GEOCHEMICAL AND SCLEROCHRONOLOGICAL ANALYSES OF THE LOWER PLEISTOCENE MACROFAUNA OF WESTERN EMILIA (NORTHERN ITALY): PALAEOENVIRONMENTAL AND PALAEOCLIMATIC IMPLICATIONS

The Early Pleistocene is a time interval characterized by several climatic oscillations which has its lower and upper boundaries coinciding respectively with the beginning of the Northern Hemisphere Glaciation and the Middle Pleistocene Transition. The Mediterranean area was strongly affected by the Early Pleistocene climatic changes. One of the most important biotic event is here represented by the appearance of the boreal guests (e.g. the bivalve Arctica islandica and the foraminifer Hyalinea balthica) at the beginning of the Calabrian Stage, suggesting significant cooling of the Mediterranean Sea, which is also confirmed by a change in the pollen flora indicating cold climatic conditions. The Arda River marine succession, cropping out in Western Emilia, Northern Italy, is very rich in macrofossils and it covers without significant gaps the Early Pleistocene. It thus represents an ideal setting where to study the climatic oscillations of this time interval. This study, combining sedimentology, taxonomy, palaeoecology, biostratigraphy and geochemistry and sclerochemistry of bivalve shells, provides an integrated investigation of the palaeoenvironmental and palaeoclimatic conditions accompanying these major climatic changes in the Arda section. The Arda marine succession is 237.40 m-thick and consists of sandstones, siltstones and mudstones deposited in a tectonically active setting during phases of advance of fan deltas; it is bounded at the top by continental conglomerates indicating a major sea level drop and the establishment of a continental environment with vertebrate faunas and fresh water mollusks. The taxonomic analysis of the macrofauna allows to identify 159 taxa, of which bivalves are dominant with 105 taxa, followed by gastropods (44 taxa) and a few corals (3 taxa) and serpulids (2 taxa); brachiopods, echinoids, barnacles, bryozoans and scaphopods do also occur. The comparative sedimentological and palaeoecological analysis shows that the Arda marine succession deposited in an infralittoral to a shallow circalittoral environment, where the maximum depth of the succession should not have exceeded 40-50 m. This comparative analysis confirms also the general regressive trend of the studied marine succession, punctuated by eight lower order transgressive and regressive cycles; however, no evidence of subaerial exposure or shift to water depths exceeding 50 m has been recorded, framing the depositional depth through the transgressive and regressive cycles between 5 m and 50 m of depth. According to mollusk and nannofossil biostratigraphy the Arda River section has a late Gelasian-Calabrian (Early Pleistocene) age. The main mollusk bioevents comprise the last occurrences of Chama placentina, Glycymeris inflata and Aequipecten scabrella in the basal part of the section, the first occurrence of Arctica islandica at 103.70 m from the base and the last occurrence of the gastropods Nassarius prysmaticus and Turritella tricarinata in the upper part of the section. The identified nannofossil zones range from Zone CNPL7 to the lower part of Zone CNPL9. The palaeoclimatic significance of the Arda biota has been investigated at two different levels: first the analyses of its mollusk fauna and of its pollen content and then a more detailed research involving geochemistry and sclerochemistry of the bivalve shells. The occurrence of boreal guests, such as Arctica islandica, Pseudamussium septemradiatum and possibly also Mytilus edulis, from 103 m upward, suggests that a climatic change occurred in the Arda marine succession with a shift to colder seawater temperatures. However, aside from the occurrence of boreal guests, the fauna is mainly dominated by eurythermal species having a cosmopolitan distribution, lacking the strictly arctic or tropical ones. Preliminary data from pollen analysis suggest that a climatic change occurs also in the Arda flora; in particular pollen data suggest the presence between 91 and 110 m of an interval characterized by a high seasonality with taxa preferring colder winters, which is followed by an interval in which taxa preferring milder winters are dominant, thus possibly indicating a glacial/interglacial shift. To examine in detail the palaeoclimatic evolution of the Arda section, 249 fossil specimens from 141 stratigraphic beds belonging to Glycymeris glycymeris, Glycymeris insubrica, Glycymeris inflata, Glycymeris sp., Aequipecten opercularis, Aequipecten scabrella and Arctica islandica have been analyzed for carbon and oxygen isotope analyses. In addition, ten shells collected in seven distinct stratigraphic beds and belonging to the species Glycymeris insubrica, Glycymeris inflata and Arctica islandica have been analyzed for sclerochemistry in order to examine if and how seasonality varies along the section. The geochemical and isotopic composition of calcareous skeletons has long been recognized to record past and present environmental conditions. As bivalves record in their calcium carbonate shells the primary seawater isotope composition, with little or no vital effect, the geochemical signature registered in bivalve shells can be thus used as an archive of global change in seawater composition and temperature. The equation used here to obtain palaeotemperatures from oxygen data from Arctica and Glycymeris aragonitic shells is that of Grossman & Ku (1986); the equation for the Aequipecten calcitic shell is that of O'Neil et al. (1969). As diagenetic processes may alter fossil bivalve shell geochemical composition, I have undertaken four different screening tests (SEM microstructural examination, Cathodoluminescence, X-ray Powder Diffraction and Feigl’s solution) to check if the shell is pristine, and thus effectively suitable to a correct interpretation of isotope analyses. All these tests indicate that bivalve shells of the Arda River marine succession are very well preserved. The oxygen isotope bulk data obtained from species of the genera Glycymeris, Aequipecten and Arctica have been used to construct a mean oxygen curve, which was then compared to the Mediterranean and Atlantic stacks, in order to make a correlation with the Marine Isotope Stages. The main outcome of the bulk isotope analysis is that seasurface palaeotemperatures seem to remain rather constant during the deposition of the Arda marine succession and they do not suggest any significant cooling during Gelasian and Calabrian (Early Pleistocene). Sclerochemical analyses give further detail on the palaeoclimatic evolution of the section. In the stratigraphic interval from the base of the section to 103.70 m, bivalve shells record a low seasonality (5.7°C), suggesting that probably the Northern Hemisphere Glaciation exerted a minor control on the Mediterranean climate in this moment. In the interval between 103.70 m and 110 m, which corresponds to the arrival of the boreal guests, the shells record a high seasonality (16.0° of winter-summer difference) and low palaeotemperatures (winter palaeotemperatures of 0.8-1.7°C), as also supported by other proxies in the Mediterranean region. This may be due to cold currents from the North Atlantic entering the Mediterranean Sea with annual-interannual variability causing a progressively cooling of the seawater, particularly during winter; a cooling threshold was reached and crossed, acting as a trigger factor for the arrival and successful recruitment of the boreal guests. This cold episode is followed by an interval (110 m - top of the section) in which bivalve shells record a clear increase in seasonality toward the top of the Arda section (from 9.2 to 19.0°C of seasonal change). However, they registered higher palaeotemperatures compared to the interval of the arrival of boreal guests described above, and in the case of Arctica islandica also a lower, but still increasing upward, seasonality. At this time the Apennine chain was rapidly uplifting, as suggested by a change in the pollen content, increase in sedimentation rates and terrigenous input, causing a perturbation in the Mediterranean climate system. The Apennine uplift may have acted as a shield for cold and dry continental air masses coming from the Northern Hemisphere, locally obscuring the impact of the climatic variations linked to the Northern Hemisphere Glaciation dynamics. The complex palaeoclimatic and palaeoenvironmental evolution of the Arda River succession points out that the Mediterranean region was affected by the Northern Hemisphere Glaciation dynamics, which exerted a strong control on the Mediterranean climate during most of the Early Pleistocene. The significantly cold event recorded at 103.70 m, indicates that the arrival of the boreal guests in the Mediterranean Sea was accompanied by a high seasonality and by particularly cold winters, as suggested by both sclerochemistry and pollen analyses. Seasonality increases approaching the Middle Pleistocene Transition and the beginning of the continental glaciations in the Northern Hemisphere, representing thus a clear signal of climatic deterioration in the Mediterranean Sea during the Early Pleistocene. However, no cooling trend in the average seawater temperatures has been observed both in sclerochemical and bulk shells oxygen isotope data during this interval. The changes in seasonality may well have amplified and propagated the signal of climate change toward the Middle Pleistocene Transition, representing thus a very important component in abrupt climate switches.