Australia: The Land Where Time Began

A biography of the Australian continent 

North Atlantic Climate During the Last Glacial Period - Links with Tropical Rainfall

The North Atlantic region underwent pronounced alterations, on a millennial scale, between cold stadial and mild interstadial conditions, Dansgaard-Oeschger oscillations, and periods when icebergs discharge reached massive proportions, Heinrich Events (1). Records from Greenland (2-4) contain evidence of temperature changes in the Northern Hemisphere that have been suggested to have affected the location of the intertropical convergence zone (5,6), and the strength of the Indian summer monsoon (7,8). In this article the authors¹ used high-resolution records of the colour of sediment that measures the terrigenous vs biogenic content, from the Arabian Sea and the Cariaco Basin located off the Venezuelan coast, to assess teleconnections with the climate system of the North Atlantic during the last glacial period. The intertropical convergence zone is indicated  by the Cariaco record to have migrated over the site seasonally during stadial conditions that were mild, though during peak stadials and Heinrich Events was displaced south of the basin permanently. Evidence from the Arabian Sea indicates that during stadial events Indian summer monsoons were weak. Records from the tropics show a response to the cooling in the North Atlantic that is more variable than suggested in the temperature records from Greenland. The authprs¹ therefore suggest the climate of Greenland is especially sensitive to variations of the North Atlantic system, in particular the extent of sea ice, while the intertropical convergence zone and Indian summer monsoon mainly respond to variations in mean temperature of the Northern Hemisphere.

Stadials (colder periods) and interstadials (warmer periods) have been shown by the Greenland ice core records (2,4) and North Atlantic sediment cores (1,9) to have alternated with each other over the course of the last ice age. Heinrich Events (1), episodes of massive meltwater discharges that originated from instabilities in the ice sheets in the Northern Hemisphere, have been shown to have coincided with some of the stadials that were identified in records from Greenland, as well as other records. Climate models (12,13) and palaeo-proxy data (5,7,14-16) have been used to identify Dansgaard-Oeschger oscillations global teleconnections, though their physical origins remain uncertain (10,11). Proxy records previously lacked sufficient resolution to capture fully the effect of millennial-scale glacial variability on interannual to decadal variability of tropical rainfall.

The authors¹ analysed sediments from Cariaco Basin and the northeastern Arabian Sea at unprecedennted resolution in this study. Located on the northern shelf of Venezuela the Cariaco Basin sediments preserve a detailed record of the variability of past tropical climate (5,17,18). The Cariaco Basin is within the area of the seasonal migration of the intertropical convergence zone (ITCZ) of the western Atlantic that controls the distribution of rainfall across northern and Central South America. Seasonally varying inputs of terrigenous and biogenic components recorded in the laminations of light/dark colour in the seafloor sediments (17,19) that results from the migration of the ITCZ. The lamination archives climate change variability, as they depend on rainfall and productivity. The Cariaco basin is separated from the open Caribbean Sea by a sill. At the present anoxic conditions lead to minimal bioturbation and preservation of a sediment record that has an exceptional temporal resolution (5,17-19) results from oxygen demand that is driven by remineralisation of organic material under conditions of limited deep-water renewal. The seasonal cycle is dominated by the seasonal monsoons in the northeastern section of the Arabian Sea. Bioturbation is inhibited by an intense oxygen minimum zone, the result being that high-resolution sediment, that is annually laminated, containing archives of monsoon conditions are preserved (7,8,20,21).

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Sources & Further reading

1. Deplazes, Gaudenz, Andreas Luckge, Larry C. Peterson, Axel Timmermann, Yvonne Hamann, Konrad A. Hughen, Ursula Rohl, et al. "Links between Tropical Rainfall and North Atlantic Climate During the Last Glacial Period." Nature Geosci 6, no. 3 (03//print 2013): 213-17.