RARE-EARTH ELEMENTS IN LIMESTONE MARLSTONE COUPLETS FROM THE ALBIAN-CENOMANIAN CISMON SECTION (VENETIAN REGION, NORTHERN ITALY) - ASSESSINGREE SENSITIVITY TO ENVIRONMENTAL-CHANGES/

We have studied the REE distribution through the Albian-Cenomanian int erval of a stratigraphic succession (Cismon section, Scaglia Variegata Formation, Venetian Prealps) consisting of rhythmically alternating c arbonate-rich and carbonate-poor strata which are the sedimentary expr ession of systematic variations of surface fertility and bottom-water odor conditions in the depositional basin. Limestone semicouplets show seawater-like shale-normalized patterns (HREE-enriched, with negative Ce anomalies and high Er/Nd ratios) indicating a clear dependence on the REE chemistry of surface seawaters. Marlstones and shales exhibit flatter patterns, more moderate Ce anomalies and low Er/Nd. Good corre lations between Er/Nd, Ce/Ce and Mn* suggest that, superimposed on RE E signatures imparted by detrital and biogenic phases, the REE composi tion shows evidence of their sensitivity to palaeoredox conditions res ulting in REE fractionation and Ce cycling. REE data are consistent wi th results from previous stratigraphic and geochemical (major and trac e elements) investigations in reflecting palaeoenvironmental condition s rhythmically varying from efficiently mixed seawater during the lime stone deposition to oxygen-depleted bottom waters at the time of marls tone deposition. (C) 1997 Elsevier Science B.V.