Geochimica et Cosmochimica Acta, Vol. 63, No. 5, pp. 615– 625, 1999
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4
He as a tracer of continental dust: A 1.9 million year record of aeolian flux to the west
equatorial Pacific Ocean
D. B. PATTERSON,1 K. A. FARLEY,*,1 and M. D. NORMAN2
1
Division of Geological and Planetary Sciences, MS170-25, California Institute of Technology, Pasadena, CA 91125, USA
National Key Centre for the Geochemical Evolution and Metallogeny of the Continents, School of Earth Sciences, Macquarie University,
North Ryde NSW 2109, Australia
2
(Received June 10, 1998; accepted in revised form November 18, 1998)
Abstract—The mass accumulation rates (MARs) of aeolian dust in the ocean basins provide an important
record of climate in the continental source regions of atmospheric dust and of the prevailing wind patterns
responsible for dust transport in the geologic past. The incorporation of other terrigenous components such as
volcanic ashes in seafloor sediments, however, often obscures the aeolian dust record. We describe a new
approach which uses the delivery rate of crustal 4He to seafloor sediments as a proxy for the mass
accumulation rate of old continental dust which is unaffected by the addition of other terrigenous components.
We have determined the flux of crustal 4He delivered to the seafloor of the Ontong Java Plateau (OJP) in
the western equatorial Pacific over the last 1.9 Myrs. Crustal 4He fluxes vary between 7.7 and 30 ncc cm22
kyr21 and show excellent correlation with global climate as recorded by oxygen isotopes, with high crustal
4
He fluxes associated with glacial periods over the entire interval studied. Furthermore, the onset of strong 100
kyr glacial–interglacial climate cycling is clearly seen in the 4He flux record about 700 kyrs ago. These data
record variations in the supply of Asian dust in response to climate driven changes in the aridity of the Asian
dust sources, consistent with earlier work on Asian dust flux to the northern Pacific Ocean. However, in
contrast to previous studies of sites in the central and eastern equatorial Pacific Ocean, there is no evidence
that the Inter Tropical Convergence Zone (an effective rainfall barrier to the southward transport of northern
hemisphere dust across the equator in the central and eastern Pacific) has influenced the delivery of Asian dust
to the OJP.
The most likely carrier phase for crustal helium in these sediments is zircon, which can reasonably account
for all the 4He observed in the samples. As a first order estimate, these results suggest that the mass
accumulation rate of Asian dust on the OJP over the last 1.9 Myrs varied from about 4 to 15 mg cm22 kyr21.
In contrast, previous studies show that over the same interval the total MAR of terrigenous dust (i.e. Asian
dust plus local volcanics) on OJP varied between about 34 and 90 mg cm22 kyr21. Copyright © 1999
Elsevier Science Ltd
kyrs the MARs of aeolian dust in the north Pacific show strong
100 kyr periodicity which correlates with global climate as
recorded by oxygen isotopes such that high dust fluxes are
associated with glacial periods (Hovan et al., 1989, 1991). This
dust flux record has been interpreted to reflect variations in
aridity of the Asian deserts related to 100 kyr period glacial–
interglacial climate cycling with increased aridity during glaciations. In contrast, continental dust flux records from the
equatorial central Pacific exhibit more complex and variable
relationships between dust fluxes and global climate, suggesting that dust delivery to the equatorial central Pacific has also
been influenced by such factors as the latitudinal position of the
Inter-Tropical Convergence Zone (ITCZ) which is an effective
rainfall barrier to southward transport of northern hemisphere
dust in the central equatorial Pacific (Rea, 1994).
The traditional technique to determine the abundance of
continental dust in seafloor sediments is to extract the nonauthigenic, inorganic, crystalline component by chemical leaching and to weigh the residue (Rea and Janecek, 1981). This
approach, however, is often hindered by the presence of volcanic ashes that survive the leaching process and which obscure
the record of mass accumulation of continental dust (Krissek
and Janecek, 1993). Identification of the continental dust in
such samples then requires detailed mineralogical and geo-
1. INTRODUCTION
Deserts are important sources of windblown (aeolian) continental dust which can be transported many thousands of kilometers from its source. The primary control on the supply of
such dust is the aridity of the source region (Middleton, 1985;
Rea et al., 1985; Prospero and Tees, 1986). In addition, the
delivery rates of continental dust to the deep ocean basins (at
sites far enough from continental margins that riverine and
hemipelagic deposition of crustal material is negligible) often
correlate with global climate as recorded by oxygen isotopes in
seafloor sediments (e.g. Hovan et al., 1989, 1991; Tiedemann et
al., 1994). The mass accumulation rate (MAR) of aeolian dust
in the ocean basins therefore provides a record of the climate of
the continental source regions of atmospheric dust as well as
the prevailing wind patterns responsible for dust transport
(Chuey et al., 1987; Hovan et al., 1989, 1991; Krissek and
Janecek, 1993; Rea, 1994; Tiedemann et al., 1994).
Previous studies have shown that the dominant source of
dust in the northwestern and central north Pacific Ocean is the
deserts of central and eastern China (Pye and Zhou, 1989;
Hovan et al., 1989, 1991; Kyte et al., 1993). Over the last 530
*Author to whom correspondence
([email protected]).
should
be
addressed
615
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D. B Patterson, K. A. Farley, and M. D. Norman
chemical analysis to deconvolve the relative contributions of
the various endmembers (Krissek and Janecek, 1993; Kyte et
al., 1993). This problem is particularly acute in the western
Pacific Ocean where, owing to the proximity of the volcanic
arcs of the Pacific Rim, local volcanic dust supplies can be
several orders of magnitude higher than the supply of continental dust derived from desert regions (Krissek and Janecek,
1993). As a result, there are uncertainties about the supply of
Asian dust or the possible influence of the Inter-Tropical Convergence Zone in the western equatorial Pacific in the geologic
past.
We describe a new approach which uses the delivery rate of
crustal 4He to seafloor sediments as a proxy for the mass
accumulation rate of old continental dust which is unaffected
by the addition of other terrigenous components. Specifically,
windblown Asian dust has a 4He content in excess of 2000 ncc
g21 (Farley, 1995), whereas volcanic ashes and subaerial lavas
from subduction related volcanic arcs typically have 4He contents of less than 5 ncc g21 (Patterson, unpubl. data). Thus, the
abundance of terrestrial 4He in a seafloor sediment will be
dominated by old continental dusts, and be insensitive to incorporation of volcanic materials which are effectively helium
free in comparison. Despite the presence of a dominant volcanic component, crustal 4He contents provide an unambiguous
record of the flux of Asian dust to the Ontong Java Plateau in
the equatorial western Pacific over the last 1.9 Myrs. These data
yield new insights into the aridity of the Asian dust sources, and
the influence of the ITCZ on dust transport in the western
equatorial Pacific.
of ODP 806B, as well as three other sea floor sediment cores discussed
subsequently, are shown in Fig. 1.
We determined helium abundances and isotopic compositions for 72
samples spanning the last 1.9 Myr. As this sample suite was studied as
part of a larger project initiated to investigate extraterrestrial 3He in
seafloor sediments, we have previously reported full sample details
including official ODP core designations, sample ages, sediment bulk
mass accumulation rates, and helium abundances and isotopic composition for all 72 samples studied, as well as a detailed description of the
experimental technique (Patterson and Farley, 1998).
Each sample is a thoroughly homogenized continuous strip of carbonate seafloor sediment collected from the interior of the core using a
U-channel sampling tool. The samples range between 18.7 and 29.2
kyrs duration, with age control provided by Berger et al. (1994) who
tuned the d18O record for this site to Earth’s orbital obliquity signal.
The measured compositions therefore represent true averages for each
time interval. Individual sample aliquots of 2 to 3 g were leached in 10
volume percent acetic acid to remove carbonate material which carries
no substantial amount of helium. This resulted in residues of about 250
mg suitable for handling in the gas extraction system. Helium was
extracted from the samples at approximately 1400°C, gettered to remove active gas species, then separated from neon and the heavier
noble gases on a cryogenic trap held at 34K. Abundance and isotopic
determinations were performed with a MAP 215-50 noble gas mass
spectrometer by comparison with a known volume of a helium standard
having a 3He/4He ratio of 16.45 RA. 4He hot blanks averaged 0.1 ncc,
which in nearly all cases was less than 1% of the 4He from the samples.
We routinely performed a repeat extraction and in all cases the amount
of helium in the re-extract was at blank levels, indicating complete gas
extraction in the initial heating. The 1s variation on over 100 standards
of similar size to the samples analyzed during this project was 0.26%
for 4He and 1.2% for 3He. We routinely analyzed replicates of all
samples except one; 43 samples were duplicated and 28 samples were
triplicated.
4. 4HE RESULTS
2. CALCULATION OF CRUSTAL 4HE FLUXES
Helium in seafloor sediments is primarily derived from continental material having 3He/4He ratios less than 0.1 RA (RA 5
(3He/4He)Observed/(3He/4He)Air, where (3He/4He)Air 5 1.39 3
1026), and extraterrestrial interplanetary dust particles with
3
He/4He ratios greater than 100 RA (Ozima et al., 1984; Takayanagi et al., 1987). The observed 3He/4He ratio of any sediment is then a reflection of the mixing ratio of these two
components. Assuming the 3He/4He ratios of these endmembers, the amount of crustal 4He in a sediment can be estimated
from the observed 3He/4He ratios using a simple two component mixing model:
4
Hecrustal/4HeTotal 5 (3He/4HeObserved 2 3He/4/HeET)/
(3He/4HeCrustal 2 3He/4HeET)
where ET denotes the extraterrestrial component. Previous
studies of helium in seafloor sediments have shown atmospheric helium to be negligible (Farley and Patterson, 1995).
The flux of crustal 4He (in units of 4He/area/time) can then be
determined by multiplying the calculated abundance of crustal
4
He (in units of 4He/mass) by the sediment mass accumulation
rate (in units of mass/area/time; Takayanagi et al., 1987; Farley,
1995; Farley and Patterson, 1995).
3. SAMPLES AND EXPERIMENTAL TECHNIQUE
The sediments studied are from the northeastern margin of the
Ontong Java Plateau in the equatorial western Pacific (Ocean Drilling
Program Hole 806B, 0°19.119N, 159°21.709 E, 2521 m). The location
Appendix 1 provides a summary of the averge 4He contents
of each sample. Full details of the individual replicates of each
sample (including 3He/4He ratios, sample masses, etc.) are
listed in Patterson and Farley (1998). Total 4He in the bulk
sediments varies between 3.7 and 14.9 ncc g21 (expressed as
4
He per unit mass of sediment before leaching) and 3He/4He
ratios range from 12 to 89 RA. The high 3He/4He ratios require
an extraterrestrial high 3He/4He component in these sediments.
The average variability in the total 4He amounts between
replicates of the same sample was 8.5%, reflecting variable
amounts of extraterrestrial 4He in each replicate. This is a result
of the statistics of nonrepresentative sampling of the interplanetary dust particles which carry extraterrestrial helium in these
sediments (Farley et al., 1997; Patterson and Farley, 1998).
In order to deconvolve the amounts of crustal 4He in the
samples, we have taken the 3He/4He ratio of the extraterrestrial
component to be 290 RA, similar to that observed in the solar
wind (Giess et al., 1972) and in bulk lunar fines (Nier and
Schlutter, 1990), and the 3He/4He ratio of the crustal component to be 0.015 RA as observed in Chinese loess (Farley and
Patterson, 1995). With these values, we estimate that the contribution of crustal 4He ranges between 96% and 70% of the
total 4He. The success of this deconvolution is demonstrated by
the observation that although the variability in total 4He contents between replicates averaged 8.5%, the variability in calculated amounts of crustal 4He between the replicates averaged
only 3.6%.
The average concentration of crustal 4He in each sample as
a function of age is plotted in Fig. 2 (curve A, Appendix 1).
4He as a tracer of continental dust
617
Fig. 1. Location map for sea floor sediment cores discussed in this paper: ODP 806B, V21-146, LL44-GPC3, and
RC11-210.
There are two noteworthy characteristics: (1) there is a long
term increase in the baseline concentration of crustal 4He over
the last 1.9 Myrs from about 5 ncc g21 to 10 ncc g21, and (2),
there are significant systematic variations in the 4He concentrations which over the last 700 kyrs have approximately 100
kyr periodicity. Note that the choice of 3He/4He ratios for
calculating crustal 4He amounts is not critical; for example,
studies of seafloor magnetic fines and individual stratospheric
particles suggest that a reasonable lower limit for the extraterrestrial 3He/4He is about 170 RA (Fukumoto et al., 1986; Amari
and Ozima, 1988; Matsuda et al. 1990; Nier et al., 1990) which
would reduce the fraction of crustal 4He to between 93% and
48%, but would not change the overall pattern and timing of
crustal 4He maxima and minima shown in Fig. 2.
Curve (B) in Fig. 2. plots the bulk sediment MARs (Appendix 1) as calculated from the chronostratigraphic model of
Berger et al. (1994) and the known sediment density profile.
Although there is some variation, the MARs at ODP 806B are
relatively constant over the last 1.9 Myrs with an average of
about 2.2 g cm22 kyr22. Multiplying the individual crustal 4He
amounts by the appropriate bulk sediment MARs as listed in
Appendix 1, yields crustal 4He fluxes to these sediments that
vary between 7.7 and 30 ncc cm22 kyr21 (curve C, Fig. 2).
Note that sediment MARs and 4He contents do not correlate,
implying that although changes in sedimentation rate modulate
the observed 4He abundances (via simple dilution with helium
free carbonate material), they cannot generate the observed
systematic oscillations in the crustal 4He fluxes. The overall
characteristics of the 4He signal seen in the concentration data
(curve A) are readily apparent in the 4He flux results (curve C),
specifically; there is longterm increase in 4He flux over the last
1.9 Myrs, and significant systematic variations in the 4He flux
over the last 700 kyrs.
In order to investigate the systematic variations in 4He flux,
we obtained a Fast Fourier Transform spectral analysis of our
data (R. Muller, pers. comm.). Figure 3 presents a plot of
spectral power for the time window 0 –700 kyrs ago. A statistically significant peak is clearly apparent with a period of
about 100 kyr confirming what is seen visually in Fig. 2. The
remaining smaller peaks do not stand above the expected noise
level and are not considered significant. Spectral analysis of the
interval 700 –1900 kyrs ago did not identify statistically significant periodicity in this section of the 4He flux record.
5. CRUSTAL 4HE—A DUST PROVENANCE SIGNAL
Before interpreting the 4He fluxes as a tracer of continental
dust it is necessary to demonstrate that diffusive loss of the
primary (source related) helium carried by windblown dust, or
post depositional ingrowth of radiogenic 4He from the decay of
U and Th in the sediments, has not compromised the original
continental 4He record.
In a study of pelagic sediment from the northern Pacific (core
LL44-GPC3, 30°199N, 157°49.99W, 5705 m water depth, Fig.
1), Farley (1995) observed an order of magnitude decrease with
age in the crustal 4He contents of the bulk sediment over the
last 70 Myrs. Kyte et al. (1993), however, have shown that the
fraction of the GPC3 core that is composed of aeolian dust from
618
D. B Patterson, K. A. Farley, and M. D. Norman
Fig. 2. 4He and bulk sediment mass accumulation rate data for ODP 806B as a function of age: (A) Average crustal 4He
abundance (ncc 5 1 3 1029 cm3 STP), (B), bulk sediment mass accumulation rate as listed in Patterson and Farley (1998),
and (C), flux of crustal 4He calculated by combining the data shown in curves (A) and (B). Gaps in data show positions of
core gaps were sediment was not recovered. Note the relatively high total MAR at this site reflects rapid deposition of
carbonate material (aeolian MARs for non-carbonate pelagic sediments of the northern Pacific discussed subsequently are
generally a factor of 10 lower at around 100 to 200 mg cm22 kyr21).
Asia drops from 95% to about 10% over the same period. As
the non-Asian components in the GPC3 core are almost certainly helium-poor (e.g., biogenic phosphates, authigenic minerals, various volcanic ashes), the decrease in crustal 4He
contents at GPC3 is largely the result of dilution of 4He bearing
aeolian material with 4He-free components. When corrected for
this dilution effect, the concentration of 4He in the Asian dust
component at GPC3 is constant within a factor of 2 over 70
Myrs, indicating excellent retention of primary helium. In
contrast, Farley (1995) also demonstrated that radioactive decay of U and Th in the oldest GPC3 samples would have
generated more than 2 orders of magnitude more 4He than is
actually observed. Thus, it appears that post depositional radiogenic 4He is rapidly lost, whereas the primary 4He derived
from the original source materials of wind blown continental
dusts is retained in seafloor sediments over geologic time
scales. We propose this difference in the retentivity of primary
and post depositional radiogenic helium is related to the carrier
phase (or phases) of 4He and the grain size of the dust particles
in the sediment.
First, U and Th in typical continental material is extremely
heterogeneous in distribution, being highly concentrated in
trace mineral phases (Martel et al., 1990). As a result radiogenic 4He will also be highly concentrated in U and Th rich
trace phases. For example, 4He concentrations of up to 23 3
106 ncc g21 of 4He in a zircon were reported by Damon and
Kulp (1957). The observed Zr content of Chinese loess is about
330 ppm (Taylor et al., 1983) which if contained entirely in
zircon implies a zircon content of 660 ppm. Attributing the
observed 4He content of Chinese loess of about 5000 ncc g21
(Farley and Patterson, 1995) to 660 ppm of zircon implies a
4
He concentration in the zircon of about 8 3 106 ncc g21. Thus,
it is reasonable that all the 4He in windblown Asian dust can be
attributed to zircon. Other U and Th rich trace minerals such as
apatite and sphene might also play a role, although these
minerals typically have lower U and Th contents (and hence
lower 4He abundances) and are expected to be less retentive of
4
He than zircon (Wolf, 1997). Dominant rock forming minerals
which are resistant enough to survive chemical weathering,
such as quartz, have effectively zero U and Th abundances and
can be ignored. Nor does it seem likely that He retentivity in
clay minerals will be substantial. In the strictest sense the 4He
flux is therefore a tracer not of the total flux of windblown dust,
but of the 4He rich and retentive trace component.
Second, in order for radiogenic 4He to be trapped in a U1Th
rich mineral, the grain must be larger than the typical alpha
particle recoil length which ranges between about 10 and 30
mm (Farley et al., 1995). In contrast, 95% of aeolian dust
particles in pelagic clays are less than 6 to 8 mm, and are
typically less than about 3 mm (Rea, 1994). Thus, once a zircon
4He as a tracer of continental dust
619
both in amplitude and phase, as is the gradual threefold increase
in baseline values over the last 530 kyrs. The excellent agreement between these data sets demonstrates that crustal 4He
fluxes to ODP 806B provide an unambiguous proxy record of
the MARs of Asian dust on the OJP.
6.2. Possible Transport Related Fractionation Effects?
Fig. 3. Spectral power of crustal 4He flux variations at ODP 806B
over the period 0 –700 kyrs calculated by R. Muller (pers. comm.). The
data were interpolated to evenly spaced points (every 1 kyr), detrended
and normalized to the mean, then padded with zeros (this makes the
Fast Fourier Transform calculate intermediate frequencies, and results
in a smooth curve and better estimation of the frequency of the peaks).
The spectrum was then normalized to unit area. A statistically significant single peak is clearly apparent with a frequency of 0.01 kyr21
(100 kyr period). The remaining smaller peaks do not stand significantly above the expected noise level, and are not considered statistically significant.
has been eroded and fragmented down to the grain size where
it can be transported by wind, it is too small to retain 4He atoms
produced by ongoing U and Th decay. 4He produced in windblown zircon after deposition on the seafloor will be ejected
and temporarily implanted into surrounding sediment particles
such as quartz, clays and biogenic carbonates which have lower
helium retentivities. Subsequent loss of this helium from these
phases then accounts for the highly efficient loss of post depositional radiogenic 4He from the sediment.
We therefore conclude that diffusive loss and post depositional radiogenic ingrowth are unlikely to have significantly
affected the 1.9 Myr helium record from ODP 806B. Furthermore, diffusive loss and radiogenic ingrowth are unlikely to
account for the systematic short period (5100 kyr) oscillations
in crustal 4He contents observed in Fig. 2. Thus, crustal 4He in
ODP 806 sediments indeed records a provenance related signal
derived from the Asian dust source regions.
6. DISCUSSION
4
6.1. He Fluxes and MARs of Asian Dust
Figure 4 presents a comparison of the 4He flux data from
ODP 806B with the total MARs of aeolian dust at core V21146 in the northwest Pacific Ocean east of Japan and directly
downwind of the Asian dust sources for at least the last 530
kyrs (37°419N 163°029E, 3968 m; Hovan et al., 1989, 1991,
Fig. 1). As shown, there is a striking correlation between these
two data sets; the 100 kyr periodicity is extremely well matched
As zircon is a highly resistant and dense mineral, it is
possible that some variation in crustal 4He fluxes might reflect
transport related fractionation of zircons. For example, the
observation that the crustal 4He content of the youngest pelagic
clays from core GPC3 is about a factor of 2 lower than
observed in Chinese loess (Farley, 1995; Farley and Patterson,
1995) may indicate preferential fallout and loss of zircon from
Asian dust during aeolian transport across the Pacific Ocean.
However, the excellent correlation between the 4He fluxes to
ODP 806B and the directly observed mass accumulation rates
of Asian dust at site V21-146 (Fig. 4), strongly suggests that the
dominant control on the crustal 4He fluxes to ODP 806B is the
mass accumulation rate of Asian dust. We therefore conclude
that transport related fractionation of zircon is unlikely to have
generated the observed systematic variations in the 4He fluxes
to ODP 806B. A detailed study of the provenance, grain size,
and 4He content of windblown zircons and their host sediments
from a suite of sites having different transport distances is
suggested.
6.3. Asian Dust Versus Local Volcanic Ashes
It is stressed that the 4He flux reflects only the supply of
windblown dust probably derived from Asia. In contrast, the
total MARs of terrigenous material extracted by normal wet
chemical methods on OJP will be the sum of both Asian dust
and local volcanic ashes. Previous work by Krissek and
Janecek (1993) demonstrated that total MAR of terrigenous
material on OJP over the last 2 Myrs is dominated by volcanic
material. In order to make a direct comparison between the 4He
results and Krissek and Janecek’s (1993) estimates of the total
dust MARs it is necessary to convert the 4He flux values into
absolute Asian dust flux units. Such a conversion requires
knowledge of the 4He content of the Asian dust that actually
reaches the OJP. For the following discussion we make the
assumption that the 4He content of Asian dust at OJP is the
same as for GPC3 sediments less than 2 Myrs in age (i.e., 2000
ncc g21, Farley 1995). As shown in Fig. 2, the observed crustal
4
He fluxes to ODP 806B range between 7.7 and 30 ncc cm22
kyr21. As a first order estimate, dividing these flux values by
the assumed 4He concentration in the dust of 2000 ncc g21
suggests that the mass accumulation rate of Asian dust on the
OJP over the last 1.9 Myrs varied from about 3.8 to 15 mg
cm22 kyr21. Alternatively, assuming a higher helium content
of 5000 ncc g21 as observed in Chinese loess (Farley and
Patterson, 1995) would suggest lower Asian dust accumulation
rates of between 1.5 and 6.0 mg cm22 kyr21.
For comparison, using mineralogical, geochemical and
leaching techniques, Krissek and Janecek (1993) determined
the MAR of all terrigenous material (Asian dust plus volcanic
ash) on OJP varies between 34 and 90 mg cm22 kyr21 over the
last 1.9 Myrs. These values are significantly higher than our
620
D. B Patterson, K. A. Farley, and M. D. Norman
Fig. 4. Comparison of crustal 4He flux at ODP 806B (open circles) with the total mass accumulation rate of Asian dust
at core V21-146 in the north Pacific Ocean (open squares, Hovan et al., 1991). The dotted line gives the MARs as listed
in Hovan et al. (1991). In order to remove the high frequency signal from the V21-146 data, we have binned the dust MARs
at V21-146 into the same time intervals as represented by the 4He samples from ODP 806B and plotted the averages. Gaps
in ODP 806B data represent core gaps.
first order estimate of the accumulation rate of Asian dust
alone, and probably reflect the dominance of the local volcanic
dust component in the sediments of the OJP. To further illustrate this point, we compare the 4He flux with the total Al and
Zr fluxes in the top 10 samples covering the last 238 kyrs from
ODP 806B (Fig. 5). The strong cyclic variations in crustal 4He
fluxes are not correlated with Al flux variations. The Al flux
provides a first order tracer of the total mass accumulation rate
of both local volcanic and Asian dusts, whereas the 4He flux
traces only the Asian dust component. This is consistent with
local volcanics being the dominant source of terrigenous material on OJP, and demonstrates the utility of 4He as a tracer of
continental windblown dusts in sediments containing significant volcanic contributions. In a similar fashion, the total Zr
flux does not correlate with the 4He flux, indicating the presence of a significant component of volcanic derived Zr (presumably in the form of zircon). In this regard, studies linking
the 4He and zircon contents of sediments (via the Zr concentrations) will be hampered unless the sediments have negligible
volcanic components.
Finally, we note that the estimated flux of Asian dust to OJP
(3.8 to 15 mg cm22 kyr21) is significantly lower than the
MARs of Asian dust in core V21-146 which range from about
50 mg cm22 kyr21 to in excess of 500 mg cm22 kyr21 (Fig. 4;
Hovan et al., 1989, 1991). This is in agreement with the
compilation of Rea (1994) which shows the highest Asian dust
MARs to be concentrated in a band which stretches eastward
across the northwest Pacific directly downwind from the Asian
dust sources.
6.4. Correlation with Oxygen Isotopes
Correlations between MARs of windblown dust and global
climate as recorded by oxygen isotopes in seafloor sediments
have been reported for sites in the Pacific, Indian and Atlantic
Oceans (Clemens and Prell, 1990; Rea, 1994; Tiedemann et al.,
1994). In particular, Hovan et al. (1989, 1991) previously
demonstrated that over the last 530 kyrs, the 100 kyr period
variations in the MARs of Asian continental dust at V21-146
(Fig. 3) correlate with the d18O record such that high dust
fluxes are associated with glacial maxima. As dust supply is
primarily a function of the aridity of the dust source regions
(Middleton, 1985; Rea et al., 1985; Prospero and Tees, 1986),
Hovan et al. suggested a direct link between glacial maxima
and the aridity of the deserts of Asia as the primary control on
the supply of windblown continental dust to the western Pacific
for at least the last 530 kyrs.
Figure 6 presents a comparison of crustal 4He flux with the
18
d O record for ODP 806B (Berger et al., 1994). In order to
remove high frequency variations in the oxygen record, we
have binned the observed d18O values over the same core
intervals as the sediment strip samples used for 4He determinations and plotted the averages. There is excellent correlation
between crustal 4He fluxes and the d18O climate record for this
4He as a tracer of continental dust
Fig. 5. Comparison of crustal 4He flux with total Al and Zr fluxes in the uppermost ten ODP 806B samples covering the
last 238 kyrs. Variations in crustal 4He fluxes record the variable contribution of Asian dust down the core, whereas Al and
Zr fluxes reflect the total dust flux (continental 1 volcanic). Trace element compositions of these samples were determined
by ICPMS.
Fig. 6. Comparison of crustal 4He flux from ODP 806B with the d18O record (PDB Standard) for the same core (Berger
et al., 1994). In order to remove high frequency variations in the oxygen record, we have binned the observed d18O values
over the same intervals as the sediment strip samples used for 4He determinations and plotted the averages. Note that the
discrepancy between 4He flux and d18O about 780 kyrs ago is at a major core break such that the calculated 4He flux may
be in error. Gaps in data represent core gaps.
621
622
D. B Patterson, K. A. Farley, and M. D. Norman
Fig. 7. Comparison of crustal 4He flux (open circles) and binned oxygen isotope ratios (solid circles) at ODP 806B with
the total mass accumulation rate of aeolian dust in core RC11-210 in the central equatorial Pacific Ocean (open squares,
Chuey et al., 1987). In order to remove the high frequency signals we have binned the dust MARs at RC11-210 into the
same time intervals as represented by the 4He samples from ODP 806B and plotted the averages. Note the RC11-210 results
are in phase with the ODP 806B 4He and oxygen isotope data between about 300 kyr and 500 kyr ago, but are 180 out of
phase in the times before and after this period. Gaps in ODP 806B data represent core gaps.
site with high fluxes associated with glacial maxima. This
correlation is extremely robust for at least the last 1000 kyrs,
and appears to hold (although more weakly) for the period 1000
to 1900 kyrs ago. Furthermore, the onset of strong 100 kyr
glacial–interglacial cycling in the oxygen isotope record at
about 700 kyrs ago is clearly matched with the onset of 100 kyr
periodicity in the crustal 4He flux. As 4He fluxes provide a
proxy record of the MAR of Asian dust on OJP, we conclude
that the delivery of Asian dust to OJP is correlated with glacial
maxima. This suggests climate related variation in the aridity of
the deserts of central and eastern Asia is the primary control on
the supply of windblown Asian dust for at least the last 1000
kyrs, and possibly for the entire 1.9 Myr period studied, confirming and extending the previous conclusions of Hovan et al.
(1989, 1991). As noted previously, the longterm increase in
4
He fluxes is consistent with the data of Hovan et al. for
V21-146 (Fig. 4), and may reflect a longterm trend towards
increased aridity in Asia associated with uplift of the Himalayas over the last 4 Myrs (Kutzbach et al., 1989; Pye and Zhou,
1989; Ruddiman and Kutzbach, 1989; Hovan et al., 1991, Kyte
et al., 1993).
6.5. Influence of the Inter-Tropical Convergence Zone
One of the major features of atmospheric circulation in the
Pacific Ocean is the Inter-Tropical Convergence Zone (ITCZ),
the near equatorial region where air masses from the northern
and southern hemispheres meet. Previous studies have shown
that across the central and eastern Pacific, the ITCZ forms a
narrow, well-defined rainfall barrier to dust transport between
the northern and southern hemispheres, but is broader, weaker,
and less well defined in the western Pacific (Rea, 1994).
Figure 7 presents a comparison of the 4He flux and oxygen
isotope records from ODP 806B with the aeolian MAR record
from a site in the central equatorial Pacific (core RC11-210,
1°499N140°039W, 4420 m, Fig. 1; Chuey et al., 1987). The
consistent correlation between 4He flux and glacial maxima
observed in ODP 806B is in contrast to the results from
RC11-210. At RC11-210 continental dust flux maxima correlate with glacial minima back to about 300 kyrs ago, opposite
to the relationship observed at ODP 806B. Between 300 and
500 kyrs ago, however, the pattern at RC11-210 switches to
one of dust flux maxima correlating with glacial maxima,
similar to that observed in ODP 806B. Further down RC11-210
the pattern switches back to being in phase with glacial minima.
Rea (1994) suggested this switching of phase relationships
reflects north–south migration of the ITCZ such that at different
times RC11-210 lies in either the northern or southern atmospheric hemispheres. In contrast, the consistent phase relationship between glacial maxima and high dust fluxes observed at
ODP 806B is a characteristic of northern hemisphere sites
4He as a tracer of continental dust
dominated by Asian dust (such as core V21-146, Fig. 4; Hovan
et al., 1989, 1991). The clear distinction between ODP 806B
and RC11-210 suggests that, for at least the last 1000 kyrs, the
ITCZ has not been an effective barrier to southward transport
of Asian dust in the extreme western Pacific, consistent with the
suggestions of previous workers (Rea, 1994).
7. CONCLUSIONS
4
Crustal He fluxes to seafloor sediments from the Ontong
Java Plateau show significant variations that correlate with
global climate as recorded by oxygen isotopes with high 4He
fluxes associated with glacial periods over the last 1.9 Myrs.
The excellent correlation between the 4He fluxes to ODP 806B
and the accumulation rate of Asian dusts at site V21-146
indicate that 4He fluxes can provide a reliable tracer of the
delivery of continental aeolian dust to the oceans.
We interpret the crustal 4He flux record from ODP 806B to
reflect variations in the supply of continental windblown dust to
the equatorial western Pacific Ocean caused by climate driven
changes in the aridity of the deserts of central and eastern Asia.
Based on the 4He results, the mass accumulation rate of the
Asian dust at Ontong Java Plateau over the last 1.9 Myrs is
estimated to vary between 3.8 and 15 mg cm22 kyr21, which
is notably lower than the total accumulation rate for terrigenous
material at this site. This difference reflects the predominance
of the volcanic component derived from the nearby volcanoes
of the western Pacific Rim. The primary carrier phase of crustal
4
He is probably zircon, which is known to have sufficiently
high 4He contents and retentivity to account for the observed
4
He results. Post depositional ingrowth of radiogenic 4He is not
significant owing to the ejection of 4He atoms from the small
(certainly less than 10 mm) zircon grains in windblown dust by
alpha particle recoil.
This study demonstrates the utility of 4He as a tracer of
continental dusts in sediment samples containing significant
amounts of volcanic material, and the application of this approach to mapping glaciation, aridity and atmospheric circulation patterns. Further work on the relationship between crustal
4
He fluxes and aeolian MARs is presently underway.
Acknowledgments—This work was supported by the David and Lucille
Packard Foundation and the Australian Research Council. We thank
Dr. David Rea and two anonymous referees for their comments. Special thanks to Dr. R. Muller for providing the Fourier Transform
spectral analysis. DBP wishes to also thank Mr. John Miller of ODP for
help and hospitality during sample retrieval. This is GEMOC Contribution 142.
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