Ice Age Cycles Research Articles

The residence time of Southern Ocean surface waters and the 100,000-year ice age cycle.

From 1.25 million to 700,000 years ago, the ice age cycle deepened and lengthened from 41,000- to 100,000-year periodicity, a transition that remains unexplained. Using surface- and bottom-dwelling foraminifera from the Antarctic Zone of the Southern Ocean to reconstruct the deep-to-surface supply of water during the ice ages of the past 1.5 million years, we found that a reduction in deep water supply and a concomitant freshening of the surface ocean coincided with the emergence of the high-amplitude 100,000-year glacial cycle. We propose that this slowing of deep-to-surface circulation (i.e., a longer residence time for Antarctic surface waters) prolonged ice ages by allowing the Antarctic halocline to strengthen, which increased the resistance of the Antarctic upper water column to orbitally paced drivers of carbon dioxide release.

Pronounced northward shift of the westerlies during MIS 17 leading to the strong 100-kyr ice age cycles

The MIS 17 interglacial, ∼715–675 ka, marks the end of the Mid-Pleistocene Transition as intensified, long and asymmetrical 100-kyr ice age cycles became eminently established. Increasing arrival of moisture to the Northern Hemisphere high latitudes, resulting from the northwestward migration of the Subpolar Front and the intensification of the Norwegian Greenland Seas (NGS) convection, has been put forward to explain the emergence of this quasi-periodic 100-kyr cycle. However, testing this hypothesis is problematic with the available North Atlantic precipitation data. Here we present new pollen-based quantitative seasonal climate reconstructions from the southwestern Iberian margin that track changes in the position and intensity of the westerlies. Our data compared to changes in North Atlantic deep and surface water conditions show that MIS 17 interglacial was marked by three major changes in the direction and strength of the westerlies tightly linked to oceanographic changes. In particular, we report here for the first time a drastic two-steps northward shift of the westerlies centered at ∼693 ka that ended up with the sustained precipitation over southern European. This atmospheric reorganization was associated with northwestward migration of the Subpolar Front, strengthening of the NGS deep water formation and cooling of the western North Atlantic region. This finding points to the substantial arrival of moisture to the Northern Hemisphere high latitudes at the time of the decrease in summer energy and insolation contributing to the establishment of strong 100-kyr cycles.

Combined interpretation of marine controlled source electromagnetic and reflection seismic data in the German North Sea: a case study

This paper focuses on the combined analysis and interpretation of controlled source electromagnetic (CSEM) and multichannel reflection seismic (MCS) data along one profile in the German North Sea with the goal to reduce ambiguities in interpretation. The shallow water environment of the North Sea is characterized by a complex geological development which includes rifting, several ice age cycles, a propagating shelf margin, mass-transport deposits and salt dome formation. Seismic and electromagnetic methods are sensitive to different physical properties of the seabed and therefore complement each other. We analyse the MCS data with a migration velocity tomography and an amplitude variation with offset analysis and discuss seismic velocities and densities. For true amplitude recovery the amplitude distortions are calibrated with in situ logging data. The CSEM data are analysed in 2-D, for which, for the first time, data were included that were acquired while the instrument was towed on the seafloor in addition to the stationary sites. The CSEM inversions are constrained by seismic horizons. The joint interpretation focuses on two seismic reflectors: One can be interpreted as an unconformity marking a lithological change from fresh water-bearing glacial deposits to compacted sediments below, and the other one as a layer of fine-grained deposits potentially capping patchy shallow gas occurrences. This exemplary case study shows how the combination of both methods can benefit by interpreting complex geology and eliminating ambiguous explanations.

Robust Constraints on Past CO2 Climate Forcing From the Boron Isotope Proxy

AbstractThe atmospheric concentration of the greenhouse gas carbon dioxide, CO2, is intimately coupled to the carbon chemistry of seawater, such that the radiative climate forcing from CO2 can be changed by an array of physical, geochemical, and biological ocean processes. For instance, biological carbon sequestration, seawater cooling, and net CaCO3 dissolution are commonly invoked as the primary drivers of CO2 change that amplify the orbitally paced ice age cycles of the late Pleistocene. Based on first‐principle arguments with regard to ocean chemistry, we demonstrate that seawater pH change (ΔpH) is the dominant control that effectively sets CO2 radiative forcing (ΔF) on orbital timescales, as is evident from independent late Pleistocene reconstructions of pH and CO2. In short, all processes relevant for CO2on orbital timescales, including temperature change, cause pH to change to bring about fractional CO2 change so as to yield a linear relationship of ΔpH to CO2 climate forcing. Further, we show that ΔpH and CO2 climate forcing can be reconstructed using the boron isotope pH proxy more accurately than absolute pH or CO2, even if seawater boron isotope composition is poorly constrained and without information on a second carbonate system parameter. Thus, our formalism relaxes otherwise necessary assumptions to allow the accurate determination of orbital timescale CO2 radiative forcing from boron isotope pH reconstructions alone, thereby eliminating a major limitation of current methods to estimate our planet's climate sensitivity from the geologic record.

Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages

Over the past few million years, the Earth descended from the relatively warm and stable climate of the Pliocene into the increasingly dramatic ice age cycles of the Pleistocene. The influences of orbital forcing and atmospheric CO2 on land-based ice sheets have long been considered as the key drivers of the ice ages, but less attention has been paid to their direct influences on the circulation of the deep ocean. Here we provide a broad view on the influences of CO2, orbital forcing and ice sheet size according to a comprehensive Earth system model, by integrating the model to equilibrium under 40 different combinations of the three external forcings. We find that the volume contribution of Antarctic (AABW) vs. North Atlantic (NADW) waters to the deep ocean varies widely among the simulations, and can be predicted from the difference between the surface densities at AABW and NADW deep water formation sites. Minima of both the AABW-NADW density difference and the AABW volume occur near interglacial CO2 (270–400 ppm). At low CO2, abundant formation and northward export of sea ice in the Southern Ocean contributes to very salty and dense Antarctic waters that dominate the global deep ocean. Furthermore, when the Earth is cold, low obliquity (i.e. a reduced tilt of Earth’s rotational axis) enhances the Antarctic water volume by expanding sea ice further. At high CO2, AABW dominance is favoured due to relatively warm subpolar North Atlantic waters, with more dependence on precession. Meanwhile, a large Laurentide ice sheet steers atmospheric circulation as to strengthen the Atlantic Meridional Overturning Circulation, but cools the Southern Ocean remotely, enhancing Antarctic sea ice export and leading to very salty and expanded AABW. Together, these results suggest that a ‘sweet spot’ of low CO2, low obliquity and relatively small ice sheets would have poised the AMOC for interruption, promoting Dansgaard–Oeschger-type abrupt change. The deep ocean temperature and salinity simulated under the most representative ‘glacial’ state agree very well with reconstructions from the Last Glacial Maximum (LGM), which lends confidence in the ability of the model to estimate large-scale changes in water-mass geometry. The model also simulates a circulation-driven increase of preformed radiocarbon reservoir age, which could explain most of the reconstructed LGM-preindustrial ocean radiocarbon change. However, the radiocarbon content of the simulated glacial ocean is still higher than reconstructed for the LGM, and the model does not reproduce reconstructed LGM deep ocean oxygen depletions. These ventilation-related disagreements probably reflect unresolved physical aspects of ventilation and ecosystem processes, but also raise the possibility that the LGM ocean circulation was not in equilibrium. Finally, the simulations display an increased sensitivity of both surface air temperature and AABW volume to orbital forcing under low CO2. We suggest that this enhanced orbital sensitivity contributed to the development of the ice age cycles by amplifying the responses of climate and the carbon cycle to orbital forcing, following a gradual downward trend of CO2.

Temperature correlations between the eastern equatorial Pacific and Antarctica over the past 230,000 years

Tropical sea surface temperatures (SSTs) warmed and cooled in step with the Pleistocene ice age cycles, but the mechanisms are not known. It is assumed that the answer must involve radiative forcing by CO2 but SST reconstructions have been too sparse for a conclusive test. Here I present a 230,000-yr tropical SST stack from the eastern equatorial Pacific (EEP) using two new Mg/Ca reconstructions combined with three earlier ones. The EEP stack shows persistent covariation with Antarctic temperature on orbital and millennial timescales indicating tight coupling between the two regions. This coupling however cannot be explained solely by CO2 forcing because in at least one important case, the Marine Isotope Stage (MIS) 5e–5d glacial inception, both regions cooled ∼5–6.5 thousand years before CO2 decreased. More likely, their covariation was due to advection of Antarctic climate signals to the EEP by the ocean. To explain the MIS 5e–5d event and glacial inception in general the hypothesis is advanced that the cooling signal spreads globally from the Northern Hemisphere with an active ocean circulation – first from the North Atlantic to the Southern Ocean with a colder North Atlantic Deep Water, and then to the Indian and Pacific Oceans with cooler Antarctic deep and intermediate waters.

Spratlies Archipelago as the Australasian Tektite Impact Crater, Details of Formation & Richard Muller’s Dust Cloud Explanation for the Mid-Pleistocene Ice Age Cycle Transition

Several significant events of a geological nature occurred approximately 800 ka before the present: (1) Australasian tektite fall (AA), (2) Brunhes-Matuyama geomagnetic reversal (BMR), (3) mid-Pleistocene changes in ice age cycles. Add to these the undated fault system (4) in the South-West (SW) of the South China Sea (SCS). Here we offer a unified cause for all four of these in (5), an impact in the SCS of a large, massive cosmic object, likely a comet, obliquely coming from the SW at an extremely shallow angle, striking the Sunda shelf yet unexploded with the shock of its compressed air bow wave, and causing the continual shelf and slope to collapse, resulting in the fault system (4), then traveling almost tangentially to the surface, exploding at impact with the sea surface, ejecting the tektites (1), creating the formation underlying the later atolls of Spratlies Archipelago (6), Nansha Islands in Chinese, & causing the BMR (2). An explanation of event (3) was Richard Muller’s hypothesis of planet Earth passing through an interplanetary dust cloud periodically due to ecliptic precession. Here we hypothesize this cloud actually is a belt of Australasian tektites ejected into space at super-orbital velocities that Earth encounters about every 100 ka.

Assimilating the ICE‐6G_C Reconstruction of the Latest Quaternary Ice Age Cycle Into Numerical Simulations of the Laurentide and Fennoscandian Ice Sheets

We analyze the effects of nudging 100 kyr numerical simulations of the Laurentide and Fennoscandian ice sheets toward the glacial isostatic adjustment‐based (GIA‐based) ICE‐6G_C reconstruction of the most recent ice age cycle. Starting with the ice physics approximations of the PISM ice sheet model and the SeaRISE simulation protocols, we incorporate nudging at characteristic time scales, τf, through anomalous mass balance terms in the ice mass conservation equation. As should be expected, these mass balances exhibit physically unrealistic details arising from pure GIA‐based reconstruction geometry when nudging is very strong (τf=20 years for North America), while weakly nudged (τf=1,000 years) solutions deviate from ICE‐6G_C sufficiently to degrade its observational fit quality. For reasonable intermediate time scales (τf=100 years and 200 years), we perturbatively analyze nudged ice dynamics as a superposition of “leading‐order smoothing” that diffuses ICE‐6G_C in a physically and observationally consistent manner and “higher‐order” deviations arising, for instance, from biases in the time dependence of surface climate boundary conditions. Based upon the relative deviations between respective nudged simulations in which these biases follow surface temperature from ice cores and eustatic sea level from marine sediment cores, we compute “ice core climate adjustments” that suggest how local paleoclimate observations may be applied to the systematic refinement of ICE‐6G_C. Our results are consistent with a growing body of evidence suggesting that the geographical origins of Meltwater Pulse 1B (MWP1b) may lie primarily in North America as opposed to Antarctica (as reconstructed in ICE‐6G_C).

Causes of ice age intensification across the Mid-Pleistocene Transition

During the Mid-Pleistocene Transition (MPT; 1,200-800 kya), Earth's orbitally paced ice age cycles intensified, lengthened from ∼40,000 (∼40 ky) to ∼100 ky, and became distinctly asymmetrical. Testing hypotheses that implicate changing atmospheric CO2 levels as a driver of the MPT has proven difficult with available observations. Here, we use orbitally resolved, boron isotope CO2 data to show that the glacial to interglacial CO2 difference increased from ∼43 to ∼75 μatm across the MPT, mainly because of lower glacial CO2 levels. Through carbon cycle modeling, we attribute this decline primarily to the initiation of substantive dust-borne iron fertilization of the Southern Ocean during peak glacial stages. We also observe a twofold steepening of the relationship between sea level and CO2-related climate forcing that is suggestive of a change in the dynamics that govern ice sheet stability, such as that expected from the removal of subglacial regolith or interhemispheric ice sheet phase-locking. We argue that neither ice sheet dynamics nor CO2 change in isolation can explain the MPT. Instead, we infer that the MPT was initiated by a change in ice sheet dynamics and that longer and deeper post-MPT ice ages were sustained by carbon cycle feedbacks related to dust fertilization of the Southern Ocean as a consequence of larger ice sheets.

Mid-Pleistocene climate transition: Characteristic, mechanism and perspective

Milankovich theory suggested that orbitally induced summer insolation change at high-latitude region of Northern Hemisphere played a key role in driving the ice cycles during the Pleistocene. However, a significant shift of the Pleistocence climate from 41-ka to 100-ka cycles (namely mid-Pleistocene transition, MPT) cannot be simply attributed to the astronomical forcing due to insignificant insolation change induced by eccentricity forcing. Here we summarized multiple proxies generated from the ocean and land to address Pleistocene climate changes such as global ice volume, sea surface temperature, sea level, aridity of dust sources, and monsoon-related hydrological cycles. The temporal and spatial characteristics of the MPT phenomenon, in terms of the timing, duration, amplitudes and frequencies, were addressed using spectral and wavelet results. Most records reveal that the MPT was commenced as the onset of the ~100-ka cycles at ~1.2 Ma, and ended by the establishment of the dominant 100-ka cycles at ~0.7 Ma. However, some hydroclimatic proxies from the Mediterranean Sea, South China Sea and Chinese caves show a relatively strong 23-ka cycles during the middle to late Pleistocence. Such a difference may be attributable to varied sensitivity of proxy indicators to the insolation and glacial forcing. Globally, temperature and sea level signals that are affected by changing ice volume usually demonstrate a distinctive and consistent MPT. By contrast, regional hydroclimatic proxies at mid-to-low latitudes are likely dominated by the precession cycles (i.e. high-resolution, absolutely dated Chinese speleothem records). The MPT associated with the onset of ~100-ka cycles has inspired the paleocommunities to address its driving mechanism from both data and model perspectives. Several hypotheses have been proposed to explain the MPT in the past two decades, including nonlinear response to external forcing and complicated internal feedbacks. The nonlinear response to the astronomical forcing including eccentricity (100-ka cycle), interplay between obliquity (41-ka cycle) and precession (23- and 19-ka cycles) can produce the 100-ka ice-age cycles. However, model results suggest that the MPT and subsequent ″100 ka world″ are likely caused by changes in atmospheric CO2 concentration, bedrock exposure or ocean circulations. Taking these different hypotheses together, it was suggested that the solar insolation caused by changing Earth′s orbital parameters as external forcing, drives the glacial-interglacial climate cycles, whilst the nonlinear responses to atmospheric CO2 decrease and to different regolith boundary conditions as internal feedbacks have played key roles in triggering the climate transition and amplifying the glacial-interglacial oscillations. While most proxies from the land and ocean demonstrate a remarkable MPT from 41- to 100-ka cycles, regional high-resolution proxy records indicated that hydroclimatic changes at mid-to-low latitudes exhibit insignificant MPT and weak ~100-ka cycles after the MPT. In the future, more data assimilation should be focused on the hydroclimatic changes particularly in monsoon-affected regions to decipher the diverse responses in a regional scale. Meanwhile, simulations using high-resolution regional climate system models should be performed to quantify the responses of different climate parameters to external forcing and internal feedback. Through intensively data-model comparison, new insights into past climate variability and dynamics will permit a better projection of future climate change under the global warming context.

Overestimate of committed warming.

Palaeoclimate variations are an essential component in constraining future projections of climate change as a function of increasing anthropogenic greenhouse gases. The Earth System Sensitivity (ESS) describes the multi-millennial response of Earth (in terms of global mean temperature) to a doubling of CO2 concentrations. A recent study used a correlation of inferred temperatures and radiative forcing from greenhouse gases over the past 800,000 years to estimate the ESS from present day CO2 is about 9 degrees C, and to imply a long-term commitment of 3-7 degrees C even if greenhouse gas levels remain at present-day concentrations. However, we demonstrate that the methodology of ref. 2 does not reliably estimate the ESS in the presence of orbital forcing of ice age cycles and therefore conclude that the inferred present-day committed warming is considerably overestimated.

The smoking gun of the ice ages.

Forty years ago, a seminal paper showed that small changes in Earth's orbit are key to understanding ice age cycles

In retrospect: Forty years of linking orbits to ice ages.

In 1976, it was demonstrated that tiny wobbles in Earth's orbit led to the great ice-age cycles of the past few million years. This finding had wide implications for climate science and the details remain hotly debated today.

A generalization of the taxon cycle

A generalization of the taxon cycle

Global ice volume during MIS 3 inferred from a sea-level analysis of sedimentary core records in the Yellow River Delta

Abstract Estimates of global ice volume during the glacial phase of the most recent ice age cycle are characterized by significant uncertainty, reflecting the relative paucity of geological constraints on sea level relevant to this time interval. For example, during the middle stages of Marine Isotope Stage 3, published estimates of peak global mean sea level (GMSL) relative to the present range from −25 m to −87 m. The large uncertainty in GMSL at MIS 3 has significant implications for estimates of the rate of ice growth in the period leading to the Last Glacial Maximum (∼26 ka). We refine estimates of global ice volume during MIS 3 by employing sediment cores in the Bohai and Yellow Sea that record a migration of the paleoshoreline at ∼50–37 ka through a transition from marine to brackish conditions. In particular, we correct relative sea level at these sites for contamination due to glacial isostatic adjustment using a sea-level calculation that includes a gravitationally self-consistent treatment of sediment redistribution and compaction, and estimate a peak global mean sea level of −38 ± 7 m during the interval 50–37 ka. With suitable sedimentary core records, the approach described herein can be extended to refine existing constraints on global ice volume across the entire glacial period.

Disentangling the impacts of climate and human colonization on the flora and fauna of the Australian arid zone over the past 100 ka using stable isotopes in avian eggshell

Abstract Throughout the Quaternary, the flora and fauna of Australia evolved and adapted to the high-amplitude, low- and high-frequency climate changes that characterized the ice-age cycles. However, during the last glacial cycle, between ∼120 and 15 ka, unprecedented irreversible changes in flora and fauna occurred, and in that same interval modern humans established their first firm presence in the landscape. Disentangling the impacts of the first-order trend toward a colder, drier planet through the Late Quaternary from the impacts of human colonization has been challenging, from both the chronological and paleoenvironmental perspectives. We utilize the stable isotopes of carbon and oxygen preserved in near-continuous time series of Dromaius (emu) eggshell from five regions across Australia to provide independent reconstructions of ecosystem status and climate over the past 100 ka. Carbon isotopes are determined by the diet consumed by the female bird, whereas oxygen isotopes record the status of local moisture balance in the months prior to breeding. Together, δ13C and δ18O provide ecosystem status and climate from the same dated sample, reducing correlation uncertainties between proxies. Combined with recent improvements in the chronologies of Late Quaternary shorelines fringing inland lake basins and deflation during arid times, these data collectively reaffirm that Australia generally became increasingly, albeit irregularly, drier from the last interglaciation through to the last glacial maximum. Dromaius eggshell δ18O documents peak aridity between 30 and 15 ka, but shows no evidence of exceptional climate change between 60 and 40 ka. In contrast, Dromaius δ13Cdiet documents an irreversible loss of the majority of palatable summer-rainfall-related C4 grasses across the Australian arid zone between 50 and 45 ka, about the same time that the giant megafaunal bird, Genyornis, became extinct, and coincident with human dispersal across the continent. Our data indicate that changes unique to Australia occurred between 50 and 45 ka that led to a new climate-vegetation relationship and an overall reduction in effective moisture across much of the continent. The large summer-rainfall-dominated lakes of interior Australia failed to re-fill subsequently, despite a wide range of global climate states. A full explanation for the mechanisms behind these changes remains elusive, but they are almost certainly related to human agency. Plausible explanations include a change in fire regime resulting from human-lit fires, a change in fire regime following extinction of megafaunal browsers, and/or a threshold response to increasing aridity. Of these, the climate change explanation is least likely, given the lack of evidence for unprecedented aridity between 60 and 40 ka, and the successful adaptation of Australian ecosystems to 2.5 Ma of similar changes.

Paleoclimate record connects Asian monsoons and ice-age cycles

A new analysis of cave stalagmites almost doubles the span of known variations in rainfall over East Asia.

Asian monsoons and ice-age cycles

Isotopic analysis of cave stalagmites connects the intensity of rainfall over East Asia and ice-age terminations.

The Asian monsoon over the past 640,000 years and ice age terminations.

Oxygen isotope records from Chinese caves characterize changes in both the Asian monsoon and global climate. Here, using our new speleothem data, we extend the Chinese record to cover the full uranium/thorium dating range, that is, the past 640,000 years. The record's length and temporal precision allow us to test the idea that insolation changes caused by the Earth's precession drove the terminations of each of the last seven ice ages as well as the millennia-long intervals of reduced monsoon rainfall associated with each of the terminations. On the basis of our record's timing, the terminations are separated by four or five precession cycles, supporting the idea that the '100,000-year' ice age cycle is an average of discrete numbers of precession cycles. Furthermore, the suborbital component of monsoon rainfall variability exhibits power in both the precession and obliquity bands, and is nearly in anti-phase with summer boreal insolation. These observations indicate that insolation, in part, sets the pace of the occurrence of millennial-scale events, including those associated with terminations and 'unfinished terminations'.

Earth science: An extended yardstick for climate variability.

Decoded and precisely dated information encrypted in stalagmites from a cave in China reveal past climatic changes and provide insight into the complex interactions in today's climate system. See Letter p.640 Prior records of the Asian monsoon have revealed cyclic variations over hundreds of thousands of years, probably driven by variations in insolation caused by the precession of Earth's orbit. Hai Cheng and colleagues now provide a speleothem record from Chinese cave samples that extends earlier records to 640,000 years ago, close to the maximum age possible with uranium/thorium dating. This spectacular record confirms that the characteristic '100,000-year' ice age cycle corresponds to an integral number (four or five) of precession cycles, and that insolation influences millennial-scale variations in monsoon strength.