The use of carbon isotope stratigraphy to construct time lines for stratigraphic correlation requires synchronous changes in carbon isotope ratios (δ13C) to be preserved in carbonate-dominated strata. Such changes are commonly interpreted to reflect primary secular variation in ocean chemistry. However, negative δ13C anomalies developed in Pliocene–Pleistocene carbonate platforms following glacioeustatic sea-level fall due to remineralization of terrestrial biomass during meteoric diagenesis. These anomalies are similar in structure and magnitude to some Neoproterozoic δ13C records, opening the possibility that the Neoproterozoic δ13C anomalies have a meteoric origin derived from a large terrestrial biosphere. Here we test the hypothesis that a large terrestrial biosphere existed prior to the Silurian–Devonian land-plant radiation by examining δ13C records of subaerial exposure surfaces formed in a shallow-water carbonate platform during the Ordovician–Silurian icehouse. The exposure surfaces include an unconformity at the Ordovician-Silurian boundary with terra rossa and dissolution-collapse breccia, and a lower Silurian quartz sand layer feeding a 50-m-deep system of karst pipes. There is no evidence for δ13C depletion beneath either exposure surface. Strontium concentrations in the rocks are low (10–120 ppm) and covary with δ18O; oxygen isotope ratios, however, do not positively correlate with δ13C. Our results suggest that there was no significant terrestrial biosphere during Ordovician–Silurian time, and by extension, that Neoproterozoic negative carbon isotope anomalies cannot be explained by meteoric diagenesis.
Carbon isotope stratigraphy is a fundamental tool for stratigraphic correlation, particularly in Precambrian strata lacking precise biostratigraphic constraints (Halverson et al., 2005; Kaufman and Knoll, 1995). Secular changes in carbon isotope ratios of inorganic carbon (δ13C) are used as stratigraphic tie points under the assumption that δ13C records represent global ocean chemistry at the time of deposition, with little postdepositional alteration during water-rock interactions (e.g., Banner and Hanson, 1990). Many interpretations of Proterozoic environmental and evolutionary history are informed by global correlations made using δ13C records; therefore, it is critical to understand mechanisms governing δ13C in ancient and modern carbonate successions.
Modern shallow-water environments have served as natural laboratories for investigating effects of early diagenesis on δ13C. Pliocene and Pleistocene shallow carbonate platforms commonly do not retain δ13C values of open ocean sediment due in large measure to the effects of exposure to meteoric water during glacioeustatic sea-level minima (Swart and Eberli, 2005). In this well-understood diagenetic process (Allan and Matthews, 1982; Lohmann, 1988), δ13C is depleted when diagenetic fluids incorporate isotopically light remineralized terrestrial organic matter into the host rock, whereas depletion of oxygen isotope ratios (δ18O) is a result of recrystallization from meteoric waters. As a result, negative anomalies in both δ13C and δ18O are observed in carbonate sediments underlying modern shallow-water carbonate platforms; positive correlation between δ13C and δ18O is often a geochemical hallmark of this style of meteoric alteration.
Although the meteoric diagenetic processes that have altered modern carbonate platforms are well understood, their relevance to the isotope records of carbonate platforms in deep time is not clearly established, especially for time periods prior to the development of a robust terrestrial biosphere. However, broad similarities in the architecture of negative δ13C anomalies in Pliocene–Pleistocene and Neoproterozoic strata associated with global glaciations have called into question the interpretation of the Neoproterozoic carbon isotope record as a seawater signal (Swart and Kennedy, 2012) and therefore chronologies that use this record for correlation. These Pliocene–Pleistocene and Neoproterozoic anomalies span tens to hundreds of meters, commonly below subaerial exposure surfaces, and have magnitudes as high as 15‰. Furthermore, positive δ13C-δ18O covariation has been highlighted as a putative indicator of an extensive terrestrial biosphere in the Neoproterozoic (Knauth and Kennedy, 2009), present long before the colonization of land by complex plants in the Silurian and Devonian (Kenrick and Crane, 1997). Because substantial terrestrial biomass would be required if the carbon isotopic depletion observed in Neoproterozoic carbonates were the result of meteoric diagenesis, this interpretation makes profound predictions about the early evolution of terrestrial life (Knauth and Kennedy, 2009).
If the terrestrial biosphere had developed in the Neoproterozoic to the extent that it could cause extensive meteoric alteration of carbonate δ13C (Knauth and Kennedy, 2009), it is reasonable to expect such alteration in younger strata subject to meteoric diagenesis. One test of the Neoproterozoic greening hypothesis therefore is to examine stable isotope records of meteorically altered carbonates deposited between the Neoproterozoic glaciations and the commonly accepted greening of the continents by land plants in the late Silurian and Devonian (Kenrick and Crane, 1997). We perform that test by examining the sedimentology and stable isotope geochemistry of a carbonate platform that underwent subaerial exposure and meteoric flushing during multiple episodes of glacioeustatic sea-level change during the Ordovician–Silurian icehouse.
The Ordovician (Hambrey, 1985) was the first return to glacial conditions after the Neoproterozoic (Arnaud et al., 2011). Long-term cooling accompanied the Ordovician biodiversification (Trotter et al., 2008) and led to the development of ice sheets on south polar Gondwana (Vaslet, 1990; Ghienne et al., 2014). A protracted icehouse possibly spanning more than 30 m.y. began in the Middle Ordovician (Saltzman and Young, 2005; Pope and Steffen, 2003) and lasted into the early Silurian, with a global sea-level fall during the Hirnantian Stage that may have equaled or exceeded that of the last glacial maximum (Finnegan et al., 2011). Sea-level changes resulting from Late Ordovician (Katian–Hirnantian) glacial cycles have been inferred based on transgressive-regressive stratigraphic sequences from the margins of Laurentia (e.g., Harris and Sheehan, 1997; Desrochers et al., 2010).
Sediments in the Great Basin of North America comprising the Ordovician Ely Springs Dolostone and the Silurian Laketown Dolostone were deposited in an equatorial shallow carbonate ramp setting (Fig. 1A). Ramp facies represent tidal flat, burrowed subtidal, and subtidal shoal environments (Harris and Sheehan, 1997). The ramp succession is divided into depositional sequences linked to glacioeustatic sea-level fluctuation on the basis of facies stacking patterns and recognition of subaerial exposure surfaces (Harris and Sheehan, 1997; Finney et al., 1997). We report data from stratigraphic sections in the Pancake and South Egan Ranges (Fig. 1A).
The depositional context of the Ely Springs and Laketown formations provides a useful if imperfect analogy to that of Neoproterozoic carbonate strata deposited immediately prior to global glaciation, including the Ombaatje Formation in Namibia (Halverson et al., 2005), the Keele Formation in the Canadian Cordillera (Narbonne et al., 1994), and the Trezona Formation in South Australia (McKirdy et al., 2001; Rose et al., 2013). In each case, glacioeustatic sea-level fall exposed a shallow-water carbonate platform. While glacial deposits directly overly the Neoproterozoic carbonates, attesting to the global extent of the Cryogenian glaciations, the Paleozoic strata are marked by paleosols and karst features (Harris and Sheehan, 1997; Finney et al., 1997).
Samples were collected at ∼1 m intervals from measured sections in the Pancake and South Egan Ranges (Fig. 1; Harris and Sheehan, 1997). Dolomite powders were drilled from fine-grained components of slabs cut from those samples. Aliquots of powders were analyzed for δ13C/δ18O on a Thermo Scientific Delta V Plus isotope ratio mass spectrometer following dissolution in phosphoric acid. Elemental concentrations were measured on a Teledyne Leeman inductively coupled plasma spectrometer from powders dissolved in weak acetic acid. (For a description of detailed methods, see the GSA Data Repository1.)
RESULTS AND DISCUSSION
Field evidence for subaerial exposure and meteoric diagenesis at the Ordovician-Silurian boundary in the Pancake Range includes a <10-m-thick interval of sand- and breccia-filled karst dissolution pockets within ooid grainstones of the uppermost Ely Springs Dolostone (sequence O5; Fig. 1B); the unit is capped by a terra rossa exposure surface with <0.5 m of relief (Fig. 1C). Karst solution-collapse breccia (South Egan Range) and a 50-m-deep system of sand-filled karst pipes (Pancake Range) occur higher in the succession, beneath a lower Silurian sequence boundary (S2-S3) (Harris and Sheehan, 1997) (Fig. 1D). These field observations provide evidence of meteoric diagenesis independent of geochemical records (e.g., James and Choquette, 1984).
Elemental and isotope geochemistry support field evidence for meteoric diagenesis of the Ely Springs and Laketown formations. Strontium concentrations are highly depleted (10–120 ppm) and correlate with δ18O at both sections (Fig. 2A). This pattern is consistent with diagenetic alteration by meteoric waters (Brand and Veizer, 1980), although it can also be produced during marine diagenesis and dolomite recrystallization (Kupecz et al., 1993). Despite the depletion in δ18O with decreasing Sr, there is no concomitant depletion in δ13C as would be predicted if remineralized terrestrial organic matter played a significant role in the early diagenesis of these rocks (Fig. 2B). A weak but negative correlation between δ13C and Sr is observed in Pancake Range samples, and is antithetical to the influence of remineralized terrestrial organic matter during meteoric diagenesis. A positive correlation between δ13C and δ18O is often cited as evidence of the influence of remineralized terrestrial organic matter during meteoric diagenesis (Allan and Matthews, 1982; Knauth and Kennedy, 2009). No positive correlation is observed (Fig. 2C; see also Gouldey et al., 2010) despite independent evidence for subaerial exposure and meteoric diagenesis, although processes other than meteoric diagenesis can alter δ18O.
Considered in stratigraphic context, the isotope records of the Great Basin strata show no negative δ13C excursion associated with sequence boundaries during the Ordovician–Silurian icehouse (Fig. 3) despite multiple episodes of subaerial exposure during glacioeustatic sea-level changes. In particular, neither the strata immediately below the Ordovician-Silurian unconformity nor the heavily karsted strata below the S2-S3 sequence boundary contains a negative excursion (Fig. 3). The lone negative excursion in the upper S2 sequence (Pancake Range) is not associated with an exposure surface, spans <10 m of stratigraphy, and has a magnitude <2‰; none of these characteristics is consistent with the sedimentologic context, scale, or magnitude of Pliocene–Pleistocene δ13C anomalies produced by meteoric alteration, or of the negative δ13C anomalies associated with Neoproterozoic strata (Swart and Kennedy, 2012). This anomaly may relate to remineralization of sedimentary organic carbon in shallow-marine environments (Lloyd, 1964; Patterson and Walter, 1994).
These results extend previous isotopic studies of Ordovician–Silurian icehouse carbonates. Limestones of the Nashville Dome deposited during the Ordovician–Silurian icehouse contain subaerial exposure surfaces but lack Pliocene–Pleistocene–style geochemical fingerprints of meteoric diagenesis; these strata host small-magnitude negative δ13C excursions (<2.4‰) developed over <1.5 m of stratigraphy below sequence boundaries (Railsback et al., 2003). Well-preserved Hirnantian limestones from Anticosti Island lack negative δ13C anomalies below the Ordovician-Silurian boundary (Fig. 3; Desrochers et al., 2010), despite generally covariant δ13C and δ18O (Long, 1993; Jones et al., 2011). Prolonged subaerial exposure and extensive karst development below the Middle Ordovician Knox unconformity in the Appalachians significantly depleted δ18O but had a minimal effect on δ13C (Mussman et al., 1988).
The δ13C records of the Ordovician–Silurian Great Basin carbonate strata do not contain hallmarks of the remineralization of terrestrially derived organic matter during meteoric diagenesis. The terrestrial biosphere may not have exerted a significant geochemical influence during meteoric diagenesis prior to the radiation of land plants in the Silurian and Devonian (Kenrick and Crane, 1997). By extension, we find it unlikely that Neoproterozoic negative carbon isotope anomalies are the products of meteoric diagenesis. That carbon and oxygen isotope records of altered and karsted exposure surfaces become more complex in the Late Devonian (Myrow et al., 2013) and Pennsylvanian–Permian (Koch and Frank, 2012) may reflect the growing influence of terrestrial organic matter on the geochemical records of platform carbonates. Meteoric diagenesis may not have left a consistent isotopic imprint on carbonate strata exposed during glacioeustatic sea-level fall at different times in Earth history. Future work should expand the record of secular variation in the geochemistry of meteorically altered carbonates in response to the evolving terrestrial biosphere.
Global δ13C Correlation
Latest Ordovician (Hirnantian) strata across the globe record two positive excursions in δ13C, referred to as the Elkhorn and the Hirnantian isotopic carbon excursions (Melchin et al., 2013). The origin of these excursions, which range in magnitude from +2‰ to +8‰, is currently unresolved. Sequence O5 of the Ely Springs Dolostone preserves a positive Hirnantian excursion, tentatively identified as the Elkhorn; the exposure surface marking the Ordovician-Silurian unconformity truncates the full Hirnantian isotope record (Fig. 3). The presence of this chemostratigraphic marker demonstrates that a primary global δ13C signal is recoverable from the meteorically altered dolostones from the Great Basin. Anomalies in older sequences could be useful tie points for high-resolution global correlation to well-preserved fossiliferous limestone strata, including rocks of the Cincinnati Arch, Estonia, and Anticosti Island (Bergström et al., 2010; Desrochers et al., 2010; Jones et al., 2011).
The geochemical records of meteorically altered platform carbonates deposited during the Ordovician–Silurian icehouse do not conform to stable isotope patterns observed in Pliocene–Pleistocene strata that had similar histories of subaerial exposure during glacioeustatic sea-level fluctuations. These results are inconsistent with late Precambrian greening of the continents and suggest a nonmeteoric origin for Neoproterozoic negative δ13C anomalies. The Ordovician–Silurian data provide a snapshot of secular variation in the dominant processes affecting both the marine carbon cycle and carbonate diagenesis over the past 800 m.y.
We are grateful to P. Sheehan and M. Harris for sharing expertise in Great Basin stratigraphy and to D. Fike and A. Maloof for discussions. We thank A. Martini and M. Kopicki at Amherst College and D. Fike and D. McCay at Washington University (St. Louis, Missouri) for laboratory help. The manuscript was improved by reviews from M. Saltzman, M. Harris, and an anonymous reviewer. This work was funded by Amherst College.