Two major Cenozoic episodes of phosphogenesis recorded in equatorial Pacific seamount deposits

Abstract

Seamount 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 seamounts 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-normalized 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 phosphatization: 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 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 glaciation 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, CO 2 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 phosphorite 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 phosphorites are exposed at the surface of the seamounts and have been for most of the Neogene and Oligocene. The phosphorites 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 × 10 12 g of P 2 O 5 are locked up in equatorial Pacific seamount phosphorites. That amount is equivalent to about 2-7 years of the present annual input from rivers.