Chromium speciation in marine carbonates and implications on atmospheric oxygenation

May 15, 2020

The oxidation of Earth's early atmosphere and ocean played an important role in the evolution of life. Reconstructing the paleo-redox conditions is crucial for the understanding of the coevolution of life and environment. The Cr isotopic composition in sedimentary rocks have been increasingly used as an emerging paleo-redox indicator. It is largely based on the assumption that when the atmospheric oxygen level is relatively high, oxidation of Cr(III) in terrestrial rocks will result in soluble Cr(VI). This process can lead to positively fractionated Cr isotopic composition of Cr(VI), which will ultimately be transported to the ocean and preserved in sedimentary rocks. Carbonates are widely distributed sedimentary rocks and are an important geological archive. Previous studies suggested that Cr(VI) in seawater can be directly incorporated into carbonate crystal lattice, and marine non-skeletal carbonate could directly record the Cr isotopic composition of the contemporaneous seawater. However, there is hitherto no direct evidence for the presence of Cr(VI) in sedimentary carbonates.

To address whether Cr(VI) is present in sedimentary carbonates, Ziyao Fang and colleagues applied synchrotron based X-ray absorption near edge structure (XANES) spectra to geochemical studies. They examined the Cr valence states and Cr isotopic compositions of sedimentary carbonates formed in different geological periods. Their results showed that Cr(III) dominates in all sedimentary marine carbonates, even in those which have positively fractionated Cr isotopic compositions. This is in apparent contrast with the previous assumption.

Fang and colleagues proposed two possible mechanisms for the absence of Cr(VI) in carbonates. One is that Cr(VI) in seawater might have been reduced by microbes or porewater during carbonate precipitation or early diagenesis. In this case, the reduction of Cr(VI) is likely not quantitative in the relatively oxic carbonate deposition environment, and as a result, the isotopic composition of carbonate would be lighter than that of contemporaneous seawater. The other possible mechanism is that carbonate preferentially directly uptake Cr(III) in seawater, especially those formed in the early geological times when the oxygen level is widely regarded low. Trivalent Cr can come from non-redox Cr cycling, and recent studies suggested that dissolution and adsorption of Cr(III) can also cause positive Cr isotopic fractionation. Therefore, the slightly positively fractionated Cr isotopic values recorded in some sedimentary carbonates do not necessarily correspond to the presence of oxidative Cr(III) weathering as previously suggested.
-end-
This work has important implications for the Cr cycling and the correct interpretation of the Cr isotope data recorded in sedimentary rocks, and aids a more comprehensive understanding of the coevolution between the oxygenation of the atmosphere and the ocean, and life.

See the article:

Ziyao Fang, Liping Qin, Wei Liu, Tao Yao, Xiaoyan Chen and Shiqiang Wei. Absence of hexavalent chromium in marine carbonates: implications for chromium isotopes as paleoenvironment proxy. National Science Reivew 2020, https://doi.org/10.1093/nsr/nwaa090

Science China Press

Related Environment Articles from Brightsurf:

Detecting SARS-CoV-2 in the environment
Researchers have outlined an approach to characterize and develop an effective environmental monitoring methodology for SARS CoV-2 virus, that can be used to better understand viral persistence in built environments.

Can your diet help protect the environment?
If Americans adhere to global dietary recommendations designed to reduce the impact of food production and consumption, environmental degradation could be reduced by up to 38%, according to a new paper published in the journal Environmental Justice.

How do we disconnect from the environment during sleep and under anesthesia?
A series of new studies by researchers at Tel Aviv University's Sackler Faculty of Medicine and Sagol School of Neuroscience finds, among other important discoveries, that noradrenaline, a neurotransmitter secreted in response to stress, lies at the heart of our ability to ''shut off'' our sensory responses and sleep soundly.

Our pupil moves to the rhythm of the environment
Regular processes in the environment improve our eyesight.

New self-forming membrane to protect our environment
A new class of self-forming membrane has been developed by researchers from Newcastle University, UK.

COVID-19 and the built environment
Social distancing has Americans mostly out of the places they usually gather and in their homes as we try to reduce the spread of COVID-19.

A broad look at plant-environment interactions
Three plant science journals---the American Journal of Botany (AJB), Applications in Plant Sciences (APPS), and the International Journal of Plant Sciences (IJPS)---have joined efforts to provide a broad look at how plants interact with their environment.

New research looks at type 1 diabetes and changes in the environment
Studies have shown a rapid increase in new cases of type 1 diabetes worldwide.

Chemicals in the environment: A focus on mixtures
The real world is marked by multiple stressors, among them cocktails of chemicals.

Rubber in the environment
The tread on the tyre is worn out, new tyres are needed.

Read More: Environment News and Environment Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.