Diagenetic development of rhizoliths in the Ironshore Formation (Pleistocene) of the Cayman Islands

Abstract

Rhizoliths in the Ironshore Formation on the Cayman Islands, up to 12 cm in diameter and 1 m long, developed in skeletal grainstones that accumulated on sand flats above high tide level. Internally, these well-preserved rhizoliths are characterized by well-defined concentric zones, centered around the central root opening, that reflect radial variations in calcite–aragonite content, decreases in micrite content, increases in porosity, and radial variations in trace element (e.g., Ca, Ni, S, Si, and Sr) concentrations. The stable isotopes, δ¹³C and δ¹⁸O, which vary in accord with the internal zones are more variable than the δ¹³C and δ¹⁸O of the host sediments. For the rhizoliths there is a high correlation between the δ¹³C and δ¹⁸O values and the calcite content. The contrast between the rhizoliths and their host sediments is largely a reflection of the more intense diagenetic processes that were largely mediated by the microbial populations that were concentrated in the rhizosphere that developed around each plant root. Microbial activity, which mediated micritization of the skeletal sands and micrite precipitation in the intergranular pores, was largely responsible for development of the internal zonation in the rhizoliths. Use of the δ¹³C values to determine if the rhizoliths developed in association with C₃ or C₄ vegetation failed because the δ¹³C values correlate with the calcite content that developed largely as diagenetic products.