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

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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 cor- relate 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 proxies. Here we present the fi rst complete high- tope data are available in Table DR2; see foot- in the cultural landscape of the southwestern resolution 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.
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