Redefining the Stable Oxygen and Carbon Isotopic Composition of the Mixing Zone

Abstract

The landmark work of Allan and Matthews recognized a series of patterns in the stable O and C isotopic composition of carbonates which could be used to identify exposure surfaces, the vadose zone, the freshwater phreatic zone, the mixing zone, and the marine phreatic zone. In the case of the mixing zone, an interval common in many islands and coastal zones in which freshwater mixes with marine fluids, the pattern was proposed to be identifiable by strong positive correlation between carbon and oxygen isotopic values measured in the carbonate record. This covarying pattern was evident between 110 and 150 m depth in cores drilled into Neogene sediments in the Bahamas and Florida and has been interpreted by various workers as being produced by the mixing zone associated with Pleistocene sea-level variations. However, this pattern could not have been produced by the mixing of fresh and marine water as during the last glacial sea level would have been at least 120 below the present level. As a result of the principle associated with Ghyben Herzberg lens, there must have been 40 meters of freshwater lens for every 1 meter of elevation of the water table. Therefore with a water table elevation of at least 2 meters, similar to that seen in the Everglades today, the freshwater lens would have extended to a depth of 200 m or more. This depth would have included the entire interval in which the stable isotopes of carbon and oxygen co-varied or what is currently being interpreted as the mixing zone. Hence the positive correlation between carbon and oxygen isotopes in the carbonates cannot have been produced by alteration by waters with varying salinities. Rather the covarying pattern probably corresponds to the thickness of the freshwater lens (rather than the mixing zone) with greater amounts of diagenesis taking place in the upper portion of the profile, close to the interface between the vadose and phreatic zones and lower amounts of alteration at the bottom, close to the transition into the mixing zone. Such interpretations are supported by work on modern vadose and phreatic zones as well as petrographic examination of the rocks from these sequences. If correct these data would necessitate a reinterpretation of carbon and oxygen trends in ancient carbonates.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016