engleski

Evaluating the impact of Cladocora caespitosa association in the taphonomic condition of foraminiferal assemblage from the Veliko jezero sediments (Mljet Is., Adriatic Sea)

The taphonomic conditions of foraminiferal tests from shallow-water environments are the result of multiple taphonomic processes caused by the age of the sediments, sedimentary processes, depositional environment and burial history. The most important taphonomic processes affecting the composition of shallow, benthic foraminiferal assemblages are transport and destruction of tests. To evaluate these effects, we analyzed surface sediment samples collected at the Cladocora caespitosa bank in Veliko jezero, Island of Mljet, Adriatic Sea. The C. caespitosa bank ranges from depths of 6–18 m and covers an area of 650 m2, forming one of the largest banks of C. caespitosa found in the Mediterranean Sea. Veliko jezero is a karstic depression filled with saline water, connected to the open sea through a shallow, narrow channel, characterized by microtidal amplitudes and restricted antropogenic influence, while meteorological effects (surge, caused by wind and barometric pressure) are rare. Hand samples were collected by scuba diving at different water depths. In 2012 samples were recovered from 5 sites (7, 11, 12, 14 and 16 m of water depth ; 420 46 0'N, 170 22 8'E), while in 2005, samples were taken only at 16 m water depth. Grain size analysis of the sediments (Folk, 1954) showed that the structure of the samples at 7, 11 and 12 m mostly consist of sand while samples at 14 and 16 m are classified as slightly gravelly sand. The total foraminiferal assemblages (including both living and dead) were assessed, following standard procedure for foraminiferal analysis. The studied foraminiferal assemblages are low to moderately diverse. Shannon-Wiener index values obtained in this study are consistent with those typical of marginal marine environments (Murray, 2006). All samples from 2012 are characterized by dominance of three genera: Peneroplis (P. pertosus, P. planatus making up from 51 to 64% of the total assemblage), Elphidium (E. crispum varying from 18 to 30% of the total assemblage) and Quinqueloculina (reaching a maximum proportion of 12%). The dominant taphonomic characteristics of the assemblages include high fragmentation, high abundance of coloured tests (reworked) and low corrosion. According to taphonomic modifications, all types are classified into six groups: pristine tests, tests with the youngest whorl exfoliated, broken tests (a great portion of test is missing, but genus is still possible to define), tests with minor physical damages on the last chamber wall, abraded and coloured tests (yellowish or blackish). The Elphidium crispum population has a great proportion of pristine tests in all studied samples except from samples collected at 12 m. Broken tests are most common at the shallowest depth and coloured tests were very rare. By contrast, the tests of representatives of Peneroplis are coloured, either yellowish or blackish. The blackish tests dominate at shallower samples (57% of the population at sample from 7 m water depth), while yellowish tests are more common at greater depths (reaching up to 43% of the population in samples from 12 m of water depth). A low proportion of tests are well preserved. Broken and coloured tests of Quinqueloculina sp. show uniform distribution at all studied samples. The assemblage collected in 2005 at 16 m depth, apart for having lower species richness and diversity indices, shows a better preserved foraminiferal assemblage. In this well preserved assemblage, only broken Elphidium tests have been recorded. Our analysis shows a complex relationship between taphonomic modification of tests and water depth. Well preserved Elphidium tests representing about 80% of the population are found in the deepest sample (gravely sands) and in samples from 12 m water depth abraded tests prevail (61%). Strong domination of peneroplids seems to be related to Cladocora rubbly substrate, reflecting biological preferences of species. Robust tests of P. pertusus are abundant in deeper settings down to 16 m depth, and flat forms of P. planatus prefer shallow water settings. Clinging to small filamentous macroalgae, in almost vertical position with the aperture face turned toward algal filaments, these “purple gardeners” are successfully hiding from strong wave breakers as well as dwelling in geographically confined areas where bank construction offers somewhat protected areas. The studied assemblages are para-autochthonous, and taphonomic modifications observed are transport of tests from life position due to storms and tidal currents. Comparison with the previous study, conducted in 2005, revealed that along with expansion of the bank, the overall foraminiferal diversity increased, arenaceous (Textularia sp., Ammodicus sp.) and planktonic species (making up to 25% of the total assemblage) disappeared, and peneroplids flourished and dominated the assemblage.

Taphonomy; Forminifera; Cladocora bank; Mljet Island

Rad je bio predstavljen kao predavanje na kongresu o tafonomiji i fosilizaciji, 7th International Meeting on Taphonomy and Fossilization, Taphos 2014 koji je održan u Ferrari od 10.-13.09.2014.