Combined Stalagmite and Limpet-Based Holocene Paleoclimate Reconstructions From Coastal Portugal

April 11th, 2011

Few paleoclimate records preserve high-resolution information for the middle Holocene in (near) coastal Portugal, but this region contains many caves within a few kilometers of the coast.  In addition, shells of shallow marine invertebrates record sea surface conditions and some of these were harvested and preserved in archaeological middens.

We attempted to develop a paleoclimate record from coeval, middle Holocene limpets and a single stalagmite from coastal Portugal.  The stalagmite was collected approximately 30 m from the 1 m2 entrance to the shallow but poorly ventilated Glory Hole cave near the town of Nazare.  U/Th dating demonstrated that the stalagmite grew uninterrupted from 8.2-4.2 ka, and stable isotopic analysis revealed multiple episodes of oxygen isotopic variability in excess of 5% in as little as 100 years, changes that are unlikely to reflect shifts in mean annual temperature.  Strong covariance between stalagmite d13C and d18O values argues for non-equilibrium crystallization, and thus we interpret these data as reflecting secondary controls on stalagmite isotopic ratios through kinetic effects and/or the evaporation and 18O-enrichment of cave dripwater.

Comparison with the GISP2 Greenland ice oxygen isotopic time series reveals increases in North Atlantic air temperature that were coincident with increases in evapokinetic influences at Glory Hole, suggesting a possible climatic link between the two sites.  Limpet shells were cracked and in some places degraded, and despite careful preparation and delicate microsampling, we were unable to extract a stable isotopic time series of sufficiently high resolution to define seasonal shallow marine temperature variability.

Megan Denner, ’11
Algona, IA
Majors: Geology, Classical Studies

Sponsor: Rhawn Denniston

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