The origin of organic matter in Holocene sediments in the Bay of Koper (Gulf of Trieste, northern Adriatic Sea)

Abstract

Three cores, V-3, V-5 and MK-6, drilled in the inner part of the Bay of Koper were used to reconstruct the paleoenvironmental conditions occurring during the Holocene. Based on stable isotope results two depositing environments can be distinguished in the cores: the upper, marine and lower, brackish sedimentation. Marine sedimentation prevailed over flu-vial sedimentation at depths of 15 m, 19 m and 10 m in the V-3, MK-6 and V-5 cores respec-tively. The marine part of the core V-3 was influenced by varying amounts of land-derived organic carbon transported by the RiverRìana, while in MK-6 and V-5 cores the marine algae and/or microphytes constituted the main source of sedimentary organic carbon. The fluvial sedimentation has typically lower δ 13 C org and higher C/N ratios, ∼ – 26 ‰ and > 12 re-spectively and based on stable carbon isotope mass balance the terrestrial organic carbon present up to 70 to 100 % of the sedimentary organic carbon. On the other hand, the higher δ 15 N values observed at some depths in the fluvial sedimentation indicated that nitrogen could be of marine origin. The observed data in parallel with previous studies are in good agreement with the simultaneous rise of the sea level in the Northern Adriatic. Izvle~ek Jedra vrtin V-3, V-5 in MK-6, izvrtanih v notranjem delu Koprskega zaliva smo upora-bili za rekonstrukcijo paleookoljskih razmer v ~asu holocena. Na osnovi rezultatov izotop-skih meritev 13 C in 15 N v organski snovi lahko v njih dolo~imo dve sedimenatcijski okolji – zgornje morsko in spodnje braki~no. Morsko sedimentacijsko okolje prevladuje do globi-ne 15 m v vrtini V-3, 19 m v MK-6 in 10 m v vrtini V-5. Morska sedimentacija v vrtini V-3 je bila pod variabilnim vplivom vnosa kopenske organske snovi z reko Rižano, medtem ko opažamo, da izvira organska snov v vrtinah MK-6 in V-5 pretežno iz morskih mikroalg in makrofitov. Okolje re~ne sedimentacije karakterizirata nižja vrednost δ 13 C org ∼ – 26 ‰ in vi{je razmerje C/N > 12. Delež kopenskega organskega ogljika v teh delih sedimenta, dolo~en na osnovi masne bilance, zna{a 70–100 %. Povi{ane vrednosti δ 15 N ugotovljene v nekaterih globinah re~ne sedimenatcijske sekvence kažejo, da je N tudi morskega izvora. Na{e ugotovitve, vzporedno s predhodnimi raziskavami, so ujemajo z dvigovanjem morja v severnem Jadranu v holocenu.