Geo-Mar Lett
DOI 10.1007/s00367-011-0267-6
ORIGINAL
Evidence for climatic and oceanographic controls
on terrigenous sediment supply to the Indian Ocean sector
of the Southern Ocean over the past 63,000 years
Madhusudhanan C. Manoj & Meloth Thamban &
Natani Basavaiah & Rahul Mohan
Received: 15 April 2011 / Accepted: 17 November 2011
# Springer-Verlag 2011
Abstract Multiple proxy studies on sediment core SK
200/22a from the sub-Antarctic sector of the Indian
Ocean revealed millennial-scale fluctuations in terrigenous input during the last 63,000 years. The marine
isotope stages 1 (MIS 1) and MIS 3 are characterized
by generally low concentrations of magnetic minerals,
being dominated by fine-grained magnetite and titanomagnetite. Within the chronological constraints, periods
of enhanced terrigenous input and calcite productivity
over the last 63,000 years are nearly synchronous with
the warming events recorded in Antarctic ice cores. An
equatorward shift of the westerly wind system in association with a strengthening of the Antarctic Circumpolar Current (ACC) system may have promoted windinduced shallow-water erosion around oceanic islands,
leading to enhanced terrigenous input to the core site.
Major ice-rafted debris events at 13–23, 25–30, 45–48
and 55–58 ka BP are asynchronous with δ18O and carbonate productivity records. This out-of-phase relation suggests that ice-sheet dynamics and ACC intensity were the
primary factors influencing ice rafting and terrigenous input
to the Indian sector of the Southern Ocean, with only limited
support from sea-surface warming.
M. C. Manoj (*) : M. Thamban : R. Mohan
National Centre for Antarctic and Ocean Research,
Headland Sada, Vasco-da-Gama,
Goa 403 804, India
e-mail: [email protected]
N. Basavaiah
Indian Institute of Geomagnetism,
New Panvel,
Navi Mumbai, India
Introduction
Palaeoceanographic studies over the last few decades have
shown that processes occurring in the Southern Ocean and
in the large Antarctic ice sheets have played a crucial role in
defining Earth’s climate (Kennett and Barron 1992). The
Antarctic Circumpolar Current (ACC), regarded as a key
system in understanding the influence of biogeochemical
cycling on the global climate, connects three large ocean
basins (Atlantic, Indian and Pacific) and allows transfer of
water, salts, heat and other properties and constituents
(Nozaki and Alibo 2003). An important feature of the
southern hemisphere is the mid- to high-latitude westerly
wind system which drives the clockwise ACC. This has a
large impact on the Southern Ocean hydrography, sea ice
distribution and biologic productivity, and hence plays a
significant role in modulating atmospheric CO2 concentrations (Sigman and Boyle 2000; Toggweiler et al. 2006).
The ACC is characterized by several circumpolar fronts,
which separate regions of relatively uniform water-mass
properties and coincide with strong current cores (Orsi et
al. 1995; Sparrow et al. 1996). Like the other parts of the
Southern Ocean, the Indian Ocean sector is characterized by
three major fronts: the Subtropical Front (STF), the SubAntarctic Front (SAF) and the Polar Front (Rintoul and
England 2002; Anilkumar et al. 2007; Fig. 1). Both the
STF and the SAF are located north of the Crozet and
Kerguelen plateaus where they form a frontal zone
(Gamberoni et al. 1982; Dezileau et al. 2000), the advection
of water masses being driven by the ACC. The major water
masses playing a significant role in this region are the
Antarctic Bottom Water (AABW), the Circumpolar Deep
Water (CDW) and the Antarctic Intermediate Water. The
AABW originates near the Antarctic continent and flows
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northwards to infill the deep basins at a depth greater than
3,600 m (Orsi et al. 1999). The CDW is the main water mass
involved in the formation of all other water masses occurring between 2,000 and 3,600 m water depth, and it rises
sharply to shallower depths south of the frontal zone. The
CDW within the ACC (ACC-CDW) allows stronger advection with widespread effects on sediment transport and
deposition in the Indian sector of the Southern Ocean
(Dezileau et al. 2000).
Because the Southern Ocean encircles the West and East
Antarctic ice sheets, and thus represents a junction box of
the global conveyor-belt circulation, marine proxy records
from the Southern Ocean provide information on changes in
biogenic productivity, sea ice cover, iceberg rafting and
deep-ocean circulation (e.g. Berger and Wefer 1996;
Filippelli et al. 2007; Anderson et al. 2009). Enhanced input
of magnetic materials during cold marine isotope stages, for
example, has been attributed to more efficient erosion and
transportation by the ACC (Pudsey and Howe 1998;
Mazaud et al. 2010). Terrigenous sediment composition in
the Atlantic and Indian Ocean sectors of the Southern Ocean
is dominated by silt- and clay-sized material with small
proportions of sand and gravel delivered by icebergs and/
or sea ice (Diekmann et al. 2004; Diekmann 2007). Within
the Indian Ocean sector, much of the observed ice-rafted
debris (IRD) consist primarily of volcanic tephra which are
transported and deposited by sea ice/icebergs, with quartz
fragments forming an additional component (Kanfoush et al.
2002; Nielsen et al. 2007). Accordingly, besides the Antarctic ice-sheet dynamics, changes in IRD deposition in the
Southern Ocean may also reflect iceberg survivability and
sea ice melting (Carter et al. 2002). Using a wide array of
proxies, elevated detrital fluxes to the region have been
found to be related to the combined effects of low sea level,
increased glaciogenic input, long-distance aeolian supply
and enhanced current transport (e.g. Kolla et al. 1976;
Bareille et al. 1994; Gaiero et al. 2004; Latimer et al. 2006;
Diekmann 2007; Hillenbrand et al. 2008).
Environmental magnetic parameters of deep-sea sediments have been used as proxy indicators for changes in
palaeoclimate and marine palaeoenvironments before (e.g.
Robinson et al. 1995; Verosub and Roberts 1995). However,
knowledge about the sources of the magnetic minerals, the
influence of diagenesis and environmental conditions is also
important for reconstructing the palaeoclimatic history
(Bloemendal et al. 1992). The magnetic parameters, in combination with other palaeoenvironmental proxies such as
IRD, calcium carbonate content and oxygen isotope records,
are useful palaeoceanographic indicators in marine sedimentary records (Bloemendal et al. 1988, 1992). Within this
global palaeoclimatic and palaeoenvironmental context, the
purpose of the present study was to evaluate environmental
magnetic parameters as palaeoclimatic proxy indicators,
along with other proxy parameters, to reconstruct the
late Quaternary climate history of the region and to
compare this with other global palaeoclimatic records
on the basis of a sediment core (SK 200/22a) collected
from the sub-Antarctic regime of the Indian Ocean
sector of the Southern Ocean as part of a long-term
multidisciplinary program.
Materials and methods
A 7.54-m-long sediment core (SK 200/22a) was collected
from the Indian Ocean sector of the Southern Ocean at 43°
42′S/45°04′E from a water depth of 2,730 m onboard ORV
Sagar Kanya (Fig. 1). The core was sampled onboard at
1 cm intervals for the first 200 cm and at 2 cm intervals
below this. To a depth of 115 cm the core consisted of
alternating layers of calcareous white to grey sandy silt/clay,
followed by a dark greyish band dominated by silty clay
between 130 and 148 cm, and by light-grey clays below this
band. Subsamples were stored in clean, labelled low-density
polyethylene bags, proper care being taken to avoid external
and cross-sample contamination.
Measurements of mass-specific magnetic susceptibility
(χ) were carried out at the Indian Institute of Geomagnetism
(IIG), Navi Mumbai, using a MFK1-FA Multi-function
Kappabridge. Duplicate measurements were performed at
low (0.976 kHz) and high (15.616 kHz) frequencies on all
samples. Frequency-dependent susceptibility (χfd) was calculated from the difference in χ at the two frequencies divided
by the low-frequency χ, i.e. c fd ¼ ½ðc L c H Þ=c L 100%.
Anhysteretic remanent magnetization (ARM), with a peak
alternating field of 100 mT and decreasing amplitude, was
imposed on a steady field of 0.1 mT, the remanence being
measured with a Molspin spinner magnetometer. Isothermal
remanent magnetization (IRM) was achieved by placing the
samples in increasing magnetic fields at room temperature
using a Molspin pulse magnetizer. The IRM acquired at
1.5 T is referred to here as the saturation isothermal remanent magnetization (SIRM). A MMPM9 pulse magnetizer
(Magnetic Measurements Ltd.) was used for inducing IRMs.
All remanent magnetization measurements were made using
an AGICO Molspin spinner magnetometer. The S-ratio
f½ð1 IRM0:3T =IRM1T Þ=2 100g reflects variations in
the coercivity spectrum of magnetic mineral assemblages
and, therefore, the magnetic mineralogy (Bloemendal et al.
1988). The magnetic remanence acquired at low (soft IRM)
and high (hard IRM) applied magnetic fields of the sediment
was also measured. The L-ratio was calculated by taking the
ratio between hard IRM and IRMAF@100mT (hard IRM/
IRMAF@100mT; Liu et al. 2007). The magnetic mineralogy
was investigated on representative samples by (1) measuring
the temperature dependence of magnetic susceptibility using
Geo-Mar Lett
Fig. 1 Study area and location
map showing the position of
core SK 200/22a relative to
those of important oceanographic fronts and the winter sea ice
limit in the Indian Ocean sector
of the Southern Ocean (based on
Park et al. 1993; Labeyrie et al.
1996; Anilkumar et al. 2007).
PF Polar Front, SAF
Sub-Antarctic Front, STF
Subtropical Front
an AGICO KLY-4S Kappabridge with an attached CS3
furnace in an argon atmosphere, and (2) stepwise thermal
demagnetization of SIRM using a MMTD80 furnace (Magnetic Measurements Ltd.) between 100 and 700°C in steps
of 25 and 50°C.
Detailed sedimentological and microscopic studies were
carried out on the coarse fraction to determine the origin and
transport modes of the detrital flux. For this purpose, the
coarse fraction (grain size >125 μm) in every 2 cm interval
sample was studied under a binocular microscope, and 300–
400 terrigenous grains were counted to quantify the IRD
fraction in the sediment (Baumann et al. 1995; Stoner et al.
1996; Nielsen et al. 2007). Inorganic carbon (IC) analyses
were carried out using a TOC Analyser (Shimadzu TOC-V
series SSM-5000A), and the CaCO3 content (weight%) was
calculated with high precision (±5%) using the equation %
CaCO3 0%IC×8.33. Oxygen isotope measurements were
performed at selected depths on ~20 clean Neogloboquadrina pachyderma tests using an Isotope Ratio Mass Spectrometer (Isoprime, GV Instruments). The estimated
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external precision using a laboratory standard (Z-Carrara
marble) was ±0.05‰.
Chronological control was obtained by accelerator
mass spectrometry (AMS) radiocarbon (14C) dating at
selected intervals using the planktonic foraminifers Globigerina bulloides and/or Neogloboquadrina pachyderma.
The age model is based on linear interpolation between the
dated horizons. Because the radiocarbon ages at the depth
intervals of 248–250 and 448–450 cm were close to or
above the critical limit of 14C dating (≥40 ka BP), they were
omitted in the chronological reconstruction. All radiocarbon
ages were calibrated to calendar ages using the Calib 6
program of Stuiver et al. (2005), and all dates cited in the
text thus refer to calendar years (Table 1, Fig. 2). The depth
versus age relationships along with the estimated sedimentation rates are illustrated in Fig. 2. Age controls
beyond the limits of the conventional radiocarbon dating
technique were estimated by correlating the oxygen isotope
data with the SPECMAP (Martinson et al. 1987) and LR04
δ18O stack (Lisiecki and Raymo 2005), and also with the
age models derived from the geomagnetic correlation and
foraminiferal assemblages at site MD94-103 from the midlatitudinal southern Indian Ocean to the east of the
Kerguelen Plateau (Mazaud et al. 2002; Sicre et al.
2005), as well as the stacked δ18O record from the Drake
Passage (Bae et al. 2003). As evident from Fig. 3, the 7.54
metres of the sediment core represent the past ~63,000 years,
comprising the marine isotope stages 1 (MIS 1) to early MIS
4. Thus, the sedimentation rates of various intervals in the
core vary from 4 to 16 cm/1,000 years, and are highest for
sediments deposited during the last glacial maximum
(LGM; Fig. 2).
Results
Bulk magnetic properties
Magnetic concentration
The Holocene (MIS 1) is characterized by low values of χ,
χARM and SIRM (Fig. 4), whereas the glacial intervals (MIS
Table 1 AMS 14C ages determined on planktonic foraminifer
samples from core SK 200/22a,
and calibrated ages after Stuiver
et al. (2005)
2 and MIS 4) show relatively high χ, χARM and SIRM values.
The last deglaciation (Termination I) indicates a rapid increase in χ, χARM and SIRM values, with the most substantial increase between 18.5 and 13 ka BP. The χARM values
fluctuate between 9 and 24×10–8/kg during the early Holocene but are very high (up to 347×10–8/kg) during the early
deglacial period (Fig. 4). The χARM and SIRM values show
similar variations, but with extremely high values during the
early deglacial period in MIS 2 and during the late MIS 4.
To investigate the glacial–interglacial change in the concentration of magnetite, the magnetite accumulation rate
(AR(mag)) was calculated using the age model. The AR(mag)
record resembles the bulk χ record (Fig. 5), suggesting that
the variations in magnetic concentration principally reflect
changes in the flux of magnetite to the study area.
Magnetic grain size
The variations in the magnetic grain-size parameters traced
by χfd%, χARM/χ, SIRM/χ and χARM/SIRM show similar
trends (Fig. 4). The χfd%, which reflects the concentration
of superparamagnetic grains in the sediment, fluctuates between 3 and 6%, and largely follows the trends of the χARM/
χ and SIRM/χ profiles. A uniform effective magnetic grain
size is attested by the strong nonlinear relation between high
and low values of χ with SIRM and ARM (Fig. 6a, b). Such
strong relationships indicate that the systematic changes in
χARM/χ and SIRM/χ with magnetite content are not due to
the changes in the magnetic grain size.
Magnetic mineralogy
Throughout the core, the S-ratio remains high, ranging from
0.92 to 0.96 (Fig. 4). The hard IRM and soft IRM values
vary in similar manner as the magnetic mineral concentration parameters such as χ, χARM and SIRM. The high Sratio and high soft IRM values suggest that the mineralogy
is dominated by fine-grained low-coercivity magnetite and
titano-magnetite. Liu et al. (2007) have demonstrated that
the estimation of the L-ratio (IRMAF@300mT/IRMAF@100mT)
helps to decipher the actual mineral contributing to the hard
IRM records. In the present study, the L-ratio is seen to be
Depth interval in core (cm)
Lab code
Radiocarbon age (years BP)
Calibrated age (years BP)
8–10
76–78
128–130
144–146
180–182
X8398
B7710A
X8399
X8400
B7711
2,635±36
8,822±46
10,943±70
13,881±68
17,186±88
2,764±16
9,929±151
12,885±98
17,124±200
20,636±295
248–250
448–450
B7712
B7714
35,090±630
45,400±2,300
40,051±953
49,279±2,953
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Fig. 2 Age model for core
SK200/22a. Radiocarbon control points are shown with error
bars. The circle denotes the
SPECMAP control point
independent of the hard IRM, as revealed by the relatively
constant values in Fig. 7. The lack of correlation between
the L-ratio and hard IRM indicates that there is no significant change in the coercivity of antiferromagnetic minerals;
in turn, this implies common sediment provenance. Therefore, the hard IRM record of the core is predominantly
controlled by fluctuations in the concentration of hematite,
whereas the contribution of goethite is insignificant.
Thermal studies of representative samples of MIS 1 to MIS
4 display similar χ–T curves (Fig. 8a–d), χ decreasing gradually with increasing heating before dropping sharply at
580°C, which is typical for magnetite. Only in the Holocene
sediment sample (Fig. 8a) does the trend line for heating
initially increase slightly (up to 260°C) before reversing. The
trend lines also show two minor changes in slope at ~300°C
and above 580°C. The three changes in the slope of χ at ~300,
580 and ~675°C suggest the presence of titano-magnetite,
magnetite and hematite respectively. On cooling, the curves
are shown to be irreversible, the formation of magnetite now
being indicated at temperatures less than 400°C and that of
hematite at temperatures above 400°C. During SIRM thermal
demagnetization (Fig. 8e), the sudden change in the IRM
slope at 580°C reflects magnetite, whereas the presence of
titano-magnetite and hematite is revealed by smaller
changes in the IRM slope at ~300 and ~675°C respectively.
events at 13–23 ka BP, as well as less prominent events at
25–30, 45–52 and 55–58 ka BP (Fig. 9). During MIS 2 the
ice-rafting events are synchronous with the bulk magnetic
record. The microscopic study of the IRD fractions in core
SK200/22a revealed a dominance of tephra/ash material
with additional quartz fragments. The tephra mostly comprise volcaniclastic materials consisting of sub-rounded to
elongate vesicular black and brown fragments. The IRD
record shows a good correlation with the high-coercivity
hematite during the ice-rafting events (Fig. 10).
Carbonate and δ18O records
The carbonate content varies between 0.03 and 53.12%, with
low values during the MIS 2 and late MIS 4, and high values
during the MIS 1 and MIS 3 (Fig. 10). Carbonate content
increases during the warming events, as represented by the
δ18O record, and shows similar fluctuations with magnetic
concentration during the MIS 2 and MIS 3. The δ18O record
of N. pachyderma varies between 1.37 and 3.65‰, with
heavier δ18O values during the glacial period (Fig. 10).
Discussion and conclusions
Ice-rafted debris
Changes in bulk magnetic properties and their relation
to terrigenous input during the late Quaternary
Several IRD pulses were recorded in the sediment core
SK200/22a. The IRD record revealed major ice-rafting
The temporal variations in the values of χ, χARM and SIRM
indicate that MIS 1 and MIS 3 are characterized by low
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Fig. 3 Comparison between the
age records of core SK 200/22a,
core MD94-103 from the Indian
sector of the Southern Ocean
(Mazaud et al. 2002; Sicre et al.
2005) and the LR04 δ18O stack
(Lisiecki and Raymo 2005)
magnetic concentrations, whereas higher concentrations are
observed during the early deglacial period of MIS 2 and
during the MIS 4 (Fig. 4). Although the hard IRM and soft
IRM records show similar temporal variations (Fig. 4)
throughout the core, hard IRM values are much lower,
suggesting that low-coercivity minerals like magnetite dominate the magnetic record compared to the high-coercivity
hematite. Therefore, the MIS 4, and certain intervals of MIS
3, is characterized by increased magnetic concentrations of
coarser-grained magnetite and titano-magnetite, whereas the
contribution by hematite is reduced. Such periods of increased concentration of magnetic minerals in the Southern
Ocean have been attributed to enhanced terrigenous input
and ice-rafting episodes (Diekmann 2007). Therefore, the
downcore profiles of bulk magnetic parameters at the core
site suggest that the total terrigenous influx to the Indian
Ocean sector of the Southern Ocean increased substantially
during the late MIS 2, MIS 4, and the early intervals of MIS 3.
Fine-grained magnetic minerals dominate during MIS 1
and early MIS 3, suggesting a reduced erosion capacity of
the currents or increased supply of fine-grained magnetic
minerals. Lower values of magnetic grain-size parameters
within MIS 2, MIS 4, and certain intervals of MIS 3 may
also be indicative of selective dissolution of fine-grained
ferromagnetic components during early diagenesis (Karlin
1990; Rolph et al. 2004). The nonlinear positive
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Fig. 4 Trends of the environmentally significant environmental magnetic parameters
which represent magnetic concentration, grain size and mineralogy in core SK200/22a. The
isotope stages are shaded
relationships of χ with SIRM and ARM (Fig. 6a, b) suggest
that the magnetic parameters principally depict the variations in concentration of magnetic minerals, and only to a
very limited extent the changes in magnetic mineralogy and
grain size during the past ~63,000 years. A comparison
between the proxy records for magnetic concentration (χ,
χARM and SIRM) and magnetic grain size (χfd%, χARM/χ,
SIRM/χ and χARM/SIRM) suggests that increased magnetic
mineral concentrations are associated with the coarsening of
the magnetic grain size and vice versa. Mazaud et al. (2007)
have proposed that changes of the magnetic grain size, with
smaller grain sizes at the time of minimal deposition, are due
to modulations in the ACC-CDW current which erodes and
transports the magnetic grains in this region. The sediment
grain-size distribution of the core supports this contention:
the silt fraction, which is the major component, shows
increased percentages of up to 90% during the MIS 2,
MIS 4, and certain intervals of MIS 3, suggesting strong
bottom current activity in these periods.
Relationships between terrigenous input, ice rafting
and carbonate productivity
A comparison of the magnetic record with the N. pachyderma δ18O, IRD and carbonate records reveals that terrigenous influx, ice rafting, sea-surface temperature (and/or
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Fig. 5 Magnetite accumulation rate (AR(mag)) and magnetic susceptibility (χ) versus age in core SK 200/22a
salinity) and carbonate productivity at the core site are
apparently interrelated. Within the chronological uncertainties, the major ice-rafting events recorded in the present core
are nearly synchronous with those of other studies from the
Southern Ocean (Labeyrie et al. 1986; Grobe and Mackensen
1992; Kanfoush et al. 2000; Carter et al. 2002). Whereas
major ice-rafting events are clearly reflected in the magnetic
concentration record (tracing the terrigenous influx to the
core site), some events are not synchronous with it, few of
these not even showing up in the IRD record (Fig. 10). Such
out-of-phase relations between terrigenous flux and IRD is
not surprising because IRD represents only one component
of the terrigenous influx, the total terrigenous input to the
Southern Ocean being controlled by several oceanographic
and climatic factors. Low-coercivity minerals like magnetite
are the dominant component of the magnetic record representing the total terrigenous input to the study site. Other
studies have reported that such magnetic minerals are generally not associated with coarse-grained IRD materials
(Bareille et al. 1994; Mazaud et al. 2007). Such an interpretation is consistent with the grain size of the core, the IRD
fraction being a minor (<15%) and the silt fraction a dominant (up to 95%) component of the sediment (Fig. 9).
However, records of high-coercivity hematite (represented
by hard IRM) show better correlation with the IRD profile
(Fig. 10), suggesting that the coarse-grained hematite
minerals are more associated with ice rafting. During the
largest ice-rafting event of the last deglaciation, the IRD and
the magnetic mineral concentration (both magnetite and
hematite) are positively correlated (p<0.001), suggesting
that the sea-surface warming (and related ice rafting) and
the current strength simultaneously increased significantly.
It is therefore proposed that the early deglacial warming and
associated strengthening of the ACC-CDW in the Southern
Ocean (Dezileau et al. 2000; Mazaud et al. 2007) had a
strong influence on the terrigenous input to the Indian sector
of the Southern Ocean.
Comparisons between magnetic records and IRD data
also support the contention that ice rafting explains only a
minor part of the terrigenous influx, and that a significant
portion appears to have been derived either from enhanced
aeolian activity or from wind enhancement of the ACC. In
this context, aeolian input to the Indian Ocean sector of the
Southern Ocean has been reported to be subordinate to
material supplied by wind forcing of the ACC (Diekmann
2007). However, Pugh et al. (2009) revealed that periods of
increased magnetic susceptibility of sediments from the
circum-ACC belt in the Southern Ocean are well correlated
with the dust records from Antarctic ice cores. The comparison between magnetic proxy data of the present core and
dust concentration at ODP site 1090, which is located in the
Atlantic sector of the sub-Antarctic zone (Martínez-Garcia
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a
Fig. 7 Correlation between the L-ratio and hard IRM of core SK200/22a
b
Fig. 6 Correlations of a anhysteretic remanent magnetization (ARM)
versus magnetic susceptibility (χ) and b saturation isothermal remanent magnetization (SIRM) versus magnetic susceptibility (χ) of core
SK200/22a
et al. 2011), suggests that, with the exception of MIS 4 and
early MIS 3, the terrigenous record from the present site was
not influenced by direct dust input (Fig. 9). The most significant dust source to Antarctica and the Southern Ocean is
the Patagonian desert (Gaiero et al. 2007; Sugden et al.
2009; Wolff et al. 2010). The results of the present study
are consistent with earlier reports suggesting that aeolian
dust was not a major contributor to the enhanced terrigenous
supply to the southern Indian Ocean in the past (Bareille et
al. 1994; Mazaud et al. 2010). Wind-driven current erosion
in shallow waters around volcanic islands and ridges during
periods of strengthened ACC could be an important additional mechanism for the enhanced terrigenous input to the
core site. It should, however, not be discounted that during
MIS 4 and early MIS 3 Patagonian dust may have been
increasingly mobilized by enhanced Westerlies (Gaiero et al.
2004, 2007) and deposited in the study region.
The variations in the magnetic grain size, with coarser
material at the time of maximum concentration during MIS
2 and MIS 4, and finer material at the time of minimal
concentration during MIS 1 and MIS 3, also suggest a
modulation of the terrigenous input in the study region by
the intensity of the ACC-CDW current. In fact, Mazaud et
al. (2007, 2010) proposed that erosion processes associated
with sea-level fluctuations had significantly contributed to
changes in terrigenous input to the southern Indian Ocean.
During glacial periods (MISs 2 and 4), the velocity of the
ACC seems to have increased (Carter et al. 2000; Neil et al.
2004; Mazaud et al. 2010), which may have led to the
winnowing of finer magnetic grain sizes. During the last
deglaciation the ACC-CDW flow stabilized and was accompanied by an increased sorting of the sediment (Howe and
Pudsey 1999). Such a process could have increased the
supply of terrigenous material to the study site. The low
and uniform values in magnetic concentration parameters
during MIS 1 indeed reveal a reduced terrigenous input
during the postglacial period.
In the Southern Ocean, especially north of the Polar
Front, temporal variations in carbonate contents as well as
accumulation rates were shown to have increased during
interglacials compared to glacial stages (Howard and Prell
1994; Bareille et al. 1998; Chase et al. 2003). The comparison between δ18O and carbonate records reveals that
throughout the last ~63,000 years the calcite record fluctuated in tandem with the sea-surface temperature and/or
salinity (Fig. 10). Studies from the SAF region of the Indian
sector of the Southern Ocean report that the carbonate
records are essentially indicative of calcite productivity
and terrigenous dilution (Marsh et al. 2007). In general, an
inverse relationship between magnetic concentration and
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Fig. 8 Magnetic mineralogy of dated samples from SK200/22a, showing the relationships between susceptibility and temperature during
heating (solid lines) and cooling (dotted lines) in panels a–d, and the
relationship between the thermal demagnetization of saturation isothermal remanent magnetization (SIRM) and temperature in panel e
carbonate content during MIS 1 and MIS 4 indicates a
possible dilution of terrigenous material by carbonates during these intervals. The carbonate content and magnetic
mineral concentration records show similar fluctuations during MIS 2 and MIS 3 (Fig. 10). Although the amplitude of
the variability between the carbonate and magnetic record is
different during the early deglaciation within MIS 2, the
timing of the increase in carbonate content matches well
with the magnetic and IRD records. The synchronicity
between the carbonate and magnetite records during MIS 2
and MIS 3 suggests that a common factor influenced both
the magnetic and the carbonate profile during these time
intervals. This factor seems to be the millennial climate
oscillation which evidently controlled the input of finegrained terrigenous material and calcite productivity at the
core site over the past ~63,000 years.
In contrast, the carbonate content and IRD records during
MIS 2 and MIS 3 do not show any significant relationships.
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Fig. 9 Relationships of N. pachyderma δ18O, saturation isothermal remanent magnetization (SIRM), ice-rafted debris (IRD) and silt records of
SK200/22a with dust accumulation rate at ODP site 1090 (Martínez-Garcia et al. 2011)
The major ice-rafting events at the core site are characterized by significantly lower carbonate content, indicating
decreased calcite productivity, carbonate dilution by enhanced terrigenous sediments and/or enhanced carbonate
Geo-Mar Lett
Fig. 10 Correlations between
the SK200/22a records (δ18O,
SIRM, hard IRM, IRD and carbonate content) and the Antarctic (Byrd and EDML) as well as
Greenland (NGRIP) δ18O ice
core records. Climatic events
like the Antarctic Cold Reversal
(ACR), Antarctic Isotope Maximum (AIM), Antarctic warming
events (A1–A4) as well as
Heinrich events (HEs) are also
indicated. Major events in terms
of δ18O, magnetic concentration,
IRD, and carbonate content of
SK 200/22a are shaded
dissolution during these intervals. The reduced carbonate
contents during IRD events seem to be the result of enhanced dilution by terrigenous material, as the relation between ice rafting and biological productivity in the Southern
Ocean seems to be equivocal, with many IRD events
asynchronous with increased foraminiferal abundances
(Kanfoush et al. 2002). Periods of increased IRD accumulation may indicate the equatorward migration of the Polar
and Sub-Antarctic fronts (Cook and Hays 1982; Howard
and Prell 1994; Bareille et al. 1994). Equatorward shifting of
the frontal regimes, and its influence on ice rafting in the
Southern Ocean during glacial periods, has previously been
reported by Howard and Prell (1994), Bareille et al. (1994)
and Kanfoush et al. (2002). While sea ice could be a dominant delivery mechanism of tephra/ash to the study region
from volcanic islands, quartz grains could have been transported by icebergs (Bareille et al. 1998; Thamban et al.
2005). A stronger ACC due to stronger glacial winds would
thus have transported increased amounts of sedimentary
material to the region. A strengthening of the wind-driven
Geo-Mar Lett
system would also have enhanced the eddy system in this
part of the Southern Ocean, leading to increased erosion and
transport of terrigenous material to the core site.
Timing of depositional events and Antarctic climate
linkages
The correlation between Southern Ocean sedimentary
records and ice records from Greenland and Antarctica have
been used to interpret millennial-scale climate changes as
well as climatic synchronicity or asynchronicity (Mazaud et
al. 2002, 2007; Lamy et al. 2004; Sachs and Anderson 2005;
Barrows et al. 2007). Records from the SE Pacific show a
clear ‘Antarctic timing’ of the last glacial and deglacial
events (Lamy et al. 2004, 2007), whereas the records from
other parts of the Southern Ocean are equivocal (Andres et
al. 2003). The oxygen isotope record and the carbonate
content of core SK 200/22a show similar fluctuations
throughout the past ~63,000 years. To understand the timing
of the events with respect to global climate records, the SK
200/22a records are compared here with Antarctic (Byrd and
EPICA Dronning Maud Land) and Greenland (North GRIP)
ice core records (Fig. 10; Blunier and Brook 2001; EPICA
Community Members 2006). Within the chronological
uncertainties, the δ18O and carbonate records have revealed
the presence of the Antarctic Cold Reversal at 12.4–14.1 ka
BP, the early deglacial warming event (Termination I) as
well as the major interstadials depicted in Antarctic ice core
records. Major warming events in these records are well
correlated with the Byrd ice core δ18O record for warm
events A2–A4 but not for A1. However, the records of the
present study are not in phase with the Greenland ice core
records (Fig. 10), and it is therefore suggested that the
climatic evolution and calcite production at the core site
essentially followed the Antarctic climatic events.
Compared to the oceanic proxy data (δ18O, carbonate
content), the terrigenous proxy data (magnetic records,
IRD) revealed certain differences during MIS 4 and MIS
3. The magnetic parameters (dominantly represented by fine
sand and silt particles) fluctuate in similar manner, suggesting a remarkably increased terrigenous input during the
early deglaciation (13–19 ka BP), and less prominent but
nevertheless significant pulses at 21–23, 26–30, 33–35, 45–
48.5, 54–56 and 59–62 ka BP. Within the chronological
uncertainties, many of these events can be considered as
being synchronous with the Antarctic ice core records. In
general, increased terrigenous input to the core site occurred
during southern hemispheric warming events. Recent studies have identified that north–south linkages were widespread during the late Quaternary through a bipolar seesaw
mechanism (Blunier and Brook 2001; Lamy et al. 2007).
The bipolar seesaw is manifested by an out-of-phase
millennial-scale climatic pattern of atmospheric and oceanic
changes which are systematically linked to rapid meridional
displacements of sea ice, westerly winds and the ACC
system (Stocker and Johnsen 2003; Lamy et al. 2007;
Mazaud et al. 2007). However, it is not possible to confirm
the hemispheric synchronicity/asynchronicity in the present
case because the prominent terrigenous event at 54–56 ka
BP was out of phase with the Antarctic warming event A4
(Fig. 10). Moreover, there are no significant ice-rafting
events after the Antarctic warming events A1 and A2, which
indicates that ice-rafting events are predominantly influenced by the ice-sheet dynamics. Studies from the South
Atlantic region showed that, unlike the northern hemisphere, the southern hemispheric IRD records for the last
glacial period were controlled by changes in sea-surface
temperature and sea ice conditions, rather than Antarctic
ice-sheet dynamics (Carter et al. 2002; Nielsen et al.
2007). However, the findings of the present study suggest
that most of the IRD pulses occurred when the sea-surface
temperatures were possibly colder (more positive δ18O
values), some of the ice-rafting events even preceding the
warming events (Fig. 10). Only the last deglacial ice-rafting
event (13–19 ka BP) correlates with the early warming after
the LGM. The present study thus suggests that the ice-sheet
dynamics and the intensity of the ACC dominantly controlled IRD deposition in the Indian sector of the Southern
Ocean and that the sea-surface temperature had only a
secondary influence. Additional well-dated records, along
with independent sea-surface temperature proxy records
from the Southern Ocean, would greatly help in resolving
the timing of the events as well as the physical mechanisms
involved in controlling the terrigenous input to the Indian
sector of the Southern Ocean.
Acknowledgements We thank the Director, National Centre for Antarctic and Ocean Research, and Dr. M. Sudhakar and members of the pilot
expedition to the Southern Ocean for their excellent support. Thanks are
due to S. Shanavas, Anayat A. Quarshi, B.L. Redkar and Sunayna S.
Wadekar for laboratory help and analyses. We also thank two anonymous
reviewers and the editors for valuable suggestions which have helped us
to improve the manuscript. This is NCAOR Contribution No. 36/2011.
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