Browsing by Subject "Paleocene"
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Item Lead Isotopic Variations of the Pacific and Implications for Paleogene Water Mass Composition(2013-11-13) Subt, CristinaTo understand the effects changes in the meridional overturning circulation (MOC) played on past climate we need to determine where convection occurred. The late Cretaceous and early Paleogene interval (~75 to 35 Ma) was characterized by low meridional temperature gradients. Nd isotopes indicate high-latitude convection in the Pacific Ocean during this interval, with deep waters mixing in the tropical Pacific. Here we investigate the evolution of the Pb isotopic composition of water masses in the Pacific to constrain variability in weathering inputs into the inferred convection regions from ~75 to 35 Ma. We generated ^(206),^(207),^(208)Pb/^(204)Pb isotope records for North Pacific DSDP and ODP Sites 192, 464, and 883, and South Pacific DSDP Sites 323, 463, and 596. South Pacific deep waters increasing in contributions. Pb composition in the subtropical South Pacific also suggests a stronger influence of dust dissolution whereas the Nd composition was dominated by water mass composition. North Pacific Pb values may also have reflected shifting sources during the EECO. The Pb and Nd composition at Site 883 shows a short radiogenic excursion in Pb values ~40 Ma, possibly cased by a short period of strengthened North Pacific Deep Water (NPDW) influence on the isotopic composition during the Mid-Eocene Climatic Optimum (MECO). ^(206)Pb/2^(04)Pb trends differ from ^(207),^(208)Pb/^(204)Pb values, which typically show similar trends to each other?resulting from the mixing of multiple Pb sources. North Pacific sites typically exhibit relatively lower Pb compositions than South Pacific, and evidence from detrital analyses suggests sources of Pb to North Pacific sites received strong contributions of continental dust. Pb and Nd are coupled in North Pacific sites between ~62 and 50 Ma, as well as in the Southern Ocean throughout the study period, indicating the Pb composition in this region was influenced by the composition of advected deep water masses. North Pacific sites also show decoupling of Pb and Nd ~50 Ma, during the Early Eocene Climatic Optimum (EECO), which may be indicative of more distalItem Provenance of the south Texas Paleocene-Eocene Wilcox Group, western Gulf of Mexico basin : insights from sandstone modal compositions and detrital zircon geochronology(2009-08) Mackey, Glen Nelson; Milliken, Kitty L.; Horton, Brian K.; Steel, Ronald J.Sandstone modal compositions and detrital zircon U-Pb analysis of the Paleocene-Eocene Wilcox Group of the southern Gulf Coast of Texas indicate long-distance sediment transport primarily from volcanic and basement sources to the west, northwest and southwest. The Wilcox Group of south Texas represents the earliest series of major post-Cretaceous pulses of sand deposition along the western margin of the Gulf of Mexico (GoM). Laramide basement uplifts have long been held to be the provenance of the Wilcox Group, implying that initiation of basement uplifts was the driving factor for this transition from carbonate sedimentation to clastic deposition. To determine the provenance of the Wilcox Group and test this conventional hypothesis, 40 thin sections were point-counted using the Gazzi-Dickinson method to determine sandstone composition and 10 detrital zircon samples were analyzed by LA-ICP-MS to determine U-Pb age spectra for each of the sampled areas. Modal data for sand grain populations suggest mixed sources including basement rocks, magmatic arc rocks and subordinate sedimentary rocks for the Wilcox Group. Zircon age spectra for these sandstones reveal a complex grain assemblage derived from older sediments and crystalline rocks ranging in age from Archean to Cenozoic. Sediment was primarily derived from Laramide uplifted crystalline blocks of the central and southern Rocky Mountains, the Cordilleran arc of western North America, and arc related extrusive and intrusive igneous rock of northern Mexico. Comparisons of Upper and Lower Wilcox zircon age spectra show that more arc related material was deposited in the Lower Wilcox, whereas more basement material was deposited in the Upper Wilcox.