A 2200 to 2600 year quasi-periodicity is present in atmospheric delta C-14 records after removal of long-term trends due to the geomagnetic dipole amplitude variation. This periodicity consists of both a long-term variation of the mean and a superposed, approximately recurring pattern of century-scale variations. The strongest of these latter variations occur near maxima of the approx. 2400 year delta C-14 cycles. The residual record can be modeled to first order as an amplitude modulation of a century-scale periodic forcing function by a approx. 2400 year periodic forcing function. During the last millennium, the largest century-scale variations (occurring near the most recent 2400 year delta C-14 maximum) are known to be mainly a consequence of the pronounced Maunder, Sporer, and Wolf solar activity minima, as verified by independent proxy solar activity records. Therefore, during this period, amplitude modulation has been occurring primarily in the sun and not in the terrestrial radiocarbon system. It is therefore inferred that the approx. 2400 year forcing function is mainly solar although some secondary terrestrial feedback into the delta C-14 record is likely. This conclusion has implications for the predictability of future pronounced solar activity minima and for the interpretation of certain minor Holocene climatic variations

Hood, L. L.

Jirikowic, J. L.

English

NASA Technical Reports Server

A PROBABLE -- 2400 YEAR SOLAR QUASI-CYCLE IN ATMOSPHERIC A14C 
L. L. Hood and J. L. Jiriko~vic 
Lunar and Planetary Laboratory, University of Arizona 
Tucson, Arizona 85721 USA 
ABSTRACT 
A 2200-2600 year quasi-periodicity is present in atmospheric A14C records after re- 
moval of long-term trends due to the geomagnetic dipole amplitude variation. This period- 
icity consists of both a long-term variation of the mean and a superposed, approximately 
recurring pattern of century-scale variations. The strongest of these latter variations occur 
near maxima of the - 2400 year A14C cycles. The residual record can be modeled to 
first order as an amplitude modulation of a century-scale periodic forcing function by a 
- 2400 year periodic forcing function. During the last millennium; the largest century- 
scale variations (occurring near the most recent 2400 year A14C maximum) are known 
to be mainly a consequence of the pronounced Maunder, Sparer, and Wolf solar activity 
minima, as verified by independent proxy solar activity records. Therefore, during this 
period, amplitude modulation has been occurring primarily in the sun and not in the ter- 
restrial radiocarbon system. It is therefore inferred that the - 2400 year forcing function 
is mainly solar although some secondary terrestrial feedback into the A14C record is likely. 
This conclusion has implications for the predictability of future pronounced solar activity 
minima and for the interpretation of certain minor Holocene climatic variations. 
INTRODUCTION 
Deviations of atmospheric 14C versus time (A14C) are produced in part by solar- 
induced changes in galactic cosmic ray flux which in turn modulate the radiocarbon pro- 
duction rate [Stuiver, 1961; Stuiver and Quay, 1080; Sonett, 1984; Damon, 1988; and 
references therein]. Directly dated tree ring A14C records covering the last - 8000 years 
therefore represent one of the best available prosy measures of solar magnetic variability. 
However, an important problem in the interpretation of such records is the separation of 
solar variability contributions from other potential terrestrial sources of radiocarbon vari- 
ability. In this paper, empirical methods are used to investigate whether millennium-scale 
variations in the radiocarbon record are primarily solar or terrestrial in origin. 
ANALYSIS 
Figure 1 shows a high-precision A'" record resulting from the 12th International Ra- 
diocarbon Conference [Stuiver and Icra, 19861. A long-term trend is present that has been 
shown to be largely consistent with a modeled response to the - 11,000 year geomagnetic 
dipole moment amplitude variation as derived from archeomagnetic records [Sternberg and 
Damon, 1983; Damon, 19881. Figure 2 sho~irs the same record after detrending by removal 
of a least-squares-fitted cubic polynomial representing long-term changes in geomagnetic 
dipole moment intensity. Note that the residual radiocarbon variations are now plotted 
with an inverse scale so that decreases in A14C are upward and correspond to increases in 
https://ntrs.nasa.gov/search.jsp?R=19910003157 2020-03-24T06:52:09+00:00Z
Years 
Figure 1. 12th International Radiocarbon Conference calillibration record produced by com- 
bining high-precision records from a series of lahoratorics [Stuiver and Icra, 19861. (Data 
courtesy of W1. Stuiver and R. Icra) 
I I I I 
-6000 -4000 -2000 0 2000 
years 
Figure 2. Residual Conference A14C record after detrending to remove the long-term ge- 
omagnetic variation and smoothing to minimize variations with periods less than approx- 
imately 100 years. Note that the vertical scale has been inverted so that A14C maxima 
map solar activity minima. A least-squares-fitted sinusoid is superposed to indicate the 
approximate locations of maxima and minima of the N 2400 year quasi-cycle. 
solar activity. 
The residual variations of Figure 2 exhibit a 2200-2600 year quasi-periodicity that has 
been recognized in power spectral analyses and visual examinations of various radiocarbon 
records [Houtermans, 1971; Denton and Karlkn, 1973; Suess, 1980; Sonett and Finney, 
19891. To indicate the approximate locations of maxima and minima of this quasi-cycle, a 
sinusoid with a N 2400 year period was least-squares-fitted to the residual record (fitted 
parameters: period = 2310 years; amplitude 4.08 per mil) and is superposed on the record 
in Figure 2. A geomagnetic origin for this quasi-periodicity in A14C has been considered 
unlikely since there is no dipole moment amplitude variation near a period of 2400 years 
[Damon and Sonett, 19901. However, there is a well-known climatic period near 2500 years 
[Mitchell, 1976; Pestiaux et al., 19873. Several earlier investigators have reported that 
this N 2500 year climate variation has been nearly in phase with the inverted A14C record 
during the last 8000 years [Bray, 1968; 1970; Denton and I<arlkn, 19731. Figure 3 compares 
the residual A14C record of Figure 2 to well-dated proxy northern hemispheric climate 
indicators including S180 from the Camp Century Greenland ice core [Dansgaard et al., 
19841 and the Devon Island Canada ice core [Fisher, 19821 as well as to the temperature- 
sensitive Campito Mountain bristlecone pine tree ring width record of LaMarche [1974]. 
A tendency for climatic minima to be associated with A14C maxima is evident. The last 
of these climatic minima was the Little Ice Age [Eddy, 19771. 
There are two "end-member" candidate sources of the N 2400 year quasi-periodicity 
in atmospheric A14C. First, the A14C could be responding entirely to a terrestrial climate 
cycle of uncertain origin through a redistribution of 14C between the atmospheric and 
oceanic reservoirs. Second, the A14C could be responding entirely to a solar quasi-cycle of 
the same period and the N 2500 year climate period could be independently driven by solar 
variability. In order to distinguish between these possibilities, it is helpful to consider the 
detailed characteristics of the radiocarbon record of Figure 1 together with independent 
prosy solar activity records covering the last 1000 years. 
Century-scale variations in the A14C record are believed to be dominantly of solar 
origin as evidenced by their correlation with the independently verified Maunder and Sporer 
solar activity minima of the last millennium [Eddy, 1976; 1977; Stuiver and Quay, 1980; 
Stuiver and Braziunas, 19891. As can be seen in Figure 2, the strongest century-scale 
variations tend to occur near successive maxima of the N 2400 year A14C cycles. In 
addition, it has been shown that century-scale variations in the most recent N 2400 year 
cycle in A14C are positively correlated with similar short-term variations in each of the two 
previous cycles [Hood and Jilikowic, 19901. Thus, the quasi-periodicity consists of both 
a long-term variation of the mean and a superposed, approximately recurring pattern of 
century-scale variations. 
The behavior of the residual A14C record can be interpreted in terms of amplitude 
modulation of a century-scale solar forcing function by a longer-term forcing function 
[Sonett, 19841. Figure 4 shows a simplified example of amplitude modulation in which a 
200-year sinusoid is modulated by a 2400-year sinusoid. Pronounced short-term maxima 
(note the inverted scale) occur in the resulting time series (Figure 4c) at intervals of the 
longer period. If the 2400-year forcing function (represented in Figure 4b) were 
domi~la~ltly terrestrial in origin, then we would expect to be able to observe 
+1.0  r Carrlp Ccntury Ice Core 
U 
&, 4-1.0 Devon Island Ice Core 
a 
,+0.25 
E Campito Mountain Tree Ring 
E 
- 
5 
a 
* -0.25 
Figure 3. Comparison between the residual Conference A1*C record and several well-dated 
proxy northern hemispheric climatic indicators for the last 8000 years (see the text). 
I I 
I I I 1 1 I I 1 
-5 -4 -3 -2 - 1  0 1 2 
Years (thousands) 
where A = 5 ,  B = 0.6, wl = 2~12400  p - 1 ,  w2 = 
2 ~ 1 2 0 0  yr-', and t is the time in years. 
Figure 4. Simple model of amplitude modulation of a 200 year sinusoid (a) by a 2400 year 
sinusoid (b). The product is shown in (c) and has characteristics that are qualitatively 
similar to those of the residual A14C record. 
years 
Figure 5. A comparison between the last 1000 years of the residual Conference A14C series 
and two independent proxy solar activity indicators (see the text). 
the unmodulated solar signal (represented in Figure 4a) in independent proxy 
solar records covering the last 1000 years. However, the latter records show the 
occurrence of pronounced minima in the form of the Wolf, SpGrer, and Maunder solar 
activity minima. 
Figure 5 compares the residual A14C record for the current millennium with the 
auroral frequency record compiled by Schove [I9871 and with mean annual sunspot number 
since 1670 [Waldmeier, 1961; Eddy, 19761. Within the recent interval covered by the 
sunspot record, both the Dalton and Maunder sunspot minima are approximately reflected 
in the residual inverted A14C record. At earlier times, the auroral record remains positively 
correlated with the inverted A14C time series (R = 0.55 at 20 years lag versus the smoothed 
auroral record for the entire series) and exhibits minima that can be identified with the 
Maunder, Sporer, and Wolf minima in the A14C time series. A broad maximum centered 
on A.D. 1150, known as the medieval maximum, is also evident in both records. From 
the existence of pronounced minima in these independent proxy solar records 
near the last A1*C maximum, it is clear that amplitude modulation of century- 
scale solar variations has been occurring in the sun during tlie last 1000 years 
and not just in tlie terrestrial radiocarbon system. By inference, the earlier strong 
variations occurring at N 2400 year intervals are most probably produced mainly by solar 
changes although some secondary climatic feedback effects on A14C can not be excluded. 
On this basis, the N 2400 year forcing function that is modulating the atmospheric A14C 
record is suggested to be primarily solar. 
IMPLICATIONS 
Clearly, the construction of more complete quantitative models for A14 C production 
and exchange appropriate for millennium scale variations would be valuable and may 
elucidate further the relative roles of terrestrial and solar forcing mechanisms. However, 
the empirical evidence discussed here for a dominantly solar origin of both the century-scale 
and longer-term (N 2400 year) residual variations in the Conference A14C record does allow 
several provisional implications to be stated. The first of these relates to the predictability 
of future long-term activity changes. Although the Babcock-Leighton models for the Hale 
magnetic cycle (involving distortion of an initially poloidal field by differential rotation in 
the convection zone) have been developed, these models along with dynamo theories for 
the origin of the magnetic field itself are not yet sufficiently advanced to allow extensions 
to longer time scales [e.g., Gough, 19771. From a purely empirical standpoint, however, 
it is reasonable to expect that the inferred quasi-cyclicity will persist in the future. In 
particular, it can be projected that large Maunder- and SpGrer-type minima are not likely 
to occur again until ca. A.D. 4000 and that these will be preceded by a lengthy period of 
relatively high solar activity. In addition, the above analysis (indicating that the N 2400 
year A14C periodicity is dominantly of solar origin) supports a possible solar origin for the 
Holocene climatic cycle of the same approximate period (Figure 3). 
Acknowlednments. M. Stuiver and R. I<ra kindly provided a digital form of the 12th 
International Radiocarbon Conference calibration record. We thank C. P. Sonett, S. A. 
Finney, and P. E. Damon for helpful discussions. 
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A Probable Approx. 2400 Year Solar Quasi-cycle in Atmospheric Delta C-14

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