Meltwater injections and their impact on Atlantic meridional overturning circulation and climate during the time period of Heinrich Event 1 and the last deglaciation

The temporal relationship between meltwater pulse 1a (mwp-1a) and the climate<br/>history of the last deglaciation remains a subject of debate. By combining the<br/>GRIP ?18O ice core record on the new Greenland Ice Core Chronology 2005<br/>(GICC05) timescale with the U/Th-dated Barbados coral record, it is<br/>conclusively derived that mwp-1a did not coincide with the sharp Bølling<br/>warming, but with the abrupt cooling of the Older Dryas. To evaluate whether<br/>there is a relationship between meltwater injections, North Atlantic Deep Water<br/>(NADW) formation and climate change (i.e., the long term change in the average<br/>weather), a high-resolution magnetic (?ARM/?) proxy record of NADW flow<br/>intensity from Eirik Drift, south of Greenland, is presented. A record of mean<br/>sortable silt grain sizes (an established proxy for near bottom current flow<br/>speed), obtained from the same samples on which the ?ARM/? was measured,<br/>shows remarkable similarity to the magnetic record and validates ?ARM/? as a<br/>proxy for NADW flow intensity. The record of ?ARM/? indicates only a relatively<br/>minor 200-yr weakening of NADW flow, coincident with mwp-1a. This<br/>compilation of records also indicates that during Heinrich event 1 (H1) and the<br/>Younger Dryas there were no discernible sea-level rises, and yet these periods<br/>were characterised by intense NADW slowdowns. Records of planktonic<br/>foraminiferal ?18O, as well as lithic and foraminiferal counts from Eirik Drift are<br/>combined with previous studies from the Nordic seas and the ‘Ice Rafted Debris<br/>(IRD) belt’, and portray a sequence of events through the interval of H1. These<br/>events progressed from an onset of meltwater release around 19 ka BP, through<br/>the ‘conventional’ H1 phase from ~17.5 ka BP, to a final phase between 16.5 and<br/>14.6 ka BP, characterised by a pooling of fresh waters in the Nordic Seas, which<br/>were injected hyperpycnally. This build up of fresh waters was purged from the<br/>Nordic Seas, preconditioning the Nordic Seas for convective deep-water<br/>formation. This allowed the abrupt re-start of NADW formation in the Nordic<br/>Seas at the Bølling warming. In contrast to previous estimates for the duration of<br/>H1 (i.e., 1000 years to only a century or two), the total, combined composite<br/>signal of H1 presented here had a duration of nearly 4000 yrs (~19–14.6 ka BP),<br/>now spanning the established period of NADW shutdown. Clearly, deep-water<br/>formation and climate are not simply controlled by the magnitude or rate of<br/>meltwater addition. Instead, the results presented here emphasise that the location<br/>of meltwater pulses may be more important, with NADW formation being<br/>particularly sensitive to surface freshening in the Arctic/Nordic Seas.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2008

Author dc:creator

• Stanford, Jennifer D.