Mangrove ecosystem changes during the Holocene at Spanish Lookout Cay, Belize

Mangroves are halophytic plants living at the land-sea interface and are therefore natural trackers of sea-level. Multiple proxies of a continuous (8 m) mangrove peat core (BT-79) from Spanish Lookout Cay, Belize illustrate mangrove ecosystem changes during the Holocene. Radiocarbon measurements show this site was colonized by mangroves similar to 8000 cal. yrs BP, with a significant decrease in the peat accumulation rate from similar to 6000 to 1000 cal. yrs BR Stratigraphic characteristics of this peat core such as bulk density, magnetic susceptibility, and loss on ignition show relative uniformity, inferring an uninterrupted mangrove ecosystem during a majority of the Holocene. This is supported by pollen data from BT-79 that show that the site has been consistently dominated by Rhizophora mangle (red mangrove), with Avicennia germinans (black mangrove) and Laguncularia racemosa (white mangrove) present as well. Subfossil R. mangle leaves are used for stable nitrogen, carbon, and oxygen isotope (delta N-15, delta C-13, and delta O-18) analyses. delta N-15 and delta C-13 values provide a proxy of this plant's past physiology and stand structure showing that dwarf (delta N-15< - 3 parts per thousand) and tall (delta C-13 < - 27 parts per thousand) R. mangle stands were previously present at the site, which are a result of nutrient limitations that we equate with seawater inundation. delta O-18 values show differences in source water of R. mangle, with higher values attributed to the source water being composed of a greater proportion of seawater relative to precipitation. A decrease in inundation at the site is shown by lower delta O-18 values (<19 parts per thousand) from similar to 7000 to similar to 1000 cal. yrs BP that covary with the decreased sedimentation rate. Existing Caribbean sea-level data do not show evidence of a decrease in the rate of relative sea-level rise or fluctuations that we take to be the major causes of environmental changes at site BT-79. (C) 2009 Elsevier B.V. All rights reserved.