5:15 PM - 7:15 PM
[SCG55-P19] The temporal variation of Sr-Nd isotopic composition through Cenozoic recorded in subseafloor sediment core from south Atlantic Ocean
Keywords:South Atlantic Transect , Sediment Core, Sr-Nd Isotope, Cenozoic Climate Change, IODP Exp. 390/393
1. Introduction
In the IODP Expedition 390/393, subseafloor cores were collected from the South Atlantic Ocean. The core from the Hole C in the site U1556 (280.3 m length) was almost continuous sediments. The age was estimated Paleocene (ca. 61.2 Ma)–Quaternary by calcareous nannofossils. This core included much amount of terrigenous sediments (Coggon et al., 2022, 2024), Kaplan et al. (2024) suggests that they were originated from the South America and the Africa continents.
In this study, we measured Sr-Nd isotopic compositions of these sediments and compared the results with the clay mineral compositions by Kim et al. (2024).
2. Analytical samples and methods
We collected 28 samples from U1556C core, which include over 10% of total clay minerals. These samples cover the following climatic events; Paleocene/Eocene Thermal Maximum and following Early Eocene hyperthermals (ca. 56–49 Ma), Eocene/Oligocene Transition (Oi-1 glaciation, ca. 34 Ma), Oligocene/Miocene Transition and following Mi-1 glaciation (ca. 23-22 Ma), Middle Miocene Climatic Optimum and following glaciation (ca. 17–14 Ma), and Quaternary cooling. For the removing the seawater Sr isotopic evolution, the powdered samples were firstly leached carbonate materials using 1 mol/L HCl then the samples were perfectly dissolved using HF, HNO3 and HCl. Elements Sr and Nd were purified through Sr-spec and Ln-spec resins. Finally, Sr and Nd isotopic ratios were measured by the MC-ICP-MS installed in the University of the Ryukyus, Japan (Thermo Scientific, Neptune Plus).
3. The temporal variations of Sr-Nd isotopic ratios and clay mineral contents
The results of 87Sr/86Sr and 143Nd/144Nd show the reverse correlation and they suggest to originate from continental crust. The thermal periods tend to display high 87Sr/86Sr (low 143Nd/144Nd) and the cold periods show opposite trend. This trend is apparently different to Sr isotopic evolution of ocean thus the results reflect the provenance change of terrigenous grains.
The clay mineral compositions (Kim et al., 2024), especially illite and smectite, are also correlated with Sr-Nd isotopic ratios. The contents of illite and smectite show the reverse correlation (r2=0.5), and illite vs. 87Sr/86Sr and smectite vs. 143Nd/144Nd also have positive correlation. These correlations mean that the Sr-Nd isotopic compositions reflect the source change of illite and smectite.
In general, illite is showing the physical weathering under cold period without chemical weathering on the hinterland and smectite is well formed by chemical weathering under the seasonal rainy climate (e.g., Robert & Kennett, 1994; Diester-Haass et al., 1996). However, illite content is greater during thermal period and smectite content is greater during rapid cooling event, such as Oi-1 and Mi-1, in U1556C core. These mismatches suggest that the types of clay minerals may reflect the source geology. For example, smectite might be derived from Mesozoic mafic intrusive rocks in central South America and illite was derived from a craton in the south Africa. In the Sr-Nd isotopic data, the terrigenous sediments tend to be lower 87Sr/86Sr and higher 143Nd/144Nd in South America than Africa (Grousset & Biscaye, 2005). The positive correlation between 87Sr/86Sr and illite content (or 143Nd/144Nd and smectite content) supports this hypothesis.
Furthermore, the strength of correlation between isotopic composition and illite/smectite contents are changed around 35 Ma. This implies that the influxes of sediments from source regions were changed around that time.
<References>
Coggon, R.M. et al. (2022), IODP Exp.390 Preliminary Report.
Coggon, R.M. et al. (2024), Proceedings of IODP 390/393.
Diester-Haass, L. et al. (1996), Marine Geology, 131, 123-149.
Gousset, F.E. & Biscaye, P.E. (2005), Chemical Geology, 222, 149-167.
Kaplan, M. et al. (2024), Abstract for The South Atlantic Transect 2nd Post-Expedition Meeting.
Kim, S. et al. (2024), Abstract for The South Atlantic Transect 2nd Post-Expedition Meeting.
Robert, C. & Kennett, J.P. (1994), Geology, 22, 211-214.
In the IODP Expedition 390/393, subseafloor cores were collected from the South Atlantic Ocean. The core from the Hole C in the site U1556 (280.3 m length) was almost continuous sediments. The age was estimated Paleocene (ca. 61.2 Ma)–Quaternary by calcareous nannofossils. This core included much amount of terrigenous sediments (Coggon et al., 2022, 2024), Kaplan et al. (2024) suggests that they were originated from the South America and the Africa continents.
In this study, we measured Sr-Nd isotopic compositions of these sediments and compared the results with the clay mineral compositions by Kim et al. (2024).
2. Analytical samples and methods
We collected 28 samples from U1556C core, which include over 10% of total clay minerals. These samples cover the following climatic events; Paleocene/Eocene Thermal Maximum and following Early Eocene hyperthermals (ca. 56–49 Ma), Eocene/Oligocene Transition (Oi-1 glaciation, ca. 34 Ma), Oligocene/Miocene Transition and following Mi-1 glaciation (ca. 23-22 Ma), Middle Miocene Climatic Optimum and following glaciation (ca. 17–14 Ma), and Quaternary cooling. For the removing the seawater Sr isotopic evolution, the powdered samples were firstly leached carbonate materials using 1 mol/L HCl then the samples were perfectly dissolved using HF, HNO3 and HCl. Elements Sr and Nd were purified through Sr-spec and Ln-spec resins. Finally, Sr and Nd isotopic ratios were measured by the MC-ICP-MS installed in the University of the Ryukyus, Japan (Thermo Scientific, Neptune Plus).
3. The temporal variations of Sr-Nd isotopic ratios and clay mineral contents
The results of 87Sr/86Sr and 143Nd/144Nd show the reverse correlation and they suggest to originate from continental crust. The thermal periods tend to display high 87Sr/86Sr (low 143Nd/144Nd) and the cold periods show opposite trend. This trend is apparently different to Sr isotopic evolution of ocean thus the results reflect the provenance change of terrigenous grains.
The clay mineral compositions (Kim et al., 2024), especially illite and smectite, are also correlated with Sr-Nd isotopic ratios. The contents of illite and smectite show the reverse correlation (r2=0.5), and illite vs. 87Sr/86Sr and smectite vs. 143Nd/144Nd also have positive correlation. These correlations mean that the Sr-Nd isotopic compositions reflect the source change of illite and smectite.
In general, illite is showing the physical weathering under cold period without chemical weathering on the hinterland and smectite is well formed by chemical weathering under the seasonal rainy climate (e.g., Robert & Kennett, 1994; Diester-Haass et al., 1996). However, illite content is greater during thermal period and smectite content is greater during rapid cooling event, such as Oi-1 and Mi-1, in U1556C core. These mismatches suggest that the types of clay minerals may reflect the source geology. For example, smectite might be derived from Mesozoic mafic intrusive rocks in central South America and illite was derived from a craton in the south Africa. In the Sr-Nd isotopic data, the terrigenous sediments tend to be lower 87Sr/86Sr and higher 143Nd/144Nd in South America than Africa (Grousset & Biscaye, 2005). The positive correlation between 87Sr/86Sr and illite content (or 143Nd/144Nd and smectite content) supports this hypothesis.
Furthermore, the strength of correlation between isotopic composition and illite/smectite contents are changed around 35 Ma. This implies that the influxes of sediments from source regions were changed around that time.
<References>
Coggon, R.M. et al. (2022), IODP Exp.390 Preliminary Report.
Coggon, R.M. et al. (2024), Proceedings of IODP 390/393.
Diester-Haass, L. et al. (1996), Marine Geology, 131, 123-149.
Gousset, F.E. & Biscaye, P.E. (2005), Chemical Geology, 222, 149-167.
Kaplan, M. et al. (2024), Abstract for The South Atlantic Transect 2nd Post-Expedition Meeting.
Kim, S. et al. (2024), Abstract for The South Atlantic Transect 2nd Post-Expedition Meeting.
Robert, C. & Kennett, J.P. (1994), Geology, 22, 211-214.