Underwater sinkhole sediments sequester Lake Huron’s carbon

Abstract

Lake Huron’s submerged sinkhole habitats are impacted by high-conductivity groundwater that allows photosynthetic cyanobacterial mats to form over thick, carbon-rich sediments. To better understand nutrient cycling in these habitats, we measured the stable isotopic content of carbon and nitrogen in organic and inorganic carbon pools in Middle Island sinkhole, a ~23 m deep feature influenced by both groundwater and overlying lake water. Two distinct sources of dissolved CO2 (DIC) were available to primary producers. Lake water DIC (δ 13C = −0.1 ‰) differed by +5.9 ‰ from groundwater DIC (δ 13C = −6.0 ‰). Organic carbon fixed by primary producers reflected the two DIC sources. Phytoplankton utilizing lake water DIC were more enriched in 13C (δ 13C = −22.2 to −23.2 ‰) than mat cyanobacteria utilizing groundwater DIC (δ 13C = −26.3 to −30.0 ‰). Sinkhole sediments displayed an isotopic signature (δ 13C = −23.1 ‰) more similar to sedimenting phytoplankton than the cyanobacterial mat. Corroborated by sediment C/N ratios, these data suggest that the carbon deposited in sinkhole sediments originates primarily from planktonic rather than benthic sources. 210Pb/137Cs radiodating suggests rapid sediment accumulation and sub-bottom imaging indicated a massive deposit of organic carbon beneath the sediment surface. We conclude that submerged sinkholes may therefore act as nutrient sinks within the larger lake ecosystem.