Drilling of
two ice cores on the summit of Greenland in the early 1990s provided dramatic
confirmation of numerous large and abrupt reversals of climate, especially in
the equivalent of marine oxygen isotope stages (MIS) 3-4 , about 25-80 ka.
These findings provide support for the conjecture of Broecker et al. (1985, and
earlier comments by Stommel) of possible multiple stable states in the ocean-atmosphere
system that could be due to abrupt changes in the strength of the North
Atlantic thermohaline circulation (THC), "The Great Ocean Conveyor".
Since these findings a wealth of new
information has provided considerable additional insight into the patterns of
climate change during these intervals. Many records of environmental change
covering the last 80,000 years from polar, marine and terrestrial systems show
that there were rapid changes with "Dansgaard-Oeschger" (DO) -style
and higher-frequency oscillations in, for example, atmospheric temperature as
inferred from d18O in ice records, sea surface temperature as inferred from
planktic foraminifera faunas in marine sediments, and in terrestrial
vegetation as deduced from lacustrine pollen. DO cycles occur in subpolar North
Atlantic marine records nearby to Greenland and European pollen profiles as
well as in a number of more remote regions (South Atlantic, circum-North
Pacific, and south and east Asia) and in atmospheric trace gas (CH4 and CO2)
records of Antarctica.
The pattern of climate change
observed in a number of sites supports the idea of a north-south anti-phasing
of responses in the Atlantic Basin. In the north ~700-1500 year long DO
stadials, including the large Heinrich Events, are marked by massive meltwater
and iceberg discharges. In the last phase of the stadials brine discharges may
have triggered the subsequent prominent, but only 3-15 years long DO warming
events. There is also an unusual and not-well-understood ~7200 year
oscillation, the saw-toothed Bond Cycles of Heinrich events in many of these
records. These cyclicities show the largest amplitudes during cold marine
isotope stages, when the sea level dropped below -45 m. In most cases the
stadials appear to greatly reduce North Atlantic Deep Water (NADW) production
rates and the global conveyor, associated with major changes in atmospheric
14C. It has also been suggested that the cycles may be due to low-frequency
variations in the equatorial Pacific.
Prerequisites for the study of the mechanisms which control rapid
decadal to millennial environmental changes and leads and lags between
different global environmental systems are high-resolution stratigraphic
records with accurate calendar year chronologies. Precisely this issue of
creating a joint global time scale of decadal-to-millennial-scale climate
oscillations was discussed by more than 50 top scientists at the SCOR-IMAGES
Workshop in Trins, Austria, February 16-19, 2000. Their broad spectrum of
scientific expertise comprised spanned from climate modelling, paleoceanography,
ice core and terrestrial paleoclimatology, trace gas analysis to radiometric
and geomagnetic dating).
Task Group
1: Calendar year chronologies of the
last 80,000 years
Task Group
2: Physical dating techniques
The techniques for establishing
calendar year chronologies for stratigraphic records of the last 80 ka fall
into three categories:
1.Incremental
dating techniques which provide continuous and independent calendar year
chronologies throughout a record.
- Annual
rings laid down by long-lived trees in temperate regions of the world provide
one of the best annual records - dendrochronology. Ring-width variations can be
cross-matched between trees and a composite dendrochronological (calendar
year) timescale can be constructed. A continuous dendrochronology now extends
to 11,800 calendar years BP. Older "floating" sections of
dendrochronological records have yet to be tied to the continuous record; once
this is achieved, a detailed calibration curve will extend to ca 40 ka.
However, it is rarely possible to apply dendrochronological dates to other
records without recourse to cross matching between tree-ring measurements and
other proxies.
- The ice
cores from the polar ice sheets have yielded, and will continue to yield incredibly
valuable palaeoclimatic archives. One great strength of the records is their
extremely high stratigraphic resolution (for the upper parts, sub-annual) and
that many different types of record may be derived from the same core. Thus
there is not the phase uncertainty between the various climate records from a
single ice core that so often affects different geological records (although
uncertainties exist in transferring a solid ice time scale to the atmospheric
gas records). The stratigraphically best-resolved records for the last climate
cycle come from two cores drilled at the summit of Greenland. The GISP2 and
GRIP data sets were published together in a single issue of J.G.R. (Vol 102,
1997). However, several different time scales exist for each ice core and to
date no attempt has been made to develop an optimal timescale based on the data
in both the ice cores.
The GISP2 time scale of Meese et al.
(1997) was derived by continuous counting of several properties exhibiting a
seasonal cycle; conservatively estimated errors range from 2% over the last 40
kyr to about 20% at 110 ka. Older than about 50 ka the "official"
GISP2 timescale uses a comparison of d18O of oxygen in trapped air with that in
the Vostok record and is consistent with the marine SPECMAP timescale. This
provides a time scale with probably somewhat smaller uncertainties at 100 ka.
An independent time scale for the GISP2 core older than 40 ka and back to the
last interglacial was based on the record of laser light scattering by dust
particles, which preserves annual layering.
The GRIP time scale back to ca 7000
years BP is based on a correlation with the counted Dye-3 record using volcanic
acidity layers as tie lines. Continuous layer counting in GRIP was used between
7000 and the Oldest Dryas/Bolling transition close to 14,500 BP. Ice flow
modeling, incorporating smoothly varying accumulation estimates based on the
d18O record, and using age control points at the Younger Dryas/Preboreal
transition (11 500 BP) and MIS 5d at 110 ka, was used to construct the
remainder of the GRIP "ss08 timescale" (Dansgaard et al., 1993).
Hammer et al. (1997) have given an independent time scale for GRIP back to
~60,000 years, based on annual dust layers.
- Annually laminated or varved deposits occur
in both the marine and lacustrine realms. Marine varved chronologies are
usually fragmentary covering only parts of the last 80 ka, e.g. 9-15 calendar
ka in the Cariaco Basin record. Lacustrine environments can provide ideal
conditions for varved sedimentation throughout their existence. Few such sites
are Lake Gosciaz (Poland), Lake Holzmaar, Meerfelder Maar (Germany), Lake
Monticchio (Italy), Elk Lake (USA), and Lake Suigetsu (Japan), but none of them
covers the complete last glacial cycle.
- Corals and speleothems can have annual
growth bands which provide a chronological framework for subsequent analyses.
The counting of growth bands, however, produces calendar years only for modern
corals and speleothems, usually restricted to the last few centuries. Fossil
corals can provide records from any older time window providing
"floating" chronologies that need to be dated by other techniques
such as 14C and U-series dating. Speleothems might capture much longer time
intervals but in long speleothems the growth bands tend to fade and loose
their annual resolution.
The consensus of the Trins workshop
is that the records from the Greenland ice cores have a value that extends far
outside the "ice core community" for many reasons:
1. These two
records certainly provide the longest archives within which features such as
the 11-year sunspot cycle can be studied.
2. Some hypotheses for explaining climatic
variability rely on time-series analyses which are critically dependent on the
integrity of the timescale.
3. The
important paleoceanographic records available can only obtain a precise
timescale sufficient for time-series analysis through correlation to the Greenland
ice core record.
4.
Correlation to Greenland obviously provides an essential link for ensuring the
accuracy of the Antarctic ice core chronologies.
5. The
accuracy of time scales based on annual layer counting in lake sediments and
tree rings can ultimately only be checked by comparison with other annually
resolved time scales. Again the Greenland records will probably remain an ultimate
means of checking.
6. More and
more important paleoclimatic data series are based on "floating"
annually resolved timescales: stalagmite sections, pieces of wood, lake
sediments not extending to the present, and so on. Their value will be enhanced
if they can be fixed in time through correlation to the Greenland ice record
with its broad diversity of proxy data sets. Thus
it is imperative that a significant coordinated effort should be made to
develop a new calendar time scale for the Greenland GISP2 and GRIP records,
that is documented in sufficient detail that it can in the future be readily
upgraded incrementally as new informations (e.g. from NGGRIP) become available.
2.
Radiometric techniques which provide discrete dates for subsamples from a
record, with the continuous chronology being constructed by interpolation
between these dates.
- Uranium-Thorium (230U/234Th) dating using
thermal-ionisation mass spectrometry (TIMS) can be used to date materials from
speleothems, corals, carbonate deposits, peat and bones from 5-350 ka. The
crucial prerequisite for this technique
is that there has been no loss or gain of nucleides since depostion, i.e. the
system is closed. Unlike 14C the formation of these nuclides is not dependent
on variations in solar activity or reorganizations in the global carbon
cycle. When correcting for various sources of error (deviation from the closed
system, inherited Th) this technique provides calendar chronologies.
- Radiocarbon (14C) dating is the much widely
used technique for dating materials up to ca. 50 ka. 14C dating by decay
counting or Accelerator mass spectrometry (AMS) has an age uncertainty ranging
from about ñ20-30 years for samples back to 10.5 ka, up to ~100 years back to
20 ka, and to >~1000 years near to the limits of the counting statistics and
background due to counting statistics depending on the age and size of a
sample. Since the atmospheric 14C/12C ratio was not constant in the past as a
consequence of short-term changes in the 14C production rate and/or in the
properties of the oceanic carbon system (e.g. deep-water formation and
upwelling) the radiocarbon age has to be calibrated.
This calibration which is necessary
to compare 14C dated records to other calendar year records leads to a further
increase of the error range. The problem is exacerbated by plateaux in the
calibration curves resulting from a great temporal variations in the atmospheric
14C content. Back to 11,800 years the calibration uses 14C datings of dendrochronologically
dated tree rings and will be extended into the late Weichselian when floating
Late Glacial dendrochronologies are tied to the Holocene master record. Beyond
these limits back to about 55,000 years the attempts to extend the calibration
curve are based on (1.) 230U/234Th ages of corals, (2.) annually laminated
sediments from marine (Cariaco Basin, off Venezuela) and lacustrine realms
(Suigetsu, Japan; Lisan, Israel). (3.) An indirect calibration approach is to
measure high-resolution 14C ages on records for which a climate proxy exist,
that allows detailed cross-matching with well-dated climate records, e.g. from
Greenland ice cores.
In addition, the dates in marine
sediment records may be biased by the local 14C reservoir effect, i.e. the
14C/12C ratio of CO2 in ambient sea water, that is different from the
atmospheric ratio. This local effect may vary by the same magnitude as
potential leads and lags between climate signals of interest and thus add large
uncertainties to the interpretation of marine proxy records.
To overcome these calibration
problems we need to measure 14C ages at high resolution in further annually
laminated records (e.g. Lake Monticchio, Italy; Lake Suigetsu, Japan) which
have independent calendar-year chronologies and finally, to assemble a single
14C calibration curve from these records for the past 50 ka.
3.
Correlation techniques which facilitate the synchronous cross-matching of
proxies that can be used to transfer the chronology of a dated to an undated
climate record.
- Geomagnetic palaeointensity is a global and
synchronous signal in many geological records and is used to correlate them. A
prime candidate for this purpose is the newly available high-resolution paleointensity
record from the North Atlantic (NAPIS-75) covering the interval 10-75 ka. The
stacked record has been dated by detailed correlation of one of the NAPIS-75
cores to the GISP2 ice core via d18O- meltwater signals. Geomagnetic
excursions and events are used as master tie points. Refined techniques are
necessary for precise and objective correlations using the entire shape of the
paleointensity records which will soon enable global correlations with an
uncertainty of less than 5 ky.
- Volcanic ash layers, once well defined in
terms of geochemistry and mineralogy, can be also utilized as event markers to
correlate ice and sediment cores. Furthermore, such events are suitable for
39Ar/40Ar dating with an error of 2 ka or less for the last 80 ka.
- Changes of sea level directly express
changes in the volume of continental ice sheets. Such changes can be
determined from coral reef deposits, dated by 14C and/or 230U /234Th methods.
Coral terraces at sites with high tectonic uplift provide detailed records of past sea level changes. However, sea
level stands as global chronological markers may be problematic because of
local tectonics, effects of storm events, and tsunamis.
Based on the outlined dating
techniques the Trins Workshop is recommending a number of joint research
efforts across various disciplines, necessary in the near future to
- establish a common 14C-time scale based on
the existing annually laminated sediment and ice records and to locate
additional laminated lacustrine and marine records,
- calibrate the common 14C-time scale back to
55 ka,
- define variations in the 14C reservoir
effect in various ocean basins,
- validate and promote the geomagnetic
palaeointensity record as dating tool,
- establish a set of global time markers.
Task Group
3: Sites and possiblities to test mechanisms proposed to explain
centennial-millennial-scale Dansgaard-Oeschger-Bond oscillations
At present there are a number of
sites of varying resolution (Figure 1) that record, or seem to record, the
centennial-millennial-scale DO and Bond oscillations. These records describe
variability in atmospheric/ocean circulation through proxies reflecting changes
in polar temperature, SST, sea ice and salinity (meltwater), productivity,
intermediate/deep-ocean convection, monsoon intensity and precipitation.
Figure 2 shows examples of some particularly high-resolution time series.
Several different hypotheses have
been proposed to explain, at least in part, such
phenomena,
the most prominent being variations of the Atlantic conveyor and low-frequency
variability in the eastern equatorial Pacific upwelling system. Different mechanisms
may be triggering such variations Ð in the case of the conveyor e.g. meltwater
pulses linked to internal (binge-purge) ice sheet instabilities, variations in
the tropical Atlantic sector, or solar irradiance changes. The equatorial
Pacific mode might be some resonant time scale for the Pacific Basin. The two
mechanisms may be interdependent Ð variations in the equatorial Pacific should
affect the North American and Greenland ice sheets which, in turn, may affect
the conveyor. Conversely, changes in the conveyor may trigger trade wind
changes in the eastern equatorial Pacific or changes in the strength of the
Siberian high may influence westerly wind bursts in the western equatorial
Pacific.
Each of the above mechanisms has a
different "fingerprint" in the time- and space-domain. For example,
the conveyor is associated with an anti-phase SST pattern in the North and
South Atlantic, possibly also in the northwestern Pacific, with variations in
the southern Indian Ocean and the Pacific south of 30-40 degrees N being muted.
There are some of the same regions that should record the strong imprint of
equatorial Pacific warming.
The consensus of the Trins Workshop
supports two major initiatives:
1. Acquisition of new records to test proposed
mechanisms
One of the goals of future
high-resolution climate studies should be to test among these different
mechanisms by more specific targeting of "high payoff" sites.
Although additional projects of opportunity may justify addition of samples
from other regions, the tremendous time and costs of acquiring and analyzing
such records requires some priority be given to sites that yield the highest
payof for the effort involved. However, because our understanding of such
processes is still at an early stage, there should be a delicate balance
between these targeted sites and 'serendipity'. One criterion to select other
sites would be the sheer quality of proposed sites. As geoscience
investigations have repeatedly shown, high quality records consistently yield
surprises as to how the climate system works. "Incomplete" records of
climate change such as varves and speleothems should also be collected for the
last 80,000 years. Such information can often be very valuable.
Some specific tests of the conveyor
hypothesis would involve:
- North Atlantic studies examining phase
relations between deep- and surface water hydrography and origin, ice-rafting
events, meltwater and brine pulses, and indications of tropical humidity
changes (e.g. at tropical Atlantic/Indian Ocean and Mediterranean sites);
- South Atlantic sites where the Atlantic
see-saw would predict increased SST and southern African precipitation;
- Regions affected by meltwater-induced
changes in the strength of the Siberian high and its effect on the south and
east Asian monsoons, e.g. to test whether subpolar North Pacific SSTs are
controlled by coeval atmospheric forcing or rather by thermocline ventilation
at the terminus of the 'global conveyor' in anti-phase with North Atlantic
changes;
- High-latitude sites in the Southern Ocean
and Antarctica, that record anti-phase information; since the 'conveyor' also
predicts a west-east see-saw in Antarctic Circumpolar flow, cooling in the Ross
Sea may be in-phase with North Atlantic cooling; additional testing from these sites
would be most useful.
Equatorial Pacific fluctuations can
be tested with key records of
- Eastern equatorial Pacific warming;
- Drying in Australia that should be
detectable with pollen records;
- SST increases along the west coast of the
USA (e.g., Santa Barbara Basin);
- Increased precipitation in the U.S.
southwest;
- Peru margin upwelling;
- Potential links to central Canada.
Holocene studies imply some support
for the role of low-frequency solar irradiance changes as contributing to
decadal-to-millennial-scale variability. If so, such variability may be also
detected in MIS 2-4. Better testing of the solar hypothesis requires:
- More 10Be and 36Cl data that would enable
separation of the cosmogenic effect from the 14C record, which is also
influenced by THC changes;
- More detailed records and improved age
control of continental climate change, particularly in the southern hemisphere,
for models suggest that the solar signal is stronger over land, with the same
sign of response in both hemispheres.
2. Analysis of new and existing records as a
guide to testing mechanisms
A number of complementary / overlapping approaches can provide a
better understanding of the processes involved in centennial/millennial-scale
climate change:
- "Zereo'th order" best-fit
correlations between different time series to determine more objectively
(independent of phase) the linear relationship of different records with the
Greenland ice core "type sections".
- Time slice studies of selected DOB
oscillations to map the spatial and temporal evolution of patterns associated
with the atmosphere and ocean (both surface and subsurface). ù Effect of such changes on methane and carbon
dioxide variations and potential feedbacks of atmospheric trace gas changes on
centennial-to-millennial-scale oscillations.
- Intensive statistical examination of the
1500-year cycle to determine its robustness, band with, and coherence and phase
relationships between different regions, moreover, an examination of
periodicities of the DOB band other than the 1500-year cycle.
- Analysis of centennial-millennial-scale
fluctuations in non-glacial sections (Holocene, MIS 5.5 and 11). Even though
such oscillations are of lower amplitude than their glacial cousins, the
removal of the greatly complicating effect of most of the northern hemisphere
ice sheets may enable isolation of the relative importance of other sources of
forcing especially during the last 1000 years, where time and space control of
samples is very good.
- Intercomparisons of climate models to assess
robustness of model predicted responses to e.g., meltwater pulses or equatorial
Pacific oscillations.
Authors:
M.
Sarnthein, J.P. Kennett,
MAIN
CO-AUTHORS: J. Chappell, T. Crowley, W. Curry, P. Grootes, I. Hendy, C. Laj, J.
Negendank, M. Schulz, N.J. Shackleton, A. Voelker, B. Zolitschka
For more information
contact M. Sarnthein, Institut fuer Geowissenschaften, University of Kiel
, Olshausenstr. 40, D-24098 Kiel, Germany; E-mail: ms@gpi.uni-kiel.de