RICERCA ITALIANA     

Palaeoclimatic forcing on building organism communities, carbonate productivity and depositional systems of some Italian Meso-Cenozoic shelf deposits.

Università degli Studi di Ferrara

Abstract

The forecast amplitude of the present-day atmospheric change is triggering an increasing interest for a longer time perspective, that only the palaeoclimatic study can provide. Only the stratigraphy can indeed understand an extended temporal record, quite relevant for both the interpretation of the modern modifications and the understanding of the ancient geological history. This research proposal is aimed at improving the palaeoclimatic interpretation of Tertiary and Middle Triassic times, through the multidisciplinary, co-ordinated analysis of selected carbonate platform cases history from Italy. The calcareous production supporting the platforms growth is indeed so strongly related to the biological processes that these stratigraphic bodies can be thought as the end product of an ancient ecological dynamics. Due to the strong genetic linkage between climatic fluctuations, depositional, geochemical and biological processes, the research on carbonate platforms needs the interdisciplinary integration of sedimentological and palaeontological analysis, performed by the four involved research teams. The very same carbonate platform outcrops will therefore be analysed from both sedimentological and palaeontological points of view. To make a more comprehensive palaeoclimatic reconstruction, palaeobotanic information preserved within basinal successions and correlative Tertiary continental deposits and palaeosoils will also be investigated. The sedimentological and physical stratigraphy research will be based on the reconstruction of the depositional geometry, and on facies analysis, followed by thin section, and epifluorescence observations. Physical evidence will be integrated by the palaeoecological interpretation of climate sensitive fossil faunae, such as mollusc and coral associations in the Triassic, zooxanthellate corals, and benthic macroforaminifera assemblages in the Tertiary. The palaeoecological interpretation will be based on the statistical analysis of the thanatocenosis and on the dimensional and morphological study of specific taxa. In the Triassic, the research effort will be focused on Anisian and Ladinian platforms from the Southern Alpine Dolomites. The Middle Triassic generally records arid climatic conditions, but evidences of moister conditions are available both in the Anisian and upper Ladinian. The isolated nature of the well outcropping, poorly deformed platforms is valuable for a quantitative estimation of the volumetric depositional rates, within an accurate time framework. The examined units (Mt Rite, Upper Serla, Latemar-Schlern formations) record the first reappearing of buildups following the extinction at the Permo-Triassic boundary, followed by the development of small isolated platforms, and then of high relief, rimmed edifice, affected by a strong pervasive syndepositional cementation. This depositional trend was followed by the evolutionary radiation of corals. An other Triassic carbonate platfor-basin system will be analysed in the Southern Apennines, in northern Calabria. The Mediterranean Tertiary records a globally cooling trend, from the warmest conditions developed near the Paleocene-Eocene boundary, toward the Pleistocene preglacial environments, with a first sharply cooling event at the Eocene-Oligocene Boundary. This trend strongly affected the depositional evolution of the platforms, dominated by macroforaminifera, algae and coral associations. These successions will be studied in the Central Apennines (Maiella), Apulia, and Sardinia. Specific attention will be paid to the production of a coral faunae data set, and its relationships with the palaeoclimatic fluctuations. <<<

Principal Investigator

Alfonso BOSELLINI Università degli Studi di FERRARA

Research Objectives

The forecast amplitude of the present-day climatic change is triggering a new awareness and an increasing interest toward palaeoclimatic modification. The stratigraphy can provide a long temporal record useful both in the interpretation of the modern modifications and to the understanding of the ancient geological history. The research project hereafter proposed is aimed at reconstructing the dynamic relationships between palaeoclimatic fluctuations, carbonate production and depositional dynamics of selected carbonate platform systems form the Meso-Cainozoic of Italy. To achieve a more comprehensive view of the palaeoclimatic record, a few terrigenous and mixed terrigenous carbonate systems will be also analysed. The research is based on the interdisciplinary integration of physical stratigraphy, sedimentology, macro- and micro-palaeontology and selected geochemical analysis, to achieve a somewhat holistic approach to the palaeoenvironmental dynamic reconstruction. The sedimentological analysis will be performed both through field work and microfacies analysis, the palaeontological investigation will be mainly focused on the palaeoecological interpretation of taxa sensitive to environmental fluctuations, such as corals and terrestrial plants. To improve the lateral tracing of palaeoclimatic events, selected intervals will be analysed also from a biostratigraphic point of view (ammonoids, conodonts or planctonic foraminifera). Integrated through workshops involving the whole of the four operational unit researchers.

TRIASSIC
The research work will be aimed at investigating the impact of the climatic fluctuations on the carbonate production, in platform top, margin and slope settings. The Dolomites platforms provide us with quite valuable an opportunity to evaluate volumetric depositional rates, being small isolated systems, developed within a particularly well constrained chronological framework, spectacularly outcropping, and comparatively poorly affected by tectonic deformation. The platform record will be integrated by the palaeoclimatic and chnonostratigraphic information stored within the coeval basinal successions.
The lower Middle Triassic of the Dolomites records the first reappearing of carbonate buildups in the Alpine realm, following the Permo-Triassic boundary mass extinction. This radiation event was matched in the region with the activation of palaeotectonic movements and the development of isolated carbonate edifices. The earliest carbonate platforms of the Dolomites have been recently ascribed to the Monte Rite Fm. We suggest to study this early platform formation in its eastern Dolomites type locality.
The next Anisian depositional sequence frames the Upper Serla Fm, again recording the development of an archipelago of independent carbonate edifices, in the eastern and northern Dolomites. We shall analyse this carbonate system in the Pragser Dolomiten, in the area between Piz da Peres and Lake Braies; emphasis will be focused on the nice Pra de la Vacca outcrops; coeval carbonate systems will be examined also in the southern slope of the Cernera Massif, above Selva di Cadore. This carbonate platform units interfingers with the terrigenous-carbonate basinal succession of the Dont Formation, which provides valuable palaeoclimatological information, through its rich and diversified palaeobotanic and palynological content, suggesting a comparatively moist framework, in sharp contrast with the underlying and overlying units, recording an arid climate.
The thick, high relief carbonate platforms grown during latest Anisian and early Ladinian times, under arid conditions, record a massive carbonate production and a even faster creation of accommodation space, supported by an accelerate tectonic subsidence. The Latemar and the Cernera are particularly suited for an evaluation of the volumetric accumulation rate. Beside studying the platform-margin facies, a reviewing of the slope sediment is important to test the role of the deeper water environments in the carbonate production dynamics. The two platforms record different depositional evolutionary trends, since the Cernera was terminated by an early drowning, giving us a precise estimation of the carbonate production potential and demonstrating that the main carbonate factory was active in shallow water only. A shallow water carbonate factory was indeed particularly sensitive to palaeoclimatic fluctuations. The Latemar Platform was on the contrary able to keep pace with the fast creation of accommodation space and to catch up with the sea level, surviving for a longer time span in very shallow water, euphotic conditions. Both platforms, together with the Marmolada one, escaped the facies destroying dolomitisation so pervasively affecting the surrounding buildups. The good preservation of the primary depositional features will enable the microfacies and geochemical investigation.
A short lived but violent Ladinian volcanic event dramatically affected the environmental framework, generating siliciclastic emerging areas and sharply increasing the basinward input of both continental sediments and nutrients, in the meantime, the region experienced a temporary shift toward moister conditions, as witnessed by the rich palaeobotanic assemblages preserved within basinal successions . The whole of these factors suddenly changed the depositional dynamics of the syn-postvolcanic platforms. We plan to analyse these system in the classical Schlern Plateaux area, where both platformtop, margin and slope facies are preserved, in geometric continuity with basinal sediments (Wengen and St. Cassiano Formations), outcropping in the Alpe di Siusi-Seiser Alm area. These successions record important sedimentological and palaeontological events, such as the reappearance of ooid grains, lacking since the Early Triassic, and the occurrence of Techosmilia-like branchy coral colony in near margin settings. Additional information on the early post-volcanic platforms will be gathered in the Sella and Duerrenstein areas, the latter poorly study, as far as the upper Ladinian interval is concerned.

TERTIARY
The evolution of the Tertiary platforms of Italy was strongly affected by a climatic shift from warm tropical to cooler conditions. The palaeoecological signals recorded by the shallow water palaeontological associations, dominated by macroforaminifera, green algae and corals, can be compared with the sedimentological and some geochemical evidence. Study of the Oligocene and lower Miocene carbonate facies and biota from the Maiella Platform. This area is particularly suitable for the analysis of the foraminifera and coral biota in a reef environment, within an accurate chronological and geometric framework. The Oligo-Miocene of the Apulia Platform is relevant for this research topic for its recording the transition from tropical to temperate conditions, a modification affecting the evolution from highly diversified corallofaunae to monotipic assemblages of Porites, associated with Halimeda bioerms and vermetids trottoirs, whereas the macroforaminfera disappeared. The taxonomic analysis will be mainly aimed at assembling a comprehensive data-set, to be checked on the regional palaeoclimatic evolution. The direct recording of the shallow marine environmental evolution provided by the carbonate platform margin will be integrated by the analysis of coeval, correlative continental deposits, such as the upper Miocene calclithic fan conglomerates of the Gargano Peninsula, preserving their primary geometric relationships with shallow marine deposits. The palaeoclimatic meaning of mature palaeosoils and continental stromatolitic and tufa carbonate mounds from Sardinia will also be analysed, from both sedimentological and geochemical points of view. <<<

First Results

Coordination between the various units and definition of a common schedule;
Construction of a preliminary database based on the previous bio-cronostratigraphical knowledges;
Definition of priorities and methodologies in the approach to the areas and the sections key to study;First acquisition of paleontological data in order to define a high resolution biostratigraphic framework for paleoclimatic signals. Refinig of facies analyses and preliminary outline of depositional geometries of chosen terrigenous-carbonatic systems. First semi-quantitative models of designated areas and of preliminary carbonate productivity estimates.Reconstruction of facies associations and of fossil tanatocenosis: palaeoecological characterization of the analized systems.Organization of the stratigraphic successions analyzed in respect of existing biozonation standards and/or sharpening of existing biozonations. First isotopic curves and their use for high resolution correlations and to give estimations of the productivity and paleo-temperature. Acquisition of geochronological data in the Middle and Upper Triassic successions for temporal quantification of the duration of biozone with the consequent possibility to estimate with great precision the aggradational and progradational rates in various paleoclimatic contexts.Integration of acquired field data with synthetic stratigraphic models.
Integration between acquired data and the sequence stratigraphy framework.
Elaboration of a depositional dynamic model of platforms.Final synthesis of all given sedimentological, biostratigraphical, paleontological data to clarify the answer of depositional systems to climatic pulsations.
For every single temporal interval examined a qualitative and, if possible, quantitative esteem of paleoclimate. A better understanding on the answer of the different community of bioconstructors to climatic forcing and an acknowledgment of the organisms that better recorded the climatic mutations. All it would go therefore linked to high resolution chronostratigraphic scales to the aim to define productivity models. Organization of a convention of international importance where to discuss the results obtained. <<<

Timescale

24 months

National and international background

The scientific attention for the dynamic relationships between palaeoclimatic fluctuation, eustatic variation and carbonate production has been growing through the last 10-20 years, both because of the importance for fossil fuel research and under of the pressure of the present day climatic fluctuations, generating a strong impact on human activity and economics. Good palaeoclimatic information is provided by the integrated sedimentological analysis of carbonate platform and the palaeoecological interpretation of the associated fossil content. The calcareous precipitation supporting the carbonate platforms growth is largely associated with and supported by biological processes, such as the direct biocontrolled growth of skeletal parts or the bioinduced precipitation supported by biological macromolecule. The carbonate sediments are therefore much strongly related to the biological processes to the point that carbonates are said to be born and not made. This intimate association with biological processes makes the carbonate systems much more sensitive to palaeoclimatic fluctuations than their terrigenous counterparts. A carbonate platform can be thus considered as the end product of an ecological system dynamic, and the depositional processes can be therefore understood only within an accurate palaeobiologic and palaeoclimatic framework.

MESOZOIC PALAEOCLIMATE
The available sedimentological, palaeontological and isotopic evidence suggests that during Mesozoic times climate was generally significantly warmer and latitudinally more uniform that the Quaternary ones. The Triassic atmospheric circulation patter was also influenced by the largely different arrangement of the continental masses, characterised by the development of the Pangea, inducing a sharply different precipitation pattern (Parrish, 1993). The Triassic Earth is generally though as dominated by a green-house situation, lacking any significant polar ice accumulation, but nevertheless characterised by a low stand situation, because of the continent-ocean distribution pattern. Two different palaeoclimatic models do exist in the literature on the Triassic environments, a firs one emphasising the role of the monsoon circulation (Robinson 1973, Blodgett, 1988; Kutzbach and Gallimore, 1989; Dubiel et al., 1991; Ash & Creber, 1992; Mutti & Weissert, 1995; Reinhardt and Ricken, 2000), whereas the second one put a greater emphases on the latitudinal belts variations (Tucker & Benton, 1982; Ziegler et al., 1994). The Alpine Middle Triassic is often seen as a transitional interval between the Lower Triassic aridity and somewhat moister conditions, as recorded by the spreading of karstification phenomena and an increased caolinite abundance (Tucker and Benton, 1982; Ziegler et al., 1994). Floral remain are particularly suitable as palaeoclimatic indicators, and sometimes also as palaeorelief markers (e.g. Wolfe, 1995). The palaeoecological interpretation of fossil florae relies on two methodologies, i.e. the comparison with the nearest living relatives and on the morphometric analysis of taxa. The application of the first methods to the Mesozoic taxa is obviously made complex by the large evolutionary modification, but was nevertheless attempted, for examples on ancient ferns (Barnard, 1973); firmer conclusion could derive from quantitative morphometric analysis, particularly on the Cretaceous angiosperms (e.g., Traiser, et al., 2002).

THE IMPACT ON THE CARBONATE PLATFORM DYNAMICS
The climatic fluctuations recorded in the Meso-Cenozoic certainly affected the carbonate production and therefore the platform sedimentary dynamics and depositional geometry. A comparison of the estimated fossil production and accumulation rates with the modern one, under different climatic conditions, looks like a quite interesting one. Significant data on the present-day carbonate production rates are now available, particularly on intertropical warm systems, such as the Bahamas ones. Several models considering the carbonate production as function of the depth (Bosscher and Schlager 1992), as well as the distance from the platform edge (Bosence and Waltham 1990, Bowman and Vail 1999). According to recent publications (Demicco and Hardie 2002) total carbonate production is also function of the residence time of oversaturated waters in respect of calcium carbonate. These estimation on the modern accumulation rates provide high value, since hiatus, post depositional erosion, redistribution and solution processes

THE ESTIMATION OF THE CARBONATE PRODUCTION
Estimation on the carbonate productivity of ancient platforms have also be successfully carried out, following different research lines. Both the vertical aggradation and lateral progradation rates have to be considered (Bosscher and Schlager 1993), but the crucial factor is often the estimation of the difficult estimation of the time gaps developed within the sedimentary bodies. This factor is the main causing factor of the apparently decreasing accumulation rates in the past ("scaling rate" sensu Schlager, 1999). The average completeness of the stratigraphic record for ancient carbonate platform over a 2 Myr time interval is estimate in between 15 an 80%, in the majority of cases probably being nearer to the lower value (e.g.. Burgess e Wright 2003). The preservation potential of carbonate systems have been also related to the climatic framework, the tropical systems being more active during highstand times, more prone to a good stratigraphic record, whereas the temperate platforms are often more active during lowstand times, acting in a more "siliciclastic way". This complex behaviour has to be analysed on systems recording different climatic, palaeoecologic and evolutionary framework. The Triassic of the Dolomites and the Cretaceous and Tertiary of the Apulian Forland successions are particularly suitable for such an analysis. The Triassic examples are particularly interesting, since the carbonate production was mainly active on platform top or at margin settings, but automicrite production was also develloped in slope settings (Keim e Schlager 2001).

TERTIARY
During Tertiary times, significant climatic variations took place in the Mediterranean region, within a generalised cooling trend, leading from the temperature climax, developed by the Palaeocene-Eocene boundary, to the Plio-Pleistocene icehouse condition. This climatic evolution largely impacted on the shallow water biological communities, particularly so on the taxa sensitive to the environmental fluctuations, such as zooxanthellate corals and large foraminifera . Kiessling (2001) observed that different biotic reef types show a strong correlation with palaeolatitudes: the lowest latitudes being occupied by coralline sponge and microbial reefs, the intermediate latitudes by coral reefs and the highest latitudes by bryozoans reefs. Rosen (1999) obtained palaeotemperatures through the analysis of the zooxanthellate coral diversity in the Neogene of the Mediterranean region, according to the so-called "energy hypothesis" of Fraser & Currie (1996). This hypothesis provides a quantitative relationship between taxonomic richness and mean sea-water surface temperatures. In the Tertiary (Perrin, 2002), Eocene, Oligocene and Miocene Times record extensive reef development and are characterised by a number of important changes in the carbonate-producing biota. The Mediterranean region was characterised during this time interval by a complex palaeogeographic and climatic history, marked by the stepwise transition towards cooler climates, associated with the disappearance of zooxanthellate corals and reefs, at the very ending of the Miocene (Esteban, 1996; Rosen,1999; Perrin, 2002).

PALEOECOLOGICAL INTERPRETATION OF BENTHIC TAXA
Foraminifers are among the most important palaeoenvironmental indicators and palaeoclimatic proxies in the marine environments. A major role in the palaeoclimatic reconstruction is played in the Mediterranean area by planktic foraminifera (Sbaffi et al., 2001).
Shallow-water smaller and larger benthic foraminifers have often been used as palaeoecological indicators. The paleobiogeography and generally stenotherm ecology of foraminifera appear to be related to sea water temperature. For example, large eurytherm zooxanthellate foraminifera are broadly confined to intertropical regions; in other climatic regions different types of faunal assemblages have been identified (Adams et al., 1990). Among the shallow-water benthos with higher fossilisation potential, foraminifera are particularly suited for investigating ancient climatic variations, because the ecological depth gradient can be assessed.
However, the full potential of shallow-water benthic foraminifera is still limited. because of the insufficient knowledge on their palaeobiogeography, often based on older literature data only, which must be carefully assessed especially as concerns taxonomic definitions and identifications, in particular at species level. As previously discussed, corals palaeocological interpretation can also provides powerful tools for the palaeoclimatic interpretation of the Tertiary evolution, particularly so in a region like Italy rich in well preserved and well exposed fossil reefs, associated with platform margins. The palaeoclimatic information derived form marine fossil and sediments can be useful integrated by information on the continental environments, such as the ones deriving from palaeosoil investigation. <<<