HURRICANES ALTER SHORT-TERM CONCENTRATION OF DETRITAL 10BE,210PBEX, 137CS, AND 7BE, BUT DO NOT CHANGE LONG-TERM EROSION RATE ESTIMATES

Tropical islands are relatively understudied for erosion rates despite them providing important ecosystem services including reefs, biodiversity, and sustenance which are highly susceptible to climate change. Large storms may change the concentration of isotopic indicators of erosion rates and depths inferred through analysis of river sediment including in situ ¹⁰Be (¹⁰Be_i) and meteoric ¹⁰Be (¹⁰Be_m), ²¹⁰Pb_ex, ¹³⁷Cs, and ⁷Be. To test for such dependence on episodic events, we measured these nuclides in sediment collected at 29 sites in 20 watersheds in Dominica before and after Hurricane Maria (2017).

Isotopic concentration in individual pairs of samples varied greatly before and after the storm but median concentration of all samples did not change. Concentration of ¹⁰Be_m in fine grained (<63 µm) sediment, ¹⁰Be_i, and ²¹⁰Pb_ex do not change systematically from Hurricane Maria (concentration after Maria is 0.25 to 2 times concentration before Maria). However, ¹⁰Be_m and ¹³⁷Cs concentration either remained the same or increased in coarse-grained (250-850 µm) samples from the same sites, suggesting some increase in shallow erosion. ⁷Be decreased after Maria, indicating that erosion from Maria was greater than the penetration depth of ⁷Be (~5 cm). The increased ¹⁰Be_i, ¹⁰Be_m, ²¹⁰Pb_ex, and ¹³⁷Cs in post-Maria samples may indicate an increase of shallow erosion in previously stable areas. Overall, we find that the effects of Maria are not systematic; there is no change in the median values suggesting that inferred long-term erosion rates are not biased by large storms.

Erosion rates in Dominica are low (median = 0.082 ± 0.132 mm/yr), increase to the south of the island, and correlate well with elevation but not with basin average slope. The strong orographic precipitation gradient on Dominica suggests that elevation may be a proxy for rainfall. The independence of slope and erosion rates is similar to other tropical locations and may be because basin average slope varies little across the island. ¹⁰Be_m concentrations are highest in the northeast part of the island and correlate best with grain coating concentration and agricultural land use, suggesting that modern land use is affecting erosion in some parts of the landscape. Overall we find that erosion rates on tropical islands may be lower than previously assumed.