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1. Impacts of climate and tree morphology on tree-ring stable isotopes in central Mongolia

   https://doi.org/10.1093/treephys/tpac142  

   Leland, Caroline; Andreu-Hayles, Laia; Cook, Edward R; Anchukaitis, Kevin J; Byambasuren, Oyunsanaa; Davi, Nicole; Hessl, Amy; Martin-Benito, Dario; Nachin, Baatarbileg; Pederson, Neil (December 2022, Tree Physiology)

   Cernusak, Lucas (Ed.)

   Abstract Recent climate extremes in Mongolia have ignited a renewed interest in understanding past climate variability over centennial and longer time scales across north-central Asia. Tree-ring width records have been extensively studied in Mongolia as proxies for climate reconstruction, however, the climate and environmental signals of tree-ring stable isotopes from this region need to be further explored. Here, we evaluated a 182-year record of tree-ring δ13C and δ18O from Siberian Pine (Pinus sibirica Du Tour) from a xeric site in central Mongolia (Khorgo Lava) to elucidate the environmental factors modulating these parameters. First, we analyzed the climate sensitivity of tree-ring δ13C and δ18O at Khorgo Lava for comparison with ring-width records, which have been instrumental in reconstructing hydroclimate in central Mongolia over two millennia. We also compared stable isotope records of trees with partial cambial dieback (‘strip-bark morphology’), a feature of long-lived conifers growing on resource-limited sites, and trees with a full cambium (‘whole-bark morphology’), to assess the inferred leaf-level physiological behavior of these trees. We found that interannual variability in tree-ring δ13C and δ18O reflected summer hydroclimatic variability, and captured recent, extreme drought conditions, thereby complementing ring-width records. The tree-ring δ18O records also had a spring temperature signal and thus expanded the window of climate information recorded by these trees. Over longer time scales, strip-bark trees had an increasing trend in ring-widths, δ13C (and intrinsic water-use efficiency, iWUE) and δ18O, relative to whole-bark trees. Our results suggest that increases in iWUE at this site might be related to a combination of leaf-level physiological responses to increasing atmospheric CO2, recent drought, and stem morphological changes. Our study underscores the potential of stable isotopes for broadening our understanding of past climate in north-central Asia. However, further studies are needed to understand how stem morphological changes might impact stable isotopic trends. 

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2. An abrupt temperature and hydroclimate transition in eastern Africa during Termination V

   Ramirez, B.; Castañeda, I.S.; Salacup, J.M.; Johnson, T.C. (July 2023, XXI INQUA Congress)

   Over the last few million years, Africa’s climate exhibits a long-term drying trend with episodes of high climate variability coinciding with the intensification of glacial-interglacial cycles. Of particular interest, is a shift to drier and more variable conditions noted in the Olorgesailie Formation (Kenya) between 500 and 300 thousand years ago (ka) in which Potts et al. (2018) observed a turnover of ~85% of large-body mammalian fauna to smaller-body related taxa and suggested that the shift was an evolutionary response to better adapt to the changing climate. However, an erosional gap in the Olorgesailie record during this time interval means that the cause of this faunal shift is still an outstanding question. To understand East African climate variability during the Mid-Pleistocene, we analyze Lake Malawi drill core MAL 05–1 (~11ºS, 34ºE) to investigate if a specific climatic event stands out as a possible driver of the dramatic change observed in the East African mammal community. We use organic geochemical proxies including branched glycerol diaklyl glycerol tetraethers (brGDGTs; the MBT′5ME index) andleaf wax carbon and deuterium isotopes to develop high-resolution temperature, vegetation, and precipitation records, respectively, between 600 and 200 ka. Results show an abrupt temperature increase of ~9°C occurring in less than 3000 years during Glacial Termination V, which is the Marine Isotope Stage (MIS) 12 to MIS 11 transition at ~330 ka. Preliminary leaf wax deuterium isotopic values show an enrichment that coincides with deglacial warmings suggesting a shift to more arid conditions during interglacial than in glacial periods. This change from a cold/wet glacial to a warm/dry interglacial contrast with the cool/dry pattern of the Last Glacial Maximum (LGM) in East Africa which transitioned to a warm/wet Holocene. Leaf wax carbon isotopes are presently being analyzed to understand past shifts in C3 vs C4 vegetation type, which can be related to climatic conditions. We propose that the major warming and drying during Termination V in East Africa represents a significant abrupt change in the climate of eastern Africa and was a likely driver of the major faunal turnover noted in the region. 

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3. Orbital Forcing Drives Both the South American Monsoon and Local Water Balance in the Central Andes During Interglacials

   https://doi.org/10.1029/2025GL116249  

   Katz, Sarah A; Levin, Naomi E; Rodbell, Donald T; Abbott, Mark B; Passey, Benjamin H; Katz, Scott A (August 2025, Geophysical Research Letters)

   Abstract South American summer monsoon (SASM) strength tracks insolation on orbital timescales, linking global climate and continental hydrology. However, whether local water availability also responds to global climate forcings is unclear. Here, we present water balance records from Lake Junín, an Andean lake within the SASM domain. Local water balance and SASM strength is inferred from triple oxygen isotopes of lake carbonates during two interglacial periods (Marine Isotope Stage (MIS) 15, 621–563 ka; the Holocene, 11.7–0 ka). We find SASM strength and water balance both follow the precession‐pacing of local summer insolation, with the driest conditions occurring at Lake Junín under weakened SASM conditions (and vice versa). Further, the largest variations occurred during MIS 15, when insolation was more variable than the Holocene. These results suggest that global climate influences South American hydrology on both the local and continental scales, with implications for tropical water resources, the atmospheric greenhouse effect, and ecosystem dynamics. 

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4. Exceptional variability in historical fire regimes across a western Cascades landscape, Oregon, USA

   https://doi.org/10.1002/ecs2.4735  

   Johnston, James D.; Schmidt, Micah R.; Merschel, Andrew G.; Downing, William M.; Coughlan, Michael R.; Lewis, David G. (December 2023, Ecosphere)

   Abstract Detailed information about the historical range of variability in wildfire activity informs adaptation to future climate and disturbance regimes. Here, we describe one of the first annually resolved reconstructions of historical (1500–1900 ce) fire occurrence in coast Douglas‐fir dominated forests of the west slope of the Cascade Range in western Oregon. Mean fire return intervals (MFRIs) across 16 sites within our study area ranged from 6 to 165 years. Variability in MFRIs was strongly associated with average maximum summer vapor pressure deficit. Fire occurred infrequently in Douglas‐fir forest stands seral to mountain hemlock or silver fir, but fire frequency was much shorter than predicted by theory in other forest types. MFRIs within Douglas‐fir stands seral to western hemlock or grand fir ranged from 19 to 45 years, and MFRIs in stands seral to Douglas‐fir ranged from 6 to 11 years. There was little synchrony in fire occurrence or tree establishment across 16 sites separated by 4 km. The lack of synchrony in fire suggests that large, wind‐driven fire events that are often considered to be characteristic of coast Douglas‐fir forests were not an important driver of succession in our study area during the last ~400–500 years. Climate was more arid than normal during fire years in most forest types, but historical fire in stands seral to Douglas‐fir was strongly associated with antecedent moisture and less strongly associated with drought. We interpret the extraordinary tempo of fire we observed in stands seral to Douglas‐fir and the unique climate pattern associated with fire in these stands to be indicative of Indigenous fire stewardship. This study provides evidence of far more frequent historical fire in coast Douglas‐fir forests than assumed by managers or scientists—including some of the most frequent fire return intervals documented in the Pacific Northwest. We recommend additional research across the western Cascades to create a comprehensive account of historical fire in highly productive forests with significant cultural, economic, and ecological importance. 

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5. PP41D-1176 A 0.5-Million-Year Benthic Stable Isotope Record of Climate and Deep North Atlantic Circulation at the Iberian Margin

   Matthews, Macy; McManus, JF; Pallone, CT; Arellano, A (December 2025, American Geophysical Union)

   Climate cycles following the mid-Pleistocene transition include the coldest and longest glaciations of the Quaternary, punctuated by repeated rapid climate oscillations and sharp transitions into exceptionally warm, atmospheric CO₂-rich interglacial intervals. Such dramatic climate shifts reflect major reorganizations of the Earth system, with complex feedbacks between North Atlantic ice sheets and ocean–atmosphere carbon cycling believed to have played a central role in the pacing and amplitude of past climate variability. Yet, the response of the deep-ocean, the Earth’s largest reservoir of exchangeable carbon, to abrupt climate change remains elusive, posing a significant challenge to our understanding of the relationship between deep-ocean circulation and past, present, and future climate changes. The recently recovered sedimentary sequence from International Ocean Discovery Program Site U1587 (37°35′N, 10°22′W, 3479n meters below sea level [mbsl]), positioned in a mixing zone of northernand southern-sourced deep waters, offers a valuable archive for reconstructing orbital- to millennial-scale variations in climate and deepocean structure. This study presents new, high-resolution (~1 kyr) benthic foraminifera δ¹⁸O and δ¹³C records from Site U1587, spanning ~ 800 - 300 ka, with a focus on intense glaciations (Marine Isotope Stage [MIS] 16, 12, 10) and subsequent deglaciations (MIS 15, 11, 9). The U1587 δ¹⁸O record was aligned to the established chronology of the nearby Site U1385 (37°34′N, 10°08′W, 2578 mbsl), providing a refined age model for Site U1587 and enabling calculation of vertical δ¹³C gradients across the water column. Site U1587 consistently recorded more negative δ¹³C than the shallower Site U1385, with an average offset of –0.455‰ overall. The vertical δ¹³C gradient between sites is most pronounced during glacial intervals, reaching –1.64‰ at ~465 ka during MIS 12, and is notably reduced during interglacials, with an average of –0.264‰ in MIS 11. These results suggest enhanced deep-ocean stratification and carbon storage during intense glaciations, while the subsequent gradient collapse during deglaciations underscores the link between abrupt changes in deep-ocean circulation and rapid climate transitions into interglacial states. 

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