Composition and sources of sedimentary organic matter in the deep eastern Mediterranean Sea

R Pedrosa-Pàmies,A Sanchez-Vidal,N Lampadariou,I Bouloubassi,M Canals,A Gogou,C Parinos,A Calafat

doi:10.5194/bg-12-7379-2015

Abstract

Abstract. Surface sediments collected from deep slopes and basins (1018–4087 m depth) of the oligotrophic eastern Mediterranean Sea have been analysed for bulk elemental and isotopic composition of organic carbon, total nitrogen and selected lipid biomarkers, jointly with grain size distribution and other geochemical proxies. The distribution and sources of sedimentary organic matter (OM) have been subsequently assessed and general environmental variables, such as water column depth and physical circulation patterns, have been examined as causative factors of deep-sea sediment characteristics. Lithogenic and biogenic carbonates are the dominant sedimentary fractions, accounting for up to 85.4 and 66.5 % of the total weight respectively. The low OC and TN contents in the surface sediments of the study area, which ranged from 0.15 to 1.15 % and 0.06 to 0.11 % respectively, reflect the oligotrophic character of the eastern Mediterranean Sea. Both bulk and molecular organic tracers reflect a mixed contribution from autochthonous and allochthonous sources for the sedimentary OM, as indicated by relatively degraded marine OM, terrestrial plant waxes and anthropogenic OM (e.g. degraded petroleum by-products) respectively. Wide regional variations have been observed amongst the studied proxies, which reflect the multiple factors controlling sedimentation in the deep eastern Mediterranean Sea. Our findings highlight the role of deep eastern Mediterranean basins as depocentres of organic-rich fine-grained sediments (mean 5.4 ± 2.4 μm), with OM accumulation and burial being attributed to aggregation mechanisms and hydrodynamic sorting. A multi-proxy approach is applied aiming to investigate the biogeochemical composition of sediment samples, which sheds new light on the sources and transport mechanisms along with the impact of preservation vs. diagenetic processes on the composition of sedimentary OM in the deep basins of the oligotrophic eastern Mediterranean Sea.

Highlights

The burial of organic matter (OM) in marine sediments constitutes the main link between “active” pools of carbon in the oceans, atmosphere and landmasses and carbon pools that cycle on much longer, geological, timescales (Burdige, 2007)
Sub-samples collected for grain size and elemental and stable isotopic composition were stored in sealed plastic bags at 4 ◦C, while those collected for the analysis of lipid biomarkers were stored in pre-combusted aluminium foils at −20 ◦C
The top δ13C values found in Surface sediments collected from deep basins of the oligotrophic eastern Mediterranean Sea (EMS) were investigated using a multi-proxy approach that involved elemental composition, grain size, stable isotopes and selected lipid biomarkers’ analyses resulting in a robust database to determine sediment sources, the degradation and preservation state of OM and processes that affect sediment dispersal and deposition

Summary

Introduction

The burial of organic matter (OM) in marine sediments constitutes the main link between “active” pools of carbon in the oceans, atmosphere and landmasses and carbon pools that cycle on much longer, geological, timescales (Burdige, 2007). Investigating the processes that control the composition of sedimentary OM that is buried in deep-sea sediments is crucial for understanding carbon cycling on a global scale. The deep sea receives inputs of organic particles from multiple sources, both autochthonous (e.g. biogenic particulate matter from primary production in ocean surface waters) and allochthonous (i.e. land-sourced OM from soils, plant debris, riverine phytoplankton and man-made compounds transported by runoff and atmospheric deposition into the marine domain) The combined use of bulk geochemical indicators such as total nitrogen (TN) to organic carbon (OC) ratios, stable isotope of OC (δ13C) and molecular proxies such as lipid biomarkers can aid to gain knowledge on the origin, delivery and preservation of OM in marine sediments (Bouloubassi et al, 1997; Meyers, 1997; Goñi et al, 2003; Volkman, 2006)

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