UPPER SLOPE 3D MORPHOLOGIES ALONG THE LIGHTHOUSE REEF MARGIN (BELIZE): PUNCTUATED GLOBAL RECORD OF LAST DEGLACIAL SEA LEVEL FLUCTUATIONS?

In the past 500 ky, Earth has witnessed unprecedented extreme glacial/interglacial cyclic changes in climate that translated into unusually high amplitude sea-level fluctuations. During the most recent glacial to interglacial, or deglacial, transition, sea level rose ~125 m since the Last Glacial Maximum (LGM), or during the last 20 kyr! The Belize offshore isolated carbonate platforms, one of them the Lighthouse Reef (LR), and the Barrier Reef itself were fully exposed during the LGM, forming islands and coastlines with imposing coastal cliffs, contrasting with the thriving coral reef systems we know today. High-Resolution Multi-beam Echo Sounder (MBES) data sets were recently acquired along segments of the LR leeward and windward margins, from a few meters to 400 m of water depth. Once the data sets were processed, high-resolution 3D bathymetric and slope models, and hypsometric analysis, display a series of characteristic morphological features on the LR upper slopes: (a) a marked change of slope angle at ~120-130 m at the base of a subvertical wall, (b) the wall itself ubiquitously occurs between ~80 and ~120 m, (c) two major terraces centered at ~70 m and ~50 m, separated by steep cliffs, and, (d) one or two terraces in front of the modern reef crest centered at ~20 and ~10 m. The slope terraces are more developed on the LR leeside, while its windward margin is systematically much steeper. Because these morphologies compare well with similar ones observed along the upper slopes of atolls in the Maldives Archipelago or drowned reefs offshore the Gulf of Papua and south Texas shelf edge, they are assumed to be related to well established punctuated global sea level rises. The abrupt change of the slope at ~120-130 m water depth marks where sea-level was located during the Last Glacial Maximum 21 ka. The two terraces, centered at water depths ~70 and ~50 m, most likely drowned by the fast rates of sea level rise (m/century) during Meltwater Pulses 1-b and 1-c (11.5 and 9.5 ka). The rapid sea level rise during MW-1d (~7.5 ka) could explain the drowning of the ~ 20 m terrace. Alternatively, these terraces could correspond to the re-occupation of older terraces formed during the sea-level stepwise lowering at sub-glacial and interglacial Marine Isotope Stage (MIS) 5 d-c-b-a and MIS 3, following the MIS 5e interglacial sea level highstand.