Dust, iron, life
Boulder, Colo., USA – Dust begets life, and Earth's atmosphere 300 million years ago was perhaps the dustiest of all time, with large consequences for carbon cycling and the climate system. In a new paper for Geology, Sohini Sur and colleagues examine the bioavailability of iron in dust from Earth's penultimate icehouse of the late Paleozoic. Dust links to carbon because of the iron — a key nutrient for nearly all life, so atmospheric dust acts as a fertilizer.
Vast volumes of dust deposits dating from the late Paleozoic, from both land areas and marine reefal deposits, record a remarkably dusty atmosphere. Moreover these dust deposits contain unusually high concentrations of reactive iron. This concomitance — dust with exceptionally high values of bioavailable iron — implies major ecosystem fertilization and an associated massive drawdown of atmospheric carbon.
The biogeochemical impacts of iron-rich dust to the oceans are known for Earth's recent record but unexplored for deep time, despite recognition of large ancient dust fluxes, particularly during the late Paleozoic. Sur and colleagues report a unique iron relationship for Upper Pennsylvanian mudrock of eolian origin that records lowstand (glacial) conditions within a carbonate buildup of western equatorial Pangaea (now the western United States).
With iron fertilization proposed as a serious geoengineering scheme to control future atmospheric carbon and attendant climate change, authors Sur and colleagues write, "It behooves us to study the consequences of analogous events as archived in Earth's deep-time record."
Extreme eolian delivery of reactive iron to late Paleozoic icehouse seas
Sohini Sur et al., School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019, USA. This paper is online at http://dx.doi.org/10.1130/G37226.1. Corresponding author: Gerilyn S. Soreghan, [email protected]
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Radiometric dating and temperature history of banded iron formation-associated hematite, Gogebic iron range, Michigan, USA
K.A. Farley and R. McKeon, Division of Geological and Planetary Sciences, California Institute of Technology, MS 170-25, Pasadena, California 91125, USA. This paper is online at http://dx.doi.org/10.1130/G37190.1.
A new dating technique was applied to hematite specimens obtained from iron mines in the Upper Peninsula of Michigan to better understand when and how these high grade iron deposits developed. The dates obtained indicate multiple episodes of hematite crystallization spanning hundreds of millions of years. The samples formed at elevated temperatures of 60 degrees C to more than 80 degrees C, suggesting the deposit was buried to between 3 and 5 km depth between 775 and 450 million years ago. The mechanism by which fluids likely responsible for hematite growth penetrate so deeply into the crust remains enigmatic. Over the last half billion years the deposit was brought slowly to the surface, most likely by steady erosion. There is no evidence that recent glaciation caused significant additional erosion of the region.
Seismic anisotropy beneath the Juan de Fuca plate system: Evidence for heterogeneous mantle flow
Miles Bodmer et al., Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA. This paper is online at http://dx.doi.org/10.1130/G37181.1.
The Cascadia subduction zone, where the Juan de Fuca plate subducts beneath North America, dominates the tectonics and seismic hazards of the Pacific Northwest. Miles Bodmer of the University of Oregon and colleagues present seismic results from the Cascadia Initiative, an onshore-offshore seismic and geodetic experiment designed to study plate formation and subduction zone processes. The group has analyzed SKS shear wave splitting at more than 100 ocean bottom seismometers, making this the largest study of its type in the ocean and providing one of the first major results from the initiative. Bodmer and colleagues find that upper mantle flow is surprisingly heterogeneous beneath the subducting Juan de Fuca plate. In northern Cascadia, entrained upper mantle is dragged into the subduction zone. However, in southern Cascadia, upper mantle flow is controlled by fragmentation of the internally deforming Gorda plate, as opposed to the toroidal flow that is predicted by some geodynamic models. The study provides new insights to the processes driving deformation beneath oceanic plates and in regions like the Cascadia subduction zone.
A sub-crustal piercing point for North Atlantic reconstructions and tectonic implications
Christian Schiffer et al., Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, DK-8000 Aarhus C., Denmark. This paper is online at http://dx.doi.org/10.1130/G37245.1.
In this paper, authors Christian Schiffer and colleagues suggest a correlation of two geometrically and seismologically similar east dipping structures in the lithosphere of East Greenland and northern Scotland. They write, "Their similarity and location in paleogeographic maps suggest that these structures might have been formed during the same event, probably during the closure of the Iapetus Ocean and subsequent continent-continent collision, approx. 450-425 million years ago. Approx. 55 million years ago, this once contiguous lineament would have broken apart, leaving remnants on both sides of the forming North Atlantic Ocean. Accordingly, these two remnants may be used as a tool to fit the European and North American continents back together." Further, the authors suggest that the identified upper mantle structure may have provided an important control on some major geodynamic processes in the North Atlantic region, such as the occurrence of substantial magmatic anomalies and the formation of Iceland. The identification of this "sub-crustal piercing point" attempts to stimulate the discussion about the evolution and formation processes of the North Atlantic region.
Widespread shoaling of sulfidic waters linked to the end-Guadalupian (Permian) mass extinction
Guijie Zhang et al., Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China. This paper is online at http://dx.doi.org/10.1130/G37284.1.
The end-Guadalupian mass extinction about 257-260 million years ago may represent the first phase of Permian mass extinctions, one of the most severe extinction events in Earth history. During this extinction event, numerous shallow marine taxa such as fusulinacean foraminifera, brachiopods, corals, and bivalves were killed. Leading hypotheses invoking environmental perturbations include rapid sea-level fall, the eruption of the large igneous province, and long term global cooling have been proposed to account for the end-Guadalupian extinction. However, these hypotheses have been challenged by new geological and geochemical data and therefore the causes of the end-Guadalupian mass extinction remain mysterious. Zhang and colleagues took a new approach by measuring multiple sulfur isotopes on pyrites from sedimentary rocks from South China and west Texas (USA). They found minor sulfur isotopic anomaly with negative D33S values both in the South China and west Texas. These sulfur isotopic anomalies reflect a complex of sulfur cycling in the oceans which may have been driven by shoaling of deep seawaters containing toxic hydrogen sulfide. Importantly, the sulfur isotopic anomalies coincide temporally with the end-Guadalupian mass extinction and these results suggest a causal link between widespread shoaling of toxic sulfidic waters and the mass extinction.
Marine organic matter cycling during the Ediacaran Shu-ram excursion
Carina Lee et al., Department of Earth Sciences, University of California Riverside, 900 University Avenue, Riverside, California 92521, USA. This paper is online at http://dx.doi.org/10.1130/G37236.1.
The largest negative carbon isotope excursion in Earth history — the Shuram excursion — is found in marine carbonate rocks from around the world. The causes of this excursion have provoked a number of hypotheses that try to explain its origin. An unusual aspect of this excursion–in which carbonate rocks record carbon isotope signatures of -12‰ from a typical 0‰ baseline — is that there is no carbon isotopic excursion of similar magnitude preserved in coexisting bulk organic matter. Previous studies that examine the carbon isotopes of organic matter through the Shuram excursion measured bulk organic matter values in organic-lean sections. Here we studied a section from the South Oman Salt Basin that captures the Shuram excursion in organic-rich marine rocks. We measured the carbon isotopic signature of individual hydrocarbons in order to capture the source contributions to bulk values. We discovered two end-member compositions that contribute to bulk isotope values, one that is 13C-depleted and the other 13C-enriched. The excursion that was at least 5-7‰ in the hydrocarbons, showing for the first time that the organic matter pool experienced a primary perturbation of significant size during the Shuram excursion in carbonate.
Extrinsic forcing of plant ecosystems in a large igneous province: The Columbia River flood basalt province, Washington State, USA
Alena Ebinghaus et al., Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB24 3UE, UK. This paper is online at http://dx.doi.org/10.1130/G37276.1.
Large igneous province (LIP) volcanism is implied to have affected regional and global environment and ecosystems significantly. Vegetation inhabiting modern and ancient volcanic terrains is assumed to show more mature plant communities with increasing volcanic hiatuses. This hypothesis is tested by examining interbasaltic palynoflora and ecological conditions from interbed intervals of the Miocene Columbia River flood basalt province (CRBP) in north western United States. The expected succession trajectory is contradicted by a successional decline during the phase of waning CRBP volcanism and prolonged interbed intervals. The assessment of Miocene climate change and CRBP volcanic activity affecting local ecological conditions and the hydrological regime does not show an obvious influence on intralava field vegetation. Frequent deposition of volcanic ash derived from adjacent Snake River Plain hotspot imply significant disturbance of CRBP flora by frequently re-setting seral succession. Volcanic ashes may cause serious damage by such as burial of soil and plants, defoliation, and acidification. This implies that LIPs are not only interactive open environments, but also that LIPs comparable to the CRBP are likely to have had limited impact on past global and regional ecosystems. This work shows that past major global environmental changes were driven by multiple factors.
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