STRATIGRAPHIC EVIDENCE FOR HOLOCENE ENVIRONMENTAL CHANGE IN A BERMUDIAN COASTAL UNDERWATER CAVE (PALM CAVE SYSTEM): IMPLICATIONS FOR BERMUDA’S SEA LEVEL HISTORY

Cave and sinkhole sediment records provide an important perspective of climate and environmental change on karst landscapes, but the sedimentary deposits in underwater caves remain poorly understood. Previous research from Bermuda suggests that (a) Holocene sea-level rise plays a critical role in driving environmental change in underwater caves by providing the aquatic (i.e., groundwater) framework for the cave system, and (b) that cave sedimentary deposits may provide useful information related to Holocene sea-level rise on carbonate platforms. Here we present a multi-proxy stratigraphic analysis of 14 sediment cores that were collected from 2 to 20 m below groundwater level in the Palm Cave System, Bermuda (e.g., textural analysis, x-radiographs, microfossil analysis, radiocarbon dating). The rate of deposition at each core site was variable during the Holocene, and dependent upon the proximity of core locations to cave openings (‘karst windows’) and conduit geometry. The oldest recovered sediment was Pleistocene-aged, terra-rosa soil deposits that predate the Holocene inundation of the cave by the coastal aquifer. By ~9500 Cal yrs BP, deposition in the cave was dominated by an organic-rich sapropel facies containing brackish foraminifera (Trochammina, Polysaccammina) and bivalves, which indicates that a meteoric lens (low salinity, oligohaline) of the coastal aquifer flooded the cave benthos by ~9200 Cal yrs BP. A coherent pattern of Holocene sea-level rise in Bermuda emerges when the sedimentary results from the Palm Cave System, which is a minimum sea-level indicator, is compared to other Bermudian sea-level indicators (marsh deposits, cave deposits elsewhere). A system-wide shift to carbonate deposition occurred in the Palm Cave System at ~6500 Cal yrs BP, which indicates that the onset of oxygenated marine water entering the cave and development of a marine-dominated (i.e., submarine) cave environments. These results indicate that sea-level change is an important driver of environmental change in coastal underwater caves, and where available, their sedimentary records preserve useful information on Holocene sea-level rise.