THE MEANING OF DOLOMITE IN THE DEEP-SEA SEDIMENTARY RECORD: IMPLICATIONS FOR GLOBAL CARBON CYCLE (Invited Presentation)

Although dolomite is not abundant in the Modern, it is a minor and persistent component (< 1 to 20%) of many deep-sea sediments. In this presentation we show data on dolomite abundance and δ¹³C values from sediments deposited in deep water (> 500 m) adjacent to several carbonate platforms (Bahamas, Maldives, Great Barrier Reef, and Great Australian Bight) that suggest that higher concentrations of dolomites generally are associated with periods of low rates of deposition. The low rate of deposition is critical as it provides time for the diffusion of Mg²⁺ and SO₄^2- from the over lying seawater. Magnesium is necessary for the process of dolomitization while SO₄^2- is needed to oxidize organic material and thereby raise the alkalinity to promote the dolomitization process. In turn the oxidation of organic material alters the carbon isotope balance and, depending upon the amount of SO₄^2- supplied, the organic content, and the carbonate content of the sediments, can produce a negative δ¹³C value in the bulk sediments. Such changes in the δ¹³C values do not represent a global signal and the extent to which they develop is very much dependent upon local sedimentary conditions, such as sediment supply, insoluble content, and slope geometry. A typical dolomite profile beneath a hardground might show a low concentrations immediately at the hardground surface (representing a more open system), but increase with depth, reaching a maximum at a distance dependent upon the time period represented by the hiatus (thus controlling the supply of Mg²⁺ and SO₄^2-) and the amount of organic material available for oxidation (closed system). An interval of reduced sedimentation may show a similar profile, although the maximum concentration of dolomite and influence on the δ¹³C values would be reduced. Further support for these processes is provided by additional geochemical tracers such as δ³⁴S values of the carbonate associated sulfate, δ¹⁸O, δ⁴⁴Ca, and δ²⁶Mg values, and concentrations of strontium in the dolomite.