Abstract
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Diagenesis is now recognized as a potentially major source of error in paleoclimatic reconstructions
from fossil and modern coral geochemical records. Key to avoiding spurious results caused by diagenesis
is thorough screening of coral material prior to geochemical analysis. In this data brief we present color
images from thin sections of fossil and modern Porites corals and demonstrate the effectiveness of thin
sections in detecting low levels of diagenesis. The images presented here cover a range of coral
preservation levels from pristine aragonite to 100% calcite. We particularly focus on samples containing
around 1% diagenetic material, a level known to create artifacts in key climate parameters such as sea
surface temperature, and close to the detection limits of other screening methods such as X-ray diffraction
(XRD). A qualitative scheme is also presented to rate the degree of diagenesis in a coral, where XRD
results are not available or where secondary aragonite is present. Overall, this collection of images is
designed as a starting point, in combination with other techniques, to assist in identifying and screening
corals for diagenesis.