Diagenesis in Pleistocene (80 to 500 ka) corals from the Ironshore Formation: Implications for paleoclimate reconstruction

Abstract

Fossil corals provide a unique opportunity for paleoclimate research because they can be dated and they incorporate climate controlled seawater proxies into their aragonitic skeletons during growth. Coral skeletons, however, can only produce reliable environmental information if they have not been diagenetically altered, an issue that becomes increasingly probable as the age of the corals increase. Although commonly used analytical techniques, such as thin section, Scanning Electron Microscopy, and X-Ray diffraction, can highlight obvious mineralogical changes, subtle changes in the trace element and isotopic composition of the skeletons may not be evident. This premise is assessed by examining Orbicella annularis and Montastrea cavernosa corals from the Pleistocene Ironshore Formation (Units A-F) on Grand Cayman (80–500 ka) to determine the diagenetic effects on both physical and chemical properties of corals. In the Ironshore Formation, the corals in Units A and C were altered in a semi-open diagenetic system by seawater diluted by meteoric water, which resulted in chemical diagenesis that changed the elemental and isotopic composition of the corals even though their aragonitic skeletons show no obvious signs of change. In contrast, the limestones in Unit B have been pervasively recrystallized in an open diagenetic system by meteoric waters that caused both chemical and physical changes. The corals in Units D to F, however, underwent minimal diagenesis and show no evidence of alteration. For altered corals from the Ironshore Formation, elevated Mg/Ca and low Sr/Ca ratios provide evidence of subtle chemical diagenetic changes even in corals that have retained their primary aragonitic skeleton. Thus, corals that are formed of >95 wt% aragonite, show no evidence of cementation, and have Mg/Ca ratios <12.0 mmol mol−1, Sr/Ca ratios >8.0 mmol mol−1, δ18O values >−5.7‰, and δ13C values >−3.0‰, are ideal candidates for paleoclimate reconstruction. Adoption of these criteria shows that the corals from Units D to F can be used to calculate sea surface temperatures (SST), whereas those from Units A to C cannot be used for this purpose. The calculated SST and the corresponding temperature profiles derived from the corals in Units D to F are consistent with global SST reconstructions during the Pleistocene. Unit D corals record an overall increase in SST, which is consistent with elevated temperatures associated with Marine Isotope Stage (MIS) 5e, whereas Units E and F record overall cooling, which is consistent with the end of the peak interglacials of MIS 5c and 5a, respectively.