Study of Os isotopic variations in marine deposits has revealed evidence of a nearly eight-fold increase in the 187Os/186Os of seawater during the Cenozoic (Pegram et al., 1992; Ravizza, 1993; Peucker-Ehrenbrink et al., 1995; Esser et al., 1994). However, the existing record has several
limitations. First, the majority of data are from analyses of Pacific samples. Thus the interpretation of the existing data as an ocean wide record is tentative. Second, the materials presently used as recorders of past variations in the marine Os isotope record (pelagic clays, manganese crusts and metalliferous sediments) have slow to moderate accumulation rates, and often, poorly constrained chronologies. This hampers efforts to correlate between records and impedes study of high frequency variation in the marine Os isotope record. Third, very few analyses of sediments older than 65 Ma are available. Consequently, current data reveals
the range of marine Os isotopic variations over a relatively small portion of the varied climatic and tectonic conditions represented during the Phanerozoic.
To extend the geographic coverage and temporal resolution the marine Os record, Os isotopic variations from rapidly accumulating Neogene organic-rich sediments are being examined. If Os isotope records from this sediment type can be demonstrated to be reliable, significant improvements in geographic coverage and temporal resolution become possible. Analyses of organic-rich nannofossil oozes from the Eastern Tropical Atlantic (ODP Site 658) and the Oman margin (ODP Site 723) have yielded mixed results. Data from bulk analyses of sediments from Site 658 reveal an increase in 187Os/186Os ratio from 7.5 to 8.7 over the past 3.5 Ma, similar to the variation reported in the Pacific. This pattern of increase is also consistent with a recent description of Os analysis of a manganese crust from the western North Atlantic (Burton et al., 1995), and support the contention that the current marine Os isotope record is representative of the global ocean. In contrast, bulk analyses of sediments from ODP Site 723 on the Oman Margin yield 187Os/186Os ratios lower than the inferred value of contemporaneous seawater. These low ratios are attributed to the influence of unradiogenic Os derived from the Semail ophiolite. Preliminary experiments indicate that combustion of bulk sediment samples from Site 723 volatilizes a more radiogenic fraction of Os, possibly a seawater derived component. Refinement of this methodology may provide a means of selectively isolating a seawater-derived Os component from bulk sediment samples by organic matter combustion.
To extend the marine Os record farther back in time, metalliferous sediments associated with obducted oceanic crust are being investigated. Six samples from the Late Cretaceous Perapedhi Formation, which directly overlies pillow lavas of the Troodos ophiolite, have been analyzed. Five samples cluster within a narrow range in Os isotopic composition (187Os/186Os between 4.2 and 4.5) while the sixth is distinctly less radiogenic (187Os/186Os = 2.0). Based on analogies to modern metalliferous sediments from the base of the active TAG hydrothermal mound on the Mid-Atlantic Ridge, the cluster of 187Os/186Os between 4.2 and 4.5 are interpreted as a measure of the Os isotopic composition of Late Cretaceous seawater. Similar deposits of Paleozoic age exist and may provide a record of variations in the Os isotopic spanning Phanerozoic time.
Burton et al., Terra Nova 7, 22 (1995).
Esser, B. K. et al., EOS 74, 368 (1993).
Pegram, W. J. et al., Earth planet. Sci. Lett. 113, 569-576 (1992).
Peucker-Ehrenbrink, B. et al., Earth planet. Sci. Lett. 130, 5155-167 (1995).
Ravizza, G., Earth planet. Sci. Lett. 118, 335-348 (1993).