Elsevier

Chemical Geology

Volume 161, Issues 1–3, 30 September 1999, Pages 37-57
Chemical Geology

The Sr, C and O isotopic evolution of Neoproterozoic seawater

https://doi.org/10.1016/S0009-2541(99)00080-7Get rights and content

Abstract

Sr and C isotopic data obtained on stratigraphic suites of well-preserved marine limestone from Siberia, Namibia, Canada, Svalbard and East Greenland provide a relatively detailed first-order record of isotopic variation in seawater through the late Neoproterozoic Era. This data is used to revise the 87Sr/86Sr and δ13C curves of this important interval, during which several discrete global ice ages occurred and the first macroscopic animals evolved. Through this time, the lowest 87Sr/86Sr values (ca. 0.7056) characterize the interval between about 750–800 Ma and have been interpreted to reflect a major hydrothermal event. From 750 to 600 Ma, the Sr isotope values oscillate between highs and lows, ranging between 0.7063 and 0.7074. Between 600 Ma and the Early Cambrian (ca. 535 Ma), 87Sr/86Sr values rise sharply from 0.7063 to 0.7087. This is thought to reflect enhanced continental input to the oceans associated with a Pan-African continental collision. This small subset of limestone samples (dolomites dominate the Neoproterozoic record) shows the δ13C curve rises from values close to 0 prior to 800 Ma to about +6‰ at 750 Ma and about +8‰ for the time between 600 and 730 Ma. During the time between 600 and 542 Ma, the highest values are about +4‰ (higher values in each interval are preserved in little-altered dolomites). Strong positive-to-negative excursions to values of −5‰ are associated with both Vendian glaciations estimated at about 575 and 590 Ma and with Sturtian glaciations estimated at about 720 and 740 Ma. In strong contrast, based on our view of least altered samples, there are no distinct changes in 87Sr/86Sr across Neoproterozoic glacial intervals. The duration of these global refrigeration events is a subject of considerable debate. However, consideration of Sr residence times based on elemental partitioning, and the relationship between δ13C and 87Sr/86Sr variations, suggest that these negative carbon isotope excursions would have lasted at least 350,000 years and no more than about one million years, assuming modern diagenetic fluxes of Sr to the oceans and total absence of continental fluxes.

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 87Sr/86Sr variation in seawater through Earth history has improved dramatically over the past two decades. The secular 87Sr/86Sr and δ13C 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 87Sr/86Sr and high initial 143Nd/144Nd 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 87Sr/86Sr and δ13C 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 87Sr/86Sr isotope record

Veizer and Compston (1976) obtained a seawater 87Sr/86Sr 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 87Sr/86Sr 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 δ13C and δ18O isotope records

Schidlowski et al. (1975) obtained a Precambrian δ13C 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, δ13C fluctuated with frequencies comparable to Phanerozoic variations but with far greater magnitudes (reviewed in Kaufman and Knoll, 1995).

The δ13C and δ18O values for the same carbonate samples used to construct the 87Sr/86Sr curve in Fig. 4 are

Box model for Sr and C cycles

A box model for evaluating 87Sr/86Sr and δ13C 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 87Sr/86Sr

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 87Sr/86Sr and δ13C 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)

  • L.M. Heaman et al.

    Nature and timing of Franklin igneous events, Canada: implications for a Late Proterozoic mantle plume and the break-up of Laurentia

    Earth Planet. Sci. Lett.

    (1992)
  • S.S. Iyer et al.

    Highly 13C-enriched carbonate and organic matter in the Neoproterozoic sediments of the Bambui Group, Brazil

    Precambrian Res.

    (1995)
  • S.B. Jacobsen

    Isotopic constraints on crustal growth and recycling

    Earth Planet. Sci. Lett.

    (1988)
  • S.B. Jacobsen et al.

    A Nd isotopic study of the Hamersley and Michipicoten banded iron formations: the source of REE and Fe in Archean oceans

    Earth Planet. Sci. Lett.

    (1988)
  • A.J. Kaufman et al.

    Neoproterozoic variations in the C-isotopic composition of seawater: stratigraphic and biogeochemical implications

    Precambrian Res.

    (1995)
  • A.J. Kaufman et al.

    Isotopic compositions of carbonates and organic carbon from upper Proterozoic successions in Namibia: stratigraphic variation and the effects of diagenesis and metamorphism

    Precambrian Res.

    (1991)
  • A.J. Kaufman et al.

    The Vendian record of Sr- and C-isotopic variations in seawater: implications for tectonics and paleoclimate

    Earth Planet. Sci. Lett.

    (1993)
  • H. Kimura et al.

    The Vendian–Cambrian δ13C record, North Iran: evidence for overturning of the ocean before the Cambrian explosion

    Earth Planet. Sci. Lett.

    (1997)
  • A. Misi et al.

    Neoproterozoic carbonate sequences of the Una Group, Irece Basin, Brasil: chemostratigraphy, age and correlations

    Precambrian Res.

    (1998)
  • P.I. Nabelek

    General equations for modeling fluid/rock interaction using trace elements and isotopes

    Geochim. Cosmochim. Acta

    (1987)
  • M.R. Palmer et al.

    The strontium isotope budget of the modern ocean

    Earth Planet. Sci. Lett.

    (1989)
  • F.M. Richter et al.

    Simple models for the geochemical response of the ocean to climatic and tectonic forcing

    Earth Planet. Sci. Lett.

    (1993)
  • M. Schidlowski et al.

    Precambrian sedimentary carbonates: carbon and oxygen isotopic geochemistry and implications for the terrestrial oxygen budget

    Precambrian Res.

    (1975)
  • I.G. Stanistreet et al.

    Sedimentary response to a Late Proterozoic Wilson Cycle: the Damara Orogen and Nama Foreland, Namibia

    J. Afr. Earth Sci.

    (1991)
  • J. Veizer et al.

    87Sr/86Sr in Precambrian carbonates as an index of crustal evolution

    Geochim. Cosmochim. Acta

    (1976)
  • J. Veizer et al.

    87Sr/86Sr in Late Proterozoic carbonates: Evidence for a “mantle event” at 900 Ma ago

    Geochim. Cosmochim. Acta

    (1983)
  • J.D. Aitken

    Two Late Proterozoic glaciations, MacKenzie Mountains, northwestern Canada

    Geology

    (1991)
  • Bartley, J.K., Kaufman, A.J., Semikhatov, M.A., Pope, M.C., Podkovyrov, N., Knoll, A.H., Jacobsen, S.B., 1999....
  • J.K. Bartley et al.

    A Vendian–Cambrian boundary succession from the northwestern margin of the Siberian Platform: stratigraphy, palaeontology, chemostratigraphy and correlation

    Geol. Mag.

    (1998)
  • J. Bertrand-Sarfati et al.

    First Ediacaran fauna found in western Africa and evidence for an Early Cambrian glaciation

    Geology

    (1995)
  • S.A. Bowring et al.

    Calibrating rates of Early Cambrian evolution

    Science

    (1993)
  • S.A. Bowring et al.

    U/Pb zircon geochronology and tempo of the end-Permian mass extinction

    Science

    (1998)
  • Bowring, S.A., Martin, M.W., Grotzinger, J.P., Myrow, P., Landing, E., 1998b. Geochronological constraints on the...
  • U. Brand et al.

    Chemical diagenesis of a multicomponent carbonate system: 1. Trace elements

    J. Sedim. Petrol.

    (1980)
  • U. Brand et al.

    Chemical diagenesis of a multicomponent carbonate system: 2. Stable isotopes

    J. Sedim. Petrol.

    (1981)
  • M.D. Brasier et al.

    Integrated chemo- and biostratigraphic calibration of early animal evolution: Neoproterozoic–early Cambrian of southwest Mongolia

    Geol. Mag.

    (1996)
  • W.M. Burke et al.

    Variations of seawater 87Sr/86Sr throughout Phanerozoic time

    Geology

    (1982)
  • S.J. Burns et al.

    Carbon isotopic record of the latest Proterozoic from Oman

    Eclogae Geol. Helv.

    (1993)
  • G.R. Dickens et al.

    A blast of gas in the latest Paleocene: simulating first-order effects of massive dissociation of oceanic methane hydrate

    Geology

    (1997)
  • Dubreuilh, J., Bechennec, F., Berthiaux, A., Le Metour, J., Platel, J.P., Roger, J., Wyns, R., 1992. Geological Map of...
  • I.J. Fairchild et al.

    Petrological and isotopic implications of some contrasting Late Precambrian carbonates. NE Spitsbergen

    Sedimentology

    (1987)
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