* Corresponding author. Fax: #39-06-4991-2279. E-mail addresses: magri@uniromal.it (D. Magri), pct11@cus.cam. ac.uk (P.C. Tzedakis). Quaternary International 73/74 (2000) 69}78 Orbital signatures and long-term vegetation patterns in the Mediterranean D. Magri*, P.C. Tzedakis Dipartimento di Biologia Vegetale, University **La Sapienza++, P.le Aldo Moro, 5, 00185 Rome, Italy Department of Geography, Godwin Institute for Quaternary Research, University of Cambridge, Downing Place, Cambridge CB2 3EN, UK Abstract An attempt is made to explore the relation between vegetation phases and orbital con"gurations by comparing long pollen sequences from the Mediterranean region and astronomical curves on the basis of their independent chronologies. Three recurring patterns emerge from this comparison: (i) all intervals with perihelion passage occurring during northern winter were associated with signi"cant contractions of tree populations; (ii) all temperate-stage expansions of certain Mediterranean vegetation elements occurred when perihelion passage was in summer; and (iii) temperate-stage forests reached maximum extent at times of autumn perihelion. Of these patterns, the "rst corresponds to extreme dry/cold episodes culminating at times of March perihelion, and the second, to maximum summer temperature and evaporation regimes following June perihelion. The third, associated with the occurrence of September perihelion, appears to be more di!use both in time and space, possibly re#ecting local variations in hydrological regimes. The consistency of at least the "rst two cases provides some support for the notion that certain recurring vegetation patterns appear to be a result of climate changes linked to speci"c orbital signatures. This could eventually lead to the development of improved terrestrial chronologies through astronomical tuning.  2000 Elsevier Science Ltd and INQUA. All rights reserved. 1. Introduction It has long been suggested that orbital variations exert a signi"cant in#uence on climate (e.g. Milankovitch, 1930). The e!ects of insolation perturbations on the sea- sonal temperature cycle and global ice volume have re- ceived much attention, but the in#uence on the behaviour of atmospheric features, such as the intensity of monsoons, has also been taken into consideration (e.g. Hays et al., 1976; Kutzbach and Street-Perrot, 1985; COHMAP Members, 1988). However, the relation be- tween vegetation phases and orbital con"gurations has not been explored in any great depth. Evidence sugges- ting that components of vegetation have responded sys- tematically to insolation forcing associated with speci"c orbital geometries through time would have at least two signi"cant implications: (i) the identi"cation of the eco- logical processes underlying the vegetation}orbital links could lead to an improved understanding of the interac- tions between the biosphere and the climate system; and (ii) an opportunity to develop improved terrestrial chro- nologies, based on an astronomical clock which may be much more precise than other methods in dating Pleistocene sediments. So far, the application of an orbital chronology to long pollen records has mostly been done indirectly, through correlation to marine oxygen isotope records, themselves tuned to astronomical variations. For example, Tzedakis et al. (1997) aligned the four longest pollen records from southern Europe to the SPECMAP O record of Imbrie et al. (1984), assigning marine ages to the begin- nings of forest periods. In the Funza I pollen record from Colombia (Hooghiemstra et al., 1993), the orbital peri- odicities were studied through frequency spectra. The geochronological framework of the sequence was based on "ssion-track ages on zircon in combination with cor- relation of the pollen record with the oxygen isotope record of ODP site 677 in the Eastern Paci"c. In such cases, once ages are assigned to pollen se- quences on the basis of correlations with other records tuned to astronomical parameters, it is no longer possible to explore the relationship between vegetational events and orbital situations without introducing an element of circularity. Thus, ideally what is necessary for a proper assessment of the orbital-vegetation relationship is 1040-6182/00/$20.00  2000 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 0 ) 0 0 0 6 5 - 3