Xing Tian, Yuanhong Gao, T. Kukla, O. Lenz, He Huang, D. Ibarra, Shouliang Sun, Chengshan Wang
Solar cycles are important moderators of the Earth’s global climate system. Although modern-day solar cycles are well known, they have been less studied over geological time. High-resolution records such as varves have been previously used for reconstructing solar cycles from the Paleoproterozoic through Quaternary. In this paper, very fine (<1 mm) sedimentary laminations of the Early Cretaceous Yixian Formation in Xiushui Basin were studied in Northern Liaoning Province, North China. Two different microfacies of the striped shale in the Third Member of the Yixian Formation were identified. These include the light-gray to gray siltstone (Mf 1) and the gray to black organic-rich mudstone (Mf 2). Laminations of Mf 2 are mainly made of biofilms. Sub-millimeter scaled couplets of biofilm and siliciclastic-rich sublamina record seasonal growth and withering of microbial mats during the warm season (summer) and cold season (winter), respectively. Evolutionary spectral analyses of three binary rank series (the binary boxcar series, triangle series, and midpoint-triangle series), varve couplet thickness and gray scale image data (gray data) show multiple periodicities consistent with solar cycles, including the robust Schwabe sunspot cycle (10.0–10.6 year) and solar Bruckner cycle (31.0–40.6 year), and relatively weaker signals for the solar Hale cycle (21.9 year) and 16.5-year solar cycles that have been linked to solar magnetic activity. Solar cycles recognized in this paper indicate the total solar irradiance (TSI) influenced microbial mat growth in the Early Cretaceous in North China. Further, we extend our new record with a compilation of varve-recorded sunspot cycles throughout geological time to show that the 11-year Schwabe sunspot cycle and the 22-year Hale cycle have persisted since the Paleoproterozoic.
{"title":"Early Cretaceous solar cycles recorded in lacustrine laminations in North China","authors":"Xing Tian, Yuanhong Gao, T. Kukla, O. Lenz, He Huang, D. Ibarra, Shouliang Sun, Chengshan Wang","doi":"10.2475/09.2021.01","DOIUrl":"https://doi.org/10.2475/09.2021.01","url":null,"abstract":"Solar cycles are important moderators of the Earth’s global climate system. Although modern-day solar cycles are well known, they have been less studied over geological time. High-resolution records such as varves have been previously used for reconstructing solar cycles from the Paleoproterozoic through Quaternary. In this paper, very fine (<1 mm) sedimentary laminations of the Early Cretaceous Yixian Formation in Xiushui Basin were studied in Northern Liaoning Province, North China. Two different microfacies of the striped shale in the Third Member of the Yixian Formation were identified. These include the light-gray to gray siltstone (Mf 1) and the gray to black organic-rich mudstone (Mf 2). Laminations of Mf 2 are mainly made of biofilms. Sub-millimeter scaled couplets of biofilm and siliciclastic-rich sublamina record seasonal growth and withering of microbial mats during the warm season (summer) and cold season (winter), respectively. Evolutionary spectral analyses of three binary rank series (the binary boxcar series, triangle series, and midpoint-triangle series), varve couplet thickness and gray scale image data (gray data) show multiple periodicities consistent with solar cycles, including the robust Schwabe sunspot cycle (10.0–10.6 year) and solar Bruckner cycle (31.0–40.6 year), and relatively weaker signals for the solar Hale cycle (21.9 year) and 16.5-year solar cycles that have been linked to solar magnetic activity. Solar cycles recognized in this paper indicate the total solar irradiance (TSI) influenced microbial mat growth in the Early Cretaceous in North China. Further, we extend our new record with a compilation of varve-recorded sunspot cycles throughout geological time to show that the 11-year Schwabe sunspot cycle and the 22-year Hale cycle have persisted since the Paleoproterozoic.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"1285 - 1307"},"PeriodicalIF":2.9,"publicationDate":"2021-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47157312","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Archean basement in the Central Asian Orogenic Belt (CAOB) is relatively rare, but it has the potential to provide additional information on the processes of lithospheric mantle enrichment and crust extraction processes during the early history of the Earth. We identified Neoarchean amphibolite (2537−2565 Ma) and metadiorite (2481−2539 Ma) in the Biliya area of the Erguna Terrane in the southeast CAOB. The amphibolite is geochemically MORB-like and has a weakly left-leaning REE pattern, and low zircon εHf(t) (−0.7−+6.2), and whole-rock εNd(t) (−1.7–+4.5) and εHf(t) (−1.9) values. Our petrogenetic modeling reveals that the amphibolite is derived from ∼20 % partial melting of the lithospheric mantle in the spinel stability field (∼65 km depth). The metadiorite shows near-zero εNd(t) (−0.5–+3.6) and εHf(t) (+0.5–+1.4) values and is likely derived from partial melting of mafic lower crust. The metadiorite and amphibolite likely formed in an extensional continental arc/back-arc setting and represent the Archean crystalline basement of the microcontinents within the CAOB. Three-staged mantle segregation and crust extraction processes have been proposed: (a) 20 % melt extraction from primitive mantle-like lithospheric mantle, leaving behind a depleted mantle; (b) subduction-related fluid/melt metasomatism of the lithospheric mantle and its partial melting, generating the arc-type enriched mantle and mafic lower crust; and (c) partial remelting of the mafic lower crust produced the Tonalite-trondhjemite-granodiorite (TTG) crust.
{"title":"Neoarchean basement, mantle enrichment and crustal extraction in central Asia: petrogenesis of 2.5 Ga amphibolite and metadiorite in NE China","authors":"Huichuan Liu, Jun Shao, G. Zhu, Yinglei Li","doi":"10.2475/09.2021.03","DOIUrl":"https://doi.org/10.2475/09.2021.03","url":null,"abstract":"Archean basement in the Central Asian Orogenic Belt (CAOB) is relatively rare, but it has the potential to provide additional information on the processes of lithospheric mantle enrichment and crust extraction processes during the early history of the Earth. We identified Neoarchean amphibolite (2537−2565 Ma) and metadiorite (2481−2539 Ma) in the Biliya area of the Erguna Terrane in the southeast CAOB. The amphibolite is geochemically MORB-like and has a weakly left-leaning REE pattern, and low zircon εHf(t) (−0.7−+6.2), and whole-rock εNd(t) (−1.7–+4.5) and εHf(t) (−1.9) values. Our petrogenetic modeling reveals that the amphibolite is derived from ∼20 % partial melting of the lithospheric mantle in the spinel stability field (∼65 km depth). The metadiorite shows near-zero εNd(t) (−0.5–+3.6) and εHf(t) (+0.5–+1.4) values and is likely derived from partial melting of mafic lower crust. The metadiorite and amphibolite likely formed in an extensional continental arc/back-arc setting and represent the Archean crystalline basement of the microcontinents within the CAOB. Three-staged mantle segregation and crust extraction processes have been proposed: (a) 20 % melt extraction from primitive mantle-like lithospheric mantle, leaving behind a depleted mantle; (b) subduction-related fluid/melt metasomatism of the lithospheric mantle and its partial melting, generating the arc-type enriched mantle and mafic lower crust; and (c) partial remelting of the mafic lower crust produced the Tonalite-trondhjemite-granodiorite (TTG) crust.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"1350 - 1379"},"PeriodicalIF":2.9,"publicationDate":"2021-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49143291","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. K. Deb, D. Saha, S. Patranabis‐Deb, A. Banerjee
Differentiation of rock suites related to mid-ocean ridge and subduction zone in Archean greenstone belts is important in tracing back tectonic processes related to evolution of these belts. The late Neoarchean – early Paleoproterozoic Sonakhan greenstone belt (SGB) lying between Mesoarchean gneisses of the Bastar craton and the Mesoproterozoic Chattisgarh Supergroup in central India was earlier interpreted to have arc-like affinity. New data from the SGB is presented to reinterpret the Archean tectonic setting. NNW-SSE trending SGB is constituted of three domains. The Baghmara domain in the west is dominantly a mafic metavolcanic rock succession (BGMV group), with repeated cycles of massive to pillowed basalts, pillow breccia and thin chert-BIF-shale and greywacke interlayers, representing an oceanic back-arc system. The Bilari domain in the east, with mixed mafic and felsic metavolcanic rocks (BLMV group) and minor clastic metasediments, presents an ancient magmatic arc. Overlapping these, a polymictic conglomerate-sandstone (greywacke) intercalation of the Arjuni Formation occurs in the central part of steep fold-fault belt of the SGB. Basic to intermediate intrusives (SMI group) and syn- to late-tectonic granitoids occur in all three domains. The BGMV group samples are low-K tholeiites and characterized by modern MORB like major element composition and near-flat REE patterns, reminiscent of some basalts of back-arc spreading centres, such as Parece Vela off Mariana arc. These features together with plots in Sm/Yb versus La/Sm diagram suggest derivation of their parental magmas from primitive spinel lherzolite mantle source with an N-MORB affinity that subsequently fractionated under low-pressure conditions. The BLMV and SMI samples with calc-alkaline major element composition are characterized by E-MORB type REE profiles, with enriched LREE and fractionated HREE patterns, and enrichment in trace elements more incompatible than Ti, relative to N-MORB. In addition, plots in Sm/Yb versus La/Sm diagram indicate derivation of parental magmas from partial melting of enriched garnet lherzolite mantle source at different depths, less and more deep for the BLMV and SMI groups, respectively. The BLMV magmas evolved via crystal fractionation under high water pressure conditions. The intermediate to acidic intrusives of the SGB are calc-alkaline and metaluminous, similar to I-type granites. Although in Th/Yb versus Nb/Yb diagram all the SGB mafic rocks plot above MORB array, restriction of the BGMV samples near N-MORB – PM field and distribution of the BLMV and SMI samples along AFC curve above the MORB array confirm juxtaposition of two contrasting suites, with oceanic back-arc and arc affinities, in the SGB. The Arjuni Formation apparently represents an accretionary wedge lodged in between the Baghmara and Bilari domains. Based on geological and geochemical characteristics, we suggest influence of subduction rollback and oceanic back-arc spreading in t
{"title":"Coexisting arc and MORB signatures in the Sonakhan greenstone belt, India: late Neoarchean – early Proterozoic subduction rollback and back-arc formation","authors":"G. K. Deb, D. Saha, S. Patranabis‐Deb, A. Banerjee","doi":"10.2475/09.2021.02","DOIUrl":"https://doi.org/10.2475/09.2021.02","url":null,"abstract":"Differentiation of rock suites related to mid-ocean ridge and subduction zone in Archean greenstone belts is important in tracing back tectonic processes related to evolution of these belts. The late Neoarchean – early Paleoproterozoic Sonakhan greenstone belt (SGB) lying between Mesoarchean gneisses of the Bastar craton and the Mesoproterozoic Chattisgarh Supergroup in central India was earlier interpreted to have arc-like affinity. New data from the SGB is presented to reinterpret the Archean tectonic setting. NNW-SSE trending SGB is constituted of three domains. The Baghmara domain in the west is dominantly a mafic metavolcanic rock succession (BGMV group), with repeated cycles of massive to pillowed basalts, pillow breccia and thin chert-BIF-shale and greywacke interlayers, representing an oceanic back-arc system. The Bilari domain in the east, with mixed mafic and felsic metavolcanic rocks (BLMV group) and minor clastic metasediments, presents an ancient magmatic arc. Overlapping these, a polymictic conglomerate-sandstone (greywacke) intercalation of the Arjuni Formation occurs in the central part of steep fold-fault belt of the SGB. Basic to intermediate intrusives (SMI group) and syn- to late-tectonic granitoids occur in all three domains. The BGMV group samples are low-K tholeiites and characterized by modern MORB like major element composition and near-flat REE patterns, reminiscent of some basalts of back-arc spreading centres, such as Parece Vela off Mariana arc. These features together with plots in Sm/Yb versus La/Sm diagram suggest derivation of their parental magmas from primitive spinel lherzolite mantle source with an N-MORB affinity that subsequently fractionated under low-pressure conditions. The BLMV and SMI samples with calc-alkaline major element composition are characterized by E-MORB type REE profiles, with enriched LREE and fractionated HREE patterns, and enrichment in trace elements more incompatible than Ti, relative to N-MORB. In addition, plots in Sm/Yb versus La/Sm diagram indicate derivation of parental magmas from partial melting of enriched garnet lherzolite mantle source at different depths, less and more deep for the BLMV and SMI groups, respectively. The BLMV magmas evolved via crystal fractionation under high water pressure conditions. The intermediate to acidic intrusives of the SGB are calc-alkaline and metaluminous, similar to I-type granites. Although in Th/Yb versus Nb/Yb diagram all the SGB mafic rocks plot above MORB array, restriction of the BGMV samples near N-MORB – PM field and distribution of the BLMV and SMI samples along AFC curve above the MORB array confirm juxtaposition of two contrasting suites, with oceanic back-arc and arc affinities, in the SGB. The Arjuni Formation apparently represents an accretionary wedge lodged in between the Baghmara and Bilari domains. Based on geological and geochemical characteristics, we suggest influence of subduction rollback and oceanic back-arc spreading in t","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"1308 - 1349"},"PeriodicalIF":2.9,"publicationDate":"2021-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47843093","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
P. Frings, Franziska Schubring, M. Oelze, F. von Blanckenburg
Silicon (Si) is an important nutrient for many plant and algae species, and the ultimate source of Si is silicate mineral weathering reactions. These topics have inspired the application of Si isotope geochemistry to quantifying Si cycling in the Critical Zone, though the interpretations are often equivocal. Because germanium (Ge) geochemistry is similar to that of Si, the Ge/Si ratio is considered a tracer that provides additional constraints on Si cycling. Here, we provide Ge/Si ratios for three sites that span a gradient of erosion rates and thus time that material spends in the weathering zone before being removed. We present Ge/Si ratios in bulk rock, soil and saprolite, clay-size fractions, plant biomass, and river water from the Central Swiss Alps, the southern Californian Sierra Nevada, and the highlands of Sri Lanka. Our data perform two functions. First, they provide insight into the Ge/Si system. In particular, we document the presence of a substantial pool of Ge in plant biomass that is not associated with phytoliths, suggesting that overall plants do not discriminate against Ge relative to Si during uptake. We also quantify the preferential incorporation of Ge into clay minerals. We show that Ge/Si ratios in secondary clays may be a better proxy for weathering intensity (the fraction of denudation achieved chemically) than the Ge/Si ratio of river solutes. Ge/Si ratios in secondary clay minerals also perform as well as or even better than silicon isotopes as weathering intensity proxies. Second, the Ge/Si data are used in conjunction with silicon isotope data to develop a catchment Si mass-balance model. It suggests that the export of secondary, fractionated solids (largely clays and plant material) becomes increasingly important at shorter regolith residence times: 80−24+15% of total solubilized Si in the rapidly eroding Alps site, vs. 32−20+22% in the slowly eroding Sri Lanka site. The results also suggest that plant material is a surprisingly large contributor to Si export from these catchments, likely equivalent to 25 to110 % of dissolved Si export.
{"title":"Quantifying biotic and abiotic Si fluxes in the Critical Zone with Ge/Si ratios along a gradient of erosion rates","authors":"P. Frings, Franziska Schubring, M. Oelze, F. von Blanckenburg","doi":"10.7185/gold2021.4653","DOIUrl":"https://doi.org/10.7185/gold2021.4653","url":null,"abstract":"Silicon (Si) is an important nutrient for many plant and algae species, and the ultimate source of Si is silicate mineral weathering reactions. These topics have inspired the application of Si isotope geochemistry to quantifying Si cycling in the Critical Zone, though the interpretations are often equivocal. Because germanium (Ge) geochemistry is similar to that of Si, the Ge/Si ratio is considered a tracer that provides additional constraints on Si cycling. Here, we provide Ge/Si ratios for three sites that span a gradient of erosion rates and thus time that material spends in the weathering zone before being removed. We present Ge/Si ratios in bulk rock, soil and saprolite, clay-size fractions, plant biomass, and river water from the Central Swiss Alps, the southern Californian Sierra Nevada, and the highlands of Sri Lanka. Our data perform two functions. First, they provide insight into the Ge/Si system. In particular, we document the presence of a substantial pool of Ge in plant biomass that is not associated with phytoliths, suggesting that overall plants do not discriminate against Ge relative to Si during uptake. We also quantify the preferential incorporation of Ge into clay minerals. We show that Ge/Si ratios in secondary clays may be a better proxy for weathering intensity (the fraction of denudation achieved chemically) than the Ge/Si ratio of river solutes. Ge/Si ratios in secondary clay minerals also perform as well as or even better than silicon isotopes as weathering intensity proxies. Second, the Ge/Si data are used in conjunction with silicon isotope data to develop a catchment Si mass-balance model. It suggests that the export of secondary, fractionated solids (largely clays and plant material) becomes increasingly important at shorter regolith residence times: 80−24+15% of total solubilized Si in the rapidly eroding Alps site, vs. 32−20+22% in the slowly eroding Sri Lanka site. The results also suggest that plant material is a surprisingly large contributor to Si export from these catchments, likely equivalent to 25 to110 % of dissolved Si export.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"1204 - 1245"},"PeriodicalIF":2.9,"publicationDate":"2021-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42413775","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
At Earth's surface the stable isotope ratio of strontium (88Sr/86Sr) is predominantly set by biological uptake of Sr and its storage in plant litter. This conclusion was reached from a stable isotope mass balance that was independently validated by direct determination of elemental fluxes between the Critical Zone compartments (rock, soil, vegetation, and stream water) of three field sites located in the Swiss Alps, the US Sierra Nevada, and the tropical highlands of Sri Lanka. These sites cover a gradient in erosion rates, which is inversely related to the residence time of solids in the Critical Zone thereby constituting an “erodosequence”. For eroding landscapes, previous stable isotope models predicted that isotope ratios are set by the rate at which secondary solids form during the conversion of rock to regolith. Counter to this expectation we found that, after release from primary minerals, Sr is partitioned into one fraction taken up by plants and the remainder into dissolved Sr flux. The formation of secondary weathering products such as clays and oxides plays a subordinate role in determining the Sr budget. A Sr isotope fractionation factor for biological uptake was determined for each of the three ecosystems from the average Sr stable isotope composition in bulk plants and its dissolved counterpart in stream water. This fractionation factors range from ca. −0.3 ‰ for the Alps and Sierra Nevada to ∼0 ‰ for the tropical Sri Lanka site. That these isotope fingerprints caused by biologic uptake are preserved means that more Sr was physically removed in plant litter than recycled. Such Sr removal in plant litter appears to be strongest at the slowly-eroding site, whereas the dissolved Sr export by streams is highest at the site with the fastest erosion rate. There, all Sr taken up by plants is returned from litter back into solution. The site with short residence time of solids is the only one at which parent material and dissolved export differ in their Sr isotope composition. Our study shows that the behavior of Sr in the Critical Zone is in stark contrast to that of metals of which the isotope fractionation is not affected by biological uptake (for example lithium, mostly set by formation of secondary solids) or affected by both secondary solid formation and biological uptake (for example silicon). Strontium stable isotope signatures offer the new opportunity to quantify nutrient cycling in the Critical Zone as a function of environmental and ecological parameters.
{"title":"The role of vegetation in setting strontium stable isotope ratios in the Critical Zone","authors":"J. Bouchez, F. von Blanckenburg","doi":"10.2475/08.2021.04","DOIUrl":"https://doi.org/10.2475/08.2021.04","url":null,"abstract":"At Earth's surface the stable isotope ratio of strontium (88Sr/86Sr) is predominantly set by biological uptake of Sr and its storage in plant litter. This conclusion was reached from a stable isotope mass balance that was independently validated by direct determination of elemental fluxes between the Critical Zone compartments (rock, soil, vegetation, and stream water) of three field sites located in the Swiss Alps, the US Sierra Nevada, and the tropical highlands of Sri Lanka. These sites cover a gradient in erosion rates, which is inversely related to the residence time of solids in the Critical Zone thereby constituting an “erodosequence”. For eroding landscapes, previous stable isotope models predicted that isotope ratios are set by the rate at which secondary solids form during the conversion of rock to regolith. Counter to this expectation we found that, after release from primary minerals, Sr is partitioned into one fraction taken up by plants and the remainder into dissolved Sr flux. The formation of secondary weathering products such as clays and oxides plays a subordinate role in determining the Sr budget. A Sr isotope fractionation factor for biological uptake was determined for each of the three ecosystems from the average Sr stable isotope composition in bulk plants and its dissolved counterpart in stream water. This fractionation factors range from ca. −0.3 ‰ for the Alps and Sierra Nevada to ∼0 ‰ for the tropical Sri Lanka site. That these isotope fingerprints caused by biologic uptake are preserved means that more Sr was physically removed in plant litter than recycled. Such Sr removal in plant litter appears to be strongest at the slowly-eroding site, whereas the dissolved Sr export by streams is highest at the site with the fastest erosion rate. There, all Sr taken up by plants is returned from litter back into solution. The site with short residence time of solids is the only one at which parent material and dissolved export differ in their Sr isotope composition. Our study shows that the behavior of Sr in the Critical Zone is in stark contrast to that of metals of which the isotope fractionation is not affected by biological uptake (for example lithium, mostly set by formation of secondary solids) or affected by both secondary solid formation and biological uptake (for example silicon). Strontium stable isotope signatures offer the new opportunity to quantify nutrient cycling in the Critical Zone as a function of environmental and ecological parameters.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"1246 - 1283"},"PeriodicalIF":2.9,"publicationDate":"2021-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46113133","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Metal isotopes as markers of biogeochemical processes and fluxes in the eroding Critical Zone","authors":"C. Chamberlain","doi":"10.2475/08.2021.05","DOIUrl":"https://doi.org/10.2475/08.2021.05","url":null,"abstract":"","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"iii - iv"},"PeriodicalIF":2.9,"publicationDate":"2021-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47408367","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
F. von Blanckenburg, J. Schuessler, J. Bouchez, P. Frings, D. Uhlig, M. Oelze, D. Frick, T. Hewawasam, Jean L. Dixon, K. Norton
How flowing water and organisms can shape Earth's surface, the Critical Zone, depends on how fast this layer is turned over by erosion. To quantify the dependence of rock weathering and the cycling of elements through ecosystems on erosion we have used existing and new metrics that quantify the partitioning and cycling of elements between rock, saprolite, soil, plants, and river dissolved and solid loads. We demonstrate their utility at three sites along a global transect of mountain landscapes that differ in erosion rates – an “erodosequence”. These sites are the Swiss Central Alps, a rapidly-eroding, post-glacial mountain belt; the Southern Sierra Nevada, USA, eroding at moderate rates; and the slowly-eroding tropical Highlands of Sri Lanka. The backbone of this analysis is an extensive data set of rock, saprolite, soil, water, and plant geochemical and isotopic data. This set of material properties is converted into process rates by using regolith production and weathering rates from cosmogenic nuclides and river loads, and estimates of biomass growth. Combined, these metrics allow us to derive elemental fluxes through regolith and vegetation. The main findings are: 1) the rates of weathering are set locally in regolith, and not by the rate at which entire landscapes erode; 2) the degree of weathering is mainly controlled by regolith residence time. This results in supply-limited weathering in Sri Lanka where weathering runs to completion in the regolith, and kinetically-limited weathering in the Alps and Sierra Nevada where soluble primary minerals persist; 3) these weathering characteristics are reflected in the sites' ecosystem processes, namely in that nutritive elements are intensely recycled in the supply-limited setting, and directly taken up from soil and rock in the kinetically settings; 4) the weathering rates are not controlled by biomass growth; 5) at all sites we find a deficit in river solute export when compared to solute production in regolith, the extent of which differs between elements. Plant uptake followed by litter export might explain this deficit for biologically utilized elements of high solubility, and rare, high-discharge flushing events for colloidal-bound elements of low solubility. Our data and new metrics have begun to serve for calibrating metal isotope systems in the weathering zone, the isotope ratios of which depend on the flux partitioning between the compartments of the Critical Zone. We demonstrate this application in several isotope geochemical companion papers.
{"title":"Rock weathering and nutrient cycling along an erodosequence","authors":"F. von Blanckenburg, J. Schuessler, J. Bouchez, P. Frings, D. Uhlig, M. Oelze, D. Frick, T. Hewawasam, Jean L. Dixon, K. Norton","doi":"10.2475/08.2021.01","DOIUrl":"https://doi.org/10.2475/08.2021.01","url":null,"abstract":"How flowing water and organisms can shape Earth's surface, the Critical Zone, depends on how fast this layer is turned over by erosion. To quantify the dependence of rock weathering and the cycling of elements through ecosystems on erosion we have used existing and new metrics that quantify the partitioning and cycling of elements between rock, saprolite, soil, plants, and river dissolved and solid loads. We demonstrate their utility at three sites along a global transect of mountain landscapes that differ in erosion rates – an “erodosequence”. These sites are the Swiss Central Alps, a rapidly-eroding, post-glacial mountain belt; the Southern Sierra Nevada, USA, eroding at moderate rates; and the slowly-eroding tropical Highlands of Sri Lanka. The backbone of this analysis is an extensive data set of rock, saprolite, soil, water, and plant geochemical and isotopic data. This set of material properties is converted into process rates by using regolith production and weathering rates from cosmogenic nuclides and river loads, and estimates of biomass growth. Combined, these metrics allow us to derive elemental fluxes through regolith and vegetation. The main findings are: 1) the rates of weathering are set locally in regolith, and not by the rate at which entire landscapes erode; 2) the degree of weathering is mainly controlled by regolith residence time. This results in supply-limited weathering in Sri Lanka where weathering runs to completion in the regolith, and kinetically-limited weathering in the Alps and Sierra Nevada where soluble primary minerals persist; 3) these weathering characteristics are reflected in the sites' ecosystem processes, namely in that nutritive elements are intensely recycled in the supply-limited setting, and directly taken up from soil and rock in the kinetically settings; 4) the weathering rates are not controlled by biomass growth; 5) at all sites we find a deficit in river solute export when compared to solute production in regolith, the extent of which differs between elements. Plant uptake followed by litter export might explain this deficit for biologically utilized elements of high solubility, and rare, high-discharge flushing events for colloidal-bound elements of low solubility. Our data and new metrics have begun to serve for calibrating metal isotope systems in the weathering zone, the isotope ratios of which depend on the flux partitioning between the compartments of the Critical Zone. We demonstrate this application in several isotope geochemical companion papers.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"1111 - 1163"},"PeriodicalIF":2.9,"publicationDate":"2021-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41607943","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
P. Frings, M. Oelze, Franziska Schubring, D. Frick, F. von Blanckenburg
Metal and metalloid stable isotope ratios have emerged as potentially powerful proxies for weathering, element cycling and export in the Critical Zone. The simplest possible interpretative framework for these isotope ratios has three parameters: (i) the isotope ratio of the parent minerals undergoing weathering, (ii) the partitioning of the element between solute and the new secondary phases, and (iii) the fractionation factors associated with the formation of new secondary phases. Using the example of silicon, we show how all three of these parameters vary along a gradient of erosion rate and regolith residence time defined by three sites located on granitoid bedrock. These sites run from the kinetically limited Rhone Valley in the Central Swiss Alps to the tectonically inactive and supply-limited Sri Lankan highlands, with the Sierra Nevada mountains as a site of intermediate weathering intensity. At each site, primary mineral specific 30Si/28Si ratios span >0.4‰. These minerals weather differentially, such that the isotope ratio of silicon solubilised from rock differs at the three sites and is not necessarily equal to bulk bedrock composition. The partitioning of silicon between secondary clay and solute is reflected in the clay mineralogy and chemical composition: more intense weathering produces Si-poor clays. The clay composition thus comprises a first-order mass-balance control on the extent to which any fractionation factor can be expressed. Finally, the Si isotope fractionation factor associated with clay formation varies systematically with clay mineralogy: the formation of Si-deplete clay minerals is associated with larger fractionation factors. The magnitude of the fractionation may be mechanistically linked to relative aluminium availability. These findings provide the framework needed to use Si isotope ratios as a quantitative proxy to explore Si cycling and reconstruct weathering in the present and past.
{"title":"Interpreting silicon isotopes in the Critical Zone","authors":"P. Frings, M. Oelze, Franziska Schubring, D. Frick, F. von Blanckenburg","doi":"10.2475/08.2021.02","DOIUrl":"https://doi.org/10.2475/08.2021.02","url":null,"abstract":"Metal and metalloid stable isotope ratios have emerged as potentially powerful proxies for weathering, element cycling and export in the Critical Zone. The simplest possible interpretative framework for these isotope ratios has three parameters: (i) the isotope ratio of the parent minerals undergoing weathering, (ii) the partitioning of the element between solute and the new secondary phases, and (iii) the fractionation factors associated with the formation of new secondary phases. Using the example of silicon, we show how all three of these parameters vary along a gradient of erosion rate and regolith residence time defined by three sites located on granitoid bedrock. These sites run from the kinetically limited Rhone Valley in the Central Swiss Alps to the tectonically inactive and supply-limited Sri Lankan highlands, with the Sierra Nevada mountains as a site of intermediate weathering intensity. At each site, primary mineral specific 30Si/28Si ratios span >0.4‰. These minerals weather differentially, such that the isotope ratio of silicon solubilised from rock differs at the three sites and is not necessarily equal to bulk bedrock composition. The partitioning of silicon between secondary clay and solute is reflected in the clay mineralogy and chemical composition: more intense weathering produces Si-poor clays. The clay composition thus comprises a first-order mass-balance control on the extent to which any fractionation factor can be expressed. Finally, the Si isotope fractionation factor associated with clay formation varies systematically with clay mineralogy: the formation of Si-deplete clay minerals is associated with larger fractionation factors. The magnitude of the fractionation may be mechanistically linked to relative aluminium availability. These findings provide the framework needed to use Si isotope ratios as a quantitative proxy to explore Si cycling and reconstruct weathering in the present and past.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"1164 - 1203"},"PeriodicalIF":2.9,"publicationDate":"2021-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49437503","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Patrick J. Frings,Franziska Schubring,Marcus Oelze,Friedhelm von Blanckenburg
Silicon (Si) is an important nutrient for many plant and algae species, and the ultimate source of Si is silicate mineral weathering reactions. These topics have inspired the application of Si isotope geochemistry to quantifying Si cycling in the Critical Zone, though the interpretations are often equivocal. Because germanium (Ge) geochemistry is similar to that of Si, the Ge/Si ratio is considered a tracer that provides additional constraints on Si cycling. Here, we provide Ge/Si ratios for three sites that span a gradient of erosion rates and thus time that material spends in the weathering zone before being removed. We present Ge/Si ratios in bulk rock, soil and saprolite, clay-size fractions, plant biomass, and river water from the Central Swiss Alps, the southern Californian Sierra Nevada, and the highlands of Sri Lanka. Our data perform two functions. First, they provide insight into the Ge/Si system. In particular, we document the presence of a substantial pool of Ge in plant biomass that is not associated with phytoliths, suggesting that overall plants do not discriminate against Ge relative to Si during uptake. We also quantify the preferential incorporation of Ge into clay minerals. We show that Ge/Si ratios in secondary clays may be a better proxy for weathering intensity (the fraction of denudation achieved chemically) than the Ge/Si ratio of river solutes. Ge/Si ratios in secondary clay minerals also perform as well as or even better than silicon isotopes as weathering intensity proxies. Second, the Ge/Si data are used in conjunction with silicon isotope data to develop a catchment Si mass-balance model. It suggests that the export of secondary, fractionated solids (largely clays and plant material) becomes increasingly important at shorter regolith residence times: 80−24+15% of total solubilized Si in the rapidly eroding Alps site, vs. 32−20+22% in the slowly eroding Sri Lanka site. The results also suggest that plant material is a surprisingly large contributor to Si export from these catchments, likely equivalent to 25 to110 % of dissolved Si export.
{"title":"Quantifying biotic and abiotic Si fluxes in the Critical Zone with Ge/Si ratios along a gradient of erosion rates","authors":"Patrick J. Frings,Franziska Schubring,Marcus Oelze,Friedhelm von Blanckenburg","doi":"10.2475/08.2021.03","DOIUrl":"https://doi.org/10.2475/08.2021.03","url":null,"abstract":"Silicon (Si) is an important nutrient for many plant and algae species, and the ultimate source of Si is silicate mineral weathering reactions. These topics have inspired the application of Si isotope geochemistry to quantifying Si cycling in the Critical Zone, though the interpretations are often equivocal. Because germanium (Ge) geochemistry is similar to that of Si, the Ge/Si ratio is considered a tracer that provides additional constraints on Si cycling. Here, we provide Ge/Si ratios for three sites that span a gradient of erosion rates and thus time that material spends in the weathering zone before being removed. We present Ge/Si ratios in bulk rock, soil and saprolite, clay-size fractions, plant biomass, and river water from the Central Swiss Alps, the southern Californian Sierra Nevada, and the highlands of Sri Lanka. Our data perform two functions. First, they provide insight into the Ge/Si system. In particular, we document the presence of a substantial pool of Ge in plant biomass that is not associated with phytoliths, suggesting that overall plants do not discriminate against Ge relative to Si during uptake. We also quantify the preferential incorporation of Ge into clay minerals. We show that Ge/Si ratios in secondary clays may be a better proxy for weathering intensity (the fraction of denudation achieved chemically) than the Ge/Si ratio of river solutes. Ge/Si ratios in secondary clay minerals also perform as well as or even better than silicon isotopes as weathering intensity proxies. Second, the Ge/Si data are used in conjunction with silicon isotope data to develop a catchment Si mass-balance model. It suggests that the export of secondary, fractionated solids (largely clays and plant material) becomes increasingly important at shorter regolith residence times: 80−24+15% of total solubilized Si in the rapidly eroding Alps site, vs. 32−20+22% in the slowly eroding Sri Lanka site. The results also suggest that plant material is a surprisingly large contributor to Si export from these catchments, likely equivalent to 25 to110 % of dissolved Si export.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"850 ","pages":"1204-1245"},"PeriodicalIF":2.9,"publicationDate":"2021-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138504452","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Benjamin J. W. Mills, S. Tennenbaum, D. Schwartzman
The long-term carbon cycle regulates Earth's climate and atmospheric CO2 levels over multimillion-year timescales, but it is not clear that this system has a single steady state for a given input rate of CO2. In this paper we explore the possibility for multiple steady states in the long-term climate system. Using a simple carbon cycle box model, we show that the location of precipitation bands around the tropics and high mid-latitudes, coupled with the response of the terrestrial biosphere to local surface temperature, can result in system bi-stability. Here, maximum CO2 drawdown can occur when either the tropics or high mid-latitudes are at the photosynthetic optimum temperature of around 25°C, and a period of instability can exist between these states. We suggest that this dynamic has influenced climate variations over Phanerozoic time, and that higher steady state surface temperatures may be easier to reach than is commonly demonstrated in simple ‘GEOCARB style’ carbon cycle models.
{"title":"Exploring multiple steady states in Earth's long-term carbon cycle","authors":"Benjamin J. W. Mills, S. Tennenbaum, D. Schwartzman","doi":"10.2475/07.2021.01","DOIUrl":"https://doi.org/10.2475/07.2021.01","url":null,"abstract":"The long-term carbon cycle regulates Earth's climate and atmospheric CO2 levels over multimillion-year timescales, but it is not clear that this system has a single steady state for a given input rate of CO2. In this paper we explore the possibility for multiple steady states in the long-term climate system. Using a simple carbon cycle box model, we show that the location of precipitation bands around the tropics and high mid-latitudes, coupled with the response of the terrestrial biosphere to local surface temperature, can result in system bi-stability. Here, maximum CO2 drawdown can occur when either the tropics or high mid-latitudes are at the photosynthetic optimum temperature of around 25°C, and a period of instability can exist between these states. We suggest that this dynamic has influenced climate variations over Phanerozoic time, and that higher steady state surface temperatures may be easier to reach than is commonly demonstrated in simple ‘GEOCARB style’ carbon cycle models.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"1033 - 1044"},"PeriodicalIF":2.9,"publicationDate":"2021-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47339289","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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