Greenhouse Gas and Ice Volume Drive Pleistocene Indian Summer Monsoon Precipitation Isotope Variability

Abstract

Orbital‐scale Indian Summer Monsoon variability is often interpreted as a direct response to northern hemisphere summer insolation. Here we present a continuous (0–640 kyr) orbital scale precipitation isotope (δDprecip) record using leaf wax δD from the core monsoon zone of India. The δDprecip record is quantitatively coherent with, and δDprecip minima in phase with, greenhouses gas maxima, and ice volume minima across all orbital bands. The δDprecip record is also coherent and in phase with the two existing orbital‐scale Indian speleothem δ18O records, demonstrating a consistent regional response among independent proxies. These findings preclude interpretation of Indian precipitation isotope records as a direct response to northern hemisphere summer insolation. Rather, they dominantly reflect changes in moisture source and transport paths associated with changes in greenhouse gases and ice volume. The orbital‐scale precipitation isotope responses of the Indian and East Asian monsoon systems are uncoupled and are driven by different forcings.

Glacial‐Interglacial Shifts Dominate Tropical Indo‐Pacific Hydroclimate During the Late Pleistocene

“…A recent study similarly showed that  18 O speleo records from eastern China are spectrally distinct from precipitation isotope records in the Indian monsoon domain, including the Bay of Bengal record used here (McGrath et al, 2021). Our SSA results confirm the dominance of 100 ky variability in the Bay of Bengal as presented by McGrath et al (2021), who conclude that the Indian monsoon and EAM are decoupled during the late Pleistocene. The distinct spectral configurations between these regions (Figures 3e and 3f) contradicts the theory that eastern China  18 O speleo reflects upstream moisture sources from the Indian monsoon region (Pausata et al, 2011;Battisti et al, 2014).…”

“…Our SSA highlights that the Chinese cave isotopic signal is unique in the context of data from not only the Indian monsoon region (McGrath et al, 2021), but also the IPWP and South China Sea, which are dominated by 100 ky cycles (Figure 3). While a full diagnosis of the dynamics behind the nearly "pure" precessional signature of China  18 O speleo is outside the scope of this paper, we hypothesize that it must be related to a mid-latitude mechanism; otherwise, we would expect its tempo to more closely match that of tropical moisture from the IPWP and Indian monsoon regions.…”

“…fluxes and pathways (DiNezio et al, 2018;Griffiths et al, 2009). Glacial cycles have a 100 ky pacing over the late Pleistocene, related to the rhythmic timing of high latitude ice-volume growth and termination (Lisiecki & Raymo, 2005 from sediment cores are powerful hydrologic tracers and are commonly used to reconstruct hydroclimate in the IPWP (Ayliffe et al, 2013;Niedermeyer et al, 2014), the Indian- (Kathayat et al, 2016;McGrath et al, 2021), and the East Asian-monsoon regions (Liu et al, 2014;Wang et al, 2001). Precipitation isotopes vary with moisture transport, rainfall, and convective processes (Conroy et al, 2013;Kurita, 2013;Midhun et al, 2018;Moore et al, 2014), thus complicating interpretations of  18 O speleo and  D wax .…”

Remote and local drivers of Pleistocene South Asian summer monsoon precipitation: A test for future predictions

“…4C). The three continental precipitation isotope records cluster near IVGHG (−71° ± 9°), indicating strong sensitivity to these internal drivers (49). This phasing is consistent with previous interpretations of these records and with simulations indicating more distal sources, longer transport paths, and lighter precipitation  18 O during Pmin compared to Pmax (52), predominantly reflecting large-scale circulation versus more local to regional rainfall on June 13, 2021 http://advances.sciencemag.org/ Downloaded from amount.…”

“…Bittoo and XBL  18 O are interpreted as representative of both the South and East Asian (EA) summer monsoons and as responding directly to NH summer insolation (12). In contrast, a longer, more continuous NW BoB reconstruction of South Asian precipitation isotopic composition derived from leaf wax  2 H ( 2 H precip ) documents strong sensitivity to internal drivers including CO 2 and IV (49). Below, we examine phase relationships and model simulations, providing evidence that South Asian precipitation and runoff lag changes in the isotopic composition of precipitation (speleothem  18 O and leaf wax  2 H) and are sensitive to both IVGHG boundary conditions and SH moisture export associated with large-scale wind patterns.…”