Diagenesis and sediment-water exchanges in organic-poor sediments of Lake Superior.

Abstract

To investigate early diagenetic processes and the spatial and temporal variability in organic-poor sediments of Lake Superior, we have repeatedly sampled sediments at 8 locations across the lake. Sediment geochemistry was characterized by the penetration depths and uptake rates of oxygen, and the distributions of organic carbon, dissolved Fe(II), nitrate, ammonium, soluble reactive phosphate (SRP), and solid phase Fe(III)/Fe(II) and phosphorus. Oxygen penetrated deeply into the sediments at all locations: from ~3.5 cm at near-shore stations to >12 cm in the deep basins. The total oxygen uptake ranged from 4.44 mmol m-2 d-1 to 7.68 mmol m-2 d-1, averaging 6.10 mmol m-2 d-1. Diffusive oxygen flux averaged 2.92 ± 0.75 mmol m-2 d-1. Aerobic respiration accounted for >90-95% of the total carbon degradation, with denitrification and iron reduction contributing <5% of the total carbon degradation. Reactivity of the organic carbon in the upper 1 cm of sediment was calculated to be ~ 1.2 yr-1, which is typical for organic material less than a year old. Sediment carbon degradation rate of 5.29± 1.20 mmol m-2 d-1 corresponds to ~ 19% of the recently estimated primary production. More than ~ 90% of carbon reaching the lake floor is mineralized, with a sediment carbon burial flux of 0.49 mmol m-2 d-1. Diffusive fluxes of nitrate from sediment into the overlying water averaged 0.17± 0.07 mmol m-2 d-1, recycling ~ 40% of nitrogen sedimentation flux. Phosphorus cycling was strongly associated with the diagenetic cycling of iron. Sediment diffusive effluxes of SRP were small (< ~ 1.5 x 10-3 mmol m-2 d-1) and similar among stations. The efficient trapping of phosphorus in the sediment is interpreted as a result of strong adsorption of phosphorus by iron (hydro)oxides. Sediments in Lake Superior exhibit strong spatial heterogeneity on spatial scales down to hundreds of meters. The presence of multiple Fe- and Mn-rich layers, forming dense crusts and often visible to the naked eye, suggests decadal or longer variations in the sediment’s physical or redox environment. Oxygen diffusive fluxes and carbon degradation rates exhibited strong seasonality, with higher oxygen fluxes and carbon degradation rates in July, and the depth of oxygen penetration varied by several mm to cm.