Stratigraphic Trends in Cerium Anomaly in Authigenic Marine Carbonates and Phosphates: Diagenetic Alteration or Seawater Signals?

Abstract

Modem marine authigenic carbonates, barytes and phosphates incorporate seawater rare earth element (REE) patterns, which display an enrichment in heavy REE and a pronounced relative depletion in cerium (a negative cerium anomaly). The Ce anomaly relates to the decrease in solubility which accompanies the oxidation of Ce(III) to Ce(IV), while the size of the Ce anomaly may correlate with the oxidation potential, age (German and Elderfield, 1990), pH (Tricca et al., 1997) and/or depth of a water body (Piepgras and Jacobsen, 1992). Several authors have attempted to use Ce anomaly to proxy ocean anoxia through time by measuring REE concentrations in authigenic minerals. However, all such studies have run against apparently irreconcilable problems of early diagenetic alteration. Although, late diagenetic alteration of REE distributions is considered uncommon in carbonates (Banner et al., 1988) due to the high fluid/rock ratios required, recycling of REE and reduced forms of Fe, Mn and Ce related to organic decay may reset Ce anomalies in authigenic minerals during early diagenesis. Elderfield and Pagett (1986) reported that although apatitic icthyoliths in marine sediments retained superficially similar REE patterns to seawater, those deposited in 'shallow, nearshore, anoxic sediments underlying high productivity environments' recorded positive Ce anomalies, while those from 'cores of deep ocean sediment undergoing oxic diagenesis' exhibited negative Ce anomalies. Presumably, all these icthyoliths possessed original ly a seawater derived Ce anomaly. The authors conclude that authigenic minerals will only preserve a negative Ce anomaly if no suboxic or anoxic diagenesis has taken place, and this appears to have become the prevailing view among marine geochemists. However, if this is true, how can we explain the preservation of marked Ce anomalies in ancient, often sulphidic sediments and robust stratigraphic trends (data herein). It appears that these 'exceptions' have managed to retain a seawater Ce anomaly despite the ubiquitously anoxic or suboxic nature of shallow marine diagenesis. We report Ce anomaly and REE data for two s e p a r a t e s e d i m e n t a r y s e c t i o n s f rom the Neoproterozoic (Tsagaan Gol, W Mongolia) and the early Cambrian (Meishucun, Yunnan, S China). For Tsagaan Gol, calcite was measured from >95% pure limestones and for Meishucun, we measured carbonate fluorapatite from dolomitic phosphorites. Both studies reveal robust stratigraphic trends in Ce anomaly. Unlike the Elderfield and Pagett study, pronounced Ce anomalies were found in samples deposited in shallow waters, whereas those from laminated, more organic rich (also Fe, Mn, REE rich), deeper water samples showed little fractionation among the light REE. Instead of concluding that diagenesis was oxic in shallow waters and anoxic in deeper waters, we interpret this relation as suggesting the existence of redox stratification in late Precambrian/Cambrian seawater. In the case of Tsagaan Gol, that the REE, Fe and Mn are really contained within the calcite phase has been confirmed by sequential leaching procedures using K6nigswasser, HNO3, HC1, acetic acid and alpha-hiba reagent buffered to a pH of 4.5, which reveal no change in Ce anomaly of the leachate. Initial results of this work are published in Shields et al. (1997). In this presentation, we describe a second more detailed data set from Tsagaan Gol, which although based on unrelated samples from a second sampling excursion shows the same features as the first. We argue that correlation of Ce anomaly with other primary parameters implies that trends in Ce anomaly in this study reflect changes in seawater chemistry: 1. Preservation of primary Sr isotope ratios, and Sr contents up to 3000 ppm in these samples confirm that the diagenetic system was closed to isotopic