Morphology of the Belize Barrier Reef as indicators for postglacial Atlantic sea-level changes

The mixed carbonate-siliciclastic barrier and atoll reef system offshore Belize is the largest modern tropical reef complex in the Atlantic Ocean, highly sensitive to past and future sea-level changes. The deglaciation and increasing temperatures after the Last Glacial Maximum caused a rise in sea level characterized by multiple melt-water pulses and stillstands, which left their characteristic marks in the morphology and growth pattern of the Belize Barrier Reef. Such postglacial sea-level change indicators provide thus critical details to reconstruct how sea level rose from the full glacial to Late Holocene levels. We present a study that was done within the framework of the active IODP proposal “Postglacial Atlantic sea-level and climate reconstruction through drilling the Belize Barrier Reef (BBRdrill)”. To gain better insight into the morphological details, we acquired a high-resolution (1 x 1 m) topographic dataset of the Belize Barrier Reef with a state-of-the-art multibeam bathymetric device. Moreover, by investigating the entire point cloud of sonar reflections, we were even able to visualize the rarely investigated overhanging reef walls in great detail.

Concise morphological features indicating stagnant or slow-change phases were mapped in detail. They comprise elongated ridges at various water depths, indicating reef build-up to past sea level, which are aligned in single or multiple parallel lines, connecting hook-like structures, or complex honeycomb patterns. We hypothesize that older, postglacial and glacial reefs are stacked more or less vertically below the outermost ridge and the wall. The walls contain various erosional notches indicating still stands of sea level causing enhanced erosion in the quasi-vertical structure. This vertical stacking of the barrier reef crests gets affected towards the submarine outflow area of the English Cay Channel, where turbid waters likely challenged reef growth so that the aggradation eventually stopped and reefs drowned forming a reef line deepening towards the channel.

We provide a statistical distribution of features indicative for sea levels over 100 km length of Belize Barrier Reef, indicating the different slow-downs or stillstands of sea levels since the last glacial maximum. Several levels of erosional notches could be mapped at water depths of ~ -60 to -110 m, whereas the single or multiple reef crest occurs within a range of ~ -15 to -40 m water depth relating to sea levels ~13-16 ka and ~8-11 ka, respectively. The bathymetric distribution of notches and reefs suggests also the existence of a vertical tectonic displacement in the reef.

"BBRdrill" proposes to drill these morphological features in order to i) reconstruct the LGM and postglacial sea-level rise in the western Atlantic ii) reconstruct environmental parameters using corals, coralline algae, and cryptic microbialites; iii) elucidate reef paleoecology in relation to postglacial sea-level rise and associated environmental changes; and iv) assess microbial life in a barrier-reef system.