Warming and glacier recession in the Rakaia valley, Southern Alps of New Zealand, during Heinrich Stadial 1

The termination of the last ice age featured a major reconfiguration of Earth's climate and cryosphere, yet the underlying causes of these massive changes continue to be debated. Documenting the spatial and temporal variations of atmospheric temperature during deglaciation can help discriminate among potential drivers. Here, we present a ¹⁰Be surface-exposure chronology and glaciological reconstruction of ice recession following the Last Glacial Maximum (LGM) in the Rakaia valley, Southern Alps of New Zealand. Innermost LGM moraines at Big Ben have an age of 17,840 ± 240 yrs, whereas ice-marginal moraines or ice-molded bedrock surfaces at distances up-valley from Big Ben of 12.5 km (Lake Coleridge), ∼25 km (Castle Hill), ∼28 km (Double Hill), ∼43 km (Prospect Hill), and ∼58 km (Reischek knob) have ages of 17,020 ± 70 yrs, 17,100 ± 110 yrs, 16,960 ± 370 yrs, 16,250 ± 340 yrs, and 15,660 ± 160 yrs, respectively. These results indicate extensive recession of the Rakaia glacier, which we attribute primarily to the effects of climatic warming. In conjunction with geomorphological maps and a glaciological reconstruction for the Rakaia valley, we use our chronology to infer timing and magnitude of past atmospheric temperature changes. Compared to an overall temperature rise of ∼4.65 °C between the end of the LGM and the start of the Holocene, the glacier recession between ∼17,840 and ∼15,660 yrs ago is attributable to a net temperature increase of ∼4.0 °C (from -6.25 to -2.25 °C), accounting for ∼86% of the overall warming. Approximately 3.75 °C (∼70%) of the warming occurred between ∼17,840 and ∼16,250 yrs ago, with a further 0.75 °C (∼16%) increase between ∼16,250 and ∼15,660 yrs ago. A sustained southward shift of the Subtropical Front (STF) south of Australia between ∼17,800 and ∼16,000 yrs ago coincides with the warming over the Rakaia valley, and suggests a close link between Southern Ocean frontal boundary positions and southern mid-latitude climate. Most of the deglacial warming in the Southern Alps occurred during the early part of Heinrich Stadial 1 (HS1) of the North Atlantic region. Because the STF is associated with the position of the westerly wind belt, our findings support the concept that a southward shift of Earth's wind belts accompanied the early part of HS1 cooling in the North Atlantic, leading to warming and deglaciation in southern middle latitudes.