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1. Scientific drilling of Lake Chalco, Basin of Mexico (MexiDrill)

   https://doi.org/10.5194/sd-26-1-2019  

   Brown, Erik T. ; Caballero, Margarita ; Cabral Cano, Enrique ; Fawcett, Peter J. ; Lozano-García, Socorro ; Ortega, Beatriz ; Pérez, Liseth ; Schwalb, Antje ; Smith, Victoria ; Steinman, Byron A. ; et al ( January 2019 , Scientific Drilling)

   Abstract. The primary scientific objective of MexiDrill, the Basin of MexicoDrilling Program, is development of a continuous, high-resolution∼400 kyr lacustrine record of tropical North Americanenvironmental change. The field location, in the densely populated,water-stressed Mexico City region gives this record particular societalrelevance. A detailed paleoclimate reconstruction from central Mexico willenhance our understanding of long-term natural climate variability in theNorth American tropics and its relationship with changes at higher latitudes.The site lies at the northern margin of the Intertropical Convergence Zone(ITCZ), where modern precipitation amounts are influenced by sea surfacetemperatures in the Pacific and Atlantic basins. During the Last GlacialMaximum (LGM), more winter precipitation at the site is hypothesized to have beena consequence of a southward displacement of the mid-latitude westerlies. Itthus represents a key spatial node for understanding large-scalehydrological variability of tropical and subtropical North America and isat an altitude (2240 m a.s.l.), typical of much of western North America. In addition, its sediments contain a rich record of pre-Holocene volcanichistory; knowledge of the magnitude and frequency relationships of thearea's explosive volcanic eruptions will improve capacity for riskassessment of future activity. Explosive eruption deposits will also be usedto provide the backbone of a robust chronology necessary for fullexploitation of the paleoclimate record. Here we report initial resultsfrom, and outreach activities of, the 2016 coring campaign. 

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2. Global climate disruption and regional climate shelters after the Toba supereruption

   https://doi.org/10.1073/pnas.2013046118  

   Black, Benjamin A. ; Lamarque, Jean-François ; Marsh, Daniel R. ; Schmidt, Anja ; Bardeen, Charles G. ( July 2021 , Proceedings of the National Academy of Sciences)

   The Toba eruption ∼74,000 y ago was the largest volcanic eruption since the start of the Pleistocene and represents an important test case for understanding the effects of large explosive eruptions on climate and ecosystems. However, the magnitude and repercussions of climatic changes driven by the eruption are strongly debated. High-resolution paleoclimate and archaeological records from Africa find little evidence for the disruption of climate or human activity in the wake of the eruption in contrast with a controversial link with a bottleneck in human evolution and climate model simulations predicting strong volcanic cooling for up to a decade after a Toba-scale eruption. Here, we use a large ensemble of high-resolution Community Earth System Model (CESM1.3) simulations to reconcile climate model predictions with paleoclimate records, accounting for uncertainties in the magnitude of Toba sulfur emissions with high and low emission scenarios. We find a near-zero probability of annual mean surface temperature anomalies exceeding 4 °C in most of Africa in contrast with near 100% probabilities of cooling this severe in Asia and North America for the high sulfur emission case. The likelihood of strong decreases in precipitation is low in most of Africa. Therefore, even Toba sulfur release at the upper range of plausible estimates remains consistent with the muted response in Africa indicated by paleoclimate proxies. Our results provide a probabilistic view of the uneven patterns of volcanic climate disruption during a crucial interval in human evolution, with implications for understanding the range of environmental impacts from past and future supereruptions.

    

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3. Latest Pleistocene to Holocene hydroclimates from Lake Elsinore, California

   https://doi.org/10.1016/j.quascirev.2013.05.023  

   Kirby, Matthew E. ; Feakins, Sarah J. ; Bonuso, Nicole ; Fantozzi, Joanna M. ; Hiner, Christine A. ( September 2013 , Quaternary Science Reviews)

   The hydroclimate of the southwestern United States (US) region changed abruptly during the latest Pleistocene as the continental ice sheets over North America retreated from their most southerly extent. To investigate the nature of this change, we present a new record from Lake Elsinore, located 36 km inland from the Pacific Ocean in Southern California and evaluate it in the context of records across the coastal and interior southwest United States, including northwest Mexico. The sediment core recovered from Lake Elsinore provides a continuous sequence with multi-decadal resolution spanning 19e9 ka BP. Sedimentological and geochemical analyses reveal hydrologic variability. In particular, sand and carbonate components indicate abrupt changes at the Oldest Dryas (OD), BøllingeAllerød (BA), and Younger Dryas (YD) transitions, consistent with the timing in Greenland. Hydrogen isotope analyses of the C28 nalkanoic acids from plant leaf waxes (dDwax) reveal a long term trend toward less negative values across 19 9 ka BP. dDwax values during the OD suggest a North Pacific moisture source for precipitation, consistent with the dipping westerlies hypothesis. We find no isotopic evidence for the North American Monsoon reaching as far west as Lake Elsinore; therefore, we infer that wet/dry changes in the coastal southwest were expressed through winter-season precipitation, consistent with modern climatology. Comparing Lake Elsinore to other southwest records (notably Cave of Bells and Fort Stanton) we find coincident timing of the major transitions (OD to BA, BA to YD) and hydrologic responses during the OD and BA. The hydrologic response, however, varied during the YD consistent with a dipole between the coastal and interior southwest. The coherent pattern of hydrologic responses across the interior southwest US and northwest Mexico during the OD (wet), the BA (drier), and YD (wet) follows changes in the Atlantic Meridional Overturning Circulation, presumably via its combined influence on North Pacific winter storm tracks and the extent/magnitude of the North American Monsoon. In contrast, Lake Elsinore and the coastal southwest experiences a deglacial drying trend punctuated by abrupt change at the OD to BA and BA to YD transitions. This trend tracks rising greenhouse gases through the deglacial, with an apparent southward shift in westerly moisture sources adjusting to the retreating ice sheet. 

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4. Volcanic Eruption Signatures in the Isotope‐Enabled Last Millennium Ensemble

   https://doi.org/10.1029/2019PA003625  

   Stevenson, S. ; Otto‐Bliesner, B. L. ; Brady, E. C. ; Nusbaumer, J. ; Tabor, C. ; Tomas, R. ; Noone, D. C. ; Liu, Z. ( August 2019 , Paleoceanography and Paleoclimatology)

   Explosive volcanic eruptions are one of the largest natural climate perturbations, but few observational constraints exist on either the climate responses to eruptions or the properties (size, hemispheric aerosol distribution, etc.) of the eruptions themselves. Paleoclimate records are thus important sources of information on past eruptions, often through the measurement of oxygen isotopic ratios (δ¹⁸O) in natural archives. However, since many processes affectδ¹⁸O, the dynamical interpretation of these records can be quite complex. Here we present results from new, isotope‐enabled members of the Community Earth System Model Last Millennium Ensemble, documenting eruption‐inducedδ¹⁸O variations throughout the climate system. Eruptions create significant perturbations in theδ¹⁸O of precipitation and soil moisture in central/eastern North America, via excitation of the Atlantic Multidecadal Oscillation. Monsoon Asia and Australia also exhibit strong precipitation and soilδ¹⁸O anomalies; in these cases,δ¹⁸O may reflect changes to El Niño‐Southern Oscillation phase following eruptions. Salinity and seawaterδ¹⁸O patterns demonstrate the importance of both local hydrologic shifts and the phasing of the El Niño‐Southern Oscillation response, both along the equator and in the subtropics. In all cases, the responses are highly sensitive to eruption latitude, which points to the utility of isotopic records in constraining aerosol distribution patterns associated with past eruptions. This is most effective using precipitationδ¹⁸O; all Southern eruptions and the majority (66%) of Northern eruptions can be correctly identified. This work thus serves as a starting point for new, quantitative uses of isotopic records for understanding volcanic impacts on climate.

    

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5. Dipole Response of Millennial Variability in Tropical South American Precipitation and δ18Op during the Last Deglaciation. Part I: Rainfall Response

   https://doi.org/10.1175/JCLI-D-22-0172.1  

   Bao, Yuntao ; Liu, Zhengyu ; He, Chengfei ( July 2023 , Journal of Climate)

   Oxygen isotope speleothems have been widely used to infer past climate changes over tropical South America (TSA). However, the spatial patterns of the millennial precipitation and precipitationδ¹⁸O (δ¹⁸O_p) response have remained controversial, and their response mechanisms are unclear. In particular, it is not clear whether the regional precipitation represents the intensity of the millennial South American summer monsoon (SASM). Here, we study the TSA hydroclimate variability during the last deglaciation (20–11 ka ago) by combining transient simulations of an isotope-enabled Community Earth System Model (iCESM) and the speleothem records over the lowland TSA. Our model reasonably simulates the deglacial evolution of hydroclimate variables and water isotopes over the TSA, albeit underestimating the amplitude of variability. North Atlantic meltwater discharge is the leading factor driving the TSA’s millennial hydroclimate variability. The spatial pattern of both precipitation andδ¹⁸O_pshow a northwest–southeast dipole associated with the meridional migration of the intertropical convergence zone, instead of a continental-wide coherent change as inferred in many previous works on speleothem records. The dipole response is supported by multisource paleoclimate proxies. In response to increased meltwater forcing, the SASM weakened (characterized by a decreased low-level easterly wind) and consequently reduced rainfall in the western Amazon and increased rainfall in eastern Brazil. A similar dipole response is also generated by insolation, ice sheets, and greenhouse gases, suggesting an inherent stability of the spatial characteristics of the SASM regardless of the external forcing and time scales. Finally, we discuss the potential reasons for the model–proxy discrepancy and pose the necessity to build more paleoclimate proxy data in central-western Amazon.

   We want to reconcile the controversy on whether there is a coherent or heterogeneous response in millennial hydroclimate over tropical South America and to clearly understand the forcing mechanisms behind it. Our isotope-enabled transient simulations fill the gap in speleothem reconstructions to capture a complete picture of millennial precipitation/δ¹⁸O_pand monsoon intensity change. We highlight a heterogeneous dipole response in precipitation andδ¹⁸O_pon millennial and orbital time scales. Increased meltwater discharge shifts ITCZ southward and favors a wet condition in coastal Brazil. Meanwhile, the low-level easterly and the summer monsoon intensity reduced, causing a dry condition in the central-western Amazon. However, the millennial variability of hydroclimate response is underestimated in our model, together with the lack of direct paleoclimate proxies in the central-west Amazon, complicating the interpretation of changes in specific paleoclimate events and posing a challenge to constraining the spatial range of the dipole. Therefore, we emphasize the necessity to increase the source of proxies, enhance proxy interpretations, and improve climate model performance in the future.

    

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