An Oxygen-Isotope Study of Weathering in the Eastern Amazon Basin, Brazil

Bird, Michael I.; Longstaffe, Fred J.; Fyfe, W. S.; Kronberg, B.I.; Kishida, A.

doi:10.1029/gm078p0295

Cited by 18 publications

(5 citation statements)

References 35 publications

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“…About 250 g of bulk rock sample was suspended in DI water and the <0.2 μm size fraction was separated out via a Thermo IEC centrifuge. The <0.2 μm fraction was subsequently sequentially cleaned to remove contaminants: (a) 0.5 mol/L sodium acetate buffer solution to remove carbonates when necessary (Savin & Epstein, 1970); (b) 3% hydrogen peroxide solution to remove organic matter; and (c) ammonium citrate/sodium dithionite solution (80°C) to remove iron oxyhydroxides (Bird et al., 1993; Stern et al., 1997). Samples were then rinsed 5 times with deionized water and dried in oven at 60°C for at least 40 hr.…”

Section: Methodsmentioning

confidence: 99%

Drier Winters Drove Cenozoic Open Habitat Expansion in North America

et al. 2022

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The shift from denser forests to open, grass‐dominated vegetation in west‐central North America between 26 and 15 million years ago is a major ecological transition with no clear driving force. This open habitat transition (OHT) is considered by some to be evidence for drier summers, more seasonal precipitation, or a cooler climate, but others have proposed that wetter conditions and/or warming initiated the OHT. Here, we use published (n = 2,065) and new (n = 173) oxygen isotope measurements (δ18O) in authigenic clays and soil carbonates to test the hypothesis that the OHT is linked to increasing wintertime aridity. Oxygen isotope ratios in meteoric water (δ18Op) vary seasonally, and clays and carbonates often form at different times of the year. Therefore, a change in precipitation seasonality can be recorded differently in each mineral. We find that oxygen isotope ratios of clay minerals increase across the OHT while carbonate oxygen isotope ratios show no change or decrease. This result cannot be explained solely by changes in global temperature or a shift to drier summers. Instead, it is consistent with a decrease in winter precipitation that increases annual mean δ18Op (and clay δ18O) but has a smaller or negligible effect on soil carbonates that primarily form in warmer months. We suggest that forest communities in west‐central North America were adapted to a wet‐winter precipitation regime for most of the Cenozoic, and they subsequently struggled to meet water demands when winters became drier, resulting in the observed open habitat expansion.

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Section: Methodsmentioning

confidence: 99%

Drier Winters Drove Cenozoic Open Habitat Expansion in North America

et al. 2022

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“…hydrogen peroxide solution to remove organic matter; and (3) ammonium citrate/sodium dithionite solution (80°C) to remove iron oxyhydroxides (Bird et al, 1993;Stern et al, 1997). Samples were then rinsed 5 times with deionized water and dried in oven at 60°C…”

Section: Clay Stable Isotopesmentioning

confidence: 99%

Drier winters drove Cenozoic open habitat expansion in North America

Kukla¹,

Rugenstein²,

Ibarra³

et al. 2021

Preprint

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The shift from denser forests to open, grass-dominated vegetation in west-central North America between 26 and 15 million years ago is a major ecological transition with no clear driving force. This open habitat transition (OHT) is considered by some to be evidence for drier summers, more seasonal precipitation, or a cooler climate, but others have proposed that wetter conditions and/or warming initiated the OHT. Here, we use published (n=2065) and new (n=173) oxygen isotope measurements (δ18O) in authigenic clays and soil carbonates to test the hypothesis that the OHT is linked to increasing wintertime aridity.Oxygen isotope ratios in meteoric water (δ18Op) vary seasonally, and clays and carbonates often form at different times of the year. Therefore, a change in precipitation seasonality can be recorded differently in each mineral. We find that oxygen isotope ratios of clay minerals increase across the OHT while carbonate oxygen isotope ratios show no change or decrease. This result cannot be explained solely by changes in global temperature or a shift to drier summers. Instead, it is consistent with a decrease in winter precipitation that increases annual mean δ18Op (and clay δ18O) but has a smaller or negligible effect on soil carbonates that primarily form in warmer months. We suggest that forest communities in west-central North America were adapted to a wet-winter precipitation regime for most of the Cenozoic, and they subsequently struggled to meet water demands when winters became drier, resulting in the observed open habitat expansion.

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“…In many cases, a vertical distribution of several generations of secondary minerals is observed in the same profile (e.g. Bird et al, 1993;Muller et al, 1995;Girard et al, 2000Girard et al, , 2002Balan et al, 2005Balan et al, , 2007Fritsch et al, 2011). Absolute dating of laterites from previous studies revealed that they have Mesozoic and Cenozoic ages in India, Africa or South America (Vasconcelos et al, 1994(Vasconcelos et al, , 2015Balan et al, 2005;Bonnet et al, 2016;Allard et al, 2018).…”

Section: Introductionmentioning

confidence: 95%

Unraveling weathering episodes in Tertiary regoliths by kaolinite dating (Western Ghats, India)

et al. 2019

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Secondary minerals in soils can record climatic changes affecting continental surfaces over geological times. Their dating should refine our present knowledge about their potential periods of formation as well as their relations with the ongoing change of climate and erosion/weathering regimes. In the present study, twenty kaolinite samples from two lateritic profiles of the Karnataka plateau, an intensively studied area in the southern India, have been dated using electron paramagnetic resonance (EPR) spectroscopy. Kaolinite ages vary between 0.229 ±0.24 Ma to 40.73 ±15.37 Ma. Four different groups of age can be identified with ages clustered around 1.0, 3.5, 9.0 and 39.0 Ma. These groups of age indicate local preferential weathering periods that coincide with distinct Indian climatic events described in independent studies, such as monsoon strengthening. Thus, regional or subcontinental factors likely prevailed over global forcing in the imprint of climatic events in the regolith profiles. These results confirm that despite their simple mineralogy, laterites can contain several relictual and coexisting generations of secondary minerals and that EPR dating of kaolinite contributes to unraveling the complex history of continental surfaces over geological periods.

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An Oxygen-Isotope Study of Weathering in the Eastern Amazon Basin, Brazil

Cited by 18 publications

References 35 publications

Drier Winters Drove Cenozoic Open Habitat Expansion in North America

Drier Winters Drove Cenozoic Open Habitat Expansion in North America

Drier winters drove Cenozoic open habitat expansion in North America

Unraveling weathering episodes in Tertiary regoliths by kaolinite dating (Western Ghats, India)

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