Browsing by Author "Li, Jiying"
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Item Diagenesis and sediment-water exchanges in organic-poor sediments of Lake Superior.(2011-06) Li, JiyingTo 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.Item Sediment diagenesis in large lakes Superior and Malawi, geochemical cycles and budgets and comparisons to marine sediments(2014-09) Li, JiyingLarge freshwater lakes, despite their socioeconomic importance, are insufficiently characterized in terms of their geochemical cycling. In systems such as Lake Superior, contributions of several important processes, including those affecting biological productivity, remain poorly quantified. To understand the geochemical controls on sediment diagenesis, we investigated sediments in well-oxygenated temperate Lake Superior and tropical meromictic Lake Malawi. We characterized solid-sediment and porewater geochemistry, calculated diagenetic rates and fluxes, and investigated temporal and geographic variability for the cycles of carbon, nitrogen, phosphorus, iron, and sulfur. Revised nutrient budgets (for N and P) were constructed for both sediment and water column, suggesting a significant contribution of sediments to the geochemical cycling in both lakes. Sedimentation rate and the depth of oxygen penetration (OPD) were found to strongly affect the dynamics of carbon and nutrients. In Lake Superior, the deep (>4 cm) oxygenation of sediments in low-sedimentation areas regulates the remineralization rates of carbon and phosphorus, controls denitrification rates, and creates an unusual sulfur cycle driven by the oxidation of organic sulfur to sulfate. It also makes these deeply oxygenated sediments qualitatively distinct from sediments in nearshore high-sedimentation areas, necessitating their separate consideration in geochemical budgets. Comparisons against data from marine environments suggest that sediment processes in large lakes (both temperate and tropical) can be described by the same quantitative relationships as in marine sediments, facilitating the transfer of knowledge.