Magnesium and calcium isotopes in modern and ancient carbonate sediments


a.) Ca and Mg isotope variability in carbonate sediments associated with large and extreme C isotope excursions

i.) Stratigraphic Expression of Earth's δ13C Excursion in the Wonoka Formation of South Australia

The most negative carbon isotope excursion in Earth history is found in carbonate rocks of the Ediacaran Period (635-541 Ma). Known colloquially as the “Shuram” excursion, workers have long noted its broad concordance with the rise of abundant macro-scale fossils in the rock record, collectively known as the “Ediacaran Biota.” Thus, the Shuram excursion has been interpreted by many in the context of a dramatically changing redox state of the Ediacaran oceans—for example, a result of methane cycling in a low O2 atmosphere, the final destruction of a large pool of recalcitrant dissolved organic carbon (DOC), and the step-wise oxidation of the Ediacaran oceans. More recently, diagenetic interpretations of the Shuram excursion have challenged the various redox models, with the very negative δ13C values of Ediacaran carbonates explained via sedimentary in-growth of very δ13C depleted authigenic carbonates, meteoric alteration or late-stage burial diagenesis. A stratigraphic and sedimentological context is required to discriminate between these explanatory models, and to determine whether the Shuram excursion can be used to evaluate oxygenation in terminal Neoproterozoic oceans. Here we present chemostratigraphic data (δ13C, δ13O, and trace element abundances) from 15 measured sections of the Ediacaran-aged Wonoka Formation (Fm.) of South Australia. In some locations, the Wonoka Fm. is ~700 meters (m) of mixed shelf limestones and siliciclastics that record a 17 permil δ13C excursion (12 to 5‰). Further north in the basin, the Wonoka Fm. is host to deep (1 km) paleocanyons, which are partly filled by tabular-clast carbonate breccias. Canyon-filling ceased during ongoing sedimentation on the shelf interior (the “canyon-shoulder”), as evidenced by upper canyonshoulder units that overlie and cap certain canyon-fill sequences. The unprecedented size of the chemostratigraphic dataset presented here (2671 δ13C-δ13O measurements from the canyon-shoulder, 1393 δ13C-δ13O measurements from canyon clasts, and 11 different trace elements measured on 247 Wonoka Fm. carbonate samples), when coupled with the unique canyon-shoulder to canyon-fill depositional system of the Wonoka Fm., allows for new insights into Ediacaran carbon isotope systematics. The excursion is preserved in a remarkably consistent fashion across 12,000 km2 of basin area; fabric-altering diagenesis, where present, occurs at the sub-meter vertical scale, results in < 1 permil offsets in δ13C and cannot be used to explain the full δ13C excursion. Multi-variate analysis of the dataset allows for rigorous assessment of different potential carbonate sources for the Wonoka canyon-fill breccias. Eroded and transported canyon-shoulder carbonates are the most likely source, thus requiring a syn-depositional age for the extraordinary range of δ13C values (-12 to +5‰) observed in both the Wonoka Fm. canyon-shoulder and canyon-fill breccias. Geological observations (for example, excellent preservation of sedimentary structures in Wonoka carbonates, absence of top-down alteration profiles associated with exposure surfaces) do not provide first-order evidence for the meteoric or authigenic carbonate models. Thus, the balance of evidence supports either a primary origin or syndepositional, fabric-retentive diagenesis for the deep negative δ13C excursion hosted in the Wonoka Formation of South Australia.

J. M. Husson, Maloof, A. C. , Schoene, B. , Chen, C. Y. , and Higgins, J. A. , “Stratigraphic expression of Earth's deepest δ13C excursion in the Wonoka Formation of South Australia”, American Journal of Science, vol. 315, no. 1, pp. 1 - 45, 2015.

Figure 1.  “Shuram”  excursion δ13C carb  (A) and  δ13C carb (B)  data  from  Oman (Fike  and  others,  2006), south  China  (McFadden and  others,   2008),  Siberia  (Pokrovskii  and  others,  2006),  southwestern USA (Verdel  and  others,  2011),  the  Yukon (Macdonald and  others,  2013),  and  South  Australia  (this  work) plotted by continent and  by lithology (limestone vs. dolomite). The complete recovery from the excursion has been dated  in only one section [551.1 ± 0.7 Ma in south China; Condon and others  (2005)], but there is almost  no  geochronologic control on  its initiation, and  duration estimates  range  from  5 (Bowring  and others,  2007) to 50 Myr (Le Guerroue´ and others,  2006c). Thus, the similar δ13C carb profiles have been used to correlate the sections. (C) The width of the bars, color-coded by continent and lithology and labeled  with true section thicknesses (in meters), correlates with 1/S, where S is the stretch  factor applied to each dataset. If the Shuram excursion is an expression of secular change in global DIC, then  the width of the bars would also correlate with relative  sedimentation rates  of each  section.  (D)  It is widely assumed  that  all Shuram excursion datasets  display  strong  covariation between  δ13C carb  and  δ18O carb, especially  for  samples  with δ13C carb  < -5‰. A cross-plot  of this compiled Ediacaran dataset,  however,  with correlation coefficients between δ13C carb  and  δ18O listed by section  [for  δ13C carb values < -5‰; p values <<0.001 except  for Siberia (p = 0.01) and southwestern USA (p = 0.025)],  shows that the correlation ranges from strong (r2 = 0.84 in South Australia)  to weak (r2 = 0.12 in southwestern USA) with linear slopes ranging from 1.5 to 0.1. (E) World map showing present-day  location of all sections used in this compilation. Figure courtesy of John M. Husson

Figure 1. Shuram excursion data.

ii.) Ca and Mg isotope stratigraphy of the Trezona C isotope excursion – Geochemical record of the descent into a Snowball?

Extreme negative carbon isotope (δ13C) excursions below the canonical mantle value are found globally in carbonate rocks from the Neoproterozoic Era. One of the more spectacular is the 16-18‰ negative excursion in carbonate sediments, observed on 5 continents, which precede the younger of the two Cryogenian glaciations. Recent research suggests that coupled measurements of δ44Ca and δ26Mg values in marine carbonates (limestones and dolomites) can provide unique constraints on diagenesis in carbonate sediments [1-3]. Here we present 330 δ44Ca and 120 δ26Mg measurements of pre-Marinoan carbonate sediments from Australia, Namibia, and North America. We show that the δ44Ca, δ26Mg, δ18O, and δ13C co-vary and that the large range in Ca and Mg isotope values are unlikely to be due to changes in global Ca and Mg cycles. We explore alternative explanations which invoke post or syn-depositional water-rock interaction and recrystallization associated with sea-level change during the descent into a global glaciation.

Ahm A-S, Bjerrum C, Hoffman P, Macdonald F, Maloof A, Rose C and Higgins J. A., Ca and Mg Isotope Stratigraphy of the Trezona C Isotope Excursion – Geochemical Record of the Descent into a Snowball? (presentation, Goldschmidt2015, Prague, CZ, August 20, 2015).

Figure 2. Anne-Sophie Ahm's figure.


b.) Ca and Mg isotope systematics of modern shallow water carbonate sediments

i.) Modern shallow water carbonate systems

Modern shallow carbonate environments, like that of Triple Goose Creek on Andros Island in the Bahamas, serve as an analogue for the shallow carbonate sequences that are preserved for much of Earth history. By analyzing the major ion and isotopic composition of sediments and pore-fluids collected on and around Andros Island, we hope to constrain geochemical signatures of early diagenesis (e.g., meteoric alteration, dissolution and recrystallization). Furthermore, by integrating this study with other bank-top datasets, we hope to determine the viability of shallow water carbonates as recorders of global seawater chemistry, which has implications for how we interpret geochemical and isotopic signatures preserved in such sequences over geologic time.

Figure 3. δ13C and δ18O for all sediment samples collected on Andros Island, including Triple Goose Creek (TGC) muds, shells and crusts; Morgan’s cave and aeolionite deposits and ooids from Joulter’s Cay. Figure courtesy of Alliya A. Akhtar.

Figure 4. δ44/40Ca and Ca concentrations for TGC pore fluids. Samples labeled ‘Core-37’ and ‘Core-41’ are from deep-sea cores from the western Equatorial Pacific and represent recrystallization (i.e. no net dissolution or precipitation). TGC data mostly follows modeled dissolution pathways, but evidence for recrystallization is seen in samples that fall below the Halimeda and inorganic aragonite lines. Figure courtesy of Alliya A. Akhtar.

Figure 3. Composition of Bahama sediments.
FIgure 4. Concentrations for TGC pore fluids.

ii.) Bahamas Drilling Project

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c.) Ca and Mg isotope systematics of authigenic carbonates

i.) Mg and Ca isotope signatures of authigenic dolomite in siliceous deep-sea sediments

Authigenic carbonates in marine sediments frequently have carbon isotope ratios that reflect local organic carbon processing rather than the δ13C of the global DIC (dissolved inorganic carbon) reservoir, but their contributions to ancient sedimentary sections are difficult to assess. In this study of authigenic dolomite from the Miocene-age Monterey Formation of offshore California, Mg and Ca isotopes are shown to vary with stratigraphic depth as a result of early diagenetic processes. The dolomite is a pre-compaction authigenic phase that occurs as beds and nodules with δ13C ranging from −16 to +9‰. Light δ13C values were likely acquired from the sedimentary zone of microbial sulfate reduction, whileheavy δ13C values were acquired from the zone of methanogenesis. Mg and Ca isotopes are roughly anti-correlated, with intervals of negative δ13C associated with low δ26Mg and higher δ44/40Ca values. The variability is observed over a wide range of length-scales, from 10−2 meters within individual authigenic beds/nodules, to 102 meters over the entire stratigraphic column, and can be understood as the consequence of dolomite precipitation in pore fluids where Mg supply is limited by diffusive transport. The relationship of δ26Mg and δ44/40Ca to the more common stable isotope measurements of δ13C and δ18O represents a new, diagenetically robust, geochemical fingerprint for identifying synsedimentaryauthigenic carbonates in the geological record.

C. L. Blättler, Miller, N. R. , and Higgins, J. A. , “Mg and Ca isotope signatures of authigenic dolomite in siliceous deep-sea sediments”, Earth and Planetary Science Letters, vol. 419, pp. 32 - 42, 2015.

Figure 6. Isotopic data for bulk dolomite samples from the Monterey Formation, plotted against stratigraphic depth. Delta values are reported relative to VPDB (C and O), DSM-3 (Mg), and modern seawater (Ca). The dataset includes 45 analyses each of Mg isotopes and Ca isotopes. Figure courtesy of Clara L.Blättler

Figure 6. Bulk dolomite samples.