The Mt Logan Holocene–late Wisconsinan isotope record

12 Pages·2008·1.32 MB·English

The Holocene18,5 (2008) pp. 667–677
The Mt Logan Holocene–late Wisconsinan
isotope record: tropical Pacific–Yukon
connections
David Fisher,1* Erich Osterberg,3 Art Dyke,1 Dorthe Dahl-Jensen,5
Mike Demuth,1 Christian Zdanowicz,1 Jocelyne Bourgeois,1 Roy M.
Koerner,1 Paul Mayewski,3 Cameron Wake,2 Karl Kreutz,3 Eric
Steig,4 James Zheng,1 Kaplan Yalcin,6 Kumiko Goto-Azuma,7 Brian
Luckman8 and Summer Rupper9
(1Geological Survey of Canada, (NRCan), 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada;
2Climate Change Research Center, EOS, Morse Hall, University of New Hampshire,Durham NH
03824, USA; 3Climate Change Institute and Department of Earth Sciences, University of Maine,
Orono ME 04469, USA; 4Quaternary Research Center, ESS, 19 Johnson Hall, Box 1360, University of
Washington, Seattle WA 98195-1360, USA; 5Niels Bohr Institute, Juliane Maries Vej 30, University of
Copenhagen, DK-2100, Copenhagen East, Denmark; 6Department of Geosciences, 102D Wilkinson
Hall, Oregon State University, Corvallis OR 97331-5506, USA; 7NIPR, Kaga 1-9-10, Itabashi-ku,
National Institute of Polar Research,Tokyo 173-8515, Japan; 8Department of Geography, University of
Western Ontario, London, Ontario N6F 5C2, Canada; 9Department of Geological Sciences, Brigham
Young University, S389, Provo UT 84602, USA)
Received 13th November 2007; revised manuscript accepted 26 February 2008
Abstract: The ice core recovered from Prospector Russell Col on Mt Logan (5.4 km a.s.l.), in the Yukon spans
over 20000 years. This unique record offers a Pacific view of the stable isotope and chemical record from the
Lateglacial to the present. The timescale is based on seasonal counted years, the largest known volcanic acid
signatures and the major shift in stable isotopes and chemistry at the end of the Younger Dryas. There are large
and sustained changes in the stable isotopic record that are anti-correlated with marine and continental
chemistry series. The oxygen-18 in this area is not a proxy for palaeotemperature but rather for source region.
The last major isotope shift in AD1840 in δ(18O) and chemistry is compared with the Quinn’s ENSO record.
During periods of more frequent La Niña (stronger tropical easterlies) there is more zonal flow of water vapour
transport to the Pacific Northwest, δ(18O) values are larger and the deuterium excess dsmaller. These periods
coincide with periods of lower accumulation/precipitation in southern Yukon. The Holocene δ(18O) record
indicates many large shifts between the meridional (strong El Niño) and zonal (La Niña). Comparison of the
Logan isotopic record and the moisture/temperature-sensitive time series of peat bog inception dates for the
Northwest shows a strong correlation (0.36) that points to high accumulation rates coincident with low δ(18O)
and enhanced meridional flow. Major changes in the core at 4200 BP and 7000–8000 BP point to enhanced
meridional flow, which coincide with big changes in the Pacific palaeorecords of the balance between El Niño
and La Niña. 4200 BP seems to have inaugurated the ‘modern’ ENSO world.
Key words: Mt Logan, stable isotopes, Holocene, ENSO, peat, N Pacific, sudden change.
*Author for correspondence (e-mail: [email protected])
© 2008 SAGE Publications 10.1177/0959683608092236
668 The Holocene18,5 (2008)
Introduction (Clague et al., 1995; Clausen et al., 1995; Zheng et al., 1998;
Yalcin et al., 2007). The unidentified 1516-event must be ‘local’,
In Holocene times outside the tropics, stable isotopes of water because, although it is the largest sulfate concentration peak in the
given in terms of oxygen (δ(18O)) and deuterium (δ(D)) are usu- last 550 years in the PRCol and Eclipse cores (Yalcin et al., 2007),
ally correlated geographically and temporally with the precipita- it has no prominence in the eastern Arctic ice cores (Clausen et al.,
tion weighted temperature of the site (Dansgaard et al., 1973; 1995; Zheng et al., 1998). The White River candidate peak is
Jouzel et al., 1984). The standard unitless measure of ratio within a metre of where it is expected. Thus, it is assigned the age
[D]/[H], denoted δ, is defined: of the most recent large eruption (AD803) that deposited the white
ash layer over most of southern Yukon (Clague et al., 1995). The
δ(D) =1000* (([D]/[H]) −([D]/[H]) )/([D]/[H]) (1) timescale presented here is very close to the tentative one used in
sample SMOW SMOW
Fisher et al. (2004), with differences smaller than 5%.
where SMOW denotes ‘standard mean ocean water’. Stable iso- Using an approximate theoretical timescale the annual layer
topic ratios are also affected by the water cycle history between thicknesses were estimated and the cores cut into at least six sam-
sources and the site and a generous amount of stratigraphic noise ples per year (Figure 2). The samples were measured for a host of
(Fisher et al., 1985, 1996). The deuterium excess, d, is a derived chemical species that vary seasonally. Over 8000 samples were
variable (d = δ (D)−8* δ(18O)) that is a conserved quantity for obtained and measured. The high resolution depth series were then
each source region (Merlivat and Jouzel, 1979; Johnsen et al., adjusted for densification and the years counted. Layer counting
1989). Although it is also sensitive to water cycle history that was accomplished independently by two groups in an objective
bears the moisture to the sites (Fisher, 1992), dis an indicator of way. The data series were put on an ice-equivalent depth scale and
the source ocean temperature. The geographic distribution of run through a band pass filter that only passed periodicities that
δ(18O) and δ(D) has been modelled with the help of GCMs could be annual in nature. For example, in the upper 40 m (ice
(Jouzel et al., 1997; Sturm et al., 2005) and intermediate com- equivalent) only power in the period range 30 cm to 90 cm was
plexity models (Fisher, 1990, 1992; Kavanaugh and Cuffey, passed. The near-end points were filtered with the help of adjacent
2003). The storm by storm models of Holdsworth (2008) hold the mirror reflections. The resulting smooth wave forms were easily
promise of eventually unravelling the synoptic scale processes at and unambiguously countable by both groups and the resulting
work in the Logan region but, for now, we will use the well-tested ages were identical and consistent with the largest volcanic hori-
intermediate complexity model as our interpretive tool. zons such as Katmai (AD 1912). As a check, the earlier NWCol
Previous work on two Mt Logan δ(18O) series (Holdsworth data sets of Holdsworth were passed through the same procedure
etal., 1992; Fisher et al., 2004) demonstrated that it is out of phase and the resulting timescale for the NWCol series was always
with other proxy temperature series, showing a ‘Little Ice Age’ within a year or two of that reported by Holdsworth et al. (1992).
with ‘warmer’, more positive, δvalues. At 5400 m a.s.l., δ(18O) is Figure 2 shows the measured layer thicknesses (averaged) and a
not a palaeotemperature indicator, but rather one of palaeosource theoretical layer thickness for a model that correctly places the
(Fisher et al., 2004). Katamai eruption (AD1912) and the transition at 11703 BP with
A number of temporal records are available from this region respect to (wrt) AD2000 (Vinther et al., 2006). Down to about 122
and their locations are given in Table 1. m depth (110 m ice equivalent) annual layers are countable. This
The top of the Mt Logan massif is a 5400 m a.s.l. plateau about goes back about 300 years. Deeper than that volcanic horizons and
10 km × 25 km dotted with several ~700 m high cones, the tallest the transition are used. The insert in the upper right of Figure 2
of which is Mt Logan (Figure 1b). Between the cones are rela- shows the bottom detail and the unfortunate loss of core quality
tively flat areas with low ice velocities (Fisher et al., 2004). Given marked by the grey bar near 170 m depth. The poor core quality
the low annual temperature and lack of uncovered area for ice to resulted in coarser temporal resolution. The grey bar in Figure 3
expand into, this situation has probably been stable over the shows where this is manifested, ie, from 7500 to 9500 BP.
Holocene. Older ice is within the bottom-most 5% of the thickness Ages beyond AD803 were initially pinned by a very clear tran-
and possibly affected by flow discontinuities. sition into ice-age ice with a sudden reduction in electrical con-
ductivity (ECM) coinciding with a large drop in δ(18O) (Wolff
et al., 1997) and increase in various chemical species (eg, sulfate)
Timescale for PRCol Logan Holocene (Figure 3e and f). This transition is assigned the Greenland
GICC05-date of 11703 cal. BP (Johnsen et al., 1997; Vinther
The PRCol record is placed on a timescale using layer counting, a et al., 2006 ). In between AD803 and the transition the SO−time
4
model, recent accumulation rates and the major acid peaks; eg, series was compared to the Smithsonian ‘Large Holocene Eruption’
Katmai, Laki, a large unknown peak at AD1516 and White River data base, parsed for only the eruptions in Alaska, Aleutian Islands
Table 1 Specifics of the core sites
Name Elevation Lat. Long. Mean temp. (°C) Main Max. age accumulation Depth reached (m)
(m a.s.l.) institutionsa (yr BP) (m ice/yr)
PRCol 5340 60.595 140.50 −29 GSC UMaine 30000 ~0.65 188 (bed)
NWCol 5340 60.617 140.50 −29 EC 350 ~0.6 103
Eclipse 3017 60.51 139.47 ~ −5 UNH, UMaine ~1000 1.38 345
King Col 4135 60.58 140.60 −17 NIPR ~300 ~1.0 220.5
Jelly Bean Lake 800(1650) 60.351 134.805 −1 Umass, U Pitt, ~7500 n/a n/a
UAF
aGSC, Geological Survey of Canada; EC, Environment Canada; NIPR, National Institute of Polar Research (Japan); U Pitt, University of Pittsburg;
Umaine, University of Maine; UAF, University of Alaska Fairbanks.
David Fisher et al.: Mt Logan Holocene–late Wisconsinan isotope record 669
Figure 2 (a)Annual Layer thicknesses, measured is black and mod-
elled is grey. The sampling interval (shown in black) down to 110 m
(ice equivalent) is fine enough relative to the annual layer thickness to
allow at least six samples per year and annual layer counting. Annual
layer counting for PRCol and NWCol go back about 300 years, after
which PRCol relies on the transition signature at 11703 BP and major
Figure 1 (a) Location map of the Alaska and the Yukon sites. (b) Logan volcanic events of the N Pacific region, as described in Figure 3. The
and S Yukon sites. The Logan massif is indicated by the ellipse and is about insert covers the bottom 25 m. The grey vertical bar shows where the
5400 m a.s.l. The white areas are glaciated, and the dark grey is water sampling interval became large, because of poor core quality. The res-
olution in part of the Holocene is poor because of this grey zone. Its
effects are seen in the grey bar shown in Figure 3 in the poor resolu-
and the Kamchatka Peninsula. We compared the Smithsonian VEI tion in part of the early Holocene. (b) Age ice–depth relationship for
(Volcanic Explosivity Index) for North Pacific eruptions as posted the PRCol
on the internet (Smithsonian, 2007) to sulfate time series from
PRCol then made small (< 10%) smooth shifts in PRCol ages to et al.’s (1997) GISP2 volcanic SO−series appears in Figure 3d.
4
accommodate the Smithsonian series. The Smithsonian time series Similar high levels are found in the tentative early Holocene of the
of these large eruptions appears in Figure 3c. The ice core sources PRCol core, Figure 3b. The typical Greenland high SO−Younger
4
of SO−are from ocean salts, continental mineral dust or volca- Dryas signature combined with low δ(18O) (Stuiver et al., 1997)
4
noes. The [Ca] plotted in Figure 3b (darker shade) is taken as a appears in Figure 3e and f and is very close to that for PRCol, pre-
measure of the non-volcanic input of sulfate and thus the volcanic sented in Figure 3a and b. When all the above age assignments are
sulfate peaks are those for which there are no Ca peaks. These are made for the PRCol the resulting annual layer thickness profile is
back high-lighted with vertical bars. The correspondence between that presented in Figure 2.
major ‘swarms’ of large eruptions and volcanic SO−is good with Between ~ 4ka BP and the transition, there are few constraints
4
the exception of the time interval 7500–9500 BP (real ages), except the theoretical (smooth and monotonic) timescale that goes
which corresponds to the interval of poor core quality. In particu- through the existing tie points. Nevertheless, the large changes in
lar the great swarm of North Pacific eruptions from 3500 to 4000 stable isotopes and chemistry throughout the Holocene will be
BP matches one of the largest SO−peaks in the PRCol middle present even if the assigned ages have errors much larger than
4
Holocene. In the early Holocene (9500 to 11500 BP) of the 10%. We estimate the errors in the upper 300 years to be ± 2 years,
GISP2 Greenland core there are extremely high volcanic SO−lev- in the 300 BP to 600 BP interval about ±5%, from 600 to 4000 BP
4
els attributed by Zielinski et al. (1997) to enhanced volacanism set 15%. The tie point at the transition looks very solid but between
off by mantle unloading at the end of the ice age. Zielinski 4000 and ~12000 BP we have no tie points and we assume the
670 The Holocene18,5 (2008)
Figure 3 The Holocene of PRCol, Mt Logan using the timescale described in the text. (a) The oxygen 18 profile. (b) The SO−(lighter shading) and
4
Ca++concentrations in ppb (parts per billion by mass). (c) Large volcanic eruptions in Alaska, Aleutian Islands and Kamchatka Peninisula, taken from
the Smithsonian internet-posted VEI listing. (d) Shows 25-yr averages of the GISP2 volcanic sulfate record (Zielinski et al., 1997). (e) GISP2 oxygen
18 record (Stuiver et al., 1997) and (f) the GISP2 total sulfate record (Zielinski et al.,1997). The grey bar in (b) indicates where core quality was poor
layer thickness always decreases smoothly. The correlations length in Fisher et al. (2004) and is one of many during the
between the PRCol Holocene and nearby absolutely dated palaeo- Holocene. Probably all the large Holocene changes share a similar
records will be shown to be very significant. This supports the explanation. The recent 300 years of both Logan cores show this
PRCol timescale. shift (Holdsworth et al., 1992; Fisher et al., 2004) and since they are
both accurately co-registered they are stacked in order to reduce
noise (Fisher et al., 1985, 1996). Fisher et al. (2004) noted that high-
Interpretation of the PRCol Holocene
resolution stable isotope records at three elevations (800 (JBL)
and comparison with other records (Anderson et al., 2005), 3000(E) and 5400(PRCol) m a.s.l. (see
Table 1)), show different manifestations of this shift. It is largest at
The large differences between the Logan δ(18O) series Figure 3a and the top (PRCol), clear at the bottom (JBL), but not present at the
that of Greenland as represented by the GISP2 series, Figure 3e, are middle elevation(E). The size of the AD1840 shift at these three sites
now discussed. Figure 3a shows at the extreme left the large shift in appears as squares in Figure 4 along with the results of the model
AD 1840. This AD 1840 shift in the PRCol δ(18O), is discussed at presented in Fisher et al.(2004) used to explain these differences.
David Fisher et al.: Mt Logan Holocene–late Wisconsinan isotope record 671
Figure 4 The size of (a) the δ(18O) and (b) the deuterium excess dshifts in AD1840 at four sites (elevations) in the Southern Yukon. ‘PRCol’, ice
core from Logan; E, ice core from Eclipse Dome; and JBL, a lake record from Jelly Bean Lake (Anderson et al., 2005); see Table 1. ML is the dif-
ference from Marcella Lake 100 km SE of JBL (Anderson et al., 2007). The line indicates model predictions of the differences when water sources
are switched between zonal and meridional, see Fisher et al. (2004). Grey shading is the model error range
The model posits that such shifts are driven by alternations between Pacific tropical ENSO system. Zhao and Moore used ice core
mainly zonal sources of moisture and meridional sources, and that series from Asia (Thompson et al.,2000) to assert that the Pacific
the different elevations ‘see’ different source distributions. The tropical easterlies (La Niña) were strong prior to AD1840 and that
model’s shifting between moisture sources can successfully repro- after AD1840 there was a shift to weaker easterlies and an increase
duce the 1840 shift at the three elevations, but what is the physical in the ENSO oscillations. They assert that Asian monsoon became
change in the Pacific climate to cause the meridional-zonal switch- weaker after 1840 as a result of the diminished easterlies. The
ing? The suspected switch controlling which is the source of mois- Quinn and Neal (1992) record of El Niño appears in Figure 5d.
ture is the state of the Pacific tropical Easterly winds, ie, the state of Their ENSO record is based on documentary evidence. We have
balance between El Niño and La Niña, (Moore et al., 2001; Fisher given their assessment of El Niño strength numerical values (with
et al., 2004; Zhao and Moore, 2006). their El Niño’s rated Very Strong = 10, Strong+ = 6, Strong = 4,
What else changes in or close to AD 1840 and how does this Moderate+ = 2, Moderate = 1, Moderate − = 0.7). The 50 year
relate to the Logan record? The Yukon temperature proxy records averages of the Quinn El Niño strength record is shown in Figure
derived from tree rings show a very modest temperature decrease. 5e, with weak El Niño periods shaded and strong ones unshaded.
For example, temperature-sensitive Yukon and Gulf of Alaska It is clear that periods of sustained weak El Niño’s (shaded) coin-
tree ring chronologies suggest that the temperature prior to AD cide with the more positive δ(18O) on Mt Logan. This is made
1840 was colder (Wiles et al., 2004; Youngblut and Luckman, obvious in Figure 5f where the statistical distribution of annual
2008). What does show a very big shift in the south Yukon (near δ(18O) values is plotted for the whole series (black) and for the
Kluane lake) is the precipitation rate proxy from Luckman’s shaded intervals (weak El Niño). The distribution for the whole
‘Landslide’ chronology (Luckman et al.,2002; Van Dorp, 2004) series has a double maximum and the weak El Niño δ(18O) set is
(Figure 5a). The annual average Logan stack for δ(18O), Figure 5c, significantly different than the strong El Niño set.
shows the shift in AD1840. The Logan δ(18O) annual series corre- The SW Yukon’s precipitation-sensitive tree ring record, Figure
lates significantly (−0.21) over 300 years with the Landslide-pre- 5a, from the Kluane region is (anti-) correlated with Logan δ(18O)
cipitation series. When it is dry in the south Yukon, Logan δ(18O) and matches low Yukon proxy-precipitation with reduced ENSO
is more positive (and dis smaller) and according to the theory the activity as shown in Quinn’s record, Figure 5d. Figure 5b shows
moisture flow more zonal for PRCol Logan (Fisher et al.,2004). the Logan sodium concentration [Na] record and demonstrates what
What changes occurred in the Pacific in the mid-1800s that is even clearer throughout the Holocene record, that [Na] and δ(18O)
could have caused such changes in the Yukon and Alaska? As are in anti-phase with each other (r = −0.41, see Figure 7e and 7f).
suggested in Mann et al. (2000), Fisher et al. (2004) and Zhao and In the 105 year calibration interval, [Na] PRCol annual series cor-
Moore (2006), there was a significant shift in the nature of the relates with its δ(18O) series (−0.17) and with the NPI (North Pacific
672 The Holocene18,5 (2008)
Index measures the strength of the Aleution low) for autumn and
winter (−0.38). Unfortunately, the calibration interval does not
span the most recent large shift in AD1840. For example, nobody
knows if the calibration-based relationships between various mete-
orological indices (such as NPI) and ice core variables hold across
the 1840 transition. So we have had to rely on the more fragmen-
tary palaeorecords to interpret this proto-type shift.
From Figure 5, the generalization is that strong La Niña (or
strong easterlies) coincide with lower accumulation in the Yukon
and more positive δ(18O) Mt Logan. One possible physical driver
for shifts such as AD 1840 is a slight change in the ‘deep’ water
temperature and/or its depth. By ‘deep’ is meant the water below
the warm mixed layer, which is of the order of 100 m thick. Sun’s
(2000) ENSO oscillation solution is chaotic, whereby small
changes in Tc (the deep water temperature of the tropical Pacific)
can push the tropical Pacific from permanent La Niña to the oscil-
latory La Niña–El Niño or ENSO state. Alterations in the easter-
lies of this sort have very large effects on the transport of water
vapour (monsoons) (Zhao and Moore, 2006). We note in passing
that the strength of the Kuroshio current, is largely powered by the
coupled Pacific tropical easterlies and mid-latitude westerlies and
thus sensitive to changes in these easterly winds (Munk, 1950;
Stommel, 1958).
Holocene comparison of PRCol
δδ(18O), d and Na with peat inception
rates in the northwest
A rich source of information about climate is found in the peat
data base assembled by A. Dyke (personal communication, 2007)
and reported in (Gorman et al.,2007). These bogs presently exist
over most of North America (outside the driest biomes). Peat bogs
start or expand when the climate is sufficiently wet and warm.
They do not start in dry and/or cold conditions and once the ice
departed they started to grow. After the departure of the ice there
is a necessary lag, while regional ecosystems are re-established
(Gorman et al., 2007). The multithousand-year trends in the in-
peat inception date series are partly caused by this lag, combined
with longer climate trends (Gorman et al., 2007). There are ~2100
C14 peat inception dates in the data base. Figure 6c shows all the
inception dates divided into C14 age bins, subdivided by longitude
range. Looking at the whole data set of 2054 inception dates
(Figure 7c, top panel), there is a clear 8.2 ka(calibrated) dry/cold
event and there is a strong oscillation from very wet to dry after
4.1 ka (calibrated). Comparison of the eastern sites (Figure 6a) to
the western (Figure 6b) shows the 8.2 ka event is not outstanding
in the west and the 4.1 ka oscillation in the eastern record is
replaced in the west by a wet/warm episode near 4.0 ka.
Examination of the east-to-west data series shows the effects of
the earlier departure of the Wisconsinan ice in the western regions.
The maximum in the inception rate is earlier in the west and it
becomes more subdued. For the Yukon and Alaska, which were
largely ice free, there is very little long-term trend in the inception
date series (not shown) and only the residuals are left. The residu-
als we take as related to the climate of the time. Figure 6a and b
Figure 5 (a) Preciptation-sensitive tree ring chronology for the shows the whole data set divided at longitude 100°W and the dates
Kluane Lake region near Mt Logan (Luckman et al.,2002; Van Dorp, converted to real ages using a smooth ninth order polynomial
2004) (elevation ~ 1000 m a.s.l.). (b) Na concentration stack for the through the Stuiver et al.(1998) C14 calibration set. The PRCol
PRCol and NWCol Mt Logan ice cores. (c) δ(18O) stack for the PRCol
and NWcol Logan series over the common 300 years. (d) Quinn
and Neal (1992) series of El Niño strengths based on documentary Figure 5 (continued)
evidence and given numerical values by us. (e) 50 yr averages of distributions of the annual δ(18O) of (c) for the whole 300 values
the Quinn and Neal series. The unshaded intervals are times of numer- (black) and for just the low El Niño (shaded) intervals. The signifi-
ous El Niño’s and the shaded intervals those of scarce El Niño or cance of the difference between the weak El Niño (strong La Niña)
more constant and stronger tropical easterlies. (f) Shows the probability δ(18O) population from the overall population is close to 100%
David Fisher et al.: Mt Logan Holocene–late Wisconsinan isotope record 673
Figure 6 Peat Inception dates of peat bogs, data from Gorman et al. (2007) (a) Calibrated dates for the eastern half of North America 100>
log.>50. (b) For the western half 100<long<170 W. (c) Carbon 14 dates for various longitudinal subsets of the peat inception data set
Holocene should intuitively relate to the more westerly sites north this history at ML must be largely in anti-phase with that indicated
of 55°N. But, in order to maintain peat sample size (N > 600) sim- by the peat records for most of the Yukon and Alaska. ML is in
ilar to the ice core’s resolution, we use the whole western subset the ‘rain shadow’ and could possibly give a local history different
but look only at the residuals of the inception date series. The from the peat history. It is also tempting to suggest that, since ML
residuals are much the same as for the extreme western subset. receives rain and snow from a very narrow elevation band near
The residual series is compared with the Logan isotope and 700 m a.s.l., its δ(18O) history should be in anti-phase to that of
sodium series in Figure 7. JBL and PRCol (see Figure 4).
The PRCol δ(18O) record follows that of the (meteoric water For the Holocene, Figure 7 shows that 25 year averages of
fed) nearby Jellybean Lake (JBL) (Anderson et al., 2005). δ(18O) and Na are anti-correlated (−0.41). For the calibration inter-
However, Anderson et al. (2007) report that δ(18O) for Marcella val, we know larger Na values go along with stronger Aleution
Lake (‘ML’ at 697 m a.s.l.) 100 km SE of JBL is in anti-phase to lows, which pull in more distant moisture. In the Holocene the
that of JBL and PRCol. They have argued that ML δ(18O) records lower δ(18O) goes with the higher Na, higher little-dand high peat
the local (precipitation–evaporation) balance. If this is true, then bog creation rates. The model interpretation in Fisher et al.(2004)
674 The Holocene18,5 (2008)
Figure 7 (a) and (b) The peat inception ages for the western sites (b, long = >100), and the extreme western sites (a, 170>long>120 W ). A low
pass filter (~4500 a) is run through the western data and the residuals ploted (inverted ) in (c). The green residuals indicate wet/warm and the tan
the opposite. The series in (d) is the inverted Logan-PRCol little-d series (deuterium excess). Larger d (green) indicates more distant/warmer
sources for the water (Dansgaardet al.,1973; Merlivat and Jouzel, 1979; Johnsen et al.,1989; Fisher, 1990). The good coherence between Logan
dand peat series (correlation 0.36, N = 400) shows that wetter/warmer conditions are coincident with more distant/warmer sources. (e) and (f) The
PRCol δ(18O) and Na (salt) series
suggests the high d and low δ(18O) indicates more distant water high precipitation and periods of strong El Niño ( or periods of
sources via enhanced meridional water flux. We harken back to weak La Niña and weaker tropical easterlies) go together. It may
the AD1840 d, and δ(18O) shifts and the relationship suggested in be the case that many of the shifts in the Holocene have these
Figure 5. There we concluded that more negative δ(18O), large d, co-relations.
David Fisher et al.: Mt Logan Holocene–late Wisconsinan isotope record 675
Discussion: tropical easterlies, Asian has a Younger Dryas isotopic signature that is in anti-phase with the
chemical impurities. Thus, during the late Wisconsin, Logan shares
monsoons and the Kuroshio current
a typical multivariable signature with eastern Arctic ice cores. In the
Holocene, the major isotopic and chemistry variations are in anti-
It is thought that a more active period of El Niño inhibits and phase and have a dominant periodicity of ~950 years. Even given
weakens the Asian monsoon (Morrill et al., 2003) that provides the possible errors in the timescale, this period must have this order
water to India, China and Indo China. Stronger and persistent La of magnitude. The main physical drivers for the Logan Holocene
Niñas (tropical easterlies) strengthen this monsoon. The interval record must have a timescale of about 950 years and in the chain of
about 4200 BP (3700 to 5000 BP), which is defined in PRCol by physical processes there should be at least one amplifier that makes
low δ(18O), high dand high [Na], has shown up in many Pacific the Logan record’s isotope changes much larger than those of
and Asian proxy records as one of weak or failed Asian monsoons Greenland and eastern Arctic Canada.
and drought conditions in Australia (Morrill at al., 2003, 2006; There are three main categories of suggested drivers for the
Shulmeister and Lees, 1995). Chen et al. (1999) have reported millennial-scale variations in the Holocene. They have been summa-
from lake studies that 4.2 ka BP, 7.6 ka BP and 11.3 ka BP were rized by Maslin et al.(2003) and are: (1) internal instability of the
dates that marked beginnings of extreme desiccation events in the Greenland Ice Sheet or Arctic sea ice; (2) ocean oscillator and the
arid part of China. The Dongge Cave record also records the 4.2 bipolar seesaw; (3) North Atlantic changes forced by solar variations.
ka and 7 to 7.5 ka BP ‘weak monsoon’ events (as well as many Whatever combination of these physical processes produces the
others that align with the Logan isotope and chemistry series) large millennial-scale variations of the PRCol ice core, we strongly
(Wang et al., 2005). Wang et al. (2005) noted that a long- suspect the tropical Pacific ocean and its subsurface temperature
established agriculture in central China collapsed because of the must be involved through the easterly wind system and the
4.2 ka BP desiccation, and Drysdale et al. (2006) have blamed the Kuroshio current. This will have to be explored in other papers.
4.2 ka BP event for crippling droughts in many sites in Africa and
Asia. The demise of the middle-eastern Akkadian empire has also
Acknowledgements
been linked to a disastrous drought at 4025 BP (cal.) (de Menocal,
2001) so this ~4100 BP climate event was global in extent and
probably culturally more significant than the 8200 BP event. The field and laboratory work for these projects have been gen-
Booth et al.(2005) reported a North American-wide drought 4 erously supported by International Arctic Research Center
ka BP and the peat bog inception study of Gorham et al. (2007) (IARC), the National Institute for Polar Research (Japan), the
also captures this event (see Figure 6). A regional analysis of the Geological Survey of Canada, the Niels Bohr Institute of the
peat data (Figure 6 ) shows, however, that there was in fact no 4.1 University of Copenhagen and the National Science Foundation
ka drought in the extreme west coast, Yukon and Alaska but rather (NSF grants OPP-0136005, OPP-0240878, 0094587). The assis-
the opposite. tance of the Kluane Research Station manager Andy Williams
Other large Asian monsoon failures have been reported by (Arctic Institute of North America) was invaluable. The active
Morrill et al. (2006) in 7000 to 7500 BP, which align with another help and cooperation of Gerrold Holdsworth is also gratefully
coincidence of very low δ(18O), high dand high salt in the Logan acknowledged. The paper was improved through discussion with
Holocene. Earlier than 7500 the Holocene has been characterized Lloyd Keigwin.
as a time of strong monsoons, driven partly by the early Holocene
northern insolation maximum enhancement of terrestrial tempera-
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