Investigating molecular changes in organic matter composition in two Holocene lake‐sediment records from central Sweden using pyrolysis‐GC/MS

Abstract

Organic matter (OM) is a key component of lake sediments, affecting carbon, nutrient, and trace metal cycling at local and global scales. Yet little is known about long‐term (millennial) changes in OM composition due to the inherent chemical complexity arising from multiple OM sources and from secondary transformations. In this study we explore how the molecular composition of OM changes throughout the Holocene in two adjacent boreal lakes in central Sweden and compare molecular‐level information with conventional OM variables, including total carbon, total nitrogen, C:N ratios, δ13C, and δ15N. To characterize the molecular OM composition, we employed a new method based on pyrolysis‐gas chromatography/mass spectrometry (Py‐GC/MS), which yields semiquantitative data on >100 organic compounds of different origin and degradation status. We identify large changes in OM composition after deglaciation (circa 8500 ± 500 B.C.), associated with early landscape development, and during the most recent 40–50 years, driven by degradation processes. With molecular‐level information we can also distinguish between natural landscape development and human catchment disturbance during the last 1700 years. Our study demonstrates that characterization of the molecular OM composition by the high‐throughput Py‐GC/MS method is an efficient complement to conventional OM variables for identification and understanding of past OM dynamics in lake‐sediment records. Holocene changes observed for pyrolytic compounds and compound classes known for having different reactivity indicate the need for further paleo‐reconstruction of the molecular OM composition to better understand both past and future OM dynamics and associated environmental changes.

Late Holocene volcanic and anthropogenic mercury deposition in the western Central Andes (Lake Chungará, Chile)

“…3 and 4) were in the reported range for algae (diatom, green and brown algae) and/or macrophytes (e.g., Characeae) found in Lake Chungará (Pueyo et al, 2011). In addition, lignin oligomers [a specific proxy for higher plants (Meyers and Ishiwatari, 1993)] always occurred at low levels, whereas the ratio of N-compounds to carbohydrates [a proxy for algal vs plant OM (Bianchi and Canuel, 2011)] was high compared to sediment from boreal lakes [S.I.4, Table S3 - (Ninnes et al, 2017;Tolu et al, 2017)]. Both independent tracers confirm the hypothesis made by Pueyo et al (2011) based on the carbon:nitrogen ratio (C/N), on the dominance of algal OM as a source of organic matter (OM) for offshore sediment.…”

“…Both, proteins and chlorophyll are typically found in larger quantities in algae than in plants, the plants being instead richer in carbohydrates and polysaccharides (Bianchi and Canuel, 2011). Moreover, the relative abundances or contents of proteins and chlorophyll in sediment have been shown to be good proxies of algal production when determined by Py-GC/MS (Peulvé et al, 1996;Ninnes et al, 2017;Tolu et al, 2017) or other methods (Michelutti et al, 2005;Ady and Patoine, 2016). While the abundances of proteins and chlorophyll declined, the abundance of (alkyl)pyrroles, which are indicators of degradation products of proteins, chlorophylls, and other aquatic organism Ncompounds (Damsté et al, 1992;Jokic et al, 2004;Schellekens et al, 2009;Tolu et al, 2017), remained stable or increased (S.I.4., Fig.…”

How Does Environmental Inter-annual Variability Shape Aquatic Microbial Communities? A 40-Year Annual Record of Sedimentary DNA From a Boreal Lake (Nylandssjön, Sweden)

“…The py-GC/MS analysis, peak integration and identification follow the method optimized by Tolu et al (2015), and is described in detail in Supplementary Material. The pyrolytic organic products used in this study include: (i) 16 lignin oligomers and 4 phenolic compounds described as biomarkers for terrestrial OM (Meyers and Ishiwatari, 1993); and (ii) 6 specific Py-products of chlorophyll (i.e., prist-1-ene, phytenes, and phytadienes; Nguyen et al, 2003), 6 specific Py-products of proteins (i.e., 2,5diketopiperazines; Fabbri et al, 2012) and 13 steroids described as biomarkers for algal OM (Ishiwatari et al, 1991;Nguyen et al, 2003;Ninnes et al, 2017;Tolu et al, 2017). Changes in their fluxes to the sediment were estimated by dividing their signal intensity (i.e., peak area) by the dry mass accumulation rate (DM-AR).…”

Assessing the effects of lake‐dredged sediments on soil health: Agricultural and environmental implications for northwestern Ohio

“…Dredged sediments can improve soil health by adding organic matter (OM) and nutrients, lowering bulk density, and slightly increasing soil pH (Daniels et al., 2007; Darmody & Ruiz Diaz, 2017; Sigua et al., 2004). Dredged sediments can contain OM in the form of lignin oligomers, marine and terrestrial humic acids, chlorophylls, carbohydrates, and other compounds (Ninnes, et al., 2017; Zhou et al., 2016). Soil OM (SOM) has high surface area, provides C and energy to soil microorganisms, and provides nutrients for plants (Lal, 2006, 2016).…”