Palmer Jonathan G.

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Palmer
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Jonathan G.
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Now showing 1 - 6 of 6
  • Article
    Rapid global ocean-atmosphere response to Southern Ocean freshening during the last glacial
    (Nature Publishing Group, 2017-09-12) Turney, Christian S. M. ; Jones, Richard ; Phipps, Steven J. ; Thomas, Zoë ; Hogg, Alan ; Kershaw, Peter ; Fogwill, Christopher J. ; Palmer, Jonathan G. ; Bronk Ramsey, Christopher ; Adolphi, Florian ; Muscheler, Raimund ; Hughen, Konrad A. ; Staff, Richard A. ; Grosvenor, Mark ; Golledge, Nicholas ; Rasmussen, Sune O. ; Hutchinson, David K. ; Haberle, Simon ; Lorrey, Andrew ; Boswijk, Gretel ; Cooper, Alan
    Contrasting Greenland and Antarctic temperatures during the last glacial period (115,000 to 11,650 years ago) are thought to have been driven by imbalances in the rates of formation of North Atlantic and Antarctic Deep Water (the ‘bipolar seesaw’). Here we exploit a bidecadally resolved 14C data set obtained from New Zealand kauri (Agathis australis) to undertake high-precision alignment of key climate data sets spanning iceberg-rafted debris event Heinrich 3 and Greenland Interstadial (GI) 5.1 in the North Atlantic (~30,400 to 28,400 years ago). We observe no divergence between the kauri and Atlantic marine sediment 14C data sets, implying limited changes in deep water formation. However, a Southern Ocean (Atlantic-sector) iceberg rafted debris event appears to have occurred synchronously with GI-5.1 warming and decreased precipitation over the western equatorial Pacific and Atlantic. An ensemble of transient meltwater simulations shows that Antarctic-sourced salinity anomalies can generate climate changes that are propagated globally via an atmospheric Rossby wave train.
  • Article
    The Intcal20 Northern Hemisphere radiocarbon age calibration curve (0-55 cal kBP)
    (Cambridge University Press, 2020-08-12) Reimer, Paula J. ; Austin, William E. N. ; Bard, Edouard ; Bayliss, Alex ; Blackwell, Paul G. ; Bronk Ramsey, Christopher ; Butzin, Martin ; Cheng, Hai ; Edwards, R. Lawrence ; Friedrich, Michael ; Grootes, Pieter M. ; Guilderson, Thomas P. ; Hajdas, Irka ; Heaton, Timothy J. ; Hogg, Alan G. ; Hughen, Konrad A. ; Kromer, Bernd ; Manning, Sturt W. ; Muscheler, Raimund ; Palmer, Jonathan G. ; Pearson, Charlotte ; van der Plicht, Johannes ; Reimer, Ron W. ; Richards, David A. ; Scott, E. Marian ; Southon, John R. ; Turney, Christian S. M. ; Wacker, Lukas ; Adolphi, Florian ; Büntgen, Ulf ; Capano, Manuela ; Fahrni, Simon M. ; Fogtmann-Schulz, Alexandra ; Friedrich, Ronny ; Köhler, Peter ; Kudsk, Sabrina ; Miyake, Fusa ; Olsen, Jesper ; Reinig, Frederick ; Sakamoto, Minoru ; Sookdeo, Adam ; Talamo, Sahra
    Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.
  • Article
    Punctuated shutdown of Atlantic Meridional Overturning Circulation during Greenland Stadial 1
    (Nature Publishing Group, 2016-05-19) Hogg, Alan G. ; Southon, John R. ; Turney, Christian S. M. ; Palmer, Jonathan G. ; Bronk Ramsey, Christopher ; Fenwick, Pavla ; Boswijk, Gretel ; Friedrich, Michael ; Helle, Gerhard ; Hughen, Konrad A. ; Jones, Richard ; Kromer, Bernd ; Noronha, Alexandra ; Reynard, Linda ; Staff, Richard ; Wacker, Lukas
    The Greenland Stadial 1 (GS-1; ~12.9 to 11.65 kyr cal BP) was a period of North Atlantic cooling, thought to have been initiated by North America fresh water runoff that caused a sustained reduction of North Atlantic Meridional Overturning Circulation (AMOC), resulting in an antiphase temperature response between the hemispheres (the ‘bipolar seesaw’). Here we exploit sub-fossil New Zealand kauri trees to report the first securely dated, decadally-resolved atmospheric radiocarbon (14C) record spanning GS-1. By precisely aligning Southern and Northern Hemisphere tree-ring 14C records with marine 14C sequences we document two relatively short periods of AMOC collapse during the stadial, at ~12,920-12,640 cal BP and 12,050-11,900 cal BP. In addition, our data show that the interhemispheric atmospheric 14C offset was close to zero prior to GS-1, before reaching ‘near-modern’ values at ~12,660 cal BP, consistent with synchronous recovery of overturning in both hemispheres and increased Southern Ocean ventilation. Hence, sustained North Atlantic cooling across GS-1 was not driven by a prolonged AMOC reduction but probably due to an equatorward migration of the Polar Front, reducing the advection of southwesterly air masses to high latitudes. Our findings suggest opposing hemispheric temperature trends were driven by atmospheric teleconnections, rather than AMOC changes.
  • Article
    Obliquity-driven expansion of North Atlantic sea ice during the last glacial
    (John Wiley & Sons, 2015-12-10) Turney, Christian S. M. ; Thomas, Zoë ; Hutchinson, David K. ; Bradshaw, Corey J. A. ; Brook, Barry W. ; England, Matthew H. ; Fogwill, Christopher J. ; Jones, Richard T. ; Palmer, Jonathan G. ; Hughen, Konrad A. ; Cooper, Alan
    North Atlantic late Pleistocene climate (60,000 to 11,650 years ago) was characterized by abrupt and extreme millennial duration oscillations known as Dansgaard-Oeschger (D-O) events. However, during the Last Glacial Maximum (LGM) 23,000 to 19,000 cal years ago (23 to 19 ka), no D-O events are observed in the Greenland ice cores. Our new analysis of the Greenland δ18O record reveals a switch in the stability of the climate system around 30 ka, suggesting that a critical threshold was passed. Climate system modeling suggests that low axial obliquity at this time caused vastly expanded sea ice in the Labrador Sea, shifting Northern Hemisphere westerly winds south and reducing the strength of meridional overturning circulation. The results suggest that these feedbacks tipped the climate system into full glacial conditions, leading to maximum continental ice growth during the LGM.
  • Article
    Cold tropical Pacific Sea surface temperatures during the late sixteenth-century North American megadrought
    (John Wiley & Sons, 2018-09-21) Cook, Benjamin I. ; Williams, A. Park ; Smerdon, Jason E. ; Palmer, Jonathan G. ; Cook, Edward R. ; Stahle, David W. ; Coats, Sloan
    The late 16th‐century North American megadrought was notable for its persistence, extent, intensity, and occurrence after the main interval of megadrought activity during the Medieval Climate Anomaly. Forcing from sea surface temperatures (SSTs) in the tropical Pacific is considered a possible driver of megadroughts, and we investigate this hypothesis for the late 16th‐century event using two new 600‐year long hydroclimate field reconstructions from Mexico and Australia. Areas represented by these reconstructions have strong teleconnections to tropical Pacific SSTs, evidenced by the leading principal component in each region explaining ∼40% of local hydroclimate variability and correlating significantly with the boreal winter (December‐January‐February) NINO 3.4 index. Using these two principal components as predictors, we develop a skillful reconstruction of the December‐January‐February NINO 3.4 index. The reconstruction reveals that the late 16th‐century megadrought likely occurred during one of the most persistent and intense periods of cold tropical Pacific SST anomalies of the last 600 years (1566–1590 C.E.; median NINO 3.4 = −0.79 K). This anomalously cold period coincided with a major filling episode for Kati Thanda‐Lake Eyre in Australia, a hydroclimate response dynamically consistent with the reconstructed SST state. These results offer new evidence that tropical Pacific forcing was an important driver of the late 16th‐century North American megadrought over the Southwest and Mexico, highlighting the large amplitude of natural variability that can occur within the climate system.
  • Preprint
    Decadally resolved lateglacial radiocarbon evidence from New Zealand kauri
    ( 2016-10) Hogg, Alan G. ; Southon, John R. ; Turney, Christian S. M. ; Palmer, Jonathan G. ; Bronk Ramsey, Christopher ; Fenwick, Pavla ; Boswijk, Gretel ; Büntgen, Ulf ; Friedrich, Michael ; Helle, Gerhard ; Hughen, Konrad A. ; Jones, Richard ; Kromer, Bernd ; Noronha, Alexandra ; Reinig, Frederick ; Reynard, Linda ; Staff, Richard ; Wacker, Lukas
    The Last Glacial-Interglacial Transition (LGIT; 15,000-11,000 cal BP) was characterized by complex spatiotemporal patterns of climate change, with numerous studies requiring accurate chronological control to decipher leads from lags in global paleoclimatic, -environmental and archaeological records. However, close scrutiny of the few available tree-ring chronologies and 14C-dated sequences composing the IntCal13 radiocarbon calibration curve, indicates significant weakness in 14C calibration across key periods of the LGIT. Here, we present a decadally-resolved atmospheric 14C record derived from New Zealand kauri spanning the Lateglacial from ~13,100 - 11,365 cal BP. Two floating kauri 14C time series, curve-matched to IntCal13, serve as a radiocarbon backbone through the Younger Dryas. The floating Northern Hemisphere (NH) 14C datasets derived from the YD-B and Central European Lateglacial Master tree-ring series are matched against the new kauri data, forming a robust NH 14C time series to ~14,200 cal BP. Our results show that IntCal13 is questionable from ~12,200 - 11,900 cal BP and the ~10,400 BP 14C plateau is approximately five decades too short. The new kauri record and re-positioned NH pine 14C series offer a refinement of the international 14C calibration curves IntCal13 and SHCal13, providing increased confidence in the correlation of global paleorecords.