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

Abstract

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

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