Stable Organic Isotopic (δ^15Norg, δ^13Corg) Evidence for Climate-Forced Landscape and Aquatic Change During the Late Holocene in Abaco Island, The Bahamas

Abstract

The sediments that have accumulated in Blackwood Sinkhole preserve a 3000-year record of environmental change on Abaco Island, Northern Bahamas. Previous palynological data reveal that Abaco’s forest structure has not been constant during the late Holocene (last 3000 years), likely in response to local precipitation changes from southern migration of the Intertropical Convergence Zone (ITCZ) at 1000 Cal yrs BP. This thesis investigates the geochemical record of forest structure change with geochemical signals (δ^13Corg and δ^15Norg, C:N and Lignin-derived CuO Oxidation Products (LOP)) preserved by the bulk sedimentary organic matter (OM) within Blackwood Sinkhole.

Currently, the northern Bahamian islands (Abaco, Andros, Grand Bahamas, and New Providence) receive greater annual precipitation than those further to the south, which may be generating sufficiently mesic conditions for the northern islands to support Pinus forests. Geochemical proxies reveal shifts in OM sources throughout the core that can be classified under three distinct groupings. Measured variation in the geochemical proxies remains consistent with the effects of a southern displacement of the ITCZ at 1000 Cal yrs BP. Group 3 (approximately, 1500-3000 YBP (1700 YBP in core 3)) is characterized by a depleted δ^13Corg signature (relative to group 1), a higher mean S/V ratio relative to group 1, the highest mean C:N ratio, and a dominance of Myrtaraceae and Arecaceae. Group 2 (approximately 1000-1500 YBP) is defined by a higher mean δ^15Norg signature, the most depleted δ^13Corg signature, an increase in soft tissue LOP signatures (Cinnamyl/Vanillyl, C/V ratio), and a presence of Lemna spores within the interval. Group 1 (approximately 0-1000 YBP) is predominantly defined by a relatively enriched δ^13Corg signature, a lower Syringyl/Vanillyl (S/V) LOP ratio, and an emergence of Conocarpus, Pinus, and Typha angustifolia. OM source changes, defined by the measure of δ^13Corg and δ^15Norg, C:N and LOP signatures, within Blackwood Sinkhole support the previously compiled palynological reconstruction of forest structure change, and record regional terrestrial and aquatic response (of the surrounding watershed and in the sinkhole) to larger scale climatic changes through time.