RHIZOGENIC LAMINAR CALCRETE DEVELOPED ON PLEISTOCENE SUBTIDAL CARBONATES, AMBERGRIS CAYE, BELIZE

Pleistocene MIS 5e subtidal carbonates from the Grand Belizean Estates area, ~5 miles North of San Pedro Town on Ambergris Caye, Belize are capped by a prominent subaerial laminated caliche crust or laminar calcrete that marks the Pleistocene/Holocene boundary. This layer was examined in the field, and via hand specimens, petrographic thin sections, XRD and stable isotope analyses to: 1) assess the extent of meteoric diagenesis on subtidal carbonate deposits; and 2) document the composition and origin of extensive laminar calcrete for subsequent comparisons with coeval examples throughout the wider Caribbean.

Caliche crust capping the succession has brown color, wavy lamination, and thickness up to 9 cm. It is composed of iron-oxide impregnated microcrystalline calcite with common rhizolithic or root molds and fenestral fabric. Some pores are partially occluded with remains of plant roots. This crust represents rhizogenic laminar calcrete that formed by dissolution and reprecipitation of fine-crystalline carbonate around plant roots during subaerial exposure.

Shelly subtidal limestone (peloidal-skeletal packstone) below the surficial crust is noticeably altered and contains dark irregular patches of iron-oxide rich micrite with root molds. Visually the least altered subtidal limestone from the lower part of the succession (~1.5 m below the crust) also contains evidence for modification during exposure in the form of dissolution of skeletal fragments and iron-oxide staining. This is supported by XRD analyses, which indicate calcitic composition and the absence of aragonite. Stable isotope data mark an “inverted J trend” with: 1) mollusk shells having the most positive (i.e., least altered) compositions; 2) bulk subtidal sediment of extremely variable degrees of alteration; and 3) laminar calcrete with very negative carbon isotope values, indicating formation from meteoric fluids under the influence of soil derived organic matter. In conjunction with common rhizolithic structures, this suggests that the study area had a climate suitable for terrestrial plant growth and preservation of root traces, and that plant-sediment interactions during subaerial exposure resulted in development of thick rhizogenic laminar calcrete that serves as a useful paleoenvironmental and paleoclimate indicator.