Sebastian is an environmental researcher working at the interface of environmental chemistry, hydrology, climatology, and climate policy. Much of his work is field-based with a focus on hydro-chemical processes in karst landscapes and climate-related ‘proxy’ signals preserved in cave carbonates (e.g., stalagmites). Since 2019, he has been involved in the long-term monitoring of several remote cave and spring systems across Aotearoa New Zealand and the South Pacific island of Niue to help inform local decision-making and support stalagmite-based reconstructions of past climate variability. 

As of September 2025, Sebastian is a Marie Skłodowska-Curie Postdoctoral Fellow in the Climatology and Biosphere group, leading the project GROTTOS (Groundwater and climate through time – modern observations and stalagmite-based recharge estimates on a raised coral atoll). Together with Niuean and international partners, GROTTOS involves extensive hydrological and chemical monitoring of surface environments, caves and groundwater boreholes across Niue. The project aims to quantify spatio-temporal groundwater recharge dynamics and help assess natural and anthropogenic threats to the island’s groundwater quality and sustainability. Using stalagmite-based climate records, GROTTOS further investigates Holocene climate variability and its drivers in the wider South Pacific region.

For more information on GROTTOS: https://cordis.europa.eu/project/id/101205765

• Environmental monitoring
• Aqueous and carbonate chemistry
• Climate variability and extremes
• Groundwater quality and sustainability

• isotopes

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• Perera, M.G.N., Torres-Rojas, D., Höpker, S.N., Olsen, G., Craggs, R., Hartland, A. (2024). Iron-based composites for in-field phosphorus removal from agricultural drainage.  Surfaces and Interfaces, 51, 104566. https://doi.org/10.1016/j.surfin.2024.104566
• Hartland, A., Farant, M., Höpker, S.N., Torres-Rojas, D., Saeed, H., Rivas, A., La Croix, A., Grainger, M., O’Neill, T. (2024). Mines to moana: hydrochemical legacies in a historically mined watershed. Applied Geochemistry, 171, 106104. https://doi.org/10.1016/j.apgeochem.2024.106104
• Höpker, S.N.  (2024).  Organic complexation in karst cave systems:  implications for transition metal incorporation into speleothems. Dissertation, The University of Waikato. https://hdl.handle.net/10289/16496
• Höpker, S.N., Breitenbach, S.F.M., Grainger, M., Stirling, C.H., Hartland, A. (2024). Characterising the decay of organic metal complexes in speleothem-forming cave waters. Geochimica et Cosmochimica Acta, 373, 98-108. https://doi.org/10.1016/j.gca.2024.03.024
• Pearson, A.R., Fox, B.R.S., Hellstrom, J.C., Vandergoes, M., Breitenbach, S.F.M., Drysdale, R.N., Höpker, S.N., Wood, C.T., Schiller, M., Hartland, A. (2024). Warming drives dissolved organic carbon export from pristine alpine soils. Nature Communications, 15(1), 3522. https://doi.org/10.1038/s41467-024-47706-6
• Nava-Fernandez, C., Braun, T., Pederson, C.L., Fox, B.R.S., Hartland, A., Kwiecien, O., Höpker, S.N., Bernasconi, S., Jaggi, M., Hellstrom, J.C., G´azquez, F., French, A., Marwan, N., Immenhauser, A., Breitenbach, S.F.M. (2024). Mid-Holocene rainfall seasonality and ENSO dynamics over the south-western Pacific. The Depositional Record, 10, 176-194.
• Warken, S.F., Kuchalski, L., Schröder-Ritzrau, A., Vieten, R., Schmidt, M., Höpker, S.N., Hartland, A., Spötl, C., Scholz, D., Frank, N. (2022).  The impact of seasonal and event-based infiltration on transition metals (Cu, Ni, Co) in tropical cave drip water. Rapid Communications in Mass Spectrometry, 36(10), e9278. https://doi.org/10.1002/rcm.9278
• Höpker, S.N., Wu, H.C., Lucassen, F., Sadio, O., Brochier, T., Nuworkpor, I.Y., Kasemann, S.A., Merschel, P., Westphal, H. (2022). Sr Isotope Ratios (87Sr/86Sr) in Water and Fish Otoliths as Estuarine Salinity Tracers: Case Studies from Three NW African Rivers. Estuaries and Coasts, 45(6), 1780-1802. https://doi.org/10.1007/s12237-021-01041
• Höpker, S.N., Wu, H.C., Müller, P., Barusseau, J.-P., Vernet, R., Lucassen, F., Kasemann, S.A., Westphal, H. (2019). Pronounced Northwest African monsoon discharge during the mid- to late Holocene. Frontiers in Earth Science, 7, 314. https://doi.org/10.3389/feart.2019.00314