How Two Unassuming Elements, Re and Os, Assumed Acclaim in the Geosciences

2021 ◽  
Author(s):  
Holly Stein
Keyword(s):  
Organic Matter ◽  
Isotope Composition ◽  
Ore Deposits ◽  
Black Shales ◽  
Radiometric Dating ◽  
Accessory Mineral ◽  
Temperature Sensitive ◽  
Mass Extinctions ◽  
High Definition ◽  
Os Isotope

<p>Re and Os (rhenium and osmium) are chalcophile-siderophile elements (transition metals) with a unique position in isotope geochemistry.  Unlike other commonly used decay schemes for radiometric dating, these metals take residency in resource-related media, for example, sulfide minerals, the organic component in black shales, coals, and bitumens and oils.  In sum, the reducing environment is their haven whereas under oxidizing conditions, Re and Os become unmoored and the radiometric clock becomes compromised.  The clock is not temperature sensitive, and its applicability spans Early Archean to Pleistocene. </p><p>This Bunsen Medal lecture will explore and review the challenges in bringing Re-Os from the meteorite-mantle community into the crustal environment.  At the center of it all is our ability to turn geologic observation into a thoughtful sampling strategy.  The potential to date ore deposits was an obvious application and molybdenite [Mo(Re)S<sub>2</sub>], rarely with significant common Os and rarely with overgrowths, became an overnight superstar, yielding highly precise, accurate, and reproducible ages.  Yet, molybdenite presented our first sampling challenge with recognition of a puzzling parent-daughter (<sup>187</sup>Re-<sup>187</sup>Os) decoupling in certain occurrences.  A strategic sampling procedure was employed.  From there, the diversity of applications spread, as molybdenite is also an accessory mineral in many granitoids, and can be a common trace sulfide in metamorphic rocks.  Whether conformable with and/or crosscutting foliation, molybdenite ages define the timing of deformational events.  Pyrite and arsenopyrite can also be readily dated. </p><p>Applications jumped from sulfides to organic matter.  The hydrogenous component from organic matter in black shales gives us Re-Os ages in the sedimentary record for the Geologic Time Scale.  This led to construction of an Os isotope seawater curve – an ongoing process.  Unlike the well-known Sr seawater curve, the short residence time of Os in the oceans creates a high-definition time record with unambiguous high-amplitude swings in <sup>187</sup>Os/<sup>188</sup>Os.  Re-Os puts time pins into the biostratigraphic record, and we have even directly dated fossils.  Re-Os opened the door for a new generation of paleoclimate studies to evaluate seawater conditions at the time of organic blooms and organic sequestration in bottom mud.  Uplift and continental erosion can be balanced with hydrothermal input into oceans based on changes in the Os isotope composition of seawater.  The timing and connectivity of opening seaways can be determined, and the timing of glaciation and deglaciation events can be globally correlated.  The timing and instigators of mass extinctions are carried in the Re-Os record.  A major meteorite impact places an enormous scar in the Os isotope record as seawater drops toward mantle values but recovers in just a few thousand years.  Most recently, Re-Os has transformed our understanding of the events and fluids involved in construction of whole petroleum systems. </p><p>Looking to the future, what kinds of data sets will be explored and what are the interdisciplinary skill sets needed to interpret those data?  Re-Os will continue to provide us with new ways to dismantle geologic media for new scientific understanding of processes that have shaped our lithosphere, biosphere and hydrosphere, recording their intersection and exchange. </p>

How Two Unassuming Elements, Re and Os, Assumed Acclaim in the Geosciences

2020 ◽  
Author(s):  
Holly Stein
Keyword(s):  
Isotope Composition ◽  
Sampling Strategy ◽  
Ore Deposits ◽  
Black Shales ◽  
Radiometric Dating ◽  
Accessory Mineral ◽  
Temperature Sensitive ◽  
Mass Extinctions ◽  
High Definition ◽  
Os Isotope

<p>Re and Os (rhenium and osmium) are chalcophile-siderophile elements (transition metals) with a unique position in isotope geochemistry. Unlike other commonly used decay schemes for radiometric dating, these metals take residency in resource-related media, for example, sulfide minerals, the organic component in black shales, coals, and bitumens and oils. In short, the reducing environment is their haven whereas under oxidizing conditions, Re and Os become unmoored and the radiometric clock becomes compromised. The clock is not temperature sensitive, and its applicability spans Early Archean to Pleistocene.</p><p>This Bunsen Medal lecture will explore and review the challenges in bringing Re-Os from the meteorite-mantle community into the crustal environment. At the center of it all is our ability to turn geologic observation into a thoughtful sampling strategy. The possibility to date ore deposits was an obvious application, and molybdenite [Mo(Re)S<sub>2</sub>], rarely with significant common Os and lacking overgrowths, became an overnight superstar, yielding highly precise, accurate, and reproducible ages. Yet, molybdenite presented our first sampling challenge with recognition of a puzzling parent-daughter (<sup>187</sup>Re-<sup>187</sup>Os) decoupling in certain occurrences. A strategic sampling procedure was employed. From there, the diversity of applications spread, as molybdenite is also an accessory mineral in many granitoids, and can be a common trace sulfide in metamorphic rocks conformable with and/or cross-cutting foliation linking timing and deformation. Pyrite and arsenopyrite can also be readily dated.</p><p>Applications jumped from sulfides to organic matter. Extracting and dating the organic (hydrogeneous) component in black shales gives us Re-Os ages for sedimentary units in the Geologic Time Scale. This led to construction of an Os isotope seawater curve – an ongoing process. Unlike the well-known Sr seawater curve, the short residence time of Os in the oceans creates a high-definition time record with unambiguous high-amplitude swings in <sup>187</sup>Os/<sup>188</sup>Os. Re-Os puts time pins into the biostratigraphic record, and we have even directly dated fossils. Re-Os opened the door for a new generation of paleoclimate studies to evaluate seawater conditions at the time of organic blooms and organic sequestration in bottom mud. Uplift and continental erosion can be balanced with hydrothermal input into oceans based on changes in the Os isotope composition of seawater. The timing and connectivity of opening seaways can be determined, and the timing of glaciation and deglaciation events can be globally correlated. The timing and instigators of mass extinctions are carried in the Re-Os record. A meteorite impact places an enormous scar in the Os isotope record as seawater drops toward mantle values and recovers in a few thousand years. Most recently, Re-Os has transformed our understanding of the events and fluids involved in the construction of whole petroleum systems.</p><p>Looking to the future, what kinds of data sets will be explored and what are the interdisciplinary skill sets needed to interpret those data? Re-Os will continue to provide us with new ways to dismantle geologic media for new scientific understanding of processes that have shaped our lithosphere, biosphere and hydrosphere and their intersection and exchange.</p>

scholarly journals An open marine record of the Toarcian oceanic anoxic event

2011 ◽  
Vol 3 (1) ◽  
pp. 385-410 ◽  
Author(s):  
D. R. Gröcke ◽  
R. S. Hori ◽  
J. Trabucho-Alexandre ◽  
D. B. Kemp ◽  
L. Schwark
Keyword(s):  
Organic Matter ◽  
Carbon Cycle ◽  
Carbon Isotope ◽  
Isotope Composition ◽  
Global Carbon Cycle ◽  
Open Ocean ◽  
Black Shales ◽  
Oceanic Anoxic Event ◽  
Anoxic Event ◽  
Global Carbon

Abstract. Oceanic anoxic events were time intervals in the Mesozoic characterized by widespread distribution of marine organic-rich sediments (black shales) and significant perturbations in the global carbon cycle. The expression of these perturbations is globally recorded in sediments as excursions in the carbon isotope record irrespective of lithology or depositional environment. During the Early Toarcian, black shales were deposited on the epi- and peri-continental shelves of Pangaea and these sedimentary rocks are associated with a pronounced (ca. 7‰) negative (organic) carbon isotope excursion (CIE) which is thought to be the result of a major perturbation in the global carbon cycle. For this reason, the Early Toarcian is thought to represent an oceanic anoxic event (the T-OAE). Associated with this event, there were pronounced perturbations in global weathering rates and seawater temperatures. Although it is commonly asserted that the T-OAE is a global event and that the distribution of black shales is likewise global, an isotopic and/or organic-rich expression of this event has as yet only been recognized on epi- and peri-continental Pangaean localities. To address this issue, the carbon isotope composition of organic matter (δ13Corg) of Early Toarcian cherts from Japan that were deposited in the open Panthalassa Ocean was analysed. The results show the presence of a major (>6‰) negative excursion in δ13Corg that, based on radiolarian biostratigraphy, is a correlative of the Early Toarcian negative CIE known from European epicontinental strata. Furthermore, a secondary ca. −2‰ excursion in δ13Corg is also recognized lower in the studied succession that, within the current biostratigraphical resolution, is likely to represent the excursion that occurs close to the Pliensbachian/Toarcian boundary and which is also recorded in European epicontinental successions. These results from the open ocean realm suggest that, in conjunction with other previously published datasets, these major Early Jurassic carbon cycle perturbations affected all active global reservoirs of the exchangeable carbon cycle (deep marine, shallow marine, atmospheric). An extremely negative δ13Corg value (−57‰) during the peak of the T-OAE is also reported, which suggests that the inferred open ocean mid-water oxygen minimum layer within which these sediments are thought to have been deposited was highly enriched in methanotrophic bacteria, since these organisms are the only plausible producers of such 12C-enriched organic matter.

Sulfur isotope of pyrite response to redox chemistry in organic matter-enriched shales and implications for components of shale gas

2018 ◽  
Vol 6 (4) ◽  
pp. SN71-SN83 ◽  
Author(s):  
Dongya Zhu ◽  
Quanyou Liu ◽  
Bing Zhou ◽  
Zhijun Jin ◽  
Tianyi Li
Keyword(s):  
Organic Matter ◽  
Sulfate Reduction ◽  
Shale Gas ◽  
Sulfur Isotope ◽  
Isotope Composition ◽  
Redox Chemistry ◽  
Gas Production ◽  
Black Shales ◽  
Upper Ordovician ◽  
Longmaxi Formation

The Sichuan Basin has achieved breakthroughs in shale gas production from the Upper Ordovician Wufeng and Lower Silurian Longmaxi Formation black shales. Large amounts of pyrite commonly occur in the organic matter (OM)-enriched black shales, but [Formula: see text] has not been detected in the shale gas. The genetic mechanism of pyrite, its implications for redox chemistry, and the main controlling factors for the absence of [Formula: see text] are unclear. The [Formula: see text] values of the pyrite are extremely high. In particular, the nodular pyrite has [Formula: see text] values as high as 38.6‰. The high sulfur isotopic values indicate that the Wufeng-Longmaxi Formation shales were deposited in an anaerobic sulfide euxinic environment where the limited [Formula: see text] in the stagnant bottom water was completely reduced to pyrite by bacterial sulfate reduction (BSR). The heavy sulfur isotope composition of the pyrite is indicative of organic-rich intervals, which are also the high-yielding intervals for shale gas. Shale gas from the Wufeng-Longmaxi Formation is dominated by alkanes, with an average [Formula: see text] content of 97.91%. The shale gas contains a small amount of [Formula: see text], with an average of 0.34%. However, no [Formula: see text] was detected. The [Formula: see text] values have a range of 4.7‰–11.5‰, with an average of 7.8‰, which is significantly different from the [Formula: see text] related to thermochemical sulfate reduction (TSR) but similar to the [Formula: see text] from the decomposition of carbonate minerals. The black shales experienced high burial temperatures and were rich in OM, which met part of the necessary conditions for the occurrence of TSR. However, TSR did not occur. The reason for the lack of TSR process is that no sulfate mineral was available in the shales because the [Formula: see text] in the seawater was fully consumed by BSR. As a result, [Formula: see text] associated with TSR was not detectable in the shale gas.

scholarly journals An open ocean record of the Toarcian oceanic anoxic event

Solid Earth ◽  
2011 ◽  
Vol 2 (2) ◽  
pp. 245-257 ◽  
Author(s):  
D. R. Gröcke ◽  
R. S. Hori ◽  
J. Trabucho-Alexandre ◽  
D. B. Kemp ◽  
L. Schwark
Keyword(s):  
Organic Matter ◽  
Carbon Cycle ◽  
Carbon Isotope ◽  
Isotope Composition ◽  
Global Carbon Cycle ◽  
Open Ocean ◽  
Black Shales ◽  
Oceanic Anoxic Event ◽  
Anoxic Event ◽  
Global Carbon

Abstract. Oceanic anoxic events were time intervals in the Mesozoic characterized by widespread distribution of marine organic matter-rich sediments (black shales) and significant perturbations in the global carbon cycle. These perturbations are globally recorded in sediments as carbon isotope excursions irrespective of lithology and depositional environment. During the early Toarcian, black shales were deposited on the epi- and pericontinental shelves of Pangaea, and these sedimentary rocks are associated with a pronounced (ca. 7 ‰) negative (organic) carbon isotope excursion (CIE) which is thought to be the result of a major perturbation in the global carbon cycle. For this reason, the lower Toarcian is thought to represent an oceanic anoxic event (the T-OAE). If the T-OAE was indeed a global event, an isotopic expression of this event should be found beyond the epi- and pericontinental Pangaean localities. To address this issue, the carbon isotope composition of organic matter (δ13Corg of lower Toarcian organic matter-rich cherts from Japan, deposited in the open Panthalassa Ocean, was analysed. The results show the presence of a major (>6 ‰) negative excursion in δ13Corg that, based on radiolarian biostratigraphy, is a correlative of the lower Toarcian negative CIE known from Pangaean epi- and pericontinental strata. A smaller negative excursion in δ13Corg (ca. 2 ‰) is recognized lower in the studied succession. This excursion may, within the current biostratigraphic resolution, represent the excursion recorded in European epicontinental successions close to the Pliensbachian/Toarcian boundary. These results from the open ocean realm suggest, in conjunction with other previously published datasets, that these Early Jurassic carbon cycle perturbations affected the active global reservoirs of the exchangeable carbon cycle (deep marine, shallow marine, atmospheric).

Dissolution and recrystallization in modern shelf carbonates: evidence from pore water and solid phase chemistry

1993 ◽  
Vol 344 (1670) ◽  
pp. 27-36 ◽  
Keyword(s):  
Organic Matter ◽  
Pore Water ◽  
Solid Phase ◽  
Chemical Exchange ◽  
Isotope Composition ◽  
Geochemical Data ◽  
Carbonate Sediments ◽  
Pore Waters ◽  
Carbonate Minerals ◽  
Carbonate Production

We present an overview of geochemical data from pore waters and solid phases that clarify earliest diagenetic processes affecting modern, shallow marine carbonate sediments. Acids produced by organic matter decomposition react rapidly with metastable carbonate minerals in pore waters to produce extensive syndepositional dissolution and recrystallization. Stoichiometric relations among pore water solutes suggest that dissolution is related to oxidation of H 2 S which can accumulate in these low-Fe sediments. Sulphide oxidation likely occurs by enhanced diffusion of O 2 mediated by sulphide-oxidizing bacteria which colonize oxic/anoxic interfaces invaginating these intensely bioturbated sediments. Buffering of pore water stable isotopic compositions towards values of bulk sediment and rapid 45 Ca exchange rates during sediment incubations demonstrate that carbonate recrystallization is a significant process. Comparison of average biogenic carbonate production rates with estimated rates of dissolution and recrystallization suggests that over half the gross production is dissolved and/or recrystallized. Thus isotopic and elemental composition of carbonate minerals can experience significant alteration during earliest burial driven by chemical exchange among carbonate minerals and decomposing organic matter. Temporal shifts in palaeo-ocean carbon isotope composition inferred from bulk-rocks may be seriously compromised by facies-dependent differences in dissolution and recrystallization rates.

scholarly journals Composition changes of eroded carbon at different spatial scales in a tropical watershed suggest enrichment of degraded material during transport

2014 ◽  
Vol 11 (12) ◽  
pp. 3299-3305 ◽  
Author(s):  
C. Rumpel ◽  
V. Chaplot ◽  
P. Ciais ◽  
A. Chabbi ◽  
B. Bouahom ◽  
...  
Keyword(s):  
Organic Matter ◽  
Chemical Composition ◽  
Soil Organic Matter ◽  
Isotope Composition ◽  
Spatial Scales ◽  
Suspended Sediments ◽  
Resonance Spectroscopy ◽  
Vertical Decomposition ◽  
Degraded Material ◽  
Soil Units

Abstract. In order to assess whether eroded carbon is a net source or sink of atmospheric CO2, characterisation of the chemical composition and residence time of eroded organic matter (EOM) at the landscape level is needed. This information is crucial to evaluate (1) how fast EOM can be decomposed by soil microbes during its lateral transport and (2) its impact at deposition sites. This study considers a continuum of scales to measure the composition of EOM across a steep hillslope landscape of the Mekong basin with intense erosion. We sampled suspended sediments eroded during rainfall events from runoff plots (1 and 2.5 m2) and the outlets of four nested watersheds (0.6 × 104 to 1 × 107 m2). Here we show that changes in the chemical composition of EOM (measured by nuclear magnetic resonance spectroscopy) and in its 13C and 15N isotope composition from plot scale through to landscape scale provide consistent evidence for enrichment of more decomposed EOM across distances of 10 km. Between individual soil units (1 m2) to a small watershed (107 m2), the observed 28% decrease of the C/N ratio, the enrichment of 13C and 15N isotopes as well as O-alkyl C in EOM is of similar magnitude as changes recorded with depth in soil profiles due to soil organic matter "vertical" decomposition. Radiocarbon measurements indicated ageing of EOM from the plot to the watershed scale. Therefore transport of EOM may lead to enrichment of stabilised soil organic matter compounds, eventually being subject to export from the watershed.

Sign in / Sign up

Export Citation Format

Share Document