Seawater chemistry, climate, and the global carbon and oxygen cycles on geologic timescales

a.) Mg isotope evidence for a link between seawater Mg/Ca and climate

Cooling of Earth’s climate over the Cenozoic has been accompanied by large changes in the magnesium and calcium content of seawater whose origins remain enigmatic. The processes that control these changes affect the magnesium isotopic composition of seawater, rendering it a useful tool for elucidating the processes that control seawater chemistry on geologic timescales. Here we present a Cenozoic magnesium isotope record of carbonate sediments and use a numerical model of seawater chemistry and the carbon cycle to test hypotheses for the covariation between Cenozoic seawater chemistry and climate. Records are consistent with a 2–3×increase in seawater Mg/Ca and little change inthe Mg isotopic composition of seawater. These observations are best explained by a change in the cycling of Mg-silicates. We propose that Mg/Ca changes were caused by a reduction in removal of Mg from seawater in low-temperature marine clays, though an increase in the weathering of Mg-silicates cannot be excluded. We attribute the reduction in the Mg sink in marine clays to changes in ocean temperature, directly linking the major element chemistry of seawater to global climate and providing a novel explanation for the covariation of seawater Mg/Ca and climate over the Cenozoic.

Higgins, J. A. and Schrag, D. P., “The Mg isotopic composition of Cenozoic seawater - evidence for a link between Mg-clays, seawater Mg/Ca, and climate”, Earth and Planetary Science Letters, vol. 416, pp. 73 - 81, 2015.

Figure 1. Model output. — Figure 1. Model output for our preferred scenario for reconstructed changes in the concentration and isotopic composition of Mg in seawater over the Cenozoic. Recon-structions of Mg/Ca ratios and [Mg]swover the Cenozoic taken from the published literature.

b.) Records of seawater chemistry over the last 200 Myr from exceptionally preserved fossil corals

Reconstructing the chemistry of ancient seawater from well preserved aragonitic corals

Robust records of the major ion and isotopic composition of seawater have the potential to help identify causative mechanisms and elucidate the relationship between seawater chemistry and climate over million year timescales. Using an archive of extremely well preserved fossil scleractinian corals (Gothmann et al., 2015), we are working on reconstructing the elemental and isotopic composition of seawater since the Mesozoic (e.g., Mg/Ca, Sr/Ca, U/Ca ẟ²⁶Mg, and ẟ⁷Li). Together, these records may help provide insight into the mechanisms driving secular change in seawater chemistry and climate since the Mesozoic (e.g., silicate weathering, carbonate weathering and precipitation, and hydrothermal alteration).

We are also studying patterns of ‘vital effects’ in ancient corals. Vital effects are defined as geochemical signatures of biological control over calcification, which appear as departures in the geochemistry of coral aragonite relative to the composition predicted based on inorganic distribution coefficients. While vital effects can be problematic for paleoenvironmental reconstructions, they can also give important insight into coral calcification and how it responds to changes in seawater boundary conditions across a range of timescales. We have assembled a record of Ca isotopes in fossil corals, which appears to reflect changes in coral Ca isotope discrimination and biomineralization dynamics since the Jurassic. We are interested in continuing to investigate the origin of the apparent change in coral Ca isotope discrimination and how it may relate to changes in the major ion and/or carbonate chemistry of seawater.

Figure 2. Fossil scleractinian coral. — Figure 2. Well preserved fossil scleractinian coral. Photo courtesy of Anne Gothman

Figure 3. Jurassic Coral. — Figure 3. Thin section image of a Jurassic age fossil coral from Ostromice, Poland. Photo courtesy of Anne Gothman and Jarek Stolarski

Figure 4. Eocene Coral. Photo courtesy of Anne Gothman

c.) Records of seawater SO₄/Ca from Ca isotopes in marine evaporites

Marine evaporites can provide a useful record of past seawater composition, if the relationship between the original seawater and the preserved minerals can be established. Fluid inclusions in halite, for example, have been used to constrain the major ion composition of the ancient oceans. We have developed a new test to constrain the calcium to sulfate ratio of past seawater by analyzing Ca isotopes in gypsum and anhydrite sequences (Blättler and Higgins, 2014). We are using this method to explore sulfate concentrations during critical intervals of Earth history, and are also investigating additional analyses in evaporite minerals (e.g. to determine the magnesium isotope composition of seawater).

FIgures 6 Marine Evaporites

Photos courtesy of Clara L. Blättler.

FIgures 8 Marine Evaporites in a testing glass — FIgures 8 Marine Evaporites

d.) The global CaMg(CO₃) cycle over Earth History

i.) The Higgins laboratory has many active projects exploring the calcium and magnesium isotope composition of marine carbonates.

As the major cations in carbonate rocks (both limestone and dolomite), Ca and Mg isotopes have good preservation potential and numerous applications over a wide range of timescales

1. 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 9. Composition of Bahama sediments — 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 10. Concentrations for TGC pore fluids — 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.

2. Investigating carbon isotope excursions: Ca and Mg isotopes can provide additional dimensions of geochemical data to help constrain the causes of δ¹³C excursions, which are often linked to dramatic climate perturbations in Earth history. We have projects involving the Neoproterozoic Shuram excursion (Husson et al., 2015), the Trezona anomaly, the Lomagundi interval, and the Carboniferous. Carbonate sedimentary sections are often used to interpret the history of the carbon cycle and climate, but additional geochemical analyses are required to test these hypotheses and fully interpret δ¹³C and δ¹⁸O records.

Figure 11. Canyon-fill breccias — Figure 11. Photographs of canyon-fill breccias of Mount Thomas (A) and Saint Ronan (B) paleocanyons. Both are composed of tabular clasts of carbonate that have been eroded from canyon-shoulder sections and transported downslope into the bases of paleocanyons. At Mount Thomas (A), the breccias are composed of both limestone (grey clasts) and dolostones (brown clasts), whereas Saint Ronan breccias (B) are composed entirely of limestone clasts. These breccia units have been studied extensively for δ13Ccarb and δ18Ocarb values (Husson et al., 2012, 2015), and a subset of individually sampled clasts from both Mount Thomas and Saint Ronan have been analyzed for trace elements, δ44=40Ca and δ26Mg. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article. Figure courtesy of John M. Husson.

3. Authigenic carbonates: Authigenic carbonates may constitute an important part of many sedimentary records, but may be difficult to identify. We have analyzed a suite of authigenic dolomites from the Monterey Formation to establish a geochemical fingerprint for this style of authigenesis (Blättler et al., 2015).

Figure 12. Authigenic carbonates. Photo courtesy of Clara Blättler

4. Carbonates through time: We are working on compiling data on Ca and Mg isotopes in many ancient carbonate deposits to assess the isotopic composition of seawater and the state of the Ca and Mg isotopic cycles. Constraining inputs and outputs of Ca and Mg has important implications for the carbon cycle, seawater chemistry, and global geochemical cycles. We can identify different styles of dolomitization.

Figure 13. Dissolution-Collapse Breccia Hand Sample.

ii.) Oxygenation of the ocean and sediments: Consequences for the seafloor carbonate factory

Observed changes in the source of CaCO₃ sediments since Archean time suggest a first order pattern of decreasing abundance of carbonate cements precipitated directly on the seafloor. We propose that the observed reduction in CaCO₃ precipitation on the seafloor is caused by a decrease in CaCO₃ saturation in sediments related to increased oxic cycling of organic carbon and a decline in the size of the marine DIC reservoir. Using a simple model of CaCO₃ saturation in the ocean, we show that changes in ocean–atmosphere redox and the size of the marine carbon reservoir strongly influence the ability of sediments to dissolve or precipitate CaCO₃. Oxic oceans like the modern are characterized by large gradients in CaCO₃ saturation. Calcium carbonate precipitates where CaCO₃ saturation is high (surface ocean) and dissolves where CaCO₃ saturation is low (sediments). In contrast, anoxic respiration of organic carbon and/or a large ocean carbon reservoir leads to a more homogeneous distribution of CaCO₃ saturation in the ocean and sediments. This effect suppresses CaCO₃ dissolution and promotes CaCO₃ precipitation on the seafloor. Our results suggest that the growth or contraction of gradients in CaCO₃ saturation in the ocean and sediments may explain the observed trends in carbonate precipitation on the seafloor in the Precambrian and changes in the global CaCO₃ cycle, such as the reappearance of seafloor precipitates and the drowning of carbonate platforms during episodes of widespread anoxia in the Phanerozoic marine basins. Our work provides novel insights into the consequences of the long-term geochemical evolution of the ocean and atmosphere for the global CaCO₃cycle.

Higgins, J. A., Fischer, W. W., and Schrag, D. P., “Oxygenation of the ocean and sediments: Consequences for the seafloor carbonate factory”, Earth and Planetary Science Letters, vol. 284, no. 1-2, pp. 25 - 33, 2009.

Figure 15. Chart that depicts Proposed changes in the global CaCO3 cycle since 2.5 Ga due to the oxidation of the ocean and sediments and long-term decline in atmospheric CO2 — Figure 15. Proposed changes in the global CaCO3 cycle since 2.5 Ga due to the oxidation of the ocean and sediments and long-term decline in atmospheric CO2. Upper panel depicts changes in the partitioning of the global CaCO3 flux between precipitation in the surface ocean, precipitation on the seafloor, and dissolution as a function of ΔΩ⁎ assuming constant kps, kpb, and kd. Bottom panel illustrates the evolution of CaCO3 sediments since 2.5 Ga. A: If CaCO3 dissolution is important, a decline in ΔΩ⁎ must be accompanied by a decline in the rate of CaCO3 precipitation so that the rate of CaCO3 burial remains in balance with alkalinity inputs(W). This type of response may be consistentwith observed changes in the global CaCO3 cycle during Cretaceous OAEs. B: A very large drop in ΔΩ⁎ will re-start the seafloor CaCO3 factory at the expense of CaCO3 precipitation in the surface ocean. This result could explain the return of seafloor precipitates and the delayed recovery of skeletal CaCO3 producers in the early Triassic (Payne et al., 2004).

iii.) Authigenic carbonate and the history of the global carbon cycle

We present a framework for interpreting the carbon isotopic composition of sedimentary rocks, which in turn requires a fundamental reinterpretation of the carbon cycle and redox budgets over Earth’s history. We propose that authigenic carbonate, produced in sediment pore fluids during early diagenesis, has played a major role in the carbon cycle in the past. This sink constitutes a minor component of the carbon isotope mass balance under the modern, high levels of atmospheric oxygen but was much larger in times of low atmospheric O₂ or widespread marine anoxia. Waxing and waning of a global authigenic carbonate sink helps to explain extreme carbon isotope variations in the Proterozoic, Paleozoic, and Triassic.

Schrag, D. P., Higgins, J. A. , Macdonald, F. A. , and Johnston, D. T. , “Authigenic Carbonate and the History of the Global Carbon Cycle”, Science, vol. 339, pp. 540 - 543, 2013.

Figure 16. Calcite and Dolomite Carbon Isotope Measurements. — Figure 16. ¹³C_carb measurements on Early Paleozoic and Proterozoic calcites (open circles) and dolomites (triangles) from the compilation of (Prokoph et al, 24).