Nearshore microfossil assemblages in a Caribbean reef environment show variable rates of recovery following Hurricane Irma

Abstract

Modern microfossil distributions reflect site-specific habitats and provide an opportunity to assess sediment transport pathways in the nearshore environment. When applied to overwash deposits in the geological record, they provide insight into sediment provenance and transport, factors important for understanding patterns of frequency and intensity of past storms and tsunamis. Modern distribution studies are rare and often the first established ones occur immediately after an overwash event as part of a post-event field survey. This is problematic because it is unclear what effect overwash events have on nearshore microfossil assemblages and what time interval is necessary for them to return to pre-event conditions. This study documents the impacts of Hurricane Irma on nearshore sediments off the coast of Anegada, British Virgin Islands, using distributions of Homotrema rubrum, an encrusting foraminifer with a defined provenance in coral reefs. At four sampling intervals spanning two years, from six months pre-Hurricane Irma to eighteen months after, surface sediment was collected from three transects on the northern and southern shores of the island. Partitioning Around Medoids cluster analysis revealed that Hurricane Irma introduced an influx of wellpreserved fragments into the reef flat and made the sediments more uniform, limiting the foraminifer's utility as a known sediment transport indicator. The mixing of sediments along the two northern transects (reef proximal) persisted for seven to eighteen months before returning to near pre-hurricane conditions. However, the southern transect (absence of reef), where Homotrema rubrum concentrations are significantly less, failed to recover within the time period assessed by this study, indicating a variable recovery period between Atlantic Ocean and Caribbean Sea facing shorelines. Results from this study suggest that a waiting period of at least eighteen months after a major storm is recommended before collecting surface sediment from the nearshore environments of reef-dominated coastlines.

Coastal erosion and sediment reworking caused by hurricane Irma – implications for storm impact on low‐lying tropical islands

“…ferred into offshore environments (below the storm wave base) or alongshore, allowing the nor'easter to transport this sand back onshore. The observations on Anegada are congruent with the study ofBrenner et al (2018) who documented forebeach accretion and widening following a series of consecutive nor'easters, and byCheng & Wang (2019) who reported recovery related to modest storms during the winter season.Assemblages of Homotrema rubrum provide supporting evidence for the nearshore storage of hurricane-eroded sediments and subsequent remobilization by the nor'easter-like storm Mitchell et al (2021). document an increase in well-preserved Homotrema rubrum that was detached from the reef by Hurricane Irma.…”

“…Mitchell et al (2021) report well-mixed and heavily homogenized nearshore sediments following Hurricane Irma on Anegada and therewith support that resuspended and eroded coastal sediments were stored in the shallow water.Sediment resuspension was also observed on St John after the passage of hurricane Earl in 2010(Edmunds & Gray, 2014).Browning et al (2019) conducted bathymetric surveys to document changes in seafloor morphology on St John after the passage of hurricanes Irma, Jose and Maria. They found an average seafloor aggradation of 25 cm (maximum 62 cm).…”

“…In addition, nearshore sediments were resuspended by the storm waves. Mitchell et al (2021) report well‐mixed and heavily homogenized nearshore sediments following Hurricane Irma on Anegada and therewith support that resuspended and eroded coastal sediments were stored in the shallow water.…”

“…Assemblages of Homotrema rubrum provide supporting evidence for the nearshore storage of hurricane‐eroded sediments and subsequent remobilization by the nor'easter‐like storm. Mitchell et al (2021) document an increase in well‐preserved Homotrema rubrum that was detached from the reef by Hurricane Irma. In repeat surveys of the same sample locations spanning 1.5 years after Hurricane Irma impacted Anegada (i.e., bracketing the time period of the nor'easter‐like storm), Homotrema rubrum assemblages were found to contain relatively high concentrations of well‐preserved individuals.…”

“…These sands are slightly coarser compared to the underlying strata as they contain a higher content of shell fragments and the reefal‐dwelling foraminifer Homotrema rubrum (e.g., Pilarczyk & Reinhardt, 2012). The hurricane deposits on Anegada contain well‐preserved nearshore taxa and lack specimen from offshore environments (Mitchell et al, 2021). This indicates that waves during Hurricane Irma were able to detach living Homotrema rubrum from the fringing reef and transport it landward into the reef flat, but material from greater water depths farther offshore was not scoured.…”