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Ice loss in the Southern Hemisphere has been greatest over the past 30 years in West Antarctica. The high sensitivity of this region to climate change has motivated geologists to examine marine sedimentary records for evidence of past episodes of West Antarctic Ice Sheet (WAIS) instability. Sediments accumulating in the Scotia Sea are useful to examine for this purpose because they receive iceberg-rafted debris (IBRD) sourced from the Pacific- and Atlantic-facing sectors of West Antarctica. Here we report on the sedimentology and provenance of the oldest of three cm-scale coarse-grained layers recovered from this sea at International Ocean Discovery Program Site U1538. These layers are preserved in opal-rich sediments deposited ∼1.2 Ma during a relatively warm regional climate. Our microCT-based analysis of the layer's in-situ fabric confirms its ice-rafted origin. We further infer that it is the product of an intense but short-lived episode of IBRD deposition. Based on the petrography of its sand fraction and the Phanerozoic 40Ar/39Ar ages of hornblende and mica it contains, we conclude that the IBRD it contains was likely sourced from the Weddell Sea and/or Amundsen Sea embayment(s) of West Antarctica. We attribute the high concentrations of IBRD in these layers to "dirty" icebergs calved from the WAIS following its retreat inland from its modern grounding line. These layers also sit at the top of a ∼366-m thick Pliocene and early Pleistocene sequence that is much more dropstone-rich than its overlying sediments. We speculate this fact may reflect that WAIS mass-balance was highly dynamic during the ∼41-kyr (inter)glacial world.

Marine fallout ash beds can provide continuous, time‐precise records of highly explosive arc volcanism that can be linked with the climate record. An evaluation of revised Plio‐Pleistocene (0–4 Myr) tephrostratigraphies from Ocean Drilling Program Sites 881, 882, and 884 confirms cyclicity of the Kamchatka‐Kurile arc volcanism and a marked increase just after the intensification of the Northern Hemisphere glaciation at 2.73 Ma. The compositional constancy of the Kamchatka‐Kurile volcano‐magma systems through time points to external modulation of volcanic cyclicity and frequency. The stacked tephra record reveals periodic peaks in arc volcanicity at ∼0.3, ∼1.0, ∼1.6, ∼2.5, and ∼3.8 Myr that coincide with maxima of the global ice volume variability that have been linked with the amplitude modulation of the precession (0.3, 1.0 Myr) and obliquity (1.6, 2.5 and 3.8 Myr) bands. A simple model of a decreasing obliquity variance across the mid‐Pleistocene Transition at constant precession variance produces an excellent correlation of ash bed cycles with the variability of global benthic δ18O (r2 = 0.75), which implies that climate, and not direct orbital forcing, modulates Kamchatka‐Kurile arc volcanism. The rising influence of precession variance in the Kamchatka‐Kurile ash bed record after the mid‐Pleistocene Transition contrasts with the dominant 100 kyr signal in the benthic δ18O global ice volume variability, which may either reflect limitations of the ash bed record or an regional rather than global influence of ice volume variability. Our results indicate that climate influences the Kamchatka‐Kurile arc volcanism, which may influence climate only by feedback. Plain Language Summary: Volcanic ash and dust produced during catastrophic explosive volcanic eruptions, such as those of Mount Pinatubo or El Chichón, can cause short‐term global cooling on the scale of a few years. It has long been speculated whether the Earth's long‐term cooling over the past few million years has been augmented by an increase in explosive volcanism about 2.58 million years ago. In order to investigate causal links between the climate evolution and volcanism during the past 4 million years, we obtained a time‐precise and temporally highly resolved record of the Kamchatka‐Kurile arc volcanism from the centimeter‐thick ash beds that were embedded in marine sediments after large eruptions downwind the volcanic sources. When the ash bed record is compared to climate evolution, it clearly shows that explosive volcanic eruptions—regardless of their short‐term effects—do not contribute directly to the long‐term global cooling. Instead, the variations of the Earth's powerful climate system modulate these explosive volcanic eruptions, as the periodic waxing and waning of the large ice shields affect the magma‐producing systems deep in the Earth's interior. However, climate‐active gases and particles produced during periods with more vigorous arc volcanism may still enhance the ice cycles. Key Points: Marine fallout ash beds record cyclicity and acceleration of the Plio‐Pleistocene (0–4 Myr) explosive Kamchatka‐Kurile arc volcanismAsh bed cyclicity correlates with the obliquity and precession variance of the global ice volumeClimate, and not direct orbital forcing, modulates the Plio‐Pleistocene volcanicity of the Kamchatka‐Kurile arc [ABSTRACT FROM AUTHOR]

Over the last 3.3 million years, the Antarctic Ice Sheet (AIS) has undergone phases of ice sheet growth and decay, impacting sea level and climate globally. Presently, the largely marine‐terminating AIS loses mass primarily by iceberg calving and basal melt of ice shelves. Quantifying past rates and timing of AIS melt is vital to understanding future cryosphere and sea level changes. One proxy for past ice sheet instabilities is iceberg rafted debris (IRD) fluxes. However, traditional methods of IRD quantification are labor‐intensive. Here, we present a new method of identifying IRD grains in sediment core X‐ray images using a convolutional neural network machine learning algorithm. We present a 3.3‐million‐year record of AIS IRD melt events using sediment cores from International Ocean Discovery Program Sites U1536, U1537, and U1538 in the Southern Ocean's "Iceberg Alley." We identify two increases in the IRD fluxes throughout this period, at ∼1.8 and 0.43 Ma. We propose that after 1.8 Ma, the AIS expanded and transitioned from a primarily terrestrial‐terminating to a primarily marine‐terminating ice sheet. Therefore, after 1.8 Ma, glacial terminations and AIS iceberg discharge are associated with variations in global ice volume, presumably through the mechanism of sea level and, therefore, grounding line change. The second AIS regime change occurs during the Mid‐Brunhes Event (∼0.43 Ma). After this time, there are heightened and continuous IRD fluxes at each glacial termination, indicating increased AIS size and instability after this time. Plain Language Summary: Understanding the timing and magnitude of the melt and retreat of the Antarctic Ice Sheet (AIS) in the past can help us understand its sensitivity to future global climate change. One way to understand ice sheets in the past is to identify and quantify sand to pebble‐sized rock and mineral grains incorporated into ice sheets and exported to the ocean by icebergs. These so‐called iceberg rafted debris (IRD) grains get deposited on the seafloor when icebergs melt, and they are easily identifiable in deep‐sea sediment core X‐ray images. Here, we present an artificial intelligence image detection method to identify IRD in sediment cores collected in the heart of "Iceberg Alley," a region that today receives the majority of Antarctic icebergs. We find that over the past 3.3 million years, there have been two increases in the amount of IRD deposited. We propose that the first major increase in IRD just after 1.8 Ma is evidence of an expansion of the AIS to an ice sheet with marine margins and ice shelves. We further propose that after 430,000 years ago, the AIS undergoes additional expansion and sees consistent, rapid retreat at the end of each global glaciation. Key Points: We present a machine learning method to quantify iceberg rafted debris (IRD) in deep‐sea sediment coresWe quantify IRD at three International Ocean Discovery Program sites in the heart of Iceberg Alley over 3.3 million yearsWe propose that Antarctica transitions from a primarily terrestrial‐terminating to marine‐terminating ice sheet 1.8 million years ago [ABSTRACT FROM AUTHOR]

Global mean sea level during the mid-Pliocene epoch (∼3 Ma), when CO2 and temperatures were above present levels, was notably higher than today due to reduced global ice sheet coverage. Nevertheless, the extent to which ice sheets responded to Pliocene warmth remains in question owing to high levels of uncertainty in proxy-based sea level reconstructions as well as solid Earth dynamic models that have been used to evaluate a limited number of data constraints. Here, we present a global dataset of 10 wave-cut scarps that formed by successive Pliocene sea level oscillations and which are observed today at elevations ranging from ∼6 to 109 m above sea level. The present-day elevations of these features have been identified using a combination of high-resolution digital elevation models and field mapping. Using the MATLAB interface TerraceM, we extrapolate the cliff and platform surfaces to determine the elevation of the scarp toe, which in most settings is buried under meters of talus. We correct the scarp-toe elevations for glacial isostatic adjustment and find that this process alone cannot explain observed differences in Pliocene paleo-shoreline elevations around the globe. We next determine the signal associated with mantle dynamic topography by back-advecting the present-day three-dimensional buoyancy structure of the mantle and calculating the difference in radial surface stresses over the last 3 Myr using the convection code ASPECT. We include a wide range of present-day mantle structures (buoyancy and viscosity) constrained by seismic tomography models, geodynamic observations, and rock mechanics laboratory experiments. Finally, we identify preferred dynamic topography change predictions based on their agreement with scarp elevations and use our most confident result to estimate a Pliocene global mean sea level based on one scarp from De Hoop, South Africa. This inference (11.6 ± 5.2 m) is a downward revision and may imply that ice sheets were relatively resistant to warm Pliocene climate conditions. We also conclude, however, that more targeted model development is needed to more reliably infer mid-Pliocene global mean sea level based on all scarps mapped in this study. [ABSTRACT FROM AUTHOR]

The sequence of feedbacks that characterized 100-kyr glacial cycles of the past million years remains uncertain, hampering an understanding of the interconnections between insolation, ice sheets, greenhouse gas forcing, and climate. Critical to addressing this issue is an accurate interpretation of the marine δ18O record, the main template for the Ice Ages. This study uses a global compilation of 49 paired sea surface temperature-planktonic δ18O records to extract the mean δ18O of surface ocean seawater over the past 800 kyr, which we interpret to dominantly reflect global ice volume. The results indicate that global surface temperature, inferred deep ocean temperature, and atmospheric CO2 decrease early during each glacial cycle in close association with one another, whereas major ice sheet growth occurs later in glacial cycles. These relationships suggest that ice volume may have exhibited a threshold response to global cooling, and that global deglaciations do not occur until after the growth of large ice sheets. This phase sequence also suggests that the ice sheets had relatively little feedback on global cooling. Simple modeling shows that the rate of ice volume change through time is largely determined by the combined influence of insolation, temperature, and ice sheet size, with possible implications for the evolution of glacial cycles over the past three million years.

Benthic δ 18O is often used as a stratigraphic tool to place marine records on a common age model and as a proxy for the timing of ice volume/sea level change. However, Skinner and Shackleton (2005) found that the timing of benthic δ 18O change at the last termination differed by 3900 years between one Atlantic site and one Pacific site. These results suggest that benthic δ 18O change may not always accurately record the timing of deglaciation. We compare benthic δ 18O records from 20 Atlantic sites and 14 Pacific sites to evaluate systematic differences in the timing of terminations in benthic δ 18O. Analysis of sedimentation rates derived from the alignment of benthic δ 18O suggests a statistically significant Atlantic lead over Pacific benthic δ 18O change during the last six terminations. We estimate an average Pacific benthic δ 18O lag of 1600 years for Terminations 1-5, slightly larger than the delay expected from ocean mixing rates given that most glacial meltwater probably enters the North Atlantic. We additionally find evidence of ∼4000-year Pacific δ 18O lags at approximately 128 ka and 330 ka, suggesting that stratigraphic correlation of δ 18O has the potential to generate age model errors of several thousand years during terminations. A simple model demonstrates that these lags can be generated by diachronous temperature changes and do not require slower circulation rates. Most importantly, diachronous benthic δ 18O responses must be taken into account when comparing Atlantic and Pacific benthic δ 18O records or when using benthic δ 18O records as a proxy for the timing of ice volume change. Copyright 2009 by the American Geophysical Union.

We describe the evolution of climate system dynamics by examining the climate response to changes in obliquity and precession over the last 5.3 Myr. In particular, we examine changes in the shape of glacial cycles and the power of obliquity and precession response in benthic δ18O. When the exponential trend in δ18O variance is removed, its spectral power exhibits strong, proportional responses to amplitude modulations in orbital forcing over most of the Plio-Pleistocene. Precession responses correlate with modulations in forcing for the last 5 Myr, but 41-kyr response is sensitive to obliquity modulation only before 1.4 Myr. Where responses are sensitive to modulations in forcing, we demonstrate that glacial cycles are orbitally forced rather than being self-sustained or paced by orbital changes. The shapes of glacial cycles have several nonlinear properties, which may be indicative of glacial-interglacial differences in climate sensitivity or response time. The "saw-tooth" asymmetry of glacial cycles first appears shortly after the onset of major northern hemisphere glaciation, and the relative duration of interglacial stages decreases at 1.4 Myr. Collectively, trends in the shape of glacial cycles and the sensitivity of δ18O to obliquity and precession are suggestive of major transitions in climate dynamics at approximately 2.5 and 1.4 Myr but show no significant change associated with the appearance of strong 100-kyr cycles during the mid-Pleistocene transition. © 2006 Elsevier Ltd. All rights reserved.

We present a 5.3-Myr stack (the "LR04" stack) of benthic δ18O records from 57 globally distributed sites aligned by an automated graphic correlation algorithm. This is the first benthic δ18O stack composed of more than three records to extend beyond 850 ka, and we use its improved signal quality to identify 24 new marine isotope stages in the early Pliocene. We also present a new LR04 age model for the Pliocene-Pleistocene derived from tuning the δ18O stack to a simple ice model based on 21 June insolation at 65°N. Stacked sedimentation rates provide additional age model constraints to prevent overtuning. Despite a conservative tuning strategy, the LR04 benthic stack exhibits significant coherency with insolation in the obliquity band throughout the entire 5.3 Myr and in the precession band for more than half of the record. The LR04 stack contains significantly more variance in benthic δ18O than previously published stacks of the late Pleistocene as the result of higher-resolution records, a better alignment technique, and a greater percentage of records from the Atlantic. Finally, the relative phases of the stack's 41- and 23-kyr components suggest that the precession component of δ18O from 2.7-1.6 Ma is primarily a deep-water temperature signal and that the phase of δ 18O precession response changed suddenly at 1.6 Ma. Copyright 2005 by the American Geophysical Union.

The growth and reduction of Northern Hemisphere ice sheets over the past million years is dominated by an approximately 100,000-year periodicity and a sawtooth pattern (1,2) (gradual growth and fast termination). Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles (3), and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession cycles (4,5). Yet insolation alone cannot explain the strong 100,000-year cycle, suggesting that internal climatic feedbacks may also be at work (4-7). Earlier conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere 'excess ice' (5,8-10), but the physical mechanisms underpinning the 100,000-year cycle remain unclear. Here we show, using comprehensive climate and ice-sheet models, that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere-asthenosphere system explain the 100,000-year periodicity. The responses of equilibrium states of ice sheets to summer insolation show hysteresis (11-13), with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles. The hysteresis loop of the North American ice sheet is such that after inception of the ice sheet, its mass balance remains mostly positive through several precession cycles, whose amplitudes decrease towards an eccentricity minimum. The larger the ice sheet grows and extends towards lower latitudes, the smaller is the insolation required to make the mass balance negative. Therefore, once a large ice sheet is established, a moderate increase in insolation is sufficient to trigger a negative mass balance, leading to an almost complete retreat of the ice sheet within several thousand years. This fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound (14-16), which is the lithosphere-asthenosphere response. Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles., Several internal feedback mechanisms have been suggested as crucial in 100-kyr glacial cycles, such as delayed bedrock rebound (14-16), the calving of ice-sheet margins (15), C[O.sub.2] variations (17,18), ocean feedback [...]

Contentious observations of Pleistocene shoreline features on the tectonically stable islands of Bermuda and the Bahamas have suggested that sea level about 400,000 years ago was more than 20 metres [...]

The factors driving glacial changes in ocean overturning circulation are not well understood. On the basis of a comparison of 20 climate variables over the past four glacial cycles, the [...]

Author(s): Kenji Kawamura (corresponding author) [1, 2, 9]; Frédéric Parrenin [3]; Lorraine Lisiecki [4]; Ryu Uemura [5]; Françoise Vimeux [6, 7]; Jeffrey P. Severinghaus [2]; Manuel A. Hutterli [8]; Takakiyo [...]