During the last ice age sea level fell over 100 m below the present level and the Bahamas became an elevated flat topped carbonate terrain in which there were abundant cave formations.  Within these caves different types of speleothems were formed recording the temperature and thechemistry of the waters which formed them. The stalagmites stopped forming as sea level increased and the caves became flooded by seawater.  Using advanced diving methods, stalagmites have been retrieved from these caves from depths of up to 50-100 m.  The stalagmites forming within these caves can be dated using uranium-thorium methods and the climate detailed from the chemical information contained within the calcium carbonate forming the stalagmite.  An example of one of these stalagmites and the position of the dates is shown in the figure 1. This figure shows a sectioned stalagmite revealing dark bands and laminations.  The darker bands generally correspond to the Heinrich events and are characterized by higher concentrations of Fe. During the last glacial time period the northern hemisphere was subjected to a series of extremely cold intervals (Heinrich events) during which the glacial ice sheets extending into the northern Atlantic Ocean.  The extent of temperature variation in the sub-tropics is not known, but generally it is believed that only minimal changes in temperature occurred. This notion is challenged by our work. The Heinrich events are characterized in the Bahamas by changes in the stable C and O isotopic composition of the stalagmites.  Previous research would have interpreted these changes as reflecting a change in the source of precipitation.  However, using a newly developed method at the University of Miami we have been able to extract fluids from the speleothem which represent trapped water preserved from the time of formation.  We have then analyzed these fluids using a cavity ring down spectrometer.  Combining these results with analyses of the carbonates in the speleothems our work suggests that these changes in oxygen isotopes are actually related to lower temperature at this time period rather than a change in the source of the precipitation.  An example of the changes in the isotope within one stalagmite is shown in figure 2.   In this example the temperature dropped by approximately 6^oC within Heinrich event 1 (H1).  The termination of H1 was marked by a similar rise in temperature. Furthermore, these results support the teleconnection between the subtropical Atlantic and cooling in the North Atlantic and the sensitivity of the Bahamas to changes in the global oceanic circulation.   Similar changes were discovered associated with other Heinrich events in these stalagmites and are replicated across multiple specimens obtained from different depths.

We also noticed increases in the concentration of Fe associated with all of the terminations of the Heinrich stadials.  We initially believed that this was a result of increased dust flux, a finding supported by higher than normal dust fluxes during these time periods. To evaluate the mechanisms linking this enhanced dust radiative forcing during HS and the AMOC, we parameterized the dust radiative effects in the University of Victoria Intermediate Complexity (UVic) earth system model, and explored the sensitivity to the amount of atmospheric dust loading, and freshwater (FW) forcing. Our modelling results suggest that the western subtropical Atlantic temperatures is sensitive to the amount of North African dust emitted during HS and we are comparing these results with proxy records from Bahamian caves to determine which forcing scenarios lead to the best agreement between model and data for different HS.  However, our current thinking is that the high Fe present in the stalagmites is a results of an increase in water supply at the termination of the Heinrich events, a change which also corresponds with a pronounced change in the carbon isotopic composition.

Last Modified: 11/15/2016
Modified by: Peter K Swart