Soil Science Journal Club : geochemistry

Small data set, high impact

Andrew.Rate — Thu, 11 Jun 2009 01:52:00 GMT

The first article for the revived Journal Club was ‘Jarosite as an indicator of water-limited chemical weathering on Mars' (Elwood Madden et al. [2004] Nature, 431:821-823), chosen by Talitha because it was a short and reasonably simple article about soils on another planet. The authors used observations of jarosite and gypsum as alteration products of the basaltic parent rock at the Meridiani Planum landing site on Mars to argue a case for chemical weathering on Mars being water-limited. Using geochemical modelling software, a basaltic mineral assemblage (using data from Rosenbauer et al. [1983]) was titrated into a fluid containing SO₄^2-, Na⁺, K⁺, Ca²⁺, Fe²⁺, Mg²⁺, Al³⁺, and dissolved SiO₂, under current Martian atmospheric O₂ and CO₂ fugacities, at 298 K and 10⁴Pa total atmospheric pressure. The authors modelled both mineral assemblages as basalt weathering progressed, and the final mineral assemblages at different water:rock ratios. Modelling indicated that jarosite could only be present as a result of basalt weathering if (a) a large quantity of water reacted completely with a small amount of rock (for example, water creating an alteration rind on rock surfaces); or (b) a small amount of water reacted only partially with a large amount of rock. It was concluded that once jarosite formed, water must have been removed quickly in order to halt chemical weathering before pH increased and jarosite was converted to an iron (oxy)hydroxide - hence the article's title.

Once we started examining the article, we realized how little data were required to have an article published in Nature. The authors used only five pieces of information about Mars (the presence of a basaltic parent rock, the presence of jarosite and gypsum, and Martian atmospheric O₂ and CO₂ fugacities and atmospheric pressure) and a geochemical model designed for Earth surface conditions. This got us thinking about having a look at some Martian soil data from NASA (presented in Rieder et al. [1997]) and publishing our own articles. It's a good example of researchers considering data critically and extracting as much information as possible with the tools they have available to them. The tools we have available aren't perfect though, and one of the criticisms of this article was the ‘Earth-centric' modelling - that is, the use of Earth surface conditions to simulate Martian weathering processes. Where possible, the authors used (current) Martian conditions; however, in the case of temperature and pressure, current Earth conditions were used.

The ‘suppression of mineral phases...at the discretion of the operator' also raised debate about using geochemical models to simulate weathering. The problem is that models generally predict phase assemblages at thermodynamic equilibrium, which may not actually occur in the field. The authors cite the example of slow goethite formation hindering its occurrence in acid mine drainage environments despite being the thermodynamically stable iron phase. Kinetics were only included in this model ‘through the suppression of mineral phases unlikely to form in a geologically relevant time period.'

So why should we care about soils on Mars, anyway? We came up with a few answers to this: for the pure basic science objectives of understanding the properties and history of Martian surface materials and understanding how soils develop under different environmental conditions; because soil on another planet is inherently interesting; and because we may even want to annex Mars one day as a replacement Earth, in which case an understanding of its soils would be pretty important.

The next article will be chosen by Bree, with the next meeting scheduled for July 1.

Image from www.andersonfreepress.net

Journal club revived

Andrew.Rate — Mon, 18 May 2009 03:05:00 GMT

[Edit at 29 May 2009 - the date has been shifted to Wednesday 3 June]

The poster at right was prepared by Talitha to publicise the first meeting of the Soil Science Journal Club for 2009...

...we'll be discussing the article: Madden MEE, Bodnar RJ, Rimstidt JD. 2004. Jarosite as an indicator of water-limited chemical weathering on Mars. Nature, 431:821-823.

Maybe we'll see you there?

[The second-floor lunch area is on the top floor of the School of Earth & Environment (South) building (i.e. the Soil Science building).]

Digging for gold

Andrew.Rate — Thu, 06 Nov 2008 05:36:00 GMT

Musings on:
Anand RR, Cornelius M, Phang C, 2007. Use of vegetation and soil in mineral exploration in areas of transported overburden, Yilgarn Craton, Western Australia: a contribution towards understanding metal transportation processes.Geochemistry-Exploration Environment Analysis, 7: 267-288.

The use of soils and vegetation as sample media for geochemical exploration is not new. In recent years there seems to have been a resurgence of interest in sampling soils, and particularly plants, with the expectation that these media carry a signature of underlying ore bodies. In many cases the rock or regolith which hosts the ore body may be masked by transported sediments, so that even the weathered ghostly imprint of an ore may lie well below the surface.

Several mechanisms may allow the formation of signatures of ore bodies in surface soils, even in a transported overburden. In addition to purely chemical or physical processes, it is possible that plants can access ore-forming elements at depth, as deep roots search the regolith for water. Not only can this give anomalous concentrations of trace elements in plant tissues, but biological re-cycling can subsequently enrich surface soils with a signature of what lies beneath.

These ideas are the basis for Anand et al's. article. They took vegetation (mulga; Acacia aneura and related species) and soil samples across known but unexploited gold and/or base-metal (Cu, Zn, Ag) deposits in Western Australia, where much of the bedrock is covered by deeply weathered regolith profiles and, in many cases, transported sediments as well. They found that analysis of soils did not clearly delineate the vertical projection of underlying ores (even using selective extractions for trace elements which commonly improve signal:noise ratios). In contrast the living plant tissues, and particularly the litter layer, showed clear signatures of underlying mineralization, with peak concentrations in both media vertically over the ore body.

These results are of obviously practical importance to exploration geochemists, demonstrating that (at least in some instances) vegetation sampling and analysis offers an additional tool to "see through" weathered and/or transported material to ore bodies beneath. It is possibly more generally significant, from an "understanding the Earth" point of view, that vegetation may be important in redistributing trace elements in earth surface environments, particularly in arid regions where plants' search for water requires root growth to great depth. In such environments, geochemical signatures in soils may well be the result of millennia of plant uptake.

Image of the Moolart Well prospect from www.regisresources.com

the soil on Mars

Andrew.Rate — Tue, 02 Sep 2008 03:15:00 GMT

...musings on Amundson R, Ewing S, Dietrich W, Sutter B, Owen J, Chadwick OA, Nishiizumi K, Walvoord M, McKay C. 2008. On the in situ aqueous alteration of soils on Mars. Geochimica et Cosmochimica Acta 72:3845-3864.

It's great when an article related to one's own discipline is about something exotic, and it would be hard to imagine a more exotic environment than the surface of Mars. I enjoy highlighting interesting developments to students, so this year's Introduction to Geochemistry students had the data from this article as an example when we learned about mass balance during weathering. And this was my first crack at teaching this geochemical topic to students as well, so I learned a lot too. We used the subject matter, if not the Martian data, in a prac class, using an excellent dataset published by Oh & Richter (2005).

Amundson et al.'s hypothesis is that liquid water must have existed on Mars at some stage in that planet's history, based on the mineralogical record (minerals which need water to form, such as smectites and jarosite, have been identified on the Martian surface). Amundson et al. further tested this hypothesis by using elemental analysis data from conveniently exposed "soil profiles" on Mars, within the Gusev and Endurance craters investigated by the Mars Exploration Rover (Opportunity) mission. They were looking for evidence of absolute loss or gain of elements which might reflect transport by liquid water - and they found exactly that. Soils were depleted in major rock-forming elements (Si, Al, Mg, Ca, Fe, etc.) relative to the likely parent materials (Gusev basalt, or aeolian dust), probably representing earlier weathering mediated by water. A key result was the enrichment in the soil profiles of sulfur, chlorine and bromine, consistent with the aqueous transport of sulfate, chloride and bromide salts followed by drying.

The point is also made that comparable environments (very dry and cold; e.g., some of Antarctica) exist on Earth, and similar soil-forming processes have occurred here (on Earth, that is!) as well. The authors refer to such terrestrial soils as being "abiotic"; a bit of a misnomer, I thought, where perhaps they meant the absence of higher organisms (surely some microorganisms were present in all the Earth examples - were they there on Mars?). Plus there is the usual issue with mass-balance approaches (fully acknowledged) of matching the weathered/altered material to its assumed parent material.

This was a fun article to read. It's not often that I get to read and use a publication that contains so much language usually reserved (in my reading experience) for science fiction - Mars landers, differences in gravity - all wonderful stuff.

Image from www.nasa.gov

So... that was that

Andrew.Rate — Thu, 16 Aug 2007 06:10:00 GMT

Just rode my bike back from what will be my last lecture of the Semester. Just over half the class of Introduction to Geochemistry students gained (hopefully!) some benefit from working through past exam questions.

I reminded myself, inadvertently, that there is not really that much to be said about a topic in three weeks of lectures. Particularly when I'm trying to get across some fundamental material about my discipline - when it comes down to asking exam questions, they're mostly variations on a theme or even repetitive from year to year. Students with good exam technique will recognise this and exploit it - more power to them.

Oh yes - this is supposed to be a journal club blog. The club meetings have been at best sporadic this year and attendance is dwindling. Although I did talk to one of the PhD students who attends sometimes who mentioned that she used one of the papers (Davidson & Janssens 2006 - see here) we discussed for teaching first-year ecology. That's where I've gotten the most benefit so far from the Journal Club - it has enhanced my teaching.