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The February issue of Nature Geoscience features a couple of interesting articles on the origin of water in the Earth-Moon system. By way of an introduction, Robert (2011) reviews the known ratios of deuterium (heavy hydrogen; one proton, one neutron) to hydrogen (one proton) of various planetary bodies in the solar system: the proto-Sun, Earth, and Moon, along with carbonaceous chondrite meteorites and comets (Fig. 1).

Robert (2011)

Fig. 1. Deuterium/hydrogen ratios of the proto-Sun (peach), Earth (blue), Moon (red), carbonaceous chondrites (black), and comets (green). The D/H ratio is multiplied by 10^6 reflecting parts-per-million quanities of deuterium with respect to hydrogen. Adapted from Robert (2011).

As you can see, there are a couple of interesting isotopic associations. The Earth overlaps strongly with carbonaceous chondrites, while the Moon spans a range of D/H values, potentially indicating a significant affinity with comets.

Why might this disparity between the Earth and Moon exist? The prevailing hypothesis for the formation of the Moon is the impact of Theia with Earth, so within this framework, it stands to reason that water on Earth appeared after the formation of the Moon via significant contributions of water from carbonate chondrites.

By comparison, Greenwood et al. (2011) discovered that abundant lunar water exists bound up within the hydrous mineral apatite, which represents a mafic phase within the mare basalts and anorthositic highlands. The interesting thing about the Moon, however, is that a number of sources are identified including solar protons, the lunar mantle, and comets (D/H ratios increasing respectively, with the solar fraction as the lightest). These different sources potentially explain the wide range of D/H ratios observed in lunar rocks.

This appears to be a tidy hypothesis, but as Robson (2011) hints, how do you explain the prominent influence of comets on lunar water, and its apparent absence in terrestrial water? The Earth and Moon are next-door neighbours in the context of the solar system, and Greenwood et al. (2011) predict a likewise cometary bombardment of the Earth at this time.

So where is the terrestrial D/H isotopic signature reflecting this cometary bombardment interval? Or is it there, but just obscured by the lighter, carbonaceous chondrite fraction? That may be the case following the research of Kulikov et al. (2006) which indicates that the relatively high D/H ratio on Venus arises from the equivalent loss of a terrestrial ocean; something which most certainly did not occur on Earth.

References

Greenwood, J.P., Itoh, S., Sakamoto, N., Warren, P., Taylor, L., and Yurimoto, H., 2011: Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon. Nature Geoscience, vol. 4, p. 87-92.

Robert, F., 2011: Planetary science: A distinct source for lunar water? Nature Geoscience, vol. 4, p. 74-75.

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I’ll admit it: I spent more time in the field this fall mapping a trench for my B.Sc. thesis than was actually necessary. Part of the reason was that I wanted to make sure I collected quality field data for the project, but another part of it was that I enjoyed the excuse to get out there into the near wilderness and spend some time alone. However, now that a respectable layer of snow is blanketing southeastern Manitoba, I suppose it’s time to face reality and start treating some of that data.

The first task? Take my structural measurements – foliations, lineations, joints, and veining – and plunk them into a stereonet. For my joints I’d already done this by hand a few weeks ago (mainly to see how they looked), but for ease of use I opted for GEOrient – stereonet software which is free for academic use. Using software is a treat because you simply export a spreadsheet of your structural measurements into a tab delimited .TXT file from Excel or Open Office, and import it in GEOrient, which itself is an easy task.

Unfortunately, while GEOrient does a nice job of plotting your points on the stereonet, it’s not very robust when it comes to the visual display of pole contours. Consider this plot, for example:

GEOrient foliation plot

It does an excellent preliminary plot, but I notice two things: 1) visually it needs to be retouched A LOT, and 2) there’s a potential problem of the 8% contour continuity across the great circle. The first is no real problem, and no real criticism of the software itself – it aims to plot data, not to be a state-of-the-art graphics package. Things can easily be touched up with other graphics programs such as Illustrator, as exemplified by the final version:

Final version using Illustrator

The second, however, is a potential problem, and I’m definitely going to talk to my advisor. In the original, the contour interval crosses the great circle on the NE quadrant, but does not in the SW quad. In the final version I’ve gone ahead and manually traced it. Additionally, the retraced contours were smoothed in Illustrator to provide a better presentation.

However, this potentially raises another issue: I’ve essentially tampered with the data. By extending and smoothing contours, I’ve taken a representation of the plotted data and altered in a manner that looks better and “makes sense”. That said, I’ve only “tampered” with the data if the original GEOrient plots were correct in the first place. Pole positions are certainly correct – it’s quantitative strike/dip data – but the automated contouring is where the trickiness comes in. Although the final stereonet looks pretty slick from a design standpoint, I have to wonder if the contouring can be accepted with a degree of confidence. I’m not sure it can – I’ll have to give it some thought. I think I’ll also try other software packages, as well as hand plots, to compare results between the two.

Ultimately, from a practical standpoint, a stereonet plot can overcome these minor issues with ease. From my final contours I can confidently assert a general, preferred orientation of my foliations. Yes, there is some scatter, and yes, there are some odd things going on, but this is geology after all – and just a first treatment of the data. I’m thinking the next step will be to break down foliations between lithologies in the trench to see if there are distinct generations, as well as potentially identify different structural sub-domains throughout the linear extent of the outcrop.

One enjoyable aspect of school is the opportunity to explore new ideas which keeps your field of study fresh. Now that may seem like a funny statement, if only for its obviousness alone. Isn’t that what school is for? Well, sure, but in the grind of labs, term papers, presentations, and exams, the experience of discovery for its own sake is often put on the backburner for other things such as good grades and late-night coffee runs.

Geomorphology is a subfield of geology that I’ve not had much exposure to in the past few years, and I regret that. Not only is it useful for seeing first hand the way wind, water, and ice enact weathering and erosion to sculpt the land in a plethora of unique features to better our understanding as geoscientists, it just so happens to be interesting, too. For example, just this evening I was working on a chapter summary assignment on periglacial environments – high latitude or high altitude environments that are not permanently glaciated, but where seasonal ices and permafrost features act as a major control on landforms – and stumbled across the idea of nivation.

Simply stated, nivation is a feature of the periglacial environment where small snow patches not large enough to be considered glaciers act as a control on weathering and erosion (Fig. 1). Weathering occurs at the margin of the snow bank, and meltwater acts as the erosianl agent. It’s a slow and subtle process, but effective, forming so-called ‘nivation hollows’ in the side of hills.

Fig. 1. Idealized model of a nivation hollow in the side of a hill. Note the erosional alluvial fan at the base of the snowpatch. Adapted from Thorn and Hall (2002).

The reason this struck so pointedly was because it reminded me of the sort of features I saw while hiking the Bald Hills this summer near Jasper, Alberta:

 

(Nivation hollow with snow patch on the northeast face.)

Granted, it’s minimal in extent compared to the model in Figure 1, but it’s at the proper elevation for a periglacial setting above the treeline at approximately 2225 m. Secondly, keep in mind this photo was taken in early August with snow cover at a minimum. I would like to visit this site in late spring once the seasonal snow cover has melted. I can easily imagine the process of nivation going on; field confirmation would be the icing on the cake.

Additionally, just down slope is an impressive talus field:

(Looking east, downslope on a periglacial talus field. Nivation hollow to the right, Maligne Lake in the background.)

I cannot provide a date on this feature (I suspect the last glacial maximum) but I think it provides a pretty good case for a periglacial environment. In any event, it’s a veritable playground for any geomorphologist. I think I can even provide evidence for tors:

 

(Tors near the ‘summit’ of the Bald Hills at 2300 m – note the vertical and horizontal joints between close-packed, yet separate weathered blocks of rock, esp. near centre of photo.) 

References

Thorn, C.E., and Hall, K. (2002), Nivation and cryoplanation: the case for scrutiny and integration. Progress in Physical Geography, vol. 26, no. 4, p. 533-550

Athabasca Glacier(Click to enlarge)

The Athabasca Glacier as seen from the Icefields Parkway which runs between Banff and Jasper in Alberta, Canada. This particular ‘toe’ is one of six of the much larger Columbia Icefield (higher in elevation beyond horizon) which spans 325 square km.

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