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 fluvial sedimentation at depths of 15 m, 19 m and 10 m in the V-3, MK-6 and V-5 cores respectively. The marine part of the core V-3 was influenced by varying amounts of land-derived organic carbon transported by the River Ri`ana, 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 respectively 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~ekJedra vrtin V-3, V-5 in MK-6, izvrtanih v notranjem delu Koprskega zaliva smo uporabili za rekonstrukcijo paleookoljskih razmer v ~asu holocena. Na osnovi rezultatov izotopskih 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 globine 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.

Thickness of marine Holocene sediment in the Gulf of Trieste (northern Adriatic Sea)

“…1). After the Last Glacial Maximum with the onset of the Holocene transgression, marine sediment started depositing approximately 10 000 years ago in this area (Marocco, 1991;Ogorelec et al, 1991Ogorelec et al, , 1997Lambeck et al, 2004;Covelli et al, 2006;Ogrinc et al, 2007;Trincardi et al, 2011b).…”

Sound Velocity in a Thin Shallowly Submerged Terrestrial-Marine Quaternary Succession (Northern Adriatic Sea)

“…In the northern Adriatic seabed, gas seeps are commonly observed and are attributed to both deep and shallow sources [94][95][96][97][98]. Since the diffraction hyperbolas in our sonar profiles are constrained only to a narrow zone in the uppermost part of the Quaternary sequence (Figures 3 and 4), we propose that the gas (probably methane) originates from a degradation of organic matter contained in the Holocene paralic and marine sediments and/or Late Pleistocene terrestrial sequences [62,97,99]. Gas production related to biological processes in seagrass meadows can also greatly hinder the propagation of acoustic signals [100][101][102]; however, the contribution of this effect is difficult to determine from our dataset since only a single borehole (V-5/95) is located within a meadow (Figure 3a).…”

“…The post-LGM sea-level rise induced significant changes in the sedimentary environments of the northern Adriatic Sea with terrestrial environments transitioning in paralic and later shallow marine environments [22,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56]. In the Gulf of Trieste, where our study area is located (Figure 1), the Late Pleistocene alluvial plain transitioned into a paralic environment until open marine conditions finally prevailed approximately 10,000 years ago [22,42,44,45,[55][56][57][58][59][60][61][62][63][64].…”

“…The hinterland of the Bay of Koper is composed of Eocene turbidites (flysch) comprising interbedded sandstones and marlstones, which are overlain by Quaternary alluvial and paralic sediments in the valleys of Rižana and Badaševica (Figure 1b; [65,66]). Offshore the Eocene succession is unconformably overlain by Quaternary terrestrial and paralic sediments topped by Holocene marine sediments [55,56,59,62,[67][68][69][70][71]. The Quaternary succession in the Bay of Koper, which was recognized as a submerged fluvial valley of the Rižana river (Figure 1b, [68][69][70][71]), is up to a few tens of meters thick and is composed of a lower alluvial part and an upper paralic part; however, alternations of terrestrial and paralic sedimentary environments have also been observed [68,72].…”

“…The alluvial sediments are generally composed of fine-grained clastic sediments with occasional gravelly and sandy horizons, whereas the paralic sediments are mostly composed of silty clay [68][69][70]. The Holocene marine cover comprises fine-grained bioclastic sediment with the surficial sediments showing a clear zonation: sandy silt near the coastline, clayey silt in the central part, and silt in the outer part of the Bay [62,[68][69][70][71]. Holocene sedimentation rates in the Gulf of Trieste are relatively low and amount to a few millimeters per year [44,57,69,73].…”