Caterina Morigi

Deep-Sea Biodiversity in the Mediterranean Sea: The Known, the Unknown, and the Unknowable

Deep-sea ecosystems represent the largest biome of the global biosphere, but knowledge of their biodiversity is still scant. The Mediterranean basin has been proposed as a hot spot of terrestrial and coastal marine biodiversity but has been supposed to be impoverished of deep-sea species richness. We summarized all available information on benthic biodiversity (Prokaryotes, Foraminifera, Meiofauna, Macrofauna, and Megafauna) in different deep-sea ecosystems of the Mediterranean Sea (200 to more than 4,000 m depth), including open slopes, deep basins, canyons, cold seeps, seamounts, deep-water corals and deep-hypersaline anoxic basins and analyzed overall longitudinal and bathymetric patterns. We show that in contrast to what was expected from the sharp decrease in organic carbon fluxes and reduced faunal abundance, the deep-sea biodiversity of both the eastern and the western basins of the Mediterranean Sea is similarly high. All of the biodiversity components, except Bacteria and Archaea, displayed a decreasing pattern with increasing water depth, but to a different extent for each component. Unlike patterns observed for faunal abundance, highest negative values of the slopes of the biodiversity patterns were observed for Meiofauna, followed by Macrofauna and Megafauna. Comparison of the biodiversity associated with open slopes, deep basins, canyons, and deep-water corals showed that the deep basins were the least diverse. Rarefaction curves allowed us to estimate the expected number of species for each benthic component in different bathymetric ranges. A large fraction of exclusive species was associated with each specific habitat or ecosystem. Thus, each deep-sea ecosystem contributes significantly to overall biodiversity. From theoretical extrapolations we estimate that the overall deep-sea Mediterranean biodiversity (excluding prokaryotes) reaches approximately 2805 species of which about 66% is still undiscovered. Among the biotic components investigated (Prokaryotes excluded), most of the unknown species are within the phylum Nematoda, followed by Foraminifera, but an important fraction of macrofaunal and megafaunal species also remains unknown. Data reported here provide new insights into the patterns of biodiversity in the deep-sea Mediterranean and new clues for future investigations aimed at identifying the factors controlling and threatening deep-sea biodiversity.

BENTHIC FORAMINIFERAL FAUNAS IN SURFACE SEDIMENTS OFF NW AFRICA: RELATIONSHIP WITH ORGANIC FLUX TO THE OCEAN FLOOR

Benthic foraminiferal thanatocoenoses were studied in 29 sediment surface samples off NW Africa, between 19° and 27°N, and from water depths between 506 and 3314 m. The results were compared with estimated values of the downward organic flux and measured bottom water oxygen concentrations. Although the latter 2 parameters show a strong negative correlation in the study area, we argue that the organic flux is the main controlling ecological factor. On the basis of an R-mode multivariate statistical analysis, 6 species clusters are recognized, which are separated geographically by differences in organic flux and water depth. The combined sample scores on the first 2 principal component axes show a strong positive correlation with the estimated downward organic flux (R 2 = 0.83). It is suggested that this relationship can be used on a local scale as a proxy for paleo-export production, as long as bottom water oxygenation does not fall below a critical level.

Foraminiferal ecozones, a high resolution proxy for the late Quaternary biochronology in the central Mediterranean Sea

The planktic foraminiferal distribution identified in 60 cores collected in different basins of the Mediterranean Sea allowed to establish an ecostratigraphical scheme which provides a very important tool for the biochronological subdivision of the uppermost Quaternary. We identified a succession of ten ecozones during the last 23 ka in the Tyrrhenian basin and eight ecozones in the Adriatic Sea during the last 15 ka. The ecozones boundaries have been calibrated by 14C AMS radiometric data and by the stable oxygen isotope record. The chronological framework defined by the successive bioevents shows a very high resolution (millenary scale) and evidences that the changes in the planktic microfauna occurred more or less synchronously throughout the central Mediterranean Sea. Differences due to different oceanographic settings of the basins do not affect the general distributional pattern of planktic foraminifera.

Mid-late Pleistocene glacimarine sedimentary processes of a high-latitude, deep-sea sediment drift (Antarctic Peninsula Pacific margin)

The effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the Antarctic Peninsula Pacific margin. Nineteen sediment cores were visually described, logged for magnetic susceptibility, and X-radiographed. About 1000 analyses were performed for grain size, clay minerals and biostratigraphy (foraminifera, nannofossils and diatoms). Four sediment types associated with distinct sedimentary processes are recognised based on textural/compositional analysis. (1) Hemipelagic mud forms the bulk of the interglacial sediment, and accumulated from the pelagic settling of bioclasts and ice-rafted/wind-transported detritus. (2) Terrigenous mud forms the bulk of the glacial sediment, and accumulated from a combination of sedimentary processes including turbidity currents, turbid plumes, and bottom current reworking of nepheloid layers. (3) Silty deposits occurring as laminated layers and lenses, represent the lateral spillout of low-density turbidity currents. (4) Lastly, glacial/interglacial gravelly mud layers derive from settling of ice-rafted detritus. Five depositional settings are interpreted within sediment Drift 7, each characterised by the dominance/interaction of one or several depositional processes. The repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages (glaciation, glacial, deglaciation, and interglacial), each one characterised by a distinctive clay mineral assemblage and bioclastic content. Variations in clay mineral assemblage within interglacial stage 5 (core SED-06) suggest minor colder climatic fluctuations, possibly correlatable with substages 5a to 5e.

Coccolithophorid ecostratigraphy and multi-proxy paleoceanographic reconstruction in the Southern Adriatic Sea during the last deglacial time (Core AD91-17)

A very detailed environmental history of the last deglaciation and the Holocene is recorded in a high sedimentation rate core collected in the Southern Adriatic Sea (Core AD91-17). The stratigraphic framework, based on radiocarbon dating (14C AMS) and the oxygen isotope record, allows recognition of the paleoceanographic changes of the last 16 200 years. Fluctuations within the coccolithophorid assemblage identify five intervals that can be correlated to major changes in the planktonic foraminifera records. Recognition of the same zonation in the eastern Mediterranean Sea suggests that they are truly basin-wide ecozones rather than local events. Interpretation of the paleoceanographic meaning of these coccolithophorid ecozones, together with planktonic and benthic foraminifera, dinocyst, pollen, magnetic parameters, oxygen isotopes and alkenone unsaturation indexes, outlines fluctuations related to different paleoclimatic phases. In particular, from 16 200 to about 11 670 yr BPnc all proxies register cold conditions with a well-ventilated sea bottom. A gradual sea-surface temperature (SST) increase characterized the period between 11 670 and 10 800 yr BPnc, followed by a slight cooling coincident with the beginning of the Younger Dryas. Between 8650 and 6560 yr BPnc, corresponding to sapropel S1 formation, all proxies register a transition to warmer climate. Sea-surface productivity reached maximum values, while evidence for development of low salinity superficial waters and of a deep chlorophyll maximum is observed. The multi-proxy analysis suggests the presence of at least three phases in the S1 sapropel itself. The first part of the sapropel is characterized by high nutrient availability, warm stratified waters and severe bottom anoxia. Between 7650 and 7500 yr BPnc, corresponding to an interruption of the sapropel, we observe a rapid reoxygenation at the seafloor when SST warmed. Just after the sapropel interruption, and particularly between 7400 and 7250 yr BPnc, a slight climatic deterioration, an increase of salinity and a decrease of runoff are observed. At the top part of the sapropel, eutrophic environments, a well-stratified euphotic zone and dysoxic conditions at the bottom are identified. From 6560 to 5080 yr BPnc, conditions became more oligotrophic and SST reached a maximum, while a slight increase of superficial water salinity may suggest the end of stratified waters and the beginning of water column mixing. Finally, between 5080 and 2240 yr BPnc all proxies indicate warm and normal salinity waters. The bottom environment returned to normal oxygenated conditions.

Are productivity and stratification important to sapropel deposition? Microfossil evidence from late Pliocene insolation cycle 180 at Vrica, Calabria

We present the results of a micropaleontological study performed on the sapropel sequence associated with insolation cycle 180 from the Plio–Pleistocene Vrica sequence (Calabria, Italy). We performed a high-resolution study on the 3.38-m-thick layer c from a core drilled close to the classical outcrop section in which we analyze fluctuations in the abundance and composition of calcareous nannofossils and planktic and benthic foraminifera. Changes in the fossil assemblages reveal at least three major paleoenvironmental phases in layer c. The base of the sapropel contains an abrupt decrease in benthic fauna that continues through all of layer c. It also has an increase of the coccolithophorids species Coccolithus pelagicus. Planktic foraminifera show at the same depth a peak of the cold species Globorotalia scitula. These changes are followed by decreases in the carbonate preservation index and in abundances of Globigerinita glutinata, Globigerinita uvula and Neogloboquadrina pachyderma (sinistral), which suggest cold and highly productive upwelling waters. A short interval in the middle of the sapropel is characterized by low values of C. pelagicus, a fluctuating increase of Pseudoemiliania lacunosa and among the foraminifera an increase of Globigerinoides ruber together with the presence (although decreased) of G. glutinata, G. uvula and N. pachyderma (sinistral). We interpret these features as suggesting high seasonality with warm stratified and probably oligotrophic waters during summer and relatively cold conditions during winter. Finally, the topmost interval of the Vrica layer c exhibits the re-appearance of P. lacunosa together with abundant siliceous phytoplankton. Planktic microfauna show the disappearance of the cold species G. glutinata, G. uvula and N. pachyderma (sinistral). Thus this interval appears to be characterized by warmer temperature. The transition from the laminated to the massive sediment displays a sequence of events, including a decrease of the carbonate preservation index and peaks of Globorotalia inflata and G. scitula, suggesting again upwelling and mixing of the whole water column and, thus, transition to the oxygenated conditions characterizing the massive layer. Neither increased productivity nor stratification appear to characterize the whole sapropel interval, which is, however, always dysoxic.

Biostratigraphic characterization and Quaternary microfossil palaeoecology in sediment drifts west of the Antarctic Peninsula – implications for cyclic glacial–interglacial deposition

Abstract The SEDANO Project recovered 19 gravity cores on sediment drifts from the Pacific continental margin of the Antarctic Peninsula [Camerlenghi et al. (1997a) High-resolution terrigenous sedimentary record of the sediment drifts on the Antarctic Peninsula Pacific margin. In: Ricci, C.A. (Ed.), The Antarctic region. Museo Nazionale dell’Antartide, Siena, pp. 705–710]. Fifteen cores were sampled with the aim of developing an integrated biostratigraphy and palaeoecology based on calcareous nannofossils, diatoms, planktonic and benthic foraminifera, framed in a depositional process reconstruction. Barren gray laminated and brown bioturbated hemipelagic sediments characterize glacial and interglacial cycles, respectively. Analyses from both intervals allow comparison between microfossil occurrence in glacial and interglacial cycles. The unit boundaries were drawn more accurately by means of the microfossil distribution. On the basis of micropalaeontological and sedimentological evidence, Interglacial Unit C is correlated to Oxygen Isotope Stage 5, thus dating the unit boundaries at 127 and 70 ka. Peaks in diatom abundance correlate well with interglacial units and indicate high productivity and an open ocean environment. A calcareous nannofossil cold-taxa association is present in most cores examined, and its consistent distribution within Interglacial Unit C indicates key environmental relationships. The occurrence of calcareous nannofossils has been related to temperature tolerance, sea-ice cover reduction, nutrient availability, and factors limiting primary productivity. Our results confirm that coccolithophorids occurred at southern high latitudes, in the western marginal basins of the Antarctic Peninsula, during short periods of the Late Quaternary. A foraminiferal assemblage, made up of sinistral Neogloboquadrina pachyderma and few benthics, occurs only in interglacial units. Correlation of microfossil occurrences with climatic cycles adds information on their palaeoecology and palaeoproductivity in the southern high latitudes.

Survival of benthic foraminifera under hypoxic conditions: results of an experimental study using the CellTracker Green method.

We present results of an experimental study, in which benthic foraminiferal faunas have been kept under strongly hypoxic conditions. Sixteen short sediment cores from a 35m deep site in the Adriatic Sea were incubated for a maximum of 69days. Some of the cores were air-bubbled and remained well oxygenated throughout the experiment. The other cores were bubbled with nitrogen; the overlying waters of these cores became strongly hypoxic, whereas the sediment remained virtually without oxygen. Live foraminifera have been inventoried with the CellTracker Green method. Our results show that all dominant taxa survive strongly hypoxic conditions. Nouria polymorphinoides and Nonionella turgida show a clear tendency to move to the sediment surface in the nitrogen-bubbled cores, whereas Bulimina spp. and Eggerella scabra do not show such a migrational response. We suggest that this is a response to the concentration of nutritional resources at the sediment-water interface.

A possible record of the Younger Dryas event in deep-sea sediments of the Southern Ocean (Pacific sector)

Abstract The oxygen isotope record combined with radiocarbon dating from two deep-sea cores collected along a transect between New Zealand and the Ross Sea are used to establish a reliable chronostratigraphy for the last 14 kyr. After an integrated geochemical and micropaleontological analysis in this timeframe we detected a cooling interval dated between 12.5 cal kyr BP and 11.4 cal kyr BP. The age control suggests that this event started 1.5 kyr after the onset of the Antarctic Cold Reversal previously observed in several Antarctic ice cores. We infer that the observed cool event corresponds to the Younger Dryas event defined in Northern Europe. This suggests that climate change recorded in this sector of the Southern Hemisphere still shows some synchronicity with Northern Hemisphere variations and that the decoupling of climate change between the two hemispheres likely occurred south of the Polar Front.

Soft-shelled benthic foraminifera from a hadal site (7800 m water depth) in the Atacama Trench (SE Pacific): preliminary observations

Soft-shelled foraminifera (organic-walled allogromiids and agglutinated saccamminids) are an important component of the deep-sea meiofauna. Although these largely monothalamous taxa are common at bathyal and abyssal sites in almost all oceans, there are only two records from hadal depths. Here we report the occurrence of numerous allogromiids and saccamminids in a sample collected at 7800 m water depth in the Atacama Trench. The >20 μm fraction of the core sample (0–6 cm layer) yielded a total of 546 soft-walled specimens, the vast majority of them Rose Bengal stained, belonging to 20 morphospecies. Most specimens were allogromiids (82.0%), followed by saccamminids (11.0%) and psammosphaerids (6.0%). Allogromiids, particularly Nodellum- and Resigella-like forms, were responsible for a distinct peak around 120–160 μm in the size distribution, while the spherical Allogromiid sp. 1 dominated the larger-size classes. This sample provides further evidence for the widespread occurrence of soft-walled monothalamous foraminifera in marine habitats. A form resembling Resigella is common in the Atacama Trench sample but has not been observed at abyssal sites in the Pacific Ocean or Atlantic Ocean.