Preservation of successive diagenetic stages in Middle Triassic bonebeds: Evidence from in situ trace element and strontium isotope analysis of vertebrate fossils

• Trace elements and strontium isotope ratios in vertebrate remains
• REE contents measured by LA-ICP-MS show late diagenetic incorporation.
• Diagenetic stages deciphered by combining REE and 87Sr/86Sr with SEM-CL imaging
• REE preserve earlier diagenetic signals in diagenetically more resistant tissue.
• Positive and negative europium anomalies are found in the same vertebrate clast.

Bonebeds comprise reworked and time averaged constituents of different phosphatic vertebrate remains and, hence, provide an ideal substrate for the study of long and short term diagenetic processes. To test whether trace elements (U, Sr, REE) and 87Sr/86Sr ratios can be used for distinguishing between successive diagenetic signals, we performed geochemical analyses on vertebrate remains (bones, teeth, scales and coprolites) from two Triassic bonebeds located near Palzem (Germany). Trace element analysis was done on 170 vertebrate remains by LA-ICP-MS while 87Sr/86Sr ratios were determined for 39 bioclasts using LA-MC-ICP-MS. Although a low inter- and intra-bioclast REE variability indicates long-term open system behaviour, the observed REE patterns suggest the preservation of different diagenetic stages.A distinct difference in the geochemical composition exists between dark luminescent SEM-CL domains and greyish ones. Dark domains represent dense, diagenetically more resistant material of low porosities such as enamel and ganoine. These domains display distinctly lower ƩREE contents and a higher inter-clast variability than those of various grey tones. Together with lower, hence more seawater-like 87Sr/86Sr ratios this argues for a preservation of an earlier diagenetic signal. Notably, most dark domains are characterised by negative Eu anomalies while all grey ones display positive Eu anomalies. With the exception of Eu both domains display almost identical REE patterns with different concentrations indicating that late diagenetic REE uptake is not necessarily accompanied by fractionation processes in vertebrate bioclasts. Negative Eu anomalies are most likely inherited from Eu-depleted surface waters, possibly as a result of Eu-depleted aeolian input. Conversely, positive Eu anomalies were presumably affected by Eu-enriched late diagenetic fluids with a possible high temperature origin causing a fractionation of europium. Our study therefore demonstrates the potential of high-resolution geochemical analysis in bonebed strata to disentangle different stages of diagenesis and to detect least altered areas in vertebrate fossils for recovering pristine palaeoenvironmental signals.