Millennial to Centennial-Scale Karst Records #2
Millennial-scale climate shifts (e.g., Dansgaard-Oeschger cycles) are one of the most notorious features of glacial climate, affecting the temperature and the hydrological regime of tropical and subtropical areas during the glacial cycles of the Pleistocene. Centennial-scale climate shifts in the Holocene (e.g., 8.2 ka BP and 4.2 ka BP events) have also been identified in many regions, and some authors suggest that these events significantly affected human civilizations. Benefiting from U-Th dating and a worldwide distribution, speleothems have yielded climate records from different key climate areas during the late Pleistocene, providing highly-precise timing and high-resolution structures of abrupt climate changes. This session invites contributions presenting new records of millennial- or centennial-scale climate shifts, including but not limited to those featuring temperature, precipitation, sea level, and monsoon intensity, from speleothem, tufa, and travertine archives. We welcome studies investigating the timing, phases, amplitude and mechanisms of these climatic changes across the Pleistocene and Holocene.
10:30am - 10:45am
A speleothem record of Last Interglacial cooling in New Zealand and its potential link to a West Antarctic meltwater release
1University of Melbourne, Australia; 2Université Savoie Mont Blanc, France; 3Victoria University Wellington, New Zealand; 4University of NSW, Australia
The Last Interglacial (LIG) was the last time that Earth’s temperatures were warmer than present, making it an interesting target for better understanding the consequences of current global warming. Of particular interest is the smaller global ice volume: in combination with ocean thermal expansion, excess melting of polar ice sheets contributed to sea levels of the order of 6 to 9 metres above present. However, the source and timing of the excess meltwater release(s) are debated, and their resolution is probably beyond the use of coral-based records of sea-level change. An alternative way of addressing this issue is to examine datable proxy records from the middle latitudes that could record a local climatic response to a high-latitude meltwater release.
Here we report a precisely dated speleothem (flowstone drill-core) record from Nettlebed Cave, located in the northwest tip of New Zealand’s South Island. The flowstone contains an uninterrupted growth phase that spans most of the LIG (~132 - 118 ka). It was drilled from a chamber situated at ~800 m above sea level, and was fed by recharge received above the current tree line (>1200 m). Based on interpretation of growth-rate changes and the carbon and oxygen isotope time series, the most prominent feature of the record is a millennial-scale cooling dated to between 127.5 and 124. 5 ka. The timing of this cooling is consistent with a sea-surface temperature reversal preserved in a nearby Tasman Sea ocean-sediment record, and is broadly synchronous with cooling over Antarctica immediately following the local Last Interglacial thermal optimum. A marine-sediment record of authigenic uranium from the South Atlantic sector of the Southern Ocean links this latter Antarctic cooling to an ice-sheet meltwater pulse. Recent ice-sheet modelling also implicates meltwater as a plausible trigger for this Antarctic cooling.
To test whether collapse of the West Antarctic ice sheet (WAIS, the most vulnerable of Antarctica’s two ice sheets) could trigger a cooling over New Zealand, we ran a climate-model (LOVECLIM) hosing experiment. The model results reveal a cooling of ocean temperatures south of the cave site (the main moisture source region), which is consistent with both the Tasman Sea SST decrease and SST anomalies in the source region of Antarctic precipitation (based on EPICA Dome C deuterium-excess data).
Taken together, the consistent timing of the early-mid LIG cooling anomaly in the regional proxy records, and both the ice-sheet and climate modelling results not only constrain the timing of a complete or partial melting of the WAIS during the LIG, but also indicate a southern mid-latitude climatic response to such melting. This has implications for regional climatic impacts of a future WAIS collapse.
10:45am - 11:00am
Towards a quantitative precipitation history in Central-Eastern Europe since the last Glacial
1Institute of Earth Sciences, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany; 2Institute of Environmental Physics, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany; 3Institute for Geosciences, Johannes Gutenberg-University Mainz, Johann-Joachim-Becher-Weg 21, 55128 Mainz, Germany; 4German Federal Office for Radiation Protection, Köpenicker Allee 120-130, 10318 Berlin, Germany; 5Institute of Geology, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria; 6Department of Climate Geochemistry, Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany; 7Institute of Speleology "Emil Racovita", strada frumosa 31, 010986 Bucuresti, Romania
Speleothems from well-monitored Cloşani Cave, SW Romania, have provided excellent records of past rainfall amount and European (autumn/winter) hydro-climate variability (Warken et al., 2018). Here we present an extended precipitation history of Central-Eastern Europe, dating back to the Last Glacial Maximum (LGM) and also covering parts of MIS 3. High-precision 230Th/U-ages show that the two analysed speleothems (C09-1 and C09-2) grew nearly continuously during the past 20 ka, with only one growth interruption between ca. 11 and 9.7 ka BP. In addition, C09-2 covers the Dansgaard/Oeschger (D/O) events 16 to 14 between 60 and 52 ka BP. The multiple speleothem proxies (δ18O and δ13C values as well as laser ablation ICPMS trace element profiles) provide a comprehensive picture of local and regional hydro-climate variability on centennial to glacial/interglacial timescales.
Preliminary results show that over the course of the record, all proxies reveal millennial-scale features associated with prominent transitions during MIS 3, i.e., D/O events 16 to 14, as well as during the deglaciation, i.e., the Bølling-Allerød (BA) and Younger Dryas (YD). Local wetness as indicated by Mg/Ca ratios exhibits large variability, including pronounced swings between dry conditions during MIS 3 interstadials, and wetter conditions during stadials. After speleothem growth re-initiated at 20 ka BP, Mg/Ca ratios indicate a progressive drying until 15 ka BP, followed by an interval with enhanced variability from the Late Glacial to the Early Holocene. During the subsequent mid to late Holocene, from 8 ka BP to present, the local hydroclimate was characterized by relatively stable conditions.
Warken, S.F., Fohlmeister, J., Schröder-Ritzrau, A., Constantin, S., Spötl, C., Gerdes, A., Esper, J., Frank, N., Arps, J., Terente, M., Riechelmann, D.F.C., Mangini, A., Scholz, D., (2018). Reconstruction of late Holocene autumn/winter precipitation variability in SW Romania from a high-resolution speleothem trace element record. Earth and Planetary Science Letters 499, 122-133. DOI: 10.1016/j.epsl.2018.07.027
11:00am - 11:15am
Glacial changes in sea level modulated millennia-scale variability of the Southeast Asian autumn monsoon
1University of California, Irvine, United States of America; 2Department of Environmental Science, William Paterson University, Wayne, NJ 07470, USA; 3Berkeley Geochronology Center, Berkeley, CA 94709, USA; 4Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; 5Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI 02912, USA; 6Department of Geography and Environmental Sciences, Northumbria University, Newcastle, NE1 8ST, UK; 7Department of Geography and Environmental Sciences, Northumbria University, Newcastle, NE1 8ST, UK; 8Department of Archaeology, University of York, York, UK; 9Department of Science and International Cooperation, Phong Nha Ke Bang National Park, Phong Nha, 511860, VN
Relatively small changes to the seasonal rains of Mainland Southeast Asia (MSEA) can have broad reaching impacts for the densely populated region. Thus, characterizing past rainfall change using paleoclimate records provides key insight into how MSEA hydroclimate may change in the future. Furthermore, most paleostudies of MSEA hydroclimate center on the summer monsoon, with little work focusing on rainfall in other seasons. Here we use a multiproxy stalagmite record (45-4 ka) from central Vietnam, a region that primarily receives autumn moisture (September-November), to investigate autumn rainfall variability. Our record reveals a prolonged dry period spanning the last glacial maximum that is punctuated by an abrupt shift to wetter conditions during the deglaciation at ~14 ka. The isotope-enabled Transient Climate Evolution (iTRACE) experiment, a climate model simulating the deglaciation (20-11 ka), exhibits a similarly timed abrupt increase in autumn rainfall amount, which is driven by glacially modulated sea level rise. Lower glacial sea level exposes land in the Gulf of Tonkin and along the South China shelf, reducing convection and moisture delivery to central Vietnam causing dry conditions. When sea level rises and these landmasses flood at ~14ka, moisture delivery to central Vietnam increases causing an abrupt shift from dry to wet conditions. Other well-known millennial-scale events such as Heinrich Events (dry conditions) and Dansgaard-Oeschger Events (wet conditions) also appear in our record. Notably, sea level modulates the magnitude of these events by muting dry events and enhancing wet events during periods of low sea level, highlighting the dominance of this mechanism on autumn monsoon variability.
11:15am - 11:30am
Zonal Indian Ocean sea surface temperature gradients drive deglacial millennial-scale hydroclimate variability in southeast Africa
1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, USA; 2Department of Geosciences, University of Massachusetts Amherst, USA; 3Irish Climate and Analysis Research Units, Maynooth University, Ireland; 4Mention Bassins sédimentaires, Evolution, Conservation (BEC), Université d’Antananarivo, Madagascar
Due to a relative paucity of instrumental and paleoclimate records, the Indian Ocean is one of the least understood ocean basins. The tropical Indian Ocean is warming faster than any other basin, and its interannual climate variability is projected to become more extreme under future emissions scenarios with substantial impacts on developing Indian Ocean rim countries. Therefore, it has become increasingly important to understand the drivers of regional precipitation in a changing climate. Here we present AB12, a new speleothem record from Anjohibe, a cave in NW Madagascar. An age model was constructed from 30 U-Th age determinations, dating speleothem growth from 27 to 14 ka. δ18O, δ13C, and trace metal proxies were analyzed to reconstruct decadal to millennial-scale hydroclimatic change through the Last Glacial Maximum and deglaciation. Stable isotope and trace metal proxies reconstruct drier conditions during Heinrich Stadial 1, and wetter conditions during the Bølling–Allerød. This is surprising considering hypotheses arguing for southward (northward) ITCZ shifts during North Atlantic cooling (warming) events, which would be expected to result in wetter (drier) conditions at Anjohibe in the Southern Hemisphere tropics. An analysis of previously published sea surface temperature (SST) reconstructions from the eastern and western flanks of the basin indicates that the Indian Ocean zonal SST gradient is in close agreement with hydroclimate proxies in NW Madagascar, with periods of increased precipitation correlating with relatively warmer conditions in the western Indian Ocean and cooler conditions in the eastern Indian Ocean. Such gradients could drive long-term shifts in the strength of the Walker circulation with widespread effects on hydroclimate across East Africa. These results suggest that during abrupt millennial-scale climate changes, the tropical Indian Ocean SST gradient and Walker circulation drive East African hydroclimate variability rather than meridional ITCZ shifts alone.
11:30am - 11:45am
Spatial and temporal ITCZ dynamics in the last three millennia in Northeastern Brazil
1Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, Cidade Universitária, São Paulo-SP, 05508-090, Brazil; 2Geo- and Environmental Research Center, University of Tübingen, Tübingen, Germany; 3Interdisciplinary Research Center of Earth Science Frontier, Beijing Normal University, Beijing, China; 4Institute of Global Environmental Change, Xi’an Jiaotong University, Xi’an, China; 5Department of Earth Sciences, University of Minnesota, Minneapolis, MN, USA
The behavior of the Intertropical Convergence Zone (ITCZ) have been attributed to latitudinal movements between hemispheres. In most recent studies, new evidences suggest that contraction and expansion of the rainbelt are also responsible for ITCZ variability on a centennial time scale. Over South America (SA) few records point to the same interpretation, however the scarcity of data prevents an accurate interpretation of the ITCZ behavior over the continent. In Northeast (NEB) Brazil the ITCZ is located in its southernmost position between March and May (~5°S) which determines the wet period at the region. During this period, greater convective activity provides lower δ18O values in precipitation (~ -4.4‰) contrasting with a more positive values from the other seasons (~ -1.6‰). In order to improve the knowledge about the equatorial rainbelt, we reconstructed precipitation in the Northeast of Brazil during the last 3,200 years using the δ18O data obtained in four speleothems. These records were integrated in a unique paleoprecipitation curve based on iscam composite method. The composite record shows two periods of intense droughts from 250 to 550 CE years and from 1070 to 1570 CE years. Two others periods are characterized by wetter phases, from 850 to 1050 CE, equivalent to the Medieval Climate Anomaly (MCA), and from 1570 to 1850 CE, equivalent to the Little Ice Age (LIA). During the MCA, the ITCZ was expanded providing increased precipitation in both ITCZ margins, NEB and Cariaco Basin, and in Eastern Amazon (EA), indicated by Paraíso Cave record. The transition between MCA and LIA in NEB was the longest and driest period in the last 3200 yr in NEB and EA, suggesting a north migration of the ITCZ, when Cariaco Basin in Venezuela was wetter. Abruptly, the southernmost position of ITCZ occur during the LIA inducing increased precipitation over NEB and dryness in Cariaco, however in EA there was a long drought record, probably associated with reduced continental projection of the ITCZ. Our results in comparison with other proxies indicate that similar climate conditions over a large equatorial region was due to ITCZ expansion or a prolonged seasonal migration over each hemisphere. Thus, the ITCZ in centennial scale was 1) controlled by north-south migration and expansion of the ranbelt and 2) not symmetrically distributed around the Equator.
11:45am - 12:00pm
ITCZ AND ENSO SIGNATURE FOR HYDROCLIMATE OVER THE WESTERN PACIFIC DURING THE PAST 3000 YEARS
1Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan, ROC; 2Xi'an Jiaotong University, Xi'an, PRC; 3Department of Geology, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia; 4Earth Observatory of Singapore, Nanyang Technological University, Singapore; 5Asian School of the Environment, Nanyang Technological University, Singapore; 6Sarawak Museum, Campus Project Jalan Barrack, Kuching, Malaysia
The Intertropical Convergence Zone (ITCZ) indicates the region with most intense precipitation in the tropics. In other words, nature dynamics of the ITCZ is related to the energy balance between the two Hemispheres. On orbital to millennial timescales, ITCZ main position closely follows the Northern Hemisphere summer insolation and the meridional ocean circulations in the North Atlantic [1, 2, 3]. The ITCZ variations on shorter timescales are, however, not well understood. For instance, recent studies [4, 5, 6] raise the debate between the meridional displacement and expansion/contraction of the ITCZ.
Here, a stalagmite was collected from Padang Kawad cave in the central Malay Peninsula. We have obtained an oxygen isotope ratio (δ18O)-inferred hydroclimate record over the past ~3000 years at decadal resolution. The δ18O variability generally consists with the Southern Oscillation Index , which demonstrates the influence of ENSO on the climate of western Pacific. In detail, an increasing trend in precipitation is shown throughout the entire Medieval Warm Period (MWP). The climate remained humid until the mid Little Ice Age (LIA) and then became drier afterwards, consistent with other hydroclimate proxy records in the region. Combining precisely-dated stalagmite records from southern China , eastern Indonesia , and northern Australia , we further developed an index to reconstruct the meridional variation of the ITCZ over the past 2000 years. It implies a relatively stable and more fluctuated condition in the first and second millennium, respectively. In addition, the ITCZ rapidly contracted at the beginning of LIA and gradually restored to modern condition in the end of the 19th century. To summarize, we revealed frequent meridional expansion/contraction in the ITCZ, as well as its mean position migration.
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