Extended megadroughts in the southwestern United States ...

Extended megadroughts in the southwestern United States during Pleistocene interglacialsLETTER doi:10.1038/nature09839
Extended megadroughts in the southwestern United States during Pleistocene interglacials Peter J. Fawcett1, Josef P. Werne2,4,5, R. Scott Anderson6,7, Jeffrey M. Heikoop8, Erik T. Brown3, Melissa A. Berke3, Susan J. Smith7, Fraser Goff1, Linda Donohoo-Hurley1, Luz M. Cisneros-Dozal8, Stefan Schouten9, Jaap S. Sinninghe Damste9, Yongsong Huang10, Jaime Toney8, Julianna Fessenden6, Giday WoldeGabriel6, Viorel Atudorei1, John W. Geissman1 & Craig D. Allen11
The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern1. Multi-year droughts during the instrumental period2 and decadal-length droughts of the past two millennia1,3 were shorter and climatically different from the future permanent, ‘dust-bowl-like’ mega- drought conditions, lasting decades to a century, that are predicted as a consequence of warming4. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeo- temperature proxies5 to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C4 plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles 2 6C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the
muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4–6 6C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase.
The hydroclimatology of the southwestern US shows significant natural variability including major historical droughts1. Models of climate response to anthropogenic warming predict future dust- bowl-like conditions that will last much longer than historical droughts and have a different underlying cause, a poleward expansion of the subtropical dry zones4. At present, no palaeoclimatic analogues are available to assess the potential duration of aridity under a warmer climate or to evaluate its effect on the seasonality of precipitation.
Here we present a high-resolution climate record from an 82-m lacustrine sediment core (VC-3) from the Valles Caldera (Fig. 1) that spans two mid-Pleistocene glacial cycles from MIS 14 to MIS 10 (552 kyr ago to ,368 kyr ago; see Supplementary Information). MISs 11 and 13 are long interglacials that may have been as warm as
1Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. 2Large Lakes Observatory and Department of Chemistry and Biochemistry, University of Minnesota Duluth, Duluth, Minnesota 55812, USA. 3Large Lakes Observatory and Department of Geological Sciences, University of Minnesota Duluth, Duluth, Minnesota 55812, USA. 4Centre for Water Research, University of Western Australia, Crawley, Western Australia 6009, Australia. 5WA-Organic and Isotope Geochemistry Centre, Curtin University of Technology, Bentley, Western Australia 6845, Australia. 6School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA. 7Laboratory of Paleoecology, Bilby Research Center, Northern Arizona University, Flagstaff, Arizona 86011, USA. 8Earth and Environmental Sciences Division, EES-14, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. 9NIOZ Royal Netherlands Institute for Sea Research, Department of Marine Organic Biogeochemistry, PO Box 59, 1790 AB Den Burg, Netherlands. 10Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA. 11USGS Fort Collins Science Center, Jemez Mountains Field Station, Los Alamos, New Mexico 87544, USA.
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Figure 1 | Location map of the Valles Caldera. a, Location in northern New Mexico. b, Digital elevation model of the Valles Caldera showing the location of South Mountain rhyolite, Valle Grande, the drilling location of core VC-3 (black square) and a photograph of the drilling site.
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the Holocene epoch, and MIS 11 is a good analogue for future natural climate variability with similar, low-amplitude precessional cycles6,7. We used novel organic geochemical proxies (the cyclization ratio of branched tetraethers (CBT, related to pH) and the methylation index of branched tetraethers (MBT, related to temperature and pH5,8)) to reconstruct the annual MAT of the Valles Caldera watershed, and compared these with proxies of hydrologic balance to evaluate the relationship between warmth and aridity.
Interglacial MATs in the VC-3 record range from ,0 to 7 uC, with the highest temperatures occurring in MIS 13 and early in MIS 11 (Fig. 2). The highest temperatures (5–7 uC) are similar to modern MATs, of ,5 uC. The glacial stages have multiple millennial-scale temperature oscillations with amplitudes as large as 7 uC; approximately seven oscillations are preserved in MIS 12 (B1–B7), three in late MIS 14 (C1–C3) and one in early MIS 10 (A1). The frequency of these oscillations (2–10 kyr) is similar to those recorded in contemporaneous Atlantic Ocean sediment records9. All VC-3 stadials correlate with high percentages of Picea 1 Abies pollen, whereas interstadials have lower Picea 1 Abies pollen percentages and many correlate with local maxima in Juniperus and Quercus (Fig. 2). Increased percentages of Cyperaceae (sedge) pollen during several interstadials suggest a shallower lake rimmed by a broad marshy zone, which would have been minimized during stadials, when the lake was deeper. Interstadial shallowing probably resulted from increased evaporation and/or a reduction in the winter precipitation that dominates regional glacial- stage precipitation10.
Glacial terminations VI and V in the VC-3 record show temperature increases of ,7 and ,8 uC, respectively. The d13C record of TOC (Fig. 2) shows negative isotopic shifts of 2.5–3.5% at the terminations that we interpret as biotic responses to global increases in atmospheric CO2, similar to the Termination I d13C response in Lake Baikal11.
We subdivide MIS 11 into five distinct substages, three warm and two cool, on the basis of MAT estimates, warm (lower-elevation) versus boreal (higher-elevation) pollen taxa, and variation in aquatic productivity proxies (Fig. 2). The warm substages (MISs 11a, c and e) are separated by intervals in which the temperature is ,2 uC lower (MISs 11b and d). Although these small temperature variations are within the error limits of the MBT/CBT calibration, their timing is supported by decreases in warm pollen taxa and increases in boreal pollen taxa (with the exception of MIS 11a). The warmest substage, MIS 11e, occurs early in the interglacial, and has peak MATs of 6–7 uC and the highest percentages of Juniperus pollen. After MIS 11e, the warm substages become progressively cooler.
The preservation of five MIS 11 substages in VC-3 is unusual. Most published records recognize only three substages, although a weak MIS 11e was noted in the Lake Baikal biogenic silica record12 and there are three distinct (warm) peaks in MIS 11 pollen influx from Greenland preserved in ODP Site 646 sediments13. The VC-3 MIS 11a substage is cooler than the extended warm phases of MIS 11, similar to other mid- Pleistocene climate records12, and is defined mainly by elevated lacustrine productivity (Si/Ti and TOC), more-positive d13C values, slightly higher temperature estimates and a combination of Quercus, Picea and Abies pollen that may not have a good modern climatic analogue. Within the limits of the VC-3 age model and the calibration uncertainty in the MBT/CBT proxies, the warm substages seem to correspond to the three precessional peaks of MIS 11, suggesting that the temperature response of this region to low-amplitude precessional cycles is ,2 uC. On the basis of the MIS 11 orbital forcing similarity with the Holocene, we suggest that in the absence of anthropogenic forcing future southwestern US climate should see a cooling of ,2 uC relative to the early Holocene.
Large parts of MIS 13 seem to have been warmer than most of MIS 11, as shown by MATs of up to 7 uC, higher Juniperus pollen percentages and the absence of Picea 1 Abies pollen (Fig. 2). Only MIS 11e had temperatures approaching the peak warmth of MIS 13. Other Northern Hemisphere records suggest that MIS 13 was warmer than
MIS 1114,15, and a smaller Greenland ice sheet13 and a lack of ice rafting in the North Atlantic9 also indicate Northern Hemisphere warmth during MIS 13, although not necessarily more than during MIS 11. In contrast, Southern Hemisphere records uniformly show a cooler MIS 1314,16.
The higher MIS 13 temperatures in the southwestern US occur despite lower interglacial values of atmospheric CO2 and CH4 (ref. 17). However, the amplitude of precessional cycles and, hence, extremes in
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Figure 2 | Multi-proxy profiles of VC-3 plotted versus calendar age. Age model age–depth tie points are shown as arrows at the top and possible sedimentary hiatuses are indicated by positions of mud cracks (below). Shading indicates interglacial periods including odd-numbered MISs 11 and 13 and substages within MIS 11 (a, c and e). a, June insolation at latitude 30uN (ref. 6). b, Quercus (warm) pollen percentages. c, Juniperus (warm) pollen percentages. d, MAT estimates from MBT/CBT, with data size marker equivalent to 2 uC (blue). Red line shows the modern MAT, of 4.8 uC, in the Valle Grande. MBT/ CBT temperature estimates have an absolute uncertainty of 5 uC based on uncertainties in the global calibration (see further discussion in Supplementary Information). Millennial-scale events within the three glacial periods, defined by local maxima in MAT and Cyperaceae and local minima in Picea and Abies, are indicated (A for MIS 10, B for MIS 12 and C for MIS 14). e, Picea 1 Abies (boreal) pollen percentages. f, Cyperaceae pollen percentages; mud cracks indicated with orange diamonds. g, Si/Ti ratios from core scanning X-ray fluorescence (XRF). Large peaks in MIS 14 correspond to pumiceous gravels. h, Total organic carbon (TOC). i, d13CTOC 5 ((13C/12C)sample/ (13C/12C)standard 2 1) 3 1,000%, relative to the Vienna PeeDee Belemnite (VPBD) standard. j, Watershed soil pH estimate from CBT. k, Calcium concentration in sediments from core scanning XRF.
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Northern Hemisphere insolation were larger in MIS 13 than MIS 116
and may have led to higher continental temperatures during parts of MIS 13. Temperature variability during MIS 13 was as much as 6 uC, which is significantly larger than that during MIS 11 (Fig. 2). In combination with the apparent precessional timing of MIS 11 warm substages, this suggests that the southwestern US responded more strongly to insolation variations than to interglacial trends in greenhouse gases or global ice volume.
Mud cracks present in the warmest phases, MIS 11e and MIS 13, are unambiguous indicators of drier conditions. One 70-cm mud crack occurs within MIS 11e, and ,3 m of section within the upper portion of MIS 13 sediments contains multiple, centimetre-scale mud cracks, making this portion of the VC-3 age model less certain (Fig. 2). Also, the presence or absence of calcite in VC-3 sediments provides a con- tinuous indicator of closed-basin or, respectively, open-basin conditions in the lake. No calcite precipitated during freshwater open-basin conditions, whereas during drier (closed-basin) conditions, evaporative concentration led to calcite precipitation and preservation. XRF core scanning results show two intervals with high calcium concentrations during MISs 11e and MIS 13 (Fig. 2) that correlate with elevated (1–2%) total inorganic carbon (not shown), whereas core sections with low calcium concentrations have essentially no total inorganic carbon. Significant increases in calcium mark the onsets of closed-basin conditions coincident with rapid temperature increases a few thousand years after Terminations V and VI (Fig. 2). Mud cracks develop later within these closed-basin periods.
Long-term changes in watershed hydrology are also reflected in the CBT-derived soil pH record. Changes in soil pH are assumed to reflect changes in total precipitation18; greater soil leaching and acidification occurs with more precipitation, whereas drier conditions result in weaker soil acidification. The most alkaline soils occur within the interglacial mud-cracked facies (Fig. 2) and are more basic for longer periods in MIS 13. In contrast, soil pH shows a progressive acidification through MIS 12, consistent with progressively wetter conditions through that glacial stage, and possibly also caused by increases in boreal tree vegetation.
During MIS 11e, Si/Ti (a proxy for diatom productivity) is initially very high and then declines to average interglacial values, whereas TOC increases to ,5% in MIS 11e and is also high during early MIS 11d and MISs 11c and 11a. After the glacial termination, d13CTOC
rapidly increases to ,220%, indicating an expansion of C4 plants in the watershed and higher lacustrine productivity levels19. Increases d13CTOC also occurs in MISs 11c and 11a, and early in MIS 11d. Continued high Si/Ti, TOC and more-positive d13CTOC values during early stages of closed-basin conditions in MIS 11e and MIS 13 indicate periods of robust summer precipitation and productivity related to insolation forcing of monsoon strength, even as reduced winter precipitation led to less precipitation overall and closed-basin conditions.
Mud-cracked facies in MIS 11e and MIS 13 are characterized by negative shifts in d13CTOC of 6–7% and dramatic decreases in percentage TOC (Fig. 2). Si/Ti ratios, however, remain elevated relative to glacial values, suggesting that low percentage TOC values are due to organic degradation in shallow, oxidized sediments rather than lower aquatic productivity. The large negative shifts in d13CTOC are best explained by a collapse of the interglacial C4 plant community. Variations in C3 and C4 plant communities are a complex function of temperature, atmospheric CO2, and growing-season precipita- tion20,21. These dry intervals include some of the highest MATs in the VC-3 record that should favour C4 plants, and the relatively high interglacial levels of atmospheric CO2 during MISs 11 and 13 vary by less than 20–30 p.p.m.v. Thus, the best explanation for the decline of C4 plants in the watershed is a significant decrease in summer precipitation. In contrast to the early interglacial closed-basin phases where significant C4 plant growth provided evidence for robust summer precipitation, we interpret the extended arid periods later in MIS 11e and MIS 13 to be the result of greatly reduced summer precipitation.
Following the aridity of MIS 11e, the lake expanded during MIS 11d as shown by well-laminated sediments and open-basin conditions (low calcium values). Despite this interval being ,2 uC cooler, sufficient summer rainfall early in MIS 11d allowed renewed C4 plant growth.
Northern New Mexico at present receives ,40–50% of its annual precipitation total during the summer monsoon22. During the warmest phases of the interglacials, we would expect greater summer precipitation, as the monsoon is primarily driven by land surface heating22. Indeed, linkages among interglacial warmth, robust summer precipitation and precessional variations are indicated by the presence of C4
plants in early MIS 13, early MIS 11e and MISs 11c and 11a, when MATs were similar to or slightly less than modern values, but the warmest intervals did not have robust summer precipitation. As possible analogues for interglacial aridity, both historical droughts and pre- historical megadroughts were characterized by reductions in winter precipitation as a consequence of more-frequent La Nina events22,3,23, with summer precipitation reduced also.
In contrast, the extended arid episodes (centuries to millennia) of MIS 11e and MIS 13 lasted much longer than pre-historical megadroughts. An analogous relationship between peak interglacial warmth and extended aridity was also noted in a mid-Holocene bog record from the margin of the Valles Caldera24. Here, ,2 kyr of desiccation occurred contemporaneously with the highest temperatures of the Holocene in the southwestern US25 and with the northernmost extent of the inter- tropical convergence zone in the Gulf of Mexico26. The timing of this dry episode in the Holocene interglacial following the deglaciation is very similar to that of the arid episode in MIS 11e; subsequent late- Holocene conditions became wetter in the southwestern US, with increased winter precipitation27 similar in timing to wetter conditions during MIS 11d in the VC-3 record.
The strong correspondence between the warmest temperatures and extended aridity during at least three interglacials (MIS 13, MIS 11e and the early Holocene) in the southwestern US suggests a stable climate state fundamentally different from conventional drought conditions. These periods of aridity are related to lower winter precipitation (as mid-latitude westerlies shifted polewards during warmer periods), but reductions in summer precipitation seem to be critical to their development. Unlike the temporary summer blocking high over the southwestern US thought to partly explain the 1950s drought28, these longer periods of aridity indicate a more permanent change in atmospheric circulation. Climate model analysis shows that the dust-bowl-like conditions predicted for the southwestern US over the next century in response to anthropogenic warming arise from a poleward shift of the mid-latitude westerlies and the poleward branch of the Hadley cell4. This response to warming is not transient and would result in a more arid southwestern US as long as the underlying conditions (warming) remained in place. Our palaeoclimate record shows that extended interglacial aridity is strongly linked to higher- than-modern temperatures and reduced summer rainfall, and we suggest that a similar expansion of the subtropical dry zone has occurred several times in the past in response to natural warming, even though MIS 11 and MIS 13 had different orbital and atmospheric CO2 forcings. Our results strongly indicate that interglacial climates in the southwestern US can experience prolonged periods of aridity, lasting centuries to millennia, with profound effects on water avail- ability and ecosystem composition. The risk of prolonged aridity is likely to be heightened by anthropogenic forcing1,4.
METHODS SUMMARY Measurement of fossil branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids from soil bacteria were conducted at the Royal Netherlands Institute for Sea Research (NIOZ) and Brown University following procedures outlined in Supplementary Information. At NIOZ we analysed GDGTs on an Agilent 1100 series LC-MSD SL, and at Brown University we analysed GDGTs on an HP 1200 series LC- MS. Both labs used an Alltech Prevail Cyano column (2.13 150 mm, 3mm) with the same solvent elution scheme and instrument operating conditions. GDGTs were detected using atmospheric-pressure chemical ionization mass spectrometry. All
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liquid chromatography/mass spectrometry runs were integrated at NIOZ by the same technician to ensure consistency. To evaluate the compatibility between the Brown and NIOZ measurements, representative samples were analysed on both machines and the resulting MBT/CBT indices were found to be identical within analytical uncertainty.
Processing for pollen included suspension in KOH, dilute HCL, hydrofluoric acid and acetolysis solution. The pollen sum included all terrestrial pollen types; Cyperaceae percentages were calculated outside the sum. We identified pollen grains to the lowest taxonomic level using the modern pollen reference collection at Northern Arizona University. Analysis for organic carbon elemental concentrations and d13CTOC included samples being dried, ground and pretreated twice with 6 N HCL at 60 uC to remove the carbonate fraction. TOC and d13CTOC were analysed using a Costech Elemental Analyser coupled to a Thermo-Finnigan Delta Plus isotope ratio mass spectrometer. The bulk elemental composition of core VC- 3 sediments was determined using an ITRAX X-ray Fluorescence Scanner (Cox Analytical Instruments). XRF scanning was conducted at 1-cm resolution with 60- s scans using a molybdenum X-ray source set to 30 kV and 15 mA.
Received 11 June 2010; accepted 12 January 2011.
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Supplementary Information is linked to the online version of the paper at www.nature.com/nature.
Acknowledgements We thank A. Mets for analytical support, W. McIntosh for the Ar–Ar age determination, T. Wawrzyniec and A. Ellwein for drilling help, and the Valles Caldera Trust for permission to drill in the Valle Grande. Core assistance was provided by LRC/ LacCore. This work was supported by the NSF Paleoclimate and P2C2 programs, IGPP LANL and the USGS Western Mountain Initiative. Support from the Gledden Fellowship is acknowledged. This work forms contribution 2399-JW at the Centre for Water Research, TheUniversityofWesternAustralia andcontribution131at theLaboratory of Paleoecology, Northern Arizona University.
Author Contributions Writing and interpretation was done by P.J.F. with significant contributions from J.P.W., R.S.A., J.M.H. and E.T.B. MBT/CBT analyses were conducted by J.P.W., M.A.B., J.S.S.D., S.S., Y.H. and J.T. Organic carbon/nitrogen analyses were conducted by P.J.F., J.M.H., L.M.C.-D., J.F. and V.A. XRF core scanning analyses were conducted by E.T.B. Pollen analyses and palaeovegetation analyses were conducted by R.S.A., S.J.S. and C.D.A., and F.G., G.W. and P.J.F. conducted core sediment and stratigraphic analyses. L.D.-H. and J.W.G. investigated palaeomagnetic and rock magnetic core properties. All authors discussed the results and commented on the manuscript.
Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Readers are welcome to comment on the online version of this article at www.nature.com/nature. Correspondence and requests for materials should be addressed to P.J.F. ([email protected]).
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Supplementary Figures
Supplementary Figure 1 Photograph of mudcracked core section in MIS 13 from
46.4 m to 47.05 m depth. Scale bar in cm with dm marked by alternating yellow and
black bars. Note multiple mudcrack horizons, including prominent horizons ~5 cm from
the top (left) and ~56 cm from the top.
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Supplementary Figure 2: Additional proxy profiles from core VC-3. Rosaceae and
Artemisia pollen counts from core VC-3 plotted vs. age (ka B.P.), bulk organic matter
δ15N (‰),C:N ratios of bulk organic matter, and bulk magnetic susceptibility (SI)
measured by a Geotek multisensor core logging system. MIS shading as in Figure 2.
Supplementary Methods
Measurement of fossil Glycerol Dialkyl Glycerol Tetraether (GDGT) membrane
lipids from soil bacteria was conducted at 20 cm to 1 m intervals in VC-3 sediments, with
most spaced at 40 cm (~1200 yrs). All VC-3 samples were carefully chosen to avoid
SUPPLEMENTARY INFORMATIONRESEARCHdoi:10.1038/nature09839
turbidites (based on visual and X-radiograph core inspection) and these are most common
in the lowest section of the core where our sampling density was lowest. GDGT analyses
were conducted at both NIOZ and Brown University following supplementary references
1 and 2. Freeze-dried, homogenized sediment was extracted with dichloromethane
(DCM)/methanol (2:1) using Soxhlet or 9:1 DCM/methanol using Dionex™accelerated
solvent extraction (ASE) techniques to obtain the total lipid extract (TLE). An aliquot of
the TLE was further separated into neutral, free fatty acid, and phospholipid fatty acid
fractions using Alltech Ultra-Clean SPE Aminopropylsilyl bond elute columns. Eight mL
each of 1:1 methylene chloride:2-propanol, 4% glacial acetic acid in ethyl ether, and
methanol were used to elute the neutral lipid, fatty acid, and phospholipid fatty acid
fractions, respectively. The neutral lipid fraction was further separated into apolar and
polar fractions using an activated Al2O3 column and eluting with hexane/DCM 9:1 and
DCM/methanol 1:1 (v/v) at NIOZ. At Brown University, the neutral lipid fraction was
separated into alkane, ketone and polar fractions using a silica gel column with four mL
each of hexane, DCM, and methanol (all GDGTs are found in the polar fractions eluted
with methanol). The polar fraction (DCM/methanol) was dried, dissolved in 99:1
hexane/2-propanol and filtered over a 0.45 µm PTFE filter prior to analysis.
At NIOZ, GDGTs were analyzed on an Agilent 1100 series LC-MSD SL, using
an Alltech Pevail Cyano column (2.1 x 150 mm, 3µm). The compounds were eluted
isocratically with 90% A and 10% B for 5min (flow rate 0.2ml/min), and then with a
linear gradient to 16% B for 34min, where A = hexane and B = hexane:isopropanol (9:1,
v/v). At Brown University GDGTs were analyzed with the same column, solvent elution
scheme and intrument operating conditions as at NIOZ. For both instruments, GDGTs
were detected using atmospheric pressure chemical ionization mass spectrometry (ACPI-
MS) according to supplementary references 1 and 2. Injection volume was 10 μl and
analyses were performed in selective ion monitoring mode to increase sensitivity and
reproducibility. Relative quantification of the compounds was accomplished by
integrating the [M+H]+ (protonated molecular ion) peaks in the mass chromatograms. All
LC/MS runs were integrated at NIOZ by the same technician using Agilent ChemStation
software. To evaluate the compatibility between the Brown and NIOZ measurements,
representative samples were analyzed on both machines and the resulting MBT/CBT
indices were found to be identical within analytical uncertainty. The analytical
reproducibility of results in this study based on replicate sample processing is 0.5°C for
the MAT estimate and 0.1 for pH.
The Branched and Isoprenoid Tetraether (BIT) index is based on the relative
abundance of terrestrially derived branched GDGT lipids vs. crenarchaeol (an isoprenoid
GDGT lipid derived from aquatic Crenarchaeota) and represents a measure for the
relative fluvial input of soil organic matter in sediments3-5. In VC-3 sediments, the BIT
index for the relative input of soil organic matter ranges from 0.83 to 1.0, with an average
value of 0.96 indicating a large input of soil organic matter1,3,5 as would be expected in
this lacustrine setting. The residence time of these branched tetraether lipids in soils
surrounding the paleolake is likely to be short relative to our sampling resolution as we
see no significant lag between MBT/CBT temperature estimates (lipids delivered
fluvially to the lake) and climatically sensitive pollen types primarily delivered to the
lake via wind (e.g. Quercus, Juniperus or Picea and Abies). There is no systematic
difference in BIT index between glacial and interglacial periods, although the lowest
values are all located near the bottom of the core in MIS 14. Branched GDGT membrane
lipids are thought to be derived predominantly from soil bacteria1,4,5, though recent
studies suggest that these compounds may also be produced in lacustrine and/or marine
sediments6-8. The relative distribution of branched GDGT lipids, expressed as the
Methylation index of Branched Tetraethers (MBT) and the Cyclisation ratio of Branched
Tetraethers (CBT) is a function of annual mean air temperature (MAT) and soil pH1. The
calibration equations for MBT and CBT are taken from supplementary reference 1, in
which a significant linear correlation with modern MAT ranging from -6 to 27oC was
shown:
MBT = 0.122 + 0.187 × CBT + 0.020 × MAT [2]
The original three dimensional global soil calibration for MBT and CBT had an r2
of 0.77, corresponding to a total calibration error of ~5°C and 1 pH unit1. However,
recent studies on Mt Kilimanjaro9 as well as in East African lakes10 have demonstrated
that local calibrations, particularly for lacustrine sediment reconstructions, are typically
much better than the global calibration. In the global calibration, both vegetation type and
soil type differences appear to contribute to the data scatter, and therefore to the
calibration error. The East African Lake study10 showed that with consistent soil and
vegetation types, the local calibration had an RMS error of ~2oC in the MBT/CBT
temperature estimate. The Mt Kilimanjaro study9 also showed a smaller error than the
global calibration, and noted that a dramatic vegetation change at the highest elevations
in the study altered the calibration relative to the lower elevation sites.
We therefore expect that the actual error in our temperature estimates is better
than 5°C, but at present there is no local calibration for lakes in the southwest of North
America. Based on the results of the East African Lake study10, we estimate that our
calibration error will be closer to ~2oC for the Valle Grande sediments once a local
calibration is developed. The documented vegetation changes in the VC-3 record (where
we estimate ~600 m of vegetation zone changes between glacial and interglacials) could
potentially alter this (cf. supplemental reference 9) and raises the possibility that the
absolute error in calibration is larger between glacial and interglacial periods than within
either a glacial or an interglacial. Furthermore, these and other studies 7,8 have also
identified a “cold bias” in the MBT-reconstructed temperatures from lake sediments
compared to soils. Thus, our temperatures should be taken as minimum values. For
these reasons, we believe that our MBT-reconstructed temperature trends are quite
robust, but the absolute values of the temperatures reconstructed are likely to be under-
estimates.
In the VC-3 record, two parts of the section are partially oxidized (described in
VC-3 core chronology and sediment characteristics section) which raises the possibility
of complicating the MBT/CBT proxy. However, recent studies show that the branched
GDGTs used in the MBT/CBT proxy are not significantly affected by oxidation of
organic material in sediments11,12. First, isoprenoid GDGT concentrations used in the
TEX86 paleothermometer are not sensitive to oxidizing conditions11, and more
importantly, a study of branched GDGTs used in the MBT/CBT proxy in oxidized marine
sediments show that they are much more resistant to oxidation than the isoprenoid
GDGTs12. Thus, the oxidation in portions of the Valles Caldera sediments (resulting in
TOC values of ~0.5%) is unlikely to affect the distribution of branched GDGTs within
those sediments, and therefore the MBT/CBT based reconstructions of temperature and
soil pH.
VC-3 sediments were sampled continuously for pollen and spore content at
intervals ranging from 10 cm to 100 cm, with most spaced at 40 cm (~1200 yrs).
Processing for pollen followed supplementary reference 13 including suspension in
KOH, dilute HCL, HF and acetolesis solution. The pollen sum included all terrestrial
pollen types; Cyperaceae percentages were calculated outside the sum. Pollen grains were
identified to the lowest taxonomic level using the modern pollen reference collection at
Northern Arizona University.
The bulk elemental composition of core VC-3 sediments was determined using an
ITRAX X-ray Fluorescence Scanner (Cox Analytical Instruments) at the Large Lakes
Observatory, University of Minnesota Duluth. XRF core scanning was conducted at 1-cm
resolution (approximately 30 yrs) with 60 second scans using a Mo x-ray source set to 30
kV and 15 mA. For this work, we focus on Si:Ti, an indicator of the abundance of
biogenic opal in lacustrine sediments14. In core VC-3, diatom abundances estimated from
smear slides taken every 50 cm down the length of the core show good agreement with
the Si:Ti measurements. The high Si:Ti values in the lowest portions of the core
correspond directly with pumiceous gravels which have little to no diatom content. In
general, Si:Ti and TOC trends follow each other with two significant exceptions during
the interglacials – the later stage of MIS 13 and late MIS 11e where desiccation and
oxidation has removed significant amounts of organic carbon. The ITRAX XRF scanner
also provided 0.2 mm resolution x-radiographs of the core.
Sediments were sampled continuously at a 20 cm resolution (~600 yrs) and
analyzed for organic carbon and nitrogen elemental concentration and δ15N and δ13C
values. Samples were dried, ground and pretreated twice with 60oC 6N HCL in order to
remove the carbonate fraction. The hot acid treatment was applied because some of the
samples contain small rhombs of diagenetic siderite, which may not be entirely removed
by standard HCl treatment15. Total Organic Carbon (TOC), Total Organic Nitrogen
(TON) and bulk δ13C and δ15N were analyzed using a Costech Elemental Analyzer
coupled to a Thermo-Finnigan Delta Plus Isotope Ratio Mass Spectrometer. Laboratory
standards were run at the beginning, at intervals between samples and at the end of
analytical sessions. The analytical precision calculated from duplicate measurements of
the standards is ± 0.1‰ for δ13C and ± 0.15‰ for δ15N (1σ standard deviation).
Physical and Climatic Setting of the Valle Grande
The Valles Caldera complex formed after a large shallow magma chamber erupted, at
1.23 Ma16. Over the next million years, a series of rhyolite domes erupted from ring
fracture vents following the caldera collapse and initial resurgence. Several lakes formed
in the moat valleys after 0.8 Ma when the post-caldera eruptive domes temporarily
blocked drainages out of the caldera17. A particularly long-lived lake formed in the Valle
Grande (35o 52’ N, 106o 28’ W, 2553 m asl) following the eruption of the South
Mountain rhyolite dome17,18 which dammed the drainage to the southwest from the basin
(Figure 1). The Valle Grande catchment has a total area of ~100 km2 (Figure 1) and the
dominant bedrock within the catchment is almost exclusively rhyolite16. The largest lake
present had a maximum surface area of ~20 km2.
Vegetation communities in the Jemez Mountains are strongly affected by aspect19.
At the highest elevations (ca. 2900 to 3500 m), north-facing slopes contain forests of
Picea engelmannii and Abies lasiocarpa var. arizonica, while local areas of high
elevation grasslands are found on south-facing slopes20. Mixed conifer forests occur
from ca. 2300 to 2900 m, with Pinus ponderosa, Pseudotsuga menziesii, A. concolor,
Populus tremuloides and Pinus flexilis. P. ponderosa with shrub oak (Quercus gambelii)
dominates elevations 2100 to ca. 2600 m, while piñon (Pinus edulis) – juniper (Juniperus
monosperma or J. scopulorum) woodlands occur between ca. 1900 and 2100 m elevation.
Juniper grasslands (J. monosperma, big sagebrush [Artemisia tridentata], Bouteloua sp.)
occur from ca. 1600 to 1900 m, while shrub steppe with members of the goosefoot and
aster families (i.e., A. tridentata, Sarcobatus) and joint-fir (Ephedra) occur at elevations
below this.
Modern pollen studies within southern Colorado and northern New Mexico by
Susan Smith, Scott Anderson and others help to elucidate the fossil pollen record. The
percentages of piñon (Pinus edulis), juniper (Juniperus) and oak (Quercus) from MIS 11
and 13 are very similar to those from the modern piñon – juniper (PJ) woodland of today.
For instance, in modern PJ, piñon, juniper and oak constitutes about 20% or more, 10%
and 10%, respectively, of the pollen sum21. The difference between the modern and
fossil assemblages is that juniper is much higher. Clearly, if these species were not
growing directly around the lake, they probably grew very close to the site, perhaps
below the rim of the Caldera.
Similarly, the percentages of dominant species during MIS 10, 12 and 14 are very
similar to those from modern spruce (Picea) – fir (Abies) forest in northern New Mexico
(Jicarita Bog, 3207 m elevation22; Stewart Bog, 3100 m23) and southern Colorado (Little
Molas Lake, 3370 m24; Hunters Lake, 3516 m25). This includes percentages of Picea to
20%; Abies to 5%; Pinus 20-40%, and Poaceae 5-10%. On these bases, we ascribe
changes in apparent elevation of the vegetation within the VC-3 core (2553 m elevation)
of at least 600 m between interglacial and glacials.
Modern climate data for the Valle Grande was derived from a USCRN co-op
weather station (NM Los Alamos 13 W) located at the Valles Caldera National Preserve
headquarters, on the flank of the VC-3 paleolake valley. The Mean Annual Temperature
of this site is 4.8oC with summer (JJA) temperatures averaging 14.0oC and winter (DJF)
temperatures averaging -4.5oC. Most of the annual average precipitation (~670 mm total)
occurs during the summer and winter with drier spring and fall seasons. Summer (JAS)
precipitation totals average 260 mm (~40% of the annual total) and is mainly convective
(monsoon), while winter (DJF) frontal precipitation totals average 180 mm for the three
months (~26% of the annual total).
Core VC-3 Chronology and Sediment Characteristics
Previous work on sediments from the Valle Grande included a low-resolution pollen
study on cuttings from a deep USGS water well, where two major cycles of wet to dry
climates (consistent with two glacial cycles) were interpreted from ratios of Picea
(spruce), Abies (fir), Pinus (pine) and Quercus (oak) pollen26. As numerical age control
was not available at the time of that study, the uppermost lacustrine sediments were
assumed to be Holocene to late Pleistocene in age. Subsequent Ar-Ar dating of the South
Mountain rhyolite27 dam forming the paleolake provided a middle Pleistocene age and an
updated mid-Pleistocene age for VC-3 lacustrine sediment was shown by an Ar-Ar date
of 552 ± 3 kyr from a basal tephra18.
The VC-3 core hole was drilled within the floor of the caldera away from the
steep slopes of the topographic margin, the resurgent dome, and the post-caldera eruptive
center to the east, west, and to the north of the well, respectively. Volcaniclastic
sediments eroded from these topographic highs were directly carried into the ancestral
lake. At least five compositionally discrete tephra stratigraphic markers within the VC-3
sequence represent sporadic pulses of volcaniclastic input directly into the lake without
transient sedimentation traps. The earliest marker tuffs within the core originated to the
south of VC-3 and are probably related to fallout eruptions, whereas subsequent units
were of fluvial origin supplied from the north.
Sediments recovered in core VC-3 consist of lacustrine silts and clays with
occasional gravels in the lower strata, and pumiceous gravels and indurated muds at the
base that probably represent a fluvial/meadow environment prior to the eruption of the
South Mountain rhyolite and damming of the Valle Grande drainage18. Above the basal
gravels and turbidites, two major lithologic cycles of about the same thickness are
defined by transitions from silty diatomaceous clays with well-defined, thin (mm)
laminations to silty diatomaceous clays with thicker and less well defined laminations (to
massive units lacking lamination) with occasional mm-scale silts and sands. Within this
latter facies, cm scale mudcracks, bioturbation and color mottled, gleyed horizons are
also locally present from 47.5 to 44 m depth (e.g. Supplementary Figure 1) and again at
~23 m depth. The basal transitions from thinly laminated to thickly laminated massive
clays are relatively abrupt (cm-scale) while the transitions back to thinly laminated clays
are more gradational (m-scale). We interpret the well-laminated sediments as forming
under deep lake conditions during the glacial periods and the less well-laminated to
mudcracked sediments forming in shallower to desiccated lake conditions during drier
interglacials. The mid-Pleistocene glacial deep and interglacial shallow pattern is
consistent with late Pleistocene, LGM-Holocene lake level changes in the Southwest28.
Two basal tephra Ar-Ar dates of 552 ± 3 ka from a glassy tephra and a well-sorted
pumiceous gravel at 75.8 and 76.0 m depth respectively constrain the onset of lacustrine
sedimentation in core VC-3 to MIS 14 in the middle Pleistocene18 (Supplementary Table
1). Trace element analyses of the tephra and pumiceous gravel show a close match with
the South Mountain Rhyolite, as well as with a tephra mapped outside the caldera dated
at 551 ± 2 ka29,30, further confirming the basal date for VC-3. This is overlain by ~20 m
of sediment that accumulated rapidly, with deposition of numerous turbidites and gravels
as unstable volcanic landforms eroded. Higher in the core, within the finer grained
lacustrine sequence, there are abrupt and dramatic increases in TOC, Si/Ti ratios, MAT
estimates, δ13CTOC, C:N and warm taxa pollen types (Quercus, Juniperus, Rosaceae) at
51.5 m depth and again at 27.5 m (Figure 2 and Supplementary Figure 2). We correlate
these dramatic climate shifts with glacial terminations VI and V and their globally
constrained ages of 533 ka and 426 ka respectively31. Paleomagnetic work (progressive
alternating field demagnetization) has identified two short-lived magnetic field “features”
at 17.25 m and at 52.3 m depth in the core, where negative inclination magnetizations
have been either well-resolved or implied by demagnetization trajectories. We correlate
these negative inclination features with globally recognized events 11α (400 ka) and 14α
(536 ka)32 (Supplementary Table S1). The positions of these dated magnetic field events
are consistent with the age model based on glacial termination correlations.
At 43.1 m depth, dramatic decreases in Si:Ti, warm taxa pollen types, and MAT
estimates occur, indicating the end of MIS 13 warmth. A simple linear interpolation
assuming constant sedimentation rates between Glacial Terminations VI and V would
result in the end of MIS 13 occurring at ~505 ka. However, the end of MIS 13 / onset of
MIS 12 has a very consistent age of 480 ka in several long Pleistocene climate records
from different environments31,33-35. Additionally, MIS 13 is an extraordinarily long
interglacial that spans ~50 kyr (e.g.,; ~50 kyr Dome C Antarctica35; ~50 kyr in Lake
Baikal33,34; ~52 kyr deep sea ice volume record31; but ~65 kyr in the Devils Hole
record36). MIS 11 spans a similar amount of time (~50 ka)31,33-35, but in VC-3 it is
represented by 14 m of sediment whereas MIS 13 is represented by 9 m of sediment. This
5 m sediment preservation difference between these two interglacials is probably due to
sediment hiatuses as shown by the presence of multiple (12 to 15) mudcracks
(Supplementary Figure 1) in the upper half of MIS 13 sediments (from 47.5 m to 44.0 m
depth), although we cannot absolutely rule out lower sedimentation rates during MIS 13.
Abrupt changes in MAT estimates, specific pollen taxa counts and other proxies occur
across several of the more prominent mudcracks suggesting that these do represent
hiatuses. To account for this ~5 m of missing sediment, we assign an age of 480 ka to the
43.1 m depth corresponding to the end of MIS 13. Because we cannot assign precise ages
to the mudcracked sediments between 47.45 m and 43.1 m, we apply a simple linear
interpolation between these depths as the most parsimonious age model for this portion of
the core.
As the lower half of MIS 13 does not contain any obvious mudcracks/hiatuses, a
linear interpolation from the start to end of MIS 13 would result in anomalously high
sedimentation rates the upper interval with hiatuses and anomalously low sedimentation
rates in the lower interval without hiatuses. To determine an approximate age for the
47.45 m depth tie-point (just below the lowest mudcrack), we extrapolated the upper core
average sedimentation rate of 0.32 mm/yr from glacial termination VI (51.5 m depth) to
this point. This gives an age of 520 ka at 47.45 m depth.
The VC-3 lacustrine record terminates in the late middle Pleistocene, likely
following a breach of the South Mountain rhyolite dam as the available accommodation
space in the caldera moat filled. Because there are no available age-depth tie-points near
the top of the core, we apply the upper core average sedimentation rate to the sediments
between 17.25 m depth (400 Ma) and the lacustrine core top at 5.5 m depth. This
extrapolation results in an age estimate for the top of the core of ~363 ka.
Depth (m) Age (ka) Nature of age tie-point
17.25 400 Geomagnetic Field Event 11α32
27.50 426 Termination V31
43.1 480 End of MIS 13 (see text)
47.45 520 Onset of mudcracked interval in MIS 13 (see text)
51.50 533 Termination VI31
75.60 552 Ar-Ar age date18
Additional VC-3 Core Properties
Artemisia (sage) pollen tends to be higher during glacial periods, and increases
through the complete MIS 12 glacial as conditions build toward the glacial maximum
(Supplementary Figure 2). It is almost absent during MIS 13, consistent with the warm
temperatures occurring during this interglacial, and it is highly variable during MIS 11
with low values during the warm substages and very high values during the cool
substages (11b and d). Rosaceae pollen (probably mostly Cercocarpus, but also Cowania
or Amelanchier) is highest during the interglacials, particularly evident during the first
two warm substages of MIS 11 (c and e). Rosaceae pollen is low during MIS 11a,
consistent with this being the coolest of the three warm substages (Supplementary Figure
2).
C:N ratios in bulk organic matter parallel other indicators of lacustrine aquatic
productivity. Average glacial values range between 5 and 10 while interglacial values
range between 10 and 15 (Supplementary Figure 2). Decreases in C:N ratios in during the
mudcracked, low TOC intervals during MIS 11e and MIS 13 are consistent with intense
decomposition of organic matter37. The degradation of organic matter leads to enrichment
in 15N, explaining the high δ15N values during these intervals (Supplementary Figure 2),
but does not have a large effect on δ13C values38.
The rock magnetic signature of the upper dry MIS 13 interval is exceptional with
high magnetic susceptibility (MS) values (Supplementary Figure 2). The normal
interglacial MS values are much lower (MIS 11a,c,e; early MIS 13) with biogenic silica
and organic carbon dilution of the clastic sediments. This dilution effect is also evident in
the bulk sediment density (not shown). The high MS values, which coincide with the
mudcracked portion of MIS 13, are probably caused by diagenetic formation of
metastable iron oxides and minor amounts of greigite in a shallow lake with fluctuating
water tables39. The presence of multiple gleyed horizons over this interval indicates rapid
changes in sediment redox conditions, consistent with shallow fluctuation water tables.
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of continental climate during the Pleistocene: a complete astronomically tuned climatic record from Lake Baikal, SE Siberia. Quat. Sci. Rev. 25, p. 3431-3457 (2006).
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Duration and structure of the past four interglaciations. Quat. Res. 48, 141-154 (1997).
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measures of organic matter alteration during decomposition. Ecology 84, 2021- 2025 (2003).
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A 200 kyr lacustrine record from the Valles Caldera: Insight from environmental magnetism and paleomagnetism, New Mexico Geological Society Guidebook, 58th Field Conference, Geology of the Jemez Mountains Region II, 424-432 (2007).
different geometry from that described by Wat- son and Crick (Fig. 1a, b). Similarly, an alterna- tive base-pairing geometry can occur for G•C pairs. Hoogsteen pointed out that if the alterna- tive hydrogen-bonding patterns were present in DNA, then the double helix would have to assume a quite different shape. Hoogsteen base pairs are, however, rarely observed.
Nikolova and colleagues’ key finding1 is that, in some DNA sequences, especially CA and TA dinucleotides, Hoogsteen base pairs exist as transient entities that are present in thermal equilibrium with standard Watson– Crick base pairs. The detection of the transient species required the use of NMR techniques that have only recently been applied to macromolecules6.
Why is this finding important? Hoog- steen base pairs have, after all, previously been observed in protein–DNA complexes7–9 (Fig. 1c). But it has not been possible to determine whether Hoogsteen base pairs are present in free DNA. Nikolova and colleagues’ study1 reveals that the ability to flip between Watson–Crick and Hoogsteen base pairing in free DNA is an intrinsic property of indi- vidual sequences. This implies that some proteins have evolved to recognize only one base-pair type, and use intermolecular interactions to shift the equilibrium between the two geometries9.
DNA has many features that allow its sequence-specific recognition by proteins. This recognition was originally thought to primarily involve specific hydrogen-bonding interactions between amino-acid side chains and bases. But it soon became clear that there was no identifiable one-to-one correspondence — that is, there was no simple code to be read. Part of the problem is that DNA can undergo conformational changes that distort the classical double helix. The resulting variations in the way that DNA bases are presented to proteins can thus affect the recognition mechanism.
More significantly, it has become evident that distortions in the double helix are them- selves dependent on base sequence. This enables proteins to recognize DNA shape in a manner reminiscent of the way that they recognize other proteins and small ligand molecules. For example, stretches of A and T bases can narrow the minor groove of DNA (the narrower of the two grooves in the double helix), thus enhancing local negative electrostatic potentials and creating binding sites for appropriately placed, positively charged arginine amino-acid residues3.
Nikolova and colleagues’ discovery1 that DNA base pairs can so easily leave their favoured Watson–Crick conformation comes as a surprise. But viewed in another way, the phenomenon is not so different from protein side chains undergoing a conformational change so as to optimize binding with another protein. The real surprise where DNA is
C L I M AT E C H A N G E
Old droughts in New Mexico A long climate record reveals abrupt hydrological variations during past interglacials in southwestern North America. These data set a natural benchmark for detecting human effects on regional climates. See Letter p.518
J O H N W I L L I A M S
Southwestern North America is a pretty dry place, and is likely to get drier this century because of anthropogenic
climate warming. On page 518 of this issue1, Fawcett et al. provide a climate record from deep in the past that will help in assessing the future hydrological regime for the area.
Climate models consistently project declines in winter precipitation for the southwest, in response to rising greenhouse gases as the subtropical dry zones expand polewards2,3. This precipitation decline, combined with expected increases in evaporation rates and reduced snowpack, would severely strain the region’s capacity to adapt to climate change. Moreover, the southwest is historically prone to droughts, with six multi- year droughts in the nineteenth and twenti- eth centuries, including the infamous 1930s
Dust Bowl4. These droughts are linked to yearly-to-decadal variations in sea surface temperatures in the tropical Pacific, enhanced by local soil-moisture feedbacks5. Further understanding of the mechanisms of hydrological variability, together with efforts to limit societal vulnerability to climate change, are priorities in global-change research4.
Palaeoclimatic studies have made an essen- tial, if not particularly reassuring, contribution to this effort, by providing insight into the natural behaviour of hydrological sys- tems and a longer-term context for historical and projected changes. Tree-ring records offer sobering evidence of widespread and decades-long ‘megadroughts’ in the western United States over the past several millennia that dwarfed recorded historical droughts6. Records spanning the past 11,000 years (the Holocene interglacial) demonstrate long-term shifts in southwestern monsoon
concerned is that the constraints of the double helix don’t preclude this possibility. The presence of Hoogsteen base pairs in detectable amounts — even in free DNA — therefore provides a notable example of the remarkable plasticity of the canonical double helix. It also implies that, if DNA bases are regarded as letters, each letter potentially has two mean- ings that determine both hydrogen-bonding patterns and structural variations in the double helix.
Structural biologists have long recognized that there is no second code in which certain amino acids recognize complementary DNA bases in protein–DNA interactions. Never- theless, protein–DNA binding is still com- monly thought of purely in terms of codes and sequence motifs, rather than as the binding of two large macromolecules that have complex shapes and considerable conformational flexibility10. Nikolova and colleagues’ discovery reminds us that DNA offers proteins not only an enticing linear alphabet, but also a set of conformations that can be recognized in a sequence-dependent way. Understanding how the linear sequence of bases in DNA is recognized by proteins is therefore a problem that must be solved in three dimensions. This will require structural, biochemical, genomic and
computational studies on both naked double helices and protein–DNA complexes.
Barry Honig is at the Howard Hughes Medical Institute, Center for Computational Biology and Bioinformatics, Department of Biochemistry and Molecular Biophysics, Columbia University, New York, New York 10032, USA. Remo Rohs is in the Molecular and Computational Biology Program, Department of Biological Sciences, University of Southern California, Los Angeles, California 90089, USA. e-mails: [email protected]; [email protected]
1. Nikolova, E. N. et al. Nature 470, 498–502 (2011). 2. Parker, S. C. J., Hansen, L., Abaan, H. O., Tullius, T. D.
& Margulies, E. H. Science 324, 389–392 (2009). 3. Rohs, R. et al. Nature 461, 1248–1253 (2009). 4. Watson, J. D. & Crick, F. H. C. Nature 171, 737–738
(1953). 5. Hoogsteen, K. Acta Crystallogr. 16, 907–916 (1963). 6. Palmer, A. G. & Massi, F. Chem. Rev. 106,
1700–1719 (2006). 7. Aishima, J. et al. Nucleic Acids Res. 30, 5244–5252
(2002). 8. Nair, D. T., Johnson, R. E., Prakash, S., Prakash, L. &
Aggarwal, A. K. Nature 430, 377–380 (2004). 9. Kitayner, M. et al. Nature Struct. Mol. Biol. 17,
423–429 (2010). 10. Rohs, R. et al. Annu. Rev. Biochem. 79, 233–269
(2010). 11. Chen, Y., Dey, R. & Chen, L. Structure 18, 246–256
(2010).
NEWS & VIEWS RESEARCH
© 2011 Macmillan Publishers Limited. All rights reserved
intensity7. Fawcett et al.1 now present a beautifully detailed record of climatic variability between 370,000 and 550,000 years ago from lake sediments at Valles Caldera, New Mexico (Fig. 1). This time interval spans two earlier interglacial periods, known as Marine Isotope Stage (MIS) 11 and 13.
Among palaeoclimatic aficionados, MIS 11 is of great interest, because Earth’s orbital con- figuration, and hence the insolation regime (amount of solar radiation reaching Earth), closely resembles that during the Holocene8. Usually, interglacial periods are short, lasting half of one precessional cycle (about 10,000 years), and end when summer cooling in the Northern Hemisphere triggers a new round of continental glaciations. However, during MIS 11, Earth’s orbit was nearly circular for an unusually long time, dampening the effect of precessional variations on seasonal insolation and causing MIS 11 to last 50,000 years. (MIS 13 is comparably long and hence also of interest.) The similar orbital configura- tion today raises the fascinating possibility that, even without rising greenhouse gases, the current interglacial might have another 40,000 years to go9. Thus, palaeoclimatic records from MIS 11 offer unique insights into the potential trajectory for the con- temporary climate system, in the absence of humanity’s effects.
Unfortunately, most MIS 11 records are from Antarctic ice cores or marine sediments; terrestrial records are scarce. The Valles Cal- dera record is remarkable for the wealth of palaeoclimatic proxy data collected by Fawcett and colleagues1. The proxies include both new geochemical indicators of past temperatures and tried-and-true proxies for terrestrial vegetation (pollen and δ13C [13C/12C]), lake productivity (silica/titanium ratios) and
lake hydrology (total organic carbon and calcium concentrations). The age model is well constrained by a basal, radiometrically dated ash layer and by palaeomagnetic reversals of known age, and the temporal resolution is fine enough to resolve millennial-scale climate variations.
The Valles Caldera record reveals three warm and two cool stages within MIS 11, with a 2 °C difference between stages. Warm peaks probably correspond to peaks in summer insolation.
The first warm stage (MIS 11e) is the counterpart to the past 11,000 years, and its trajectory is documented in exquisite detail. MIS 11e began with a rapid 8 °C warming, increased abundances of vegetation such as oak and juniper that prefer warm conditions, and increased lake productivity. A few thousand years later, abundances of grasses with the C4 photosynthetic system increased, suggesting warm and at least seasonally wet conditions. Simultaneously, however, the Valles Caldera lake became hydrologically closed (there were no outflowing streams), a signal of negative water balance. This somewhat paradoxical result can be explained by differing seasonal precipitation signals, with the C4 grasses responding to continued summer precipitation and lake hydrology responding to reduced winter precipitation. Summer and winter precipitation also diverged during the early Holocene, probably due to the effects of insolation on southwestern monsoon intensity10.
The next phase within MIS 11e is the most striking: abrupt shifts in δ13C and lowered total organic carbon suggest a rapid collapse of C4 grasses and oxidation of lake sediments; mud cracks in the sediments show that the lake dried out. This arid episode lasted several
thousand years, and apparently began and ended abruptly. At least two other multi- millennial dry periods occurred during MIS 11, and higher-frequency variations in total organic carbon and δ13C hint at sub- millennial hydrological variability during the warm stages of MIS 11.
Mud cracks and low total organic carbon also characterize much of MIS 13, implying that this interglacial was similarly marked by recurring episodes of high temperatures and aridity. Interestingly, MIS 13 seems to have been warmer and drier than MIS 11, despite lower concentrations of atmospheric carbon dioxide and methane during MIS 13 (refs 11, 12). This suggests that the larger variations in insolation during MIS 13 strongly regulated southwestern aridity.
What lessons can we draw from the Valles Caldera record? First, in southwestern North America, hydrological variability seems to be the rule rather than the exception during interglacial periods. Second, on timescales of 103 to 104 years, orbital precession strongly influences southwestern monsoon intensity and seasonal precipitation. Third, insofar as MIS 11 is a good analogue for the Holocene, we should now be at a time roughly equivalent to the transition between warm MIS 11e and cool MIS 11d. If so, southwestern climates might naturally be trending towards a somewhat cooler and wetter stage — except that other factors (us) are affecting climate. Knowing the likely natural trends thus helps us to diagnose the causes of twenty-first-century hydrological trends. And perhaps, just perhaps, these natural trends will partially mitigate the projected drying in the southwest.
John (Jack) Williams is in the Department of Geography and Center for Climatic Research, University of Wisconsin-Madison, Madison, Wisconsin 53706-1695, USA. e-mail: [email protected]
1. Fawcett, P. J. et al. Nature 470, 518–521 (2011). 2. Intergovernmental Panel on Climate Change
Climate Change 2007: The Physical Science Basis (Cambridge Univ. Press, 2007).
3. Seager, R. & Vecchi, G. A. Proc. Natl Acad. Sci. USA 107, 21277–21282 (2010).
4. Cook, E. R. et al. in Abrupt Climate Change. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research (eds Clark, P. U. et al.) 143–257 (US Geol. Surv., 2008).
5. Schubert, S. D., Suarez, M. J., Pegion, P. J., Koster, R. D. & Bacmeister, J. T. Science 303, 1855–1859 (2004).
6. Cook, E. R., Seager, R., Cane, M. A. & Stahle, D. W. Earth Sci. Rev. 81, 93–134 (2007).
7. Thompson, R. S., Whitlock, C., Bartlein, P. J., Harrison, S. P. & Spaulding, W. G. in Global Climates Since the Last Glacial Maximum (eds Wright, H. E. Jr et al.) 468–513 (Univ. Minnesota Press, 1993).
8. Loutre, M. F. & Berger, A. Global Planet. Change 36, 209–217 (2003).
9. Berger, A. & Loutre, M. F. Science 297, 1287–1288 (2002).
10. Harrison, S. P. et al. Clim. Dyn. 20, 663–688 (2003). 11. Siegenthaler, U. et al. Science 310, 1313–1317
(2005). 12. Spahni, R. et al. Science 310, 1317–1321 (2005).
Figure 1 | An aerial view of Valles Caldera, site of the lake sediments from which Fawcett et al.1 extracted their climate record.
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Extended megadroughts in the southwestern United States ...

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