Alessandra Negri

Integrated stratigraphy and astrochronology of the Messinian GSSP at Oued Akrech (Atlantic Morocco)

A much improved high-resolution integrated stratigraphy (calcareous plankton biostratigraphy, magnetostratigraphy, cyclostratigraphy) is presented for the classic section of Oued Akrech (Atlantic Morocco) straddling the Tortonian^Messinian boundary. Magnetobiostratigraphic correlations with time-equivalent and astronomically dated sections in the Mediterranean indicate that cyclic alternations of indurated light beige coloured marls and softer, more clayey and reddish coloured marls are dominantly precession-controlled. Characteristic sedimentary cycle patterns, in particular those reflecting precession^obliquity interference, allow for one possible tuning, thus providing accurate astronomical ages for cycles, calcareous plankton events and magnetic reversals. The tuning further indicates that the reddish layers are the equivalent of sapropels in the Mediterranean. The Messinian Global boundary Stratotype Section and Point (GSSP) has recently been formally defined at the base of the reddish layer of cycle No. 15 in section Oued Akrech. This level coincides closely with the first regular occurrence of the Globorotalia miotumida group and is astronomically dated at 7.251 Ma. The global correlation potential is guaranteed by the straightforward calibration of the Oued Akrech magnetostratigraphy to the geomagnetic polarity time scale, locating the GSSP within C3Br.1r. In the marine realm the calcareous nannofossil genus Amaurolithus provides a series of extremely useful events to delimit the boundary on a global scale. The astronomical tuning guarantees a direct first-order calibration of the Messinian GSSP to the standard geological time scale once, as anticipated, the late Miocene part of the astronomical time scale has been incorporated. fl 2000 Elsevier Science B.V. All rights reserved.

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.

Calcareous nannofossil paleoecology in the sapropel S1 of the eastern Ionian sea: paleoceanographic implications

A high-resolution study performed by means of calcareous nannofossils on two box cores located in the South Eastern Ionian basin allowed recognition of fluctuations in the assemblage, which can be related to the climatic and paleoceanographic changes that occurred during the deposition of the Holocene Sapropel S1. This study reports a decrease of the coccolith abundance and of the genus Reticulofenestra in the S1 sapropel interval considered as whole with its Mn and Fe oxidized intervals. Florisphaera profunda shows an increase at the base of the dark Corg rich interval, while Helicosphaera carteri and Rhabdosphaera claviger show an increase in abundance within the Fe enriched interval of the oxidized sapropel. Braarudosphaera bigelowii is very rare in the interval studied as a whole, however, it shows a small but significant increase in abundance in the Mn enriched interval of the oxidized sapropel. Sapropel deposition is probably related to important climatic changes that influenced the abundance of the various nannofossil species as a function of their paleoecological preferences. Our results indicate that three different paleoceanographic phases exist in S1, suggesting eutrophication at the beginning of S1 deposition, followed by an intermediate stage characterized by nutrient depletion and, finally, by stagnation.

The Monte del Casino section (Northern Apennines, Italy) - A potential Tortonian/Messinian boundary stratotype

Abstract Results are presented from a high-resolution integrated stratigraphic study of the Monte del Casino section, which is considered as a candidate boundary stratotype section for the Tortonian/Messinian ( T M ) boundary. The section yields a good to excellent cyclostratigraphy, tephrastratigraphy and planktonic foraminiferal and nannofossil biostratigraphy. It can be correlated in detail to other Mediterranean sections using cyclostratigraphic patterns in combination with the planktonic foraminiferal biostratigraphy. These correlations indicate that the succession of the Monte del Casino composite section is continuous in the T M boundary interval and that all sedimentary cycles, volcanic ash layers and bio-events can be accurately dated by tuning the sedimentary cycles to astronomical target curves. The quality of the paleomagnetic signal varies considerably throughout the section. A reliable magnetostratigraphy cannot be established for the T M boundary interval because of a secondary magnetisation carried by iron sulphides. Cyclostratigraphic correlations indicate that discrepancies exist between the position of the reversals recorded here and those found in earlier magnetostratigraphic studies. Comparison of 40 Ar 39 Ar ages of ash layers with their astronomical ages, show that biotite ages give a good approximation of the age of the section, but are not suitable for high-resolution chronology. Despite these short-comings the Monte del Casino section remains a candidate to define the T M boundary, but its qualities should be critically weighted against those of other sections.

Unraveling tectonic and climatic controls on synorogenic growth strata (Northern Apennines, Italy)

We develop a new high-resolution stratigraphic age model to unravel the contributions of tectonic and climatic processes on early to late Pleistocene synorogenic growth strata. We capitalize on excellent, continuous exposures along the flank of the Po foreland in northern Italy to elucidate hydrologic, geomorphic, and sedimentologic processes that are regularly attributed to, but rarely proven to be caused by, glacial-interglacial climatic changes and unsteady rock uplift. We perform our analysis on the Enza section, a succession of marine and terrestrial strata exposed along the Enza River, between Parma and Reggio Emilia, northern Italy. Bedding in the Enza section displays synorogenic growth strata geometry, with bedding dips that range from 2° to 55°, that becomes progressively shallower upsection. We develop an age model that incorporates biostratigraphy, magnetostratigraphy, rock-magnetic cyclostratigraphy, cosmogenic radionuclide burial dating, and optically stimulated luminescence dating and shows that the Enza section spans the interval between 0.04 and 1.65 Ma. Furthermore, the age model pins the time of deposition for several lithostratigraphic units of regional significance and shows that sediment accumulation was unsteady, ranging from 14–31 cm/k.y. in the marine part of the section to 5–362 cm/k.y. in the overlying littoral and terrestrial part of the section. Unsteady deposition is most pronounced in the terrestrial deposits where thick fluvial gravel packages accumulated in short (∼10–15 k.y.) time periods that coincide with Quaternary glacial intervals. There is direct evidence for a dominant tectonic control in the older, marine part of the section. Here, sediment accumulation rates on the limb of the fold growing along this portion of the Northern Apennine mountain front show that between 1.07 and 1.65 Ma, repetitive progradation of neritic sand units directly followed pulses of rapid, punctuated uplift. In contrast, the cyclic terrestrial facies variations in the Enza section reveal that once the section became emergent at ca. 1 Ma and uplift slowed, climate was the dominant control on sediment production and deposition.

Late Pleistocene–Holocene planktonic assemblages in three box-cores from the Mediterranean Ridge area (west–southwest of Crete): palaeoecological and palaeoceanographic reconstruction of sapropel S1 interval

Planktonic foraminiferal and calcareous nannofossil assemblages of three box-cores from the western–central part of the Mediterranean Ridge area were investigated and correlated. In particular we focused on the most significant faunal and floral signals recorded before, during and after sapropel S1 deposition. The interval preceding S1 is characterised by an increasing trend of the two planktonic foraminiferal species, Globorotalia inflata and Truncorotalia truncatulinoides, that are usually related to a well developed cold and deep mixed layer. This change is supported also by the rarity of warm water coccolithophorid species and by the presence of the deep dwelling species Florisphaera profunda. The beginning of S1 is marked by the disappearance of G. inflata and T. truncatulinoides and by the significant increase in abundance of Globigerinoides ruber, especially the rosea variety usually considered indicative of warmer conditions. Warm water species increase also within the coccolithophorid assemblage, while the upwelling species Reticulofenestra spp. suddenly decreases and remains low across the whole sapropel interval. The sedimentary expression of the end of anoxia is the upper boundary of the oxidised level observed above S1 in the three box-cores. Across this boundary we detect a small but well defined increase of Braarudosphaera bigelowii, the decrease of warm water coccolithophorid species and the increase of Reticulofenestra spp. This boundary is marked also by the reoccurrence of the planktonic foraminiferal species G. inflata and Neogloboquadrina pachyderma dextral and by an evident decrease of G. ruber var. rosea, indicating the end of the Holocene Climatic Optimum and the beginning of a cold and wet phase, probably corresponding to the Atlantic–Subboreal continental transition.

Calcareous nannofossil biostratigraphy of the M. del Casino section (northern Apennines, Italy) and paleoceanographic conditions at times of Late Miocene sapropel formation

A detailed quantitative calcareous nannofossil analysis has been performed on 138 samples from the astronomically dated Monte del Casino section with the aim to identify and precisely date the most important calcareous nannofossil events across the Tortonian=Messinian boundary in the Mediterranean, and to unravel paleoceanographic conditions at times of sapropel formation during the Late Miocene. From the biostratigraphic perspective, the genus Amaurolithus provides three successive first occurrences (FOs): A. primus, A .c f.amplificusand A. delicatus, dated at 7.446, 7.434 and 7.226 Ma, respectively. Other bioevents include the base and top of the ‘small reticulofenestrids’ Acme, dated at 7.644 and 6.697 Ma, and the FO, FCO and LO of R. rotaria, dated at 7.405, 7.226 and 6.771 Ma. These events appear to be useful in improving biostratigraphic resolution in the Tortonian‐Messinian boundary interval, at least for the Mediterranean. Quantitative analysis revealed changes in the calcareous nannofossil assemblage associated with the sapropels. The observed fluctuations suggest a single mechanism for sapropel formation in the Mediterranean during the late Neogene. Sapropels are characterized by a decrease in the total number of coccoliths, interpreted mainly as a reduction in calcareous nannofossil production due to increased siliceous plankton production during spring blooms; and an increase in reworked specimens, interpreted to reflect enhanced continental input via river run-off. An increase in abundance of the genus Rhabdosphaeracan be explained by opportunistic behavior at the end of the spring bloom when nutrient levels start to become impoverished. As far as sea surface water temperature indicators are concerned, warm water D. pentaradiatus shows positive fluctuations in sapropels while cooler water D. intercalaris and C. pelagicus show negative fluctuations.

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.

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.

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.