Is the Shuram Event Still Younger Than the 580 Million Years agoEdiacaran NeoProteorozoic Gaskiers Glaciation Though?

Silicified glendonites in the Ediacaran Doushantuo Formation (South China) and their potential paleoclimatic implications

Authors:

Wang et al

Abstract:

The Ediacaran Period is punctuated by the ca. 580 Ma Gaskiers glaciation in Newfoundland. However, paleoclimatic data are scarce in Ediacaran successions in South China, where abundant geochemical and paleobiological data are shaping current understanding of Ediacaran evolutionary and environmental history. Here, we report the occurrence of silicified glendonites in the Ediacaran Doushantuo Formation deposited in an inner-shelf environment on the South China block. Petrographic evidence suggests that these silicified glendonites are pseudomorphs after syndepositional or early authigenic ikaites formed at near-freezing temperatures. The glendonite-bearing stratigraphic interval is characterized by positive δ13C values. It predates both the negative δ13C excursion EN3 (widely believed to be an equivalent of the Shuram negative excursion) and excursion EN2. Although alternative interpretations may be possible, these glendonites may be related to and correlated with the Gaskiers glaciation. If confirmed, this correlation suggests that the Shuram event postdates the Gaskiers glaciation, thus having important implications for Ediacaran climate changes, carbon cycles, and biological evolution.

There was Significantly Less Sediment Laid Down During the Cryogenian NeoProterozoic (Snowball Earth) Glaciations

Slow net sediment accumulation sets snowball Earth apart from all younger glacial episodes

Authors:

Partin et al

Abstract:

During the Sturtian and Marinoan "snowball Earth" episodes, ice cover is thought to have extended from polar to tropical latitudes. We test the supposition that such an extreme glacial climate, not repeated in the subsequent ∼635 m.y. of Earth history, would have reduced the vigor of the hydrologic cycle and thus diminished sediment flux to the oceans. With >500 sediment accumulation rates to characterize Sturtian and Marinoan deposits, we find median accumulation rates at least four to 15 times slower than expected for Phanerozoic glaciomarine deposits as characterized by >10,000 rates. Our comparison is conservative with respect to time span, latitude, and distance from the ice margin. Phanerozoic accumulation rates decrease systematically when averaged over longer time spans. Comparisons were drawn, therefore, at 5 and 57 m.y. time spans to match minimum Marinoan and Sturtian durations, respectively. Cenozoic glaciomarine accumulation also slows with increasing latitude from temperate to polar climates and with increasing distance from the ice margin. After accounting for time span, snowball Earth deposits at low latitude are found to be thinner than would be expected either for high-latitude Cenozoic glacial deposits or for very distal glaciomarine abyssal muds with ice-rafted debris. The rate discrepancy is not readily attributed to overestimates of the total Marinoan or Sturtian durations. If sediment fluxes during warm melt intervals did approach Phanerozoic rates, these intervals must have occupied a much smaller proportion of snowball Earth episodes than in younger glacial climates.

Did Earth Narrowly Escape a Third Snowball Earth Episode?

Dodging snowballs: Geochronology of the Gaskiers glaciation and the first appearance of the Ediacaran biota

Authors:

Pu et al

Abstract:

The snowball Earth hypothesis predicts that low-latitude glaciation lasted millions of years while CO2 built up to critical levels to culminate in catastrophic deglaciation in a supergreenhouse climate. The Gaskiers Formation of eastern Newfoundland (Canada) has been attributed to a snowball glaciation event, but the lack of robust paleomagnetic data and precise geochronological constraints has precluded tests of the hypothesis. Here we present high-precision U-Pb zircon geochronology (chemical abrasion–isotope dilution–thermal ionization mass spectrometry) from eight tuffs from multiple distant stratigraphic sections that bracket glacial diamictites and the first appearance of large Ediacaran fossils. Including internal error, deposition of the Gaskiers diamictite on the Avalon Peninsula is constrained to have been between 580.90 ± 0.40 and 579.88 ± 0.44 Ma, and the Trinity diamictite on Bonavista Peninsula was deposited between 579.63 ± 0.15 and 579.24 ± 0.17 Ma. Assuming approximately synchronous deglaciation, these results imply a maximum duration for deposition of the Trinity diamictite of ≤340 k.y.; this is inconsistent with the multimillion year duration predicted by the snowball Earth hypothesis. Our geochronologic data also constrain the first appearance datum of Ediacaran fossils to

Explaining the Iron Drop Stones of the Sturtian Glaciation (Snowball Earth)

Sub–ice shelf ironstone deposition during the Neoproterozoic Sturtian glaciation

Authors:

Lechte et al

Abstract:

The Neoproterozoic Sturtian glaciation is considered to be among the most severe glaciations in Earth history, possibly encompassing the entire planet and lasting for more than 50 m.y. Iron formations are globally associated with Sturtian glacial successions, although the influence of glaciation on the genesis of these iron formations remains contentious. Here we examine the Sturtian iron formations of Namibia and Australia that feature finely laminated ironstones containing up to 55% total iron. These ironstones are repeatedly interbedded with massive diamictites, yet dropstones and other clastic input are nearly absent in the laminated ironstone facies. Intercalated diamictites are variably ferruginous and characterized by a strong glacial influence with evidence of glaciotectonism. The ferruginous facies are laterally discontinuous and commonly occupy paleobathymetric depressions. Rare earth element signatures from these iron formations are similar to those from modern seawater but lack cerium anomalies. The paradox of dropstone-free, laminated sediments intimately interlaminated with massive ice-proximal diamictites can be resolved by deposition under an ice shelf. Polynya activity and the mixing of cold, oxygenated glacial fluids with ferruginous seawater via an ice pump mechanism may explain the deposition of these iron formations and their restriction to Sturtian glacial successions globally

Was the Cryogenian Marinoan Glaciation REALLY a Snowball Earth?

Duration and nature of the end-Cryogenian (Marinoan) glaciation

Authors:

Prave et al

Abstract:

The end-Cryogenian glaciation (Marinoan) is portrayed commonly as the archetype of snowball Earth, yet its duration and character remain uncertain. Here we report U-Pb zircon ages for two ash beds from widely separated localities of the Marinoan-equivalent Ghaub Formation in Namibia: 639.29 ± 0.26 Ma and 635.21 ± 0.59 Ma. These findings verify, for the first time, the key prediction of the snowball Earth hypothesis for the Marinoan glaciation, i.e., longevity, with a duration of ≥4 m.y. They also show that the nonglacial interlude of Cryogenian time spanned 20 m.y. or less and that glacigenic erosion and sedimentation, and at least intermittent open-water conditions, occurred 4 m.y. prior to termination of the Marinoan glaciation.

Evidence of Amazonian era ice Ages From Terra Cimmeria, Mars

Amazonian-aged fluvial system and associated ice-related features in Terra Cimmeria, Mars

Authors:

Adeli et al

Abstract:

The Martian climate throughout the Amazonian is widely believed to have been cold and hyper-arid, very similar to the current conditions. However, ubiquitous evidence of aqueous and glacial activity has been recently reported, including channels that can be tens to hundreds of kilometres long, alluvial and fluvial deposits, ice-rich mantles, and glacial and periglacial landforms. Here we study a ∼340 km-long fluvial system located in the Terra Cimmeria region, in the southern mid-latitudes of Mars. The fluvial system is composed of an upstream catchment system with narrow glaciofluvial valleys and remnants of ice-rich deposits. We observe depositional features including fan-shaped deposits, and erosional features such as scour marks and streamlined islands. At the downstream section of this fluvial system is an outflow channel named Kārūn Valles, which displays a unique braided alluvial fan and terminates on the floor of the Ariadnes Colles basin. Our observations point to surface runoff of ice/snow melt as the water source for this fluvial activity. According to our crater size–frequency distribution analysis the entire fluvial system formed during early to middle Amazonian, between View the MathML source∼1.8−0.2+0.2 Ga to View the MathML source510−40+40 Ma. Hydraulic modelling indicates that the Kārūn Valles and consequently the alluvial fan formation took place in geologically short-term event(s). We conclude that liquid water was present in Terra Cimmeria during the early to middle Amazonian, and that Mars during that time may have undergone several episodic glacial-related events.

Mars is Emerging From an Ice Age

Radar measurements of Mars' polar ice caps reveal that the mostly dry, dusty planet is emerging from an ice age, following multiple rounds of climate change. Understanding the Martian climate will help determine when the planet was habitable in the past, how that changed, and may inform studies of climate change on Earth. Models have suggested that Mars has undergone ice ages in the past, but empirical data to confirm this has been sparse. Here, Isaac Smith and colleagues used radar to analyze layers of ice within the planet's polar ice caps, using the Shallow Radar instrument onboard the Mars Reconnaissance Orbiter spacecraft. As ice erodes, wind can create spiral troughs and other distinct features. Tracing the layers of these features within the ice can reveal changes in ice accumulation and flow - and thus changes in climate - in the past. While the southern ice cap is relatively small and altered by meteorite impacts, the researchers were able to trace the layers within the northern ice cap. They found layers and migration paths that increase in slope abruptly, reverse direction, or are completely buried. Their analysis suggests that the planet is currently emerging from an ice age, in a retreat that began approximately 370,000 years ago.

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Ice Ages Really Triggered by Tectonic Activity?

For hundreds of millions of years, Earth's climate has remained on a fairly even keel, with some dramatic exceptions: Around 80 million years ago, the planet's temperature plummeted, along with carbon dioxide levels in the atmosphere. The Earth eventually recovered, only to swing back into the present-day ice age 50 million years ago.

Now geologists at MIT have identified the likely cause of both ice ages, as well as a natural mechanism for carbon sequestration. Just prior to both periods, massive tectonic collisions took place near the Earth's equator -- a tropical zone where rocks undergo heavy weathering due to frequent rain and other environmental conditions. This weathering involves chemical reactions that absorb a large amount of carbon dioxide from the atmosphere. The dramatic drawdown of carbon dioxide cooled the atmosphere, the new study suggests, and set the planet up for two ice ages, 80 million and 50 million years ago.

"Everybody agrees that on geological timescales over hundreds of millions of years, tectonics control the climate, but we didn't know how to connect this," says Oliver Jagoutz, associate professor of Earth, Atmospheric and Planetary Sciences (EAPS) at MIT. "I think we're the first ones to really link large-scale tectonic events to climate change."

Jagoutz and his colleagues, EAPS Professor Leigh Royden, and Francis McDonald of Harvard University, have published their findings in the Proceedings of the National Academy of Sciences.

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Ecological Changes During the Glacial & Interglacials of the late Paleozoic ice age of Pennsylvanian Carboniferous Argentina

Compositional turnover and ecological changes related to the waxing and waning of glaciers during the late Paleozoic ice age in ice-proximal regions (Pennsylvanian, western Argentina)

Author:

Balseiro

Abstract:

The late Paleozoic ice age (LPIA) had a profound effect on the biota. Despite much research having been focused on paleotropical regions or global-scale analyses, regional ecological changes have seldom been studied in ice-proximal basins. Here, I study the compositional turnover and diversity structure across the main Carboniferous glacial event recorded in western Argentina and the subsequent nonglacial interval. Brachiopod and bivalve data from western Argentina suggest that the transition from glacial to nonglacial climates caused major compositional changes. Turnover, however, was not uniform across the bathymetric gradient, being higher in deep environments. Because extirpation was concentrated in brachiopods, but immigration was similar in both clades, the taxonomic structure of the region was significantly modified. Although regional hierarchical diversity structure and occupancy distributions remained stable, dissecting the analysis in brachiopods and bivalves underscores that both clades had different responses to climate change. Brachiopods, on the one hand, show stability in the diversity structure and a very slight decrease in occupancies of intermediate genera, while bivalves show an important rise in diversity, both at the environment and regional scale, and an increase in genera with intermediate occupancies. The bathymetric diversity gradient was also modified from hump shaped with maximum diversity in the deep subtidal to a linear gradient with maximum values toward the offshore. However, relative compositional differences within environments remained stable, with maximum values at intermediate depths both in glacial and nonglacial intervals. Moreover, local-scale coexistence between brachiopods and bivalves changed in the nonglacial interval, showing significant segregation, which indicates relevant modifications in community assembly dynamics. Results from western Argentina highlight the magnitude of regional-scale ecological changes during the LPIA in ice-proximal regions, suggesting that the waxing and waning of glaciers was able to cause regional taxonomic turnover and medium-scale ecological changes even during intervals of relative macroevolutionary quiescence.

Quaternary glaciation and the Great American Biotic Interchange

Quaternary glaciation and the Great American Biotic Interchange

Authors:

Bacon et al

Abstract:

Recent geological studies demonstrate that the Isthmus of Panama emerged some 10 m.y. earlier than previously assumed. Although absent today in Panama, Central American savanna environments likely developed in connection with the onset of Northern Hemisphere glaciations. As is widely recognized, most of the mammals crossing the isthmus since 2.5 Ma lived in savannas. Could climate-induced vegetational changes across Panama explain the delayed migration of mammals, rather than terrestrial connectivity? We investigate the congruence between cross-continental mammal migration and climate change through analysis of fossil data and molecular phylogenies. Evidence from fossil findings shows that the vast majority of mammals crossed between South and North America after ca. 3 Ma. By contrast, dated mammal phylogenies suggest that migration events started somewhat earlier, ca. 4–3 Ma, but allowing for biases toward greater ages of molecular than geologic dating and uncertainties in the former, we consider this age range not to be significantly earlier than 3 Ma. We conclude that savanna-like environments developed in response to the vast Laurentide ice sheet at the first Quaternary glaciation triggered the initiation of the Great American Biotic Interchange in mammals.

The Importance of Atmospheric Carbon in the end of the Last Glaciation

As the Earth emerged from its last ice age several thousand years ago, atmospheric carbon dioxide increased and further warmed the planet. Scientists have long speculated that the primary source of this CO2 was from the deep ocean around Antarctica, though it has been difficult to prove.

A new study published this week in Proceedings of the National Academy of Sciences confirmed that the ocean played a significant role in the rise of atmospheric carbon dioxide, but also documents the signature of land-based carbon sources in Antarctic ice cores that contributed to abrupt increases in CO2.

"There wasn't a steady rate of rising carbon dioxide during the last deglaciation," said Edward Brook, an Oregon State University paleoclimatologist and co-author on the PNAS study. "It happened in fits and starts. With the new precise techniques we developed to fingerprint the sources, it is apparent that the early carbon largely came from the ocean, but we think the system got a jolt from an influx of land-based carbon a few times as the climate warmed."

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At the end of the Pleistocene Ice Age, Antarctica had a Massive Ice-Shelf Collapse

In a new study that provides clues about how Antarctica's nation-sized Ross Ice Shelf might respond to a warming climate, U.S. and Japanese oceanographers have shown that a 100,000-square-mile section of the ice shelf broke apart within 1,500 years during a warming period after the last ice age.

The Ross Ice Shelf is the world's largest ice shelf, a vast floating extension of the West Antarctic Ice Sheet that is about the size of France. But at the end of the last ice age, it extended much farther north and covered the entire Ross Sea.

A study in this week's Proceedings of the National Academy of Sciences details how the ice shelf shrank during a period of climate warming following the ice age. The paper was co-authored by Rice University oceanographer John Anderson, postdoctoral research associate Lauren Simkins, graduate student Lindsay Prothro and colleagues at the University of Tokyo.

"At the height of the last ice age, we know that the sheet of ice covering the Antarctic continent was larger and thicker than it is today," said Anderson, Rice's Maurice Ewing Professor of Oceanography and professor of Earth science. "This continent-enveloping ice sheet extended all the way to the continental shelf, and in western Antarctica it filled the entire Ross Sea basin."

While people typically think of continents as landmasses that rise above the sea, the margins of all continents, including Antarctica, extend well beyond their shores to include continental shelves, subsea aprons that are far more shallow than the deep ocean abysses that mark the continental boundary.

In western Antarctica, the Ross Sea is characterized by a continental shelf that extends nearly 1,000 miles from the coast and is as much as 3,500 feet deep. Anderson said the geologic record shows that as recently as 18,000 years ago the entire Ross basin was filled with ice that was so thick and heavy it was grounded on the seafloor all the way to the edge of the continental shelf.

"We found that about 10,000 years ago, this thick, grounded ice sheet broke apart in dramatic fashion," Anderson said. "The evidence shows that an armada of icebergs -- each at least twice as tall as the Empire State Building -- was pushed out en masse. We know this because this part of the Ross Sea is about 550 meters (1,804 feet) deep, and the icebergs were so large and so tightly packed that they gouged huge furrows into the seafloor as they moved north."

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Were the Exposed Continental Shelves Important Refugia for the Neotropical Rainforests During the Last Ice Age?

Neotropical forest expansion during the last glacial period challenges refuge hypothesis

Authors:

Leite et al

Abstract:

The forest refuge hypothesis (FRH) has long been a paradigm for explaining the extreme biological diversity of tropical forests. According to this hypothesis, forest retraction and fragmentation during glacial periods would have promoted reproductive isolation and consequently speciation in forest patches (ecological refuges) surrounded by open habitats. The recent use of paleoclimatic models of species and habitat distributions revitalized the FRH, not by considering refuges as the main drivers of allopatric speciation, but instead by suggesting that high contemporary diversity is associated with historically stable forest areas. However, the role of the emerged continental shelf on the Atlantic Forest biodiversity hotspot of eastern South America during glacial periods has been ignored in the literature. Here, we combined results of species distribution models with coalescent simulations based on DNA sequences to explore the congruence between scenarios of forest dynamics through time and the genetic structure of mammal species cooccurring in the central region of the Atlantic Forest. Contrary to the FRH predictions, we found more fragmentation of suitable habitats during the last interglacial (LIG) and the present than in the last glacial maximum (LGM), probably due to topography. We also detected expansion of suitable climatic conditions onto the emerged continental shelf during the LGM, which would have allowed forests and forest-adapted species to expand. The interplay of sea level and land distribution must have been crucial in the biogeographic history of the Atlantic Forest, and forest refuges played only a minor role, if any, in this biodiversity hotspot during glacial periods.

Human Caused Global Warming has Postponed the Next Glacial Cycle by at Least 50,000 Years

Humanity has become a geological force that is able to suppress the beginning of the next ice age, a study now published in the renowned scientific journal Nature shows. Cracking the code of glacial inception, scientists of the Potsdam Institute for Climate Impact Research found the relation of insolation and CO2 concentration in the atmosphere to be the key criterion to explain the last eight glacial cycles in Earth history. At the same time their results illustrate that even moderate human interference with the planet's natural carbon balance might postpone the next glacial inception by 100.000 years.

"Even without man-made climate change we would expect the beginning of a new ice age no earlier than in 50.000 years from now - which makes the Holocene as the present geological epoch an unusually long period in between ice ages," explains lead author Andrey Ganopolski. "However, our study also shows that relatively moderate additional anthropogenic CO2-emissions from burning oil, coal and gas are already sufficient to postpone the next ice age for another 50.000 years. The bottom line is that we are basically skipping a whole glacial cycle, which is unprecedented. It is mind-boggling that humankind is able to interfere with a mechanism that shaped the world as we know it."

For the first time, research can explain the onset of the past eight ice ages by quantifying several key factors that preceded the formation of each glacial cycle. "Our results indicate a unique functional relationship between summer insolation and atmospheric CO2 for the beginning of a large-scale ice-sheet growth which does not only explain the past, but also enables us to anticipate future periods when glacial inception might occur again," Ganopolski says.

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The Bashkirian Pennsylvanian Carboniferous was the Peak of the Late Paleozoic Ice Age

Ice volume and paleoclimate history of the late Paleozoic ice age from conodont apatite oxygen isotopes from naqing (Guizhou, China)

Authors:

Chen et al

Abstract:

A high-resolution and continuous conodont apatite oxygen isotope record spanning the late Viséan to Middle Permian is reported from South China, which is interpreted with respect to the ice volume and/or tropical sea water temperature history of the Late Paleozoic Ice Age (LPIA). The presented δ18O record shows significant fluctuations in δ18O from the late Viséan to Middle Permian with highest values observed in the Bashkirian (Early Pennsylvanian). The δ18O maximum coincides with a major eustatic sea level fall recorded in low-latitudinal successions, and postdates the significant increases in 87Sr/86Sr and δ13Ccarb measured on well-preserved brachiopod calcite, which can be interpreted as reflecting intensified weathering as consequence of the closure of Rheic Ocean as well as enhanced carbon burial. Both processes may have contributed to lower greenhouse gas levels and cooled down the Earth's surface, triggering the maximum glaciation. The high Bashkirian δ18O values are interpreted to represent the glacial maximum of the LPIA. A coeval change in faunal composition and a decreasing diversity in climate-sensitive marine invertebrates can be ascribed to icehouse cooling and/or loss of habitat. Despite inconsistenceies with earlier interpretations that the Early Permian represented the glacial maximum of the LPIA as inferred from Gondwanan glacial sediments records, the suggested Bashkirian glacial maximum agrees well with ice extent estimates based on the regional tectonic history in Gondwana, which suggests that the Bashkirian glaciation occurred during Gondwana interior uplift promoting maximum ice cover of the entire LPIA. However, maximum glaciation is only poorly represented in the depositional record because large parts of the glacial deposits were possibly removed by erosion as outlined by a major regional unconformity.

The Race Between Orogeny and Glacial Erosion in the St Elias Mountains, Alaska During the Pleistocene Quaternary

Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska

Authors:

Gulick et al

Abstract:

Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8–1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2–0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50–80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

Congo's Permo-carboniferous Paleoclimate had a Long Term Warming Trend With Five/Six Frigid Snaps

Permo-carboniferous paleoclimate of the Congo Basin: Evidence from lithostratigraphy, clay mineralogy, and stable isotope geochemistry

Authors:

Milleson et al

Abstract:

Approximately 1000 m of strata in the Upper Paleozoic Lukuga Formation in the Dekese core in the central Congo Basin provide lithostratigraphic, mineralogical, and isotopic evidence for substantial climatic variation within a long-lived lacustrine basin. Lithostratigraphic indicators of cold climate include polymictic strata (dropstone deposits) and coupled laminations of fine clay-size material and coarse silt (glacial varves). Dropstones are concentrated in three stratigraphic zones in the lower ~ 425 m of the Lukuga Formation, and varved strata occur in two broad stratigraphic zones in the lower ~ 700 m of the formation. These sedimentological indicators suggest that the lower ~ 2/3 of the Lukuga Formation was strongly influenced by frigid conditions and glacial-like processes.

The clay-size fraction of 97 samples is dominated by detrital minerals, including quartz, feldspar, chlorite, illite, and poorly ordered expansible 2:1 phyllosilicates. Based on variation in the mineralogy of these samples, the Lukuga Formation is divisible into three Clay Mineral Zones (CMZs), numbered in ascending stratigraphic order. CMZ 1 and CMZ 3 include several horizons of expansible 2:1 phyllosilicates that represent warmer/wetter intervals. Intervening CMZ 2 is a long (~ 500 m) zone of chlorite and illite with no expansible phyllosilicates and is interpreted as a continuous cold/frigid interval.

There are numerous calcite-cemented layers, including spar-filled veins that cross depositional bedding and represent postburial alteration, radiaxial fibrous cements that displace detrital grains, and micrite that crystallized near the time of deposition. Eighty-two stable isotope analyses of micrite yield δ13C values that range from –44.6‰ to –4.1‰ and δ18O values that range from –20.0‰ to 5.0‰ (VPDB). The carbon isotope data likely reflect a range of local carbon sources derived from bacterial activity and are unrelated to paleoclimatic conditions. In contrast, stratigraphic patterns in the oxygen isotope data suggest five or six intervals of frigid conditions conducive to glacial processes.

How Much of the Antarctic Marine Ice Retreat in the Ross Sea at the end of the Pleistocene Quaternary Ice Age

Antarctic marine ice-sheet retreat in the Ross Sea during the early Holocene

Authors:

McKay et al

Abstract:

Geological constraints on the timing of retreat of the Last Glacial Maximum (LGM) Antarctic Ice Sheets provide critical insights into the processes controlling marine-based ice-sheet retreat. The over-deepened, landward-sloping bathymetry of Antarctica's continental shelves is an ideal configuration for marine ice-sheet instability, with the potential for past and future ice-sheet collapse and accelerated sea-level rise. However, the chronology of retreat of the LGM ice sheet in the Ross Sea is largely constrained by imprecise radiocarbon chronology of bulk marine sediments or by coastal records that offer more reliable dating techniques but which may be influenced by local piedmont glaciers derived from East Antarctic outlet glaciers. Consequently, these coastal records may be ambiguous in the broader context of retreat in the central regions of the Ross Sea. Here, we present a sedimentary facies succession and foraminifera-based radiocarbon chronology from within the Ross Sea embayment that indicates glacial retreat and open-marine conditions to the east of Ross Island before 8.6 cal. (calibrated) kyr B.P., at least 1 k.y. earlier than indicated by terrestrial records in McMurdo Sound. Comparing these data to new modeling experiments, we hypothesize that marine-based ice-sheet retreat was triggered by oceanic forcings along most of the Pacific Ocean coastline of Antarctica, but continued Holocene retreat into the inner shelf region of the Ross Sea occurred primarily as a consequence of bathymetric controls on marine ice-sheet instability.

Extreme Dust Storms Delivered Very Reactive Iron to the Late Paleozoic Oceans

Extreme eolian delivery of reactive iron to late Paleozoic icehouse seas

Authors:

Sur et al

Abstract:

The biogeochemical impacts of iron-rich dust to the oceans are known for Earth's recent record but unexplored for deep time, despite recognition of large ancient dust fluxes, particularly during the late Paleozoic. We report a unique Fe relationship for Upper Pennsylvanian mudrock of eolian origin that records lowstand (glacial) conditions within a carbonate buildup of western equatorial Pangaea (western United States) well removed from other detrital inputs. Here, reactive Fe unambiguously linked to dust is enriched without a corresponding increase in total Fe. More broadly, data from thick coeval loess deposits of western equatorial Pangaea show the same marked enrichment in reactive Fe. This enrichment—atypical compared to modern marine, fluvial, glacial, loess, and soil sediments—suggests an enhancement of the reactivity of the internal Fe pool that increased the bioavailability of the Fe for marine primary production. Regardless of the mechanism behind this enhancement, our data in combination with other evidence for high dust fluxes imply delivery of extraordinarily large amounts of biogeochemically reactive Fe to glacial-stage late Paleozoic seas, and modeling of this indicates major impacts on carbon cycling and attendant climatic feedbacks.

Snowball Earth Era Sedimentary Formations Found in China

U–Pb age and Hf isotope composition of detrital zircons from Neoproterozoic sedimentary units in southern Anhui Province, South China: Implications for the provenance, tectonic evolution and glacial history of the eastern Jiangnan Orogen

Authors:

Cui et al

Abstract:

Neoproterozoic sedimentary units at the southeastern margin of the Yangtze Block, South China, record a complex geological history including development of the Jiangnan Orogen during assembly of the Yangtze and Cathaysia blocks and later episodes of glacial activity. The timing of these events is controversial, and we have used U–Pb–Hf compositions of detrital zircons from the Lantian type section to constrain their ages in southern Anhui Province, at the eastern end of the Jiangnan Orogen. This section comprises the Xikou Group below an angular unconformity, and the Xiuning Formation and overlying glacial Leigongwu Formation above the unconformity. Detrital zircons in all three units are dominated by 2.6–2.4 Ga, 2.1–1.9 Ga and 960–740 Ma age populations suggesting erosion from similar source regions, and an increasing proportion of older grains up through the sequence reflects a change in depositional environment from syn-collisional to extensional. Pre-Neoproterozoic zircon grains were derived from the Yangtze Block basement, much of which is now concealed by younger rocks, while the Neoproterozoic population was eroded from local igneous rocks in the Jiangnan Orogen. Zircon Hf isotope compositions indicate that 2.6–2.4 Ga source rocks were a mix of juvenile and reworked crust, while 2.1–1.9 Ga source rocks were dominated by reworked crust. Neoproterozoic sources show a switch from 960–860 Ma juvenile crust to 860–740 Ma juvenile and reworked crust, reflecting a transition in the Jiangnan Orogen from subduction to collision and extension. The youngest detrital grains in the Xikou Group and overlying Xiuning Formation indicate deposition after ca. 810 Ma and ca. 732 Ma, respectively, correlating closely with comparable sequences elsewhere in the Yangtze Block. This demonstrates that the unconformity in South Anhui is part of a regional erosion surface that formed more or less synchronously throughout the orogen at 830–800 Ma, consistent with it dating the end of collision between the Yangtze and Cathaysia blocks. The age of the overlying glacial Leigongwu Formation is difficult to constrain from detrital zircons because of a paucity of post-720 Ma source rocks, but our data suggest the Leigongwu diamictite in the Lantian section is a single Marinoan-age glacial unit deposited after 649 ± 13 Ma. It follows that a carbonate layer in the middle of the Leigongwu diamictite in the Lantian section most likely reflects later faulting, rather than two separate glacial sequences, although we cannot rule out the presence of both Sturtian and Marinoan diamictite elsewhere in South Anhui.