Coral microatoll based sea level records from the Sunda shelf
Majewski, Jędrzej Marcin
Date of Issue2018
Interdisciplinary Graduate School (IGS)
Earth Observatory of Singapore
Coral microatolls are coral colonies which grow within the tidal zone and which are periodically limited in their upward growth by exposure during low water levels such as lowest astronomical tides, sea-level anomalies caused by weather or oceanographic phenomena. Vertical constraints on growth cause coral microatolls to develop a dead upper surface with growth continuing predominantly horizontally, thus forming characteristic disc shaped colonies. Coral microatolls are an important sea-level proxy. They have been used to reconstruct Holocene relative sea-level (RSL) change, land-level changes related to the earthquake deformation cycle and oceanographic processes such as the El Niño/Southern Oscillation. Yet not all aspects of the coral microatoll methodology have been fully resolved. There are few studies that define the relationship between coral microatolls and sea level. In particular, ponding, a process whereby a microatoll is separated from the low tides behind a rampart of stone, shingle or sand, remains a source of uncertainties within microatoll reconstructions. In my thesis, I have studied RSL of 25 coral microatolls from 8 sites in Indonesia and Malaysia, from the Sunda Shelf to address four research questions. First, I investigated the accuracy of coral microatoll RSL reconstructions and reliability. I demonstrate that open ocean microatolls track sea level and may be used to reconstruct sea-level trends. Ponded microatolls are only partially influenced by the tidal cycle and cannot be used to recover sea level trends. Second, I applied the coral microatoll methodology to reconstruct RSL from Mapur and Belitung using living, open ocean coral microatolls. Relative sea-level trend reconstructed from coral microatolls in Mapur (1986 to 2005) shows agreement with mean sea-level trend (1972 to 2016) reconstructed from the available tide gauge data in Singapore and Malaysia. A ninety year long (1915-2005) open-water microatoll record from Mapur shows RSL change of 0.01 ±0.89 mm/yr. In Belitung two openwater microatolls show a range from 1.6 ±2.11 to 4.7 ±2.16 mm/yr over the period from 1967 to 2005. Third, I reconstructed Holocene RSL from western Sarawak, Malaysia using surveyed and U-Th dated fossil microatolls. RSL data from western Sarawak was compared the data to a glacio-isostatic adjustment (GIA) model for the region that was calibrated to a dataset just from Thailand and Malay Peninsula. My data shows that RSL rose to 1.45 ±0.8 m above present by 7439 BP and remained stable until 7006 BP. After a hiatus RSL is reconstructed from younger coral microatolls at 2.3 +1.15/-0.8 m above present between 6128 and 6042 BP. My comparison shows a disagreement between the RSL reconstructions from coral microatolls and the GIA model. I infer up to 1.4 meter vertical motion post 7000 yr. BP on the Serabang fault, previously considered inactive. The motion is suggested by the offset between microatolls on the Talang Islands and the Satang Islands either side of it. Finally, I reconstruct 115 years of a mid-Holocene RSL from Mapur, Indonesia by combining seven coral microatoll sequences from two sites on the Island. They demonstrate RSL reached ~0.4 m above present by 7282 cal yr BP and rose at least a further ~0.5 m over the following 115 years. Here I also define Sea-level index sequences, a data format more suitable to continuous RSL records such as those reconstructed from coral microatolls. This thesis contributes to our understanding of coral microatoll methodology, and demonstrates the utility of coral microatolls for long composite sequences of relative sea level. The thesis also contributes RSL records from the Sunda Shelf, in the recent pre- instrument past, and in the mid Holocene.