Clemens Steven C.

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Clemens
First Name
Steven C.
ORCID
0000-0002-1136-7815

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Preprint

Processes controlling the geochemical composition of the South China Sea sediments during the last climatic cycle

2008-09 , Sun, Youbin , Wu, Feng , Clemens, Steven C. , Oppo, Delia W.

Sediments of the upper 28.2 meters of Ocean Drilling Program (ODP) Site 1145 from the northern South China Sea (SCS) were analyzed for their geochemical composition. Most of the major and trace elements exhibit significant fluctuations at glacial-interglacial scales, implying a close relation with regional and global climate change. Al-normalized elemental ratios can be subdivided into three principal components (PC). PC1 (e.g., Ca/Al, Ba/Al, Sr/Al) displays significant glacial-interglacial variation and is related to paleoproductivity in the northern SCS. PC2 (e.g., K/Al, Mg/Al, Rb/Al) is associated with the degree of chemical weathering in the source regions and shows little glacial-interglacial variation. PC3 (e.g., Ti/Al, Zr/Al) reflects the relative contribution of coarse- and fine-grained materials in the terrigenous components of the SCS sediments, likely associated with changes in sea level and monsoon-induced fluvial input. Spectral analyses indicate that paleoproductivity (i.e., Ba/Al) in the South China Sea lags Hulu/Sanbao speleothem δ18O record (a indicator of annual average meteoric precipitation) by 102° and Indian summer monsoon (multi-proxy stack) by 23° at the precession band, indicating a close relationship with the Indian summer monsoon. However, the chemical weathering degree in the source area (PC2) is not sensitive to monsoon-related changes at the precession band during the last climatic cycle.

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Article

Remote and local drivers of Pleistocene South Asian summer monsoon precipitation: a test for future predictions

2021-06-04 , Clemens, Steven C. , Yamamoto, Masanobu , Thirumalai, Kaustubh , Giosan, Liviu , Richey, Julie N. , Nilsson-Kerr, Katrina , Rosenthal, Yair , Anand, Pallavi , McGrath, Sarah M.

South Asian precipitation amount and extreme variability are predicted to increase due to thermodynamic effects of increased 21st-century greenhouse gases, accompanied by an increased supply of moisture from the southern hemisphere Indian Ocean. We reconstructed South Asian summer monsoon precipitation and runoff into the Bay of Bengal to assess the extent to which these factors also operated in the Pleistocene, a time of large-scale natural changes in carbon dioxide and ice volume. South Asian precipitation and runoff are strongly coherent with, and lag, atmospheric carbon dioxide changes at Earth’s orbital eccentricity, obliquity, and precession bands and are closely tied to cross-equatorial wind strength at the precession band. We find that the projected monsoon response to ongoing, rapid high-latitude ice melt and rising carbon dioxide levels is fully consistent with dynamics of the past 0.9 million years.

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Article

A synthesis of monsoon exploration in the Asian marginal seas

2022-10-28 , Clift, Peter D , Betzler, Christian , Clemens, Steven C , Christensen, Beth , Eberli, Gregor P , France-Lanord, Christian , Gallagher, Stephen J. , Holbourn, Ann , Kuhnt, Wolfgang , Murray, Richard W , Rosenthal, Yair , Tada, Ryuji , Wan, Shiming

The International Ocean Discovery Program (IODP) conducted a series of expeditions between 2013 and 2016 that were designed to address the development of monsoon climate systems in Asia and Australia. Significant progress was made in recovering Neogene sections spanning the region from the Arabian Sea to the Sea of Japan and southward to western Australia. High recovery by advanced piston corer (APC) has provided a host of semi-continuous sections that have been used to examine monsoonal evolution. Use of the half-length APC was successful in sampling sand-rich sediment in Indian Ocean submarine fans. The records show that humidity and seasonality developed diachronously across the region, although most regions show drying since the middle Miocene and especially since ∼ 4 Ma, likely linked to global cooling. A transition from C3 to C4 vegetation often accompanied the drying but may be more linked to global cooling. Western Australia and possibly southern China diverge from the general trend in becoming wetter during the late Miocene, with the Australian monsoon being more affected by the Indonesian Throughflow, while the Asian monsoon is tied more to the rising Himalaya in South Asia and to the Tibetan Plateau in East Asia. The monsoon shows sensitivity to orbital forcing, with many regions having a weaker summer monsoon during times of northern hemispheric Glaciation. Stronger monsoons are associated with faster continental erosion but not weathering intensity, which either shows no trend or a decreasing strength since the middle Miocene in Asia. Marine productivity proxies and terrestrial chemical weathering, erosion, and vegetation proxiesare often seen to diverge. Future work on the almost unknown Paleogene is needed, as well as the potential of carbonate platforms as archives of paleoceanographic conditions.

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Article

Southern Hemisphere forcing of South Asian monsoon precipitation over the past ~1 million years

2018-11-08 , Gebregiorgis, Daniel , Hathorne, Ed C. , Giosan, Liviu , Clemens, Steven C. , Nürnberg, Dirk , Frank, Martin

The orbital-scale timing of South Asian monsoon (SAM) precipitation is poorly understood. Here we present new SST and seawater δ18O (δ18Osw) records from the Bay of Bengal, the core convective region of the South Asian monsoon, over the past 1 million years. Our records reveal that SAM precipitation peaked in the precession band ~9 kyrs after Northern Hemisphere summer insolation maxima, in phase with records of SAM winds in the Arabian Sea and eastern Indian Ocean. Precession-band variance, however, accounts for ~30% of the total variance of SAM precipitation while it was either absent or dominant in records of the East Asian monsoon (EAM). This and the observation that SAM precipitation was phase locked with obliquity minima and was sensitive to Southern Hemisphere warming provides clear evidence that SAM and EAM precipitation responded differently to orbital forcing and highlights the importance of internal processes forcing monsoon variability.

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Article

What can we learn from X-ray fluorescence core scanning data? A paleomonsoon case study

2020-01-12 , Gebregiorgis, Daniel , Giosan, Liviu , Hathorne, Ed C. , Anand, Pallavi , Nilsson-Kerr, Katrina , Plass, Anna , Luckge, Andreas , Clemens, Steven C. , Frank, Martin

X‐ray fluorescence (XRF) core scanning of marine and lake sediments has been extensively used to study changes in past environmental and climatic processes over a range of timescales. The interpretation of XRF‐derived element ratios in paleoclimatic and paleoceanographic studies primarily considers differences in the relative abundances of particular elements. Here we present new XRF core scanning data from two long sediment cores in the Andaman Sea in the northern Indian Ocean and show that sea level related processes influence terrigenous inputs based proxies such as Ti/Ca, Fe/Ca, and elemental concentrations of the transition metals (e.g., Mn). Zr/Rb ratios are mainly a function of changes in median grain size of lithogenic particles and often covary with changes in Ca concentrations that reflect changes in biogenic calcium carbonate production. This suggests that a common process (i.e., sea level) influences both records. The interpretation of lighter element data (e.g., Si and Al) based on low XRF counts is complicated as variations in mean grain size and water content result in systematic artifacts and signal intensities not related to the Al or Si content of the sediments. This highlights the need for calibration of XRF core scanning data based on discrete sample analyses and careful examination of sediment properties such as porosity/water content for reliably disentangling environmental signals from other physical properties. In the case of the Andaman Sea, reliable extraction of a monsoon signal requires accounting for the sea level influence on the XRF data.

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High-resolution and high-precision correlation of dark and light layers in the Quaternary hemipelagic sediments of the Japan Sea recovered during IODP Expedition 346

2018-03-26 , Tada, Ryuji , Irino, Tomohisa , Ikehara, Ken , Karasuda, Akinori , Sugisaki, Saiko , Xuan, Chuang , Sagawa, Takuya , Itaki, Takuya , Kubota, Yoshimi , Lu, Song , Seki, Arisa , Murray, Richard W. , Alvarez-Zarikian, Carlos A. , Anderson, William T. , Bassetti, Maria-Angela , Brace, Bobbi J. , Clemens, Steven C. , da Costa Gurgel, Marcio H. , Dickens, Gerald R. , Dunlea, Ann G. , Gallagher, Stephen J. , Giosan, Liviu , Henderson, Andrew C. G. , Holbourn, Ann E. , Kinsley, Christopher W. , Lee, Gwang Soo , Lee, Kyung Eun , Lofi, Johanna , Lopes, Christina I. C. D. , Saavedra-Pellitero, Mariem , Peterson, Larry C. , Singh, Raj K. , Toucanne, Samuel , Wan, Shiming , Zheng, Hongbo , Ziegler, Martin

The Quaternary hemipelagic sediments of the Japan Sea are characterized by centimeter- to decimeter-scale alternation of dark and light clay to silty clay, which are bio-siliceous and/or bio-calcareous to a various degree. Each of the dark and light layers are considered as deposited synchronously throughout the deeper (> 500 m) part of the sea. However, attempts for correlation and age estimation of individual layers are limited to the upper few tens of meters. In addition, the exact timing of the depositional onset of these dark and light layers and its synchronicity throughout the deeper part of the sea have not been explored previously, although the onset timing was roughly estimated as ~ 1.5 Ma based on the result of Ocean Drilling Program legs 127/128. Consequently, it is not certain exactly when their deposition started, whether deposition of dark and light layers was synchronous and whether they are correlatable also in the earlier part of their depositional history. The Quaternary hemipelagic sediments of the Japan Sea were drilled at seven sites during Integrated Ocean Drilling Program Expedition 346 in 2013. Alternation of dark and light layers was recovered at six sites whose water depths are > ~ 900 m, and continuous composite columns were constructed at each site. Here, we report our effort to correlate individual dark layers and estimate their ages based on a newly constructed age model at Site U1424 using the best available paleomagnetic datum and marker tephras. The age model is further tuned to LR04 δ18O curve using gamma ray attenuation density (GRA) since it reflects diatom contents that are higher during interglacial high-stands. The constructed age model for Site U1424 is projected to other sites using correlation of dark layers to form a high-resolution and high-precision paleo-observatory network that allows to reconstruct changes in material fluxes with high spatio-temporal resolutions.

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