‘Silica Pills’ is a dissemination initiative promoted by the Silicofossil Group of The Micropalaeontological Society. It is aimed at spreading knowledge of the work of early career and more advanced researchers on siliceous microfossils and related fields, through the regular publication of invited short online articles.

Radiolarians in Costa Rica: from taxonomy to paleoceanography

By Alexis Rodríguez-Jiménez

Polycystine radiolarians play a key role in reconstructing past ocean conditions. Their skeletons, preserved in deep-sea sediments, record valuable information about suface water fertility, temperature and circulation. Costa Rica’s Pacific Ocean offers an exceptional natural laboratory for studying radiolarian fossil record.

Radiolarian research in Costa Rica

Studies on radiolarians in Costa Rican deep-sea sediments began with Baldessin (2012), who identified 116 species from a core collected at 3120 m depth. Later, Sandoval et al. (2017) and Sandoval (2018) analyzed two cores from the southeastern Pacific margin, covering the late Middle Miocene to the Middle Pliocene (~12–3 Ma), revealing 116 species and demonstrating how tectonics and global climate influenced ocean currents and microfossil preservation.

More recently, Rodríguez-Jiménez et al. (2024) examined 13 Holocene cores from the Costa Rican Central Pacific margin and seamounts north of the Cocos Volcanic Ridge. Collected between 659 m and 1873 m depth, these cores yielded 119 radiolarian taxa preserved in excellent condition (Fig. 1), highlighting the region’s taxonomic richness.

Figure 1. Selected Polycystina radiolarians from deep-sea sediments of the Costa Rican Pacific. Scanning electron microscope images taken from Rodríguez-Jiménez et al. (2024). Scale bar = 50 µm. (A) Acrosphaera murrayana (Haeckel, 1887), (B) Siphonosphaera polysiphonia Haeckel, 1887, (C) Hexacontium heracliti (Haeckel, 1887), (D) Theocorythium trachelium (Ehrenberg, 1872), (E) Cladococcus cervicornis Haeckel, 1860, (F) Euchitonia elegans-furcata group (Ehrenberg, 1872), (G) Botryostrobus auritus-australis Nigrini, 1977, (H) Lophospyris pentagona hyperborea (Jørgensen, 1905), (I) Zygocircus productus group Bütschli, 1882, (J) Pterocanium praetextum praetextum (Ehrenberg, 1872).

Study area and research context

My ongoing thesis research focuses on late Quaternary radiolarians (last ~30,000 years) from the Costa Rican Pacific. The sediment cores come from the Pampa Submarina, a region located 200 km to the east from the north coast of Costa Rica (Fig. 2), influenced by the Intertropical Convergence Zone. Despite its scientific value, the area has only recently been explored from a paleoceanographic perspective (Kessler, 2006; Fernández-Álamo & Farber-Lorda, 2006).

Samples were collected during the Octopus Odyssey expedition in 2023 using the ROV SuBastian. This material allows me to investigate surface water fertility, upwelling intensity, and sea surface temperatures—key variables for understanding the Eastern Tropical Pacific’s past and supporting sustainable marine management.

Figure 2. Map showing the location of the Pampa Submarina and the sediment core sites sampled during the Octopus Odyssey expedition (2023) using the ROV SuBastian. Bathymetric contours are shown every 500 m, with thicker lines every 2000 m below sea level. Bathymetric data from GeoMapApp.

First paleoceanographic insights

Although some results are still under review, this research has yielded the first quantitative estimates of sea surface temperatures derived from radiolarians in Costa Rican waters at high resolution (each core was sampled each centimeter).

Reconstructed Late Holocene temperatures are consistent with modern averages and other paleotemperature records from the Eastern Tropical Pacific (Koutavas & Sachs, 2008). A cooling event during the so-called Younger Dryas (~12,900–11,700 years ago) could represent the first quantitative marine evidence of this episode in Costa Rican waters, with a magnitude comparable to terrestrial records from the region (Islebe et al., 1995; Partin et al., 2015).

As with any pioneering study, there are limitations. Future work will focus on developing regional calibrations to strengthen transfer functions. Despite these challenges, the results are promising and demonstrate the potential of radiolarians as paleoceanographic tools in the Eastern Tropical Pacific.

Concluding remarks

Costa Rica’s Pacific Ocean holds great potential for radiolarian-based paleoceanographic research. The combination of exceptional fossil preservation, high species diversity, and a complex oceanographic setting makes this region ideal for reconstructing past climate variability.

About the author

Alexis Rodríguez-Jiménez holds a degree in Geology and is currently M.Sc. student in the Graduate Program in Biology at the University of Costa Rica. His research focuses on Quaternary Polycystina radiolarians preserved in deep-sea sediments (>200 m depth), with an emphasis on taxonomy, systematics, and paleoceanography.

References

Baldessin, E. (2012). Le microplancton siliceux dans le Pacifique oriental tropical: relation entre morphologie, écologie et système des courants océaniques [Unpublished Master’s thesis]. University of Lausanne.

Fernández-Álamo, M. A. & Färber-Lorda, J. (2006). Zooplankton and the oceanography of the eastern tropical Pacific: A review. Progress in Oceanography, 69(2–4), 318–359. https://doi.org/10.1016/j.pocean.2006.03.003

Islebe, G. et al. (1995). A cooling event during the Younger Dryas Chron in Costa Rica. Palaeogeography, Palaeoclimatology, Palaeoecology, 117(1–2), 73–80. https://doi.org/10.1016/0031-0182(95)00124-5

Kessler, W. S. (2006). The circulation of the eastern tropical Pacific: A review. Progress in Oceanography, 69(2–4), 181–217. https://doi.org/10.1016/j.pocean.2006.03.009

Koutavas, A. & Sachs, J. P. (2008). Northern timing of deglaciation in the eastern equatorial Pacific from alkenone paleothermometry. Paleoceanography, 23(4). https://doi.org/10.1029/2008pa001593

Partin, J. et al. (2015). Gradual onset and recovery of the Younger Dryas abrupt climate event in the tropics. Nature Communications, 6(1), 8061. https://doi.org/10.1038/ncomms9061

Rodríguez-Jiménez, A. et al. (2024). Radiolarians (Class Polycystina) in deep-sea sediments from the Pacific Ocean off Costa Rica. Revista de Biología Tropical, 72(S2). https://doi.org/10.15517/rev.biol.trop.v72iS2.61522

Sandoval, M. I. (2018). Miocene to Recent Radiolarians from Southern Pacific Coast of Costa Rica. Revista Geológica de América Central, 58, 115–169. https://doi.org/10.15517/rgac.v58i0.32846

Sandoval, M. I. et al. (2017). Neogene paleoceanography of the eastern equatorial Pacific based on the radiolarian record of IODP drill sites off Costa Rica. Geochemistry, Geophysics, Geosystems, 18, 889–906. https://doi.org/10.1002/2016GC006623