Marine carbon cycling, biomarker 14C

This project seeks to detect perturbations in the carbon cycle in large (sub-)tropical watersheds during the last deglaciation and the Holocene and to understand what controls their occurrence. In particular, we investigate whether hydrologic variations affect the re-mobilization of pre-aged terrigenous organic carbon previously locked in terrigenous carbon reservoirs. We test the hypothesis that during arid intervals the land-ocean transport of pre-aged terrigenous organic carbon increased as river-associated wetlands dry out. Once mobilized pre-aged terrigenous organic matter reaches the ocean via fluvial discharge and potentially is introduced into the marine carbon cycle. The pre-aged organic matter can be re-locked in marine sediments and thus withdrawn from short-term carbon cycling. However, exposed to biogeochemical cycling prior to burial, the discharged organic matter in dissolved and particulate forms is a potential substrate to heterotrophic respiration by bacteria and after oxidation to CO₂, for primary producers. We aim at testing the hypothesis that during episodes of enhanced land-ocean transfer of pre-aged organic matter, pre-aged carbon is incorporated into the marine carbon cycle through heterotrophic respiration and primary production.

We work on two sediment cores from the Mediterranean Sea offshore the Nile River (GeoB7702-3 and GeoB7723-3) which cover the last deglaciation and the Holocene. These paired high- and low-resolution records were chosen to allow investigation of different climate states and short-term variations. In order to identify intervals of intensified mobilization of pre-aged terrigenous organic matter we perform compound-specific radiocarbon analysis (CSRA) on terrigenous biomarkers. We combine these data with compound-specific stable hydrogen isotopes (δD) on the same compounds to reconstruct past hydrologic conditions and their effects on mobilization of pre-aged organic carbon. In order to track the pathways of pre-aged carbon in the marine carbon cycle we will perform CSRA on bacterial and marine lipid biomarkers.

Hypothesis

1) During arid intervals the land-ocean transport of pre-aged terrigenous organic carbon increased as river-associated wetlands dry out.

2) During episodes of enhanced land-ocean transfer of pre-aged organic matter, pre-aged carbon is incorporated into the marine carbon cycle through heterotrophic respiration and primary production.