Introduction
The key to gaining information on climate
analogues and periodicities, on decadal to multi-centennial and millennial timescales, is the measurement of proxy records over recent millennia, with
multi-annual resolution and matching accuracy in dating.
Among the different timescales of natural climatic variability, the
centennial scale is particularly interesting, being comparable to the scale of
human life and to the modern variation related to anthropogenic forcing
.
The instrumental observations, covering only a couple of centuries
, are
influenced by human activity and are also too short to study
centennial variability. In order to overcome this problem, several
large-scale temperature reconstructions have been proposed, from both
single-proxy (tree rings, corals, varved sediments, cave deposits, ice cores,
boreholes, glaciers, and ocean and lake sediments) and multi-proxy records
deriving from different
geographical locations: ice cores for high latitudes, tree
rings for mid-latitudes and corals
for low latitudes. However, palaeoclimatic
reconstructions depend on multiple, often uncontrolled, factors, e.g.
multi-proxy weighting and proxy calibration. These factors may lead to
non-robust reconstructions .
Marine cores with very high sedimentation rates allow for investigation of climate
variations on scales of decades to millennia. In order to avoid possible
artefacts produced by the composition of different proxies, we measured the
oxygen isotopic ratio δ18O in the shells of the
surface-dwelling planktonic foraminifera Globigerinoides ruber in
a high-resolution, well-dated central Mediterranean core. The isotopic
composition of the shell, deposited on the sea bottom after the death of the
organism, reflects the chemical and physical properties of marine surface
waters, and therefore can give information about the environmental conditions
in which the shell grew.
In a previous paper we presented a 2200-year-long
foraminiferal δ18O series and detected significant modes of
variability from decadal to multicentennial scales, using singular-spectrum
analysis (SSA) and other spectral methods. The isotopic profile showed
features related to particular climatic periods, such as the low
δ18O values around AD 1000 (corresponding to the Medieval Warm
Period – MWP), the high δ18O values during the 18th century
(corresponding to the Little Ice Age – LIA), the sudden decrease in
δ18O values starting from the 19th century (related to the
temperature increase during the industrial era, and the high δ18O
values at the beginning of the Common Era, suggesting a local decrease in
temperature.
The record has now been extended to cover the last 2700 years. The aim of the
present work is to investigate the spectral features of the prolonged series
in order to detect the modes describing the climate variability over the
interval 707–200 BC, in comparison with the following two millennia.
Moreover, the results of a recent study by our group concerning Northern Hemisphere
(NH) temperature and based on a reliable and extended data set
allow for the local variability in the central Mediterranean to be compared with that
characterizing NH.
Experimental procedure
Since the 1990s, the Torino cosmogeophysics group has been studying
shallow-water Ionian Sea sediment cores, drilled from the Gallipoli Terrace
in the Gulf of Taranto, and has carried out their absolute dating. The
Gallipoli Terrace is a particularly favourable site for high-resolution
climatic studies, due to a high sedimentation rate and to the possibility of
accurate dating, offered by the presence along the cores of volcanic markers
related to eruptive events that occurred in the Campanian area, a region for which
documentation of the major eruptions is available. Historical documents are
quite detailed for the last 350 years (a complete catalogue of
eruptive events, starting from 1638, is given by ,
), while they are rather sparse before that date.
The markers of the eruptions were identified along the cores as peaks of the
number density of clinopyroxene crystals, carried by the prevailing westerly
winds from the volcano to the Ionian Sea, and deposited there as part of
marine sediments. The time–depth relation for the cores retrieved from the
Gallipoli Terrace was
obtained by tephroanalysis, which confirmed, improved and extended, to the
deeper part of the core, the dating obtained in the upper 20 cm by the
radiometric 210Pb method .
further confirmed this dating by applying advanced statistical procedures
.
The cores were sampled every 2.5 mm, and the number density of
clinopyroxenes of clear volcanic origin, characterized by skeletal morphology
and sector zoning, was determined for the last two millennia. Twenty-two sharp
pyroxene peaks, corresponding to historical eruptions of the Campanian area,
starting from the Pompeii event in AD 79 and ending with the last Vesuvius
eruption in AD 1944, were found. The depth h in centimetres at which
a volcanic peak is found turned out to be related to the historical date of
the corresponding eruption, expressed in years counted backward from AD 1979
(hence years before top, yBT) by h=(0.0645±0.0002)yBT, with a very high correlation coefficient
(r=0.99). The linearity of this relationship demonstrates that the
sedimentation rate has remained constant over the last two millennia to
a very good approximation. Moreover, the measurements performed in different
cores retrieved from the same area showed that this rate is also uniform
across the whole Gallipoli Terrace . The
very sharp pyroxene peaks indicate that bioturbation by bottom-dwelling
organisms is quite limited; thus we were able to conclude that the climatic
information obtained from these cores is not significantly affected by
sediment mixing.
The series presented here was measured in the GT90/3 core
(39∘45′53′′ N,
17∘53′33′′ E). In order to
obtain the δ18O value of each sample, we soaked 5 g of
sediment in 5 % calgon solution overnight, then treated it in
10 % H2O2 to remove any residual organic material, and
subsequently washed it with a distilled-water jet through a sieve with
a 150 µm mesh. The fraction > 150 µm was kept and
oven-dried at 50 ∘C. The planktonic foraminifera
Globigerinoides ruber were picked out of the samples under the
microscope. For each sample, 20 to 30 specimens were selected from the
fraction comprised between 150 and 300 µm. The use of
a relatively large number of specimens for each sample removes the isotopic
variability of the individual organisms, giving a more representative
δ18O value. The stable isotope measurements were performed
using a VGPRISM mass spectrometer fitted with an automated ISOCARB
preparation device. Analytical precision based on internal standards is
better than 0.1 ‰. Calibration of the mass spectrometer to VPDB scale
was done using NBS19 and NBS18 carbonate standards.
δ18O profile (707 BC–AD 1979) measured in the
Ionian GT90/3 core (grey line). In order to agree in tendency with
temperatures, the isotopic ratio is plotted “upside down”. The sampling
interval is Δt=3.87y, the raw data mean is xm=0.47‰ and their standard deviation is σ=0.23‰.
δ18O signal reconstruction obtained by summing up its first 12
significant components extracted by SSA (blue line). The signal
reconstruction obtained from the SSA analysis of the shorter, previously
published δ18O time series (Taricco et al., 2009) is shown as
a black line. Except for a negligible border effect, the agreement between
the two smooth curves is excellent over their common section (r=0.99).
Results and discussion
In a previous paper we presented the
δ18O measurements performed in the upper 173 cm of the
GT90/3 core (560 samples). The δ18O series has now been
extended, obtaining a continuous record of 694 points covering the last
2700 years (707 BC–AD 1979), shown in Fig. (grey
line). The high sampling rate (Δt=3.87 years) makes this
palaeoclimatic record suitable for the study of both long- and short-term
variability components.
In Fig. , δ18O is plotted “upside down” in order to
agree in tendency with temperature. Since δ18O reflects
changes both in sea surface temperature (SST) and sea water isotopic
composition, it is necessary to reliably extract independent
components of variability and identify the temperature-driven ones.
Thus, several classical and advanced spectral methods were applied to the
δ18O time series, such as classical Fourier analysis, the maximum
entropy method (MEM), singular-spectrum analysis (SSA) and the multi-taper method
(MTM). Two review papers and references therein cover
these methodologies. The application of more than one spectral method ensures
that reliable information is extracted from the δ18O record,
in spite of its low signal-to-noise ratio. Here we focus on the SSA results
that were obtained using an embedding dimension M=150, equivalent to
a time window MΔt≈600 years, but we will also show that
these results are stable to varying M over a wide range of values. We refer
the interested reader to the Appendix for technical details on both SSA and
Monte Carlo SSA (MC-SSA).
The SSA spectrum is shown in the main panel of Fig. ,
where the 150 eigenvalues are plotted in decreasing order of power.
Eigenvalue spectrum from the SSA of the δ18O record
(window length M=150). Each eigenvalue describes the fraction of total
variance in the direction specified by the corresponding eigenvector
(empirical orthogonal function – EOF). Inset: Monte Carlo SSA test using
EOFs 1–12+AR(1) as the null-hypothesis model. The Monte Carlo ensemble size
is 5000. The empty squares highlight the eigenvalues corresponding to the
EOFs included in the null hypothesis, while the blue squares represent the
eigenvalues corresponding to the remaining EOFs. No excursions occur outside
the 99 % limits, indicating that the series is well explained by this
model.
At a first glance, we can notice a break between the initial steep slope
(first 12 eigenvalues) and an almost flat floor. However, to reliably extract
signal from noise, an MC-SSA test MC-SSA;
was applied, showing that the first 12 eigenvalues are statistically
significant at the 99 % confidence level (c.l.) and explain about
46 % of the δ18O total variance.
The inset in Fig. shows the results of the MC-SSA test.
The error bars bracket 99 % of the eigenvalues obtained by the SSA of
5000 surrogate series, all of them generated by a null-hypothesis model that
superposes empirical orthogonal functions (EOFs) 1–12 onto a red-noise process, i.e. an auto-regressive
process of order 1, or AR(1). We can notice that only the eigenvalues
associated with EOFs 1–12, the ones included in the null hypothesis and
represented by empty squares, lie outside the 99 % error bars. This
confirms that the model AR(1) + EOFs 1–12 captures the δ18O
variability at the 99 % c.l.; we drew this conclusion after rejecting,
at the same confidence level, several null hypotheses, including different
combinations of EOFs. Moreover, we chose red noise to accommodate the usual
background assumption in geophysical applications, where the intrinsic
inertia of the system leads to greater power at lower frequencies.
The significant components are a trend (EOF 1) explaining 17.7 % of
total variance, and five oscillatory components of about 600 (EOFs 2–3), 380
(EOFs 4–5), 170 (EOFs 6–8), 130 (EOFs 9-12) and 11 years (EOFs
10–11), respectively explaining 12.0, 6.7, 4.6, 2.3 and 2.4 % of the
total variance. The periods associated to each oscillation were evaluated by
MEM. Figure displays the reconstructions
of the trend and the individual significant
oscillations. In the same figure, these components (coloured lines) are
compared with those obtained by the SSA of the shorter (N=560)
δ18O time series, represented by black lines .
The agreement between the old and new reconstructed components is good;
moreover, the small differences balance out if we consider the total
reconstruction (RCs 1–12) of both the shorter and extended
δ18O time series (Fig. , black and blue smooth
curves, respectively): the match between the two total reconstructions is
excellent over their common time span, with a correlation coefficient r=0.99. Only around the first century BC does the shorter series show a small
border effect.
Significant components extracted by SSA from the δ18O
record: RC1 (trend), RC 2–3 (600 years), RC 4–5 (380 years), RCs
6–8 (170 years), RCs 9-12 (130 years) and RCs 10–11
(11 years). The black curves represent the reconstructions of the same
oscillations provided by the analysis of the shorter, previously published
δ18O time series (Taricco et al., 2009).
Reconstructed components from the SSA of the δ18O
time series, obtained adopting different values for the window length M.
Thus, the SSA analysis of the longer δ18O time series
strengthens the results presented in our previous paper , both
detecting the same significant oscillations and, as a consequence, leading to
the same signal reconstruction. In order to test the robustness of these
results, we repeated the analysis, letting M vary over a wide range of
values (100–250). Figure shows the reconstructions of
the 600-, 380- and 11-year oscillations for three
values of M (150, 200 and 230). We notice that there is good agreement
between the reconstructions corresponding to different values of M, so that
the robustness of our analysis with respect to changes of the window is
ensured.
The long-term variability features characterizing the δ18O
time series are captured by the trend (upper panel of Fig. ),
showing the pronounced maximum near AD 0, the minimum during the MWP
(AD 900–1100) and the increase from the MWP toward the LIA. The 170-year
oscillation, shown in the same figure, exhibits relative maxima around AD 1500,
1700 and 1900, possibly associated with the Spörer (AD 1460–1550),
Maunder (AD 1645–1715) and modern minima of solar activity.
In order to compare the variability detected in the δ18O
profile with that characterizing Northern Hemisphere (NH) temperature, we
constructed and analysed a data set of 26 temperature-proxy records,
extending back at least to AD 1000 and having decadal or better resolution
. In order to ensure careful temperature calibration of the
proxy data , our data set contains only series satisfying the
requirement that the temperature calibration of each proxy record be provided
by the authors who published the record itself. The properties of the 26
records are listed in Table .
This data set was analysed using multi-channel singular-spectrum analysis
MSSA; see, a multivariate extension of SSA, with
each channel corresponding to one of the time series of interest.
Application of SSA requires uniformly spaced time series; therefore, all the
time series were interpolated to a common annual resolution. We then applied
MSSA over the largest-possible common interval, spanning AD 1000 to 1935
(936 years; N=936). We used a window length of 300 years, i.e.
M=300≤N/3.
High variance was found in the NH data set at both multi-decadal and
centennial timescales, relative to what would be expected under the
red-noise hypothesis. The significant reconstructed components are RCs 1–2
(trend), 6–8 (170 years), 9–10 (110 years), 12–13 (80 years), 16–17 (45 years) and 18–19
(60 years) .
In our previous paper , thanks to an alkenone-derived SST time
series measured in cores extracted from the Gallipoli Terrace ,
we suggested that the long-term trend and the 200-year oscillation in the
δ18O record are temperature-driven. Here we notice that these
two components dominate the spectrum of the NH temperature data set, which
not only confirms that they are temperature-related but also that they
characterize the dominant variability of the whole NH. These two modes also
give the most important contributions to the net modern NH temperature rise
.
Characteristics of the 26 temperature time series in the NH data
set. The columns in the table give a two-letter acronym; a full name based
on the location; longitude; latitude; the archive from which the series was
extracted; the proxy type; the time span; the sampling interval, Δt; and the published reference. The identification of the archives uses the
following abbreviations: LS, lake sediments; IC, ice core; TR, tree rings;
MS, marine sediments; MP, multi-proxy composite; ST, speleothems; DO, documentary.
Acronym
Name
Long.
Lat.
Archive
Proxy type
Time span (yr)
Δt (yr)
Reference
ML
Lower Murray Lake
-69.32
81.21
LS
Mass accumul. rate
3236 BC–AD 1969
1
G2
GISP2
-38.5
72.6
IC
δ15N and δ40Ar
2000 BC–AD 1993
1
FL
Finnish Lapland
25
69
TR
Ring width
2000 BC–AD 2005
1
FE
Fennoscandia (Laanila)
27.3
68.5
TR
Height increment
AD 745–2007
1
TR
Torneträsk
19.80
68.31
TR
Density
AD 500–2004
1
SV
Northern Scandinavia
25
68
TR
Density
138 BC–AD 2006
1
VP
Vøring Plateau
7.64
66.97
MS
Foraminifer
883 BC–AD 1995
Irregular
DL
Donard Lake
-61.35
66.66
LS
Varve thickness
AD 752–1992
1
NI
North Icelandic Shelf (MD99-2275)
-19.3
66.3
MS
UK37′ alkenone
2549 BC–AD 1997
Irregular
IL
Iceberg Lake
-142.95
60.78
LS
Varve thickness
AD 442–1998
1
GA
Gulf of Alaska
-145
60
TR
Ring width
AD 724–2002
1
GD
Gardar Drift (RAPiD21-3K)
-27.91
57.45
MS
UK37′ alkenone
5 BC–AD 1959
Irregular
RP
Russian Plains
45
45
MP
–
AD 5–1995
10
HL
Hallet Lake
-146.2
61.5
LS
Biogenic silica
AD 116–2000
Irregular
TL
Teletskoye Lake
87.61
51.76
LS
Biogenic silica
1018 BC–AD 2002
1
CE
Central Europe (Alpine arc)
8
46
TR
Ring width
499 BC–AD 2003
1
SC
Spannagel Cave
11.40
47.05
ST
δ18O
90 BC–AD 1932
Irregular
AL
The Alps (Lötschental)
8.0
46.3
TR
Density
499 BC–AD 2003
1
FA
French Alps
9
46
TR
Ring width
AD 751–2008
1
NS
Northern Spain
-3.5
42.9
ST
δ13C
1949 BC–AD 1998
Irregular
SH
Shihua Cave
115.56
39.47
ST
Layer thickness
665 BC–AD 1985
1
SS
Southern Sierra Nevada
-118.9
36.9
TR
Ring width
AD 800–1988
1
TI
Tibet
98.5
36.5
TR
Ring width
AD 1000–2000
1
SP
Southern Colorado Plateau
-111.4
35.2
TR
Ring width
250 BC–AD 1996
1
EC
East China
114
35
DO
Historical
AD 15–1977
Irregular
CS
China Stack
100
35
MP
–
AD 0–1990
10
A centennial-scale periodicity is also in common between our local proxy
record and NH temperature anomalies. However, spectral analysis of the
700-year-long local alkenone-based temperature record does not detect
a centennial component (figure not shown) and therefore we can deduce that
this climatic variability mode is not present locally. Its presence in the
δ18O record should thus derive from changes in the isotopic
composition of sea water.
Focusing on the two dominant modes of NH temperature, in
Figs. and we show their time behaviour
reconstructed at each site, plotted in the upper panels of the figures in
order of increasing latitude. In the lower panels we show the corresponding
RC pairs averaged over two different latitude bands (30–60
and 60–90∘ N), as well as over the whole NH.
The trend (RCs 1–2) marks the MWP and the LIA climatic features and is
present in both latitude belts. The cooler temperatures associated with the
LIA appear first in mid-latitudes and propagate on to higher latitudes. The
bicentennial oscillation (RCs 6–8; 170-year period), when averaged over the
two different latitude belts, exhibits comparable amplitudes and a good phase
agreement, as shown especially by the lower panel of Fig. .
Figure a compares the δ18O and NH
temperature trends. The two oscillations are in fair phase agreement: they
exhibit nearly contemporary MWP features, while the LIA temperature minimum
(δ18O maximum) seems to have occurred slightly later at
Gallipoli in respect to the whole NH. The average NH temperature decrease
between the MWP and the LIA is of about 0.4 ∘C (black curve;
also visible in the lower panel of Fig. ). At
mid-latitudes (30–60∘ N; orange curve in the lower panel of
Fig. ), the MWP–LIA temperature difference appears to be
of the same order.
The individual series of the NH data set show, however, a certain difference
in trend amplitudes (see the upper panel of Fig. ). If we
focus on the central Europe record , which is representative of
a relatively large European area extending latitudinally from the Alps to
northern Germany, we find a MWP–LIA decrease of the order of
0.3 ∘C. The alkenone-derived SST measurements from the
Gallipoli Terrace , covering AD 1306–1979, show a local
temperature decrease from ∼ 1300 to ∼ AD 1700 of about
0.5 ∘C, in agreement with NH temperature, as may be
expected for a long-term, global variation.
Reconstructed components RCs 1–2 of the NH temperature data set,
representing the long-term trend; colour bar is for amplitude from -0.60 to
0.60 ∘C. Upper panel: RC pair of temperature anomalies
from MSSA analysis as a function of increasing latitude; lower panel:
the same RC pair averaged over two latitude bands, namely
30–60∘ N (orange) and 60–90∘ N (green), as well
as over the entire NH (black). The red curve represents the trend of the
central Europe series.
Reconstructed components RCs 6–8 of the NH temperature data set,
representing a bicentennial oscillation; colour bar for amplitude from -0.20
to 0.20 ∘C. Upper panel: RC pair of temperature
anomalies from MSSA analysis as a function of increasing latitude; lower
panel: the same RC pair averaged over two latitude bands, namely
30–60∘ N (orange) and 60–90∘ N (green), as well
as over the entire NH (black). The red curve represents the bicentennial
oscillation of the central Europe series.
On the other hand, the MWP–LIA increase in the trend component of
δ18O (Fig. a, dark-red curve) is about
0.025 ‰: according to the Shackleton equation , assuming
a nearly constant oxygen isotopic ratio of sea water during the considered
time interval, this variation would correspond to a cooling of
∼ 0.1 ∘C only. Thus at the Ionian Sea scale,
δ18O indicates a MWP–LIA temperature difference that is
smaller than that found locally in the alkenone series, as well as
hemispherically in the NH data set. This could be due to a contemporary change
in the hydrological balance of the Ionian basin: a decrease in evaporation,
accompanying the temperature decrease, would imply a reduction of the
δ18O of sea water and therefore a salinity increase
. Therefore the Ionian temperature MWP–LIA decrease,
calculated from the Shackleton equation, would be greater than the one calculated
assuming the δ18O of sea water to be constant.
Using the alkenone-based MWP–LIA temperature variation of
0.5 ∘C, from the Shackleton equation we find that the
+0.025 ‰ variation observed in the calcite δ18O of
foraminifera shells would be justified if the δ18O of Ionian
Sea water had varied, over the same time interval, by -0.1‰. This
change would correspond, according to , to a salinity decrease
of about 0.4 PSU, a value that is of the order of the salinity variability
range measured at Gallipoli during the last 60 years .
We can thus state that the trend component of δ18O reflects
the long-term variations of NH temperature, provided that plausible changes
in the hydrological balance of the Ionian basin are taken into account.
Comparison between the reconstructed components extracted by SSA
from the δ18O profile and the corresponding oscillations
extracted by MSSA from the NH temperature data set. (a) Long-term
trend: δ18O RC 1 (dark-red line) and NH temperature RCs 1–2
(black line); (b) 170-years oscillation: δ18O
RCs 6–8 (green line) and NH temperature RCs 6–8 (black line).
Turning now to the bicentennial component, we compare the δ18O
and NH temperature 170-year oscillations in Fig. b
(green and black curves, respectively). The average amplitude for NH
temperature is about 0.06 ∘C, but, as shown by the upper panel
of Fig. , the amplitude of this component varies
considerably from record to record in the NH data set. Among the individual
local records we actually notice larger amplitudes, as in the case of central
Europe, for which the 170-year oscillation amplitude is as large as
0.2 ∘C. This is not surprising, considering the shorter timescale considered here. The δ18O amplitude is of the order of
0.4–0.5 ‰, which according to the Shackleton equation and in the absence
of salinity variations would correspond to 0.2 ∘C, in
agreement with central Europe. On the other hand, the Ionian alkenone-derived
SST record has a bicentennial variation amplitude of about
1 ∘C , which would imply a local amplification
effect in respect to European variability at this scale. This suggests that, also at this scale, it may be necessary to invoke salinity variations to
explain the observed δ18O variations.