3:30pm - 3:45pm
Holocene extreme precipitation frequency and paleoclimate reconstruction based on high-resolution multi-proxy records from speleothems from south-east Brazil
1Max Planck Graduate Center (MPGC-Mainz), Germany; 2Institute of Geosciences, Johannes-Gutenberg Universität Mainz, Germany; 3Max Planck Institute for Chemistry Mainz, Germany; 4Geochemistry department, Universidade Federal Fluminense, Niterói, Brazil; 5Institute of Geosciences, Universidade de São Paulo, Brazil
Observational evidence and future projections have shown the increase in frequency and intensity of extreme rainfall events worldwide. However, despite of its extreme importance, the data available is scarce and mainly restricted to the last century. In Brazil, these events trigger floods and landslides, accounting for 74% of natural disaster-related deaths, almost 50% of unsheltered and 47% of material damage with an impact of 31 billion of reais (ca. 6.2 billion dollars) between 2010 and 2019. The lack of data going beyond the instrumental record hampers robust assessments of the effects of climate change on those events (frequency, intensity, spatial extent, duration, and timing), precluding the inquiry of whether they are part of natural variability or influenced by human activities (increase in greenhouse gas emissions, land use changes and deforestation or intense urbanization). Thus, reconstructing the frequency of extreme rainfall events beyond the period of instrumental data, is essential to understand how the current warming trend affects the frequency and magnitude of such events.
We use detrital layers within stalagmites from flooded cave galleries as records of flood activity in caves and as archives for past extreme hydrological events, such as hurricanes and storms. In this context, we investigate stalagmites from three different caves (Malfazido, Varzeão and Lage Branca cave) in southeastern Brazil, where modern flood activity is related to extreme rainfall precipitation as shown by cave monitoring, along with conventional multi-proxy analyses to reconstruct past changes in paleohydrology. A representative set of samples is used to generate a robust cave flooding record for the Holocene, and coeval speleothems from different galleries and different caves are used to replicate the record. The extreme rainfall events in southeastern Brazil are mainly associated with the South American Monsoon System (SAMS) between late-autumn and early-spring (between November and April) and extratropical excursions between late-spring and early-autumn (between May and October). Stalagmite δ18O records are used to reconstruct regional changes in the moisture source related to seasonal shifts in atmospheric circulation from a monsoonal regime (more negative δ18O values) to a more extratropical regime (less negative δ18O values). Correlation between trace element ratios shows that prior-calcite/aragonite precipitation is the main processes controlling the trace element concentrations. Therefore, high-resolution records of trace element ratios along with δ13C values are used to reconstruct local hydrological conditions at the cave sites. This multi-proxy approach provides an exceptional opportunity to understand how past climate and environmental changes influenced the occurrence of extreme rainfall events during the Holocene in southeastern Brazil.
3:45pm - 4:00pm
The speleothem water content as a proxy for past moisture variability in Milandre Cave, Switzerland
1Department of Environmental Sciences, University of Basel, Switzerland; 2Physics Institute and Oeschger Centre for Climate Change Research, University of Bern, Switzerland
Speleothem fluid inclusions contain relics of past precipitation water, which constitute a powerful tool for studying the past hydrological cycle. Analyses of hydrogen and oxygen isotopes of the water stored in fluid inclusions allow the reconstruction of paleoclimate information such as the temperature or moisture source. There is however a need for new proxies related to past moisture availability, which would allow reconstructions especially in Europe, where such records are lacking. The quantitative estimation of the speleothem water content (hereafter SWC) remains rare as it is generally a collateral result of more challenging analyses such as isotope determinations of fluid inclusions or noble gases. Using a recently developed method to analyse speleothem fluid inclusion water isotopes, we obtained a record of more than 250 SWC data covering the Younger Dryas and Holocene intervals with a decadal to multi-decadal resolution measured on two Swiss stalagmites from Milandre Cave, NW Switzerland. The crushing of samples in the measuring line resulted in a mean SWC of 1.9 microlitre of water per gram of crushed calcite from both stalagmites. The comparison with other paleohumidity-related indicators from central Europe suggests that the SWC is related to past moisture variability. At the Quaternary geology group of the University of Basel, we have recently measured on the same samples the trace elements strontium (Sr) and magnesium (Mg), which are proxies for water residence time and growth rate, respectively. The close correlation between Sr, Mg and the SWC also suggests that the water availability in the karst system above the cave is an important factor for SWC. New reconstruction of past moisture variability together with speleothem fluid inclusion temperature estimates would allow a better understanding of the central European climate variability during the Holocene.
4:00pm - 4:15pm
Drivers of Southeast Asian monsoon variability during the Holocene
1Department of Earth System Science, University of California, Irvine, USA; 2Northumbria University Newcastle, Engineering and Environment, Geography and Environmental Sciences, Newcastle Upon Tyne, UK; 3Potsdam Institute for Climate Impact Research, Potsdam, Germany; 4Environmental Research Institute, Waikato University, Hamilton, New Zealand; 5Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, USA; 6Department of Earth Sciences, ETH Zurich, Switzerland; 7Department of Environmental Science, William Paterson University, Wayne, USA; 8Federal Office for Radiations Protection, Berlin, Germany; 9Centre ESCER (Étude et la Simulation du Climat à l’Échelle RÉgionale) and GEOTOP (Research Center on the dynamics of the Earth System); 10Department of Geosciences, University of Connecticut, Storrs, USA; 11Institute for Chemistry and Biology of the Marine Environment (ICBM), University of Oldenburg, Oldenburg, Germany; 12Department of Physics, University of Toronto, Canada; 13British Cave Research Association (BCRA), Buxton, UK; 14Vietnam Atomic Energy Institute, Nuclear Training Center, Ha Noi, Viet Nam
The Northeast Winter Monsoon leads to intensive rainfalls in parts of Southeast Asia, supporting but also threating the lives of millions in these regions. A detailed history of Northeast Winter Monsoon rainfall is largely unknown, especially for the Holocene, where proxy and modelling studies give inconclusive results. We present the first high-resolution multi-proxy speleothem record from central Vietnam, reconstructing Northeast Winter Monsoon rainfall variability. Local hydroclimate proxies (Mg/Ca ratios and δ13C) show that winter monsoon rainfall weakened substantially during the mid-Holocene, coinciding with a decline in summer monsoon rainfall. Comparing our paleoclimate record to climate simulations, we conclude that changes in the mean thermal state of the Pacific and Indian oceans, amplified by the end of the Green Sahara interval and increased airborne dust load associated with drier conditions, caused an eastward shift in the Walker Circulation. This shift led to a decline in rainfall across Southeast Asia affecting both summer and winter monsoonal rainfall. We find evidence that winter and summer monsoon in Southeast Asia had a positive relationship during the early and mid-Holocene, suggesting a common forcing. Our findings highlight that the variability of the winter and summer monsoon in Southeast Asia is complex and does not follow a simple anti-phase relationship for most of the Holocene.
4:15pm - 4:30pm
A new speleothem record from Ascunsă Cave, Romania: insight in climate during the 8.2 ka event
1Université Savoie Mont Blanc, France; 2University of Melbourne, Australia; 3Romanian Academy, Romania; 4University of Bergen, Norway; 5Universidad de Granada, Spain; 6Australian Nuclear Science and Technology Organisation, Australia
The 8.2 ka event was the most prominent climate anomaly of the Holocene and was probably triggered by the drainage of lakes Agassiz and Ojibway during the final retreat of the Laurentide ice sheet. The meltwater pulse was of sufficient magnitude to disrupt the Atlantic Meridional Overturning Circulation (AMOC). The climatic effects of this meltwater release is most clearly captured in the Greenland ice-core records, where a cold and dry anomaly lasting ~160 years (8.25 ± 0.05 ka BP until 8.09 ± 0.05 ka BP) is recorded (Thomas et al., 2007).
The impact of the 8.2 ka event in the Carpathian-Balkan region is generally weak, and conclusions as to the regional climatic response vary. Some palaeoclimate archives, such as lake and marine records, indicate a climatic change through the event that are more or less synchronous with those around the North Atlantic Ocean, but significantly smaller in magnitude. However, the region’s speleothem records do not consistently capture the event, in part reflecting the quality of the underlying chronologies and temporal resolution. Most of the region’s speleothem δ18O records have been interpreted in terms of temperature, with an increasing trend from the earliest Holocene towards the late Holocene, with no change through the 8.2 ka event. Potentially this suggests that there were no major temperature anomalies in the region at that time. However, the distinction between temperature and hydrologically driven changes are complex and difficult to resolve based solely on the stable isotope records.
Here we present a new speleothem record (POM10) from Ascunsă cave in Romania. We explore the climate impact of the 8.2 ka event in the Carpathian-Balkan region using a multi-proxy approach. By coupling stable carbon and oxygen isotopes, trace elements (Mg/Ca and Sr/Ca), growth rate, fluid inclusion isotopes and clumped isotopes, we explore the potential role of climatic forcing factors in the region. In stalagmite POM10, only minor changes in the proxies are observed through the 8.2 ka event, suggesting no major temperature or hydrological changes. Thus, stalagmite from this cave site, or even this region, are potentially not sensitive enough to capture climate changes triggered by modest perturbations to North Atlantic meridional overturning circulation. However, this could also be explained by cave site-specific effects, such as kinetic effects or, more likely counter-balancing influences of hydrological and temperature effects, resulting in muted signals.