INVITED: NOVEL MOLECULAR TRACERS OF SOIL AND PEAT ORGANIC MATTER IN AQUATIC SEDIMENTS

How important is the transport of organic matter from the terrestrial to the marine domain and its subsequent burial and preservation? – this question is of fundamental importance to the global carbon cycle (organic carbon budgets and CO₂ sinks) as well as models of palaeoclimatic change.

To date soil input to the ocean has largely been ignored in marine sciences. If at all, soil OM has been identified and characterised by organic carbon isotope analysis and C/N ratios, bulk parameters that are not source specific and thus more detailed information is required. Organic molecules (lipid biomarkers) can provide this information making them valuable tools to trace the origin and transfer of organic matter through the bio- and geospheres. Certain lipids can be unambiguously related to a specific group or groups of source organisms (e.g. bacteria, archaea, phytoplankton, land plants etc.), and in some cases to specific species (or a few related species).

The presentation will focus on our recent work investigating bacterial hopanoid (cell membrane components) composition of various soils (urban garden, organic farm, natural pasture) including the intact biological hopanoids (i.e. bacteriohopanepolyols), their natural degradation products (hopanols and hopanoic acids) and hopanoids bound into macromolecular organic matter. A recent case study tracking the sources of hopanoids deposited in lake sediments in the English lake district (as a model for input to the marine environment) has highlighted a novel group of hopanoids (from as yet unidentified or unconfirmed bacterial species) which have been found to be present in all soils studied to date and which can be traced into the aquatic sedimentary environment for at least the last 8000 years. Such compounds have great potential for tracing soil input to marine sediments.

Combined with other potential markers and ancillary bulk geochemical data, bacterial hopanoids will be discussed as novel and promising soil markers for future quantitative studies addressing sources, fluxes and diagenetic fate, in order to work towards quantitative markers of soil OM in aquatic sediments.