Diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies: an isotopic and REE study of Cambrian phosphorites

Seawater rare-earth element (REE) distributions show heavy REE enrichment a nd cerium depletion but these characteristics are only rarely found in anci ent, marine authigenic minerals such as carbonate fluorapatite and calcite. This enigma may be due to changing REE distributions in seawater through t ime or to post-depositional redistribution of REE. Differentiating between these two factors is, however, far from straightforward as their relative i mportance is likely to vary and evidence is often ambiguous. In this articl e, we present REE as well as strontium acid sulphur isotope data for basal Cambrian phosphorites of South China, which confirm the emerging rule of th umb that granular phosphorites seldom reveal extreme departures from the se awater norm, and tend to undergo, therefore, less diagenetic alteration of REE spectra than skeletal apatites and other phosphate-types. This study re veals stratigraphic trends in Ce anomalies (Ce/Ce+) that could be taken to imply the preservation of a palaeoenvironmental signal. However, further in vestigation of our data shows that such trends in phosphorites can also be caused by post-depositional alteration, which tends to (I) increase REE con tents, (2) reverse Ce depletion, (3) alter Eu anomalies, and (4) concentrat e preferentially the middle REE (MREE). Further alteration, which may be du e to weathering, has led to HREE depletion, and positive La. and Y anomalie s (tetrad effect). This is the first study where Ce-anom,Ce- and phosphate- bound sulphate S-isotope values could be compared for one particular strati graphic section, which provides us with an additional tool for the interpre tation of diagenetic redox conditions. Least altered phosphorites from Meis hucun that yield a strongly negative Ce-anom,Ce- constrain early Cambrian s eawater delta S-34 to about +33 parts per thousand. We conclude that phosph orite Ce-anom,Ce- primarily records variations in the local conditions of d iagenesis, while outlining some special cases in which diagenetic redox con ditions may broadly reflect local bottom-water redox conditions. (C) 2001 E lsevier Science B.V. All rights reserved.