ACS Earth and Space Chemistry: What Fractionates Oxygen Isotopes During Respiration? Insights from Multiple Isotopologue Measurements and Theory

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Jeanine L. Ash, Huanting Hu, and Laurence Y. Yeung

Abstract

The precise mass dependence of respiratory O2 consumption underpins the “oxygen triple-isotope” approach to quantifying gross primary productivity in modern and ancient environments. Yet, the physical-chemical origins of the key 18O/16O and 17O/16O covariations observed during respiration have not been tied to theory; thus the approach remains empirical. First-principles calculations on enzyme active-site models suggest that changes in the O-O bond strength upon electron transfer strongly influence respiratory isotopic fractionation. However, molecular diffusion may also be important. Here, we use measurements of the relative abundances of rare isotopologues 17O18O and 18O18O as additional tracers of mass dependence during dark respiration experiments of lacustrine water. We then compare the experimental results to first-principles calculations of O2 interacting with heme-oxidase analogues. We find a significantly steeper mass dependence, supported by theory, than has been previously observed. Enrichments of 17O18O and 18O18O in the O2 residue suggest that θ values are strongly influenced by chemical processes, rather than being dominated by physical processes (i.e. by bond alteration rather than diffusion). In contrast, earlier data are inconsistent with theory, implying that analytical artifacts may have biased those results. Implications for quantifying primary productivity are discussed.

doi: 10.1021/acsearthspacechem.9b00230

Nature Geoscience: Great Oxidation and Lomagundi events linked by deep cycling and enhanced degassing of carbon

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Great Oxidation and Lomagundi events linked by deep cycling and enhanced degassing of carbon

James Eguchi, Johnny Seales, and Rajdeep Dasgupta

Abstract

For approximately the first 2 billion years of the Earth’s history, atmospheric oxygen levels were extremely low. It was not until at least half a billion years after the evolution of oxygenic photosynthesis, perhaps as early as 3 billion years ago, that oxygen rose to appreciable levels during the Great Oxidation Event. Shortly after, marine carbonates underwent a large positive spike in carbon isotope ratios known as the Lomagundi event. The mechanisms responsible for the Great Oxidation and Lomagundi events remain debated. Using a carbon–oxygen box model that tracks the Earth’s surface and interior carbon fluxes and reservoirs, while also tracking carbon isotopes and atmospheric oxygen levels, we demonstrate that about 2.5 billion years ago a tectonic transition that resulted in increased volcanic CO2 emissions could have led to increased deposition of both carbonates and organic carbon (organic C) via enhanced weathering and nutrient delivery to oceans. Increased burial of carbonates and organic C would have allowed the accumulation of atmospheric oxygen while also increasing the delivery of carbon to subduction zones. Coupled with preferential release of carbonates at arc volcanoes and deep recycling of organic C to ocean island volcanoes, we find that such a tectonic transition can simultaneously explain the Great Oxidation and Lomagundi events without any change in the fraction of carbon buried as organic C relative to carbonate, which is often invoked to explain carbon isotope excursions.

 

James Eguchi, Johnny Seales, & Rajdeep Dasgupta (2019). Great oxidation and Lomagundi events linked by deep cycling and increased degassing of carbon. Nature Geoscience. https://doi.org/10.1038/s41561-019-0492-6

AJS: The contribution to exogenic CO2 by contact metamorphism at continental arcs: A coupled model of fluid flux and metamorphic decarbonation

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The contribution to exogenic CO2 by contact metamorphism at continental arcs: A coupled model of fluid flux and metamorphic decarbonation

Xu Chu, Cin-Ty A. Lee, Rajdeep Dasgupta, & Wenrong Cao

ABSTRACT. Recent work has suggested a possible temporal coincidence between
greenhouse intervals and enhanced arc volcanism, motivating the hypothesis that magmatic
and metamorphic CO2 emissions from volcanic arcs, particularly those intersecting
crustal carbonates, may play a strong role in modulating the long-term carbon budget of
the exogenic system. When hot fluid exsolving from arc magmas interacts with carbonate
sequences on active margins, contact metamorphism releases CO2 to metasomatic fluids
that transport carbon to shallow reservoirs. To estimate the magnitude of CO2 release,
here we model how the infiltration of silica-saturated magmatic water into a porous
medium facilitates the decarbonation reaction in contact aureoles. Analytical scalings and
numerical simulations show that the propagation rate of the reaction front scales with the
ratio of the infiltration flux to the mass of the rate-limiting reactant, and accordingly the
CO2 flux increases linearly with the infiltration flux. This simple relationship allows for
scaling to predict regional and global scale CO2 release at continental arcs if magma
emplacement rate is known. Using the global rate of continental arc magma emplacement,
we estimate that the present-day contact-metamorphic CO2 release range from 0.06 to
0.9 Tmol/yr, half-to-one orders of magnitude smaller than the field-based estimates of
carbon output in modern arcs (1.5–3.5 Tmol/yr). Yet, the extrapolated CO2 release from
Cretaceous continental arcs via simple infiltration-induced decarbonation is comparable to
the release from mid-ocean ridges. CO2 released from continental arcs amplifies the
background flux of CO2 from direct degassing of the magma, and therefore may have
been key in causing the climatic greenhouse interval in the Cretaceous when there was
heightened arc activity. Thus, our result supports the hypothesis that global arc flare-ups at
continental margins effectively increase CO2 outgassing coinciding with green-house
intervals in the geological past. The contribution by arcs to the tectonic CO2 input could be
significant, which needs field-based studies to revise long-term climate models.

 

Chu, X., Lee, C-T. A., Dasgupta, R. & Cao, W. (2019). The contribution to exogenic CO2 by contact metamorphism at continental arcs: A coupled model of fluid flux and metamorphic decarbonation. American Journal of Science 319, 631-657. doi:10.2475/08.2019.01

SCIENCE: Illuminating seafloor faults and ocean dynamics with dark fiber distributed acoustic sensing

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Nathaniel J. Lindsey, T. Craig Dowe, and Jonathan B. Ajo-Franklin

Abstract

Distributed fiber-optic sensing technology coupled to existing subsea cables (dark fiber) allows observation of ocean and solid earth phenomena. We used an optical fiber from the cable supporting the Monterey Accelerated Research System during a 4-day maintenance period with a distributed acoustic sensing (DAS) instrument operating onshore, creating a ~10,000-component, 20-kilometer-long seismic array. Recordings of a minor earthquake wavefield identified multiple submarine fault zones. Ambient noise was dominated by shoaling ocean surface waves but also contained observations of in situ secondary microseism generation, post–low-tide bores, storm-induced sediment transport, infragravity waves, and breaking internal waves. DAS amplitudes in the microseism band tracked sea-state dynamics during a storm cycle in the northern Pacific. These observations highlight this method’s potential for marine geophysics.

doi: 10.1126/science.aay5881

GRL: Size of the atmospheric blocking events: Scaling law and response to climate change

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Nabizadeh, E., Hassanzadeh, P., Yang, D., & Barnes, E. A. ( 2019). Size of the atmospheric blocking events: Scaling law and response to climate change. Geophysical Research Letters, 46. https://doi.org/10.1029/2019GL084863

 

Abstract

Understanding the response of atmospheric blocking events to climate change has been of great interest in recent years. However, potential changes in the blocking area (size), which can affect the spatiotemporal characteristics of the resulting extreme events, have not received much attention. Using two large‐ensemble, fully coupled general circulation model (GCM) simulations, we show that the size of blocking events increases with climate change, particularly in the Northern Hemisphere (by as much as 17%). Using a two‐layer quasi‐geostrophic model and a dimensional analysis technique, we derive a scaling law for the size of blocking events, which shows that area mostly scales with width of the jet times the Kuo scale (i.e., the length of stationary Rossby waves). The scaling law is validated in a range of idealized GCM simulations. Predictions of this scaling law agree well with changes in blocking events’ size under climate change in fully coupled GCMs in winters but not in summers.

GCA: Sulfur extraction via carbonated melts from sulfide-bearing mantle lithologies – Implications for deep sulfur cycle and mantle redox

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Proteek Chowdhury and Rajdeep Dasgupta

Abstract

Transport of sulfur via mantle-derived partial melts from deep Earth to the surface reservoirs is a critical step in the deep

global sulfur cycle. Given that sulfur is stored mostly in sulfide phases in mantle lithologies, the critical parameter is sulfur

concentration at sulfide saturation (SCSS) of mantle-derived magmas. CO2 and H2O-induced melting beneath oceanic and

continental mantle produces incipient CO2-rich melts. Although, SCSS of silicate melts of a variety of compositions is extensively

studied, the SCSS of carbonatitic and carbonated silicate melts have not received much attention. Here we present

experiments in graphite capsules at pressures (P) of 2.5–6.0 GPa and temperatures (T) of 1350–1650 C investigating the SCSS

of carbonatitic and carbonated silicate melts. All experiments produced quenched Fe +- Ni-sulfide melt blobs + carbonated

melt matrix +- ol +- cpx +- opx +- gt, with melt composition on a CO2-free basis varying from 7 to 40 wt.% SiO2, 0.5 to 7

wt.% Al2O3, and 9 to 17 wt.% FeO* (total FeO). SCSS measured using EPMA increases with SiO2 and T but is not affected

by P; the effect of composition being more pronounced than P-T. The composition of sulfide melt phase also affects SCSS.

With increasing Ni in the molten sulfide phase, the SCSS changes from 2000 to 4000 ppm (Ni-free) to is 800–3000 ppm (33 wt.

% Ni). Comparison of our measured SCSS with the existing SCSS models for nominally CO2-free silicate melts and with one

study for carbonated melts show that these parameterizations fail to capture the sulfide saturation values in CO2-rich melts

from our study. Using our new SCSS data and previous SCSS data for melt compositions that span the range from carbonatite

to basalts via carbonated silicate melts, we develop a new empirical SCSS parameterization. Unlike a previous model,

which suggested SCSS of carbonated melt is only affected by melt FeO* (other than P-T) and did not constrain how SCSS

evolves from low-silica carbonatitic melt to low-CO2 basaltic melt, our new parameterization captured complex effects of

many melt compositional parameters, including silica on SCSS. Using our new SCSS model, we constrained the efficiency

of S extraction from the mantle beneath mid-oceanic ridges and continents via low-degree carbonated melts. Deep carbonated

melts beneath ridges are expected to mobilize 5–15% of the initial sulfur before nominally-volatile-free peridotite melting

begins. In continental mantle, deep kimberlitic melt can act as an agent to mildly enrich the shallow mantle in sulfide as it

evolves to a carbonatitic melt upon reactive cooling. Application of our data to subduction zones suggests that low degree

carbonatitic melt is not an efficient agent to extract residual sulfide from the subducting oceanic crust.

 

Geochimica et Cosmochimica Acta: https://doi.org/10.1016/j.gca.2019.11.002

Frontiers in Marine Science: The Future of Reef Ecosystems in the Gulf of Mexico: Insights From Coupled Climate Model Simulations and Ancient Hot-House Reefs

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Sylvia G. Dee1*, Mark A. Torres1, Rowan C. Martindale2, Anna Weiss2 and Kristine L. DeLong3
  • 1Department of Earth, Environmental, and Planetary Science, Rice University, Houston, TX, United States
  • 2Department of Geological Sciences, The University of Texas at Austin, Austin, TX, United States
  • 3Department of Geography and Anthropology, The Coastal Studies Institute, Louisiana State University, Baton Rouge, LA, United States

Shallow water coral reefs and deep sea coral communities are sensitive to current and future environmental stresses, such as changes in sea surface temperatures (SST), salinity, carbonate chemistry, and acidity. Over the last half-century, some reef communities have been disappearing at an alarming pace. This study focuses on the Gulf of Mexico, where the majority of shallow coral reefs are reported to be in poor or fair condition. We analyze the RCP8.5 ensemble of the Community Earth System Model v1.2 to identify monthly-to-decadal trends in Gulf of Mexico SST. Secondly, we examine projected changes in ocean pH, carbonate saturation state, and salinity in the same coupled model simulations. We find that the joint impacts of predicted higher temperatures and changes in ocean acidification will severely degrade Gulf of Mexico reef systems by the end of the twenty-first century. SSTs are likely to warm by 2.5–3°C; while corals do show signs of an ability to adapt toward higher temperatures, current coral species and reef systems are likely to suffer major bleaching events in coming years. We contextualize future changes with ancient reefs from paleoclimate analogs, periods of Earth’s past that were also exceptionally warm, specifically rapid “hyperthermal” events. Ancient analog events are often associated with extinctions, reef collapse, and significant ecological changes, yet reef communities managed to survive these events on evolutionary timescales. Finally, we review research which discusses the adaptive potential of the Gulf of Mexico’s coral reefs, meccas of biodiversity and oceanic health. We assert that the only guaranteed solution for long-term conservation and recovery is substantial, rapid reduction of anthropogenic greenhouse gas emissions.

 

Front. Mar. Sci., 20 November 2019 | https://doi.org/10.3389/fmars.2019.00691

JGR Solid Earth: Phase relations of a depleted peridotite fluxed by a CO2-H2O fluid – Implications for the stability of partial melts versus volatile-bearing mineral phases in the cratonic mantle

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Phase relations of a depleted peridotite fluxed by a CO2-H2O fluid – Implications for the stability of partial melts versus volatile-bearing mineral phases in the cratonic mantle
Sriparna Saha and Rajdeep Dasgupta

Abstract

We present phase‐equilibria experiments of a K‐bearing, depleted peridotite (Mg# 92) fluxed with a mixed CO2‐H2O fluid (0.5 wt.% CO2 and 0.94 wt.% H2O in the bulk) to gain insight into the stability of volatile‐bearing partial melts versus volatile‐bearing mineral phases in a depleted peridotite system. Experiments were performed at 850–1150 °C and 2–4 GPa using a piston‐cylinder and a multianvil apparatus. Olivine, orthopyroxene, clinopyroxene, and spinel/garnet are present at all experimental conditions. Textural confirmation of partial melt is made at temperatures as low as 1000 °C at 2 GPa, 950 °C at 3 GPa, and 1000 °C at 4 GPa marking the onset of melting at 900–1000 °C at 2 GPa, 850–950 °C at 3 GPa, and 950–1000 °C at 3 GPa. Phlogopite and magnesite breakdown at 900–1000 °C at 2 GPa, 950–1000 °C at 3 GPa, and 1000–1050 °C at 4 GPa. Comparison with previously published experiments in depleted peridotite system with identical CO2‐H2O content introduced via a silicic melt show that introduction of CO2‐H2O as fluid lowers the temperature of phlogopite breakdown by 150–200 °C at 2–4 GPa and stabilizes partial melts at lower temperatures. Our study thus, shows that the volatile‐bearing phase present in the cratonic mantle is controlled by bulk composition and is affected by the process of volatile addition during craton formation in a subduction zone. In addition, volatile introduction via melt versus aqueous fluid, leads to different proportion of anhydrous phases such as olivine and orthopyroxene. Considering the agent of metasomatism is thus critical to evaluate how the bulk composition of depleted peridotite is modified, leading to potential stability of volatile‐bearing phases as the cause of anomalously low shear wave velocity in mantle domains such as mid lithospheric discontinuities beneath continents.

Saha, S. & Dasgupta, R. (2019). Phase relations of a depleted peridotite fluxed by a CO2-H2O fluid – Implications for the stability of partial melts versus volatile-bearing mineral phases in the cratonic mantle. Journal of Geophysical Research: Solid Earth 124. doi:10.1029/2019JB017653

 

Cambridge University Press: A framework for understanding whole Earth carbon cycling

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A Framework for Understanding Whole-Earth Carbon Cycling

Summary

This chapter explores how the cycling of carbon in subduction zones and orogenic belts varies with supercontinent cycles and mountain building. It discusses the processes that link short-term and long-term carbon cycling and the timescales of these processes, such as the response times of weathering and atmospheric drawdown at periods of enhanced volcanism. This chapter covers topics of potential fluctuations in the long-term CO2 content of Earth’s atmosphere because of mantle–climate feedback.

Lee, C-T. A., Jiang, H., Dasgupta, R. & Torres, M. (2019). A framework for understanding whole Earth carbon cycling. In Orcutt, B., Daniel, I., and Dasgupta, R. (Eds.) Deep Carbon: Past to Present. Cambridge University Press, Cambridge, pp. 313-357. doi:10.1017/9781108677950.011

 

Cambridge University Press: Origin and Early Differentiation of Carbon and Associated Life-Essential Volatile Elements on Earth

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Rajdeep Dasgupta, Damanveer S. Grewal

Summary

This chapter reviews what is known about the fate of carbon during early differentiation of inner solar system planets. It reviews the nature of carbon fractionation in a magma ocean as compared to the core, mantle, and atmosphere, and how this may have varied between planetary bodies in the solar system. It discusses whether magma ocean processes could have established the present-day budget of carbon in Earth’s bulk silicate, and also reviews possibilities for the early temporal evolution of the mantle carbon budget through core formation, later veneer addition, and magma ocean crystallization processes.

Dasgupta, R., & Grewal, D. (2019). Origin and Early Differentiation of Carbon and Associated Life-Essential Volatile Elements on Earth. In B. Orcutt, I. Daniel, & R. Dasgupta (Eds.), Deep Carbon: Past to Present (pp. 4-39). Cambridge: Cambridge University Press. doi:10.1017/9781108677950.002