Sub-zero temperature acid-weathering experiments with implications for Mars

The Meridiani Planum, Mars, is the landing site of the hugely successful Mars Exploration Rover, Opportunity (MER 1). Meridiani Planum is comprised of a vast deposit of sulfate-rich sediment that is characterized by fine layering, cross-bedding, and hematite spherules, or as they are better known, “blueberries”. The origin of the deposit is debated, but the most popular hypothesis is that it was a groundwater fed evaporative playa. It has recently been suggested, however, that the sediments were altered via acid-weathering in a massive dust-ice deposit and later reworked by aeolian processes. To test this hypothesis, sub-zero temperature acid-weathering must first be understood. Because Mars temperatures are typically sub-zero (about -90 to -5 °C), this process may be an important means to weather sediments on Mars. Reaction kinetics are necessarily slower at temperatures below 0 °C, but acid-weathering and the effect of temperature on pH dependence are not as well understood as they are at >0 °C. The situation is complicated by the formation of ice. As ice forms, the remaining acidic solution becomes more and more concentrated. The acid concentration can therefore increase as temperature decreases, which may mitigate the effect of the cold.


Top: Meridiani Planum, as seen by the Opportunity rover, in foreground. In the background is the rim of Endurance Crater, 8 km away. Bottom: The fine layering and cross-bedding of the sediments can only be seen inside craters. This image was taken inside Endurance Crater. Both images are photo-mosaics, courtesy NASA/JPL Photojournal.

A preliminary study was conducted in order to investigate the rate of acid-weathering at very low temperatures. In this study, forsteritic olivine, was exposed to sulfuric acid in capped test tubes under different temperature conditions (-52°, -20°, 4°, and 20 °C) for varying amounts of time. For the sub-zero temperatures, the acid concentration and the temperature condition for a particular sample were chosen so that the solution and ice would be stable together. Atomic absorption spectroscopy (AAS) was used to analyze the solutions for their concentrations of dissolved Mg²⁺ and Fe²⁺ ions from the dissolution of the olivine. The solid residue was examined for evidence of weathering with a scanning electron microscope (SEM) using backscatter electron (BSE) imaging and energy dispersive spectroscopy (EDS), which gives qualitative chemical analyses.

BSE images of two olivine grains. A. is a residue grain from the longest duration -20 °C sample. B. is from the least weathered sample (-52 °C, 1 hour). EDS are also shown, showing differing Mg:Si between the two samples.

The results of the analyses suggest that acid-weathering occurred in even the coldest temperatures and rates and magnitudes comparable to those at higher temperatures. Both Mg²⁺ and Fe²⁺ were found in all the solutions, with the samples in the coldest conditions having the lowest concentrations. Additionally, the concentration of the acid seemed to compensate for the colder temperatures in the first 24 to 40 hours. However, when the rates of the reactions were calculated, the rate drops off very quickly in the coldest temperatures (where initially it was relatively close to the warmer reaction rates), indicating that the temperature does play a large role in slowing the reaction kinetics of olivine weathering. We hypothesized that this temperature effect may be due to slow ion diffusion around grains in the coldest temperatures. Furthermore, the BSE images and EDS of the solid residue give compositional and textural evidence of sub-zero temperature acid-weathering. The ratio of Mg to Si is much lower than typical forsterite composition in darker colored areas and spots (color differences indicate compositional differences in BSE images), showing that some degree of weathering has occurred, possibly in conjunction with the genesis of a weathering product.

Additional information about this study can study can be found here: http://www.lpi.usra.edu/meetings/lpsc2010/pdf/2599.pdf

Animals are older than we thought!

The global radiation of the Ediacara biota, which occurred between 575 and 542 million years ago, marks the transition from a microbial-dominated world to a world shaped by animal (metazoan) life. The Ediacara biota, which first make an appearance about 580 million years ago (Canfield et al., 2007), represent the earliest known large bodied, morphologically complex metazoans in Earth’s history (Narbone, 2005).

The Ediacara biota are a now extinct group of soft-bodied animals that lived in deep water environments at the end of the Neoproterozoic era (1,000 to 542 million years ago). Modified from Narbone, 2005.

However, molecular clock studies estimate the divergence of the metazoan lineage to have occurred as long as 1,400 million years ago (Skorokhod et al., 1999). Therefore, there should be a long history of animal life that preceded the rise of the Ediacara biota.

Calcarea, Demospongiae, and Hexactinellida sponges are the basal metazoans and are estimated to have diverged from the last common metazoan ancestor between 1,100 million years ago and 1,400 million years ago. Modified from Skorokhod et al., 1999.

Until recently however, definitive evidence for animals before 580 million years ago has been lacking. A 2009 study that analyzed molecular fossils (biomarkers) from the Huqf Supergroup in South Oman, identified a biomarker indicative of marine sponges (sponges are at the base of the animal tree of life). The biomarker, 24-isopropylcholestane, was identified throughout the Huqf Supergroup which ranges in age from more than 635 million years old at the base to less than 541 million years old at the top of the section (Love et al., 2009).

The stratigraphic distribution of samples containing the 24-isopropylcholestane biomarker are plotted on the right. Absolute age constraints that were measured from U-Pb of detrital zircons and ash within the Huqf group are shown in black and absolute age constraints based on stratigraphic correlation with other dated units are shown in red. Modified from Love et al., 2009.

This is significant not only because it pushes back the animal fossil record by more than 30 million years but it also suggests that the dissolved oxygen concentration, at least in the shallow ocean, was high enough to support animal life.

If you would like to learn more please read the original paper Love et al., 2009

References:

Love, G.D., Grosjean, E., Stalvies, C., Fike, D.A., Grotzinger, J.P., Bradley, A.S., Kelly, A.E., Bhatia, M., Meredith, W., Snape, C.E., Bowring, S.A., Condon, D.J., Summons, R.E. (2009). Nature, vol. 457, p. 718.

Narbone, G.M. (2005). Annual Review of Earth and Planetary Science, vol. 33, p. 421.

Skorokhod, A., Gamulin, V., Gundacker, D., Kavsan, V., Müller, I.M., Müller, W.E.G. (1999). Biological Bulletin, vol. 197, p. 198.

Tracing hydrothermal sulfur cycling at the Mid-Atlantic Ridge and in the Tyrrhenian Sea

Many hydrothermal systems are located at zones where oceanic crust is produced (mid-ocean ridges) or recycled and reincorporated into Earth’s mantle (subduction zones). These systems are characterized by seawater circulating through the uppermost part of the oceanic crust driven by deeply seated magma as the ultimate heat source. The analysis of sulfur extracted from fluids, minerals and animals reveals different organic and inorganic reactions occurring along the flowpath of the seawater through the crust and provide valuable information about the physical and chemical conditions of the hydrothermal subsurface.

Hot hydrothermal vents at the ocean floor are often represented by remarkable chimney structures: the black smokers (courtesy of MARUM, Bremen, Germany).

Our investigations were focused on multiple mid-ocean ridge and subduction related hydrothermal fields in the Atlantic Ocean and in the Tyrrhenian Sea near Italy. We sampled these sites during several research cruises using highly sophisticated sampling technology, such as remotely operated vehicles (ROV).

Remotely operated vehicle (ROV) at the stern of the research vessel

(courtesy of MARUM, Bremen, Germany).

The sulfur data of samples from the Mid-Atlantic Ridge were used for the identification and quantification of so far undetected chemical processes in the subseafloor. Moreover, the results indicate fundamental differences with respect to subsurface structures (e.g. depth of magma chamber, seawater flowpath) and fluid evolution compared with other mid-ocean ridges, especially at the Pacific Ocean. Chemical data from the Tyrrhenian Sea suggest the existence of a microbial community consisting of diverse types of collaborating sulfur bacteria and show their significant role in the formation of metal and sulfur-rich hydrothermal deposits. In summary, these investigations improve our understanding of hydrothermal systems in different geological settings and draw a much more detailed picture of biogeochmical cycles in hydrothermal environments.

A paper referring to the Tyrrhenian Sea study is available at doi:10.1016/j.chemgeo.2010.11.011

Does Oceanic Crust Support a Seafloor Biosphere?

Life on Earth may be even more ubiquitous than previously thought.  At least that is one of the main implications of a recently emerged hypothesis that states that exposed oceanic crust (composed of basalt) may host a substantial “rock-eating” microbial biosphere.   This biosphere may even extend deep into oceanic crust, far from sunlight and substances traditionally thought of as food.  This hypothetical zone of life—termed the Seafloor (or Subseafloor) Biosphere—is thought to subsist solely on reduced rock chemicals found in basalt and other inorganic substances.  In order to investigate the potential for oceanic crust to host life, a group of colleagues and I investigated the initial stages of alteration of one of the most chemically reactive components of oceanic crust—basaltic glass—at the Loihi Seamount, a young and active submarine volcano in the Hawaiian Islands.

Collecting altered fragment of pillow basalt from Loihi using ROV Jason II on the FeMO 2008 research cruise to the seamount.  Where did those orange alteration materials come from?

The Loihi Seamount is covered with fresh pillow basalts.  The surfaces of these basalts are glassy and often covered with a layer of orange (iron-rich) secondary materials.  These materials have associated with them a large diversity of microbial life, including microbes that can fix inorganic carbon for growth by oxidizing (i.e., “rusting”) the iron and manganese compounds found in basalt.  It has been supposed that much of this biomass is living off of basaltic glass and, thus, taking part in basalt weathering.  However, it isn't clear whether these materials are weathering from the underlying basalt or are deposited by the low-temperature, iron- and manganese-rich hydrothermal fluids found all over Loihi.  We aimed to test the hypothesis that the biomass associated with these alteration materials was directly supported by basalt weathering by determining the source of this secondary material. 

Hydrothermal vent site within Pele's Pit, a pit crater formed by the collapse of Loihi's summit during a violent eruption in 1997.  The orange chimney structures and coatings are made primarily of iron minerals (amorphous to poorly crystalline Fe-oxyhydroxides) precipitated out of iron-rich vent fluids.  Iron- and manganese-oxidizing microorganisms make their living off of the redox gradient between the reduced vent fluids and the oxidizing ambient seawater.  Distance between laser points is approximately 10 cm.

Using various techniques (many of them synchrotron-based), we examined and characterized the glassy surfaces and associated secondary materials of a variety of basalts collected at numerous sites along the length of the seamount.  This included sites adjacent to hydrothermal vents and sites where no obvious venting was occurring.  We found little to no evidence that any significant chemical alteration of our glasses occurred, and that there was simply too much iron and manganese in the secondary materials to have been sourced from the underlying glass.  We also found that many of the alteration materials structurally resembled those known to have precipitated from vents, further lending support to the notion that these materials were externally sourced.  Thus, it appears that at Loihi the microbes in question aren’t directly eating the rocks, but rather living off of chemicals leached from rocks deeper in the volcano by hot fluids.  This might mean that any Seafloor (or Subseafloor) Biosphere may be restricted to volcanically active areas of oceanic crust (e.g., in proximity to mid-oceanic ridges) where shallow subseafloor hydrothermal circulation is an important process.

The full study can be found here: http://www.nature.com/ngeo/journal/v2/n12/full/ngeo696.html.  A short video about the Big Island of Hawaii and nearby Loihi can be found here.  Loihi is considered to be the location for the next Hawaiian Island.  To snag some of this ‘hot’ future real estate, see: http://www.petroglyphs.com/loihi/real-estate-future-value.html.

An 800-Year Record of Atmospheric Circulation and Climate Change from Kepler Lake, South-Central Alaska

Retrieval of the two short cores from Kepler Lake, Alaska. Cores were taken using a plastic tube corer fit with a piston.

Two short cores (85 cm and 101 cm long) were taken from Kepler Lake, a groundwater-fed lake in south-central Alaska, with the goal of reconstructing climate and environmental changes in recent centuries for the region. Previous studies claimed a dominant relationship between δ¹⁸O (a ratio of the heavy and light isotopes of oxygen) and local atmospheric circulation patterns, rather than the conventional temperature interpretation (positive shifts reflect warmer temperatures, and negative colder).

Top of the first short core taken from Kepler Lake. Marl sediment is primarily calcium carbonate precipitated dominantly by: Charophytes (an algae which forms a calcium carbonate encrustation around the stalk), and inorganic calcite precipitation. Inorganic precipitation occurs during peak heating in the summer when lake waters reach super saturation, also known as “whiting events”.

The cores were analyzed for stable isotopes of carbon and oxygen from marl (calcium carbonate rich) sediment. This analysis served to help interpret changes in moisture balance (the amount of precipitation versus evaporation) as well as changing patterns of atmospheric circulation. During periods of evaporation, the lighter ¹⁶O isotope preferentially enters the vapor phase and leaves the system, leaving the lake water relatively enriched in ¹⁸O, causing a positive shift in δ¹⁸O values. Age constraints were provided by ²¹⁰Pb analysis of sediment samples and AMS ¹⁴C dating of terrestrial macrofossils, such as fragments of leaves, twigs, and other debris from the vegetation surrounding the lake. Utilizing these two dating methods we are able to assign tight age control for the post 1950-period during which radiocarbon dating is not useful (due to bomb testing) and for the previous millennium, which is outside of the limitations of ²¹⁰Pb dating (approximately 100 years before present).

Based on the results of our dating analysis, an 800-year record was established for the two cores. The most positive values of δ¹⁸O occurred during the Little Ice Age (LIA) period, contrary to the conventional temperature interpretation. The LIA is well established as a period of colder temperatures in the region, with studies showing reduced tree ring width and glacial advance; however moisture balance is still relatively unconstrained. Our record from Kepler Lake supports the hypothesis of a changing source of moisture input to the region and through comparison with other regional records suggests spatial complexity to changes in moisture during the last millennium.

More details on this study are forthcoming in a paper which is currently in review with the Journal of Paleoliminology.

Gonyo, A., Yu, Z., Bebout, G. (In Review) An An 800-Year Multiple-Proxy Record of Atmospheric Circulation and Climate Change from Kepler Lake, South-Central Alaska. Journal of Paleoliminology.

Stitching together North America 1.9 Billion Years Ago

Running up the center of North America from south to north is a cryptic boundary that represents an enormous ancient mountain belt. Now eroded away, this vast mountain range was formed when two continents collided 1.8 Billion years ago and stitched together the greater part of North America. In Arctic Canada the parentage of Southampton Island - an island the size of Switzerland - to the eastern or western continents that collided during this mountain-building event has been debated for some time. The greatest implication of this conundrum is that in northern Canada, the western continent has a much greater potential to host rich mineral deposits. In this study rock samples were collected and analyzed to investigate the parentage of Southampton Island.

Fieldwork in Arctic Canada: Outcrops of rock situated amongst a vast expanse of lichens and glacial sediments. Southampton Island is situated in northern Hudson's Bay between Baffin Island and mainland Nunavut and is the 34th largest island in the world.

Caught in the act. A snapshot of mineral transformation or metamorphosis. This is a picture of a paper-thin slice of rock underneath a microscope. This 'sun-ray' like texture is the result of the sun (pink mineral in the center, which is about 1 mm across) being transformed into the rays (brown, black, and white minerals). These minerals grew in a natural pressure cooker at temperatures of 850-900°C at depths of 28-30 km beneath the surface of the Earth. To put it in perspective, these depths are equivalent to the length of 330 American or 300 Canadian Football fields!

The temperatures and depths these rocks attained can be compared with those of rocks from the eastern and western continents that collided 1.8 billion years ago. It turns out that the results of this study are compatible with similar results from the western continent, also know as the Churchill Province (named after the town of Churchill in Manitoba, Canada, which is the Polar Bear capitol of the world). This suggests that Southampton Island has the potential to host rich mineral deposits that are found predominately in the western continent.

For more information on the 'sun-ray' textures (aka. symplectite textures), the interested reader is referred to Dave Waters' website on pressure-temperature paths from clinopyroxene-hornblende-plagioclase symplectites.