Carbon and Nitrogen isotope compositions of organic matter in coastal marine sediments (the Gulf of Trieste, N Adriatic Sea): indicators of sources and preservation.

Terrestrial organic carbon (OC) preserved in marine sediments is an important reservoir in the global carbon cycle, but remains poorly constrained. We determined the elemental and C- and N-isotopic compositions of organic material (OM) in surface sediments and sediment cores from the Gulf of Trieste (N Adriatic), an area characterized by relatively high riverine input. This multiple tracer approach permits evaluation of how sensitive each tracer is for constraining the terrestrial component of OM. Using the relevant terrestrial and marine OM nd-members, we estimate that the terrestrial contribution ranges from about 90% at the mouth of the Isonzo River and decreases to V10% in the center of the Gulf. In the southern part of the Gulf, 32–44% of terrestrial OC is contributed from smaller rivers, while the d15N values of OM reflect more effective nutrient utilization. Analyses of different size fractions show that smaller particles are consistently more organic-rich and tend to be transported further from the river mouth, depending on river discharge. In the size fraction 2–16 um, elemental ratios and d13Corg and d15N isotopic signatures are consistent with a sedimentary OM pool dominated by bacterially derived biomass. Radiocarbon analyses of sedimentary OC yield depleted D14C values (-338.7 %o to -429.4%o), indicating a significant fraction of relatively old OC. The natural abundance of 13C and 14C tracers suggests a two-component mixture of ancient and modern C in the sediments down to approximately 60 cm sub-bottom, with an estimated ~45% of sedimentary OC probably being derivedfrom ancient sedimentary OC (kerogen). In addition, there is a significant change in the source of organic material downcore inthese sediments which, along with geological evidence, indicate contributions from freshwater swamps typical of coastal lakeswith intermittent connections to the sea (brackish) prior to ~9000 BP. Depth variations in a core from a salt marsh furtherindicate temporal variability of OM deposition and transformations in a changing brackish-marine environment in the past.