Antipodean glaciers confirm complementary southern warming during the Younger Dryas

Studies of air-temperature proxies in cores from the Antarctic ice cap show a roughly mirrored climate record to that found in the Greenland ice. While the Northern Hemisphere underwent a sudden climate collapse into almost full-glacial conditions around 12.9 ka and an equally dramatic warming around 11.7 ka, Antarctica steadily warmed over the same period to reach full interglacial conditions by 11.5. That this climatic inversion reached relatively low southern latitudes is confirmed by a record of the changing size of glaciers on mountains in New Zealand’s South Island (Kaplan, M.R. and 9 others 2010. Glacier retreat in New Zealand during the Younger Dryas stadial. Nature, v. 467, p. 194-197). The US-New Zealand-Norwegian-French partnerships used detailed geomorphological mapping, and cosmogenic isotope studies of exposed rock fragments to show that after about 13 ka glaciers retreated by more than a kilometre in the succeeding 1500 years in contrast to the dramatic glacial advances in northern areas such as the Scottish Highlands.

Record of rising sea-level in the tropics

Areas beyond the zones of isostatic depression by ice-loading and recovery during glacial-interglacial cycles passively undergo sea-level fall and inundation. They best record the progress of Holocene ice-sheet melting and sea-level rise since 11.5 ka, especially if they are tectonically stable. The island state of Singapore, 1.5 º north of the Equator, is a near-ideal place for study (Bird, M.I. et al. 2010. Punctuated eustatic sea-level rise in the early mid-Holocene. Geology, v. 38, p. 803-806). The Australian and British geoscientists analysed a core through sediments in a mangrove swamp now just below sea level. The top 14 m penetrated a uniform though laminated sequence of marine muds, calibrated to time by radiocarbon dating of mollusc shells, mainly focused on the period from 9 to 6ka period that the global oxygen-isotope record of ice volume suggests to have been the main period of final melting after the Younger Dryas.

Sedimentation was very rapid (~1 cm y-1) from  8.5 to 7.8 ka, probably as sea level rose too rapidly for the coast to be protected by mangrove growth.  Then for 400 years it slackened off to ~0.1 cm y-1 to rise again to 0.5 cm y-1 by 6.5 ka. The last date is the time of the mid-Holocene sea level highstand, after which sedimentation rate soon declined to 0.05 cm y-1, when mangroves became established at the site. Stable isotopes of carbon in the core (δ13C) show how the relative input of marine and terrestrial (mainly mangroves) organisms shifted over the period and are a proxy for the distance to the coastline and hence sea level. From 8.5 to 6.5 ka this was erratic from a starting point about 10 m lower than nowadays, showing rapid rises and falls that culminated in a sea level in Singapore about 3 m above present during the mid-Holocene sea level highstand that slowly declined to that of the present.

The team’s findings tally with evidence for the melting record of the North American ice sheet. An interesting aspect is that they also cover the period when rice cultivation in swampy areas of SE Asia got underway (~7.7 ka). Very rapid sedimentation would have encouraged development of the substrate for the highly fertile delta plains that now support the largest regional population densities on Earth. In turn they culminated in a series of early south and east Asian civilisations based on class societies.

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