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1. Pre-Columbian agriculture, fire, and Spanish contact: A 4200-year record from Laguna Los Mangos, Costa Rica

   https://doi.org/10.1177/0959683619862032  

   Johanson, Erik_N; Horn, Sally_P; Lane, Chad_S (July 2019, The Holocene)

   We present a lake-sediment record of pre-Columbian agriculture and fire history from the lowlands of southern Pacific Costa Rica that captures the arrival of maize agriculture at ca. 3360 cal yr BP in the Diquís subregion of the Gran Chiriquí archeological region. Our 4200-year record from Laguna Los Mangos begins 1000 to 2000 years earlier than other lake records from the region and provides the first microfossil and geochemical evidence of vegetation and fire prior to the establishment of maize agriculture. This early portion of the record shows evidence of fire events associated with land clearance or field preparation and maintenance for subsistence activities. Alternatively, these were wildfires ignited unintentionally by people or naturally by lightning or volcanism. Evidence of early maize by ca. 3200 cal yr BP was found at Laguna Zoncho in the southeastern section of the Diquís subregion. Our discovery of early maize agriculture at ca. 3360 cal yr BP in the Laguna Los Mangos watershed in the northwestern portion of the Diquís subregion indicates a rapid adoption of maize agriculture in the region after initial introduction. Pre-Columbian agriculture and fire activity at Los Mangos is nearly continual until historic times, but with a decline after ca. 1170 cal yr BP, coincident with the early Terminal Classic Drought (TCD). We infer a pronounced drying of the lowland environment at Laguna Los Mangos based on a depositional hiatus in the record at ca. 950 during late TCD. Agricultural proxies indicate reduced watershed activity during the ‘Little Ice Age’ following Spanish contact in southern Central America until the 20th century. 

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2. Response of diatom communities to climate and human disturbance: A 4200-year record from Costa Rica

   https://doi.org/10.1177/09596836231197775  

   Bitting, Madeleine; Johanson, Erik N; Haberyan, Kurt A; Horn, Sally P (December 2023, The Holocene)

   This study integrates diatom analysis with existing records of pollen, charcoal, elemental composition, and stable light isotopes to expand upon the 4200-year history of human activity and climate change from Laguna Los Mangos in southern Pacific Costa Rica. We counted diatoms in peroxide-treated samples and analyzed community composition using cluster analysis, revealing four distinct assemblage zones with diatom variability most closely correlated with phosphorus, titanium, and organic content. The earliest assemblage (Zone D, 4150–3430 cal yr BP) was dominated by Encyonema silesiacum and Nitzschia incognita and aligned with a period of deforestation, erosion, and abundant macrophytes. Gomphonema affine proliferated in Zone C (3430–2450 cal yr BP), reflecting increased pH and productivity likely caused by agriculture-induced nutrient loading. We attributed the preservation gap from 3290 to 2970 cal yr BP in Zone C to silica depletion and erosional deposition that induced decline in diatom abundance by diluting valve concentrations in lake sediments. Nitzschia incognita and G. affine became the dominant taxa in Zone B (2450–1400 cal yr BP), likely reflecting eutrophy, increasing conductivity, and drying climate. Dominance of Diadesmis confervacea indicated reduced lake level in Zone A (1400 cal yr BP–modern) at the onset of the Terminal Classic Drought (TCD). A hiatus in the record indicates lake desiccation from 950 to 450 cal yr BP. During the Little Ice Age (LIA), diatoms reflect conditions similar to Zone B indicating increased lake level, circumneutral pH, and eutrophy. Refilling of the lake indicates increased precipitation during the LIA despite evidence of severe regional drought reported at other sites. Variable precipitation during this period likely resulted from the combined effects of Spanish contact, agricultural collapse, forest recovery, and shifts in Atlantic and Pacific climate forcing mechanisms. Overall, the Los Mangos diatom record reflects shallow, slightly alkaline, eutrophic conditions influenced by nutrient enrichment, erosion, and deforestation associated with maize agriculture. 

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3. Holocene geo-ecological evolution of Lower Geyser Basin, Yellowstone National Park (USA)

   https://doi.org/10.1017/qua.2021.42  

   Schiller, Christopher M.; Whitlock, Cathy; Brown, Sabrina R. (January 2022, Quaternary Research)

   Abstract Changes in climate and fire regime have long been recognized as drivers of the postglacial vegetation history of Yellowstone National Park, but the effects of locally dramatic hydrothermal activity are poorly known. Multi-proxy records from Goose Lake have been used to describe the history of Lower Geyser Basin where modern hydrothermal activity is widespread. From 10,300 cal yr BP to 3800 cal yr BP, thermal waters discharged into the lake, as evidenced by the deposition of arsenic-rich sediment, fluorite mud, and relatively high δ 13 C sediment values. Partially thermal conditions affected the limnobiotic composition, but prevailing climate, fire regime, and rhyolitic substrate maintained Pinus contorta forest in the basin, as found throughout the region. At 3800 cal yr BP, thermal water discharge into Goose Lake ceased, as evidenced by a shift in sediment geochemistry and limnobiota. Pollen and charcoal data indicate concurrent grassland development with limited fuel biomass and less fire activity, despite late Holocene climate conditions that were conducive to expanded forest cover. The shift in hydrothermal activity at Goose Lake and establishment of the treeless geyser basin may have been the result of a tectonic event or change in hydroclimate. This record illustrates the complex interactions of geology and climate that govern the development of an active hydrothermal geo-ecosystem. 

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4. Precipitation variability, vegetation turnover, and anthropogenic disturbance over the last millennium in the Atacama highlands of northern Chile (19°S)

   https://doi.org/10.1177/09596836231151834  

   Domic, Alejandra I; de Porras, María Eugenia; Capriles, José M; Zamora-Allendes, Andrés; Ivory, Sarah J; Maldonado, Antonio (May 2023, The Holocene)

   The Late-Holocene history of hydroclimatic variability in the Atacama Desert offers insights into the effects of precipitation and humans on ecosystems in one of the most extremely arid regions of the world. However, understanding the effects of regional precipitation variability in relation to local ecological stressors remains to be fully resolved. Here, we present a pollen-based qualitative precipitation reconstruction derived from fossil rodent middens recovered from two sites near Laguna Roja (LRO; n = 25) and Isluga (ISL; n = 15) in the Atacama highlands (19°S) of northern Chile. At LRO, the fossil pollen record shows multi-centennial hydroclimatic anomalies during the last millennium, with wetter than present phases at 1155–1130, 865–670, and 215–80 cal yrs BP, and similar to present conditions between 1005 and 880 cal yrs BP. In contrast, the ISL record shows a wet phase during 1115–840 cal yrs BP, suggesting that meso-ecological processes were as important in vegetation turnover as regional hydroclimate anomalies. Wetter conditions derived from LRO partially overlap with the Medieval Climate Anomaly (865–670 cal yrs BP) and with the latest part of the Little Ice Age (215–80 cal yrs BP). Furthermore, no strong anthropogenic signal was identified possibly related to the remote location of the records. Palynological diversity analyses evidence increasing diversification of plant communities during wet events at both sites. In correlation to existing regional hydroclimatic records from the Western Andes, our precipitation reconstruction verifies that centennial-scale changes in the strength of the South American Summer Monsoon (SASM) and partial influence of El Niño-like (ENSO) conditions drove vegetation turnover in the Atacama Desert during the last millennium. 

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5. A late Wisconsin (32–10k cal a BP) history of pluvials, droughts and vegetation in the Pacific south‐west United States (Lake Elsinore, CA)

   https://doi.org/10.1002/jqs.3018  

   Kirby, M. E.; Heusser, L.; Scholz, C.; Ramezan, R.; Anderson, M. A.; Markle, B.; Rhodes, E.; Glover, K. C.; Fantozzi, J.; Hiner, C.; et al (February 2018, Journal of Quaternary Science)

   ABSTRACT Continuous, sub‐centennially resolved, paleo terrestrial records are rare from arid environments such as the Pacific south‐west United States. Here, we present a multi‐decadal to centennial resolution sediment core (Lake Elsinore, CA) to reconstruct late Wisconsin pluvials, droughts and vegetation. In general, the late Wisconsin is characterized by a wetter and colder climate than during the Holocene. Specifically, conditions between 32.3 and 24.9k cal a BP are characterized by large‐amplitude hydrologic and ecologic variability. Highlighting this period is a ∼2000‐year glacial mega‐drought (27.6–25.7k cal a BP) during which the lake shallowed (3.2–4.5 m depth). This period is approximately coeval with a Lake Manix regression and an increase in xeric vegetation in the San Bernardino Mountains (Baldwin Lake). The Local Last Glacial Maximum (LLGM) is bracketed between 23.3 and 19.7k cal a BP − a ∼3000‐year interval characterized by reduced run‐off (relative to the glacial), colder conditions and vegetative stability. Maximum sustained wetness follows the LLGM, beginning at 19.7 and peaking by 14.4k cal a BP. A two‐step decrease in runoff characterizes the Lateglacial to Holocene transition; however, the vegetation change is more complex, particularly at the beginning of the Younger Dryas chronozone. By 12.6–12.4k cal a BP, the climate achieved near Holocene conditions. 

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