Calcareous termite mounds in South Africa are ancient carbon reservoirs

Abstract

Ecosystems that offer carbon sequestration by leaching bicarbonate to groundwater are valuable natural capital. One region that may offer this service is the west coast of South Africa. Over 20 % is covered by soil mounds ("heuweltjies") up to 40 m diameter, 2 m high, inhabited by the southern harvester termite Microhodotermes viator and enriched in soil organic and inorganic carbon and soluble minerals. We aimed to generate radiogenic and stable isotope data for soils and groundwater in a region where these data are absent, to 1) verify the atmosphere-soil-groundwater link, and 2) resolve the timing and pattern of calcite dissolution and water infiltration in the landscape. Results show that soil and groundwater sulfate have the same marine aerosol source. Episodic calcite dissolution in mound centers, which increased during periods of global cooling, has been set against background input of marine aerosols since before the Last Glacial according to radiocarbon (¹⁴C) ages. Our data push back soil organic carbon ¹⁴C ages of inhabited termite mounds to 13-19 ka (kiloannum, thousand years before present), nest carbonate ¹⁴C ages to 33 ka, and mound soil carbonate ¹⁴C ages to 34 ka, making these the oldest active termite features ever dated. These ages are consistent with soil organic carbon and carbonate ¹⁴C ages of regional, non-mound, coastal petrocalcic horizons formed by accumulation of carbonate leached from their overlying aeolian dune fields. Harvesting activities of termites inject younger organic material around nests >1 m deep, leading to continuous renewal of important soil carbon reservoirs at depth. Termite bioturbation increases the system's ability to dissolve carbonate. The central, bioturbated part of the mounds have greater infiltration depths and greater calcite dissolution, whereas surrounding soils experienced more surface runoff. Calcareous termite mounds offer a mechanism to sequester CO₂ through dissolution and leaching of soil carbonate-bicarbonate to groundwater.

Keywords: Radiogenic carbon (14)C; Stable carbonate isotopes; Sulfate isotopes; δ(13)C; δ(18)O; δ(34)S.