A Dual Speleothem Perspective on Glacial–Interglacial Hydroclimate in Northern Namibia

Northern Namibia lies in a climatically sensitive transition zone, yet its hydroclimate across multiple glacial–interglacial cycles remains poorly constrained. We present a regional speleothem dataset from ten caves in the Otavi Mountains, comprising about 55 speleothems and ~200 U-Th dates, spanning four glacial–interglacial cycles. Vadose speleothem deposition is strongly biased toward glacial periods, with no growth recorded during pre-Holocene interglacials and only limited Holocene deposition restricted to a few caves.

In contrast, submerged caves from the same region document a pronounced rise in groundwater levels beginning around 15 ka BP, providing compelling evidence for increased effective infiltration since the late glacial. The near-absence of vadose speleothem growth during interglacials despite elevated groundwater levels presents a fundamental paradox. We propose that high cave-air pCO₂ during warm interglacial conditions suppressed calcite precipitation in the vadose zone by limiting CO₂ degassing, even under increased recharge. This interpretation is supported by modern observations from caves in the Otavi Mountains today, which exhibit high to very high cave-air CO₂ concentrations and little to no active stalagmite growth.

By integrating vadose speleothems with subaqueous records of groundwater-level change, this dual-archive approach provides a powerful framework to reconstruct Namibia’s paleo-hydroclimate and to disentangle the roles of effective infiltration and cave-atmosphere dynamics on centennial to orbital timescales.