Bonardi, M., Percival, J.B. and Tosi, L. The compositional relation of selected clay sediments to late Pleistocene-Holocene depositional environments from Italy and China. In: Clay for our future. Proceeding of the 11th International Clay Conference, June 15-21, 1997, Ottawa, Canada, pp. 767-774. The compositional relation of selected clay sediments to late Pleistocene-Holocene depositional environnlents from Italy and China M. Bonardi 1 , J.B. Percival 2 and L. Tosi' 1 Consiglio Nazionale delle Ricerche Istituto per 10 Studio della Dinamica de/Ie Grandi Masse, San Polo 1364, 30125 Venezia, Italy; 2 Mineral Resources Division, Geological Survey of Canada, 601 Booth St., Ottawa, Ont., Canada, K1A OE1 Mineralogical and textural variations in Upper Pleistocene and Holocene clay sediments from three cores taken in different depositional environments: the Venetian lagoonal littoral zone, the South Yellow Sea and the Yangtze River Delta, are reported. The compositional variations of clay layers from the three sites are attributed to major global climatic changes. Although the compositional difference of clay sediments of the three sites reflects the petrology of their different areas of provenance, the relative abundance and textural variations show a correlation with climatic-driven changes in their depositional environment. In the Venice lagoon the carbonate content decreases from cold to warm periods whereas in the Yangtze River delta and in the South Yellow Sea carbonates are ab sen t or not signiticant, and the most noticeable changes are given by the total clay mineral content variations (chlorite, smectite, mixed-layer clay minerals and muscovite) and feldspar. There is a general increase in abundance of clay minerals and a decrease in feldspar content during warm periods. The cores from all three study sites have overconsolidated continentally-delived clay layers at the PleistocenelHolocene (ca. 10,000 BP) boundary. The overconsolidated nature of these hard clay layers is due to long subaerial exposure under a cold and dry climatic regime before the Holocene (Flandrian) transgression. Key words: Paleoclimate; clay minerals; overconsolidated clay; Venice lagoon; South China Sea, Yangtze River Delta INTRODUCTION Tllis study forms part of a multidisciplinary investigation of the impact of climatic changes on depositional environments. The aim of this study was to contribute to the study of paleoclimatic changes using mineralogical and textural information to complement existing paleontological, palynological and dendrochronological data. This paper provides a general overview of regional paleoclimatic changes that have occurred in the Venice lagoon (Italy) , South Yellow Sea and Yangtze River delta (China) since the end of the Pleistocene. The paper also discusses the mineralogical variations in clay sediments, in particular on the samples from the overconsolidated clay layer considered to be the last continental deposit before the Holocene (Flandrian) transgression. Study Areas The Veni.ce lagoon, in the northwestern Adriatic Sea, forms an arch-shaped area of about 500 km 2 that is separated from the sea by a littoral sand bar about 50 km long (Fig. 1). Lagoonal waters freely exchange with the sea through the Lido, Malamocco and Chioggia inlets. The lagoon is underlain by about 1000 m of Quaternary sediments of sand, silt, clay and peat. Within this sequence are calcareous mud layers a few centimetres to a few metres thick . One of these mud layers, the Caranto unit, is overconsolidated, marks the Pleistocene-Holocene boundary and is linked to the evolution and origin of the lagoon. The natural evolution of the lagoon has been modified since 1500 A.D. with the redirection of the major tributaries into the open sea, the construction of breakwaters for inlet protection, and excavation of canals for inland navigation (Gatto and Carbognin 1981). The South Yellow Sea (Fig. 2) located east of Qingdao and Shanghai can be divided into three depositional regions: the 1101th slope, the central bottom and the south slope. The north slope is underlain by 30 to 40 m of muddy sediments formed by sediment derived from the Yellow River whereas bedrock is exposed on the south slope. The central bottom, where the water depth is more than 70 m, is covered by less than 10 m of sediments (Zhao and Li 1991). The Yangtze River (Fig. 2) flows into the East China Sea and has formed one of the word's largest deltas about 300 km long with a radius of about 250 km. The Yangtze River delta is classified as a tide and fluvial dominated system. Paleoclimatic History In Europe, during the WUrmian glaciation, ca. 25,000-17,000 BP, the climate was cold and dry . The Adriatic Sea was