Temporal variations in Rare Earth Element distributions in the Cenozoic succession and modern sediments of the Cayman Islands

Abstract

The Rare Earth Elements (REEs) in limestones and dolostones that develop on isolated oceanic islands, like the Cayman Islands, were acquired from the seawater in which they formed and various diagenetic processes that led to their lithification and, in some cases, pervasive dolomitization. Modern sediments from North Sound, Grand Cayman, contain up to 30.93 ppm ∑REEs and 45.23 ppm ∑REEs + Y, whereas modern corals contain up to 2.30 ppm ∑REEs and 3.65 ppm ∑REEs + Y. The limestones and dolostones in the Brac Formation (late Oligocene), the Cayman Formation (Miocene), and Ironshore Formation (Pleistocene) generally contain <7 ppm and <10 ppm, ∑REEs and ∑REEs + Y, respectively. In contrast, the Pedro Castle Formation (Pliocene) contains up to 14.18 ppm ∑REEs and 20.81 ppm ∑REEs + Y. The Distribution Coefficients for the REEs from the modern Cayman corals are similar to those determined for corals elsewhere in the world. In the modern sediments from North Sound there is a high correlation between the ∑REEs + Y and Fe and Al but a low correlation with Na and Mn. In contrast, the correlation between the ∑REEs + Y and other elements in the limestones of the Ironshore Formation and dolostones/limestones of the Brac Formation and Cayman Formation is low. The light REEs were >Limit of Quantification (LoQ) in most samples, whereas the medium REEs and heavy REEs were commonly <LoQ. There is no recognizable pattern to the presence/absence of the light, medium, and heavy REEs. Collectively, these data show that (1) the REEs in the limestone/dolostone succession on Grand Cayman are similar to those from Cayman Brac, (2) dolomitization does not seem to have modified the REEs of the original limestones, and (3) the content of the REEs of the limestones and dolostones of the Pedro Castle Formation is higher than those in the Brac Formation and Cayman Formation. This may indicate that the concentration of the REEs in the seawater during the Pliocene highstand was higher than those when the original sediments of the Brac Formation and Cayman Formation were deposited.