October 2020 Issue Table of Contents
Time is a big deal for us geologists. Rates of Earth processes range from the mind-numbingly slow (mantle convection) to the catastrophically fast (volcanic eruptions) with everything in between. Geologists move effortlessly from units of seconds to giga years in a way that often confounds scientists in other disciplines; no geologist is unaware of humanitys’ fleetingly brief tenure of the planet in the grand scheme of things.
Not surprisingly, devising ways of measuring time is a central plank of the Earth sciences. Most so-called chronometers fall broadly into two categories: those involving radioactive decay and those involving diffusion. The former clock ticks relentlessly at a rate dependent only on the abundance of the parent isotope undergoing decay – a classic first-order process. The latter clock ticks sporadically at a rate governed by the gradient in chemical potential and the prevailing temperature – a second-order process.
The original concept of applying radioactive decay to dating rocks is attributed to Ernest (Lord) Rutherford who proposed, in the early 1900s, that the concentration of helium (a proxy for alpha particles) in uranium-bearing materials could be used to estimate time. Over the subsequent century or so, many variants of the radiometric clock have been devised, exploiting our ever-increasing ability to measure different isotopes using mass spectrometers. Diffusion chronometry has had a slightly less steady trajectory, although its roots may be traced to Joseph Fourier’s seminal work on conductive heat flow (also a diffusive process) published in 1822. The challenge in applying diffusion chronometry has been to know both diffusion coefficients (less tractable and more variable than radioactive decay constants) and temperature, which is rarely constant throughout any geological process. Here, complex numerical modelling, as well as analytical sophistication, is required.
This issue of Elements elegantly draws together these two ways of telling the (geological) time. To exploit the radiometric chronometer, radiogenic helium is measured in rocks and minerals: the diffusive clock is exploited by measuring the spatial distribution of helium, either by high-resolution microbeam analysis or by progressive heating of the sample, and monitoring the resultant step-wise helium release. The result is a cutting-edge time piece that is applicable over a very wide time range and to problems that do not always lend themselves to one or other chronometer used in isolation. Examples in this issue include landscape evolution (page 311), fault movement (page 319), and extra-terrestrial processes (page 331). To this lay reader, the potential is considerable and is set to be enhanced by new generations of numerical models applied to large datasets, as well as new developments in mass spectrometry. Lord Rutherford would be most impressed.
Time is not the unique province of the geologist. Historians, archaeologists and anthropologists also rely on chronometers to reconstruct a younger, but no less important, swathe of Earth history. To both radiometric tools (e.g., radiocarbon dating of organic matter) and diffusive tools (e.g., water diffusion in obsidian) these scholars add written and verbal testimony. Often Bayesian statistical methods are used to bring these different approaches into conformity, but cases remain where the written testimony is not entirely reliable and geological chronometers are not entirely suitable. This is an exciting interdisciplinary frontier for both Earth sciences and archaeology, and one destined to stir up controversy and debate.
As I write this editorial on the Ionian Greek island of Ithaca, I am reminded of a particular opportunity at this new frontier. Ithaca is widely held to have been the home of Odysseus, the Greek king whose exploits are documented in fantastical detail in the epic poems of Homer. After ten long years fighting the Trojans and a further ten action-packed years getting lost on the way home, Odysseus was eventually reunited with his patient bride, Penelope. It is one of the more celebrated homecomings in literature. But did it really take place on Ithaca?
There are several inconsistencies in Homer’s description of Ithaca. Unlike today’s rugged island, Odysseus’ Ithaca is flat and the westernmost of the Ionian islands: “lies low, furthest to sea” according to Homer. This might seem a mere detail to the casual holidaymaker, but it has piqued the interest of some scholars who propose an alternative, controversial, home for Odysseus: the Paliki Peninsular west of Kefalonia. Flat-lying and westward of all other Ionian islands, Paliki has a lot going for it in terms of geography, but beyond the wrong name there’s another catch. Paliki is connected to the rest of Kefalonia by a thin strip of land, up to 180 m above sea-level. This at first insuperable problem has been explained by a group of geologists and archaeologists, known as the Odysseus Unbound Foundation, to be the result of a giant landslip filling a narrow seaway sometime between Odysseus’ homecoming and the present day (read more at http://www.geolsoc.org.uk/Geoscientist/Archive/May-2018/Ithaca-the-story-continues). Such landslides are not uncommon in tectonically active regions. And Kefalonia straddles a plate boundary.
Resolving the hypothesis that Odysseus returned to modern-day Paliki rather than modern-day Ithaca is crying out for a suitable chronometer. Dating faulted surfaces and landslide deposits is no easy matter. The Odysseus Unbound Foundation is no doubt exploring possible dating methods as I write. I rather hope they might read this issue of Elements to see just what insights thermochronology can offer. This is just one of a number of exciting interdisciplinary opportunities in this rapidly evolving field.
Jon Blundy, Principal Editor