120 Peter Stille and Graham Shields
these data and compared with those from Fe-Mn-rch sediments investigated by
Hooker et al. (1981) and Chyi et a1.(1982). Independently varying Nd isotope
evolution curves can be observed for the last 200 million years. 200 Ma ago, at the
opening of the central Atlantic, both oceans, the Pacific (Panthalassa and the
Paleotethys) and the Atlantic had the same isotopic characteristics. At that time,
the 'super ocean' Panthalassa, which surrounded the supercontinent Pangaea, may
still have existed. The further opening of the Atlantic Ocean went hand in hand
with the development of its unique isotopic signature. This means that the Pacific
and the Atlantic received their rare earth element intake from isotopicaily different
sources. These observations made it clear for the first time that, unlike Sr, the Nd
isotope system is particularly well suited to support paleo-oceanographic as well
as paleo-tectonic models. We will come back to this later on. In the next part, we
discuss the suitability of carbonate and phosphate material for the derivation of Nd
isotopic composition in the paleo-oceans.
Carbonates
The Nd concentrations in modern biogenic calcites and aragonites are low and
vary between 0.2 and 9.6 ppb. Inorganic aragonite precipitate can have
concentrations of up to 65 ppb. Limestones and dolomites likewise show low Nd
concentrations of a few hundred ppb. The Sm/Nd ratios of carbonates vary
stronger than those in seawater and allow us to surmise that there must be some
fractionation during the incorporation of the REE. Banner et al. (1988) point out
that the REE can be mobilized and fractionated during carbonate diagenesis. In
contrast to recent carbonates, carbonate fossils show far higher Nd concentrations
(1-30 ppm). They are therefore more greatly enriched in Nd by a factor of 103- l0 a
over recent carbonate bivalves.
The observations of Turekian et al. (1973) allow us to explain this enrichment
process. These authors were the first to be able to show that the rare earth
elements are preferentially incorporated into sub-microscopic Fe-hydroxides on
Fe-flakes. These flakes are deposited along with the carbonate. Palmer and
Elderfield (t986) observed something similar. They could show that less than
10% of the REE were actually found in the calcite lattices of Tertiary
foraminifera. More than 90% were found in Fe-Mn oxides coating the
foraminifera skeletons. These crusts build up after the death of the foram during
the sedimentation process. This means that the REE contents of recent
foraminifera are determined by the compositions of these crusts and not by the
calcite of the foraminifera themselves. Yet still these recent foraminifera show Nd
isotope signatures characteristic of today's seawater. How do fossil foraminifera
behave? Can they retain the Nd isotope signatures of ancient oceans?
Palmer and Elderfield (1986) showed that Mn/Fe ratios in Tertiary foraminifera
are far higher than in foraminifera today. This implies either that the accumulation
rates of Mn and Fe have changed over time or that the Mn and Fe crusts have been
these data and compared with those from Fe-Mn-rch sediments investigated by
Hooker et al. (1981) and Chyi et a1.(1982). Independently varying Nd isotope
evolution curves can be observed for the last 200 million years. 200 Ma ago, at the
opening of the central Atlantic, both oceans, the Pacific (Panthalassa and the
Paleotethys) and the Atlantic had the same isotopic characteristics. At that time,
the 'super ocean' Panthalassa, which surrounded the supercontinent Pangaea, may
still have existed. The further opening of the Atlantic Ocean went hand in hand
with the development of its unique isotopic signature. This means that the Pacific
and the Atlantic received their rare earth element intake from isotopicaily different
sources. These observations made it clear for the first time that, unlike Sr, the Nd
isotope system is particularly well suited to support paleo-oceanographic as well
as paleo-tectonic models. We will come back to this later on. In the next part, we
discuss the suitability of carbonate and phosphate material for the derivation of Nd
isotopic composition in the paleo-oceans.
Carbonates
The Nd concentrations in modern biogenic calcites and aragonites are low and
vary between 0.2 and 9.6 ppb. Inorganic aragonite precipitate can have
concentrations of up to 65 ppb. Limestones and dolomites likewise show low Nd
concentrations of a few hundred ppb. The Sm/Nd ratios of carbonates vary
stronger than those in seawater and allow us to surmise that there must be some
fractionation during the incorporation of the REE. Banner et al. (1988) point out
that the REE can be mobilized and fractionated during carbonate diagenesis. In
contrast to recent carbonates, carbonate fossils show far higher Nd concentrations
(1-30 ppm). They are therefore more greatly enriched in Nd by a factor of 103- l0 a
over recent carbonate bivalves.
The observations of Turekian et al. (1973) allow us to explain this enrichment
process. These authors were the first to be able to show that the rare earth
elements are preferentially incorporated into sub-microscopic Fe-hydroxides on
Fe-flakes. These flakes are deposited along with the carbonate. Palmer and
Elderfield (t986) observed something similar. They could show that less than
10% of the REE were actually found in the calcite lattices of Tertiary
foraminifera. More than 90% were found in Fe-Mn oxides coating the
foraminifera skeletons. These crusts build up after the death of the foram during
the sedimentation process. This means that the REE contents of recent
foraminifera are determined by the compositions of these crusts and not by the
calcite of the foraminifera themselves. Yet still these recent foraminifera show Nd
isotope signatures characteristic of today's seawater. How do fossil foraminifera
behave? Can they retain the Nd isotope signatures of ancient oceans?
Palmer and Elderfield (1986) showed that Mn/Fe ratios in Tertiary foraminifera
are far higher than in foraminifera today. This implies either that the accumulation
rates of Mn and Fe have changed over time or that the Mn and Fe crusts have been