Environmental Controls on Alternating Aragonite-calcite Laminations in Notch-speleothems from Cayman Brac, Cayman Islands, British West Indies

Abstract

Speleothems that grow in semi-exposed surface environments, such as wave-cut notches, are not well understood despite their widespread distribution. The +6.4 m-high wave-cut notch on Cayman Brac, that formed ~ 125 ka ago, is decorated with stalactites, stalagmites, and columns that grew between ~50 ka and ~45 ka ago. These speleothems are formed of calcite, aragonite, and mixed calcite-aragonite laminae, that are commonly separated by unconformities. The mineralogy, crystal morphologies, and fabrics vary between laminations in the same speleothem and from speleothem to speleothem. The calcite and aragonite laminae in the Cayman notch-speleothems are primary precipitates with minimal alteration by diagenesis or microbial processes. The δ18O values of these precipitates range from -6.5 to +0.3‰ VPDB, and the δ13C values range from -11.4 to +1.9‰ VPDB. The strong positive co-variation between δ18O and δ13C indicates that evaporation and/or kinetic fractionation was involved in the precipitation processes. Growth of the speleothems spans the warm and wet Dansgaard/Oeschger interstadial 13 (D/O-13), the cooler and drier Heinrich stadial 5 (HS-5), and the warm and wet Dansgaard/Oeschger interstadial (D/O-12), that are millennial-scale climate events. Comparison of stable-isotopic variations from the notch-speleothems with other paleoenvironmental proxies suggests that their growth was controlled by the paleoclimate variation, and confirms the global teleconnection between the tropical Caribbean and the high-latitude North Atlantic. The cooler and drier HS-5 period with enhanced evaporation potentially led to the elevated Mg/Ca and elevated saturation states, that may have been the main control over aragonite precipitation. In contrast, precipitation of calcite was typically favored during the warm and wet periods (D/O-13 and D/O-12) when evaporation was lower.