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Centre for Earth and Environmental Science Research

Phosphate Deposition in the Cretaceous Chalk Sea: Trigger or Response to Environmental Change?

Executive Summary
ResearchersMr Kevin Attree (PhD Student): CEESR
Dr Ian Jarvis (Director of Studies): CEESR
Professor Rory Mortimore, University of Brighton
Funding Body/SourceSelf-funded
DurationSeptember 2004 - ongoing
Project SummaryDuring 2003, a borehole near the Stonehenge monument on the World Heritage Site in Wiltshire intersected Santonian - Campanian chalks containing 20 m of dark brown phosphatic chalk. Phosphorus availability is a limiting factor in biological productivity, and hence influences the demand for CO2, a major greenhouse gas. This research project is based on a study of the Stonehenge phosphatic chalk, and its aim is to identify possible relationships between changes in bioproductivity/enviromental conditions and episodes of elevated phosphate deposition.

Background

Phosphorus (P) is present in most rocks in minor to trace quantities; phosphorites are sediments that include significant portions of authigenic and biogenic phosphate minerals (Follmi, 1996). Marine sedimentary phosphate deposits, phosphorites (c. 18% P2O5), are a major source of phosphate rock. Marine phosphate deposits occur globally in rocks of Precambrian to Holocene age, and are forming today on continental margins off SW Africa, NW Africa, SE United States, southern California, northern South America, eastern Australia, and eastern New Zealand (Cook, 1976). Francolite, a carbonate-fluorapatite with >1% F, plus appreciable amounts of CO2, is considered to be the most common phosphate mineral that forms in early diagenetic marine environments (Fig. 1)

Fig. 1. Francolite chemistry. A number of stable and radiogenic isotope substitutions may occur in the structural components of francolite (from Jarvis, 1994).

Phosphorites are of interest due to the essential role of phosphorus as a component of DNA, ATP, and the phospholipid molecules of cell membranes. Changes in the availability of phosphorus are likely to have controlled the size and activity of the biosphere over geological time (Schlesinger, 1997). Such fluctuations in biological productivity influences the conversion of atmospheric CO2 into organic matter. Therefore, variations in P availability may mediate climatic, environmental, and ecological change.

Project objectives

  1. Document the stratigraphy, sedimentology, palaeontology, and geochemistry of the Stonehenge phosphate deposit based primarily on material obtained from reference core R142
  2. Delimit the vertical and lateral extent of the phosphate deposit based on borehole data, investigate its burial and diagenetic history, and assess its present-day environmental and archaeological significance to the Stonehenge World Heritage Site
  3. Compare the Stonehenge deposit to other known phosphogenic episodes from late Proterozoic age to the present, and develop models that explain their distribution, size and extent, and the environmental, geographical and geological factors influenced their formation
  4. Utilise geochemical and other palaeoenvironmental proxies to assess relationships between changing rates of phosphate accumulation and periods of environmental change
  5. Study relationships between Cretaceous phosphate accumulation, weathering, climate and sea-level change, and model their impact on global biogeochemical cycles.

Climate, Sea-levels and Ocean Circulation Patterns

An important component of this research is the possible relationship between Santonian - Campanian phosphogenesis and climate change (Jarvis, 1992, 2006): did one event trigger the other, or are their co-occurrence in the geological record purely coincidental?

Any attempt to answer such questions will need to consider, amongst others, the following factors:

  1. the global climate of the Late Cretaceous
  2. the influence of tectonics on ocean basin size, and landmass configuration and topography
  3. changes in ocean circulation patterns in light of the above
  4. changes in atmospheric circulation patterns and pressure systems
  5. local palaeogeography
  6. what was global distribution of phosphate deposition at this time
  7. how do their depositional environments compare with the Stonehenge deposit

Jarvis et al. (2002) identified the Campanian as representing a major turning point in Earth history, with high but declining sea-levels, falling global surface temperatures and increasing oceanic turnover (Fig. 2).

Fig. 2. Late Cretaceous climate change, based on d18O in southern hemisphere ODP cores. Note the cooling trend that follows the late Santonian - early Campanian warm episode (modified from Clarke & Jenkyns, 1999).

Work to date

The detailed sedimentological and geochemical study of the Stonehenge phosphates is based to date on a 45 m long core (R142). The R142 core displays three lithological units: the lower section consists of white chalk with marl seams; the middle section comprises dark brown, coarse granular phosphate grains; the upper section is white chalk interspersed with flint and marl seams. Figure 3 shows part of the lower white chalk, although the resolution and contrast of the image does not allow the marls seams to be identified. Figure 4 shows the brown phosphatic chalk. A study of the complete core reveals that the transition from the white chalk to the phosphatic chalk is quite abrupt, as is the return to white chalk at the top of the core, part of which is shown in Figure 5.

Fig. 3. Core R142 lithofacies, white chalk (lower). Depth: 34.65 - 34.40 m. This bottom section also contains thin marl seams, which cannot be distinguished in this image. The core is approximately 15 cm wide.

Fig. 4. Core R142 lithofacies, phosphatic chalk. Depth: 20.15 - 19.95 m. This section of the core consists of dark brown, coarse granular phosphate. The sediment is extremely friable, and in places is prone to disintegration when touched. The core is approximately 15 cm wide.

Fig. 5. Core R142 lithofacies, white chalk (upper). Depth: 9.90 - 9.70 m. The top section of the core is interspersed with marl and flint seams, which are not visible in this section. The core is approximately 15 cm wide.

Twenty-three samples have been taken from the R142 core at 1 m intervals and thin sections have been prepared for petrographic analysis. Thin section petrographic analysis reveals P-rich horizons consists mainly of phosphatized foraminifera, with subordinate intraclasts, bioclasts, and faecal pellets (Fig. 6).

Fig. 6. Cross-polarised-light thin-section image of phosphatic chalk composed predominantly of foraminifera tests (f). Most tests have a P-rich infill, and are externally coated by fine laminar phosphate of probable microbial origin. A large faecal pellet (fp) is apparent near the centre of the image; this is also phosphatised, note the unreplaced components of the pellet include foraminiferal tests. Angular shell fragments (sf) are also visible. Core R142 19.95 - 20.15 m.

From the P-rich core samples between 19.95 - 20.15 m, polished thin sections have been produced for cathodoluminescence (CL), as shown in Fig. 7, and Scanning Electron Microscope (SEM) analysis (Fig. 8).

Fig. 7. Cathodoluminescence image of phosphatic chalk from Stonehenge. The image shows luminescent foraminifera calcite tests (dull orange), with dark brown very weakly-luminescent phosphatic infills (f). Also visible are spar-filled (bright orange) tests and altered shell fragments (a). The foraminifera are approximately 300 µm across. Core R142 19.95 - 20.15 m.

Fig. 8. Elemental phosphorus map produced of phosphatic chalk from Stonehenge. The foraminifera tests (f) exhibit phosphatic infills and external coatings (bright areas). The foraminifera are approximately 300 µm across. Core R142 19.95 - 20.15 m.

Research in Progress

A number of sub-samples have been prepared by LiBO2 fusion-HNO3 digestion, and HF - HClO4 acid digestion, these are to be analysed for major-, minor-, and trace-element content. Stable isotope analysis of d13C, d18O in structural carbonate, and d34S of the structural sulphate, will also be performed. The unique fossil assemblages that occur in these deposits will be collected and used to define the biostratigraphy. Laboratory results will be compared with cm-scale resolution down-hole gamma-ray logs to improve understanding of variation through the deposit.

Further work will focus on Santonian - Campanian white chalks that occur at Seaford Head, Sussex. It is believed that these chalks display lithological and geochemical cyclicity on Milankovitch-type time scales of 20 thousand to 2.4 million years. Closely spaced samples from the Seaford chalks will be used to identify Milankovitch cycles through geochemical analysis. Identification of these cycles will be used to calculate sedimentation rates and sedimentation rate variation in the successions and to relate these to phosphorus bulk accumulations rates.

References

Cook, P. 1976. Sedimentary phosphate deposits. In: Wolf, K. Handbook of Strata-Bound and Strataform Ore Deposits. Elsevier Amsterdam, p 505-535.

Follmi, K. 1996. The phosphorus cycle, phosphogenesis and marine phosphate-rich deposits. Earth-Science Reviews, 40, 55-124.

Jarvis, I. 1992. Sedimentology, geochemistry and origin of phosphatic chalks: the Upper Cretaceous deposits of NW Europe. Sedimentology, 39, 55-97.

Jarvis, I. 2006. The Santonian-Campanian phosphatic chalks of England and France. Proceedings of the Geologists' Association. 117, 219-237.

Jarvis, I., Mabrouk, A., Moody, R.T.J. & De Cabrera, S.C., 2002. Late Cretaceous (Campanian) carbon isotope events, sea-level change and correlation of the Tethyan and Boreal realms. Palaeogeography, Palaeoclimatology, Palaeoecology, 188: 215-248.

Schlesinger, W.H. 1997. Biogeochemistry: An analysis of Global Change. Academic Press, New York, p. 398.

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