Fires did not destroy (as fast as we thought)
Boulder, Colorado, USA – Today, people are major agents of landscape change and catalysts for erosion, but what did people do to the environment before the industrial revolution — before mechanized agriculture? The impact that indigenous peoples had on their landscapes, and when, are often difficult to determine. In this article for Geology, Eric W. Portenga and colleagues show that the use of fire by native Australians had little impact on the erosion of the landscapes they inhabited.
By counting atoms of beryllium-10 in rock and stream sediment samples from Australia's southeastern Tablelands and comparing them to the total amount of beryllium-10 the team was predicted they would find, they were able to model the effect of Aboriginal burning-practices on the rate of erosion in this landscape.
Portenga and colleagues found that Aboriginal burning was not intense enough, or used for long enough, to change significantly the Tablelands' natural long-term rate of landscape erosion over the preceding thousands to millions of years. They also concluded that Aboriginal burning started affecting natural erosion processes in the Tablelands only in the past few thousand years.
These findings show that in southeastern Australia, Aboriginal impact on the landscapes was much less and much more recent than previously thought.
A late Holocene onset of Aboriginal burning in southeastern Australia
Eric W. Portenga et al., School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK. This article is OPEN ACCESS online at http://dx.doi.org/10.1130/G37257.1. For more information, please contact Eric Portenga, University of Glasgow, [email protected]
Other recently posted GEOLOGY articles are highlighted below:
- Gas pathways and remotely triggered earthquakes beneath Mount Fuji, Japan
- Mercury anomaly, Deccan volcanism, and the end-Cretaceous mass extinction
- Tracking Adria indentation beneath the Alps by detrital zircon U-Pb geochronology: Implications for the Oligocene-Miocene dynamics of the Adriatic microplate
- A climatic control on reorganization of ocean circulation during the mid-Cenomanian event and Cenomanian-Turonian oceanic anoxic event (OAE 2): Nd isotope evidence
- Stronger or longer: Discriminating between Hawaiian and Strombolian eruption styles
- Rhenium-osmium isotope fractionation at the oceanic crust-mantle boundary
Gas pathways and remotely triggered earthquakes beneath Mount Fuji, Japan
Koki Aizawa et al., Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan. This article is online at http://dx.doi.org/10.1130/G37313.1.
Large earthquakes sometimes trigger local seismicity that is distal to their rupture zones. Various mechanisms for this triggered seismicity have been proposed, based on either the static stress change or ground shaking from seismic waves, but local geological structure is rarely studied to discern why this seismicity is remotely induced. In this study, Koki Aizawa and colleagues present the results of a joint 3-D resistivity and isotopic analysis of the groundwater system surrounding Mount Fuji, Japan, where increased seismicity was observed following the 2011 Tohoku-Oki megathrust earthquake. Their results suggest a previously unknown N20°E-S20°W-trending structure that they interpret to be a fracture zone that allows magmatic gas to preferentially move upward through the groundwater network. The local seismicity triggered by the Tohoku-Oki megathrust earthquake occurred within this gas pathway. Upwelling of gas-rich hydrous fluids and/or gas bubbles are considered to promote earthquake generation. These types of gas pathways are also present beneath active faults, and may therefore be important not only for triggering seismicity beneath volcanoes, but for generating inland earthquakes.
Mercury anomaly, Deccan volcanism, and the end-Cretaceous mass extinction
Eric Font et al., IDL-FCUL (Instituto Dom Luís, Faculdade de Ciências da Universidade de Lisboa), Campo Grande, Edifício C1, Piso 1, 1749-016 Lisbon, Portugal. This article is online at http://dx.doi.org/10.1130/G37451.1.
The direct correlation between the Cretaceous-Paleogene (KPg) mass extinction and Deccan Trap volcanism in India inevitably leads to questions concerning the role volcanic gas emissions played in this global die-off. Well documented is the role of greenhouse gases (CO2, SO2) leading to rapid global climate warming of 4-8 degrees C and cooling, but also acid rain and ocean acidification. Less well known are the toxic effects of volcanic gases such as Cl, N, and Hg across the KPg boundary. Mercury (Hg) is extremely toxic, easily transported globally and persists in the environment. Here, Eric Font and colleagues report anomalous concentrations of mercury (Hg) in the KPg boundary at Bidart (France) within stratigraphic layers that are coeval with major Deccan eruption Phase-2. The Hg-enhanced level spans from 80 cm below the KPg boundary to 30 cm above in the early Danian. This interval also reveals high fragmentation of calcareous planktic foraminiferal shells that indicates ocean acidification, one of the leading adverse effects of greenhouse warming and possibly the major cause for the diversity decline and mass extinction. The discovery of anomalous Hg concentrations in the same interval suggests toxicity as important contributor to Deccan-related environmental changes leading up to the mass extinction and delayed recovery.
Tracking Adria indentation beneath the Alps by detrital zircon U-Pb geochronology: Implications for the Oligocene-Miocene dynamics of the Adriatic microplate
Marco G. Malusà et al., Department of Earth and Environmental Sciences, University of Milano-Bicocca, Piazza della Scienza 4, 20126 Milan, Italy. This article is online at http://dx.doi.org/10.1130/G37407.1.
In this article, Marco G. Malusà and colleagues constrain the timing of progressive indentation of the Adriatic microplate beneath the European Alps by utilizing detrital zircon U-Pb geochronology on Adriatic foredeep turbidites. They compared the geochronologic fingerprints of the exhuming tectonic domes of the Central Alps (Ticino and Toce subdomes) with those of the Oligo-Miocene turbidites chiefly derived from their erosion. The team found that the ratio between Variscan and Caledonian zircon grains (which are dominant in the Toce and Ticino subdomes, respectively) sharply increases approx. 23 to 24 million years ago. This major provenance change marks the westward shift of the Adriatic indenter beneath the Central Alps, and the associated right-lateral activity of the Insubric Fault. Coexistence of strike-slip motion at the northern boundary of the Adriatic microplate approx. 23 to 24 million years ago, and of trench retreat during scissor-type backarc opening to the west, requires a near-vertical rotation axis located at the northern tip of the Ligurian-Provençal basin. They propose that the rotation axis position was controlled by the interaction between the European and the Adriatic slabs, which may have collided at depth by the end of the Oligocene triggering the westward shift of the Adriatic indenter beneath the Central Alps.
A climatic control on reorganization of ocean circulation during the mid-Cenomanian event and Cenomanian-Turonian oceanic anoxic event (OAE 2): Nd isotope evidence
Xin-Yuan Zheng et al., Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. This article is online at http://dx.doi.org/10.1130/G37354.1.
The warm Cretaceous had witnessed dramatic climate changes, including an extreme one named oceanic anoxic event 2 (OAE 2), during which the deep ocean was devoid of oxygen on a global scale for almost half a million years. The role of ocean circulation in these climate events remains unclear. Isotopic ratios of the element neodymium (Nd) are fingerprints of water masses in the ocean, and, in this study, Xin-Yuan Zheng and colleagues used this proxy, mostly recorded in fossil fish teeth, to track changes in ocean circulation in the European epicontinental sea across a climate event analogous to OAE 2 — the mid-Cenomanian event I (MCE I). They found recurrent circulation reorganizations in response to transient climate cooling during the event. Episodic invasions of high-latitude marine species to the mid-latitudes always started after circulation changes had occurred, implying a slower biological response to climate change. This sequence of change in climate, ocean circulation and biology is identical to that seen during OAE 2. The high sensitivity of ocean circulation to carbon cycle changes during MCE I and OAE 2 may be critical in replenishing upper ocean nutrients from the deep ocean or volcanic sources, fueling these events for long periods of time.
Stronger or longer: Discriminating between Hawaiian and Strombolian eruption styles
B.F. Houghton et al., Department of Geology and Geophysics, University of Hawai'i, Honolulu, Hawaii 96822, USA. This article is online at http://dx.doi.org/10.1130/G37423.1.
Kilauea, Stromboli, and Etna are the three most intensely monitored, continually active volcanoes in the world and also locations of large and growing volcano-tourism operations. Small eruptions at these volcanoes create large problems because of their high frequency and high accessibility. This study reexamines how to distinguish the two classical styles of small eruptions, Strombolian explosions and Hawaiian "fountains," using new sets of data from these volcanoes. It proposes that they key difference is neither the size nor the strength of the eruptions but their duration, with a gap between 40 and 1200 seconds that separates short-lived Strombolian events from prolonged Hawaiian ones. This is a first-order distinction in volcanology, linked to the extent to which gas released by the molten rock, or magma, in the plumbing system of the volcanoes remains linked to, or escapes from, the magma during its final ascent to the surface.
Rhenium-osmium isotope fractionation at the oceanic crust-mantle boundary
Alessio Sanfilippo et al., Dipartimento di Scienze della Terra e dell'Ambiente, Università degli Studi di Pavia, Via Ferrata 1, 27100 Pavia, Italy. This article is online at http://dx.doi.org/10.1130/G37428.1.
This study by Alessio Sanfilippo and colleagues shows that rocks from the oceanic crust-mantle boundary in the Central Indian Ridge have variable rhenium-osmium isotope compositions. Motivated by the assumption that mantle rocks (source of magmatism) and crust (products of crystallization) must preserve similar isotopic ratios, the team argues that the variability of the rocks of this study is related to reactive processes occurring at the crust-mantle boundary. In particular, they infer that the chemistry of the lowermost sector of the oceanic crust is shaped by interactions between primary melts and mantle peridotites. Under this light, they propose that the crust-mantle boundary acts as a "reactive filter," which may modify the isotopic composition of the melts delivered to the surface.
GEOLOGY articles are online http://geology.gsapubs.org/. Representatives of the media may obtain complimentary articles by contacting Kea Giles at the e-mail address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Non-media requests for articles may be directed to GSA Sales and Service, [email protected]