Coherent deglacial changes in western Atlantic Ocean circulation

Abrupt climate changes in the past have been attributed to variations in Atlantic Meridional Overturning Circulation (AMOC) strength. However, the exact timing and magnitude of past AMOC shifts remain elusive, which continues to limit our understanding of the driving mechanisms of such climate variability. Here we show a consistent signal of the 231Pa/230Th proxy that reveals a spatially coherent picture of western Atlantic circulation changes over the last deglaciation, during abrupt millennial-scale climate transitions. At the onset of deglaciation, we observe an early slowdown of circulation in the western Atlantic from around 19 to 16.5 thousand years ago (ka), consistent with the timing of accelerated Eurasian ice melting. The subsequent weakened AMOC state persists for over a millennium (~16.5–15 ka), during which time there is substantial ice rafting from the Laurentide ice sheet. This timing indicates a role for melting ice in driving a two-step AMOC slowdown, with a positive feedback sustaining continued iceberg calving and climate change during Heinrich Stadial 1

Germination and Establishment of Pearl-Millet and Sorghum of Different Seed Qualities Under Controlled High-Temperature Environments

Larger and/or denser pearl millet [Pennisetum glaucum (L.) R. Br.] and sorghum [Sorghum bicolor (L.) Moench] seed have improved field establishment under temperate conditions. Because soil temperatures are higher in the semi-arid tropics and often are a constraint to successful establishment, effects of seed size and density on germination and establishment under high temperatures (up to 50°C) were evaluated. Effects of seed size and seed density on germination of Senegal Bulk millet and Segaolane sorghum were examined at temperatures of 26-50°C using a thermogradient plate. Seed size and density combinations and timing and duration of 40-45°C stress were evaluated for germination in an incubator. Effects of millet seed size and seed density on establishment were determined at constant temperatures of 35, 43, and 50°C. Sorghum had highest germination at 32-40°C and 32-42°C for medium and large seeds, respectively. Medium-sized millet seed showed higher germination over a wider temperature range than small or large seed. Millet germination rate and plumule emergence had optima at 37-38°C. High-density or large sorghum seed had greater radicle emergence at 24 h with 1 or 2-h high-temperature stress; however, with 4 or 8 h of stress, there was no difference among seed sizes or seed densities. Millet germination at 24 h was lower following 8 h of stress than for 1 or 2 of stress. Millet germination at 48 h was lower for large seed than small seed. High-density millet seeds had higher emergence than low-density seeds at a constant 43°C. At 7 days after planting, seedlings from large and medium-sized high-density seeds were taller than from small or low-density seeds.</p