Evidence of Glacial Meltwater/Volcanic Interaction From Rifting of Supercontinent Rodinia During Tonian/Crogenian NeoProterozoic

High temperature glacial meltwater-rock reaction in the Neoproterozoic: evidence from zircon in-situ oxygen isotopes in granitic gneiss from the Sulu orogen

Authors:

He et al

Abstract:

To seek the occurrence of negative δ18O magmas in the Neoproterozoic, we conducted in-situ zircon O isotope analysis and U-Pb dating for granitic gneisses from the northeastern Sulu orogen, east-central China. Zircon U-Pb dating yields protolith ages of 753±15 Ma to 780±13 Ma and metamorphic ages of 209±3 to 244±7 Ma. The Neoproterozoic cores with concordant U-Pb ages exhibit a wide δ18O range from -11.0 to 5.8‰, which is nearly the same as those for cores with discordant U-Pb ages. The Triassic rims of some samples have homogeneous δ18O values of around -10‰ whereas the rims of the other samples show a wider range from -9.8 to 5.0‰. The δ18O values as negative as -11.0‰ for zircons with concordant Neoproterozoic U-Pb ages are reported for the first time, representing the primary record of negative δ18O magma in the Neoproterozoic. The continental subduction-zone metamorphism in the Triassic did not erase the abnormal δ18O record in the protolith cores despite metamorphic dehydration and partial melting under high-pressure to ultrahigh-pressure conditions. A conservative estimate suggests that the hydrothermal fluid reacted with the rocks should have δ18O values lower than -9.2‰, corresponding to the meteoric water in cold paleoclimate or the meltwater of local continental glaciation. The spatial variation in the O isotope compositions of Neoproterozoic zircons is a manifestation of the O isotope heterogeneity in the extinct hydrothermal-magmatic system. The hydrothermal alteration during the Neoproterozoic was incongruent, which was lately recorded by the wide range of δ18O values in the metamorphic zircons of Triassic age. The extensive O isotope exchange between the surface water and the deep rock requires high temperature and high water-rock ratios in continental rifting zones. This is ascribed to Neoproterozoic splitting of the South China Block from the Rodinia supercontinent.

Modeling glacial flow on and onto Pluto's Sputnik Planum

Modeling glacial flow on and onto Pluto's Sputnik Planum

Authors:

Umurhan et al

Abstract:

Observations of Pluto's surface made by the New Horizons spacecraft indicates present-day nitrogen ice glaciation in and around the basin known as Sputnik Planum. Motivated by these observations, we have developed an evolutionary glacial flow model of solid nitrogen ice taking into account its published thermophysical and rheologies properties. This model assumes that glacial ice layers flow laminarly and have low aspect ratios which permits a vertically integrated mathematical formulation. We assess the conditions for the validity of laminar nitrogen ice motion by revisiting the problem of the onset of solid-state buoyant convection of nitrogen ice for a variety of bottom thermal boundary conditions. Subject to uncertainties in nitrogen ice rheology, nitrogen ice layers are estimated to flow laminarly for thicknesses less than 400-1000 meters. The resulting mass-flux formulation for when the nitrogen ice flows as a laminar dry glacier is characterized by an Arrhenius-Glen functional form. The flow model developed is used here to qualitatively answer some questions motivated by observed glacial flow features found on Sputnik Planum. We find that the wavy transverse dark features found along the northern shoreline of Sputnik Planum may be a transitory imprint of shallow topography just beneath the ice surface suggesting the possibility that a major shoreward flow event happened relatively recently within the last few hundred years. Model results also support the interpretation that the prominent darkened features resembling flow lobes observed along the eastern shoreline of the Sputnik Planum basin may be a result of wet nitrogen glacial ice flowing into the basin from the pitted highlands of eastern Tombaugh Regio.

Trace Evidence of Alps-like Glaciers on Mars

Former extent of glacier-like forms on Mars

Authors:

Brough et al

Abstract:

Mars’ mid-latitude glacier-like forms (GLFs) have undergone substantial mass loss and recession since a hypothesised last martian glacial maximum (LMGM) stand. To date, there is a lack of knowledge of the nature and timing of the LMGM, the subsequent mass loss and whether this mass loss has been spatially variable. Here, we present the results of a population-scale inventory of recessional GLFs, derived from analysis of 1293 GLFs3 identified within Context Camera (CTX) imagery, to assess the distribution and controls on GLF recession. A total of 436 GLFs were identified showing strong evidence of recession: 197 in the northern hemisphere and 239 in the southern hemisphere. Relative to their parent populations, recessional GLFs are over-represented in the low latitude belts between 25 and 40° and in areas of high relief, suggesting that these zones exert some control over GLF sensitivity and response to forcing. This analysis is complemented by the reconstruction of the maximum extent and morphology of a specific GLF for which High Resolution Imaging Science Experiment (HiRISE) derived digital elevation data are available. Using Nye's (Nye, J.F. [1951] Proc. Roy. Soc. Lond, Ser. a—Mat. Phys. Sci., 207, 554–572) perfect plastic approximation of ice flow applied to multiple flow-lines under an optimum yield strength of 22 kPa, we calculate that the reconstructed GLF has lost an area of 6.86 km2 with a corresponding volume loss of 0.31 km3 since the LMGM. Assuming the loss reconstructed at this GLF occurred at all mid-latitude GLFs yields a total planetary ice loss from Mars’ GLFs of 135 km3, similar to the current ice volume in the European Alps on Earth.

The Behavior of the Antarctic Ice Sheet Across the Eocene-Oligocene Boundary Climate Change

Antarctic Ice Sheet variability across the Eocene-Oligocene boundary climate transition

Authors:

Galeotti et al

Abstract:

About 34 million years ago, Earth’s climate cooled and an ice sheet formed on Antarctica as atmospheric carbon dioxide (CO2) fell below ~750 parts per million (ppm). Sedimentary cycles from a drillcore in the western Ross Sea provide direct evidence of orbitally controlled glacial cycles between 34 million and 31 million years ago. Initially, under atmospheric CO2 levels of ≥600 ppm, a smaller Antarctic Ice Sheet (AIS), restricted to the terrestrial continent, was highly responsive to local insolation forcing. A more stable, continental-scale ice sheet calving at the coastline did not form until ~32.8 million years ago, coincident with the earliest time that atmospheric CO2 levels fell below ~600 ppm. Our results provide insight into the potential of the AIS for threshold behavior and have implications for its sensitivity to atmospheric CO2 concentrations above present-day levels.

The Predicted Interaction of Lava and Glaciers on Early Mars

Lava heating and loading of ice sheets on early Mars: Predictions for meltwater generation, groundwater recharge, and resulting landforms

Authors:

Cassanelli et al

Abstract:

Recent modeling studies of the early Mars climate predict a predominantly cold climate, characterized by the formation of regional ice sheets across the highland areas of Mars. Formation of the predicted “icy highlands” ice sheets is coincident with a peak in the volcanic flux of Mars involving the emplacement of the Late Noachian – Early Hesperian ridged plains unit. We explore the relationship between the predicted early Mars “icy highlands” ice sheets, and the extensive early flood volcanism to gain insight into the surface conditions prevalent during the Late Noachian to Early Hesperian transition period. Using Hesperia Planum as a type area, we develop an ice sheet lava heating and loading model. We quantitatively assess the thermal and melting processes involved in the lava heating and loading process following the chronological sequence of lava emplacement. We test a broad range of parameters to thoroughly constrain the lava heating and loading process and outline predictions for the formation of resulting geological features. We apply the theoretical model to a study area within the Hesperia Planum region and assess the observed geology against predictions derived from the ice sheet lava heating and loading model. Due to the highly cratered nature of the Noachian highlands terrain onto which the volcanic plains were emplaced, we predict highly asymmetrical lava loading conditions. Crater interiors are predicted to accumulate greater thicknesses of lava over more rapid timescales, while in the intercrater plains, lava accumulation occurs over longer timescales and does not reach great thicknesses. We find that top-down melting due to conductive heat transfer from supraglacial lava flows is generally limited when the emplaced lava flows are less than ∼10 m thick, but is very significant at lava flow thicknesses of ∼100 m or greater. We find that bottom-up cryosphere and ice sheet melting is most likely to occur within crater interiors where lavas accumulate to a sufficient thickness to raise the ice-melting isotherm to the base of the superposed lavas. In these locations, if lava accumulation occurs rapidly, bottom-up melting of the ice sheet can continue, or begin, after lava accumulation has completed in a process we term “deferred melting”. Subsurface mass loss through melting of the buried ice sheets is predicted to cause substantial subsidence in the superposed lavas, leading to the formation of associated collapse features including fracture systems, depressions, surface faulting and folding, wrinkle-ridge formation, and chaos terrain. In addition, if meltwater generated from the lava heating and loading process becomes trapped at the lava flow margins due to the presence of impermeable confining units, large highly pressurized episodic flooding events could occur. Examination of the study area reveals geological features which are generally consistent with those predicted to form as a result of the ice sheet lava heating and loading process, suggesting the presence of surface snow and ice during the Late Noachian to Early Hesperian period.

Galaxias Mons may Have Been Formed From a Subglacial Volcanic Eruption

Possible sub-glacial eruptions in the Galaxias Quadrangle, Mars

Authors:

Mouginis-Mark et al

Abstract:

We have identified several landforms in the Galaxias Quadrangle of Mars (MTM 35217), 33.0–35.5°N, 216.0–218.0°W which are consistent with this area having been covered by an ancient ice sheet concurrent with volcanic eruptions. Volcanic activity was initiated by the intrusion of several large dikes measuring ∼50–100 m wide and protruding up to ∼35 m above the present-day surface. These dikes appear to have originated from Elysium Planitia ∼600 km to the SE. In one instance, a dike (at an elevation of −3750 m) appears to have produced a subglacial mound (referred to here as “Galaxias Mons 2”) that evolved into an extrusive eruption and produced copious volumes of melt water that carved an outflow channel that extends almost 300 km to the north. At a lower elevation (−3980 m), a second putative dike may have failed to break the surface of the ice sheet and formed Galaxias Mons as a laccolithic intrusion. We numerically model the formation of Galaxias Mons and find that at least 200 m of ice may once have existed at this latitude at the time of the dike intrusions. Such a conclusion supports the idea that enigmatic small domes in the area may be pingoes. Collectively, these observations suggest that the previous interpretations for the origin of near-by Hrad Vallis as a sub-aerial eruption may need to be revised.

What is REALLY Happening With Antarctica's Ice Sheet?

There have been quite few big media stories related to Antarctica recently, including a paper on the irreversible collapse of the marine portion of the West Antarctic Ice Sheet and a NASA-funded study that finds, contrary to numerous previous results, that the Antarctic ice sheet as a whole has been gaining mass between 1992 and 2008. This most recent study received a lot of media attention because it runs counter to what was said in the last IPCC Report. Certain parts of the media hailed this as another sign that the impacts of climate change had somehow been exaggerated a risk that the lead author Jay Zwally was concerned about before the research was published.

So what did Zwally and his colleagues do, what did they find, and why does it contradict a plethora of previous studies that suggest Antarctica has been losing mass over the same time period?

link.

Antarctic ice Growth Before and After the Eocene-Oligocene Transition

Antarctic ice growth before and after the Eocene-Oligocene Transition: New estimates from clumped isotope paleothermometry

Authors:

Petersen et al

Abstract:

Across the Eocene-Oligocene transition, the oxygen isotopic composition (δ18O) of benthic and planktonic foraminifera increased by over 1‰. This shift is thought to represent a combination of global cooling and the growth of a large ice sheet on the Antarctic continent. To determine the contribution of each of these factors to the total change in δ18O, we measured the clumped isotopic composition of planktonic foraminifera tests from ODP Site 689 in the Southern Ocean. Near-surface temperatures were ~12°C in the intervals 0-1.5 Myr before and 1-2 Myr after the major (Oi-1) transition, in agreement with estimates made using other proxies at nearby sites. Temperatures cooled by 0.4 ± 1.1°C between these intervals, indicating that the long-term change in δ18O seen in planktonic foraminifera at this site is predominantly due to changes in ice volume. A larger instantaneous cooling may have occurred during Oi-1, but is not captured in this study due to sampling resolution. The corresponding change in the isotopic composition of seawater (δ18Osw) is 0.75 ± 0.23‰, which is within the range of previous estimates, and represents global ice growth equivalent to roughly ~110-120% of the volume of the modern Antarctic Ice Sheet, or ~80-90m of eustatic sea level change.

More Incredible Pictures of Pluto: Close up of Glacial Flow [5/5]

More Incredible Pictures of Pluto: Glacial Flow in the Sputnik Planum [4/5]

Pluto's Active Nitrogen Ice Flows

link.

Mars is Still in an Ice Age? Remnant ice Found on Tropical Latitude Arsia Mons

on mars (left), on earth (right)

Remnant buried ice in the equatorial Regions of Mars: Morphological Indicators Associated with the Arsia Mons Tropical Mountain Glacier Deposits

Authors:

Scanlon et al

Abstract:

The fan-shaped deposit (FSD) on the western and northwestern flanks of Arsia Mons is the remnant of tropical mountain glaciers, deposited several tens to hundreds of millions of years ago during periods of high spin-axis obliquity. Previous workers have argued that the Smooth Facies in the FSD contains a core of ancient glacial ice. Here, we find evidence that additional glacial ice remains preserved within several other landforms in the Smooth Facies and Ridged Facies. These include landforms that we interpret as kame and kettle topography on the basis of their distribution, size, and morphologies ranging progressively from knobs to degraded knobs to pits. We argue that some moraines in the Ridged Facies are ice-cored on the basis of their interactions with lava flows and the axial troughs at the crests of some moraines. We also argue that dunes with axial troughs, found in and surrounding the FSD, are the remnants of sediment-covered snow dunes formed by reworking of snow or glacial ice, and that the axial troughs form as tension cracks in the sediment and deepen by sublimation of the underlying ice. Long-term preservation of water ice in equatorial environments is assisted by a meters- to decameters-thick debris cover (lag) formed from sublimation of dirty ice, as well as burial beneath volcanic tephra and eolian deposits. This ancient ice could contain preserved biosignatures, provide information on Martian climate and atmospheric history, and serve as a resource for human exploration.

Mountains of Martian Ice: An Extensive System of Martian High Latitude, Dust Covered Glaciers Found

 example of dust covered glacier

 High latitude locales of dust covered glaciers

Mars has distinct polar ice caps, but Mars also has belts of glaciers at its central latitudes in both the southern and northern hemispheres. A thick layer of dust covers the glaciers, so they appear as surface of the ground, but radar measurements show that underneath the dust there are glaciers composed of frozen water. New studies have now calculated the size of the glaciers and thus the amount of water in the glaciers. It is the equivalent of all of Mars being covered by more than one meter of ice. The results are published in the scientific journal, Geophysical Research Letter.

Several satellites orbit Mars and on satellite images, researchers have been able to observe the shape of glaciers just below the surface. For a long time scientists did not know if the ice was made of frozen water (H2O) or of carbon dioxide (CO2) or whether it was mud. Using radar measurements from the NASA satellite, Mars Reconnaissance Orbiter, researchers have been able to determine that it is water ice. But how thick is the ice and do the glaciers resemble glaciers on Earth? A group of researchers at the Niels Bohr Institute have now calculated this using radar observations combined with ice flow modelling.

link.

Evidence of air Pollution From Spanish Conquest of the Inca Found in Glacier Ice Cores

In the 16th century, during its conquest of South America, the Spanish Empire forced countless Incas to work extracting silver from the mountaintop mines of Potosí, in what is now Bolivia--then the largest source of silver in the world. The Inca already knew how to refine silver, but in 1572 the Spanish introduced a new technology that boosted production many times over and sent thick clouds of lead dust rising over the Andes for the first time in history.

Winds carried some of that pollution 500 miles northwest into Peru, where tiny remnants of it settled on the Quelccaya Ice Cap.

There it stayed--buried under hundreds of years of snow and ice--until researchers from The Ohio State University found it in 2003.

In the early online edition of the Proceedings of the National Academy of Sciences, they report discovery of a layer within a Quelccaya ice core that dates to the Spanish conquest of the Inca, contains bits of lead and bears the chemical signature of the silver mines of Potosí.

The core provides the first detailed record of widespread human-produced air pollution in South America from before the industrial revolution, and makes Quelccaya one of only a few select sites on the planet where the pre-industrial human impact on air quality can be studied today.

"This evidence supports the idea that human impact on the environment was widespread even before the industrial revolution," said Paolo Gabrielli, a research scientist at the Byrd Polar and Climate Research Center at Ohio State and corresponding author of the study.

Lonnie Thompson, Distinguished University Professor of earth sciences at Ohio State and co-author of the study, called the find "another keyhole into the past of human activity in that part of the world," and suggested that further investigation could ultimately help us better understand the fate of pollution circulating in the atmosphere today.

Previously, Thompson has called the Quelccaya ice cores a "Rosetta Stone" for gauging Earth's climate history. The samples were cut from ice that formed over 1,200 years as snow settled on the Peruvian Andes. Layer by layer, the ice captured chemicals from the air and precipitation during wet and dry seasons for all those years. Today, researchers analyze the chemistry of different layers to measure historical changes in climate.

For this study, the researchers used a mass spectrometer to measure the amount and type of chemicals present in the ice dating back to 800 AD. They looked for antimony, arsenic, bismuth, molybdenum and especially lead. That's because the refining process that the Spanish introduced to South America involved grinding silver ore--which contains much more lead than silver--into powder before mixing it with mercury in a process called amalgamation. So atmospheric pollution from silver production would chiefly contain traces of lead particulates.

The mass spectrometer revealed some spikes in the concentrations of these elements in the years before Spanish rule, but those layers all likely coincide with natural contamination sources, such as volcanic eruptions. Starting just before 1600, however, the Quelccaya ice began capturing much larger quantities of these elements, and the high amounts persisted until the early 1800s, when South American countries declared independence from Spain.

link.

Washington's Scablands Have a Cousin in Iceland

A massive canyon that is home to Europe's most powerful waterfall was created in a matter of days by extreme flooding, new research reveals.

The Jökulsárgljúfur canyon in Iceland, which is 28 km long and 100 metres deep in places, was formed by a series of distinct floods that occurred thousands of years apart, a study shows.

Scientists analysed rocks along a 5km stretch of the canyon - which contains the Jökulsá á Fjöllum river and the mighty Dettifoss waterfall - to create a timeline of how the landscape was created.

They used geochemical analysis to determine how long rocks on the canyon walls had been exposed to the elements. This helped the team pinpoint how the position and shape of the landscape had changed over time.

Researchers connected major shifts in the landscape to a series of extreme floods, which took place 9,000, 5,000 and 2,000 years ago. The floods were caused by volcanic activity under glaciers, and each was powerful enough to tear up bedrock. They formed the canyon's 100-metre walls and pushed three waterfalls, including Dettifoss, back upstream by as much as 2km during each flood.

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paper link.

Craters From Two Subglacial Lakes Found in Greenland

Researchers who are building the highest-resolution map of the Greenland Ice Sheet to date have made a surprising discovery: two lakes of meltwater that pooled beneath the ice and rapidly drained away.

One lake once held billions of gallons of water and emptied to form a mile-wide crater in just a few weeks. The other lake has filled and emptied twice in the last two years.

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Where did the Water Come From in the Martian Late Noachian “Icy Highlands” Model

Sources of water for the outflow Channels on Mars: Implications of the Late Noachian “Icy Highlands” Model for Melting and Groundwater Recharge on the Tharsis Rise

Authors:

Cassenelli et al

Abstract:

From the Late Noachian period, through the Hesperian, and into the Amazonian periods on Mars, large outflow channels were formed. Many are interpreted to have originated through the catastrophic discharge of groundwater from martian aquifers, involving the release of up to millions of cubic-kilometers of water. Such a mechanism for outflow channel formation requires that martian aquifers were supplied with significant quantities of water some time prior to the discharge events. Typical groundwater recharge occurs due to the infiltration of surficial waters through a permeable substrate down into aquifers. However, some climate models predict an early martian climate dominated by generally “cold and icy” conditions. In this scenario, a globally continuous, impermeable cryosphere prevents infiltration of liquid water (that might be generated at the surface through anomalous heating conditions), leaving the martian aquifers without an apparent source of recharge to supply later outflow channel formation by groundwater discharge. More recent global climate modeling of an early, thicker CO2 martian atmosphere predicts that, when coupled with a full water cycle, the atmosphere of Mars will behave adiabatically causing temperatures to decrease with elevation. The high standing areas of Mars, such as the southern highlands and the Tharsis region, then act as cold traps. This leads to the preferential accumulation of snow and ice, resulting in the formation of regional ice sheets throughout the highlands that characterize the Late Noachian “icy highlands” early Mars climate model (LNIH). We make the initial assumption that the LNIH model is representative of the early Mars climate, and seek to test the model against the presence of the Hesperian and Amazonian outflow channels to determine if it can be consistent. In order to reconcile the LNIH early Mars climate model with the presence of the later outflow channels a groundwater recharge mechanism that can operate under the predicted “cold and icy” conditions is required. We test basal melting of surface snow and ice in response to a regionally elevated geothermal heat flux throughout the Tharsis rise (resulting from widespread volcanic and magmatic activity during the Noachian) as a mechanism that can provide: (1) liquid water generation at the surface of Mars under generally “cold and icy” conditions, and (2) potentially large scale integration of the hydrological system (through thinning or breaching of the cryosphere), allowing for infiltration of meltwater to provide groundwater recharge during the Late Noachian to supply the later formation of outflow channels. We find: (1) Regional scale basal melting of LNIH ice sheets is not likely to occur at the predicted nominal average ice sheet thicknesses, even in the presence of the anomalous bottom-up heating conditions expected in the Tharsis region (although the increased baseline heating will render the LNIH ice sheets more susceptible to melting through additional anomalous heating conditions introduced by top-down and bottom-up processes). (2) Local scale basal melting and groundwater recharge through a “heat-pipe drain pipe” mechanism is likely to occur, but is not predicted to produce sufficient groundwater recharge to supply the water needed to form the outflow channels. (3) Under the assumption of an ice saturated cryosphere, regional scale melting of the cryosphere due to the insulating effect of the LNIH ice sheets does not provide enough water to explain the formation of all of the outflow channels. Therefore, if the LNIH model is correct, the groundwater recharge that supplied outflow channel formation requires a source that operated earlier in martian history, or the recharge was supplied by other mechanisms.

Volcanic Induced Deglaciation on Arsia Mons During Late Amazonian

Volcanism-induced, local wet-based glacial conditions recorded in the Late Amazonian Arsia Mons tropical mountain glacier deposits

Authors:

Scanlon et al

Abstract:

The tropical mountain glacial fan-shaped deposit (FSD) to the northwest of the Arsia Mons volcano on Mars contains numerous glacial and volcanic landforms. While most of the glacial landforms are interpreted to have formed by cold-based glacial processes, several glacial landforms near glaciovolcanic edifices are more consistent with localized wet-based glacial processes. These landforms include ribbed moraines, which suggest local, thermal transitions between wet- and cold-based ice; thrust-block moraines, whose formation is typically assisted by the presence of subglacial water; streamlined knobs that we interpret to have been sculpted by ice sliding along its base; and a braided outflow channel. The presence and association of these features, together with evidence of both subglacial volcanic eruptions and local ice-marginal advances, favor polythermal glaciers with localized wet-based conditions. We propose that lava-to-ice heat transfer during the eruption of the glaciovolcanic edifices caused the Arsia Mons paleoglacier to melt at its base in some areas, resulting in these locally wet-based glacial conditions. A polythermal glacier provides more potential microbial habitats and more connectivity between habitats than does a cold-based glacier, and we review glacial and glaciovolcanic habitats on Earth that may provide insight into the likelihood of potential microbial habitats within the Arsia Mons FSD on Mars.

Evidence the Martian South Polar Plateau Radically Shrank During the Late Amazonian

New insights into the Late Amazonian zonal shrinkage of the martian south polar plateau

Authors:

Rodriguez et al

Abstract:

The martian south polar plateau, Planum Australe, comprises the largest known water–ice surface deposit on the planet. Here, we present evidence for an episode of extensive polar plateau retreat during the Late Amazonian, which affected regions flanking circum-polar terrains located between Cavi Angusti and Sisyphi Montes.

Southern Laurentide Ice-sheet Retreat Synchronous due to Rising Boreal Summer Insolation

Southern Laurentide ice-sheet retreat synchronous with rising boreal summer insolation

Authors:

Ullman et al

Abstract:

Establishing the precise timing for the onset of ice-sheet retreat at the end of the Last Glacial Maximum (LGM) is critical for delineating mechanisms that drive deglaciations. Uncertainties in the timing of ice-margin retreat and global ice-volume change allow a variety of plausible deglaciation triggers. Using boulder 10Be surface exposure ages, we date initial southern Laurentide ice-sheet (LIS) retreat from LGM moraines in Wisconsin (USA) to 23.0 ± 0.6 ka, coincident with retreat elsewhere along the southern LIS and synchronous with the initial rise in boreal summer insolation 24–23 ka. We show with climate-surface mass balance simulations that this small increase in boreal summer insolation alone is potentially sufficient to drive enhanced southern LIS surface ablation. We also date increased southern LIS retreat after ca. 20.5 ka likely driven by an acceleration in rising isolation. This near-instantaneous southern LIS response to boreal summer insolation before any rise in atmospheric CO2 supports the Milanković hypothesis of orbital forcing of deglaciations.