Abstract
Traditional provenance techniques (Nd isotopes and clay mineralogy) are combined with recently developed bulk sediment (super 40) Ar- (super 39) Ar radiometric methods to determine how the terrestrial sources of sediment to the Oregon continental margin have changed over the last 25,000 years. Both Pacific Northwest river-borne detritus, and sediment from piston coring site EW9504-17PC (2671m water depth) offshore southern Oregon have been analyzed. Nd isotopic analyses of river silts show a range of 10 units in epsilon (sub Nd) . North of the core site, the Columbia River has epsilon (sub Nd) =-7.6, while the Coos River has a value of epsilon (sub Nd) =-10.8. Rivers proximal to the core site have more radiogenic values from north to south, of epsilon (sub Nd) =-5.0 (Umpqua River), epsilon (sub Nd) =-1.3 (Rogue River), epsilon (sub Nd) =-0.6 (Klamath River) and epsilon (sub Nd) =-3.0 (Eel River). Measured epsilon (sub Nd) in core sediments show subtle downcore changes, between epsilon (sub Nd) =-0.9 and -2.5. The bulk sediment (super 40) Ar- (super 39) Ar plateau ages show more notable downcore variation between 25 and 14 ka, ranging from 113.5 to 124.0 Ma, but are still within the range of bulk ages previously measured on river mouth sediments. The Nd isotopic analyses are combined with bulk sediment (super 40) Ar- (super 39) Ar plateau ages into a ternary mixing model to quantitatively assess the sources of terrigenous material. Mixtures are best described by three sources proximal to the core site (the Umpqua, Rogue+Klamath and Eel Rivers) from approximately 14 ka to Present. Sediment deposited in the interval from 22 to 25 ka is not adequately described by the present-day rivers and requires an additional source. This additional source is best explained by an enhanced contribution from the interior Cascade volcanic arc, probably due to glaciation in the Cascade Range and the presence of pluvial Lake Modoc in the Upper Klamath Basin at this time. From 22 to 14 ka, the influence of Cascade Range sediment at the core site was overprinted by contemporaneous glaciation and sediment production in the Klamath Mountains, and possibly addition of sediment from the Eel River region as well. Thus, differential erosion appears to play a primary role in the provenance changes seen at the core site, and is more significant than sediment transport changes due to ocean circulation during this time period. Overall, the combined Ar-Nd isotopic technique provides insight into the coupling of land surface processes and ocean circulation related to climate change and will be a useful provenance tool in a variety of geologic/climatic settings. Abstract Copyright (2008) Elsevier, B.V.