『Abstract
The evolution of Tethyan phosphogenesis during the Cretaceous-Eocene
is examined to try to explain fluctuations of phosphogenesis through
time, and whether or not they reflect long-term changes in ocean
circulation or in continental weathering. Twenty-seven time-stratigraphic
phosphate levels in various Tethyan sites, covering a time span
of about 90 Myr from the Hauterivian to the Eocene, were analyzed
for 44Ca/42Ca and 143Nd/144Nd
in their carbonate fluorapatite (CFA) fraction. P and Ca accumulation
rates and bulk sedimentation rates were quantified throughout
the Cretaceous-Eocene Negev sequence to examine how changes in
44Ca/42Ca and 143Nd/144Nd
are reflected in the intensity of phosphogenesis.
A clear-cut change occurs in εNd(T )
and δ44Ca and in the rates of P and Ca accumulation
and bulk sedimentation through the time analyzed. εNd(T
) is much lower in the Hauterivian-Lower Cenomanian (-12.8
to -10.9) than in the Upper Cenomanian-Eocene (-7.8 to -5.9).
Much lower δ44Ca values occur in the Hauterivian-Turonian
(-0.22 to +0.02) than in the Coniacian-Eocene (+0.23 to +0.40).
P accumulation rates in the Negev steeply increase from <200μmol
cm-2 k yr-1 in the Albian-Coniacian to 〜1500
μmol cm-2 k yr-1 in the Campanian, whereas
a strong decrease is concomitantly recorded in the rates of Ca
accumulation and bulk sedimentation. In addition, distinct εNd(T ) values are shown by the phosphorites
of the Negev (-6.7 to -6.4) and Egypt (-9.0 to -7.6) during the
Campanian, and by those of the Negev (-7.8 to -6.3) and North
Africa (-10.1 to -8.9) during the Maastrichtian-Eocene.
The culmination of P accumulation rates in the Negev during the
Campanian, occurring with a high in εNd(T
) and δ44Ca and a low in sedimentation rates,
indicates that paleoceanographic and paleogeographical factors
mostly governed phosphorite accumulation in this area. The abrupt
εNd(T ) rise after the Cenomanian
is attributed to increased incursion of Pacific (radiogenic) water
masses into the Tethys, driven by the Late Cretaceous global sea-level
rise, the connection between North and South Atlantic, the global
post-Santonian cooling, and the progressive widening of the Caribbean
threshold, all acting in combination to significantly intensify
the Tethyan circumglobal current (TCC). It also reflects a weakening
of the continental Nd signal due to a reduction of exposed landmasses
caused by increased flooding of continental shelves. High δ44Ca
values at those times also point to a decrease in weathering Ca+2
fluxes and expansion of carbonate sedimentation in shelves, both
enriching seawater with isotopically heavy Ca+2. Deep
ocean circulation intensified by the post-Santonian cooling of
high latitudes increased P inventory in the Tethys basin, whereas
the strengthened TCC and the folded shelf likely resulted in coastal
and topographically-induced upwelling, supplying P-rich intermediate
waters to southeastern Tethys shelves. Only in the Paleocene-Eocene,
following major changes in global circulation produced by narrowing
Tethys and widening of the Atlantic, did phosphogenesis shifts
its locus of high intensity to the western (Atlantic) Tethys and
West African Atlantic coasts. This change in paleocirculation
is expressed by distinctly differing εNd(T
) in the Middle East and the North and West African phosphorites,
suggesting different oceanic P sources and current systems for
these two major groups of phosphorites. Our Nd isotope results
further suggest a weaker TCC during the Mid-Cretaceous, becoming
more intense in Late Cretaceous times. They also point to the
North Pacific Ocean as major source of deep water formation for
the intermediate-deep waters in the Tethys Basin during the Late
Cretaceous.
Keywords: phosphogenesis; south Tethys margins; Cretaceous-Eocene;
εNd(T ) ; δ44/42Ca; P accumulation
rates; paleoceanography; TCC; deep water formation』
Contents
1. Introduction
2. Material
3. Methods
3.1. Separation of the CFA fraction
3.2. Sr and Nd isotope analysis
3.3. Ca isotope analysis
3.4. Quantification of P and Ca accumulation rates
4. Results
5. Interpretation of the results and discussion
5.1. Temporal variations of Nd isotopes
5.2. Temporal variations of Ca isotopes
5.3. Variations of P accumulation: the main pulse of massive
Cretaceous phosphogenesis
6. Summary and conclusions
Acknowledgments
Appendix A. Data on the analyzed phosphate samples and 87Sr/86Sr,
143Nd/144Nd and δ44/42Ca compositions
of the separated CFA fractions
References