Two-phase structure of tropical hydroclimate during Heinrich Stadial 1 and its global implications

Abstract

Previous studies have proposed that the two-phase structure of Heinrich Stadial 1 (HS1) is a ubiquitous feature. However, few studies have focused on the internal structure of HS1 over a tropical domain; thus, the interhemispheric ocean-atmosphere teleconnections during HS1 are poorly defined. Here, we present high-resolution sedimentological, geochemical, and palaeoceanographic records from the core CG2 in the southern South China Sea (SCS), off the northern Sunda Shelf. The core continuously spans 9.0–22.0 ky BP. The records suggest two episodes of increased monsoon rainfall during the pre-HS1 (∼19.0–18.0 ky BP) and in the early HS1 (17.5–16.1 ky BP), along with a weaker monsoon rainfall during the late HS1 (16.1–14.7 ky BP). In contrast, a lower precipitation over the Flores Sea occurred during the early HS1, off the southern Sunda Shelf, followed by an enhanced precipitation runoff during the late HS1 (Muller et al., 2012). Notably, these two different precipitation phases characterise a southward Intertropical Convergence Zone (ITCZ) shift over the Sunda Shelf during HS1. These changes are also consistent with other monsoon records, as well as other available data from the tropical to high latitudes of both hemispheres, supporting the global scope of the twofold HS1. We propose that the two episodes of intense monsoon rainfall over the northern Sunda Shelf during the pre-HS1 and the early HS1 observed in our study are closely linked to two different melting events associated with European ice sheet (EIS) retreat. We further propose that the subsequent maximum Atlantic meridional overturning circulation (AMOC) reduction and the migration of the ITCZ to its southernmost position during the late HS1 are tightly connected with the destabilization of the marine-terminating ice-streams of the Laurentide ice sheet (LIS) in the North Atlantic. Tropical positive feedbacks during HS1, in turn, should have played an important role in sustaining the AMOC reduction and Northern Hemisphere (NH) stadial conditions, by continuously lowering the salinity of the low-latitude currents in the Atlantic Ocean. In addition, the tropical ITCZ appears to have been a key bridge in promoting interhemispheric climate connections, by which North Atlantic cooling was rapidly linked to increased Southern Ocean surface westerlies. This, in turn, drove upwelling and CO2 ventilation from the Southern Ocean, and thereby facilitated deglacial warming. Finally, these findings show that tropical climate variation is an important component of deglacial climate changes; understanding how it responds to millennial events like HS1 may help us better understand global teleconnections.