Origin and consequences of cyclic ice rafting in the Northeast Atlantic Ocean during the past 130,000 years

From the time that detailed oxygen isotope records derived from foraminifera living in the constant-temperature environment of the abyssal ocean became available, there has been a discrepancy between the ice volume record that these records imply, and that derived from the altitude of dated coral terraces around the world. Here, we re-examine the data and conclude that the temperature of the abyssal ocean has been an actively varying component of the climate system.

Oxygen isotopes and sea level

238U234U230Th232Th systematics and the precise measurement of time over the past 500,000 years

The subterranean estuary: a reaction zone of ground water and sea water

THE study of the sea level record during the last glacial cycle has primarily proceeded indirectly by means of oxygen isotope measurements on foraminifera from deep-sea sediments1,2. The direct approach of dating sea level indicators stagnated during the past decade, mainly because the samples required to complete our knowledge of the past glaciations are below the present-day sea level3–7. Using the 14C ages of Acropora palmata samples collected by drilling offshore the island of Barbados, Fairbanks8 presented the first detailed chronology for the last deglaciation. This radiocarbon chronology is limited to the past 30 kyr because of the short half-life of 14C (5,730 yr); we must therefore rely on other dating methods to obtain information for the whole last glacial cycle. During the past four years 230Th–234U dating of corals by thermal-ionization mass spectrometry has been shown to be significantly more precise and accurate than the classical α counting method9–14. We have used this technique to measure U–Th ages in coral samples from the Barbados collection and from subaerially exposed outcrops (see also ref. 15). Here we present results bearing on the sea level record for the past 130,000 years; we conclude that the last deglaciation started 3,000 years earlier than previously thought and confirm that there were two surges in melt water8 at about 11 kyr and 14 kyr BP (before present).

U-Th ages obtained by mass spectrometry in corals from Barbados: sea level during the past 130,000 years

Many models of the origin of marine phosphorites require a sediment rich in organic matter, which by decomposition releases phosphorus for the precipitation of carbonate fluorapatite1–7. This concept is useful in explaining contemporary phosphorite in areas of very high productivity off the coasts of Peru–Chile and South West Africa8–10 where there are high organic matter fluxes to the sea floor4–7,11. It does not explain the origin of marine phosphorite deposits which formed in regions of limited oceanic upwelling and productivity12–14. The East Australian continental margin, an area of phosphogenesis throughout the late Pleistocene and Holocene13,14, is a modern analogue of an ‘East Coast’ phosphogenic province15–17 with low productivity over the upper slope region where the most recent phosphorites have been found18. Evidence is reported here for the carbonate fluorapatite in the East Australian phosphorites being located within bacterial cellular structures, and a model proposed for the origin of these deposits through the slow bacterial assimilation of phosphorus from seawater in an area of restricted sedimentation.

Bacterial origin of East Australian continental margin phosphorites

Uranium-series disequilibrium studies in phosphorite nodules from the west coast of South America

Abstract Detailed sedimentological and geochemical studies of phosphorites and sediments from the East Australian continental margin have shown that both apatite and glauconite are forming at a transition zone between relict, iron oxyhydroxide-rich, organic-poor (TOC<0.3%) outer shelf (200–350 m) sediments and relatively rapidly accumulating, iron oxyhydroxide-deficient, organic-rich (TOC>0.8%) deep water (460–650 m) sediments. The interaction between sediment mixing and Fe-P cycling processes (between the pore waters and the solid phase) appear critical to the formation of modern phosphorites in this area. The phosphate nodules form within the anoxic zone in the sediments at depths of approximately 10–18 cm below the sediment-seawater interface. Nodules which remain in the sediment mixed layer after they form continue to accumulate both P and Fe for up to 60 ka; during this time their apatite and iron oxyhydroxide contents more than double and the nodules become denser and more lithified. Apatite and glauconite formation are favoured by periods of high sea-level and low current velocities, as these conditions allow a relatively high organic carbon input to the sediments and thereby the maintenance of anoxia at shallow depths within the sediments. During periods of low sea-level and high current velocities, the carbon flux into the sediments decreases and the sediments become oxic. Consequently the Fe-cycling processes cease and apatite and glauconite formation stops: the glauconite is progressively transformed to goethite, and phosphorite nodules are concentrated into lag deposits and ferruginized. Alternations of high and low sea-level cycles eventually result in the formation of the massive ferruginous Neogene phosphorites that mantle much of the outer shelf. The iron enrichment processes observed in the modern to Neogene phosphorites on the East Australian continental margin provide explanations for many of the features seen in ferruginous Neogene deposits in the world’s oceans.

Sedimentation dynamics and redox iron-cycling: controlling factors for the apatite—glauconite association on the East Australian continental margin

Metal accumulation rates in the Southeast Pacific and the origin of metalliferous sediments

Carbonate–fluorapatite, the major component of marine phosphorite, forms within organic-rich diatomaceous sediments in present day conditions along the continental margin of Peru–Chile and South-West Africa (Namibia)1–4. Both of these areas are ‘West-Coast’ phosphogenic provinces5,6, characterized by an eastern boundary current, with strong coastal upwelling and associated high organic productivity. We report here the first radiometric evidence supporting Holocene apatite formation in an ‘East-Coast’ phosphogenic province, the East Australian continental margin, an area of only moderate seasonal upwelling7,8.

H. Herbert Veeh

Papers

Bacterial origin of East Australian continental margin phosphorites

Uranium-series disequilibrium studies in phosphorite nodules from the west coast of South America

Sedimentation dynamics and redox iron-cycling: controlling factors for the apatite—glauconite association on the East Australian continental margin

Metal accumulation rates in the Southeast Pacific and the origin of metalliferous sediments

Holocene phosphorite on the East Australian continental margin