Climate Change thematic programme, project NE/C509158/1 : ‘Precise chronology of the timing of changes
in behaviour of the North Atlantic THC and their forcing effects, 16-8 ka BP’
is concerned with an important transitional period (the Last
Glacial-Interglacial Transition: LGIT) which spans the final stages of
ice-sheet glaciation in Britain and the early phase of the present
interglacial. This period, between c 16
and 8 ka BP, witnessed a number of abrupt and pronounced climatic changes
throughout the North Atlantic region and elsewhere, events which are clearly
reflected in, and precisely dated by, the Greenland ice-core records. The overall aim of our project is to test the
extent to which the impacts of those abrupt events, as experienced in the
British Isles and environs, were the direct consequence of changes in North
Atlantic THC. To do this, we need to
demonstrate the precise temporal relationships between North Atlantic THC
changes on the one hand, and marked changes in environmental conditions in the
British Isles on the other. This is
something which is presently difficult to achieve at better than centennial
precision, because of a number of technical difficulties that compromise
radiocarbon dating, the principal geochronological method employed for this
is exploring the potential of three methods which, in combination, may lead to
significant improvement in the precision and accuracy with which we can date
climatic events, and synchronise the proxy-climate records obtained from marine
and terrestrial contexts, and from ice cores: these are varve chronology, tephrochronology
and new statistical approaches to age modeling.
The key outcomes to emerge so far are outlined below.
varve chronology for the Lochaber district has been constructed which spans 515
years (Roy and Spean lake systems) while an embryonic chronology for Glacial
Lake Blane spanning 295 years has been defined.
series of varves show decadal-scale variations in internal composition and
structure that match climatic variations reflected in the Greenland ice-core
record. We believe that the Greenland
ice and sedimentation in ice-dammed lakes in Scotland were responding to a
common climate forcing factor – perhaps decadal migrations of the North
Atlantic Polar Front, or variations in solar radiation output.
the varves and comparison of these laminated sequences with the Greenland records
enables us to date events in Scotland with a remarkable degree of precision. For example, the records show that the varves
in the Glen Roy-Glen Spean area accumulated over a period of 515 years between
12,119 and 11,506 years ago, and that the glacier ice that impounded the lakes
in Glens Roy and Spean reached their maximum extent ca. 840 years after the
onset of a major cooling episode which, according to the Greenland ice-core
records, commenced around 11,797 years ago.
The onset of
climate warming and melting of the ice that impounded the lakes in Glen Roy is dated
to 11,570, which is very close to the age of the onset of the present
interglacial in the Greenland ice-core records, again suggesting that Greenland
and Scotland were dancing to the same climatic tune.
glass shards, originating from volcanoes on Iceland (see below), have been found
in the varved sediments. This is a very important discovery for it will enable
us to validate the above conclusions, and to synchronise the Scottish records
with those in the North Atlantic, mainland Europe and Greenland more precisely.
sequence provides an important test of numerical models of the timing of the
last glacier ice advance in Scotland, and of the rate of response to climatic
triggers, such as changes in the North Atlantic THC
Discovery of important new cryptotephras in sites in the British Isles
(Pyne-O-Donnell et al., 2007, 2008) which fall within the period of interest to
our project (Figure 7).
Discovery of non-visible, discrete cryptotephra layers of probable
Icelandic origin in marine basins in the Labrador Sea and in the vicinity of
the Laurentian Fan, as well as in other parts of the North Atlantic (Figure 8).
These tephra layers were probably disseminated by icebergs or melting sea ice,
and hence provide important information on the conditions in, and circulation
of, the North Atlantic during the study period.
Extension of the distribution of one of the most important ash beds, the Vedde
Ash (dated to c. 12.0 k cal yrs BP), across Europe to sites in southern Germany
and Switzerland, the most southerly finds so far (Blockley et al., 2007), and
to Lake Yamozero on the Timan Ridge in northern Russia. This is 1200 km beyond
the previous most easterly find (St. Petersburg area), and 3,000 km from the
source volcano, Katla, in southern Iceland.
The use of tephrochronology to provide a more precise chronology of the
retreat of the last British ice sheet.
Discovery of the first cryptotephras in varved records in the British
Isles (see ‘varve chronology’ above).
An extensive radiocarbon dating programme at Croftamie has allowed revision
of the timing of maximum ice extent at the type-site for the Loch Lomond
Readvance (equivalent to the Younger Dryas).
Research in Southern Europe has indicates that the onset and termination of
abrupt climate events may be asynchronous with Greenland.
The Royal Holloway RAPID team
Professor John Lowe
Dr Simon Blockley
Dr Adrian Palmer
Dr Sean Pyne O’Donnell
Dr Alison MacLeod