How to choose optimal samples for U-Th dating

U-Th dating can be used to date calcium carbonate (CaCO₃) bearing samples, including cave deposits / speleothems, corals, shells, bones (in some cases), and carbonate layers in sedimentary successions. The method assumes that the sample does not exchange ²³⁰Th or ²³⁴U with the environment and therefore the sample occurs within a closed system. As a result, it’s best to choose samples that are considered a closed system to ensure the dating is as accurate as possible. Contribution from detrital ²³⁰Th will alter the true U-Th age of the sample, therefore, samples need to be free from detrital materials as well to achieve accurate results. 

Cave deposits / speleothems: when choosing a cave deposit sample for paleoclimate reconstructions, it’s best to choose samples that are in-tact with few growth hiatus periods. Ideally, choose samples that have limited influence from outside conditions, found deeper in the cave environment in an area undergoing active growth. In the cases of cave art, samples close to a cave entrance can still be good candidates for U-Th dating as long as they’re covered with a calcite layer.   

Corals: While isotopic fractionation does not seem to occur after growth in most environments, recrystallization can introduce foreign uranium and thorium during periods of low sea-level. Any potential recrystallization should be analysed and considered prior to U-Th dating (and completed via XRD). Broadly, a comparison of U content between fossil coral and modern counterpart corals (nearest living relative) can provide some information as to whether the fossil existed in a closed or open system. Coral samples with more than 95% aragonite are ideal for the U-Th chronology.

Shells: U-Th dating remains difficult in these environments due to the introduction of thorium into the partially open system (usually via detrital material). This can potentially be corrected for by measuring ²³⁰Th/²³²Th to estimate the quantity of detrital material (see Chen et al. 2020 for details). Mollusc shells and foraminifera found within sediment cores have been shown to exchange uranium after deposition, challenging the use of U-Th dating in this context.

Bones: Due to the porous nature of bones, uptake and release of uranium creates an open system. When it comes to fossil bones, studies have found that fossil bones contain much higher levels (10-100x) of uranium than modern bone material. As a result, it is assumed that bones take up a significant amount of uranium after death and deposition, but it is still unclear when this occurs. It would be preferable to test U-Th at multiple portions of the bone to investigate the degree of infiltration and the quality of the U-Th dating results. Furthermore, denser bones with high preservation – such as those from permafrost – will yield more accurate results.