Tracking fluvial response to climate change in the Pacific Northwest: a combined provenance approach using Ar and Nd isotopic systems on fine-grained sediments

Abstract

Traditional provenance techniques (Nd isotopes and clay mineralogy) are combined with recently developed bulk sediment 40Ar– 39Ar 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 (2671 m water depth) offshore southern Oregon have been analyzed. Nd isotopic analyses of river silts show a range of 10 units in ε Nd. North of the core site, the Columbia River has ε Nd=–7.6, while the Coos River has a value of ε Nd=−10.8. Rivers proximal to the core site have more radiogenic values from north to south, of ε Nd=−5.0 (Umpqua River), ε Nd=−1.3 (Rogue River), ε Nd=−0.6 (Klamath River) and ε Nd=−3.0 (Eel River). Measured ε Nd in core sediments show subtle downcore changes, between ε Nd=−0.9 and −2.5. The bulk sediment 40Ar– 39Ar 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 40Ar– 39Ar 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 ∼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.