Project

Discipline

stable isotopes; sedimentary organic matter; microbial degradation; isotope effects; nitrogen compounds; bacteria; PON; biomarker; amino acids; amino sugars; lake sediments; sedimentary diagenesis; incubation experiments; nitrogen; isotope alteration; early diagenesis

Lead
Lay summary
Organic nitrogen (N) preserved in marine, estuarine and lacustrine sediments retains substantial information about paleoenvironmental conditions. Sedimentary diagenesis and bacteria-mediated decomposition of organic material can markedly modify its biochemical and isotopic composition, and thus is a pressing concern in paleoceanographic and paleolimnological studies. Field data indicate that microbial degradation of phytoplankton is associated with a significant N isotope effect, potentially masking primary N isotope signals and, hence, compromising the interpretation of N isotope records in sedimentary archives. The exact mechanisms that lead to N-isotope alteration during organic matter (OM) degradation, the susceptibility of different organic materials to post-burial isotopic alteration, and the role of bacterial biomass adding to bulk isotope signals are still poorly understood. The prime objectives of the first ISOALT project is to characterize the N isotope effects during OM decomposition, and to gain a mechanistic understanding of the processes and bio-(geo)chemical patterns underlying early diagenetic alteration of bulk N isotope signals. Towards these objectives, a series of laboratory experiments is designed, in which the decay of algal OM (15N-labeled and unlabeled) is simulated under controlled environmental conditions. The anticipated changes in the concentration and isotopic composition of particulate (PON) and dissolved (DON, NO3-, NH4+) N species are monitored, and the (isotope) fluxes between the different N pools during in vitro OM hydrolysis and degradation are assessed. In addition, the change in concentration and N isotopic composition of specific nitrogenous compounds/compound classes (e.g., amino acids, chlorophyll derivatives) is determined during the incubation experiments, and their proneness to isotopic alteration is studied. Moreover, the role of bacterial biomass potentially modulating the N isotope signals is assessed. Combining state of the art organic geochemistry, isotope geochemistry and molecular techniques to measure N isotope ratios in various organic and inorganic N species, the proposed research promises the first comprehensive assessment of the controls on, and mechanisms behind, N isotope alteration during microbial organic matter remineralization on different time scales. Thus, it will yield invaluable information regarding the use of N isotope ratios for paleoenvironmental reconstructions.

Direct link to Lay Summary

Last update: 21.02.2013

Number	Title	Start	Funding scheme

Organic nitrogen (N) preserved in marine, estuarine and lacustrine sediments retains substantial information about paleoenvironmental conditions. Sedimentary diagenesis and bacteria-mediated decomposition of organic material can markedly modify its biochemical and isotopic composition, and thus is a pressing concern in paleoceanographic and paleolimnological studies. Field data indicate that microbial degradation of phytoplankton is associated with a significant N isotope effect, potentially masking primary N isotope signals and, hence, compromising the interpretation of N isotope records in sedimentary archives. The exact mechanisms that lead to N-isotope alteration during organic matter (OM) degradation, the susceptibility of different organic materials to post-burial isotopic alteration, and the role of bacterial biomass adding to bulk isotope signals are still poorly understood. The prime objectives of the proposed study is to characterize the N isotope effects during OM decomposition, and to gain a mechanistic understanding of the processes and bio-(geo)chemical patterns underlying early diagenetic alteration of bulk N isotope signals. Towards these objectives, we propose to design a series of laboratory experiments, in which we simulate the decay of algal OM (15N-labeled and unlabeled) under controlled environmental conditions. We will monitor the anticipated changes in the concentration and isotopic composition of particulate (PON) and dissolved (DON, NO3-, NH4+) N species, and assess the (isotope) fluxes between the different N pools during in vitro OM hydrolysis and degradation. In addition, we propose to examine the change in concentration and N isotopic composition of specific nitrogenous compounds/compound classes (e.g., amino acids, chlorophyll derivatives) during the incubation experiments, and to study their proneness to isotopic alteration, as well as to assess the role of bacterial biomass potentially modulating the N isotope signals. In a second step, we will study decadal diagenetic effects on the N isotopic composition of sediments and sedimentary components in natural systems. Through the analysis of archived sediment-trap and sediment-core material from the last two decades from Swiss lakes, we will assess the organic-geochemical and N-isotopic compositional changes in lake sediments during recent sedimentary diagenesis. Results from both laboratory experiments and field data will be integrated, and will allow us to constrain the key N-isotope fractionating steps during organic matter remineralization and reworking.Combining state of the art organic geochemistry, isotope geochemistry and molecular techniques to measure N isotope ratios in various organic and inorganic N species, the proposed research promises the first comprehensive assessment of the controls on, and mechanisms behind, N isotope alteration during microbial organic matter remineralization on different time scales. The planned study will not only provide insight into processes that predetermine the element recycling and preservation potential of sedimentary organic nitrogen (bulk and compound-specific) in modern aquatic environments. It will also, and most importantly, yield invaluable information regarding the use of N isotope ratios for paleoenvironmental reconstructions.