Clemens
Steven C.
Clemens
Steven C.
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ArticleWhat can we learn from X-ray fluorescence core scanning data? A paleomonsoon case study(American Geophysical Union, 2020-01-12) Gebregiorgis, Daniel ; Giosan, Liviu ; Hathorne, Ed C. ; Anand, Pallavi ; Nilsson-Kerr, Katrina ; Plass, Anna ; Luckge, Andreas ; Clemens, Steven C. ; Frank, MartinX‐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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ArticleRemote and local drivers of Pleistocene South Asian summer monsoon precipitation: a test for future predictions(American Association for the Advancement of Science, 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.