Foraminifera reveal a shallow nearshore origin for overwash sediments deposited by Tropical Cyclone Pam in Vanuatu (South Pacific)

Tropical cyclone inundation is a major threat to the highly exposed islands of the South Pacific. This vulnerability was highlighted in March 2015 when Tropical Cyclone (TC) Pam made landfall on Vanuatu as a Category 5 storm, impacting coastlines with storm surges that produced high water marks up to 7 m above mean sea level (MSL) and deposited overwash sediments up to 400 m inland. We examined the foraminiferal assemblages contained within TC Pam sediments at two locations in Vanuatu: a mixed-carbonate embayment at Manuro on Efate Island and a volcaniclastic beach at Port Resolution Bay on Tanna Island. At Manuro, the TC Pam sediments were up to 10 cm thick and composed of coarse to medium sand that contained abundant foraminifera (955 to 2015 individuals per 5 cm3) and fragments of corals and mollusks. At Port Resolution Bay, TC Pam sediments were up to 44 cm thick and composed of medium sand-sized volcaniclastics with low to moderate abundances of foraminifera (27 to 206 individuals per 5 cm3). TC Pam sediments could be discriminated from underlying units by a sharp basal contact, an abrupt decrease in organic matter, and an increase in the concentration of foraminifera. Foraminiferal assemblages between the two sites varied in terms of taxonomy and taphonomy. At both sites, the TC Pam assemblage was generally dominated by intertidal (e.g., Amphistegina spp., Baculogypsina sphaerulata, Calcarina mayori, Elphidium spp., Pararotalia spp.) and subtidal (e.g., Peneroplis pertusus, Quinqueloculina spp.) foraminifera that are characteristic of beach, reef flat, and reef crest environments. The TC Pam assemblage at Manuro was characterized by individuals that were dominantly unaltered (i.e., pristine), but also those that showed signs of abrasion (including edge rounded fragments). By contrast, TC Pam sediments at Port Resolution Bay contained fewer unaltered and more corraded (i.e., combined influence of corrosion and abrasion) foraminifera. We compared modern surface foraminiferal distributions with those from TC Pam sediments to assess provenance. Partitioning Around a Medoid (PAM) cluster analysis discriminated six subenvironments within the modern coastal zone: open bay, forereef, reef crest, reef flat, mangrove, and beach. Discrete intervals sampled from TC Pam sediments at Manuro were individually clustered with the surface samples and revealed a shallow nearshore to supratidal (reef crest to beach; − 4.9 to 1.3 m above MSL) source for the sand.