Miocene to Pleistocene Palaeoceanography of the Andaman Region: Evolution of the Indian Monsoon on a Warmer-Than-Present Earth

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

“…For example, the 1/24 kyr heterodyne, prominent in all our records, could result from the interference between eccentricity and precession, and the 1/30 kyr heterodyne seen in Ba records from an interaction between obliquity and precession. Several of these heterodynes have been previously identified in spectra of seawater δ 18 O that reflect Asian monsoon precipitation and runoff, both in the Andaman Sea (30 and 130 kyr during the Pleistocene, 27 and 30 kyr in the latest Miocene) (Gebregiorgis et al, 2018;Jöhnck et al, 2020) and in the East China Sea (29 and 69 kyr during the Pleistocene) (Clemens et al, 2018), suggesting high sensitivity of the monsoon to orbital forcing. We favour the interpretation that the prominent 24 kyr variability in our records reflects a primary period of precession, because precession filters of [Ba] xs and log(Ba/Fe) spanning 18 to 26 kyr show strong amplitude modulation of the precession signal at a period of ∼ 405 kyr, which results from the interaction of the 23.6 and 22.3 kyr periods (1/[(1/22.3) − (1/23.6)] = 404.8 kyr) (Fig.…”

“…Significant variability at obliquity and precession periods has been identified in high-resolution late Miocene-early Pliocene records of precipitation-runoff based on planktic foraminiferal δ 18 O and seawater δ 18 O in the nearby Andaman Sea (Jöhnck et al, 2020). These authors suggest that, prior to a distinct switch to obliquity-driven variability around 5.55 Ma, their records reflect strong precession (insolation) control on South Asian monsoon rainfall from 6.2-5.55 Ma, with significant phase lags between proxy variations and precession.…”

“…These records suggest that summer monsoon proxies significantly lag Northern Hemisphere summer insolation maxima in the precession and obliquity bands due to climate feedbacks. Orbital control on past SASM strength in the pre-Pleistocene, when boundary conditions were different, has so far only been investigated in the Andaman Sea over the latest Miocene-early Pliocene, where seawater oxygen isotope data suggest high-amplitude precession and obliquity forcing of monsoon rainfall with significant phase lags (Jöhnck et al, 2020).…”

Variability of the Indian Monsoon in the Andaman Sea Across the Miocene‐Pliocene Transition

“…The pre‐Pleistocene variability of Indian monsoonal circulation and precipitation patterns in the Bay of Bengal, the Earth's strongest monsoonal regime, is still poorly understood (e.g., Clemens et al, 2016a; Kuhnt et al, 2020). In particular, the sensitivity of the East Asian and Indian monsoon systems to internal feedback processes such as changes in atmospheric greenhouse gases and ice volume, which affect zonal and meridional temperature gradients and moisture transport, remains a matter of intense debate (e.g., Clemens et al, 2018; Gebregiorgis et al, 2018).…”

“…The deep waters in the Andaman Sea are composed of a mixture of Indian Central and Deep Water from the equatorial and Southern Hemisphere Indian Ocean and local thermocline water including Indonesian Throughflow water generated from North Pacific Intermediate Water modified by intense mixing while flowing through the Indonesian seaways (Gordon, 2005; Tomczak & Godfrey, 2002). Pacific‐derived intermediate and deep water masses may have played a more important role in the Andaman Sea during the late Cenozoic when the connection between the Pacific and Indian Oceans was less restricted (Kuhnt et al, 2020).…”