Solar Forcing of Holocene Climate: New Insights from a Speleothem Record, Southwestern United States

Solar forcing of Holocene climate: New insights from a speleothem record, southwestern United States

Yemane Asmerom Victor Polyak Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA Stephen Burns Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA Jessica Rassmussen Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA

ABSTRACT Holocene climate change has likely had a profound infl uence on ecosystems and culture. A link between solar forcing and Holocene climate, such as the Asian monsoon, has been shown for some regions, although no mechanism for this relationship has been suggested. Here we present the fi rst high-resolution complete Holocene climate record for the North American monsoon region of the southwestern United States (southwest) in order to address the nature and causes of Holocene climate change. We show that periods of increased solar radiation correlate with decreased rainfall, the opposite to that observed in the Asian monsoon, and suggest that a solar link to Holocene climate is through changes in the Walker circulation and the Pacifi c Decadal Oscillation and El Niño–Southern Oscillation systems of the tropical Pacifi c Ocean. Given the link between increased warming and aridity in the southwest, additional warming due to greenhouse forcing could potentially lead to persistent hyperarid conditions, similar to those seen in our record during periods of high solar activity.

Keywords: Solar forcing, Holocene climate, monsoon, speothem, U-series, southwestern United States.

INTRODUCTION beyond the tree-ring chronology, which in the tory1; complete uranium-series chronology data The climate of the Holocene, while relatively southwest only covers the past 2 k.y. (Grissino- are in Table DR1). We measured oxygen stable stable in comparison to variability during the Mayer, 1996). Speleothems provide a unique isotope ratios at 200 µm intervals for a total of last glacial period, has varied enough to dramat- opportunity because it is possible to obtain abso- 681 samples, an average time resolution of 17 yr ically affect ecosystems and human develop- lute chronology at any interval, and they con- over the entire period. The δ18O values vary ment in many parts of the world. For example, tain multiple chemical, physical, and isotopic from −5.81‰ to −2.66‰ (Fig. 2A; stable iso- the Holocene was a time of profound changes p roxies. Here we present the fi rst complete high- tope data are available in Table DR2; see foot- in the cultural landscape of the southwestern r esolution climate proxy for the southwest in the note 1). The time series of δ18O does not show United States (herein southwest), including form of δ18O variations in a speleothem cover- a clear trend through the Holocene, but displays expansions and contractions of ancestral Ameri- ing the entire Holocene, to 12.3 ka. Our results rapid variations of (a few per mil) on millennial can communities that were in part responses show that periods of greater solar activity are and centennial time scales. The variation due to changes in climate (Polyak and Asmerom, associated with decreased moisture in the south- to temperature of calcite-water fractionation 2001; Polyak et al., 2004). The causes of Holo- west. Over the same time period, the response (~0.25‰/°C) probably does not contribute sig- cene climate variation are not known with cer- of the Asian monsoons to solar variability was nifi cantly to variation in the δ18O values. Tem- tainty, but three forcing mechanisms have been in the opposite direction. We suggest that these perature variations of 4–5 °C would cause only suggested as possible factors: ocean circulation, relationships may be explained if solar activity 1‰ variation in δ18O, and there are no indica- volcanic activity, and solar variability (Bianchi acts to modulate the El Niño–Southern Oscilla- tors of such large temperature changes during and McCave, 1999; Bond et al., 2001; Crowley, tion (ENSO) and or Pacifi c Decadal Oscillation the Holocene. Variations in the O and C isotopic 2000). Recent studies have suggested a link (PDO), and in particular the Walker circulation, values could result from kinetic fractionation. In between solar variability, expressed as changes on decadal and centennial time scales. this case O and C values would covary. A plot in the carbon 14 content of the atmosphere of δ18O versus δ13C shows no overall signifi - (∆14C), and North Atlantic climate (Bond et al., CHRONOLOGY AND 2001) and the Asian monsoon (Neff et al., 2001; ISOTOPE GEOCHEMISTRY 1GSA Data Repository item 2007010, Figures Fleitmann et al., 2003; Wang et al., 2005). It is Sample PP1, a 14-cm-long stalagmite, was DR1A (photo of speleothem PP1), DR1B (photo of still not clear how solar variability modulates collected from Pink Panther Cave in the Gua- Pink Panther Cave) and DR1C (δ18O vs. δ13C fi gure), climate changes in these regions and whether dalupe Mountains, New Mexico (Fig. 1A). A and Tables DR1 (complete data and methods for this link is global. In arid regions such as the series of 22 230Th-234U dates demonstrates that uranium-series chronology), and DR2, (δ18O and δ13C data), is available online at www.geosociety. southwest, the main challenge to identifying the sample grew continuously but nonlinearly org/pubs/ft2007.htm, or on request from editing@ causes of Holocene climate variability has been from ca. 12,330 yr B.P. to the present (Fig. 1B; geosociety.org or Documents Secretary, GSA, P.O. the lack of high- resolution records that extend Figs. DR1A and DR1B in the Data Reposi- Box 9140, Boulder, CO 80301, USA.

© 2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGYGeology, January, January 2007; 2007 v. 35; no. 1; p.1–4; doi: 10.1130/G22865A.1; 3 ﬁ gures; Data Repository item 2007010. 1 Figure 2. A: Raw (not detrended) δ18O record A for stalagmite PP1. For two segments where A GUADALUPE MOUNTAINS, NM we have detailed speleothem growth data -3 UT CO [between 0–4 ka (Polyak and Asmerom, 18 AZ 2001) and 9–12 ka (Polyak et al., 2004)], δ O NM variations correspond to variations in amount -4 (VPDB) Winter Precipitation. TX

of precipitation. Overall, early Holocene, for O

18 8

δ O ~ –11 18 1 which δ O values are highest, was driest δ -5 Summer Precipitation segment of Holocene, characterized by des- δ18O ~ –3.0 iccation of regional lakes (Anderson et al., 2002), after wet period that started after the beginning of Younger Dryas (Polyak et al., -6 0 2 4 6 8 10 12 14 2004). Large negative excursions during late B Holocene, starting ca. 3.5 ka, such as at 3.3 Age (ka) 12 and 2.7 ka, correspond to unusually wet peri- 10 ods, possibly initiating migration of agricul- 8 ture into southwest (Polyak and Asmerom, 100 699 6 2001). Long-term trend from higher δ18O in B Age (ka) 18 e 4 early Holocene to lower δ O values in late d

2 Holocene (calculated using simple linear ﬁ t to data) is opposite to trend observed in 10 128

Amplitu 86 0 104 78 0 20 40 60 80 100 120 140 Asian monsoon (Wang et al., 2005) and is 65–68 unlikely to represent strengthening of North Distance from top (mm) American monsoon. B: Univariate spectra of Spectral 1 18 Figure 1. A: Guadalupe Mountains, New entire raw δ O record estimated using com- 0.000 0.005 0.010 0.015 Mexico, site of Pink Panther Cave and typi- puter program Redfi t (Schulz and Mudelsee, Frequency (1/yr) cal of other parts of U.S. southwest, has 2002), which utilizes Lomb-Scargle Fourier two rainy seasons. Summer North American transform for unevenly spaced data. Number of overlapping (50%) segments (nseg) of 6 and monsoon (NAM) rainfall (Adams and Com- Welch I type spectral window were used. Dashed line shows red-noise boundary (upper rie, 1997), derived from Gulf of Mexico, is 90% χ2 bound). Most signifi cant peaks (above red-noise background) within 6 dB bandwidth characterized on average by heavy δ18O val- frequency resolution (0.451 k.y.–1) are 128, 104, 86, 78, and 65–68 yr, and are similar to peaks ues (~–3‰), while winter Pacifi c-derived pre- observed in ∆14C spectra and have been attributed to solar variability (Sonnett and Finney, cipitation is characterized by low δ18O values 1990; Stuiver and Braziunas, 1993). Resolution of our δ18O data, on average, is 17 yr, thus (~–11‰) (Hoy and Gross, 1982; Yapp, 1985). peaks younger than 50 yr were disregarded. VPDB—Vienna Peedee belemnite. B: Chronology of PP1, showing distance from top of stalagmite to bottom vs. U-Th ages. Solid curve is age model based on 9 degree polynomial fi t of chronology data, ~−3‰) compared to the Pacifi c-derived winter and in particular at 2700 yr B.P. correlate with covering entire period. Error bars refl ect precipitation (δ18O ~−11‰) (Fig. 1A) (Hoy and increased stalagmite growth (high moisture) and analytical errors, including uncertainties Gross, 1982; Yapp, 1985). Thus, the balance are often the ages above hiatuses (initiation of related to estimate of initial 230Th/232Th ratio between winter and summer precipitation is one growth after middle Holocene aridity) (Polyak (details on chronology, including analytical factor that may infl uence the 18O value of mean and Asmerom, 2001). Conversely, the large methods, are in Table DR1; see footnote 1). δ annual precipitation. In addition, the amount of positive excursion in δ18O values ca. 10,000 yr rainfall in many monsoon regions is observed B.P. represents the termination of growth for a cant fractionation trend (Fig. DR1C). Secular to be inversely correlated with the δ18O value of number of speleothems and a period of desicca- variation in δ18O for a given speleothem, even precipitation (termed the amount effect) during tion for regional lakes (Anderson et al., 2002), in arid regions more arid than our sample site the monsoon months (Dansgaard, 1964). Wright marking the end of a wet period that started (Bar-Matthews et al., 2003; Neff et al., 2001), et al. (2001) documented amount effect in the soon after the initiation of the Younger Dryas has been shown to primarily refl ect variations North American monsoon. Analysis of instru- and lasted until ca. 10 ka (Polyak et al., 2004). in the isotopic composition of precipitation, the mental data for the past 100 yr shows that, even We have virtually no speleothems (except for source of the cave drip water. We similarly attrib- though the annual moisture budget is dominated PP1) that show any growth during the period ute the great majority of the observed variability by summer precipitation, years with above- consistent with the high δ18O values in PP1. This in PP-1 to be due to changes in the isotopic com- average annual precipitation are most often years dry period persisted to ca. 7 ka. After 7 ka, lower position of rainfall through time. with above-average winter precipitation (Liles, δ18O values suggest a return to wetter conditions. 1999; McCabe et al., 2004). To a fi rst approxi- This is corroborated by the initiation of growth DISCUSSION AND CONCLUSIONS mation, we interpret lower δ18O values in sample in a number of our speleothems, although not Typical of much of the southwest, rainfall PP1 as indicating an increase in either summer to the extent observed during the late Holocene in the study region is concentrated in two peri- or winter precipitation, or an increase in both. (Polyak and Asmerom, 2001). The prominent ods. From July through September, rainfall in The high δ18O values seen during the early negative δ18O excursions ca. 6.2 ka likely rep- the area is associated with the North Ameri- Holocene and the early half of the middle Holo- resent a brief episode of greater than normal can monsoon (NAM) (Adams and Comrie, cene are consistent with previous observations effective precipitation. Evidence for such an 1997), moisture being derived primarily from that these were very arid periods for the region event is noted in the nearby Lake Estancia sedi- the Gulf of Mexico. From December through (Anderson et al., 2002; Polyak et al., 2004). For ment record ( Menking and Anderson, 2003) and March precipitation is derived from westerly the late Holocene (Polyak and Asmerom, 2001) a northern Mexico lake record (Castiglia and low-pressure systems from the Pacifi c Ocean and the Pleistocene-Holocene transition (Polyak Fawcett, 2006). by the jet stream. On average these two seasons et al., 2004), we have an independent, growth To test for the presence of cyclical variation and sources of rainfall also have very distinct thickness–based record with which to compare in our record, we performed spectral analysis oxygen isotope ratios. Summer precipitation is our δ18O data. The large negative excursions of the raw δ18O data using the program Redfi t much more enriched in the heavy isotope (δ18O in the δ18O record (Fig. 2A) ca. 3300 yr B.P. (Schulz and Mudelsee, 2002). Results show

2 GEOLOGY, January 2007 signifi cant (above the red-noise curve) peaks (INTCAL 98; Stuiver et al., 1998). Periods of is overridden in the southwest by some other centered at ~128, 104, 86, 78, and 65–68 yr. increased solar activity and likely higher irradi- effect of solar forcing that results in a contrast- A number of these peaks closely match previ- ance (Stuiver and Braziunas, 1993), expressed ing relationship between solar variability and ously reported periodicities in the 14C content as negative ∆14C, are well correlated with posi- moisture in the two regions. of the atmosphere, which have been attributed tive δ18O anomalies. If our interpretation of the One possible mechanism for the observed to periodicities in the solar cycle (Stuiver and isotopic record is correct, greater solar activity climate responses is that solar variability is Braziunas, 1993). Shorter periods are not con- results in decreased winter or summer precipi- affecting climate via the Pacifi c Ocean and sidered because our δ18O record has an average tation (or both) and a drier climate in southern the PDO–ENSO system, because PDO–ENSO 17 yr sampling resolution. Our δ18O data and the New Mexico. Our δ18O record and the global affects rainfall in the Asian monsoon region detrended ∆14C data (INTCAL 98; Stuiver et al., ∆14C record show a remarkable correlation, R = and the southwest in opposite ways. Drier than 1998) have a noticeable visual match (Fig. 3A). −0.34 to −0.38 (20–50 yr interpolation) for the normal winters in the southwest almost without The chronology for the δ18O data is tuned to the entire record, considering that the proxies are so exception follow a La Niña episode, while wet ∆14C record using a polynomial curve (shown dissimilar. winters and less often wet summers typically in Fig. 1B), taking into consideration and fi t- Cross-spectral analysis of the ∆14C data occur during El Niño years, in particular during ting within the 2 σ analytical errors, plus small and δ18O data (Fig. 3B) confi rms that the two positive PDO years (Cook et al., 2004; Liles, errors associated with the ∆14C chronology records have matching periodicities at 1533, 1999; McCabe et al., 2004). While the relation- 444, 170, 146, and 88 yr above the 95% confi - ship between ENSO and the Asian monsoon is dence interval. The coherent peaks, within the less clear, in the summers following an El Niño

A –20 bandwidth of the Welch window, are identical event the Asian monsoon is typically weakened 14 –3 to the spectral peaks of the ∆ C record, and and the area is often characterized by droughts –10 y ) l

a some of these, including the 1533 (1500) yr (Chan and Zhou, 2005). Comparison of mod- B D m P –4 0 o n

V (Bond), 440 (400–500) yr, the 146 yr, and the ern rainfall data from China and the southwest (

O C 8 4

1 88 yr (Gleissberg) cycles, have been attributed shows an opposite relationship (Rasmussen

10 1 δ –5 ∆ to solar variability (Bond et al., 2001; Sonnett et al., 2006). If this relationship between the two 20 and Finney, 1990; Stuiver and Braziunas, 1993). regions holds true for the rest of the Holocene, –6 30 The coherence between our δ18O and the ∆14C then it suggests that solar forcing might be steer- 0 2 4 6 8 10 12 Age (ky) data (Figs. 3A, 3B) suggests a link between ing the PDO–ENSO system on millennial and southwest climate change and solar variability centennial time scales via changes in the Walker 0.8 1533 (1500) 142–150(146) B over the past 12 k.y. circulation. If periods of increased solar activ- 170 88 (88) Previous studies of the Asian monsoon show ity lead to a stronger Walker circulation and 0.6 444 (500) a relationship between rainfall and solar activity generally more La Niña–like conditions (and 95%

y 74

c that is opposite in sign to the results of our study. vice versa), then the antiphase behavior of the n 90% e r 0.4 e In speleothem records from Oman and China, responses of precipitation in the Asian monsoon h o

C increased solar activity correlates with more nega- and the southwest could be explained. Such a 18 0.2 tive stalagmite δ O values and greater monsoon relationship between solar variability and the rainfall. The variance in solar irradiance associ- ENSO system has been observed in recent mod-

0.0 ated with historical solar cycles is estimated to eling studies (Mann et al., 2005). 0.000 0.005 0.010 0.015 be between 0.1% and 0.3% (1–2 mW/m2) (Hoyt This salient anticorrelation of the two climatic Frequency (1/yr) and Schatten, 1993; Lean et al., 1995), which regimes, currently expressed in their seesaw Figure 3. A: Covariation of raw δ18O record seems too small to directly affect the sensible responses to PDO–ENSO, suggests that solar (black line) and detrended ∆14C data heating component of the Asian monsoon. It is forcing of continental climate is likely modu- (INTCAL 98) (red line) (Stuiver et al., 1998). not clear how such minor changes in irradiance lated through the Pacifi c Ocean. Temperature We detrended ∆14C data by subtracting six degree polynomial long-term trend from are amplifi ed by the ocean- atmosphere system anomalies in the Pacifi c Ocean seem capable of data. Scales for two records are in opposite so that they affect large-scale components of inducing globally synchronous climate change direction to each other to show negative climate such as the monsoons. Nonetheless, if (Clement et al., 1999). The link between south- 18 correlation. In this case, higher δ O values some direct (though amplifi ed) solar forcing of west climate, Asian monsoon, and solar variabil- correspond to lower ∆14C values but higher solar intensity. Zero reference is A.D. 1955. the NAM was the dominant control on south- ity described here shows that the teleconnections Two records have excellent visual match. west precipitation, we would expect a positive observed in the modern climate regime may Correlation coeffi cient for two records relationship between solar activity and moisture have operated throughout the Holocene and that for entire Holocene is R = −0.34 to –0.38 (positive correlation between δ18O and ∆14C), solar forcing of the tropical Pacifi c may explain (20–50 yr interpolation). We do not know similar to the relationship observed with the much of the variability observed in Holocene cause of mismatch ca. 7.2 ka. B: Cross- spectral analysis, using computer pack- Asian monsoon. Reanalysis of modern instru- climate both in the southwest and elsewhere. A age SPECTRUM (Schulz and Stattegger, mental data shows some degree of intensifi ca- relationship between historical southwest pre- 1997), of raw δ18O and raw ∆14C time series. tion of the Asian monsoon and the NAM (van cipitation and the Atlantic Multidecadal Oscil- Parameters for coherency calculations were Loon et al., 2004). Thus we do not rule out a lation (AMO) has been reported (McCabe et al., similar to those used for univariate spectral analysis (Fig. 2B). 95% and 90% false alarm component in our record that refl ects positive 2004). AMO variability is related to changes limits are shown as solid and dashed lines, solar feedback on the NAM, especially on short in sea surface temperature (SST) in the North respectively. Signifi cant peaks (in black) time scales. What is clear from our record is Atlantic; Holocene paleoclimatic data also show include 1533, 444, 170, 146, and 88 yr. Cor- that the dominant climate-solar relationship in strong modulation of North Atlantic climate responding numbers in red are peaks previ- the southwest is opposite to that observed in the (Bond et al., 2001). Moreover, Hoerling et al. ously associated with solar peaks (Sonnett and Finney, 1990; Stuiver and Braziunas, Asian monsoon regions, and implies that any (2001), based on climate data since 1950, has 1993). VPDB—Vienna Peedee belemnite. direct solar intensifi cation of the two monsoons suggested that North Atlantic climate, including

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4 GEOLOGY, January 2007