Rare earth elements in limestone/marlstone couplets from the Albian-Cenomanian Cismon section (Venetian region, northern Italy): assessing REE sensitivity to environmental changes

We have studied the REE distribution through the Albian-Cenomanian interval of a stratigraphic succession (Cismon section, Scaglia Variegata Formation, Venetian Prealps) consisting of rhythmically alternating carbonate-rich and carbonate-poor strata which are the sedimentary expression 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 correlations between Er/Nd, Ce/Ce* and Mn* suggest that, superimposed on REE signatures imparted by detrital and biogenic phases, the REE composition shows evidence of their sensitivity to palaeoredox conditions resulting in REE fractionation and Ce cycling. REE data are consistent with results from previous stratigraphic and geochemical (major and trace elements) investigations in reflecting palaeoenvironmental conditions rhythmically varying from efficiently mixed seawater during the limestone deposition to oxygen-depleted bottom waters at the time of marlstone deposition. (C) 1997 Elsevier Science B.V.