Glaciation

The Palaeoproterozoic Earth experienced a global glacial event at 2400 Ma that occurred during the transitional period from anoxic to aerobic conditions in the atmosphere and oceans. The Transvaal Supergroup in the Griqualand West Basin, South Africa, hosts glacial deposits and associated major iron and manganese deposits that are apparently related to these global changes. The focus of this study is to assess the stratigraphy and geochemistry of the glaciogenic Makganyene Formation, in order to constrain its palaeoenvironmental settings.

During Pleistocene, the Laurentide ice sheet rearranged and diversified biotic distributions in eastern North America, yet had minimal physical impact in western North America where lineage diversification is instead hypothesized to result from climatic changes. If Pleistocene climatic fluctuations impacted desert species, the latter would reflect patterns of restricted gene flow concomitant with indications of demographic bottlenecks. Accordingly, molecular evidence for refugia should be present within these distributions and for subsequent range expansions as conditions improved. We sought answers to these questions by evaluating mitochondrial DNA (mtDNA) sequences from four species of rattlesnakes [ Crotalus mitchellii (speckled rattlesnake), Crotalus cerastes (sidewinder), Crotalus tigris (tiger rattlesnake), Crotalus ruber (red diamond rattlesnake)] with distributions restricted to desert regions of southwestern North America. We inferred relationships using parsimony and maximum likelihood, tested intraspecific clades for population expansions, applied an isolation-with-migration model to determine bi-directional migration rates ( m ) among regions, and inferred divergence times for species and clades by applying a semiparametric penalized likelihood approach to our molecular data. Evidence for significant range expansion was present in two of eight regions in two species ( Crotalus mitchellii pyrrhus , C. tigris region north). Two species ( C. cerastes , C. mitchellii ) showed a distribution concomitant with northward displacement of Baja California from mainland México, followed by vicariant separation into subclades. Effects of Pleistocene climate fluctuations were found in the distributions of all four species. Three regional diversification patterns were identified: (i) shallow genetic diversity that resulted from Pleistocene climatic events ( C. tigris, C. ruber ); (ii) deep Pleistocene divisions indicating allopatric segregation of subclades within refugia ( C. mitchellii, C. cerastes) ; and (iii) lineage diversifications that extended to Pliocene or Late Miocene ( C. mitchellii, C. cerastes) . Clade-diversifying and clade-constraining effects impacted the four species of rattlesnakes unequally. We found relatively high levels of molecular diversification in the two most broadly distributed species ( C. mitchellii, C. cerastes ), and lower levels of genetic diversification in the two species ( C. tigris, C. ruber ) whose ranges are relatively more restricted. Furthermore, in several cases, the distributions of subspecies were not congruent with our molecular information. We suggest regional conservation perspectives for southwestern deserts cannot rely upon subspecies as biodiversity surrogates, but must instead employ a molecular and deep historical perspective as a primary mechanism to frame biodiversity reserves within this region.

We describe and discuss the glacial and climate variations in the Polar Ural Mountains in northern Russia over the last 100 000 years. A series of optically stimulated luminescence (OSL) and radiocarbon ages from sediment cores demonstrate that there has been continuous deposition of lacustrine sediments throughout the last 65 ka in Lake Gerdizty, a lake basin situated on the eastern foothills of the Urals. Below the lacustrine sequence are two till beds; the uppermost (unit B) was probably deposited by a large mountain-centred glacier complex that covered the lake basin during Marine Isotope Stage (MIS) 4. This till bed might be a counterpart of the Usa Moraine in the western foothills of the Polar Urals, which is dated with OSL-and 10 Be cosmogenic nuclide methods to between 68 and 58 ka, consistent with the OSL ages from the lowermost lacustrine sediments in Lake Gerdizty. During this glaciation some of the northern Uralian outlet glaciers probably merged with the adjacent BarentseKara Ice Sheet that reached onto the adjacent lowlands forming large ice-dammed lakes on both sides of the Ural Mountains. The equilibrium line altitude (ELA) on the local ice cap was then at least 1200 m lower than at present. The pollen stratigraphy in Lake Gerditzy reflects an open tundra and steppe-like vegetation throughout MIS 3e2 (60e11,7 ka). The first significant change in vegetation, shown by an increase of dwarf-shrub communities of Salix and Betula, took place during the Lateglacial at around 15 ka or shortly after. Birch and spruce trees approached the area for the first time soon after the Holocene transition w11.7 ka, and became dominant elements in the vegetation after a few hundred years. Our reconstruction suggests a very cold summer climate during MIS 4 and presumably also during the preceding MIS 5b (95e85 ka) glaciation, probably amplified by the large ice-dammed lakes that existed during these two glaciations. A somewhat milder climate prevailed during MIS 3 (60e25 ka), but a treeless vegetation nevertheless suggests that mean summer temperatures did not exceed 10e12 C before the earliest Holocene.

The Neoproterozoic Quruqtagh Group in eastern Chinese Tianshan consists of, in ascending order, the Bayisi, Zhaobishan, Altungol, Tereeken, Zhamoketi, Yukkengol, Shuiquan, and Hankalchough Formations. Diamictite units occur in the Bayisi-Altungol, Tereeken, and Hankalchough Formations. Our field observations confirm the glacial origin of the Tereeken and Hankalchough diamictites, but the glacial origin of diamictites in the Bayisi and Altungol Formations remain ambiguous. While siliciclastic strata dominate the succession, carbonate units are recognized between the diamictites and immediately atop the Tereeken and Hankalchough diamictites. Carbonates in the Altungol Formation are characterized by highly positive ␦ 13 C values (up to +10.4‰ versus PDB), comparable to those of pre-Marinoan but post-Sturtian carbonates. High-resolution sampling of the pinkish cap dolostone (ca. 10 m-thick) overlying the Tereeken diamictite shows a consistent ␦ 13 C chemostratigraphic pattern (ca. −5‰ at the base and slightly decreasing upsection) similar to that of Marinoan cap carbonates. This cap dolostone is also characterized by macropeloids similar to those in the Marinoan-age Raventhroat cap carbonate, Mackenzie Mountains. Chemostratigraphic and sedimentary features are suggestive of a Marinoan age for the Tereeken glaciation. Thus, the younger Hankalchough diamictite may represent a post-Marinoan glaciation. This interpretation is consistent with the occurrence of vendotaenid fossils in the Shuiquan Formation. The alternative interpretation-the Tereeken and Hankalchough diamictites are, respectively, Sturtian and Marinoan in age-is not consistent with the above evidence or the chemostratigraphic features of the Shuiquan Formation and the Hankalchough cap carbonate. A post-Marinoan age for the Hankalchough and other glaciogenic deposits indicates that the disappearance of Doushantuo-type acritarchs and the subsequent radiation of macroscopic animals are closely linked, in geochronological terms, with a Neoproterozoic glaciation.