Organic matter biogeochemical characteristics in Lake Superior: insights into composition, source and reactivity

Abstract

Organic matter (OM) composition and its relationship with OM source and dynamics in Lake Superior were investigated with the combination of advanced spectroscopic techniques and multiple-geochemical signatures. Studies of water-column dissolved, water-column particulate, and sedimentary organic matter (DOM, POM, and SOM, respectively) were performed. The composition of DOM obtained with two different solid phase extraction resins (C18 vs SDB-XC) was investigated using Uv-visible spectrometry and negative-ion electrospray Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). Lake Superior raw offshore water was found to be very clear; its DOM is low in aromaticity, with primarily non-humic, hydrophilic and low molecular weight materials. Radiocarbon signatures reveal the primarily modern (post-bomb) nature of Lake Superior DOM. With such Lake Superior water, we found SDB-XC disks to outperform C18 disks in the isolation of DOC in terms of both higher recovery and less degree of fractionation to the initial DOM composition. Extracts of the same samples obtained with the different extraction disks share 70% of compounds, which were dominated by lignin-CRAM-like material but also include a variety of other functional groups including lipids, proteins, carbohydrates and condensed hydrocarbon. To assess settling particulate organic matter seasonality and availability to the benthic community, settling particulate matter was studied in terms of mass fluxes and main biochemical characteristics and composition at two Lake Superior offshore sites over the course of a year. Increase in sinking flux was variably associated with sediment resuspension and enhanced surface production. The combination of PCHO-C%, THAA and Fourier transform infrared spectroscopy (FTIR) data revealed that the relative bioavailability and nutritional values of POM to benthic microbes should be lower in spring than summer, although both periods exhibited high sinking fluxes. Isotopic and elemental analyses, FTIR, principal components analysis (PCA), and two dimensional (2D) correlation analysis, where core depth was used as perturbation, were used to study the diagenesis of organic matter (OM) in Lake Superior sediments. Depth-related changes among sites were found to be similar, leading to an increased contribution from inorganic (and possibly refractory aromatic organic) components at each site, and a loss of contribution from other organic components. Synchronous spectra reveal that aliphatic esters and carbohydrates degrade significantly with increasing depth, leading to an increased contribution from clays/ biogenic silica/ inactive carbohydrates. Asynchronous spectra show that, generally, carboxyl groups, including aliphatic ester and amide/protein, degrade first, followed by a group of carbohydrates, and then aromatic compounds and/ or the Si-O framework in clays and biogenic silica.