scholarly journals The Laurentian Neoproterozoic Glacial Interval: reappraising the extent and timing of glaciation

2020 ◽  
Vol 113 (1) ◽  
pp. 59-70
Author(s):  
Daniel Paul Le Heron ◽  
Nicholas Eyles ◽  
Marie Elen Busfield
Keyword(s):  
North America ◽  
Time Window ◽  
Careful Consideration ◽  
Snowball Earth ◽  
Glacial Cycles ◽  
Glacial Epoch ◽  
Rodinia Supercontinent ◽  
Mass Flow Deposits ◽  
Time Band ◽  
Age Constraints

AbstractOne of the major issues in Neoproterozoic geology is the extent to which glaciations in the Cryogenian and Ediacaran periods were global in extent and synchronous or regional in extent and diachronous. A similarly outstanding concern is determining whether deposits are truly glacial, as opposed to gravitationally initiated mass flow deposits in the context of a rifting Rodinia supercontinent. In this paper, we present 115 publically available, quality-filtered chronostratigraphic constraints on the age and duration of Neoproterozoic glacial successions, and compare their palaeocontinental distribution. Depositional ages from North America (Laurentia) clearly support the idea of a substantial glacial epoch between about 720-660 Ma on this palaeocontinent but paradoxically, the majority of Australian glacial strata plot outside the previously proposed global time band for the eponymous Sturtian glaciation, with new dates from China also plotting in a time window previously thought to be an interglacial. For the early Cryogenian, the data permit either a short, sharp 2.4 Ma long global glaciation, or diachronous shifting of ice centres across the Rodinia palaeocontinent, implying regional rather than global ice covers and asynchronous glacial cycles. Thus, based on careful consideration of age constraints, we suggest that strata deposited in the ca. 720-660 Ma window in North America are better described as belonging to a Laurentian Neoproterozoic Glacial Interval (LNGI), given that use of the term Sturtian for a major Neoproterozoic glacial epoch can clearly no longer be justified. This finding is of fundamental importance for reconstructing the Neoproterozoic climate system because chronological constraints do not support the concept of a synchronous panglacial Snowball Earth. Diachroneity of the glacial record reflects underlying palaeotectonic and palaeogeographic controls on the timing of glaciation resulting from the progressive breakup of the Rodinian supercontinent.

Where Do New Ideas (About Class) Come From?

2000 ◽  
Vol 57 ◽  
pp. 53-59 ◽  
Author(s):  
Barbara Weinstein
Keyword(s):  
North America ◽  
Latin American ◽  
Western Europe ◽  
Labor History ◽  
Careful Consideration ◽  
The State ◽  
The Political ◽  
Working Class History ◽  
New Ideas ◽  
American Labor

In my comment I raise two main questions about the Eley/Nield essay. First, I express some doubts about whether the issues discussed in their essay can be unproblematically transposed to historiographical debates in areas beyond Western Europe and North America. Certain themes, such as the need to reemphasize the political, are hardly pressing given the continual emphasis on politics and the state in Latin American labor history. Closely related to this, I question whether the state of gender studies within labor history can be used, in the way these authors seem to be doing, as a barometer of the sophistication and vitality of labor and working-class history. Despite recognizing the tremendous contribution of gendered approaches to labor history, I express doubts about its ability to help us rethink the category of class, and even express some concern that it might occlude careful consideration of class identities. Instead, pointing to two pathbreaking works in Latin American labor history, I argue that the types of questions we ask about class, and primarily about class, can provide the key to innovative scholarship about workers even if questions such as gender or ethnicity go unexamined. Finally, I point out that class will only be a vital category of analysis if it is recognized not simply as “useful,” but as forming a basis for genuinely creative and innovative historical studies.

The Proterozoic Record of Eukaryotes

Paleobiology ◽  
2015 ◽  
Vol 41 (4) ◽  
pp. 610-632 ◽  
Author(s):  
Phoebe A. Cohen ◽  
Francis A. Macdonald
Keyword(s):  
Large Scale ◽  
Snowball Earth ◽  
Geographic Variability ◽  
The Past ◽  
Past Half Century ◽  
Age Constraints ◽  
Composition Changes ◽  
Current Record ◽  
Past Half ◽  
Early Neoproterozoic

AbstractProterozoic strata host evidence of global “Snowball Earth” glaciations, large perturbations to the carbon cycle, proposed changes in the redox state of oceans, the diversification of microscopic eukaryotes, and the rise of metazoans. Over the past half century, the number of fossils described from Proterozoic rocks has increased exponentially. These discoveries have occurred alongside an increased understanding of the Proterozoic Earth system and the geological context of fossil occurrences, including improved age constraints. However, the evaluation of relationships between Proterozoic environmental change and fossil diversity has been hampered by several factors, particularly lithological and taphonomic biases. Here we compile and analyze the current record of eukaryotic fossils in Proterozoic strata to assess the effect of biases and better constrain diversity through time. Our results show that mean within assemblage diversity increases through the Proterozoic Eon due to an increase in high diversity assemblages, and that this trend is robust to various external factors including lithology and paleogeographic location. In addition, assemblage composition changes dramatically through time. Most notably, robust recalcitrant taxa appear in the early Neoproterozoic Era, only to disappear by the beginning of the Ediacaran Period. Within assemblage diversity is significantly lower in the Cryogenian Period than in the preceding and following intervals, but the short duration of the nonglacial interlude and unusual depositional conditions may present additional biases. In general, large scale patterns of diversity are robust while smaller scale patterns are difficult to discern through the lens of lithological, taphonomic, and geographic variability.

scholarly journals Evolution of andrenine bees reveals a long and complex history of faunal interchanges between the Americas during the Mesozoic and Cenozoic

2021 ◽  
Author(s):  
Kelli S Ramos ◽  
Aline C Martins ◽  
Gabriel A R Melo
Keyword(s):  
North America ◽  
South America ◽  
Time Window ◽  
Distributional Pattern ◽  
Long Distance ◽  
Early Diversification ◽  
Palearctic Region ◽  
Long Time ◽  
The Times ◽  
Dated Phylogeny

Bees are presumed to have arisen in the early to mid-Cretaceous coincident with the fragmentation of the southern continents and concurrently with the early diversification of the flowering plants. Among the main groups of bees, Andreninae sensu lato comprise about 3000 species widely distributed with greatest and disjunct diversity in arid areas of North America, South America, and the Palearctic region. Here, we present the first comprehensive dated phylogeny and historical biogeographic analysis for andrenine bees, including representatives of all currently recognized tribes. Our analyses rely on a dataset of 106 taxa and 7952 aligned nucleotide positions from one mitochondrial and six nuclear loci. Andreninae is strongly supported as a monophyletic group and the recovered phylogeny corroborates the commonly recognized clades for the group. Thus, we propose a revised tribal classification that is congruent with our phylogenetic results. The time-calibrated phylogeny and ancestral range reconstructions of Andreninae reveal a fascinating evolutionary history with Gondwana patterns that are unlike those observed in other subfamilies of bees. Andreninae arose in South America during the Late Cretaceous around 90 Million years ago (Ma) and the origin of tribes occurred through a relatively long time-window from this age to the Miocene. The early evolution of the main lineages took place in South America until the beginning of Paleocene with North American fauna origin from it and Palearctic from North America as results of multiple lineage interchanges between these areas by long-distance dispersal or hopping through landmass chains. Overall, our analyses provide strong evidence of amphitropical distributional pattern currently observed in Andreninae in the American continent as result at least three periods of possible land connections between the two American landmasses, much prior to the Panama Isthmus closure. The andrenine lineages reached the Palearctic region through four dispersal events from North America during the Eocene, late Oligocene and early Miocene, most probably via the Thulean Bridge. The few lineages with Afrotropical distribution likely originated from a Palearctic ancestral in the Miocene around 10 Ma when these regions were contiguous, and the Sahara Desert was mostly vegetated making feasible the passage by several organisms. Incursions of andrenine bees to North America and then onto the Old World are chronological congruent with distinct periods when open-vegetation habitats were available for trans-continental dispersal and at the times when aridification and temperature decline offered favorable circumstances for bee diversification.

scholarly journals Antipodean fugitive terranes in southern Laurentia: How Proterozoic Australia built the American West

Lithosphere ◽  
2019 ◽  
Vol 11 (4) ◽  
pp. 551-559 ◽  
Author(s):  
G.M. Gibson ◽  
D.C. Champion
Keyword(s):  
North America ◽  
American West ◽  
Ocean Basin ◽  
Backarc Basin ◽  
Left Behind ◽  
Marginal Sea ◽  
Depleted Mantle ◽  
Rodinia Supercontinent ◽  
Laurentian Margin ◽  
Archean Basement

Abstract Paleoproterozoic arc and backarc assemblages accreted to the south Laurentian margin between 1800 Ma and 1600 Ma, and previously thought to be indigenous to North America, more likely represent fragments of a dismembered marginal sea developed outboard of the formerly opposing Australian-Antarctic plate. Fugitive elements of this arc-backarc system in North America share a common geological record with their left-behind Australia-Antarctic counterparts, including discrete peaks in tectonic and/or magmatic activity at 1780 Ma, 1760 Ma, 1740 Ma, 1710–1705 Ma, 1690–1670 Ma, 1650 Ma, and 1620 Ma. Subduction rollback, ocean basin closure, and the arrival of Laurentia at the Australian-Antarctic convergent margin first led to arc-continent collision at 1650–1640 Ma and then continent-continent collision by 1620 Ma as the last vestiges of the backarc basin collapsed. Collision induced obduction and transfer of the arc and more outboard parts of the Australian-Antarctic backarc basin onto the Laurentian margin, where they remained following later breakup of the Neoproterozoic Rodinia supercontinent. North American felsic rocks generally yield Nd depleted mantle model ages consistent with arc and backarc assemblages built on early Paleoproterozoic Australian crust as opposed to older Archean basement making up the now underlying Wyoming and Superior cratons.

scholarly journals U–Pb zircon geochronology and depositional history of the Montresor group, Rae Province, Nunavut, Canada

2017 ◽  
Vol 54 (5) ◽  
pp. 512-528 ◽  
Author(s):  
John A. Percival ◽  
William J. Davis ◽  
Michael A. Hamilton
Keyword(s):  
Detrital Zircon ◽  
Time Window ◽  
Depositional History ◽  
Tectonic Events ◽  
Density Peaks ◽  
Minimum Age ◽  
History Of ◽  
Igneous Zircon ◽  
Definition Of ◽  
Age Constraints

Paleoproterozoic metasedimentary successions of the northwestern Canadian Shield provide records of tectonic events, but the definition of depositional ages has proved elusive. Although previously poorly understood, the Montresor belt of western Nunavut yields new insight into the 2.2–1.8 Ga time window. On the basis of U–Pb analyses of detrital zircon in sedimentary rocks and igneous zircon in sills, we conclude that arenite of the lower Montresor group was deposited between 2.194 and 2.045 Ga, and arkose of the upper Montresor group after 1.924 Ga, adding constraints on the Rae cover sequence. The lower Montresor arenite yielded an older group (3.05–2.58 Ga) and a younger, more tightly constrained group (2.194 ± 0.014 Ga). Four of six zircon grains analyzed from a gabbro sill within the lower Montresor have discordant 207Pb/206Pb ages (2.71, 2.66, 2.53, and 2.39 Ga) and are considered to be inherited, whereas two grains provide an age of 2045 ± 13 Ma, interpreted to date crystallization and providing a minimum age for the lower Montresor package. Upper Montresor arkose contains detrital zircon with probability density peaks at 2.55–2.25 and 2.1–1.92 Ga, together with scattered older grains (3.8–2.65 Ga). The youngest grain yields an age of 1924 ± 6 Ma, establishing a maximum age for sandstone deposition. Provenance is inferred to have been from the west, where igneous sources of 2.5–2.3 Ga (Queen Maud block) and 2.03–1.89 Ga (Thelon orogen) are known. Collectively, the new ages suggest a minimum 120 million year gap between deposition of the pre-2045 ± 13 Ma lower and post-1924 ± 6 Ma upper parts of the Montresor group. Similar age constraints may apply to other parts of the Rae cover sequence.

scholarly journals Timing and tempo of the Great Oxidation Event

2017 ◽  
Vol 114 (8) ◽  
pp. 1811-1816 ◽  
Author(s):  
Ashley P. Gumsley ◽  
Kevin R. Chamberlain ◽  
Wouter Bleeker ◽  
Ulf Söderlund ◽  
Michiel O. de Kock ◽  
...  
Keyword(s):  
Atmospheric Oxygen ◽  
Kaapvaal Craton ◽  
Large Igneous Province ◽  
Snowball Earth ◽  
Low Latitude ◽  
Great Oxidation Event ◽  
Carbon Isotope Excursion ◽  
Igneous Province ◽  
Age Constraints ◽  
Exact Timing

The first significant buildup in atmospheric oxygen, the Great Oxidation Event (GOE), began in the early Paleoproterozoic in association with global glaciations and continued until the end of the Lomagundi carbon isotope excursion ca. 2,060 Ma. The exact timing of and relationships among these events are debated because of poor age constraints and contradictory stratigraphic correlations. Here, we show that the first Paleoproterozoic global glaciation and the onset of the GOE occurred between ca. 2,460 and 2,426 Ma, ∼100 My earlier than previously estimated, based on an age of 2,426 ± 3 Ma for Ongeluk Formation magmatism from the Kaapvaal Craton of southern Africa. This age helps define a key paleomagnetic pole that positions the Kaapvaal Craton at equatorial latitudes of 11° ± 6° at this time. Furthermore, the rise of atmospheric oxygen was not monotonic, but was instead characterized by oscillations, which together with climatic instabilities may have continued over the next ∼200 My until ≤2,250–2,240 Ma. Ongeluk Formation volcanism at ca. 2,426 Ma was part of a large igneous province (LIP) and represents a waning stage in the emplacement of several temporally discrete LIPs across a large low-latitude continental landmass. These LIPs played critical, albeit complex, roles in the rise of oxygen and in both initiating and terminating global glaciations. This series of events invites comparison with the Neoproterozoic oxygen increase and Sturtian Snowball Earth glaciation, which accompanied emplacement of LIPs across supercontinent Rodinia, also positioned at low latitude.

scholarly journals Orbital control of western North America atmospheric circulation and climate over two glacial cycles

2014 ◽  
Vol 5 (1) ◽  
Author(s):  
Matthew S. Lachniet ◽  
Rhawn F. Denniston ◽  
Yemane Asmerom ◽  
Victor J. Polyak
Keyword(s):  
North America ◽  
Atmospheric Circulation ◽  
Western North America ◽  
Glacial Cycles ◽  
Orbital Control

scholarly journals Moose genomes reveal past glacial demography and the origin of modern lineages

BMC Genomics ◽  
2020 ◽  
Vol 21 (1) ◽  
Author(s):  
Nicolas Dussex ◽  
Federica Alberti ◽  
Matti T. Heino ◽  
Remi-Andre Olsen ◽  
Tom van der Valk ◽  
...  
Keyword(s):  
North America ◽  
De Novo ◽  
Demographic History ◽  
Glacial Refugia ◽  
Population Declines ◽  
Glacial Cycles ◽  
Climate Fluctuations ◽  
Northern Latitudes ◽  
Genomic Studies ◽  
History Of

Abstract Background Numerous megafauna species from northern latitudes went extinct during the Pleistocene/Holocene transition as a result of climate-induced habitat changes. However, several ungulate species managed to successfully track their habitats during this period to eventually flourish and recolonise the holarctic regions. So far, the genomic impacts of these climate fluctuations on ungulates from high latitudes have been little explored. Here, we assemble a de-novo genome for the European moose (Alces alces) and analyse it together with re-sequenced nuclear genomes and ancient and modern mitogenomes from across the moose range in Eurasia and North America. Results We found that moose demographic history was greatly influenced by glacial cycles, with demographic responses to the Pleistocene/Holocene transition similar to other temperate ungulates. Our results further support that modern moose lineages trace their origin back to populations that inhabited distinct glacial refugia during the Last Glacial Maximum (LGM). Finally, we found that present day moose in Europe and North America show low to moderate inbreeding levels resulting from post-glacial bottlenecks and founder effects, but no evidence for recent inbreeding resulting from human-induced population declines. Conclusions Taken together, our results highlight the dynamic recent evolutionary history of the moose and provide an important resource for further genomic studies.

scholarly journals Lions and brown bears colonized North America in multiple synchronous waves of dispersal across the Bering Land Bridge

2020 ◽  
Author(s):  
Alexander T Salis ◽  
Sarah C E Bray ◽  
Michael S Y Lee ◽  
Holly Heiniger ◽  
Ross Barnett ◽  
...  
Keyword(s):  
Population Dynamics ◽  
North America ◽  
Glacial Cycles ◽  
Land Bridge ◽  
Large Herbivores ◽  
Brown Bears ◽  
Sea Levels ◽  
North West ◽  
North East ◽  
Bering Land Bridge

AbstractThe Bering Land Bridge connecting North America and Eurasia was periodically exposed and inundated by oscillating sea levels during the Pleistocene glacial cycles. This land connection allowed the intermittent dispersal of animals, including humans, between Western Beringia (far north-east Asia) and Eastern Beringia (north-west North America), changing the faunal community composition of both continents. The Pleistocene glacial cycles also had profound impacts on temperature, precipitation, and vegetation, impacting faunal community structure and demography. While these paleoenvironmental impacts have been studied in many large herbivores from Beringia (e.g., bison, mammoths, horses), the Pleistocene population dynamics of the diverse guild of carnivorans present in the region are less well understood, due to their lower abundances. In this study, we analyze mitochondrial genome data from ancient brown bears (Ursus arctos; n = 103) and lions (Panthera spp.; n = 39), two megafaunal carnivorans that dispersed into North America during the Pleistocene. Our results reveal striking synchronicity in the population dynamics of Beringian lions and brown bears, with multiple waves of dispersal across the Bering Land Bridge coinciding with glacial periods of low sea levels, as well as synchronous local extinctions in Eastern Beringia during Marine Isotope Stage 3. The evolutionary histories of these two taxa underscore the crucial biogeographic role of the Bering Land Bridge in the distribution, turnover, and maintenance of megafaunal populations in North America.

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