Appraisal of a molecular approach to infer variations in surface ocean freshwater inputs into the North Atlantic during the last glacial

The thermohaline circulation is considered responsible for regulating climate variability at a global scale [Geochim. Cosmochim. Acta 53 (1989) 2465]. It has been argued that it can be disrupted by changes in surface ocean salinity in the sites of deepwater formation [J. Geophys. Res. 96 (1991) 16811]. However, to date reconstruction of sea surface salinity (SSS) has lacked the accuracy to identify the location, extent and amplitude of past meltwater events [Science 282 (1998) 61]. Recently, a measurement based on alkenones (%37:4) has been shown to be related to types of water masses and perhaps surface salinity in the Nordic Seas [Paleoceanography 13 (1998) 694; Terra Nova 10 (1998) 86]. Here, we present new data from surface sediments that further confirm that it is possible to obtain a linear correlation between %37:4 and salinity. Records of %37:4 from three nearby sediment cores in the NE Atlantic show equivalent profiles, with the highest values occurring in the Heinrich layers (HLs). We propose that downcore %37:4 data can be interpreted in terms of decreases in surface ocean salinity (freshening of surface waters) in response to incoming freshwater from sea-ice and/or icebergs to the core relation. The %37:4 data provides further indication of the locally lower surface ocean salinity that prevailed in the North Atlantic during the last glacial period, and particularly during the Heinrich episodes due to the influx of iceberg meltwater.