STABLE ISOTOPE ANALYSIS OF HOLOCENE CARBONATE EOLIANITES WITH MULTIPLE PENETRATIVE CALICHE HORIZONS AND A COMPARISON WITH PLEISTOCENE EXAMPLES, SAN SALVADOR, BAHAMAS

Carbonate eolianites of the Holocene Rice Bay Fm exposed at North Point on San Salvador, Bahamas have multiple horizons of mm-scale penetrative caliche with common rhizoliths. Caliche coats bedding planes in the upper part of the exposure and separates wind-ripple laminated strata into discrete ~3 to 30 cm thick beds. Stable isotope analysis was aimed at gaining insights into the origin of caliche and diagenesis of host sediment, and a comparison is made with Pleistocene eolianites characterized by thicker beds separated by better-developed paleosols.

Isotopic values (‰ VPDB) of micritic Holocene caliche (n=29) are highly variable and relatively negative (δ¹³C = -9.18 to -4.31; δ¹⁸O = -14.33 to -8.22), as expected for meteoric precipitate. Substantial variations and no apparent trends were observed among 12 caliche horizons spanning over 2 m of stratigraphic thickness. Associated rhizoliths (n=7) have similar δ¹³C (-9.22 to -4.76 ‰), but less negative δ¹⁸O values (-12.20 to -6.30 ‰) than host caliche, suggesting similar origin and minor differences related to precipitation around plant roots. In contrast, the thicker and better-developed Pleistocene caliches (n=8) have less variable composition (δ¹³C = -8.74 to -7.71; δ¹⁸O = -8.48 to -6.50), related to a lower number of samples analyzed, less variable conditions, and/or more buffering to host-rock values. Generally more positive Pleistocene δ¹⁸O values indicate greater influence of evaporation on or close to the surface.

Composition of Holocene ooid-skeletal eolian grainstone (whole-rock; n=39) varies greatly (δ¹³C = -9.82 to +2.63; δ¹⁸O = -12.33 to -4.57). The most positive values resemble primary marine composition, and the most negative values represent most severe alteration by meteoric water. Comparison with the most positive values of Pleistocene (MIS 5e) grainstone (n=9; δ¹³C = -6.44 to +1.26; δ¹⁸O = -7.18 to -4.70) indicates similar primary composition of these interglacial marine deposits. The most negative values are not as low as Holocene ones, which is consistent with caliche data. This reflects variations in terrestrial environmental conditions and less alteration of Pleistocene marine carbonate. The results reveal important differences in stratification pattern and diagenesis of Pleistocene vs. Holocene eolianites in the Bahamas.