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1. Reconstructing Pliocene West Pacific Warm Pool Hydroclimate Using In Situ Microanalyses on Fossil Planktic Foraminifer Shells

   https://doi.org/10.1029/2019PA003772  

   Wycech, J. ; Kelly, D. C. ; Fournelle, J. ; Kitajima, K. ; Kozdon, R. ; Orland, I. J. ( June 2020 , Paleoceanography and Paleoclimatology)

   A controversial aspect of Pliocene (5.3–2.6 Ma) climate is whether El Niño‐like (El Padre) conditions, characterized by a reduced trans‐equatorial sea‐surface temperature (SST) gradient, prevailed across the Pacific. Evidence for El Padre is chiefly based on reconstructions of sea‐surface conditions derived from the oxygen isotope (δ¹⁸O) and Mg/Ca compositions of shells belonging to the planktic foraminiferTrilobatus sacculifer. However, fossil shells of this species are a mixture of multiple carbonate phases—pre‐gametogenic, gametogenic (reproductive), and diagenetic calcites—that formed under different physiological and/or environmental conditions and are averaged in conventional whole‐shell analyses. Through in situ measurements of micrometer‐scale domains within Pliocene‐aged shells ofT. sacculiferfrom Ocean Drilling Program Site 806 in the western equatorial Pacific, we show that the δ¹⁸O of gametogenic calcite is 0.6–0.8‰ higher than pre‐gametogenic calcite, while the Mg/Ca ratios of these two phases are the same. Both the whole‐shell and pre‐gametogenic Mg/Ca records indicate that average early Pliocene SSTs were ~1°C warmer than modern, with present‐day SSTs being established during the latest Pliocene and early Pleistocene (~3.0–2.0 Ma). The measurement of multiple calcite phases by whole‐shell δ¹⁸O analyses masks a late Pliocene to earliest Pleistocene (3.6–2.2 Ma) decrease in seawater δ¹⁸O (δ¹⁸O_sw) values reconstructed from in situ pre‐gametogenic δ¹⁸O and Mg/Ca measurements. Our novel δ¹⁸O_swrecord indicates that sea‐surface salinities in the west Pacific warm pool were higher than modern prior to ~3.5 Ma, which is consistent with more arid conditions under an El Padre state.

    

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2. Monitoring of Sierra Nevada Caves Reveals the Potential for Stalagmites to Archive Seasonal Variability

   https://doi.org/10.3389/feart.2021.781526  

   Wortham, Barbara E. ; Montañez, Isabel P. ; Bowman, Kimberly ; Kuta, Daphne ; Contreras, Nora Soto ; Brummage, Eleana ; Pang, Allison ; Tinsley, John ; Roemer-Baer, Greg ( December 2021 , Frontiers in Earth Science)

   In the southwestern United States, California (CA) is one of the most climatically sensitive regions given its low (≤250 mm/year) seasonal precipitation and its inherently variable hydroclimate, subject to large magnitude modulation. To reconstruct past climate change in CA, cave calcite deposits (stalagmites) have been utilized as an archive for environmentally sensitive proxies, such as stable isotope compositions (δ¹⁸O, δ¹³C) and trace element concentrations (e.g., Mg, Ba, Sr). Monitoring the cave and associated surface environments, the chemical evolution of cave drip-water, the calcite precipitated from the drip-water, and the response of these systems to seasonal variability in precipitation and temperature is imperative for interpreting stalagmite proxies. Here we present monitored drip-water and physical parameters at Lilburn Cave, Sequoia Kings Canyon National Park (Southern Sierra Nevada), CA, and measured trace element concentrations (Mg, Sr, Ba, Cu, Fe, Mn) and stable isotopic compositions (δ¹⁸O, δ²H) of drip-water and for calcite (δ¹⁸O) precipitated on glass substrates over a two-year period (November 2018 to February 2021) to better understand how chemical variability at this site is influenced by local and regional precipitation and temperature variability. Despite large variability in surface temperatures and precipitation amount and source region (North Pacific vs. subtropical Pacific), Lilburn Cave exhibits a constant cave environment year-round. At two of the three sites within the cave, drip-water δ¹⁸O and δ²H are influenced seasonally by evaporative enrichment. At a third collection site in the cave, the drip-water δ¹⁸O responds solely to precipitation δ¹⁸O variability. The Mg/Ca, Ba/Ca, and Sr/Ca ratios are seasonally responsive to prior calcite precipitation at all sites but minimally to water-rock interaction. Lastly, we examine the potential of trace metals (e.g., Mn²⁺and Cu²⁺as a geochemical proxy of recharge and find that variability in their concentrations has high potential to denote the onset of the rainy season in the study region. The drip-water composition is recorded in the calcite, demonstrating that stalagmites from Lilburn Cave, and potentially more regionally, could record seasonal variability in weather even during periods of substantially reduced rainfall.

    

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3. Warm High‐Elevation Mid‐Latitudes During the Miocene Climatic Optimum: Paleosol Clumped Isotope Temperatures From the Northern Rocky Mountains, USA

   https://doi.org/10.1029/2020PA003991  

   Methner, K. ; Mulch, A. ; Fiebig, J. ; Krsnik, E. ; Löffler, N. ; Bajnai, D. ; Chamberlain, C. P. ( June 2021 , Paleoceanography and Paleoclimatology)

   Interrupting a long‐term Cenozoic cooling trend, the Miocene Climatic Optimum (MCO; ca. 17–15 Ma) represents a time interval characterized globally by warmer than present temperatures, lower ice volume, and elevated pCO₂levels. Establishing quantitative Neogene temperature estimates is an important element in the effort to explore the long‐term changes in the carbon cycle and associated climate feedbacks, yet terrestrial temperature records are still sparse. Here, we present a clumped isotope (Δ₄₇) temperature record of the MCO from intermontane basins in the Northern Rocky Mountain (NRM) region. Arikareean (22.7–21.5 Ma) to Barstovian (16.9–14.7 Ma) paleosol carbonates from the Hepburn's Mesa Formation (Montana), supplemented with data from fossil localities in western Idaho. These records yield Δ₄₇‐temperatures ranging from 17°C to 24°C, which are rather warm given the high elevation sites and are further relatively stable (mean of 21 ± 2°C) leading into and during the MCO until ca. 14.7 Ma. At ca. 14.7 Ma, we observe low Δ₄₇‐temperatures (8°C–10°C) concomitantly with elevated Δ₄₇‐temperatures (ca. 22°C). In line with recently suggested climate stability in the NRM region leading into the MCO, our Δ₄₇‐temperature record, combined with carbon isotope (δ¹³C) and reconstructed soil water oxygen isotope (δ¹⁸O_sw) values, indicates rather stable climate and environmental conditions throughout the MCO. Combining available records from inland sites in the western United States (NRM, Mojave region) points to prevailing stable continental climates even during the MCO.

    

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4. Synoptic and Mesoscale Mechanisms Drive Winter Precipitation δ 18 O/δ 2 H in South‐Central Alaska

   https://doi.org/10.1029/2018JD030050  

   Bailey, Hannah L. ; Klein, Eric S. ; Welker, Jeffrey M. ( April 2019 , Journal of Geophysical Research: Atmospheres)

   Measurements of oxygen and hydrogen stable isotopes in precipitation (δ¹⁸O_Pand δ²H_P) provide a valuable tool for understanding modern hydrological processes and the empirical foundation for interpreting paleoisotope archives. However, long‐term data sets of modern δ¹⁸O_Pand δ²H_Pin southern Alaska are entirely absent, thus limiting our insight and application of regionally defined climate‐isotope relationships in this proxy‐rich region. We present and utilize a 13‐year‐long record of event‐based δ¹⁸O_Pand δ²H_Pdata from Anchorage, Alaska (2005–2018,n = 332), to determine the mechanisms controlling precipitation isotopes. Local surface air temperature explains ~30% of variability in the δ¹⁸O_Pdata with a temperature‐δ¹⁸O slope of 0.31 ‰/°C, indicating that δ¹⁸O_Parchives may not be suitable paleo‐thermometers in this region. Instead, back‐trajectory modeling reveals how winter δ¹⁸O_P/δ²H_Preflects synoptic and mesoscale processes in atmospheric circulation that drive changes in the passage of air masses with different moisture sources, transport, and rainout histories. Specifically, meridional systems—with either northerly flow from the Arctic or southerly flow from the Gulf of Alaska—have relatively low δ¹⁸O_P/δ²H_Pdue to progressive cooling and removal of precipitation as it condenses with altitude over Alaska's southern mountain ranges. To the contrary, zonally derived moisture from either the North Pacific and/or Bering Sea retains relatively high δ¹⁸O_P/δ²H_Pvalues. These new data contribute a better understanding of the modern Alaska water isotope cycle and provide an empirical basis for interpreting paleoisotope archives in context of regional atmospheric circulation.

    

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5. Constraints on Marine Isotope Stage 3 and 5 Sea Level From the Flooding History of the Karimata Strait in Indonesia

   https://doi.org/10.1029/2021PA004361  

   Weiss, Thomas L. ; Linsley, Braddock K. ; Gordon, Arnold L. ; Rosenthal, Yair ; Dannenmann‐Di Palma, Stefanie ( September 2022 , Paleoceanography and Paleoclimatology)

   Global mean sea level (GMSL) during intermediate interglacial Marine Isotope Stage 3 (MIS 3) (60–26 ka) has proven difficult to constrain. Paleo‐sea level estimates based on ice margin, modeling, and paleo‐shoreline reconstructions indicate that MIS 3 GMSL was substantially higher than reconstructed from deep‐ocean benthic foraminifera oxygen isotope (δ¹⁸O) and coral records, implying much smaller ice sheets during MIS 3. Here, we use the δ¹⁸O and Mg/Ca chemistry of surface and thermocline dwelling foraminifera in the Sulu Sea in the western Pacific margin to estimate relative changes of the influx of South China Sea surface flow through the Sulu Sea over the last 140 ka. We show that this South China Sea throughflow is controlled in part by changes in GMSL modulating the depth of the 36 m deep Karimata Strait at the southern end of the South China Sea. We constrain maximum allowable GMSL at the beginning and end of MIS 3 to −22 ± 6 and −29 ± 5 m, respectively, and minimum allowable GMSL during interglacial stages MIS 5c and 5a (117–72 ka) to range from −3 ± 8 to −8 ± 8 m and −11 ± 7 to −12 ± 7 m, respectively. Our results constrain MIS 3 GMSL, but do not rule out higher MIS 3 ice margin, modeling, and paleo‐shoreline‐based MIS 3 GMSL estimates or lower coral and seawater δ¹⁸O‐based estimates. Our results favor the highest MIS 5a and 5c GMSL estimates and confirm that the Sunda Shelf served as a land‐bridge for human and megafauna migration during MIS 3 when humans first arrived in Borneo.

    

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