The preservation of conglomerates far from mountainous sources are commonly interpreted in terms of tectonic or climatic forcing. To relate a depositional signal to changing conditions in source areas, the process and duration of sediment routing from source to sink need to be determined. For the first time, we quantify sediment transport histories using cosmogenic 21 Ne in quartzite pebbles from modern river gravels and Neogene conglomerates from the modern and ancient North Platte River of the Great Plains of Nebraska. We demonstrate that at ~400 km from the Rockies mountain front, the majority of pebbles must have been stored in older channel deposits for up to several millions of years before being recycled; this is enabled by very slow to zero basin subsidence rates. This implies that upstream tectonic or climatic controls on surface processes are decoupled from the downstream depositional record; a result supported by the similarities in cosmogenic 21 Ne between Miocene, Pliocene and modern river channel pebbles despite known changes in tectonic and climatic forcing.
Noble gas isotopes have been measured in fluid inclusions in sulphides spanning 25 000 years of hydrothermal activity at 13~ on the East Pacific Rise. The 3He/aHe ratios are typical of mid-ocean ridge hydrothermal fluids, albeit slightly higher than contemporary vent waters, and reveal no temporal variation or correlation with the ~-US of the host sulphide. The absence of radiogenic He in fluids from the 25 000 year old mineralization on the SE Seamount suggests that the hydrothermal circulation occurred within an active magmatic system and not within the underlying 130ka oceanic crust. This implies that seamount volcanism and hydrothermal activity occurred simultaneously off-ridge, and that magmatic activity shifted approximately 5 km off-ridge at this time. Helium concentrations in fluid inclusions from three samples are significantly greater than the end-member hydrothermal fluids at mid-ocean ridges. Small excesses of 4~ in the included fluids demonstrate that mantle-derived 4~ has been degassed along with primordial helium. Both are consistent with the direct addition of magmatic volatiles into the hydrothermal system at times during the history of hydrothermal activity at the site.
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