Late Pleistocene to Holocene sedimentation in the Great Blue Hole (Lighthouse Reef, Belize): Results from a 30 m long core

Abstract The Great Blue Hole is a prominent flooded karst sinkhole, located in the lagoon of Lighthouse Reef atoll off the coast of Belize. Short cores recovered from varved bottom sediments have been used in previous studies as climate and cyclone archives covering as much as the past ca 1.7 ka BP. A new 30 m long sediment core, encompassing the entire Holocene and the latest Pleistocene, allows a reconstruction of the development of this geoscientifically significant site in the light of the postglacial and Holocene sea‐level rise. The sedimentary succession in the core is tripartite. The lowermost sedimentary unit A (30.0–28.6 m) comprises grey‐brown to black organic‐rich carbonate sediments, which contain freshwater snails ( Pyrgophorus ), tropical forest pollen (Myrtaceae) and a low‐diversity dinocyst assemblage. The intermediate unit B (28.6–24.65 m) is a dark greyish‐green to greyish‐brown, weakly laminated carbonate silt, which comprises marine fossils, that is, euryhaline foraminifera ( Elphidium ) and Halimeda (codiacean algal) platelets. Unit B contains abundant red mangrove ( Rhizophora ) pollen and a dinocyst assemblage indicating high productivity in surface waters. The uppermost unit C (24.65–0 m) is an annually layered buff and light green carbonate silt with abundant marine fossils. A total of 574 intercalated, coarser‐grained and lighter‐coloured event (storm) beds usually rich in Halimeda and coral fragments were identified in unit C. Fully marine conditions, including trophic seasonality, are also indicated by the dinocyst spectrum. The pollen spectrum derives from a variety of trees (largely pine, oak, podocarp), shrubs and herbs. The sedimentary succession represents the transition from an initial terrestrial cenote phase on a vegetated limestone island (unit A: 12.5–7.2 ka BP), via a subsequent restricted marine phase on an initially flooded carbonate platform with mangrove swamps (unit B: 7.2–5.7 ka BP), to a fully marine phase in an open, well‐circulated atoll lagoon (unit C: 5.7–0 ka BP).