ALBERT

Description: U-Pb zircon data from the uppermost Cottons Breccia, representing the Marinoan glacial-postglacial transition on King Island, Tasmania, provide the first direct age constraint on the Cryogenian-Ediacaran boundary in Australia. Zircons in four samples from the topmost meter of the Cottons Breccia, dated by sensitive high-resolution ion microprobe, exhibit two modes ca. 660 Ma and ca. 635 Ma. The younger component predominates in the uppermost sample, a possibly volcanolithic dolomitic sandstone, apparently lacking glacially transported debris, in the transition to cap carbonate. Chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb dating of euhedral zircons from that sample yields a weighted-mean age of 636.41 ± 0.45 Ma. Equivalence to published TIMS ash bed dates from Cryogenian-Ediacaran transitional strata in Namibia (635.51 ± 0.82 Ma, within glacial deposit) and China (635.23 ± 0.84 Ma, 2 m above glacial deposit) supports correlation of those strata to the Australian type sections and globally synchronous deglaciation at the end of the Cryogenian Period.

Description: Twenty-five years ago, initial plans for reconstructing the Rodinia supercontinent were being drafted, based on the growing recognition of correlatable mid-Neoproterozoic (0.8–0.7 Ga) rifted passive margins, many of which were established on the eroded remnants of late Mesoproterozoic (1.3–1.0 Ga) orogenic belts. The 1990s witnessed a surge of interest in Rodinia, with many regional studies of tectonostratigraphy and U-Pb geochronology generally conforming to the "inside-out" reconstruction model: juxtaposition of west Laurentia with east Australia/Antarctica, north Laurentia with Siberia, and east Laurentia with Baltica and cratons that would later form West Gondwana. This standard model of Rodinia appeared to be converging toward a solution with only minor variations by the turn of the millennium, but new paleomagnetic data and tectonostratigraphic information obtained in the succeeding decade chipped away at various aspects of the reconstruction; several cratons seemed to require exclusion from the supercontinent (thus questioning its very validity), or the landmass might have assembled much later (≤0.9 Ga) than originally envisaged (thus weakening the link to global Mesoproterozoic orogenesis). Although a consensus model of Rodinia’s assembly and fragmentation has arisen from the International Geoscience Programme Project 440 working group, the reconstruction is supported by rather sparse definitive-quality data. As the quest for Rodinia matures to a third decade of scrutiny, the search for its predecessor Nuna (a.k.a. Hudsonland or Columbia) is only now reaching a stage of global synthesis between tectonostratigraphic and paleomagnetic data. According to most definitions, Nuna assembled at 1.9–1.75 Ga, or perhaps as late as 1.6 Ga, and fragmented during the interval 1.5–1.2 Ga. Because mafic dike swarms are ideal targets for paleomagnetic study, and because they are now amenable to routine dating by U-Pb on baddeleyite, the global abundance of Paleo-Mesoproterozoic dike swarms might make Nuna more imminently solvable than Rodinia. Prior to the assembly of Nuna, various "supercraton" connections such as Vaalbara, Superia, and Sclavia are only beginning to take form. Unmetamorphosed, early Paleoproterozoic (2.5–2.0 Ga) mafic dike swarms are commonplace features across the interiors of Archean cratons, and their joint paleomagnetic and geochronologic study can help reassemble the cratons into their supercraton parent landmasses. Progressively older geologic times require consideration of a greater number of potentially independent terranes, each needing individual kinematic constraints. Furthermore, the initial stabilizing events of most extant cratons during Neoarchean time (3.0–2.5 Ga) therefore render global reconstructions older than that interval improbable.

Description: The configuration of subglacial meltwater is a critical control on ice sheet dynamics, and the presence of pressurized water distributed across the bed can induce dynamic instabilities. However, this process can be offset by efficient evacuation of water within large subglacial channels, and drainage systems beneath alpine glaciers have been shown to become increasingly channelized throughout the melt season in response to the increased production of meltwater. This seasonal evolution has recently been inferred beneath outlet glaciers of the Greenland Ice Sheet, but the extent to which this process occurs across much larger spatial and temporal scales is largely unknown, introducing considerable uncertainty about the evolution of subglacial drainage networks at the ice sheet scale and associated ice sheet dynamics. This paper uses an unprecedented data set of over 20,000 eskers to reconstruct the evolution of channelized meltwater systems during the final deglaciation of the Laurentide Ice Sheet (13–7 kyr B.P.). We demonstrate that eskers become more frequent during deglaciation and that this coincides with periods of increased rates of ice margin recession and climatic warming. Such behavior is reminiscent of the seasonal evolution of drainage systems observed in smaller glaciers and implies that channelized drainage became increasingly important during deglaciation. An important corollary is that the area of the bed subjected to a less efficient pressurized drainage system decreased, which may have precluded dynamic instabilities, such as surging or ice streaming.