Indications of low macrobenthic activity in the deep sediments of the eastern Mediterranean Sea

Abstract

SUMMARY:The fluxes and budget of organic matter from the oligotrophic surface waters of the eastern Mediterranean to the deep waters are poorly known, and little information is available on past and present macrobenthic activity on the sea floor. Evidence of macrobenthic activity can be direct, through recovery of living organisms or their autochthonous skeletal remains, or indirect, through bioturbation and trace fossils. The evidence of biological activity in deep eastern Mediterranean sediments has been evaluated and compared through 210 Pb profiles from box-cores and study of dredge samples from sites on Medina Rise (1374 m water depth), the Messina Abyssal Plain (4135 m) and several sites along the Mediterranean Ridge, SW and S of Crete (1783 to 3655 m). All these sites are remote from the continental shelves, so the biological benthic activity is expected to depend primarily on primary production from surface waters. The results show that present-day macrobenthos and trace fossils are generally scarce, especially at depths >2500 m. This observation is supported by surface sediment 210 Pb excess distributions that show a surface mixed layer (SML) <6 cm deep in cores from water depths <2500 m and only 2-3 cm at greater depths, and estimated bioturbation intensities similarly fall off with water depth. These values are much lower than the average SML of ~10 cm in marine sediments elsewhere. It is unlikely that the present-day primary production in the eastern Mediterranean can sustain any structured macrobenthic activity at >2500 m. The historical layer of some box-cores and the Pleistocene hardgrounds collected in the Cleft area (Mediterranean Ridge) do, however, record a macrobenthic activity that is apparently more intense than at present, which may be related to higher primary production of the Pleistocene glacial intervals. In contrast with most areas of the present-day deep eastern Mediterranean which depend on surface primary production based on photosynthesis, a relatively dense and diversified macrobenthic community based on chemosynthesis has been recognised at depths >1100 m on the Napoli Dome mud volcano in the Olimpi area, and on the Kazan and other mud volcanoes in the Anaximander Mountains.Key words: deep macrobenthos, eastern Mediterranean, 210 Pb profiles, surface mixed layer, trace fossils, chemosynthesis, sapropels, Pleistocene. RESUMEN: INDICACIONES DE BAJA ACTIVIDAD EN EL MACROBENTOS DE SEDIMENTOS PROFUNDOS EN EL MEDITERRÁNEO ORIEN-TAL. -En el Mediterráneo oriental los flujos de materia orgánica desde las aguas superficiales oligotróficas hasta las aguas profundas son poco conocidos y en general hay poca información sobre la actividad del macrobentos en el fondo del mar. La evidencia de la actividad macrobentónica puede ser directa, a través de los organismos vivos o de los restos de esqueletos autóctonos, o indirecta, a través de la bioturbación y fósiles. La evidencia de la actividad biológica en los sedimentos profundos del Mediterráneo oriental se ha evaluado y comparado por medio de ...

A quantitative reconstruction of organic matter and nutrient diagenesis in Mediterranean Sea sediments over the Holocene

“…These values are comparable to other pelagic eastern Mediterranean sites (Basso et al, 2004), but are small compared to non-Mediterranean oceanic sites (cf. Boudreau, 1994).…”

“…Boudreau, 1994). This is attributed to the scarcity of macrobenthos in the deep Mediterranean; it has been suggested that meiofauna are responsible for the majority of particle mixing that is observed (Basso et al, 2004). This meagre benthic community is likely the result of a low export of organic matter from oligotrophic surface waters.…”

“…3). At site SL114, the depth and intensity of bioturbation are relatively small at 2 cm and 5 cm 2 kyr À1 , respectively (Basso et al, 2004), and consequently have only a very minor impact on sapropel oxidation. The rate of aerobic organic matter decomposition also has a negligible influence on sapropel oxidation, because, as seen in the preceding section, organic matter remineralisation is for the most part limited by oxygen availability.…”

“…Bioturbation parameters for the model are taken from Basso et al (2004) who estimated the mixed layer depth and biodiffusion coefficient at SL114 by means of the radioisotope tracer 210 Pb, determining values of 2 cm and 5 cm 2 kyr À1 , respectively. These values are comparable to other pelagic eastern Mediterranean sites (Basso et al, 2004), but are small compared to non-Mediterranean oceanic sites (cf.…”

“…The initial goal of the model is to verify that the current conceptualisation of sapropel formation and oxidation is indeed quantitatively coherent, which is achieved by comparing model output and published pore water and sediment data. Furthermore, since the vast majority of model parameters are known either from the modelled dataset (i.e., Slomp et al, 2002) or from other literature (e.g., Basso et al, 2004), any parameters that cannot be measured directly can be inferred by fitting the model to observed sediment profiles. Following successful groundtruthing, a sensitivity analysis is undertaken to examine the influence that various environmental parameters exert on the preservation and remineralisation of sapropels.…”