The rare earth element (REE) composition of bioapatite has long been used as a proxy for ancient seawater chemistry and paleomarine environmental reconstruction, based on the assumption of preservation of a hydrogenous (seawater-derived) REE signal. Recent work, however, has begun to question the provenance of REEs in conodonts, emphasizing the importance of REEs released by the lithogenous fraction of the sediment and subsequently adsorbed onto conodont apatite in the burial environment. Here, we investigate patterns of REE and trace-element abundance in conodonts and their host sediments from the Early to Late Ordovician Huanghuachang and Chenjiahe sections of Hubei Province, South China. Several lines of evidence indicate that REEs in the conodont samples were acquired mainly from clay minerals in the host sediment during burial diagenesis: (1) REEs in conodonts show a strong positive correlation to Th and other lithogenic elements; (2) conodonts and whole-rock samples show general patterns of REE and trace-element enrichment that are highly similar to each other and bear no resemblance to seawater elemental concentrations; (3) similar patterns are observed in Triassic conodonts and whole-rock samples; and (4) Y/Ho ratios in conodonts are mostly <40 (mean ~33), values that are consistent with derivation of >90% of REEs from lithogenous sources. Conodonts show pronounced middle rare earth element (MREE) enrichment, a pattern that is unambiguously of diagenetic origin owing to its association with lower Y/Ho ratios. With increasing MREE enrichment of conodont samples, U concentrations and La N /Yb N ratios shift from high to low, and Mn concentrations from low to high. These patterns suggest that conodont diagenesis was initiated at shallow burial depths under suboxic conditions (i.e., in the zone of Mn(IV) and Fe(III) reduction) but continued at greater burial depths, with most acquisition of secondary REEs at later diagenetic stages. Our findings indicate that (1) conodont apatite frequently does not preserve a recognizable hydrogenous REE signal, and (2) the results of many earlier studies in which REEs in bioapatite were used as a paleo-seawater proxy may need 47 48 1．Introduction 49 The rare earth element (REE) composition of bioapatite has long been used for 50 reconstruction of paleoceanographic conditions based on Ce anomalies, Nd-isotope 51 127 are younger than the Baota Formation (i.e., the top of the Huanghuachang section).
128We therefore generated a ~85-m-thick composite section consisting of the full ~260 m 129 thickness of the Huanghuachang section as well as the upper ~10 m of the Chenjiahe 130 section. Detailed lithologic descriptions are given in Table 1, and the two study 131 sections are correlated in Figure 2. The entire succession is characterized by a gradual 132 shift from carbonate platform facies in the Early Ordovician to neritic facies in the 133 Middle Ordovician, reflecting a long-term sea-level rise during this period (Fig. 2; Hu 134 et al., 1996). Deep basinal facies accumulate...
