Three-phased Heinrich Stadial 4 recorded in NE Brazil stalagmites

The growth intervals of stalagmites from Northeast (NE) Brazil provide precise information on the timing of pluvial periods associated with the southward shift of the Intertropical Convergence Zone (ITCZ) during Heinrich Stadials (HS). Using ²³⁰ Th dating and annual band counting, we constrained the timing of the pluvial period associated with HS4 to occur between 40.06 ± 0.11 and 38.38 ± 0.10 thousand years before present (kyr B.P.), coinciding with the interval between the end of Greenland Interstadial (GIS)/Chinese Interstadial (CIS) 9 and GIS/CIS 8. Oxygen isotope analysis shows a close anti-correlation between NE Brazil wet periods and East Asian monsoon intensity recorded in Hulu Cave, supporting the hypothesis of a southerly migration of the global ITCZ during HS4. The pluvial anomaly can be divided into three phases, starting with a precursor, less intense pluvial interval (phase 1) between 40.06 ± 0.11 and 39.59±0.10 kyr B.P., likely correlative to the cool phase of the North Atlantic immediately before the partial collapse of the Laurentide Ice Sheet (LIS). This phase ends abruptly (<30 yr) and is followed by an intense pluvial phase (phase 2) that spans approximately 5 centuries. Luminescent couplets are identified in all samples between 39.50 ± 0.10 and 39.41 ± 0.10 kyr B.P., and are interpreted as a period of two rainy seasons per year during which the southernmost extent of the inland West Atlantic ITCZ reached south of our study site (10°S). Following the end of phase 2 at 39.07 ± 0.32 kyr B.P., intermittent speleothem growth suggests intermittent rainfall over NE Brazil (phase 3) until the abrupt onset of GIS/CIS 8 at 38.38 ± 0.10 kyr B.P. The phases identified in NE Brazilian stalagmites agree with the three-phased variation in low-latitude proxies registered in Northern Greenland, and are consistent with changes in methane concentrations recorded in the West Antarctic Ice Sheet (WAIS) Divide. The synchronicity of these distant records suggests a multi-phased response of tropical atmospheric circulation during HS4. The timescales surrounding the LIS collapse, as inferred from our precisely dated stalagmites, are on the same order as recent predictions for the impending collapse of the WAIS and may therefore serve as a past analogue.