ALLOCHTHONOUS EOLIAN ADDITIONS TO GRANITOID-HOSTED SURFACES IN THE MEDITERRANEAN

Several studies have suggested that Critical Zone weathering of even coarse-grained (e.g. granitoid) bedrock produces abundant silt. Here, we test this hypothesis on granitoid-hosted weathering profiles in Mediterranean regions of southern France and northern Spain—regions susceptible to autochthonous dust influx from the Saharan region.

We selected four localities hosting surface exposures of granitoid bedrock, as granitoids are typically dominated by crystals greatly exceeding silt size. At each locality we identified a road cut exposure and excavated to obtain fresh samples collected in 10 cm vertical intervals from the surface to one meter depth. We also collected fresh and weathered bedrock from each locality. Additionally, two dust traps were deployed over a ten-month period near the site of one of the weathering profiles in north coastal Spain to characterize windblown detrital dust influx to the area. All samples were sieved to obtain gravel-sand-mud fractions, and the mud fractions were treated with buffered acetic acid for 24 hours to remove carbonates, and 30% hydrogen peroxide for 48 hours to remove organics. The mud fractions were then analyzed using a Malvern Mastersizer 3000 Laser Size Particle Analyzer (LPSA) after adding dispersant. Geochemical analysis was done on the mud fractions from the weathering profiles and dust traps, as well as the bedrock samples, using ICP-MS.

Granulometric analyses of weathering profile samples reveal that the majority of sediment is gravel (8-68%) and sand (28-86%), with limited mud (2-13%). The modes of the mud are typically 20-40 µm (silt), and profiles generally fine upward. Similarly in comparison, the primary modes of the mud from the dust traps are 29.5 µm and 34 µm. Geochemical analyses reveal that most samples are generally depleted in mobile elements. Ratios of generally immobile elements normalized to Zr or Ti are also relatively depleted; however Zr, when normalized to Ti, shows up-profile enrichment. Using the same technique on the dust traps, analyses also revealed the dust traps are relatively depleted in mobile and immobile elements as well. Upward increases in silt, and positive mass fluxes of Zr suggest allochthonous additions of fines to these granitoid profiles, rather than production by in situ weathering. These results challenge the hypothesis of significant production of silt by weathering.