CPDClimate of the Past DiscussionsCPDClim. Past Discuss.1814-9359Copernicus GmbHGöttingen, Germany10.5194/cpd-11-1407-2015Millennial-scale precipitation variability over
Easter Island (South Pacific) during MIS 3:
inter-hemispheric teleconnections with North Atlantic
abrupt cold eventsMargalefO.omargalefgeo@gmail.comCachoI.https://orcid.org/0000-0002-6512-0770Pla-RabesS.Cañellas-BoltàN.PueyoJ. J.SáezA.PenaL. D.Valero-GarcésB. L.RullV.GiraltS.Ecological Research Center and Forestry Applications
(CREAF), Campus de Bellaterra (UAB)
08193 Cerdanyola del Vallès,
Barcelona, SpainFaculty of Geology, Universitat de
Barcelona, Martí Franquès s/n, 08028 Barcelona, SpainInstitute of Earth Sciences Jaume Almera
(ICTJA-CSIC), Sedimentary Geology and Geohazards, Lluís Solé i Sabarís
s/n, 08028 Barcelona, SpainPyrenean Institute of Ecology
(IPCSIC), Avda, de Montañana 1005,
50059 Zaragoza, SpainO. Margalef (omargalefgeo@gmail.com)17April20151121407143513March201523March2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://cp.copernicus.org/preprints/11/1407/2015/cpd-11-1407-2015.htmlThe full text article is available as a PDF file from https://cp.copernicus.org/preprints/11/1407/2015/cpd-11-1407-2015.pdf
Marine Isotope Stage 3 (MIS 3, 59.4–27.8 kyr BP) is
characterized by the occurrence of rapid millennial-scale climate
oscillations known as Dansgaard–Oeschger cycles (DO) and by abrupt
cooling events in the North Atlantic known as Heinrich events.
Although both the timing and dynamics of these events have been
broadly explored in North Atlantic records, the response of the
tropical and subtropical latitudes to these rapid climatic
excursions, particularly in the Southern Hemisphere, still remains
unclear. The Rano Aroi peat record (Easter Island, 27∘ S)
provides a unique opportunity to understand atmospheric and oceanic
changes in the South Pacific during these DO cycles because of its
singular location, which is influenced by the South Pacific
Anticyclone (SPA), the Southern Westerlies (SW), and the
Intertropical Convergence Zone (ITCZ) linked to the South Pacific
Convergence Zone (SPCZ). The Rano Aroi sequence records 6 major
events of enhanced precipitation between 38 and
65 kyr BP. These events are compared with other
hydrological records from the tropical and subtropical band
supporting a coherent regional picture, with the dominance of humid
conditions in Southern Hemisphere tropical band during Heinrich
Stadials (HS) 5, 5a and 6 and other Stadials while dry conditions
prevailed in the Northern tropics. This antiphased hydrological
pattern between hemispheres has been attributed to ITCZ migration,
which in turn might be associated with an eastward expansion of the
SPCZ storm track, leading to an increased intensity of cyclogenic
storms reaching Easter Island. Low Pacific Sea Surface Temperature
(SST) gradients across the Equator were coincident with the
here-defined Rano Aroi humid events and consistent with
a reorganization of Southern Pacific atmospheric and oceanic
circulation also at higher latitudes during Heinrich and
Dansgaard–Oeschger stadials.
Introduction
With On suborbital timescales, climate in the Northern Hemisphere
during MIS 3 was dominated by rapid millennial-scale temperature
oscillations defined as the Dansgaard–Oeschger (DO)
stadial-interstadial cycles (Dansgaard et al., 1993). During some of
the DO stadials, large armadas of icebergs covered the North
Atlantic Ocean, causing the so-called Heinrich events, which induced
a rapid weakening of the Atlantic Meridional Overturning Circulation
(AMOC) (Heinrich, 1988; Broecker, 1994; Ganopolski and Rahmstorf, 2002;
Hemming, 2004).
Although most of the records documenting this rapid climate
variability are concentrated in the North Atlantic region, a number of
studies from the tropical Atlantic and Pacific Oceans point towards
a linkage between cooling episodes in the North Atlantic records and
millennial-scale changes in Sea Surface Temperature (SST), humidity
and marine productivity in the tropics (Baker et al., 2001; Haug
et al., 2001; Wang et al., 2004, 2007; Muller, 2006; Clement and
Peterson, 2008). This evidence suggests a connection between high and
low latitudes to rapid climatic oscillations during last glacial
cycle, which likely involved a tied ocean–atmosphere coupling (Oppo
et al., 2003). Some tropical and subtropical records of MIS 3 from
Africa, South America and Australia show changes in precipitation
patterns during Heinrich Stadials (HS) (Wang et al., 2004; Muller,
2006; Tierney et al., 2008). MIS 3 changes in SST and salinity in the
West Equatorial Pacific have been interpreted as changes in the super
El Niño–Southern Oscillation (ENSO) state (Stott et al., 2002),
whereas others have regarded changes in tropical rainfall as evidence
of latitudinal displacements of the Intertropical Convergence Zone
(ITCZ) (Wang et al., 2007; Leduc et al., 2009).
Currently, there are few records available with an appropriate
temporal resolution to characterize millennial-scale changes in
tropical and subtropical areas (Arz et al., 1998; Peterson et al.,
2000; Rosenthal et al., 2000; Haug et al., 2001; Wang et al., 2004,
2007; Cruz et al., 2005; Muller et al., 2006; Conroy et al., 2008).
However, none of these records is adequate for tracking changes over
Central Pacific, an area were paleoceanographical records are absent
due to the extremely low marine primary productivity which makes the
sediments unsuitable for such climatic reconstructions. Easter Island
is situated in a key area for understanding South Pacific climate,
filling a regional gap without proper paleoclimatic registers suitable
to understand MIS 3 climate. The Rano Aroi peatland is located in the
highest area of Easter Island and its environmental history and has
been extensively studied from an interdisciplinary approach (Margalef
et al., 2013, 2014). The hydroclimatic sensitivity of this mire, the
broad multiproxy dataset available and the adequate resolution of its
record makes Rano Aroi an excellent location to provide
a comprehensive reconstruction of the South Pacific Convergence Zone
(SPCZ) evolution during the MIS 3.
In order to better understand the regional ocean-atmosphere
connections, the Rano Aroi record dataset (Margalef et al., 2013,
2014) is compared to a new suite of equatorial SST gradient
estimations based on previously published SST reconstructions. We also
compare the Rano Aroi record to high latitude datasets from the
Northern and Southern Hemispheres to discuss the inter-latitudinal
connections responsible for the propagation of rapid climate
variability during MIS 3.
Study site
Easter Island (Chile, 27∘07′ S,
109∘22′ W), or Rapa Nui in the local indigenous
language, is a small Miocene volcanic island in the South Pacific
Ocean, 3510 km from the South American continent (Fig. 1). The
climate at the study site is subtropical, with average monthly
temperatures oscillating between 18 and 24 ∘C and with
a highly variable annual rainfall (mean value 1130 mm) (Junk
and Claussen, 2011).
The climate on Easter Island displays low seasonality. However,
a seasonal latitudinal migration of the ITCZ, SPCZ and westerly storm
track is responsible for higher precipitation rates between March and
June. Two processes are responsible for rainfall formation over Easter
Island: (1) cyclonic storms associated with SPCZ dynamics and pushed
eastwards by the Southern Westerlies (Sáez et al., 2009, Junk and
Claussen, 2011), and (2) land-sea breeze convection storms (Junk and
Claussen, 2011). An analysis of the 1987–2005 satellite data
performed within the framework of the HOAPS-3 project indicates that
Easter Island lies at the active edge of the SPCZ, with associated
precipitation rates between 657 and 803 mmyr-1 (Andersson
et al., 2007; Junk and Claussen, 2011). This analysis of the HOAPS-3
dataset excluded topographic effects, and for this reason, the
disparity between estimated and recorded rainfall rates has been
attributed to the contribution of island topography to convective
storms (Junk and Claussen, 2011).
Paleoenvironmental studies from Easter Island have traditionally been
based on pollen analyses (Flenley and King, 1984; Flenley et al.,
1991; Dumont et al., 1998; Butler et al., 2004; Gossen, 2007; Azizi
and Flenley, 2008; Horrocks and Wozniak, 2008; Mann et al., 2008;
Cañellas-Boltà et al., 2013) and macrofossils remains (Dumont
et al., 1998; Orliac and Orliac, 1998, 2000; Peteet et al., 2003; Mann
et al., 2008; Cañellas-Boltà et al., 2012), which have allowed
the reconstruction of regional paleoclimatic and paleoenvironmental
conditions from the last glacial period to the Holocene (Flenley
et al., 1991; Azizi and Flenley, 2008; Rull et al., 2010a). Recent
multiproxy studies which combined sedimentological, mineralogical,
geochemical and biological data, have also documented hydrological
changes in Easter Island since ca. 34 kyrcalBP (Sáez
et al., 2009; Cañellas-Boltà et al., 2012, 2013) and since MIS
4 (Margalef et al., 2013).
The Rano Aroi mire
(27∘5′36′′ S–109∘22′25′′ W,
430 m elevation) is located in a volcano crater near the
highest summit of the island, Mauna Terevaka (511 ma.s.l.)
(Fig. 1). The chemical composition of the flowing Rano Aroi outlet
(lightly acidic, pH =5.5–6.5) is similar to that of the region's
groundwater. Water isotopic data (δ18O and
δ2H) indicates that waters are renewed through
discharge from an aquitard, which is quite sensitive to seasonal
variations in precipitation (Herrera and Custodio, 2008). The
hydrology configuration in this region indicates that Rano Aroi is
a self-sealing mire fed by deeper discharging groundwater rather than
by interflow (Margalef et al., 2013).
Methodology
Detailed information about field campaign, peat sampling and chemical
analyses (TC, TN, δ13C, Fe, Ti and Ca) can be found in
Margalef et al. (2013). δ13C variability
(δ13Cres) was analyzed by subtracting
a 19-sample mean running average from the raw δ13C data
to highlight high-frequency events compared to long-term
tendencies. This 6-variables dataset was selected because it permits
to reconstruct the main environmental processes controlling Rano Aroi
geochemical evolution (Margalef et al., 2013).
A new age model for the bottommost part of the record has been set up
using the ages provided at Margalef et al. (2013). Chronologic
uncertainties are one of the most common troubles on studies
comprising the MIS 3 period, because they are situated beyond the
radiocarbon limit. In this study we estimate age-depth relationship
using a mixed-effect model and constant variance (Heegaard et al.,
2005) using R software (R Development Core Team, 2011). This method,
highlights not only the error estimate of the sample, but also the
uncertainty related to how representative the obtained age is in
relation to the object level. The procedure combines two random
effects (within-object variance and between-object variance) obtaining
better confidence intervals than other methodologies for modeling ages
beyond the dating limit (Heegaard et al., 2005). This mixed regression
method has been run to model the age-depth relation in the well
constrained part of the record (235–750 cm depth). The
obtained model, was used to determine the age of the bottommost part
of the register, older than the radiocarbon dating limit.
Statistical treatment of the data was performed with R software (R
Development Core Team, 2011) and the “vegan” package (Oksanen
et al., 2005). Principal component analysis (PCA) on the data that
represented the MIS 3 was run to extract the main components of
variability of the geochemical data (TC, TN,
δ13Cres, Fe, Ti and Ca), standardizing and
omitting samples with missing values. As in Margalef et al. (2013) and
because of the different sampling resolution of the XRF dataset
(2 mm) and of the geochemical data (5 cm), the XRF
dataset was resampled at 5 cm intervals to make both datasets
comparable. The resampling involved obtaining the mean values of the
Ca, Ti and Fe measurements in every 5 cm.
A new gradient estimation between the Western and Eastern Equatorial Pacific
have been calculated using previously published SST datasets from sites ODP
1240 site in the Eastern Equatorial Pacific Ocean (Pena et al., 2008) and
core MD97-2141 in the Western Pacific Ocean (Dannenmann et al., 2003). The
temperature calibrations were those used by authors in their original
publications and have been interpreted to reflect mostly annual average
temperature (Dannenmann et al., 2003; Pena et al., 2008). Previously to the
calculation, all the SST records were resampled at common age interval (every
250 years) using R software (R Development Core Team, 2011). For an
extended explanation of the criteria followed to choose these sites to the
gradient see the Supplement.
ResultsGeochemistry and peat facies
According to Margalef et al. (2013, 2014), the Rano Aroi sequence ARO 06 01
is composed of radicel peat sensu Succow and Joosten (2001), made of fine
roots (diameter <1mm), with <10 % of larger remains,
(Cyperaceae, Poaceae and Polygonaceae). General
description based on geochemistry, peat type, plant components and color has
been used to define four facies (Margalef et al., 2013). The facies A
(reddish peat) is characterized by low TN, Fe and Ti values, elevate carbon
to nitrogen (C / N) ratio and a δ13C signature close to C3
plants values (Fig. 2). On the other hand, facies B (granulated muddy peat)
is described as coarse and brown peat, mainly made of roots and rootlets and
characterized by low terrigenous content.
High C/N ratios, low Fe and Ti
content and δ13C values ranging from -14 to -26 ‰
differentiate this facies (Fig. 2). Facies C (organic mud) is characterized by
thin layers interbedding Facies B and displaying high Fe and Ti values, high
TN and relatively light δ13C values (-14 to
-22 ‰). Facies D (sapric peat) appears as dark and fine grains and
contains organic matter with advanced degradation signs. This facies is
primarily defined by high Fe and Ca content (Fig. 2).
SEM analysis of Facies C sand grains revealed the presence of
plagioclase and quartz grains in the coarse fraction (>500µm). Silt particles (<50µm) were
present on Facies B and C, mainly compound of ilmenite, rutile and
silica. SEM analysis of the terrigenous content of the Facies D showed
that the mineral fraction below 30 µm consisted of
a mixture of Al, Fe, Mn oxides and organic bounded Ca as well as other
organic compounds.
Age model
A detailed description of main features of Rano Aroi age model can be
found in Margalef et al. (2013). The application of mixed-effect model
(Heegaard et al., 2005), instead a an extrapolation, introduces slight
changes, preserving general patterns but improving the age
determination and associated errors of the record bottom part, that
lies beyond the radiocarbon limit (Fig. 3).
We have restricted our study of millennial-scale variability
(including stadial-interstadial oscillations) to the time window
between an stratigraphic discontinuity at 38 kyrcalBP (see
Margalef et al., 2013, 2014) and 65 kyrBP, so the MIS3/MIS4
transition (59.4 kyrBP, Svensson et al., 2008) is also
included in our study (Fig. 2). The facies and stratigraphy present in
the well constrained part of the record selected – between 38.5 and
55 kyr (4.31–8.75 m) – and the portion that is older
than radiocarbon limit are homogenous. Moreover, there were no
evidence of drought episodes by any geochemical, microscopic or
macroscopic observations in the lower part of the record, what
supports age extrapolation by the mixed-effect model approach
(Heegaard et al., 2005).
Principal component analysis result
Principal Component Analysis (PCA) was performed on a dataset composed
of 6 variables (TN, TC, δ13Cres, Ti, Fe and
Ca) and 142 samples, which represented the aforementioned 38.5 to
65 kyrBP period (Figs. 2 and 4). The first component
explained 34.7 % and the second component explained an additional
30.6 % of the total variance (Fig. 4). Ti, Fe and Ca contributed
positively to the first component, whereas TN and
δ13Cres values are found at the opposite end
of the first component. TN and Ti are found at the positive end of PC2
(Figs. 2 and 4), whereas Ca and δ13Cres
contributed negatively to the second component. Facies C scores are
related to Ti, TN and δ13Cres variability,
indicating that they are well represented by PC2 (Figs. 2 and 4).
SST gradient across the equatorial Pacific
The current SST gradient between the E–W locations of the equatorial
Pacific Ocean oscillates from 1.2 ∘C in boreal winter to
6 ∘C in boreal summer when the eastern equatorial upwelling
system is fully developed, while the annual average SST gradient is
3.3 ∘C (World Ocean Data 2009; Locarnini et al., 2010). The
calculated SST gradient between the western and eastern sites shows
average values of 4.4 ∘C, with maximum values reaching
6 ∘C and minimum values at approximately 3 ∘C
(Fig. 5). According to their SST-calibrations the error of these SST
gradients is considered to be better than ±0.6∘C while
the discussion of this SST gradient mostly focuses on those changes
which are above 1 ∘C or even larger (>2∘C).
As stated in Margalef et al. (2013) and Margalef et al. (2014) three
main environmental and hydrologic phases can be distinguished on the
basis of the characterization of the four facies:
Open water phase represented by Facies C. During this phase, coarse
(> 500 m) sandy particles from volcanic soils were transported
through dense mire vegetation and deposited at the center of this
sedimentary deposit. The presence of these sandy particles would
involve high precipitation rates. Low δ13Cres
isotopic values also reinforce the hypothesis that Facies C are
tracking enhanced precipitation events (Margalef et al., 2014). This
open water phase is tracked by high PC2 values (Figs. 2 and 4) what
allows us to use this component as rainfall indicator.
Long-term stable and near-surface water table phase evidenced by
Facies A and B. This phase led the constant accumulation of peat
sediments which were deposited in a kettle hole mire
(Margalef et al., 2014), under accumulation conditions similar to the
present Rano Aroi. Peat presented a low degree of humidification owing
to lower rainfall conditions with respect to the previous ones. This
stable and sub-surficial water table conditions are reflected in the
PC2 component by intermediate values.
A third environmental phase is represented by Facies D and can be
interpreted as a result of diagenesis of previously accumulated peat
(Fig. 2, Margalef et al., 2013). Iron and calcium were incorporated
into the mire as terrigenous particles, but affected by
post-depositional remobilization with water movement and redox
changes. Both chemical elements were incorporated into organic matter
by complexation as Fe–Ca-humates under oxic conditions (Shotyk
et al., 1996; Margalef et al., 2014).
Precipitation patterns over the tropical and subtropical
Pacific during MIS 3
As previously stated in Margalef et al. (2014), the present second
component of variability (PC2) adequately summarizes the occurrence of
a waterlogged environment and higher precipitation periods and,
therefore, it can be used as a non-quantitative
humidity-index. Following this premise, the occurrence of 6 wet events
– labeled Ar1 to Ar6 – have been identified between 38.5 and
65 kyrBP based on their high PC2 values (Fig. 2). Three of
these wet periods (Ar2, Ar4 and Ar6) are particularly
outstanding. These periods are characterized by an abrupt onset and
last for approximately 2000 years. The other three wet events
(Ar1, Ar3 and Ar5) are also characterized by an abrupt start but are
of minor intensity and duration, lasting approximately
1000 years or less (Figs. 5–6). A comparison of the PC2
scores of the Rano Aroi record with other well-established climate
records from the Northern and Southern Hemispheres indicates that the
three major Rano Aroi events can tentatively be correlated with the
North Atlantic HS 5 (ca. 47 kyrBP), 5a
(ca. 53 kyrBP) and 6 (ca. 60 kyrBP), whereas the
three minor events can be correlated with other DO stadials
(Figs. 5–6). Wet events Ar1, Ar2 and Ar3, that are located within the
well-constrained part of the age model, show a good correlation with
cold phases at North Atlantic coinciding with DO stadials and Heinrich
events. This pattern is maintained in the bottommost part of the
record, therefore supporting the Rano Aroi chronological framework
(Figs. 5–6).
A strong argument that reinforces the link between the Rano Aroi wet
events and the DO stadials is that is mechanistically coherent with
the regional atmospheric and oceanographic reconstructions from
independent proxy records. Some of the most solid evidence of
atmospheric teleconnections for the DO oscillations come from Northern
Hemisphere records such as speleothem records from the Hulu cave
speleothems in China (32∘30′ N; Wang et al., 2001),
the reflectance record from the Cariaco basin, northern Venezuela
(10∘43′ N, Peterson et al., 2000; Haug et al.,
2001) and changes in surface salinity in the Sulu sea indicated by
core MD 97-2141 (8∘48′ N, Oppo et al., 2003;
Dannenmann et al., 2003; Rosenthal et al., 2000). All of these records
consistently indicate the dominance of dry conditions during the HS
and other DO stadials (Fig. 6). These events have been associated with
the opposite behavior in the Southern Hemisphere, as documented by wet
events recorded in Northern Brazil travertine formations (Wang et al.,
2007) and in marine cores offshore of the South American Atlantic
coast (3∘40′ S, Arz et al., 1998), which correlate
with the Rano Aroi major wet events (Fig. 6). A similar climatic
pattern has been described by Muller et al. (2008b) in the Lynch
crater (17∘ S) peat record in North Australia over the last
45 kyrcalBP. This tropical asynchrony between the two
hemispheres has been explained by latitudinal migrations of ITCZ
(Peterson et al., 2000; Tierney et al., 2008; Leduc et al.,
2009). Changes in the AMOC and atmospheric coupling during DO stadials
(interstadials) seems to provoke the southward (northward)
displacement of ITCZ (Zhang and Delworth, 2005; Clement and Peterson,
2008; Chiang and Bitz, 2005; Timmermann et al., 2005). Other authors
propose that main changes in ITCZ migration and in SST temperatures of
tropical Pacific were also linked to monsoon fluctuations. For
example, Oppo and Sun (2005) suggested the connection between periods
of reduced precipitation due to a weaker monsoon over South China Sea
during HS.
Although several studies support the occurrence of these ITCZ
migrations, changes in the position of the SPCZ low-pressure belt are
less well known. In spite of both structures are intimately related,
precipitation over Easter Island is primarily determined by the
arrival of cyclogenic storms generated on the SPCZ (Junk and Claussen,
2011). Easter Island hydrologic changes might record then,
millennial-scale oscillations in the expansion of the SPCZ, coupled
with ITCZ migrations. This could be explained by an eastward expansion
of the SPCZ associated to the southwards shift in the ITCZ during MIS
3, in a Pacific configuration resembling the austral summer. However,
stronger efforts on finding records capable to track SPCZ activity
during MIS 3 and modeling support are required to completely
understand MIS 3 Central Pacific configuration. Rano Aroi record
suggests that abrupt hydrological oscillations were present not only
during HS but also during other shorter DO stadials (Fig. 6). Rano
Aroi record also shows how hydrological conditions underwent abrupt
changes meaning a rapid response to AMOC. These abrupt shifts differed
from the Southern Hemisphere thermal response, which involved gradual
onsets and terminations, as seen in Antarctic ice core records
(Schmittner et al., 2003; Blunier et al., 1998; Blunier and Brooks,
2001; EPICA, 2006) (Fig. 6).
Changes in the E–W equatorial SST gradient of the
Pacific Ocean during MIS 3
Some climate models propose that North Atlantic cold events eventually
lead to changes in the Walker circulation (Zhang and Delworth,
2005). Following Zhang and Delworth (2005) modeling, the southeastern
trade winds were weakened during North Atlantic melting events, and,
consequently, the upwelling in the eastern equatorial Pacific was
drastically reduced what could be related to ITCZ migration and to
ENSO-state changes. This hypothesis can be tested by examining
a reconstruction of the SST gradient across the equatorial Pacific
using the temperature differences between the eastern cold tongue and
the western warm pool. Low values in the SST gradient would be
consistent with a weakening of the Walker circulation and should
reflect periods of diminished upwelling activity in the eastern
equatorial Pacific. Eastern Pacific upwelling is responsible for the
upward transport of relatively cold and salty waters, whereas the very
warm and fresh waters are dominant in the western Pacific warm pool
(Kessler, 2006). By contrast, higher SST gradient values should be
consistent with intensified upwelling in the eastern equatorial
Pacific, whereas a reduced gradient indicates more homogenous
hydrographic conditions along the Equator.
The obtained gradient present lower values of zonal SST gradient
coincident with the Rano Aroi wet events (Fig. 5). Consequently, the
lower equatorial SST gradient associated to DO stadials is consistent
with a muted upwelling in the eastern Equatorial Pacific and a weaker
Walker circulation, which ultimately would favors a southward shift in
the ITCZ and an eastward expansion of the SPCZ, as is interpreted from
the Rano Aroi record. This configuration would be in line with the
already proposed ENSO-like state during cold North Atlantic periods
and the southward migration of the ITCZ based in climate models
(Clement and Cane, 1999) and also in proxy reconstructions (Haug
et al., 2000; Fedorov and Philander, 2000; Koutavas et al., 2006;
Zuraida et al., 2009; Bolliet et al., 2011). An analogous correlation
can be described based on present day instrumental data: extreme El
Niño events are characterized by a mean southward migration of the
Pacific ITCZ (Haug et al., 2000; Fedorov and Philander, 2000) and by
an eastward and northward migration of the SPCZ (Vincent et al.,
2009).
Nevertheless, other models consider these atmospheric mechanisms less
relevant and instead highlight the role of the ocean's circulation
through a global baroclinic adjustment when the North Atlantic cools
and when there is a reduction in the AMOC. These models suggest that
North Atlantic water density variations can lead to changes in the
global thermocline within a few years to decades (Huang et al., 2000).
These authors describe changes in the Pacific thermocline and argue
that, despite the occurrence of climatic fluctuations that can be
explained without invoking a link between El Niño and stadials,
the existence of such a linkage cannot be excluded (Huang et al.,
2000; Timmermann et al., 2005).
Tropical connections to Southern Hemisphere high latitudes
The southward migration of the ITCZ results in both a reinforcement of
the equator-to-pole pressure gradient over the Southern Hemisphere and
in an intensification of the Southern Westerlies (SW) (Toggweiler
et al., 2006; Anderson and Carr, 2010; Heirman, 2011). Changes in the
position and intensity of the SW during MIS 3 should have promoted
storminess over Central Pacific and contributed to an eastward
movement of storms generated under the SPCZ. Another process
intimately linked with the SW is the formation of intermediate water
masses in the Southern Ocean. Changes in the formation rate of
Antarctic Intermediate Water (AAIW) associated with DO cycles have
been described in a marine record from Chatman Rise, East New Zealand
(MD97–2120, 45∘32.06′ S,
174∘55.85′ E, Pahnke and Zahn, 2005) on the basis
of the benthic foraminiferal δ13Crecord (Figs. 1 and
6). It has been demonstrated that periods of increased AAIW production
were in phase with Southern Hemisphere warming and southward shifts of
the ITCZ (Pahnke and Zahn). During DO stadials, the Antarctic
continent and Southern Ocean warmed as a result of the bipolar seesaw,
and, consequently, Antarctic sea ice retreated (Anderson and Carr,
2010; Skinner et al., 2010). This reduced sea ice extent would
contribute to enhanced upwelling of circumpolar deep water and to
a more efficient downwelling of AAIW (Toggweiler et al., 2006;
Anderson and Carr, 2010; Skinner et al., 2010) (Fig. 1). A recent
study using Nd isotopes as a proxy for water mass provenance
demonstrated that an increased export of AAIW from the Southern Ocean
to tropical regions occurred during Northern Hemisphere cold periods
such as HS (Pena et al., 2013). This oceanic circulation scenario also
induced the release of oceanic CO2, which was stored in poorly
ventilated deep-water masses, to the atmosphere (Anderson and Carr,
2010; Skinner et al., 2010). In the context of the Rano Aroi wet
events, these specific oceanic conditions in the Southern Ocean during
DO stadials could have cause increased precipitation over Central
Pacific.
Conclusions
The Rano Aroi peat record provides a unique opportunity to understand
the evolution of South Pacific climate during the late
Pleistocene. This record contains information concerning climate
variability during MIS 3 and is located thousands of kilometers away
from other continental and marine paleoclimatic records. Six main
humid events occurred in Rano Aroi during MIS 3 as a result of an
eastward expansion of the SPCZ and they have been associated to the
North Atlantic HS but also to other minor DO stadials.
Anti-phase changes in precipitation and hydrology have been observed
in low-latitude areas of the Northern and Southern Hemispheres. These
changes have already been linked to North Atlantic cold stadials
through a southward displacement of the ITCZ, as described by several
studies based on both numerical climate models and environmental
reconstructions from Circum-Pacific sites. The Rano Aroi record
allowed us to propose that these stadials were also associated with an
eastward expansion of the SPCZ, highlighting a close coupling between
the migration of the ITCZ and potentially the SPCZ on millennial
timescales. The abrupt character of the Rano Aroi humid events
demonstrates the rapid atmospheric response of the tropical regions to
the DO-related sudden changes in the AMOC, in contrast to the more
progressive heat redistribution in the Southern Ocean led by the
bipolar seesaw. The Rano Aroi wet events have been correlated with
periods of a reduced SST gradient along the Equator, suggesting that
more humid conditions over the Easter Island region occurred when the
Walker circulation was reduced. These atmosphere-ocean connections in
the tropical Pacific could be considered analogous to modern El
Niño conditions. Associated changes in the Southern Ocean with
strengthened Southern Westerlies, enhanced AAIW production and sea ice
retreat during DO stadials, could have also reinforced the SPCZ
extension over Easter Island.
The Supplement related to this article is available online at doi:10.5194/cpd-11-1407-2015-supplement.
Acknowledgements
This research was funded by the Spanish Ministry of Science and
Education through the projects LAVOLTER (CGL2004–00683/BTE),
GEOBILA (CGL2007–60932/BTE), PALEONAO (CGL2010–15767) and RAPIDNAO
(CGL2013–40608R) and through undergraduate CSIC JAEPre grant (BOE
04 March 2008) to Olga Margalef. We would like to thank Hans
Joosten, Alex Barthemles, John Couwenberg, Martin Theuerkauf,
Susanne Abel, Almut Spangenberg, Dirk Michaelis and René Dommain
for their help and contributions to the peatland characterisation
during O.M.'s stay at the University of Greifswald.
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Oceanic and atmospheric spatial patterns of millennial-scale
climate change events (see discussion, Sect. 5) that acted as
climatic teleconnection mechanisms or feedbacks during MIS 3
interstadials (Map A) and HS (Map B). Black dots indicate the most
relevant sites mentioned in the text, double circles indicate
records also represented in Fig. 6 and Rano Aroi is designated as
a red star.
Geochemical proxies analyzed in ARO 06 01 core
vs. depth. Peat facies and radiocarbon ages are indicated in the
first column. Geochemical proxies: TC, TN, (in percentages),
C/N ratio, and δ13C (‰) is
indicative of the origin of organic matter. Residual values of
δ13Cres (‰) are used to enhance the
presence of δ13C dips. Fe, Ti and Ca FRX measurements
(in cps) and Fe/Ti, Ca/Ti ratios are
also shown together with the scores of PC2.
Rano Aroi age model. Samples from ARO 06 01. Ages in red
were rejected by reflecting inversions (Margalef et al., 2013). Ages
in grey lied beyond radiocarbon limit. Error bars for each point are
shown. Black lines shows the result of the mixed-effect model
performed between 235 and 750 cm depth and extrapolated
until the bottommost part of the record. Green dashed line are
showing the confidence limits.
Principal component analysis of the geochemical data
(δ13Cres Ti, Fe, Ca, TN, TC and
C/N). Two principal component axes explain more than
60 % of the variability (Axis 1: 34.7 %, Axis 2:
30.6 %). Variable loadings and sample scores are presented in
the plane defined by the first two axes. Wet events are associated
with high Ti and TN and with low δ13Cres,
which are representative of flood conditions.
Comparison between Rano Aroi PC2 humidity index (green line)
and W–E temperature (∘C) gradient over the Equatorial
Pacific and the corresponding 5 point running average (thick black
and line). The E–W gradient was obtained from the difference
between MD97-2141-SST (red line, Dannenmann et al., 2003) and ODP
1240-SST (blue line, Pena et al., 2008). Wetter events in the Rano
Aroi record coincide with a lower E–W temperature gradient, which
implies that a displacement of the ITCZ and the SPCZ is associated
with weaker oceanic circulation over the Equatorial Pacific,
resembling an El Niño-like state.
Records of marine sediment cores from the Cariaco Basin (ODP
1002C, Peterson et al., 2000), Sulu Sea (MD97 21–41, Dannenmann
et al., 2003; Oppo et al., 2003; Rosenthal et al., 2003), South
Pacific (MD97 21–20, Pahnke and Zahn, 2005) and Atlantic Caribbean
(GeoB3912, GeoB3104; Arz et al., 1998); and the ice core datasets of
NorthGRIP (Svensson et al., 2008) and of Byrd (Blunier and Brooks,
2001). The data presented and the correlations between each dataset
have been reproduced from their original publications. North
Atlantic temperature variability during DO oscillations is
correlated with low-latitude millennial changes in
precipitation. Northern Hemisphere records indicate the occurrence
of dry periods during Heinrich stadials and other stadials and
during Southern Hemisphere wet events.