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Reports: ND252863-ND2: Testing the 'Clumped Isotope Method' Under Conditions of Well-Constrained Diagenesis

Peter K. Swart, University of Miami

Stable oxygen (d18O) and carbon (d13C) isotopes have become standard tools for understanding the formation and diagenesis of carbonate reservoir rocks.  Normally such studies are carried out with the hope that the data can provide information on the environment and the temperature of diagenesis and therefore help with the interpretation of the reservoir development.  However, in all studies the measured d18O of the carbonate rocks is a composite of the d18O of the fluids and the temperature at which the rocks were original formed or recrystallized.  Precise interpretation of the results is therefore usually problematic unless accompanied by other independent information of either recrystallization temperature of fluid composition.

However, there has been a recent revolution in the application of stable oxygen isotopes in the understanding of the temperature of carbonate precipitation and recrystallization, namely the recognition that deviations in the measured abundance of the 13C18O in CO2 released through the digestion of carbonate minerals, as measured at mass 47, differs from that predicted by the rule of means and is solely as a function of temperature.

In this project we carried out a detailed study of the behavior of the clumped isotope signal during (i) formation of non-biologically produced sediments, (ii) freshwater and marine alteration and (ii) dolomitization.

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Figure 1:  Relationship between mean temperatures of formation of the surface samples measured using clumped isotopes and facies variations (1= Mud, 1.5= Mud-rich wackestone, 2= Wackestone, 2.5= Mud-rich packestone, 3= Packstone, 3.5= Mud-lean packstone, 4= Grainstone, 5= Rudstone).

Non-biologically produced sediments

We measured the clumped isotopic compositions of approximately 35 non-diagenetically altered recent biogenic and non-biogenic carbonate samples collected from GBB. Such data are expected to serve as a reference line for future diagenetic studies and is crucial to determine the influence of early diagenesis on clumped signatures Δ47 in marine environments.

There is a correlation between clumped signals and temperature estimates in the original mainly non-biogenic sediments of GBB (Figure 1). However, there may also be some facies control as the clumped signals increase and temperature values decrease from muddy facies to grainy facies.

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Figure 2: The calculated formation temperature (using clumped isotopes) and the oxygen isotopic composition of water from Hog Cay (HC) and New Providence (NP) with pathways of potential change during meteoric diagenesis.

Meteoric Diagenesis

The d18O values of meteorically altered are valuable proxies for the reconstruction of paleoclimate and paleoenvironmental conditions recording both temperature and the d18O values of rainfall. In the Bahamas calcretes form under different rainfall regimes and therefore samples were taken from a relatively wet area (New Providence) and a drier area (Hog Cay) in order to investigate understanding the role of rainfall in interpreting calcrete formation. These samples were analyzed using the clumped isotope method which allows the determination of formation temperature as well as the d18O of the precipitating water.

The formation temperatures are higher or equal than the mean annual summer temperature (MAST) in Hog Cay, while formation temperature values are consistent with mean annual temperature (MAT) under the subtropical conditions of New Providence. The amount of rainfall between Hog Cay and New Providence is a prevailing factor formation temperatures in calcrete formation where the difference between MAT and MAST is restricted by approximately 3°C few degrees. The d18O of calcrete complex is influenced by small temperature effect and mostly d18O of the water directly related to rainfall and intensive evaporation demonstrated by co-varying of the traditional isotopes.

Dolomitization

Samples have been analyzed from several localities in the Bahamas including San Salvador and Clino.  The samples from San Salvador generally give expected temperatures, provided that the acid fractionation factor developed at the University of Miami is used.

Dolomite samples from Clino give warmer than normal temperatures, a feature which we believe is a result of fractionation association with formation under conditions of bacterial sulfate reduction.

Marine Diagenesis

Samples are being analyzed from several Ocean Drilling Program Sites (1003 and 807).  The goal will be do investigate changes in the clumped isotope signature as a function of depth.  These analyses are continuing and will part of a PhD dissertation which will be completed in three years time.