The Sr, C and O isotopic evolution of Neoproterozoic seawater
Introduction
Major questions relating to the evolution of the Earth's oceans, atmosphere, climate and sedimentary shell are most likely resolvable through the study of high resolution isotopic variations in seawater through time. The seawater Sr and C isotopic curves are also important tools for stratigraphic correlation, in particular for intervals like the Neoproterozoic (544–1000 Ma) that lack an adequate biostratigraphic framework (Kaufman et al., 1997).
Our understanding of variation in seawater through Earth history has improved dramatically over the past two decades. The secular and trends in Phanerozoic seawater are now well-established (Burke et al., 1982; Veizer et al., this volume). Extending the Sr isotope record back in time, this laboratory has focused on detailed studies of stratigraphic suites from Early Cambrian and late Neoproterozoic (ca. 800–530 Ma) successions in Svalbard, East Greenland, Canada, Siberia and Namibia (Derry et al., 1989, Derry et al., 1992, Derry et al., 1994; Asmerom et al., 1991; Kaufman et al., 1993, Kaufman et al., 1996).
The demonstration that both the Sr and C isotopic composition of seawater varied systematically through this interval allows, for the first time, formation-level correlations worldwide. This stratigraphic framework provides us with the potential to resolve fundamental questions of the relative timing of biological, environmental and climatic events in the Neoproterozoic Earth. In particular, chemical stratigraphy may allow for the correlation of thick glacial blankets, which are preserved in all regions with major Neoproterozoic sedimentary basins. Glacial diamictites of Neoproterozoic age are recognized at two broad levels, identified as the Varangian (ca. 600 Ma, Knoll and Walter, 1992) and Sturtian (ca. 700–760 Ma, Hoffman et al., 1998a) ice ages. Each of these in turn consists of at least two discrete pulses of glaciation (cf. Young, 1995, but see Kennedy et al., 1998 for a different point of view).
We have developed simple models to interpret these records (cf. Jacobsen, 1988; Asmerom et al., 1991; Derry et al., 1992; Kaufman et al., 1993) and will build on these previous efforts here. The Sr isotopic balance of the modern oceans is dominated by the erosional flux of material from continental sources, with a smaller but significant contribution from mantle sources via submarine hydrothermal systems (Goldstein and Jacobsen, 1987; Palmer and Edmond, 1989). At present, other fluxes (e.g., diagenesis of marine carbonates) are insignificant, but may have been more important in past oceans. Low and high initial ratios for Archean chemical precipitates both argue for a much stronger mantle input into the Archean oceans (Veizer and Compston, 1976; Jacobsen and Pimentel-Klose, 1988a, Jacobsen and Pimentel-Klose, 1988b). There are also strong suggestions of major hydrothermal events during parts of the Neoproterozoic (Veizer et al., 1983; Asmerom et al., 1991). On the other hand, most of the geochemical cycles that control the ocean system today were probably in place by the early Paleozoic (cf. Holland, 1984). Thus, the Proterozoic must have been a period of transition from early “mantle-buffered” to modern environments.
The purpose of this paper is to highlight the transitional Neoproterozoic interval by (i) reporting our present compilation of data on Sr and C isotope variations in marine carbonates deposited between ∼520–800 Ma, (ii) evaluating the data in light of diagenetic and or metamorphic alteration, and (iii) presenting a more elaborate geochemical box model for interpreting Sr and C isotopic variations in seawater, with particular focus on the periods of global glaciation (cf. Hoffman et al., 1998b).
Section snippets
Geology, stratigraphy and samples
The samples used for this study are from sedimentary basins which have been described in detail in earlier publications (Knoll et al., 1986, Knoll et al., 1995; Fairchild and Spiro, 1987; Kaufman et al., 1991, Kaufman et al., 1993, Kaufman et al., 1996, Kaufman et al., 1997; Derry et al., 1989, Derry et al., 1992, Derry et al., 1994; Asmerom et al., 1991; Kaufman and Knoll, 1995; Grotzinger et al., 1995; Pelechaty et al., 1996; Hoffman et al., 1998a, Hoffman et al., 1998b; Saylor et al., 1998).
Ages
The absolute chronology of few Neoproterozoic sedimentary sequences are well determined and biostratigraphy for this time period still provides only broad constraints. For assigning absolute ages, we have relied on a few precise ages obtained by U–Pb isotopic analyses of zircons or baddeleyite. These minerals are isolated from volcanic layers interbedded with sedimentary rocks (cf. Bowring et al., 1993, Bowring et al., 1998a) that could be correlated based on recognizable isotopic features or
Simple models of fluid–rock interaction
The isotopic compositions of carbonates can be substantially changed from primary seawater equilibrated values by meteoric diagenesis, dolomitization, and metamorphism. Various models for fluid–rock interaction (e.g., Taylor, 1977; Nabelek, 1987; Banner and Hanson, 1990; Schrag, this volume) have been used to established parameters for determining the degree of alteration of Sr, C and O isotopic compositions in carbonates.
The response of the concentration of an element i (Ci) to fluid–rock
Identification of primary isotopic signatures
Many samples of Proterozoic carbonates are altered to some degree, and therefore, cannot be used to determine secular trends in and of coeval seawater. Various investigators established parameters for determining the degree of alteration of Sr and C isotopes in Precambrian carbonates (e.g., Brand and Veizer, 1980, Brand and Veizer, 1981; Veizer et al., 1983; Derry et al., 1989, Derry et al., 1992; Asmerom et al., 1991; Kaufman et al., 1993). We believe that our earlier isotopic
Neoproterozoic isotope record
Veizer and Compston (1976) obtained a seawater curve for the whole Precambrian and Veizer et al. (1983) later improved on this record for the Neoproterozoic (∼1000–540 Ma). These authors observed a sharp increase in the ratio of marine carbonates between 2.5–2.0 Ga ago that is consistent with a significant decrease in the post-Archean hydrothermal flux of Sr to seawater. This isotopic event may be closely linked to the agglomeration of an early supercontinent and a
Neoproterozoic and isotope records
Schidlowski et al. (1975) obtained a Precambrian record, which suggested that Proterozoic carbonates lacked strong temporal variation. However, more recent work has demonstrated that from about 850 Ma ago until the end of the Proterozoic Eon, fluctuated with frequencies comparable to Phanerozoic variations but with far greater magnitudes (reviewed in Kaufman and Knoll, 1995).
The and values for the same carbonate samples used to construct the curve in Fig. 4 are
Box model for Sr and C cycles
A box model for evaluating and variations in Neoproterozoic carbonates is shown in Fig. 7. This is a more elaborate version of a model for the Sr and C cycles that we have used earlier (Goldstein and Jacobsen, 1987; Jacobsen, 1988; Asmerom et al., 1991; Derry et al., 1992; Kaufman et al., 1993). The main difference is for the C cycle. Previously, we used a model that primarily involved the crustal rock reservoirs, with the ocean as a rapid mixer. This was sufficient for
Implications for Neoproterozoic ice ages
As shown in Fig. 4, Fig. 5, there are at least four Neoproterozoic glacial intervals that appear to be correlative between sedimentary successions worldwide, all with characteristic positive-to-negative C isotope excursions (Kaufman et al., 1997). In an earlier publication (Kaufman et al., 1993), we compared these records with those of the Cenozoic to investigate if there is a general relationship between global tectonics and glaciations. We record a spectacularly large change in
Conclusions
Sr and C isotopic data obtained on samples of marine carbonates provide a relatively detailed record of isotopic variation in seawater through the Neoproterozoic, and allow direct correlation of these isotopic changes for this time period. This data set was used to revise the and curves of Neoproterozoic seawater.
The frequency and magnitude of variation in the Sr isotope record appear comparable in the Neproterozoic to typical Phanerozoic values, however, the record also includes
Acknowledgements
This research was funded by NSF grant EAR 94-18445 to SBJ and NSF grants 96-30928 and 97-14070 to AJK. We thank M. Brasier and I. Fairchild for their helpful reviews.
References (82)
- et al.
Strontium isotopic variations of Neoproterozoic seawater: implications for crustal evolution
Geochim. Cosmochim. Acta
(1991) - et al.
Calculations of simultaneous isotopic and trace element variations during water–rock interaction with applications to carbonate diagenesis
Geochim. Cosmochim. Acta
(1990) The variation of the marine ratio during Phanerozoic time: interpretation using a flux model
Geochim. Cosmochim. Acta
(1976)- et al.
The strontium isotopic composition of carbonates from the Late Precambrian (∼560–540 Ma) Huqf Group of Oman
Chem. Geol.
(1994) - et al.
values, and Sr/Mg ratios of late Devonian abiotic marine calcite: implications for the composition of ancient seawater
Geochim. Cosmochim. Acta
(1992) - et al.
values, and Sr/Mg ratios of late Devonian abiotic marine calcite: implications for the composition of ancient seawater
Geochim. Cosmochim. Acta
(1991) - et al.
Sr isotopic variations in Upper Proterozoic carbonates from Svalbard and East Greenland
Geochim. Cosmochim. Acta
(1989) - et al.
Sedimentary cycling and environmental change in the Late Proterozoic: evidence from stable and radiogenic isotopes
Geochim. Cosmochim. Acta
(1992) - et al.
Sr and C isotopes in Lower Cambrian carbonates from the Siberian craton: a paleoenvironmental record during the `Cambrian explosion'
Earth Planet. Sci. Lett.
(1994) - et al.
The Nd and Sr isotope systematics of river water dissolved material: implications for the source of Nd and Sr in seawater
Chem. Geol.
(1987)