Ancient minerals: Which gave rise to life?

The magnesium silicate forsterite was one of the most abundant minerals in the Hadean Eon, and it played a major role in Earth's near-surface processes. The green color of this mineral (which is also known as the semi-precious gemstone peridot, the birthstone of August) is caused by small amounts iron. The iron can react with seawater to promote chemical reactions that may have played a role in life's origins. -  Robert Downs, University of Arizona, Ruff Project
The magnesium silicate forsterite was one of the most abundant minerals in the Hadean Eon, and it played a major role in Earth’s near-surface processes. The green color of this mineral (which is also known as the semi-precious gemstone peridot, the birthstone of August) is caused by small amounts iron. The iron can react with seawater to promote chemical reactions that may have played a role in life’s origins. – Robert Downs, University of Arizona, Ruff Project

Life originated as a result of natural processes that exploited early Earth’s raw materials. Scientific models of life’s origins almost always look to minerals for such essential tasks as the synthesis of life’s molecular building blocks or the supply of metabolic energy. But this assumes that the mineral species found on Earth today are much the same as they were during Earth’s first 550 million years-the Hadean Eon-when life emerged. A new analysis of Hadean mineralogy challenges that assumption. It is published in American Journal of Science.

Carnegie’s Robert Hazen compiled a list of every plausible mineral species on the Hadean Earth and concludes that no more than 420 different minerals-about 8 percent of the nearly 5,000 species found on Earth today-would have been present at or near Earth’s surface.

“This is a consequence of the limited ways that minerals might have formed prior to 4 billion years ago,” Hazen explained. “Most of the 420 minerals of the Hadean Eon formed from magma-molten rock that slowly crystallized at or near Earth’s surface-as well as the alteration of those minerals when exposed to hot water.”

By contrast, thousands of mineral species known today are the direct result of growth by living organisms, such as shells and bones, as well as life’s chemical byproducts, such as oxygen from photosynthesis. In addition, hundreds of other minerals that incorporate relatively rare elements such as lithium, beryllium, and molybdenum appear to have taken a billion years or more to first appear because it is difficult to concentrate these elements sufficiently to form new minerals. So those slow-forming minerals are also excluded from the time of life’s origins.

“Fortunately for most origin-of-life models, the most commonly invoked minerals were present on early Earth,” Hazen said.

For example, clay minerals-sometimes theorized by chemists to trigger interesting reactions-were certainly available. Sulfide minerals, including reactive iron and nickel varieties, were also widely available to catalyze organic reactions. However, borate and molybdate minerals, which are relatively rare even today, are unlikely to have occurred on the Hadean Earth and call into question origin models that rely on those mineral groups.

Several questions remain unanswered and offer opportunities for further study of the paleomineralogy of the Hadean Eon. For example, the Hadean Eon differs from today in the frequent large impacts of asteroids and comets-thousands of collisions by objects with diameters from a mile up to 100 miles. Such impacts would have caused massive disruption of Earth’s crust, with extensive fracture zones that were filled with hot circulating water. Such hydrothermal areas could have created complex zones with many exotic minerals.

This study also raises the question of how other planets and moons evolved mineralogically. Hazen suggests that Mars today may have progressed only as far as Earth’s Hadean Eon. As such, Mars may be limited to a similar suite of no more than about 400 different mineral species. Thanks to the Curiosity rover, we may soon know if that’s the case.

Extreme climate change linked to early animal evolution

This photo shows researchers studying exposures of the Doushanto Formation. Located in China, the formation is most notable for its scientific contributions in the hunt for Precambrian life. -  M. Kennedy.
This photo shows researchers studying exposures of the Doushanto Formation. Located in China, the formation is most notable for its scientific contributions in the hunt for Precambrian life. – M. Kennedy.

An international team of scientists, including geochemists from the University of California, Riverside, has uncovered new evidence linking extreme climate change, oxygen rise, and early animal evolution.

A dramatic rise in atmospheric oxygen levels has long been speculated as the trigger for early animal evolution. While the direct cause-and-effect relationships between animal and environmental evolution remain topics of intense debate, all this research has been hampered by the lack of direct evidence for an oxygen increase coincident with the appearance of the earliest animals – until now.

In the Sept. 27 issue of the journal Nature, the research team, led by scientists at the University of Nevada, Las Vegas, offers the first evidence of a direct link between trends in early animal diversity and shifts in Earth system processes.

The fossil record shows a marked increase in animal and algae fossils roughly 635 million years ago. An analysis of organic-rich rocks from South China points to a sudden spike in oceanic oxygen levels at this time – in the wake of severe glaciation. The new evidence pre-dates previous estimates of a life-sustaining oxygenation event by more than 50 million years.

“This work provides the first real evidence for a long speculated change in oxygen levels in the aftermath of the most severe climatic event in Earth’s history – one of the so-called ‘Snowball Earth’ glaciations,” said Timothy Lyons, a professor of biogeochemistry at UC Riverside.

The research team analyzed concentrations of trace metals and sulfur isotopes, which are tracers of early oxygen levels, in mudstone collected from the Doushantuo Formation in South China. The team found spikes in concentrations of the trace metals, denoting higher oxygen levels in seawater on a global scale.

“We found levels of molybdenum and vanadium in the Doushantuo Formation mudstones that necessitate that the global ocean was well ventilated. This well-oxygenated ocean was the environmental backdrop for early animal diversification,” said Noah Planavsky, a former UCR graduate student in Lyons’s lab now at CalTech.

The high element concentrations found in the South China rocks are comparable to modern ocean sediments and point to a substantial oxygen increase in the ocean-atmosphere system around 635 million years ago. According to the researchers, the oxygen rise is likely due to increased organic carbon burial, a result of more nutrient availability following the extreme cold climate of the ‘Snowball Earth’ glaciation when ice shrouded much of Earth’s surface.

Lyons and Planavsky argued in research published earlier in the journal Nature that a nutrient surplus associated with the extensive glaciations may have initiated intense carbon burial and oxygenation. Burial of organic carbon – from photosynthetic organisms – in ocean sediments would result in the release of vast amounts of oxygen into the ocean-atmosphere system.

“We are delighted that the new metal data from the South China shale seem to be confirming these hypothesized events,” Lyons said.

The joint research was supported by grants from the National Science Foundation, the NASA Exobiology Program, and the National Natural Science Foundation of China. Besides Lyons and Planavsky, the research team includes Swapan K. Sahoo (first author of the research paper) and Ganqing Jiang (principal investigator of the study) of the University of Nevada, Las Vegas; Brian Kendall and Ariel D. Anbar of Arizona State University; Xinqiang Wang and Xiaoying Shi of the China University of Geosciences (Beijing); and UCR alumnus Clint Scott of United States Geological Survey.

Large bacterial population colonized land 2.75 billion years ago

A drill core from the 2.5 billion-year-old Mount McRae Shale formation in Western Australia, which originally was fine-grained ocean sediment, shows high concentrations of sulfide and molybdenum. That supports the idea that most of the sulfate came from land, likely freed by microbial activity on rocks. Some data for the research came from the Mount McRae formation. -  Roger Buick/U. of Washington
A drill core from the 2.5 billion-year-old Mount McRae Shale formation in Western Australia, which originally was fine-grained ocean sediment, shows high concentrations of sulfide and molybdenum. That supports the idea that most of the sulfate came from land, likely freed by microbial activity on rocks. Some data for the research came from the Mount McRae formation. – Roger Buick/U. of Washington

There is evidence that some microbial life had migrated from the Earth’s oceans to land by 2.75 billion years ago, though many scientists believe such land-based life was limited because the ozone layer that shields against ultraviolet radiation did not form until hundreds of millions years later.

But new research from the University of Washington suggests that early microbes might have been widespread on land, producing oxygen and weathering pyrite, an iron sulfide mineral, which released sulfur and molybdenum into the oceans.

“This shows that life didn’t just exist in a few little places on land. It was important on a global scale because it was enhancing the flow of sulfate from land into the ocean,” said Eva Stüeken, a UW doctoral student in Earth and space sciences.

In turn, the influx of sulfur probably enhanced the spread of life in the oceans, said Stüeken, who is the lead author of a paper presenting the research published Sunday (Sept. 23) in Nature Geoscience. The work also will be part of her doctoral dissertation.

Sulfur could have been released into sea water by other processes, including volcanic activity. But evidence that molybdenum was being released at the same time suggests that both substances were being liberated as bacteria slowly disintegrated continental rocks, she said.

If that is the case, it likely means the land-based microbes were producing oxygen well in advance of what geologists refer to as the “Great Oxidation Event” about 2.4 billion years ago that initiated the oxygen-rich atmosphere that fostered life as we know it.

In fact, the added sulfur might have allowed marine microbes to consume methane, which could have set the stage for atmospheric oxygenation. Before that occurred, it is likely large amounts of oxygen were destroyed by reacting with methane that rose from the ocean into the air.

“It supports the theory that oxygen was being produced for several hundred million years before the Great Oxidation Event. It just took time for it to reach higher concentrations in the atmosphere,” Stüeken said.

The research examined data on sulfur levels in 1,194 samples from marine sediment formations dating from before the Cambrian period began about 542 million years ago. The processes by which sulfur can be added or removed are understood well enough to detect biological contributions, the researchers said.

The data came from numerous research projects during the last several decades, but in most cases those observations were just a small part of much larger studies. In an effort to provide consistent interpretation, Stüeken combed the research record for data that came from similar types of sedimentary rock and similar environments.

“The data has been out there for a long time, but people have ignored it because it is hard to interpret when it is not part of a large database,” she said.

Poisonous oceans delayed animal evolution

Sedimentary rocks from Grand Canyon, USA, show evidence for widespread anoxic and sulfidic waters 750 million years ago. Since sulfide is poisonous to animals, it can explain why animals had not evolved on Earth at that time. -  PD photo (www.pdphoto.org)
Sedimentary rocks from Grand Canyon, USA, show evidence for widespread anoxic and sulfidic waters 750 million years ago. Since sulfide is poisonous to animals, it can explain why animals had not evolved on Earth at that time. – PD photo (www.pdphoto.org)

“We have investigated the cycling of molybdenum (Mo) in ancient oceans by studying the elemental and isotopic composition of Mo in sedimentary rocks from Grand Canyon that formed in the oceans 750 million years ago”, explains Tais W. Dahl, who did this research in collaboration with researchers from Arizona State University, Harvard University and the Nordic Center of Earth Evolution in Denmark (NordCEE).

Molybdenum tracks the presence of poisonous sulfide in ancient oceans

The study uses a new method to determine the extent of anoxia and presence of sulfide in the world oceans. Geochemical analyses of the trace element, molybdenum, in 750 million year-old rocks from Grand Canyon suggest oceans contained enormous amounts of lethal sulfide.

Molybdenum is relatively rich in today’s seawater, because it is soluble in water in the presence of O2, and therefore it accumulates in modern oxygenated oceans. Conversely, molybdenum becomes insoluble in anoxic waters where sulfide is present, so it precipitates out of the oceans. The new results show that oceans contained less Mo in the past, because sulfide-rich waters extended over much greater areas than today.

Vast areas of animal-inhospitable seafloor

Today, oceans are nearly fully oxygenated and sulfide is only present in restricted areas of the ocean, such as the deepest parts of the Black Sea and the Baltic Sea. According to a hypothesis established by Donald Canfield (NordCEE) in 1998 sulfide was a much more common constituent in the oceans 1900-750 million years ago.

The new study is first to quantify the expansion of sulfide in the ‘Canfield-ocean’. Model calculations for the oceanic molybdenum cycle suggest that 10-50% of the shallow oceans were covered with sulfidic waters. This is 400-800 times more than in today’s oceans. The vast areas of poisonous seafloor would have made oceans inhospitable for animals. Expansive anoxic and poisonous oceans are now held responsible for the late appearance of animal life forms on Earth.

Low Oxygen and Molybdenum Levels in Ancient Oceans Delayed Evolution of Life by Two Billion Years





Shale
Shale

UCR-led study tracked biogeochemical signatures preserved in ancient sedimentary rocks to establish nature and timing of oxygenation of Earth’s atmosphere



A deficiency of oxygen and the heavy metal molybdenum in the ancient deep ocean may have delayed the evolution of animal life on Earth by nearly two billion years, a study led by UC Riverside biogeochemists has found.



The researchers arrived at their result by tracking molybdenum in black shales, which are a kind of sedimentary rock rich in organic matter and usually found in the deep ocean. Molybdenum is a key micronutrient for life and serves as a proxy for oceanic and atmospheric oxygen amounts.



Study results appear in the March 27 issue of Nature.



Following the initial rise of oxygen in the Earth’s atmosphere 2.4 billion years ago, oxygen was transferred to the surface ocean to support oxygen-demanding microorganims. Yet the diversity of these single-celled life forms remained low, and their multicellular descendants, the animals, did not appear until about 600 million years ago, explained Timothy Lyons, a professor of biogeochemistry in the Department of Earth Sciences and one of the study’s authors.



Suspecting that deficiencies in oxygen and molybdenum might explain this evolutionary lag, Lyons and his colleagues measured abundances of molybdenum in ancient marine sediments over time to estimate how much of the metal had been dissolved in the seawater in which the sediments formed.



The researchers found significant, firsthand evidence for a molybdenum-depleted ocean relative to the high levels measured in modern, oxygen-rich seawater.



“These molybdenum depletions may have retarded the development of complex life such as animals for almost two billion years of Earth history,” Lyons said. “The amount of molybdenum in the ocean probably played a major role in the development of early life. As in the case of iron today, molybdenum can be thought of as a life-affirming micronutrient that regulates the biological cycling of nitrogen in the ocean.



“At the same time, molybdenum’s low abundance in the early ocean tracks the global extent of oxygen-poor seawater and implies that the amount of oxygen in the atmosphere was still low.



“Knowing the amount of oxygen in the early ocean is important for many reasons, including a refined understanding of how and when appreciable oxygen first began to accumulate in the atmosphere,” Lyons said. “These steps in oxygenation are what gave rise ultimately to the first animals almost 600 million years ago – just the last tenth or so of Earth history.”

Earth’s oxygenation



For animal life to commence, survive and eventually expand on Earth, a threshold amount of oxygen – estimated to be on the order of 1 to 10 percent of present atmospheric levels of oxygen – was needed.



Past research has shown that Earth’s oxygenation occurred in two major steps:



The first step, around 2.4 billion years ago, took place as the ocean transitioned to a state where only the surface ocean was oxygenated by photosynthesizing bacteria, while the deep ocean was relatively oxygen-free.



The second step, around 600 million years ago, marked the occasion when the entire ocean became fully oxygenated through a process not yet fully understood.



“We wanted to know what the state of the ocean was between the two steps,” said Clinton Scott, a graduate student working in Lyons’s lab and the first author of the research paper. “By tracking molybdenum in shales rich in organic matter, we found the deep ocean remained oxygen- and molybdenum-deficient after the first step. This condition may have had a negative impact on the evolution of early eukaryotes, our single-celled ancestors. The molybdenum record also tells us that the deep ocean was already fully oxygenated by around 550 million years ago.”



According to Scott, the timing of the oxygenation steps suggests that significant events in Earth history are related. Scientists have long speculated that the evolution of the first animals was linked somehow to the so-called Snowball Earth hypothesis, which posits that the Earth was covered from pole to pole in a thick sheet of ice for millions of years at a time. “The second oxygenation step took place not long after the last Snowball Earth episode ended around 600 million years ago,” Scott said. “So one question is: Did this global glaciation play a role in the increasing abundance of oxygen which, in turn, enabled the evolution of animals?”



Scott and Lyons were joined in the research by A. Bekker of the Carnegie Institution of Washington, DC; Y. Shen of the Université du Québec à Montréal, Canada; S.W. Poulton of Newcastle University, Newcastle upon Tyne, United Kingdom; X. Chu of the Chinese Academy of Sciences, Beijing, China; and A.D. Anbar of Arizona State University, Tempe, Ariz.



The research was supported by grants from the U.S. National Science Foundation Division of Earth Sciences and the NASA Astrobiology Institute.


More about molybdenum as a proxy for ocean chemistry



Molybdenum, a metal abundant in the ocean today but less so at times in the past, is an excellent tracer of ancient chemistry for two reasons. First, the primary source of molybdenum to the ocean is oxidative weathering of continental crust, requiring oxygen in the atmosphere. Second, molybdenum is removed primarily in marine sediments where oxygen is absent and sulfide is abundant. Thus the enrichment of molybdenum in ancient organic-rich shales requires oxygen in the atmosphere but high sulfur and very low or no oxygen in the deep ocean. This combination is relatively rare today but may have been common when oxygen was less abundant in the earlier atmosphere.



When oxygen is available in the atmosphere, the amount of dissolved molybdenum in seawater is determined by the extent of hydrogen-sulfide-containing sediments and bottom waters (the colder, more isolated, lowermost layer of ocean water). Where sulfidic environments are widespread, the pool of molybdenum remaining in seawater is small, growing as the sulfidic environments shrink. The amount of molybdenum in the seawater is reflected in the magnitude of molybdenum enrichment in shales deposited in the deep ocean.



The UCR-led team of researchers estimated the size of the oceanic reservoir, and thus the extent of sulfidic bottom waters and sediments, based on the concentration of molybdenum in ancient black shales. They did so by dissolving the samples in a cocktail of acids and analyzing the dissolved rock for concentration using a mass spectrometer. The amount of this metal in the shales tracks the oxygen state of the early ocean and atmosphere and also points to the varying abundance of this essential ingredient of life. Molybdenum limitations may have delayed the development of eukaryotes, including the first animals, our earliest multicellular cousins.