Geology: Continental-scale geographic change across Zealandia during Paleogene subduction initiation

Data from International Ocean Discovery Program (IODP) Expedition 371 reveal vertical movements of 1–3 km in northern Zealandia during early Cenozoic subduction initiation in the western Pacific Ocean. Lord Howe Rise rose from deep (~1 km) water to sea level and subsided back, with peak uplift at 50 Ma in the north and between 41 and 32 Ma in the south. The New Caledonia Trough subsided 2–3 km between 55 and 45 Ma. We suggest these elevation changes resulted from crust delamination and mantle flow that led to slab formation. We propose a “subduction resurrection” model in which (1) a subduction rupture event activated lithospheric-scale faults across a broad region during less than ~5 m.y., and (2) tectonic forces evolved over a further 4–8 m.y. as subducted slabs grew in size and drove plate-motion change. Such a subduction rupture event may have involved nucleation and lateral propagation of slip-weakening rupture along an interconnected set of preexisting weaknesses adjacent to density anomalies.

R. Sutherland ; G.R. Dickens ; P. Blum ; C. Agnini ; L. Alegret ; G. Asatryan ; J. Bhattacharya ; A. Bordenave ; L. Chang ; J. Collot ; M.J. Cramwinckel ; E. Dallanave ; M.K. Drake ; S.J.G. Etienne ; M. Giorgioni ; M. Gurnis ; D.T. Harper ; H.-H.M. Huang ; A.L. Keller ; A.R. Lam ; H. Li ; H. Matsui ; H.E.G. Morgans ; C. Newsam ; Y.-H. Park ; K.M. Pascher ; S.F. Pekar ; D.E. Penman ; S. Saito ; W.R. Stratford ; T. Westerhold ; X. Zhou
Geology (2020)
https://doi.org/10.1130/G47008.1

EPSL: How to make porphyry copper deposits

Much of the world’s economic copper resources are hosted in porphyry copper deposits (PCDs), shallow level magmatic intrusions associated mostly with thick (>45km) magmatic arcs, such as mature island arcs and continental arcs. However, a well-known, but unresolved paradox, is that arc magmas traversing thick crust, particularly in continental arcs, are generally depleted in Cu whereas in island arcs, where PCDs are less common, magmas become enriched in Cu. Here, we show that the formation of PCDs requires a complex sequence of intra-crustal magmatic processes, from the lower crust to the upper crust. PCDs form when the crust becomes thick (>45km) enough to crystallize garnet. Garnet fractionation depletes Fe from the magma, which drives sulfide segregation and removal of most of the magma’s Cu into the lower crust, leaving only small amounts of Cu in the residual magma to make PCDs. However, because garnet is depleted in ferric iron, the remaining Fe in the magma becomes progressively oxidized, which eventually oxidizes sulfide to sulfate, thereby releasing sulfide bound Cu from the magma into solution. This auto-oxidation of the magma, made possible by deep-seated garnet fractionation, increases the ability of endogenic magmatic fluids to self-scavenge Cu from large volumes of otherwise Cu-poor magmas and then transport and concentrate Cu to the tops of magmatic bodies. Examination of the occurrence of PCDs in the central Andes shows that ore formation occurs when continental arcs reach their maximum thickness (>60km), just before the termination of magmatism.

 

Cin-Ty A.Lee and MingTang, Volume 529, 1 January 2020, 115868

https://doi.org/10.1016/j.epsl.2019.115868

Computational Geosciences: A numerical study of multi-parameter full waveform inversion with iterative regularization using multi-frequency vibroseis data

We study the inverse boundary value problem for time-harmonic elastic waves, for the recovery of P– and S-wave speeds from vibroseis data or the Neumann-to-Dirichlet map. Our study is based on our recent result pertaining to the uniqueness and a conditional Lipschitz stability estimate for parametrizations on unstructured tetrahedral meshes of this inverse boundary value problem. With the conditional Lipschitz stability estimate, we design a procedure for full waveform inversion (FWI) with iterative regularization. The iterative regularization is implemented by projecting gradients, after scaling, onto subspaces associated with the mentioned parametrizations yielding Lipschitz stability. The procedure is illustrated in computational experiments using the continuous Galerkin finite element method of recovering the rough shapes and wave speeds of geological bodies from simple starting models, near and far from the boundary, that is, the free surface.

 

Shi, J., Beretta, E., Maarten, V., Francini, E., & Vessella, S. (2019). A numerical study of multi-parameter full waveform inversion with iterative regularization using multi-frequency vibroseis data. Computational Geosciences, 1-19. https://link.springer.com/article/10.1007/s10596-019-09897-6

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

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

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

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

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

Nabizadeh, E.Hassanzadeh, P.Yang, D., & Barnes, E. A. ( 2019). Size of the atmospheric blocking events: Scaling law and response to climate changeGeophysical Research Letters46https://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

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

  • 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