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1. Orbital-pacing of South American Monsoon Strength and Regional Water Balance: New Evidence from Triple Oxygen Isotopes

   Katz, SA; Levin, N E; Rodbell, D T; Abbott, M B; Passey, B; Katz, S (December 2024, 2024 Fall Meeting, AGU)

   none (Ed.)

   Abstract The South American monsoon is central to the continent’s water and energy cycles, however, the relationships between the monsoon, regional water balance, and global climate change is poorly understood. Sediment records at Lake Junín (11°S, 76°W) provide an opportunity to explore these connections over the last 650 ka. Here, we focus on two interglacials, the Holocene (11.7–0 ka) and MIS 15 (621–563 ka), when sediment proxies suggest rapid regional hydroclimate fluctuations occurred. Clumped isotope distributions of lake carbonates reveal that interglacial water temperatures were similar to present, though analytical limitations preclude detecting the small temperature differences expected in the tropics (<2 °C). Combining the reconstructed water temperatures with carbonate oxygen (δ18O) and triple oxygen (Δ′17O) isotope values, we reconstruct precipitation δ18O values and lake water Δ′17O values. Precipitation δ18O values, a proxy of monsoon strength, range from -18.6 to -12.3 ‰ with lower values reflecting a stronger monsoon. Lake water Δ′17O values are -14 to 43 per meg and indicate the extent of lake water evaporation; lower values reflect a higher proportion of evaporation to inputs (i.e., more negative P-E). The precipitation δ18O and lake water Δ′17O values from both interglacials vary with the pacing of local summertime insolation, which follows an orbital pacing. These data document the close connection between Andean water balance, the South American monsoon, and global climate. Further, we analyze the relationship between precipitation δ18O and insolation, and we find that the relationship is consistent among interglacials, suggesting a similar response of the monsoon to orbital forcings over time. In contrast, while lake water Δ′17O and insolation are also correlated during both interglacials, water balance was overall more positive during MIS 15 than the Holocene. This suggests that either other global forcings or local basin dynamics can also contribute to water balance at Lake Junín. Together, these data provide new evidence of the connections between global climate, monsoon strength, and regional water balance. 

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2. Holocene Temperature and Water Stress in the Peruvian Andes: Insights From Lake Carbonate Clumped and Triple Oxygen Isotopes

   https://doi.org/10.1029/2023PA004827  

   Katz, Sarah A; Levin, Naomi E; Abbott, Mark B; Rodbell, Donald T; Passey, Benjamin H; DeLuca, Nicole M; Larsen, Darren J; Woods, Arielle (May 2024, Paleoceanography and Paleoclimatology)

   Abstract Global climate during the Holocene was relatively stable compared to the late Pleistocene. However, evidence from lacustrine records in South America suggests that tropical latitudes experienced significant water balance variability during the Holocene, rather than quiescence. For example, a tight coupling between insolation and carbonate δ¹⁸O records from central Andean lakes (e.g., Lakes Junín, Pumacocha) suggest water balance is tied directly to South American summer monsoon (SASM) strength. However, lake carbonate δ¹⁸O records also incorporate information about temperature and evaporation. To overcome this ambiguity, clumped and triple oxygen isotope records can provide independent constraints on temperature and evaporation. Here, we use clumped and triple oxygen isotopes to develop Holocene temperature and evaporation records from three central Andean lakes, Lakes Junín, Pumacocha, and Mehcocha, to build a more complete picture of regional water balance (P–E). We find that Holocene water temperatures at all three lakes were stable and slightly warmer than during the latest Pleistocene. These results are consistent with global data assimilations and records from the foothills and Amazon basin. In contrast, evaporation was highly variable and tracks SASM intensity. The hydrologic response of each lake to SASM depends greatly on the physical characteristics of the lake basin, but they all record peak evaporation in the early to mid‐Holocene (11,700 to 4,200 years BP) when regional insolation was relatively low and the SASM was weak. These results corroborate other central Andean records and suggest synchronous, widespread water stress tracks insolation‐paced variability in SASM strength. 

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3. Paleoclimate Changes in the Pacific Northwest Over the Past 36,000 Years From Clumped Isotope Measurements and Model Analysis

   https://doi.org/10.1029/2021PA004266  

   Lopez‐Maldonado, Ricardo; Bateman, Jesse Bloom; Ellis, Andre; Bader, Nicholas E.; Ramirez, Pedro; Arnold, Alexandrea; Ajoku, Osinachi; Lee, Hung‐I; Jesmok, Gregory; Upadhyay, Deepshikha; et al (February 2023, Paleoceanography and Paleoclimatology)

   Abstract Since the last glacial period, North America has experienced dramatic changes in regional climate, including the collapse of ice sheets and changes in precipitation. We use clumped isotope (∆₄₇) thermometry and carbonate δ¹⁸O measurements of glacial and deglacial pedogenic carbonates from the Palouse Loess to provide constraints on hydroclimate changes in the Pacific Northwest. We also employ analysis of climate model simulations to help us further provide constraints on the hydroclimate changes in the Pacific Northwest. The coldest clumped isotope soil temperaturesT(₄₇) (13.5 ± 1.9°C to 17.1 ± 1.7°C) occurred ∼34,000–23,000 years ago. Using a soil‐to‐air temperature transfer function, we estimate Last Glacial Maximum (LGM) mean annual air temperatures of ∼−5.5°C and warmest average monthly temperatures (i.e., mean summer air temperatures) of ∼4.4°C. These data indicate a regional warming of 16.4 ± 2.6°C from the LGM to the modern temperatures of 10.9°C, which was about 2.5–3 times the global average. Proxy data provide locality constraints on the boundary of the cooler anticyclone induced by LGM ice sheets, and the warmer cyclone in the Eastern Pacific Ocean. Climate model analysis suggests regional amplification of temperature anomalies is due to the proximal location of the study area to the Laurentide Ice Sheet margin and the impact of the glacial anticyclone on the region, as well as local albedo. Isotope‐enabled model experiments indicate variations in water δ¹⁸O largely reflect atmospheric circulation changes and enhanced rainout upstream that brings more depleted vapor to the region during the LGM. 

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4. Expression of the “4.2 ka event” in the southern Rocky Mountains, USA

   https://doi.org/10.5194/cp-18-1109-2022  

   Liefert, David T.; Shuman, Bryan N. (January 2022, Climate of the Past)

   Abstract. The use of the climatic anomaly known as the “4.2 ka event” as thestratigraphic division between the middle and late Holocene has prompteddebate over its impact, geographic pattern, and significance. The anomalyhas primarily been described as abrupt drying in the Northern Hemisphere atca. 4 ka (ka, thousands of years before present), but evidence of thehydroclimate change is inconsistent among sites both globally and withinNorth America. Climate records from the southern Rocky Mountains demonstratethe challenge with diagnosing the extent and severity of the anomaly.Dune-field chronologies and a pollen record in southeastern Wyoming revealseveral centuries of low moisture at around 4.2 ka, and prominent low standsin lakes in Colorado suggest the drought was unique amid Holocenevariability, but detailed carbonate oxygen isotope (δ18Ocarb) records from Colorado do not record drought at the sametime. We find new evidence from δ18Ocarb in a smallmountain lake in southeastern Wyoming of an abrupt reduction in effectivemoisture or snowpack from approximately 4.2–4 ka, which coincides in timewith the other evidence of regional drying from the southern Rocky Mountainsand the western Great Plains. We find that the δ18Ocarb inour record may reflect cool-season inputs into the lake, which do not appearto track the strong enrichment of heavy oxygen by evaporation during summermonths today. The modern relationship differs from some widely appliedconceptual models of lake–isotope systems and may indicate reduced winterprecipitation rather than enhanced evaporation at ca. 4.2 ka.Inconsistencies among the North American records, particularly in δ18Ocarb trends, thus show that site-specific factors can preventidentification of the patterns of multi-century drought. However, theprominence of the drought at ca. 4 ka among a growing number of sites in theNorth American interior suggests it was a regionally substantial climateevent amid other Holocene variability. 

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5. Profiling interactions between the Westerlies and Asian summer monsoons since 45 ka: Insights from biomarker, isotope, and numerical modeling studies in the Qaidam Basin

   https://doi.org/https://doi.org/10.1130/B35751.1  

   Hou, M.; Zhuang, G.S.; Ji, J.; Xiang, S.; Kong, W.; Cui, X.; Wu, M.; and Hren, M. (December 2020, Geological Society of America bulletin)

   Singer, B.; Jiang, G. (Ed.)

   The Qaidam Basin marks a crucial boundary between the Westerlies and the Asian summer monsoons. Previous studies in the Qaidam Basin have advanced our knowledge of the paleoclimate over glacial to interglacial cycles. However, our understanding of the paleoclimatic sensitivity of the Qaidam Basin to the relative strength of these two climatic driving forces remains limited due to the lack of regional paleoclimatic reconstructions. The Qaidam Basin is proposed as a regional and global eolian dust source during the glacial periods, during which a cold, dry climate is associated with the equatorward shift of the jet stream. On the contrary, paleoshoreline records suggest that a highstand lake stage prevailed in late Marine Isotope Stage 3 (MIS 3) and lasted until 15 ka. To address this conundrum, we have applied an integrated approach to reconstructing the regional paleoclimatic history by combining compound-specific isotope analysis, lake temperature reconstruction, and numerical modeling. Our results show varying paleoclimate associated with the dynamic climate boundary since 45 ka: (1) a wet climate during late MIS 3, when the Asian summer monsoons are strengthened under high summer insolation and penetrate further into Central Asia; (2) a general cold, dry but wetter than at present climate in the Last Glacial Maximum (LGM), when the Asian summer monsoons retreat and the Westerlies become dominant; and (3) three short periods of extreme aridity corresponding to the Younger Dryas and Heinrich 2 and 4 events, when the normal moisture transport via the Westerlies and Asian summer monsoons is interrupted. The numerical modeling supports an increase in the effective precipitation during the LGM due to reduced evaporation under low summer insolation. These results suggest that the Westerlies and Asian summer monsoons alternately controlled the climate in the Qaidam Basin in response to precessional forcing during the late Pleistocene. 

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