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9 isotopes fractionating

Posted on January 2, 2010 by Chris Rowan

On the 9th day of Christmas my true love sent to me: 9 isotopes fractionating…

Many elements exist in the form of two or more isotopes. Different isotopes have the same number of protons and electrons, so chemically will react in the same way, but their nuclei contain different numbers of neutrons, giving them different atomic masses. For example, the simplest and most abundant element in the Universe, hydrogen, has three isotopes with zero, one and two neutrons in their nuclei, giving the isotopes atomic masses of one (protium), two (deuterium), and three (tritium), respectively.

500px-Hydrogen_Deuterium_Tritium_Nuclei_Schematic.svg.png
Hydrogen, Deuterium, Tritium. Source: Wikipedia

The different atomic weights of isotopes allows mass spectrometers to separate them from each other (their ions are deflected slightly different amounts by an applied magnetic field) and measure their relative proportions in a sample. And, as it turns out, the ability to measure isotopic ratios is a rather useful tool, and not just for dating zircons. Just as in a mass spectrometer, physical processes can fractionate many isotopes, so that they are distributed in different proportions in different reservoirs (physically or chemically distinct parts of the Earth’s biogeochemical systems), such as the ocean and the biosphere, the crust and the mantle, or even the Earth and the rest of the solar system. This enables the influence and contribution of different reservoirs to be measured, and new ones to be identified. Changes in the distribution of isotopes between different reservoirs over time can be used to track the rates of different geological processes that have produced the fractionation. Here is just a flavour of the sort of things you can investigate using the isotopic ratios of different elements, some of which have been discussed on this blog before.

Carbon. Life, or more specifically, the enzymes that drive cell biochemistry, generally prefer to use lighter isotopes at the expense of heavier ones. Because of this, organic matter is enriched in ¹²C relative to ¹³C. Isotopically light carbon is thought to be a distinctive product of biological activity, and such material found in 3.85 billion year old rocks in western Greenland, is possibly evidence for very early life: the only evidence that could possibly be found, given how much the rocks they are found in have been altered by heat and pressure.

Oxygen. Water cycle processes that involve change of state between solid, liquid, and gas fractionate oxygen isotopes. Evaporation from the ocean favours lighter isotopes, so when the Earth’s ice caps grow, they form from snow that is enriched in ¹⁶O relative to ¹⁸O. This in turn enriches the ocean water that is left behind. The ratio of ¹⁸O to ¹⁶O in the oceans therefore increases and decreases as the polar ice caps wax and wane, and these variations are recorded by carbonate in the shells of marine organisms such as foraminifera, allowing climatic fluctuations to be measured and dated.

Source: INGV

Sulphur. Purely physical processes in the upper atmosphere cause mass independent fractionation of ³²S, ³³S, and ³⁴S, in contrast to reactions involving sulphur at the Earth’s surface. In today’s oxygenated atmosphere, all the compounds fractionated in the upper atmosphere quickly recombine with each other, but when the Earth’s atmosphere was anoxic, they remained separate. Mass independent fractionation of sulphur isotopes is observed in most sediments older than about 2.5 billion years, and the disappearance of this signal in younger rocks provides a highly sensitive marker of when oxygen first began to accumulate in the Earth’s atmosphere.

Osmium. On a planetary scale, isotopic studies take advantage of the fact that the Earth is differentiated, meaning that when the planet formed a lot of heavy elements ended up in virtually inaccessible reservoirs such as the core and lower mantle, with most of the surface stocks of these elements being produced by radioactive decay of other elements since the Earth’s formation. Thus reservoirs on the Earth’s surface have markedly different isotopic ratios than the deep Earth, or undifferentiated extra-terrestrial material like chondritic meteorites. Following this principle, excursions in the ratio of ¹⁸⁷Os to ¹⁸⁸Os of seawater can be used to detect asteroid impacts, and even estimate the size of the impactor. Most of the osmium on the Earth’s surface is ¹⁸⁷ Os which has been produced by radioactive decay of rhenium since the planet’s formation; an asteroid contains much more primordial ¹⁸⁸Os. Thus an impact leads to large negative excursions in the marine ¹⁸⁷Os/188^Os record.

Source: Francois et al., 2008

Isotope geochemistry is an extremely versatile and powerful tool – once you’re armed with a mass spectrometer, you can use it to attack a wide variety of geological problems, and many diverse processes, although in terms of my abiding interest – tectonics – they are perhaps less informative than my palaeomagic, even though I suspect that a rock’s magnetism is reset much more easily than its isotopic ratios…

…8 streams reversing,

7 glaciers melting,

6 fields a-flipping,

5 focal mechanisms,

4 index fossils,

3 Helmholtz coils,

2 concordant zircons,

and an APWP.

Categories: geochemistry, geology, planets

8 streams reversing

Posted on January 1, 2010 by Chris Rowan

On the 8th day of Christmas my true love sent to me: 8 streams reversing…

Wind gaps are fossil rivers: water once flowed through these valleys, but now that water has been diverted to flow elsewhere.

Wind Gap in Wheeler Ridge, California. Source (HT to my co-blogger Anne)

Each of the triangles on the drainage map of northern California (from a 2006 paper by Lock et al.) below marks a wind gap that occurs mid-way along a continuous north-south trending channel.

F4.large_.jpg
Figure 4 of Lock et al. (2006)

On either side of these dry sections, water in the channels to the north flow north, and water in the channels to the south flow south. But this doesn’t always seem to have been the case: the deposits left by these stream systems contain signs of their flow directions in the past, paleocurrent indicators that show that a few million years ago, water in the currently south flowing sections also flowed north, as did water in the wind gaps before they were dry. At some point, the streams south of these wind gaps have reversed themselves.

In their paper discussing the origin of these features, Lock et al. argue that their formation was the result of a relatively recent uplift event altering the drainage patterns of pre-existing streams and rivers. Initially, the high ground was in the south, and water drained northwest all the way from here into the sea. But over the last few million years, something has caused the ground in the middle of the area to rise up as the ground in the south subsided. This created a wind gap-bearing barrier to water flowing from further south, and by reversing the topographic slope in this area, forced the streams there to turn back on themselves, so that they now flowed south.

So, what caused this uplift? Northern California is actually parked right on top of a rather interesting tectonic situation: it marks the spot where the boundaries of three tectonic plates intersect with each other. To the north is a subduction boundary, where the Juan de Fuca plate is being thrust beneath the Cascades. To the south, the Pacific and North American plate are sliding past each other along the San Andreas Fault. Trending west out to sea from Cape Mendecino (CM in the northwest of the drainage map above), the Gorda and Pacific plates move past each other along a ridge transform fault (the Mendecino fracture zone). Due to the presence of this triple junction, Northern California marks the boundary between two distinctive styles of deformation on the coast of west North America: subduction thrusting to the north, and strike slip motion to the south.

The net result of the relative motions between the North American, Pacific, and Juan de Fuca plates is that the triple junction does not stay in the same place: in the last few million years, it has gradually moved northwards along the North Californian coast, lengthening the San Andreas Fault at the expense of the Cascade Trench. This migration squashes the crust ahead of it, causing uplift, and stretches the crust behind it, causing subsidence, causing a region of transient uplift to also gradually move up through northern California, reversing streams as it goes.
By tracking the changes in drainage by mapping, correlating and dating the river deposits in northern California, you can chart the passage of the triple junction with much higher resolution than is possible with other geological indicators. And given the interests of myself and my co-blogger, a study revealing an interesting and informative link between tectonics and hydrology is sort of perfect for this blog , don’t you think? Although if the hydrology stuff makes sense, it’s really only thanks to Anne’s editorial assistance – and if it doesn’t, it’s only thanks to me.
Lock, J., Kelsey, H., Furlong, K., & Woolace, A. (2006). Late Neogene and Quaternary landscape evolution of the northern California Coast Ranges: Evidence for Mendocino triple junction tectonics Geological Society of America Bulletin, 118 (9) DOI: 10.1130/B25885.1

…7 glaciers melting,

6 fields a-flipping,