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

Abstract

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 in the form of 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 monsoon variability 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 part of the Bay of Bengal spanning the late Miocene (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. Results 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 in the equatorial sector of the Indian Ocean, with no apparent intensification over the latest Miocene.

Drivers of Late Miocene Tropical Sea Surface Cooling: A New Perspective From the Equatorial Indian Ocean

“…All SST records were recalculated using the same calibration and Mg/Ca sw correction as in Figure 2, the thick curves are 10% Lowess filters. (d) Site U1443 benthic ( Cibicidoides wuellerstorfi ) δ 13 C (Bolton et al., 2022) and (e) Site U1443 benthic δ 18 O (Bolton et al., 2022) with Thvera–Gilbert (TG) cold stages marked. Blue shading represents the Late Miocene Global Cooling (LMGC) interval and gray shading represents the late Miocene carbon isotope shift (LMCIS).…”

“…Site U1443 samples used in this study are from the revised shipboard splice comprised between 122.6 and 70.06 m CCSF‐A (Core Composite depth below Sea Floor), from Core U1443B‐7H‐5W to Core U1443C‐15H‐4W, spanning the late Miocene and earliest Pliocene (9–5 Ma; Bolton et al., 2022). One‐centimeter‐thick half‐rounds were sampled from the core working halves.…”

“…Sediments are dominated by calcareous nannofossil ooze with well-preserved shells of foraminifera (∼70-80% CaCO 3 with slightly increasing clay content upcore; Clemens et al, 2016). A detailed analysis of carbonate content and biogenic sediment mass accumulation for the same late Miocene splice interval is presented in Bolton et al (2022). We apply the age model of Bolton et al (2022), based on revised biostratigraphy and tuning of benthic foraminiferal δ 18 O to an eccentricity-tilt target, which results in sedimentation rates between 0.5 and 1.9 cm/kyr.…”

Enhanced Late Miocene Chemical Weathering and Altered Precipitation Patterns in the Watersheds of the Bay of Bengal Recorded by Detrital Clay Radiogenic Isotopes

“…Long-term precision is better than ±0.05‰ for δ 13 C based on an analysis of international and in-house carbonate standards and results calibrated using the National Institute of Standard and Technology (NIST) carbonate isotope standard and NBS (National Bureau of Standard) 19, and are reported on the Vienna PeeDee Belemnite (VPDB) scale. The full resolution benthic stable isotope record for the late Miocene is presented in Bolton et al (2021).…”

“…The full resolution benthic stable isotope record for the late Miocene is presented in Bolton et al. (2021).…”