Microfossils as Hydrographic and Hydroclimate Indicators in Bahamian Blue Holes on Carbonate Tidal Flats

Abstract

Rainfall in the western sub-tropical North Atlantic region is strongly seasonal and modulated by large-scale climate forcing. Problematically, current regional hydroclimate records are inconsistent or incomplete over the late Holocene due to a lack of high-resolution records, poor or negligible overlap with the instrumental period, or limited geographic coverage. The hydrographic conditions within groundwater-fed Caribbean blue holes are linked to local hydroclimate, and blue hole stratigraphy can preserve long-term records of external forcing via in-situ hydrographic changes. In succession, this dissertation (i) first explores the modern foraminiferal distributions and hydrography in two subtidal blue holes on Great Abaco Island, The Bahamas. Then, this information is leveraged to (ii) develop a high-resolution record of hydroclimate during the Common Era, and (iii) document evidence for regionally pluvial conditions from 4100-3200 years ago. The first study presents evidence that the unique hydrographic conditions of the blue holes support an in-situ blue hole foraminiferal assemblage, characterized by benthic foraminiferal taxa tolerant to ‘stressful’ environmental conditions of brackish waters and dysoxia. In both blue holes, modern meteorological and water column profiling document the sensitivity of blue hole hydrography to rainfall any time during the year, as opposed to simple covariance of blue hole stratification with boreal summer rainfall maximums. In the next study, a well-dated sedimentary and benthic foraminiferal record (i.e., multi-decadal to sub-decadal) from Freshwater River Blue Hole documents 2000 years of hydroclimate variability over Abaco Island. More specifically during the instrumental period, enhanced wintertime rainfall increases fine-grained organic matter (OM) deposition in an otherwise carbonate succession. In addition, detailed benthic foraminiferal assemblages provide a more time-averaged perspective of increased OM deposition and brackish conditions at the sediment-water interface. Taken collectively, hydroclimate during the Common Era on Abaco Island appears highly sensitive to the thermal state of the Atlantic, especially during the Little Ice Age. In the last study, ostracodes document an abrupt loss of water column stratification, oxygenation or ventilation of the benthos, and salinity decrease from 4100 to 3200 years ago. The appearance of oligohaline ostracodes following a prolonged period of benthic anoxia suggest increased submarine groundwater discharge through the blue holes, most likely associated with increased regional rainfall. Overall, this dissertation highlights that sedimentary and micropaleontological records (i.e., ostracodes and benthic foraminifera) in blue holes are useful for evaluating paleo hydroclimate variability in the western subtropical North Atlantic. Further, this work also cautions that careful evaluation of modern hydrographic behavior of blue holes is required to fully understand paleo hydroclimate records.