Constraining sediment provenance for tsunami deposits using distributions of grain size and foraminifera from the Kujukuri coastline and shelf, Japan

Abstract

Tsunami deposits preserved in the geological record provide a more comprehensive understanding of their patterns of frequency and intensity over longer timescales; but recognizing tsunami deposits can prove challenging due to post‐depositional changes, lack of contrast between the deposits and surrounding sedimentary layers, and differentiating between tsunami and storm deposition. Modern baseline studies address these challenges by providing insight into modern spatial distributions that can be compared with palaeotsunami deposits. This study documents the spatial fingerprint of grain size and foraminifera from Hasunuma Beach and the Kujukuri shelf to provide a basis from which tsunami deposits can be interpreted. At Hasunuma Beach, approximately 50 km east of Tokyo, the spatial distribution of three common proxies (foraminiferal taxonomy, foraminiferal taphonomy and sediment grain size) for tsunami identification were mapped and clustered using Partitioning Around Medoids cluster analysis. Partitioning Around Medoids cluster analysis objectively discriminated two coastal zones corresponding to onshore and offshore sample locations. Results show that onshore samples are characterized by coarser grain sizes (medium to coarse sand) and higher abundances of Pararotalia nipponica (27 to 63%) than offshore samples, which are characterized by finer grain sizes (fine to medium sand), lower abundances of Pararotalia nipponica (2 to 19%) and Ammonia parkinsoniana (0 to 10%), higher abundances of planktonics (15 to 58%) and species with fragile tests including Uvigerinella glabra. When compared to grain‐size and foraminiferal taxonomy, foraminiferal taphonomy; i.e. surface condition of foraminifera, a proxy not commonly used to identify tsunami deposits, was most effective in discriminating modern coastal zones (identified supratidal, intertidal and offshore environments) and determining sediment provenance for tsunami deposits at Kujukuri. This modern baseline study assists the interpretation of tsunami deposits in the geological record because it provides a basis for sediment provenance to be determined.

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

“…The average silhouette width and percent accuracy (i.e. ecological appropriateness: percentage of samples that clustered in the appropriate subenvironment) for a set of clustered data assists in determining which scenario is most appropriate (Kosciuch et al ., 2018; Pilarczyk et al ., 2020).…”

“…Several studies have documented modern foraminiferal distributions using a combination of taxonomic and taphonomic data (Pilarczyk et al, 2011(Pilarczyk et al, , 2020Strotz et al, 2016;Kosciuch et al, 2018), yet it remains unclear the degree to which large storms and tsunamis impact the distributions and concentrations of the microfossil assemblage, and whether or not they ever recover to pre-disturbance conditions. Modern distributions of microfossil and sediment assemblages are used to determine provenance for overwash deposits because they serve as a baseline for comparison with the palaeorecord (Pham et al, 2017).…”

“…Surface distribution studies document the specific microfossil assemblages of coastal, nearshore and offshore environments (e.g. Strotz et al ., 2016; Culver et al ., 2019; Pilarczyk et al ., 2020), and can often be different than the broad‐scale ecological niches described in atlases that cover larger areas (e.g. Debenay, 2012; Foerderer & Langer, 2018).…”

Diatom (Bacillariophyceae) assemblages in tidal environments of Vancouver Island, British Columbia, Canada

“…2016; Pilarczyk et al . 2020). Hierarchical clustering, one of the most common methods of cluster analyses, enable researchers to make dendrograms without a fixed number of clusters.…”

Tsunami deposits: Present knowledge and future challenges

“…Also focusing on higher resolution studies, Pilarczyk et al . () proved that taphonomy of foraminifera, which is not commonly used in overwash studies, is a useful parameter in constraining sediment provenance and aiding in the interpretation of additional anomalous sand layers. In contrast, foraminiferal taxonomy, a common proxy used to assess overwash deposits, proved to be the less effective parameter in differentiating offshore and onshore zones when used in isolation of other proxies.…”