Donatella Magri

Comparison of terrestrial and marine records of changing climate of the last 500,000 years

A broad correspondence between long pollen sequences and the deep-sea oxygen isotope record has been noted for some time, but there has been little effort to explore just how similar the two types of evidence are in terms of their overall structure on glacial-interglacial timescales and also how they may differ. These questions have profound importance both for how we view the stratigraphic record of changing climate in different regions and for our understanding of the climate system. Here we link the four longest European pollen records and derive a terrestrial sequence of vegetation events and a coherent stratigraphic scheme for the last 500,000 years. Comparison of the terrestrial and marine records shows good agreement, but it also reveals that the pollen sequences contain a higher degree of climate sensitivity than the oxygen isotope record. In addition, it suggests that neither an oxygen isotope record nor a Milankovitch-forced ice volume model may provide an appropriate template for fine-tuning the terrestrial record and that better chronologies will depend on an improved understanding of controls on sedimentation rates in individual sedimentary basins

Climate refugia: joint inference from fossil records, species distribution models and phylogeography

Climate refugia, locations where taxa survive periods of regionally adverse climate, are thought to be critical for maintaining biodiversity through the glacial-interglacial climate changes of the Quaternary. A critical research need is to better integrate and reconcile the three major lines of evidence used to infer the existence of past refugia - fossil records, species distribution models and phylogeographic surveys - in order to characterize the complex spatiotemporal trajectories of species and populations in and out of refugia. Here we review the complementary strengths, limitations and new advances for these three approaches. We provide case studies to illustrate their combined application, and point the way towards new opportunities for synthesizing these disparate lines of evidence. Case studies with European beech, Qinghai spruce and Douglas-fir illustrate how the combination of these three approaches successfully resolves complex species histories not attainable from any one approach. Promising new statistical techniques can capitalize on the strengths of each method and provide a robust quantitative reconstruction of species history. Studying past refugia can help identify contemporary refugia and clarify their conservation significance, in particular by elucidating the fine-scale processes and the particular geographic locations that buffer species against rapidly changing climate.

Late Pleistocene and Holocene pollen stratigraphy at Lago di Vico, central Italy

A new pollen record from Lago di Vico (core V1) provides fundamental new information towards reconstruction of flora and vegetation history in central Italy during the last 90 000 years. The chronological framework is secured by seventeen AMS14C dates, one40Ar/39Ar date and tephra analyses. At the base of the pollen record, i.e. shortly after the40Ar/39Ar date 87 000±7000 B.P., three phases with significant expansion of trees are recorded in close succession. These forest phases, which stratigraphically correspond to St Germain II (and Ognon?) and precede pleniglacial steppe vegetation, are designated by the local names Etruria I, Etruria II and Etruria III. During the pleniglacial, a number of fluctuations of angiosperm mesophilous trees suggest the presence of tree refugia in the area. The lowest pollen concentration values are recorded at ca. 22 000 B.P. which corresponds with other pollen records from the region. The late-glacial is characterized by an expansion in the arboreal pollen curves that is less pronounced, however, than in other pollen profiles from Italy. The Holocene part of the profile is consistently dominated by deciduous oak pollen. No major changes in arboreal pollen composition are recorded but several marked and sudden declines of the tree pollen concentration suggest that the forest cover underwent dramatic changes. Clear evidence for human impact is recorded only when cultivated crops became important which dates to ca. 2630±95 B.P.

Late Quaternary vegetation history at Lagaccione near Lago di Bolsena (central Italy)

Abstract The palynological study of the lacustrine sediments from Lagaccione, a maar-lake near Lago di Bolsena in central Italy, has provided a palaeoenvironmental record for the last 100,000 years. The chronology of the sequence is based on sixteen radiocarbon dates and a tephra layer at the base of the St Germain II s.l. The pollen record, starting during the St Germain I forest phase and ending about 3000 years ago, shows that remarkable changes occurred in the floristic composition, in the structure of the vegetation and in the plant biomass, including also long-lasting periods of open woodlands, often with an appreciable floristic diversity, during the last pleniglacial. Three new vegetational oscillations (Etruria I, Etruria II and Etruria III), characterized by a well-defined vegetation composition and dynamics, have been recognized after the St Germain I forest phase and before the middle pleniglacial interstadials. New evidence has been obtained for a significant diffusion of deciduous trees in the Italian peninsula during the late-glacial.

PALYNOSTRATIGRAPHY OF THE LAST GLACIAL PERIOD IN THE VOLCANIC REGION OF CENTRAL ITALY

Abstract The summarized pollen records from the crater lakes of Valle di Castiglione, Lagaccione, Lago di Vico and Stracciacappa, in the Lazio region, are presented and correlated with each other. They outline the history of the vegetation during the last glacial period. The local name ‘Lazio Complex’ is used to describe seven main fluctuations of trees during the Pleniglacial. The succession of the main forest and non-forest periods of the last climatic cycle is generally similar to what is known in central and northern Europe, but the floristic composition and the vegetational configurations seem much more complex and sensitive to climatic changes. The history of Zelkova , Abies , Picea , Corylus , Ulmus , Carpinus betulus , Tilia and Fagus in central Italy is outlined.

Establishing a terrestrial chronological framework as a basis for biostratigraphical comparisons.

The palynological signature of interglacial deposits in the fragmentary European terrestrial record has often been used as the basis for determining their chronostratigraphical position and ultimately their age. This has placed emphasis on the presence/absence and abundance of certain characteristic taxa, but given the lack of continuous stratigraphies and independent chronologies, it has been difficult to assess the extent to which this strategy has produced reliable schemes. Here, an alternative approach is adopted whereby a chronological framework is developed for long and continuous pollen sequences from southern Europe. This in turn allows the emergence of a complete stratigraphical scheme of major vegetation events for the last 430 thousand years (ka) and the evaluation of the stage record of different taxa and their potential diagnostic value for biostratigraphical correlation. The comparison shows distinct similarities among some temperate stages of the terrestrial equivalent complexes of Marine Isotope Stages (MIS) 5 and 7 and also of MIS 9 and 11, but examination of combined records of taxa provides a possibility to differentiate between individual stages. A numerically-derived dichotomous key for the terrestrial stages based on the palynological records of 10 taxa is presented. Carpinus, Fagus, Abies, Pterocarya and Buxus emerge as the best ‘indicator pollen types’ because of their variable behaviour from one stage to the next, possibly a result of their late expansion within a temperate stage or reduced genetic variability. The analysis shows that the palynological signature of a temperate deposit can constrain the range of chronostratigraphical possibilities, but vegetation and palynological variability arising from local factors could result in difficulties in making a definite assignment at individual sites.

Predictability of biomass burning in response to climate changes

Climate is an important control on biomass burning, but the sensitivity of fire to changes in temperature and moisture balance has not been quantified. We analyze sedimentary charcoal records to show that the changes in fire regime over the past 21,000 yrs are predictable from changes in regional climates. Analyses of paleo- fire data show that fire increases monotonically with changes in temperature and peaks at intermediate moisture levels, and that temperature is quantitatively the most important driver of changes in biomass burning over the past 21,000 yrs. Given that a similar relationship between climate drivers and fire emerges from analyses of the interannual variability in biomass burning shown by remote-sensing observations of month-by-month burnt area between 1996 and 2008, our results signal a serious cause for concern in the face of continuing global warming.

Holocene drought, deforestation and evergreen vegetation development in the central Mediterranean: a 5500 year record from Lago Alimini Piccolo, Apulia, southeast Italy

Pollen analysis from Lago Alimini Piccolo provides the first record of mid- and late-Holocene vegetation history of a coastal area in the easternmost region of southern Italy (Salento Peninsula). Terrestrial pollen taxa document expansions and declines of the Mediterranean forest, in relation to human activity and climate changes. Between 5200 and 4350 cal. BP a dense evergreen oak forest dominated the landscape; then a distinct opening of the forest is recorded (4350—3900 cal. BP). A new forest expansion (3900—2100 cal. BP) is characterized by an increase of Olea and evergreen shrubs, indicating a development of mediterranean climate conditions and increasing human disturbance. The Roman occupation period (2100—1500 cal. BP) shows a significant opening of the forest, expansion of halophytes and modest values of Olea. After 1500 cal. BP human impact causes a further decrease of the natural woodland in favour of an extraordinary expansion of Olea . The vegetation development at Lago Alimini Piccolo, interpreted in the light of other pollen records, provides new insights into climate evolution and evergreen vegetation development in the central Mediterranean region: (1) a temporary mid-Holocene deforestation at 4000 cal. BP, involving many Italian sites south of 43°N, was possibly caused by drought associated with an expansion or northward displacement of the North African high pressure zone; (2) the Bronze Age increase of Olea, coupled with a widespread increase of Mediterranean shrubs, suggests management of wild trees, while the beginning of intensive cultivation of olive trees is only found after the Roman time.

Late Quaternary western Mediterranean pollen records and African winds

Late Quaternary vegetation changes of regional and global significance in the Mediterranean region are generally interpreted as being controlled by changes in the circulation patterns of North Atlantic air masses. However, the possibility cannot be excluded that they may also be related to winds blowing from Africa. Long pollen records from southwest Europe show that Cedrus pollen of northwest African provenance is found during the glacial periods, and occasionally during abrupt deforestation events in the forest-dominated periods of the interglacials. A pollen concentration record from central Italy shows that during the Holocene the presence of Cedrus pollen coincides with two abrupt deforestation events, around 8.1 and 4.2 cal kyr BP. These observations raise the question of a possible influence of eolian activity on arboreal vegetation during phases of climatic stress towards aridity, and suggest the use of modern pollen monitoring as a strategy for revealing tendencies towards aridification in southwestern Europe.

The Tiber river delta plain (central Italy): Coastal evolution and implications for the ancient Ostia Roman settlement

Geomorphologic, stratigraphic, faunistic, palynological and carbon isotope analyses were carried out in the area of the Tiber river mouth. The results depict a complex palaeoenvironmental evolution in the area of the Roman town of Ostia, ascertain the changes of the Tiber river delta over the last 6000 years and support a re-interpretation of some archaeologic issues. The wave-dominated Tiber delta evolved through three distinct phases. In the first step (5000–2700 yr BP) a delta cusp was built at the river mouth, which was located north of the present outlet. Subsequently (2700–1900 BP), an abrupt southward migration of the river mouth determined the abandonment of the previous cusp and the progradation of a new one. The third step, which is still in progress, is marked by the appearance of a complex cusp made up of two distributary channels. The transition from the first to the second evolution phase occurred in the seventh century bc and was contemporary to the foundation of Ostia, as suggested by historical accounts. However, the oldest archaeological evidence of the town of Ostia dates to the fourth century bc, when human activity is clearly recorded also by pollen data. We suggest that the first human settlement (seventh century bc) consisted of ephemeral military posts, with the aim of controlling the strategic river mouth and establishing the Ostia saltworks. Only after the fourth century bc the coastal environment was stable enough for the foundation and development of the town of Ostia.