Abrasion of common tropical reef dweller halimeda : implications for the preservation of atoll islands, a Maldives case study

Abstract

The aim of this thesis was to calculate the abrasion rate for post-mortem disarticulated Halimeda segments collected from a lagoonal reef platform in the south Maldives. Further, to assess the relative taphonomic breakdown signatures of segments through time. Calcifying macroalgae species Halimeda has been long recorded as a significant tropical reef dweller and represent a key contributor to tropical reef sediment complexes, particularly in post-bleaching scenarios due to ecological and environmental shifts from coral to algae dominated tropical reefs. Preliminary data was first collected on the living species assemblages and combined with sediment size data to understand the source and abrasion environments. Counts of 2672 H. macrophysa and H. micronesica living plants present across Kandahalagalaa reef platform, South Huvadhu Atoll, Maldives, on both the north (windward) and south (leeward) reef crest were performed with combines species presence ranging from 65 plants per m2 to 38 plants per m2 respectively. Spatial distributions of Halimeda across the Kandahalagalaa reef were significantly different between north and south sample sites (p = 0.00). Results then analysed 57 sediment samples taken from eight radial transects extending around the island across the reef platform and current beach face through a grain size analysis. Using the Wentworth (1922) sediment size standard there was a general trend of fining grain size from medium and coarse sand with distance from the reef crest to the inner island, with medium sand the main contributor to Kandahalagalaa platform. Sediment ranged from 749.4 µm – 232.8 µm on average at the north reef crest and north east inner lagoon respectively. Kandahalagalaa island sediment is largely composed of medium sand clustering around 250 µm (66.51 % of all sand collected from island zones). Recognisable Halimeda segments were not found incorporated within the island complex, there was also a significant lack of sediment smaller than 90 µm across all sites, with the two finest size classes containing less than 0.19 % of all sediment measured. Using locally sourced sediment as abrasive in tumbling barrel experiments a comprehensive weight reduction and types of taphonomic breakdown occurring through time were then recorded. 2500 hours of swash zone simulated abrasion of Halimeda were performed in total. In contrast to recent research on Halimeda durability under tumbling barrel abrasion studies, plate segments remained in this environment for 500 hours with 82.27 ± 0.45 % of samples remaining at the end of experiments. Post-mortem Halimeda segments were more likely to persist in the reef platform system and is not a soft, or easily erodible material as when abraded using source environment material Halimeda showed remarkable durability and remained largely intact. Types of taphonomic breakdown seen was largely pitting, scalloping, and flaking of individual segments, and occurring across most Halimeda segments by the end of tumbling experiments. This presents an issue where Halimeda segments are persistent but were not found within the associated reef island sediment complex. The pathway between Halimeda breakdown and transport to sediment reservoirs The approach used in this study and findings from this research highlight the importance of replicating likely abrasive environments where in field studies could not be conducted and the importance in identifying natural abrasion rates of species for estimating time between death and deposition, to integration of Halimeda into reef island complexes. This thesis also identifies the need for understanding Halimeda breakdown rates for AMS radiocarbon dating to estimate the age of sediments due to an extended period between disarticulation from plants, the need for identifying the critical pathway Halimeda takes when undergoing abrasion due to a lack of identifiable segments and fine sand and silt stored within the island, as well as understanding the process of recrystallisation and microstructural changes during abrasion.