SIZE MATTERS! (Or the crucial importance of small foraminifera in interpreting tsunami sediments)

Benthic foraminiferal studies were hardly comparable for several decades because of the absence of standardised size criteria. Actually, sample wash and foraminifera investigations in different studies addressed >63µm, >125µm, >150µm or even >250µm fractions. The turning point arrived with Schröder et al. (1987) and Sen Gupta et al. (1987). Both reported significant loss in the foraminifera and species abundances in the >125µm fraction, when compared with the >63µm. Dominant species in oceanic environment became non-significant or disappear, and the larger sieves record became obviously less informative. Schönfeld et al. (2012) consider that >125µm is adequate for ecological monitoring but point that, in some environments, to prevent losing smaller species and juveniles it is required to use the >63µm fraction. Recently, a worrying trend argues that solely the >150μm residue should be investigated to save time, even if it results on assemblages bias. Such trend represents an unacceptable step back. In fact 1) the analysis of coarser fractions reduces representativity of small, but relevant, adult species, effectively biasing both the associations and interpretations, 2) up to 50% (in cases 99%) of foraminiferal fauna may be lost, 3) this constrains comparison with published research and jeopardizes future work and 4) the contribution of juveniles (regardless of their identification) for sedimentary dynamic interpretations is lost. This is clearly the case of foraminiferal studies on tsunami deposits, where small species and juveniles often represent an important proxy to understand tsunami flow dynamics. For instance, in the Algarve 1755AD tsunami deposits juveniles represent up to 22% of the assemblage (e.g. Quintela et al., 2016).

Furthermore, >150µm fraction does not correspond to any Wentworth’s grain-size classes, precluding correlation between foraminifera and sediment textural features in tsunami deposits analysis (e.g., Hawkes et al., 2007;Mamo et al., 2009; Pilarczyk et al., 2019). Consequently it must be assumed that foraminiferal research is a time consuming task, and that “Yes, size matters!” thus small foraminifera cannot be disregarded and fraction >63µm should be mandatory in multiproxy analyses.

 

Authors acknowledge the financial support of FCT through projects OnOff – PTDC/CTAGEO/28941/2017 and  UIDB/50019/2020–IDL.

Hawkes, AD et al. (2007). Sediments deposited by the 2004 Indian Ocean Tsunami along the Malaysia-Thailand Peninsula. Marine Geology 242, 169-190.

Mamo, B et al (2009). Tsunami sediments and their foraminiferal assemblages. Earth-Science Reviews 96, 263-278.

Pilarczyk, J et al. (2019).Constraining sediment provenance for tsunami deposits using distributions of grain size and foraminifera from the Kujukuri coastline and shelf, Japan. Sedimentology doi: 10.1111/sed.12591

Quintela, M et al. (2016). The AD 1755 tsunami deposits onshore and offshore of Algarve (south Portugal): Sediment transport interpretations based on the study of Foraminifera assemblages. Quaternary International, 408: 123-138.

Schönfeld, J and FOBIMO group (2012). The FOBIMO (FOraminiferal BIo-MOnitoring) initiative—Towards a standardized protocol for soft-bottom benthic foraminiferal monitoring studies. Marine Micropaeontology 94-95, 1-13.

Schröder, CJ et al. (1987). Can smaller benthic foraminifera be ignored in Paleoenvironmental analysis? Journal of Foraminiferal Research 17, 101-105.

Sen Gupta, BK et al. (1987). Relevance of specimen size in distribution studies of deep-sea benthic foraminifera. Palaios 2, 332-338.