REFLUX CIRCULATION AND GROUNDWATER AGE BELOW THE GREAT AUSTRALIAN BIGHT

Drilling during ODP Leg 182 revealed Na-Cl brines with salinities exceeding 80 g/L within 500 m of the seafloor below the Great Australian Bight. The brines most likely represent the remnants of a reflux circulation system that was active during past sea-level low stands, but flow rates and the ages of these fluids are unknown. We developed numerical models based on available geophysical data to see how long reflux brines might persist in this system and at what rate they could be discharging to the ocean. Coupled fluid flow, heat transport and solute transport equations were solved using COMSOL. Preliminary results suggest that the brines formed less than 1 My ago, and dispersive mixing with younger fluids has reduced the apparent age to a few hundred thousand years. Brines are likely discharging to the ocean at rates of cm/yr. Per unit volume, this type of discharge delivers a large solute flux to the ocean compared to freshwater discharge. This type of flow may also have been much more common in the past, and may have contributed to seawater composition, depending on the degree of diagenetic alteration during flow.