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

Abstract

We reconstructed the variability of the Earth's strongest hydrological system, the Indian monsoon, over the interval 6.24 to 4.91 Ma at International Ocean Discovery Program (IODP) Expedition 353 Site U1448 in the Andaman Sea. We integrated high-resolution benthic and planktic foraminiferal carbon and oxygen isotopes with Mg/Ca measurements of the mixed layer foraminifer Trilobatus sacculifer to reconstruct the isotopic composition of seawater (δ 18 O sw) and the gradient between planktic and benthic foraminiferal δ 13 C. A prominent increase in mixed layer temperatures of~4°C occurred between 5.55 and 5.28 Ma, accompanied by a change from precession-to obliquity-driven variability in planktic δ 18 O and δ 18 O sw. We suggest that an intensified cross-equatorial transport of heat and moisture, paced by obliquity, led to increased summer monsoon precipitation during warm stages after 5.55 Ma. Transient cold stages were characterized by reduced mixed layer temperatures and summer monsoon failure, thus resembling late Pleistocene stadials. In contrast, an overall cooler background climate state with a strengthened biological pump prevailed prior to 5.55 Ma. These findings highlight the importance of internal feedback processes for the long-term evolution of the Indian monsoon.

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

“…11h), potentially linked to an increase in monsoon rainfall intensity and global cooling . Between 6.2 and 5 Ma, our equatorial Indian Ocean wind records show good long-term agreement with a seawater δ 18 O record 690 from the Andaman Sea (Jöhnck et al, 2020), with a minimum in export productivity at ~5.3 Ma at Site U1443 coinciding with a maximum in seawater δ 18 O at Site U1448 (Fig. 11i, j).…”

“…8 as one grey band spanning 22-24 kyr) are primary periods of Earth's precession, whereas the 53-kyr and 41-kyr periods are related to 560 Earth's obliquity (Laskar et al, 2004). seawater δ 18 O in the nearby Andaman Sea (Jöhnck et al, 2020). These authors suggest that, prior to 565 a distinct switch to obliquity-driven variability around 5.55 Ma, their records reflect strong precessional (insolation) control on South Asian monsoon rainfall from 6.2-5.55 Ma, with significant phase lags between proxies 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 110 suggest high-amplitude precession and obliquity forcing of monsoon rainfall with significant phase lags (Jöhnck et al, 2020).…”

“…For example, the 1/24 kyr heterodyne, prominent in all our records, could result from the interference of eccentricity 590 with 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 595 al., 2018), interpreted to suggest a strongly non-linear response of the monsoon to orbital forcing.…”

“…8a-c) could also suggest obliquity control on monsoon runoff into the BOB at this time, inferring a partial decoupling between monsoon winds (controlling open-ocean productivity) and runoff (controlling terrigenous 620 sedimentation and salinity/seawater δ 18 O) on orbital timescales, although additional records from regions closer to river sediment and freshwater sources are needed to corroborate this idea. It is important to note that whilst SAM expression above Site U1443 in the southern BOB is dominated by summer monsoon winds driving surface ocean currents and deeper mixing, oceanographic conditions in the northern and eastern BOB (e.g., Sites U1447 and U1448) are instead primarily 625 controlled by summer monsoon freshwater inputs (Jöhnck et al, 2020;. Runoff and direct precipitation during the SASM lead to strong salinity stratification in the northern parts of the BOB in the late summer and autumn that prevents upper ocean mixing (e.g., Sengupta et al, 2016).…”