For hundreds of millions of years, Earth's climate has remained on a fairly even keel, with some dramatic exceptions: Around 80 million years ago, the planet's temperature plummeted, along with carbon dioxide levels in the atmosphere. The Earth eventually recovered, only to swing back into the present-day ice age 50 million years ago.
Now geologists at MIT have identified the likely cause of both ice ages, as well as a natural mechanism for carbon sequestration. Just prior to both periods, massive tectonic collisions took place near the Earth's equator -- a tropical zone where rocks undergo heavy weathering due to frequent rain and other environmental conditions. This weathering involves chemical reactions that absorb a large amount of carbon dioxide from the atmosphere. The dramatic drawdown of carbon dioxide cooled the atmosphere, the new study suggests, and set the planet up for two ice ages, 80 million and 50 million years ago.
"Everybody agrees that on geological timescales over hundreds of millions of years, tectonics control the climate, but we didn't know how to connect this," says Oliver Jagoutz, associate professor of Earth, Atmospheric and Planetary Sciences (EAPS) at MIT. "I think we're the first ones to really link large-scale tectonic events to climate change."
Jagoutz and his colleagues, EAPS Professor Leigh Royden, and Francis McDonald of Harvard University, have published their findings in the Proceedings of the National Academy of Sciences.
In a new paper published this week in the Proceedings of the National Academy of Sciences (PNAS), San Francisco State University Professor of Earth and Climate Sciences Leonard Sklar and colleagues show how two established geochemical techniques can be combined in a novel way to reveal both the altitude where river rocks were originally produced and the rate of erosion that led them to crumble into the river.
Geologists have long dreamed of interviewing the rocks on the bed of a river to learn the story of where they were born and how they came to be the size they are. This is because the size of river rocks influences how rivers behave, from the habitat they provide to the speed with which they carve canyons. Yet, until now, the rocks have withheld their secrets.
Sklar, along with lead author Cliff Riebe and doctoral student Claire Lukens from the University of Wyoming and David Shuster from the University of California, Berkeley, wanted to understand how climate, which varies with altitude, controls the size and flux of sediments in rivers.
The National Science Foundation-funded research team's key breakthrough came when it used two techniques to query river rocks. First, researchers used cosmogenic nuclides to trace erosion rates in sediment samples. This common method of measuring erosion rates uses rare isotopes formed in minerals exposed to cosmic rays at the earth's surface. A higher concentration of isotopes means the rock has spent a longer time exposed at the surface, indicating a slower erosion rate.
They then combined this technique with detrital thermochronometry, another sediment tracing tool, which pinpoints where on a mountain sediment was produced. This is done by laboriously isolating tiny crystals of the mineral apatite and using ultraprecise machines to count the number of helium atoms contained in the crystals. The helium is formed by radioactive decay of uranium and is more abundant in rocks at higher elevations in the study area.
3806 Ma Isua rhyolites and dacites affected by low temperature Eoarchaean surficial alteration: Earth's earliest weathering
Authors:
Nutman et al
Abstract:
This paper reports evidence for Earth's oldest-recognised low temperature alteration, at ∼3800 Ma. Potassic felsic schists with a protolith age of 3806 ± 2 Ma form a ∼30 km long unit in the amphibolite facies, deformed, Isua supracrustal belt (West Greenland). At a single locality, boudinaged layers (nodules) within the schists are low strain zones: they are fine-grained, weakly feldspar-phyric, contain quartz amygdules and have fiamme-like structures, all supporting a volcanic protolith.
The nodules and surrounding schistose matrix contain abundant, 100–50 μm, euhedral, oscillatory zoned 3806 Ma zircons. The rare earth patterns of the zircons indicate crystallisation was magmatic. Some zircons contain axial lobate voids indicating that they grew at low pressure as the magma exsolved a fluid. Ti-in-zircon thermometry indicates crystallisation temperatures of 750–650 °C. Taken together, these zircon features indicates growth at eutectic temperatures in a hypabyssal chamber as the magma was exsolving a fluid phase. The magmatic zircons have ɛHf initial values of ∼0 and δ18OVSMOW of +5.0‰ ( Hiess et al., 2009), showing that the source of the volcanic rocks was devoid of assimilated markedly older or weathered crustal material, and probably essentially juvenile. In contrast, the whole rock δ18OVSMOW values are elevated at +14.7 to +16.2‰, indicative of superimposed low-temperature alteration processes.
The nodules and matrix schists have non-igneous bulk compositions, exemplified by strong enrichment in K2O and depletion in Na2O. They are depleted in Sr, have no negative Eu anomalies, but have high Rb/Sr, with an Rb–Sr age of 3760 ± 140 Ma (Jacobsen and Dymek, 1988). This indicates that the alteration involving strong degradation of plagioclase occurred in the Eoarchaean. Trace element compositions and establishment of alteration vectors suggest the protoliths were likely rhyolitic and dacitic in composition.
The strongest-modified matrix schist compositions contain biotite ± calcite ± dolomite with increase in MgO relative to the nodules, which indicates early magnesian carbonate growth. The whole-rock chemistry, decoupling of the igneous zircon and whole-rock oxygen isotope signatures and the Rb–Sr dating indicate that after eruption, the 3806 Ma felsic volcanic rocks underwent Eoarchaean low-temperature potassic alteration with weathering and groundwater circulation the most likely process. The geochemistry of the Isua felsic schists is compared with that of better-preserved volcanic rocks where the alteration conditions are known. This suggests a subaerial environment. The carbonatisation of the Isua felsic schists demonstrates drawdown of atmospheric CO2 into rocks made porous by the weathering.
Investigating Devonian trees as geo-engineers of past climates: linking palaeosols to palaeobotany and experimental geobiology
Authors:
Morris et al
Abstract:
We present the rationale for a cross-disciplinary investigation addressing the ‘Devonian plant hypothesis’ which proposes that the evolutionary appearance of trees with deep, complex rooting systems represents one of the major biotic feedbacks on geochemical carbon cycling during the Phanerozoic. According to this hypothesis, trees have dramatically enhanced mineral weathering driving an increased flux of Ca2+ to the oceans and, ultimately, a 90% decline in atmospheric CO2 levels through the Palaeozoic. Furthermore, experimental studies indicate a key role for arbuscular mycorrhizal fungi in soil–plant processes and especially in unlocking the limiting nutrient phosphorus in soil via Ca-phosphate dissolution mineral weathering. This suggests co-evolution of roots and symbiotic fungi since the Early Devonian could well have triggered positive feedbacks on weathering rates whereby root–fungal P release supports higher biomass forested ecosystems. Long-standing areas of uncertainty in this paradigm include the following: (1) limited fossil record documenting the origin and timeline of the evolution of tree-sized plants through the Devonian; and (2) the effects of the evolutionary advance of trees and their in situ rooting structures on palaeosol geochemistry. We are addressing these issues by integrating palaeobotanical studies with geochemical and mineralogical analyses of palaeosol sequences at selected sites across eastern North America with a particular focus on drill cores from Middle Devonian forests in Greene County, New York State.
Ubiquitous occurrence of basaltic-derived paleosols in the Late Archean Fortescue Group, Western Australia
Authors:
Teitler et al
Abstract:
The 2.76 Ga old Mount Roe Basalt paleosols (MR#1 and MR#2), recognized near the base of the 2.76–2.69 Ga Fortescue Group in Western Australia, represent some of the oldest definite examples of Archean paleoweathering profiles. The loss of Fe and the absence of pedogenic carbonates in these reference paleosols have been considered as strong evidence for low oxygen and moderate carbon dioxide in the Late Archean atmosphere, respectively. However, the robustness of such interpretations suffers both from the scarcity of paleosol exposures and the superposition of post-weathering alteration over primary soil profiles. Here we report new exposures of the MR#1 paleosol as well as a number of new basalt-derived paleosol occurrences distributed in the Mount Roe Basalt and the 2.73 Ga old Kylena Formations of the Fortescue Group. We show that all these paleosols, including MR#1 and MR#2, were strongly affected by post-weathering reductive alteration. Nevertheless, we discovered early lithologic units, locally preserved within the hydrothermally altered paleosols, which feature distinct chemical and mineralogic compositions. These include: (i) 13C-depleted carbonaceous-rich, diaspore–pyrophyllite boudins likely inherited from the hydrolysis and bauxitization of the parent basalt and (ii) green hard core material, mostly composed of Fe–sericite associated with authigenic sphene and containing early relics of sulfate-bearing iron-rich smectite or berthierine. We argue that smectite relics may constitute a part of the primary pedogenic mineral assemblage, while berthierine and sphene formed during diagenesis through the circulation of reductive fluids. Because the depletion of iron observed in the Fortescue paleosols is not solely due to pedogenesis, but also to post-weathering alteration, particular care has to be taken when bulk chemical profiles of iron in such paleosols are used as atmospheric paleobarometer. Recognition of the regional-scale, syn-depositional alteration of the Fortescue subaerial basalts over the north Pilbara suggests the onset of intense continental weathering associated with the uplift and emergence of the Pilbara craton during continental break-up and rifting. Such a regional onset of continental weathering may have provided large amounts of nutrients to nearby marine and/or lacustrine systems, favoring the development of microbial life in shallow waters.
Metasaprolite in the McGrath Gneiss, Minnesota, USA: viewing Paleoproterozoic weathering through a veil of metamorphism and metasomatism
Authors:
Medaris et al
Abstract:
A 2100 Ma, 420 cm–thick metasaprolite, which is devoid of plagioclase but contains abundant microcline, occurs in the Archean McGrath Gneiss beneath the overlying Paleoproterozoic Denham Formation. Despite being recrystallized under amphibolite–facies conditions and chemically modified by potassium metasomatism, mass fluxes related to weathering and metasomatism of the McGrath metasaprolite can be estimated by judicious application of several geochemical parameters and the A–C*N–K plot (Fedo et al., 1995). The metasaprolite yields values of 67–94 for the Plagioclase Index of Alteration, 62–72 for the Chemical Index of Alteration, and 47 for the Feldspar Index of Weathering, and the total mass flux removed by weathering is a minimum of 2.4 moles/cm2. Such values correspond to an intermediate degree, or intensity, of weathering compared to other, more intensely weathered paleosols in the region, such as those beneath the Baraboo and Sioux quartzites. Potassium metasomatism occurred at 1742 Ma during the geon 17 Yavapai tectonothermal event, when a minimum of 0.33 moles/cm2 K2O was added to the metasaprolite. Although the absence of pedogenic textures precludes interpretation of climatic conditions during weathering of the McGrath Gneiss, the level of atmospheric pCO2 can be estimated by application of Sheldon's (2006) method, which yields a minimum of 3.7 × PAL for a weathering duration of 100,000 years.
Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering
Authors:
Mills et al
Abstract:
A shift toward higher atmospheric oxygen concentration during the late Proterozoic has been inferred from multiple indirect proxies and is seen by many as a prerequisite for the emergence of complex animal life. However, the mechanisms controlling the level of oxygen throughout the Proterozoic and its eventual rise remain uncertain. Here we use a simple biogeochemical model to show that the balance between long-term carbon removal fluxes via terrestrial silicate weathering and ocean crust alteration plays a key role in determining atmospheric oxygen concentration. This balance may be shifted by changes in terrestrial weatherability or in the generation rate of oceanic crust. As a result, the terrestrial chemical weathering flux may be permanently altered—contrasting with the conventional view that the global silicate weathering flux must adjust to equal the volcanic CO2 degassing flux. Changes in chemical weathering flux in turn alter the long-term supply of phosphorus to the ocean, and therefore the flux of organic carbon burial, which is the long-term source of atmospheric oxygen. Hence we propose that increasing solar luminosity and a decrease in seafloor spreading rate over 1,500–500 Ma drove a gradual shift from seafloor weathering to terrestrial weathering, and a corresponding steady rise in atmospheric oxygen. Furthermore, increased terrestrial weatherability during the late Neoproterozoic may explain low temperature, increases in ocean phosphate, ocean sulfate, and atmospheric oxygen concentration at this time.
Geochemists have calculated a huge rise in atmospheric CO2 was only avoided by the formation of a vast mountain range in the middle of the ancient supercontinent, Pangea. This work is being presented to the Goldschmidt geochemistry conference in Sacramento, California.
Around 300 million years ago, plate tectonics caused the continents to aggregate into a giant supercontinent, known as "Pangea". The sheer size of the continent meant that much of the land surface was far from the sea, and so the continent became increasingly arid due to lack of humidity. This aridity meant that rock weathering was reduced; normally, a reduction in rock weathering means that CO2 levels rise, yet in spite of this CO2 levels – which had been falling prior to the mountain formation- continued to drop, eventually undergoing the most significant drop in atmospheric CO2 of the last 500 million years. This phenomenon has remained unexplained, until now.
Now a group of French scientists from the CNRS in Toulouse have produced a model which seems to explain this contradiction. The period coincides with the rise of a vast series of mountains in the interior of Pangea, the "Hercynian" mountains". These mountains arose in a wide belt, running from what is now the Appalachians, through to Ireland, South-Western England, through Paris and the Alps into Germany, and on further East.
According to team leader, Dr Yves Godderis (CNRS, Toulouse, France):
"The formation of these mountains meant that the rock weathering, which was threatening to slow to a walk through much of the supercontinent, was able to continue. The steep slopes of these Hercynian mountains produced physical erosion. Occurring in a humid equatorial environment, this physical erosion promoted rock weathering and removing CO2 from the atmosphere".
He continued, "We believe that it is this which led to the dramatic drop in atmospheric levels of CO2. We estimate that if it hadn't been for the formation of the Hercynian mountains, the atmospheric CO2 levels would have reached around 25 times the pre-industrial level, meaning that CO2 levels would have reached around 7000 ppm (parts per million). Let me put that into a present-day context; the current atmospheric CO2 levels are around 400 ppm, so this means that we would have seen CO2 rise to a level around 17 times current levels. This would obviously have had severe effects on the environment of that time. But the formation of the mountains in fact contributed to the greatest fall in atmospheric CO2 in the last 500 million years".
The team believes that even if the mountains had not formed and CO2 levels rose sharply, this would not have led to a runaway greenhouse effect as happened on Venus, because the increasing temperatures would have led to rocks being ultimately weathered, heat compensating for the scarcity of water. Rock weathering would have removed CO2 from the atmosphere, thus stopping the rising temperatures.
"So it would eventually have been self-correcting" said Dr Godderis, "but there's no doubt that this would have stalled Earth's temperature at a high level for a long, long time. The world would look very different today if these mountains had not developed when they did.
40Ar/39Ar dating of exceptional concentration of metals by weathering of Precambrian rocks at the Precambrian-Cambrian boundary
Authors:
Parnell et al
Abstract:
The sub-Cambrian surface, including diverse metalliferous deposits, shows evidence of intense weathering of Precambrian rocks to form supergene-enriched ores and metalliferous placers, followed by widespread peneplanation. Much of the metal would have been flushed to the Cambrian ocean during peneplanation. An 40Ar/39Ar age of 542.62 ± 0.38 Ma (1 sigma, full external precision, Renne et al., 2011) for metalliferous alteration clays in Scotland shows that this event occurred immediately prior to the Precambrian-Cambrian boundary. A negative δ53Cr isotopic signature for the clay is consistent with mobilization on land of redox sensitive metals by oxidative terrestrial weathering. This unprecedented flushing of metals from the weathered Precambrian surface would have contributed to the chemistry of the earliest Cambrian ocean at a time of marked faunal evolution.
Favorable conditions for life on Earth are enabled in part by the natural shuttling of carbon dioxide from the planet's atmosphere to its rocky interior and back again. Now Stanford scientists have devised a pair of math equations that better describe how topography, rock compositions and the movement of water through a landscape affects this vital recycling process.
Scientists have long suspected that the so-called the geologic carbon cycle is responsible for Earth's clement and life-friendly conditions because it helps regulate atmospheric concentrations of carbon dioxide, a greenhouse gas that acts to trap the sun's heat. This cycle is also thought to have played an important role in slowly thawing the planet during those rare times in the past when temperatures dipped so low that the globe was plunged into a "snowball-Earth" scenario and glaciers blanketed the equator.
"Our equations suggest that different landscapes have different potentials for regulating the transfer of carbon dioxide," said Kate Maher, an assistant professor of geological and environmental sciences who developed the equations along with her colleague, Environmental Earth System Science Professor Page Chamberlain. The research, which was supported by the National Research Foundation, is described in the March 14 issue of the journal Science.
The geologic carbon cycle begins when volcanoes release carbon dioxide into the atmosphere. Some of the carbon dioxide (CO2) mixes with rainwater and falls back to Earth as carbonic acid. On land, the carbonic acid chemically erodes, or "weathers," silicate rocks exposed at the Earth's surface to produce bicarbonate and release elements such as calcium and magnesium that eventually wash into the ocean.
Over millions of years, these elements are transformed into rocks such as limestone. When plate tectonics push the carbonate-loaded seafloor down into the Earth's mantle, the carbon is released again as CO2, which is vented back into the atmosphere through volcanic eruptions, thereby completing the cycle.
The equations developed by Maher and Chamberlain address the weathering component of the geologic carbon cycle. The amount of weathering that occurs depends on several factors. One is the makeup of the soil: older soils that have already been weathered dissolve more slowly compared to soils made of fresh rock. "As you weather soil and sediment over time, they become less and less chemically reactive," Maher said. "Physical erosion, which is often associated with mountainous regions, replenishes the soil with reactive minerals."
Late Neoproterozoic Baltic paleosol: Intense weathering at high latitude?
Authors:
Liivamägi et al
Abstract:
The Neoproterozoic was a time when repeated global cooling events, interrupted by supergreenhouse phases, preceded environmental change toward a modern oxygen-rich atmosphere and the eventual emergence of animal life. Cyclically increased atmospheric CO2 levels intensified weathering of continental silicates, but little is known about the influence of Neoproterozoic climates on soil morphogenesis, which acts as a direct proxy of conditions at the time of formation. However, being typically fragmented in time and space, these mineralic soils (paleosols) had a low preservation potential. An exceptionally well preserved Neoproterozoic deeply weathered paleosol on the Baltica paleocontinent provides n ew information on weathering during the Precambrian. The kaolinite-Fe-oxyhydroxide composition of this paleosol is indicative of intense weathering in a stable landscape at a time when Baltica was positioned between intermediate to high southern latitudes. It is plausible that this paleosol developed over long steady-state weathering under temperate climatic conditions, or alternatively during an intensified weathering event triggered by elevated CO2 levels and transient greenhouse climatic conditions at the termination of Ediacaran glaciations, or possibly the Ediacaran Shuram-Wonoka isotope event.
How accurate are rivers as gauges of chemical denudation of the Earth surface?
Authors:
Bouchez et al
Abstract:
Examination of the behavior of oxygen and hydrogen during weathering reactions shows that river dissolved load, although widely used, is an imperfect tracer of chemical denudation. At the current state of knowledge, none of the metrics for river total dissolved loads (such as the silicate-derived total dissolved solids, TDSsil = Ca2+ + Mg2+ + Na+ + K+ + SiO2, converted or not to equivalent oxides) account, in a mechanistic manner, for the transfer of oxygen and hydrogen between the solid and fluid phase during weathering reactions. We assess that chemical denudation derived from TDSsil will significantly overestimate the true chemical denudation for weathering of Ca-feldspar to kaolinite, whereas weathering of water-rich sedimentary rocks will be characterized by an underestimation of chemical denudation by TDSsil. For a handful of field sites, we estimate that the bias is lower than ±10%. The sign and extent of the bias depends on the nature of bedrock and on weathering conditions. Altogether, our analysis questions the broadly accepted concept of chemical denudation rate.
