1:30pm - 1:45pm
Surface-cave thermal decoupling and its impact on the speleothem oxygen isotope records during a full glacial cycle
1Universidad de Salamanca, Department of Geology, Spain; 2Universidad de Zagreb, Department of Soil Science, Croatia
Temperature in inner sections of most caves has limited variability and its value is close to the mean annual temperature above the cave. Nevertheless, any cave atmosphere temperature has long-term changes. Away from the cave entrances and in the absence of streams, the long-term thermal variability of caves is controlled by the surface atmosphere temperature and its transfer to the cave is the result of thermal conduction. The surface thermal signal takes time in its propagation underground by conduction, causing a temporary decoupling between surface and cave atmosphere temperatures. Such thermal decoupling depends on the cave depth and the duration of the thermal anomaly. Since temperature is a main control on the oxygen isotope composition of speleothems, we explored the potential impact of this thermal decoupling in speleothem records from a series of hypothetical caves located at different depths.
We have explored the theoretical impact of thermal decoupling in speleothem oxygen isotope records during the last glacial cycle. A synthetic surface atmosphere temperature record was constructed from sinusoidal signals inspired in alkenone paleotemperature ocean records from the North Atlantic off-shore Portugal and the Western Mediterranean. The synthetic surface atmosphere temperature includes 15 stadial periods and 4 interstadial periods during the MIS5 that are superimposed to a full glacial cycle.
The result of the underground thermal model suggests that thermal decoupling is very limited in shallow caves (e.g., 10 m), whereas in deeper caves (e.g., 500 m) can reach anomalies (either positive or negative) in the order of 4 ºC. The largest long-term temperature change is simulated during the Termination II (i.e., up to 8 ºC within <2 ka). The delay of the recorded signal is proportional to the duration of the anomaly (i.e., period of the cycle), and the thermal anomalies of a glacial cycle can take hundreds/thousands of years to reach a cave teens/hundreds of meters underground. Our simulations suggest that a full glacial cycle could take several thousand meters before being fully attenuated with depth.
We explore the impact of thermal decoupling on speleothem oxygen isotope records considering the thermal impact during rain condensation and calcite precipitation, but not to any other isotope change related to the hydrological cycle, that are likely to be specific to every location. Speleothem oxygen isotope anomalies are more significant in speleothems from deeper caves and the largest anomalies are recorded during Termination II. The impact of the thermal decoupling on the oxygen isotope composition of speleothems also depends on the on the thermal relationship at the time of rain condensation, but anomalies (either positive or negative) can exceed 1 to 2 ‰.
This research suggests that temperature can be a significant control on the speleothem oxygen isotope glacial records due to the thermal decoupling between surface and cave atmospheres, especially, but not exclusively in deep caves located in mountainous areas.
1:45pm - 2:00pm
Asynchronous Holocene Optimum in East Asia monsoon region recorded by stalagmites and its underlying climate dynamics
1Université catholique de Louvain, Belgium; 2National Taiwan University, Taiwan; 3State Key Laboratory of Tibetan Plateau Earth System, Resources and Environment (TPESRE), Institute of Tibetan Plateau Research, Chinese Academy of Sciences, China
Reconstructions of Holocene Optimum (HO) in East Asian summer monsoon (EASM) regime from speleothem versus other proxy records have yielded divergent phase relationships with the EASM and local precipitation. This apparent discrepancy has been partly attributed to the uncertainties in the climatic representation of Chinese speleothem oxygen isotope (δ18O) records. Here we conducted a data-model comparison along with a water moisture budget analysis to assess the role of thermodynamic and dynamic components in controlling mid-summer and spring rainfall during early and mid-Holocene, and to compare with the precipitation changes referred by the stalagmite δ18O records. Our results show that 1) a marked southward shift of the HO period from 10500~6500 yr BP in North China (NC) to 9000~5000 yr BP in Yangtze river valley (YRV). During the Holocene, the variation of the summer precipitation is dominated by precession in NC, ice sheet in YRV. 2) An incoherent orbital-scale speleothem δ18O variability in EASM regime indicate that speleothem δ18O is largely controlled by the large-scale circulation and concomitant latitude shifts of monsoon rain belt.3) The intensified hydroclimate in YRV in mid-Holocene was contributed to excessive rainfall in spring, especially for increasing the large-scale/total precipitation ratio, which leads to the lightest speleothem δ18O during the mid-Holocene. The excessive rainfall in spring is mainly from the enhancement of horizontal monsoonal moisture transport that is caused by the anticyclone over Western North Pacific.
2:00pm - 2:15pm
Insolation triggered abrupt climate changes confirmed by speleothem records
Georges Lemaître Centre for Earth and Climate Research, Earth and Life Institute, Université Catholique de Louvain, Louvain-la-Neuve, Belgium
Various paleoclimate records show that the end of interglacials of the late Pleistocene was marked by abrupt cooling events. Strong abrupt cooling occurring when climate was still in a warm interglacial condition is puzzling. Our transient climate simulations for all the eleven interglacial (sub)stages of the past 800,000 years show that there exists a threshold in the astronomically induced insolation below which abrupt changes at the end of interglacials occur (Yin et al., 2021). When the summer insolation in the Northern Hemisphere (NH) high latitudes decreases to a critical value, it triggers a strong, abrupt weakening of the Atlantic meridional overturning circulation and a strong cooling in the NH followed by high-amplitude variability. The mechanism involves sea ice feedbacks in the Northern Nordic Sea and the Labrador Sea. Similar abrupt oscillations happen in the simulated temperature, precipitation and vegetation from low to high latitudes. The simulated results are supported by observations from different marine and terrestrial records. In particular, the simulated abrupt events at the end of interglacials are confirmed by high-resolution speleothem records from China. Strong, abrupt shifts in oxygen and carbon isotope compositions are observed in the speleothem records, which is suggested to indicate abrupt weakening of the Asian monsoon and abrupt environmental changes. Similar abrupt changes are also observed in speleothem records from Europe. The timings of the abrupt events observed in the speleothem records are highly consistent with those simulated by the model, validating the model results and revealing that the astronomically-induced slow variation of insolation could trigger abrupt climate events.
The model results show that the insolation threshold occurred at the end of each interglacial of the past 800,000 years, suggesting its fundamental role in terminating the warm climate conditions of the interglacials. The next insolation threshold will occur in 50,000 years, suggesting an exceptionally long interglacial ahead, which is in line with what has been suggested by previous modelling studies.
Reference: Yin Q.Z., Wu Z.P., Berger A., Goosse H., Hodell D., 2021. Insolation triggered abrupt weakening of Atlantic circulation at the end of interglacials. Science, 373, 1035-1040, DOI: 10.1126/science.abg1737
2:15pm - 2:30pm
Pronounced climate reversals following the 8.2 ka cooling event
1IBS Center for Climate Physics, Pusan National University, South Korea; 2Laboratoire des Sciences du Climat et de l'Environnement, France; 3Université de Bordeaux, France; 4Xi’an Jiaotong University, China; 5University of Minnesota, USA; 6Stockholm University, Sweden; 7Universität Innsbruck, Austria; 8Heidelberg University, Germany; 9Université Claude Bernard Lyon 1, France; 10Critt Materiaux Alsace, France; 11Université Paris-Saclay, France
About 8 thousand years ago (ka) proglacial lakes in North America, which had formed in the aftermath of a rapid Laurentide ice-sheet retreat, suddenly drained into the North Atlantic. The associated oceanic freshwater flux temporarily weakened the Atlantic Meridional Overturning Circulation (AMOC) and meridional heat transport, causing global climate disruptions. Although a centennial cooling anomaly was frequently reported in northern high latitudes, higher resolution climate records from Europe (e.g. annual-to-decadal) reveal a more complex picture. Using high-resolution multiproxy stalagmite records from Sweden and France and climate simulations with an isotope-enabled Community Earth System Model, we provide further evidence for an overlooked aspect of the dynamics of the 8.2 ka event - the post-event overshoot.
Constrained by multiple precise U/Th dating around 8 ka, the δ18O in both stalagmites shows two distinct periods of negative anomalies (up to -1 ‰ at 8.20 +/- 0.06 ka), similar to those recorded in Greenland ice cores. Our model simulation largely reproduces these isotopic variations as well as those in a global proxy data compilation and suggests a century-long cooling of ~ -1-2 ℃ in Europe and northern Asia, and ~ -2-4 ℃ in the Arctic, Greenland and its surrounding seas. The cooling was followed by a century long recovery to warmer conditions with a temperature overshoot of ~ 0.5-2 ℃ in wide regions north of ~ 40 °N. Mg/Ca and δ13C data extracted from the Swedish and French speleothems suggest that moisture at these sites changed only slightly during the cold event, defined by the δ18O variability, but increased significantly in the subsequent warming period. These post-cooling wetter phases are also visible in few regional varved lake sediment proxies. These observations are supported by our model, which simulates a ~ 5-30 % post-event precipitation increase in Atlantic Europe, Greenland, the Arctic, northern Asia, and in the Northern Hemisphere summer monsoon domain. The Southern Hemisphere monsoon proxies typically show opposite anomalies. This overshoot seesaw pattern in global monsoons can be explained by a rapid two-phase resumption of the AMOC in our simulation, and a quick retreat of sea ice in the Greenland-Iceland-Norwegian Seas. Our study highlights the connection between rapid freshwater forcing and alternating cold/warm responses in the Northern Hemisphere, thereby providing a blueprint mechanism for the generation of centennial-scale Holocene climate variability.
2:30pm - 3:00pm
Overcoming data and model uncertainties to understand hydroclimate changes during Heinrich stadials
Massachusetts Institute of Technology, United States of America
Periods of Northern Hemisphere cooling known as Heinrich stadials produced some of the most pronounced hydroclimate changes of the late Pleistocene. As such, they present unique opportunities for testing the abilities of climate models to represent atmospheric circulation and precipitation changes. In this talk, I’ll explore efforts to compare and integrate stalagmite records of Heinrich stadials with model simulations of these climates. I note three possible reasons for data-model discrepancies: first, errors in the interpretation of observational datasets from stalagmites and other archives (‘data’); second, uncertainties in the forcings and boundary conditions provided to models (‘forcings’); and third, problems with models’ representations of the climate (‘biases’). I’ll explore examples interrogating data, forcings and model biases to build a better understanding of precipitation changes during Heinrich stadials, with the aim of identifying approaches that can be useful across a range of data-model integration efforts.