Holocene climate variability revealed using geochemistry and Computed Tomography scanning of stalagmites from the North Atlantic Basin

Abstract

The reliable prediction of the future climate change requires high-quality densely distributed climate records that clarify natural climate variability and its recent modulation by anthropogenic impacts. Stalagmites can meet those requirements; however, conservation concerns require new non-destructive techniques to preserve the natural environment of the caves. This thesis discusses two high-resolution Holocene climate reconstructions built using i) geochemical analyses of a Bermudan stalagmite and ii) novel and non-destructive Computed Tomography data from a Spanish stalagmite, as well as iii) supporting cave monitoring data. Considered together, these efforts provide a complementary view of diverse aspects of climate change across the North Atlantic, and provide insights into how anthropogenic CO2 emissions may further affect climate.

Monitoring of environmental conditions took place at Leamington Cave (Bermuda) and Conch Bar Caves (Turks and Caicos Islands). To our knowledge, these are the first long-term monitoring efforts at coastal sites near sea level. The results of continuous monitoring of these two near shore caves in the tropical western North Atlantic are not dissimilar to previous results derived from inland caves. The microclimate and ventilation dynamics of Leamington Cave and Conch Bar Caves are controlled mainly by seasonal temperature differences between outside and cave air, wind, and rainfall totals. However, because hydrological connections with the sea, the microclimate of these near shore caves is also modulated by the seawater temperature, and sea level fluctuations.

Following from the cave monitoring efforts, one stalagmite from Leamington Cave was collected. The 18O of stalagmite BER-SWI-13 provides an annually-resolved rainfall record for the last 700 years. The record suggests that the tropical cyclone activity in Bermuda following the Industrial Revolution has not exceeded its longer-term natural variability. Higher tropical cyclone activity during Bermuda’s pre-colonisation period is associated with more El Niño events and positive NAO phase and reflects the northeast-ward tropical cyclone tracks migration that coincides with the low TC activity in the Caribbean Sea.

Finally, one stalagmite from southern Spain was used in a pilot study to assess the applicability of CT scanning to speleothem-based climate reconstruction. Internal density variability of stalagmite REF-07 (from Refugio Cave) was used to create a time-series record of stalagmite density, which was then linked to climate.
The record reveals Holocene climate variability in southern Iberia, and suggests that the current Mediterranean climate in southern Iberia was initially emplaced at approximately 5.3 ka BP following gradual southward migration of the North Atlantic Subtropical High.