The Origin of Organic Matter in Holocene Sediments in the Bay of Koper (Gulf of Trieste, Northern Adriatic Sea)

GEOLOGIJA 50/1, 179–187, Ljubljana 2007 doi:10.5474/geologija.2007.014

The origin of organic matter in Holocene sediments in the Bay of Koper (Gulf of Trieste, northern Adriatic Sea) Izvor organske snovi v holocenskem sedimentu Koprskega zaliva

Nives OGRINC1, Jadran FAGANELI2, Bojan OGORELEC3 & Branko ^ERMELJ2 1Department of Environmental Sciences, ”J. Stefan” Institute, Jamova 39, 1000 Ljubljana, Slovenia, nives.ogrinc�ijs.si 2Marine Biological Station, National Institute of Biology, Forna~e 41, 6330 Piran, Slovenia, faganeli�mbss.org; cermelj�mbss.org 3Geological Survey of Slovenia, Dimi~eva ul. 14, 1000 Ljubljana, Slovenia, bojan.ogorelec�geo-zs.si

Key words: marine sediments, organic matter, carbon, nitrogen, stable isotopes, northern Adriatic Klju~ne besede: morski sedimenti, organska snov, ogljik, du{ik, stabilni izotopi, severni Jadran 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 Riàna, while in MK-6 and V-5 cores the marine algae and/or microphytes constituted the main source of sedimentary organic carbon. The 13 fluvial sedimentation has typically lower δ Corg 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 δ15N 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 13C in 15N 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 13 in makroﬁtov. Okolje re~ne sedimentacije karakterizirata nižja vrednost δ Corg ∼ – 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 δ15N 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. 180 Nives Ogrinc, Jadran Faganeli, Bojan Ogorelec & Branko ^ermelj

Introduction nner et al., 1999; Meyers & Teranes, 2001; Huon et al., 2002). A large source of the uncertainty in The study of boreholes drilled in the Bay present understanding of the global carbon of Koper, inner part of the Gulf of Trieste cycle is the distribution and dynamics of (Fig. 1) has enabled us to reconstruct the the coastal sediment organic carbon (OC) re- sedimentation environment through the servoir, where total organic carbon (TOC) is Holocene. The changes observed in the sedi- composed of material derived from both ma- ment are also closely related to the global rine and terrestrial sources. Identification of rise of sea levels. Analyses at a depth of environmental factors, especially those re- 26 m showed that, during the most intensive lated to degradation and preservation of OC, sea transgression marine sedimentation pre- allow reconstruction of paleo-productivity, vailed over fluvial sedimentation (Faganeli terrigenous OC input and anthropogenic im- et al., 1987; Ogorelec et al., 1997), as il- pact in relation to global environmental and lustrated in the lithology of cores V-3 and climate changes. The organic matter (OM) in MK-6 presented in Figure 2. In addition, it marine sediments typically accumulates at can be seen that the V-3 core fluvial sedi- slower rates and over longer periods of time mentation is replaced by marine sedimenta- and consequently has the potential ability to tion and, in parallel a brackish environment record longer paleoceanographic histories changes into a marine environment in the and to reveal the effects of slow-acting dia- MK-6 core. genetic alteration. A variety of geochemical approaches have been employed to define the marine to terrestrial ratio of OC buried in sediments. On the basis of elemental and isotopic values it is possible to distinguish two end-members: land-derived OM, with 13 15 δ Corg ~ − 26 ‰, δ N ~ 0.4 ‰ C/N ~ 18, and 13 marine OM, with δ Corg = − 20 ‰ to − 22 ‰, δ15N ~ 8.6 ‰, C/N ~ 8 (Meyers, 1994, 1997). Despite the extensive early diagenetic processes which have influenced the OM, the C/N ratio and especially 13C/12C ratio appear, in general, not to have changed extensively. It was found that the C/N ratio of OM depends on sediment grain size. Lower C/N ratios were found in fine-sized sediments than in coarse sediments in which a larger proportion of intact land-plant debris was observed. Unlike C/N ratio, δ13C values are not significantly influenced by sediment grain size making them useful in reconstructing Figure 1. Sampling locations of the studied cores past sources of OM. The δ15N values can give in the Bay of Koper additional information to distinguish the source of OM and to reconstruct past pro- The aim of the present study was to de- ductivity rates. The dynamics of nitrogen in termine stable carbon and nitrogen isotopic sediments are more complicated than those composition in three deep cores, V-3, V-5 of carbon and make interpretation of sedi- and MK-6, drilled in the inner part of the mentary δ15N records more difficult. The iso- Bay of Koper in order to better understand topic signature depends on the source of the the paleoenvironmental conditions obtain- nitrogen, the rates of primary production ing during the Holocene in this area. and respiration, and the nitrification-deni- trification processes (Macko et al., 1993; de Lange et al., 1994; Prahl et al., 1997; Materials and methods Sigman et al., 1999; Brenner et al., 1999). However, δ15N values of sedimentary organic Site description – The Bay of Koper co- matter are widely used to complement δ13C vers an area of about 35 km2. At present it values in paleoproductivity studies (Bre- is a wide submarine plateau up to 20 m deep The origin of organic matter in Holocene sediments in the Bay of Koper (Gulf of Trieste, ... 181

Figure 2. Changes in the Holocene sedimentary environments in the inner part of the Bay of Koper (from Ogorelec et al. 1991) with restricted sea water circulation. In the from flysch sandstones and marl and its con- west, i.e. along the Izola–Debeli rti~ line, the tent ranges from 20 to 35 %. The carbonate Bay slopes downwards towards the open content composed of calcite, dolomite and part of the Gulf of Trieste, while in the east skeletous of various organisms molluscs, fo- it slopes upwards towards the inflow of the raminifers and echinoids ranges from 20 to River Rižana. The Bay of Koper has a fairly 30 %. Calcite is mainly of terrigenous origin, steep coast which, at depths 5 and 10 me- however a rather large proportion is associ- tres, quickly grades into a sloping underwa- ated with organic skeletons. Dolomite, as the ter plain. The coast is composed of Eocene second most abundant carbonate mineral, is flysch layers with alternating solid sandstone present at only about 5 %. The clay is com- and soft marl. The flysch coast gives the Bay posed of illite, chlorite and illite/montmo- its characteristic form, particularly between rillonite, the last as a mineral with mixed Izola and Koper, at Cape Ronek and between composition. The OC content in the surficial Valdoltra and Debeli rti~. The surficial sedi- 5 cm sediment layer ranges from 0.44 to 2.72 ment of the Bay of Koper can be divided into % with an average value of 1.38 ± 0.50 %. three distinct zones according to grain size The total nitrogen content (TN) ranges from distribution, mineral composition and car- 0.05 to 0.46 %, with an average value of 0.18 bonate content: a) coastal sediment b) sedi- ± 0.13 %. Higher OC and TN contents are ment of the inner part and c) sediment of the found in the fine clayey silt fraction, while open part of the Bay (Ogorelec et al., 1987). the coarse sediment along the shore, influ- All three cores are located in the coastal part enced by tides and sediment resuspension, of the Bay. In this zone the sediment is com- contains less. posed of dark grey-green silt and sandy silt Samples and analyses – A coring plat- with up to 40 % of sand and with less than form was set up in the sea to take the cores 15 % of clay (grain size < 2 µm). The most from the boreholes located in the cargo port abundant minerals in the recent sediments of Koper – V-3, 200 m off Koper – V-5 and of the Bay of Koper are quartz and calcite. 200 m off Žusterna – MK-6 (Fig. 1). The V-3 Quartz is present in all fractions originating core was 41 m deep, taken at a sea water depth 182 Nives Ogrinc, Jadran Faganeli, Bojan Ogorelec & Branko êrmelj of 4.5 m, while V-5 and MK-6 cores were with uniform grain size and mineral compo- 43 m deep, taken at a sea water depths of sition. 5 and 7 m, respectively. The sediments was In this upper part, many foraminifers, subsequently divided into several subsam- gastropods, molluscs and fragments of sea ples, which were freeze-dried and homog- urchin remains are present. The mineral enized to a fine powder with a mortar. These composition consists of quartz, calcite, illite, samples were analysed for OC and TN con- chlorite, illite/montmorillonite, feldspars, tents and for stable isotopic composition of dolomite and pyrite, while the last is absent 13 15 OC (δ Corg) and TN (δ N). in the fluvial deposit (Ogorelec et al., 1984, OC contents were determined using a Car- 1997). The carbonate content ranged from lo Erba elemental analyzer (model EA 1108) 20 to 30 % except below the depth of 26 m after acidification with 1M HCl (Hedges & where only 5 to 6 % of carbonate is present Stern, 1984). Weight percentages of TN and due to the higher influence of land derived total C were determined similarly, but with- terrestrial material. out acidification. The precision of measure- The OC content varied from 0.23 to ments was ± 3 %. 1.62 % in the upper marine part of the core, The isotopic composition of sedimen- while in the lower fluvial part it range ranged tary OC was determined after treatment from 0.2 to 1.1 %. The vertical distribution with 1M HCl to remove carbonate mate- of total nitrogen follows the distribution of rial. Sample were treated on a sand plate OC contents. The TN concentrations ranged at 70 °C at least twice, until no further gas from 0.10 to 0.30 % in the marine part and bubbling was observed. A washing step was from 0.01 to 0.07 % in the fluvial part of the used prior to sample analysis to remove core. A plot of % TN vs. % OC showed a dissolved salts (Schubert & Nielsen, rather good correlation (% TN = 0.13 (± 0.03) 2000; Ogrinc et al., 2005). δ15N of TN was * % OC + 0.013 (± 0.03); r = 0.77), suggesting determined directly on bulk powdered sam- that there is close relationship between these ples. The δ13C and δ15N of OC and TN frac- two parameters. The intercept value indi- tions were determined by a Europa 20–20 cates that some inorganic N may be present continuous-flow isotope ratio mass spec- in these sediments. Previous estimates of ex- trometer with ANCA-SL preparation mo- changeable ammonium, nitrite and nitrate dule for solid and liquid samples. Isotopic comprised up to 15 % of TN, while fixed am- ratios were expressed in the usual δ-nota- monium comprised up to 56 % of TN in the tion in parts per mil (‰). For carbon, the sediments (Faganeli et al., 1987, 1991). 13 standard is the V-PDB carbonate, while The δ Corg values ranged from − 21.5 to for nitrogen the standard is atmospheric − 26.2 ‰, and from 1.7 to 7.8 ‰ for δ15N va- 13 15 (air) nitrogen. Data quality control was lues. Higher variability in δ Corg and δ N controlled by running a reference standard values was found in the upper 12 m of the after every 8 samples: IAEA-CH-7 polyethy- core and they do not follow the OC pattern. lene and NBS22 were used for carbon, and These variations correspond to different IAEA-N-1 and IAEA-NO-3 were used for proportions of terrestrial OC discharged by nitrogen. The overall analytical precision the River Rižana to sediments. The highest 13 15 13 was ± 0.2 ‰ and ± 0.3 ‰ for δ C and δ N δ Corg of − 21.5 ‰, a value typical for marine values respectively. microalgae, was determined at a depth of 10 m, but the δ15N value of 4.5 ‰ was lower than the expected value of 7.3 ‰, probably Results and discussion due to selective degradation of organic N. Higher δ15N values of 7.8 ‰ and 6.0 ‰ were The results from core V-3 are presen- found at depths of 15 and 25 m, respectively. 13 ted in Figure 3. The borehole reached the At these depths a prevalent terrestrial δ Corg flysch basement at a depth of approximately signature of − 25.7 ‰ was observed, but the 40 m. The core can be divided into two parts. higher 15N contents in the sediment suggest The bottom section of the sediment starting that N is mainly of microalgal origin. Iso- at the depth of 25 m comprised alluvial de- topic analysis below 15 m from the top of 13 15 posit from the River Rižana with alternat- the core gave values of δ Corg ~ − 25 ‰, δ N ing layers of sand, silt and gravel. The sedi- ~ 2 ‰ and C/N ~ 12 which, together with ment in the top 25 m of the core is mainly of lithostratigraphic evidence, reflects sedi- marine origin and consists of dark grey silt ments of mainly terrestrial origin. The origin of organic matter in Holocene sediments in the Bay of Koper (Gulf of Trieste, ... 183

Figure 3. Vertical proﬁles of lithology, grain size, carbonate content, (Ogorelec et al., 1984, 1987), 13 15 organic carbon (Corg) (Faganeli et al. 1987), δ Corg, total nitrogen (TN), δ N values and C/N ratios in the sediment core V-3.

15 13 13 A plot of δ N vs. δ Corg values were con- δ Corg values provide a better indication of structed, since the linear relationship is the potential sources of OC than δ15N values, usually interpreted as a mixing trend be- a semi-quantitative estimate of the propor- tween terrestrial and marine source of OM tion of terrestrially derived OC of the cores (Thornton & McManus, 1994; Muzuka was based on the stable carbon isotope mass & Hillarie-Marcel, 1999; Huon et al., balance: 13 13 13 2002; Ogrinc et al., 2005). However, a low δ Corg,x = Fm · δ Corg,m + Ft · δ Corg,t (1) correlation (r = 0.05) and considerable spread 1 = F + F (2) in the distribution of values determined in the m t 13 13 13 core V-3 suggests that other factors influence where δ Corg,x, δ Corg,m and δ Corg,t are the 15 13 13 δ N to a greater extent than δ Corg. Since the δ Corg values for the sample, marine and 184 Nives Ogrinc, Jadran Faganeli, Bojan Ogorelec & Branko êrmelj terrestrial source materials, respectively, values of some microphytes were found to and Ft and Fm are the respective terrigenous be higher than those of phytoplanktonic and and marine OC fractions in the sample. For microphytobenthic OC: Ulva rigida: δ13C = 13 13 this calculation we assumed constant δ Corg − 17.9 ‰; Fucus virsoides: C = − 15.6 ‰; end-member values for terrestrial and ma- Cymodocea nodosa: δ13C = − 9.6 ‰ and δ15N rine OC supply, since the effect of selective = 4.8 ‰ (Ogrinc et al., 2005). diagenesis of OC fractions that are isotopi- The depth distribution of the two para- cally heavy or light appears to be small, meters is not so variable as it is in the V-3 13 usually less than 2 ‰ (Meyers, 1997). A core. Using δ Corg values, we can clearly dis- 13 δ Corg value of – 26 ‰ was used as the ter- tinguish two depositing environments with restrial end-member, while for the marine two different sources of OC. The OC is main- end-member the value of – 21 ‰ was ap- ly of marine origin in the upper 17 m of the 13 plied. The δ Corg for terrestrial end-mem- core, with only 40 % of terrestrial OC. Like 13 ber was based on the δ Corg determined in the V-3 core, more than 70 % of OC is of ter- the sediments of the River Rižana outflow. restrial origin in the lower brackish part. 13 This value is similar to terrestrial end-mem- The depth distributions of OC, δ Corg, bers identified in other studies (Wada et TN, δ15N and C/N ratios, together with the al., 1987; Thornton & McManus, 1994; lithology of the core V-5 are presented in Middelburg & Nieuwenhuize, 1998). Figure 5 which also shows the changes be- 13 Further, the average δ Corg value of – 20.7 ± tween marine and brackish environments 1.6 ‰ for marine derived OC is well-estab- during the Holocene deposition. OC and con- lished for the Gulf of Trieste (Faganeli et centrations ranged from 0.54 to 1.85 wt. % al., 1994). According to our estimates, be- and TN from 0.04 to 0.12 wt. %. The high- tween 10 and 92 % of OC in sediments is of est OC content was usually associated with 13 terrestrial origin in the upper 10 m of the the lowest δ Corg values, but not necessarily cores, while in the lower part the terrestrial with the highest TN concentration. The C/N OC comprises more than 70 %. ratios ranged from 11.6 to 31.0, the highest 13 The results from the core MK-6, drilled found at a depth of 13 m. The δ Corg values in the location of the planned Koper marina varied by over 11.0 ‰, from – 27.2 to − 17.1 (Žusterna), are shown in Figure 4. In this ‰, the highest being found at the depth of core no fluvial deposit was observed. The 2.5 m. The δ15N values were less variable, sediment is homogeneous, being composed ranging from 3.7 to 6.5 ‰. A good correla- of grey clayey silt with mean grain size less tion between OC and TN contents in core than 10 µm, similar to the composition of sediments (% TN = 0.036 (± 0.01) * % OC surface sediment in the central part of the + 0.033 (± 0.013); r = 0.59) suggests that Bay of Koper. Several fossil remains are there was a close relationship between OC present such as foraminifers, molluscs and and TN, while the intercept value of 0.033 ostracods. The mineral composition is rath- indicates that there was more inorganic N er uniform and consists of quartz, calcite, present in the sediment than in the core V-3. illite, chlorite, illite/montmorillonite, feld- This is also the reason why a slope-derived spars, dolomite and pyrite. The carbonate C/N value could not be representative of the content was around 20 % in the upper 20 m bulk OC in these samples. Larger variations 13 of the core, but lower content (approx. 10%) in δ Corg values were present in the upper deeper in the core. The OC content ranged 4 m of the core. These variations reflect a dif- from 0.58 to 2.59 %, while nitrogen content ferent cause of the changes of OC between varied from 0.07 to 0.13%. marine algae and microphyte derived OC. In 13 15 13 The δ C and δ N values ranged from addition to a higher δ Corg values, the micro- − 19.0 to − 27.0 ‰ and from 2.3 to 6.3 ‰, phyte derived OC also had higher C/N ratios respectively. The highest δ15N value of 6.3 ‰ (> 20). Deeper in the core the terrestrial sig- was found at a depth of 34 m together with nature, with more than 70 % of OC, was first 13 the δ Corg of − 25.0 ‰, similar to the core V-3 observed at depths of 5 and 6 m, then at 8 m, at the depths of 17 m and 25 m. Again, the and in all samples below 10 m. source of N is different from that of C and Changes in the marine and brackish en- appears to be of marine origin. The highest vironments during the Holocene deposition 13 δ Corg value of − 19‰ indicates that benthic were also reflected in the downcore profile macrophytes could represent an additional of the V-6 core drilled in the salt marsh 13 source of OC at a depth of 4 m. The δ Corg Se~ovlje near the inflow of River Dragonja. The origin of organic matter in Holocene sediments in the Bay of Koper (Gulf of Trieste, ... 185

Figure 4. Vertical proﬁles of lithology, grain size, carbonate content (Ogorelec et al., 1987), organic 13 15 carbon (Corg), δ Corg , total nitrogen (TN) (Faganeli et al., 1991), with δ N values and C/N ratios in the sediment core MK-6.

This was characterized by the changing of the other hand, in the core GT2, collected the carbon and nitrogen isotopic compo- near the River So~a/Isonzo outflow, varia- sitions of OC at depth intervals of 6–8 m, tions of terrestrial OC are similar to those 8–14 m, 16–24 m (Ogrinc et al., 2005). in the upper part of the V-3 core, indicating However, the presence of terrestrial OC in variable inputs of allochthonous material the upper part of core V-3 is more abundant to sediment relative to in situ production than in sediments of the core V-6, indicating (Ogrinc et al., 2005). that the River Rižana has a greater influence on sediments than the River Dragonja Conclusions with lower mean discharge. In addition, the 13 15 changes in δ Corg and δ N values observed in Due to their tight isotope-environment the upper part of the core V-6 are compara- relation, carbon and nitrogen stable isotopes ble to those obtained in the core V-5, but the were used to draw a more detailed picture C/N ratios are different. The data indicate of the changes occurring in the sediments of changing variations in organic δ13C values the Bay of Koper during the Holocene. The affected by marine algae production and/or considerable spread in the distribution of δ15N 13 microphyte presence in the sediments. On vs. δ Corg values suggests that C and N were of 186 Nives Ogrinc, Jadran Faganeli, Bojan Ogorelec & Branko êrmelj

13 15 Figure 5. Vertical profiles of lithology, organic carbon (Corg), δ Corg, total nitrogen (TN), δ N values and C/N ratios in the sediment core V-5. different origin and that additional processes tion of carbon and nitrogen indicate that influence δ15N values to a greater extent than most of the sediment OC was from marine 13 δ Corg. Variations in parameters determined algae, while at some depths the influence of in the depth profiles of the V-3, MK-6 and OC derived from microphytes was observed, 13 V-5 cores show variable inputs of alloch- as is evident from δ Corg values ranging from thonous material relative to in situ biologi- − 17.9 to − 19.8 ‰ and high C/N ratios of cal production in the upper, marine part of > 20. There are significant changes in the the core. The marine sediments in core V-3 source of OC at depths of 15 m, 19 m and 10 were additionally influenced by transport of m in V-3, MK-6 and V-5 cores respectively, terrestrial OC by River Rižana and OC origi- indicating fluvial depositional environment. nating mainly from algal production was According to stable isotope mass balance observed only at the depth of 9 and 10 m. the terrestrial supply was estimated to con- The MK-6 and V-5 sedimentary OC showed tribute up to 70 to 100 % of the OC. In the a different pattern. The isotopic composi- fluvial sedimentation the highest δ15N values The origin of organic matter in Holocene sediments in the Bay of Koper (Gulf of Trieste, ... 187

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