Benthic foraminiferal stable carbon isotopes as a proxy for organic matter fluxes to the sea floor
Final Report Abstract
The stable carbon isotope composition of benthic foraminiferal tests contains a wide range of environmental and biological information. Although δ13C is commonly used in paleoceanographic studies our knowledge on its inter- and intraspecific variability is still insufficient. In the frame of the project we have studied the stable carbon and oxygen isotope composition of live (Rose Bengal stained) and dead (unstained) tests of selected benthic foraminiferal species from different microhabitats and regions of the Mediterranean Sea covering a wide range of trophic conditions and related isotopic pore water gradients. We observed up to -2.3 ‰ lower δ13C values with increasing living depth within the sediment, reflecting the pore-water gradient δ13CDICof dissolved inorganic carbon (DIC). Ontogenetic shifts towards higher δ13C and δ18O ratios with increasing test sizes of 1.2 and 1.3 ‰, respectively, were observed in buliminid (Globobulimina spp., Uvigerina spp.), but not in rotaliid taxa (Lobatula lobatula, Cibicidoides pachydermus, Melonis barleeanum). Uvigerina peregrina revealed a strong metabolic fractionation for small tests, suppressing the pore water signal towards more negative ratios. In contrast, the likewise shallow infaunal Uvigerina mediterranea appears to be less biologically influenced and reflects the more eutrophic conditions of the Alboran Sea by a general shift of the ontogenetic function towards lower δ13C values. The Δδ13CUmed-Epi and the Corg flux data showed a good correlation for open marine sites (Alboran Sea, Mallorca Channel, Strait of Sicily, Southern Aegean Sea), while the Corg supply is underestimated for regions influenced by lateral organic matter fluxes, due to intense resuspension (Canyons at the Gulf of Lions, isolated basins in the Northern and Central Aegean Sea). Considering these local biases, a first transfer function for Corg fluxes based on the stable carbon isotope differences of benthic foraminifera could be established. This transfer function was then applied to down-core δ13C records from the Strait of Sicily and the Ligurian Sea. Generally enhanced food fluxes by about 5 gC m^-2 a^-1 and 8.5 gC m-2 a^-1 were reconstructed for the Last Glacial Maximum in the Strait of Sicily and the Ligurian Sea, respectively. The open-ocean setting of the Strait of Sicily revealed long-term trophic changes for the past 23 kyr. In contrast, the more proximal location of the Ligurian Sea in the vicinity of the Alps revealed a stronger millennial-scale trophic variability suggesting the influence of short-term climate changes on the local vertical and lateral Corg fluxes. Although the applicability of the transfer function is hampered in environments with low dissolved oxygen concentrations and enhanced lateral Corg fluxes, species-specific δ13CFor deviations appear a useful proxy for quantitative paleoceanographic reconstructions.
Publications
- 2013. Stable isotope signal of benthic foraminifera from the western Mediterranean Sea: Biological and environmental effects. 11th International Conference on Paleoceanography, Sitges – Barcelona, P-170
Theodor, M., Jorissen, F., Mackensen, A., Schmiedl, G.
- 2014. Stable isotope composition and onthogenetic trends of deep-sea benthic foraminifera under contrasting trophic conditions in the western Mediterranean Sea. GeoFrankfurt 2014, Schriftenreihe der Deutschen Gesellschaft für Geowissenschaften, 85
Theodor, M., Mackensen, A., Schmiedl, G.
- 2016. Benthic foraminiferal stable isotope record of organic carbon fluxes during deposition of Mediterranean sapropel S1. EGU General Assembly Conference Abstracts, 18, 4689
Theodor, M., Mackensen, A. Schmiedl, G.
- 2016. Productivity changes reconstructed from species-specific benthic foraminiferal stable carbon isotope deviations. 12th International Conference on Paleoceanography, Utrecht, P-263
Theodor, M., Schmiedl, G., Mackensen, A., Jorissen, F.
- 2016. Stable carbon isotope gradients in benthic foraminifera as proxy for organic carbon fluxes in the Mediterranean Sea. Biogeosciences, 13, 6385–6404
Theodor, M., Schmiedl, G., Jorissen, F., Mackensen, A.
(See online at https://doi.org/10.5194/bg-13-6385-2016) - 2016. Stable isotope composition of benthic foraminifera: Species-specific differences and their application. PhD thesis, University of Hamburg, 115 pp.
Theodor, M.
- 2016. Stable isotope composition of deep-sea benthic foraminifera under contrasting trophic conditions in the western Mediterranean Sea. Marine Micropaleontology, 124, 16-28
Theodor, M., Schmiedl, G., Mackensen, A.
(See online at https://doi.org/10.1016/j.marmicro.2016.02.001)
