagmite records from northern China. This finding implies that low-latitude tropical climate likely played a key role in triggering abrupt millennial-scale events. Moreover, the transition into GIS 14 lasted 1.49 kyr in our record, which is in contrast to the abrupt temperature rise observed in NGRIP (the North Greenland Ice Core Project, 0.02 kyr). Compared with previous stalagmite records, we identified two transitional patterns into GIS 14 in monsoonal China: 1) a rapid transition in northern China and 2) a gradual transition in southeastern China. The gradual transition in our Xianyun record is analogous to a sea surface temperature (SST) record in the western tropical Pacific, highlighting a possible inherent connection between the changes in western tropical Pacific SSTs and East Asian summer monsoon variability in southeastern China. Paleoclimate records in southeastern China are therefore critical for understanding the mechanisms of abrupt millennial-scale climate events, as they can act as a bridge between high- and low-latitude records.

Timing and structure of Termination II in north China constrained by a precisely dated stalagmite record

The timing and structure of Termination II (T-II) is still debatable as the number of high-quality records is insufficient. This study presents precisely dated stalagmite δ18O data between 133.4±0.3 to 126.6±0.3 ka BP (covering most of T-II) from north China, near the northern limit of the East Asian summer monsoon (EASM), an area sensitive to climate change. The onset of the last interglacial is constrained at 129.4±0.3 ka BP, consistent with south Chinese speleothem records, further supporting the idea that Northern Hemisphere summer insolation plays an important role in initiating glacial terminations. An extended interval of heavy δ18O is observed immediately prior to this abrupt transition, named as the “Weak Monsoon Interval” (WMI) in south Chinese cave records, which was associated with the Heinrich Stadial 11 (H11) in the North Atlantic. A significant millennial-scale interstadial peaking at 132.7±0.3 ka BP is identified preceding the weakest phase of the WMI. A synchronous counterpart is also found in some south Chinese speleothem and North Atlantic alkenone Mg/Ca SST records. It is inferred that the main H11 freshwater penetration possibly occurred after 131.5 ka BP, leaving some aspects of ocean and atmospheric circulation still in interstadial mode from 134 to 131.5 ka BP. Following the onset of the last interglacial, there was a millennial-scale “pause” in our δ18O record, synchronized with a “slowdown” in the rate of decrease in δ18O in south Chinese cave records. This is possibly a large regional climate oscillation, as it is also identified in some North Atlantic records, but one which is too weak to be a Younger Dryas (YD)-type event.In summary, compared to south Chinese cave records, significant millennial-scale climate oscillations before and after the synchronous onset of the last interglacial is the main feature of our record. Nevertheless, the sequence of these climate events remains very different from Termination I (T-I), which is possibly ascribed to the stronger insolation, higher atmosphere CO2, and the resultant faster and greater meltwater pulse to the North Atlantic during T-II than during T-I.

High-resolution foraminifer stable isotope record of MIS 19 at Montalbano Jonico, southern Italy: A window into Mediterranean climatic variability during a low-eccentricity interglacial

Understanding millennial and sub-millennial climate variability during past low eccentricity interglacials similar to the Holocene is important for forecasting the evolution of climate and natural variability. The Ideale section (Montalbano Jonico, Southern Italy) studied here provides one of the best records of MIS 19c, the closest orbital analog to the Holocene. This exposed marine series covers Termination IX to the inception of MIS 18 with very high sedimentation rates (i.e. 90–200 cm/ka). We present 1) benthic δ18O and δ13C records at 90–200 year time-resolution, 2) a new 40Ar/39Ar age of 774.1 ± 0.9 ka for tephra layer V4 (Matuyama-Brunhes transitional period) and 3) new calcareous plankton, palynological and authigenic 10Be/9Be data. Our new Bayesian depth-age model suggests a 11.5 ± 3.4 ka (95% confidence) duration for the climatic optimum. The δ18O series reveals millennial-scale oscillations (with sharp transitions < 200 years) between ∼774.0 and the onset of MIS 18 (∼757.0 ka) with a cyclicity of about 5.4 ka. Spectral analysis and band-pass filtering indicate that these climate oscillations existed throughout the entire MIS 19 period, although they were dampened during MIS 19c, which is chiefly controlled by orbitally-driven insolation. The amplitude of those sub-orbital oscillations increased towards MIS 18 as the climate became drier and cooler. The Ideale section reveals, with unprecedented detail, millennial-scale climatic oscillations of MIS 19b-a that have been observed worldwide. They highlight the response of the central Mediterranean area to North Atlantic climatic variation (i.e. oceanic circulation and atmospheric processes related to ice-sheet dynamics) during this low eccentricity interglacial.

The timing and cause of glacial activity during the last glacial in central Tibet based on 10Be surface exposure dating east of Mount Jaggang, the Xainza range

Mountain glaciers are sensitive to climate change, and can provide valuable information for inferring former climates on the Tibetan Plateau (TP). The increasing glacial chronologies indicate that the timing of the local Last Glacial Maximum (LGM) recorded across the TP is asynchronous, implying different local influences of the mid-latitude westerlies and Asian Summer Monsoon in triggering glacier advances. However, the well-dated sites are still too few, especially in the transition zone between regions controlled by the two climate systems. Here we present detailed last glacial chronologies for the Mount Jaggang area, in the Xainza range, central Tibet, with forty-three apparent 10Be exposure-ages ranging from 12.4 ± 0.8 ka to 61.9 ± 3.8 ka. These exposure-ages indicate that at least seven glacial episodes occurred during the last glacial cycle east of Mount Jaggang. These include: a local LGM that occurred at ∼61.9 ± 3.8 ka, possibly corresponding to Marine Isotope Stage 4 (MIS 4); subsequent glacial advances at ∼43.2 ± 2.6 ka and ∼35.1 ± 2.1 ka during MIS 3; one glacial re-advance/standstill at MIS3/2 transition (∼29.8 ± 1.8 ka); and three glacial re-advances/standstills that occurred following MIS 3 at ∼27.9 ± 1.7 ka, ∼21.8 ± 1.3 ka, and ∼15.1 ± 0.9 ka. The timing of these glacial activities is roughly in agreement with North Atlantic millennial-scale climate oscillations (Heinrich events), suggesting the potential correlations between these abrupt climate changes and glacial fluctuations in the Mount Jaggang area. The successively reduced glacial extent might have resulted from an overall decrease in Asian Summer Monsoon intensity over this timeframe.

Millennial-scale climate oscillations inferred from visible spectroscopy of a sediment core in Qarhan Salt Lake of Qaidam Basin between 40 and 10 cal ka BP

Little information about millennial-scale change is known in large extremely arid area of northwest China, which is less influenced by the Asian monsoon than regions to the southeast. Visible spectroscopy of a 35 m long core taken from the Qarhan Salt Lake is used to reconstruct humidity changes of Qaidam Basin between 40 and 10 cal ka BP. High redness values (low brightness values) reflect moist climates, whereas low redness values (high brightness values) indicate dry climates. On millennial-scale, Qaidam Basin was relatively wet during the Younger Dryas and Heinrich H1-H4 events. Dry periods appear to have occurred at nearly the same time as warm periods in the North Atlantic region. These millennial-scale climate oscillations in Qaidam Basin are different from those in Asian monsoon regions that show a relatively dry climate during the occurrence of the Younger Dryas and Heinrich H1-H4 events. Our data indicate that the cooling of the North Atlantic caused a downstream cooling of the atmosphere by westerly jet and influenced the climate in Qaidam Basin.

Heinrich events triggered by ocean forcing and modulated by isostatic adjustment.

During the last glacial period, the Laurentide Ice Sheet sporadically discharged huge numbers of icebergs through the Hudson Strait into the North Atlantic Ocean, leaving behind distinct layers of ice-rafted debris in the ocean sediments. Perplexingly, these massive discharge events-Heinrich events-occurred during the cold portion of millennial-scale climate oscillations called Dansgaard-Oeschger cycles. This is in contrast to the expectation that ice sheets expand in colder climates and shrink in warmer climates. Here we use an ice sheet model to show that the magnitude and timing of Heinrich events can be explained by the same processes that drive the retreat of modern marine-terminating glaciers. In our model, subsurface ocean warming associated with variations in the overturning circulation increases underwater melt along the calving face, triggering rapid margin retreat and increased iceberg discharge. On millennial timescales, isostatic adjustment causes the bed to uplift, isolating the terminus from subsurface warming and allowing the ice sheet to advance again until, at its most advanced position, it is poised for another Heinrich event. This mechanism not only explains the timing and magnitude of observed Heinrich events, but also suggests that ice sheets in contact with warming oceans may be vulnerable to catastrophic collapse even with little atmospheric warming.