Two Major Cenozoic Episodes of Phosphogenesis Recorded in Equatorial Pacific Seamount Deposits

Abstract

Sea_mount phosphorites have been recognized since the 1950s, but this is the first study to provide an in depth exploration of the origin and history of these widespread deposits. Representative samples from equatorial Pacific Cretaceous s•mounts were analyzed for chemical, mineralogical, and stable isotope compositions. The phosphorites occur in a wide variety of forms, but most commonly carbonate fluorapatite (CFA) replaced middle Eocene and older carbonate sediment in a deep water environment (>1000 m). Element ratios distinguish seamount phosphorites from continental margin, plateau, and insular phosphorites. Uranium and thorium contents are low and total rare earth element (REE) contents are generally high. REE ratios and shale-normaiized patterns demonstrate that the REEs and host CFA were derived from seawater. Strontium isotopic compositions compared with inferred Cenozoic seawater curves define two major episodes of Cenozoic phosphafization: Late Eocene/early Oligocene (39-34 Ma) and late Oligocene/early Miocene (27-21 Ma); three minor events are also indicated. The major episodes occurred at times of climate transition, the first from a nonglacial to glacial earth and the second from a predominantly glacial to warm earth. The paleoceanographic conditions that existed at those times initiated and sustained development of phosphorite by 1U. S. Geological Survey, Paper number 93PA00320 0883-8305/93/93PA-00320510.00 accumulation of dissolved phosphorus in the deep sea during relatively stable climatic conditions when oceanic circulation was sluggish. Fluctuations in climate, sealevel, and upwelling that accompanied the climate transitions may have driven cycles of enrichment and depletion of the deep-sea phosphorus reservoir. As temperature gradients in the oceans increased, Antarctic glaciafion expanded and oceanic circulation and upwelling intensified. Expansion and intensification of the oxygen minimum zone may have increased the capacity for midwater storage of phosphorus supplied by dynamic upwelling around seamounts; however, the bottom waters never became anoxic during the phosphogenic episodes. Fluctuations in the CCD and lysocline, CO2 fluxes, and changes in bottom water circulation and temperatures may have bathed the seamount carbonates in more corrosive waters which, coupled with increased supplies of dissolved phosphorus, promoted replacement processes. The late Eocene/early Oligocene phosphogenic episode recorded in seamount deposits is not matched by large phosphofite deposits in the geologic record, whereas the late Oligocene/early Miocene episode and middle Miocene event are matched by large deposits distributed globally. The seamount phosphorires are exposed at the surface of the seamounts and have been for most of the Neogene and Oligocene. The phosphorires do not show signs of etching that would indicate substantial undersaturation of seawater phosphate with respect to CFA. Mass balance calculations indicate that about 5.4-19 x 1012 g of P205 are locked up in equatorial Pacific seamount ...

Miocene phosphorites from the Murray Ridge, northwestern Arabian Sea

“…It may also be mentioned that evidence for open-system evolution of 87 Sr/ 86 Sr isotopic ratios in phosphorites resulting in resetted and unusually young Sr-stratigraphic ages has been reported by e.g., McArthur et al (1990), Compton et al (1990), Hein et al (1993), and Stille et al (1994). Modeling the behavior of Sr during diagenesis of carbonate rocks, DePaolo (1987, 1988) found that solution-reprecipitation rates decrease significantly with the age and depth of burial of the sediment and that increasing diagenesis resulted in a shift of the 87 Sr/ 86 Sr towards older stratigraphic ages.…”

Cretaceous and Paleogene Manganese-Encrusted Hardgrounds from Central Pacific Guyots

“…The late Cretaceous to early Paleogene event may correspond to the 71 m.y. "minor" event of Hein et al (1993). A third event that occurred during the early middle Eocene seems to correspond temporally with the deposition of Horizon A c biosiliceous sediments.…”

“…The latest Eocene to early Oligocene event corresponds precisely with a period of major phosphogenesis for the Pacific, as documented by Hein et al (1993). The late Cretaceous to early Paleogene event may correspond to the 71 m.y.…”

“…The pervasive phosphatization of the Eocene pelagic limestone and its host rock suggest a major phosphatization event. Hein et al (1993) identified such a major event, in the western Pacific, spanning the Eocene/Oligocene boundary (34-39 m.y.). The postulated age of this event corresponds almost exactly with the biostratigraphically inferred date from Limalok Guyot.…”

“…Assuming that this was the result of one event, it must have occurred after the early late Campanian (CC20) and before the early late Paleocene (CP4) deposition of the (still calcareous) nannofossil limestone in Sample 144-872B-4R-1, 13-16 cm. Hein et al (1993) report a minor phosphatization event dated as 71 m.y. from the central western Pacific.…”