Preprints
https://doi.org/10.5194/cp-2021-77
https://doi.org/10.5194/cp-2021-77

  29 Jun 2021

29 Jun 2021

Review status: a revised version of this preprint is currently under review for the journal CP.

Secular and orbital-scale variability of equatorial Indian Ocean summer monsoon winds during the late Miocene

Clara T. Bolton1, Emmeline Gray1,a, Wolfgang Kuhnt2, Ann E. Holbourn2, Julia Lübbers2, Katharine Grant3, Kazuyo Tachikawa1, Gianluca Marino3,4, Eelco J. Rohling3,5, Anta-Clarisse Sarr1, and Nils Andersen6 Clara T. Bolton et al.
  • 1Aix Marseille Univ, CNRS, IRD, INRAE, Coll France, CEREGE, Aix-en-Provence, France
  • 2Institute of Geosciences, University of Kiel, D-24118 Kiel, Germany
  • 3Research School of Earth Sciences, Australian National University, Canberra ACT 2601, Australia
  • 4Centro de Investigación Mariña, Universidade de Vigo, GEOMA, Palaeoclimatology Lab, Vigo, 36310, Spain
  • 5Ocean and Earth Science, University of Southampton, Southampton SO14 3ZH, UK
  • 6Leibniz Laboratory for Radiometric Dating and Stable Isotope Research, University of Kiel, D-24118 Kiel, Germany
  • anow at: School of Environment, Earth and Ecosystem Sciences, The Open University, Milton Keynes, UK

Abstract. In the modern northern Indian Ocean, biological productivity is intimately linked to near-surface oceanographic dynamics forced by the South Asian, or Indian, monsoon. In the late Pleistocene, this strong seasonal signal is transferred to the sedimentary record as strong variance in the precession band (19–23 kyr) because precession dominates low-latitude insolation variations and drives seasonal contrast in oceanographic conditions. In addition, internal climate system feedbacks (e.g. ice-sheet albedo, carbon cycle, topography) play a key role in monsoon variability. Little is known about orbital-scale variability of the monsoon in the pre-Pleistocene, when atmospheric CO2 levels and global temperatures were higher. In addition, many questions remain open regarding the timing of the initiation and intensification of the South Asian monsoon during the Miocene, an interval of significant global climate change that culminated in bipolar glaciation. Here, we present new high-resolution (< 1 kyr) records of export productivity and sediment accumulation from International Ocean Discovery Program Site U1443 in the southernmost Bay of Bengal spanning the late Miocene and earliest Pliocene (9 to 5 million years ago). Underpinned by a new orbitally-tuned benthic isotope stratigraphy, we use X-Ray Fluorescence-derived biogenic barium variations to discern productivity trends and rhythms. Our data show strong eccentricity-modulated precession-band productivity variations throughout the late Miocene, interpreted to reflect insolation forcing of summer monsoon wind strength in the equatorial Indian Ocean. On long timescales, our data support the interpretation that South Asian monsoon winds were already established by 9 Ma, with no apparent intensification over the late Miocene.

Clara T. Bolton et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2021-77', David De Vleeschouwer, 13 Jul 2021
  • RC2: 'Comment on cp-2021-77', Mitch Lyle, 28 Aug 2021

Clara T. Bolton et al.

Clara T. Bolton et al.

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Short summary
The timing of the initiation and evolution of the South Asian monsoon in the geological past is a subject of debate. Here, we present a new age model spanning the late Miocene (9 to 5 million years ago) and high-resolution records of past open-ocean biological productivity from the equatorial Indian Ocean that we interpret to reflect monsoon wind strength. Our data show no long-term intensification, however strong orbital periodicities suggest insolation forcing of monsoon wind strength.