The benthic manganese cycle along the Oregon-California continental margin

High rates of sedimentary organic carbon degradation within continental margin sediments combined with abundant supply of reactive Mn-oxides from continental weathering result in a sedimentary environment where Mn is readily solubilized through microbial or abiotic Mn-oxide reduction. Here we examine the consequences of the interplay among the delivery of terrigenous material, high organic carbon oxidation rates, and Mn reduction within the sediments along the Oregon-California continental margin. In areas of high terrigenous sediment discharge, the benthic Mn efflux is enhanced over typical continental margin rates—even at sites with comparable organic carbon oxidation rates. Our preliminary flux estimates suggest that the highest benthic Mn effluxes may exceed ~50 µmol m-2 day-1 off the Eel River; prior work and our new results show that open margin (i.e., non-shelf) environments typically have Mn efflux rates < 10 µmol m-2 day-1. A survey of surface sediments from the Umpqua River discharge area supports the notion that shallow (~ < 100 m) shelf sediments may be a net source of Mn to the water column. These sediments, which are bathed intermittently by hypoxic waters, have Mn to Al ratios as low as ~ 2 x 10-3 g g-1; for comparison, Umpqua river sediments have Mn to Al values of ~6 - 7 x 10-3 g g-1 for the 20 - 63 micron size fraction (VanLaningham, 2007). Mn to Al ratios of ~ 8 - 11 x 10-3 g g-1 (implying possible Mn enrichment relative to river sediments) occur between ~125 and 150 m water depth. Ratios decrease again at greater depths to values of ~7 x 10-3 g g-1, close to that of river sediment. Collectively our data support the idea of an efflux of Mn from shelf sediments. Although the benthic incubation chamber data all indicate a Mn efflux from the sediments, the sedimentary Mn distributions indicate the possibility of re-deposition at depths of ~125 - 150 m. Further work will need to evaluate diagenetic processes and provenance effects on the sediment distribution of Mn. VanLaningham, S. 2007. The Fluvial Response to Glacial-Interglacial Climate Change in the Pacific Northwest, USA. Ph.D. Thesis. Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis Oregon, USA