New Spruce (Picea spp.) Macrofossils from Yukon Territory: Implications for Late Pleistocene Refugia in Eastern Beringia

New radiocarbon-dated plant macrofossils provide evidence for black spruce (Picea mariana) and white spruce (Picea glauca) within the unglaciated Yukon Territory at the onset of glacial conditions during the Marine Isotope Stage 3/2 transition, between about 26 000 and 24500 14C yr BP. These data indicate that spruce trees were able to reproduce sexually and grow to maturity within a glacial environment characterized by widespread steppe-tundra vegetation, loess aggradation, and icewedge formation. These trees may have been restricted to rare valley-bottom habitats that provided adequate shelter and moisture similar to those at the present latitudinal tree line. Previously published hypotheses suggest that low Picea frequencies in regional Beringian pollen data point to the local persistence of spruce trees through the last glaciation. Although our data provide evidence for local spruce trees at the onset of the last glaciation, the available macrofossil record is inconclusive regarding the survival of spruce through the Last Glacial Maximum in Eastern Beringia. These new plant macrofossil data require palynologists to reexamine the relationship between Picea pollen frequency and local trees and highlight the importance of integrated pollen- and macrofossil-based paleoecological reconstructions.De nouveaux macrofossiles de plantes datés au C14 attestent de la présence d’épinette noire (Picea mariana) et d’épinette blanche (Picea glauca) dans le territoire non glaciaire du Yukon au début des conditions glaciaires, pendant la transition Marine Isotope Stage 3/2, et s’étendant entre environ 26 000 et 24 500 années avant le présent, daté au C14. Selon ces données, les épinettes étaient capables de se reproduire par voie sexuée et de croître jusqu’à maturité dans un milieu glaciaire caractérisé par une végétation à forte densité de steppe et de toundra, par l’aggradation de loess et par une formation de glace fossile. Il se peut que ces arbres se limitaient à de rares habitats au fond de vallées, habitats qui leur procuraient un abri adéquat et un degré d’humidité similaires à ceux qui existent dans la limite actuelle transversale de végétation des arbres. D’après des hypothèses déjà publiées, la faible fréquence de Picea dans les données régionales de pollen bérégien laissent supposer la persistance locale des épinettes pendant la dernière glaciation. Bien que nos données fournissent la preuve de l’existence d’épinettes locales au début de la dernière glaciation, les données macrofossiles disponibles ne sont pas concluantes en ce qui a trait à la survie de l’épinette pendant le dernier maximum glaciaire dans la Béringie de l’Est. Ces nouvelles données macrofossiles de plantes impliquent que les palynologues doivent réexaminer la relation entre la fréquence du pollen de Picea et les arbres locaux, et font ressortir l’importance des reconstructions paléoécologiques intégrées du pollen et des macrofossiles

Re-Os isotope characteristics of postorogenic lavas: Implications for the nature of young lithospheric mantle and its contribution to basaltic magmas

Re-Os isotopes have been measured on postorogenic potassic lavas from the Tibetan Plateau, the Betic domain of southeastern Spain, and the Colorado Plateau of the southwestern United States. Previous work has established that these lavas were all derived from parts of the subcontinental lithospheric mantle that had undergone metasomatic enrichment in incompatible elements, following various degrees of melt depletion. Cratonic depleted subcontinental lithospheric mantle peridotites typically have subchondritic 187Os/188Os; however, the postorogenic lavas are characterized by radiogenic 187Os/188Os ratios (0.139-0.559). Simple modeling shows that only very large degrees of melt depletion (>25-30) can lower source Re/Os ratios sufficiently to permit time-integrated development of subchondritic 187Os/188Os ratios. Such processes may have been largely restricted to the older Precambrian, and the peridotite component of the postorogenic lavas source was probably depleted by <25. The more radiogenic values may reflect increasing contributions from metasomatic components or possibly crustal contamination. Our findings imply the need for caution in the use of Os isotopes as a diagnostic tracer of subcontinental lithospheric mantle contributions to lavas erupted through younger Proterozoic and Phanerozoic lithosphere