Uncoupled organic matter burial and quality in boreal lake sediments over the Holocene

Abstract

Boreal lake sediments are important sites of organic carbon (OC) storage, which have accumulated substantial amounts of OC over the Holocene epoch; the temporal evolution and the strength of this Holocene carbon (C) sink is, however, not well constrained. In this study we investigated the temporal record of carbon mass accumulation rates (CMARs) and assessed qualitative changes of terrestrially derived OC in the sediment profiles of seven Swedish boreal lakes, in order to evaluate the variability of boreal lake sediments as a C sink over time. CMARs were resolved on a short-term (centennial) and long-term (i.e., over millennia of the Holocene) timescale, using radioactive lead ( 210 Pb) and carbon ( 14 C) isotope dating. Sources and degradation state of terrestrially derived OC were identified and characterized by molecular analyses of lignin phenols. We found that CMARs varied substantially on both short-term and long-term scales and that the variability was mostly attributed to sedimentation rates and uncoupled from the OC content in the sediment profiles. The lignin phenol analyses revealed that woody material from gymnosperms was a dominant and constant OC source to the sediments over the Holocene. Furthermore, lignin-based degradation indices, such as acid-to-aldehyde ratios, indicated that postdepositional degradation in the sediments was very limited on longer timescales, implying that terrestrial OC is stabilized in the sediments on a permanent basis.

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

“…With an estimated time of deglaciation of 8500 ± 500 B.C. [ Geological Survey of Sweden , ], the development for the conventional OM variables is characteristic for recently deglaciated lakes [ Engstrom et al , ; Fritz and Anderson , ] and can be linked to the establishment and development of catchment vegetation, reduced catchment erosion, and increased in‐lake productivity [e.g., Reuss et al , ; Ilyashuk et al , ; Chmiel et al , ; Meyer‐Jacob et al , ; Rantala et al , ]. Declines in δ 13 C may be the result of increased aquatic respiration and supply of δ 13 C‐depleated plant OM but can also represent a reduced influence of inorganic C (IC) from glacial debris [ Hammarlund , ; Wolfe et al , ].…”

The role of sediments in the carbon budget of a small boreal lake

“…When calculating sediment C budgets there is an inherent mismatch in timescales, since OC burial is calculated over timescales of past decades, while OC mineralization or OC deposition in sediment traps is determined at daily or weekly timescales. However, sediment core data suggest that in Lake Gäddtjärn, OC mass accumulation rates have varied very little over the past century and that the OC, once buried in the sediment, undergoes very little degradation (Chmiel et al ). Accordingly, even if we calculate the OCBE only using the most recent OC burial estimate of 9 g C m −2 yr −1 (which was excluded from calculations because of presumably incomplete degradation), very similar numbers of whole‐lake OC burial (0.36 t C yr −1 ) and burial efficiency (28%, if calculated via OC mineralization) are returned.…”

“…Leaf litter input and direct inputs via precipitation were considered of minor importance in the C budget (Sobek et al ) and therefore not included in the mass balance equation. The presence of particulate inorganic carbon was also considered negligible due to the low pH in the streams (pH 4–6; Kokic et al ) and since no solid‐phase carbonate was detected in the sediment of Lake Gäddtjärn (Chmiel et al ).…”

High terrestrial carbon load via groundwater to a boreal lake dominated by surface water inflow

“…Land‐derived (i.e., allochthonous) DOC is an important energy source for the aquatic microbial community and either accumulates in the food web or mineralized to carbon gases (CO 2 and CH 4 ) that can be emitted to the atmosphere. In addition, allochthonous DOC can flocculate and sink to the sediments [ von Wachenfeldt and Tranvik , ], where it is preferentially buried [ Sobek et al ., ; Chmiel et al ., ]. The contribution of allochthonous DOC is therefore essential for the metabolic state of lakes, most often resulting in boreal lakes being net heterotrophic systems [ Pace et al ., ].…”