A neotropical perspective on the uniqueness of the Holocene among interglacials

Abstract

Understanding how tropical systems have responded to large-scale climate change, such as glacial-interglacial oscillations, and how human impacts have altered those responses is key to current and future ecology. A sedimentary record recovered from Lake Junín, in the Peruvian Andes (4085 m elevation) spans the last 670,000 years and represents the longest continuous and empirically-dated record of tropical vegetation change to date. Spanning seven glacial-interglacial oscillations, fossil pollen and charcoal recovered from the core showed the general dominance of grasslands, although during the warmest times some Andean forest trees grew above their modern limits near the lake. Fire was very rare until the last 12,000 years, when humans were in the landscape. Here we show that, due to human activity, our present interglacial, the Holocene, has a distinctive vegetation composition and ecological trajectory compared with six previous interglacials. Our data reinforce the view that modern vegetation assemblages of high Andean grasslands and the presence of a defined tree line are aspects of a human-modified landscape.

Fig. 1. Map showing the location of Lake Junín relative to other sites mentioned in text and the 2015 coring location (red circle).

The pink circle denotes the sediment core raised in 1996. Black arcuate lines are approximate extent of glaciers during Marine Isotope Stages 2 and 3; downvalley topographic ridges are pre-Marine Isotope Stage 3 moraines. Inset map: circles indicate Andean records discussed in the text: J Junín, F Fúquene, High Plain of Bogotá, T Lake Titicaca. Maps are derived from NASA Shuttle Radar Topography Mission and mapped in Esri ArcGIS Pro.

Fig. 2. Habitat representation around Lake Junín, Peru, based on fossil pollen recovered from core JUN 15.

For taxa assigned to each habitat see Table S1. Aquatic plants, Isöetes and Alnus are excluded from the pollen sum. Source data are provided as a Source Data file.

Fig. 3. Selected fossil pollen taxa from Lake Junín, Peru, showing vegetation changes across seven glacial-interglacial cycles.

Aquatic plants, Isöetes and Alnus are excluded from the pollen sum. Source data are provided as a Source Data file.

Fig. 4. Schematic diagram illustrating vegetation and lake-level changes, and fire histories at the lakes Junín, Fúquene, and Titicaca.

Fúqene shows the most predictable biome response with glacial-interglacial transitions from Paramó to Andean forest. Junín shows some ecological variability but does not go through full biome transitions as seen at Fúqene and does not exhibit the ecological instability of Lake Titicaca. The arrival of people in the terminal Pleistocene influenced Holocene histories at all three sites. All icons courtesy of Nina Witteveen.

Fig. 5. Modern pollen abundance compared with documented plant occurrences across elevation for four Andean trees.

The occurrence of modern Podocarpus, Hedyosmum, Weinmannia and Alnus pollen in samples collected in Peru, Ecuador and Bolivia between 3000 and 4600 m elevation, source relative to the number of occurrences per 100 m vertical increment. Source: Global Biodiversity Information Facility (GBIF.org) accessed August 1st, 2023. Triangles mark the highest elevation record from GBIF. Stars indicate the highest pollen value from the JUN 15 core. Modern pollen data are based on n = 93 ecologically independent samples collected from Peru and Ecuador.

Fig. 6. Changes in paleoclimate proxies from Lake Junín, Peru, relative to changes in ocean temperature across seven glacial cycles.

The relative timing of environmental variables, peaks of charcoal, Podocarpus pollen, relative to the stacked ocean isotope record. Interglacial peaks are marked by colored blocks as in Figs. 1, 2. GI Glacial Index a proxy for regional ice cover, OM Organic matter, high OM is interpreted as peat deposition in a shallow lake, Low CO₃ is a proxy for low runoff. Green dotted line is the maximum value for Podocarpus in MIS 1. Mauve bars highlight periods when Podocarpus in prior interglacials exceeded that of MIS 1. Source data are provided as a Source Data file.

Fig. 7. The trajectory of vegetation changes at Lake Junín, Peru.

Charcoal and Detrended Correspondence Analysis (DCA) Axis 1 scores are plotted. DCA scores are calculated on the entire dataset and standardized by setting to zero at the start of each interglacial. Higher DCA values represent pollen assemblage changes toward landscapes with more arboreal elements and greater productivity. Plots run from 5000 years before the onset of the interglacial through the first 25,000 years of the interglacial. The Marine Isotope Stage (MIS) 1 curve is shown for comparison with each interglacial. Northern Hemispheric (primarily) warm periods, i.e. the LR04 benthic stack data, which is a proxy for deep sea temperature, are plotted for comparison. Source data are provided as a Source Data file.